Mechanistic Studies of the Thermolysis of Tetraneopentyltitanium(IV). 1. Solution Evidence That Titanium Alkylidenes Activate Saturated Hydrocarbons

Abstract

Studies of the thermolysis of Ti(CH2CMe3)4 in solution have been carried out in parallel with studies of the chemical mechanism responsible for its conversion to titanium carbide under CVD conditions. In hydrocarbon solutions, the neopentyl complex thermolyzes to eliminate 2.1 equiv of neopentane as the principal organic product. A deuterium kinetic isotope effect (kR(H)/kR(D) ) 5.2 ( 0.4) upon deuterating the alkyl groups at the R positions provides clear evidence that the initial step in the thermolysis is an R-hydrogen abstraction reaction to form neopentane. The activation parameters for this R-hydrogen abstraction process are ¢H q ) 21.5 ( 1.4 kcal/mol and ¢S q )- 16.6 ( 3.8 cal/(mol K). The titanium-containing product of this reaction is a titanium alkylidene, which in solution activates C-H bonds of both saturated and unsaturated hydrocarbon solvents such as benzene and cyclohexane. No activation of the C-F bonds of hexafluorobenzene is seen, however. Under special circumstances, a second thermolysis pathway for TiNp4 can be detected, A-hydrogen activation, but this pathway is intrinsically about 25 times slower than the R-hydrogen abstraction process.