Evidence that steric factors modulate reactivity of tautomeric iron–oxo species in stereospecific alkane C–H hydroxylation

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Evidence that steric factors modulate reactivity of tautomeric iron–oxo species in stereospecific alkane C–H hydroxylation Mitra, Mainak; Lloret-Fillol, Julio; Haukka, Matti; Costas, Miquel; Nordlander, Ebbe Mitra, M., Lloret-Fillol, J., Haukka, M., Costas, M., & Nordlander, E. (2014). Evidence that steric factors modulate reactivity of tautomeric iron–oxo species in stereospecific alkane C–H hydroxylation. Chemical Communications, 50(12), 1408- 1410. https://doi.org/10.1039/C3CC47830K 2014 This journal is ©The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 2539--2542 | 2539 Cite this: Chem. Commun., 2014, 50,2539 Manganese carbonyl terpyridyl complexes: their synthesis, characterization and potential application as CO-release molecules† Jean-Daniel Compain, a Marc Bourrez, a Matti Haukka, b Alain Deronzier a and Sylvie Chardon-Noblat* a Mn I carbonyl terpyridyl complexes have been synthesized and characterized. The tricarbonyl derivative exhibits interesting behaviors for controlled CO-release by both thermal and photosynthetic pathways. Metal carbonyl complexes incorporating nitrogen heterocyclic ligands, especially those based on rhenium like for instance [Re I (L)(CO) 3 X] (L = bidentate diimine ligand), have been the subject of extensive investigation for applications ranging from the photo or electrocatalysis of CO 2 reduction 1 to the design of new active CO-release molecules (CO-RMs) as therapeutic agents. 2 The interest in these compounds stems from their various physico-chemical properties which depend largely on the ligand structure. Less is known about dicarbonyl Re equivalent complexes. 3 The strategy of developing studies looking at the effects of the ligand structure and the nature of metals in this type of complex greatly expands the range of properties and thus enhances their potential applications. Considering the general trend of replacing unsustainable noble metals by abundant first-row transition metals, manganese complexes aremoreandmoreenvisionedaspromising and safe substitutes to their Re counterparts. Whereas a significant number of Mn–carbonyl complexes are already known as CO-RMs, 2,4 our group 5 and the group of C. P. Kubiak 6 have recently demonstrated that complexes like [Mn(bipyridyl)(CO) 3 Br] could be used as selective electrocatalysts for CO 2 reduction, as efficient as their Re tricarbonyl analogues. As a continuation of our interest in exploring Mn I –carbonyl– polypyridyl complexes, in order to find new molecules for catalysis or CO-RM applications, we have prepared novel Mn I complexes featuring the ambidentate 2,20:60,200 -terpyridine (terpy) ligand (Scheme 1). We expected that an incoming pre-organized ligand close to the metal in the tri-CO terpy-k 2 N 1 ,N 2 Mn complex 1b would help in the decoordination of a CO ligand and lead to the di-CO terpy-k 3 N 1 ,N 2 ,N 3 species 2. Such decarbonylation reaction is essential for both electrocatalysis of CO 2 reduction and CO-release reactions utilizing carbonyl metallic complexes. The effect induced by a pre-organized chelating group close to the metallic center, on CO photodissociation reaction, has been recently demonstrated for cyclopentadienyl Mn–tricarbonyl derivatives. 7 It should be underlined that [Re(terpy-k 2 N 1 ,N 2 )(CO) 3 Cl] was reported in the 1990s, 8a,b and the corresponding dicarbonyl species was synthesized only recently by a thermal CO substitution under very harsh conditions 8c but, as far as we know, never by a photochemical pathway. Herein we report both synthetic pathways, which we have developed to synthesize 1a and 1b and to convert 1b into 2by CO substitution reactions under mild experimental conditions, as well as their characterization and potential applications. fac-[Mn(terpy-k 2 N 1 ,N 2 )(CO) 3 Br] (1a), in which the terpy acts as a bidentate ligand (NN-chelate), was synthesized with almost 90% yield from [Mn(CO) 5 Br] (see ESI†for experimental details and full characterization). The FTIR spectrum of 1a features three distinct n CRO stretching bands, in agreement with those observed for other fac-Mn tricarbonyl complexes. 