Unprecedented Isomerism–Activity Relation in Molecular Electrocatalysis

Abstract

The role of electrocatalysts in energy storage/conversion, biomedical and environmental sectors, green chemistry, and much more has generated enormous interest in comprehending their structure–activity relations. While targeting the surface-to-volume ratio, exposing reactive crystal planes and interfacial modifications are time-tested considerations for activating metallic catalysts; it is primarily by substitution in molecular electrocatalysts. This account draws the distinction between a substituent’s chemical identity and isomerism, when regioisomerism of the −NO2 substituent is conferred at the “α” and “β” positions on the macrocycle of cobalt phthalocyanines. Spectroscopic analysis and theoretical calculations establish that the β isomer accumulates catalytically active intermediates via a cumulative influence of inductive and resonance effects. However, the field effect in the α isomer restricts this activation due to a vanishing resonance effect. The demonstration of the distinct role of isomerism in substituted molecular electrocatalysts for reactions ranging from energy conversion to biosensing highlights that isomerism of the substituents makes an independent contribution to electrocatalysis over its chemical identity.

Isomerism‐Activity Relation in Molecular Electrocatalysis: A Perspective

“…Furthermore, to have an in-depth understanding of isomerism-activity relation we resorted to spectroscopic analysis and detailed theoretical calculations. Spectroscopic analyses mainly revealed a kind of oxidative activation of the catalytic metal centre and comparatively extensive electronic delocalization in the β isomer compared to the α isomer [21]. To understand the root of this, we performed DFT calculations which predicted that when the -NO 2 substituent resides at the β position of the N 4 macrocycle, it forces the entire molecule to be planar, which in turn allows oxidative activation of the catalytic metal center via the cumulative influence of electron withdrawing inductive (À I) and electron withdrawing resonance effects (À R).…”

“…To understand the root of this, we performed DFT calculations which predicted that when the -NO 2 substituent resides at the β position of the N 4 macrocycle, it forces the entire molecule to be planar, which in turn allows oxidative activation of the catalytic metal center via the cumulative influence of electron withdrawing inductive (À I) and electron withdrawing resonance effects (À R). On the other hand, the field-effect originating from the lone pair-lone pair electronic repulsion between the secondary N atom (at the meso position) and the oxygen atom of À NO 2 group in the α isomer prompts the -NO 2 plane to cascade away from the N 4macrocyclic plane [21]. This subsequently restricts the electron delocalization between the N 4 -macrocycle and À NO 2 group leading to a diminishing resonance effect in the α isomer which in turn limits the oxidative activation of the catalytic metal centre.…”

“…We note that this investigation was specific to regioisomers of substituted metal phthalocyanines (MPcs) when regioisomerism of substituents are effected at the α and β positions on the N 4 macrocycle of the molecule (Figure 1). We have shown that isomerism of the functionality impacts the electronic arrangement and electrocatalytic activity and hence makes a distinct contribution to molecular electrocatalysis over the contribution of its chemical identity [21,22].…”

“…The existing accounts fail to attach any significance to the latter aspect i. e., the role of isomerism of functionality in molecular electrocatalysis and to the best of our knowledge, no literature has been found that paid attention to this isomerism-activity relation. Our group recently shed some light in this direction where a distinction is made between substituent's chemical identity and regioisomerism towards elec-trocatalysis [21,22]. We note that this investigation was specific to regioisomers of substituted metal phthalocyanines (MPcs) when regioisomerism of substituents are effected at the α and β positions on the N 4 macrocycle of the molecule (Figure 1).…”

“…Cyclic voltammograms recorded with unsubstituted CoPc and αand β-NO 2 isomers at a scan rate of 10 mV/s in the presence of 1 mM concentrations of (b) arsenite (As(III)) in phosphate buffer (pH-8), (c) ascorbic acid in acetate buffer (pH-2), (d) uric acid in acetate buffer (pH = 4), (e) hydrogen peroxide in phosphate buffer (pH-7), and (f) L-cysteine in acetate buffer (pH-4). Reproduced with permission from The American Chemical Society[21].…”