Metal Azolate/Carboxylate Frameworks as Catalysts in Oxidative and C–C Coupling Reactions

The five metal azolate/carboxylate (MAC) compounds [Cd­(dmpzc)­(DMF)­(H2O)] (Cd-dmpzc), [Pd­(H2dmpzc)2Cl2] (Pd-dmpzc), [Cu­(Hdmpzc)2] (Cu-dmpzc), [Zn4O­(dmpzc)3]·Solv (Zn-dmpzc·S), and [Co4O­(dmpzc)3]·Solv (Co-dmpzc·S) were isolated by coupling 3,5-dimethyl-1H-pyrazol-4-carboxylic acid (H2dmpzc) to cadmium­(II), palladium­(II), copper­(II), zinc­(II), and cobalt­(II) salts. While Cd-dmpzc and Pd-dmpzc had never been prepared in the past, for Cu-dmpzc, Zn-dmpzc·S, and Co-dmpzc·S we optimized alternative synthetic paths that, in the case of the copper­(II) and cobalt­(II) derivatives, are faster and grant higher yields than the previously reported ones. The crystal structure details were determined ab initio (Cd-dmpzc and Pd-dmpzc) or refined (Cu-dmpzc, Zn-dmpzc·S, and Co-dmpzc·S) by means of powder X-ray diffraction (PXRD). While Cd-dmpzc is a nonporous 3D MAC framework, Pd-dmpzc shows a 3D hybrid coordination/hydrogen-bonded network, in which Pd­(H2dmpzc)2Cl2 monomers are present. The thermal behavior of the five MAC compounds was investigated by coupling thermal analysis to variable-temperature PXRD. Their catalytic activity was assessed in oxidative and C–C coupling reactions, with the copper­(II) and cadmium­(II) derivatives being the first nonporous MAC frameworks to be tested as catalysts. Cu-dmpzc is the most active catalyst in the partial oxidation of cyclohexane by tert-butyl hydroperoxide in acetonitrile (yields up to 12% after 9 h) and is remarkably active in the solvent-free microwave-assisted oxidation of 1-phenylethanol to acetophenone (yields up to 99% at 120 °C in only 0.5 h). On the other hand, activated Zn-dmpzc·S (Zn-dmpzc) is the most active catalyst in the Henry C–C coupling reaction of aromatic aldehydes with nitroethane, showing appreciable diastereoselectivity toward the syn-nitroalkanol isomer (syn:anti selectivity up to 79:21).