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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

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A Carborane-Derived Proton-Coupled Electron Transfer Reagent.

Enric H Adillon¹, Jonas C Peters¹

¹Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech), Pasadena, California 91125, United States.

Journal of the American Chemical Society

|October 28, 2024

Summary

This summary is machine-generated.

Researchers explored carboranes as cobaltocene substitutes for reductive electrocatalysis. They found these carboranes facilitate multi-electron/proton reductions via electron transfer, not concerted proton-electron transfer pathways.

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Area of Science:

Inorganic chemistry
Electrochemistry
Organic synthesis

Background:

Concerted proton-electron transfer (CPET) offers lower reaction barriers than stepwise electron transfer (ET) and proton transfer (PT).
Coupling cobaltocene ET sites with arylamine PT sites enables reductive electrocatalysis.
Exploring alternative frameworks for CPET-like reactivity is crucial for advancing reductive transformations.

Purpose of the Study:

To investigate C,C'-diaryl-o-carboranes as substitutes for cobaltocene in reductive electrocatalysis.
To characterize the redox properties and protonation behavior of diaryl-o-carboranes.
To determine the mechanistic pathways (CPET vs. ET/PT) for carborane-mediated reductions.

Main Methods:

Synthesis and characterization of diaryl-o-carboranes.
Electrochemical reduction and protonation studies.
Thermodynamic measurements to determine effective bond dissociation free energy (BDFEeff).
Crystallographic analysis of reduced carborane species.
Mechanistic studies of multi-ET/PT reductions and hydrogen evolution reaction (HER).

Main Results:

Diaryl-o-carboranes were synthesized and characterized, exhibiting stable redox behavior.
The protonated carborane generated an N-H bond with a low BDFEeff of 31 kcal/mol.
Solid-state structures of reduced carboranes provided insight into their redox characteristics.
Carborane mediators facilitated multi-ET/PT reductions of azoarenes, diphenylfumarate, and nitrotoluene.
Mechanistic data indicated rate-limiting ET steps, distinct from CPET pathways.

Conclusions:

Diaryl-o-carboranes serve as effective mediators for multi-ET/PT reductions.
The observed mechanisms proceed via stepwise ET/PT, not CPET, differing from cobaltocene systems.
The carborane core demonstrates stability towards protonation, relevant for hydrogen evolution reaction studies.