One-electron Reduction Processes Research Articles

Photoexcitation and One-Electron Reduction Processes of a CO2 Photoreduction Dyad Catalyst Having a Zinc(II) Porphyrin Photosensitizer.

We have explored the photophysical properties and one-electron reduction process in the dyad photocatalyst for CO2 photoreduction, ZnP-phen=Re, in which the catalyst of fac-[Re(1,10-phenanthoroline)(CO)3Br] is directly connected with the photosensitizer of zinc(II) porphyrin (ZnP), using time-resolved infrared spectroscopy, transient absorption spectroscopy, and quantum chemical calculations. We revealed the following photophysical properties: (1) the intersystem crossing occurs with a time constant of ∼20 ps, which is much faster than that of a ZnP single unit, and (2) the charge density in the excited singlet and triplet states is mainly localized on ZnP, which means that the excited state is assignable to the π-π* transition in ZnP. The one-electron reduction by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole occurs via the triplet excited state with the time constant of ∼100 ns and directly from the ground state with the time constant of ∼3 μs. The charge in the one-electron reduction species spans ZnP and the phenanthroline ligand, and the dihedral angle between ZnP and the phenanthroline ligand is rotated by ∼24° with respect to that in the ground state, which presumably offers an advantage for proceeding to the next CO2 reduction process. These insights could guide the new design of dyad photocatalysts with porphyrin photosensitizers.

Cation-Stacking Approach Enabling Interconversion between Bis(xanthylium) and its Reduced Species.

Cyclophane-type dications with two units of xanthylium were designed, with the expectation that intramolecular interaction between cation units could induce changes in absorption and redox behavior. The desired dications were synthesized via the macrocyclic diketone as a key intermediate, which was efficiently obtained by a stepwise etherification. X-ray and UV/Vis measurements revealed that the cyclophane-type dications adopt a stacking structure in both the crystal and solution. Due to the intramolecular interaction caused by π-π stacking of the xanthylium units, a considerable blue shift compared to the corresponding monocations and a two-stage one-electron reduction process were observed in the dications. Furthermore, upon electrochemical reduction of dications, the formation of biradicals via radical cation species was demonstrated by UV/Vis spectroscopy with several isosbestic points at both stages. Therefore, the cation-stacking approach is a promising way to provide novel properties due to perturbation of their molecular orbitals and to stabilize the reduced species even though they have open-shell characters.

Introducing Novel Redox-Active Bis(phenolate) N-Heterocyclic Carbene Proligands: Investigation of Their Coordination to Fe(II)/Fe(III) and Their Catalytic Activity in Transfer Hydrogenation of Carbonyl Compounds.

A simple and efficient procedure for synthesizing novel pincer-type tridentate N-heterocyclic carbene bisphenolate ligands is reported. The synthesis of pincer proligands with N,N'-disubstituted imidazoline core, 5 and 6, was carried out via triethylorthoformate-promoted cyclization of either N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)cyclohexanediamine, 3, or N,N'-bis(2-hydroxyphenyl)cyclohexanediamine, 4, in the presence of concentrated hydrochloric acid. Cyclic voltammograms of the ligands revealed ligand-centered redox activity, indicating the noninnocent nature of the ligands. The voltammograms of the ligands exhibit two successive one-electron oxidations and two consecutive one-electron reductions. In contrast to previous reports, the redox-active ligands in this study exhibit one-electron oxidation and reduction processes. All products were thoroughly characterized by using 1H and 13C NMR spectroscopy. The base-promoted deprotonation of the proligands and subsequent reaction with iron(II) and iron(III) chlorides yielded compounds 7 and 8. These compounds are binuclear and tetranuclear iron(III) complexes that do not contain carbene functional groups. Complexes 7 and 8 were characterized by using elemental analysis and single-crystal X-ray crystallography. At low catalyst loadings, both 7 and 8 exhibited high catalytic activity in the transfer hydrogenation of selected aldehydes and ketones.

