Biomimetic hydrogen generation catalyzed by triironnonacarbonyl disulfide cluster

SHORT REPORT

Biomimetic hydrogen generation catalyzed by triironnonacarbonyl disulfide cluster

Charles A. Mebi^*; Kyra E. Brigance; Robert B. Bowman

Department of Physical Sciences, College of Natural and Health Sciences, Arkansas Tech University, 1701 N. Boulder Ave, Russellville, Arkansas 72801 USA

ABSTRACT

Triironnonacarbonyl disulfide cluster, a stable and synthetic precursor for active site models of [Fe-Fe] hydrogenase enzyme, has been evaluated as catalyst for the electrochemical generation of hydrogen by cyclic voltammetry. In the presence of acetic acid, Fe₃S₂(CO)₉ catalyzes the reduction of proton to hydrogen at -2.24 V (vs. Fc/Fc⁺) with an overpotential of -0.78 V (acetonitrile as solvent). The overpotential is comparable to those reported for diironcarbonyl models of [Fe-Fe] hydrogenase.

Keywords: electrocatalysis, hydrogen, iron-cluster, metalloenzyme, transition metal

RESUMO

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. The chemistry of iron-sulfur clusters has been associated with the development of life on planet Earth.^1-3 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 [Fe₃S₂(CO)₉], Figure 1.⁴

The chemistry of Fe₃S₂(CO)₉ has drawn a lot of attention over the past few decades.¹ Triironnonacarbonyl disulfide was first prepared in 1958 by Hieber and Beck⁵ by the reaction of [HFe(CO)₄]^- and sulfite ion. However, Fe₃S₂(CO)₉ has been identified as a common product from reactions between iron-carbonyls and sulfur supplying agents.¹ Wei and Dahlz⁶ structurally characterized Fe₃S₂(CO)₉ using X-ray crystallography revealing its nido-type square pyramidal or trigonal bipyramidal arrangement. Fe₃S₂(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.¹⁰ The reactivity of Fe₃S₂(CO)₉ has been explored and continues to be of great interest.^1,11-14 Some important reactions of Fe₃S₂(CO)₉ are depicted in Scheme 1: (a) the reaction of Fe₃S₂(CO)₉ with formaldehyde and amines to form diiron clusters of the type, Fe₂[(SCH₂)₂NR](CO)₆, models for the active site of [Fe-Fe] hydrogenase enzyme (Figure 2)¹⁴ and (b) the photochemical transformation of Fe₃S₂(CO)₉ in the presence of Fe(CO)₅ to the tetrairon cluster, Fe₄S₂(CO)₁₁.¹³

Although the reactivity, electrochemical properties, and electronic structure of Fe₃S₂(CO)₉ have been investigated, to the best of our knowledge, its catalytic properties remain unexplored. Giving the synthetic and structural relationship between Fe₃S₂(CO)₉ and models for the active site of [Fe-Fe] hydrogenase enzyme, it is important to examine the ability of Fe₃S₂(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⁺/H₂ couple for [Fe-Fe] hydrogenase has been determined to be -0.36 V vs. SCE at pH 6 and 30 ºC.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 Fe₃S₂(CO)₉. The catalytic properties of Fe₃S₂(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 Fe₃S₂(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 CH₃CN solution of Bu₄NPF₄ was used as supporting electrolyte. The concentration of Fe₃S₂(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 Fe₃S₂(CO)₉ as a byproduct from the reaction of Fe₃(CO)₁₂ and phenanthrene-4,5-disulfide.¹⁵ The identify of Fe₃S₂(CO)₉ was confirmed by infrared and X-ray crystallography.

Results and Discussion

The electrochemical properties of Fe₃S₂(CO)₉ have been previously investigated in benzonitrile by cyclic voltammetry.⁹ Fe₃S₂(CO)₉ is reported to undergo two one-electron reduction processes at -0.43 V (reversible, E_1/2) and -1.38 V (irreversible, E_pc). An irreversible oxidation of Fe₃S₂(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 Fe₃S₂(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 Fe₃S₂(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.¹⁶ The overpotential was determined to be -0.78 V using the standard reduction (Eº_HA) of -1.46 V (vs. Fc/Fc⁺) for acetic acid.¹⁷ The overpotential and potential for the reduction of acetic acid to hydrogen (E_cat) by Fe₃S₂(CO)₉ are contained in Table 2 alongside those of some diironcarbonyl models of [Fe-Fe] hydrogenase.^17,18 As observed in Table 2, the overpotential and E_cat for Fe₃S₂(CO)₉ are comparable to those of the diironcarbonyl models.

