C-H bond activation processes in cationic and neutral titanium benzyl compounds with cyclopentadienyl-arene ligands

Titanium tribenzyl complexes with cyclopentadienyl-arene ligands, eta(5)-C-5-H1CMe2Ar)Ti(CH2Ph)(3) Ar = 3,5-Me2C6H3 (1), Ph (2)), were synthesized. Reaction of 1 with either B(C6F5)(3) or [Ph3C] [B(C6F5)(4)] affords the cation [(eta,(5)eta-C5H4CMe2-3,5-Me2C6H3)Ti(eta-CH2Ph)(2)](+) (3), in which the pendant arene group and the benzyl ligands show fluxional behavior. Reaction of 2 with B(C6F5)(3), leads to rapid ortho cyclometalation of the pendant arene to give the contact ion pair [(eta(5):eta(1)-C5H4CMe2C6H4)Ti(eta(1)-CH2Ph)] [eta(6)-PhCH2B(C6F5)(3)] (4) and toluene. Reaction of 2 with [Ph3C][B(C6F5)(4)] also leads to ligand ortho nietalation, giving [eta(5):eta(1)-C5H4CMe2C6H4)Ti(eta(2)-CH2Ph)] [B(C6F5)(4)] (5) with a eta(2)-benzyl group. Thermolysis of compounds 1 and 2 (50 degreesC, 50 h) results in the ortho-cyclometalated dibenzyl species (eta(5):eta(1)-C5H4CMe2Ar')Ti(CH2Ph)(2) (Ar' = 3,5-Me2C6H2 (6), C6H4 (7)) and toluene. The thermolysis of 1 follows first-order kinetics (k approximate to 10(-5) s(-1) at 333 K) with DeltaH(double dagger) = 24 +/- 2 kcal mol(-1) and DeltaS(double dagger) = -5 +/- 5 cal mol(-1) K-1. For the thermolysis of the related titanium trialkyl (eta(5)-C5H4CMe2Ph)Ti(CH2SiMe3)(3) (9), activation parameters of DeltaH(double dagger) = 21 +/- 2 kcal mol(-1) and DeltaS(double dagger) = -18 +/- 4 cal mol-1 K-1 were found. Deuterium labeling studies with (eta(5)-C5H4CMe2C6D5)Ti(CH2Ph)(3), (2-d(5)) show that thermolysis of the neutral compound involves initial formation of all alkylidene intermediate, followed by o-CH addition to the Ti=C bond. In the corresponding cationic species, ortho cyclometalation proceeds via direct sigma-bond metathesis.