Synthesis of new [6]helicene derivatives for OLED applications. Experimental photophysical and chiroptical properties and theoretical investigation

Six-membered helicenes with specific functionalities were synthesized in good overall yields (40 %-73 %) through a concise photochemical method, allowing for their comprehensive characterization. The racemic mixtures were effectively resolved into their P- and M-enantiomers with high optical purity (97.56 %-100 % ee) by using chiral HPLC. This separation showcased remarkable optical activity, revealing substantial optical rotations (e.g., +2500–3000 for the (P)-enantiomer at λ = 589 nm) and notable electronic circular dichroism (ECD) signals. The organic helical species showed a strong absorption in the UV region and exhibited a red shifted emission, resulting in a quantum yield of 0.12–0.21. Moreover, they demonstrated irreversible one-electron oxidation and reduction processes, and their HOMO and LUMO energy levels were estimated via electrochemical methods. Utilizing DFT and TD–DFT computational approaches, we accurately predicted the electronic absorption and ECD spectra, as well as the frontier molecular orbitals (FMOs), aligning closely with experimental observations. Furthermore, in-depth analyses of absorption and ECD spectra, molecular electrostatic potential (MEP), and reduced density gradient (RDG) through quantum chemical calculations offered insights into the fundamental characteristics of these materials. The collective properties strongly indicate the potential application of these helicenes in electroluminescent and OLED devices, underscoring their promise as candidates in optoelectronic applications.

Electronic and Magnetic Interactions in 6-Oxoverdazyl Diradicals: Connection through N(1) vs C(3) Revisited.

Four isomeric di-6-oxoverdazyl diradicals connected at their N(1) or C(3) positions with either 1,3- or 1,4-phenylene linkers were obtained and characterized by spectroscopic, electrochemical, magnetic, and structural methods. These results were compared to those for the corresponding 6-oxoverdazyl monoradicals. UV-vis spectroscopy demonstrated that only the N(1)-connected para-through-benzene diradical has a distinct spectrum with significant bathochromic and hypsochromic shifts relative to the remaining species. Electrochemical analysis revealed two one-electron reduction processes in all diradiacals, while only the N(1)-connected para-through-benzene diradical exhibits two one-electron oxidation processes separated by 0.10 V. Variable temperature EPR measurements in polystyrene solid solutions gave negative mean exchange interaction energies J for all diradicals, suggesting the dominance of conformers with significant intramolecular antiferromagnetic interactions for the meta-through-benzene isomers. DFT calculations predict a small preference for the triplet state with the ΔES-T of about 0.25 kcal mol-1 for both meta-through-benzene connected diradicals.

The bonding situations in ruthenium chalcogenonitrosyl compounds: a physical reasoning.

This research presents, for the first time, a comprehensive and rigorous investigation of ruthenium(II) chalcogenonitrosyl bonding situations in two sets of coordination compounds: [Ru(NE)Cl2(LOEt)] (1a-4a) and [Ru(NE)Cl2(LOEt)]- (1b-4b), where E = O, S, Se, Te. Prior to and following the one-electron reduction, the Ru-NE bonding situations were subjected to analysis. The calculated geometric parameters indicate that both the Ru-NE and N-E bond lengths are susceptible to variation depending on the nature of the chalcogen employed. Furthermore, the results demonstrate that the one-electron reduction process serves to diminish the NE double bond character. The generalized Kohn-Sham energy decomposition analysis (GKS-EDA) was conducted to illustrate the Ru-NE bonding scenarios prior to and following the one-electron reduction. The results provide valuable insights into the nature of Ru(II)-NE (E = O, S, Se, Te) bonds, the influence of chalcogens on ruthenium compounds, as well as how the one-electron reduction affects the release of NE groups. The main findings indicate that the total interaction energy, prior to the one-electron reduction, is three to four times more stabilizing than in the reduced analogs, confirming that the reduction unequivocally enhances the lability of the Ru-NE bond even when heavier chalcogen analogues are employed.

