Syntheses, structures and catalytic activities of dinuclear copper complexes with tetradentate diaminebis (phenolate) ligands

Abstract

Two new Cu(II) complexes were synthesized from Cu(NO3)2·3H2O and salan ligands, H2[H4]L1 and H2[H4]L2 (H2[H4]L1 = N,N′-bis(o-hydroxybenzyl)-1,2-diaminocyclohexane; H2[H4]L2 = N,N′-bis(o-hydroxybromobenzyl)1,2-diaminocyclohexane). They were characterized by various spectroscopic methods and structures of the complexes determined by X-ray diffraction analyses. Interestingly, H2[H4]L1 coordinates to copper centers as monoanionic in a tetradentate mode in complex 1, [Cu(H[H4]L1)]2·2NO3·2H2O, whereas H2[H4]L2 behaves as a dianionic ligand in complex 2, [Cu([H4]L2)]2·0.4H2O, via the O,N,N′,O′-donor atoms. The asymmetric unit in the complexes is dimerized about a center of inversion by asymmetric bridging of the phenoxide-O atom between two metal centers. The electronic spectra studies of the complexes in various solvents with different coordination numbers proved the stability of the dinuclear complexes in solvents other than DMSO and DMF. Complexes 1 and 2 showed high catalytic activities with good-to-excellent selectivities in the oxidation of olefins, benzyl alcohol and ethyl benzene with H2O2 in acetonitrile. A probable mechanism is discussed.