91 Hand 13 CNMRspectra are characteristic of an unsymmetrically coordinated terpy. Single crystals of 1a (yellow blocks) have been grown by slow vapor diffusion of diisopropyl ether in a DMF solution. The crystallographic structure shows that both enantiomers of 1a coexist in the unit cell, which is thus racemic. The asymmetric unit is presented Scheme 1 The synthesized terpyridyl complexes. a Universite ´Joseph Fourier Grenoble 1/CNRS, De ´partement de Chimie Mole ´culaire, UMR 5250, Equipe Chimie Inorganique Redox, BP53, 38041 Grenoble Cedex 9, France. E-mail: [email protected] b Department of Chemistry, University of Jyva ¨skyla ¨, P.O. Box 35, 40014 Jyva ¨skyla ¨, Finland †Electronic supplementary information (ESI) available: Detailed experimental procedures, further characterization, and crystallographic data. CCDC 961705. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc00197d Received 9th January 2014, Accepted 10th January 2014 DOI: 10.1039/c4cc00197d www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 10 January 2014. Downloaded by Jyvaskylan Yliopisto on 14/01/2016 07:05:38. View Article Online View Journal | View Issue 2540 |Chem. Commun., 2014, 50, 2539--2542 This journal is ©The Royal Society of Chemistry 2014 in Fig. 1; it confirms the fac configuration of the complex and the bidentate coordination mode of the terpy to the Mn center, leaving one of the outer pyridyl rings uncoordinated (see Fig. S1 and Table S1 in ESI†for crystal data, atom numbering and characteristic bond lengths). The dihedral angle between the two coordinated pyridyl rings is 16.51, between the middle ring and the noncoordinated ring is 143.21, a distortion which is higher than the one observed in the Re analogue [Re(terpy-k 2 N 1 ,N 2 )(CO) 3 Cl] (6.91and 130.01, respectively). 10 Thenon-coplanarityofthefreepyridylis obviously due to steric hindrance of the neighboring CO ligand. 1a solubilizes well in MeCN, in which the bromide is gradually exchanged for an MeCN molecule, but this substitution is incomplete and slow (Fig. S2 in ESI†). Thereby, [Mn(terpy-k 2 N 1 ,N 2 )(CO) 3 - (MeCN)](PF 6 )(1b) has been quantitatively prepared by reaction of 1a with AgPF 6 .1b is soluble in MeCN, and moderately soluble in H 2 O (saturation concentration E0.5 mM). When dissolved in MeCN, it shows two strong absorption bands in the UV region at l max = 280 and 296 nm (e=1.410 4 M 1 cm 1 for both), attributed to intra-ligand (IL) transitions (p–p* terpy) in accordance with those observed for Re-terpyridyl tri-CO analogues. 8a A single, weaker band is observed in the visible region, at l max = 370 nm (e=2800M 1 cm 1 ); it has been attributed to a metal-to- ligand charge transfer (MLCT). 9a In water, the latter is red-shifted (l max = 390 nm), presumably due to the exchange of the coordinated MeCN by H 2 O. No change in the UV-Vis spectra was observed after leaving the solutions for 24 h in the dark, confirming the good stability of 1b for an extended period of time in both organic and aqueous media. However, selective photolysis (l irr =372nm)ofa MeCN solution of 1b quickly leads to the apparition of two new broad absorption bands, centered at 460 and 610 nm. These are characteristic of the mer,cis-[Mn(terpy-k 3 N 1 ,N 2 ,N 3 )(CO) 2 (MeCN)] + dicarbonyl complex (2), in which one CO is replaced by the initially non-coordinated pyridyl of the terpy ligand (eqn (1)). Similar photoconversion has been proposed for instance for fac-[Mn(iPr-DAB)(CO) 3 Br], 11 and for a trispyridyl amine Re complex, 8b but, as far as we know, never for the Re-terpy-k 2 N 1 ,N 2 equivalent compound. [Mn(terpy-k 2 N 1 ,N 2 )(CO) 3 (MeCN)] + -[Mn(terpy-k 3 N 1 ,N 2 ,N 3 )(CO) 2 (MeCN)] + + CO (1) To characterize 2more accurately we have developed a synthetic pathway, alternative to the photo-activated loss of CO, by refluxing an acetonitrile solution of 1b, in the absence of