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Theoretical study of the mechanism of an interfacial oxygen reduction reaction in the presence of a hydrazone ligand, its cobalt (II) complex and their conjugate acids as catalyst
Abstract
Study of oxygen (O2) reduction reaction and its mechanism in the presence of different catalysts are the most interesting topics for electrochemists and biochemists especially from the energy conversion view point. Protonation of O2 by conjugate acids of some hydrazone-based ligands, 1, and their transition metal complexes, 2, and reduction of the activated O2 by ferrocene at the water/dichloromethane interface are the main aims of this theoretical study. A DFT method at the B3LYP/6-31G∗∗ level of theory is used for the calculations in the gas and solution phases. The results show that the catalytic reaction can proceed through direct interaction of O2 with the most acidic conjugate acid of 1 or 2 and the formation of a proton bound complex, i.e., an electrophilic cation-diradical, to promote oxygen reduction by Fc. Also, the findings confirm that hydrogen bonding interaction acts more effective than proton transfer reaction at both aqueous and organic phases.
In order to resist the growth of human pathogenic microorganisms, a new N-heterocyclic ligand of (E)-N′-((2-hydroxynaphthalen-1-yl)methylene)-4-oxopiperidine-1-carbohydrazide was synthesised from the condensation reaction of 4-oxopiperidine-1-carbohydrazide and 2-hydroxynaphthalene-1-carbaldehyde in ethanol. Reaction of metal salt of (M(II/III)·Cl2)·nH2O M = Cr(III), Fe(III), Mn(II), Co(II), Ni(II) and Cu(II) with ligand resulted in the formation the two types of complexes [M(III)C17H19N3O5Cl] and [M(II)C17H20N3O5Cl] which adopted octahedral geometry. The ligand was thoroughly characterized by elemental analysis, FT-IR, UV–Vis, NMR (¹H, ¹³C) and HR-mass spectroscopy while the structure of metal complexes was confirmed on the basis of elemental analysis, UV–Vis spectra, molar conductivity, magnetic susceptibility, and TGA-DTA analysis. The ligand behaves as dibasic tridentate, linkages via phenolic-O, azomethine-N, enolic-O atoms in metal (III) complexes and as monobasic tridentate in metal (II) complexes. The decomposition pattern were ascertained by TG analysis, and kinetics accountability from Coats–Redfern relation. The compounds were excited at λex = 380 nm and observed intense emission intensity at λem = λ527–533 nm. SEM analysis was performed to observe their surface analysis. The molecular geometry optimization and quantum chemical properties have been retrieved from DFT. ADMET score have been predicted as a drug-likeness prospect from admetSAR. The molecular docking outcomes revealed the good binding score of ligand with Adenylate kinase, Peptide deformylase (bacterial enzymes) and DNA polymerase (fungal enzyme). The in vitro antimicrobial potency of ligand and its metal complexes were carried against the bacterial species (Escherichia coli, Salmonella typhi, Staphylococcus aureus, Bacillus substilis), and fungal species (Candida albicans, Aspergillus niger) and showed their promising activity.
Graphical abstract
Research on hydrogen evolution (HER), oxygen evolution (OER) and oxygen reduction (ORR) reactions at polarizable liquid|liquid interfaces was reviewed. A number of biphasic systems based on hydrophobic molecular solvents and hydrophobic ionic liquids was proposed to carry out these reactions. For HER and ORR metallocenes were extensively explored as electron donors and metallocene hydrides were found as intermediate product. The number of approaches to increase the efficiency of these reactions was proposed. They include assembly/electrodeposition of nanocatalyst at liquid|liquid interface, dissolution of molecular catalyst in organic phase, electro/photochemical regeneration of electron donor and application of light. For biphasic OER light driven systems employing nanoparticles assembled at liquid|liquid interface were reported.
