Heterodinuclear Ni/M (M = Mo, W) Complexes Relevant to the Active Site of [NiFe]-Hydrogenases: Synthesis, Characterization, and Electrocatalytic H₂ Evolution

Abstract

As the active site mimics of [NiFe]-H2ases, the first heterodinuclear Ni/M (M = Mo, W)-based complexes that contain both azadiphosphine and cyclopentadienyl ligands have been prepared by simple and convenient methods in quite high yields. Thus, treatment of the azadiphosphine-chelated Ni dithiolates R1N(PPh2)2Ni(SEt)2 (R1 = p-MeC6H4CH2, EtO2CCH2) with an equimolar parent or acetyl-substituted cyclopentadienyl Mo halides (η5-R2C5H4)(CO)3MoY (R2 = H, MeCO; Y = Cl, Br) gave dinuclear Ni/Mo halide complexes [R1N(PPh2)2Ni(μ-SEt)2Mo(η5-R2C5H4)(CO)2]Y (1a, R1 = p-MeC6H4CH2, R2 = H, Y = Br; 1b, R1 = EtO2CCH2, R2 = H, Y = Cl; 1c, R1 = EtO2CCH2, R2 = MeCO, Y = Br), while further treatment of 1a–c with a slight excess of NaBPh4 afforded dinuclear Ni/Mo tetraphenylboron complexes [R1N(PPh2)2Ni(μ-SEt)2Mo(η5-R2C5H4)(CO)2]BPh4 (2a, R1 = p-MeC6H4CH2, R2 = H; 2b, R1 = EtO2CCH2, R2 = H; 2c, R1 = EtO2CCH2, R2 = MeCO). Similarly, while dinuclear Ni/W halide complexes [R1N(PPh2)2Ni(μ-SEt)2W(η5-R2C5H4)(CO)2]Y (3a, R1 = p-MeC6H4CH2, R2 = H, Y = Cl; 3b, R1 = EtO2CCH2, R2 = H, Y = Cl; 3c, R1 = EtO2CCH2, R2 = MeCO, Y = Br) could be prepared by treatment of mononuclear Ni dithiolates R1N(PPh2)2Ni(SEt)2 with mononuclear W halides (η5-R2C5H4)(CO)3WY, further treatment of 3a–c with NaBPh4 resulted in formation of dinuclear Ni/W tetraphenylboron complexes [R1N(PPh2)2Ni(μ-SEt)2W(η5-R2C5H4)(CO)2]BPh4 (4a, R1 = p-MeC6H4CH2, R2 = H; 4b, R1 = EtO2CCH2, R2 = H; 4c, R1 = EtO2CCH2, R2 = MeCO). The X-ray crystallographic study of dinuclear Ni/Mo and Ni/W tetraphenylboron complexes 2a–c and 4a,b confirmed that they have a butterfly [NiIIMII(μ-S)2] (M = Mo, W) core in which the azadiphosphine R1N(PPh2)2-chelated Ni fragment is combined with an (η5-R2C5H4)(CO)2M moiety via two thiolate ligands. In addition, the electrochemical and electrocatalytic properties of representative dinuclear complexes 2a,b and 4b,c have been investigated, and particularly, they have been found to be catalysts for proton reduction to H2 from TFA under CV conditions.