Synthesis, reactions and structure of a hydroxo-bridged dinuclear Zn(II) complex: Modeling the hydrolytic zinc enzymes

The dinuclear complex [Zn(2)L(mu-OH)](ClO4)(3) . CH3CN (L = the dinucleating ligand alpha,alpha'-bis(bis(2-pyridylethyl)amino)-m-xylene) (1) has been prepared and characterized structurally by X-ray diffraction crystallography. The complex crystallizes in the monoclinic crystal system, space group P2(1)/n, cell dimensions a = 13.109(2), b = 13.339(2), c = 24.317(5) Angstrom and Z=4. The cation [Zn(2)L(mu-OH)](3+) contains a dinuclear zinc complex with hydroxide bridging, showing a Zn-mu-OH-Zn angle of 140.2 degrees, and Zn ... Zn distance of 3.625 Angstrom. Each zinc ion is in a distorted tetrahedral coordination environment, but with slightly different bond angles and Zn-OH distances. Thus the two Zn ions in the dinuclear unit are slightly different and invequivalent. In aqueous solution the pK(a) of the Zn-coordinated H2O (the conjugate acid form of 1) is 7.55 +/- 0.05, which is one of the lowest values reported for zinc complexes modeling the hydrolytic enzymes. The rate of hydrolysis of bis-p-nitrophenylphosphate catalyzed by complex 1 is shown to increase from pH 7 to 8 but to decline at pH 9. Complex 1 reacts reversibly in acetonitrile solution with CO2 as shown by the IR peak observed at 1613 cm(-1) assigned to the coordinated bicarbonate ion.