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

Abstract

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.