Homoleptic group 12 metal bis(mercaptoimidazolyl)borate complexes M(BMR)2 (M = Zn, Cd, Hg)

The sodium salt of the bis(2-mercapto-1-methylimidazolyl)borate anion [Bm(Me)](-) and those of the new bis(2-mercapto-1-alkylimidazolyl)borates [BMR](-) (R = Bz, Bu-t, p-Tol) have been readily obtained from NaBH4 and the appropriate 2-mercapto-1-alkylimidazoles. To contrast the binding preferences of the group 12 metals in a sulfur-rich environment, the four complete series of homoleptic complexes M[Bm(R)](2) (M = Zn, Cd, Hg), including the first bis(mercaptoimidazolyl)borate derivatives of cadmium and mercury, have been prepared. X-ray diffraction studies of Cd[Bm(Me)](2) and M[Bm(tBu)](2) (M = Zn, Cd, Hg) show the presence of distorted tetrahedral [MS4] central cores supplemented by two weak vicinal M...H-B bonds, interactions which appear to be a common feature in the coordination chemistry of Bm(R) ligands. In the case of zinc, it has been found that only in the presence of bulky ligands, as in Zn[Bm(tBu)](2), may an unexpected expansion in the coordination number from four to six be induced. This observation suggests the viability of octahedral intermediates in the processes whereby certain zinc enzymes transfer or exchange metal ions.