Metal nanoparticles (Fe, Co, Ni, Ru) stabilised by SNS and NHC ligands. Synthesis, Characterisation and Catalytic Applications

i! C S IC C ONSE J O SUPERIOR DE INVESTIGACIONES C IENTfF ICA S Metal nanoparticles (Fe, Co, Ni, Ru) stabilised by SNS and NHC ligands. Synthesis, Characterisation and Catalytic Applications PABLO MOLINILLO FERNÁNDEZ SEVILLA 2024 INSTITUTO DE INVESTIGACIONES QUÍMICAS Tesis Doctoral Metal nanoparticles (Fe, Co, Ni, Ru) stabilised by SNS and NHC ligands. Synthesis, Characterisation and Catalytic Applications. Pablo Molinillo Fernández Sevilla, 2024 Metal nanoparticles (Fe, Co, Ni, Ru) stabilised by SNS and NHC ligands. Synthesis, Characterisation and Catalytic Applications. por Pablo Molinillo Fernández Memoria presentada para optar al Título de Doctor en Química MOLINILLO Firmado digitalmente por MOLINILLO FERNANDEZ FERNANDEZ PABLO PABLO - - Fecha: 2024.12.04 10:35:06 +01'00' Fdo. Pablo Molinillo Fernández LARA MUÑOZ PATRICIA - Firmado digitalmente por LARA MUÑOZ PATRICIA – Fecha: 2024.12.04 10:32:29 +01'00' Directoras RENDON MARQUIEZ NURIA - Firmado digitalmente por RENDON MARQUIEZ NURIA – Fecha: 2024.12.04 10:30:13 +01'00' Patricia Lara Muñoz Nuria Rendón Márquez Profesora Titular de la Profesora Titular de la Universidad de Sevilla Universidad de Sevilla A mi familia. A quien ya es parte de ella, y a quien llegue en el futuro. I Table of contents Acknowledgments ..................................................................................................... III List of abbreviations ................................................................................................... V Consideraciones generales ...................................................................................... VII Chapter I: General Introduction .................................................................................. 1 Chapter II: Objectives ............................................................................................... 53 Chapter III: Reduction of N2O with hydrosilanes catalysed by Ru∙SNS NPs ........... 57 Chapter IV: 1st row transition metal NPs stabilised by NHC ligands as catalysts for the methanolysis of ammonia-borane ................................................................. 103 Chapter V: Ru and Ni∙MIC NPs as catalysts for chemoselective H/D exchange on hydrides of main group elements ........................................................................ 159 Chapter VI: Conclusions ......................................................................................... 223 VIII El Capítulo III describe la síntesis y caracterización de nanopartículas de rutenio estabilizadas mediante ligandos de tipo SNS (azufre-nitrógeno-azufre), así como su aplicación catalítica en la reducción de óxido nitroso mediante silanos. Este capítulo se encuentra a su vez dividido en tres secciones. En la primera se realiza una breve introducción al sistema estudiado, especificando la aplicación catalítica que se abordará posteriormente. En la segunda sección se desarrollan y discuten los resultados obtenidos, tanto en relación con la síntesis de los ligandos y nanopartículas mencionados, como con el procedimiento catalítico estudiado. Finalmente, en la tercera sección se describen los aspectos experimentales, incluyendo las técnicas empleadas y los protocolos seguidos en cada caso. De manera similar, el Capítulo IV está dedicado al estudio de nanopartículas basadas en metales de la primera serie de transición, concretamente hierro, cobalto y níquel, estabilizadas mediante ligandos de tipo NHC (Carbenos N-Heterocíclicos), que se han empleado como catalizadores en la obtención de hidrógeno mediante metanólisis del aducto de borano del amoniaco. La primera sección de este capítulo consiste en una breve introducción al campo, especificando los distintos procedimientos de obtención de hidrógeno. La segunda sección describe el proceso de síntesis y caracterización de las mencionadas nanopartículas de hierro, cobalto y níquel, así como las diferentes pruebas catalíticas realizadas. Finalmente, la tercera sección explica el procedimiento experimental seguido en cada caso. El Capítulo V trata la síntesis y caracterización de nanopartículas tanto de rutenio como de níquel, estabilizadas por ligandos de tipo MIC (Carbenos Mesoiónicos), así como su aplicación catalítica en la deuteración de hidrosilanos y otros hidruros de elementos de los grupos principales (Ge, Sn, B). Siguiendo la misma estructura descrita para los capítulos anteriores, en la primera sección se introduce el sistema, realizando un breve repaso de los procesos de intercambio isotópico descritos en la bibliografía. En la segunda sección se discuten los resultados obtenidos en la preparación y evaluación del comportamiento catalítico de las nanopartículas de rutenio y las de níquel. Finalmente, en la tercera sección se especifican los procedimientos experimentales empleados. IX Por último, el Capítulo VI recopila las principales conclusiones derivadas de la investigación llevada a cabo en este trabajo. Con la finalidad de optar a la Mención Internacional en el título de Doctor (RD 99/2011; artículo 15), la presente Tesis Doctoral, con excepción de estas Consideraciones Generales, se ha redactado en inglés. Además, como requisito imprescindible para optar a dicha Mención, se realizó en el año 2023 una estancia de tres meses en el grupo de investigación de la Dra. Karine Philippot, perteneciente al “Laboratoire de Chimie de Coordination-Centre National de la Recherche Scientifique” (LCC-CNRS) en Toulouse, Francia. Esta estancia se financió a través del programa iMOVE del Consejo Superior de Investigaciones Científicas y fue supervisada por la Dra. Nuria Romero, Maître de conférences de la Universidad Toulouse III-Paul Sabatier, e investigadora de dicho grupo. La investigación llevada a cabo durante la estancia se enmarca en la utilización de nanopartículas de metales de la primera serie de transición en procesos electrocatalíticos de Water Splitting. Las nanopartículas utilizadas como electrocatalizadores son las descritas en el Capítulo IV de la presente Tesis Doctoral. Los resultados de electrocatálisis se encuentran actualmente en proceso de análisis y no han sido incluidos en esta memoria. Las figuras, tablas, esquemas y referencias bibliográficas se encuentran enumeradas de forma independiente para cada capítulo. Cada una de las fuentes consultadas se encuentran especificadas como notas a pie de página la primera vez que son utilizadas en un capítulo determinado y, de manera adicional, al final de cada capítulo. El análisis mediante Espectroscopía Fotoelectrónica de rayos X (XPS) incluido en los Capítulos III, IV y V de esta memoria ha sido llevado a cabo por la Dra. Florencia Vattier Lagarrigue del Instituto de Ciencia de Materiales de Sevilla (ICMS) y Profesora Asociada del Departamento de Química Inorgánica de la Universidad de Sevilla. De manera similar, la obtención y posterior tratamiento de las imágenes de Microscopía Electrónica de Transmisión de Alta Resolución (HRTEM) presentadas en los Capítulos III y V han sido realizados por el Dr. Bertrand Lacroix, X del grupo Tribología y Protección de Superficies del Departamento de Física Aplicada I de la Universidad de Sevilla, y por la Dra. Ana Beltrán Custodio, Profesora Titular del Departamento de Ingeniería y Ciencia de los Materiales y del Transporte de la Universidad de Sevilla. Las imágenes análogas presentadas en el Capítulo IV han sido realizadas por Vincent Collière, responsable del Servicio de Microscopía Electrónica del “Laboratoire de Chimie de Coordination” de Toulouse, Francia. Parte de los resultados obtenidos durante la realización de esta Tesis Doctoral han sido publicados en revistas científicas del ámbito de la Química, y específicamente de la Nanociencia y la Catálisis. Además, al menos dos artículos se encuentran en proceso de redacción. Los artículos ya publicados se indican a continuación: Reduction of N2O with hydrosilanes catalysed by RuSNS nanoparticles. Pablo Molinillo, Bertrand Lacroix, Florencia Vattier, Nuria Rendón, Andrés Suárez, Patricia Lara, Chem. Commun., 2022, 58, 7176-7179. DOI: 10.1039/d2cc01470j. Ruthenium nanoparticles stabilized by 1,2,3-triazolylidene ligands in the hydrogen isotope exchange of E-H bonds (E = B, Si, Ge, Sn) using deuterium gas. Pablo Molinillo, Maxime Puyo, Florencia Vattier, Bertrand Lacroix, Nuria Rendón, Patricia Lara, Andrés Suárez. Nanoscale, 2023, 15, 14488-14495. DOI: 10.1039/d3nr02637j. Chapter I: General Introduction Chapter I of this PhD thesis serves as a general introduction to the fundamental concepts of nanoscience and nanotechnology, with a focus on the study of nanoparticles. It covers aspects such as their classification, synthesis, characterisation and applications, particularly in the field of catalysis. General Introduction 3 Table of Contents 1.1 Nanoscience and nanotechnology ......................................................................... 5 1.2 Nanoparticles. General aspects ............................................................................. 8 1.2.1 Organic nanoparticles .................................................................................... 9 1.2.2 Carbon-based nanoparticles ......................................................................... 10 1.2.3 Inorganic nanoparticles ................................................................................ 12 1.3 Synthesis of metal nanoparticles ......................................................................... 13 1.3.1 Formation mechanism .................................................................................. 13 1.3.2 Top-down and bottom up methodologies ..................................................... 16 1.3.3 Organometallic approach in nanoparticle synthesis ..................................... 18 1.4 Stabilisation of nanoparticles .............................................................................. 21 1.4.1 Electrostatic stabilisation ............................................................................. 22 1.4.2 Steric stabilisation ........................................................................................ 22 1.4.3 Electrosteric stabilisation ............................................................................. 23 1.4.4 Stabilisation by solid supports ..................................................................... 23 1.4.5 Stabilisation by ligands. ............................................................................... 23 1.5 Characterisation of metal nanoparticles .............................................................. 33 1.5.1 Transmission Electron Microscopy (TEM and HRTEM) ............................... 33 1.5.2 Scanning Transmission Electron Microscopy (STEM)................................ 34 1.5.3 Inductively Coupled Plasma (ICP) ............................................................... 34 1.5.4 X-ray Photoelectron Spectroscopy (XPS) .................................................... 35 1.6 Catalysis by metal nanoparticles ......................................................................... 35 1.7 References ........................................................................................................... 43 General Introduction 5 1.1 Nanoscience and nanotechnology The modern concept of nanoscience was introduced in 1959 during Richard Feynman’s lecture “There’s Plenty of Room at the Bottom” presented at the annual meeting of the American Physical Society at Caltech (California Institute of Technology).1 In this lecture, the author proposed various methods and tool sets to transform individual atoms or molecules into nanoscale materials, including the possibility of building machines at a molecular level or even smaller. Due to these groundbreaking ideas, he is regarded as the father of nanotechnology. A decade and a half later, in 1974, the term of nanotechnology was coined by Norio Taniguchi, discussing the concept in the context of materials processing. He stated that “nanotechnology mainly consists of the processing of separation, consolidation and deformation of materials by one atom or one molecule”.2 Nowadays, it is widely accepted that nanoscience involves the study of structures and molecules at the nanometre scale, typically defined as ranging from 1 to 100 nanometres (1 nm = 10-9 m).3 Nanotechnology, in turn, focuses on the development of practical applications of nanoscience. In that sense, nanoscience is envisaged as a multidisciplinary discipline that covers the areas of physics, chemistry, biology, medicine and material science. It enables the study of unprecedented phenomena occurring at the atomic and molecular level, while nanotechnology is primarily concerned with the controlled assembly of nanomaterials. The importance of nanotechnology lies in the fact that in the nanometric size, materials exhibit different properties compared to their macroscopic counterparts. Nanomaterials, with small size and high surface to volume ratio, exhibit exceptional physical (optical, electrical, mechanical and magnetic) and chemical properties (such as catalytic 1 R. P. Feynman, “There’s Plenty of Room at the Bottom”, Engineering and Science 1960, 23, 22-36. 2 N. Taniguchi, C. Arakawa, T. Kobayashi, “On the basic concept of nano-technology”, Proceedings of the International Conference on Production Engineering, Tokyo, Japan, 26-29 August 1974. 3 S. Bayda, M. Adeel, T. Tuccinardi, M. Cordani, F. Rizzolio, Molecules 2020, 25, 112-126. Chapter I 6 activity).4 These properties can be precisely tuned by controlling the size, shape, synthesis conditions, and appropriate functionalisation of the nanomaterials. Nanomaterials have been used for centuries. Historical examples of nanomaterial use can be found in various ancient civilizations, with the Lycurgus cup being one of the most famous cases (Figure 1). This Roman chalice demonstrates the unique optical properties of gold and silver nanoparticles. Under reflected light, it exhibits a pea-green colour, while illumination with transmitted light, reveals a redwine hue.5 Figure 1. The Lycurgus Cup, British Museum. Image used under non-commercial Creative Commons (CC BY-NC-SA 4.0) license. In the last few decades, nanoscience has attracted the attention of the scientific community and, subsequently, recent advances in preparation and characterisation of nanomaterials have produced a boom in science and industry, contributing to nearly every field of science and technology, including human health, computer science, and catalysts development. The latter will be explored in depth in following sections.3,6,7 Nanomaterials, which are key elements of nanotechnology, exhibit at least one 4 N. Baig, I. Kammakakam, W. Falath, Mater. Adv. 2021, 2, 1821-1871. 5 D. J. Barber, I. C. Freestone, Archaeometry 1990 32, 33-45. 6 S. P. Forster, S. Olveira, S. Seeger, Int. J. Nanotechnol. 2011, 8, 592-612. 7 M. R. Axet, K. Philippot in Nanoparticles in Catalysis: Advances in Synthesis and Applications, Chapter 4, 73-97, K. Philippot, A. Roucoux (Eds), Wiley-VCH, 2021. 7 General Introduction dimension measuring between 1 and 100 nm, and can be classified based on various criteria, including their dimensions, shape, and structure. In terms of the number of dimensions on the nanoscale, nanomaterials can be classified into four categories, as represented in Figure 2.8,9 Figure 2. Nanomaterials classified in 0D, 1D, 2D and 3D categories. Zero-Dimension nanomaterials (0D): These nanomaterials possess all dimensions at the nanoscale. Classical examples include nanoparticles, fullerenes, and quantum dots.8 One-Dimension nanomaterials (1D): Nanomaterials in this category have two nanoscale dimensions and the third one in the microscale. They include structures such as nanofibers, nanotubes, nanorods, and nanowires.10 Two-Dimension nanomaterials (2D): These nanomaterials have only one dimension in the nanoscale. Graphene is perhaps the most representative example in this category, which also includes materials like nanosheets or nanolayers.11,12 8 N. Joudeh, D. Linke, Journal of Nanobiotechnology 2022, 20, 262. 