In this study, the catalytic effect of 2,2′-dipyridylamine (DPA) on the reduction of oxygen (O2) at the polarized water/1,2-dichloroethane (DCE) interface was investigated. Ferrocene (Fc) and tetrathiafulvalene (TTF) were weak electron donors used in this study. Slow reduction of O2 at the interface containing Fc and TTF was significantly accelerated upon the addition of DPA. Voltammetry and biphasic shake flask experiments revealed that DPA acts as a proton ionophore to transfer protons between the aqueous and organic phases. The PA, GB, and pKa values of all possible conjugate acids of DPA were calculated. Then, a mechanism was suggested to explain the interaction between protonated DPA and oxygen molecular. The mechanism was computationally analyzed by using density functional theory (DFT). Furthermore, DFT calculations at the B3LYP/6-31G** level of theory showed that the conjugate acid species of DPA transfer proton to O2 at the interface. The results show that DPA-H2+ and DPA-H1+ are the best species to transfer proton to molecular oxygen.
The catalytic reduction of oxygen (O2) by using oxidovanadium(IV)-4-methyl salophen (VOL) indicated that this complex can efficiently catalyze this reaction in the presence of ferrocene (Fc) as a weak electron donor at the interface between two immiscible electrolyte solutions (ITIES). The oxidovanadium complex has four positions for transferring proton at interface. Protonated complex (VOL-OH+) is as an active intermediate specie in this catalytic process. The catalytic oxygen reduction by VOL at liquid-liquid interface was also studied by density functional theory (DFT) computations in order to find the effect of structural parameters on this catalytic reaction. The results showed that the catalytic reaction can proceed by direct interaction between molecular oxygen and most acidic position in the structure of VOL.
Oxygen reduction at the polarized water/1,2‐dichloroethane (DCE) interface catalyzed by a Cu (II) coordination polymer (Cu–pol) was studied with two lipophilic electron donors ferrocene (Fc) and tetrathiafulvalene (TTF). The results of the ion transfer voltammetry and two‐phase shake flask experiments suggest proceeding of the catalytic reaction as proton‐coupled electron transfer reduction of oxygen to hydrogen peroxide and water. In this process, while the protons supplied from the aqueous phase, the electrons provided from the organic phase by the weak electron donor, Fc. The O2 molecule takes a superoxide structure with Cu–pol which resulted to hydrogen peroxide or water on reduction. Furthermore, the results revealed that the apparent rate constant of TTF + Cu‐pol is higher than that of Fc + Cu‐pol system due to the faster kinetic reaction of TTF with respect to Fc.
A new dinuclear cobalt(III) coordination compound, [Co2L(μ‐N3)(N3)2]·CH3OH (1), was synthesized and characterized by elemental analysis, spectroscopic methods, and single‐crystal X‐ray analysis in which H3L is a heptadentate ligand obtained by the condensation of triethylenetetramine with 5‐bromo‐2‐hydroxybenzaldehyde. X‐ray analysis revealed that two cobalt(III) ions have distorted octahedral geometry and are connected together by a phenoxy and an azide bridging ligand. The catalytic activity of compound 1 for oxygen (O2) reduction reaction was investigated. Compound 1 can efficiently catalyze the reduction of O2 by a weak electron donor, ferrocene (Fc), at the polarized water–1,2‐dichloroethane interface. It was found that compound 1 can catalyze O2 reduction to H2O2, whereas in the presence of Fc, it can catalyze the reduction of O2 to water. Syntheses, structure, and spectroscopic studies of a new dinuclear Co(III) coordination compound were performed. Electrocatalysis of oxygen reduction at the liquid–liquid interface and hydrogen peroxide generation by the coordination compound alone were achieved. Electrocatalytic reduction of oxygen to water at the liquid–liquid interface in the presence of both the coordination compound and ferrocene was performed.
