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(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][BArF 4] or BArF 3 (ArF = C6F5) in C6D5Br a...
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| Published in: | Organometallics 2006-05, Vol.25 (11), p.2806-2825 |
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| Main Authors: | , , , , , |
| Format: | Article |
| Language: | eng |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | 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][BArF 4] or BArF 3 (ArF = C6F5) in C6D5Br afforded the methyl cation [Ti(NtBu)(Me3[9]aneN3)Me]+ (6 +), whereas with half an equivalent of [Ph3C][BArF 4] the fluxional methyl-bridged homo-binuclear cation [Ti2(NtBu)2(Me3[9]aneN3)2Me2(μ-Me)]+ (10 +) was formed. Reaction of 1 with [Ph3C][BArF 4] 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][BArF 4]. 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][BArF 4] 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][BArF 4] in CH2Cl2 led to Ti(NtBu)(Me3[9]aneN3)Cl2 and [PhNMe2(CH2Cl)][BArF 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 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 1 J CH 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 1 J CH for the α-agostic methyl in [Cp2TiMe]+. The difference between the Ti−Me 1 J CH values in 1 (111 Hz) and isolobal Cp2TiMe2 (124 Hz) was also attributed to differences in Ti |
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| ISSN: | 0276-7333 1520-6041 |