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

A series of seven mixed-ligand coordination polymers, namely {[Cd-4(H(2)L1)(4)(suc)(4)]center dot 3H(2)O}(n) (1), {[Cd-4(H(2)L1)(3)(suc)(4)]center dot 5H(2)O}(n) (2), {[Cd-2(HL1)(2)(suc)]center dot 8H(2)O}(n) (3), {[Cd(H(2)L2)(suc)]center dot 3H(2)O}(n) (4), {[Cd(H(2)L3)(suc)]center dot 3H(2)O}(n) (5), {[Co(H(2)L1)(chdc)(H2O)]center dot H2O}(n) (6), {[Co(H(2)L2)(chdc)(H2O)]}(n) (7) have been produced by the reaction of three 2-amino-4,6-dipyridylpyrimidine isomers (H(2)L1, H(2)L2, H(2)L3) and secondary ligand 1,4-succinic acid (H(2)suc) or 1,4-cyclohexanedicarboxylic acid (H(2)chdc) 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 H(2)L1-H(2)suc-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 H(2)L2 and H(2)L3 were used instead of H(2)L1, 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 pi-pi stacking interactions are different. When H(2)L1 and H(2)L2 were introduced into the Co(NO3)(2)-H(2)chdc 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 H(2)chdc 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)(10) cluster in 2, a 2-D water net formed by (H2O)(8) clusters in 3, a 1-D undulated water chain in 4 and a chair-shaped (H2O)(6) cluster in 5. The solid-state properties of compounds 1-7 such as thermal stability and fluorescence have also been investigated.