Reversible Oxidation Processes Research Articles

Ultrafast Near‐Infrared Luminescence from Cyclometalated Iridium(III) Complexes with Indirubin as Ancillary Ligand

AbstractBeing a constitutional isomer of the indigo dye, indirubin has been known as a purple colourant for a long time. On the other hand, its structural and photophysical properties as ligand in metal complexes are virtually unexplored. Herein, for the first time, we utilized indirubin as ancillary chelator to develop two new heteroleptic iridium(III) complexes equipped with two 2‐phenylpyridine units as archetypical cyclometalating ligands. These new complexes display fully reversible oxidation and reduction processes, and show a panchromatic absorption extending up to 900 nm in acetonitrile solution at 298 K. Moreover, an unexpected broad and unstructured emission band is detected for both complexes in the near‐infrared region, peaking at approx. 1100 nm and having a lifetime of approx. 60 ps; such an emission is attributed to a ligand‐centred triplet excited state (3LC) located on the indirubin ligand itself, as proved by DFT calculations. These findings may pave the way for further exploration of the indirubin dye as a ligand for different types of metal centres to create efficient near‐infrared triplet emitters without the need for difficult synthetic procedures.

Synthesis, Characterization, and Properties of a Titanium(IV)-Tetrathiafulvalene-Based Complex.

To deeply investigate the interaction between a tetrathiafulvalene (TTF) unit and a Ti(IV) center, a monomeric heteroleptic octahedral Ti(IV) complex containing a diimine ligand composed of a 1,10-phenanthroline core fused with a TTF fragment (ligand 2a) was prepared. The stable complex formulated as Ti(1)2(2a), where 1 is a 2,2'-biphenolato derivative, was efficiently synthesized by following a one-step approach. This complex and its model species [Ti(1)2(2b)] were fully characterized in solution, and their solid-state structures were established by single-crystal X-ray diffraction analysis. Density functional theory calculations allowed the assignment of the frontier orbitals involved in the electronic transitions characterized by ultraviolet-visible absorption spectroscopy. Electrochemical and spectroelectrochemical studies revealed that the TTF unit within Ti(1)2(2a) can undergo two reversible one-electron oxidation processes; a reversible one-electron reduction of the Ti(IV) atom was highlighted. The photophysical measurements performed for this donor-acceptor molecular system indicated that an electron transfer process upon light excitation occurred within Ti(1)2(2a).

Hydrogen atom abstraction as a synthetic route to a square planar CoII complex with a redox-active tetradentate PNNP ligand.

Redox-active ligands improve the reactivity of transition metal complexes by facilitating redox processes independent of the transition metal center. A tetradentate square planar (PNCH2CH2NP)CoII (1) complex was synthesized and the ethylene backbone was dehydrogenated through hydrogen atom abstraction to afford (PNCHCHNP)CoII (2), which now contains a redox-active ligand. The ligand backbone of 2 can be readily hydrogenated with H2 to regenerate 1. Reduction of 1 and 2 with KC8 in the presence of 18-crown-6 results in cobalt-based reductions to afford [(PNCH2CH2NP)CoI][K(18-crown-6)] (3) and [(PNCHCHNP)CoI][K(18-crown-6)] (4), respectively. Cyclic voltammetry revealed two reversible oxidation processes for 2, presumed to be ligand-based. Following treatment of 2 with one equivalent of FcPF6, the one-electron oxidation product {[(PNCHCHNP)CoII(THF)][PF6]}·THF (5) was obtained. Treating 5 with an additional equivalent of FcPF6 affords the two-electron oxidation product [(PNCHCHNP)CoII][PF6]2 (6). Addition of PMe3 to 5 produced [(PNCHCHNP)CoII(PMe3)][PF6] (7). A host of characterization methods including nuclear magnetic resonance (NMR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, cyclic voltammetry, magnetic susceptibility measurements using SQUID magnetometry, single-crystal X-ray diffraction, and density functional theory calculations were used to assign 5 and 6 as ligand-based oxidation products of 2.

