Electron Transfer and Electron Exchange between [Cp*(dppe)Fe]ⁿ⁺ (n = 0, 1) Building Blocks Mediated by the 9,10-Bis(ethynyl)anthracene Bridge

A novel bis(iron) alkynyl-bridged complex, Cp*(dppe)Fe(C⋮C-9,10-ant-C⋮C)Fe(dppe)Cp*, 10 (ant = anthracene), and its oxidized forms, 10·PF₆, 10·TCNQ, and 10·2PF₆, were synthesized and characterized by X-ray crystal structures. The cyclic voltammogram of 10 shows two well-reversible redox couples at −0.40 and −0.04 V (vs SCE) and a third redox process close to the solvent edge. Density-functional theory (DFT) calculations carried out on the substituted model complex (η⁵-C₅H₅)(η²-dpe)Fe−C⋮C-9,10-ant-C⋮C−Fe(η²-dpe)(η⁵-C₅H₅), 10-H (dpe = H₂P−(CH₂)₂−PH₂), suggest that the HOMO, which is depopulated upon oxidation, has a dominant anthracene character. The ¹H NMR and the magnetic susceptibility measurements indicate that the complex 10·2PF₆ is diamagnetic, in contrast with that of its congeners of the bis(iron) series, which do not contain the anthracene fragment in the bridge. Experimental measurements and DFT calculations reveal a large energy gap between the singlet ground state and the triplet excited state (ΔE_ST(exp) < −1200 cm^-1). Mössbauer spectroscopy reveals that the two iron centers are spectroscopically equivalent for the three oxidation states. The parameters found for 10·2PF₆ are not typical of an iron(III) center, but rather characteristic of iron(II). The spectrum of 10·TCNQ is in accord with a detrapped mixed-valence (MV) electronic structure with a large contribution of the delocalized unpaired electron on the carbon bridge. In addition, Mössbauer and UV−vis data and DFT calculations indicate also that the electronic structure of the MV complex is much closer to the dication 10·2PF₆ than the parent complex 10. The analysis of the NIR absorption band allows the determination of a large electronic coupling parameter, as expected for a class III MV complex (V_ab = 2180 cm^-1). Taken as a whole, the experimental and theoretical data emphasize the specific role of the anthracene fragment inserted in the carbon bridge, which allows good electronic communication between the iron centers, but significantly contributes to the displacement of the spin density from the metal centers onto the α and β sp carbon atoms in the vicinity of the metal and the ipso carbon of the anthracene.