Diferrous Cyanides as Models for the Fe-only Hydrogenases

Abstract

The first systematic study of diferrous dicyano dithiolates is described. Oxidation of [Fe2(S2C2H4)(CN)2(CO)4](2-) in the presence of cyanide and tertiary phosphines and of Fe2(S2C2H4)(CO)4(PMe3)2 in the presence of cyanide affords a series of diferrous cyanide derivatives that bear a stoichiometric, structural, and electronic relationship to the H(ox)(air) state of the Fe-only hydrogenases. With PPh3 as the trapping ligand, we obtained an unsymmetrical isomer of Fe2(S2C2H4)(mu-CO)(CN)2(PPh3)2(CO)2, as confirmed crystallographically. This diferrous cyanide features the semibridging CO-ligand, with Fe-muC bond lengths of 2.15 and 1.85 A. Four isomers of Fe2(S2C2H4)(mu-CO)(CN)2(PMe3)2(CO)2 were observed, the initial product again being unsymmetrical but more stable isomers being symmetrical. DFT calculations confirm that the most stable isomers of Fe2(S2C2H4)(mu-CO)(CN)2(PMe3)2(CO)2 have cyanide trans to mu-CO. Oxidative decarbonylation also afforded the new tetracyanide [Fe2(S2C2H4)(mu-CO)(CN)4(CO)2]2-. Insights into the oxidative decarbonylation mechanism of these syntheses come from the spectroscopic characterization of the tetracarbonyl [Fe2(S2C2H4)(mu-CO)(CN)3(CO)3](-). This species reacts with PEt3 to produce the stable adduct [Fe2(S2C2H4)(mu-CO)(CN)3(CO)2(PEt3)](-).

Electron‐Rich Diferrous–Phosphane–Thiolates Relevant to Fe‐only Hydrogenase: Is Cyanide “Nature's Trimethylphosphane”?

“…Each consisted of a face-sharing bioctahedral core, not unlike previous members of this series of diferrous-dithiolates, that is, [Fe 2 (S 2 C 2 H 4 )(mCNMe)(CNMe) 6 ] 2 + , [15] [Fe 2 (S 2 C 2 H 4 )(m-CO)(CNMe) 6 ] 2 + , [22] and [Fe 2 (S 2 C 2 H 4 )(m-CO)(CN) 2 (CO) 2 (PPh 3 ) 2 ]. [23] The Fe À Fe distances fall in the range 2.5135(12)-2.6006(7) , comparable to other diferrous-thiolates as well as the Fe-only hydrogenase active site. [4,26] The bridging CO ligand in 3 and 4 is coordinated in a symmetric fashion, indicated by the similar bond lengths for Fe1ÀC1 and Fe2ÀC1 in each case.…”

“…The effect is more pronounced in these two compounds than in the previously reported diferrous-dithiolates, for example, [Fe 2 (S 2 C 2 H 4 )(m-CO)(CN) 2 (CO) 2 (PPh 3 ) 2 ], in which D(FeÀmC) = 0.295 (Table 3). [23] The bending of the m-CO ligands in 5 and 6 is evident from the angles aFe2-C1-O1 of 165.0(4) (5) and 160.0(2)8 (6), which Crabtree [31] has classified as "bent semibridging". [32,33] Cotton described semibridging CO coordination "as a means for a metal atom, otherwise tending to be excessively negative, to transfer electron density to a CO group on a less negatively charged metal atom."…”

“…[22] A recent extension of the oxidative decarbonylation methodology led to the diferrous-dicyanide [Fe 2 (S 2 C 2 H 4 )(m-CO)(CN) 2 (CO) 2 (PMe 3 ) 2 ] (Figure 1). [23] During these investigations, we realized that the bisphosphane [Fe 2 -(S 2 C 2 H 4 )(CO) 4 (Figure 1). [23] The specific issue that we begin to address is the complicated reactivity of cyanide, the naturally occurring donor ligand that supports the active site of the Fe-only hydrogenases.…”

“…[23] During these investigations, we realized that the bisphosphane [Fe 2 -(S 2 C 2 H 4 )(CO) 4 (Figure 1). [23] The specific issue that we begin to address is the complicated reactivity of cyanide, the naturally occurring donor ligand that supports the active site of the Fe-only hydrogenases. In particular, the oxidation and protonation of the cyano derivatives has proven difficult because of the high reactivity inherent in this bifunctional ligand, [24] that is, the ability of FeCN to bridge to other metals and the Brønsted basicity of the FeCN fragment.…”

Computational approaches to shed light on molecular mechanisms in biological processes

“…As an example, the systematic experimental and computational study of diferrous dicyano dithiolates showed that the oxidation of [Fe 2 (S 2 C 2 H 4 ) (CN) 2 (CO) 4 ] 2− in the presence of cyanide and tertiary phosphines and of Fe 2 (S 2 C 2 H 4 )(CO) 4 (PMe 3 ) 2 in the presence of cyanide affords a series of diferrous cyanide derivatives that bear a stoichiometric, structural, and electronic relationship oxidized state of Fe-only hydrogenases. DFT calculations revealed that the most stable isomers of Fe 2 (S 2 C 2 H 4 )(μ-CO)(CN) 2 (PMe 3 ) 2 (CO) 2 have cyanide trans to μ-CO, an relevant observation to understand the properties of the CO inhibited form of the enzyme [151].…”