Supramolecular assemblies of trinuclear triangular copper(II) secondary building units through hydrogen bonds. Generation of different metal-organic frameworks, valuable catalysts for peroxidative oxidation of alkanes

Formation of the trinuclear triangular copper derivative [Cu3(μ3-OH)(μ-pz)3(EtCOO)2(H2O)]·H2O, 1b (Hpz = pyrazole), has been simply achieved by addition of Hpz to a water solution of Cu(EtCOO)2·H2O and leaving the resulting solution to crystallize at ca. 12 °C. When the reaction and crystallization were carried out at a slightly higher temperature (18−22 °C), the compound [Cu3(μ3-OH)(μ-pz)3(EtCOO)2(H2O)], 1c, formed. Single-crystal X-ray molecular structure determinations show that both compounds have analogous trinuclear triangular structures, but very different supramolecular assemblies, due mainly, but not only, to the crystallization molecule of H2O in 1b. In particular, contrarily to the previously reported, strictly related, [Cu3(μ3-OH)(μ-pz)3(EtCOO)2(EtOH)], 1a, the propionate ions in 1b and 1c do not bridge different triangular units, whereas they are involved in intra- and intermolecular H-bonds, generating complex supramolecular 2-D MOFs. Compounds 1a and 1c act as remarkably active and selective catalysts or catalyst precursors for liquid biphasic (MeCN/H2O) peroxidative oxidation of cyclohexane and cyclopentane to the corresponding alcohols and ketones.