9 J-H. Son, Y-U. Kwon, Bull. Korean. Chem. Soc 2001, 22, 1224-1230. 10 J. J. Ramsden in Nanotechnology: An Introduction, Chapter 6, 101-124, J. J. Ramsden (Ed), William Andrew Publishing, 2011. 11 P. Feng, Y. Kong, M. Liu, S. Peng, C. Shuai, Materials Today Nano 2021, 15, 100127. 12 T. Imae in Nanolayer Research: Methodology and technology for green chemistry, Chapter 1, 1-34, T. Imae (Ed), Elsevier, 2017. Chapter I 14 In stage I, the concentration of monomer increases over time because of the metal precursor decomposition until the solution becomes saturated, and reaches the supersaturation concentration (Cmin). At this point, individual atoms start to aggregate homogeneously, but no nanoparticles are present in suspension. In stage II, the monomer concentration exceeds the supersaturation level, triggering the nucleation of nanoparticles and a corresponding decrease in the monomer concentration to values below Cmin. From stage III onward, no new particle nucleation occurs, only the growth of existing nanoparticles continues until the precursor is exhausted. In this context, the maximum particle size is determined by the amount of precursor employed and the number of nuclei generated in stage II. The LaMer mechanism is used to explain reactions that take place in closed systems, where the number of nanoparticles formed strongly depends on the nucleation process. Typically, nanoparticles that follow the LaMer mechanism present a wide size distribution due to the complex nucleation and growth steps, where the coalescence of two nuclei can lead to a loss of monodispersity. In recent years, a modification of the original LaMer mechanism has been proposed by Huber et al., represented in Figure 10.35 Figure 10. Extended LaMer mechanism, adapted from Huber et al. 35 E. C. Vreeland, J. Watt, G. B. Schober, B. G. Hance, M. J. Austin, A. D. Price, B. D. Fellows, T. C. Monson, N. S. Hudak, L. Maldonado-Camargo, A. C. Bohorquez, C. Rinaldi, D. L. Huber, Chem. Mater. 2015, 27, 6059-6066. General Introduction 15 This proposed mechanism, known as the Extended LaMer mechanism (Figure 10), suggests that if the reaction occurs in an open system with continuous addition of precursor, stages I and II remain unchanged. However, the concentration of monomer decreases in a modified stage III until it stabilises over time. This introduces a new stage IV, where nanoparticles growth occurs, allowing for a better control over size distribution. The LaMer mechanism and its derivatives are not the only mechanisms proposed to explain nanoparticle formation, as mentioned at the beginning of this section. Finke et al. described a two-step mechanism in which a slow nucleation process, initiated by species A (the monomer), is followed by a faster autocatalytic surface growth of species B (the nanoparticle), as illustrated in Scheme 1.36 Although this method does not align with the classical nucleation concepts of the LaMer mechanism, both prioritise understanding nanoparticle growth through monomer attachment in solution. Scheme 1. Nanoparticles formation according to the two-step mechanism proposed by Finke et al. Considering the different proposed mechanisms, nanoparticles formation can be simplified into two essential steps: nucleation and growth. Both steps can be influenced by factors such as temperature, time and precursor employed. In this context, the use of preparation methodologies that enables the reproducible formation of nanoparticles in terms of size, shape, and surface state is crucial.37 Some examples of existing methodologies for nanoparticle preparation will be discussed in the next section. 36 M. A. Watzky, R. G. Finke, Chem. Mater. 1997, 9, 3083-3095. 37 M. Sajid, J. Plotka-Wasylka, Microchem. J. 2020, 154, 104623. Chapter I 16 1.3.2 Top-down and bottom-up methodologies Metal nanoparticles can be synthesised through various routes, which can be classified into two main groups: top-down and bottom-up methodologies (Figure 11). In top-down methodologies (Figure 11a), a bulk material, typically a solid, is miniaturised using physical techniques until it reaches the nanoscale. These topdown techniques are considered destructive approaches to nanofabrication. While they are well-suited for large-scale production of nanoparticles, they may not be ideal for producing very uniform, regular and small particles. Some examples of techniques employing a top-down approach include nanolithography, sputtering deposition and laser ablation.38 Figure 11. Schematic representation of top-down (a) and bottom-up (b) methodologies for nanoparticle synthesis. Nanolithography: This group of techniques enables the modification of a substrate at the nanoscale using patterns derived from a template.39,40 Nanolithography is commonly used to prepare structures around 20 nm, while sizes below 10 nm can 38 N. Abid, A. M. Khan, S. Shujait, K. Chaudhary, M. Ikram, M. Imran, J. Haider, M. Khan, Q. Khan, M. Maqbool, Adv. Colloid Interface Sci. 2022, 300, 102597. 39 D. Wouters, U. S. Schubert, Angew. Chem. Int. Ed. 2004, 43, 2480-2495. 40 D. Resnick in Nanolithography: The Art of Fabricating Nanoelectronic and Nanophotonic Devices and Systems, Chapter 9, 315-347, M. Feldman (Ed), Woodhead Publishing, 2014. General Introduction 17 be achieved employing specific techniques such as Extreme Ultraviolet Interference Lithography (EUV-IL).41 Sputtering deposition: In this process, a target surface is bombarded with ions in the gas phase, resulting in the physical expulsion of small particles.42 This technique has been used to prepare stable colloidal metal nanoparticles in ionic liquids, showing a clear size dependence based on the ionic liquid employed.43 Laser Ablation: In this technique, a laser serves as energy source to remove surface atoms from a solid. When the laser is focused on a specific spot, the temperature rapidly increases, leading to localised vaporisation of the starting material. Subsequent collisions between the vaporised species (free atoms, molecules, or ions) create a plasma plume at very high temperature (> 5000 K) which is then quenched to room temperature. Nucleation of the saturated vapor ultimately results in the formation of nanosized particles.44 On the other hand, bottom–up techniques (Figure 11b) use very small particles (free atoms or molecules) as building blocks to form nanoparticles.37 This construction occurs in a controlled manner, enabling the production of nanoparticles that are homogeneous in size, shape and surface state. Some examples of bottom-up synthesis procedures include chemical vapour deposition (CVD), metal salt reduction, sol-gel synthesis and the organometallic approach. The latter method is the one selected for the preparation of metal nanoparticles in this PhD thesis. Chemical Vapour Deposition/Chemical Vapour Synthesis: In CVD techniques, thin films are prepared on a substrate via chemical reactions involving precursors released in the gas phase.45 However, nanoparticles can also be produced 41 W. Karim, S. A. Tschupp, M. Oezaslan, T. J. Schmidt, J. Gobrecht, J. A. van Bokhoven, Y. Ekinci, Nanoscale 2015, 7, 7386-7393. 42 H. Wender, P. Migowski, A. F. Feil, S. R. Teixeira, J. Dupont, Coord. Chem. Rev. 2013, 257, 2468-2483. 43 T. Torimoto, K. Okazaki, T. Kiyama, K. Hirahara, N. Tanaka, S. Kuwabata, Appl. Phys. Letters 2006, 89, 243117. 44 E. Mzwd, N. M. Ahmed, N. Suradi, S. K. Alsaee, A. S. Altowyan, M. A. Almessiere, A. F. Omar, Scientific Reports 2022, 12, 10549-10559. 45 L. Sun, G. Yuan, L. Gao, J. Yang, M. Chhowalla, M. H. Gharahcheshmeh, K. K. Gleason, Y. S. Choi, B. H. Hong, Z. Liu, Nat. Rev. Methods Primers 2021, 1, 5-24. 18 Chapter I by adjusting specific reaction conditions such as very high temperatures, high partial pressures of monomers, the use of small molecules as substrates and long residence times of the vapour in the reactor. These longer residence times can be achieved by employing low gas flows or using long reactors. Under these conditions, the technique is known as Chemical Vapour Synthesis (CVS). The mean sizes of the particles prepared through CVS are strongly dependent on the nature of the nanoparticles, and often exhibit significant size dispersity. For example, CVS has been employed to prepare ZnO nanoparticles ranging from 6 to 30 nm, as well as WS2 NPs with mean sizes between 20 and 70 nm.46 Metal Salt Reduction: In this methodology, a metal salt is reduced employing a reducing agent (such as H2, CO, or NaBH4) in the presence of an appropriate stabilising agent. This stabiliser controls the nucleation process of the metal atoms and facilitates the formation of nanoparticles with a narrow size distribution and in high yield.47,48 Sol-Gel: In a typical sol-gel synthesis, a molecular precursor is suspended in water or alcohol, forming a sol rich in colloidal structures. This sol undergoes a hydrolysis/alcoholysis to form a nanoporous structure (gel), which is subsequently calcined to achieve the final nanomaterial.49 1.3.3 Organometallic approach in nanoparticle synthesis The organometallic approach, primarily developed by Chaudret and collaborators,50 is a synthetic procedure based on the decomposition of an organometallic precursor under mild pressure and temperature conditions, and in the 46 C. Dhand, N. Dwivedi, X. J. Loh, A. N. J. Ying, N. K. Verma, R. W. Beuerman, R. Lakshminarayanan, S. Ramakrishna, RSC Adv. 2015, 5, 105003-105037. 47 A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 2002, 102, 3757-3778. 48 Y. Yu, W. Yang, X. Sun, W. Zhu, X. Z. Li, D. J. Sellmyer, S. Sun, Nano Lett. 2014, 14, 27782782. 49 F. Hu, Z. Hu, Y. Liu, K. C. Tam, R. Liang, Q. Xie, Z. Fan, C. Pan, J. Tang, G. Yu, W. Zhang, J. Am. Chem. Soc. 2023, 145, 27718-27727. 50 K. Philippot, B. Chaudret, in Comprehensive Organometallic Chemistry III, R. H. Crabtree & M. P. Mingos (Eds-in-Chief); Applications III: Functional Materials, Environmental and Biological Applications, D. O´Hare (Volume Ed.), Vol 12, Chapter 03, 71-99, Elsevier, 2007. 19 General Introduction presence of a stabilising agent. This stabiliser can be a polymer, an organic ligand, solid supports, or the solvent of the reaction in specific cases. The main advantage of this methodology is the production of metal nanoparticles (MNPs) with wellcontrolled sizes and shapes, and clean surfaces. Scheme 2 summarises this process, distinguishing three steps: a) decomposition of the organometallic precursor by H2 or another reducing agent, releasing naked metallic atoms into the reaction medium, b) nucleation of these naked atoms, and c) growth of the nanoparticles, which is controlled by the stabiliser agent. Scheme 2. Organometallic approach for the synthesis of metal nanoparticles. The use of organometallic complexes containing zero or low-valent metal as precursors facilitates their decomposition under milder conditions compared to metal salts. Various types of complexes can be employed, with olefinic ones being the most common. Under hydrogen pressure, the unsaturated ligands in the coordination sphere of the metal are reduced to alkanes, allowing for the easy release of naked atoms into the medium, even at room temperature.51 These atoms form clusters during the nucleation process and subsequently grow to form MNPs. Stabilising ligands play a key role in the formation of the particles, preventing metal coalescence and enabling the formation of well-controlled nanoparticles in terms of size, shape, and surface 51 C. Amiens, B. Chaudret, D. Ciuculescu-Pradines, V. Collière, K. Fajerwerg, P. Fau, M. Kahn, A. Maisonnat, K. Soulantica, K. Philippot, New J. Chem. 2013, 37, 3374-3401. 20 Chapter I state.52 Additionally, the use of H2 as a reducing agent results in the presence of hydrides on the surface of these organometallic nanoparticles, which significantly influences their reactivity. Different organometallic precursors have been employed in the literature for the preparation of MNPs. Some examples of organometallic precursors that have been successfully used as starting materials include [Pt(dba)2] (dba = dibenzylideneacetone) and [Ru(COD)(COT)] (COD = 1,5-cyclooctadiene; COT = 1,3,5-cyclooctatriene), both of which yield spherical and small nanoparticles (1-2 nm).53,54 Complexes containing aryl or alkyl substituents, such as [Pt(CH3)2(COD)], are more challenging to decompose due to their higher stability. However, this property can be employed to prepare large nanoparticles (25-75 nm) with shapes like cubes or arrows, exposing (100) or (111) faces.55 Carbonyl complexes are also suitable as organometallic precursors for the synthesis of metal NPs. However, since CO acts as both a σ-donor and a π-acceptor ligand, its coordination energy increases, making complete elimination from the metal coordination sphere challenging. Therefore, the synthesis of nanoparticles involving CO complexes as precursors, such as [Fe(CO)5], may require harsher conditions, including high temperatures or the use of ultrasounds, and controlling their size can be complicated. Despite these challenges, [Fe(CO)5] has been successfully utilised in the preparation of nanoparticles with magnetic hyperthermia properties.56 Although not strictly organometallic species, coordination complexes containing amide ligands can also be employed as precursors, as they can be 52 B. Cormary, F. Dumestre, N. Liakakos, K. Soulantica, B. Chaudret, Dalton Trans. 2013, 42, 12546-12553. 53 C. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P. Lecante, M. J. Casanove, J. Am. Chem. Soc. 2001, 123, 7584-7593. 54 A. Rodríguez, C. Amiens, B. Chaudret, M. J. Casanove, P. Lecante, J. S. Bradley, Chem. Mater. 1996, 8, 1978-1986. 55 M. R. Axet, K. Philippot, B. Chaudret, M. Cabié, S. Giorgio, C. R. Henry, Small 2011, 7, 235-241. 56 A. Meffre, B. Mehdaoui, V. Kelsen, P. F. Fazzini, J. Carrey, S. Lachaize, M. Respaud, B. Chaudret, Nano Lett. 2012, 12, 4722-4728. General Introduction 21 decomposed under H2 pressure. Some of the most characteristic complexes in this category include Fe[N(SiMe3)2]2 and Co[N(SiMe3)2]2.52 Ni,57 Ru,58 Pt,59 Pd,60 Co,61 or Ir62 NPs have been successfully obtained applying the organometallic methodology. While the choice of the precursor is essential for determining the nature of the nanoparticles, the stabilising agent employed is crucial, as it was mentioned previously.63 These agents can vary significantly in their nature, composition and stabilisation modes. 1.4 Stabilisation of nanoparticles Nanoparticles, like other nanomaterials, exhibit high surface to volume and surface to mass ratios compared to their bulk materials. As mentioned, the properties of NPs are largely dependent on their size, particularly in catalytic applications. For such purposes, it is desirable to obtain NPs as small as possible; however, smaller particles tend to have low thermodynamic stability and can easily agglomerate, due to Van der Waals forces.64 This behaviour leads to the formation of larger, more stable particles, which consequently lose some of their unique properties. To counter this, the addition of an agent capable of inhibiting the formation of excessively large particles or metal agglomerates is essential for synthesising nanoparticles. Stabilisation modes of nanoparticles are commonly classified into five categories: (a) electrostatic, (b) steric, (c) electrosteric, (d) stabilisation by solid supports, and (e) stabilisation by ligands (Figure 12).47,65 57 D. Bouzouita, J. M. Asensio, V. Pfeifer, A. Palazzolo, P. Lecante, G. Pieters, S. Feuillastre, S. Tricard, B. Chaudret, Nanoscale 2020, 12, 15736-15742. 58 P. Molinillo, B. Lacroix, F. Vattier, N. Rendón, A. Suárez, P. Lara, Chem. Commun. 2022, 58, 7176-7179. 59 P. Lara, K. Philippot, A. Suárez, ChemCatChem 2019, 11, 766-771. 60 N. J. S. Costa, M. Guerrero, V. Collière, E. Teixeira-Neto, R. Landers, K. Philippot, L. M. Rossi, ACS Catal. 2014, 4, 1735-1742. 