Bis(mercaptoimidazolyl)(pyrazolyl)hydroborato Complexes of Zinc, Cadmium, and Cobalt:  Structural Evidence for the Enhanced Tendency of Zinc in Biological Systems to Adopt Tetrahedral M[S4] Coordination

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.