Reduction-induced CO dissociation by a [Mn(bpy)(CO)₄][SbF₆] complex and its relevance in electrocatalytic CO₂ reduction

[Mn(bpy)(CO)₃Br] is recognized as a benchmark electrocatalyst for CO₂ 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 CO₂ 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][SbF₆] (1) was directly synthesized and found to be an efficient electrocatalyst for the reduction of CO₂ to CO in the presence of H₂O. 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)₃Br] (2) and [Mn(bpy)(CO)₃(MeCN)][PF₆] (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 CO₂ reduction.