Dithiolato- and halogenido-bridged nickel-iron complexes related to the active site of [NiFe]-H2ases: preparation, structures, and electrocatalytic H2 production

Abstract

A new series of the structural and functional models for the active site of [NiFe]-H₂ases has been prepared by a simple and convenient synthetic route. Thus, treatment of diphosphines RN(PPh₂)₂ (1a, R = p-MeC₆H₄CH₂; 1b, R = EtO₂CCH₂) with an equimolar NiCl₂·6H₂O, NiBr₂·3H₂O, and NiI₂ in refluxing CH₂Cl₂/MeOH or EtOH gave the mononuclear Ni complexes RN(PPh₂)₂NiX₂ (2a, R = p-MeC₆H₄CH₂, X = Cl; 2b, R = EtO₂CCH₂, X = Cl; 3a, R = p-MeC₆H₄CH₂, X = Br; 3b, R = EtO₂CCH₂, X = Br; 4a, R = p-MeC₆H₄CH₂, X = I; 4b, R = EtO₂CCH₂, X = I) in 67-97% yields. Further treatment of complexes 2a,b-4a,b with an equimolar mononuclear Fe complex (dppv)(CO)₂Fe(pdt) and NaBF₄ resulted in formation of the targeted model complexes [RN(PPh₂)₂Ni(μ-pdt)(μ-X)Fe(CO)(dppv)](BF₄) (5a, R = p-MeC₆H₄CH₂, X = Cl; 5b, R = EtO₂CCH₂, X = Cl; 6a, R = p-MeC₆H₄CH₂, X = Br; 6b, R = EtO₂CCH₂, X = Br; 7a, R = p-MeC₆H₄CH₂, X = I; 7b, R = EtO₂CCH₂, X = I) in 60-96% yields. All the new complexes 3a,b-4a,b and 5a,b-7a,b have been characterized by elemental analysis and spectroscopy, and particularly for some of them (3a,b/4a,b and 5b/6b) by X-ray crystallography. More interestingly, the electrochemical and electrocatalytic properties of such halogenido-bridged model complexes are first studied systematically and particularly they have been found to be pre-catalysts for proton reduction to H₂ under CV conditions.