Why the Chromyl Bond Is Stronger Than the Perchromyl Bond in High-Valent Oxochromium(IV,V) Complexes of Tris(pentafluorophenyl)corrole

Resonance Raman (RR) spectroscopy and density functional theory (DFT) calculations of oxochromium(IV,V) derivatives of 5,10,15-tris(pentafluorophenyl)corrole (tpfpc) are shown to provide useful, information about the relative strength of the metal-oxo bond in high-valent Cr-IV versus Cr-V corroles. Isotope labeling of the terminal oxo group with O-18 revealed that the Cr-V-oxo (perchromyl) stretch of (tpfpc)(CrO)-O-V vibrates at a frequency of 986 cm(-1) in carbon disutfide, consistent with a triply bonded Cr-V=O unit. In contrast, an acetonitrile solution produced RR scattering that rapidly changed with the number of scans collected and eventually became dominated by an O-18-sensitive vibration at a significantly higher frequency of 1002 cm(-1). On the basis of DFT calculations and the observed O-18/16 isotopic shift, we assigned this new RR band at 1002 cm(-1) in acetonitrite as the Cr-IV-oxo (chromyl) stretch of the autoreduced [(tpfpc)(CrO)-O-IV](-) product, which previously has been shown to form only during the course of the oxygen atom transfer (OAT) reaction with triphenylphosphine in acetonitrite or in the presence of a reducing chemical (cobaltocene) and electrochemical agents in other solvents. Consequently, RR observations indicate that the pi-bonding character of the chromyl bond is actually increased relative to that of the perchromyl bond, which is of interest if the beneficial rote of acetonitrile in OAT catalysis by high-valent oxochromium(IV,V) corroles is to be elucidated.