Synthesis and characterization of copper(ii) 4′-phenyl-terpyridine compounds and catalytic application for aerobic oxidation of benzylic alcohols

Abstract

The reactions between 4′-phenyl-terpyridine (L) and nitrate, acetate or chloride Cu(II) salts led to the formation of [Cu(NO₃)₂L] (1), [Cu(OCOCH₃)₂L]·CH₂Cl₂ (2·CH₂Cl₂) and [CuCl₂L]·[Cu(Cl)(μ-Cl)L]₂ (3), respectively. Upon dissolving 1 in mixtures of DMSO–MeOH or EtOH–DMF the compounds [Cu(H₂O){OS(CH₃)₂}L](NO₃)₂ (4) and [Cu(HO)(CH₃CH₂OH)L](NO₃) (5) were obtained, in this order. Reaction of 3 with AgSO₃CF₃ led to [CuCl(OSO₂CF₃)L] (6). The compounds were characterized by ESI-MS, IR, elemental analysis, electrochemical techniques and, for 2–6, also by single crystal X-ray diffraction. They undergo, by cyclic voltammetry, two single-electron irreversible reductions assigned to Cu(II) → Cu(I) and Cu(I) → Cu(0) and, for those of the same structural type, the reduction potential appears to correlate with the summation of the values of the Lever electrochemical E_L ligand parameter, which is reported for the first time for copper complexes. Complexes 1–6 in combination with TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl radical) can exhibit a high catalytic activity, under mild conditions and in alkaline aqueous solution, for the aerobic oxidation of benzylic alcohols. Molar yields up to 94% (based on the alcohol) with TON values up to 320 were achieved after 22 h.