Turning on the Protonation-First Pathway for Electrocatalytic CO₂ Reduction by Manganese Bipyridyl Tricarbonyl Complexes

Abstract

Electrocatalytic reduction of CO to CO is reported for the complex, {fac-Mn([(MeO)Ph]bpy)(CO)(CHCN)}(OTf), containing four pendant methoxy groups, where [(MeO)Ph]bpy = 6,6'-bis(2,6-dimethoxyphenyl)-2,2'-bipyridine. In addition to a steric influence similar to that previously established [Sampson, M. D. et al. J. Am. Chem. Soc. 2014, 136, 5460-5471] for the 6,6'-dimesityl-2,2'-bipyridine ligand in [fac-Mn(mesbpy)(CO)(CHCN)](OTf), which prevents Mn-Mn dimerization, the [(MeO)Ph]bpy ligand introduces an additional electronic influence combined with a weak allosteric hydrogen-bonding interaction that significantly lowers the activation barrier for C-OH bond cleavage from the metallocarboxylic acid intermediate. This provides access to the thus far elusive protonation-first pathway, minimizing the required overpotential for electrocatalytic CO to CO conversion by Mn(I) polypyridyl catalysts, while concurrently maintaining a respectable turnover frequency. Comprehensive electrochemical and computational studies here confirm the positive influence of the [(MeO)Ph]bpy ligand framework on electrocatalytic CO reduction and its dependence upon the concentration and pK of the external Brønsted acid proton source (water, methanol, trifluoroethanol, and phenol) that is required for this class of manganese catalyst. Linear sweep voltammetry studies show that both phenol and trifluoroethanol as proton sources exhibit the largest protonation-first catalytic currents in combination with {fac-Mn([(MeO)Ph]bpy)(CO)(CHCN)}(OTf), saving up to 0.55 V in overpotential with respect to the thermodynamically demanding reduction-first pathway, while bulk electrolysis studies confirm a high product selectivity for CO formation. To gain further insight into catalyst activation, time-resolved infrared (TRIR) spectroscopy combined with pulse-radiolysis (PR-TRIR), infrared spectroelectrochemistry, and density functional theory calculations were used to establish the v(CO) stretching frequencies and energetics of key redox intermediates relevant to catalyst activation.

Solvent and Ligand Substitution Effects on the Electrocatalytic Reduction of CO2 with [Mo(CO)4(x,x′‐dimethyl‐2,2′‐bipyridine)] (x=4–6) Enhanced at a Gold Cathodic Surface

“…Currently, much of the literature regarding molecular electrochemical activation of CO 2 still continues to explore the catalytic properties of the heavier members of Group‐7 (Re), Group‐8 (Ru and Os) and Group‐9 (Rh, and Ir) triads. These “first‐generation” electrocatalysts must be phased out; encouragingly, cheaper alternatives exist within the same groups – Mn substituting Re, Fe substituting Ru and Os, and Co substituting Rh and Ir . However, the community's focus on late transition metals is leaving the Group‐6 metals (Cr, Mo, W) rather neglected, despite the high potential for the comparatively abundant metal triad to function as both high‐performance alternatives to the noble metals and photostable alternatives to the hot‐topic manganese carbonyls .…”

Recent Trends, Benchmarking, and Challenges of Electrochemical Reduction of CO2 by Molecular Catalysts

“…2019, 9,1900090 Figure 32. [90][91][92][93][94][95][96] While electroreduction of CO 2 involving copper has mainly focused on multielectron transfers at copper electrodes to produce products such as methanol (6 electron reduced) as compared to CO and HCOOH (2 electron reduced), Cu molecular catalysts have also been researched in a heterogeneous form to produce similar products. HER at pH = 1 (black curve), pH = 2 (red curve) and pH = 3 (blue curve) on Co protoporphyrin-modified PG electrode in the absence of CO 2 .…”

“…[32,92,[105][106][107] Work by Sampson et al pushed the boundaries of manganese bipyridine aided electro-reduction; the team synthesized, for the first time in reported literature, Mn(mesbpy)-(CO) 3 Br and [Mn(mesbpy)(CO) 3 (MeCN)]-(OTf) molecular catalysts (Figure 40) (mesbpy = Br6,6′-dimesityl-2,2′-bipyridine). [32,92,[105][106][107] Work by Sampson et al pushed the boundaries of manganese bipyridine aided electro-reduction; the team synthesized, for the first time in reported literature, Mn(mesbpy)-(CO) 3 Br and [Mn(mesbpy)(CO) 3 (MeCN)]-(OTf) molecular catalysts (Figure 40) (mesbpy = Br6,6′-dimesityl-2,2′-bipyridine).…”

RhI‐Catalyzed PIII‐Directed C−H Bond Alkylation: Design of Multifunctional Phosphines for Carboxylation of Aryl Bromides with Carbon Dioxide

“…[7] This strategy has been recently applied for the reduction of CO 2 using metal catalysts with nitrogen-based ligands bearing af unctional group (amine,a mide,e ther, imidazolium, phenol, or thiourea) that is able to activate the CO 2 or stabilize intermediates via secondary-coordinationsphere interactions. [8] However,t he preparation of phosphines with pendant functional groups remains challenging owing to their synthetic schemes,w hich rely on multistep synthesis with polar reagents (i.e., Grignard or organolithium).…”