Dimerization of [Fe^III(bpy)₃]³⁺ in Aqueous Solutions: Elucidating a Mechanism Based on Historical Proposals, Electrochemical Data, and Computational Free Energy Analysis

Abstract

Iron­(II) tris-bipyridine, [FeII(bpy)3]2+, is a historically significant organometallic coordination complex with attractive redox and photophysical properties. With respect to energy storage, it is a low-cost, high-redox potential complex and thus attractive for use as a catholyte in aqueous redox flow batteries. Despite these favorable characteristics, its oxidized Fe­(III) form undergoes dimerization to form μ-O-[FeIII(bpy)2(H2O)]2 4+, leading to a dramatic ∼0.7 V decrease during battery discharge. To date, the energetics and complete mechanism of this slow, sequential electrochemical–chemical (EC) process, which includes electron transfer, nucleophilic attack, ligand cleavage, μ-oxo bond formation, and spin state transition, have not been elucidated. Using cyclic voltammetry, redox flow battery data, and density functional theory calculations guided by previously proposed mechanisms, we modeled more than 100 complexes and performed more than 50 geometry scans to resolve the key steps dictating these complex chemical processes. Quantitative free energy surfaces are developed to model the mechanism of dimerization accounting for the spins and identities of any possible Fe­(II), Fe­(III), or Fe­(IV) intermediates. Electrochemical reduction of the dimer regenerates [FeII(bpy)3]2+ in an overall reversible process. Computational electrochemistry interrogates the influence of spin state, coordination environment, and molecular conformation at the electrode–electrolyte interface through a proposed stepwise dimer reduction process. Experimentally, we show that the considerable overpotential associated with this event can be catalytically mitigated with disparate materials, including platinum, copper hexacyanoferrate, and activated carbon. The findings are of fundamental and applied significance and could elevate [FeII(bpy)3]2+ and its derivatives to play a vital role in the burgeoning renewable energy economy.

A High Potential, Low Capacity Fade Rate Iron Complex Posolyte for Aqueous Organic Flow Batteries

“…Although the solubility limit of Fe(Bhmbpy) 3 is higher than 0.5 m in 1 m NaCl solution (Figure 1d), we observed formation of small precipitates at the end of battery cycling (Figure S5e, Supporting Information). We propose that the precipitates are due to the dimerization of the oxidized form of the posolyte [ 46 ] (Scheme 1, route 1). To investigate the structure of the newly formed precipitate during high‐concentration cell cycling, we chemically synthesized the ferric dimer [Fe 2 O(Bhmbpy) 4 Cl 2 ] 2+ (compound 7 ) (see Scheme 1) according to a modified synthetic method [ 36 ] (see Supporting Information for details) and obtained its single‐crystal XRD data (CCDC number: 2165674) for comparison and further analysis (Figure S5d, Tables S3 and S4, Supporting Information).…”

“…Additionally, with longer hold durations, the discharge voltage profile dropped to the constant voltage phase earlier (Figure S8c, Supporting Information), providing the majority of the discharge capacity at the lower constant voltage of 0.5 V, which is due to the formation of ferric dimer of posolyte active species. [46] Notably, the full cell voltage dropped by ≈0.7 V, which is consistent with the potential difference between Fe(bpy) 3 2+/3+ and the newly appearing cathodic peak of synthesized ferric dimer 8 (Figure S8e, Supporting Information, single-crystal data are shown in Tables S5 and S6, Supporting Information, CCDC number: 2165675) shown at 0.27 V versus SHE (Figure S8d, Supporting Information), indicating that dimerization occurs in the fully charged state with longer hold durations. Although dimerization of the tris(bipyridyl)iron complex has previously been reported, [32,46] the self-discharge phenomena has not been identified, given that commonly used characterization techniques comprise cycling with immediate charge/discharge phases without hold time.…”

“…[46] Notably, the full cell voltage dropped by ≈0.7 V, which is consistent with the potential difference between Fe(bpy) 3 2+/3+ and the newly appearing cathodic peak of synthesized ferric dimer 8 (Figure S8e, Supporting Information, single-crystal data are shown in Tables S5 and S6, Supporting Information, CCDC number: 2165675) shown at 0.27 V versus SHE (Figure S8d, Supporting Information), indicating that dimerization occurs in the fully charged state with longer hold durations. Although dimerization of the tris(bipyridyl)iron complex has previously been reported, [32,46] the self-discharge phenomena has not been identified, given that commonly used characterization techniques comprise cycling with immediate charge/discharge phases without hold time. Our results show that at least for redox active compounds with highly positive reduction potentials, the possibility of self-discharge needs to be explored as part of the characterization method.…”

Critical Roles of pH and Activated Carbon on the Speciation and Performance of an Archetypal Organometallic Complex for Aqueous Redox Flow Batteries

“…These parameters were rigorously benchmarked for the iron tris-bipyridine monomer and μ-oxo dimer system with experimental data in our previous publication. 27 As in that work, frequency calculations with geometry optimizations were performed under implicit solvation using the CPCM (water) keyword.…”

“…Indeed, a recent report from the Aziz group demonstrated a [Fe(bpy) 3 ] 2+ derivative that exhibits superlative performance as an aqueous RFB catholyte, aside from its susceptibility to dimerization and ligand dissociation. 29 We consider [Fe(bpy) 3 ] 2+ a model complex for flow battery studies because there is a widespread appeal to aqueous and nonaqueous organometallic complexes based on Fe, 27,28,30 Cr, 31 and Co 32 that use bipyridine-based ligands in homoleptic and heteroleptic (mixed ligand) configurations. 33,34 The polypyridine moiety can serve as a substrate for derivatization to enhance its complexes' solubility, reduction potential, and stability.…”