Reliable reduction potentials of diaryliodonium cations and aryl radicals in acetonitrile from high-level ab initio computations

Abstract

The experimental values of the standard reduction potentials of diaryliodonium cations (Ar2I+) and aryl radicals (Ar•) are generally not deemed reliable owing to the irreversibility of electrode processes, often aggravated by blocking the electrode by the grafted layer. They are, however, required for the prediction of suitability of photoinitiators and the course of arylation reactions or aryl radical surface modifications. Here, we present accurate E° values of reduction of symmetrical and unsymmetrical Ar2I+, and of Ar• in MeCN, obtained from calculations based on the gas-phase redox energetics from the high-level explicitly correlated coupled-cluster method (CCSD(T)-F12) combined with the SMD solvation free energies. Plausible paths of C–I bond cleavage in unsymmetrical Ar2I+ cations were also predicted from reference values of the C–I bond dissociation energies (BDEs) in Ar2I+. The computed reduction potentials were compared with available experimental data and with results from other quantum chemical methods including other implicit solvation models. A perfect linear correlation with Hammett σ constants was found for the computed potentials of Ar2I+ reduction. This correlation equation may be used for rapid estimation of reduction potentials of other Ar2I+ cations, but also for the prediction of the fate of other processes, in which diaryliodonium cations take part, like aryl exchange reactions or the ability to initiate photopolymerisation.