Controlled Potential Electrolysis Experiments Research Articles

Electro- and photochemical studies of gold (III) bromide towards a novel laser-based method of gold patterning

In this report, we demonstrate a novel technique for the microscopic patterning of gold by combining the photoreduction of AuIIIBr4 − to AuIBr2 − and the electrochemical reduction of AuIBr2 − to elemental gold in a single step within solution. While mask-based methods have been the norm for electroplating, the adoption of direct laser writing for flexible, real-time patterning has not been widespread. Through irradiation using a 405 nm laser and applying a voltage corresponding to a selective potential window specific to AuIBr2 −, we have shown that we can locally deposit elemental gold at the focal point of the laser. In addition to demonstrating the feasibility of the technique, we have collected data on the kinetics of the photoreduction reaction in ethanol and have deduced its rate law. We have confirmed the selective deposition of AuIBr2 − within a potential window through controlled potential electrolysis experiments and through direct measurement on a quartz crystal microbalance. Finally, we have verified local deposition through scanning electron microscopy.

Deciphering Distinct Overpotential-Dependent Pathways for Electrochemical CO2 Reduction Catalyzed by an Iron–Terpyridine Complex

[Fe(tpyPY2Me)]2+ ([Fe]2+) is a homogeneous electrocatalyst for converting CO2 into CO featuring low overpotentials of <100 mV, near-unity selectivity, and high activity with turnover frequencies faster than 100 000 s-1. To identify the origins of its exceptional performance and inform future catalyst design, we report a combined computational and experimental study that establishes two distinct mechanistic pathways for electrochemical CO2 reduction catalyzed by [Fe]2+ as a function of applied overpotential. Electrochemical data shows the formation of two catalytic regimes at low (ηTOF/2 of 160 mV) and high (ηTOF/2 of 590 mV) overpotential plateaus. We propose that at low overpotentials [Fe]2+ undergoes a two-electron reduction, two-proton-transfer mechanism (electrochemical-electrochemical-chemical-chemical, EECC), where turnover occurs through the dicationic iron complex, [Fe]2+. Computational analysis supports the importance of the singlet ground-state electronic structure for CO2 binding and that the rate-limiting step is the second protonation in this low-overpotential regime. When more negative potentials are applied, an additional electron-transfer event occurs through either a stepwise or proton-coupled electron-transfer (PCET) pathway, enabling catalytic turnover from the monocationic iron complex ([Fe]+) via an electrochemical-chemical-electrochemical-chemical (ECEC) mechanism. Comparison of experimental kinetic data obtained from variable controlled potential electrolysis (CPE) experiments with direct product detection with calculated rates obtained from the energetic span model supports the PCET pathway as the most likely mechanism. Moreover, we build upon this mechanistic understanding to propose the design of an improved ligand framework that is predicted to stabilize the key transition states identified in our study and explore their electronic structures using an energy decomposition analysis. Taken together, this work highlights the value of synergistic computational/experimental approaches to decipher mechanisms of new electrocatalysts and direct the rational design of improved platforms.

Computational Study for CO2-to-CO Conversion over Proton Reduction Using [Re[bpyMe(Im-R)](CO)3Cl]+ (R = Me, Me2, and Me4) Electrocatalysts and Comparison with Manganese Analogues