The results of theoretical studies on structures and energetics are presented for the proton-bound complexes Ar-n-N2H+ (n = 1-12). Complexes are formed due to the consecutive filling of shells. The largest studied cation possesses. the slightly deformed icosahedral geometry. For shells formed outside the Ar-N2H+ core, increasing binding energies are observed during the coordination of consecutive Ar atoms. Such an anomalous behavior is also observed for consecutive values of enthalpy and entropy of ligand binding, Mulliken atomic charges, and Ar-H+ stretching vibrations. The analysis of physical components resulting from interaction energy decomposition allows the observed phenomenon to be rationalized.
A new set of penta-coordinated copper(ii) hydrazone complexes containing solvated methanol were synthesized by reacting the hydrazone ligands, 2-acetylpyridine benzoyl hydrazone () and 2-acetylpyridine thiophene-2-carboxylic acid hydrazone (), with [CuCl2(DMSO)2] and characterized by different spectral methods. Single crystal X-ray diffraction studies of the complexes revealed that both of them, [CuCl(L1)(MeOH)] () and [CuCl(L2)(MeOH)] (), have square pyramidal geometry around the cupric ion, in which the hydrazone is coordinated through NNO atoms along with a molecule of methanol in the apical position. Interaction of the ligands and along with the corresponding copper complexes and with calf thymus DNA (CT-DNA) has been estimated by absorption and emission titration methods which revealed that the compounds interacted with CT-DNA through intercalation. Binding of the compounds, i.e., free ligands and complexes () and () with bovine serum albumin (BSA) protein investigated using UV-visible, fluorescence and synchronous fluorescence spectroscopic methods indicated that there occurred strong binding of copper complexes to BSA over the ligands. Further, the cytotoxicity of the compounds examined in vitro on a panel of cancerous cell lines such as a human cervical cancer cell line (HeLa), a pancreatic cancer cell line (PANC-1), an Ehrlich ascites cancer cell line (EAC) and Dalton's lymphoma ascitic cancer cells (DLA) and a normal mouse embryonic fibroblasts cell line (NIH3) demonstrated that the complexes and possessed superior cytotoxicity than that of well-known commercial anticancer drug cisplatin to the tumor cells but are less toxic to the normal cell line and have emerged as potential candidates for further studies.
Reactions of tridentate chelating hydrazone Schiff bases benzoic acid (2-hydroxyl-benzylidene)-hydrazide (H 2 L) (1) obtained from salicylaldehyde and benzhydrazide with [NiCl 2 (PPh 3) 2 ] and [CoCl 2 (PPh 3) 2 ] afforded respective metal hydrazone complexes of the composition [Ni(L)(PPh 3)] (2) and [Co1(L) 2 ] 2 [Co2(H 2 O) 4 (OPPh 3) 2 ] (3). The molecular structure of both complexes 2 and 3 determined by single crystal X-ray diffraction revealed that complex 2 is neutral in charge with distorted square planar geometry. However, complex 3 was found to have distorted octahedral geometry. All of the synthesised compounds 1–3 were studied by interaction with calf thymus DNA (CT DNA) and bovine serum albumin (BSA). In addition in vitro free radical scavenging and cytotoxic potential of all the synthesised compounds were also investigated.
We present a computer simulation study of the temperature dependence of the structural and dynamical properties of dilute O2 aqueous solutions. A clathrate-like solvation shell, in line with other apolar gas solutions, emerged from the present simulations. The average number of water molecules in the first hydration shell decreases with temperature, and, in the investigated temperature range (291–348 K), a net transfer of one water molecule from the hydration shell to the bulk has been detected. We have found oscillations of both water density and electrostatic charges in the neighborhood of the apolar solute, which is surrounded by shells of water at different density, and with water molecules oriented in such a way as to form shells with alternating net electrostatic charges. In the O2, first hydration shell water–water interactions are stronger and water diffusional and rotational dynamics slower than in the bulk. A hydrogen bond’s mean lifetime is affected by the apolar solute as well, being shorter in the first hydration shell. Differences between shell and bulk water properties are smoothed by increasing temperature. Suggestions for the molecular mechanism relevant to the more general problems of the hydrophobic effects are deduced from the simulations. A possible microscopic explanation for the lowering of solubility of oxygen in water with temperature is given. © 1999 American Institute of Physics.
