Benzene Hydroxylation by Bioinspired Copper(II) Complexes: Coordination Geometry versus Reactivity

A series of bioinspired copper(II) complexes of N-4-tripodal and sterically crowded diazepane-based ligands have been investigated as catalysts for functionalization of the aromatic C-H bond. The tripodalligand-based complexes exhibited distorted trigonal-bipyramidal (TBP) geometry (tau, 0.70) around the copper(II) center; however, diazepaneligand-based complexes adopted square-pyramidal (SP) geometry (tau, 0.037). The Cu-Np-y bonds (2.003-2.096 angstrom) are almost identical and shorter than Cu-N-amine bonds (2.01-2.148 angstrom). Also, their Cu-O (Cu-O-water, 1.988 angstrom; Cu-O-triflate,O- 2.33 angstrom) bond distances are slightly varied. All of the complexes exhibited Cu2+-> Cu+ redox couples in acetonitrile, where the redox potentials of TBP-based complexes (-0.251 to -0.383 V) are higher than those of SP-based complexes (-0.450 to -0.527 V). The d-d bands around 582-757 nm and axial patterns of electron paramagnetic resonance spectra [g(parallel to), 2.200-2.251; A(parallel to),( )(146-166) x 10(-4) cm(-1)] of the complexes suggest the existence of five-coordination geometry. The bonding parameters showed K-parallel to > K-perpendicular to for all complexes, corresponding to out-ofplane pi bonding. The complexes catalyzed direct hydroxylation of benzene using 30% H2O2 and afforded phenol exclusively. The complexes with TBP geometry exhibited the highest amount of phenol formation (37%) with selectivity (98%) superior to that of diazepane-based complexes (29%), which preferred to adopt SP-based geometry. Hydroxylation of benzene likely proceeded via a Cu-parallel to-OOH key intermediate, and its formation has been established by electrospray ionization mass spectrometry, vibrational, and electronic spectra. Their formation constants have been calculated as (2.54-11.85) X 10(-2) s(-1) from the appearance of an O (pi*sigma) -> Cu ligand-to-metal charge-transfer transition around 370-390 nm. The kinetic isotope effect (KIE) experiments showed values of 0.97-1.12 for all complexes, which further supports the crucial role of Cu-OOH in catalysis. The O-18-labeling studies using (H2O2)-O-18 showed a 92% incorporation of O-18 into phenol, which confirms H2O2 as the key oxygen supplier. Overall, the coordination geometry of the complexes strongly influenced the catalytic efficiencies. The geometry of one of the Cu-II-OOH intermediates has been optimized by the density functional theory method, and its calculated electronic and vibrational spectra are almost similar to the experimentally observed values.