Carbon-hydrogen bond activation by a titanium neopentylidene complex

Abstract

The titanium neopentylidene complex (PNP)Ti=(CHBu)-Bu-t((CH2Bu)-Bu-t), PNP=N[2-(PPr2)-Pr-i-4-methylphenyl](2)(-), can activate both sp(2) and sp(3) C-H bonds under mild conditions. In this work, we studied the reaction mechanism of this complex with benzene and methane using modern density functional theory, specifically the B97XD functional which contains long-range exchange and dispersion corrections. The mechanism of the reaction is similar to that computed previously in the literature, but we describe a new conformer that is both more stable and kinetically more reactive. The four-step mechanism is very similar for both benzene and methane. However, the highest energy barriers differ; for methane, it is the last step, which elucidates the inertness of that reactant. In addition, the hydrogen exchange between alkyl and alkylidene ligands in methane's product was studied by two different mechanisms: tautomerization to form (PNP)(TiCHBu)-Bu-t(=CH2) and reverse C-H activation to form (PNP)(TiCBu)-Bu-t(CH3). The feasibility of the tautomerization, through a preliminary, accessible isomerization, suggests that these systems can be used to explore the reactivity of terminal methylidenes. Finally, methodological considerations are also discussed, as the importance of including the dispersion in the density functionals was determined by comparing several functionals. This comparison has shown that the dispersion is critical for accurate modeling, especially in the stability of the unsaturated intermediate; this has been neglected in previous studies. [GRAPHICS] .