Quasi-reversible Reduction Research Articles

Synthesis, electrochemistry and electrocatalytic activity of cobalt phthalocyanine complexes – Effects of substituents for oxygen reduction reaction

The synthesis, characterization, electrochemistry and electrocatalytic activity of the mono (pyridine-4-oxy)-tri (tert-butyl) phthalocyaninato Co(II) (Pc1) and mono (pyridine-4-oxy)-hexa (hexyl) phthalocyaninato Co(II) (Pc2) are reported here. One reversible and one irreversible oxidation couples and two quasi-reversible reduction couples are observed for two complexes on the cyclic voltammograms. Both cobalt phthalocyanine derivatives (CoPcs) and multiwall carbon nanotube (MWCNT) composite (CoPcs/MWCNT) electrodes are coated on glassy carbon electrodes and used as electrocatalysts for oxygen reduction reaction (ORR). The morphology of composite surfaces is characterized using Scanning Electron Microscope (SEM). The results obtained using rotating disk electrode (RDE) reveal that the high catalytic activity is observed with 4e− and mixed 2e− – 4e− ORR mechanisms for Pc1/MWCNT and Pc2/MWCNT hybrid catalysts in alkaline media, respectively. The tert-butyl substituent is more efficient with desired 4e− mechanism on the catalytic activity of the composite electrode compared to the hexyl substituent on the CoPc ring.

Blue-Emissive Cobalt(III) Complexes and Their Use in the Photocatalytic Trifluoromethylation of Polycyclic Aromatic Hydrocarbons.

Room-temperature luminescent CoIII complexes (1 and 2) are presented that exhibit intense ligand-to-metal and ligand-to-ligand charge transfer absorption in the low-energy UV region (λabs ≈360-400 nm) and low-negative quasi-reversible reduction events (E1/2(red) =-0.58 V and -0.39 V vs. SCE for 1 and 2, respectively). The blue emission of 1 and 2 at RT is due to the large bite angles and strong σ-donation of the ligands, the combined effect of which helps to separate the emissive 3 LMCT (triplet ligand-to-metal charge transfer) and the non-emissive 3 MC (triplet metal-centered) states. 1 and 2 were found to be powerful photo-oxidants (ECoIII*/CoII =2.26 V and 2.75 V vs. SCE of 1 and 2, respectively) and were used as inexpensive photoredox catalysts for the regioselective mono(trifluoromethylation) of polycyclic aromatic hydrocarbons (PAHs) in good yields (ca. 40-58 %).

Synthesis, Photochemical, Electrochemical and Cytotoxic Studies on Azobenzene Cored Dendrimer Decorated with Chalcone Motif

Abstractp‐Nitroazobenzene cored dendrimers decorated with chalcone surface group has been synthesized upto second generation by click reaction through convergent strategy. Photoisomerisation of the synthesized azodendrimers at 365 nm reveal the isobestic point at 296 nm with photostationary state at 11 minutes. Cyclic voltammogram exhibited quasi reversible oxidation reduction potential for all the synthesized dendrimers. Further, the second generation azodendrimer show better cytotoxic effect against MCF‐7 human breast cancer cell lines than the lower generation dendrimers.

Electrocatalytic proton reduction by thiolate-capped triiron clusters [Fe3(CO)9(μ3-SR)(μ-H)] (R = iPr, tBu)

The redox behaviour and electrocatalytic proton reduction ability of the thiolate-capped triiron clusters [Fe3(CO)9(μ3-SR)(μ-H)] (1, R = iPr; 2, R = tBu) have been investigated. In CH2Cl2, both show a quasi-reversible reduction and an irreversible oxidation. The thiol substituent has a significant influence on their reduction potentials (E1/2 = −1.24 V for 1 and E1/2 = −1.40 V for 2vs. Fc+/Fc) but less impact on oxidation potentials (E1/2 = 0.99 V for 1 and E1/2 = 0.93 V for 2vs. Fc+/Fc). Reduction is quasi-reversible and DFT studies reveal that this is due to scission of an iron-iron bond. While the clusters are not protonated by CF3CO2H or HBF4·Et2O, they can catalyse proton reduction of these acids at their corresponding reduction potentials following an ECEC mechanism.

