Binuclear Electron Reservoir Complexes: ¹Syntheses, Reactivity, and Electronic Structure of the 37- and 38-Electron Fulvalene Complexes (Fe₂(μ₂, η¹⁰-C₁₀H₈)(arene)2)ⁿ⁺, n = 0, 1

One-electron reduction of the dications (Fe₂Fv(Ar)₂)²⁺2–8 in THF with Na/Hg gives high yields of the 37e Fe^IFe^IImixed-valence complexes 9–15, The C₆Me₆complex (Fe₂Fv(HMB)₂)⁺PF₆⁻, 9, is thermally stable at 20 °C, whereas analogues with other arene ligands are not and need be isolated at lower temperatures. The symmetrical, purple complexes 9–15 show three g values around 2 by ESR spectroscopy at 77 or 4.2 K as Fe^Imonomers. Mössbauer spectra of 9 and of (Fe₂Fv(C₆H₆)₂)⁺PF₆⁻, 10, show only one quadrupole doublet at 293, 77, and 4.2 K, the parameters of which are not temperature dependent, unlike those of the Jahn-Teller active Fe^Imonomers and of the localized Fe^IFe^IImixed-valence complexes. In addition, Mössbauer spectra, under external applied magnetic field, show the presence of only one electron for the “Fe₂” unit. Thus, the mixed-valence complexes (Fe₂Fv(arene)₂)⁺are delocalized on the Mossbauer time scale (10⁷s⁻¹). EHT and SCC-Xα calculations were performed and compared for both the monomeric Fe^Iand the dimeric Fe^IFe^IIand Fe^IFe^Icomplexes. A good agreement was found with Mössbauer parameters. The MO diagram of the 37e⁻species shows a large HOMO-LUMO gap as expected from the non-variation of the quadrupole splitting values with the temperature. Two-electron reductions of 2, 3, and 8 in THF also using Na/Hg give the green organometallic 38e⁻biradicals Fe₂Fv(C₆R₆)₂(R = Me, 16; R = H, 17; R = Et, 22). Jahn-Teller active Fe^Iunits are observed by Mössbauer spectroscopy in high-temperature phase. An antiferromagnetic transition occurs at 37 K, for 16, as indicated by the Mössbauer and magnetic susceptibility data. The magnetic coupling of 16 at low temperature may be facilitated by the steric effect of the methyl substituents on the rotation around the C-C bond.