61 L. M. Martínez-Prieto, J. Marbaix, J. M. Asensio, C. Cerezo-Navarrete, P. Fazzini, K. Soulantica, B. Chaudret, A. Corma, ACS Appl. Nano Mater. 2020, 3, 7076-7087. 62 A. Zuluaga-Villamil, G. Mencia, J. M. Asensio, P. Fazzini, E. A. Baquero, B. Chaudret, Organometallics 2022, 41, 3313-3319. 63 P. Lara, K. Philippot, B. Chaudret, ChemCatChem 2013, 5, 28-45. 64 L. S. Ott, R. G. Finke, Coord. Chem. Rev. 2007, 251, 1075-1100. 65 S. Nath, S. Jana, M. Pradhan, T. Pal, J. Colloid Interface Sci. 2010, 341, 333-352. Chapter I 22 Figure 12. Stabilisation modes of nanoparticles. 1.4.1 Electrostatic stabilisation Electrostatic stabilisation arises from charged particles, such as halides, carboxylates, or similar ions and typically takes place in an aqueous solution. These anions adsorb on the nanoparticle surface, creating a double layer that generates coulombic repulsion between particles. When this coulombic repulsion balances Van der Waals forces, the formation of metal agglomerates is prevented, resulting in a stable colloid.66 An example of this stabilisation mode is the case of Ag2S nanoparticles stabilised by sulphur ions described by Kuznetsova et al.67 1.4.2 Steric stabilisation Steric stabilisation occurs when metal atoms are surrounded by macromolecules such as polymers or dendrimers. The structure of these macromolecules, which contain cavities, facilitates the nucleation process and the 66 S. Anboo, S. Y. Lau, J. Kansedo, P.-S. Yap, T. Hadibarata, A. H. Kamaruddin, Heliyon 2024, 10, e27348. 67 Y. V. Kuznetsova, I. A. Balyakin, I. D. Popov, B. Schummer, B. Sochor, S. V. Rempel, A. A. Rempel, J. Mol. Liq. 2021, 335, 116130. General Introduction 23 growth of the nanoparticles within them. In the interparticle space, the movement of the nanoparticles, influenced by Van der Waals forces, is sterically restricted thereby preventing agglomeration. Non-polar solvents are generally more suitable for this type of stabilisation.47 Various metal nanoparticles, including those based on Ni, Ru and Pt, stabilised by the polymer polyvinylpyrrolidone (PVP), have been reported in the literature.63,68,69 1.4.3 Electrosteric stabilisation Electrosteric stabilisation takes place when electrostatic and steric stabilisation modes are combined. This can be achieved, for example, by using ionic polymers that contain polar groups along with side chains capable of generating both steric and coulombic repulsion simultaneously. A notable example is the stabilising role of sodium polyacrylate in the preparation of barite nanoparticles.47,70 1.4.4 Stabilisation by solid supports In this stabilisation mode, nanoparticles are confined within a solid, which restricts their movement and prevents agglomeration. Various synthetic procedures can be used to prepare supported metal nanoparticles, such as impregnation, grafting, flame spraying, or ion exchange.71 For instance, supported ruthenium nanoparticles on different carbon nanotubes have been described by P. Serp et al.72 1.4.5 Stabilisation by ligands. Covalent interactions between the surface of the particles and the ligands prevent attractive forces between the particles and protect them from coalescence.63,65 68 D. Özhava, N. Z. Kiliçaslan, S. Özkar, Appl. Catal. B: Environ. 2015,162, 573-582. 69 F. Dassenoy, K. Philippot, T. O. Ely, C. Amiens, P. Lecante, E. Snoeck, A. Mosset, M. Casanove, B. Chaudret, New J. Chem. 1998, 22, 703-711. 70 J. Hang, L. Shi, X. Feng, L. Xiao, Powder Technology 2009, 192, 166-170. 71 C. Jia, F. Schüth, Phys. Chem. Chem. Phys. 2011, 13, 2457-2487. 72 D. M. Fernandes, M. Rocha, C. Rivera-Cárcamo, P. Serp, C. Freire, Dalton Trans. 2020, 49, 10250-10260. 30 Chapter I Å) and saturated C2-N (1.49 Å).95,99 Finally, partial aromaticity adds additional electronic stabilisation. Figure 17. Electronic structure of imidazol-2-ylidenes. Today, one of the most compelling advantages of using carbenes as ligands in organometallic chemistry relies on the ease with which structurally diverse analogues can be synthesised. In addition to imidazol-2-ylidenes (Arduengo type carbenes), exists a wide variety of N-heterocyclic carbenes including 1,2,3-triazolylidenes (representative examples of mesoionic carbenes, MIC), imidazolin-2-ylidenes, or 2pyridylidenes, among others (see Figure 18). Some of these carbenes are referred to as abnormal or remote when their structural representation requires the introduction of formal charges or lacks a heteroatom adjacent to the carbenic atom. Notably, some of these species remain non-isolable, particularly many mesoionic carbenes.100,101 It has been generally observed that MIC ligands are stronger donors than classical NHCs. This enhanced donor strength has been quantified using different techniques, primarily by spectroscopic methods. For instance, carbonyl complexes containing a carbene coordinated to the metal have been studied through IR spectroscopy. The analysis of CO stretching vibrations in complexes containing a mesoionic ligand 99 J. W. Runyon, O. Steinhof, H. V. R. Dias, J. C. Calabrese, W. J. Marshall, A. J. Arduengo, Aust. J. Chem. 2011, 64, 1165-1172. 100 O. Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev. 2009, 109, 3445-3478. 101 E. Stander-Grobler, O. Schuster, G. Heydenrych, S. Cronje, E. Tosh, M. Albrecht, G. Frenking, H. G. Raubenheimer, Organometallics 2010, 29, 5821-5833. 31 General Introduction reveals an average vibrational CO frequency that is lower than that of complexes with an Arduengo-type carbene. This behaviour reveals stronger donor properties in the case of MICs.102 Similar findings have been reported using X-ray Photoelectron Spectroscopy (XPS), which revealed that the binding energies for palladium 3d photoemission are generally lower when the metal is coordinated to mesoionic carbenes than for the complexes with imidazol-2-ylidenes, indicating a higher electron density in the metal centre due to stronger donor properties of the triazolylidene ligand.103 These stronger donor properties make mesoionic ligands particularly attractive as stabilisers for nanoparticles. Figure 18. Representative examples of NHC ligands. Most applications of carbenes involve their coordination to transition metals, whether in molecular complexes or metal nanoparticles.104 Although there are examples of the direct use of NHCs as organocatalysts in reactions involving a nucleophilic attack on esters, aldehydes or other carbon compounds,105 a discussion of that application falls outside the scope of this PhD thesis. Since this thesis focuses on nanoparticles synthesised using the organometallic approach, the discussion will centre on the reported synthetic methods involving NHC-stabilised organometallic nanoparticles. 102 A. Vivancos, C. Segarra, M. Albrecht, Chem. Rev. 2018, 118, 9493-9586. 103 T. Terashima, S. Inomata, K. Ogata, S. Fukuzawa, Eur. J. Inorg. Chem. 2012, 1387-1393. 104 S. Díez-González, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109, 3612-3676. 105 D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 2007, 107, 5606-5655. Chapter I 32 There are two main synthetic approaches for stabilising metal nanoparticles with NHC ligands. The first implies the prior isolation of the free carbene, which is then used as stabiliser in the presence of an organometallic precursor that decomposes under hydrogen gas. This approach was first reported by P. Lara, K. Philippot, B. Chaudret et al.106 The NHC-stabilised ruthenium nanoparticles described in that work, summarised in Scheme 3, were obtained by decomposition of [Ru(COD)(COT)] under 3 bar of H2 in the presence of 0.2 or 0.5 equiv. of two different imidazol-2ylidene carbenes: 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3di(tert-butyl)imidazol-2-ylidene (ItBu). This process resulted in the formation of nanoparticles with a mean size of ca 1.5 nm. Later, Godard et al. reported a similar protocol for the synthesis of rhodium nanoparticles from [Rh(C3H5)3] also stabilised by IPr, yielding nanoparticles with a mean size between 1.3 and 1.7 nm, which demonstrated catalytic activity in reduction reactions.107 Building on these precedents, the use of similar ligands has been extended to other metals. Today, carbenes are among the most commonly employed families of ligands for stabilising nanoparticles.108 106 P. Lara, O. Rivada-Wheelaghan, S. Conejero, R. Poteau, K. Philippot, B. Chaudret, Angew. Chem. Int. Ed. 2011, 123, 12286-12290. 107 F. Martínez-Espinar, P. Blondeau, P. Nolis, B. Chaudret, C. Claver, S. Castillón, C. Godard, J. Catal. 2017, 354, 113-127. 108 H. Shen, G. Tian, Z. Xu, L. Wang, Q. Wu, Y. Zhang, B. K. Teo, N. Zheng, Coord. Chem. Rev. 2022, 458, 214425. General Introduction 33 Scheme 3. Organometallic synthesis of Ru·IPr and Ru·ItBu NPs. As previously mentioned, many NHC ligands are not isolable due to their high reactivity, which limits the applicability of the procedure outlined in the previous paragraph. To overcome this inconvenience, a new approach was proposed by K. Philippot, B. Chaudret et al.,109 focused on the in-situ formation of the carbene. After deprotonating the corresponding imidazolium salt with a strong base, the resulting NHC is filtered through celite and transferred to a Fisher-Porter reactor containing the precursor. Attempts to explore this approach with isolable carbenes revealed very similar mean sizes for nanoparticles prepared by both methods, leading to its acceptance as an appropriate synthetic process for non-isolable carbenes. In addition to using isolated carbenes or generating them in-situ, decarboxylation of a carbene precursor, such as 1,3-dialkylimidazolium-2carboxylate, and its use as stabiliser for nickel nanoparticles has also been reported.110 However, due to the limited availability of suitable precursors to be decarboxylated, this method currently appears to be more anecdotal.111 109 L. M. Martínez-Prieto, A. Ferry, P. Lara, C. Richter, K. Philippot, F. Glorius, B. Chaudret, Chem. Eur. J. 2015, 21, 17495-17502. 110 M. Díaz de los Bernardos, S. Pérez-Rodríguez, A. Gual, C. Claver, C. Godard, Chem. Commun. 2017, 53, 7894-7897. 111 C. Cerezo-Navarrete, P. Lara, L. M. Martínez-Prieto, Catalysts 2020, 10, 1144. Chapter I 34 1.5 Characterisation of metal nanoparticles This section provides a brief overview of the main characterisation techniques employed in this PhD thesis. 1.5.1 Transmission Electron Microscopy (TEM and HRTEM) Transmission Electron Microscopy (TEM) is a key analytical technique used to determine the size, shape, and dispersity of nanoparticles. This technique involves depositing a drop of a colloidal nanoparticle suspension onto a copper grid, which is then introduced into the microscope. Unlike traditional optical microscopy, which relies on photons to illuminate the sample, TEM microscope uses an electron beam to irradiate the sample. A portion of the electrons is transmitted through the sample, while the rest are scattered. The resulting image is generated from the information acquired by the transmitted electrons. High Resolution Transmission Electron Microscopy (HRTEM) operates on the same principles but allows the determination of the crystalline structure of the particles by performing the analysis at a higher magnification.112,113 1.5.2 Scanning Transmission Electron Microscopy (STEM) Scanning Transmission Electron Microscopy (STEM) is a technique similar to conventional TEM, but it focuses the electron beam on a narrow spot that is scanned across the sample. The image is formed by collecting the transmitted electrons. STEM is particularly suited for High-Angle Annular Dark-Field scanning (HAADF), which captures scattered electrons at high angles. HAADF images are highly sensitive to atomic number, allowing heavier elements to be observed with greater contrast compared to lighter elements. When combined with an Energy-Dispersive X-ray spectroscopy (EDX) detector, STEM enables the obtention of elemental maps, providing valuable information about the chemical composition of the sample.112 112 S. Mourdikoudis, R. M. Pallares, N. T. K. Thanh, Nanoscale 2018, 10, 12871-12934. 113 D. Su, Green Energy Environ. 2017, 2, 70-83. General Introduction 35 1.5.3 Inductively Coupled Plasma (ICP) Inductively Coupled Plasma is a selective technique widely used for the elemental analysis of nanoparticles, thanks to its ultralow detection limit, typically in the order of ng·L-1, and minimal sample consumption. Sample preparation generally involves decomposing the nanoparticles and organic matrix through digestion with nitric, hydrochloric, sulphuric acid, or similar destructive treatments. Once a liquefied sample is obtained, a carrier gas delivers it into an argon plasma torch, where ionisation of the chemical elements occurs. A Mass Spectrometer then separates the ions based on their massto-charge ratio, enabling the identification of the different components.114,115 1.5.4 X-ray Photoelectron Spectroscopy (XPS) X-ray Photoelectron Spectroscopy (XPS) is an analytical technique employed for surface chemical analysis. The technique is based on the photoelectric effect: when a sample is irradiated with X-ray, photoelectrons are emitted from its surface. The XPS spectrum is obtained by measuring the number and kinetic energy of these photoelectrons, which can be correlated to their binding energies. By analysing these binding energies, different species present on the sample surface can be identified and quantified. The species composing the sample surface can be identified and quantified by considering variations in their oxidation state and/or chemical nature. This technique requires ultra-high vacuum conditions (around 10-9 mbar) and reaches a depth of approximately 2-5 nm.116 1.6. Catalysis by metal nanoparticles As previously mentioned, the interest of metal nanoparticles is driven by their potential applications across different fields such as optoelectronics, sensing, 114 N. V. Godoy, R. M. Galazzi, K. Chacón-Madrid, M. A. Z. Arruda, I. O. Mazali, Talanta 2021, 224, 121808. 115 B. Meermann, V. Nischwitz, J. Anal. At. Spectrom. 2018, 33, 1432-1468. 116 J. Zheng, Y. Lyu, B. Wu, S. Wang, EnergyChem 2020, 2, 100039. Chapter I 36 biomedicine, catalysis, and energy conversion or storage.117,118 The use of metal nanoparticles as catalysts in chemical reactions is expanding an area of research in contemporary chemistry. The traditional distinction between heterogeneous and homogeneous catalysis is based on the physical state of the reactants. In homogeneous catalysis, the catalyst and substrate are in the same physical state, whereas in heterogeneous catalysis, the catalyst is in a different physical state from the reactants. In the case of nanostructured catalysts, applying this classical distinction between homogeneous and heterogenous nature can be complex.119 Nanoparticles exhibit some homogeneous properties, such as mobility in solution, as well as heterogeneous properties, such as the presence of a liquid-solid interface. Since the late 20th century, researchers like Schwartz, Reetz and others have employed a definition based on the number of different active sites available on a given catalyst.120,121,122 When a catalyst possesses only one type of active site, is considered homogeneous; conversely, when multiple types of active sites are present, it is classified as heterogeneous. Given that nanoparticles inherently possess multiple active sites, such as corners, edges, or different exposed crystal faces, they are classified as heterogeneous catalysts according to this definition.123 However, it is also known that some nanoparticles in solution can generate different metallic complexes that are the actual catalytic species due to their dynamic behaviour. In this sense, nanoparticles can exhibit characteristics like homogeneous catalysts. Therefore, it is often preferable to refer to nanoparticles as colloidal catalysts, distinguishing their behaviour from that of heterogeneous and homogeneous catalysts. 