Synthesis and DSSC application of donor-acceptor stilbenoid dendrimers with triphenylamine as core and benzothiazole as surface unit

Abstract The π-conjugated donor-acceptor stilbenoid dendrimers are successfully synthesized in high yields by either Aldol-type condensation or Wittig reaction with triphenylamine as donor and benzothiazole as acceptor. The molar extinction coefficient increases with increase in the number of the conjugated π-bridges. All the stilbenoid dendrimers exhibit quasi-reversible oxidation reduction signal in cyclic voltammetry. The highly π-conjugated stilbenoid dendrimers exhibit better current generating capacity and light energy harvesting efficiency than the other dendrimers, when used as additives in dye-sensitized solar cells. Stilbenoid dendrimer 7 shows the maximum power conversion efficiency (ɳ) of 5.08%.

Tuning the photophysical properties of carboranyl luminophores by closo- to nido-carborane conversion and application to OFF-ON fluoride sensing.

A family of closo-carborane-appended luminophores (closo-OXD1-2 and closo-DPS1-2) in which 2-R-o-carboranes (R = H, Me) are attached to the diphenyl-1,3,4-oxadiazole (OXD) or diphenyl-sulfone (DPS) acceptor groups were prepared and characterized. Deboronation of the closo-carborane cage produced the corresponding nido-carboranyl luminophores (nido-OXD1-2 and nido-DPS1-2). Whereas the closo-compounds were poorly emissive in THF (ΦPL < 0.01), the nido-luminophores exhibited an intense fluorescence with good quantum yields (ΦPL = 0.1-0.45). Electrochemical studies showed that while the closo-OXD and -DPS compounds displayed only carborane-centred, quasi-reversible reduction, the nido-compounds exhibited the typical features for nido-carborane-centred, irreversible oxidation and acceptor-centred, reversible reduction. Theoretical studies suggested that while the 1ππ* state of closo-compounds is nonemissive due to the contribution of closo-carborane to the LUMO in the S1 excited state, the intramolecular charge transfer (ICT) state from the nido-carborane to acceptor moieties in nido-compounds leads to an efficient fluorescence. Finally, THF solutions of closo-OXD1 and -DPS1 showed strong fluorescence upon the addition of fluoride anions under mild heating, but were intact to other anions, including cyanide, allowing the selective OFF-ON fluorescence sensing of fluoride.

Titanium thiosalicylate complexes: functional metalloligands for the construction of redox-active heterometallic architectures

The titanium complex [TiCp*(thiosal)(thiosalH)] (1) has been synthesised by reaction of [TiCp*Me3], Cp* = η5-C5Me5, with thiosalicylic acid (H2thiosal). Complex 1 reacts with [M(μ-OH)(COD)]2 (M = Rh, Ir) to yield the corresponding early-late heterobimetallic complexes [TiCp*(thiosal)2M(COD)] (M = Rh (2); Ir (3)). Carbon monoxide replaces the COD ligand in 2 and 3 leading to the respective dicarbonyl complexes [TiCp*(thiosal)2M(CO)2] (M = Rh (4); Ir (5)). Compound 4 reacts with PPh3 to yield the monocarbonyl derivative [TiCp*(thiosal)2Rh(CO)(PPh3)] (6). The reaction of compound 1 with LinBu yields the tetrametallic complex [{TiCp*(thiosal)2Li}2(THF)3(H2O)] (7). Compound 7 reacts with [RuCp*Cl(COD)] yielding the heterometallic complex [TiCp*(thiosal)2RuCp*] (8). The molecular structures of compounds 4, 5 and 7 have been studied by X-ray diffraction. From cyclic voltammetric (CV) and square wave voltammetric (SWV) experiments, we observed that attachment of the titanium moiety of precursor 1 to a late transition metal moiety through the sulfur atoms has a significant influence on the reduction behaviour of the Ti(iv) metal centre. Thus, monometallic 1 exhibits an irreversible reduction process at -1.15 V vs. SCE, whereas the CVs of heterobimetallic compounds 2-6 are characterized by the reversible or quasi-reversible one-electron reduction of the Ti(iv)/Ti(iii) system, suggesting a significant stabilization of the Ti(iii) reduced species. Likewise, substitution of the M(COD) diolefin fragment in 2 and 3 by the M(CO)2 carbonyl-containing moiety (in compounds 4 and 5) leads to a significant anodic shift in the titanium E1/2 reduction redox potentials.

