C−H Bond Activation Processes in Cationic and Neutral Titanium Benzyl Compounds with Cyclopentadienyl−Arene Ligands

Abstract

Titanium tribenzyl complexes with cyclopentadienyl−arene ligands, (η5-C5H4CMe2Ar)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 [(η,5η-C5H4CMe2-3,5-Me2C6H3)Ti(η-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 [(η5:η1-C5H4CMe2C6H4)Ti(η1-CH2Ph)][η6-PhCH2B(C6F5)3] (4) and toluene. Reaction of 2 with [Ph3C][B(C6F5)4] also leads to ligand ortho metalation, giving [(η5:η1-C5H4CMe2C6H4)Ti(η2-CH2Ph)][B(C6F5)4] (5) with a η2-benzyl group. Thermolysis of compounds 1 and 2 (50 °C, 50 h) results in the ortho-cyclometalated dibenzyl species (η5:η1-C5H4CMe2Ar‘)Ti(CH2Ph)2 (Ar‘ = 3,5-Me2C6H2 (6), C6H4 (7)) and toluene. The thermolysis of 1 follows first-order kinetics (k ≈ 10-5 s-1 at 333 K) with ΔH⧧ = 24 ± 2 kcal mol-1 and ΔS⧧ = −5 ± 5 cal mol-1 K-1. For the thermolysis of the related titanium trialkyl (η5-C5H4CMe2Ph)Ti(CH2SiMe3)3 (9), activation parameters of ΔH⧧ = 21 ± 2 kcal mol-1 and ΔS⧧ = −18 ± 4 cal mol-1 K-1 were found. Deuterium labeling studies with (η5-C5H4CMe2C6D5)Ti(CH2Ph)3 (2-d5) show that thermolysis of the neutral compound involves initial formation of an alkylidene intermediate, followed by o-CH addition to the TiC bond. In the corresponding cationic species, ortho cyclometalation proceeds via direct σ-bond metathesis.