A water reduction process performed by zinc metal under very mild conditions

CREATED USING THE RSC COMMUNICATION TEMPLATE (VER. 3.1) - SEE WWW.RSC.ORG/ELECTRONICFILES FOR DETAILS ARTICLE TYPE www.rsc.org/xxxxxx | XXXXXXXX This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol] , 00–00 | 1 A water reduction process performed by zinc metal in very mild conditions María J. Romero,ᵃ Rosa Pedrido,*ᵃ Ana M. González-Noya,ᵇ Miguel Martínez-Calvo,ᵃ Guillermo Zaragozaᶜ and Manuel R. Bermejo*ᵃ Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X 5 First published on the web Xth XXXXXXXXX 200X DOI: 10.1039/b000000x Water reduction can be electrochemically promoted by zinc under very mild conditions, as demonstrated by the formation of the complex [Zn(H 2 O) 6 ][Zn 3 (SAlDs) 3 ( μ 3 -O)], 10 whose anion consists of a Zn 3 O 4 cluster incorporating a μ 3 -oxo anion. The study of polynuclear coordination compounds with novel metal cores1 has experienced increased interest in the last two 15 decades due to their potential applications as functional materials,2 catalysts3 or models for active sites in metalloenzymes.4 In particular, numerous zinc cluster complexes have been assembled in order to reproduce some enzymatic behaviours.5 For example, it is well known that zinc(II) ions can 20 promote hydrolytic reactions by coordinating a nucleophilic water to lower its pKa without the complication of radical reactions.6 Additionally, the ability of zinc(II) to adopt a variety of coordination numbers and geometries, combined with its rapid ligand exchange capacity, allows it to accommodate the 25 structural rearrangements that must occur during the course of many chemical reactions.2,4 Numerous examples of polynuclear zinc complexes featuring oxo anions coordinated to the metal centres have been reported to date. Most of these compounds were synthesized by 30 employing chemical methods that involve the introduction of dioxygen,7 water8 or carbon dioxide9 into the reaction media or/and by using different basic agents.7,10 In these cases, zinc metal or zinc coordination compounds themselves are able to carry out electron transfer or hydrolysis processes, releasing into 35 the media oxo or hydroxo anions that could act as potential ligands. On the other hand, the efficient catalytic reduction of water by zinc metal for the generation of hydrogen is one of the most challenging chemical transformations in industry at present, 40 since hydrogen could be considered as a clean energy source if it is produced using renewable electricity. This reaction is carried out by employing special conditions, which involve the careful control of factors such as pressure and temperature11 or the presence of suitable catalysts (Pd, Pt, Ni, Rh, Mn).12 45 During the last few decades our group and others have employed Schiff base ligands derived from salicylaldehyde to assemble polynuclear complexes.13 It is also well known that zinc(II) ions have a strong affinity for aromatic sulfonamides. With the aim of preparing new polynuclear zinc complexes that 50 potentially could bind oxo anions and may have catalytic activity, we have designed the new ligand 5-(dimethylamino)- N-(2-(2-hydroxybenzylideneamino)phenyl)naphthalene-1- sulfonamide, named H2L (Scheme 1). Herein we report a zinc cluster complex assembled after a 55 water reduction process carried out under mild conditions by coordinated Zn2+ ions in an electrochemical cell. Equimolecular condensation of salicylaldehyde and N-(2- aminophenyl)-5-(dimethylamino)-1-naphthalenesulfonamide gave rise to the [N2O] tridentate ligand H2L (Scheme 1). 