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 [TiCl_xNH_x-1(NH₃)_n]₂ species (x=3, 2; n=0, 1) have been considered. Dimerization of amido compounds is exothermic by about 10 kcalmol^-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 kcalmol^-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 kcalmol^-1) than Ti-Cl bond rupture (∼82.5 kcalmol^-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 kcalmol^-1. Additional coordination of two ammonia molecules to the [Cl₃TiNH₂]₂ dimer facilitates the elimination of Cl atom.