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)

The reactions of the monocyclopentadienyl titanium dimethyl compounds Cp(L)TiMe 2 (L = t Bu 2 C N; Cp = C 5 H 5, 1a; C 5 Me 5, 1b; C 5 Me 4 SiMe 3, 1c ) with the trityl borate activator [Ph 3 C] + [B(C 6 F 5 ) 4 ] - are described. Formation of μ-methyl dimers of formula {[Cp(L)TiMe] 2 (μ-Me)} + [B(C 6 F 5 ) 4 ] - as a 1:1 mixture of rac and meso diastereomers is observed when 0.5 equiv of [Ph 3 C] + [B(C 6 F 5 ) 4 ] - is employed (−25 °C, C 6 D 5 Br; Cp = C 5 H 5, rac/meso 2a; C 5 Me 5, rac/meso 2b; C 5 Me 4 SiMe 3, 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-CH 2 )(μ-CH 3 )} + [B(C 6 F 5 ) 4 ] - . For the less sterically demanding C 5 H 5 ligand, 3a is formed as a mixture of rac/meso diastereomers (7:3), but for the bulkier C 5 Me 5 and C 5 Me 4 SiMe 3 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.