Manganese Carbonyl Complexes as Selective Electrocatalysts for CO 2 Reduction in Water and Organic Solvents.

The electrochemical reduction of CO 2 provides a way to sustainably generate carbon-based fuels and feedstocks. Molecular CO 2 reduction electrocatalysts provide tunable reaction centers offering an approach to control the selectivity of catalysis. Manganese carbonyl complexes, based on [Mn(bpy)(CO) 3 Br] and its derivatives (bpy = 2,2'-bipyridine), are particularly interesting due to their ease of synthesis and the use of a first-row earth-abundant transition metal. [Mn(bpy)(CO) 3 Br] was first shown to be an active and selective catalyst for reducing CO 2 to CO in organic solvents in 2011. Since then, manganese carbonyl catalysts have been widely studied with numerous reports of their use as electrocatalysts and photocatalysts and studies of their mechanism.This class of Mn catalysts only shows CO 2 reduction activity with the addition of weak Brønsted acids. Perhaps surprisingly, early reports showed increased turnover frequencies as the acid strength is increased without a loss in selectivity toward CO evolution. It may have been expected that the competing hydrogen evolution reaction could have led to lower selectivity. Inspired by these works we began to explore if the catalyst would work in protic solvents, namely, water, and to explore the pH range over which it can operate. Here we describe the early studies from our laboratory that first demonstrated the use of manganese carbonyl complexes in water and then go on to discuss wider developments on the use of these catalysts in water, highlighting their potential as catalysts for use in aqueous CO 2 electrolyzers.Key to the excellent selectivity of these catalysts in the presence of Brønsted acids is a proton-assisted CO 2 binding mechanism, where for the acids widely studied, lower p K a values actually favor CO 2 binding over Mn-H formation, a precursor to H 2 evolution. Here we discuss the wider literature before focusing on our own contributions in validating this previously proposed mechanism through the use of vibrational sum frequency generation (VSFG) spectroelectrochemistry. This allowed us to study [Mn(bpy)(CO) 3 Br] while it is at, or near, the electrode surface, which provided a way to identify new catalytic intermediates and also confirm that proton-assisted CO 2 binding operates in both the "dimer" and primary (via [Mn(bpy)(CO) 3 ] - ) pathways. Understanding the mechanism of how these highly selective catalysts operate is important as we propose that the Mn complexes will be valuable models to guide the development of new proton/acid tolerant CO 2 reduction catalysts.