Polynuclear Copper(II) Complexes as Catalysts for the Peroxidative Oxidation of Cyclohexane in a Room-Temperature Ionic Liquid

The reaction of copper(II) nitrate hydrate with sodium 2‐[2‐(2,4‐dioxopentan‐3‐ylidene)hydrazinyl]terephthalate (Na2HL1) in the absence or presence of imidazole (im) leads to the new complexes [(H2O)Cu0.5{(μ‐L1‐1κO:2κ3N,O′,O″)Cu(H2O)2}]2 (1) and [Cu(im)(μ‐HL1‐1κO:2κ3N,O,O′)]2 (2), respectively, whereas the 1D coordination polymer [Cu2(H2O)2(μ‐H2L2‐1κ3N,O,O′:2κO″)(μ‐L2‐1κ3N,O,O′:2κ2N′,O″)]n (3) is obtained from Cu(NO3)2·2.5H2O and sodium 2‐[2‐(4,4‐dimethyl‐2,6‐dioxocyclohexylidene)hydrazinyl]terephthalate (Na2HL2). All of the compounds were characterized by ESI‐MS, IR spectroscopy, elemental analysis and X‐ray diffraction analysis. Di‐ or tri‐deprotonated ligands display different coordination modes and lead to distinct nuclearities of the CuII complexes depending on the conditions used for the syntheses. Complexes 1–3 were successfully tested as dual‐role catalysts for cyclohexane oxidation under acid‐free conditions in the ionic liquid [1‐butyl‐3‐methylimidazolium tetrafluoroborate, (bmim)(BF4)] as well as in acetonitrile. The application of the ionic liquid (IL) increased the product yield and turnover number and decreased the reaction time, in comparison with those for the use of the conventional acetonitrile solvent. Easy recycling of the catalyst, with preservation of almost the initial activity and selectivity, was also achieved by using the IL medium. Multinuclear CuII complexes with a catalytic metal centre and an acid functionality catalyze the peroxidative oxidation of cyclohexane in acid‐free water/air/CH3CN or ionic liquid media.