The tripodal bis(thioimidazolyl)(pyrazolyl)hydroborato [NSS] donor ligand, [HB(timMe)2pz]-, has been synthesized by the sequential reaction of LiBH4 with methimazole and pyrazole. Importantly, the ligand binds to zinc in a manner which resembles the coordination of the histidine and cysteine residues at the active site of liver alcohol dehydrogenase.

The tris(2-mercapto-1-mesitylimidazolyl)borate ligand, [Tm Mes ] 2 , has been used to synthesize {[Tm Mes ]Zn-(HOMe)} + , a stable monomeric tetrahedral zinc-methanol complex which resembles the proposed alcohol intermediate in the catalytic cycle of the mechanism of action of liver alcohol dehydrogenase.

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The bis(2-mercapto-1-methylimidazolyl)(pyrazolyl)hydroborato derivatives [pzBmMe]2Zn, [pzBmMe]2Co, and [pzBmMe]2Cd have been isolated and structurally characterized by X-ray diffraction. Despite their common [pzBmMe]2M composition, each of these complexes adopts a different structure. Thus, (i) the zinc complex exhibits a tetrahedral Zn[S4] structure in which only the sulfur donors coordinate to zinc, (ii) the cobalt complex exhibits a trigonal-bipyramidal Co[S3NH] structure in which one of the pyrazolyl groups and one of the B-H groups coordinate to cobalt, and (iii) the cadmium complex exhibits a six-coordinate Cd[S4H2] structure in which both B-H groups interact with the cadmium center. These comparisons emphasize that zinc has a greater preference for tetrahedral M[S4] coordination than does either cobalt or cadmium, an observation that is in accord with the prevalent role of zinc in the structural sites of enzymes.