Bimetallic Carbonyl Thiolates as Functional Models for Fe-Only Hydrogenases

The anion [Fe₂(S₂C₃H₆)(CN)(CO)₄(PMe₃)]^- (2^-) is protonated by sulfuric or toluenesulfonic acid to give HFe₂(S₂C₃H₆)(CN)(CO)₄(PMe₃) (2H), the structure of which has the hydride bridging the Fe atoms with the PMe₃ and CN^- trans to the same sulfur atom. ¹H, ¹³C, and ³¹P NMR spectroscopy revealed that HFe₂(S₂C₃H₆)(CN)(CO)₄(PMe₃) is stereochemically rigid on the NMR time scale with four inequivalent carbonyl ligands. Treatment of 2^- with (Me₃O)BF₄ gave Fe₂(S₂C₃H₆)(CNMe)(CO)₄(PMe₃) (2Me). The Et₄NCN-induced reaction of Fe₂(S₂C₃H₆)(CO)₆ with P(OMe)₃ gave {Fe₂(S₂C₃H₆)(CN)(CO)₄[P(OMe)₃]}^- (4). Spectroscopic and electrochemical measurements indicate that 2H can be further protonated at nitrogen to give [HFe₂(S₂C₃H₆)(CNH)(CO)₄(PMe₃)]⁺ (2H₂⁺). Electrochemical and analytical data show that reduction of 2H₂⁺ gives H₂ and 2^-. Parallel electrochemical studies on [HFe₂(S₂C₃H₆)(CO)₄(PMe₃)₂]⁺ (3H⁺) in acidic solutions led also to catalytic proton reduction. The 3H⁺/3H couple is reversible, whereas the 2H₂⁺/2H₂ couple is not, because of the efficiency of the latter as a proton reduction catalyst. Proton reduction is proposed to involve protonation of reduced diiron hydrides. DFT calculations establish that the regiochemistry of protonation is subtly dependent on the coligands but is more favorable to occur at the Fe−Fe bond for [Fe₂(S₂C₃H₆)(CN)(CO)₄(PMe₃)]^- than for [Fe₂(S₂C₃H₆)(CN)(CO)₄(PH₃)]^- or {Fe₂(S₂C₃H₆)(CN)(CO)₄[P(OMe)₃]}^-. The Fe₂H unit stabilizes the conformer with eclipsed CN and PMe₃ because of an attractive electrostatic interaction between these ligands.