New Titanium Borylimido Compounds: Synthesis, Structure, and Bonding.

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 HNB(NAr'CH) (Ar' = 2,6-CHPr) with Ti(NMe)Cl gave Ti{NB(NAr'CH)}Cl(NHMe), which was easily converted to Ti{NB(NAr'CH)}Cl(py). This compound is an entry point to other borylimides, for example, reacting with LiNN to form Ti(NN){NB(NAr'CH)}(py) and with 2 equiv of NaCp to give CpTi{NB(NAr'CH)}(py) (23). Borylamine-tert-butylimide exchange between HNB(NAr'CH) and CpTi(NBu)Cl(py) under forcing conditions afforded CpTi{NB(NAr'CH)}Cl(py), which could be further substituted with guanidinate or pyrrolide-amine ligands to give CpTi(hpp){NB(NAr'CH)} (16) and CpTi(NN){NB(NAr'CH)} (17). The Ti-N distances in compounds with the NB(NAr'CH) 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″Ti)(μ:η,η-N) with NBMes gave the base-free, diarylborylimide Cp″Ti(NBMes) (26) by an oxidative route; this compound has a relatively long Ti-N bond and large Cp″-Ti-Cp″ angle. Reaction of 16 with HNBu formed equilibrium mixtures with HNB(NAr'CH) and CpTi(hpp)(NBu) (ΔG = -1.0 kcal mol). In contrast, the dialkylborylimide CpTi{MeC(NPr)}(NBCH) (2) reacted quantitatively with HNBu 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 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 HNBR (R = hydrocarbyl) but not for HNB(NAr'CH) because the increased strength of the new Ti-N bond for the former is outweighed by the increased net H-N bond strengths in the borylamine. Variation of the Ti-N bond length over short distances is dominated by π-interactions with any appropriate orbital on the N 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 σ-bond character and the orthogonal π-interaction.