Experimental Redox Potentials Research Articles

Experimental and Theoretical Predictors for Redox Potentials of Bispyridinylidene Electron Donors.

Bispyridinylidenes are neutral organic molecules capable of two-electron oxidation at a range of redox potentials that are widely tunable by choice of substituent, making them attractive as homogeneous organic reductants and active materials in redox flow batteries. In an effort to readily predict the redox potentials of this important class of compounds, we have developed correlations between the experimental redox potentials and both experimental and theoretical predictors. On the experimental side, we show that multinuclear NMR chemical shifts of related pyridinium ions correlate well with the redox potentials of bispyridinylidenes, with R2 and standard errors (S) reaching 0.9810 and 0.048 V, respectively, when the 13C (N-CH3) and 1H (ortho) chemical shifts are used together. Theoretical studies of the bispyridinylidenes and their doubly oxidized bipyridinium ions gave a range of predictively valuable equations at various levels of computational cost. This ranged from a simple model using only the EHOMO of the bispyridinylidenes (R2=0.9689; S=0.060 V), to a more computationally intensive model which include solvation effects for both redox states which gave the highest predictive value for all methods (R2=0.9958; S=0.022 V). This work will guide further studies of this important class of molecules.

Exploring the Reactivity of Flavins with Nucleophiles Using a Theoretical and Experimental Approach.

Covalent adducts of flavin cofactors with nucleophiles play an important role in non-canonical function of flavoenzymes as well as in flavin-based catalysis. Herein, the interaction of flavin derivatives including substituted flavins (isoalloxazines), 1,10-ethylene-bridged flavinium salts, and non-substituted alloxazine and deazaflavin with selected nucleophiles was investigated using an experimental and computational approach. Triphenylphosphine or trimethylphosphine, 1-nitroethan-1-ide, and methoxide were selected as representatives of neutral soft, anionic soft, and hard nucleophiles, respectively. The interactions were investigated using UV/Vis and 1H NMR spectroscopy as well as by DFT calculations. The position of nucleophilic attack estimated using the calculated Gibbs free energy values was found to correspond with the experimental data, favouring the addition of phosphine and 1-nitroethan-1-ide into position N(5) and methoxide into position C(10a) of 1,10-ethylene-bridged flavinium salts. The calculated Gibbs free energy values were found to correlate with the experimental redox potentials of the flavin derivatives tested. These findings can be utilized as valuable tools for the design of artificial flavin-based catalytic systems or investigating the mechanism of flavoenzymes.

Unraveling the Effect of the Chemical and Structural Composition of ZnxNi1−xFe2O4 on the Electron Transfer at the Electrochemical Interface

In order to deepen the understanding of the role of transition metal oxides in electron transfer at the electrochemical interface, the performance of Zn x Ni1−x Fe2O4 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) nanomaterials in electrochemical sensing is studied. Nanomaterials are synthesized by simple autocombustion synthesis procedure. Field‐emission scanning electron microscopy characterization shows that the particles have a size between 30 and 70 nm with an average crystallite size between 24 and 35 nm. The bandgap energies of the nanomaterials, as estimated by UV–vis experiments, are in the 2.32–2.56 eV range. The valence band maximum is evaluated using X‐ray photoelectron spectroscopy and the position of the conduction band minimum is estimated. The ZnFe2O4 sensor has the best performances: highest rate constant (13.1 ± 2.8 ms−1), lowest peak‐to‐peak separation (386 ± 2 mV), and highest sensitivity (37.75 ± 0.17 μA mM−1). Its limit of detection (7.94 ± 0.04 μM) is second best, and its sensitivity is more than twice the sensitivity of the bare sensor (16.7 ± 0.9 μA mM−1). Nanomaterials energy bands mapping with the experimental redox potentials is performed to predict the electron transfer at the electrochemical interface, and the importance of surface states/defects is highlighted in the electron transfer mechanism.

Reducing Systematic Uncertainty in Computed Redox Potentials for Aqueous Transition-Metal-Substituted Polyoxotungstates.

