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

Abstract

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(L¹)(DMF)(H₂O)]_n (3a and 3b), [Cd(L¹)(H₂O)]_n·nH₂O (5) and two achiral coordination polymers [Zn(L¹)(H₂O)₂]_n·nH₂O (4), [Cd(L²)(H₂O)]_n·2nH₂O·nEtOH (6) were obtained. (L¹)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 (L¹)²⁻/(L²)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.