Complexes of copper(II) with 3-(ortho-substituted phenylhydrazo)pentane-2,4-diones: syntheses, properties and catalytic activity for cyclohexane oxidation

Reactions of copper(II) with 3-phenylhydrazopentane-2,4-diones X-2-C6H4–NHNC{C(O)CH3}2 bearing a substituent in the ortho-position [X = OH (H2L1) 1, AsO3H2 (H3L2) 2, Cl (HL3) 3, SO3H (H2L4) 4, COOCH3 (HL5) 5, COOH (H2L6) 6, NO2 (HL7) 7 or H (HL8) 8] lead to a variety of complexes including the monomeric [CuL4(H2O)2]·H2O 10, [CuL4(H2O)2] 11 and [Cu(HL4)2(H2O)4] 12, the dimeric [Cu2(H2O)2(μ-HL2)2] 9 and the polymeric [Cu(μ-L6)]n] 13 ones, often bearing two fused six-membered metallacycles. Complexes 10–12 can interconvert, depending on pH and temperature, whereas the Cu(II) reactions with 4 in the presence of cyanoguanidine or imidazole (im) afford the monomeric compound [Cu(H2O)4{NCNC(NH2)2}2](HL4)2·6H2O 14 and the heteroligand polymer [Cu(μ-L4)(im)]n15, respectively. The compounds were characterized by single crystal X-ray diffraction (complexes), electrochemical and thermogravimetric studies, as well as elemental analysis, IR, 1H and 13C NMR spectroscopies (diones) and ESI-MS. The effects of the substituents in 1–8 on the HOMO–LUMO gap and the relative stability of the model compounds [Cu(OH)(L8)(H2O)]·H2O, [Cu(L1)(H2O)2]·H2O and [Cu(L4)(H2O)2]·H2O are discussed on the basis of DFT calculations that show the stabilization follows the order: two fused 6-membered > two fused 6-membered/5-membered > one 6-membered metallacycles. Complexes 9, 10, 12 and 13 act as catalyst precursors for the peroxidative oxidation (with H2O2) of cyclohexane to cyclohexanol and cyclohexanone, in MeCN/H2O (total yields of ca. 20% with TONs up to 566), under mild conditions.