Abstract
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.
Highlights
Transition metal terminal organoimido complexes of the type (L)M NR (R = alkyl or aryl; L = supporting ligand or ligand set) have given rise to a wealth of new chemistry over the past three decades.[1]. Most of this interest has been centered around the activation and/or transformations of unsaturated or saturated substrates at the polar metal−nitrogen multiple bond itself, mostly for early transition metal complexes of Groups 4 and 5.2 The most notable exceptions to this general observation are the Schrock olefin metathesis catalyst families in which arylimido groups act as spectator ligands,[3] and other examples are certain types of Ziegler-type olefin polymerization and ring-opening metathesis polymerization catalysts.[4]
Our group has been interested in developing early transition metal-imido and related metal−ligand multiple bond chemistry,2a,b,d,4b,14 and during the course of these studies we recently reported the serendipitous formation of a very rare[15] titanium borylimido compound 5 (Figure 2)[16] through the reductive N−N bond cleavage reaction of the hydrazide Cp*Ti{MeC(NiPr)2}(NNR2) (R = Me or Ph) with 9-BBN
We reasoned that a borylimido ligand of the type NB(NiPr)2C6H4 formally derived from 11 would be sterically and electronically similar to the NAr′ moieties used in the work of Chen, Mindiola, Piers, and others, and related to Bettinger’s reactive NBCat
Summary
Transition metal terminal organoimido complexes of the type (L)M NR (R = alkyl or aryl; L = supporting ligand or ligand set) have given rise to a wealth of new chemistry over the past three decades.[1] Most of this interest has been centered around the activation and/or transformations of unsaturated or saturated substrates at the polar metal−nitrogen multiple bond itself (usually described as a σ2π4 triple bond), mostly for early transition metal complexes of Groups 4 and 5.2 The most notable exceptions to this general observation are the Schrock olefin metathesis catalyst families in which arylimido groups act as spectator ligands,[3] and other examples are certain types of Ziegler-type olefin polymerization and ring-opening metathesis polymerization catalysts.[4]. With isolated 1 in hand, along with a DMAP-free homologue
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