Isolation and Characterization of a Monomeric Cationic Titanium Hydride

Methyl cations 1-Cp and 1-Cp*, stabilized by the tri-tert-butylphophinimine ligand and either C₅H₅ or C₅Me₅, were generated from the neutral dimethyl precursors and [Ph₃C]⁺[B(C₆F₅)₄]^-. Reaction of these compounds with H₂ resulted in contrasting reactions. For 1-Cp, hydrogenolysis of the Ti−CH₃ group led to rapid reduction to Ti(III) and production of a cationic Ti(III) dimer, 2, presumably formed upon loss of H₂ from a transiently generated Ti(IV) hydride. Compound 2 was characterized crystallographically and via its cleavage with donor solvents such as THF to form the monomeric [Cp(L)Ti(THF)₂]⁺[B(C₆F₅)₄]^-, 3. In contrast, 1-Cp* reacted rapidly with H₂ to form a cationic Ti(IV) hydride species, 4, which was resistant to reduction. While only moderately stable in solution under H₂, a stable, isolable THF adduct preciptitated upon addition of THF, giving 4·THF, which was fully characterized, including via X-ray crystallography. Naked hydride 4 was very reactive toward haloarene solvents such as bromobenzene, giving the cationic bromide [Cp*(L)TiBr]⁺[B(C₆F₅)₄]^-, 5, which was fully characterized as its THF adduct 5·THF. The contrasting behavior of 1-Cp and 1-Cp* is a result of the greater steric protection and electron donation provided by the Cp* ligand relative to the Cp donor.