New Scandium Borylimido Chemistry: Synthesis, Bonding, and Reactivity

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-C6H3iPr2, NacNacNMe2 = 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 ScNimide) with a methyl group of the
NacNacNMe2 ligand forming {MeC­(NC6H3iPrCH­(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 ScNimide 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.