117 A. S. Galushko, A. S. Kashin, D. B. Eremin, M. V. Polynski, E. O. Pentsak, V. M. Chernyshev, V. P. Ananikov in Nanoparticles in Catalysis: Advances in Synthesis and Applications, Chapter 2, 13-42, K. Philippot, A. Roucoux (Eds), Wiley-VCH, 2021. 118 M. R. Axet, K. Philippot, Chem. Rev. 2020, 120, 1085-1145. 119 C. Tabor, R. Narayanan, M. A. El-Sayed, in Model Systems in Catalysis: Single Crystals to Supported Enzyme Mimics, Chapter 18, 395-414, R. Rioux (Ed), Springer, 2010. 120 J. Schwartz, Acc. Chem. Res. 1985, 18, 302-308. 121 M. T. Reetz, R. Breinbaur, P. Wedemann, P. Binger, Tetrahedron 1998, 54, 1233-1240. 122 J. D. Aiken, Y. Lin, R. G. Finke, J. Mol. Catal. A Chem. 1996, 114, 29-51. 123 J. A. Widegren, R. G. Finke, J. Mol. Catal. A Chem. 2003, 198, 317-341. 37 General Introduction Metal nanoparticles are attractive species in catalysis due to their high surface to volume ratio, which is especially pronounced in smaller nanoparticles, offering a vast number of potential active sites. Generally, a smaller particle size and lower surface coverage are correlated with enhanced catalytic activity, making crucial the optimisation of the balance between these factors for effective catalyst design. For a given ligand, the mean size of the resulting nanoparticles decreases with increasing ligand quantity. However, this also leads to greater surface coverage by the ligand, which can negatively impact catalytic activity.111 The orientation of the crystalline planes exposed at the nanoparticle surface, influenced by the nanoparticle shape, significantly affects catalytic properties.118,124,125 For example, it has been reported that in hydrogenation reactions employing benzene as substrate, the selectivity of the process depends on the exposed planes. In such case, platinum nanocrystals that expose both (100) and (111) crystalline planes can produce cyclohexane and cyclohexene, while nanocrystals that show only (100) plane selectively produced cyclohexane.126 In addition to the previously mentioned parameters, the composition of the nanoparticle is another critical factor that influences its catalytic performance, with two key contributions: the nature of the metallic core and the stabiliser used. The composition must be tailored based on the specific catalysis target, as the choice of the metallic core can significantly affect reactivity (certain metals are well known for specific catalytic applications while other are not). At this respect, noble metals are particularly renowned for their catalytic properties. For instance, gold nanoparticles are highly reputed for CO oxidation and alkene hydrogenation,127 while palladium nanoparticles are extensively employed in a wide range of C—C coupling reactions.128 124 A. K. Bentley, S. E. Skrabalak, J. Chem. Educ. 2023, 100, 3425-3433. 125 H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng, G. Q. Lu, Nature 2008, 453, 638-642. 126 K. M. Bratlie, H. Lee, K. Komvopoulos, P. Yang, G. Somorjai, Nano Lett. 2007, 7, 30973101. 127 G. C. Bond in Gold Nanoparticles for Physics, Chemistry and Biology, Chapter 7, 171-197, C. Louis and O. Pluchery (Eds), Imperial College Press, 2012. 128 I. Saldan, Y. Semenyuk, I. Marchuk, O. Reshetnyak, J. Mater. Sci. 2015, 50, 2337-2354. Chapter I 38 Rhodium NPs are valuable for their applications in hydrogenation and hydroformylation reactions,129 and iridium nanocatalysts are highly effective in watersplitting processes.130 However, in recent years, there has been an increasing interest for non-noble metals (Fe, Co, Ni, Mn). From a green chemistry perspective, these metals are often preferred due to their lower toxicity and greater abundance in the Earth’s crust, compared to their expensive and scarce precious metal counterparts.131 The catalytic properties of metals like iron, cobalt, or nickel have been exploited for various reactions, including hydrogenation of N-heteroarenes, hydrolysis of ammonia-borane, and oxygen reduction reactions in fuel cells, among others.132,133,134 In addition to the nature of the metal, the stability and effectiveness of metal nanoparticles are significantly influenced by the choice of the stabiliser employed, as discussed in section 1.4.5.118 Ligand interaction with metal surface (by steric or electronic effects) modulates its properties, playing a non-innocent role.135 This creates a parallelism between colloidal catalysis and homogeneous catalysis. In this context, selecting the appropriate metal, ligand and metal/ligand ratio presents a significant challenge. Since the initial reports of their use, the versatility of the organometallic approach, combined with a thorough study of the factors influencing catalytic performance, has led to the development of a wide variety of organometallic nanocatalysts.50 Research into platinum59 and ruthenium nanoparticles,106 as well as nanoparticles stabilised by NHC ligands, has been particularly fructiferous. 129 M. Guerrero, N. T. T. Chau, S. Noël, A. Denicourt-Nowicki, F. Hapiot, A. Roucoux, E. Monflier, K. Philippot, Curr. Org. Chem. 2013, 17, 364-399. 130 a) J. Quinson, Adv. Colloid Interface Sci. 2022, 303, 102643; b) G. Martí, L. Mallón, N. Romero, L. Francàs, R. Bofill, K. Philippot, J. García-Antón, X. Sala, Adv. Energy Mater. 2023, 13, 2300282. 131 V. Papa, Y. Cao, A. Spannenberg, K. Junge, M. Beller, Nature Catalysis 2020, 3, 135-142. 132 B. Sahoo, C. Kreyenschulte, G. Agostini, H. Lund, S. Bachmann, M. Scalone, K. Junge, Chem. Sci. 2018, 9, 8134-8141. 133 K. Kumar, P. Gairola, M. Lions, N. Ranjbar-Sahraie, M. Mermoux, L. Dubau, A. Zitolo, F. Jaouen, F. Maillard, ACS Catal. 2018, 8, 11264-11276. 134 H. Zhang, X. Gu, P. Liu, J. Song, J. Cheng, H. Su, J. Mater. Chem. A. 2017, 5, 2288-2296. 135 A. M. Nauth, E. Schechtel, R. Dören, W. Tremel, T. Opatz, J. Am. Chem. Soc. 2018, 140, 14169-14177. General Introduction 39 Additionally, there have been significant contributions involving 1st row transition metal nanoparticles and magnetically induced nanocatalysts.61,136,137 Some representative examples of catalytic reactions carried out with organometallic nanoparticles include hydrogenation processes, water splitting reactions, magnetically induced catalysis, hydroboration of alkynes, H2 production from amine-boranes, isotopic H/D exchange, Suzuki-Miyaura or other CarbonCarbon Coupling reactions and Fischer-Tropsch synthesis (FTS), among others.138 Hydrogenation reactions are among the most extensively studied catalytic processes involving organometallic nanoparticles. Numerous catalysts have been developed that exhibit optimal activities and selectivities. For instance, NHCstabilised platinum nanoparticles have been used for selective hydrogenation of nitroarenes without affecting other substituents on the aromatic ring.139 Similarly, bimetallic cobalt-rhodium nanoparticles stabilised in Supported Ionic Liquid Phases (SILP) show activity on hydrogenation of multifunctional substrates.140 Both examples are illustrated in Scheme 4 and Scheme 5, respectively. Scheme 4. Selective hydrogenation of nitroarenes catalysed by Pt-NHC NPs. 136 C. Cerezo-Navarrete, I. Mustieles Marin, H. García-Miquel, A. Corma, B. Chaudret, L. M. Martínez-Prieto, ACS Catal. 2022, 12, 8462-8475. 137 A. M. López-Vinasco, L. M. Martínez-Prieto, J. M. Asensio, P. Lecante, B. Chaudret, J. Cámpora, P. W. N. M. van Leeuwen, Catal. Sci. Technol. 2020, 10, 342-350. 138 Nanoparticles in Catalysis: Advances in Synthesis and Applications, K. Philippot and A. 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Trzeciak, RSC Adv. 2016, 6, 36491-36499. 143 D. O. Silva, J. D. Scholten, M. A. Gelesky, S. R. Teixeira, A. C. B. Dos Santos, E. F. Souza-Aguiar, J. Dupont, ChemSusChem 2008, 1, 291-294. 144 L. C. Moraes, R. C. Figueiredo, J. P. Espinós, F. Vattier, A. Franconetti, C. Jaime, B. Lacroix, J. Rojo, P. Lara, S. Conejero, Nanoscale 2020, 12, 6821-6831. 145 M. Kidonakis, M. Stratakis, ACS Catal. 2018, 8, 1227-1230. Chapter II: Objectives Chapter II outlines the main goals of this PhD thesis, focusing on the efficient design of nanocatalysts tailored to optimise their catalytic performance in key chemical transformations. Objectives Over the past few decades, significant advances in nanoscience have focused on controlling the characteristics of a wide range of nanomaterials based on their potential applications. In the case of metal nanoparticles, these characteristics (such as composition, size, and surface properties) are crucial factors that influence their reactivity and, consequently, their catalytic behaviour. Developing new synthetic methodologies that enable precise control over these parameters is crucial for understanding and predicting their potential applications. The backbone of this PhD thesis can be summarised by the following specific objectives: 1. Synthesis of different ligands that can serve as stabilising agents for nanoparticles: SNS pincer-type ligands, imidazole-2-ylidenes (IPr and IMes) and 1,2,3-triazolylidenes (MIC). 2. Synthesis of ruthenium nanoparticles stabilised by SNS ligands using the organometallic method, followed by an investigation of their structural and morphological characteristics. 3. Catalytic activity study of ruthenium nanoparticles stabilised with SNS pincer ligands in the reduction of nitrous oxide using hydrosilanes. 4. Synthesis of iron, cobalt and nickel nanoparticles stabilised with uNHC ligands using the organometallic approach. This involves examining their structural and morphological characteristics and analysing the differences among the three metals when stabilised by the same ligands. 5. Testing the catalytic activity of first row transition metal nanoparticles in the methanolysis of H3N·BH3, a promising process for hydrogen generation. 6. Synthesis of ruthenium and nickel nanoparticles stabilised by MIC ligands employing the organometallic method, followed by their characterisation using conventional solid-state techniques. 7. Assessment of the catalytic activity of these nanoparticles in hydrogen/deuterium (H/D) exchange reactions using hydrosilanes and related hydrides from main group elements as novel substrates for the process. 55 Ru·SNS nanoparticles 63 3.1 Introduction S-containing pincer-type ligands, such as SNS and SNN, are an easily accessible variety of ligands, although they have not received as much attention as other pincer systems. In particular, SNS ligands are characterised by a sulphurnitrogen-sulphur arrangement, which features three atoms with available electron lone pairs. This enhances their ability to act as Lewis bases, attaching to Lewis acidic metal centres and allowing them to function as tridentate pincer ligands due to their multidentate structure.1,2 As a result of their versatile applications and robust chelating ability, pincer-type ligands have become essential components of organometallic chemistry. They play a non-innocent role in some catalytic processes.3,4 Figure 1 illustrates two examples of ruthenium SNS complexes reported in the literature by Gusev et al. and Zimmerman, Wasser et al. Gusev’s complex has been used as catalyst in the hydrogenation of various unsaturated compounds, such as esters, ketones, and aldehydes.5 Similarly, Zimmerman’s complexes have been utilised both in the hydrogenation of esters and the formation of amides (Figure 1).6 Figure 1. Examples of RuSNS complexes reported in the literature. 1 A. Khanzadeh, Ligand-Assisted Catalysis Using Metal SNS Complexes, PhD Thesis, University of Ottawa, 2023. 2 H. G. Sogukomerogullari, S. P. Yalçin, U. Cylan, E. Aytar, M. Aygün, D. S. Richeson, M. Sönmez, J. Chem. Sci. 2019, 131, 32. 3 V. Singh, R. Singh, A. S. Hazari, D. Adhikari, JACS Au 2023, 3, 1213-1220. 4 K. E. Rosenkoetter, M. K. Wojnar, B. J. Charette, J. W. Ziller, A. F. Heyduk, Inorg. Chem. 2018, 57, 9728-9737. 5 D. Spasyuk, S. Smith, D. G. Gusev, Angew. Chem. Int. Ed. 2013, 52, 2538-2542. 6 J. Schörgenhumer, A. Zimmermann, M. Waser, Org. Process Res. Dev. 2018, 22, 862-870. Chapter III 64 In addition to these simple aliphatic ligand systems, pyridine-based SNS complexes have also been described by Zimmermann, Waser et al. for hydrogenation and dehydrogenation reactions (Figure 2).6 Figure 2. Examples of pyridine-based RuSNS complexes reported in the literature. The exceptional catalytic performance exhibited by pincer ruthenium complexes in various hydrogenation reactions has been also extended to the hydrogenation of nitrous oxide. This chemical remediation method involves the conversion of nitrous oxide into benign nitrogen gas and water.7 Nitrous oxide (N2O) is a potent greenhouse gas whose atmospheric concentration has significantly increased since the beginning of the industrial era. While this gas is produced both by anthropogenic and natural sources, anthropogenic emissions have risen about 30% in just four decades.8 Among the models used to compare the climate impact of different gases, the most common is Global Warming Potential (GWP), which is indexed per unit of mass of CO2. According to its GWP, each unit of mass of N2O is equivalent to nearly 300 units of carbon dioxide,9 making the decomposition of nitrous oxide an important area of research. In the context of a 7 R. Zeng, M. Feller, Y. Ben-David, D. Milstein, J. Am. Chem. Soc. 2017, 139, 5720-5723. 8 H. Tian, R. Xian, J.G. Canadell et al., Nature 2020, 586, 248-256. 9 G.A. Meehl, T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver and Z.-C. Zhao in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (Eds.), Cambridge University Press, 2007. Ru·SNS nanoparticles 65 circular economy, transforming N2O into harmless N2 serves not only to reduce its presence in the atmosphere but also to valorise it into other chemicals.10,11 Various homogeneous and heterogeneous catalysts have been employed to activate and reduce nitrous oxide, releasing nitrogen gas in the process.12-15 This activation can be achieved directly with reducing agents such as H2. The hydrogenation of N2O appears to be a promising method for reducing nitrous oxide, producing both N2 and H2O.7,16 Boranes have also been used as reductants in oxygen transfer reactions from nitrous oxide, using 1st row transition metal complexes as catalysts.17 Generally, in most reported precedents, N2O reduction occurs through transference of the oxygen atom to an acceptor such as hydrogen, phosphines, boranes, hydrocarbons, or silanes.18 As an alternative to reduction by hydrogen, silanes are commonly employed as reducing agents in a variety of processes, including the reduction of carbonyl compounds, alkyne hydrosilylation, and the activation of small molecules like CO2.19,20,21 Some advantages of using silanes include their low cost and ease of handling. However, there are few precedents for their use as reductants for nitrous oxide. 10 B. Aryal, R. Gurung, A. F. Camargo, G. Fongaro, H. Treichel, B. Mainali, M. J. Angove, H. H. Ngo, W. Guo, S. R. Puadel, Environ. Pollut. 2022, 314, 120272-120288. 11 K. Severin, Chem. Soc. Rev. 2015, 44, 6375-6386. 12 Y. Pang, M. Leutzsch, N. Nöthling, J. Cornella, J. Am. Chem. Soc. 2020, 142, 19473-19479. 13 R. Zeng, M. Feller, Y. Diskin-Posner, L. J. W. Shimon, Y. Ben-David, D. Milstein, J. Am. Chem. Soc. 2018, 140, 7061-7064. 14 V. R. Landaeta, R. E. Rodríguez-Lugo, Inorg. Chim. Acta 2015, 431, 21-47. 15 D. J. Xiao, E. D. Bloch, J. A. Mason, W. L. Queen, M. R. Hudson, N. Planas, J. Borycz, A. L. Dzubak, P. Verma, K. Lee, F. Bonino, V. Crocellà, J. Yano, S. Bordiga, D. G. Truhlar, L. Gagliardi, C. M. Brown, J. R. Long, Nature Chem. 2014, 6, 590-595. 16 I. Ortega-Lepe, P. Sánchez, L. L. Santos, P. Lara, N. Rendón, J. López-Serrano, V. SalazarPereda, E. Álvarez, M. Paneque, A. Suárez, Inorg. Chem. 2022, 61, 18590-18600. 17 X. Chen, H. Wang, S. Du, M. Driess, Z. Mo, Angew. Chem., Int. Ed. 2022, 61, e202114598. 18 J. Bösken, R. E. Rodríguez-Lugo, S. Nappen, M. Trincado, H. Grützmacher, Chem. Eur. J. 2023, 29, e202203632, 1-8. 19 B. M. Trost, Z. T. Ball, J. Am. Chem. Soc. 2005, 127, 17644-17655. 20 R. A. Pramudita, K. Motokura, ChemSusChem 2021, 14, 281-292. 