Cationic Vinylidene Complexes. Preparation and Structural Characterization of (η⁵-Cyclopentadienyl)(2-methyl-4,5-bis(diphenylphosphino)-2-penten-3-yl)iron (II). A Base-Induced Interligand Reaction in a Vinylidene Complex

The cationic vinylidene complex [(Cp)Fe(dppe)(C=C(CH₃)₂|][SO₃F] (la) (dppe = 1,2-bis(diphenylphosphino)-ethane), prepared by the methylation of (Cp)Fe(dppe)(C≡CCH₃) (II), is smoothly deprotonated by potassium hydroxide or sodium bis(trimethylsilyl)amide. The exclusive product is (Cp) Fe[Ph₂PCH₂CH(PPh₂C =C(CH₃)₂] (III), which contains a novel ferradiphospha[2.1.1]bicyclohexane ring system. This product was investigated by spectroscopic and crystal structure analysis: space group Cc, a = 14.738 (4) Å b = 12.471 (5) Å c = 16.982 (7) Å β = 106.17 (3)°, V = 2997 (4) ų, Z = 4, ρ_clacd = 1 -268 g/cm³. Refinement on 1668 reflections with F₀² > 3σ(F₀²) has yielded the final discrepancy indices R = 0.058 and R_w = 0.060. The bicyclic ring system shows geometric and spectroscopic features which indicate that significant torsional strain is present. The formation of III has been explained by the intermediacy of a zwitterionic complex containing both a deprotonated dppe ligand ([Ph₂PCH₂CHPPh₂]) and a cationic vinylidene moiety. Subsequent intramolecular cyclization produces III.