In situ ultrafast 2D NMR spectroelectrochemistry for real-time monitoring of redox reactions.

Abstract

The in situ implementation of an electrochemical cell (EC) inside a nuclear magnetic resonance (NMR) spectrometer is extremely powerful to study redox reactions in real time and identify unstable reaction intermediates. Unfortunately, the implementation of an electrochemical device near the sensitive volume of an NMR probe significantly affects the quality of the NMR signal, inducing significant line broadening resulting in peak overlap and partial loss of the multiplet structures. Two-dimensional (2D) NMR spectroscopy allows one to bypass signal overlapping by spreading the peaks along two orthogonal dimensions, while providing precious information in terms of structural elucidation. Nevertheless, the acquisition of 2D NMR data suffers from long acquisition durations which are incompatible with fast redox processes taking place in solution. Here, we present a new approach to deal with this issue, consisting of coupling EC-NMR with ultrafast 2D spectroscopy, capable of recording 2D spectra much faster than conventional 2D NMR. This approach is applied to the real-time monitoring of a model reaction. Fast correlation spectroscopy (COSY) spectra are recorded every 3 min in the course of the 80 min reaction, leading to the unambiguous identification of one reaction intermediate and two reaction products. The evolution of 2D NMR peak volumes in the course of time provides further insight into the mechanism of this reaction involving an unstable intermediate. This study demonstrates the feasibility and the relevance of coupling in situ spectroelectrochemistry with ultrafast 2D spectroscopy to monitor real-time electrochemical reactions in the NMR tube.