Supramolecular Interactions between Reduced Vitamin K1 and Dissolved CO2 and H2o in Aprotic Solvents

Vitamin K1 is a naphthoquinone (NQ) that can be electrochemically reduced at negative potentials in two one-electron steps in aprotic organic solvents. The first electron transfer at potential E 1 forms the anion radical (NQ• –) and the second electron transfer process at potential E 2 results in the formation of the dianion (NQ2–). If a hydrogen bond donor (H-D) such as water or an alcohol is progressively added to the aprotic organic solvent, both E 1 and E 2 shift towards more positive potentials. The shifts in potential are much greater for E 2 compared to E 1 as the hydrogen bond donor is added, so that a sufficiently high donor concentration, E 2 merges with E 1 and only one voltammetric wave is observed at potential E 3 due to the transfer of two-electrons. The shifts in potential with added hydrogen bond donors can be rationalized by supramolecular interactions between the strongly electronegative oxygen atoms in the quinone anions (especially the dianions) and the donor hydrogen atoms, and can be modelled according to an electrochemical square-scheme mechanism involving multiple electron transfer and chemical bonding steps. NQ + e– ⇌ NQ• – E 1 NQ• – + e– ⇌ NQ2– E 2 NQ + 2e– + nH-D ⇌ [NQ(H-D)n]2– E 3 NQ + 2e– + nCO2 ⇌ [NQ(CO2)n]2– E 4 In addition to H-bonding interactions, reduced forms of NQ also undergo supramolecular interactions with CO2(g) when it is purged into solution. Similar to the situation seen for H-bonding, when CO2 is purged into solution, the two one-electron processes merge into one two-electron process at less negative potentials (E 4). The mechanism can be considered analogous to the H-bonding case where the electronegative oxygen atoms in the reduced quinones interact with the electropositive carbon atoms in CO2. However, the supramolecular interactions appear to be stronger with CO2 than with water, since substantially less CO2 needs to be added to cause the two one-electron reduction processes of NQ to merge into one two-electron process. Controlled potential electrolysis experiments were conducted to determine the chemical reversibility of the supramolecular interactions. It was found that, (i) fully reducing NQ under a CO2 atmosphere and then applying an oxidizing potential resulted in the 100% conversion back to the starting material and, (ii) reducing NQ under a CO2 atmosphere and then purging the solution with argon resulted in the CO2 being released as CO2(g) or CO2 • – to leave NQ in its neutral form or as NQ2–. Figure 1. Cyclic voltammograms for 1,4-naphthoquinone in acetonitrile containing 0.2 M Bu4NPF6 at a scan rate of 0.1 V s–1 at a 1 mm diameter planar Pt electrode in the presence and absence of CO2. Figure 1

Utilizing Redox Transformations of Quinones to Modulate Weak Donor and Acceptor Bonding Interactions with H2o and CO2 in Organic Solvents

