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

Abstract

The hydrothermal reaction of M(NO₃)₂·6H₂O (M = Ni and Zn) with benzene-1,3,5-tricarboxylic acid (H₃btc) and 1,2-bis(1,2,4-triazol-4-yl)ethane (btre) produced the mixed-ligand coordination polymers (MOFs) ³_∞{[Ni₃(µ₃-btc)₂(µ₄-btre)₂(µ-H₂O)₂]·∼22H₂O} (1) and ³_∞{[Zn₃(µ₄-btc)₂(µ₄-btre)(H₂O)₂]·2H₂O} (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.