Pulsed‐EPR Evidence of a Manganese(II) Hydroxycarbonyl Intermediate in the Electrocatalytic Reduction of Carbon Dioxide by a Manganese Bipyridyl Derivative

Abstract

A key intermediate in the electroconversion of carbon dioxide to carbon monoxide, catalyzed by a manganese tris(carbonyl) complex, is characterized. Different catalytic pathways and their potential reaction mechanisms are investigated using a large range of experimental and computational techniques. Sophisticated spectroscopic methods including UV/ Vis absorption and pulsed-EPR techniques (2P-ESEEM and HYSCORE) were combined together with DFT calculations to successfully identify a key intermediate in the catalytic cycle of CO 2 reduction. The results directly show the formation of a metal-carboxylic acid-CO 2 adduct after oxidative addition of CO 2 and H + to a Mn 0 carbonyl dimer, an unexpected intermediate.The chemical conversion of carbon dioxide has attracted a lot of interest because simultaneously the environmental impact of CO 2 is reduced and value is created by using CO 2 as a sustainable feedstock of carbon (CO 2 recycling).[1] Electrochemical conversion of CO 2 into valuable chemicals ranging from carbon monoxide (CO) or formic acid (HCOOH), the most straightforward products from CO 2 reduction, to synthetic fuels and high-molecular polymers, could be one of the ways to store intermittent energy as chemicals that can be transported and used on demand.[2] But efficient CO 2 reduction still remains challenging. Indeed, CO 2 is a stable and inert molecule, [3] thus high-yield CO 2 conversion reactions require efficient catalysts and energy input. If the latter is a renewable source of energy the electrochemical reduction of CO 2 can be regarded as a sustainable artificial photosynthetic process. Furthermore, CO 2 reduction is a protondependent process; therefore catalysts must be kinetically selective to prevent the direct hydrogen evolution reaction (proton reduction). In the past thirty years several efficient and selective metal complex catalysts for the homogeneous electrochemical reduction of CO 2 to CO have been studied. This includes systems based on Re I , [4] Ru II , [5] and Os II [6] polypyridyl carbonyl complexes. Just a few molecular catalysts based on Fe [7] and Mn [8] (metals abundant in the earths crust) have been reported. As recently pointed out, [2c, 9] despite the large number of investigations, most reaction mechanisms of CO 2 reduction are not yet well-known and further developments are needed.Recently, to develop new molecular electrocatalysts with the goal of implementing a sustainable CO 2 reduction process, we explored Mn carbonyl bipyridyl complexes and we showed that [Mn I (L)(CO) 3 Br] (L = bpy (2,2'-bipyridine) and dmbpy (4,4'-dimethyl-2,2'-bipyridine)) are precursors to catalysts for the electroreduction of CO 2 to CO in hydro-organic electrolyte.[8a] Shortly thereafter Kubiak and co-workers In CO 2 and in the presence of H 2 O, a particular electrocatalytic property has been characterized for [Mn-(dmby)(CO) 3 Br]. Indeed when the cyclic voltammogram (CV) of this dmbpy Mn complex was recorded under CO 2 in the presence of a weak Brçnsted acid (5 % water; F...

The elusive abnormal CO2 insertion enabled by metal-ligand cooperative photochemical selectivity inversion

“…Alternatively, CO 2 insertion into a Ni–H bond has also been discussed 2 . In analogy, metallocarboxylate (MCO 2 H/M) species are also proposed as the key intermediates for synthetic CO-selective molecular catalysis, yet rarely detected 3 – 8 . Such systems are generally electro- or photochemically driven 1 , 9 – 12 as direct hydrogenation by reverse water–gas shift (RWGS) is endothermic (see equation 1 ).…”

Manganese Tricarbonyl Complexes with Asymmetric 2-Iminopyridine Ligands: Toward Decoupling Steric and Electronic Factors in Electrocatalytic CO2 Reduction

“…Two mechanisms have been proposed 10,[14][15][16][17][18] for the ultimate reduction of [Mn(CO) 3 ( -diimine)] 2 in the presence of CO 2 , which can be referred to as the anionic, and the oxidative addition 19 pathways. The anionic pathway involves reduction of the dimer [Mn(CO) 3 ( -diimine)] 2 at a more negative potential than the parent complex generating the five-coordinate anion [Mn(CO) 3 ( -diimine)] to which CO 2 coordinates and is catalyti-cally reduced in the presence of a Brönsted acid (the source of H + ).…”

“…The anionic pathway is broadly similar to the two-electron pathway observed for Re complexes. 20,21 In contrast, the uncommon second pathway identified using pulsed EPR studies 19 involves coordination of CO 2 to the dimer [Mn(CO) 3 (2,2 -bpy)] 2 in the presence of a Brönsted acid in a concerted oxidative addition step. This process is shown to generate a low-spin Mn II -COOH complex, from which CO is subsequently released.…”

Selective Electrocatalytic CO2 Reduction to CO by an NHC-Based Organometallic Heme Analogue

“…15 Such Mn catalysts were later equipped with bulky substituents to prevent Mn-Mn dimerization, 16 and this tunable platform has been widely modified to provide a deeper mechanistic understanding of CO2 reduction catalysis. [17][18][19][20][21] The complexes mentioned so far, as well as most other reported first-row transition metal electrocatalysts for the reduction of CO2, feature supporting N-donor ligands. 22 Recent work has started to address the effects of alternative ligation, in particular that of N-heterocyclic carbenes (NHCs), on the efficiency and stability of CO2 reduction catalysts.…”