Mixed-ligand coordination polymers from 1,2-bis(1,2,4-triazol-4-yl)ethane and benzene-1,3,5-tricarboxylate: Trinuclear nickel or zinc secondary building units for three-dimensional networks with crystal-to-crystal transformation upon dehydration

The hydrothermal reaction of M(NO3)2•6H 2O (M = Ni and Zn) with benzene-1,3,5-tricarboxylic acid (H 3btc) and 1,2-bis(1,2,4-triazol-4-yl)ethane (btre) produced the mixed-ligand coordination polymers (MOFs) 3a{[Ni 3(μ3-btc)2(μ4-btre) 2(μ-H2O)2]•∼22H2O} (1) and 3∞{[Zn3(μ4-btc) 2(μ4-btre)(H2O)2]•2H 2O} (3). The compounds, characterized by single-crystal X-ray diffraction, X-ray powder diffraction and thermoanalysis feature trinuclear secondary building units (SBU) within the three-dimensional frameworks. The trinuclear nickel unit in 1 exhibits an intra-trimer together with some weak inter-trimer antiferromagnetic coupling with J = -13.88(8) cm-1 from the magnetic susceptibility measurement between 1.9-300 K. The zinc coordination polymer 3 shows a strong fluorescence at 423 nm upon excitation at 323 nm (not seen in the free btre ligand). Compound 3 is thermally robust until 200 °C (ambient pressure) where loss of the water molecules starts. Careful control of the dehydration procedure (freeze-drying) for 1 and (heating to 280 °C) for 3 allowed for a solid-state reaction with single-crystal-to-single-crystal structural transformations in obtaining the largely dehydrated products 3∞{[Ni3(μ2-btc) 2(μ4-btre)2(μ-H2O) 2(H2O)2]•4H2O} (2) and 3∞{[Zn3(μ6-btc) 2(μ4-btre)2]•∼0.67H2O} (4), respectively. In the transformation from 1 to 2 the unit cell volume is reduced to about 60%. The transition from 3 to 4 involves breakage and formation of new Zn-O bonds. © The Royal Society of Chemistry.