The CBS-QB3 method was used to calculate the gas-phase free energy difference between 20 phenols and their respective anions, and the CPCM continuum solvation method was applied to calculate the free energy differences of solvation for the phenols and their anions. The CPCM solvation calculations were performed on both gas-phase and solvent-phase optimized structures. Absolute pK(a) calculations with solvated phase optimized structures for the CPCM calculations yielded standard deviations and root-mean-square errors of less than 0.4 pK(a) unit. This study is the most accurate absolute determination of the pK(a) values of phenols, and is among the most accurate of any such calculations for any group of compounds. The ability to make accurate predictions of pK(a) values using a coherent, well-defined approach, without external approximations or fitting to experimental data, is of general importance to the chemical community. The solvated phase optimized structures of the anions are absolutely critical to obtain this level of accuracy, and yield a more realistic charge separation between the negatively charged oxygen and the ring system of the phenoxide anions.
A new complex of Co(iii) using an oxidative stable hydrazone ligand, CoL, was synthesized and characterized by elemental analysis, spectroscopic methods and single crystal X-ray analysis where HL is bis-[(E)-N'-(phenyl(pyridin-2-yl)methylene)]carbohydrazide. X-ray analysis revealed that the complex is mononuclear and the coordination environment around the Co(iii) core is trans-[CoN4Cl2]. The catalytic activity of the complex in the oxygen reduction reaction was investigated. The complex is a highly oxidative resistant cobalt-hydrazone which can efficiently catalyze the reduction of oxygen (O2) by a weak electron donor ferrocene, (Fc), at the polarized water/1,2-dichloroethane (DCE) interface. Oxygen reduction is coupled with proton transfer from water to the organic phase to form hydrogen peroxide, which is extracted into the aqueous phase.
In this work we report the catalytic effect of 5,10,15,20-tetraphenylporphyrin (H2TPP) on the reduction of oxygen (O-2) by a weak electron donor, namely tetrathiafulvalene (TTF), at the polarized water/1,2-dichloroethane (DCE) interface. TIT alone, as reported previously, can reduce O-2 slowly via a four-electron transfer pathway to produce water. Herein we show that H2TPP can catalyze the reduction of O-2 by TTF to form hydrogen peroxide (H2O2) at the water/DCE interface, which was investigated by voltammetry and biphasic shake-flask experiments. The overall process was found to be essentially a proton-coupled electron transfer reaction. And the results also proved that the catalytic activity of H2TPP toward O-2 reduction under acid condition arises from the polarization activation of O-2 by H2TPP diacid, namely H4TPP2+.
Mechanistic model is presented, which allows simulating the experimentally observed catalytic effect of the 5,10,15,20-meso-tetraphenylporphyrin (H2TPP), as well as the effects of H+, tetrakis (pentafluorophenyl)borate anion (TB-) and water concentrations on the catalytic rate of the oxygen reduction reaction (ORR) with ferrocene (Fc) in 1,2-dichloroethane (DCE). Model calculations are based on the assumption that the electron transfer between the complex {(H4TPP2+)center dot(TB-)center dot O-2} and Fc is the rate determining step (r.d.s.) in both the catalytic and electrocatalytic cycle. The model calculations are performed using the reported acid dissociation constants for mono- and diprotonated H2TPP forms, and the equilibrium constants of the extraction of the ligands L=O-2, water, or TB- from the porphyrin complex {(H4TPP2+)center dot(TB-)center dot L} calculated by the DFT methods. The rate constant of r.d.s. evaluated from the initial rates is compared with that obtained by a numeric fit of the experimental Fc(+) transients. Model predictions are relevant to electrocatalysis of ORR by protonated H2TPP at the polarized water vertical bar DCE interface.