Synthesis and Characterization of a Phthalimide‐Containing Redox‐Active Polymer for High‐Voltage Polymer‐Based Redox‐Flow Batteries

AbstractA phthalimide‐containing redox‐active polymer is designed and synthesized in particular for the application in all‐organic redox‐flow batteries. 4‐Vinylbenzyl phthalimide is prepared in a one‐step reaction and copolymerized with a solubility promoting comonomer. The novel polymer is electrochemically characterized in various electrolytes via cyclic voltammetry and features a quasireversible reduction reaction at E0½ = −1.89 V versus AgNO3/Ag. Afterward, the polymer is applied as anode‐active material in a first preliminary battery test in combination with a 2,2,6,6‐tetramethylpiperdinyl‐N‐oxyl‐containing polymer as cathode‐active material and a porous membrane as separator, yielding a polymer‐based all‐organic battery system with a high cell voltage of 2.0 V.

Switching from sky blue to deep green fluorescent Zn(II) complexes for OLEDs applications

Three new Zn(II) 4′-aryl substituted 2,2′:6′,2′-terpyridine (terpy) complexes of the type [Zn(Ln)2]NO3 complexes (where, L1 = 4′-(4-methylphenyl)−2,2′:6′,2′-terpyridine, L2 = 4′-(3,4,5-trimethoxyphenyl)−2,2′:6′,2′-terpyridine, and L3 = 4′-(2-thiophenyl)−2,2′:6′,2′-terpyridine) were successfully synthesized and photophysical characterizations were done for OLEDs applications. The UV–visible absorption spectra of the complexes exhibited an intense absorption band in the range of 240–380nm owing to a spin-allowed intraligand (π–π*) charge transition. The addition of an electron-donating aryl substituent into the terpy ligand within the complexes induced a significant red-shift of the spectrum. A solid state photoluminescence (PL) analysis revealed a transition from sky blue to green, while the complexes in the DMSO solution which possessed the most emissions appeared sky to deep blue, respectively. A cyclic voltammetry investigation of the complexes revealed a quasi-reversible reduction where the estimated electrochemical data concurred with theoretical calculations. Moreover, the complexes showed an optical band gap of 3.40–3.51eV and a HOMO level from –6.64 to −6.47eV. In addition, all the complexes depicted an excellent thermal stability. Hence, the presented complexes are promising candidates for blue and green fluorescent OLEDs application as electron transport as well as electroluminescent material.

Visible-Light Photoactive, Highly Efficient Triplet Sensitizers Based on Iodinated Aza-BODIPYs: Synthesis, Photophysics and Redox Properties.

A series of novel iodinated NO2 -substituted aza-BODIPYs have been synthesized and characterized. Highly desirable photophysical and photochemical properties were induced in NO2 -substituted aza-BODIPYs by iodination of the pyrrole rings. In particular, high values of singlet oxygen quantum yields (ΦΔ ) ranging from 0.79 to 0.85 were measured. The photooxygenation process proceeds via a Type II mechanism under the experimental conditions applied. The compounds studied exhibited an absorption band within the so-called "therapeutic window", with λmax located between 645 nm to 672 nm. They were non-fluorescent at room temperature with excited singlet-state lifetimes within the picosecond range as measured by femtosecond transient absorption. Nanosecond laser flash photolysis experiments revealed T1 →Tn absorption spanning from ca. 400 nm to ca. 500 nm and allowed determination of the triplet-state lifetimes. The estimated triplet lifetimes (τT ) in deaerated acetonitrile ranged between 2.74 μs and 3.50 μs. As estimated by CV/DPV measurements, all iodinated aza-BODIPYs studied exhibited one irreversible oxidation and two quasi-reversible reductions processes. Estimation of the EHOMO gave the value of -6.06 to -6.26 eV while the ELUMO was found to be located at ca. -4.6 eV. Thermogravimetric (TGA) analysis revealed that iodinated aza-BODIPYs were stable up to approximately 300 °C. All compounds studied exhibit high photostability in toluene solution.

Cationic Nitrogen-Doped Helical Nanographenes.

Herein, we report the design and synthesis of a series of novel cationic nitrogen-doped nanographenes (CNDNs) with nonplanar geometry and axial chirality. Single-crystal X-ray analysis reveals helical and cove-edged structures. Compared to their all-carbon analogues, the frontier orbitals of the CNDNs are energetically lower lying, with a reduced optical energy gap and greater electron-accepting behavior. Cyclic voltammetry shows all the derivatives to undergo quasireversible reductions. In situ spectroelectrochemical studies prove that, depending on the number of nitrogen dopants, either neutral radicals (one nitrogen dopant) or radical cations (two nitrogen dopants) are formed upon reduction. The concept of cationic nitrogen doping and introducing helicity into nanographenes paves the way for the design and synthesis of expanded nanographenes or even graphene nanoribbons with cationic nitrogen dopants.

Electrochemical and spectroelectrochemical study on novel non-peripherally tetra 1,2,4-triazole substituted phthalocyanines

In the present study novel tetra 4-(4-fluorophenyl)-5-(4-methoxyphenyl)-4H-1,2,4-triazole-3-thio substituted non-peripherally metal free (4), zinc(II) (5), lead (II) (6) and copper(II) (7) phthalocyanines were synthesized. The obtained novel compounds were characterized by a combination of FT-IR, 1H NMR, UV–Vis and MALDI-TOF techniques. The redox properties of the complexes have been investigated via cyclic voltammetry, square wave voltammetry and in situ spectroelectrochemistry. The compounds displayed ring-based, reversible and/or quasi-reversible reduction and oxidation processes and aggregation of the complexes influenced the redox character of the processes. The color changes during the redox processes of metallo phthalocyanine were recorded by in-situ spectroelectrochemical measurements. In situ UV–vis spectroelectrochemical measurements, which was associated with color change of the complexes, showed their applicability in the fields of the electrochemical technologies.

Electrochemistry and spectroelectrochemistry of polymers based on D-A-D and D-D-D bis(N-carbazolyl) monomers, effect of the donor/acceptor core on their properties

In this work we present electropolymerization of monomers of an unusual type using N-linked carbazole units to limit their conjugation. The polymers thus obtained have limited conjugation through the backbone. Using donor-acceptor-donor (D-A-D) and donor-donor-donor (D-D-D) monomers we evaluate the effects of the presence (or absence) of charge transfer states on synthesized electropolymers. The use of a D-A-D monomer resulted in obtaining an ambipolar polymer with quasi-reversible reduction.

Electrocatalytic reduction of CO2 using rhenium complexes with dipyrido[3,2-a:2′,3′-c]phenazine ligands

A series of rhenium(I) complexes (1–4) with substituted dipyrido[3,2-a:2′,3′-c]phenazine (dppz) ligands were synthesized and analyzed as electrocatalysts for CO2 reduction. Cyclic voltammetry study shows that complexes 1–4 exhibit dppz ligand-based and metal center-based quasi-reversible reductions under N2. Under a CO2 atmosphere, complexes 1–4 exhibit electrocatalytic response consistent with CO2→CO reduction with the presence of the gaseous phase product CO confirmed by gas chromatography (GC). Re(2,2′-bpy)(CO)3Br (Lehn catalyst), a benchmark for electrocatalytic reduction of CO2 to CO, was also prepared and tested for electrocatalytic properties for comparison. The results suggest that chemical modification on the dppz ligand leads to interesting electrocatalytic properties and the large conjugation of the dppz ligand can help store multiple electrons and lower the energy barrier for CO2 reduction.

A nickel(II) thiosemicarbazonato complex: synthesis, structure, electrochemistry, and application in catalytic coupling of terminal alkynes with arylboronic acids

A simple route for the synthesis of a new square-planar nickel(II) complex of 2-((3-methylthiophen-2-yl)methylene)hydrazinecarbothioamide is described. Elemental analysis, solution conductivity, and spectral measurements were used to authenticate the composition of the complex. In addition, the X-ray crystal structure of the complex was obtained. Each of the two ligands of the centrosymmetric structure is coordinated to the metal center via the azomethine N and the thioimidate S atoms, forming five-membered chelate rings. Infrared and NMR spectroscopic features of the complex are consistent with its molecular structure. The complex displays a quasi-reversible reduction with E 1/2 of −0.83 V (ΔE p 160 mV), indicating that the ligand system can stabilize the Ni(I) oxidation state. The complex acts as an active catalyst for the C–C coupling of aryl acetylenes with arylboronic acids containing a variety of substituents. The influence of solvent, base, reaction temperature, and catalyst loading on the C–C coupling reaction was evaluated. A new square-planar nickel(II) complex bearing 2-((3-methylthiophen-2-yl)methylene)-hydrazinecarbothioamide was synthesized and characterized by elemental analysis, FTIR, NMR, and single-crystal X-ray crystallographic technique. Cyclic voltammetric study indicates stabilization of Ni(I) oxidation state. The complex acts as an effective catalyst for the carbon–carbon cross-coupling reaction of substituted phenylacetylenes with various arylboronic acids under optimized conditions.

Synthesis and Electrochemical Properties of Half-Sandwich Rhodium and Iridium Methyl Complexes

A series of complexes of the form [Cp(*)M(bpy)(CH3)]I was accessed by treatment of CpM(bpy) or Cp*M(bpy) with methyl iodide (M = Rh, Ir; Cp = cyclopentadienyl; Cp* = pentamethylcyclopentadienyl; bpy = 2,2′-bipyridyl). Solid state structures (X-ray diffraction) reveal the expected distorted octahedral geometry, with Cp or Cp* bound in the η5 mode and bpy bound in the typical κ2 mode. Electrochemical studies demonstrate that the Cp* complexes undergo a single, quasi-reversible one-electron reduction, whereas the Cp complexes undergo both a quasi-reversible one-electron reduction and a second, more negative, irreversible reduction. Electron paramagnetic resonance studies and comparisons between complexes of different metals suggest that the formulation of the singly reduced species is formally M(III) complexes with a bound bpy anion radical. The second reduction observed in the Cp complexes, on the other hand, results in cleavage of the M–C bond. Taken together, the results suggest that the compounds have st...

Electrochemical Capture and Release of CO2 in Aqueous Electrolytes Using an Organic Semiconductor Electrode.

Developing efficient methods for capture and controlled release of carbon dioxide is crucial to any carbon capture and utilization technology. Herein we present an approach using an organic semiconductor electrode to electrochemically capture dissolved CO2 in aqueous electrolytes. The process relies on electrochemical reduction of a thin film of a naphthalene bisimide derivative, 2,7-bis(4-(2-(2-ethylhexyl)thiazol-4-yl)phenyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NBIT). This molecule is specifically tailored to afford one-electron reversible and one-electron quasi-reversible reduction in aqueous conditions while not dissolving or degrading. The reduced NBIT reacts with CO2 to form a stable semicarbonate salt, which can be subsequently oxidized electrochemically to release CO2. The semicarbonate structure is confirmed by in situ IR spectroelectrochemistry. This process of capturing and releasing carbon dioxide can be realized in an oxygen-free environment under ambient pressure and temperature, with uptake efficiency for CO2 capture of ∼2.3 mmol g–1. This is on par with the best solution-phase amine chemical capture technologies available today.

Andrographis paniculata extract mediated green synthesis of CdO nanoparticles and its electrochemical and antibacterial studies

CdO nanoparticles based on Andrographis paniculata plant extract were synthesizedby biological method and characterized by XRD, FESEM, HRTEM and FTIR. XRD exhibits face centred cubic structure with an average crystallite size of 22 nm and the lattice strain (W–H plot) is 0.0023. From the HRTEM, the particles are spherical. The vibrational stretching mode of Cd–O is 456 cm−1. From the UV–Vis spectra, the optical energy bandgap is estimated as 2.33 eV. The photoluminescence emission peaks at 360, 430, 486, 528 and 616 nm are observed for the excitation at 220 nm. The electrochemical study by cyclic voltammetry reveals the quasi reversible reduction and irreversible oxidation versus Saturated Calomel Electrode (SCE). The potential is referred to the Fc/Fc + couple under the experimental conditions. The biosynthesized CdO nanoparticles are tested against gram positive (Rhodococcus rhodochrous and Staphylococcus aureus) and gram negative (Escherichia coli, Aeromonas hydrophila and Vibrio Cholera) bacterial strains. It is noted that the bioengineered CdO nanoparticles show a good antibacterial activity (zone of inhibition: E. coli-16 mm) bacteria.

Synthesis of 3-(pyridine-2-yl)-4,5-dihydro-1H-pyrazole-1-thiocarboxamides and their copper(II) complexes

New organic ligands 3-(pyridine-2-yl)-4,5-dihydro-1H-pyrazole-1-thiocarboxamides (L) have been synthesized by a two or three step reaction sequence starting from substituted benzaldehydes and 2-acetyl pyridine. Copper(II) complexes with LCuCl2 composition were obtained by the reactions of these ligands with CuCl2·2H2O. The crystal structure of two synthesized complexes has been determined by X-ray analysis. Copper atoms located in a strongly distorted square piramidal environment and coordinated by pyrazoline and pyridine nitrogen atoms, thiocarbamoyl sulfur atom and two chloride-anions. All complexes undergo reversible or quasi-reversible electrochemical reduction at 0.45–0.28V with the formation of Cu(I) containing intermediates. The cytotoxicity of copper containing complexes, comparable to cisplatin and doxorubicin, was demonstrated using cancer cell lines MCF-7, A549 and HEK293.

Coordination Chemistry and Reactivity of Bis(aldimino)pyridine Nickel Complexes in Four Different Oxidation States

A series of nickel complexes with potentially redox active bis(aldimino)pyridine ligands [NNN] ([NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-arylmethanimine), where aryl = 2,6-diisopropylphenyl, mesityl, 4-methoxyphenyl, 4-trifluoromethylphenyl, and 3,5-bis(trifluoromethyl)phenyl) were synthesized, and their properties and reactivities were investigated as a function of the overall oxidation state of the system. (Ni[NNN])2+ complexes of ligands featuring bulky electron-rich substituents (1a-Br2 and 1b-Br2, [NNN] = 1,1′-(pyridine-2,6-diyl)bis(N-(2,6-diisopropylphenyl)methanimine) and 1,1′-(pyridine-2,6-diyl)bis(N-mesitylmethanimine), respectively) demonstrated five electrochemical reduction events, the first three of which were quasi-reversible. In contrast, only two quasi-reversible reductions were observed for the less bulky and electron-deficient N-aryl substituents 4-(trifluoromethyl)phenyl and 3,5-bis(trifluoromethyl)phenyl. Chemical reduction of 1a-Br2 and 1b-Br2 with 1 equiv of KC8 or CoCp*2 forms (Ni[NNN])+ complexes of the general formula Ni[NNN]Br (2a-Br and 2b-Br). Structural, spectroscopic, and theoretical studies reveal that these complexes feature significant unpaired spin density on the metal, consistent with “nickel(I)” character. This behavior is in contrast with previously reported bis(ketimino)pyridine systems, in which at the (Ni[NNN])+ state the unpaired electron resided exclusively in the ligand. Further reduction forms a series of (Ni[NNN])0 complexes, in which all of the potentially tridentate [NNN] ligands bind via only one iminopyridine unit; the second arm is left unbound in most complexes. Variable temperature NMR spectroscopy demonstrates that bound and unbound arms exchange via a postulated tridentate intermediate. Electrochemical reduction, via three sequential one-electron reductions, of 1a-Br2 and 1b-Br2 in the presence of CO2/H+ forms an active catalyst for H2 evolution at a glassy-carbon electrode surface, again emphasizing the unique redox chemistry of the bulky bis(aldimino)pyridine nickel complexes.