Pronounced Effects of Ring Tilting on Rates of Intramolecular Electron Transfer in Polyalkyl-Substituted Mixed-Valence Biferrocenium Triiodides

Abstract

Relatively minor perturbations caused by Cp ring substituents in a series of mixed-valence biferrocenium cations have pronounced effects on the electronic structure and rate of intramolecular electron transfer. The X-ray structures of 1‘,2‘,1‘‘‘,2‘‘‘-tetrapropylbiferrocene, 1‘,2‘,1‘‘‘,2‘‘‘-tetrabenzylbiferrocene, 1‘,3‘,1‘‘‘,3‘‘‘-tetrapropylbiferrocene, 1‘,3‘,1‘‘‘,3‘‘‘-tetrabenzylbiferrocene, and their corresponding mixed-valence biferrocenium triiodide salts have been determined at 298 K. The rates of intramolecular electron transfer in the mixed-valence cations were estimated by variable-temperature 57Fe Mössbauer spectroscopy (time scale ∼107 s-1). The features in all of the 80 K spectra include two doublets, one with a quadrupole splitting (ΔE Q) of ∼2 mm s-1 (Fe(II) site) and the other with ΔE Q = ∼0.6 mm s-1 (Fe(III) site). This pattern of two doublets is expected for a mixed-valence biferrocenium cation which is valence-trapped on the time scale of the Mössbauer experiment (electron-transfer rate < ∼107 s-1 in the solid state). Increasing the sample temperature in 1‘,2‘,1‘‘‘,2‘‘‘-tetrapropylbiferrocenium triiodide, 1‘,2‘,1‘‘‘,2‘‘‘-tetrabenzylbiferrocenium triiodide, and 1‘,3‘,1‘‘‘,3‘‘‘-tetrabenzylbiferrocenium triiodide causes the two doublets to move together with no discernible line broadening, eventually becoming a single “average-valence” doublet at temperatures of 295, 265, and 190 K, respectively. In the case of 1‘,3‘,1‘‘‘,3‘‘‘-tetrapropylbiferrocenium triiodide, the two doublets at low temperatures do not coalesce into an average-valence doublet at 300 K (electron-transfer rate < ∼10 s-1 at 300 K). Thus, the micromodification of the relative positions of the alkyl substituents and the nature of the alkyl substituents has a dramatic influence on the rate of intramolecular electron transfer. The deviations of the Cp rings from the parallel position correlate quite well with the Mössbauer critical temperature for electronic delocalization−localization in mixed-valence cations. Qualitatively, we suggest that the metal nonbonding orbitals (d x 2 - y 2 , d xy ) start to interact with the ligand π orbitals as the Cp rings tilt from the parallel position. On the basis of the density functional calculations, a theoretical explanation of the influence of bending back the Cp rings on the electron-transfer rates is presented. The electrochemical measurements, IR data, and near-IR spectra for these mixed-valence biferrocenium cations are also presented.

Bis-Ferrocenyl-Pyridinediimine Trinuclear Mixed-Valent Complexes with Metal-Binding Dependent Electronic Coupling: Synthesis, Structures, and Redox-Spectroscopic Characterization

“…Indeed, there is great interest in metal–ligand redox and reactivity cooperativity and metallo-ligand mediated electronic coupling between metals in the chemistry community. Despite interest, the application of mixed valency toward exploring the effects of electronically coupled binuclear scaffolds that could be applied to catalysis has been challenging, due to issues of (A) chemical stability in different redox states; (B) establishing clear design criteria for engendering strong coupling, as similar structures often exhibit different coupling strengths as a result of the balance between geometric and electronic factors that lead to coupling; , and (C) designing scaffolds that are modular enough to retain coupling while parametrizing steric and electronic properties to optimize catalysis. Most mixed-valent catalysts capitalize on cooperative effects between metals that have different chemistries in different oxidation states (e.g., using Class I mixed valent structures with little to no delocalization between metals), or otherwise do not well characterize the effect of metal delocalization on reactivity. , However, work in direct metal–metal interactions between ferrocene and d 0 metals has revealed promising electronic structures and catalytic activity .…”

“…Under this lens, mixed valency provides an opportunity to explore chemical space in noninnocent ligand catalysis while capitalizing on well-characterized electronic structures, synthetic strategies, and established theoretical frameworks. Nevertheless, such systems are challenging to create in a reliable, modular way, and the chemical space explored remains insufficient to deduce general principles. ,,, …”

Pronounced effects of grinding on rates of intramolecular electron transfer in mixed-valence 1′,2′,1‴,2‴-tetranaphthylmethyl- and 1′,3′,1‴,3‴-tetranaphthylmethyl-biferrocenium triiodides

“…We suggested that the zero-point energy difference plays a crucial role in determining the nature of the intramolecular electron-transfer rate in biferrocenium cation. Finally, to study the influence of the alkyl substituents on the Cp ring, we prepared a series of polyethylbiferrocenium triiodide 5 and 6 [22]. We found that the deviations of the Cp rings from the parallel position correlate quite well with the Mo ¨ssbauer transition temperature of electronic delocalization-localiza- tion in mixed-valence biferrocenium cations.…”

The synthesis of binuclear half-open 1,1′-biferrocene: redox behavior and an interpretation of the 57Fe Mössbauer data

“…As opposed to closed ferrocenium [8][9][10], ESR spectra of the halfopen and open ferrocenium ions can be observed at room temperature [7]. In connection with our investigations on the electron transfer in mixed-valence biferrocenium cations [11], we became interested in the synthesis of binuclear half-open ferrocene (1).…”