Two new polar coordination polymers with diamond networks: interpenetration and thermal phase transition

Abstract

Two new polar coordination polymers, [Cu(Hmpc)2] (1) and [Cu2(mpc)2(DMA)] (2), were synthesized by solvothermal reactions of a ligand 3,5-dimethyl-1H-pyrazole-4-carboxylic acid (H2mpc) with Cu(NO3)2. Single-crystal X-ray analysis reveals that 1 crystallizes in the orthorhombic, polar space group Fdd2. Each square-planar coordinated Cu2+ ion is linked to four adjacent Cu2+ ions by mono-deprotonated Hmpc− ligands, resulting in a severely distorted 4-connected diamond network, inside which the methyl groups of the ligands are densely packed. In 2, non-centrosymmetric binuclear Cu2+ units serve as 4-connecting nodes, forming a non-porous polar structure consisting of 2-fold-interpenetrated diamond networks. A thermal structural phase transition was detected in 2 at around 323 K. At 298 K, 2 crystallizes in the tetragonal chiral space group P41 (2α), with a = 10.9931 (6) Å, c = 18.0850(9) Å, and Z = 4, whereas at a higher temperature (438 K) it is transferred to the tetragonal non-centrosymmetric space group P42nm (2β), with a = 10.9857(5) Å, c = 9.1031(9) Å, and Z = 2. By repeating in situ single-crystal X-ray diffraction, this phase transition was found to be reversible between a non-twinning non-centrosymmetric crystal (2β) and a racemic-twinning chiral crystal (2α). The symmetry of the diamond network was increased from C1 to D2d in the heating process, so that two fold interpenetrated networks become strictly parallel along the c-axis at about 323 K, and the c-axis was halved in 2β. At the same time, 2-fold disordered coordinated DMA molecules in 2α were found to be 8-fold disordered in 2β. According to aizu notation, 2 can be classified as 4mmF4 species, which are potentially secondary ferroic.