Methylidyne Transfer as a Plausible Deactivation Pathway for Ynene Metathesis

Abstract

Niobium methylidyne [(PNP)­NbCH­(OAr)] (1) (PNP– = N­[2-P i Pr2-4-methylphenyl]2 –, Ar = 2,6- i Pr2C6H3) reacts with excess ethylene to afford the propenyl-ethylene complex [(PNP)­Nb­(HCCHCH3)­(η2-H2CCH2)­(OAr)] (2), which, upon gentle heating, extrudes ethylene to yield a low-spin Nb­(III) allyl complex [(PNP)­Nb­(η3-H2CCHCH2)­(OAr)] (3). Isotopic labeling studies using [(PNP)­Nb13CH­(OAr)] (1- 13 C) have allowed us to not only observe and propose the formation of various intermediates in the conversion of 1 to 2 and then 3 but also demonstrate scrambling of the CH2 groups to only the terminal positions of the allyl moiety. The homologation of ethylene to a propenyl or isomeric allylic group might help explain why complex 1 is an inefficient catalyst for the ring-opening expansion polymerization of 2-norbornene to cyclic polynorbornene.

A niobium pentafulvene ethylene complex: synthesis, properties and reaction pathways

“…35 Compared to recently reported niobium ethylene complexes, these parameters are in good agreement, overall showing a considerable niobacyclopropane character. 14,20 To support the experimental molecular structure of 2, quantum chemical calculations at the density functional B3LYP/Def2-TZVP were used. [37][38][39][40] Comparing the optimized molecular structure with the four structures obtained by X-ray diffraction, a respectable agreement is found despite the distorted system (Table S4 †).…”

“…19 Reacting a niobium methylidyne complex with an excess of ethylene yielded the unexpected propenyl-ethylene complex, that can subsequently eliminate ethylene to yield a niobium(III) allyl complex. 20 An unusual synthesis involves reduction of tris(propofolato) niobium dichloride and dehydrogenation of one isopropyl substituent, yielding an intramolecular niobium η 2 -alkene complex that works as a one-proton acceptor and allows insertion of multiple bond containing substrates into one Nb-C bond. 21 On top of that, an in situ generated niobium η 2 -cyclopropene complex was shown to activate various C-H bonds.…”

Tantalum η2-Vinyl Complexes Bearing an [η5:σ-Me2C(C5H4)(C2B10H10)]2– Ligand: Syntheses and Structural Characterizations

“…Alternatively, they form β-CH agostic alkenyl complexes to release electron deficiency . Niobium η 2 -vinyl complexes have been postulated as intermediates in the reversible migratory insertion reaction of alkyl to alkyne and methylidyne transfer reaction on a niobium center, but trapping experiments could only give the corresponding η 1 -vinyl complexes instead, probably due to the lack of an effective stabilizing ligand …”

Reactivity and Structure of a Bis-phenolate Niobium NHC Complex

“…Niobium-based complexes are of considerable interest, − e.g. , due to their potential catalytic applications in transformations such as group atom transfer − and polymerization ,− reactions as well as the (stoichiometric) activation of nitrogen − or phosphorous. − In this context, a variety of supporting ligands have been investigated, ranging from simple phenolates ,− or amides, , to bidentate BDI ligands ,,− and guinidinates − or formamidinates over to tridentate pincer-type PNP ,,, ligands. In contrast, N -heterocyclic carbene-based ligands have been rarely used in niobium chemistry.…”