Prediction of reduction potentials from calculated electron affinities for metal-salen compounds

Sarah B Bateni,Handeep Kaur,Benjamin F Gherman,Victor A Mendiola,James A Miranda,Anthony T Galatti,Alexander R Mitchell,Michael H Vu,Kellie R England

doi:10.3762/bjoc.5.82

Abstract

The electron affinities (EAs) of a training set of 19 metal-salen compounds were calculated using density functional theory. Concurrently, the experimental reduction potentials for the training set were measured using cyclic voltammetry. The EAs and reduction potentials were found to be linearly correlated by metal. The reduction potentials of a test set of 14 different metal-salens were then measured and compared to the predicted reduction potentials based upon the training set correlation. The method was found to work well, with a mean unsigned error of 99 mV for the entire test set. This method could be used to predict the reduction potentials of a variety of metal-salen compounds, an important class of coordination compounds used in synthetic organic electrochemistry as electrocatalysts.

Highlights

The electroreductive cyclization (ERC) reaction is a process in which an electron-deficient alkene that is tethered to an acceptor undergoes an electrochemically promoted reductive cyclization leading to the formation of a new sigma bond between the β-carbon of the alkene and the acceptor unit [1]
A training set of 19 metal-salen compounds was used to build a correlation between computed electron affinities (EAs) and experimental reduction poten
This correlation was used to predict the reduction potentials of a test set of 14 metal-salen compounds with a mean unsigned error of 99 mV

Summary

Introduction

The electroreductive cyclization (ERC) reaction is a process in which an electron-deficient alkene that is tethered to an acceptor (e.g., an aldehyde or ketone) undergoes an electrochemically promoted reductive cyclization leading to the formation of a new sigma bond between the β-carbon of the alkene and the acceptor unit [1]. The postulated mechanism of the reaction operated via a Ni(II)-salen radical anion as the active catalyst and evidence for the formation of a Ni(II)-salen radical. In mediated ERC, while Ni(II)-salen (reduction potential or Epc = −2.1 V vs Ag/AgNO3) is an effective electrochemical mediator, the analogous Co(II)-salen (Epc = −1.6 V vs Ag/AgNO3) fails to promote cyclization. Direct ERC (unmediated) occurs at a reduction potential of −2.7 V vs Ag/AgNO3. It was concluded that the 1.1 V thermodynamic barrier was too large to allow electron transfer to occur from the reduced form of the Co(II)-salen to the substrate [4]

Methods

Results

Conclusion