A Structurally Characterized CuIII Complex Supported by a Bis(anilido) Ligand and Its Oxidative Catalytic Activity

Three copper(II) complexes of the (R,R)-N,N'-Bis(3,5-di-tert-butyl-2-aminobenzylidene)-1,2-diaminocyclohexane, namely [Cu(NL)], [Cu(NLH)]+ and [Cu(NLH2)]2+, were prepared and structurally characterized. In [Cu(NLH2)]2+ the copper ion lies in an octahedral geometry with the aniline groups coordinated in equatorial positions. In [Cu(NL)] the anilines are deprotonated (anilido moieties) and coordinated to an almost square metal ion. Complex [Cu(NL)] displays two oxidation waves at E1/2ox,1 = -0.14 V and E1/2ox,2 = 0.36 V vs Fc+/Fc in CH2Cl2. Complex [Cu(NLH2)]2+ displays an irreversible oxidation wave at high potential (1.22 V), but shows a readily accessible and reversible metal-centered reduction at E1/2red = -0.67 V (CuII/CuI redox couple). Oxidation of [Cu(NL)] by AgSbF6 produces [Cu(NL)](SbF6), which was isolated as single crystals. X-ray structure analysis discloses a contraction of the coordination sphere by 0.05 Ǻ upon oxidation, supporting a metal-centered process. Complex [Cu(NL)](SbF6) displays an intense NIR band at 1260 nm corresponding to an anilido-to-copper(III) charge transfer transition. This compound slowly evolves in CH2Cl2 solution towards [Cu(NLH)](SbF6), which is a copper(II) complex comprised of both anilido and aniline groups coordinated to the metal center. The copper(III) complex [Cu(NL)](SbF6) is an efficient catalyst for benzyl alcohol oxidation, with 236 TON in 24 h at 298 K, without additives other than oxygen and a base.