An experimental and computational study of sulfur-modified nucleophilicity in a dianionic NiN2S2 complex.

The dianionic NiN2S2 complex, Ni(ema)2-, ema=N,N'-ethylenebis-2-mercaptoacetamide, known as a reasonable model of the tripeptide complex Ni(CGC)2- (C=cysteine; G=glycine) with respect to the two carboxyamido nitrogens and cis-dithiolates in a (N2S2)4- ligand scaffold as found in acetyl CoA synthase, has been explored for S-based reactivity toward oxygenation and alkylation. The isolation and structural characterization of a sulfinato species, [Et4N]2[Ni(ema).O2], prepared through a unique direct reaction of molecular O2 with crystalline [Et4N]2[Ni(ema)] is described. Reaction of [Et4N]2[Ni(ema)] with Br(CH2)3Br yields a neutral N2S2 macrocyclic complex shown by DFT computations and electrostatic-potential mapping to be opposite in electron distribution from the neutral NiN2S2 complexes in which the anionic charge is localized on sulfur.