Polyoxometalates have attracted significant interest owing to their structural diversity, redox stability, and functionality at the nanoscale. In this work, density functional theory calculations have been employed to systematically study the accuracy of various exchange-correlation functionals in reproducing experimental redox potentials, U0Red in [PW11M(H2O)O39]q- M = Mn(III/II), Fe(III/II), Co(III/II), and Ru(III/II). U0Red calculations for [PW11M(H2O)O39]q- were calculated using a conductor-like screening model to neutralize the charge in the cluster. We explicitly located K+ counterions which induced positive shifting of potentials by > 500 mV. This approximation improved the reproduction of redox potentials for Kx[XW11M(H2O)O39]q-x M = Mn(III/II)/Co(III/II). However, uncertainties in U0Red for Kx[PW11M(H2O)O39]q-x M = Fe(III/II)/Ru(III/II) were observed because of the over-stabilization of the ion-pairs. Hybrid functionals exceeding 25% Hartree-Fock exchange are not recommended because of large uncertainties in ΔU0Red attributed to exaggerated proximity of the ion-pairs. Our results emphasize that understanding the nature of the electrode and electrolyte environment is essential to obtain a reasonable agreement between theoretical and experimental results.

Quantum Mechanical Calculations of Redox Potentials of the Metal Clusters in Nitrogenase.

We have calculated redox potentials of the two metal clusters in Mo-nitrogenase with quantum mechanical (QM) calculations. We employ an approach calibrated for iron-sulfur clusters with 1-4 Fe ions, involving QM-cluster calculations in continuum solvent and large QM systems (400-500 atoms), based on structures from combined QM and molecular mechanics (QM/MM) geometry optimisations. Calculations on the P-cluster show that we can reproduce the experimental redox potentials within 0.33 V. This is similar to the accuracy obtained for the smaller clusters, although two of the redox reactions involve also proton transfer. The calculated P1+/PN redox potential is nearly the same independently of whether P1+ is protonated or deprotonated, explaining why redox titrations do not show any pH dependence. For the FeMo cluster, the calculations clearly show that the formal oxidation state of the cluster in the resting E0 state is MoIIIFe3IIFe4III , in agreement with previous experimental studies and QM calculations. Moreover, the redox potentials of the first five E0-E4 states are nearly constant, as is expected if the electrons are delivered by the same site (the P-cluster). However, the redox potentials are insensitive to the formal oxidation states of the Fe ion (i.e., whether the added protons bind to sulfide or Fe ions). Finally, we show that the later (E4-E8) states of the reaction mechanism have redox potential that are more positive (i.e., more exothermic) than that of the E0/E1 couple.

Synthesis, Structure and Photochemistry of Dibenzylidenecyclobutanones.

A series of symmetrical dibenzylidene derivatives of cyclobutanone were synthesized with the goal of studying the physicochemical properties of cross-conjugated dienones (ketocyanine dyes). The structures of the products were established and studied by X-ray diffraction and by NMR and electronic spectroscopy. All the products had E,E-geometry. The oxidation and reduction potentials of the dienones were determined by cyclic voltammetry. The potentials were shown to depend on the nature, position, and number of substituents in the benzene rings. A linear correlation was found between the difference of the electrochemical oxidation and reduction potentials and the energy of the long-wavelength absorption maximum. This correlation can be employed to analyze the properties of other compounds of this type. Quantum chemistry was used to explain the observed regularities in the electrochemistry, absorption, and fluorescence of the dyes. The results are in good agreement with the experimental redox potentials and spectroscopy data.

Series of Fluorinated Benzimidazole-Substituted Nitronyl Nitroxides: Synthesis, Structure, Acidity, Redox Properties, and Magnetostructural Correlations.

