ESR Studies of Oxochromium(V) Porphyrin Complexes: Electronic Structure of the Cr(V)=O Moiety.

Abstract

ESR spectra of oxochromium(V) porphyrins were obtained to investigate the electronic structure of the Cr(V)=O moiety. At room temperature, the chlorooxochromium(V) meso-tetramesitylporphyrin complex 1-Cl exhibits an isotropic ESR signal, split into nine signals due to hyperfine coupling with four equivalent( 14)N, at g = 1.982. A small isotropic satellite signal, split into a quartet due to hyperfine coupling by the (53)Cr isotope is also observed. At 77 K, 1-Cl shows an anisotropic ESR signal with g( parallel) = 1.986 and g( perpendicular) = 1.980 and small satellite g( parallel) signals arising from hyperfine coupling with the (53)Cr isotope. The ESR parameters of 1-Cl, obtained from ESR spectral simulation, show that the pi-bonding character of the Cr(V)=O bond is not as strong as that of the Cr(V)&tbd1;N bond. To explore substituent effects on the bond character of the Cr(V)=O moiety, ESR spectra of perchloratooxochromium(V) porphyrins having various electronegative substituents were recorded. With an increase in electronegativity of the meso-substituent, the g( perpendicular) value and the (53)Cr hyperfine coupling constant (|a((53)Cr)|) are increased. These changes suggest an increase in the pi-bonding character of the Cr(V)=O moiety with an increase in the electronegativity of the meso-substituent. Furthermore, the hyperfine coupling constant of the oxo ligand, |a((17)O)|, obtained from ESR spectra of (17)O-labeled oxochromium(V) porphyrin is increased with an increase in electronegativity of the meso-substituent. This indicates that the radical character of the oxo ligand is increased by the electronegative meso-substituent. These data suggest that the pi-antibonding orbital of Cr=O is destabilized with an increase in electronegativity of the meso-substituent, which may facilitate bond breaking of the Cr=O moiety in an epoxidation reaction.