Redox chemical transformations can be used to modulate supramolecular interactions between reduced quinones and acceptor molecules, with the application of an electric potential enabling the interactions to be switched on and off. The supramolecular interactions targeted include weak solution phase processes involving H-bonding with water molecules, and interactions between dissolved molecular gases such as CO2 and solution phase electroactive species. The advantage of this approach is that it enables the supramolecular complexing abilities of one molecule to be improved and tuned simply by changing its redox state. For instance, in one redox state the compound may undergo no supramolecular interactions, but once it is oxidized or reduced it undergoes very strong interactions. This enables the development of potential sensing devices as well as the ability to gain fundamental information regarding interactions that are difficult to quantify, including H-bonding.In aprotic organic solvents, neutral quinones (Q) undergo an EE reduction mechanism (E signifies an electron transfer) to first give a radical anion or semiquinone (Q•–) at potential E Q1, followed by an aromatic dianion (Q2–) at a more negative potential, E Q2. When water is progressively added to the solvent, the second reduction wave (E Q2) progressively moves towards more positive potentials until it eventually merges with the first reduction wave (E Q1), so the two electrons transfer at close to one potential. The reason for the shift in potential is because the quinone anions, especially the dianions, undergo very strong H-bonding interactions through the highly electronegative oxygen atoms in the reduced quinones with the hydrogen atoms from the water molecules. In order to be able to quantitatively study the reactions with water, it is essential that the exact water content of the solvent is known, and the water content can be precisely controlled. By performing careful calibration experiments using Karl Fischer coulometric titrations, it was possible to establish working curves of water content versus E Q2 – E Q1 (= ΔE) that enabled the accurate calculation of the water content of an organic solvent simply by recording one voltammogram in the presence of a quinone. Therefore, using this procedure, a voltammetric method for quickly estimating water contents of organic solvents down to the low ppm levels was established.In addition to H-bonding interactions, recent experiments have demonstrated that reduced quinones undergo interactions with CO2 and this has been proposed as a method for removing CO2 from industrial gas streams.The technique involves purging the gas through an ionic liquid containing a quinone and applying a reducing potential to produce the quinone dianion which interacts strongly with CO2 in a similar supramolecular mechanism as for H2O [Figure 1(a)]. Once the CO2 is “trapped”, the [quinone–(CO2)2]2– molecules can be separated, and an oxidizing potential applied which results in the [quinone–(CO2)2]2– complex being oxidized back to the starting material and the CO2 being released.Figure 1(b) shows the results from cyclic voltammetry and electrolysis experiments for the reduction of a quinone in the presence and absence of CO2. Under an argon atmosphere, two one-electron reduction processes are evident due to the reduction to the anion radical then at more negative potentials the dianion. However, when the solution is purged with CO2 gas, the voltametric waves immediately merge into one two-electron process due to the formation of the [quinone–(CO2)2]2– supramolecular complex This species is stable under electrolysis conditions and can exist in the bulk solution for long periods (hours). The mechanism can be varied by purging the solution containing the [quinone–(CO2)2]2– complex with an inert gas such as Ar or N2, resulting in two major pathways.In one pathway, the [quinone–(CO2)2]2– complex can be made to release the CO2 •– and leave the neutral quinone, and in the other pathway the charge remains on the quinone and the neutral CO2 is released as shown in Figure 1(c). Figure 1. (a) Reduction mechanism of 1,4-napthoquinone in the presence of CO2. The counterion for the anionic species is the electrolyte cation, n-Bu4N+. (b) Cyclic voltammograms for 1,4-napthoquinone in the presence and absence of CO2. (c) Proposed pathways for the reaction of NQ(CO2)2 2 – with excess argon gas. Figure 1

Green Synthesis and Electrochemical Study of Undoped and Doped Al<sub>2</sub>O<sub>3</sub> Nanoparticles Using <i>Hibiscus rosa-sinensis</i> Leaves Extract

In the present work, nanoparticles of Al2O3, Cu-Al2O3, and Ni-Al2O3 were prepared using Hibiscus rosa-sinensis plant leaf extract through co-precipitation method. The prepared nanomaterials were characterized through TGA, EDX, SEM, UV-Vis, XRD, and FTIR instruments. The electrochemical behavior of Al2O3, Cu-Al2O3, and Ni-Al2O3 has been studied in DMF solution in the potential ranges from −1.5 to 1.5 V. The nanoparticles are thermally stable, according to the TGA, and the XRD patterns revealed that all the Al2O3, Cu-Al2O3, and Ni-Al2O3 particles were crystalline, with the mean sizes of 12.44, 34.61, and 31.63 nm, respectively. The cyclic voltammogram showed a cathodic peak (Epc) at 0.49 V with an anodic counterpart (Epa) at 0.49 V [E1/2 = 1.748 V]. The optical band gaps of Al2O3, Cu-Al2O3, and Ni-Al2O3 were 3.8, 3.2 and 3.65 eV, owed a cathode. It is observed that the electrochemical behavior of Ni-Al2O3 was identical to that of Al2O3 and Cu-Al2O3. The anodic and cathodic peak values rise with the scan rate. The one-electron oxidation and reduction processes are reversible, as seen by the shifting cathodic peak value toward higher negative values. All cycles exhibit absorption has a constant anodic current. This result indicated the diffusion-based redox process.

