Zinc−Thiolate Complexes of the Bis(pyrazolyl)(thioimidazolyl)hydroborate Tripods for the Modeling of Thiolate Alkylating Enzymes

The new tripod ligands bis(pyrazolyl)(3-tert-butyl-2-thioimidazol-1-yl)hydroborate (L1) and bis(pyrazolyl)(3-isopropyl-2-thioimidazol-1-yl)hydroborate (L2), 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 L1·Zn−SR are slowly converted to L1·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 TpPh,MeZn−SR. Alkylations with trimethyl phosphate were found to proceed very
slowly even in DMSO at 80 °C.