Biomimetic Hydrogen Generation Catalyzed by Triironnonacarbonyl Disulfide Cluster

O “cluster” dissulfeto de triferrononacarbonila, um precursor sintético e estável para modelos do sítio ativo da enzima hidrogenase [Fe-Fe], foi avaliado como catalisador para a geração eletroquímica de hidrogênio por voltametria cíclica. Na presença de ácido acético, Fe3S2(CO)9 catalisa a redução do próton para hidrogênio em 2,24 V (vs. Fc/Fc) com um sobrepotencial de -0,78 V (acetonitrila como solvente). O sobrepotencial é comparável àqueles relatados para os modelos diferrocarbonila de hidrogenase [Fe-Fe].


Introduction
Iron-sulfur clusters, diverse in composition (containing one to eight iron atoms) and structure, are ubiquitous in biological systems.These clusters are involved in electron transfer and biocatalytic processes.2][3] Iron-sulfur clusters can be coordinated to organic ligands through iron-carbon bonds to form organometallic clusters.A typical example of an organometallic iron-sulfur cluster is triironnonacarbonyl disulfide [Fe 3 S 2 (CO) 9 ], Figure 1. 4 The chemistry of Fe 3 S 2 (CO) 9 has drawn a lot of attention over the past few decades. 1 Triironnonacarbonyl disulfide was first prepared in 1958 by Hieber and Beck 5 by the reaction of [HFe(CO) 4 ] -and sulfite ion.However, Fe 3 S 2 (CO) 9 has been identified as a common product from reactions between iron-carbonyls and sulfur supplying agents. 1 Wei and Dahlz 6 structurally characterized Fe 3 S 2 (CO) 9 using X-ray crystallography revealing its nido-type square pyramidal or trigonal bipyramidal arrangement.Fe 3 S 2 (CO) 9 is stable and its electronic structure and electrochemical properties have been extensively investigated using photoelectron spectroscopy, 7 computational methods, 7,8 cyclic voltammetry, 9 and spectroelectrochemical technique. 102][13][14] Some important reactions of Fe 3 S 2 (CO) 9 are depicted in Scheme 1: (a) the reaction of Fe 3 S 2 (CO) 9 with formaldehyde and amines to form diiron clusters of the type, Fe 2 [(SCH 2 ) 2 NR](CO) 6 , models for the active site of [Fe-Fe] hydrogenase enzyme (Figure 2) 14   Although the reactivity, electrochemical properties, and electronic structure of Fe 3 S 2 (CO) 9 have been investigated, to the best of our knowledge, its catalytic properties remain unexplored.Giving the synthetic and structural relationship between Fe 3 S 2 (CO) 9 and models for the active site of [Fe-Fe] hydrogenase enzyme, it is important to examine the ability of Fe 3 S 2 (CO) 9 to mimic the enzyme.The hydrogenase enzyme efficiently catalyzes the evolution of hydrogen by proton reduction in aqueous media (TOF = 6000-9000 s -1 ).The reduction potential of the H + /H 2 couple for [Fe-Fe] hydrogenase has been determined to be -0.36V vs. SCE at pH 6 and 30 o C. 14 Studies on the enzyme and its synthetic models are useful in the development of catalysts for the production of hydrogen, a clean alternative to fossil fuels.In this study, we report on the electrocatalytic reduction of proton to produce hydrogen by Fe 3 S 2 (CO) 9 .The catalytic properties of Fe 3 S 2 (CO) 9 are compared to those of some diironcarbonyl models of the enzyme reported in the literature.

