Extended metal–organic solids based on benzenepolycarboxylic and aminobenzoic acids

This article describes the recent results obtained in our laboratory on the interaction of polyfunctional ligands with divalent alkaline earth metal ions and a few divalent transition metal ions. Treatment of MCl2nH2O (M = Mg, Ca, Sr or Ba) with 2-amino benzoic acid leads to the formation of complexes [Mg(2-aba)2] (1), [Ca(2-aba)2(OH2)3] infinity(2), [{Sr(2-aba)2(OH2)2}2H2O)]infinity(3), [Ba(2-aba)2(OH2)] infinity(4), respectively. While the calcium ions in 2 are hepta-coordinated, the strontium and barium ions in 3 and 4 reveal a coordination number of nine apart from additional metal–metal interactions. Apart from the carboxylate functionality, the amino group also binds to the metal centres in the case of strontium and barium complexes 3 and 4. Complexes [{Mg(H2O)6}(4-aba)22H2O] (5), [Ca(4-aba)2(H2O)2] (6) prepared from 4-aminobenzoic acid reveal more open or layered structures. Interaction of 2-mercaptobenzoic acid with MCl2.6H2O (M = Mg, Ca), however, leads to the oxidation of the thiol group resulting in the disulphide 2,2´-dithiobis(benzoic acid). New metal–organic framework based hydrogen-bonded porous solids [{M(btec) (OH2)4}n.n(C4H12N2)4nH2O] (btec = 1,2,4,5-benzene tetracarboxylate) (M = Co 9; Ni 10; Zn 11) have been synthesized from 1,2,4,5-benzene tetracarboxylic acid in the presence of piperazine. These compounds are made up of extensively hydrogenbonded alternating layers of anionic M-btec co-ordination polymer and piperazinium cations. Compounds 2–11 described herein form polymeric networks in the solid– state with the aid of different coordinating capabilities of the carboxylate anions hydrogen bonding interactions.