Structurally and electrochemically tunable pyrylium platforms: A new class of redox anolyte for non-aqueous organic redox flow battery operating at a high-current density

Non-aqueous all-organic redox flow batteries (NORFB) are a proficient candidate for large-scale energy storage applications. The higher operating potential window of non-aqueous solvents, diversity of redox-active organic molecules (ROMs), and independent scaling of power and energy, etc., captivate NORFB to be a potential choice for grid energy storage applications. Besides, the invention of new redox-active motifs significantly expands the development of NORFBs; thus, it is highly demanded. But, the low operational current density and slower redox kinetics are the bottlenecks for the further development of NORFB. Herein, we introduced a new class of anolyte material based on pyrylium salt (2,4,6-triphenylpyrylium tetrafluoroborate (TPT)) and demonstrated for the first time its applicability as a potential anolyte in NORFB. The substitutions at the 2, 4, and 6th positions of pyrylium salt prominently tune the structural and electrochemical behavior, thus providing a versatile platform to explore them in flow battery chemistry. TPT exhibited a one-electron reduction process with a high electron transfer rate constant of 9.94 × 10−3 cm s−1 and diffusion coefficient of 5.83 × 10−6 cm2 s−1, which is higher for the non-aqueous medium. By coupling with N-decylphenothiazine (DPTZ) catholyte, a mixed electrolyte flow cell was demonstrated with a capacity of 1.08 Ah/L and coulombic efficiency of 97 % at a high current density of 40 mA cm−2.

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.

Ester decorated 1,2,4,5-tetraphenylbenzene electrochromic materials with AEE activity, high optical contrast, fast response, and good cycling stability

Multifunctional electrochromic materials are ideal candidates for applications in various commercial fields. In this work, a series of dual-functional materials, 1,2,4,5-tetrakis (4-carboxyphenyl)benzene esters (TCPBEs), were synthesized by decorating 1,2,4,5-tetraphenylbenzene with tetra-esters. TCPBEs exhibited aggregation-induced emission and distinct phenomenon of aggregation-enhanced emission in water/N-methyl pyrrolidone due to their polyphenyl substituted structures and electrochromic properties due to surrounding esters. TCPBEs underwent a two-step one-electron reduction process in the electrochemical behavior. The electrochromic devices based on TCPBEs showed good long-term cycling stability (i.e., Hex-TCPBE remained 90% of the initial optical contrast after 2350 cycles), high optical contrast (i.e., Et-TCPBE up to 64.5%) and coloration efficiency (i.e., Hex-TCPBE up to 261 cm2/C), rapid response (<3 s), and dual colored state. Significant changes in color from colorless bleached state to orange and brown colored states were observed. Each TCPBE demonstrated distinct electrochromic properties because of the electronic effects of different R-substituted ester groups. In general, Et-TCPBE, Hex-TCPBE, and CH3Ph-TCPBE performed better than Bn-TCPBE and CF3Ph-TCPBE. In summary, this study provides a strategy for the design of novel dual-functional electrochromic materials, which have broad application prospects in the field of multifunctional devices.

Synthesis, structure and reactivity of μ3-SnH capped trinuclear nickel cluster.

Treatment of the trichlorotin-capped trinuclear nickel cluster, [Ni3(dppm)3(μ3-Cl)(μ3-SnCl3)], 1, with 4 eq. NaHB(Et)3 yields a μ3-SnH capped trinuclear nickel cluster, [Ni3(dppm)3(μ3-H)(μ3-SnH)], 2 [dppm = bis(diphenylphosphino)methane]. Single-crystal X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and computational studies together support that cluster 2 is a divalent tin hydride. Complex 2 displays a wide range of reactivity including oxidative addition of bromoethane across the Sn center. Addition of 1 eq. iodoethane to complex 2 releases H2 (g) and generates an ethyltin-capped nickel cluster with a μ3-iodide, [Ni3(dppm)3(μ3-I)(μ3-Sn(CH2CH3))], 4. Notably, insertion of alkynes into the Sn-H bond of 2 can be achieved via addition of 1 eq. 1-hexyne to generate the 1-hexen-2-yl-tin-capped nickel cluster, [Ni3(dppm)3(μ3H)(μ3-Sn(C6H11))], 5. Addition of H2 (g) to 5 regenerates the starting material, 2, and hexane. The formally 44-electron cluster 2 also displays significant redox chemistry with two reversible one-electron oxidations (E = -1.3 V, -0.8 V vs. Fc0/+) and one-electron reduction process (E = -2.7 V vs. Fc0/+) observed by cyclic voltammetry.