A systematic search for computational strategies able to accurately predict pKa values of protonated amines (HA+) has been performed. Thermodynamic cycles have been used in conjunction with the cluster-continuum model, including up to eight explicit water molecules per system. The calculations have been performed with nine different methods within the density functional theory (DFT), and with MP2. It was found that including one explicit water molecule in the vicinity of the protonated site is highly important for obtaining accurate results. We recommend the F2 reaction scheme [HA+(H2O) + 3H2O = A + H3O+(3H2O)], combined with M05-2X/6-311++G(d,p) calculations. This particular combination produces mean unsigned errors (MUE) equal to 0.54 for all the tested amines, and 0.51 pKa units for the studied neurotransmitters. Moreover it was found to be the only one that systematically leads to errors lower than, or very close to, 1 unit of pKa for every one of the studied species, with the maximum error equal to 1.09 pKa units. This scheme has the additional advantages of being computationally feasible for amines of relative large size, independent of experimental values, and free of fitting-based corrections. In addition the high influence of the reference acid of choice, in the proton exchange scheme (D), on the quality of the pKa values of amines is discussed. © 2012 Wiley Periodicals, Inc.
Bipyridine and its derivatives have been widely used as the ligands in transition metal complexes. The proton affinities of pyridine derivatives were calculated using an ab initio quantum mechanical method (B3LYP with various double zeta and triple zeta basis sets) in combination with the Poisson-Boltzmann continuum solvation model. Van der Waals radii of the atoms in the heterocyclic rings for the solvation energy calculation were set to values determined to reproduce the values of guanine and oxoguanine derivatives and that of chlorine was optimized to reproduce the experimental values of relating compounds. The values for the heterocyclic ring compounds were in agreement with the experimental values with a mean unsigned error of 0.45 units.?? ??? ?? ??? ??? ? ??? ??? ?? ??? ??? ? ??? ??? ?? ??? 낎?? ? 낎?? ??? ?? ??? 邏?? ? 邏?? ??? ?? ??? ??? ? ??? ??? ?? ??? ??? ? ??? ??? ?? ??? ゐ?? ? ゐ?? ??? ?? ??? ??? ? ??? 邑?? ?? 邑?? ??? ? ??? ??? ?? ??? ??? ? ??? ??? ?? ??? 낑?? ? 낑?? ??? ? ??? ᄐ ?돀??? ??? ??? ? ?돐 ??? ??? ??? 섏 ?덐 ??? 쀏 ? ????
Oxygen reduction reaction (ORR) by decamethylferrocene (DMFc) at a water/1,2-dichloroethane (DCE) interface proceeds as a proton coupled electron transfer process, with the supply of protons by the aqueous phase and electrons by the electron donor DMFc in the organic phase. In the present work, the reduction of oxygen by DMFc has been investigated in the presence of different aniline derivatives such as 3,5,N,N-Tetramethylaniline (TMA), N,N-Diethylaniline (DEA), 4-Nitroaniline (4-NA), 4-Dodecylaniline (4-DA) which act as proton ionophores to transfer protons from the aqueous to the organic phase. It has been found that ORR by DMFc can be catalyzed efficiently by the use of aniline derivatives depending on their basicity strength which follows an order of DEA > TMA > 4-DA > 4- NA.
Density-functional theory has been used to estimate the proton affinity (PA) and the acidity constants pKa of monoprotonated bipiperidine, bimorpholine and their derivatives in water and in acetonitrile (MeCN). Five different conformations of derivatives of bimorpholine and bipiperidine were explored. Monoprotonated bimorpholine has lower pKa values (9.1 in H2O and 17.4 in MeCN) than monoprotonated bipiperidine (12.8 in H2O and 21.0 in MeCN). For N-iPr derivatives of bimorpholine, the pKa values are 8.8 in H2O and 17.0 in MeCN and for N-iPr derivatives of bipiperidine the values are 12.5 in H2O and 20.8 in MeCN.
