Title: Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO₂ Reduction by Manganese Bipyridyl Complexes

Abstract

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)₂bpy vs mes₂bpy ligands) and Lewis basic ((Ph)₂bpy vs (MeOPh)₂bpy vs (MeSPh)₂bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO₂ reduction mechanisms with [Mn^I(R₂bpy)(CO)₃(CH₃CN)]⁺ precatalysts, and on their respective transition-state geometries/energies for rate-determining C–OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)₂bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO₂ binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mes2bpy-based precatalysts. Cyclic voltammetry conducted in CO₂-saturated electrolyte for the (Ph)₂bpy-based precatalyst [2-CH₃CN]⁺ confirms that CO₂ binding occurs at the two-electron-reduced activated catalyst [2]^– in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO₂ binding. This observation is supported by computed free energies of the parent–child reaction for [Mn–Mn]^o dimer formation, where increased steric hindrance relative to the bpy-based precatalyst correlates with favorable CO₂ binding. A critical balance must be adhered to, however, as the absence of steric bulk in the bpy-based precatalyst [1-CH₃CN]⁺ maintains a lower overpotential thanmore » [2-CH₃CN]⁺ at the protonation-first pathway with comparable kinetic performance, whereas an ~2-fold greater TOF_max is observed at its reduction-first pathway with an almost identical overpotential as [2-CH₃CN]⁺. Notably, excessive steric bulk in the mes2bpy-based precatalyst [3-CH₃CN]⁺ results in increased activation free energies of the C–OH bond cleavage transition states for both the protonation-first and the reduction-first pathways relative to both [1-CH₃CN]⁺ and [2-CH₃CN]⁺. In fact, [3-CH₃CN]⁺ requires a 1 V window beyond its onset potential to reach its peak catalytic current, which is in contrast to the narrower (<0.30 V) potential response window of the remaining catalysts here studied. Furthermore, voltammetry recorded under 1 atm of CO₂ with 2.8 M (5%) H₂O establishes [2-CH₃CN]⁺ to have the lowest overpotential (η = 0.75 V) in the series here studied, attributed to its ability to lie “on the fence” when providing sufficient steric bulk to hinder (but not prevent) [Mn–Mn]^o dimerization, while simultaneously having a limited steric impact on the free energy of activation for the rate-determining C–OH bond cleavage transition state. While the methoxyphenyl bpy-based precatalyst [4-CH₃CN]⁺ possesses an increased steric presence relative to [2-CH₃CN]⁺, this is offset by its capacity to stabilize the C–OH bond cleavage transition states of both the protonation-first and the reduction-first pathways by facilitating second coordination sphere H-bonding stabilization.« less

Authors:

Blasczak, Vanna ^[1]; McKinnon, Meaghan ^[1]; Suntrup, Lisa ^[1]; Aminudin, Nur Alisa ^[1]; Reed, Blake ^[2]; Groysman, Stanislav  ^[2];

• Search DOE PAGES for author "Groysman, Stanislav"
• Search DOE PAGES for ORCID "0000-0003-3578-7985"

Ertem, Mehmed Z.  ^[3];

• Search DOE PAGES for author "Ertem, Mehmed Z."
• Search DOE PAGES for ORCID "0000-0003-1994-9024"

Grills, David C.  ^[3];

• Search DOE PAGES for author "Grills, David C."
• Search DOE PAGES for ORCID "0000-0001-8349-9158"

Rochford, Jonathan  ^[1]

• Search DOE PAGES for author "Rochford, Jonathan"
• Search DOE PAGES for ORCID "0000-0003-2397-9162"

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+ Show Author Affiliations

1. Univ. of Massachusetts, Boston, MA (United States)
2. Wayne State Univ., Detroit, MI (United States)
3. Brookhaven National Lab. (BNL), Upton, NY (United States)

Publication Date:

Mon Sep 26 00:00:00 EDT 2022

Research Org.:

Brookhaven National Laboratory (BNL), Upton, NY (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)

OSTI Identifier:

1897506

Report Number(s):

BNL-223671-2022-JAAM
Journal ID: ISSN 0020-1669

Grant/Contract Number:  

SC0012704; CHE-1800062; CHE-1855681

Resource Type:

Accepted Manuscript

Journal Name:

Inorganic Chemistry

Additional Journal Information:

Journal Volume: 61; Journal Issue: 40; Journal ID: ISSN 0020-1669

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; Manganese; CO2 reduction; second coordination sphere; electrocatalysis; proton-coupled electron transfer; pulse radiolysis