Isolation and Characterization of a Monomeric Cationic Titanium Hydride

Methyl cations <b>1-Cp</b> and <b>1-Cp*</b>, stabilized by the tri-<i>tert</i>-butylphophinimine ligand and either C<sub>5</sub>H<sub>5</sub> or C<sub>5</sub>Me<sub>5</sub>, were generated from the neutral dimethyl precursors and [Ph<sub>3</sub>C]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>. Reaction of these compounds with H<sub>2</sub> resulted in contrasting reactions. For <b>1-Cp</b>, hydrogenolysis of the Ti−CH<sub>3</sub> group led to rapid reduction to Ti(III) and production of a cationic Ti(III) dimer, <b>2</b>, presumably formed upon loss of H<sub>2</sub> from a transiently generated Ti(IV) hydride. Compound <b>2</b> was characterized crystallographically and via its cleavage with donor solvents such as THF to form the monomeric [Cp(L)Ti(THF)<sub>2</sub>]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>, <b>3</b>. In contrast, <b>1-Cp*</b> reacted rapidly with H<sub>2</sub> to form a cationic Ti(IV) hydride species, <b>4</b>, which was resistant to reduction. While only moderately stable in solution under H<sub>2</sub>, a stable, isolable THF adduct preciptitated upon addition of THF, giving <b>4·THF</b>, which was fully characterized, including via X-ray crystallography. Naked hydride <b>4</b> was very reactive toward haloarene solvents such as bromobenzene, giving the cationic bromide [Cp*(L)TiBr]<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>, <b>5</b>, which was fully characterized as its THF adduct <b>5·THF</b>. The contrasting behavior of <b>1-Cp</b> and <b>1-Cp*</b> is a result of the greater steric protection and electron donation provided by the Cp* ligand relative to the Cp donor.