Dinuclear nickel complexes modeling the structure and function of the acetyl CoA synthase active site

A dinuclear nickel complex with methyl and thiolate ligands, Ni(dadtEt)Ni(Me)(SDmp) (2), has been synthesized as a dinuclear Nid-Nip-site model of acetyl-CoA synthase (ACS) (dadtEt is N,N'-diethyl-3,7-diazanonane-1,9-dithiolate; Dmp is 2,6-dimesitylphenyl). Complex 2 was prepared via 2 methods: (i) ligand substitution of a dinuclear Ni(II)-Ni(II) cation complex [Ni(dadtEt) Ni(tmtu)₂] (OTf)₂(1) with MeMgBr and KSDmp (tmtu is tetramethylthiourea), (ii) methyl transfer from methylcobaloxime Co(dmgBF₂)₂(Me)(Py) (5) to a Ni(II)-Ni(0) complex such as [Ni(dadtEt)Ni(cod)] (3), generated in situ from Ni(dadtEt) and Ni(cod)₂, followed by addition of KSDmp (cod is 1,5-cyclooctadiene; dmgBF₂ is difluoroboryl-dimethylglyoximate). Method ii models the formation of Nip-Me species proposed as a plausible intermediate in ACS catalysis. The reaction of 2 with excess CO affords the acetylthioester CH₃C(O)SDmp (8) with concomitant formation of Ni(dadtEt)Ni(CO)₂ (9) and Ni(CO)₄ plus Ni(dadtEt). When complex 2 is treated with 1 equiv of CO in the presence of excess 1,5-cyclooctadiene, the formation of 9 and Ni(CO)₄ is considerably suppressed, and instead the dinuclear Ni(II)-Ni(0) complex is generated in situ, which further affords 2 upon successive treatment with Co(dmgBF₂)₂(Me)(Py) (5) and KSDmp. These results suggest that (i) ACS catalysis could include the Nid(II)-Nip(0) state as the active species, (ii) The Nid(II)-Nip(0) species could first react with methylcobalamin to afford Nid(II)-Nip(II)-Me, and (iii) CO insertion into the Nip-Me bond and the successive reductive elimination of acetyl-CoA occurs immediately when CoA is coordinated to the Nip site to form the active Nid(II)-Nip(0) species.