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

Abstract

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 water) 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 μ₂-κ¹O:κ¹N 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 μ₂-κ²O,O′:κ¹N bridge, while the other acts as a μ₃-κ¹O:κ¹O′:κ¹N 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.