Abstract
We report a combined experimental and computational study of the synthesis and electronic structure of titaniumborylimido compounds. Three new synthetic routes to this hitherto almost unknown class of Group 4 imide are presented. Thedouble-deprotonation reaction of the borylamine H 2 NB(NAr′CH) 2 (Ar′ = 2,6-C 6 H 3i Pr 2 ) with Ti(NMe 2 ) 2 Cl 2 gaveTi{NB(NAr′CH) 2 }Cl 2 (NHMe 2 ) 2 , which was easily converted to Ti{NB(NAr′CH) 2 }Cl 2 (py) 3 . This compound is an entrypoint to other borylimides, for example, reacting with Li 2 N 2pyr N Me to form Ti(N 2pyr N Me ){NB(NAr′CH) 2 }(py) 2 and with 2 equivof NaCp to give Cp 2 Ti{NB(NAr′CH) 2 }(py) (23). Borylamine-tert-butylimide exchange between H 2 NB(NAr′CH) 2 andCp*Ti(N t Bu)Cl(py) under forcing conditions afforded Cp*Ti{NB(NAr′CH) 2 }Cl(py), which could be further substituted withguanidinate or pyrrolide-amine ligands to give Cp*Ti(hpp){NB(NAr′CH) 2 } (16) and Cp*Ti(N pyr N Me 2 ){NB(NAr′CH) 2 } (17).The Ti−N im distances in compounds with the NB(NAr′CH) 2 ligand were comparable to those of the corresponding arylimides.Dialkyl- or diaryl-substituted borylamines do not undergo the analogous double-deprotonation or imide-amine exchangereactions. Reaction of (Cp′′ 2 Ti) 2 (μ 2 :η 1 ,η 1 -N 2 ) with N 3 BMes 2 gave the base-free, diarylborylimide Cp′′ 2 Ti(NBMes 2 ) (26) by anoxidative route; this compound has a relatively long Ti−N im bond and large Cp′′−Ti−Cp′′ angle. Reaction of 16 with H 2 N t Buformed equilibrium mixtures with H 2 NB(NAr′CH) 2 and Cp*Ti(hpp)(N t Bu) (Δ r G = −1.0 kcal mol −1 ). In contrast, thedialkylborylimide Cp*Ti{MeC(N i Pr) 2 }(NBC 8 H 14 ) (2) reacted quantitatively with H 2 N t Bu to give the corresponding tert-butylimide and borylamine. The electronic structures and imide-amine exchange reactions of half-sandwich and sandwichtitanium borylimides have been evaluated using density functional theory (DFT), supported by quantum theory of atoms inmolecules (QTAIM) and natural bond orbital (NBO) analysis, and placed more generally in context with the well-establishedalkyl- and arylimides and hydrazides. The calculations find that Ti−N im bonds for borylimides are stronger and more covalentthan in their organoimido or hydrazido analogues, and are strongest for alkyl- and arylborylimides. Borylamine-tert-butylimideexchange reactions fail for H 2 NBR 2 (R = hydrocarbyl) but not for H 2 NB(NAr′CH) 2 because the increased strength of the newTi−N im bond for the former is outweighed by the increased net H−N bond strengths in the borylamine. Variation of the Ti−N imbond length over short distances is dominated by π-interactions with any appropriate orbital on the N im atom organic substituent.However, over the full range of imides and hydrazides studied, overall bond energies do not correlate with bond length but withthe Ti−N im σ-bond character and the orthogonal π-interaction.