The available data on gas phase basicities and proton affinities of molecules are compiled and evaluated. Tables giving the molecules ordered (1) according to proton affinity and (2) according to empirical formula, sorted alphabetically are provided. The heats of formation of the molecules and the corresponding protonated species are also listed.
This document provides an inventory of theoretical and methodological concepts in electrochemistry at the interface between two immiscible electrolyte solutions (ITIES). Definitions of basic relationships are given, together with recommendations for the preferred symbols, terminology, and nomenclature. Methods of study of ITIES are briefly described, current experimental problems are indicated, and representative experimental data are shown. The practical applications of electrochemistry at ITIES are summarized.
This work considers calculation of pKa for a series of related alcohols, carboxylic acids, and ammonium ions spanning a wide range of acidities, using quantum mechanical treatment of solute electronic structure in conjunction with a dielectric continuum model for solvation of each bare solute. The electronic structure methods used are of sufficiently high quality to give very good agreement with experimental gas phase acidities. Dielectric continuum theory of solvation is used in a recently developed form that accurately takes account of solute charge density penetrating outside the solvent cavity that nominally encloses it. The cavity surface is defined by a single parameter characterizing an electronic isodensity contour, and contours are examined at and near the value 0.001 e/ that has previously led to a good account of solvation effects on properties of neutral solutes in various solvents. In water, the pKa values calculated for alcohols and carboxylic acids are generally much higher than experiment, while for ammonium ions they are comparable to experiment. Good results in water can be obtained from linear correlations that describe the effects of different substituents in solutes sharing the same acidic functional group, but different correlations apply for different acidic functional groups. For the polar nonprotic solvents DMSO and MeCN, pKa values close to experimental results are obtained, and very good linear correlations are found that simultaneously describe well all the solutes considered. It is argued this indicates that dielectric continuum theory properly accounts for long-range bulk solvent effects on pKa without the need for special parameterization of the cavity. To achieve good pKa results in water, further account must be taken of specific short-range effects such as hydrogen bonding. Rather than distort the cavity from the physical solute−solvent interface region in order to artificially force dielectric continuum theory to serve this purpose, as is commonly done through detailed parameterization schemes, it is recommended that other complementary approaches more appropriate for describing short-range interactions should be sought to complete the treatment of solvation effects in water.
We have investigated the influence of different substitutions and solvent effects on the pKa values of the hydroxy group for a set of pyrone and dihydropyrone HIV-1 protease inhibitors. Absolute and relative pKa values were calculated for model compounds using a combination of density functional theory (DFT) and continuum solvation methods and were compared to experimental data for related compounds. The theoretical results shed light on the unusual pH dependence of the inhibition constants for these compounds and may lead to the design of new, improved pyrone and dihydropyrone inhibitors of HIV-1 protease.
Relative proton affinities are usually measured by means of high pressure mass spectrometry. In this work ion mobility spectrometry was used to determine proton affinities. The standard molar enthalpy change ΔH0M for the reaction MH++N⇌M+NH+ was found to be: , when M was ethyl acetate and N ethanol; , when M was acetophenone and N ethyl acetate; , when M was cycloheptanone and N ethyl acetate; , when M was dimethyl methyl phosphonate and N ethyl acetate; and, , when M was dimethyl methyl phosphonate and N cycloheptanone. These values are internally consistent and in good agreement with results obtained with high pressure mass spectrometry. On the basis of proton affinities of cycloheptanone and ethyl acetate, an absolute proton affinity of has been determined for dimethyl methyl phosphonate.
