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Complexes of the general form [Mn(X)(CO) 3 bpy] (X = a variety of monodentate ligands, bpy = 2,2′-bipyridine) have been reported to act as electrocatalysts for the reduction of CO 2 to CO. In this work, a series of phenol and anisole substituted bipyridine ligands were synthesized and ligated to a manganese metal center in order to probe for an intramolecular hydrogen-bonding interaction in the transition state of CO 2 reduction. Ligands without the ability to intramolecularly hydrogen bond displayed decreased catalytic current density compared to those with the ability to hydrogen bond with CO 2 . Electrocatalysis was studied by performing voltammetric and bulk electrolysis experiments under argon or CO 2 environments. Measurements of catalytic rates using hydrogen vs. deuterium for the intramolecular H/D-bonding step show that there is an isotope effect associated with the catalysis. The data presented herein suggest a mechanism involving two subsequent equilibrium isotope effects in combination with a primary kinetic isotope effect. 

[Mn(bpy)(CO) 3 Br] is recognized as a benchmark electrocatalyst for CO 2 reduction to CO, with the doubly reduced [Mn(bpy)(CO) 3 ] − proposed to be the active species in the catalytic mechanism. The reaction of this intermediate with CO 2 and two protons is expected to produce the tetracarbonyl cation, [Mn(bpy)(CO) 4 ] + , thereby closing the catalytic cycle. However, this species has not been experimentally observed. In this study, [Mn(bpy)(CO) 4 ][SbF 6 ] ( 1 ) was directly synthesized and found to be an efficient electrocatalyst for the reduction of CO 2 to CO in the presence of H 2 O. Complex 1 was characterized using X-ray crystallography as well as IR and UV-Vis spectroscopy. The redox activity of 1 was determined using cyclic voltammetry and compared with that of benchmark manganese complexes, e.g. , [Mn(bpy)(CO) 3 Br] ( 2 ) and [Mn(bpy)(CO) 3 (MeCN)][PF 6 ] ( 3 ). Infrared spectroscopic analyses indicated that CO dissociation occurs after a single-electron reduction of complex 1 , producing a [Mn(bpy)(CO) 3 (MeCN)] + species. Complex 1 was experimentally verified as both a precatalyst and an on-cycle intermediate in homogeneous Mn-based electrocatalytic CO 2 reduction. 

Manganese( i ) tricarbonyl complexes such as [Mn(bpy)(CO) 3 L] (L = Br, or CN) are known to be electrocatalysts for CO 2 reduction to CO. However, due to their rapid photodegradation under UV and visible light, these monomeric manganese complexes have not been considered as photocatalysts for CO 2 reduction without the use of a photosensitizer. In this paper, we report a cyanide-bridged di-manganese complex, {[Mn(bpy)(CO) 3 ] 2 (μ-CN)}ClO 4 , which is both electrocatalytic and photochemically active for CO 2 reduction to CO. Compared to the [Mn(bpy)(CO) 3 CN] electrocatalyst, our CN-bridged binuclear complex is a more efficient electrocatalyst for CO 2 reduction using H 2 O as a proton source. In addition, we report a photochemical CO 2 reduction to CO using the dimanganese complex under 395 nm irradiation.