A special series of nitronyl nitroxides was synthesized: 2-(benzimidazol-2'-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyls mono-, di-, tri-, or tetrafluorinated on the benzene ring. The structure of all paramagnets was unambiguously confirmed by single-crystal X-ray diffraction. It was found that in crystals, the radicals are assembled into chains due to intermolecular H-bonds between the benzimidazole moiety (H-bond donor) and the nitronyl nitroxide group or benzimidazole ring (H-bond acceptor). The magnetic properties of nitronyl nitroxides depend on the type of binding of radicals by H-bonds. The magnetic motif of 4-fluoro-, 5-fluoro-, 4,6-difluoro-, 4,5,6-trifluoro-, 4,5,7-trifluoro-, and 4,5,6,7-tetrafluoro-derivatives, as well as the nonfluorinated compound, consists of ferromagnetic chains (J/kB ≈ 20-40 K) formed by the McConnell type I mechanism. In the 5,6-difluoro- and 4,5-difluoro-derivatives, the distances between the paramagnetic centers are large, as a result of which the exchange interactions are weak. According to cyclic voltammetry, paramagnets are oxidized reversibly, while their reduction is a quasi-reversible electron transfer (EC mechanism); experimental redox potentials of radicals correlate well with the calculated values. Quantum chemical assessment of the acidity of benzimidazolyl-substituted nitronyl nitroxides revealed that the introduction of fluorine atoms into the benzene ring enhances the acidity of the paramagnets by more than 5 orders of magnitude.

Calculations of pH-Dependent Redox Potentials for Proton-Coupled Electron Transfer Reactions

We will present a computational protocol that combines density functional theory (DFT) and cheminformatics approaches to predict redox potentials of organic molecules as a function of the pH of the aqueous solution.[1] The methodology is based on an improved version of the Alberty-Legendre transform to account for varying protonation states of the redox reaction products at different pH values. Our approach circumvents the known problems with the use of DFT calculations and implicit solvation models for highly charged anions. To validate the protocol, we use a consistent set of quinones which can undergo up to two-electron two-proton redox reaction. As the reference we use the experimental redox potentials measured at pH 0, 7, and 13.[2] We demonstrate that with the proposed methodology we can achieve the MAE=0.144 V and MSE=-0.016 V at pH=13.[1] Fornari, R.P.; de Silva, P. A Computational Protocol Combining DFT and Cheminformatics for Prediction of pH-Dependent Redox Potentials. Molecules 2021, 26, 3978.[2] Wedege, K.; Dražević, E.; Konya, D.; Bentien, A. Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility. Sci. Rep. 2016, 6, 39101.

Density functional theory and CCSD(T) evaluation of ionization potentials, redox potentials, and bond energies related to zirconocene polymerization catalysts.

Quantum-mechanical-based computational design of molecular catalysts requires accurate and fast electronic structure calculations to determine and predict properties of transition-metal complexes. For Zr-based molecular complexes related to polyethylene catalysis, previous evaluation of density functional theory (DFT) and wavefunction methods only examined oxides and halides or select reaction barrier heights. In this work, we evaluate the performance of DFT against experimental redox potentials and bond dissociation enthalpies (BDEs) for zirconocene complexes directly relevant to ethylene polymerization catalysis. We also examined the ability of DFT to compute the fourth atomic ionization potential of zirconium and the effect the basis set selection has on the ionization potential computed with CCSD(T). Generally, the atomic ionization potential and redox potentials are very well reproduced by DFT, but we discovered relatively large deviations of DFT-calculated BDEs compared to experiment. However, evaluation of BDEs with CCSD(T) suggests that experimental values should be revisited, and our CCSD(T) values should be taken as most accurate.

Evaluation of an Appropriate Standard Hydrogen Electrode Potential for Computing Redox Potentials of Catechins with Density Functional Theory

Abstract Catechins are the main constituents in tea and have attracted attention because of their antioxidant properties. In this letter, to compute accurate redox potentials for catechins, an appropriate standard hydrogen electrode potential, ESHE, for catechins was evaluated by a linear fitting of experimental pKa and computed Gibbs energy difference upon deprotonation. The redox potentials of seven tea catechins were computed with the evaluated ESHE values, showing a good agreement with the experimental redox potentials of catechins by about 0.1 V.

