Photolysis of arene chromium tricarbonyls and photoinitiation of polymerization. Part 2.—Toluene, anisole, fluorobenzene and chlorobenzene chromium tricarbonyls

Abstract

The photolysis of some arene chromium tricarbonyls, arCr(CO)₃, has been studied in cyclohexane, methyl methacrylate and ethyl acetate solutions. Spectral changes occurring on irradiation are generally similar to those described for benzene chromium tricarbonyl under comparable conditions. In the ester solvents they suggest the formation of species such as arCr(CO)₂S and Cr(CO)₅S, where S represents the ester and coordination to the metal occurs through the carbonyl group. When carbon tetrachloride is present development of these spectral features is eliminated or reduced, on account of reaction between the above species and CCl₄.

As with benzene chromium tricarbonyl, the rate of arene generation is much greater in methyl methacrylate solution than in hydrocarbon media and it is further increased by the presence of CCl₄.

Systems containing arCr(CO)₃ and CCl₄ are active photoinitiators of free-radical polymerization. The primary radicals generated have been shown by spin-trapping to be trichloromethyl. At ‘high’[CCl₄](ca. 0.1 mol dm^–3) the quantum yields of photodecomposition of the carbonyl and photoinitiation are equal for all the derivatives studied, one ar—Cr bond being broken for each Ċl₃ formed. The order of quantum yields for the various arene derivatives is fluorobenzene > chlorobenzene > benzene > toluene anisole, probably the same as the order of the ar—Cr bond dissociation energies.

A reaction scheme similar to that proposed for the benzene derivative and involving exciplex formation between excited arCr(CO)₃ and methyl methacrylate is consistent with all the experimental data.