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

A new series of the structural and functional models for the active site of [NiFe]-H(2)ases 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 NiCl2.6H(2)O, NiBr(2.)3H(2)O, and NiI2 in refluxing CH2Cl2/MeOH or EtOH gave the mononuclear Ni complexes RN(PPh2)(2)NiX2 (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)(2)Ni(mu-pdt)(mu-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 H-2 under CV conditions.