21 Y. Nagai, Org. Prep. Proced. Int. 1980, 12, 13-48. Chapter III 66 Milstein et al. published one of the most interesting recent studies in this area in 2017.7 In their research, the authors described a RuPNP pincer complex capable of reducing N2O with different hydrosilanes (Me2PhSiH, MePh2SiH, and tBuMe2SiH) under mild conditions of catalyst load, pressure and temperature, as summarised in Scheme 1. Scheme 1. Nitrous oxide reduction by hydrosilanes catalysed by a RuPNP pincer complex. The ability of disilanes to perform the same process was recently described by Cantat et al. (Scheme 2),22 making the first metal-free example of N2O reduction under mild conditions. Scheme 2. Metal-free reduction of nitrous oxide. As N2O is an industrial byproduct, this reduction process could help eliminate it from the atmosphere, while also avoiding the use of H2O2 or O2 for silane 22 L. Anthore-Dalion, E. Nicolas, T. Cantat, ACS Catal. 2019, 9, 11563-11567. Ru·SNS nanoparticles 67 oxidation.23 It is important to note that oxidation of silanes with N2O leads to the formation of derivatives containing Si—O bonds (namely silanols and siloxanes), which are relevant for the synthesis of different silicon-based polymeric materials.24,25,26 Given the ability of transition metal nanoparticles to catalyse σ‑Si—H bond activation reactions, and the reactivity of ruthenium catalysts with N2O and previously mentioned silanes, there are compelling reasons to explore the reduction of nitrous oxide with hydrosilanes catalysed by ruthenium nanoparticles.27 To the best of our knowledge, there is no precedent for the use of SNS ligands as stabilising agents for transition metal nanoparticles. Chapter III of this PhD thesis focuses on the preparation of ruthenium nanoparticles stabilised by SNS ligands and the investigation of their catalytic performance in the reduction of N2O with hydrosilanes. 3.2 Results and Discussion 3.2.1 SNS ligands as stabilising agents for Ru nanoparticles Four different SNS ligands have been synthesised to be used as stabilising agents for ruthenium nanoparticles.28 Specifically, SNS ligands derived from secondary (SNS1 and SNS2, Scheme 3) or aromatic (SNS3 and SNS4, Scheme 4) amine, containing alkyl (SNS1, SNS3) or aryl (SNS2, SNS4) substituents on the sulphur atom, have been prepared with low to moderate yields following a procedure reported in the literature and detailed in the experimental section. These ligands have been isolated as light-coloured solids or oils, with yields ranging from 10 to 82 %. Due to the multidentate structure of SNS ligands, pincer-type behaviour has been 23 H. H. Moretto, M. Schulze, G. Wagner, in Silicones. Ullmann’s Encyclopedia of Industrial Chemistry, 32, 675-708. Weinheim: Wiley-VCH, 2012. 24 R. Murugavel, A. Voigt, M. G. Walawalkar, H. W. Roesky, Chem. Rev. 1996, 96, 2205-2236. 25 S. E. Denmark, C. S. Regens, Acc. Chem. Res. 2008, 41, 1486-1499. 26 Y. Abe, T. Gunji, Prog. Polym. Sci. 2004, 29, 149-182. 27 J. M. Asensio, D. Bouzouita, P. W. N. M. van Leeuwen, B. Chaudret, Chem. Rev. 2020, 120, 1042-1084. 28 P. Molinillo, B. Lacroix, F. Vattier, N. Rendón, A. Suárez, P. Lara, Chem. Commun. 2022, 58, 7176-7179. Chapter III 68 previously observed in RuSNS complexes,6 so, in principle, similar properties might be anticipated for Ru∙SNS nanoparticles. Scheme 3. Synthesis of SNS1 and SNS2 ligands. Scheme 4. Synthesis of SNS3 and SNS4 ligands. 3.2.2 Synthesis and characterisation of Ru∙SNS nanoparticles Ruthenium nanoparticles stabilised by SNS ligands were prepared following the organometallic approach.29 A solution of the olefinic complex [Ru(COD)(COT)] (COD= 1,5 cyclooctadiene, COT = 1,3,5-cyclooctatriene) in THF was decomposed at room temperature under 3 bar of H2 in the presence of non-stoichiometric amounts (0.2 or 0.5 equiv.) of an SNS ligand (Scheme 5); further details can be found in the experimental section. Scheme 5. Synthesis of ruthenium nanoparticles stabilised by SNS1-4 ligands. 29 K. Philippot, B. Chaudret, in Comprehensive Organometallic Chemistry III, R. H. Crabtree & M. P. Mingos (Eds-in-Chief); Applications III: Functional Materials, Environmental and Biological Applications, D. O´Hare (Volume Ed.), Vol 12, Chapter 03, 71-99, Elsevier, 2007. Ru·SNS nanoparticles 69 After 30 min. of reaction, the colour of the system changed from the initial yellow to dark brown. The reaction mixture was stirred overnight, and the particles were then precipitated and washed with pentane to remove the uncoordinated ligand. Different ligand/metal ratios were employed, resulting in the synthesis of five colloids. Four of them were prepared using 0.5 equiv. of SNS1-4, while the last one was prepared using 0.2 equiv. of SNS4. All samples were characterised by TEM (Figure 2-Figure 7, Table 1) and ICP (Table 1), revealing metal contents ranging from 32 to 81%wt. In all cases, mainly small and well-dispersed nanoparticles (Figure 2Figure 7) were obtained, with mean sizes between 1.5 and 2.3 nm. The use of lower amounts of SNS1-3 ligands (0.2 equiv.) resulted in the formation of bulk metal, while for SNS4, slight agglomeration was observed (Figure 7). As previously noted, increasing the ligand/metal ratio led to a decrease in the mean size of the nanoparticles.30 200 150 100 50 0 0 1 2 3 Mean size (nm) Figure 2. TEM image with size distribution histogram of Ru⋅SNS10.5. 30 P. Lara, O. Rivada-Wheelaghan, S. Conejero, R. Poteau, K. Phlippot, B. Chaudret, Angew. Chem., Int. Ed. 2011, 50, 12080-12084. d = 1.5 (0.2) nm Number of Nanoparticles Chapter III 70 200 150 100 50 0 0 1 2 3 4 Mean size (nm) Figure 3. TEM image with size distribution histogram of Ru⋅SNS20.5. 200 150 100 50 0 0 1 2 3 Mean size (nm) Figure 4. TEM image with size distribution histogram of Ru⋅SNS30.5. 200 150 100 50 0 0 1 2 3 4 Mean size (nm) Figure 5. TEM image with size distribution histogram of Ru⋅SNS40.5. d = 1.9 (0.4) nm d = 1.5 (0.2) nm d = 1.6 (0.2) nm Number of Nanoparticles Number of Nanoparticles Number of Nanoparticles Ru·SNS nanoparticles 71 200 150 100 50 0 0 1 2 3 4 Mean size (nm) Figure 6. TEM image with size distribution histogram of Ru⋅SNS40.2. Figure 7. TEM image of Ru⋅SNS40.2, showing agglomerated NPs. Table 1. Analysis of Ru·SNS nanoparticles by TEM and ICP. Colloid L/Ru ratio %wt Ru Mean size (nm) Ru⋅SNS10.5 0.5 41 1.5 (0.2) Ru⋅SNS20.5 0.5 32 1.9 (0.4) Ru⋅SNS30.5 0.5 41 1.5 (0.2) Ru⋅SNS40.5 0.5 47 1.6 (0.2) Ru⋅SNS40.2 0.2 81 2.3 (0.4) The crystalline character of the Ru∙SNS nanoparticles was confirmed by High Resolution Transmission Electron Microscopy (HRTEM) measurements conducted d = 2.3 (0.4) nm Number of Nanoparticles Chapter III 78 First, a study of the catalytic performance of Ru·SNS nanoparticles in the reduction of N2O with dimethylphenylsilane (PhMe2SiH, 1a) revealed the formation of two products: silanol (2a) and siloxane (3a) (Scheme 7). Given the significance of Si—O containing molecules in the synthesis of silicon-based materials, both products are of interest, particularly siloxane (3a) due to its ability for producing polymeric materials.38 Scheme 7. Reduction of N2O with PhMe2SiH (1a) catalysed by Ru·SNS nanoparticles. Initial experiments were performed with Ru⋅SNS10.5, using 1.0 mol% of Ru under 1 bar of N2O at 55 ºC, with PhMe2SiH as model substrate. Conversion and selectivity were determined by 1H NMR spectroscopy, using mesitylene as an internal standard. Under these reaction conditions, a conversion of 76% was observed, yielding a mixture of silanol (2a) and siloxane (3a) with a 40:60 ratio (Table 6, entry 1). After confirming that Ru·SNS nanoparticles were active for this process, the catalytic performance of the other four catalysts was examined. Table 6 summarises the conversion and selectivity values for all the colloids. Except for Ru⋅SNS10.5, all catalysts prepared with 0.5 equiv. of ligand (Ru⋅SNS20.5, Ru⋅SNS30.5 and Ru⋅SNS40.5) showed conversion values greater than 99% and silanol:siloxane ratios ranging between 12:88 and 25:75, where siloxane (3a) was the dominant product (Table 6). These catalytic results align with the expectations for catalysts with a comparable number of active sites, as inferred from the number of surface atoms (53-63%, Table 5). It is also noteworthy that the synthetic precursor [Ru(COD)(COT)] provided a very low silane conversion, less than 5% (see below). 38 S. Ananda Kumar, M. Alagar, M. Mandhakini in Concise Encyclopedia of High Performance Silicones, Chapter 3, 39-45, J. P. Lewicki, R. S. Maxwell (Eds), Wiley, 2014. Ru·SNS nanoparticles 79 Table 6. Reduction of N2O with PhMe2SiH using Ru·SNS nanoparticles. Entry Catalyst Conversion (%) 2a:3a ratio 1 Ru⋅SNS10.5 76 40:60 2 Ru⋅SNS20.5 >99 12:88 3 Ru⋅SNS30.5 >99 25:75 4 Ru⋅SNS40.5 >99 20:80 5 Ru⋅SNS40.2 28 50:50 Although all catalysts prepared with Ru/SNS ratio of 0.5 (entries 1-4) showed high conversion values, the activity decreased for Ru⋅SNS40.2 (entry 5), likely due to the presence of a slight metal agglomeration in that sample observed by TEM analysis (Figure 7). These metal agglomerates may hinder the catalytic activity of this colloid, leading to a lower conversion value. Among all the 0.5 equiv. catalysts, Ru⋅SNS10.5 showed the lowest conversion (76%), consistent with a higher degree of surface coverage for Ru⋅SNS10.5 compared to Ru⋅SNS20.5, as inferred from their XPS analyses (Table 2). To determine the selectivity of the catalytic process, the two products 2a and 3a were separated by flash chromatography on silica gel using pentane → pentane/Et2O (1:1) as eluent. Each pure product was then analysed, allowing for identification, by HRMS and 1H NMR spectroscopy. As mentioned earlier, for all the Ru·SNS0.5 catalysts, siloxane was the main product, with silanol presented in smaller quantities (Figure 13). After catalysis, TEM measurements of Ru⋅SNS40.5 colloid, obtained by depositing a drop of the crude reaction mixture on a copper grid, confirmed that the particle size remains practically unchanged. Figure 14 shows a TEM image with size distribution histogram after the catalytic process (mean size 1.5 (0.3) nm). Chapter III 80 Figure 13. 1H NMR spectrum (CD2Cl2, 400 MHz) of the crude reaction mixture after the reduction of N2O with PhMe2SiH using Ru⋅SNS20.5 nanoparticles as catalyst (* denotes mesitylene employed as internal standard and # denotes residual CH2Cl2). 150 100 50 0 0 1 2 3 Mean size (nm) Figure 14. TEM image with size distribution histogram of Ru⋅SNS40.5 after the catalytic reaction. d = 1.5 (0.3) nm Number of Nanoparticles Ru·SNS nanoparticles 81 To explore the scope of the catalytic process, various hydrosilanes were tested as reductants, using the Ru⋅SNS40.5 colloid as a representative catalyst (Table 7). Table 7. Reduction of N2O with hydrosilanes using Ru⋅SNS40.5. Entry Hydrosilane Conversion (%) Silanol:Siloxane ratio 1 Ph 2 MeSiH (1b) >99 >99:1 2a (PhCH 2 CH 2 )Me 2 SiH (1c) 98 >1:99 3 nPr 3 SiH (1d) 95 58:42 4 iPr 3 SiH (1e) 0 --- 5 (EtO) 3 SiH (1f) 98 63:37 Reaction conditions, unless otherwise noted: 1.0 mol% Ru, 1 bar N2O, 65 ºC, THF, reaction time: 24 h. aReaction time: 48 h. Complete silane conversion and high selectivity towards the formation of silanol 2b were observed when methyldiphenylsilane (1b) was used as the reductant (entry 1). In contrast, the use of dimethylphenetylsilane (1c), led primarily to the formation of siloxane 3c with >99% selectivity (entry 2). Furthermore, high conversions were also achieved with tripropylsilane (1d) and triethoxysilane (1f), with silanol:siloxane ratios of approximately 6:4 (entries 3 and 5, respectively). However, no conversion was observed for triisopropylsilane (1e), likely due to steric hindrance (entry 4). All products have been fully characterised by NMR spectroscopy and HRMS (see the Experimental Section). Figure 15 and 16 show the 1H NMR and 13C{1H} NMR spectra, respectively, for dimethylphenetylsiloxane (3c) as a representative example of the characterisation performed for each product. Chapter III 82 Figure 15. 1H NMR spectrum (CD2Cl2, 400 MHz) of [(PhCH2CH2)Me2Si]2O (3c) (# denotes residual CH2Cl2). Figure 16. 13C{1H} NMR spectrum (CD2Cl2, 101 MHz) of [(PhCH2CH2)Me2Si]2O (3c) (# denotes residual CD2Cl2). Ru·SNS nanoparticles 83 To gain further insight into the transformation of hydrosilanes into silanols and siloxanes, a series of control experiments were performed. First, the formation of N2 from N2O was detected by GC-MS analysis of the headspace gas in the Fischer Porter reactor after the catalytic reduction of N2O with dimethylphenylsilane using Ru·SNS40.5 (see Experimental Section). Next, as already noted, the organometallic precursor [Ru(COD)(COT)] was tested as a catalyst in the reduction of nitrous oxide with dimethylphenylsilane, under the same reaction conditions as those used for Ru·SNS nanoparticles (THF, 55 ºC, 1 bar N2O, 1 mol% Ru). 1H NMR analysis revealed a very low silane conversion (below 5%) (see Experimental Section), in stark contrast to the higher activity observed for the nanoparticles (Table 6). Another experiment, a mercury poisoning test, was carried out to determine whether the catalytic process was occurring due to the presence of molecular species of ruthenium.39 This test was carried out in the reduction of N2O with PhMe2SiH using Ru⋅SNS40.5, with the addition of 0.1 mmol of Hg to the reaction mixture. 1H NMR analysis of the reaction mixture after 24 hours shows that the conversion dropped to 30%, (Figure 17), a value considerably lower than the conversion observed with the non-poisoned catalyst (>99), and, in addition, siloxane was the only product detected. These control experiments provided valuable insights into the catalytic system. First, the reaction did not occur in the absence of a suitable catalyst. Second, since the metal oxidation state is the same in both the organometallic precursor and Ru·SNS NPs, Ru(0), the nature of the catalyst is critical for the activation of the N2O molecule. Finally, the possibility of catalysis being performed by well-defined ruthenium complexes cannot be ruled out. Furthermore, in order to investigate the reaction mechanism of the process conducting to the formation of siloxane, a series of experiments were carried out to test various hypotheses, which are described in the next pages. 39 I. C. Chagunda, T. Fisher, M. Schierling, J. S. McIndoe, Organometallics 2023, 42, 29382945. Chapter III 84 Figure 17. 1H NMR spectrum (CD2Cl2, 300 MHz) after the reduction of N2O with PhMe2SiH using Ru·SNS40.5 in the presence of Hg. (* denotes mesitylene added as the internal standard, # denotes residual CH2Cl2, and + denotes residual H2O). Formation of siloxane through the oxidation of disilane The formation of siloxanes could involve the oxidation of disilanes, which are formed through the dehydrogenative coupling of reacted hydrosilanes.40 To investigate this possibility, the disilane PhMe2SiSiMe2Ph was reacted with N2O employing Ru⋅SNS40.5 as catalyst (THF-d8, 55 ºC, 1 bar N2O, 1 mol% Ru, 24 h). However, no reaction was observed under these conditions and this hypothesis was ruled out (Scheme 8). Scheme 8. Control experiment for the formation of the siloxane 3a from disilane. 40 F. Neumeyer, N. Auner, Chem. Eur. J. 2016, 22, 17165-17168. Ru·SNS nanoparticles 85 Formation of siloxane via silanol-silanol condensation The formation of the siloxane 3a could occur through the condensation of two silanol molecules via dehydration (Scheme 9).41 However, no conversion was observed, nor water was detected by 1H NMR when a solution of commercial 2a in dry THF was heated to 55 ºC for 24 h in the presence of Ru·SNS40.5. Therefore, this mechanism was also discarded. Scheme 9. Control experiment for the formation of the siloxane 3a by silanol dehydration. Formation of siloxane via silanol-silane dehydrogenation The final possibility considered involved a dehydrogenative coupling of silane and silanol. Under the catalytic conditions previously proposed, this mechanism would require the initial formation of a silanol molecule from the silane, followed by the coupling of silanol and silane to form a siloxane. To test this hypothesis, the reaction of silane 1a with silanol 2a was performed under the same reaction conditions (Scheme 10) resulting in 100% conversion to the siloxane 3a and hydrogen formation, as confirmed by 1H NMR spectroscopy (Figure 18). Scheme 10. Control reaction for the formation of the siloxane 3a by dehydrogenative coupling of silanol and silane. 