Abstract
We report a combined synthetic, mechanistic,and theoretical study of the first borylimido complex of a rareearth metal, (NacNacNMe2)Sc{NB(NAr′CH)2} (25, Ar′ = 2,6-C6H3iP 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 transientimide 25 via rate-determining, first-order methaneelimination (KIE ≈ 8.7). In the absence of external substrate, 25 underwent a reversible cyclometalation reaction (sp3 C−Hbond addition to ScNimide) 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 activationoccurred, forming orthometalated complexes (NacNacNMe2)Sc{NHB(NAr′CH)2}(η2-4-NC5H3R) (R = H or NMe2). In situreaction 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-boundborylimide. The bonding in 25 and the mechanism and thermodynamics of the reactions have been studied using densityfunctional theory (DFT), supported by quantum theory of atoms in molecules and natural bond orbital analysis. Although theborylimido and arylimido dianions studied here are formally isoelectronic and possess comparable frontier molecular orbitals, theborylimido ligand is both a better π-donor and σ-donor, forming stronger and shorter metal−nitrogen bonds with somewhatreduced ionicity. Despite this, reactions of these types of borylimides with C−H or CC bonds are all more exothermic andmore strongly activating than for the corresponding arylimides. DFT calculations on model systems of differing steric bulkunpicked the underlying thermodynamic factors controlling the reactions of 25 and its reaction partners, and a detailedcomparison was made with the previously described arylimido homologues.