Chromium(V) and Chromium(VI) Nitrido Complexes of Tris(pentafluorophenyl)corrole

摘要

A unique feature of corroles relative to porphyrins and other related ligands is their ability to stabilize transition-metal ions in high oxidation states. This phenomenon can not be related to the traditional means of obtaining high-valent metal complexes, which relies on π-donating and nonoxidizable ligands such as F~-, O~(2-), and N~(3-), since the corrole macrocycle in non-transition-metal complexes is easily oxidized. Instead, recent theoretical and experimental evidence suggests that σ donation by corrole tranions raises the energy of metal d orbitals to a large extent. This leads to destabilization of low-valent oxidation states and is also manifested in the unique activation of molecular oxygen by chromium (III) and iron (III) corroles. Taken together, the cooperation of both π and σ effects in oxo- and nitridometallocorroles could be an attractive methodology for obtaining novel complexes. The validity of this hypothesis is indicated by the stability of corrole complexes with pentavalent metal ions: [(tpfc)Cr~V(O)] (1), [(tpfc)Mn~V(O)] (2), [(tpfc)Mn~V(N)]~- (3), and [(tpfc)Mn~V(NAr)] (4), where tpfc stands for the trianion of 5, 10, 15-tris (pentafluorophenyl) corrole (Scheme 1). However, complexes of the highly oxidizing Cr~(VI) ion were not reported for corroles or other related ligands. A relevant result that explains why such complexes could not be obtained is that the one-electron oxidation of 1 is corrole-rather than metal-centered and leads to [(tpfc)Cr~V(O)]~+ rather than [(tpfc)Cr~(VI)(O)]~+. Since a comparison of nitrido and oxo complexes with identical ligands and metals reveals that the former are invariably more stable, we decided to explore the possibility of obtaining stable (nitrido) chromium corroles. These attempts were successful: both [(tpfc)Cr~V(N)]~- (5a) and [(tpfc)Cr~V(N)] (5b) were isolated and characterized by a combination of spectroscopic methods and electrochemistry.