Comparisons of bpy and phen Ligand Backbones in Cr-Mediated (Co-)Electrocatalytic CO₂ Reduction

Abstract

Due to the rise in atmospheric carbon dioxide (CO2) concentrations, there is a need for the development of new strategies to enhance the selectivity and activity of the electrocatalytic conversion of CO2 to value-added products. The incorporation of redox mediators (RMs) as cocatalysts to enhance the transfer of redox equivalents during catalysis has been gaining more attention in recent years across a variety of small molecule transformations. We have shown that using Cr-centered complexes with sulfone-based RMs leads to an enhancement of CO2 reduction electrocatalysis under protic conditions via an inner-sphere mechanism. In these cocatalytic systems, an oxygen atom of the reduced RM binds to the Cr center to form a key intermediate stabilized by pancake bonding between the reduced aromatic components of the catalyst ligand backbone and the RM. This interaction facilitates the transfer of an electron and accesses a more kinetically favorable reaction pathway. Here, we show that expanding the aromatic character of the ligand backbone of the catalyst as well as the RM can cause a greater enhancement of coelectrocatalytic activity. These results suggest that further activity improvements can be achieved by focusing on the kinetic and thermodynamic parameters which control association between the catalyst and RM.

Co-electrocatalytic CO2reduction mediated by a dibenzophosphole oxide and a chromium complex

“…The observed Faradaic efficiency is comparable to the unmediated 1 + PhOH catalytic system, however, a higher TOF (15 s −1 ) is observed for the PhBPO-mediated system (Table 1). Interestingly, the activity enhancement under co-electrocatalytic conditions using PhBPO as the RM is not as significant as that observed for the DBTD-mediated system 13,14,26 even though a stronger interaction with the Cr center is anticipated, vide infra . PhBPO remains stable during electrolysis experiments as evidenced by the conserved 31 P-NMR spectral feature of the RM at about 32 ppm in pre- and post-electrolysis samples (Fig.…”

“…[22][23][24][25] Under protic conditions we proposed that a pancakebonding (PB) interaction occurred (sharing of two electrons in two p systems) that could be tuned by expanding the aromatic character of the DBTD core. 14,26 Since sulfones are generally poor ligands, with few reports on their coordination chemistry, 27,28 we became interested in how the coordinating strength of the redox mediator might alter the co-electrocatalytic response. Modifying the RM species to a different ligand type could favor the formation of the co-catalytic assembly and increase the observed activity.…”

“…30 Again, this behavior is comparable to the previously reported DBTD-mediated system. 13,14,26 A bulk electrolysis experiment at À2.58 V vs. Fc + /Fc with 1, PhBPO, and PhOH present revealed a Faradaic efficiency for CO (FE CO ) of 101 AE 4% (Table 1 and Fig. S20 and S21, ESI ‡).…”

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

“…Regarding molecular catalysis, there are a few accounts where M x + –phen catalysts surpass M x + –bpy ones thanks to the expanded aromaticity and rigidity of phen. − A case-by-case study to rationalize this feature with a consistent set of examples is out of the scope of this Review.…”

Improving co-electrocatalytic carbon dioxide reduction by optimizing the relative potentials of the redox mediator and catalyst

“…While molecular catalysts for CO 2 reduction have been studied for decades, [5][6][7][8][9][10][11][12][13][14][15][16] the incorporation of redox mediators (RMs) to assist in the transfer of electrons (often with protons) to catalyst active sites is a growing area of interest in the field. 17 While examples of RMs used with catalysts for the reduction of CO 2 are currently limited, [18][19][20][21][22][23] a few additional examples exist for the electrocatalytic conversion of a variety of small molecules. [23][24][25][26][27][28][29][30][31][32][33][34][35] Previously, the co-electrocatalytic reduction of CO 2 with Crcentered catalysts derived from dianionic N 2 O 2 ligand frameworks and sulfone-based RMs has been studied.…”