Mononuclear Ni^III alkyl complexes (Alkyl = Me and Et): Relevance to the acetyl-CoA synthase and methyl-CoM reductase

Mononuclear, distorted trigonal bipyramidal [PPN][Ni^III(R)- (P(C₆H₃-3-SiMe₃-2-S)₃)] (R) Me (1); R) =Et (2)) were prepared by reaction of [PPN][Ni_IIICl(P(C ₆H₃-3-SiMe₃-2-S)₃)] and CH ₃MgCl/C₂H₅MgCl, individually. EPR, SQUID studies as well as DFT computations Reveal that the Ni^III in 1 has a low-spin d⁷ electronic configuration in a distorted trigonal bipyramidal ligand field. The Ni-C bond lengths of 1.994(3) å in 1 and 2.015(3) å in 2 are comparable to that in the Ni^III-methyl state of MCR (̃2.04 å) (Sarangi, R.; Dey, M.; Ragsdale, S. W. Biochemistry 2009, 48, 3146). Under a CO atmosphere, CO triggers homolytic cleavage of the Ni^III-CH₃ bond in 1 to produce Ni ^II-thiolate carbonyl [PPN][Ni^II(CO)(P(C₆H ₃-3-SiMe₃-2-S)₃)] (3). Additionally, protonation of 1 with phenylthiol generates Ni^III-thiolate [PPN][Ni^III(SPh)-(P(C₆H₃-3-SiMe ₃-2-S)₃)] (4).