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

The 13 C{ 57 Fe} double resonance method has been used to investigate 57 Fe-enriched samples of ferrocene derivatives, α-ferrocenyl carbocations and carbonyl complexes with various σ- and π-hydrocarbon ligands. In the α-ferrocenyl carbocations the 57 Fe resonances span a 1200 ppm range, being a sensitive tool of direct iron participation in the stabilization. The 57 Fe resonances in the carbocations [FcCH 2 ][HSO 4 ] (I), [FcCHMe][HSO 4 ] (II), [FcCHPh][HSO 4 ] (III), [FcCHC 5 H 4 Mn(CO) 3 ][CF 3 CO 2 ] (IV), [(Fc) 2 CH][BF 4 ] (V), [FcCHC 5 H 4 RuC 5 Hs][BF 4 ] (VI) and [FcCMe 2 ][HSO 4 ] (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 57 Fe resonances, in addition to that in the previously investigated complex [(C 5 H 5 ) 2 FeH][BF 3 OH] (−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 13 C 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 13 C α resonances in carbocations IV, V and VI: δ 122.4, 147.2 and 116.9 ppm, respectively. The values of 57 Fe, 13 C 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 57 Fe resonances and 57 Fe, 13 C 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.