The Nippe group has previously reported a series of imidazolium-functionalized rhenium bipyridyl tricarbonyl electrocatalysts, [Re[bpyMe(Im-R)](CO)3Cl]+ (R = Me and Me2), for CO2-to-CO conversion using H2O as the proton source [Sung, S.; Kumar, D., Electrocatalytic CO2 Reduction by Imidazolium-Functionalized Molecular Catalysts. J. Am. Chem. Soc. 2017, 139, 40, 13993−13996. 10.1021/jacs.7b07709]. These compounds feature charged imidazolium ligands in the secondary coordination sphere and exhibit higher catalytic activities as compared to the Lehn catalyst [Re(bpy)(CO)3Cl] (where bpy = 2,2′-bipyridine). However, the reaction mechanism for the CO2 reduction reaction (CO2RR) over the competing hydrogen evolution reaction (HER) is unclear. Here, we employ density functional theory (DFT) and restricted active space self-consistent field (RASSCF) methods to study the selectivity for CO2 fixation using [Re[bpyMe(ImMe)](CO)3Cl]+ (1+) in water and compare its reactivity to [Re[bpyMe(ImMe2)](CO)3Cl]+ (2+) and [Re[bpyMe(ImMe4)](CO)3Cl]+ (3+). Our results reveal that the turnover frequency (TOF) for CO2RR using 1+ is 4 orders of magnitude higher than for proton reduction, consistent with controlled potential electrolysis (CPE) experiments in which CO was the only detectable reduction product. The imidazolium moiety in the secondary coordination sphere stabilizes the metallocarboxylate species and assists the C–O cleavage through intermolecular hydrogen-bonding stabilizations. Furthermore, our calculations imply that the strongest hydrogen-bonding interactions at the C2 position in 1+ contribute to the faster reaction rate observed experimentally with respect to 2+. More significantly, the use of the energy span model demonstrates that the turnover frequency-determining transition state (TDTS) corresponds to the formation of the Re–CO2 adduct, contrasting with manganese analogues in which the C–O bond cleavage step is the TDTS. We attribute this distinction based on the electronic structures of doubly reduced active catalysts. Indeed, RASSCF calculations indicate that rhenium compounds are best described as a rhenium(I) coupled with a doubly reduced bipyridine ligand, [ReI[bpyMe(ImMe)2–](CO)3]0. In contrast, manganese analogues feature a metal center in a formal zero oxidation state antiferromagnetically coupled with an unpaired electron on the bpy, [Mn0[bpyMe(ImMe)•–](CO)3]0.

Lowering Electrocatalytic CO2 Reduction Overpotential Using N-Annulated Perylene Diimide Rhenium Bipyridine Dyads with Variable Tether Length

We report the design, synthesis, and characterization of four N-annulated perylene diimide (NPDI) functionalized rhenium bipyridine [Re(bpy)] supramolecular dyads. The Re(bpy) scaffold was connected to the NPDI chromophore either directly [Re(py-C0-NPDI)] or via an ethyl [Re(bpy-C2-NPDI)], butyl [Re(bpy-C4-NPDI)], or hexyl [Re(bpy-C6-NPDI)] alkyl-chain spacer. Upon electrochemical reduction in the presence of CO2 and a proton source, Re(bpy-C2/4/6-NPDI) all exhibited significant current enhancement effects, while Re(py-C0-NPDI) did not. During controlled potential electrolysis (CPE) experiments at Eappl = -1.8 V vs Fc+/0, Re(bpy-C2/4/6-NPDI) all achieved comparable activity (TONco ∼ 25) and Faradaic efficiency (FEco ∼ 94%). Under identical CPE conditions, the standard catalyst Re(dmbpy) was inactive for electrocatalytic CO2 reduction; only at Eappl = -2.1 V vs Fc+/0 could Re(dmbpy) achieve the same catalytic performance, representing a 300 mV lowering in overpotential for Re(bpy-C2/4/6-NPDI). At higher overpotentials, Re(bpy-C4/6-NPDI) both outperformed Re(bpy-C2-NPDI), indicating the possibility of coinciding electrocatalytic CO2 reduction mechanisms that are dictated by tether-length and overpotential. Using UV-vis-nearIR spectroelectrochemistry (SEC), FTIR SEC, and chemical reduction experiments, it was shown that the NPDI-moiety served as an electron-reservoir for Re(bpy), thereby allowing catalytic activity at lower overpotentials. Density functional theory studies probing the optimized geometries and frontier molecular orbitals of various catalytic intermediates revealed that the geometric configuration of NPDI relative to the Re(bpy)-moiety plays a critical role in accessing electrons from the electron-reservoir. The improved performance of Re(bpy-C2/4/6-NPDI)dyads at lower overpotentials, relative to Re(dmbpy), highlights the utility of chromophore electron-reservoirs as a method for lowering the overpotential for CO2 conversion.

