863221-65-2Relevant academic research and scientific papers
Experimental and DFT Studies of cationic imido titanium alkyls: Agostic interactions and C-H bond and solvent activation reactions of isoiobal analogues of group 4 metallocenium cations
Bolton, Paul D.,Clot, Eric,Adams, Nico,Dubberley, Stuart R.,Cowley, Andrew R.,Mountford, Philip
, p. 2806 - 2825 (2006)
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 [Ph 3C][BAr4F] or BAr3F (ArF = C6F5) in C6D5Br afforded the methyl cation [Ti(NtBu)(Me3[9]aneN 3)Me]+ (6+), whereas with half an equivalent of [Ph3C][BAr4F] the fluxional methyl-bridged homo-binuclear cation [Ti2(NtBu)2(Me 3[9]aneN3)2Me2(μ-Me)] + (10+) was formed. Reaction of 1 with [Ph 3C][BAr4F] in CD2C12 formed the monochloride cation [Ti(NtBu)(Me3[9]aneN 3)Cl]+ (8+), which was also prepared from Ti(NtBu)(Me3[9]aneN3)Cl(Me) and [Ph 3C][BAr4F]. Cation 8+ reacted with pyridine to give the adduct [Ti(NtBu)(Me3[9]aneN 3)Cl(py)]+ (9+) and with Ti(N tBu)(Me3[9]aneN3)Me2 to form the chloride-bridged cation [Ti2(NtBu)2(Me 3[9]aneN3)2- Me2(μ-Cl)] + (11+). Reaction of 2 with [Ph3C][BAr 4F] gave [Ti(NtBu)(Me3[9]aneN 3)(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][BAr4 F] in CH2C12 led to Ti(NtBu)(Me 3[9]aneN3)Cl2 and [PhNMe2(CH 2Cl)][BAr2F] (12-BAr4F) 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]aneN 3)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]aneN 3)]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 [Cp 2TiMe]+ 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 1J CH 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 C α-Siβ→Ti and Siβ-C γ→Ti bond pair donation.
