Methane loss from cationic μ-methyl dimers formed via trityl borate activation of monocyclopentadienyl ketimide complexes Cp[(tBu)2C=N]Ti(CH3)2 (Cp = C5H5, C5Me5, C5Me4SiMe3)

The reactions of the monocyclopentadienyl titanium dimethyl compounds Cp(L)TiMe2 (L = (Bu2C)-Bu-t=N; Cp = C5H5, 18; C5Me5, Ib; C5Me4SiMe3, Ic) with the trityl borate activator [Ph3C](+)[B(C6F5)(4)](-) are described. Formation of Ic-methyl dimers of formula {[Cp(L)TiMe](2)(mu -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 degreesC, 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)(mu -CH2)(mu -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 mu -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.