Synthesis, X-ray structures, Hirshfeld surface analysis, and redox behaviour of 2-substituted-1H-perimidine derivatives

Peri-naptho fused pyrimidine derivatives, known as 1H-perimidines or 1H-benzo[d,e]quinazolines, have recently been studied for a diverse range of applications. A series of 2-substituted-1H-perimidine compounds (1A–1F) have been synthesized by the condensation reaction of 1,8-diaminonaphthalene with the appropriate aldehyde. The 2-substituted-1H-perimidinium iodide salts (1A+I––1F+I–) were obtained by methylating 1H-perimidine using methyl iodide in the presence of a mild base K2CO3. The perimidinium salts were reduced using NaBH4 in methanol to obtain 2-substituted-2,3-dihydro-1H-perimidine compounds (1AH–1FH). The compounds were characterized by 1H, 13C-NMR and UV–Vis absorption spectroscopy, elemental analysis, and in the case of compounds 1C, 1B+I–, 1C+I–, 1E+I–, 1F+I–, and 1BH–1EH, their molecular structures have been determined using single crystal X-ray analysis and compared to those of related 1H-perimidine derived species. The crystal packings from the solid-state structures of 1C, 1C+I–, and 1CH were compared and revealed further stability of the molecule 1C through intermolecular NH···N hydrogen bonding and π···π stacking interactions, while in the case of molecules 1C+I–, and 1CH through short interatomic CH···C contacts between two adjacent perimidine rings. To further quantify the intermolecular interactions and investigate their contributions to the crystal packing in each molecule, Hirshfeld surface analysis was performed. The redox properties of the perimidinium salts 1A+I––1F+I–, and the hydride reduced species 1AH–1FH were studied using cyclic voltammetry, and compared to that of N-DMBI-H ((4-(1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazole-2-yl)phenyl)dimethylamine). The salts exhibit an irreversible reduction wave, like those of 2-substituted-1,3-dimethyl-1H-benzo[d]imidazolium salts, while the hydrides undergo a reversible oxidation process, with the peak ratio (Ipc/Ipa) > 0.5. This shows the chemical stability of the 1H•+ species at a scan rate of 100 mV s–1.

Redox Chemistry of Pt(II) Complex with Non-Innocent NHC Bis(Phenolate) Pincer Ligand: Electrochemical, Spectroscopic, and Computational Aspects

A Pt(II) complex bearing chelating tridentate bis-aryloxide tetrahydropyrimidinium-based N-heterocyclic carbene (NHC) was synthesized and characterized by using different techniques. Both cyclic voltammetry and differential pulse voltammetry were used to study the electrochemical properties of the complex, revealing two reversible one-electron oxidation processes. The chemical generation and isolation of one-electron-oxidized species were performed oxidizing the initial complex by means of AgBF4. A combination of spectroscopic (UV-Vis/NIR- and EPR-) and theoretical (density functional theory (DFT)) studies suggests the formation of a Pt(II)-phenoxyl radical complex. The latter open-shell derivative was structurally characterized by means of X-ray diffraction analysis. Finally, the neutral platinum complex was tested as a mediator in the process of electrocatalytic oxidation of 2-(methylamino)ethanol (MEA).

Electrochemical properties of cyclen and cyclam macrocycles bearing ferrocenyl pendants and their transition metal complexes

The ligands [R-Fc(cyclen)], [Fc(cyclen)2], [Fc(cyclam)2], [Fc2(cyclen)] and [Fc4(cyclen)] (R = –H or –CH2OH; Fc = ferrocene; cyclen = 1,4,7,10-tetraazacyclododecane; cyclam = 1,4,8,11-tetraazacyclotetradecane) and their respective Cu2+, Co2+, Cd2+, Zn2+, and Ni2+ metal complexes have been synthesised and electrochemically characterised. The voltammetry of the free ligands in a CH2Cl2/CH3CN (1:4) solvent mixture containing [Bu4N][PF6] or [Bu4N](B(C6F5)4] as the supporting electrolyte yields two closely spaced oxidation processes. The first one is Fc based, and the other is related to the interaction of the Fc with the nitrogen component. Details of the mechanism were established by studying the oxidation of N,N-dimethylaminomethylferrocene by cyclic voltammetry and bulk electrolysis with product isolation. However, cyclic voltammetries exhibit a single Fc-based reversible oxidation process when the ligands form metal complexes with Cu2+, Co2+, Cd2+, Zn2+, or Ni2+. Upon metal ion binding, an important positive shift in the reversible midpoint potential, Em, is observed. The magnitude of the shift in the Em values follows the order [Fc(cyclen)2] ≈ [Fc(cyclam)2] ≫ [R-Fc(cyclen)] ≈ [Fc2(cyclen)] > [Fc4(cyclen)]. The ability of the ligands to work as electrochemical sensors for the mentioned cations is discussed.

