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(C(6)H(3)-3-SiMe(3)-2-S)(3))] (R = Me (1); R = Et (2)) were prepared by reaction of [PPN][Ni(III)Cl(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] and CH(3)MgCl/C(2)H(5)MgCl, individually. EPR, SQUID studies as well as DFT computations reveal that the Ni(III) in 1 has a low-spin d(7) electronic configuration in a distorted trigonal bipyramidal ligand field. The Ni-C bond lengths of 1.994(3) A in 1 and 2.015(3) A in 2 are comparable to that in the Ni(III)-methyl state of MCR (approximately 2.04 A) (Sarangi, R.; Dey, M.; Ragsdale, S. W. Biochemistry 2009, 48, 3146). Under a CO atmosphere, CO triggers homolytic cleavage of the Ni(III)-CH(3) bond in 1 to produce Ni(II)-thiolate carbonyl [PPN][Ni(II)(CO)(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] (3). Additionally, protonation of 1 with phenylthiol generates Ni(III)-thiolate [PPN][Ni(III)(SPh)(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] (4).