Ferrocenyl-Terminated Redox Stars: Synthesis and Electrostatic Effects in Mixed-Valence Stabilization

Abstract

A family of rigid ferrocenyl-terminated redox stars has been synthesized--by Negishi coupling, including hexa(ferrocenethynyl)benzene complexes, a dodecaferrocenyl star, and stars with extended rigid tethers--and fully characterized. Cyclic voltammetry (CV) studies of the parent complex hexa(ferrocenylethynyl)benzene, 1, show a single wave for the six-electron oxidation of 1 using Nn-Bu(4)PF(6) as the supporting electrolyte on a Pt anode in CH(2)Cl(2), whereas three distinct two-electron reversible CV waves are observed using Nn-Bu(4)BAr(F)(4) (Ar(F) = 3,5-C(6)H(3)-(CF(3))(2)). The CV of 1,3,5-tris(ferrocenylethynyl)benzene, 11, also shows only one wave for the three-electron transfer with Nn-Bu(4)PF(6) and three one-electron waves using Nn-Bu(4)BAr(F)(4). This confirms the lack of electronic communication between the ferrocenyl groups and a significant electrostatic effect among the oxidized ferrocenyl groups. This effect is not significant between para-ferrocenyl groups in 1,4-bis(ferrocenylethynyl)benzene for which only a single wave is observed even with Nn-Bu(4)BAr(F)(4) as the supporting electrolyte. The para-ferrocenyl substituents are quite independent, which explains that two para-ferrocenyl groups are oxidized at about the same potential in a single CV wave of 1. With the additional steric bulk introduced with a methyl substituent on the ferrocenyl group, however, even the para-methylferrocenyl groups are submitted to a small electrostatic effect splitting the six-electron transfer into six single-electron waves, probably because of the overall steroelectronic constraints. Contrary to 11, 1,3-bis(ferrocenylethynyl)benzene and related complexes with a third, different substituent in the remaining meta position different from a ferrocenylethynyl only show a single two-electron wave using Nn-Bu(4)BAr(F)(4), which is attributed to the transoïd conformation of the ferricinium groups minimizing the electrostatic effect. This shows that, in 11, it is the steric frustration that is responsible for the electrostatic effect, and the same occurs in 1. In several cases, ΔE(p) is much larger than the expected 60 mV value, characterizing a quasi-reversible (i.e., relatively slow) redox process. It is suggested that this slower electron transfer be attributed to conformational rearrangement of the ferrocenyl groups toward the transoïd position in the course of electron transfer. Thus both the thermodynamic and kinetic aspects of the electrostatic factor (isolated from the electronic factor), including the frustration effect, are characterized. The distinction between the electronic communication and through-space electrostatic effect was made possible in all of these complexes in which the absence of wave splitting using a strongly ion-pairing electrolyte shows the absence of significant electronic communication, and was confirmed by the new frustration phenomenon.

Mixed-Valency and Metal-Metal Coupling in Polynuclear Metal Complexes: Recent Progress and Prospects

“…[44][45][46][47][48][49] They have various structural types such as linearly-linked ferrocenyl wires, 50) star-shaped dendritic polyferrocenes, [51][52][53][54] and ring-shaped (cyclic) polyferrocenes. 55) Theoretical treatment of the stepwise oxidation processes of these polyferrocene oligomers, depending on the structural type, has been developed by Aoki and the co-workers, [56][57][58][59][60] and the theory has been successfully applied to the experimental systems.…”

“…7). 53,54) These polyferrocenyl compounds were reversibly oxidized 2 ] n (n = 2-7; Fig. 8), 55) which were separated by column chromatography, showed stepwise oxidation due to mutual "electrostatic" interactions between the redox-active units.…”

Ferrocene‐ and Biferrocene‐Containing Macrocycles towards Single‐Molecule Electronics

“…The cyclic voltammetry data does not give evidence of significant ring or spatial proximity effects owing to the structure of these molecules. Previous reports of oligoferrocenes with similar alkynyl linkers have shown no evidence for electronic communication between individual ferrocene sites through bond, however there has been some evidence of through space electrostatic interaction 17, 25, 26…”

Metallodendrimers in three oxidation states with electronically interacting metals and stabilization of size-selected gold nanoparticles

“…The redox-rich iron centres in the chosen system are Cp à Fe II (dppe)-alkynyl units (Cp à ¼ Z 5 -C 5 Me 5 , dppe ¼ 1,2-bis(diphenylphosphino) ethane) 24 that are isolobal to ferrocene and linked in meta position of an aryl bridge. In 1,3,5-trimetallaalkynylbenzene derivatives [24][25][26][27] , and in such Cp à Fe II (dppe) derivatives in particular 25,26 , it has been shown by density functional theory calculation and electrochemistry that, unlike in the 1,3,5-triferrocenylalkynylbenzene derivatives 28 , the three metals are electronically communicating. We synthesize the dendrons with only two iron-ethynyl groups in 1,3 position, leaving the third ethynyl position free for binding to the dendritic core either by Sonogashira coupling 29,30 or copper-catalysed azide-alkyne cycloaddition (CuAAC, 'click') reactions [31][32][33] with appropriately functionalized dendritic core termini.…”