Caftaric acid oxidation in the presence of cell signaling regulator glutathione: Electrochemical and chromatographic analyses

New antioxidant materials in biomedical applications are an exciting research topic. However, the lack of understanding of the products/intermediates formed and the interaction between nucleophiles and these antioxidants will prevent commercial or industrial applications. To overcome this obstacle, chromatographic techniques, mass spectroscopy, and electrochemical analyses were combined to study the mechanism and products of the expected reaction between antioxidants and nucleophiles. Grape juice contains phenolic compounds such as 2-S-glutathionylcaftaric acid, which is formed as a result of oxidative action. Through UPLC and voltammetric analysis of caftaric acid (CFT) and its quinone with glutathione (GSH), the oxidation and subsequent reactions were monitored. Electrochemical techniques such as cyclic voltammetry (CV) and chronoamperometry (CA) have been used to investigate the electrochemical oxidation characteristics of the investigated CFT in a Britton-Robinson (B-R) buffer electrolyte. As a result of the analysis of ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS) results, the oxidation products were identified to suggest the optimal mechanism for the oxidation of CFTs both in the presence and absence of GSH. A reversible oxidation process is observed at pH 1.8 for CFT without glutathione (GSH) at E1/2 = 0.52 V against Ag/AgCl 3.0 M KCl) using cyclic voltammetry. During this electrochemical process, two electrons in the CFT molecule were oxidized along with two protons, forming o-quinone. In the presence of GSH, the formed quinone either reacts with glutathione to form mono- and bi-glutathione CFT conjugates or is reduced by GSH to its original state. As a result of the UPLC-MS analysis, the oxidation products of CFT were identified, and the reaction pathways were proposed.

Electroanalytical characterization of chloroquinoline substituted redox-active phthalocyanines

In the first part of this study, the synthesis and characterization of organosoluble 5-chloroquinolin-8-yloxy substituted iron(II) (2) and oxo-titanium (IV) phthalocyanines (3) are reported for the first time. These compounds have been characterized by elemental analysis, Fourier transform infrared, electronic spectroscopy, and mass spectra. Electrochemical behaviors of metal-free and cobalt phthalocyanines and further new types of iron and oxo-titanium phthalocyanines were investigated using electroanalytical methods, such as cyclic (CV) and square wave voltammetry (SWV). According to the electrochemical results, phthalocyanines by and large showed one-electron metal- and/or ligand-based reversible or quasi-reversible reduction and oxidation processes.
 All in all, this study's results inevitably create a useful way to use them in possible future studies, which will particularly attempt to use the compound investigated in potential areas of use.

Electrochemistry and Spin‐Crossover Behavior of Fluorinated Terpyridine‐Based Co(II) and Fe(II) Complexes

AbstractDue to their ability to form stable molecular complexes that have tailor‐made properties, terpyridine ligands are of great interest in chemistry and material science. In this regard, we prepared two terpyridine ligands with two different fluorinated phenyl rings on the backbone. The corresponding CoII and FeII complexes were synthesized and characterized by single‐crystal X‐ray structural analysis, electrochemistry and temperature‐dependent SQUID magnetometry. Single crystal X‐ray diffraction analyses at 100 K of these complexes revealed Co−N and Fe−N bond lengths that are typical of low spin CoII and FeII centers. The metal centers are coordinated in an octahedral fashion and the fluorinated phenyl rings on the backbone are twisted out of the plane of the terpyridine unit. The complexes were investigated with cyclic voltammetry and UV/Vis‐NIR spectroelectrochemistry. All complexes show a reversible oxidation and several reduction processes. Temperature dependent SQUID magnetometry revealed a gradual thermal SCO behavior in two of the complexes, while EPR spectroscopy provided further insights on the electronic structure of the metal complexes, as well as site of reduction.