Construction of a low-valent thiolate-bridged dicobalt platform and its reactivity toward hydrogen activation and evolution

Low-valence transition metallic complexes have drawn longstanding attention due to their high reactivity toward catalytic transformation of various small molecules. Among these known complexes, the low-valence metal centres are commonly stabilized by neutral bulky ligands with strong electron-donating capacity. However, low-valence bimetallic complexes supported by anionic sulfur and cyclopentadienyl ligands are still difficult to obtain in high isolated yield. Herein, we report the synthesis and characterization of two scarce thiolate-bridged CoICoII and CoICoI complexes bearing sterically demanding ligands through two stepwise one-electron reduction processes. Interestingly, the CoICoII complex can facilely promote the homolytic cleavage of dihydrogen across the short Co−Co metallic bond to give a CoIICoIII dihydride bridged complex, which is capable of serving as a competent hydrogen atom transfer agent. Moreover, the anionic CoICoI complex can trigger a stepwise hydrogen generation cycle involving several isolated and structurally well-characterized intermediates.

Synthesis of First Antimony Porphycene and Electrocatalytic Hydrogen Evolution Driven by Ligand-Centered Reduction

Abstract For post-transition metal electrocatalytic hydrogen evolution reaction (HER), the new porphycene antimony complexes, Sb(III)OEPo and Sb(V)OEPo-Br2, were synthesized and characterized. Based on the electrochemical and electro-spectro measurements, the two-step one-electron reduction processes of Sb(III)OEPo were indicated to be both ligand-centered and the irreversible reduction process observed for Sb(V)OEPo-Br2 was assigned to be the reduction from Sb(V)OEPo-Br2 to Sb(III)OEPo. Electrocatalytic HER proceeded at −1.0 V (vs. Ag/AgCl) under acidic conditions via the ligand-centered reductions. The electron accepting nature of the porphycene ligand enabled the utilization of a main-group element as a central element for the ligand-centered HER at anodically shifted potentials.

Unveiling KuQuinone Redox Species: An Electrochemical and Computational Cross Study.

The study of the electrochemical properties of variegated quinones is a fascinating topic in chemistry. In fact, redox reactions occurring with quinoid scaffolds are essential for most of their applications in biological systems, in photoelectrochemical devices, and in many other fields. In this paper, a detailed investigation of KuQuinones’ redox behavior is presented. The distinctiveness of such molecules is the presence in the structure of two condensed naphthoquinone units, which implies the possibility to undergo multiple one-electron reduction processes. Solvent, supporting electrolyte, and hydrogen bond donor species effects have been elucidated. Changing the experimental parameters provoked significant shift of the redox potential for each reduction process. In particular, additions of 2,2,2-trifluoroethanol as a hydrogen bond donor in solution as well as Lewis acid coordination were crucial to obtain important shifts of the redox potentials toward more favorable values. UV–vis–NIR spectroelectrochemical experiments and DFT calculations are also presented to clarify the nature of the reduced species in solution.

1,2,5]Chalcogenadiazole-fused Dicyanonaphthoquinodiimines: Larger Contribution from Chalcogen Bond than Weak Hydrogen Bond in Determining Crystal Structures

The title electron acceptors are planar molecules undergoing reversible two-stage one-electron reduction processes. In the crystal of selenadiazole derivative (1), tetrameric structure is formed by two kinds of chalcogen bond (ChB), which is further connected by ChB and weak hydrogen bond (WHB). Although sulfur analogue (2) has only weaker ChB, it crystallizes isomorphously to 1 forming ChB-tetramer, thus contribution from ChB is more dominant than WHB in determining the crystal packing, which is different from the corresponding tetracyanoquinodimethane analogues.

Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis.

