From (3,6)-Connected kgd, chiral anh to (3,8)-connected tfz‑d Nets in Low Nuclear Metal Cluster-Based Networks with Triangular Pyridinedicarboxylate Ligand

Four novel coordination polymers based on asymmetric triangular 5-(4′-carboxylphenyl) nicotinic acid (H2cpna), namely, Ni­(cpna)­(H2O)2 (1), M2(cpna)2(H2O)3·0.5H2O·DMF [M = Ni for (2), Co for (3)] and Ni3(Hcpna)2(cpna)2(H2O)4·2H2O (4), have been synthesized under solvothermal conditions. All of the four compounds contain low nuclear metal clusters as secondary building units (SBUs). Complex 1 exhibits two-dimensional (3,6)-connected (43)2(46.66.83)-kgd topology, in which cpna ligands and edge-shared dinuclear SBUs act as 3-connected and 6-connected nodes, respectively. Compounds 2 and 3 are constructed by corner-shared dinuclear SBUs and show an unprecedented (3,6)-connected chiral (42.6)2(44.62.88.10)-anh (flu-3, P6222) topology. In 4, linear trimers are ligated by six cpna ligands to form a planar (43)2(46.66.83)-kgd layer, and adjacent layers are further linked by Hcpna– ligands through Ni–O and Ni–N bonds to form three-dimensional (3,8)-connected (43)2(46.618.84)-tfz-d net. Both 1 and 4 crystallize in the triclinic crystal system with symmetric space group P̅1, which result in an equal ratio of (R)- and (S)-cpna isomers. Isostructural 2 and 3 crystallize in a trigonal crystal system with the chiral space group P3221, and a single crystal contains a sole optical isomer of cpna, although bulk crystals are a racemic mixture. Variable-temperature magnetic susceptibility measurements reveal ferromagnetic coupling of edge-shared dimers in 1, antiferromagnetic coupling of corner-shared dimers in 2 and 3, and ferrimagnetic coupling of linear trimers in 4.