41 W. T. Grubb, J. Am. Chem. Soc. 1954, 76, 3408-3414. Chapter III 86 It is worth noting that the reaction was performed in a Fisher-Porter reactor, which resulted in a slight increase in pressure, attributed to the evolution of hydrogen gas. All these observations support a mechanism for the reduction of nitrous oxide with hydrosilanes, catalysed by Ru·SNS NPs, involving the initial oxidation of silane to silanol, the formation of nitrogen gas, and subsequent coupling of the silanol with a second molecule of hydrosilane. This reaction releases H2 and produces the siloxane derivative. Figure 18. 1H NMR spectra (THF-d8, 400 MHz) of the control experiment for the formation of the siloxane 3a by dehydrogenative coupling of silanol and silane. (* denotes mesitylene added as the internal standard). 3.3 Experimental Section 3.3.1 General considerations All experimental procedures were performed under nitrogen or argon atmosphere employing standard Schlenk techniques, Fisher-Porter tubes techniques, or a Braun MBraun Ubilab Pro glovebox. The solvents used (THF, pentane, diethyl ether) were treated with an appropriate desiccant (sodium benzophenone-ketyl in the case of THF and diethyl ether, sodium in the case of pentane) and distilled under inert Ru·SNS nanoparticles 87 atmosphere prior to their use. The precursor [Ru(COD)(COT)] (COD= 1,5 cyclooctadiene, COT = 1,3,5-cyclooctatriene),42 dimethylphenetylsilane,43 and SNS ligands6 were synthesised according to methodologies described previously in the literature. Hydrogen gas (99.99%) was purchased from Air Liquide and all other chemicals were used as received from the commercial supplier Sigma-Aldrich. Transmission Electron Microscopy (TEM) measurements were performed using a Philips CM200 and a FEI TALOS F200S working at 200 kV in the “Centro de Investigación, Tecnología e Innovación - CITIUS” (Universidad de Sevilla). The determination of the particle mean size was carried out by measuring approximately three hundred individual nanoparticles, and the statistical treatment of the measurements was performed using ImageJ and Origin, softwares commonly employed in the literature.44 The samples for microscopy have been prepared by depositing a drop of the nanoparticle dispersion in THF onto a copper grid and allowing it to dry in air. The crystal structures of the particles were also elucidated using High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Transmission Electron Microscopy simultaneously with Energy Dispersive X-Ray Spectroscopy (STEM-EDX). In the case of HRTEM, an ABSF filter was applied to enhance contrast and reduce noise.45 Various ruthenium nanoparticles were scanned to evaluate their composition by STEM-EDX. A small electron probe (size around 0.5 nm, intensity about 500 pA) was studied inside an area of 140x160 pixels, with a dwell time of 50 µs/pixel. The EDX signal was integrated over about 200 frames. These measurements were carried out by Dr. Bertrand Lacroix from the “Tribología y Protección de Superficies” group at the department of “Física Aplicada I” (Universidad de Sevilla). NMR experiments were recorded at 25 ºC on Bruker DRX-500, DRX-400, and DRX-300 spectrometers. The 1H and 13C spectra were referenced to external 42 P. Pertici, G. Vitulli, Inorg. Synt. 1983, 22, 176-181. 43 M. Ito, M. Itazaki, T. Abe, H. Nakazawa, Chem. Lett. 2016, 45, 1434-1436. 44 S. Zhang, C. Wang, Methods Protoc. 2023, 6, 63-68. 45 R. Kilaas, J. Microsc. 1998, 190, 45-51. Chapter III 94 Figure 19. N2 determination by GC-MS: a) N2O control experiment; b) N2 control experiment; c) catalytic reaction (Table 6, entry 4). [Ru(COD)(COT)] catalysis In a glovebox, 0.3 mL of a freshly prepared suspension of [Ru(COD)(COT)] (7.5 µmol) in THF and 0.2 mL of a THF solution of dimethylphenylsilane (115 µL, 0.75 mmol) were introduced into a Fisher-Porter vessel. The nitrogen atmosphere was replaced by 1 bar of nitrous oxide. The reaction mixture was stirred at 55 ºC for 24 h. After this period of time, the reaction crude was analysed by 1H NMR, revealing a conversion lower than 5% (Figure 20). Ru·SNS nanoparticles 95 Figure 20. 1H NMR spectrum (CD2Cl2, 300 MHz) after the reduction of N2O with PhMe2SiH using [Ru(COD)(COT)]. (* denotes mesitylene added as the internal standard and # denotes residual CH2Cl2). Hg test The standard procedure was repeated with the addition of Hg (20 mg, 0.1 mmol) to the reaction mixture, employing Ru⋅SNS40.5 as catalyst (7.5 µmol Ru) and Me2PhSiH as substrate (115 µL, 0.75 mmol). In this case, the conversion decreased from >99% to 30%. Formation of siloxane via the oxidation of disilane. In a glovebox, 0.2 mL of a THF-d8 solution containing disilane PhMe2SiSiMe2Ph (223 µL, 0.75 mmol) and 0.3 mL of a THF-d8 suspension of Ru⋅SNS40.5 (7.5 µmol) were introduced into a Fisher-Porter vessel, which was loaded with 1 bar of N2O and heated at 55 ºC for 24 h. After that period, 1H NMR analysis of the reaction crude revealed the absence of siloxane or any other reaction product. Chapter III 96 Formation of siloxane via condensation of silanol-silanol. In a glovebox, 0.2 mL of a THF-d8 solution containing commercial silanol PhMe2SiOH (246 µL, 1.5 mmol) and 0.3 mL of a THF-d8 suspension of Ru⋅SNS40.5 (7.5 µmol) were introduced into a J. Young valved NMR-tube. The mixture was heated at 55 ºC for 24 h. As revealed by 1H NMR analysis, no reaction occurred under these conditions. Formation of siloxane from silanol-silane dehydrogenation. In a glovebox, 0.1 mL of a THF-d8 solution containing dimethylphenylsilane (115 µL, 0.75 mmol), 0.2 mL of a THF-d8 solution containing dimethylphenylsilanol (123 µL, 0.75 mmol), and 0.2 mL of a THF-d8 suspension of Ru⋅SNS40.5 (7.5 µmol) were introduced into a J. Young valved NMR-tube, which was heated at 55 ºC for 24 h. 1H NMR analysis of the crude reaction mixture revealed the formation of tetramethyldiphenylsiloxane, (Me2PhSi)2O, as well as the presence of H2.28 NMR and HRMS data for catalytic reaction products Me2PhSiOH (2a) The spectroscopic data for this product agree with those previously reported.47 1H NMR (400 MHz, CD2Cl2): δ 7.59 (m, 2 H, 2 CHPh), 7.38 (m, 3 H, 3 CHPh), 0.36 (s, 6 H, 2 CH3) ppm. 13C{1H} NMR (101 MHz, CD2Cl2): δ 138.3 (CqPh), 133.4, 132.9, 129.3 (2:1:2, CHPh), -2.1 (CH3) ppm. (Me2PhSi)2O (3a) The spectroscopic data for this product agree with those previously reported.47 1H NMR (400 MHz, CD2Cl2): δ 7.61 (m, 4 H, 4 CHPh), 7.39 (m, 4 H, 4 CHPh), 7.38 (m, 2 H, 2 CHPh), 0.40 (s, 12 H, 4 CH3) ppm. 13C{1H} NMR (101 MHz, CD2Cl2): δ 139.8 (CqPh), 133.1, 129.3, 127.7 (2:1:2, CHPh), 0.62 (CH3) ppm. 47 R. Zeng, M. Feller, Y. Ben-David, D. Milstein, J. Am. Chem. Soc. 2017, 139, 5720-5723. Ru·SNS nanoparticles 97 HRMS (CI): m/z calcd for C16H22NaOSi2 [(M+Na)+]: 309.1107; found: 309.1100. MePh2SiOH (2b) The spectroscopic data for this product agree with those previously reported.47 1H NMR (400 MHz, CD2Cl2): δ 7.54 (m, 4 H, 4 CHPh), 7.39 (m, 2 H, 2 CHPh), 7.33 (m, 4 H, 4 CHPh), 0.6 (s, 3 H, CH3) ppm. 13C{1H} NMR (101 MHz, CD2Cl2): δ 137.7 (2 CqPh), 134.0 (4 CHPh), 129.7 (2 CHPh), 127.8 (4 CHPh), -0.74 (CH3) ppm. HRMS (CI): m/z calcd. for C13H15OSi [(M+H)+]: 215.0892; found: 215.0887. [(PhCH2CH2)Me2Si]2O (3c) 1H NMR (400 MHz, CD2Cl2): δ 7.32 (m, 4 H, 4 CHPh), 7.27 (m, 4 H, 4 CHPh), 7.20 (m, 2 H, CHPh) 2.71 (t, 3JHH = 9.1 Hz, 4 H, 2 PhCH2CH2Si), 0.96 (t, 3JHH = 9.1 Hz, 4 H, 2 PhCH2CH2Si), 0.17 (s, 12 H, 4 CH3) ppm. 13C{1H} NMR (101 MHz, CD2Cl2): δ 145.3 (2 CqPh), 128.2, 127.7, 125.4 (2:2:1, 10 CHPh), 29.4 (2 PhCH2CH2Si), 20.4 (2 PhCH2CH2Si), 0.04 (4 CH3) ppm. HRMS (CI): m/z calcd. for C20H30NaOSi2 [(M+Na)+]: 365.1733; found: 365.1724. nPr3SiOH (2d) The spectroscopic data for this product agree with those previously reported.48 1H NMR (400 MHz, CD2Cl2): δ 1.42 (m, 6 H, 3 CH2), 1.00 (m, 9 H, 3 CH3), 0.62 (m, 6 H, 3 CH2) ppm. 13C{1H} NMR (101 MHz, CD2Cl2): δ 18.0 (3 CH2), 17.76 (3 CH2), 16.6 (3 CH3) ppm. HRMS (CI): m/z calcd. for C9H23OSi [(M+H)+]: 175.1518; found: 175.1509. (nPr3Si)2O (3d) 1H NMR (400 MHz, CD2Cl2): δ 1.36 (m, 12 H, 6 CH2), 0.96 (m, 18 H, 6 CH3), 0.53 (m, 12 H, 6 CH2) ppm. 13C{1H} NMR (101 MHz, CD2Cl2): δ 18.5 (6 CH2), 18.2 (6 CH2), 16.8 (6 CH3) ppm. HRMS (CI): m/z calcd. for C18H43OSi2 [(M+H)+]: 331.2852; found: 331.2841. 48 K. Shimizu, K. Shimura, N. Imaiida, A. Satsuma, J. Mol. Catal. A: Chemical 2012, 365, 5054. Chapter III 98 (EtO)3SiOH (2f) The spectroscopic data for this product agree with those previously reported.49 1H NMR (400 MHz, CD2Cl2): δ 3.82 (q, 3JHH = 7.0 Hz, 6 H, 3 CH2), 1.25 (t, 3JHH = 7.0 Hz, 9 H, 3 CH3) ppm. 13C{1H} NMR (101 MHz, CD2Cl2): δ 59.0 (3 CH2), 18.6 (3 CH3) ppm. HRMS (CI): m/z calcd. for C6H17O4Si [(M+H)+]: 181.0896; found: 181.0888. [(EtO)3Si]2O (3f) The spectroscopic data for this product agree with those previously reported.50 1H NMR (400 MHz, CD2Cl2): δ 3.82 (q, 3JHH = 7.0 Hz, 12 H, 6 CH2), 1.22 (t, 3JHH = 7.0 Hz, 18 H, 6 CH3) ppm. 13C{1H} NMR (101 MHz, CD2Cl2): δ 59.0 (6 CH2), 17.9 (6 CH3) ppm. HRMS (CI): m/z calcd. for C12H30NaO7Si2 [(M+Na)+]: 365.1428; found: 365.1418 49 V. Kazakova, O. B. Gorbatsevich, A. M. Muzafarov, Russ. Chem. Bull. 2005, 54, 1350-1351. 50 N. Ueda, T. Gunji, Y. Abe, J. Sol-Gel Sci. Technol. 2008, 48, 163-167. Ru·SNS nanoparticles 99 3.4 References 1 A. Khanzadeh, Ligand-Assisted Catalysis Using Metal SNS Complexes, PhD Thesis, University of Ottawa, 2023. 2 H. G. Sogukomerogullari, S. P. Yalçin, U. Cylan, E. Aytar, M. Aygün, D. S. Richeson, M. Sönmez, J. Chem. Sci. 2019, 131, 32. 3 V. Singh, R. Singh, A. S. Hazari, D. Adhikari, JACS Au 2023, 3, 1213-1220. 4 K. E. Rosenkoetter, M. K. Wojnar, B. J. Charette, J. W. Ziller, A. F. Heyduk, Inorg. Chem. 2018, 57, 9728-9737. 5 D. Spasyuk, S. Smith, D. G. Gusev, Angew. Chem. Int. Ed. 2013, 52, 2538-2542. 6 J. Schörgenhumer, A. Zimmermann, M. Waser, Org. Process Res. Dev. 2018, 22, 862-870. 7 R. Zeng, M. Feller, Y. Ben-David, D. Milstein, J. Am. Chem. Soc. 2017, 139, 57205723. 8 H. Tian, R. Xian, J.G. Canadell et al., Nature 2020, 586, 248-256. 9 G.A. Meehl, T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver and Z.-C. Zhao in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (Eds.), Cambridge University Press, 2007. 10 B. Aryal, R. Gurung, A. F. Camargo, G. Fongaro, H. Treichel, B. Mainali, M. J. Angove, H. H. Ngo, W. Guo, S. R. Puadel, Environ. Pollut. 2022, 314, 120272120288. 11 K. Severin, Chem. Soc. Rev. 2015, 44, 6375-6386. 12 Y. Pang, M. Leutzsch, N. Nöthling, J. Cornella, J. Am. Chem. Soc. 2020, 142, 1947319479. 13 R. Zeng, M. Feller, Y. Diskin-Posner, L. J. W. Shimon, Y. Ben-David, D. Milstein, J. Am. Chem. Soc. 2018, 140, 7061-7064. 14 V. R. Landaeta, R. E. Rodríguez-Lugo, Inorg. Chim. Acta 2015, 431, 21-47. Chapter III 100 15 D. J. Xiao, E. D. Bloch, J. A. Mason, W. L. Queen, M. R. Hudson, N. Planas, J. Borycz, A. L. Dzubak, P. Verma, K. Lee, F. Bonino, V. Crocellà, J. Yano, S. Bordiga, D. G. Truhlar, L. Gagliardi, C. M. Brown, J. R. Long, Nature Chem. 2014, 6, 590-595. 16 I. Ortega-Lepe, P. Sánchez, L. L. Santos, P. Lara, N. Rendón, J. López-Serrano, V. Salazar-Pereda, E. Álvarez, M. Paneque, A. Suárez, Inorg. Chem. 2022, 61, 1859018600. 17 X. Chen, H. Wang, S. Du, M. Driess, Z. Mo, Angew. Chem., Int. Ed. 2022, 61, e202114598. 18 J. Bösken, R. E. Rodríguez-Lugo, S. Nappen, M. Trincado, H. Grützmacher, Chem. Eur. J. 2023, 29, e202203632, 1-8. 19 B. M. Trost, Z. T. Ball, J. Am. Chem. Soc. 2005, 127, 17644-17655. 20 R. A. Pramudita, K. Motokura, ChemSusChem 2021, 14, 281-292. 21 Y. Nagai, Org. Prep. Proced. Int. 1980, 12, 13-48. 22 L. Anthore-Dalion, E. Nicolas, T. Cantat, ACS Catal. 2019, 9, 11563-11567. 23 H. H. Moretto, M. Schulze, G. Wagner, in Silicones. Ullmann’s Encyclopedia of Industrial Chemistry, 32, 675-708. Weinheim: Wiley-VCH, 2012. 24 R. Murugavel, A. Voigt, M. G. Walawalkar, H. W. Roesky, Chem. Rev. 1996, 96, 2205-2236. 25 S. E. Denmark, C. S. Regens, Acc. Chem. Res. 2008, 41, 1486-1499. 26 Y. Abe, T. Gunji, Prog. Polym. Sci. 2004, 29, 149-182. 27 J. M. Asensio, D. Bouzouita, P. W. N. M. van Leeuwen, B. Chaudret, Chem. Rev. 2020, 120, 1042-1084. 28 P. Molinillo, B. Lacroix, F. Vattier, N. Rendón, A. Suárez, P. Lara, Chem. Commun. 2022, 58, 7176-7179. 29 K. Philippot, B. Chaudret, in Comprehensive Organometallic Chemistry III, R. H. Crabtree & M. P. Mingos (Eds-in-Chief); Applications III: Functional Materials, Environmental and Biological Applications, D. O´Hare (Volume Ed.), Vol 12, Chapter 03, 71-99, Elsevier, 2007. 30 P. Lara, O. Rivada-Wheelaghan, S. Conejero, R. Poteau, K. Phlippot, B. Chaudret, Angew. Chem., Int. Ed. 2011, 50, 12080-12084. Ru·SNS nanoparticles 101 31 R. K. Ramamoorthy, K. Soulantica, I. Del Rosal, R. Arenal, P. Decorse, J-Y. Piquemal, B. Chaudret, R. Poteau, G. Viau, Chem. Mater. 2022, 34, 2931-2944. 32 D. J. Morgan, Surf. Interface Anal. 2015, 47, 1072-1079. 33 N. Chakroune, G. Viau, S. Ammar, L. Poul, D. Veautier, M. M. Chehimi, C. Mangeney, F. Villain, F. Fiévet, Langmuir 2005, 21, 6788-6796. 34 J. B. Ernst, S. Muratsugu, F. Wang, M. Tada and F. Glorius, J. Am. Chem. Soc. 2016, 138, 10718-10721. 35 J. Wang, C. F. Mbah, T. Przybilla, B. A. Zubiri, E. Spiecker, M. Engel, N. Vogel, Nat. Commun. 2018, 9, 5259. 36 A. F. Schmidt, V. V. Smirnov, Top. Catal. 2005, 32, 71-75. 37 K. Kucinsky, H. Stachowiak-Dłużyńska, G. Hreczycho, Coord. Chem. Rev. 2022, 459, 214456. 38 S. Ananda Kumar, M. Alagar, M. Mandhakini in Concise Encyclopedia of High Performance Silicones, Chapter 3, 39-45, J. P. Lewicki, R. S. Maxwell (Eds), Wiley, 2014. 39 I. C. Chagunda, T. Fisher, M. Schierling, J. S. McIndoe, Organometallics 2023, 42, 2938-2945. 40 F. Neumeyer, N. Auner, Chem. Eur. J. 2016, 22, 17165-17168. 41 W. T. Grubb, J. Am. Chem. Soc. 1954, 76, 3408-3414. 42 P. Pertici, G. Vitulli, Inorg. Synt. 1983, 22, 176-181. 43 M. Ito, M. Itazaki, T. Abe, H. Nakazawa, Chem. Lett. 2016, 45, 1434-1436. 44 S. Zhang, C. Wang, Methods Protoc. 2023, 6, 63-68. 45 R. Kilaas, J. Microsc. 1998, 190, 45-51. 46 P. Laoharojanaphand, T. J. Lin, J.O. Stoffer, J.Appl. Polym. Sci. 1990, 40, 369-384. 47 R. Zeng, M. Feller, Y. Ben-David, D. Milstein, J. Am. Chem. Soc. 2017, 139, 57205723. 48 K. Shimizu, K. Shimura, N. Imaiida, A. Satsuma, J. Mol. Catal. A: Chemical 2012, 365, 50-54. 49 V. Kazakova, O. B. Gorbatsevich, A. M. Muzafarov, Russ. Chem. Bull. 2005, 54, 1350-1351. Chapter III 102 50 N. Ueda, T. Gunji, Y. Abe, J. Sol-Gel Sci. Technol. 2008, 48, 163-167. Chapter IV: First row transition metal nanoparticles stabilised by NHC ligands as catalysts for the methanolysis of ammonia-borane Chapter IV of this PhD thesis primarily focuses on the synthesis and characterisation of iron, cobalt and nickel nanoparticles using imidazole-2-ylidene carbenes as stabilising agents. These colloids have demonstrated activity in the methanolysis of H3N·BH3, a process that releases hydrogen gas. Chapter IV 110 focus on second or third row transition metals. To date, there are few examples in the literature describing NHC-stabilised 1st row transition metal nanoparticles, with most examples concentrating on nickel.9 Consequently, exploring the use of carbenes to stabilise cobalt or iron nanoparticles presents a promising research field. Among the various metals, 2nd and 3rd row transition metals often exhibit highly desirable catalytic properties; however, their relatively high cost and limited availability restrict their large-scale use. In contrast, 1st row transition metals offer advantages in terms of economic feasibility and affordability. They are generally safer for human health and have a lower environmental impact compared to precious metals. A major focus in contemporary catalysis is the development of catalysts based on earth abundant elements for green energy applications, including iron, cobalt or nickel, among others.10 Additionally, sustainable energy production is one of the most pressing challenges of the 21st century, driven by the depletion of fossil fuel reserves and the adverse effects of climate change associated with the continued use of non-renewable energy sources.11 Fossil fuels are responsible for unprecedented levels of atmospheric CO2, significantly contributing to the greenhouse effect and the ongoing rise of global average temperatures.12 Currently, scientific consensus in peer-reviewed literature indicates that the anthropogenic influence on global warming exceeds 99%.13 Therefore, there is an urgent need to identify new energy sources to meet the growing energy demand, which has surged since the industrial revolution. Among various studied technologies, an economy based on hydrogen as an energy vector, the socalled hydrogen economy, is viewed as a leading alternative to the predominant use of fossil fuels.14 Hydrogen (H2) serves as a viable energy storage method, with 9 D. Bouzouita, J. M. Asensio, V. Pfeifer, A. Palazzolo, P. Lecante, G. Pieters, S. Feuillastre, S. Tricard, B. Chaudret, Nanoscale 2020, 12, 15736-15742. 