Redox chemistry of bis(terpyridine)manganese(II) complexes – A molecular view

Oxidation of bis (terpyridine)manganese(II) complexes with a constrained octahedral geometry, leads to a high spin manganese(III) compression Jahn-Teller distortion geometry, while reduction results in a high spin manganese(II) complex which is antiferromagnetically coupled to one localized π-radical anion ligand. The provided linear relationships between experimental redox potentials and density functional theory calculated energies can be used to predict the redox potentials of related bis (terpyridine)manganese(II) complexes. • [Mn II (tpy) 2 ] 2+ has a constrained octahedral geometry. • [Mn III (tpy) 2 ] 3+ has a compression Jahn-Teller geometry. • [Mn IV (tpy) 2 ] 4+ has a constrained octahedral geometry. • Reduction of [Mn II (tpy) 2 ] 2+ gives the high spin [Mn II (tpy)(tpy) • ] 1+ complex. • Linear relationships between experimental redox potentials and DFT calculated energy. A density functional theory study of the oxidation and reduction of a series of bis (terpyridine)manganese(II) complexes showed that oxidation results in a high spin manganese(III) compression Jahn-Teller distortion geometry, while reduction results in a high spin manganese(II) complex which is antiferromagnetically coupled to one localized π-radical anion ligand. Experimental reports on the redox chemistry of [Mn II (tpy) 2 ] 2+ complexes, reported two oxidation and three reduction processes. The DFT results of this study, on the electronic structure of the molecules involved in the redox processes, assign the oxidation processes metal based, with the formation of [Mn III (tpy) 2 ] 3+ and [Mn IV (tpy) 2 ] 4+ respectively. Contrary to oxidation that is metal based, DFT results show that the reduction processes are ligand based, with the formation of [Mn II (tpy)(tpy) • ] 1+ and [Mn II (tpy • ) 2 ] 0 respectively, where (tpy • ) 1− is a π-radical anion that couples antiferromagnetically to the high spin Mn II ion. Correlations between experimental redox potentials and density functional theory calculated energies were established, which can be used to predict redox potential as well as electrophilic and nucleophilic reactivity of further substituted bis (terpyridine)manganese(II) complexes.

Redox Chemistry of tris(β-diketonate)cobalt(III) Complexes: A Molecular View

Reduction of low spin S = 0 tris(β-diketonate)cobalt(III) complexes leads to an intermediate spin S = 3/2 anion. This electron rearrangement upon reduction could be the reason why a large peak potential difference between the reduction and re-oxidation peaks was observed during electrochemical analysis, using cyclic voltammetry. It is shown that experimental redox potentials obtained from the following methods, namely (i) polarography, (ii) cyclic voltammetry on a hanging Hg electrode, as well as (iii) cyclic voltammetry on a carbon fibre surface, related linearly to various empirical (Gordy scale group electronegativities, Hammett meta-substituent constants, pK a and the Lever electronic parameters) and density functional theory (DFT) calculated (LUMO energy, adiabatic electron affinity, chemical potential, global electrophilicity index and molecular electrostatic potential on Co) reactivity parameters. Deviations from these linear trends were due to a resonance effect of aromatic substituent groups. The linear relationships obtained for complexes containing aliphatic substituent groups can be used to predict redox potential and the electrophilic reactivity of related tris(β-diketonate)cobalt(III) complexes.

DFT Study of bis(1,10-phenanthroline)copper complexes: Molecular and electronic structure, redox and spectroscopic properties and application to Solar Cells

A density functional theory study (DFT) on the geometry, electronic structure and properties of a series of [Cu(phen)2]n+ complexes (n = 1 and 2, and phen = differently substituted phenanthroline ligands), is presented. The DFT study determined various properties of the complexes to be used either as dyes or as redox mediators in dye-sensitized solar cells (DSSCs). The calculated properties, namely absorption wavelength, light harvesting efficiency, excited state lifetime and driving force for electron injection, and the regeneration of these complexes, are favourable for these complexes to be used as dyes in DSSCs. However, the calculated electron transfer distance is small, implying excitation would result in a serious intramolecular electron recombination and low efficiency electron injection into the semiconductor. The redox properties of these complexes compare well with the redox reaction of the iodide/triiodide (I−/I3−) redox mediator which is generally being used in DSSCs. Relationships obtained between experimental redox potentials and DFT calculated energies can be used to design and fine-tune complexes with a desired redox potential, as required for a redox mediator in DSSCs.

