Experimental and DFT Studies of Cationic Imido Titanium Alkyls: Agostic Interactions and C−H Bond and Solvent Activation Reactions of Isolobal Analogues of Group 4 Metallocenium Cations

Synthesis, reactions, and DFT studies of macrocycle-supported imido titanium alkyl cations derived from Ti(N t Bu)(Me 3 [9]aneN 3 )R 2 (R = Me ( 1 ) or CH 2 SiMe 3 ( 2 )) are described (Me 3 [9]aneN 3 = 1,4,7-trimethyltriazacyclononane). Reaction of 1 with 1 equiv of [Ph 3 C][BAr F 4 ] or BAr F 3 (Ar F = C 6 F 5 ) in C 6 D 5 Br afforded the methyl cation [Ti(N t Bu)(Me 3 [9]aneN 3 )Me] + ( 6 + ), whereas with half an equivalent of [Ph 3 C][BAr F 4 ] the fluxional methyl-bridged homo-binuclear cation [Ti 2 (N t Bu) 2 (Me 3 [9]aneN 3 ) 2 Me 2 (μ-Me)] + ( 10 + ) was formed. Reaction of 1 with [Ph 3 C][BAr F 4 ] in CD 2 Cl 2 formed the monochloride cation [Ti(N t Bu)(Me 3 [9]aneN 3 )Cl] + ( 8 + ), which was also prepared from Ti(N t Bu)(Me 3 [9]aneN 3 )Cl(Me) and [Ph 3 C][BAr F 4 ]. Cation 8 + reacted with pyridine to give the adduct [Ti(N t Bu)(Me 3 [9]aneN 3 )Cl(py)] + ( 9 + ) and with Ti(N t Bu)(Me 3 [9]aneN 3 )Me 2 to form the chloride-bridged cation [Ti 2 (N t Bu) 2 (Me 3 [9]aneN 3 ) 2 Me 2 (μ-Cl)] + ( 11 + ). Reaction of 2 with [Ph 3 C][BAr F 4 ] gave [Ti(N t Bu)(Me 3 [9]aneN 3 )(CH 2 SiMe 3 )] + ( 7 + ), which is stabilized by a β-Si−C agostic interaction characterized by a high-field-shifted 29 Si NMR resonance. Attempts to generate 7 + by reaction of 2 with [PhNMe 2 H][BAr F 4 ] in CH 2 Cl 2 led to Ti(N t Bu)(Me 3 [9]aneN 3 )Cl 2 and [PhNMe 2 (CH 2 Cl)][BAr F 4 ] ( 12-BAr F 4 ) via a series of solvent activation reactions, the details of which have been elucidated. Reaction of 6 + or 7 + with Ph 3 PO afforded the adducts [Ti(N t Bu)(Me 3 [9]aneN 3 )R(Ph 3 PO)] +, whereas with pyridine a C−H bond activation reaction occurred to give [Ti(N t Bu)(Me 3 [9]aneN 3 )(NC 5 H 4 )] + ( 17 + ) and the corresponding alkane RH. Density functional theory calculations of the isolobal d 0 fragments [Ti(NR)(R‘ 3 [9]aneN 3 )] 2+ and [Cp 2 Ti] 2+ found that their frontier orbitals, although broadly similar, featured important differences in their shapes and energies. These account for the absence of any α-C−H agostic interaction in 6 +, whereas [Cp 2 TiMe] + is stabilized by a weak interaction of this type, as judged by DFT-computed geometries. The experimentally observed increase in Ti−Me group average 1 J CH on forming either 6 + from 1 or [Cp 2 TiMe] + from Cp 2 TiMe 2 is reproduced by DFT and attributed to intrinsic global changes in carbon 2s orbital contribution to the Ti−C and C−H bonds upon cation formation. These changes were shown to mask the otherwise expected decrease in average 1 J CH for the α-agostic methyl in [Cp 2 TiMe] + . The difference between the Ti−Me 1 J CH values in 1 (111 Hz) and isolobal Cp 2 TiMe 2 (124 Hz) was also attributed to differences in Ti center electrophilicity. The experimental high-field-shifted 29 Si NMR resonance in 7 + was well reproduced in the DFT-computed β-Si−C agostic structure, and upper and lower limits for the strength of the agostic interaction were estimated. An NBO analysis of the Ti−CH 2 SiMe 3 bonding found several different contributions, including negative hyperconjugation (population of σ*(Si β −C γ )) and formal C α −Si β →Ti and Si β −C γ →Ti bond pair donation.