Intramolecular Electrostatic Effects on O₂, CO₂, and Acetate Binding to a Cationic Iron Porphyrin

Abstract

Noncovalent electrostatic interactions are important in many biological and chemical reactions, especially those that involve charged intermediates. There has been a growing interest in using electrostatic ligand designsplacing charges in the second coordination sphereto improve molecular reactivity, catalysis, and electrocatalysis. For instance, an iron porphyrin bearing four cationic ortho-trimethylanilinium groups, Fe(o-TMA), has been reported to be an exceptional electrocatalyst for both the carbon dioxide reduction reaction (CO 2 RR) and the oxygen reduction reaction (ORR). These reactions involve many different steps, and it is not evident which steps are affected by the four positive charges, or why. By comparing Fe(o-TMA) with the related iron-tetraphenylporphyrin, this work examines how covalently positioned charged groups affect substrate binding and other key pre-equilibria of both the ORR and CO 2 RR, specifically acetate, dioxygen, and carbon dioxide binding. This study is among the first to directly measure the effects of electrostatics on ligand-binding. The results show that adding electrostatic groups to a catalyst design often results in a complex interplay of multiple effects, including changes in pre-equilibria prior to substrate binding, combinations of through-space and inductive contributions, and effects of ionic strength and solution dielectric. The inverse half-order dependence of binding constant on ionic strength is proposed as a clear marker for an electrostatic effect. The conclusions provide guidance for the increasingly popular electrostatic ligand designs in catalysis and other reactivity.

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

“… 123 A dependence on solvent ionic strength was proposed as a means of separating electrostatic and inductive contributions of a R-NMe 3 + substituent on the rate of acetate binding in iron porphyrins. 34 We reasoned that a similar solvent variation approach would be useful for separating inductive and electrostatic contributions to the donor strength of 1. Specifically, the through-space electrostatic influence of the charged group should be modified by the solvent dielectric ( ε ), a measure of a medium's ability to shield a charge, 124 while the through-bond interactions should remain constant.…”

Improving Electrocatalytic Oxygen Reduction Activity and Selectivity with a Cobalt Corrole Appended with Multiple Positively Charged Proton Relay Sites

“…Molecular catalysts have the benefits of being used for the study of reaction mechanisms and structure–function relationships. − Recent efforts have led to the identification of a variety of coordination complexes of Mn, , Fe, − Co, − Ni, , and Cu − as active ORR catalysts. With the study of these complexes, valuable knowledge to improve the activity and selectivity of the 4e – ORR has been learned, including introducing hydrogen-bonding interactions, − providing rapid electron transfers, − using electron-donating axial ligands, ,, creating sterically protected O 2 -binding sites, − and making cooperative dinuclear sites. − In addition to these design strategies, it was demonstrated by Nocera, , Mayer, , Karlin, , Dey , and others that the efficient proton transfer facilitated by intramolecular proton relays can improve the selectivity for the 4e – ORR. Very recently, Warren and coworkers reported controlling the ORR selectivity by using positively charged groups, which are able to stabilize O 2 -bound intermediates through electrostatic interactions …”

Electrostatic Contributions to the Stability of an End-on Cupric Superoxide

“…7,19 Molecular-scale electrostatic effects are gaining attention for their ability to meaningfully impact the chemistry of organic, transition metal, and enzymatic systems. [22][23][24][25][26][27][28][29][30][31] We recently reported the synthesis of a Cu I complex bound by the novel tris [2-(trimethylphosphiniminato)ethyl]amine ( Me3 P3tren) ligand (1, Scheme 1). 32 The phosphinimine moieties in the secondary coordination sphere of 1 are best described by a zwitterionic resonance picture that results in the development of significant cationic electrostatic potential in the small molecule binding pocket of 1 (Figure 1, bottom).…”