Kinetics of Multielectron Transfers and Redox-Induced Structural Changes in N-Aryl-Expanded Pyridiniums: Establishing Their Unusual, Versatile Electrophoric Activity.
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.
J. Heyrovský Institute of Physical Chemistry of ASCR, v. v. i., Dolejškova 3, 182 23 Prague, Czech Republic.
Štěpánka
Š
Lachmanová
Université Paris Diderot , Sorbonne Paris Cité, ITODYS, UMR CNRS 7086, 15 rue J-A de Baïf, 75013 Paris, France.
Grégory
G
Dupeyre
Institute of Organic Chemistry and Biochemistry of ASCR, v. v. i., Flemingovo nám. 2, 166 10 Prague, Czech Republic.
Ján
J
Tarábek
Laboratoire de Cristallographie et Modélisation Moléculaire du Solide, Sanofi LGCR , 371 rue du Professeur Blayac, Montpellier 34184 Cedex 04, France.
Philippe
P
Ochsenbein
Université Paris Diderot , Sorbonne Paris Cité, ITODYS, UMR CNRS 7086, 15 rue J-A de Baïf, 75013 Paris, France.
Christian
C
Perruchot
PSL Research University , Chimie ParisTech-UMR CNRS 8247, Institut de Recherche de Chimie Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France.
Ilaria
I
Ciofini
J. Heyrovský Institute of Physical Chemistry of ASCR, v. v. i., Dolejškova 3, 182 23 Prague, Czech Republic.
Magdaléna
M
Hromadová
J. Heyrovský Institute of Physical Chemistry of ASCR, v. v. i., Dolejškova 3, 182 23 Prague, Czech Republic.|Institute of Organic Chemistry and Biochemistry of ASCR, v. v. i., Flemingovo nám. 2, 166 10 Prague, Czech Republic.
Lubomír
L
Pospíšil
Université Paris Diderot , Sorbonne Paris Cité, ITODYS, UMR CNRS 7086, 15 rue J-A de Baïf, 75013 Paris, France.
Philippe P
PP
Lainé