Low Quantum Yields of Cr(CO)6 Substitution in Fluorocarbon Solvent: Evidence That Metal-Fluorocarbon Interactions Are Very Weak


The relative heats of CO photosubstitution on Cr(CO)6 by cyclohexane, benzene, and 1,2-dichloroethane (as ligands) in perfluorocarbon solvents and in neat ligand have been determined by photoacoustic calo-rimetry. From these results and the quantum yield of CO substitution in neat ligand (0.67,0.67, and 0.62, respectively), the quantum yield in perfluorodecalin was found to be 0.3. These results are confirmed by conventional actinometry. The low quantum yield in perfluorodecalin can be attributed to its weaker coordination of Cr(CO)5. The quantum yield is independent of wavelength (254, 302, and 337 nm) in perfluorodecalin, indicating that the low quantum yield does not result from more efficient vibrational relaxation. The Cr(CO)5 generated by photolysis of Cr(CO)6 can recombine with photodissociated CO still in the solvent cage, and the relative strong coordination of cyclohexane, benzene, or 1,2-dichloroethane inhibits recombination of CO in the solvent cage, so that CO is forced to escape the cage. The results suggest a weak coordination of a fluorocarbon allows more cage CO to recombine, thus reducing the overall quantum yield of substitution. It is concluded that more efficient cage recombination of CO in a fluorocarbon solvent is primarily responsible for the lower quantum yield of substitution. © 1991, American Chemical Society. All rights reserved.

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