Synthesis and Characterization of Copper Complexes of a Tetrapyridyl Ligand, and Their Use in the Catalytic Aerobic Oxidation of Benzyl Alcohol

The synthesis and characterization of new copper complexes of the tetrapyridyl ligand N,N'-di(pyridin-2-yl)-2,2'-bipyridine- 6,6'-diamine (bapbpy) is reported. The crystal structures of the compounds [Cu(bapbpy)(H2O)] Cl-2 center dot H2O (1), [Cu(bapbpy)(H2O)] Br-2 center dot H2O (2) and [Cu(bapbpy)(H2O)](NO3)(2)center dot H2O (3) have been elucidated and show the copper(II) ions to be in a distorted square-pyramidal geometry with the tetradentate ligand in the equatorial plane. Conductivity measurements on 1 revealed that it is fully ionized in methanol, i.e., the chloride ions are not coordinated to copper(II) in methanolic solutions. The room temperature electron paramagnetic resonance (EPR) spectra of methanol solutions of the complexes show a copper signal with a g(iso) value of 2.110; it shows hyperfine splitting into four lines (A(Cu) = 77G) and superhyperfine splitting into nine lines (AN = 12G) owing to the interaction with the nuclear spin of the four nitrogen donor atoms of the bapbpy ligand. The catalytic activity of 1, 2 and 3 in the aerobic homogeneous oxidation of benzyl alcohol in presence of the TEMPO radical [(2,2,6,6-tetramethylpiperidin-1-yl) oxidanyl] was investigated. Benzaldehyde is obtained selectively and the production of benzoic acid is not observed during 24 h of reaction. EPR analyses, carried out under stoichiometric conditions, show that the TEMPO radical does not interact with the copper centre. These data suggest that oxidation of the substrate may occur in the second coordination sphere of the copper complex.