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

Abstract

The dinuclear complex [Zn2L(μ-OH)] (ClO4)3 · CH3CN (L = dinucleating ligand α,α′-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 P21/n, cell dimensions a=13.109(2), b=13.339(2), c=24.317(5) A and Z=4. The cation [Zn2L(μ-OH)]3+ contains a dinuclear zinc complex with hydroxide bridging, showing a ZnμOHZn angle of 140.2°, and Zh3Zn distances of 3.635 A. 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 pKa 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.