Methylation of neutral pseudotetrahedral zinc thiolate complexes: model reactions for alkyl group transfer to sulfur by zinc-containing enzymes

Eight scorpionate-zinc thiolate complexes, [(L1O)ZnSPh],. [(L1O)ZnSPhF5], [ (L1O)ZnSBz], [(L1O)ZnSph(2,6-Me)], [(L1O)ZnSPh2,4-Me], [(L1O)ZnSPh24-NO], [ (Tp(Ph,Ph))ZnSph], and [(L2S)ZnSPh], were reacted with methyl iodide in chl oroform, liberating the corresponding methyl thioethers as determined by H- 1 NMR. Three of these complexes are new and their synthesis and structural characterization are reported here. Weak alkylating agents such as trimethy l phosphate failed to undergo methyl transfer to the zinc thiolates under t hese conditions. Analysis of kinetic data as a function of concentration, t emperature, pK(a) of the exogenous thiolate, and donor atom of the tripodal ligand are consistent with a mechanism where the zinc-bound thiolate is th e active nucleophile in an associative-type methyl transfer reaction. Our m odel studies also provide experimental evidence to support the hypothesis t hat some enzymes can use the charge of the metal coordination site to modul ate catalytic activity.