Isomorphic coordination polymers of cobalt(ii), zinc(ii) with a flexible ligand of cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid and their molecular alloys: Crystal structures, thermal decomposition mechanisms and magnetic properties

摘要

Three new isomorphic coordination polymers of Co~(2+), Zn ~(2+) ions with flexible multicarboxylic acid ligand of the cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid (H_4L), [Co _4L_2(H_2O)_8]·3H_2O (1), [Zn_4L_2(H_2O)_8]·3H _2O (2) and [Co_(0.8)Zn_(3.2)L_2(H _2O)_8]·3H_2O (3), have been synthesized under hydrothermal conditions and by means of controlling the pH of the reaction mixtures (with an initial pH of 6.0 for 1, 4.0 for 2, and 5.0 for 3, respectively). In the crystal of 1, two crystallographically different Co ~(2+) ions (Co1 and Co2) form a negatively-charged coordination polymeric chain, which contains a centrosymmetric, linear, trinuclear Co ~(2+) cluster (Co_3L_2) subunit; another crystallographically independent Co~(2+) ion (Co3) coordinated to six water molecules acts as a counter ions to link the neighboring coordination polymeric chains via intermolecular H-bond interactions. The Co~(2+) ions in 1 were completely and partially replaced by Zn~(2+) ions to give 2 and 3, respectively. Complex 3 shows a novel molecular alloy nature, due to the random distributions of the Co~(2+) and Zn~(2+) ions. Three isomorphic complexes exhibit distinct thermal decomposition mechanisms. The deprotonated cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid ligands decompose at 420-750 °C to give the residue CoO in 1, ZnO + C in 2 and CoO + ZnO in 3. Complex 1 shows a complicated magnetic behavior with co-existence of antiferromagnetic exchange interactions between neighboring Co~(2+) ions as well as strong spin-orbital coupling interactions for each Co~(2+) ion; complex 3 exhibits a magnetically isolated high-spin Co~(2+) ion behavior with strong spin-orbital coupling interactions.