Structure-Property Relationships of Dibenzylidenecyclohexanones.

A series of symmetrical dibenzylidene derivatives of cyclohexanone were synthesized with the goal of studying the physicochemical properties of cross-conjugated dienones (ketocyanine dyes). The structures of the products were established and studied by X-ray diffraction, NMR spectroscopy, and electronic spectroscopy. All products had the E,E-geometry. The oxidation and reduction potentials of the dienones were determined by cyclic voltammetry. The potentials were shown to depend on the nature, position, and number of substituents in the benzene rings. A linear correlation was found between the difference of the electrochemical oxidation and reduction potentials and the energy of the long-wavelength absorption maximum. This correlation can be employed to analyze the properties of other compounds of this type. The frontier orbital energies and the vertical absorption and emission transitions were calculated using quantum chemistry. The results are in good agreement with experimental redox potentials and spectroscopic data.

Properties Assessment by Quantum Mechanical Calculations for Azulenes Substituted with Thiophen– or Furan–Vinyl–Pyridine

In this paper, azulenes substituted with thiophen– or furan–vinyl–pyridine are reported as heavy metal ligands in systems based on chemically modified electrodes. We undertook a computational study of their structures using density functional theory (DFT). Based on these computations, we obtained properties and key molecular descriptors related to chemical reactivity and electrochemical behavior. We investigated the correlation between some quantum parameters associated with the chemical reactivity and the complexing properties of the modified electrodes based on these ligands. The best correlations for the parameters were retained. We showed that the linear correlation between DFT-computed HOMO/LUMO energies and experimental redox potentials is very good.

Bridging the Experiment-Calculation Divide: Machine Learning Corrections to Redox Potential Calculations in Implicit and Explicit Solvent Models.

Prediction of redox potentials is essential for catalysis and energy storage. Although density functional theory (DFT) calculations have enabled rapid redox potential predictions for numerous compounds, prominent errors persist compared to experimental measurements. In this work, we develop machine learning (ML) models to reduce the errors of redox potential calculations in both implicit and explicit solvent models. Training and testing of the ML correction models are based on the diverse ROP313 data set with experimental redox potentials measured for organic and organometallic compounds in a variety of solvents. For the implicit solvent approach, our ML models can reduce both the systematic bias and the number of outliers. ML corrected redox potentials also demonstrate less sensitivity to DFT functional choice. For the explicit solvent approach, we significantly reduce the computational costs by embedding the microsolvated cluster in implicit bulk solvent, obtaining converged redox potential results with a smaller solvation shell. This combined implicit-explicit solvent model, together with GPU-accelerated quantum chemistry methods, enabled rapid generation of a large data set of explicit-solvent-calculated redox potentials for 165 organic compounds, allowing detailed investigation of the error sources in explicit solvent redox potential calculations.

Tungsten and Molybdenum Heteropolyanions with Different Central Ions-Correlation between Theory and Experiment.

Density functional theory calculations were carried out to investigate the electronic structures of Keggin-typed [XMo12O40]n− and [XW12O40]n− anions with different heteroatoms (X = Zn2+, B3+, Al3+, Ga3+, Si4+, Ge4+, P5+, As5+, and S6+). The influence of solvent on redox properties of heteropolyanions was discussed. For [XW12O40]n− systems two linear correlation: first, between the experimental redox potential and energies of LUMO orbital; and second, between the experimental redox potential and total energy interaction (calculated between internal tetrahedron (XO4n−), and rest of Kegging anion skeleton, (W12O36)) were designated. Taking into account the similarity of XW12O40n− and XMo12O40n− systems (in geometry and electronic structure), the estimated redox potential of molybdenum heteropolyanions (with X being p block elements) in different solvent were proposed.

