New Scandium Borylimido Chemistry: Synthesis, Bonding, and Reactivity
Résumé
We report a combined synthetic, mechanistic,
and theoretical study of the first borylimido complex of a rare
earth metal, (NacNacNMe2
)Sc{NB(NAr′CH)2} (25, Ar′ = 2,6-
C6H3
i
P r 2 , NacNac NMe 2 = Ar′NC(Me)CHC(Me)-
NCH2CH2NMe2). Thermolysis of the methyl-borylamide
(NacNacNMe2
)Sc(Me){NHB(NAr′CH)2} (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 (sp3 C−H
bond addition to ScNimide) with a methyl group of the NacNacNMe2 ligand forming {MeC(NC6H3
i
PrCH(Me)CH2)CHC-
(Me)NCH2CH2NMe2}Sc{NHB(NAr′CH)2} (21). In the presence of pyridine or DMAP, reversible sp2 C−H bond activation
occurred, forming orthometalated complexes (NacNacNMe2
)Sc{NHB(NAr′CH)2}(η2
-4-NC5H3R) (R = H or NMe2). In situ
reaction of 25 with HCCTol gives irreversible sp C−H bond activation under kinetic control, and with MeCCPh [2+2]
cycloaddition to ScNimide 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 CC 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.