57Fe NMR Chemical shifts and 57fe, 13C coupling constants in α-ferrocenyl carbocations. Direct metal participation in the stabilization of metallocenyl carbocations

The ¹³C{⁵⁷Fe} double resonance method has been used to investigate ⁵⁷Fe-enriched samples of ferrocene derivatives, α-ferrocenyl carbocations and carbonyl complexes with various σ- and π-hydrocarbon ligands. In the α-ferrocenyl carbocations the ⁵⁷Fe resonances span a 1200 ppm range, being a sensitive tool of direct iron participation in the stabilization. The ⁵⁷Fe resonances in the carbocations [FcCH₂][HSO₄] (I), [FcCHMe][HSO₄] (II), [FcCHPh][HSO₄] (III), [FcCHC₅H₄ Mn(CO)₃][CF₃CO₂] (IV), [(Fc)₂CH][BF₄] (V), [FcCHC₅H₄RuC₅Hs][BF₄] (VI) and [FcCMe₂][HSO₄] (VII), ?523.6, ?219.3, 221.0, 368.7, 699.0, 405.0 and 288.5 ppm, respectively, relative to ferrocene, are interpreted in terms of rehybridization of the iron non-bonding d orbitals (shielding effect and the electron withdrawing effect of the substituent in the cyclic ligand (deshielding effect). The role of rehybridization of non-bonding iron orbitals in the low-frequency shift of the ⁵⁷Fe resonances, in addition to that in the previously investigated complex [(C₅H₅)₂FeH][BF₃OH] (?1109.3 ppm), has been demonstrated for bridge-substituted [3]ferrocenophanes, whose ring tilting induces a low-frequency shift of up to 340 ppm relative to their unbridged analogues.The ¹³C NMR spectra of carbocations V and VI reveal a temperature dependence due to the rotation around the C(1)C_α exocyclic bonds. In carbocation VI the ruthenium atom effectively competes with the iron atom to bond with C_α whereas in carbocation V two equivalent metal atoms possess the same ability for such bonding; as a result, complex V has a more pronounced “carbenium ion” nature than IV and VI, as indicated by the relative positions of the ¹³C_α resonances in carbocations IV, V and VI: δ 122.4, 147.2 and 116.9 ppm, respectively.The values of ⁵⁷Fe, ¹³C coupling constants for α-ferrocenyl carbocations exclude Fe-C_α σ-bonding and support a structure in which the iron atom is π-bonded with six carbon atoms of a fulvenoid ligand. According to the data on ⁵⁷Fe resonances and ⁵⁷Fe, ¹³C coupling constants in α-ferrocenyl carbocations the strength of FeC_α bonding is markedly influenced by the electronic effect of the substituent at C_α, being even lower in carbocation I than that of Fe-cyclopentadienyl carbon atoms.