Model Complexes for the Nip Site of Acetyl Coenzyme A Synthase/Carbon Monoxide (CO) Dehydrogenase: Structure, Electrochemistry, and CO Reactivity

Aliphatic thiolato-S-bridged tri- and binuclear nickel­(II) complexes have been synthesized and characterized as models for the Nip site of the A cluster of acetyl coenzyme A synthase (ACS)/carbon monooxide (CO) dehydrogenase. Reaction of the in situ formed N2Sthiol donor ligands with [Ni­(H2O)6]­(ClO4)2 afforded the trinuclear complexes [Ni­{(LMe(S) )2Ni}2]­(ClO4)2·CH3CN (1·CH3CN) and [Ni­{(LBr(S) )2Ni}2]­(ClO4)2·5H2O (2·5H2O) following self-assembly. Complexes 1 and 2 react with [Ni­(dppe)­Cl2] and dppe [dppe = 1,2-bis­(diphenylphosphino)­ethane] to afford the binuclear [Ni­(dppe)­Ni­(LMe(S) )2]­(ClO4)2·2H2O (3·2H2O) and [Ni­(dppe)­Ni­(LBr(S) )2]­(ClO4)2·0.75O­(C2H5)2 [4·0.75O­(C2H5)2], respectively. The X-ray crystal structures of 1–4 revealed a central NiIIS4 moiety in 1 and 2 and a NiIIP2S2 moiety in 3 and 4; both moieties have a square-planar environment around Ni and may mimic the properties of the Nip site of ACS. The electrochemical reduction of both terminal NiII ions of 1 and 2 occurs simultaneously, which is further confirmed by the isolation of [Ni­{(LMe(S) )2Ni­(NO)}2]­(ClO4)2 (5) and [Ni­{(LBr(S) )2Ni­(NO)}2]­(ClO4)2 (6) following reductive nitrosylation of 1 and 2. Complexes 5 and 6 exhibit νNO at 1773 and 1789 cm–1, respectively. In the presence of O2, both 5 and 6 transform to nitrite-bound monomers [(LMe(S–S) )­Ni­(NO2)]­(ClO4) (7) and [(LBr(S–S) )­Ni­(NO2)]­(ClO4)2 (8). The nature of the ligand modification is evident from the X-ray crystal structure of 7. To understand the origin of multiple reductive responses of 1–4, complex [(LMe(SMe) )2Ni]­(ClO4)2 (9) is considered. The central NiS4 part of 1 is labile like the Nip site of ACS and can be replaced by phenanthroline. The treatment of CO to reduce 3 generates a 3 red-(CO)2 species, as confirmed by Fourier transform infrared (νCO = 1997 and 2068 cm–1) and electron paramagnetic resonance (g 1 = 2.18, g 2 = 2.13, g 3 = 1.95, and A P = 30–80 G) spectroscopy. The CO binding to NiI of 3 red is relevant to the ACS activity.