Investigation on the interconversion from DMF-solvated to unsolvated copper(ii) pyrazolate coordination polymers

Abstract

A series of copper(II) pyrazolates were synthesized employing different synthetic approaches. By refluxing either [Cu₃(μ₃-OH)(μ-pz)₃(CH₃COO)₂(pzH)] (pzH = pyrazole) or [Cu(μ-pz)₂·H₂O]_n in water, [Cu(μ-pz)(μ-OH)]_n (1) was unexpectedly obtained, whereas the reactions of copper(II) acetate hydrate with pzH or 4-XpzH (4-XpzH = 4-Xpyrazole, X = Cl, Br, I) in dimethylformamide (DMF) by microwave-assisted solvothermal syntheses yielded the compounds [Cu(μ-pz)₂(μ-DMF)]_n (2_DMF) or [Cu(μ-4-Xpz)₂(μ-DMF)]_n (3_DMF–5_DMF). As unveiled by powder X-ray diffraction, all these compounds are 1-D coordination polymers showing square planar trans-CuN₂O₂ (1) or octahedral trans-CuN₄O₂ (2_DMF–5_DMF) metal centers, reasonably involved in through-bond antiferromagnetic interactions (magnetic susceptibility in the range 0.91–1.56 B.M.). Theoretical calculations carried out on these compounds at the B3LYP-D* level confirmed the structural aspects and IR spectroscopy features unveiled experimentally. Upon heating, 2_DMF–5_DMF reversibly lose DMF to give the corresponding low-crystallinity coordination polymers [Cu(μ-pz)₂]_n (2) and [Cu(μ-4-Xpz)₂]_n (3–5), of which only 3 was characterized in the past. In order to shed light on the local-level structural features of 2, 4 and 5, UV-Vis-NIR absorption and fluorescence emission spectroscopy measurements were performed, the results suggesting a tetrahedral stereochemistry for the metal centres.