Issue 36, 2020

Redox mediators accelerate electrochemically-driven solubility cycling of molecular transition metal complexes

Abstract

The solubility of molecular transition metal complexes can vary widely across different redox states, leaving these compounds vulnerable to electron transfer-initiated heterogenization processes in which oxidation or reduction of the soluble form of the redox couple generates insoluble molecular deposits. These insoluble species can precipitate as suspended nanoparticles in solution or, under electrochemical conditions, as an electrode-adsorbed material. While this electrochemically-driven solubility cycling is technically reversible, the reverse electron transfer to regenerate the soluble redox couple state is a practical challenge if sluggish electron transfer kinetics result in a loss of electronic communication between the molecular deposits and the electrode. In this work, we present an example of this electrochemically-driven solubility cycling, report a novel strategy for catalytically enhancing the oxidation of the insoluble material using homogeneous redox mediators, and develop the theoretical framework for analysing and digitally simulating the action of a homogeneous catalyst on a heterogeneous substrate via cyclic voltammetry. First, a mix of electrochemical and spectroscopic methods are used to characterize an example of this electrochemically-driven solubility cycling which is based on the two-electron reduction of homogeneous [Ni(PPh2NPh2)2(CH3CN)]2+ (PPh2NPh2 = 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane). The limited solubility of the doubly-reduced product in acetonitrile leads to precipitation and deposition of molecular [Ni(PPh2NPh2)2]. While direct oxidation of this heterogeneous [Ni(PPh2NPh2)2] at the electrode surface is possible, this electron transfer is kinetically limited. We demonstrate how a freely diffusing redox mediator (ferrocene) – which shuttles electrons between the electrode and the molecular material – can be used to overcome these slow electron transfer kinetics, enabling catalytic regeneration of soluble [Ni(PPh2NPh2)2]2+. Finally, mathematical models are developed that describe the current–potential response for a generic EC′ mechanism involving a homogeneous catalyst and surface-adsorbed substrate. This novel strategy has the potential to enable reversible redox chemistry for heterogeneous, molecular deposits that are adsorbed on the electrode or suspended as nanoparticles in solution.

Graphical abstract: Redox mediators accelerate electrochemically-driven solubility cycling of molecular transition metal complexes

Supplementary files

Article information

Article type
Edge Article
Submitted
07 may 2020
Accepted
31 avq 2020
First published
10 sen 2020
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2020,11, 9836-9851

Redox mediators accelerate electrochemically-driven solubility cycling of molecular transition metal complexes

K. J. Lee, K. M. Lodaya, C. T. Gruninger, E. S. Rountree and J. L. Dempsey, Chem. Sci., 2020, 11, 9836 DOI: 10.1039/D0SC02592E

This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. You can use material from this article in other publications, without requesting further permission from the RSC, provided that the correct acknowledgement is given and it is not used for commercial purposes.

To request permission to reproduce material from this article in a commercial publication, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party commercial publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements