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 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 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.