On the mechanism of imine elimination from Fischer tungsten carbene complexes

(Aminoferrocenyl)(ferrocenyl)carbene(pentacarbonyl)tungsten(0) (CO)(5)W=C(NHFc)Fc (W(CO)(5)(E-2)) is synthesized by nucleophilic substitution of the ethoxy group of (CO)(5)W=C(OEt)Fc (M(CO)(5)(1(Et))) by ferrocenyl amide Fc-NH(–) (Fc = ferrocenyl). W(CO)(5)(E-2) thermally and photochemically eliminates bulky E-1,2-diferrocenylimine (E-3) via a formal 1,2-H shift from the N to the carbene C atom. Kinetic and mechanistic studies to the formation of imine E-3 are performed by NMR, IR and UV–vis spectroscopy and liquid injection field desorption ionization (LIFDI) mass spectrometry as well as by trapping experiments for low-coordinate tungsten complexes with triphenylphosphane. W(CO)(5)(E-2) decays thermally in a first-order rate-law with a Gibbs free energy of activation of ΔG(‡)(298K) = 112 kJ mol(−1). Three proposed mechanistic pathways are taken into account and supported by detailed (time-dependent) densitiy functional theory [(TD)-DFT] calculations. The preferred pathway is initiated by an irreversible CO dissociation, followed by an oxidative addition/pseudorotation/reductive elimination pathway with short-lived, elusive seven-coordinate hydrido tungsten(II) intermediates cis(N,H)-W(CO)(4)(H)(Z-15) and cis(C,H)-W(CO)(4)(H)(Z-15).