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

Two new polar coordination polymers, [Cu(Hmpc)(2)] (1) and [Cu-2(mpc)(2)(DMA)] (2), were synthesized by solvothermal reactions of a ligand 3,5-dimethyl-1H-pyrazole-4-carboxylic acid (H(2)mpc) 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 Hmpcligands, 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 P4(1) (2 alpha), with a = 10.9931 (6) angstrom, c = 18.0850(9) angstrom and Z = 4, whereas at a higher temperature (438 K) it is transferred to the tetragonal non-centrosymmetric space group P4(2)nm (2 beta), with a = 10.9857(5) angstrom, c = 9.1031(9) angstrom, 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 (2 beta) and a racemic-twinning chiral crystal (2 alpha). The symmetry of the diamond network was increased from C-1 to D-2d 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 2 beta At the same time, 2-fold disordered coordinated DMA molecules in 2 alpha were found to be 8-fold disordered in 2 beta. According to aizu notation, 2 can be classified as 4mmF4 species, which are potentially secondary ferroic.