Biologically inspired 3Fe4S cluster as structural mimics of FeMoco M‐cluster

AbstractA 3Fe4S cluster related to M‐cluster of Mo‐nitrogenase is reported. [K(THF)5][Fe3(μ‐bdt)2(μ‐PPh2)(CO)5] (1) is synthesized from photo‐assisted structural rearrangement of [K(THF)2][(μ,κ2‐bdt)Fe2(μ‐PPh2)(CO)5] under visible light irradiation. The molecular structure of 1 consists of a Fe3 core bearing one dithiolate bridge and the second dithiolate group capping onto the metallic plane. The structural motif of 1 is analogous to that of Mo‐participated 3Fe4S unit in M‐cluster of Mo‐nitrogenase, with the similar Fe‐Fe and Fe‐S bond distances. Upon protonation in 193 K, a Fe‐hydride species (1H) is generated and characterized to possess the bridging hydride group (δ = −12.57 ppm) by 1H‐NMR spectroscopy and DFT calculation. In CH3CN solution, complex 1 exhibits a reversible reduction and oxidation process at = −1.94 V and = −0.18 V, respectively, at 273 K. The reduction behavior of 1 in CH2Cl2 solution at 243 K displays a slight modification in the presence of trifluoroacetic acid, revealing a moderate anodic potential shift (~50 mV). The current amplitude of the reduction wave is linearly increased with the increasement of acid added, indicative of the catalytic event.

Multiple stable redox states and tunable ground states via the marriage of viologens and Chichibabin's hydrocarbon.

Chichibabin's hydrocarbon and viologens are among the most famous diradicaloids and organic redox systems, respectively. However, each has its own disadvantages: the instability of the former and its charged species, and the closed-shell nature of the neutral species derived from the latter, respectively. Herein, we report that terminal borylation and central distortion of 4,4'-bipyridine allow us to readily isolate the first bis-BN-based analogues (1 and 2) of Chichibabin's hydrocarbon with three stable redox states and tunable ground states. Electrochemically, both compounds exhibit two reversible oxidation processes with wide redox ranges. One- and two-electron chemical oxidations of 1 afford the crystalline radical cation 1˙+ and dication 12+, respectively. Moreover, the ground states of 1 and 2 are tunable with 1 as a closed-shell singlet and the tetramethyl-substituted 2 as an open-shell singlet, the latter of which could be thermally excited to its triplet state because of the small singlet-triplet gap.

Electrochemical Synthesis of Polycarbazole Film in Nonacidic Medium and its Electrochromic Properties

This study is about the electrochemical polymerization of the monomer of carbazole, whose anodic polymerization is very common in the literature. When the studies in the literature were examined, it was seen that the electrochromic properties of carbazole were not examined. For this purpose, electrochemical and optical properties were investigated in detail by polymerizing carbazole on both glassy carbon and ITO glass electrodes in 0.1 M TABPF6/ DCM medium using the cyclic voltammetry method. The polymer film synthesized by electroanalytical methods showed reversible electrochemical oxidation process properties as well as electrochromic properties. The polymer film synthesized by electroanalytical methods showed reversible electrochemical oxidation process properties as well as electrochromic properties. Considering that this study will form a basis for other studies, it is thought that the evaluation of carbazole monomer in terms of its electrochromic properties will provide a great deal to the literature.

Electrochemical and in-situ spectroelectrochemical properties of novel (5-(tert-butyl)-2-((3,4-dicyanophenoxy)methyl)phenyl)methanolate substituted mononuclear metal phthalocyanines