Can aluminum, a non-redox metal, alter the thermodynamics of key biological redox processes? The DPPH-QH2 radical scavenging reaction as a test case

The increased bioavailability of aluminum has led to a concern about its toxicity on living systems. Among the most important toxic effects, it has been proven that aluminum increases oxidative stress in biological systems, a controversial fact, however, due to its non-redox nature. In the present work, we characterize in detail how aluminum can alter redox equilibriums by analyzing its effects on the thermodynamics of the redox scavenging reaction between DPPH., a radical compound often used as a reactive oxygen species model, and hydroquinones, a potent natural antioxidant. For the first time, theoretical and experimental redox potentials within aluminum biochemistry are directly compared. Our results fully agree with experimental reduction and oxidation potentials, unequivocally revealing how aluminum alters the spontaneity of the reaction by stabilizing the reduction of DPPH⋅ to DPPH− and promoting a proton transfer to the diazine moiety, leading to the production of a DPPH-H species. The capability of aluminum to modify redox potentials shown here confirms previous experimental findings on the role of aluminum to interfere with free radical scavenging reactions, affecting the natural redox processes of living organisms.

Mixed RuIIComplexes Containing Diseleno‐Ligand and α,β‐Diketones Donors with Anticancer Activity. Synthesis, Characterization, Electrochemical and DFT Studies

AbstractThis work presents the synthesis, characterization, electrochemical studies, DFT calculations and in vitro evaluation of antiproliferative activity in HeLa cells of four novel direct bonded ruthenium‐selenium compounds. The complexes are [Ru(BP3Se2)Cl(PPh3)]Cl(1)and mixed [Ru(BP3Se2)(O−O)(PPh3)]Cl (2 a,2 b,2 c) systems with diseleno‐ligandBP3Se2: 1,9‐bis(2‐pyridyl)‐3,7‐diselenanonane as a primary donor and O−O: α,β‐diketones as a secondary donors. In1,BP3Se2is linked in tetradentate N2Se2form while in mixed complexes, present a tridentate NSe2linkage. Diketone nature and electroactive groups has a fundamental role in the modulation of the electronic, steric and lipophilic properties as well as in its cytotoxic activity. Among the tested compounds,2 chas the best cytotoxic performance (IC50=2.8 μmol/L) even higher than metallodrug cisplatin. In this regard, cytotoxic activity is strongly determined by experimental redox potential (E0) and calculated molar volume (VM) of the compounds.

Evaluation of Computational Chemistry Methods for Predicting Redox Potentials of Quinone-Based Cathodes for Li-Ion Batteries

High-throughput computational screening (HTCS) is an effective tool to accelerate the discovery of active materials for Li-ion batteries. For the evaluation of organic cathode materials, the effectiveness of HTCS depends on the accuracy of the employed chemical descriptors and their computing cost. This work was focused on evaluating the performance of computational chemistry methods, including semi-empirical quantum mechanics (SEQM), density-functional tight-binding (DFTB), and density functional theory (DFT), for the prediction of the redox potentials of quinone-based cathode materials for Li-ion batteries. In addition, we evaluated the accuracy of three energy-related descriptors: (1) the redox reaction energy, (2) the lowest unoccupied molecular orbital (LUMO) energy of reactant molecules, and (3) the highest occupied molecular orbital (HOMO) energy of lithiated product molecules. Among them, the LUMO energy of the reactant compounds, regardless of the level of theory used for its calculation, showed the best performance as a descriptor for the prediction of experimental redox potentials. This finding contrasts with our earlier results on the calculation of quinone redox potentials in aqueous media for redox flow batteries, for which the redox reaction energy was the best descriptor. Furthermore, the combination of geometry optimization using low-level methods (e.g., SEQM or DFTB) followed by energy calculation with DFT yielded accuracy as good as the full optimization of geometry using the DFT calculations. Thus, the proposed calculation scheme is useful for both the optimum use of computational resources and the systematic generation of robust calculation data on quinone-based cathode compounds for the training of data-driven material discovery models.