Synthesis and crystal structures of two monomeric zinc(II) complexes containing carboxylate and aqua ligands

Two monomeric zinc(II) complexes of betaine (Me3N+CH2CO2- = bet), [Zn(bet)(2)(H2O)Cl] (ClO4) (1) and [Zn(bet)(3)(H2O)] (ClO4)2 (2), have been prepared and characterized by single-crystal structural analysis. The zinc atom in both complexes is coordinated in a distorted tetrahedral geometry: the zinc atom in complex (1) is surrounded by one aqua ligand (Zn-O 1.979(3) Angstrom), one chloro ligand (Zn-Cl 2.218(1) Angstrom) and two monodentate carboxylate groups (Zn-O 1.946(2) Angstrom); the zinc atom in complex (2) is ligated by one aqua ligand (Zn-O 1.997(6) Angstrom) and three monodentate carboxylate groups (Zn-O 1.943(5)-2.106(8) Angstrom). In both complexes, the aqua ligand forms strong donor hydrogen bonds with adjacent carboxylate oxygens (for complex (1), O ... O is 2.689(2) Angstrom; for complex (2), O ... O are 2.67(1) and 2.709(8) Angstrom); these interactions are discussed in the context of zinc complexes and enzyme active sites.