Kinetics of Multielectron Transfers and Redox-Induced Structural Changes in N-Aryl-Expanded Pyridiniums: Establishing Their Unusual, Versatile Electrophoric Activity

Abstract

A combined electrochemical and theoretical study of a series of pyridinium-based electrophores, consisting of reference N-alkyl-2,4,6-triarylpyridiniums (1-3) and N-aryl-expanded pyridiniums (EPs), i.e. N-aryl-2,4,6-triarylpyridiniums (4-10), is presented with the aim of elucidating multifaceted mechanisms underpinning the complex electrophoric activity of fluxional EP systems. Series 1-10 constitutes a library of model electrophores showing an incremental variation of their composition, charge, and steric hindrance. By kinetic mapping of the first two heterogeneous electron transfers (ETs) of 1-10 and computational mapping, at the density functional theory level, of their electronic and geometrical features in various redox states, it is established that, depending on whether EPs are made of one (4, 5) or two "head-to-tail"-connected pyridinium rings (6-10), the nature of the redox-triggered distortions (when allowed) is different, namely, N-pyramidalization due to hybridization change in the former case versus saddle-shaped distortion originating from conflicting intramolecular interactions in the latter case (8-10). When skeletal relaxations are sterically hampered, zwitterionic states and electron delocalization with quinoidal features are promoted as alternative relaxation modes. It follows that "potential compression" is changed to "potential expansion" (i.e., a further separation of redox potentials) in single-pyridinium EPs (4, 5), whereas "potential inversion" (i.e., single-step two-electron transfer; 8-10) is changed to stepwise ETs of the Weitz type for two-pyridinium EPs (6, 7). Overall, kinetic rate constants not only consistently indicate the most prominent mechanistic aspects of the reduction pathways of EPs, but they are also instrumental in establishing EPs as a unique class of electrophores.

Organic Four‐Electron Redox Systems Based on Bipyridine and Phenanthroline Carbene Architectures

“…We could obtain single crystals for both neutral 2 a and 2 b and clearly establish their X‐ray solid‐state structures (Figure 2). [28] Both molecular structures are highly distorted, [29] with the N−Me groups being strongly (≈90°) tilted out of the central plane (both up for 2 a and up and down for 2 b ), clearly indicating a complete loss of aromaticity in both heterocycles. Both 2 a and 2 b feature in the central C 5 N cores significant distortions and C−C bond alternations [C2−C3 1.365(2) Å and C3−C4 1.456(2) Å ( 2 a ); C2−C3 1.354(2) Å and C3−C4 1.445(2) Å ( 2 b )] confirming a dearomatized core.…”

Exploiting Potential Inversion for Photoinduced Multielectron Transfer and Accumulation of Redox Equivalents in a Molecular Heptad

“…We hypothesized that an acceptor exhibiting redox potential inversion, i.e., a compound in which the second reduction occurs more easily than the first, could facilitate the light-driven accumulation of reduction equivalents. There are a number of possible acceptor units, , and we were inspired by prior electrochemical and computational studies of bipyridinium disulfides and dibenzo­[1,2]­dithiin (PhSSPh) compounds . We decided to incorporate the latter as a central acceptor unit between two Ru­(bpy) 3 2+ (bpy = 2,2′-bipyridine) photosensitizers equipped with peripheral triarylamine (TAA) donors (Scheme a).…”

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

“…109 In non-proton-coupled redox reactions, every additional redox step is usually thermodynamically less favored than the preceding step, except in the somewhat special cases of redox potential inversion. 71,72,[74][75][76]110 As noted in the introduction, temporary storage of redox equivalents is needed to synchronize the fast primary photoinduced electron transfer events with the slower multi-electron turnovers in catalytic reaction centers, but there is also the important issue of photon flux. Two-electron transfer is difficult to induce upon single excitation.…”