Synthesis, structure and magnetic properties of chiral and nonchiral transition-metal malates

Carboxylate-bridged complexes of transition metals, M-II=Mn-II, Fe-II, Co-II, Ni-II, Zn-II, were synthesised by reaction of M-II salts with DL-malate and L-malate under hydrothermal conditions. These complexes form four series of compounds, which have been fully characterised structurally, thermally and magnetically. The crystal structures of the new chiral compounds, [Mn(L-mal)(H2O)] (1), [Fe(L-mal)(H2O)] (2), [CO(L-mal)(H2O)] (3) and [Zn(L-mal)(H2O)] (4) as well as those of the bimetallic analogues [Mn0.63CO0.37(L-mal)(H2O)] (5) and [Mn0.79Ni0.21(L-mal)(H2O)] (6) have been solved by single-crystal X-ray diffraction. The six Lmalate monohydrates crystallise in the chiral space group P2(1)2(1)2(1) and consist in a three-dimensional network of metal(II) centres in octahedral sites formed by oxygen atoms. These structures were compared to those of the chiral trihydrate compounds [Co(L-mal)(H2O)](.)2H(2)O (7), [Ni(L-mal)(H2O)](.)2H(2)O (8) and [CO0.52Ni0.48(L-mal)(H2O)](.)2H(2)O (9), which exhibit helical chains of M-II, centres, and those of DL-malate dihydrates [CO(DL-mal)(H2O)](H2O)-H-. (10) and [Ni(DL-mal)-(H2O)-H2O (11) and trihydrate [Mn(L-mal)(H2O)](.)2H(2)O (12) highlighting the great flexibility of the coordination by the malate ligand. UV/Vis spectroscopic results are consistent with octahedral coordination geometry of high-spin transition-metal centres. Extensive magnetic characterisation of each homologous series indicates rather weak coupling interaction between paramagnetic centres linked through carboxylate bridges. Curie-like paramagnetic, antiferromagnetic, ferromagnetic or weak ferromagnetic behaviour is observed and discussed on the basis of the structural features. The bimetallic compounds 5 and 6 represent new examples of chiral magnets.