Experimental Reasons for the Variable Efficiencies of Organic Electrocatalysts Used for Converting Carbon Dioxide to Methanol

AbstractPyridinium (the protonated form of pyridine) and related compounds have been proposed to be promising homogenous electrocatalysts in the electrochemical reduction of CO2 to methanol due to the low overpotential required to achieve faradaic yields of about 20 %, although the percentage yields vary dramatically between different research groups. In this study, experimental conditions were varied during the electrolysis of CO2 at a platinum electrode in the presence of pyridinium to determine the reasons for the discrepancies in the yields of methanol reported between different research groups. Two other vitamin‐based and environmentally friendly nitrogen‐containing heterocyclic compounds (nicotinamide and nicotinic acid) which have structural similarities to the promising but toxic pyridine homogeneous electrocatalyst, were also investigated as alternative electrocatalysts for the reduction of CO2 in aqueous acidic media. Cyclic voltammetry studies suggest that nicotinamide and nicotinic acid (forms of vitamin B3) follow a similar reaction mechanism as pyridine in the reduction of CO2. Relatively low faradaic yields of methanol were obtained during controlled potential electrolysis experiments for all the electrocatalysts (0.4–1.9 %) which can be attributed to the low solubility of CO2 along with the competing hydrogen evolution reaction. It was found that adventitious sources of methanol were responsible for greatly inflating the apparent yields of methanol unless scrupulous care was taken in controlling the experimental conditions. The problem with background methanol is particularly difficult to control due to the electrochemical reactions typically generating products in the parts per million range, considerably lower than normal synthetic reactions.

Protolytic behavior of water-soluble zinc(II) porphyrin and the electrocatalytic two-electron water oxidation to form hydrogen peroxide

Two-electron water oxidation is an energy-efficient way for solar energy conversion and one of the promising candidates to get through the bottleneck of artificial photosynthesis, photon- flux density problem. In light of renewable energy factor (REF) that is defined by Energy output/Energy input, for the realization of practical systems, the energy input for catalyst preparation have much importance. Herein, we report a facile, cost-effective and environment benign synthesis of zinc (II) 5, 10, 15, 20-tetrakis(N-methylpyridinium-4′-yl)porphyrin (ZnTMPyP) in water at room temperature. We observed coordination of water molecules to the central Zn(II) ion of ZnTMPyP and four-step protolytic equilibria among five axially ligated species. Electrochemical and controlled potential electrolysis experiments, as well as theoretical DFT calculation, showed that ZnTMPyP(OH)(O−) exhibits the two-electron water oxidation to form hydrogen peroxide as the primary product initiated by one-electron oxidation process of the catalyst with a moderate turnover frequency (96.4 s-1) and a small overpotential (∼60 mV). Isotope-labeled studies clearly showed that water molecule served as an oxygen atom source in the formation of hydrogen peroxide.

Homogeneous Electrochemical Water Oxidation at Neutral pH by Water-Soluble NiII Complexes Bearing Redox Non-innocent Tetraamido Macrocyclic Ligands.

Water oxidation is the bottleneck reaction in artificial photosynthesis. Exploring highly active and stable molecular water oxidation catalysts (WOCs) is still a great challenge. In this study, a water-soluble NiII complex bearing a redox non-innocent tetraamido macrocyclic ligand (TAML) is found to be an efficient electrocatalyst for water oxidation in neutral potassium phosphate buffer. Controlled-potential electrolysis experiments show that it can sustain at a steady current of approximately 0.2 mA cm-2 for >7 h at 1.75 V versus normal hydrogen electrode (NHE) without the formation of NiOx . Electrochemical and spectroelectrochemical tests show that the redox-active ligand, as well as HPO4 2- in the buffer, participate in the catalytic cycle. More importantly, catalytically active intermediate [NiIII (TAML2- )-O. ] is formed via several proton-coupled electron transfer processes and reacts with H2 O with the assistance of base to release molecular oxygen. Thus, the employment of redox non-innocent ligands is a useful strategy for designing effective molecular WOCs.

Electrocatalytic CO2 Reduction to Formate with Molecular Fe(III) Complexes Containing Pendent Proton Relays.