This work involves the synthesis and characterization of metallic (Co, Fe, Mn, Ni, Zn) ball-type phthalocyanine precursors by using novel 4,4′-(((4-(tert-butyl)-1,2-phenylene)bis-(methylene))bis(oxy))diphthalonitrile starting compound. The classical oxo-bridge in phthalocyanines was altered with the -OCH2- bridge first time for these ball-type phthalocyanine precursors, and the effects of this change on the chemical, physical and spectral features of phthalocyanine complexes were investigated, in the study. The redox properties of phthalocyanines were researched by electrochemical and in situ spectroelectrochemical measurements on a Pt working electrode in non-aqueous medium. The color changes involved in the redox processes were observed via in situ electrocolorimetric techniques. Electrochemical and UV–Vis spectral measurements exhibited that the compounds had reversible and serial one-electron reduction and oxidation processes. Furthermore, they also had nitrile reduction processes because of opened nitrile groups located at peripheral tails of the phthalocyanine complexes. The plentiful redox conducts of the complexes such as Pc ring based and/or central redox active metal-based reduction and oxidation reactions at low potentials showed that these complexes can be used as functional materials. Because of the apparent spectral and net color changes, the complexes are suitable for utilization in electrochromic devices. Moreover, their rich redox features are a sign of their high electrocatalytic activity for oxygen reduction.

Peri-Decoration of a Tetraazaperylene with Urea Units: Chiral Octaazaperopyrenedioxides (OAPPDOs) and Their Optical and Chiroptical Properties.

Octaazaperopyrenedioxides (OAPPDOs) are a new class of fluorescent polycyclic aromatic hydrocarbons based on a tetraazaperylene core that is formally condensed with N-substituted urea units in the two opposite peri positions. Here, we report the synthesis of series of substituted OAPPDO derivatives with different N-substitution patterns (H, alkyl, benzyl) in the peri positions, including bay-chlorinated OAPPDOs. Starting from the latter, a series of bay-arylated OAPPDOs was synthesized by Suzuki cross coupling, which resulted in the formation of helically chiral OAPPDO derivatives. The electrochemical and photophysical properties were investigated by UV/Vis and fluorescence spectroscopy as well as cyclic voltammetry. The P and M enantiomers of a phenylated OAPPDO were separated by semipreparative HPLC and further analyzed by CD spectroscopy. The frontier orbital energies, the mechanism of the isomerization, the electronic excitation and the CD spectrum (TD-DFT) were computed and compared to the experimental data. The reversible 1e- oxidation of the OAPPDOs generates the corresponding radical cations, one of which was characterized by EPR spectroscopy. The reversible oxidation process was also systematically investigated by spectro-electrochemistry.

Tetrazine Chelate Ligands Bridging Two [Ru(acac)2] Fragments: Mixed Valency and Radical Complex Formation

AbstractUsing bis(3‐methyl‐2‐pyridyl)‐1,2,4,5‐tetrazine 1, 3‐(2‐pyrimidyl)‐6‐methyl‐1,2,4,5‐tetrazine 2 and bis(2‐pyrimidyl)‐1,2,4,5‐tetrazine=bmtz as ligands, the complexes 3=[Ru(acac)2(1)], 4={[Ru(acac)2]2(1)], 5={[Ru(acac)2]2(bmtz)], and 6={[Ru(acac)2]2(2)] were prepared and identified by structure analysis of crystallized material. The one‐electron oxidized form 6(PF6) was also studied structurally, suggesting a Class II mixed‐valent situation. The neutral dinuclear systems exhibit two reversible oxidation processes with comproportionation constants 109.2<Kc<1014.1 and one reduction which were analyzed UV/vis/NIR and EPR spectroscopically. Oxidation produces largely metal‐based mixed‐valent cations with very weak intervalence absorptions in the near IR whereas the electron uptake occurs at the tetrazine acceptor.

A C 54 B 2 Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity

A C ₅₄ B ₂ Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity

Recent Developments in the Syntheses of Aluminum Complexes Based on Redox-Active Ligands

Applications of aluminum as a catalyst for organic/inorganic transformation reactions are ideal because of its easy availability, inexpensiveness, and non-toxicity. However, successful implementation of Al(III) complexes as catalysts require an easy reduction-oxidation cycle between Al(III) and Al(I). Nevertheless, molecular complexes of Al(I) are not common, often require special efforts for their synthesis and isolation. Additionally, the Al(I)→Al(III) oxidation reaction is facile whereas the corresponding Al(III)→Al(I) reduction occurs only under highly reducing conditions. This creates a problem in developing Al(I)/Al(III) couple catalytic cycle for practical application. But, for aluminum complexes to be applicable in redox transformations, complexes must readily undergo reversible oxidation and reduction processes. One can envision applying a redox-active ligand where instead of aluminum the ligand would be reversibly oxidized and reduced to perform a certain electron transfer process. So, the development of a large pool of aluminum complexes supported by redox-active ligands is very important. In this review article, we have summarized recent developments on the syntheses of aluminum complexes with some redox-active ligands. This may help in designing new catalysts for future applications.

