Templating Bicarbonate in the Second Coordination Sphere Enhances Electrochemical CO2 Reduction Catalyzed by Iron Porphyrins

Abstract

Bicarbonate-based electrolytes are ubiquitous in aqueous electrochemical CO2 reduction, particularly in heterogenous catalysis, where they demonstrate improved catalytic performance relative to other buffers. In contrast, the presence of bicarbonate in organic electrolytes and its roles in homogenous electrocatalysis remain underexplored. Here, we investigate the influence of bicarbonate on iron porphyrin-catalyzed electrochemical CO2 reduction. We show that bicarbonate is a viable proton donor in organic electrolyte (pKa = 20.8 in DMSO) and that urea pendants in the second coordination sphere can be used to template bicarbonate in the vicinity of a molecular iron porphyrin catalyst. The templated binding of bicarbonate decreases its acidity, resulting in a 1500-fold enhancement in catalytic rates relative to unmodified parent iron porphyrin. This work emphasizes the importance of bicarbonate speciation in wet organic electrolytes and establishes second-sphere bicarbonate templating as a design strategy to harness this adventitious acid and enhance CO2 reduction catalysis.

CO2 Reduction by an Iron(I) Porphyrinate System: Effect of Hydrogen Bonding on the Second Coordination Sphere

“…45 The optimized geometries compare well to those calculated before using the B97-D approach with strong hydrogen-bonding interactions between bicarbonate and the two NH groups of the urea substituent and the oxygen atom of CO 2 . 36 The porphyrin scaffold is close to planarity, similar to previous calculations on porphyrin models in the gas phase. 46−50 In model III, the bicarbonate ion is locked in position through two hydrogen-bonding interactions with the urea N− H groups, while its OH group forms a hydrogen bond to the CO 2 group.…”

“…Our work, therefore, is in good quantitative agreement with the experimental work of Chang et al, which showed enhanced reactivity by 2-fold between the two complexes with either o-urea or o-2-amide in the second coordination sphere. 36 Overall, the calculations presented in this work indicate that an o-urea substituent to a TPP scaffold enables strong hydrogen-bonding interactions that hold and position distal ligands to the iron center better than o-2-amide. To fully understand the positional differences, we created an overlay of the 3 C II and 3 C III optimized geometries and present the results in Figure 4.…”

“…35 Recent work of Chang and co-workers used a system with a urea group linked to one of the mesophenyl groups. 36 In combination with bicarbonate as the proton donor, the authors measured a 1500-fold rate enhancement for CO 2 reduction compared to that of a system without bicarbonate. They also determined a crystal structure of the bicarbonate-bound o-urea-TPP with Zn 2+ as the central ion.…”

Force Decomposition Analysis: A method to decompose intermolecular forces into physically relevant component contributions

“…Examples include red-or blue-shifts in vibrational frequencies when forming hydrogen bonds, [1][2][3][4][5][6][7] wavelength tuning of organic chromophores by the solvation or protein environment, [8][9][10][11][12][13][14] and modulation of the catalytic performance of molecular CO2RR catalysts through interactions with ligands in complexes' second coordination sphere. [15][16][17][18][19][20][21][22] Decomposing the non-covalent interactions has been increasingly important to understanding the origins of these interactions as well as the development of classical force fields for the simulation of chemical and biochemical systems. [23][24][25][26][27][28][29][30][31] Moreover, to obtain statistical mechanical ensembles of a condensed phase chemical system, molecular dynamics simulations are required, for which accurate and efficient evaluation of intermolecular forces is important for systems where quantum chemical calculations are impractical.…”