S versus O alkylation and coordination in zinc complexes containing the bulky heteroscorpionate alkoxy ligand bis(3,5-dimethylpyrazol-2-yl) diphenylmethanol

A series of zinc complexes containing the tripodal heteroscorpionate ligand bis(3,5-dimethylpyrazol-2-yl)diphenylmethanol (L2H) have been synthesized and characterized by X-ray crystallography. The L2H/Zn complexes were designed to model the N2OX coordination (with the zinc-bound O being a reactive nucleophile) that is characteristic of many protease and amidase zinc enzymes. The pseudotetrahedral mononuclear complexes characterized include [(L2)ZnI] (1), [(L2)Zn(CH3)] (2), and [(L2)Zn(SPh)] (3). Alkylation of (1) with methyl iodide has revealed a modest nucleophilicity of the chelated zinc-bound alkoxide, and produces the penta-coordinate [(L2OCH3)ZnI2] (4) which contains a weakly bound ether in the fifth coordination site. However, when the coordination sphere also includes a thiolate sulfur as in (3), reaction with methyl iodide produces exclusive alkylation at the sulfur to produce thioanisole and (1). The coordination of the ether in the neutral (4) can be strengthened by reaction with various silver salts, Ag+X−, to produce other penta-coordinate complexes [(L2OCH3)ZnI(Tf)] (5) and [(L2OCH3)ZnI(H2O)]BF4 (6) which show enhanced coordination of the ether.

Graphical abstractAlkylation of Zn complexes reveals a modest nucleophilicity of a chelated zinc-bound alkoxide which become weakly bound in the fifth coordination site, however, when the coordination sphere also includes a thiolate, exclusive alkylation at the sulfur is observed.Figure optionsDownload full-size imageDownload as PowerPoint slide