Biosensing of catechol via amperometry using laccase immobilized nickel oxide/graphite modified screen-printed electrodes

In the present work, we have developed a modified screen-printed carbon electrode (SPE) based biosensor to detect catechol using nickel oxide/graphite (NiO/G) immobilized with laccase as bio-nanocomposite coating. The laccase immobilized NiO/G composite increased the effective Randles-Sevcik surface area from 0.034 cm2 to 0.083 cm2 compared to bare SPE, resulting in improved electrochemical activity and enhanced reversible catechol oxidation process. Impedance spectroscopic data for the modified SPE (MSPE) showed faster electron transfer than the bare counterpart suggesting the successful immobilization of the bio-nanocomposite onto the electrode surface. Consequently, the cyclic-voltammograms of laccase MSPE exhibited a sharp decrease in the peak-to-peak separation potential and an increase in the current responsiveness. We further, utilized the chronoamperometric method to quantify the catechol detection under optimal conditions, and found a linear biosensor response over the concentration region of 1–100 µM. The sensor's lower detection limit, response time, and sensitivity towards catechol were found to be 65 nM, 3 s, and 1.51 µAµM-1cm−2, respectively. These results suggest that the laccase immobilized NiO/G MSPE as one of the most promising customized electrodes for catechol biosensing.

Reversible electrochemical oxidation of sulfur in ionic liquid for high-voltage Al\u2212S batteries

Sulfur is an important electrode material in metal−sulfur batteries. It is usually coupled with metal anodes and undergoes electrochemical reduction to form metal sulfides. Herein, we demonstrate, for the first time, the reversible sulfur oxidation process in AlCl3/carbamide ionic liquid, where sulfur is electrochemically oxidized by AlCl4− to form AlSCl7. The sulfur oxidation is: 1) highly reversible with an efficiency of ~94%; and 2) workable within a wide range of high potentials. As a result, the Al−S battery based on sulfur oxidation can be cycled steadily around ~1.8 V, which is the highest operation voltage in Al−S batteries. The study of sulfur oxidation process benefits the understanding of sulfur chemistry and provides a valuable inspiration for the design of other high-voltage metal−sulfur batteries, not limited to Al−S configurations.

Redox Chemistry of Nickelocene-Based Monomers and Polymers

The oxidation of [n]nickelocenophanes [Ni(η5-C5H4)2(CH2)3] (3), [Ni(η5-C5H4)2(SiMe2)2] (10), [Ni(η5-C5H4)2(SiMe2)2O] (11), [Ni(η5-C5H4)2(CH2)4] (12), and poly(nickelocenylpropylene) [Ni(η5-C5H4)2(CH2)3]n (4) to both the monocationic and dicationic species was investigated in dichloromethane by cyclic voltammetry (CV) and square-wave voltammetry. The presence of acetonitrile on the oxidation potentials of 3 in dichloromethane was also investigated by CV. The [n]nickelocenophanes 3 and 10–12 exhibited two single-electron Nernstian redox processes, and the monocations [3]+, [10]+, [11]+, and [12]+ were isolable as [B(C6F5)4]− salts after chemical oxidation, and were structurally characterized. Ni–Cpcent distances in all four monomers decreased upon oxidation, with a structural distortion manifested in the ring-tilt angle, α, among other angles. CV studies of the reversible first oxidation process to the polyelectrolyte {[Ni(η5-C5H4)2(CH2)3]+}n ([4]n+) were used to estimate the molecular weight of the polymeric material (Mw = 5300 g mol–1) by comparing its diffusion coefficient with that of a monomeric analogue, and the second electrochemical oxidation of polymer 4 was found to be only partially chemically reversible.