Zinc-thiolate complexes of the bis(pyrazolyl)(thloimidazolyl)hydroborate tripods for the modeling of thiolate alkylating enzymes

The new tripod ligands bis(pyrazolyl)(3-tert-butyl-2-thioimidazol-1-yl)hydroborate (L-1) and bis(pyrazolyl)(3-isopropyl-2-thioimidazol-1-yl)hydroborate (0), together with zinc nitrate or zinc chloride and the corresponding thiolates, have yielded a total of 17 zinc-thiolate complexes. These comprise aliphatic as well as aromatic thiolates and a cysteine derivative. Structure determinations have confirmed the tetrahedral ZnN2S2 coordination in the complexes. Upon reaction with methyl iodide, the species L-1-Zn-SR are slowly converted to L-1 center dot Zn-I and the free thioethers CH3SR. A kinetic analysis has shown these alkylations to be about 1 order of magnitude slower than those of the tris(pyrazolyl) borate complexes Tp(Ph,Me)Zn-SR. Alkylations with trimethyl phosphate were found to proceed very slowly even in DMSO at 80 degrees C.