10 J. I. van der Vlugt, Eur. J. Inorg. Chem. 2012, 363-375. 11 N. Armaroli, V. Balzani, Angew. Chem. Int. Ed. 2007, 46, 52-66. 12 B. J. van Ruijven, E. De Cian, I. S. Wing, Nat. Commun. 2019, 10, 2762. 13 M. Lynas, B. Z. Houlton, S. Perry, Environ. Res. Lett. 2021, 16, 114005. 14 S. Studer, S. Stucki, J. D. Speight in Hydrogen as a Future Energy Carrier, Chapter 3, 2368, A. Zuttel, A. Borgschulte, L. Schlapbach (Eds-in-Chief), Wiley-VCH, 2008 111 1st row transition metal·NHC nanoparticles potential applications in both stationary and mobile settings. Therefore, it is not surprising that the generation of hydrogen from renewable sources, along with its safe and reversible storage, are currently very active fields of research.15 Additionally, the gravimetric energy density of hydrogen is very high, 120 MJ/kg (for comparison, gasoline is only 47 MJ/kg) and its combustion reaction produces only water.16,17 For these reasons, a transition from liquid fossil fuel energy sources to a sustainable hydrogen economy is expected to occur within this century.17 However, a hydrogen-based economy is not inherently green. Currently, about 90% of hydrogen is produced through the steam reforming process (illustrated in Scheme 1) or similar methods that utilise various organic molecules, such as glycerol or propane, resulting in carbon dioxide emissions.18,19,20 Hydrogen produced through this process is referred to as “grey hydrogen”, distinguishing it from the renewable “green hydrogen”. In recent years the term “blue hydrogen” has gained visibility, denoting the integration of carbon capture technologies (or CCUS, acronym for Carbon Capture, Utilisation and Storage) with steam reforming, thereby contributing to the reduction of CO2 emissions.21 Scheme 1. Steam reforming of methane. Although not as extensive as other processes, electrochemical water splitting (also called water electrolysis) is particularly promising in terms of hydrogen 15 A. Züttel in Catalysis for Sustainable Energy Production, Chapter 5, 109-169, P. Barbaro, C. Bianchini (Eds-in-Chief), Wiley-VCH, 2009. 16 M. Ball, M. Weeda, Int. J. Hydrogen Energy 2015, 40, 7903-7919. 17 F. Qureshi, M. Yusuf, M. A. Khan, H. Ibrahim, B. C. Ekeoma, H. Kamyab, M. M. Rahman, A. K. Nadda, S. Chelliapan, Fuel 2023, 340, 127574. 18 M. Mosinska, M. I. Szynkowska, P. Mierczynski, Catalysts 2020, 10, 896. 19 M. E. Sad, H. A. Duarte, Ch. Vignatti, C. L. Padró, C. R. Apesteguía, Int. J. of Hydrogen Energy 2015, 40, 6097-6106. 20 A. Di Nardo, M. Portarapillo, D. Russo, A. Di Benedetto, Int. J. of Hydrogen Energy 2024, 55, 1143-1160. 21 J. Incer-Valverde, A. Korayem, G. Tsatsaronis, T. Morosuk, Energy Conversion and Management 2023, 291, 117294. 112 Chapter IV production. As Scheme 2 summarises, this reaction consists of an electrolytic rupture of a water molecule, with the subsequent production of oxygen (Oxygen Evolution Reaction, OER) and hydrogen (Hydrogen Evolution Reaction, HER), depending on the particular half-reaction. However, this reaction is not thermodynamically spontaneous under standard state conditions, given that its change in free energy is ∆G0 = + 237 kJ/mol H2 or, in terms of potential, ∆E0 = -1.23 V. If this energetic requirement is satisfied by sunlight, it is possible to refer the splitting of water as a photoelectrochemical process.22 This photoelectrochemical technique, also known as “artificial photosynthesis”, could couple solar energy with hydrogen production in an easy, cheap and sustainable way, enormously contributing to the adoption of a green hydrogen economy.23,24,25 Although currently the water splitting process contributes 4% to global hydrogen production, several reported cases of different experimental settings achieve this reaction in a spontaneous way. Therefore, its relative importance is expected to grow in the near future in comparison to fossil fuels.22 Scheme 2. Half-reactions of water splitting: Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER). Given the physical properties of hydrogen, particularly its high flammability, and the necessity of producing hydrogen abundantly and cleanly, the development of 22 J. W. Ager, M. R. Shaner, K. A. Walczak, I. D. Sharp, S. Ardo, Energy Environ. Sci. 2015, 8, 2811-2824. 23 E. L. Miller, Energy Environ. Sci. 2015, 8, 2809-2810. 24 A. Raveendra, M. Chandran, R. Dhanusuraman, RSC. Adv. 2023, 13, 3843-3876. 25 H. Hou, J. Muñoz, I. J. Gómez, N. Romero, X. Sala, J. García-Antón, Mater. Today Chem. 2024, 37, 102021. 113 1st row transition metal·NHC nanoparticles safe and economical hydrogen storage and release systems is essential for implementing a large-scale hydrogen-based economy. Traditional approaches to hydrogen storage, such as compressed or liquefied hydrogen, require extreme conditions of pressure (300-700 bar) or temperature (below -252 ºC).26 However, some hydrogen applications rely on easy consumption in fuel cells, which can be challenging with existing storage techniques based on pressurised and liquefied gas.27 A fuel cell is a device that continuously converts chemical energy into electricity. While there are various types of fuel cells, hydrogen fuel cells are characterised by their ability to electrochemically convert hydrogen and oxygen into electrical energy, producing only water as a byproduct. Because they do not emit CO2 or other pollutants, hydrogen fuel cells present an attractive clean alternative to traditional internal combustion engines that use diesel or gasoline.28 Among other requirements, hydrogen fuel cells need a supply of high purity hydrogen. Therefore, it is crucial to develop safe and efficient hydrogen carriers capable of releasing H2 before use and allowing operation under milder conditions. Notably, the storage of hydrogen in chemical compounds (hydrogen carriers) through the reversible formation of covalent bonds has received considerable attention.29 In general, hydrogen carriers can be categorised into two main types: hydrides of light-weight elements (HLEs) and physical sorbents of H2. HLEs are particularly interesting due to their high gravimetric H2 contents, with notable examples including borohydrides, amide-hydrides, and ammonia-borane along with its derivatives.30-33 26 D. Clematis, D. Bellotti, M. Rivarolo, L. Magistri, A. Barbucci, Energies 2023, 16, 6035. 27 L. Fan, Z. Tu, S. H. Chan, Energy Rep. 2021, 7, 8421-8446. 28 O. Z. Sharaf, M. F. Orhan, Renewable Sustainable Energy Rev. 2014, 32, 810-853. 29 T. He, P. Pachfule, H. Wu, Q. Xu, P. Chen, Nat. Rev. Mater. 2016, 1, 16059. 30 A. Züttel, P. Wenger, S. Rentsch, P. Sudan, Ph. Mauron, Ch. Emmenegger, J. Power Sources 2003, 118, 1-7. 31 P. Chen, Z. Xiong, J. Luo, J. Lin, K. L. Tan, Nature 2002, 420, 302-304. 32 A. Gutowska, L. Li, Y. Shin, C. M. Wang, X. S. Li, J. C. Linehan, R. S. Smith, B. D. Kay, B. Schmid, W. Shaw, M. Gutowski, T. Autrey, Angew. Chem. Int. Ed. 2005, 44, 3578-3582. 33 L. Li, Q. Gu, Z. Tang, X. Chen, Y. Tan, Q. Li, X. Yu, J. Mater. Chem. A 2013, 1, 1226312269. 114 Chapter IV Ammonia-borane (H3N·BH3, AB) is a non-flammable, non-toxic white solid with a very high hydrogen content of 19.5 wt%. It is soluble in polar solvents such as water and methanol and remains stable under atmospheric pressure and room temperature. These characteristics make H3N·BH3 an excellent hydrogen carrier. For decades, the synthesis of ammonia-borane has involved the use of sodium or lithium borohydride, ammonium salts and complex isolation steps at low temperatures (around -75 ºC).34,35,36 However, in 2007, Ramachandran et al. reported a simple and rapid synthesis followed by an easy purification process that involves only a filtration step (Scheme 3). 37 This synthetic procedure is currently being utilised on a laboratory scale. Scheme 3. Synthesis of ammonia-borane. On the other hand, hydrogen stored in AB can be released through various methods, including thermolysis, dehydrogenation, hydrolysis or methanolysis.38,39 Both thermolysis and dehydrogenation share a significant drawback: to obtain the three equiv. of H2 contained in the molecule, each successive step generates multiple secondary products that are often poorly defined and characterised. In the case of thermolysis, as simplified in Scheme 4, the release of each subsequent equiv. of H2 requires progressively higher temperatures. The initial step, which requires temperatures between 90-130 ºC, produces polyaminoboranes (H2NBH2)x. The second step, which requires significantly higher temperatures, 130-350 ºC, generates polyiminoboranes (also known as borazines, (HNBH)x) as by-products. The third step 34 S. G. Shore, R. W. Parry, J. Am. Chem. Soc. 1955, 77, 6084-6085. 35 M. G. Hu, J. M. Van Paasschen, R. A. Geanangel, J. Inorg. Nucl. Chem. 1977, 39, 21472150. 36 E. Mayer, Inorg. Chem. 1973, 12, 1954-1955. 37 P. V. Ramachandran, P. D. Gagare, Inorg. Chem. 2007, 46, 7810-7817. 38 D. Sun, V. Mazumder, O. Metin, S. Sun, ACS Catal. 2012, 2, 1290-1295. 39 A. Rossin, M. Peruzzini, Chem. Rev. 2016, 116, 8848-8872. 1st row transition metal·NHC nanoparticles 115 of this thermal decomposition requires temperatures exceeding 350 ºC, making it uncommon to observe the release of the third equiv. of H2 in conjunction with the subsequent boron nitrides (NB)x.40,41 It is important to note that only the dominant products are mentioned here, as volatile by-products are challenging to characterise.42 In the case of the ammonia-borane dehydrogenation process, summarised in Scheme 5, hydrogen gas is produced along with secondary by-products such as polyaminoboranes and polyiminoboranes. Similarly to thermolysis, the release of the third hydrogen equiv. is particularly difficult and requires more severe conditions.41,43 Scheme 4. Thermolysis of ammonia-borane. On the contrary, the hydrolysis and methanolysis of AB are particularly intriguing due to their high hydrogen generation potential and the formation of a well-defined secondary product, NH4BO2 in the case of hydrolysis or NH4B(OMe)4 for the methanolysis case.43 40 A. Staubitz, A. P. M. Robertson, I. Manners, Chem. Rev. 2010, 110, 4079-4124. 41 X. Zhang, L. Kam, T. J. Williams, Dalton Trans. 2016, 45, 7672-7677. 42 P. Wang, Dalton Trans. 2012, 41, 4296-4302. 43 H. Li, Z. Yao, X. Wang, Y. Zhu, Y. Chen, Energy Fuels 2022, 36, 11745-11759. Chapter IV 116 Scheme 5. Overview of secondary products obtained by dehydrogenation of ammonia-borane. Furthermore, the ability to carry out these reactions at room temperature along with the option to regenerate AB from the secondary solid products (NH4B(OMe)4 or NH4BO2, Scheme 6) are notable advantages.37,41 Methanolysis is specially promising because it offers a wider range of applicable temperatures owing to the lower freezing point of methanol (-98 ºC) compared to that of water (0 ºC). Scheme 6. Hydrolysis and methanolysis of ammonia-borane. The first precedent of AB methanolysis was reported by Ramachandran et al. as illustrated in Scheme 7.37 Their protocol involved the use of a ruthenium salt, RuCl3, to catalyse the reaction under mild temperature conditions (25 ºC) generating nearly 3 equiv. of hydrogen within one minute. The research focused on the characterisation of a secondary solid product, ammonium tetramethoxyborate, which 1st row transition metal·NHC nanoparticles 117 was isolated as a crystalline material containing two methanol molecules of crystallisation. Scheme 7. First reported example for ammonia-borane methanolysis. Since then, a wide variety of catalytic systems for ammonia-borane methanolysis has been described in the literature. These systems primarily consist of nanoparticle-based catalysts, though there are also a few examples of homogeneous complexes capable of performing H3N·BH3 methanolysis.43,44,45 Some representative cases are discussed in the following paragraphs and summarised in Table 1.46-49 For example, Jagirdar et al. reported a series of cobalt and nickel based nanocomposites, namely Co-Co2B and Ni-Ni3B, synthesised by reducing Co2+ and Ni2+ salts, respectively.45 They obtained mostly agglomerated samples, although a small number of nanoparticles with mean sizes between 4-8 nm were also present. These nanocomposites (Table 1, entries 1 and 2) were found to catalyse ammoniaborane methanolysis, exhibiting TOF of 7.5 and 5.0 min-1 for Co-Co2B and Ni-Ni3B, respectively. Moreover, this is not the only example of a nickel nanocatalyst capable of catalysing AB methanolysis. Later, Özkar et al. described a series of nickel(0) nanoparticles stabilised by polyvinylpyrrolidone (PVP) with an average particle size of 3.0 (0.7) nm.46 This system proved to be catalytically active in both hydrazine borane and ammonia-borane methanolysis, exhibiting a TOF value of 12.1 min-1 in the latter case (Table 1, entry 3). Another noteworthy example was reported by Sun et 44 V. San Nacienceno, M. A. Garralda, J. M. Matxain, Z. Freixa, Organometallics 2020, 39, 1238-1248. 45 S. B. Kalidindi, A. A. Vernekar, B. R. Jagirdar, Phys. Chem. Chem. Phys. 2009, 11, 770-775. 46 D. Özhava, N. Z. Kiliçaslan, S. Özkar, App. Catal. B: Environmental 2015, 162, 573-582. 47 C. Yu, J. Fu, M. Muzzio, T. Shen, D. Su, J. Zhu, S. Sun, Chem. Mater. 2017, 29, 1413-1418. 48 P. Lara, A. Suárez, K. Philippot, ChemCatChem 2019, 11, 766-771. 49 N. Caner, M. Yurderi, A. Bulut, G. S. Kanberoglu, M. Kaya, M. Zahmakiran, New. J. Chem. 2020, 44, 12435-12439. Chapter IV 118 al., who described a bimetallic CuNi nanoalloy assembled on graphene (Table 1, entry 4) which exhibited a TOF value of 49.1 min-1, one of the highest values reported for non-noble metals.47 In comparison to the examples previously mentioned, catalysts based on noble metals generally exhibit superior catalytic performance. For instance, P. Lara et al. synthesised a series of small platinum nanoparticles (1.5-2.2 nm), stabilised by terphenylphosphane ligands which were found to be highly active in generating H2 from AB. Notably, the sample prepared with dimethyl-2,6-bis(2’,6’- di(isopropyl)phenyl)phenylphosphane, referred to as L3 in their work, demonstrated a TOF value of 284 min-1 (Table 1 entry 5).48 Additionally, regarding a noble metal, Zahmakiran et al. reported a heterogeneous system consisting of palladium nanoparticles stabilised by a metal organic framework; Pd@MIL-101 (MIL-101 = (Cr3F(H2O)2O{O2CC6H4(CO2)}3·nH2O). This catalyst exhibited the highest activity among all the examples described, with a TOF value of 1080 min-1 (Table 1 entry 6).49 Table 1. Representative examples of catalytic performance in AB methanolysis reported in the literature. Entry Catalyst T (ºC) TOF (min -1 ) Metal loading (mol%) 1 Nano Co−Co 2 B 25 7.5 20 2 Nano Ni−Ni 3 B 25 5.0 20 3 PVP-stabilised Ni 25 12.1 0.5 4 Cu 36 Ni 64 /Graphene 25 49.1 7.2 5 Pt∙L30.2 30 284 0.19 6 Pd@MIL-101 25 1080 0.037 As previously mentioned, while noble-metal catalysts exhibit excellent catalytic properties, they are less attractive for cost-effective applications compared to more affordable metals. To address this, various research efforts are focused on developing catalysts based on first-row transition metals. Chapter IV of this PhD Thesis focuses on the development of 1st row transition metal nanoparticles stabilised by uNHC ligands and the study of their catalytic performance in the methanolysis of H3N·BH3. 