light; however this reaction is quite slow (E20% conversion after 4 h). The kinetics of this process has been improved by keeping the solution of 1b under reduced pressure, in a H 2 O–MeCN mixture, for a few hours. The progress of this decarbonylation reaction was monitored by 1 Hand 13 CNMR.Representativespectrain Fig. 2 show the disappearance of the characteristic signals of the bidentate terpy-k 2 N 1 ,N 2 in 1b, and the concomitant apparition of the six signals of the symmetrical tridentate terpy-k 3 N 1 ,N 2 ,N 3 in 2. The reaction is quantitative, no intermediate species was ever observed under these conditions. Similarly, the 13 Cspectrum of 2exhibits in the 100–180 ppm region the eight peaks expected for a symmetrical tridentate terpy, as opposed to the fifteen distinct peaks of terpy in 1b. Moreover, the IR spectrum of 2 differs strongly from that of its parent complex, with an important shiftoftheCRO stretching bands towards lower wavenumbers (Fig. S3a in ESI†); the new pattern is consistent with two CO ligands in cis configuration as it was previously observed for [Re(tris-(2-pyridyl)-amine)(CO) 2 Cl]. 8b The UV-Vis spectrum of 2in MeCN is characterized by three strong absorption bands in the UV region at l max = 229, 280 and 323 nm (e= 2.5, 1.6 and 2.0 10 4 M 1 cm 1 , respectively), attributed to p–p* intra-ligand (IL) transitions, and three weaker ones at l max = 380, 456 and 617 nm (e= 2200, 2400 and 870 M 1 cm 1 , respectively; Fig. S4 in ESI†), confirming that the products formed by photo- and thermal dissociation of 1b are the same. The changes in the IL bands may be explained by the change in the terpy distortion between 1b and 2. The photochemical properties of 1b have been monitored by UV-vis (Fig. 3), IR, and 1 H NMR (Fig. S3 and S5 in ESI†)spectroscopies in MeCN. Under an inert atmosphere, the monodecarbonylation process is clean, quantitative and occurs with a good quantum yield (f= 0.7). This yield was determined by chemical actinometry using a solution of a dicarbonyl Ru complex as an external standard (f ref =1,seeESI†). 12 To prove the advantages in using an ambidentate terpy ligand instead of a bidentate bpy one, we have investigated fac-[Mn(bpy)(CO) 3 (MeCN)](PF 6 ) irradiation in MeCN. The quantitative mono-decarbonylation of the bpy complex could not be achieved since the di- and tri-decarbonylation takes place before the mono-decarbonylation is complete. We have compared the rate of the mono-decarbonylation process for 1b and the bpy equivalent species, under the exact same experimental irradiation conditions, and have shown that this Fig. 1 Ball-and-stick representation of the asymmetric unit in 1a. Fig. 2 Representative 1 H NMR spectra (CD 3 CN) in the aromatic region following the evolution of a solution of 1b when warmed under vacuum, showing the conversion of 1b into 2. Communication ChemComm Published on 10 January 2014. Downloaded by Jyvaskylan Yliopisto on 14/01/2016 07:05:38. View Article Online This journal is ©The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 2539--2542 | 2541 rate is roughly twice as fast for 1b; this is likely due to the chelating effect of the free pyridyl of the terpy-k 2 N 1 ,N 2 close to the Mn I center in 1b. Addition of even a large excess of pyridine in the solution of [Mn(bpy)(CO) 3 (MeCN)](PF 6 ) does not affect the decarbonylation. Maintaining the irradiation of 1b at l irr = 372 nm past the first step (eqn (1)) led to the complete decarbonylation and the formation of [Mn(terpy) 2 ] 2+ , identified by IR, EPR and electrochemistry, as well as yet unknown byproducts. Overall, this process is around ten times slower than the first monodecarbonylation step. Gas chromatography (GC) analysis of the gas phase has allowed precise detection and quantification of CO: one molar equivalent of CO is released concomitantly to the formation of 2, and two further equivalents when the irradiation is carried on beyond this point. Besides, an important feature is the possibility of interrupting the monodecarbonylation reaction at any time, by simply switching