New scandium borylimido chemistry: synthesis, bonding and reactivity

摘要

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, 1st order methane elimination (KIE ca. 8.7). In the absence of external substrate, 25 underwent a reversible cyclometallation 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 orthometallated 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 (QTAIM) and natural bond orbital (NBO) analysis. Although the borylimido and arylimido dianions studied here are formally isoelectronic and present comparable frontier molecular orbitals to a metal center, the borylimido ligand is both a better sigma-donor and pi-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.