We present a theoretical study of the four aqueous microscopic dissociation constants relating the relevant protonation forms (cation, neutral, anion, zwitterion) of the chromophore of the green fluorescent protein (GFP) in the ground and excited states. To take the protonation-state-dependent torsional flexibility around the ring-bridging bonds into account, configuration integrals in the torsion space were evaluated to yield the free energy differences. Conformational energies were calculated within a semiempirical quantum chemical scheme using a continuum solvation model. After establishing a linear regression of experimental aqueous pKa's and calculated enthalpy differences for a series of reference molecules with phenolic hydroxyl or imino nitrogen, the applied method is able to reproduce the titration behavior of bifunctional groups within an average error of 0.8 pK-units. The calculated values for the GFP chromophore agree very well (deviation < 0.2 pK-units) with known ground-state aqueous pKa's and confirm the equilibrium between neutral and anionic forms (pKa = 8.3). Freezing torsional flexibility shifts this pKa to 6.6in accordance with entropic contributions due to the enlarged configurational space of the protonated compoundand is, therefore, a dominant contribution to the adjustment of pKa's in the protein. Estimates for the excited-state pKa's were derived from vertical excitation energies under the assumption that the protolytic equilibria are faster than conformational relaxation of solute and solventcorresponding to a recent experimental model of excited-state proton transfer in GFP. The calculations reveal that increase of both the acidity of the phenolic oxygen and the basicity of the heterocycle nitrogen works in synergy. The neutral form becomes a strong photoacid and transfers a proton with a pKa* = 0.1; the imino-N becomes a strong photobase capable of accepting a proton with a pKa* = 8.9. Provided an extended hydrogen-bonded network links the two functions, phototautomerization might be a possible decay pathway.
Pushed over the edge: Proton-transfer coupled O(2) reduction catalysed by cobalt phthalocyanine ([CoPc]) was studied at the water/1,2-dichloroethane (DCE) interface (see figure). This system represents a new model of molecular electrocatalysis with the proton transfer controlled by the Galvani potential difference and the electron transfer by the molecular properties of the catalyst and electron donor.
Molecular oxygen (O2) reduction by decamethylferrocene (DMFc) was investigated at a polarized water/1,2-dichloroethane (DCE) interface. Electrochemical results point to a mechanism similar to the EC type reaction at the conventional electrode/solution interface, in which an assisted proton transfer (APT) by DMFc across the water/DCE interface via the formation of DMFcH+ corresponds to the electrochemical step and O2 reduction to hydrogen peroxide (H2O2) represents the chemical step. The proton transfer step can also be driven using lipophilic bases such as 4-dodecylaniline. Finally, voltammetric data shows that lipophilic DMFc can also be extracted to the aqueous acidic phase to react homogeneously with oxygen.
The electrochemistry of ferrocene, dimethyl ferrocene and decamethyl ferrocene at the interface of immiscible electrolytes has been investigated. The aqueous redox electrolyte was the hexacyanoferrate couple. It has been found that the study of the electron transfer reaction can be complicated by both the transfer of the ferricenium ion formed in the organic phase (1,2-dichloroethane) during oxidation, and by oxidation of the organic base electrolyte anion. However, by using a high concentration of aqueous electrolyte, the difference between ionic and electronic transfer potentials can be increased sufficiently to observe these two processes separately. Ionic transfer potentials of the ferricenium ion have been measured by preparing the corresponding FeCl−4 salts. For decamethyl ferrocene, no electron transfer reaction could be observed at the liquid/liquid interface and it is proposed that this is due to distance effects, which become more marked for this more hydrophobic ferrocene derivative.
We present a molecular dynamics simulation of solvent reorganization in the first electron transfer step in the oxygen reduction reaction, i.e. O2+e−→O2−, modeled as taking place in the outer Helmholtz plane. The first electron transfer step is usually considered the rate-determining step from many experimental studies. From our results, we conclude that solvent reorganization rather than inner-sphere reorganization provides the dominant contribution to the activation barrier, if no specific interaction of O2 or O2− with the electrode is included. The free energy surfaces for solvent reorganization are strongly non-linear owing to the effect of electrostriction, which is not incorporated in the classical Marcus electron transfer theory. In spite of the non-symmetric free energy barrier, the transfer coefficient at equilibrium is still close to 0.5, whereas it would be estimated to be much lower from harmonic model considerations.
