Syntheses, spectroscopies and structures of zinc complexes with malate

Reactions of zinc(II) ion with racemic malic acid (C4H6O5 = H3mal) result in the isolation of four new zinc(II) malato complexes: (NH4)[Zn(R-H2mal)3] · H2O (1), trans-[Zn(R-H2mal)(S-H2mal)(H2O)2] · 2H2O (2), (NH4)2[Zn(R-Hmal)(S-Hmal)] · 2H2O (3), and [Zn2(R-Hmal)(S-Hmal)(H2O)4]n · 2nH2O (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 [Zn2(R-Hmal)(S-Hmal)(H2O)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 H2O (D2O). 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.

Reactions of zinc(II) ion with racemic malic acid result in the isolation of four new zinc(II) malato complexes. 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 solution 1H and 13C NMR spectra indicate that zinc malate complexes dissociate in H2O (D2O).Figure optionsDownload as PowerPoint slide