Metal−Organic Frameworks from Homometallic Chains of Nickel(II) and 1,4-Cyclohexanedicarboxylate Connectors:  Ferrimagnet−Ferromagnet Transformation

The hydrothermal reactions of nickel(II) nitrate with a mixture of the geometric cis and trans isomers of 1,4-cyclohexanedicarboxylic acid (1,4-chdc or C6H10(COOH)2) and a base yield three structurally different complexes, Ni3(mu3-OH)2(mu4-cis-1,4-chdc)2(H2O)4.2H2O (1), Ni(3)(mu3-OH)2(mu4-trans-1,4-chdc)2(H2O)4.4H2O (2), and Ni(H2O)4(mu2-trans-1,4-chdc), depending on the reaction conditions. The single-crystal X-ray structure analyses of 1 and 2 reveal segregation of the isomers and formation of frameworks based on infinite Ni3(OH)2(H2O)4 chains, acting as secondary building units, connected by either cis- or trans-chdc for 1 and 2, respectively. The frameworks sustain channels that house two or four water molecules, respectively, according to the size and shape of the channels that depend on the particular isomer. The structure of 3 consists of chains of square-planar Ni(H2O)4 bridged by trans-chdc. Magnetic data as a function of temperature and field of the virgin samples for 1 indicate long-range ordering (LRO) to a ferrimagnetic ground state at 2.1 K that is reversibly transformed into a ferromagnet below 4.4 K upon partial dehydration and rehydration. Powder X-ray diffraction of 1, in its virgin state, after dehydration and after rehydration, confirms the stability of the framework. The magnetic data for 2 tend toward a LRO state to possibly a ferrimagnet below 2 K. The temperature dependence of the susceptibility of the two compounds is accounted for by the presence of both ferro- and antiferromagnetic exchanges within each chain via Ni-O-Ni and Ni-O-C-O-Ni pathways and weak coupling between neighboring chains via the 1,4-chdc unit. 3 is a uniform s = 1 antiferromagnetic chain (J/kB = 2.27(1) K).