Mechanism of the photochemical oxidation of Fe(CO)5 and CpW(CO)3Cl (Cp = η5-C5H5) by chlorocarbons

The photochemical oxidation of CpW(CO)₃Cl (Cp = η⁵-C₆H₅) by carbon tetrachloride and other polychlorocarbons affords CpW(CO)₂Cl₃. The sole photoprocess is dissociation of CO from the complex to give CpW(CO)₂Cl. The major pathway of the oxidation involves the net transfer of two halogen atoms from one halocarbon molecule to the unsaturated metal complex. Pathways involving the formation of trichloromethyl radical were shown to be unimportant. In a related reaction, the photooxidation of Fe(CO)₅ in carbon tetrachloride gives FeCl₂, tetrachloroethylene, tar, and minor amounts of hexachloroethane. As in the CpW(CO)₃Cl system, trichloromethyl radical is produced in, at most, only minor amounts. The reaction proceeds through an iron dichlorocarbene intermediate. Evidence for such an intermediate includes formation of isocyanides, phosgene, and thiophosgene when the oxidation is carried out in the presence of primary amines, dioxygen, and sulfur, respectively. In addition, tetrachloroethylene forms in neat carbon tetrachloride. Various alternative explanations for the above phenomena are considered and found unsatisfactory. The iron carbene species is proposed to arise via oxidative addition of carbon tetrachloride to photolytically generated Fe(CO)₄, followed by α-chlorine elimination. Degradation of the complex can then result in either dimerization or telomerization of the dichlorocarbene moiety. The CpW(LO)₃Cl system is believed to follow an analogous pathway.