Structural variability of Cd(II) and Co(II) mixed-ligand coordination polymers: effect of ligand isomerism and metal-to-ligand ratio

Abstract

A series of seven mixed-ligand coordination polymers, namely {[Cd4(H2L1)4(suc)4]·3H2O}n (1), {[Cd4(H2L1)3(suc)4]·5H2O}n (2), {[Cd2(HL1)2(suc)]·8H2O}n (3), {[Cd(H2L2)(suc)]·3H2O}n (4), {[Cd(H2L3)(suc)]·3H2O}n (5), {[Co(H2L1)(chdc)(H2O)]·H2O}n (6), {[Co(H2L2)(chdc)(H2O)]}n (7) have been produced by the reaction of three 2-amino-4,6-dipyridylpyrimidine isomers (H2L1, H2L2, H2L3) and secondary ligand 1,4-succinic acid (H2suc) or 1,4-cyclohexanedicarboxylic acid (H2chdc) with Cd(II) and Co(II) salts under hydrothermal conditions. All these compounds have been structurally determined by elemental analysis, IR and single-crystal X-ray diffraction. The three isomers are introduced in a mixed-ligand system for the first time. For the H2L1–H2suc–Cd(II) system, it is found that the structures of compounds 1–3 exhibiting a 1-D double-chain, a rare 1-D quadruple nano-chain and a 2-D network with (6,3) topology, respectively, are influenced by the ratio of metal-to-ligand. When H2L2 and H2L3 were used instead of H2L1, two 1-D chains (compounds 4 and 5) were obtained. Although they have the same molecular formula and structure motif, their supramolecular networks constructed by hydrogen-bondings and π–π stacking interactions are different. When H2L1 and H2L2 were introduced into the Co(NO3)2–H2chdc system, compounds 6 and 7 were successfully isolated, and display a zigzag chain and a helical chain, respectively. Interestingly, the cis- and trans-conformations of H2chdc were separated completely in the two compounds. The results reveal that both the coordination structure and the supramolecular assembly could be directed by the ligand isomers. Furthermore, several intriguing water clusters have been observed in these mixed-ligand compounds, such as a novel (H2O)10cluster in 2, a 2-D water net formed by (H2O)8clusters in 3, a 1-D undulated water chain in 4 and a chair-shaped (H2O)6cluster in 5. The solid-state properties of compounds 1–7 such as thermal stability and fluorescence have also been investigated.