Mechanistic studies of the thermolysis of tetraneopentyltitanium(IV). 2. Solid state and ultra-high-vacuum studies of the chemical vapor deposition of tic films

The chemical pathway responsible for the conversion of the organotitanium compound tetraneopentyltitanium to titanium carbide has been studied under chemical vapor conditions and on single crystals in ultra-high vacuum. For every equivalent of TiNp₄ consumed in the deposition process, 3.28 equiv of neopentane and 0.16 equiv of isobutane are produced; other organic species are also formed but in relatively small amounts. About 93% of the carbon and hydrogen originally present in the precursor can be accounted for in these products. Thermolysis of the specifically deuterated analogue Ti(CD₂CMe₃)4 yields a 2.25:1 ratio of neopentane-d₃ and neopentane-d₂; this result combined with a kinetic isotope effect of 4.9 at 385 K shows unequivocally that the first step in the deposition pathway under CVD conditions is α-hydrogen abstraction. The α-hydrogen abstraction step produces 1 equiv of neopentane and a titanium alkylidene, which undergoes further α- (and eventually γ-) hydrogen activation processes to generate the second and third equivalents of neopentane. In the last stages of the thermolysis sequence, neopentyl (or neopentyl-derived) organic groups evidently fragment and generate the carbon atoms that eventually form the titanium carbide phase. Spectroscopic studies with IR and HREELS techniques have also been carried out in order to provide additional evidence about the nature of the species present when single crystal surfaces dosed with TiNp₄ are heated. A hand at 1121 cm^-1 is tentatively ascribed to the υ(M=C) hand of surface-bound neopentylidene groups.