Homogeneous electrochemical catalysis of N2O reduction to N2 is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes. An activation-driving force correlation is revealed with the organic species characteristic of a redox catalysis involving an outer-sphere electron transfer from the radical anions or dianions of the reduced catalyst to N2O. Taking into account the previously estimated reorganization energy required to form the N2O radical anions leads to an estimation of the N2O/N2O˙− standard potential in acetonitrile electrolyte. The direct reduction of N2O at a glassy carbon electrode follows the same quadratic activation driving force relationship. Our analysis reveals that the catalytic effect of the mediators is due to a smaller reorganization energy of the homogeneous electron transfer than that of the heterogeneous one. The physical effect of “spreading” electrons in the electrolyte is shown to be unfavorable for the homogeneous reduction. Importantly, we show that the reduction of N2O by low valent rhenium and manganese bipyridyl carbonyl complexes is of a chemical nature, with an initial one-electron reduction process associated with a chemical reaction more efficient than the simple outer-sphere electron transfer process. This points to an inner-sphere mechanism possibly involving partial charge transfer from the low valent metal to the binding N2O and emphasizes the differences between chemical and redox catalytic processes.

Nickel(II) derivatives based on o-iminobenzoquinone-type ligands: Structural modifications, magnetism and electrochemical peculiarities

New bis-o-iminobenzosemiquinonato derivatives of nickel (imSQC(O)Ph)2Ni (1) and (imSQPh)2Ni (2) that are ligated by functionalized o-aminophenols H2LC(O)Ph and H2LPh, respectively, were synthesized. Complex 1 is distinguished by a square-planar geometry of N2O2 coordination core while the bis-o-iminobenzosemiquinonate 2 demonstrates a slight deviation from planarity. Both complexes 1 and 2 show the strong antiferromagnetic interactions between radical anion ligands as established by variable-temperature magnetic susceptibility measurements. Electrochemical behavior of 1 and 2 is represented by ligand-centered redox transitions: two quasi-reversible one-electron reduction processes and two-electron oxidation one. The salient peculiarity of cyclic voltammogram for 1 is an increase in number of redox waves owing to the quasi-reversible reduction of benzophenone fragments in ligands. Six-coordinated NiII derivative [(imQC(O)Ph)2Ni(H2O)2](OSO2CF3)2 (3) with two o-iminobenzoquinones as well as four-coordinated NiII species [(imSQC(O)Ph)Ni(APC(O)Ph)]CoCp2∙2.1toluene (4∙2.1toluene) including radical-anion and o-amidophenolate forms of redox-active ligand were prepared as products of oxidation and reduction of the initial 1, respectively, in accordance with the CV data. The value of effective magnetic moment (μeff) unambiguously indicates the high-spin state for nodal NiII ion in 3. Low-temperature EPR signal demonstrates rather the ligand-centered nature of unpaired electron in 4 that corresponds to the total doublet ground spin state (S = 1/2) for compound and diamagnetic low-spin (SNi = 0) configuration of the metal center.

Elucidating the Electroreduction Mechanism of the Monoprotonated Octaethylporphyrin. A Comparative Study with the Diprotonated Octaethyl- and meso-Tetraphenyl-porphyrins

The electrochemical reduction mechanisms of diprotonated tetraphenylporphyrin (H2TPP) and mono- and diprotonated octaethylporphyrin (H2OEP) were studied in tetrabutylammonium perchlorate/benzonitrile. The diprotonated forms of both porphyrins undergo two one-electron reversible reduction processes forming isophlorin. Contrastingly, monoprotonated H2OEP is reduced in a single process involving a two-electron one-proton transfer that yields two types of short-lived intermediates, isophlorin and neutral phlorin. The existence of intermolecular proton transfer reactions, from the parent protonated porphyrin to the isophlorin or neutral phlorin, to form phlorin cation species (isophlorin protonated at the meso-position) was demonstrated. In-situ UV–vis spectroelectrochemical experiments allowed us to identify the absorption of the isophlorin species of H2TPP but not of H2OEP. These results show that the lack of phenyl substituents increases the rate of protonation at the meso-position. Finally, it was demonstrated that the protonation of the porphyrin macrocycle not only lowers the reduction potentials but also increases the reactivity of the electrogenerated species.