Synthesis, Crystal Structures, and Properties of Molecular Squares Displaying Hydrogen and π−π Bonded Networks

We report the hydrothermal syntheses, single-crystal structure determinations, and the thermal and magnetic properties of five binuclear complexes obtained from either a mixture of P and M 2,2′-biphenyldicarboxylate, (2,2′-bpdc)2−, or a mixture of cis- and trans-1,4-cyclohexanedicarboxylate, (1,4-chdc)2−, as bridging ligands, di-2-pyridylamine (dpa) as a capping terminal ligand, and a divalent transition metal (Cu, Co, and Ni). All five compounds, [Co2(2,2′-bpdc)2(dpa)2] (1), [Cu2(2,2′-bpdc)2(dpa)2]·4H2O (2a), [Cu2(2,2′-bpdc)2(dpa)2]·2(2,2′-bpdcH2)·H2O (2b), [Co2(cis-chdc)2(dpa)2] (3), and [Ni2(cis-chdc)2(dpa)2] (4), consist of neutral molecular squares of two metal centers bridged by two elbowed dicarboxylate, and each metal is capped by the chelating di-2-pyridylamine. In 1, the Co(II) adopts a tetrahedral coordination from two oxygen atoms of monodentate 2,2′-bpdc2− and two nitrogen atoms from dpa. In 2a and 2b, the Cu(II) adopts distorted octahedral geometry from two bidentate 2,2′-bpdc2− and one bidentate dpa. 1, 2a, and 2b contain both P and M forms of 2,2′-bpdc2−. 3 and 4 contain octahedral metal coordination with only the chair form of cis-chdc2− as bridging ligands in bidentate mode and bidentate peripheral dpa. In each of these complexes, hydrogen bonds between the amine nitrogen and a carboxylate oxygen atom and π−π overlap between the pyridine rings of the dpa define the supramolecular interactions which contribute to the crystals’ stability. The magnetic properties are those of paramagnets with weak exchange interactions, as a consequence of the distant exchange pathway between nearest neighbor moment carriers.