Experimental
Electrocatalytic studies were conducted using an Epsilon BAS potentiostat.Cyclic voltammograms of Fe 3 S 2 (CO) 9 were obtained with increasing concentrations of acetic acid (0, 7, 21, 35, 42, 49, 56, 63 mmol L -1 ) using a three-electrode cell.The electrodes used are glassy carbon working electrode, platinum auxiliary electrode, and Ag/AgCl reference electrode.The platinum and glassy carbon electrodes were polished with aluminum paste and rinsed with water and acetone.A 0.1 mol L -1 CH 3 CN solution of Bu 4 NPF 4 was used as supporting electrolyte.The concentration of Fe 3 S 2 (CO) 9 was 1 mmol L -1 and the scan rate was 100 mV s -1 .We degassed the electrolyte solution by bubbling nitrogen at room temperature for 5 min before measurement.The potentials obtained with reference to Ag/AgCl electrode are quoted in this report against ferrocene/ferrocenium (Fc/Fc + ) potential except otherwise mentioned.The Fc/Fc + reference is used to allow for comparison with reported redox potential values of similar models.All reagents were obtained from commercial sources.Acetonitrile for electrochemical assay was purchased from Aldrich and used without any further purification.We obtained Fe 3 S 2 (CO) 9 as a byproduct from the reaction of Fe 3 (CO) 12 and phenanthrene-4,5-disulfide. 15he identify of Fe 3 S 2 (CO) 9 was confirmed by infrared and X-ray crystallography.

Results and Discussion
The electrochemical properties of Fe 3 S 2 (CO) 9 have been previously investigated in benzonitrile by cyclic voltammetry. 9Fe 3 S 2 (CO) 9 is reported to undergo two oneelectron reduction processes at -0.43 V (reversible, E 1/2 ) and -1.38 V (irreversible, E pc ).An irreversible oxidation of Fe 3 S 2 (CO) 9 was observed at +1.30 V vs. Ag/AgCl.These results are similar to those obtained by us in acetonitrile (Table 1).We observe ca.0.07 V shift in the electrochemical potentials with the change of solvent from benzonitrile to acetonitrile.
The electrocatalytic generation of hydrogen by Fe 3 S 2 (CO) 9 from acetic acid (a weak acid; pK a = 22.3 in acetonitrile) has been studied and the results are contained in Table 2 and Figure 3. Figure 3 contains cyclic voltammograms of Fe 3 S 2 (CO) 9 in the absence of acid and with increasing amounts of acetic acid.In the absence of acid, only the three redox events mentioned in Table 1 were observed.On addition of 7 mmol L -1 of acetic acid, a new peak at -2.24 V (vs.Fc/Fc + ) appears and its current intensity increases with sequential increment of acid concentration (Figure 4).These observations are indicative of electrocatalytic reduction of proton to molecular hydrogen. 16he overpotential was determined to be -0.78V using the standard reduction (E o HA ) of -1.46 V (vs.Fc/Fc + ) for acetic Biomimetic Hydrogen Generation Catalyzed by Triironnonacarbonyl Disulfide Cluster J. Braz.Chem.Soc.188 acid. 17The overpotential and potential for the reduction of acetic acid to hydrogen (E cat ) by Fe 3 S 2 (CO) 9 are contained in Table 2 alongside those of some diironcarbonyl models of [Fe-Fe] hydrogenase. 17,18As observed in Table 2, the overpotential and E cat for Fe 3 S 2 (CO) 9 are comparable to those of the diironcarbonyl models.
Scheme 2 shows a tentative mechanism for the electrocatalytic hydrogen production by Fe 3 S 2 (CO) 9 .The suggested electrochemical-electrochemical-chemicalchemical (EECC) mechanism is based on the results described above and similar reported examples. 16As

Conclusions
Fe 3 S 2 (CO) 9 has been shown to mimic [Fe-Fe] hydrogenase by catalyzing the electrochemical evolution of hydrogen with an overpotential of -0.78 V (vs.Fc/Fc + ).This overpotential is comparable to those of reported models of [Fe-Fe] hydrogenase.We have proposed an EECC catalytic cycle for the generation of H 2 by Fe 3 S 2 (CO) 9 .However, more studies (computational, bulk electrolysis, and spectroelectrochemical) are required to characterize the species involved and ascertain the mechanism.
and (b) the photochemical transformation of Fe 3 S 2 (CO) 9 in the presence of Fe(CO) 5 to the tetrairon cluster, Fe 4 S 2 (CO) 11 .13

Figure 4 .Scheme 2 .
Figure 4. Dependence of current heights of the electrocatalytic peaks of Fe 3 S 2 (CO) 9 on concentration of acetic acid.
a Values for diironcarbonyl models were obtained from references 17 and 18.depicted