New Titanium Borylimido Compounds: Synthesis, Structure, and Bonding
Résumé
We report a combined experimental and computational study of the synthesis and electronic structure of titanium
borylimido compounds. Three new synthetic routes to this hitherto almost unknown class of Group 4 imide are presented. The
double-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 gave
Ti{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 entry
point 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 equiv
of NaCp to give Cp 2 Ti{NB(NAr′CH) 2 }(py) (23). Borylamine-tert-butylimide exchange between H 2 NB(NAr′CH) 2 and
Cp*Ti(N t Bu)Cl(py) under forcing conditions afforded Cp*Ti{NB(NAr′CH) 2 }Cl(py), which could be further substituted with
guanidinate 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 exchange
reactions. 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 an
oxidative 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 Bu
formed 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, the
dialkylborylimide 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 sandwich
titanium borylimides have been evaluated using density functional theory (DFT), supported by quantum theory of atoms in
molecules (QTAIM) and natural bond orbital (NBO) analysis, and placed more generally in context with the well-established
alkyl- and arylimides and hydrazides. The calculations find that Ti−N im bonds for borylimides are stronger and more covalent
than in their organoimido or hydrazido analogues, and are strongest for alkyl- and arylborylimides. Borylamine-tert-butylimide
exchange reactions fail for H 2 NBR 2 (R = hydrocarbyl) but not for H 2 NB(NAr′CH) 2 because the increased strength of the new
Ti−N im bond for the former is outweighed by the increased net H−N bond strengths in the borylamine. Variation of the Ti−N im
bond 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 with
the Ti−N im σ-bond character and the orthogonal π-interaction.