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 H2NB­(NAr′CH)2 (Ar′ = 2,6-C6H3iPr2) with Ti­(NMe2)2Cl2 gave
Ti­{NB­(NAr′CH)2}­Cl2(NHMe2)2, which was easily converted to Ti­{NB­(NAr′CH)2}­Cl2(py)3. This compound is an
entry point to other borylimides, for example, reacting with Li2N2pyrNMe to form Ti­(N2pyrNMe)­{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(NtBu)­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(NpyrNMe2)­{NB­(NAr′CH)2} (17). The Ti–Nim 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″2Ti)2­(μ2:η1,η1-N2) with N3BMes2 gave the base-free, diarylborylimide Cp″2Ti­(NBMes2) (26) by an oxidative route;
this compound has a relatively long Ti–Nim bond
and large Cp″–Ti–Cp″ angle. Reaction of 16 with H2NtBu formed equilibrium mixtures
with H2NB­(NAr′CH)2 and Cp*Ti­(hpp)­(NtBu) (ΔrG = −1.0 kcal
mol–1). In contrast, the dialkylborylimide Cp*Ti­{MeC­(NiPr)2}­(NBC8H14) (2) reacted quantitatively with H2NtBu
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–Nim 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 Ti–Nim bond for the former is outweighed by the increased net H–N
bond strengths in the borylamine. Variation of the Ti–Nim bond length over short distances is dominated by π-interactions
with any appropriate orbital on the Nim 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–Nim σ-bond character and the orthogonal π-interaction.