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

Abstract

Synthesis, reactions, and DFT studies of macrocycle-supported imido titanium alkyl cations derived from Ti(NtBu)(Me3[9]aneN3)R2 (R = Me (1) or CH2SiMe3 (2)) are described (Me3[9]aneN3 = 1,4,7-trimethyltriazacyclononane). Reaction of 1 with 1 equiv of [Ph3C][BArF4] or BArF3 (ArF = C6F5) in C6D5Br afforded the methyl cation [Ti(NtBu)(Me3[9]aneN3)Me]+ (6+), whereas with half an equivalent of [Ph3C][BArF4] the fluxional methyl-bridged homo-binuclear cation [Ti2(NtBu)2(Me3[9]aneN3)2Me2(μ-Me)]+ (10+) was formed. Reaction of 1 with [Ph3C][BArF4] in CD2Cl2 formed the monochloride cation [Ti(NtBu)(Me3[9]aneN3)Cl]+ (8+), which was also prepared from Ti(NtBu)(Me3[9]aneN3)Cl(Me) and [Ph3C][BArF4]. Cation 8+ reacted with pyridine to give the adduct [Ti(NtBu)(Me3[9]aneN3)Cl(py)]+ (9+) and with Ti(NtBu)(Me3[9]aneN3)Me2 to form the chloride-bridged cation [Ti2(NtBu)2(Me3[9]aneN3)2Me2(μ-Cl)]+ (11+). Reaction of 2 with [Ph3C][BArF4] gave [Ti(NtBu)(Me3[9]aneN3)(CH2SiMe3)]+ (7+), which is stabilized by a β-Si−C agostic interaction characterized by a high-field-shifted 29Si NMR resonance. Attempts to generate 7+ by reaction of 2 with [PhNMe2H][BArF4] in CH2Cl2 led to Ti(NtBu)(Me3[9]aneN3)Cl2 and [PhNMe2(CH2Cl)][BArF4] (12-BArF4) via a series of solvent activation reactions, the details of which have been elucidated. Reaction of 6+ or 7+ with Ph3PO afforded the adducts [Ti(NtBu)(Me3[9]aneN3)R(Ph3PO)]+, whereas with pyridine a C−H bond activation reaction occurred to give [Ti(NtBu)(Me3[9]aneN3)(NC5H4)]+ (17+) and the corresponding alkane RH. Density functional theory calculations of the isolobal d0 fragments [Ti(NR)(R‘3[9]aneN3)]2+ and [Cp2Ti]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 [Cp2TiMe]+ is stabilized by a weak interaction of this type, as judged by DFT-computed geometries. The experimentally observed increase in Ti−Me group average 1JCH on forming either 6+ from 1 or [Cp2TiMe]+ from Cp2TiMe2 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 1JCH for the α-agostic methyl in [Cp2TiMe]+. The difference between the Ti−Me 1JCH values in 1 (111 Hz) and isolobal Cp2TiMe2 (124 Hz) was also attributed to differences in Ti center electrophilicity. The experimental high-field-shifted 29Si 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−CH2SiMe3 bonding found several different contributions, including negative hyperconjugation (population of σ*(Siβ−Cγ)) and formal Cα−Siβ→Ti and Siβ−Cγ→Ti bond pair donation.