New Titanium Borylimido Compounds: Synthesis, Structure, and Bonding

摘要

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 H2NB(NAr'CH)(2) (Ar' = 2,6-(C6H3Pr2)-Pr-i) with Ti(NMe2)(2)Cl-2 gave Ti{NB(NAr'CH)(2)}Cl-2(NHMe2)(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(2)pyrNMe to form Ti(N(2)pyrNMe){NB(NAr'CH)(2)}(py)(2) and with 2 equiv of NaCp to give Cp2Ti{NB(NAr'CH)(2)}(py) (23). Borylamine-tert-butylimide exchange between H2NB(NAr'CH)(2) and Cp*Ti((NBu)-Bu-t)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((NNMe2)-N-pyr){NB(NAr'CH)(2)} (17). The TiNim 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"Ti-2)(2)(mu(2):eta(1),eta(1)-N-2) with N(3)BMes(2) gave the base-free, diarylborylimide Cp"Ti-2(NBMes(2)) (26) by an oxidative route; this compound has a relatively long TiNim bond and large Cp"-Ti-Cp" angle. Reaction of 16 with (H2NBu)-Bu-t formed equilibrium mixtures with H2NB(NAr'CH)(2) and Cp*Ti(hpp)((NBu)-Bu-t) (Delta(r)G = -1.0 kcal mol(-1)). In contrast, the dialkylborylimide Cp*Ti{MeC(NiPr)(2)}(NBC8H14) (2) reacted quantitatively with (H2NBu)-Bu-t 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 TiNim 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 H2NBR2 (R = hydrocarbyl) but not for H2NB(NAr'CH)(2) because the increased strength of the new TiNim bond for the former is outweighed by the increased net HN bond strengths in the borylamine. Variation of the TiNim bond length over short distances is dominated by pi-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 TiNim sigma-bond character and the orthogonal p-interaction.