Complexation of polyaromatics by C5H5Fe+ and C5Me5Fe+ and electronic structures of the monoreduced complexes

Article abstract of DOI:10.1021/om00111a021

The synthesis of a number of known complexes [FeCp(η⁶-polyaromatic)]⁺PF₆^- has been reproduced or improved in order to examine their cathodic and LiAlH₄ reduction to Fe^I or the monoreduced state. It is found that the phenanthrene and pyrene complexes are not hydrogenated during the ligand exchange reaction with ferrocene and AlCl₃ and that pyrene does not give binuclear complexes, contrary to previous reports. The new complexes of triphenylene and of perylene have been made, and, although the latter is extremely light-sensitive, its X-ray crystal structure has been determined. It contains two different molecules in the unit cell where the two possibilities of relative conformations of the C₅ ring/C₆ ring are found. With FeCp*(CO)₂Br + AlCl₃, the new complexes [FeCp*(η⁶-polyaromatic)]⁺PF₆ ^- (Cp* = η⁵-C₅Me₅) have been made with biphenyl, phenanthrene, dihydrophenanthrene, triphenylene, and pyrene. Syntheses in the melt are suitable for both Cp and Cp* series. The first cathodic wave is chemically reversible only for polyarornatics with less than four fused rings in the Cp series, but, with Cp*, the reduction of the pyrene complex becomes reversible. The comparison of the thermodynamic redox potentials corresponding to the system FeCp(polyAr)^+/0 with those of the free ligand system (polyAr)^0/- provides, according to Vlc?ek's theory, the spin density on the polyaromatic ligand of the monoreduced complexes. Values are found to depend markedly on the number n of fused rings of the polyaromatic in the Cp series so that significant spin density remains on Fe if n ≤ 4, whereas it is mainly located on the polyaromatic if n > 4. This is not the case in the Cp* series where the Fe(I) state is stabilized, with residual spin density on the polyaromatic being about 15-30%. In order to understand the electronic structure of those compounds, calculations using both the EH and MS-SCF-Xα formalisms have been performed on a series of complexes. The results concerning the spin densities are in reasonable (EH) or good (Xα) agreement with the experimental ones. However, the EH calculations on [FeCp(naphthalene)] lead to wrong results due to a poor level ordering of two a′ levels situated in the HOMO/LUMO region. All of the cationic Fe^II complexes react with LiAlH₄ in THF at low temperature to give the neutral electron-transfer products. The ESR spectra of the latter also show a d⁷ Fe^I state for n ≤ 4 (three g values around 2) and a single line attributed to the radical anionic ligand if n > 4.