A Vanadium Methylidene

Abstract

Examples of stable 3d transition metal methylidene complexes are extremely rare. Here we report an isolable and stable vanadium methylidene complex, [(PNP)V(=NAr)(=CH₂)] (PNP = N[2-PⁱPr₂-4-methylphenyl]^-, Ar = 2,6-ⁱPr₂C₆H₃), via H atom transfer (HAT) from [(PNP)V(NHAr)(CH₃)] or [(PNP)V(=NAr)(CH₃)] using two or one equivalents of the TEMPO radical (TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl), respectively. Alternatively, the vanadium methylidene moiety can also be formed via the treatment of transient [(PNP)V=NAr] with the Wittig reagent, H₂CPPh₃. Structural and spectroscopic analysis, including ¹³C enriched labeling of the methylidene ligand, unequivocally confirmed the terminal nature of a rare 3d methylidene complex, featuring a V=CH₂ bond distance of 1.908(2) Å and a highly downfield ¹³C NMR spectral shift at 298 ppm. In the absence of the ylide, intermediate [(PNP)V=NAr] activates dinitrogen to form an end-on bridging N₂ complex, [(PNP)V(=NAr)]₂(μ₂-η¹:η¹-N₂), having a singlet ground state. Complex [(PNP)V(=NAr)(=CH₂)] reacts with H₃COTf to form [(PNP)V(=NAr)(OTf)], accompanied by the release of ethylene as evidenced by ¹H NMR spectroscopy, and reactivity studies suggest a β-hydride elimination pathway.