60 Electrochemical oxidation of a zinc plate14 in the presence of a degassed commercial acetonitrile solution of H2L yielded the neutral solid complex [Zn(H2O)6][Zn3(L)3(μ3-O)] (1), which was readily characterised by analytical and spectroscopic techniques. The MALDI-TOF mass spectrum of 1 exhibits peaks 65 corresponding to the [Zn(H2O)6][Zn3(L)3(μ3-O)] fragmentation, thus confirming the proposed molecular formula in solution. The stoichiometry of this compound was further confirmed by its molar conductivity value, which is compatible with a 1 ratio; 1 electrolyte compound.15 70 Slow evaporation of the mother liquors yielded green crystals corresponding to the complex [Zn(H2O)6][Zn3(L)3(μ3-O)]· 3CH3CN·H2O (1·3CH3CN·H2O), which was completely characterised, including by X-ray diffraction. It should be noted that this process is totally reproducible and all attempts to obtain 75 the solid complex 1 or the crystalline 1·3CH3CN·H2O under the same experimental conditions gave identical results. The structure of 1·3CH3CN·H2O consists of a trinuclear anionic complex [Zn3(L)3(μ3-O)]2- and a hexaaquo cation [Zn(H2O)6]2+, solvated by two acetonitrile and three water molecules (Fig. 1 and 80 2). The anionic entity [Zn3(L)3(μ3-O)]2- is built by three molecules of dianionic ligand [L]2- bound to three zinc(II) ions through a [N2O] donor system, together with an oxo anion coordinated in a μ3-bridging mode. Each phenol oxygen establishes a μ2-oxo bridge between two zinc centres. 85 55 90 Scheme 1 Ligand H2L. 95 2 | Journal Name, [year], [vol] , 00–00 This journal is © The Royal Society of Chemistry [year] 5 10 15 Fig. 1 Ball and stick representation of the anionic cluster [Zn3(L)3(μ3-O)]2- in 1·3CH3CN·H2O. The [Zn(H2O)6]2+ counterion, solvate acetonitrile and water molecules are omitted for clarity. 20 25 Fig. 1 Ball and stick representation of the anionic cluster 30 [Zn3(L)3(m3-O)]2_ in 1_3CH3CN_H2O. The [Zn(H2O)6]2+ counterion, solvate acetonitrile and water molecules are omitted for clarity. 35 Fig. 2 Hydrogen bonding interactions between two [Zn 3 (L) 3 ( μ 3 - O)] 2- molecules and the [Zn(H 2 O) 6 ] 2+ cation. This coordination mode allows the assembly of a Zn3O4 central 40 core, which could be described as a pseudo-cube with a vacant position at the opposite vertex to the oxygen atom O1. The metal and the phenol oxygen atoms in the Zn3O4 core describe one sixmembered metallacycle with a chair-type conformation (Fig. S2, ESI), while the coordination of the μ3-oxo anion gives rise to 45 three four-membered metallacycles, which correspond to the three faces of the distorted pseudo-cube. The zinc(II) ions are located in [N2O3] pentacoordinated environments in the trinuclear cluster, with distorted squarepyramidal geometries, according to their calculated τ parameters 50 (τ1 = 0.28, τ2 = 0.04, τ3 = 0.15).16 A weak intramolecular interaction between the Zn2 atom and one oxygen atom from the dansyl group [Zn2···O52, 2.681(3) A˚] is also observed (Fig. 1). The Zn–N and Zn–O bond distances are similar to those found in other salen polynuclear Zn(II) complexes.5d,17–19 The Zn···Zn 55 bond distances in this anionic cluster (close to 3.1 Å) are slightly longer than the sum of the van der Waals radii for two zinc atoms (2.8 Å).20,21 These distances are in the order of those found in other polynuclear zinc(II) complexes with μ2-oxo,17 μ3-oxo,22 and μ4-oxo23 bridges. 60 The hexaaquo counterion appears between two cluster molecules of [Zn3(L)3(μ3-O)]2- (Fig. 2) due to the establishment of numerous hydrogen bond contacts (Table S2, ESI). At this point the origin of the μ3-oxo anion in compound 1 must be analysed. Considering all the literature reports concerning the 65 catalytic activity of Zn2+,24 we propose that this anion was generated in situ by means of a water reduction process catalysed by zinc(II) ions during the electrochemical synthesis of the zinc(II) complex. To confirm our proposal, the electrochemical synthesis was repeated under an inert atmosphere, but employing 70 completely dry acetonitrile. Under these conditions a complex with a Zn2(L)2 stoichiometry (2) was isolated but this rapidly evolved in solution to form complex 1.25 It is clear from these findings that the absence