Synthesis and structural diversity of d10 metal coordination polymers constructed from new semi-rigid bis(3-methyl-1H-pyrazole-4-carboxylic acid) alkane ligands

Three isomeric bis(3-methyl-1H-pyrazole-4-carboxylic acid) alkane ligands (H2L1, H2L2 and H2L3) were prepared via a two-step process from ethyl 3-methyl-1H-pyrazole-4-carboxylate (1). They were introduced as bridging ligands to assemble with Zn(II) and Cd(II) ions, and two chiral coordination polymers [Zn(L-1)(DMF)(H2O)](n) (3a and 3b), [Cd(L-1)(H2O)](n)center dot nH(2)O (5) and two achiral coordination polymers [Zn(L-1)(H2O)(2)](n)center dot nH(2)O (4), [Cd(L-2)(H2O)](n)center dot 2nH(2)O center dot nEtOH (6) were obtained. (L-1)(2-) adopts a trans-conformation in 3, leading to the formation of 1D coordination chains, and the 2D coordination layers in 4-6 are constructed from (L-1)(2-)/(L-2)(2-) anions in a cis-conformation. Notably, the structures of 3a and 3b exhibit an H-bonded chiral 2D network, featuring a right-handed 3-fold helix in 3a, while a left-handed 3-fold helix in 3b. Compound 5 possesses a 3D supramolecular network with homochiral left-handed helical chains, whose chirality is due solely to the hydrogen bonds. The chiral nature of enantioenriched 3a produced by diffusion crystallization was confirmed by solid-state CD spectroscopy. The thermal and luminescence properties of 3-6 in the solid state have also been investigated.