INVESTIGATION OF THE REACTIVE OXYGEN INTERMEDIATE IN AN ARENE HYDROXYLATION REACTION PERFORMED BY XYLYL-BRIDGED BINUCLEAR COPPER-COMPLEXES

The kinetics of the reaction, via an oxygen-bound intermediate, of [Cu -2(1)(NO2-XYL)][ClO4](2). CH3-CN to [Cu-2(NO2-XYL-O-)(OH)](2+), where the bridging arene is hydroxylated, have been examined with use of res onance Raman spectroscopy. A resonance Raman peak characteristic of pe roxide bound in a side-on, mu-eta(2):eta(2) geometry is observed upon oxygenation of [Cu-2(1)(NO2-XYL)] for both intramolecularly and interm olecularly bridged complexes. The decay of the intramolecularly bridge d peroxide stretch at similar to 750 cm(-1) and the growth of the phen olate stretch of the product at 1320 cm(-1) were monitored over time w ith use of an excitation wavelength of 406.9 nm. Both the decay of the peroxide stretch and the growth of the phenolate stretch were found t o be first order, and the rate constants are consistent, within experi mental error, with the peroxide intermediate reacting directly to form the hydroxylated product. The possibility of an unobservable amount o f a bis-mu-oxo isomer which is in rapid equilibrium with the side-on p eroxide species, and that is responsible for the hydroxylation reactio n, is considered. An upper limit for the concentration of the bis-mu-o xo isomer in a solution of [CU2(NO2-XYL)(O-2)](2+) was determined. Thi s gives the lower limit for its rate of reaction to form the phenolate product, which is approximately 1000 times faster than the decay of t he peroxide intermediate. A comparison of the reactivities of the side -on peroxide and bis-mu-oxo isomers with respect to electrophilic arom atic substitution is made by using frontier molecular orbital theory. This correlation, in conjunction with the estimated, relative rates of reaction for the two isomers to form phenolate product, leads to a mo lecular mechanism in which the side-on peroxide isomer is likely to be the reactive oxygen intermediate in these systems.