Methane Loss from Cationic μ-Methyl Dimers Formed via Trityl Borate Activation of Monocyclopentadienyl Ketimide Complexes Cp[(tBu)2CN]Ti(CH3)2 (Cp = C5H5, C5Me5, C5Me4SiMe3)

Abstract

The reactions of the monocyclopentadienyl titanium dimethyl compounds Cp(L)TiMe2 (L = tBu2CN; Cp = C5H5, 1a; C5Me5, 1b; C5Me4SiMe3, 1c) with the trityl borate activator [Ph3C]+[B(C6F5)4]- are described. Formation of μ-methyl dimers of formula {[Cp(L)TiMe]2(μ-Me)}+[B(C6F5)4]- as a 1:1 mixture of rac and meso diastereomers is observed when 0.5 equiv of [Ph3C]+[B(C6F5)4]- is employed (−25 °C, C6D5Br; Cp = C5H5, rac/meso 2a; C5Me5, rac/meso 2b; C5Me4SiMe3, rac/meso 2c). Dynamic NMR and crossover experiments suggest that the dimers 2 are relatively nonlabile with respect to dissociation, intramolecular methyl group exchange, or diastereomer interconversion. Dimers 2 are observed to undergo methane loss in solution at room temperature, affording the new dimeric compounds 3a−c, {[Cp(L)Ti]2(μ-CH2)(μ-CH3)}+[B(C6F5)4]-. For the less sterically demanding C5H5 ligand, 3a is formed as a mixture of rac/meso diastereomers (7:3), but for the bulkier C5Me5 and C5Me4SiMe3 ligands, the rac isomers of 3b and 3c are formed exclusively. In contrast to μ-methyl dimers 2, in which rac/meso interconversion is not observed, the diastereomers of 3 do undergo interchange, as determined by EXSY spectroscopy, and thus the rac/meso ratios observed are thermodynamic.