Molecular Wires using (Oligo)pyrroles as Connecting Units: An Electron Transfer Study

Abstract

A series of (oligo)pyrroles featuring redox-active terminal ferrocenyl groups (Fc 2 -( c C 4 H 2 NPh) n (4, n = 1; 9, n = 2; 16, n = 3; 20, n = 4)) has been prepared using a Negishi C,C cross-coupling reaction protocol. The bi-, ter-and quarterpyrrole wire moieties have been built up by C,C cross-coupling reactions of trimethyl silyl protected pyrrole units in the presence of [Pd(CH 2 C(CH 3 ) 2 P( t C 4 H 9 ) 2 )(µ-Cl)] 2 as precatalyst. The structural properties of the title compounds were investigated by spectroscopic means and single-crystal X-ray diffraction studies (9, 16 and 20). The influence of the increasing number of N-phenyl pyrrole units on the electronic interaction between the iron centres was studied using electrochemistry (cyclic (CV) and square wave voltammetry (SWV)) as well as spectroelectrochemistry (in situ UV-Vis/NIR spectroscopy). With exception of the diferrocenyl quarterpyrrol 20, the application of [N n Bu 4 ][B(C 6 F 5 ) 4 ] as electrolyte allows the discrete oxidation of the ferrocenyl termini (∆E° ′ = 450 mV (4), ∆E° ′ = 320 mV (9), ∆E° ′ = 165 mV (16)) in cyclic and square wave voltammograms. However, the iron centres of 20 were oxidized simultaneously, generating dicationic 20 2+ . Additionally, one (9) or two (16 and 20) pyrrole-related well-defined reversible one-electron redox processes were observed. The cyclic voltammetry data reveal that the splitting of the ferrocene-based redox couples, ∆E° ′, decreases with increasing oligo-pyrrole chain length and hence, the larger metalmetal distance. The trends in ∆E° ′ with oligo-pyrrole structure also map to the electronic coupling between the ferrocene moieties as estimated by spectroelectrochemical UV-Vis/NIR measurements.Despite no direct metal-metal interaction in diferrocenyl quarterpyrrole 20, a large absorption in the NIR region is observed arising from photo-induced charge transfer from the oligopyrrole backbone to the redox-active ferrocenyl termini. These charge transfer absorptions have also been found in the dicationic oxidation state of the mono-(4), bi-(9) and terpyrroles (16). Within this series of diferrocenyl(oligo)pyrroles this CT band is shifted bathochromically with increasing chain length of the backbone motif.

Multiferrocenyl Cobalt‐Based Sandwich Compounds

“…The electrochemical properties of 3a , 5a , b , 5c , 6a – c , and 7 were studied by using cyclic voltammetry (CV) and square‐wave voltammetry (SWV) and by in situ UV/Vis/NIR spectroelectrochemistry ( 3a and 6a – c ) (Figures , , , , and S68–S70; Tables and ). The electrochemical measurements were performed under argon in dichloromethane solutions containing [ n Bu 4 N][B(C 6 F 5 ) 4 ] (0.1 mol L –1 ) as supporting electrolyte , , . The weak coordinating counterion [B(C 6 F 5 ) 4 ] – is known to stabilize highly charged species in solution.…”

Electrochemical studies of the multi‐step multi‐electron redox process of tetraferrocenyloligothiophenes with electron donor and acceptor abilities

“…The multi‐step oxidation process derived from the multi‐ferrocene system can be controlled by the π‐conjugated spacer connecting ferrocene fragments due to the fact that the strength of the electronic communication between the mixed‐valent metal centers strongly depends on the electronic nature and length of the linking π‐conjugated spacer. Thus, there have been investigations of multi‐ferrocene derivatives bridged by a wide variety of π‐conjugated spacers such as aromatics (eg, benzene), heteroaromatics (eg, pyrrole, phosphole, furan, thiophene), and their oligomers (eg, oligopyrrole:, oligofuran, oligothiophene).…”

Synthesis, characterization, and physical properties of oligo(1-(N,N-dimethylamino)pyrrole)s and their doped forms, precursors of candidates for molecular flat-band ferromagnets

“…[1][2][3] Particularly important are a,a-linked oligopyrroles, which are found in important conducting materials and also in some natural and medicinal products. 9 Experimental and theoretical studies on the redox behaviors and electronic structures of the oxidized forms of unsubstituted oligopyrroles have been reported. 9 Experimental and theoretical studies on the redox behaviors and electronic structures of the oxidized forms of unsubstituted oligopyrroles have been reported.…”

“…20 Oligo(N,N-dimethylaminopyrrole)s (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) were prepared by successive lithiation and oxidative coupling reactions using a 2,2 0 -bi Ndimethylamino)pyrrole] starting material (Scheme 1). Oligomers with controlled structures and degrees of polymerization were prepared not by oxidative polymerization but by sequential chemical synthesis.…”