Efficiency of Photosubstitution of Arenetricarbonylchromium(0) Complexes

Abstract

The quantum efficiency of the formation of (arene)Cr(CO)2(pyridine) (arene=benzene, mesitylene) is reported as a function of the irradiation wavelength of (arene)Cr(CO)3 and as a function of pyridine concentration. The quantum efficiency is independent of pyridine concentration in the range 0.008-0.17 M. The quantum efficiency is 0.72 ± 0.07 upon 313, 366, or 436 nm irradiation. The (arene)Cr(CO)3 complexes are found to quench triplet excited benzil at a diffusion controlled rate and the quenching is accompanied by reaction of the (arene)Cr(CO)3. The reaction product which presumably arises from CO release is the same as that obtained from direct irradiation of (arene)Cr(CO)3 in the presence of benzil. However, the triplet sensitized reaction occurs with a quantum efficiency of only 0.15 ± 0.05 in contrast to the high substitution efficiency (0.72 ± 0.07) of (arene)Cr(CO)3 upon direct irradiation.