New coordination polymers based on the triangular [Cu3(μ3-OH)(μ-pz)3]2+ unit and unsaturated carboxylates

Abstract

By reacting copper(II) acrylate with pyrazole (Hpz), two trinuclear copper derivatives [Cu₃(μ₃-OH)(μ-pz)₃(CH₂CHCOO)₂(H₂O)₂(Hpz)], 1, and [Cu₃(μ₃-OH)(μ-pz)₃(CH₂CHCOO)₂(CH₃OH)], 2, are obtained, in water and methanol respectively, while copper(II) methacrylate affords [Cu₃(μ₃-OH)(μ-pz)₃(CH₂C(CH₃)COO)₂], 3, independently from the solvent used. In 1 and 2 two triangular trinuclear units are connected through acrylate bridges forming hexanuclear clusters that, in the case of 2 are further connected through double syn–syn carboxylate bridges, generating a 1-D coordination polymer. In the case of 3 a different 1-D coordination polymer is obtained by alternating syn–syn and syn–anti double carboxylate bridges connecting the trinuclear clusters. In all cases H-bonds contribute both to the stabilization of these arrangements and to the formation of more extended supramolecular networks. Compounds 1–3 are valuable catalysts in the peroxidative oxidation with aqueous H₂O₂, in MeCN at 25 °C, of cycloalkanes (i.e. cyclohexane and cyclopentane) to the corresponding ketones and alcohols (overall yield up to 36%, TON = 36), following a radical mechanism as shown by radical trap experiments, and the effects of various factors are studied. Electrochemical experiments show that the copper(II) centres are reduced to copper(I) and copper(0).