Two-and three-dimensional nickel(II) and copper(II) metal-organic frameworks based on 1H-pyrazole-4-carboxylate

Abstract

Recent advances in coordination chemistry have enabled the rational design of metal–organic architectures with controlled dimensionality and topology. Metal–organic frameworks, pioneered by the seminal works of Yaghi, Kitagawa, and Eddaoudi, are constructed from extended coordination networks through the judicious choice of metal nodes and bridging ligands. The stoichiometric reactions of 1H-pyrazole-4-carboxylic acid (Hpzca) with nickel(II) or copper(II) acetate hydrates and pyrazole (Hpz) under ambient conditions (aqueous methanol) or solvothermal conditions (2-propanol) afforded two-dimensional coordination polymers, [M(pzca)2(Hpz)2]n (M = Ni (1), Cu ( 2)). The carboxylic acid group of Hpzca is deprotonated, and the resulting pzca -ligand acts as a mu2-kappa1O:kappa1N bridge. In the absence of the Hpz capping ligand, a three-dimensional coordination polymer, [Cu(pzca)2]n (3), was obtained. One pzca -ligand functions as a mu2-kappa2 O,O':kappa1N bridge, while the other acts as a mu3-kappa1O:kappa1O':kappa1N bridge. Thermogravimetric analysis revealed that compounds 1 and 3 contain no solvent molecules. Magnetic measurements indicate weak antiferromagnetic interactions between the metal centers. In addition, mononuclear cis- and trans-isomers of [Ni(pzca-N)2(H2O)4] were isolated and characterized, both of which appear to be plausible intermediates toward the formation of MOF 1.