Syntheses, spectroscopies and structures of zinc complexes with malate

Abstract

Reactions of zinc(II) ion with racemic malic acid (C 4H 6O 5 = H 3mal) result in the isolation of four new zinc(II) malato complexes: (NH 4)[Zn( R-H 2mal) 3] · H 2O ( 1), trans-[Zn( R-H 2mal)( S-H 2mal)(H 2O) 2] · 2H 2O ( 2), (NH 4) 2[Zn( R-Hmal)( S-Hmal)] · 2H 2O ( 3), and [Zn 2( R-Hmal)( S-Hmal)(H 2O) 4] n · 2 nH 2O ( 4). Three R-malic acids in 1 act as bidentate ligands via their alcoholic and the central carboxy groups with Zn(II) ion, leaving the terminal carboxylic acid groups free. The R- and S-malates of 2 coordinate in a bidentate manner with zinc ion in trans-form. In 3, Zn(II) ion is coordinated by R- and S-malates in a tridentate fashion via their alcoholic and two carboxy groups. Complex 4 forms a two-dimensional layered structure through the links of a new dimeric unit [Zn 2( R-Hmal)( S-Hmal)(H 2O) 4] with one of the oxygen atoms from the terminal carboxy group of malate ligand. The coordination of malates depends on pH variation, on Zn:malate ratio, and also on temperature. Tridentate chelation of malate in 3 is found between pH 4.5–9.0. The soluble monomeric species 1– 3 have been investigated using 13C NMR spectra by long-time acquisition. The solution NMR spectra indicate that zinc malate complexes dissociate in H 2O (D 2O). Obvious downfield shifts of the central carboxy carbon atoms in 1– 3 are observed compared with those of free malate, which indicate that these zinc malate complexes dissociate in aqueous solution.