off the irradiation (Fig. 3, inset); this can be repeated as often as needed, allowing a precisely controlled CO release, a crucial point for potential activated CO-RM applications. When kept in the dark, a deaerated solution of 2is stable for days. The reaction outcome is altered significantly in the presence of O 2 . Under aerobic conditions, the maximum conversion of 1b into 2which could ever be reached was around 60%, with further irradiation leading to the concurrent decarbonylation of 2, revealed by a decrease of all the absorption bands in the UV-Vis spectrum. This is in agreement with an observation described recently by Kurz et al. 13 They have spectroscopically demonstrated that complexes such as [Mn(tpm)(CO) 3 ] (tpm = tris(pyrazolyl)methane) first lose one CO upon irradiation, and then the last two CO in a reaction accompanied by oxidation of the Mn I center by O 2 . Following the photo-reaction of 1b in pure water is trickier, due to the very low solubility of the photogenerated product in this medium, which prevents an accurate UV-Vis study. However at the beginning of the reaction, we have clearly demonstrated that the decarbonylation process in water is similar to what was observed in an organic medium (Fig. S6 in ESI†). Eventually, in both organic and aqueous media, exposure to white light leads to complete decarbonylation in a few minutes. The complexes have been further characterized by electrochemistry, all the measurements being conducted in the dark in a MeCN electrolyte. On the initial reduction scan, the cyclic voltammograms (CVs) of 1b and 2(Fig. S7 in ESI†) show two successive irreversible reductions at the same potentials (E pc1 =1.46 V and E pc2 =1.68 V vs. Ag/Ag + ). On the oxidation side however CVs of 1b and 2differ clearly, the latter being irreversibly oxidized much more easily (E pa = 0.28 V) than the former (E pa = 0.98 V), explaining the greater instability of 2in non-deoxygenated solutions. The complete electrochemical study of these Mn–terpy complexes is underway and will be detailed elsewhere. To conclude, we have described the first quantitative synthesis of di and tricarbonyl Mn–terpy complexes and their characterization. Besides the synthetic aspect, the study of the photo-activation of CO release in the tricarbonyl Mn I terpy compound has shown the effect of the ambidentate terpy ligand on the rate of the CO-release reaction. The latter species is able to respond to an external controlled trigger (UV irradiation), in both organic and aqueous media, which could be suitable for CO-based therapeutics. Preliminary qualitative analysis of CO 2 reduction activation conducted using Mn–terpy carbonyl complexes in hydro-organic electrolytes (Fig. S8 in ESI†) has shown the potential of these Mn-based derivatives as homogeneous molecular electrocatalysts. We are currently studying in detail the redox reactions and how various substituents on the terpy ligand may influence the efficiency of the processes of CO-release and also of CO 2 electrocatalytic reduction. J.-D.C. acknowledges post-doctoral funding from the ARCANE Labex (ANR-11-LABX-0003-01). The authors thank Dr J. Chauvin for fruitful discussions and F. Molton for his technical contribution. Notes and references 1 See for instance (a) C. Costentin, M. Robert and J. M. Saveant, Chem. Soc. Rev., 2013, 42, 2423; (b) B. Kumar, M. Llorente, J. Froehlich, T. Dang, A. Sathrum and C. P. Kubiak, Annu. Rev. Phys. Chem., 2012, 63, 541; (c) C. D. Windle and R. N. Perutz, Coord. Chem. Rev., 2012, 256, 2562; (d) A. J. Morris, G. J. Meyer and E. Fujita, Acc. Chem. Res., 2009, 42, 1983; (e) A. Coleman, C. Brennan, J. G. Vos and M. T. Pryce, Coord. Chem. Rev., 2008, 252, 2585. 2Seeforinstance(a)C.C.Roma ˜o, W. A. Bla ¨ttler,J.D.SeixasandG.J.L. Bernardes, Chem.Soc.Rev., 2012, 41, 3571; (b)J.S.Ward,J.M.Lynam, J.W.B.Moir,D.E.Sanin,A.P.MountfordbandandI.J.S.Fairlamb,Dalton Trans., 2012, 41, 10514; (c) U. Schatzschneider, Inorg. Chim. Acta, 2011, 374, 19; (d)R.D.Rimmer,A.E.PierriandP.C.Ford,Coord. Chem. 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