Effects of Zero-Point Energy Difference on Intramolecular Electron Transfer in Asymmetric Polyethyl-Substituted Biferrocenium Triiodides

Abstract

The syntheses, characterizations, and physical properties of asymmetric polyethyl-substituted mixed-valence 1‘,2‘,1‘‘‘-triethylbiferrocenium triiodide (15), 1‘,3‘,1‘‘‘-triethylbiferrocenium triiodide (16), and 1‘,2‘,1‘‘‘,3‘‘‘-tetraethylbiferrocenium triiodide (17) are described. Complexes 15−17 were characterized by electrochemical measurements and by near-IR, 57Fe Mössbauer, and paramagnetic 1H NMR spectroscopy. These complexes are found to be localized on the 57Fe Mössbauer time scale (electron-transfer rates less than 107 s-1). The cations in complexes 15−17 are not in equivalent environments. This asymmetry results in a nonzero zero-point energy barrier for intramolecular electron transfer. Analysis of the sign of contact shifts suggests that the electron delocalization in 15−17 is based on competing σ and π delocalization mechanisms. The X-ray structure of 15 has been determined at 298 K: monoclinic, P21/n, a = 9.962(3) Å, b = 20.610(3) Å, c = 13.326(3) Å, β = 92.74(2)°, Z = 4, D calcd = 2.029 g cm-3, RF = 0.033, R wF = 0.036. The neutral compound 1‘,2‘,1‘‘‘-triethylbiferrocene crystallizes in the triclinic space group P1̄ with two molecules in a unit cell with dimensions a = 7.465(3), b = 8.419(2), c = 18.581(4) Å and α = 87.83(2), β = 87.98(3), γ = 64.27(2)°; RF = 0.021 and R wF = 0.029.

Biferrocene Amino Acid, a Ferrocenylogoue of Ferrocene Amino Acid: Synthesis, Cross-Linking, and Redox Chemistry

“…The IR spectrum of [7](I 3 ) additionally reveals absorptions of the typical cyclopentadienyl CH bending vibration of ferrocenium cations (838 cm −1 ) and of ferrocene units (815 cm −1 ), while the neutral complex features only the latter ferrocene Cp vibration (806 cm −1 ). ,, This points to a valence-trapped situation in the solid state on the IR time scale (ET rate below ∼10 12 s −1 at 300 K). In summary the solid-state structure of [7](I 3 ) and the solid-state IR data suggest a valence-trapped situation, as does the gas-phase DFT calculation (class I or class II in the Robin−Day classification scheme), which is quite expected due to the large zero-point energy difference of the two halves in the dissymmetric biferrocenium cation 7 + …”

“…Oxidation of the first ferrocene unit is observed at 155 to 260 mV for ferrocenes with electron-withdrawing substituents COOMe/CON 3 and at −170 to −260 mV for ferrocenes with electron-donating NHR substituents. , Oxidation of the mixed-valent cation to the bis(ferrocenium) dication in 2 , 5 , 6 , and 8 is observed at 325−380 mV higher potentials. The higher values are in a typical range for symmetric and slightly dissymmetric biferrocenes R-Fn-Fn-R′ (R = H, alkyl). ,, The significantly lower peak-to-peak separation of the donor/donor-substituted biferrocene 8 can be assigned to the delocalization of the positive charge onto the NHR substituent in the ferrocenium system, , resulting in a larger distance of the redox “centers” and thus a facilitated second oxidation. The especially large redox separation of 595 mV in donor/acceptor-substituted biferrocene 7 is certainly based on the combined effects of redox dissymmetry and electronic interaction (Figure ).…”

“…For the preparation of symmetric biferrocenes Ullmann coupling reactions or copper-mediated homocoupling reactions are typically employed. , A very elegant route to symmetrical biferrocenes R-Fn-Fn-R starting from biferrocene Fc-Fc has been recently described by Astruc et al employing Cp substitution by toluene and visible-light photolysis to exchange coordinated toluene by a substituted cyclopentadienyl ligand Cp-R . Statistical Ullmann coupling of differently substituted bromoferrocenes gives dissymmetrically substituted biferrocenes in 2−23% yield after chromatographic separation . Desymmetrization of biferrocene Fc-Fc by Friedel−Crafts acylation yields monoacylated biferrocene Fc-Fn-CO-CH 2 CH 2 COOH in 32% yield …”

Pronounced Effects of Crystal Structures on Intramolecular Electron Transfer in Mixed-Valence Biferrocenium Cations: Structural, EPR, and57Fe Mössbauer Characteristics

“…We believe that the most important factor in controlling the intramolecular electron transfer in the series of biferrocenium cations is the symmetry of the cation. We have prepared a series of asymmetric biferrocenium cations ( 9 − 12 in Chart ) to study the influence of zero-point energy difference on electron-transfer rates . The magnitudes of zero-point energy difference between the two vibronic states for asymmetric biferrocenium cations 9 − 12 are nondetectable by electrochemical measurements.…”

Mixed-Valence State of Optically Active 1‘,1‘ ‘‘-Bis(2-phenylbutyl)-1,1‘ ‘-biferrocenium Pentaiodide: Effects of Cation Symmetry and Intermolecular Interaction on Trapped-/Detrapped-Valence States

“…1‘-(( R )-2-phenylbutyl)-1‘ ‘‘-(( S )-2-phenylbutyl)-1,1‘ ‘-biferrocene was synthesized by using the equimolar mixture of 1‘-( R )-2-phenylbutyl-1-bromoferrocene and 1‘-(( R )-2-phenylbutyl)-1-bromoferrocene in the Ullman coupling reaction . After extraction with dichloromethane, the mixture was isolated by column chromatography on alumina.…”