MECHANISTIC STUDIES OF THE THERMOLYSIS OF TETRANEOPENTYLTITANIUM(IV) .1. SOLUTION EVIDENCE THAT TITANIUM ALKYLIDENES ACTIVATE SATURATED-HYDROCARBONS

Studies of the thermolysis of Ti(CH2CMe3)(4) in solution have been car ried out in parallel with studies of the chemical mechanism responsibl e for its conversion to titanium carbide under CVD conditions. In hydr ocarbon solutions, the neopentyl complex thermolyzes to eliminate 2.1 equiv of neopentane as the principal organic product. A deuterium kine tic isotope effect (k(alpha(H))/k(alpha(D))=5.2+/-0.4) upon deuteratin g the alkyl groups at the alpha positions provides clear evidence that the initial step in the thermolysis is an ct-hydrogen abstraction rea ction-to form neopentane, The activation parameters for this a-hydroge n abstraction process are Delta H-double dagger=21.5+/-1.4 kcal/mol an d Delta S-double dagger=-16.6+/-3.5 calf(mol K), The titanium-containi ng product of this reaction is a titanium alkylidene, which in solutio n activates C-H bonds of both saturated and unsaturated hydrocarbon so lvents such as benzene and cyclohexane. Na activation of the C-F bonds of hexafluorobenzene is seen, however. Under special circumstances, a second thermolysis pathway for TiNp4 can be detected, gamma-hydrogen activation, but this pathway is intrinsically about 25 times slower th an the alpha-hydrogen abstraction process.