Structural characterization of zinc bicarbonate compounds relevant to the mechanism of action of carbonic anhydrase

The tris(3-t-butyl-5-methylpyrazolyl) hydroborato and tris(2-pyridylthio) methyl derivatives, [Tp(But,Me)]ZnOCO2H and [kappa(4)-Tptm]ZnOCO2H, are the first pair of terminal zinc bicarbonate complexes to be structurally characterized by using X-ray diffraction. In both cases, the bicarbonate ligand coordinates in a unidentate manner, comparable to that in human carbonic anhydrase I. While the bicarbonate complex [kappa(4)-Tptm]ZnOCO2H is obtained by treatment of {[kappa(3)-Tptm]Zn(mu-OH)}(2) with CO2 in the presence of water, the bridging carbonate complex [Tptm]Zn(mu-CO3)Zn[Tptm] is obtained in the absence of water. The reactivity of {[k(3)-Tptm]Zn(mu-OH)}(2) towards CO2 is sufficiently high that the carbonate complex is obtained upon exposure to air.