Previously, we reported an iron(III) complex with 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(2,4-ditertbutyl-phenol) as a ligand (Fe(tbudhbpy)Cl, 1) as catalytically competent for the electrochemical reduction of CO2 to formate (Faradaic efficiency FEHCO2- = 68 ± 4%). In mechanistic experiments, an essential component was found to be a pre-equilibrium reaction involving the association of the proton donor with the catalyst, which preceded proton transfer to the Fe-bound O atoms upon reduction of the Fe center. Here, we report the synthesis, structural characterization, and reactivity of two iron(III) compounds with 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(2-methoxy-4-methylphenol) (mecrebpy[H]2, Fe(mecrebpy)Cl, 2) and 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(4-(tert-butyl)benzene-1,2-diol) (tbucatbpy[H]4, Fe(tbucatbpy), 3) as ligands, where pendent -OMe and -OH groups are poised to modify the protonation reaction involving the Fe-bound O atoms. Differences in selectivity and activity for the electrocatalytic reduction of carbon dioxide (CO2) to formate (HCO2-) between complexes 1-3 were assessed via cyclic voltammetry and controlled potential electrolysis (CPE) experiments in N,N-dimethylformamide. Mechanistic studies suggest that the O atoms in the secondary coordination sphere are important for relaying the exogenous proton donor to the active site through a preconcentration effect, which leads to the JHCO2- (partial catalytic current density for formate) increasing by 3.3-fold for 2 and 1.2-fold for 3 in comparison to the JHCO2- of 1. These results also suggest that there is a difference in the strength of the interaction between the pendent functional groups and the sacrificial proton donor between 2 and 3, resulting in quantifiable differences in catalytic activity and efficiency. CPE experiments demonstrate an increased FEHCO2- = 85 ± 2% for 2, whereas 3 had a lower FEHCO2- = 71 ± 3%, with CO and H2 generated as co-products in each case to reach mass balance. These results indicate that using secondary sphere moieties to modulate metal-ligand interactions and multisite electron and proton transfer reactivity in the primary coordination sphere through reactant preconcentration can be a powerful strategy for enhancing electrocatalytic activity and selectivity.

Hydrogen Evolution Reactions Catalyzed by a Bis(thiosemicarbazone) Cobalt Complex: An Experimental and Theoretical Study

The synthesis and characterization of a dinuclear bis(thiosemicarbazone) cobalt complex [Co2 L2 (NCS)2 ] is reported. This complex exhibits significant catalytic activity for hydrogen production in DMF by using triethylammonium (Et3 NHBF4 ) as the proton source. Cyclic voltammetry data allowed a maximum turnover frequency of 130 s-1 for 1 m proton concentration to be determined. The catalytic nature of the process and the production of dihydrogen were confirmed by gas analysis during controlled potential electrolysis experiments. Quantum chemical calculations show that the complex displays a ligand-assisted metal-centered reactivity and supports a catalytic mechanism involving ligand-based reduction and protonation steps followed by metal-centered processes.

Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring p Ka Effects and Demonstrating Electrocatalysis.

Substrate selectivity in reductive multielectron/proton catalysis with small molecules such as N2, CO2, and O2 is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, P3BFe+, for catalyzing the nitrogen reduction reaction (N2RR, N2-to-NH3 conversion) versus HER changes as a function of acid p Ka. We find that there is a strong correlation between p Ka and N2RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N2 reduction (e.g., Fe(NNH) or Fe(NNH2)) in the presence of the metallocene reductant Cp*2Co. This suggests that the interaction of acid and reductant is playing a critical role in N-H bond-forming reactions. DFT studies identify a protonated metallocene species as a strong PCET donor and suggest that it should be capable of forming the early stage N-H bonds critical for N2RR. Furthermore, DFT studies also suggest that the observed p Ka effect on N2RR efficiency is attributable to the rate and thermodynamics of Cp*2Co protonation by the different anilinium acids. Inclusion of Cp*2Co+ as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH3. The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst (up to 6.7 equiv of NH3 per Fe at -2.1 V vs Fc+/0).

Electrocatalytic Reduction of CO2 to Formate by an Iron Schiff Base Complex.

The synthesis, structural characterization, and reactivity of an iron(III) chloride compound of 6,6'-di(3,5-di-tert-butyl-2-hydroxybenzene)-2,2'-bipyridine (Fe(tbudhbpy)Cl), under electrochemically reducing conditions is reported. In the presence of carbon dioxide (CO2) under anhydrous conditions in N,N-dimethylformamide (DMF), this complex mediates the 2e- reductive disproportionation of two equivalents of CO2 to carbon monoxide (CO) and carbonate (CO32-). Upon addition of phenol (PhOH) as a proton source under CO2 saturation, catalytic current is observed; product analysis from controlled potential electrolysis experiments shows the majority product is formate (68 ± 4% Faradaic efficiency), with H2 as a minor product (30 ± 10% Faradaic efficiency) and minimal CO (1.1 ± 0.3% Faradaic efficiency). On the basis of data obtained from cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC), the release of CO from intermediate Fe carbonyl species is extremely slow and undergoes competitive degradation, limiting the activity and lifetime of this catalyst. Mechanistic studies also indicate the phenolate moieties coordinated to Fe are sensitive to protonation in the reduced state, suggesting the possibility of cooperative pendent proton interactions being involved in CO2 reduction.

