Synthesis, Molecular Structures and Electrochemical Investigations of [FeFe]-Hydrogenase Biomimics [Fe₂(CO)_6-n(EPh₃)_n(µ-edt)] (E = P, As, Sb; n = 1, 2)

A series of ethane-dithiolate (edt = S(CH₂)₂S) complexes [Fe₂(CO)₅(EPh₃)(µ-edt)] and [Fe₂(CO)₄(EPh₃)₂(µ-edt)] (E = P, As, Sb), biomimics of the core of [FeFe]-hydrogenases, have been prepared and structurally characterised. The introduced ligand(s) occupies apical sites lying trans to the iron-iron bond. NMR studies reveal that while in the mono-substituted complexes the Fe(CO)₃ moiety undergoes facile trigonal rotation, the Fe(CO)₂(PPh₃) centres do not rotate on the NMR timescale. The reductive chemistry has been examined by cyclic voltammetry both in the presence and absence of CO and the observed behavior is found to be dependent upon the nature of the substituents. With L = CO or SbPh₃ potential inversion is seen leading to a two-electron reduction, while for others (L = PPh₃, AsPh₃) a quasi-reversible one-electron reduction is observed. Protonation studies reveal that [Fe₂(CO)₅(PPh₃)(µ-edt)] is only partially protonated by excess HBF₄·Et₂O, thus ruling complexes [Fe₂(CO)₅(EPh₃)(µ-edt)(µ-H)]⁺ out as a catalytic intermediates, but [Fe₂(CO)₄(PPh₃)₂(µ-edt)] reacts readily with HBF₄·Et₂O to produce [Fe₂(CO)₄(PPh₃)₂(µ-edt)(µ-H)]⁺. While all new complexes are catalysts for the reduction of protons in MeCN, their poor stability and relatively high reduction potentials does not make them attractive in this respect.