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Spin crossover (SCO) is one of the most widely studied phenomena in transition metal complexes, in part due to the possible technological applications of such species in molecular electronics, memory storage, electrochromics, and display devices. Any transition metal complex with a d4–d7electron configuration, such as 2+ and 3+ ions of Cr, Mn, Fe, and Co, can theoretically exhibit SCO effects which depend highly on the coordination environment and ligand field strength. Redox-coupled SCO processes provide a mechanism to enhance the molecular bistability and trigger transitions between low- and high-spin states. Similar reaction mechanisms often occur in electrocatalytic and enzymatic reactions. The electrochemical response of redox-coupled SCO processes can be modeled as a square scheme containing two one-electron reactions and two chemical steps, resulting in a separation of anodic and cathodic waves. Herein we describe the influence of ligand coordination and structural changes on the redox-coupled SCO properties of several Co complexes. This structural-functional analysis allows us to understand the fundamental properties that control redox-coupled SCO processes and optimize redox bistability for a range of applications.
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Electronic Structure Distortions in Chromium Chelates Impair Redox Kinetics in Flow Batteries
Aminopolycarboxylate chelates are emerging as a promising class of electrolyte materials for aqueous redox flow batteries, offering tunable redox potentials, solubility, and pH stability through careful selection of ligands and transition metal ions. Despite their potential, the impact of molecular structure modifications on the electronic and electrochemical properties of these chelates remains underexplored. Here, we examine how introducing a hydroxyl group, often employed for its solubilizing properties, to the backbone of CrPDTA, a reference chelate material, significantly changes the thermodynamics and kinetics of the chelate’s redox process. We correlate changes in molecular and electronic structures to different electrochemical responses resulting from the hydroxyl addition and show that the introduction of this functional group leads to a distortion in the octahedral coordination of chromium. Furthermore, increased anisotropic spin density and non‐integral oxidation state changes in the Cr metal center result in a larger barrier for electron transfer in CrPDTA‐OH. We demonstrate that preserving a hexacoordinate chelate structure across a broad pH range is crucial for efficient flow battery application and emphasize that ligand modifications aimed at enhancing solubility must avoid distorting the octahedral coordination of the transition metal.
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- Award ID(s):
- 2500450
- PAR ID:
- 10585910
- Publisher / Repository:
- European Chemical Societies Publishing
- Date Published:
- Journal Name:
- Batteries & Supercaps
- ISSN:
- 2566-6223
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/batt.202500250
