Photoinduced Radical Chain Bromination of Decarbonyldimanganese

Abstract

The reaction of bromine with Mn2(CO)10 in tetrachloro methane proceeds quite rapidly and cleanly on exposure to light to form Mn(CO)5Br as the only IR detectable product. The kinetics of this reaction have been studied under a wide variety of conditions, reaction being initiated by photodissociation of bromine with light of wavelength ca. 435nm. It is concluded that the bromine atoms initiate a chain reaction by attacking the Mn2(CO)10 and forming Mn(CO)5Br and an Mn(CO)5 radical. This in turn reacts with bromine to generate Mn(CO)5Br and another bromine atom to continue the chain. The absence of appropriate effects on varying the intensity of absorbed light shows that chain termination does not involve radical recombination. It is probably effected by reaction of bromine atoms and Mn(CO)5 radicals with the solvent or, more effectively, with CBrCl3 when added in small amounts. Reaction in perfluorodecalin proceeds by a similar path but with higher quantum yields. Only bromine atoms, and not Mn(CO)5 radicals, appear to be involved in the termination step.