Formation of oligomer species in the course of the TiCl4 ammonolysis and their role in Ti(IV)–Ti(III) reduction processes: A theoretical study

Gas phase oligomer formation during TiCl4 ammonolysis is theoretically investigated at the DFT B3LYP level of theory with double-ζ quality basis set. Several possible isomers of the dimeric [TiClxNHx−1(NH3)n]2 species (x=3, 2; n=0, 1) have been considered. Dimerization of amido compounds is exothermic by about 10 kcal mol−1, but disfavored by entropy. The latter factor prevents the existence of the amido dimers in the gas phase thermodynamic equilibrium. The dimerization processes of imido compounds are much more exothermic (by about 94 kcal mol−1). A cubic [ClTiN]4 cluster is predicted to be a viable molecular form in the gas phase above 1400 °C. Reduction of Ti(IV) to Ti(III) via hydrazine formation from the [TiCl3NH2(NH3)]2 dimers is less endothermic (∼40 kcal mol−1) than Ti–Cl bond rupture (∼82.5 kcal mol−1). The overall endothermicity of the formation of N2H4 bridged Ti(III) dimer [Cl2(H3N)Ti(μ-Cl)2(μ-N2H4)Ti(NH3)Cl2] from TiCl4 and NH3 is only 6.6 kcal mol−1. Additional coordination of two ammonia molecules to the [Cl3TiNH2]2 dimer facilitates the elimination of Cl atom.