Improvement of Electrochemical Water Oxidation by Fine-Tuning the Structure of Tetradentate N4 Ligands of Molecular Copper Catalysts.

Two copper complexes, [(L1)Cu(OH2 )](BF4 )2 [1; L1=N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)-1,2-diaminoethane] and [(L2)Cu(OH2 )](BF4 )2 [2, L2=2,7-bis(2-pyridyl)-3,6-diaza-2,6-octadiene], were prepared as molecular water oxidation catalysts. Complex 1 displayed an overpotential (η) of 1.07 V at 1 mA cm-2 and an observed rate constant (kobs ) of 13.5 s-1 at η 1.0 V in pH 9.0 phosphate buffer solution, whereas 2 exhibited a significantly smaller η (0.70 V) to reach 1 mA cm-2 and a higher kobs (50.4 s-1 ) than 1 under identical test conditions. Additionally, 2 displayed better stability than 1 in controlled potential electrolysis experiments with a faradaic efficiency of 94 % for O2 evolution at 1.58 V, when a casing tube was used for the Pt cathode. A possible mechanism for 1- and 2-catalyzed O2 evolution reactions is discussed based on the experimental evidence. These comparative results indicate that fine-tuning the structures of tetradentate N4 ligands can bring about significant change in the performance of copper complexes for electrochemical water oxidation.

Rhenium Complexes Based on 2-Pyridyl-1,2,3-triazole Ligands: A New Class of CO2 Reduction Catalysts.

A series of [Re(N^N)(CO)3(X)] (N^N = diimine and X = halide) complexes based on 4-(2-pyridyl)-1,2,3-triazole (pyta) and 1-(2-pyridyl)-1,2,3-triazole (tapy) diimine ligands have been prepared and electrochemically characterized. The first ligand-based reduction process is shown to be highly sensitive to the nature of the isomer as well as to the substituents on the pyridyl ring, with the peak potential changing by up to 700 mV. The abilities of this class of complexes to catalyze the electroreduction and photoreduction of CO2 were assessed for the first time. It is found that only Re pyta complexes that have a first reduction wave with a peak potential at ca. -1.7 V vs SCE are active, producing CO as the major product, together with small amounts of H2 and formic acid. The catalytic wave that is observed in the CVs is enhanced by the addition of water or trifluoroethanol as a proton source. Long-term controlled potential electrolysis experiments gave total Faradaic yield close to 100%. In particular, functionalization of the triazolyl ring with a 2,4,6-tri-tert-butylphenyl group provided the catalyst with a remarkable stability.

A Thiosemicarbazone–Nickel(II) Complex as Efficient Electrocatalyst for Hydrogen Evolution

AbstractWe report herein the synthesis and characterization of a new mononuclear nickel complex based on a thiosemicarbazone ligand that exhibits an electrocatalytic behavior for H2 evolution in DMF using trifluoroacetic acid (TFA) as the proton source. Catalysis is observed at quite small overpotential values and a maximum turnover frequency (TOFmax) of 3080 s−1 was extrapolated for 1 m proton concentration using the foot‐of‐the wave analysis of cyclic voltammetry data. Gas analysis during controlled potential electrolysis experiments confirmed the catalytic nature of the process and production of dihydrogen with 80 % faradaic yield. Quantum chemical calculations indicate that the catalytic mechanism involves first ligand‐based reduction and protonation steps followed by metal‐centered processes.

