Outer-coordination sphere in multi-H+/multi-e–molecular electrocatalysis

Abstract

Electrocatalysis is an indispensable technique for small-molecule transformations, which are essential for the sustainability of society. Electrocatalysis utilizes electricity as an energy source for chemical reactions. Hydrogen is considered the ''fuel for the future,'' and designing electrocatalysts for hydrogen production has thus become critical. Furthermore, fuel cells are promising energy solutions that require robust electrocatalysts for key fuel cell reactions such as the interconversion of oxygen to water. Concerns regarding the rising concentration of atmospheric carbon dioxide have prompted the search for CO 2 conversion methods. One promising approach is the electrochemical conversion of CO 2 into commodity chemicals and/or liquid fuels, but such chemistry is highly energy demanding because of the thermodynamic stability of CO 2 . All of the above-mentioned electrocatalytic processes rely on the selective input of multiple protons (H + ) and electrons (e -) to yield the desired products. Biological enzymes evolved in nature to perform such redox catalysis and have inspired the design of catalysts at the molecular and atomic levels. While it is synthetically challenging to mimic the exact biological environment, incorporating functional outer coordination spheres into molecular catalysts has shown promise for advancing multi-H + and multi-eelectrocatalysis. From this Perspective, herein, catalysts with outer coordination sphere(s) are selected as the inspiration for developing new catalysts, particularly for the reductive conversion of H + , O 2 , and CO 2 , which are highly relevant to sustainability. The recent progress in electrocatalysis and opportunities to explore beyond the second coordination sphere are also emphasized.

Scaling Relation between the Reduction Potential of Copper Catalysts and the Turnover Frequency for the Oxygen and Hydrogen Peroxide Reduction Reactions

“…This noncoordinated pyridine moiety could interact with protons present in intermediate species during the ORR and HPRR reactions. Proton shuttles in the second coordination sphere have led to a significant increase in turnover frequency (TOF) for many catalytic systems. , Given that no H + transfer is involved in the rate-determining step of the Cu-mediated ORR, we do not anticipate a significant effect on the catalytic rate here. , The third complex, [Cu­(fubmpa)­(H 2 O)­(OTf) 2 ] (Cu-fubmpa; fubmpa = N -(furan-2-ylmethyl)- N -[bis­(2-pyridyl)­methyl]­amine), was designed as an analogue of the copper complex [Cu­(bmpa)­(L)] 2+ (bmpa = bis­(2-pyridylmethyl)­amine), by introduction of the noncoordinating furanyl moiety while maintaining the nature of the central tertiary amine. We have selected these complexes because their E 1/2 values range between 0.2 and 0.5 V vs reversible hydrogen electrode (RHE), and, as far as we could observe, no limitations in electron transfer rates occurred within this selected series of tmpa modifications (see below), as opposed to our previous observations in the case of rigid terpyridine catalysts …”

“…Proton shuttles in the second coordination sphere have led to a significant increase in turnover frequency (TOF) for many catalytic systems. 37,38 Given that no H + transfer is involved in the rate-determining step of the Cu-mediated ORR, we do not anticipate a significant effect on the catalytic rate here. 22,23 The third complex, [Cu(fubmpa (2-pyridyl)methyl]amine), was designed as an analogue of the copper complex [Cu(bmpa)-(L)] 2+ (bmpa = bis(2-pyridylmethyl)amine), 39 by introduction of the noncoordinating furanyl moiety while maintaining the nature of the central tertiary amine.…”

Main group elements in electrochemical hydrogen evolution and carbon dioxide reduction

“…Finding robust and efficient CO 2 RR catalysts has been of great interest, owing to their ability to mitigate atmospheric CO 2 . [9][10][11] However, most catalyst designs rely on transition metals. In 2010, Me ´nard and Stephen reported that CO 2 could be reduced to methanol (CH 3 OH) using an (Mes) 3 P(CO 2 )(AlX 3 ) 2 complex, where Mes is a mesitylene group and X is a bromide or chloride.…”

Nature‐Inspired Electrocatalysts for CO2 Reduction to C2+ Products

“…How-ever, while this approach has been successful for C 1 products, few examples exist to inform the design of electrocatalysts for C 2+ products. [48,52,53] One such example is the use of a molecular copper-porphyrin complex called copper(II)-5,10,15,20tetrakis(2,6-dihydroxyphenyl)porphyrin (PorCu), which orients functional groups toward the metal active site, resulting in excellent activity and selectivity for CO 2 RR to hydrocarbons in neutral aqueous media (Figure 3, example 1). [54] When operated at -0.976 V vs the reversible hydrogen electrode (RHE), PorCu catalyst exhibited a FE of ≈17% for ethylene production from CO 2 in a CO 2 -saturated potassium bicarbonate (KHCO 3 ) electrolyte, corresponding to a turnover frequency (TOF) of 1.8 molecules per site per second for ethylene, which is attributed to the reactivity of the Cu(I) metal center and the presence of built-in hydroxyl (OH) group in the porphyrin structure.…”