Studies in polymerization - XX. On free-radical formation by oxidation of molybdenum carbonyl with carbon tetrahalides: carbon tetrachloride

Abstract

The interaction of molybdenum carbonyl and carbon tetrachloride in a suitable medium gives rise to paramagnetic molybdenum products which can be studied by electron spin resonance (e. s. r.) spectroscopy. Most of the observations described in this paper have been made with ethyl acetate at 80 °C as solvent; in many respects methyl methacrylate gives similar results, but the rapid increase in viscosity accompanying polymerization limits investigation to short reaction times. E. s. r. spectra are consistent with the formation of Mo v species with one unpaired spin per Mo atom as the final oxidation state. Mo I and Mo III derivatives are not stable in the presence of carbon tetrachloride, but are rapidly oxidized to Mo v compounds. Kinetic investigations show that the signal-intensity versus reaction-time curve is sigmoid, the initial rate of growth of the signal being much lower than the rate of free-radical generation. It is concluded that there are at least two rate-determining processes in the reaction, the first being displacement of a CO ligand by solvent and the second the oxidation of an intermediate Mo species to a Mo v derivative. The first rate-determining reaction is followed rapidly by primary oxidation, producing one radical per Mo(CO) 6 consumed; this is essentially the only radical-generating step observed at short reaction times. The secondary oxidation, leading to Mo v , yields two radicals. Clearly some of the individual oxidation steps occur without radical formation; suggestions about the nature of the secondary oxidations are advanced. Measurements of carbon monoxide evolution indicate that all the CO ligands are ultimately released, and lead to the tentative conclusion that, in ethyl acetate, evolution of 5 molecules of CO per Mo(CO) 6 consumed accompanies formation of the primary radical. In methyl methacrylate CO evolution is slower, with a greater contribution from processes involved in secondary oxidation. The paramagnetic products of the reaction decompose to a diamagnetic black solid containing Mo IV on removal of volatile materials.