Mapping proton and carbon dioxide electrocatalytic reductions at a Rh complex by in situ spectroelectrochemical NMR

Detailed molecular level understanding of organometallic electrocatalytic systems is required to fully exploit their technological potential to store, distribute, and utilise renewable energy in chemical form. However, in situ methods providing high resolution information on the structure and reactivity of transient intermediates remain challenging due to incompatible requirements for standard electrochemical and spectroscopic cell designs. Here, we demonstrate the use of spectroelectrochemical nuclear magnetic resonance (SEC-NMR) to enable operando characterisation of molecular species during organometallic electrocatalysis. The electroreduction of a prototypical molecular rhodium(+i) diphosphine complex was studied under aprotic conditions and in the presence of H₂O and/or CO₂. By combining multinuclear SEC-NMR, chemical reductions, modelling and simulations, we determine the involved species, their relative concentrations and the competing interconversions. The bielectronic reduction leading to the highly reactive low-valent rhodium(-i) intermediate and subsequent protonation of that species into a Rh-hydride complex was followed in a time-resolved manner. Deuterium labelling and ex situ NMR analysis after SEC-NMR electrolysis revealed that under aprotic conditions the proton source substantially arises from Hofmann elimination of the nBu₄NPF₆ electrolyte in addition to the acetonitrile solvent. The reactivities of the Rh(-i) and the Rh-H complexes were further monitored under turnover conditions, providing direct molecular insights into bifurcating electrocatalytic pathways for hydrogen evolution and CO₂ reduction.