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 ligand, namely [Cu(L-N)], [Cu((LH)-L-N)](+) and [Cu((LH2)-L-N)](2+), were prepared and structurally characterized. In [Cu((LH2)-L-N)](2+) the copper ion lies in an octahedral geometry with the aniline groups coordinated in equatorial positions. In [Cu(L-N)] the anilines are deprotonated (anilido moieties) and coordinated to an almost square-planar metal ion. Complex [Cu(L-N)] displays two oxidation waves at E-1/2(ox,1)=-0.14V and E-1/2(ox,2)=0.36V vs. Fc(+)/Fc in CH2Cl2. Complex [Cu((LH2)-L-N)](2+) displays an irreversible oxidation wave at high potential (1.21V), but shows a readily accessible and reversible metal-centered reduction at E-1/2(red)=-0.67V (Cu-II/Cu-I redox couple). Oxidation of [Cu(L-N)] by AgSbF6 produces [Cu(L-N)](SbF6), which was isolated as single crystals. X-ray structure analysis discloses a contraction of the coordination sphere by 0.05 angstrom upon oxidation, supporting a metal-centered process. Complex [Cu(L-N)](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((LH)-L-N)](SbF6), which is a copper(II) complex comprised of both anilido and aniline groups coordinated to the metal center. The copper(III) complex [Cu(L-N)](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.