We report a combined synthetic, mechanistic, and theoretical study of the first borylimido complex of a rare earth metal, (NacNac)Sc{NB(NAr'CH)} (25, Ar' = 2,6-CHPr, NacNac = Ar'NC(Me)CHC(Me)NCHCHNMe). Thermolysis of the methyl-borylamide (NacNac)Sc(Me){NHB(NAr'CH)} (18) generated transient imide 25 via rate-determining, first-order methane elimination (KIE ≈ 8.7). In the absence of external substrate, 25 underwent a reversible cyclometalation reaction (sp C-H bond addition to Sc═N) with a methyl group of the NacNac ligand forming {MeC(NCHPrCH(Me)CH)CHC(Me)NCHCHNMe}Sc{NHB(NAr'CH)} (21). In the presence of pyridine or DMAP, reversible sp C-H bond activation occurred, forming orthometalated complexes (NacNac)Sc{NHB(NAr'CH)}(η-4-NCHR) (R = H or NMe). In situ reaction of 25 with HCCTol gives irreversible sp C-H bond activation under kinetic control, and with MeCCPh [2+2] cycloaddition to Sc═N takes place. These reactions represent the first substrate activation processes for any metal-bound borylimide. The bonding in 25 and the mechanism and thermodynamics of the reactions have been studied using density functional theory (DFT), supported by quantum theory of atoms in molecules and natural bond orbital analysis. Although the borylimido and arylimido dianions studied here are formally isoelectronic and possess comparable frontier molecular orbitals, the borylimido ligand is both a better π-donor and σ-donor, forming stronger and shorter metal-nitrogen bonds with somewhat reduced ionicity. Despite this, reactions of these types of borylimides with C-H or C≡C bonds are all more exothermic and more strongly activating than for the corresponding arylimides. DFT calculations on model systems of differing steric bulk unpicked the underlying thermodynamic factors controlling the reactions of 25 and its reaction partners, and a detailed comparison was made with the previously described arylimido homologues.

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 Cp*Ti(NBu)Cl(py) under forcing conditions afforded Cp*Ti{NB(NAr'CH)}Cl(py), which could be further substituted with guanidinate or pyrrolide-amine ligands to give Cp*Ti(hpp){NB(NAr'CH)} (16) and Cp*Ti(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 Cp*Ti(hpp)(NBu) (ΔG = -1.0 kcal mol). In contrast, the dialkylborylimide Cp*Ti{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.

We report a combined synthetic, mechanistic, and computational (DFT) study of the synthesis of new diamide-amine-supported titanium borylimides and their reactions with TolCCH and ArFCCH (Tol = 4-C6H4Me, ArF = C6F5). Reaction of Ti­{NB­(NAr′CH)2}­Cl2(py)3 (Ar′ = 2,6-C6H3 iPr2) with Li2N2 RNMe (N2 RNMe = MeN­(CH2CH2NR)2) or Li2N2Npy (N2Npy = (2-C5H4N)­CMe­(CH2NSiMe3)2) afforded the borylimides Ti­(N2 RNMe)­{NB­(NAr′CH)2}­(py) (R = SiMe3 (9), ArF (10), or iPr (11)) and Ti­(N2Npy)­{NB­(NAr′CH)2}­(py) (21). Compounds 9 and 10 reacted with ArCCH (Ar = Tol or ArF) via [2 + 2] cycloaddition to form the azatitanacyclobutenes Ti­(N2 RNMe)­{N­{B­(NAr′CH)2}­C­(H)­C­(Ar)}. In the case of R = ArF these underwent subsequent intramolecular C–F bond cleavage/C–C coupling processes. Reaction of 11 and 21 with TolCCH also formed azatitanacyclobutenes, whereas ArFCCH formed borylamide-acetylides via a C–H bond activation process which is endergonic in the case of TolCCH. On heating, these kinetic products rearranged via alkyne elimination to form the corresponding azatitanacyclobutenes as the thermodynamic outcomes.