The Electrochemical Study of Vanillin in Acetonitrile

A detailed electrochemical study of vanillin was performed in acetonitrile using a platinum electrode. At low scan rates≤1Vs−1, vanillin displayed an anodic peak at ca. 1.12 vs. (Fc/Fc+)/V, and a cathodic peak at ca. 0.09 vs. (Fc/Fc+)/V (on the reverse cyclic voltammetry (CV) scan after the initial oxidation) due to the reduction of a secondary reaction product. Both redox processes were found to involve a transfer of two electrons via controlled-potential electrolysis experiments, and it is surmised that vanillin first undergoes a −2e−/-H+ oxidation, followed by a hydrolysis reaction and the loss of its methoxy substituent to generate a corresponding 1,2-benzoquinone that can subsequently be reduced via +2e−/+2H+. When higher scan rates ≥2Vs−1 were employed, however, the aforementioned homogeneous reactions were partially outrun; leading to the detection of an additional cathodic peak at ca. −0.28 vs. (Fc/Fc+)/V which is ascribed to the reduction of the transient hemiketal intermediate that is formed immediately after the reaction of the phenoxonium cation with trace water. A similar trend to the fast scan rate CVs was likewise recorded at lowered temperatures (≤−10°C). Digital simulations were used to model the cyclic voltammetry data which enabled an estimation of the electrochemical and kinetic parameters associated with the electrode reactions. Independently, vanillin can also be electrochemically reduced at ca. −1.58 vs. (Fc/Fc+)/V, which is proposed to involve the formation of molecular hydrogen.

Assessing the stability of the Co(I) species of two mononuclear dichlorocobalt compounds bearing 2,2′-bipyridine and trans-2-(2′-quiolyl)methylene-3-quinuclidione via59Co NMR spectroscopy, electrochemical, and catalyzed proton electroreduction studies

The Co(I) species resulting from the reduction of two mononuclear species [CoLnCl2] (where L=2,2′-bipyridine (bpy) and n=2, or L=trans-2-(2′-quiolyl)methylene-3-quinuclidione (quin) and n=1), was studied by 59Co NMR spectroscopy, UV–visible-NIR spectroscopy, and spectroelectrochemical techniques. A quasi-stable Co(I) species produced by the reduction of [Co(bpy)2Cl2] was studied and gave a 59Co NMR spectroscopic chemical shift of 2934ppm in CD3CN-D2O (4:1, v/v) and a broad peak of λmax=600nm in the UV–visible region of the spectrum. The Co(I) species generated from [Co(quin)Cl2) (when reduced by electrochemical methods) was found to be unstable and produced a transient of λmax=530nm. Both [CoLnCl2] species were examined as electrocatalysts for the reduction of protons in an acetonitrile–water solvent mixture in the presence of p-cyanoanilinium tetrafluoroborate. The electrochemical properties of both species showed a dependence on the supporting electrolyte which also affected the electrocatalytic behavior. The simulation of the cyclic voltammograms in CH3CN-H2O (4:1, v/v) allowed for the extraction of kinetic data and suggested a homogenous reaction following the reduction to a Co(I) metal centre with rate constants k=0.01s−1 and 2Ms−1 for the Co(I) species with the ligands, bpy and quin, respectively. Analysis of the head space of controlled potential electrolysis experiments for an hour, in the presence of p-cyanoanilinium tetrafluoroborate, confirmed the production of hydrogen, and also showed that the supporting electrolyte affected the production of hydrogen at a glassy carbon electrode in CH3CN–H2O (either 1:1 or 4:1, v/v). A mechanism was postulated which involved a CoIII–H species as the most likely candidate and appeared to involve both a homolytic and a heterolytic pathway towards the production of hydrogen.

Electrocatalytic carbon dioxide reduction by using cationic pentamethylcyclopentadienyl-iridium complexes with unsymmetrically substituted bipyridine ligands.

Eight [Ir(bpy)Cp*Cl](+) -type complexes (bpy= bipyridine, Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) containing differently substituted bipyridine ligands were synthesized and characterized. Cyclic voltammetry (CV) of the complexes in Ar-saturated acetonitrile solutions showed that the redox behavior of the complexes could be fine tuned by the electronic properties of the substituted bipyridine ligands. Further CV in CO2 -saturated MeCN/H2 O (9:1, v/v) solutions showed catalytic currents for CO2 reduction. In controlled potential electrolysis experiments (MeCN/MeOH (1:1, v/v), Eapp =-1.80 V vs Ag/AgCl), all of the complexes showed moderate activity in the electrocatalytic reduction of CO2 with good stability over at least 15 hours. This electrocatalytic process was selective toward formic acid, with only traces of dihydrogen or carbon monoxide and occasionally formaldehyde as byproducts. However, the turnover frequencies and current efficiencies were quite low. No direct correlation between the redox potentials of the complexes and their catalytic activity was observed.