of water in the medium avoided the catalytic conversion of the dimer into the trinuclear cluster and 75 therefore that the presence of water is essential to generate complex 1. A possible mechanism for the formation of complex 1 would involve two different redox processes (Scheme 2). In the first stage, the zinc anode undergoes an oxidation to generate Zn2+ 80 ions, while the sulfonamide N–H and phenoxo O–H bonds from the ligand H2L are reduced to give the bideprotonated [L]2-. The coordination of two bideprotonated ligands to two Zn(II) ions would result in a dimeric compound with μ2-phenoxo bridges between the two metal centres (I).26 The particular preferences of 85 Zn(II) in terms of coordination number would provide a suitable kernel for coordination of one additional water molecule from the medium to one of the metal centres, thus one of the zinc(II) ions achieves a pentacoordinated environment (II).24 Since the electrochemical oxidation of the Zn plate would continue in the cell 90 once the Zn2(L)2 dimer is formed, aqueous Zn(II) ions {[Zn(H2O)6]2+} and two electrons are released into the solution. One of these electrons would perform a metal–water bond reduction, generating a coordinated hydroxo anion (III).27 This hydroxo anion, which acts as a μ2-oxo bridge, would be able to 95 incorporate a new zinc(II) ion and a third anionic ligand unit into the complex (IV). In the final step, the trinuclear complex would experience a second reduction on the zinc–OH bond, creating an O2- anion that would complete the pentacoordinated environment of the third metal centre ([Zn3(L)3(μ3-O)]2-). 100 This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol] , 00–00 | 3 Scheme 2 Proposed mechanism for the formation of 1. The coexistence of Zn2+, e-, H2O, Zn2L2 and L2- in solution 5 makes the formation of compound 1 possible, with the oxo anion being the key piece in the assembly of the trinuclear cluster. We must stress that all the catalytic reduction stages yield hydrogen gas, which is liberated from the cathode throughout the process. 10 An electrochemical water reduction process catalysed by Zn2+ ions in the presence of the ligand H2L leads to the formation of a μ 3 -oxo anion, which acts as the key component in the assembly of the trinuclear zinc cluster [Zn3(L)3(μ 3 -O)]2-. The main advantages of the electrochemical method to 15 achieve the water reduction in the presence of zinc metal are (i) its reproducibility and (ii) the use of very mild conditions, since the reaction occurs in the absence of reducing agents, basic reagents or external catalysts. 20 Financial support from Xunta de Galicia (INCITE09E2R209074ES), Ministerio de Educación y Ciencia and ERDF (EU) (CTQ2007-62185/BQU). We also acknowledge EU COST D31 Action “Functional Helicates” (D31/0008/04). R. Pedrido thanks Xunta de Galicia for an 25 “Isidro Parga Pondal” contract. Notes and references a Departamento de Química Inorgánica, Facultade de Química, Universidade de Santiago de Compostela, Santiago de Compostela, 30 Galicia, E-15782, Spain. Fax: +34 981597525; Tel: +34 981563100; E- mail: [email protected] b Departamento de Química Inorgánica, Facultade de Ciencias, Universidade de Santiago de Compostela, Lugo, Galicia, E-27002. 35 Crystal data for 1·3CH₃CN·H₂O: (C₈₁H₈₀N₁₂O₁₄S₃Zn₃.₅₀), Mw =1770.65, crystal dimensions: 0.25 x 0.24 x 0.24 mm, triclinic, P-1, a = 15.344(5), b = 16.139(5), c = 18.408(6) A˚ , a = 75.772(5)1, b = 81.362(5)1, g = 70.021(4)1, V = 4142(2) A˚ 3, Z = 1, m = 1.148 mm⁻¹, F(000) = 1830. Radiation l(Mo-Ka) = 0.71073 A˚, T = 293(2) K, reflections 40 collected/unique 34 313/15 033 (Rint = 0.0540), R (all data) =0.0953, wR (all data) = 0.146, GOF = 0.994, max./min. residual density 0.859/- 0.648 e A˚ -3. CCDC 766560. 1 (a) J.-M. Lehn, Supramolecular Chemistry: Concepts And Perspectives, 45 Wiley-VCH, Weinheim, 1995; (b) E. C. 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