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).