Using the Alkynyl-Substituted Rhenium(I) Complex (4,4′-Bisphenyl-Ethynyl-2,2′-Bipyridyl)Re(CO)3Cl as Catalyst for CO2 Reduction—Synthesis, Characterization, and Application

The synthesis, structure, photophysics, and spectroscopic characterization of an organometallic rhenium multichromophore compound carrying a central 2,2′-bipyridyl acceptor moiety with additional phenylethynyl substituents conjugated at the 4,4′-positions of the acceptor ligands and its effect on the electron density of the central rhenium atom as metal center for CO2 reduction is reported. The results were compared to fac-(2,2′-bipyridyl)Re(CO)3Cl and fac-(5,5′-bisphenylethynyl-2,2′-bipyridyl)Re(CO)3Cl. Cyclovoltammetric studies and rotating disk electrochemistry were performed for electrochemical characterization. Ultraviolet and visible (UV-vis) absorption, Fourier transform infrared (FTIR), and luminescence measurements were carried out for a spectroscopic characterization and compared to theoretical calculations at the density functional theory (DFT) level. In addition, the rhenium complex fac-(4,4′-bisphenyl-ethynyl-2,2′-bipyridyl)-Re(CO)3Cl was used as a novel catalyst for the electrochemical reduction of CO2 in homogeneous solution. Results showed an 11-fold increase in the current density under CO2 saturation and a catalytic second-order rate constant for CO formation of about 560 M−1 s−1 on a Pt working electrode. For further characterization of the CO2 reduction capabilities, bulk controlled potential electrolysis experiments were performed using a CO2-saturated acetonitrile electrolyte solution. The headspace product gas analysis yields CO as main reduction product with faradaic efficiencies of about 12 % over 5-h electrolysis time.

Electrochemical coating of [trans-L14CoIIICNFeII(CN)5]Na on ITO/Au electrode and its electrocatalytic properties towards nitrite reduction

The preparation of an ITO (Indium Tin Oxide) modified electrode with a mixed valence CoIII/FeII complex is described. The ITO electrode was initially modified with Au nanoparticles by dip coating in order to obtain and ITO/Au surface over which a film of the [trans-L14CoIIINCFeII(CN)5]Na (L14CoIII–FeII; L14=6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) complex was deposited electrochemically, thus obtaining an ITO/Au/L14CoIII–FeII modified electrode. The modified electrode has been characterized by cyclic voltammetry, Raman spectroscopy, AFM and SEM, thus confirming the presence of the mixed valence complex as a modifier species. The electrode prepared shows good stability in aqueous solution and its activity as electrocatalyst in the reduction of nitrites has been evaluated. Cyclic voltammetry experiments run in presence of nitrites show a current enhancement on the ITO/Au/L14CoIII–FeII modified electrodes with respect to the ITO/Au initial electrode. After controlled potential electrolysis experiments, analysis of the products indicate the presence of hydroxylamine as the main nitrite reduction species, although small quantities of hydrazine and ammonia where also obtained.

Electrochemical/chemical oxidation of bisphenol A in a four-electron/two-proton process in aprotic organic solvents

The electrochemical behavior of bisphenol A (BPA) was examined using cyclic voltammetry, bulk electrolysis and chemical oxidation in aprotic organic solvents. It was found that BPA undergoes a chemically irreversible voltammetric oxidation process to form compounds that cannot be electrochemically converted back to the starting materials on the voltammetric timescale. To overcome the effects of electrode fouling during controlled potential electrolysis experiments, NO+ was used as a one-electron chemical oxidant. A new product, hydroxylated bisdienone was isolated from the chemical oxidation of BPA with 4mol equiv of NO+SbF6− in low water content CH3CN. The structure of the cation intermediate species was deduced and it was proposed that BPA is oxidized in a four-electron/two-proton process to form a relatively unstable dication which reacts quickly in the presence of water in acetonitrile (in a mechanism that is similar to phenols in general). However, as the water content of the solvent increased it was found that the chemical oxidation mechanism produced a nitration product in high yield. The findings from this study provide useful insights into the reactions that can occur during oxidative metabolism of BPA and highlight the possibility of the role of a bisdienone cation as a reactive metabolite in biological systems.