1st row transition metal·NHC nanoparticles 119 4 4.2 Results and Discussion 4.2.1 Synthesis of IMes and IPr NHC ligands 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and 1,3-bis-(2,6diisopropylphenyl)imidazol-2-ylidene (IPr) have been synthesised following previously reported procedures as shown in Scheme 8.50 The condensation of glyoxal with the corresponding aniline (2,4,6-trimethylaniline or 2,6-diisopropylaniline) produces a diazabutadiene, which is then converted into the corresponding NHC·HCl salt through treatment with HCl and paraformaldehyde. Subsequently, treatment of the reaction mixture with HBF4 facilitates the exchange of the Clanion for BF -. Finally, deprotonation of the salt with NaH allows for the isolation of the corresponding NHC with excellent purity and high yields (see section 4.3.2 for further details). Scheme 8. Synthesis of IMes and IPr ligands. 4.2.2 Synthesis and characterisation of 1st row transition metal·NHC NPs The synthesis of carbene-stabilised iron, cobalt or nickel nanoparticles using NHCs (N-heterocyclic carbenes) was conducted following the organometallic approach outlined in Scheme 9. The corresponding organometallic precursor, {Fe[N(SiMe3)2]2}2, [Co(COE)(COD)], or [Ni(COD)2], where COE = 1-cyclooctene and COD = 1,5-cyclooctadiene, was decomposed under 3 bar of H2 in the presence of a substoichiometric amount of the appropriate uNHC ligand (Scheme 9). The reaction 50 X. Bantreil, S. P. Nolan, Nat. Protoc., 2011, 6, 69-77. Chapter IV 126 in the HRTEM images, illustrated in Figure 11, corresponds to the planes of the fcc structure of Ni(0) and Ni2O4 species, consistent with literature reports.46 In addition to HRTEM characterisation, analysis via STEM-EDX, including intensity maps (Figure 12), confirmed the nickel composition of the nanoparticles. While the presence of a peak for nitrogen may be attributed to partial decomposition of the carbene ligand, a peak corresponding to oxygen was also observed, likely due to the presence of Ni2O4. Figure 11. HRTEM image (left) and Fast Fourier Transform of spatial frequencies (right) corresponding to Ni·IMes0.2. 400 360 320 280 240 200 160 120 80 40 0 001 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 keV Figure 12. STEM-EDX analysis (left) and intensity map (right) for Ni·IMes0.2. On the other hand, HRTEM for Ni·IPr0.2 NPs revealed significant discrepancies compared to the results obtained for Ni·IMes0.2. Figure 13 presents a HRTEM image of Ni·IPr0.2 and its corresponding FFT, where the interplanar distances Counts CKa NKa OKa FKa NiLl NiLa CuLa SiKa NiKa CuKa NiKb CuKb 1st row transition metal·NHC nanoparticles 127 found correspond to the hcp structure of Ni3O6, while no distances associated with fcc structure of Ni(0) were detected. Figure 13. HRTEM image (left) and Fast Fourier Transform of spatial frequencies (right) corresponding to Ni·IPr0.2. The composition and chemical state of the nanoparticle surface was analysed using XPS by Dr. Florencia Vattier Lagarrigue at the “Instituto de Ciencia de Materiales de Sevilla” (ICMS). In all cases, the obtained spectra were quite complex, featuring peaks with various multiplets and a significant contribution from oxidised species, despite careful handling of the samples. The high resolution XPS spectrum of 2p region for Fe·IMes0.5 distinguishes two multiplets centred at 709.4 eV and 723.3 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 region, respectively. The Fe 2p3/2 signal can be fitted into three main contributors: Fe(0) at 706.3 eV (Figure 14a, top, blue line), and oxidised Fe species: FeO (Figure 14a, top, green line) and Fe2O3 (Figure 14a, top, brown line).53 Figure 14b presents the spectrum for the N 1s region of Fe·IMes0.5, showing a peak at 399.9 eV that confirms the presence of the ligand in the sample. This binding energy corresponds to a displacement of 1.1 eV relative to free IMes (401 eV). This technique enables the estimation of metal oxidation at the surface of the particles. The proportion of metallic 53 M. C. Biesinger, B. P. Payne, A. P. Grosvenor, L. W.M. Lau, A. R. Gerson, R. St.C. Smart, Appl. Surf. Sci. 2011, 257, 2717-2730. Chapter IV 128 Fe(0) accounted for only 4% of the total, and increased to 15% after surface cleaning by mild condition sputtering with ionised argon (Figure 14a, bottom). Figure 14. a) XPS spectra of the Fe 2p3/2 region of Fe·IMes0.5, prior to (bottom) and after (top) mild conditions sputtering with ionised argon. b) XPS spectra of the N 1s region for the fresh Fe·IMes0.5 (bottom) and the free IMes ligand (top). Equivalent results were obtained for Fe·IPr0.5 sample, with the corresponding spectra represented in Figure 15. In this case, the contribution of iron in the zerooxidation state was 3%, which increased to 23% after the removal of the oxidised external layer. Figure 15. a) XPS spectra of the Fe 2p3/2 region of Fe·IPr0.5, prior to (bottom) and after (top) mild conditions sputtering with ionised argon. b) XPS spectra of the N 1s region for the fresh Fe·IPr0.5 (bottom) and the free IPr ligand (top). 1st row transition metal·NHC nanoparticles 129 For Co·IMes0.2, the corresponding XPS spectrum displays two multiplets centred at 782.6 and 799.7 eV of binding energy (BE), attributed to spin orbit splitting of Co 2p3/2 and Co 2p1/2, respectively. The Co 2p3/2 signal (Figure 16) could be deconvoluted into three main components: metallic cobalt at 778.2 eV BE (blue curve) and the oxidised species (Co+2 y Co+3, green and brown line, respectively).53,54 The depth of the oxidation layer can be assessed by varying the measurement angle relative to the analyser. At a 0º angle, the metal contribution in atomic concentration (% At) is 20% (Figure 16, bottom, blue line), while at 45º, the metal contribution decreases to 8% (Figure 16, top, blue line) due to the thinning of the measured layer as the angle increases. This indicates that the oxide is confined to the surface layers of the sample. 792 788 784 780 776 BE (eV) Figure 16. XPS spectra of the Co 2p3/2 region for Co·IMes0.2 registered at 0º (bottom) and 45º (top) take off angles. The proportion of metallic Co(0) increases from 20% to 40% after the oxidised material layer was removed by ionised argon sputtering under mild conditions (Figure 17a, blue line). It can be assumed that the conditions during Ar sputtering are comparable to those in suspension under an inert atmosphere, 54 T. Mathew, S. Shylesh, B. M. Devassy, M. Vijayaraj, C. V.V. Satyanarayana, B. S. Rao, C. S. Gopinath, App. Catal. A 2004, 273, 34-45. Arbitrary Units Chapter IV 130 suggesting that cobalt presented in catalytic nanoparticles is likely in the zero oxidation state. This hypothesis is supported by the position of the Co 2p3/2 peaks for the reduced metal at 778.2 eV, a characteristic value for nanomaterials with sizes around 3 nm, which is consistent with the mean nanoparticle size found for Co·IMes0.2, 2.2 (0.4) nm.55 Figure 17. a) XPS spectra of the Co 2p3/2 region for Co·IMes0.2, prior to (bottom) and after (top) mild conditions sputtering with ionised argon. b) XPS spectra of the N 1s region for the fresh Co·IMes0.2 (bottom), the Ar sputtered sample (middle) and the free IMes ligand (top). XPS spectroscopy also confirmed the presence of IMes ligand on the nanoparticle surface (Figure 17b). For the free IMes ligand, the N 1s region shows a sharp signal (3.1 eV) centred at 401.0 eV (Figure 17b, top), while this signal is slightly broader (3.4 eV) and shifted (399.7 eV) for both Co·IMes0.2 samples (Figure 17b, 55 M. R. Narouz, C.-H. Li, A. Nazemi, C. M. Crudden, Langmuir 2017, 33 14211-14219. 1st row transition metal·NHC nanoparticles 131 bottom-fresh sample and middle-sputtered sample).56-60 This binding energy corresponds to a displacement of 1.3 eV relative to free IMes, similar to that observed for Fe·IMes0.5 nanoparticles (Figure 14b, 1.1 eV). Co·IPr0.2 nanoparticles exhibited similar behaviour to their IMes-stabilised counterparts, with the corresponding spectra shown in Figure 18. After cleaning the surface by ion Ar+ sputtering, the amount of Co in the zero-oxidation state increased from 19 to 42 %. Figure 18. a) XPS spectra of the Co 2p3/2 region for Co·IPr0.2, prior to (bottom) and after (top) mild conditions sputtering with ionised argon. b) XPS spectra of the N 1s region for the fresh Co·IPr0.2 (bottom) and the free IPr ligand (top). 56 P. Molinillo, M. Puyo, F. Vattier, B. Lacroix, N. Rendón, P. Lara, A. Suárez, Nanoscale 2023, 15, 14488-14495. 57 L.M. Martínez-Prieto, I. Cano, A. Márquez, E. A. Baquero, S. Tricard, L. Cusinato, I. del Rosal, R. Poteau, Y. Coppel, K. Philippot, B. Chaudret, J. Cámpora, P. W. N. M. van Leeuwen, Chem. Sci. 2017, 8, 2931-2941. 58 N. Bridonneau, L. Hippolyte, D. Mercier, D. Portehault, M. Desage-El Murr, P. Marcus, L. Fensterbank, C. Chanéac, F. Ribot, Dalton Trans. 2018, 47, 6850-6859. 59 L. M. Martínez-Prieto, L. Rakers, A. M. López-Vinasco, I. Cano, Y. Coppel, K. Philippot, F. Glorius, B. Chaudret, P. W. N. M. van Leeuwen, Chem. Eur. J. 2017, 23, 12779-12786. 60 A. Rühling, K. Schaepe, L. Rakers, B. Vonhören, P. Tegeder, B. J. Ravoo, F. Glorius, Angew. Chem. Int. Ed. 2016, 55, 2016, 5856-5860. Chapter IV 132 The XPS spectrum of Ni·IMes0.2 nanoparticles shows the Ni 2p region, distinguishing two main groups of signals centred at binding energies of 852.9 and 869.9 eV, associated to the Ni 2p3/2 and Ni 2p1/2 photoemission peaks, respectively. The Ni 2p3/2 signal can be deconvoluted into three main contributors: Ni(0) at 852.5 eV (Figure 14a, blue line),53,61,62,63 and oxidised Ni species: NiO2 (Figure 14a, green line) and Ni(OH)2 (Figure 14a, brown line). This Ni(0) peak significantly increased after argon sputtering (Figure 19a, top, blue line) compared to the spectrum of the fresh sample (Figure 19a, bottom, blue line), indicating that the contribution of Ni(0) rises from 11% in the fresh sample to 75% in the sputtered sample. To confirm ligand coordination on the surface of Ni·IMes0.2, a high-resolution spectrum of the N 1s region was recorded, as illustrated in Figure 19b. The position of the peak corresponding to Ni·IMes0.2 at 400.9 eV (Figure 19b, bottom) is very similar to that of the free IMes ligand, 401.0 eV (Figure 19b, top). Figure 19. a) XPS spectra of the Ni 2p3/2 region for Ni·IMes0.2, prior to (bottom) and after (top) mild conditions sputtering with ionised argon. b) XPS spectra of the N 1s region for the fresh Ni·IMes0.2 (bottom) and the free IMes ligand (top). 61 A. P. Grosvenor, M. C. Biesinger, R. St.C. Smart, N. S. McIntyre, Surf. Sci. 2006, 600, 17711779. 62 M. C. Biesinger, B. P. Payne, L. W. M. Lau, A. Gerson, R. St.C. Smart, Surf. Interface Anal. 2009, 41, 324-332. 63 A. M. López-Vinasco, L. M. Martínez-Prieto, J. A. Asensio, P. Lecante, B. Chaudret, J. Cámpora, P. W. N. M. van Leeuwen, Catal. Sci. Tech. 2020, 10, 342. 1st row transition metal·NHC nanoparticles 133 A similar behaviour was observed for Ni·IPr0.2, with the corresponding spectra shown in Figure 20. After surface cleaning by Ar+ ion sputtering, the proportion of Ni in the zero-oxidation state increases from 13% to 65%. Figure 20. a) XPS spectra of the Ni 2p3/2 region for Ni·IPr0.2, prior to (bottom) and after (top) mild conditions sputtering with ionised argon. b) XPS spectra of the N 1s region for the fresh Ni·IPr0.2 (bottom) and free IPr ligand (top). The relative intensities of the corresponding photoemission signals enabled the quantification of Fe, Co, Ni, and N atoms on the surfaces of the nanoparticles. Table 3 presents the atomic concentrations of N and the metals (Fe, Co, and Ni), along with the M/N ratio. As expected, the surface coverage degree of Fe nanoparticles is higher than that of Co and Ni NPs, due to the larger quantity of ligand employed in the Fe NPs synthesis (0.5 equiv. vs. 0.2 equiv.). Table 3. Quantitative analysis of the surface composition of 1st row transition metal·NHC nanoparticles (percentage in atomic concentration, %At). Colloid M (% At) N (% At) M/N Fe·IMes0.5 32 68 0.5 Fe·IPr0.5 34 66 0.5 Co·IMes0.2 48 52 0.9 Co·IPr0.2 35 65 0.5 Ni·IMes0.2 71 29 2.4 Ni·IPr0.2 46 54 0.9 Chapter IV 134 It is also worth mentioning that, although there was no complete overlap between the species detected by XPS and HRTEM analysis, both techniques have confirmed the presence of Fe(0), Co(0) and Ni(0) in each case, along with a clear contribution from various oxidised species. This observation underscores the significant sensitivity of these three metals to air-exposure. The percentage of surface atoms was calculated according to the magic number rule,64,65 as in previous cases, considering fcc structure (Table 4) for Fe and Ni, and hcp structure for Co samples. The estimated percentages of surface atoms for each nanoparticle are presented in Table 5. Table 4. Building of closed-shell clusters by applying the magic number rule for metals with a fcc structure. Nºumber of shell (n) 0 1 2 3 4 5 Number of atoms in shell 1 12 42 92 162 252 Surface atoms (%) -- 92 76 63 52 45 Number of shell (n) 6 7 8 9 10 11 Number of atoms in shell 362 492 642 812 1002 1212 Surface atoms (%) 39 35 31 28 26 24 Table 5. Percentage of surface atoms calculated for 1st row transition metal·NHC nanoparticles by applying the magic number rule. Colloid Number of atoms Surface atoms (%) Fe·IMes0.5 541 45 Fe·IPr0.5 975 39 Co·IMes0.2 507 52 Co·IPr0.2 659 45 Ni·IMes0.2 1424 35 Ni·IPr0.2 3060 28 64 J. Wang, C. F. Mbah, T. Pryzbilla, B. A. Zubiri, E. Spiecker, M. Engel, N. Vogel, Nat. Commun. 2018, 9, 5259. 65 A. F. Schmidt, V. V. Smirnov, Top. Catal. 2005, 32, 71-75. 1st row transition metal·NHC nanoparticles 135 4.2.3 Methanolysis of AB catalysed by 1st row transition metal·NHC NPs Ammonia-borane (H3N·BH3, AB) has gained attention as a H2 storage molecule in recent years.37,40,66 H3N·BH3 exhibits several advantageous properties: it is a non-flammable, non-toxic, and stable white solid (boiling point 104 ºC) with a very high hydrogen content (19.6 %wt). Unlike methods such as thermal decomposition or dehydrogenation, which have limited capacity to fully release the maximum available H2, methanolysis of ammonia-borane under mild conditions can yield 3 equiv. of H2.48,67 Furthermore, as shown in Scheme 10, ammonium tetramethoxyborate (NH4B(OMe)4) is the only secondary product formed during this process. This solid product can be easily reconverted to H3N·BH3 by treatment with NH4Cl and a strong reductor like LiAlH4. Additionally, the use of nanoparticles to catalyse this reaction is well documented in the literature, as previously mentioned in the introduction of this chapter.43 Scheme 10. Methanolysis of ammonia-borane. The catalytic activity of NHC stabilised first row transition metal nanoparticles was evaluated for H2 generation from methanol solutions of H3N·BH3. This reaction was performed in a closed Fisher-Porter reactor equipped with a pressure detector connected to a computer and the catalytic performance was monitored by measuring the increase in gas pressure. Once no further changes in pressure were observed, the quantitative formation of NH4B(OMe)4 was confirmed by 11B NMR spectroscopy analysis, which shows the presence of only a singlet at δ 6.9 ppm. All reactions were conducted with a catalyst loading of 0.34 mol% (S/C = 300) and an ammonia-borane concentration of 1.25 M in MeOH, with the temperature maintained at 30 ºC. Results obtained in such conditions are summarised in Table 6. 66 S. Özkar, Int. J. Hydrogen Energy 2020, 45, 7881-7891. 67 I. Ortega-Lepe, A. Rossin, P. Sánchez, L. L. Santos, N. Rendón, E. Álvarez, J. LópezSerrano, A. Suárez, Inorg. Chem. 2021, 60, 18490-18502.