Bis(Iminophosphorano)‐Substituted Pyridinium Ions and their Corresponding Bispyridinylidene Organic Electron Donors

Optimized synthetic procedures for pyridinium ions featuring iminophosphorano (−N=PR3; R=Ph, Cy) π‐donor substituents in the 2‐ and 4‐ positions are described. Crystallographic and theoretical studies reveal that the strongly donating substituents severely polarize the π‐electrons of the pyridyl rin...

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Bibliographic Details
Published in:Chemistry : a European journal 2021-06, Vol.27 (33), p.8528-8536
Main Authors: Frenette, Brandon L., Arsenault, Nadine, Walker, Sarah L., Decken, Andreas, Dyker, C. Adam
Format: Article
Language:English
Subjects:
Aromaticity
Calomel electrode
Chemistry
Crystallography
Electrochemistry
electron transfer
Electrons
iminophosphoranes
Ions
organic electron donor
Oxidation
Pyridinium
Redox potential
reduction
substituent effects
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Summary:Optimized synthetic procedures for pyridinium ions featuring iminophosphorano (−N=PR3; R=Ph, Cy) π‐donor substituents in the 2‐ and 4‐ positions are described. Crystallographic and theoretical studies reveal that the strongly donating substituents severely polarize the π‐electrons of the pyridyl ring at the expense of aromaticity. Moreover, the pyridinium ions are readily deprotonated to generate powerful bispyridinylidene (BPY) organic electron donors. Electrochemical studies show exceptionally low redox potentials for the two‐electron BPY/BPY2+ couples, ranging from −1.71 V vs the saturated calomel electrode for 3PhPh (with four Ph3P=N− groups) to −1.85 V for 3CyCy (with four Cy3P=N− groups). These new compounds represent the most reducing neutral organic electron donors (OEDs) currently known. Some preliminary reductions involving 3CyCy showed enhanced capability owing to its low redox potential, such as the thermally activated reduction of an aryl chloride, but purification challenges were often encountered. Optimized synthetic routes to pyridinium ions featuring two strongly π‐donating R3P=N− substituents are presented. These substituents cause severe disruption in aromaticity in these cations, and also impart exceptional reducing ability to the related bispyridinylidenes (neutral 2 e‐ donors). Preliminary reactivity studies show some enhanced reducing capabilities owing to the low redox potential of the bispyridinylidene.
ISSN:0947-6539
1521-3765
DOI:10.1002/chem.202100318