This work presents calculated values of the pKa for a series of carboxylic acids spanning a wide range of acidities, using quantum mechanical treatment of solute electronic structure in conjunction with a dielectric continuum model for solvation. The calculations are carried out using 3rd order Møller–Plesset perturbation theory. Solute–solvent interactions have been taken into account by employing the polarizable continuum model (PCM). The calculated pKa values are in significantly better agreement with experimental data for the majority of the acids studied than other recently published results. The mean absolute deviation of the calculated pKa values is 0.68 in pKa units, for the carboxylic acids considered.
The diprotonated form of a fluorinated free base porphyrin, namely 5-(p-aminophenyl)-10,15,20-tris(pentafluorophenyl)porphyrin (H(2)FAP), can catalyze the reduction of oxygen by a weak electron donor, namely ferrocene (Fc). At a water/1,2-dichloroethane interface, the interfacial formation of H(4)FAP(2+) is observed by UV-vis spectroscopy and ion-transfer voltammetry, due to the double protonation of H(2)FAP at the imino nitrogen atoms in the tetrapyrrole ring. H(4)FAP(2+) is shown to bind oxygen, and the complex in the organic phase can easily be reduced by Fc to produce hydrogen peroxide as studied by two-phase reactions with the Galvani potential difference between the two phases being controlled by the partition of a common ion. Spectrophotometric measurements performed in 1,2-dichloroethane solutions clearly evidence that reduction of oxygen by Fc catalyzed by H(4)FAP(2+) only occurs in the presence of the tetrakis(pentafluorophenyl)borate (TB(-)) counteranion in the organic phase. Finally, ab initio computations support the catalytic activation of H(4)FAP(2+) on oxygen.
Experimental studies and density functional theory (DFT) computations suggest that oxygen and proton reduction by decamethylferrocene (DMFc) in 1,2-dichloroethane involves protonated DMFc, DMFcH(+), as an active intermediate species, producing hydrogen peroxide and hydrogen in aerobic and anaerobic conditions, respectively.
Molecular electrocatalysis for oxygen reduction at a polarized water/1,2-dichloroethane (DCE) interface was studied, involving aqueous protons, ferrocene (Fc) in DCE and amphiphilic cobalt porphyrin catalysts adsorbed at the interface. The catalyst, (2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p-amino-phenylporphyrin) cobalt(II) (CoAP), functions like conventional cobalt porphyrins, activating O(2) via coordination by the formation of a superoxide structure. Furthermore, due to the hydrophilic nature of the aminophenyl group, CoAP has a strong affinity for the water/DCE interface as evidenced by lipophilicity mapping calculations and surface tension measurements, facilitating the protonation of the CoAP-O(2) complex and its reduction by ferrocene. The reaction is electrocatalytic as its rate depends on the applied Galvani potential difference between the two phases.
Cobalt porphine (CoP) dissolved in the organic phase of a biphasic system is used to catalyze O(2) reduction by an electron donor, ferrocene (Fc). Using voltammetry at the interface between two immiscible electrolyte solutions (ITIES), it is possible to drive this catalytic reduction at the interface as a function of the applied potential difference, where aqueous protons and organic electron donors combine to reduce O(2). The current signal observed corresponds to a proton-coupled electron transfer (PCET) reaction, as no current and no reaction can be observed in the absence of either the aqueous acid, CoP, Fc, or O(2).
[structure: see text] It is shown that the pK(a) values of strong neutral organic (super)bases in acetonitrile are well described by the density functional theory (DFT) employing the isodensity polarization continuum model (IPCM) for treating solvent-solute interactions. High pK(a) values are predicted for two model compounds, and their synthesis is strongly recommended.