Scheme 2 shows a tentative mechanism for the electrocatalytic hydrogen production by Fe₃S₂(CO)₉. The suggested electrochemical-electrochemical-chemical-chemical (EECC) mechanism is based on the results described above and similar reported examples.¹⁶ As depicted in Scheme 2, Fe₃S₂(CO)₉ first undergoes successive one-electron reductions to produce [Fe₃S₂(CO)₉]^2- which upon protonation affords [HFe₃S₂(CO)₉]^-. Subsequent protonation of [HFe₃S₂(CO)₉]^- generates hydrogen.

Conclusions

Fe₃S₂(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 H₂ by Fe₃S₂(CO)₉. However, more studies (computational, bulk electrolysis, and spectroelectrochemical) are required to characterize the species involved and ascertain the mechanism.

Acknowledgments

Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund, for support of this research.

References

1. Ogino, H.; Inomata, S.; Tobita, H.; Chem. Rev. 1998, 98, 2093.

2. Meyer, J.; J. Biol. Inorg. Chem. 2008, 13, 157.

3. Cody, G. D.; Boctor, N. Z.; Filley, T. R.; Hazen, R. M.; Scott, J. H.; Sharma, A.; Yoder, H. S. Jr.; Science 2000, 289, 1337; Koutsantonis, G. A. In Concepts and Models in Bioinorganic Chemistry; Heinz-Bernhard, K.; Nils, M-N., eds.; Wiley-VCH: Weinheim, 2006, ch. 13; Crichton, R. R.; Biological Inorganic Chemistry: An Introduction,1^st ed., Elsevier: Oxford, 2008; Ei-Ichiro, O.; Bioinorganic Chemistry: A Survey, Elsevier: Burlington, 2008.

4. Figure 4 was generated using Spartan software. 1991-2009 by Wavefunction Inc. (www.wavefun.com).

5. Hieber, W.; Beck, W.; Z. Anorg. Allg. Chem. 1958, 296, 91.

6. Wei, C. H.; Dahlz, L. F.; Inorg. Chem. 1965, 4, 493.

7. Chesky, P. T.; Hall, M. B.; Inorg. Chem. 1983, 22, 2998; Hall, M. B.; Fenske, R. F.; Inorg. Chem. 1972, 11, 768; Rives, A. B.; Xiao-Zeng, Y.; Fenske, R. F.; Inorg. Chem. 1982, 21, 2286.

8. Eveland, J. R.; Saillard, J-Y.; Whitmire, K. H.; Inorg. Chem. 1997, 36, 330.

9. Madach, T.; Vahrenkamp, H.; Chem. Ber. 1981, 114, 505.

10. Best, S. P.; Borg, S. J.; Vincent, K. A. In Spectroelectrochemistry; Kaim, W.; Klein A., eds.; Royal Society of Chemistry: Cambridge, UK, 2008, ch. 1.

11. Adams, R. D.; Babin, J. E.; Inorg. Chem. 1986, 25, 3418.

12. Fang, Z-G.; Wen, Y-S.; Wong, R. K. L.; Ng, S-C.; Liu, L-K.; Hor, T. S. A.; J. Cluster Sci. 1994, 5, 327.

13. Adams, R. D.; Babin, J. E.; Estrada, J.; Wang, J-G.; Hall, M. B.; Low, A. A.; Polyhedron 1989, 8, 1885; Mathur, P. In Advances in Organometallic Chemistry Vol. 41; West, R.; Hill, A. F., eds.; Academic Press: California, USA, 1997, ch. 5.

14. Li, H.; Rauchfuss,T. B.; J. Am. Chem. Soc. 2002, 124, 727; Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DiuBois, M. R.; DuBois, D. L.; Science 2011, 333, 863; Vincent, K. A.; Parkin, A.; Armstrong, F. A.; Chem. Rev. 2007, 107, 4366.

15. Mebi, C. A.; Noll, B. C.; Gao, R.; Karr, D. S.; Z. Anorg. Allg. Chem. 2010, 2550.

16. Song, L-C.; Yang, Z-Y.; Bian, H-Z.; Liu, Y.; Wang, H.-T.; Liu, X-F.; Hu, Q-M.; Organometallics 2005, 24, 6126.

17. Felton, G. A.; Mebi, C. A.; Petro, B. J.; Vannucci, A. K.; Evans, D. H.; Glass, R. S.; Lichtenberger, D. L.; J. Organomet. Chem. 2009, 694, 2681; Mejia-Rodriguez, R.; Chong, D.; J. Reibenspies, H.; Soriaga, M. P.; Darensbourg, M. Y.; J. Am. Chem. Soc. 2004, 126, 12004.

18. Mebi, C. A.; Felton, C. M.; J. Undergrad. Chem. Res. 2011, 10, 111.

Submitted: August 29, 2011

Published online: November 8, 2011

Publication Dates

• Publication in this collection
  02 Feb 2012
• Date of issue
  Jan 2012

History

• Received
  29 Aug 2011
• Accepted
  08 Nov 2011