Time Scale Of Cyclic Voltammetry Research Articles

Revisiting the Electrochemical Reduction Mechanism of Nitrosobenzene in Acetonitrile. Concomitant Reactions and Voltammetric Simulation

AbstractThe electrochemical reduction mechanism of nitrosobenzene in acetonitrile was reconsidered in this work. The commonly accepted mechanism assumes that the nitrosobenzene radical anion dimerize to yield a dianion, which decompose into azoxybenzene as the reduction product. Even though this proposal is consistent with preparative experiments, it does not reproduce fully the experimental voltammetric behaviour when it is simulated. Also, It does not explain why the apparent electron number is different in the time scale of cyclic voltammetry and preparative electrolysis. To understand these discrepancies, it was proposed the intervention of reactions consuming part of the starting nitrosobenzene, which allowed to reconsider the role of alternative chemical reactions like that occurring between the anion of phenylhydroxylamine with nitrosobenzene and the hydroxyl ion with nitrosobenzene. In this framework, these concomitant reactions were additionally included in the mechanism to simulate the voltammetric behaviour at different scan rates. Thermodynamic and kinetic parameters of the extended mechanism are reported. Since in the hydroxyl ion is either product or reactive in some reaction steps, the global mechanism was deemed to occurs as a kind of loop.

Electrochemical behavior of intramolecular cyclization reaction of catecholamines at carbon nanotube/carboxymethylcellulose electrode

• Intramolecular cyclization reaction of catecholamines is conducted. • A electron transfer-chemical reaction-electron transfer (ECE) process is addressed. • A carbon nanotube (CNT)/carboxymethylcellulose (CMC) electrode is used. • The electrochemical behavior in the ECE mechanism is justified by DigiElch simulation. • This work contributes to the in-depth understanding of catecholamines. Neurotransmitter catecholamines [dopamine (DA), epinephrine (EP), norepinephrine (NE), and 3,4-dihydroxyphenylalanine (DOPA)] exhibit an electron transfer-chemical reaction-electron transfer (ECE) reaction, wherein two electron transfer reactions are connected by a chemical reaction. The electrochemical behavior of catecholamines depends strongly on the kinetic property of the chemical reaction, that is, the intramolecular cyclization rate. This work provides systematic knowledge of the intramolecular cyclization reactions of catecholamines in the ECE process. We adopt a carbon nanotube (CNT)/carboxymethylcellulose (CMC) electrode because it shows an apparent ECE profile in cyclic voltammetry (CV) results, unlike other electrodes. The analysis of the electrochemical behavior in the ECE mechanism is justified by simulations using the DigiElch simulation program. The cyclization rate of various catecholamines follows a specific order: DA < NE < DOPA < EP. Because the cyclization step involves hydrogen ions, the rate depends on pH—the cyclization rate increases with a decrease in pH. The electrochemical behavior due to cyclization is verified on the basis of the CV results, particularly the change in the intensity of the cathodic peak (Peak Ic) due to the reduction of catecholamine- o -quinone. The intensity of Peak Ic is determined by (i) the type of catecholamines, (ii) pH, and (iii) the time scale of CV. This work contributes to the in-depth understanding of catecholamines and the development of novel strategies for the electrochemical sensing of catecholamines.

Spectroelectrochemical one-electron reduction product of (TPP)Zn in nonaqueous media? Not always the expected porphyrin π-anion radical

Zinc tetraphenylporphyrin (TPP)Zn was examined as to its as to its electroreduction properties by cyclic voltammetry and spectroelectrochemistry in six nonaqueous solvents containing tetrabutylammonium perchlorate (TBAP). The porphyrin undergoes an initial one-electron reduction to give a [Formula: see text]-anion radical on the cyclic voltammetry timescale but the radical was not stable on the spectroelectrochemical timescale when utilizting chlorinated alkane solvents containing 0.1 M TBAP (CH2Cl2, 1-2,dichloroethane (C2H4Cl[Formula: see text] where chloride, generated from minute amounts of reduced solvent, formed an axially coordinated complex, (TPP)Zn[Formula: see text](Cl[Formula: see text], prior to reduction of the porphyrin. This spectroelectrochemically characterized product exhibits red-shifted Soret and Q-bands as compared to that of the neutral (TPP)Zn under the same solution conditions and analytical plots obtained from recording the spectrum as a function of potential rule out the possibility of forming the superoxide complex, (TPP)Zn[Formula: see text](O[Formula: see text], in chlorinated alkane solvents.

Copper Coordination Chemistry of Sulfur Pendant Cyclen Derivatives: An Attempt to Hinder the Reductive-Induced Demetalation in 64/67Cu Radiopharmaceuticals.

The Cu2+ complexes formed by a series of cyclen derivatives bearing sulfur pendant arms, 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO4S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO3S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-10-acetamido-1,4,7,10-tetraazacyclododecane (DO3SAm), and 1,7-bis[2-(methylsulfanyl)ethyl]-4,10-diacetic acid-1,4,7,10-tetraazacyclododecane (DO2A2S), were studied in aqueous solution at 25 °C from thermodynamic and structural points of view to evaluate their potential as chelators for copper radioisotopes. UV–vis spectrophotometric out-of-cell titrations under strongly acidic conditions, direct in-cell UV–vis titrations, potentiometric measurements at pH >4, and spectrophotometric Ag+–Cu2+ competition experiments were performed to evaluate the stoichiometry and stability constants of the Cu2+ complexes. A highly stable 1:1 metal-to-ligand complex (CuL) was found in solution at all pH values for all chelators, and for DO2A2S, protonated species were also detected under acidic conditions. The structures of the Cu2+ complexes in aqueous solution were investigated by UV–vis and electron paramagnetic resonance (EPR), and the results were supported by relativistic density functional theory (DFT) calculations. Isomers were detected that differed from their coordination modes. Crystals of [Cu(DO4S)(NO3)]·NO3 and [Cu(DO2A2S)] suitable for X-ray diffraction were obtained. Cyclic voltammetry (CV) experiments highlighted the remarkable stability of the copper complexes with reference to dissociation upon reduction from Cu2+ to Cu+ on the CV time scale. The Cu+ complexes were generated in situ by electrolysis and examined by NMR spectroscopy. DFT calculations gave further structural insights. These results demonstrate that the investigated sulfur-containing chelators are promising candidates for application in copper-based radiopharmaceuticals. In this connection, the high stability of both Cu2+ and Cu+ complexes can represent a key parameter for avoiding in vivo demetalation after bioinduced reduction to Cu+, often observed for other well-known chelators that can stabilize only Cu2+.

Tailoring Valence Tautomerism by Using Redox Potentials: Studies on Ferrocene-Based Triarylmethylium Dyes with Electron-Poor Fluorenylium and Thioxanthylium Acceptors.

Three new electrochromic ferrocenyl triarylmethylium dyes with fluorenylium (1 a +, 1 b +) or thioxanthylium (1 c +) residues were selected in order to keep the intrinsic differences of redox potentials for ferrocene oxidation and triarylmethylium reduction small and to trigger valence tautomerism (VT). UV/Vis/NIR and quantitative EPR spectroscopy identified paramagnetic diradical isomers 1 a ..+–1 c ..+ alongside diamagnetic forms 1 a +–1 c +, which renders these complexes magnetochemical switches. The diradical forms 1 a ..+–1 c ..+ as well as the one‐electron‐reduced triarylmethyl forms of the complexes were found to dimerize in solution. For radical 1 a ., dimerization occurs on the timescale of cyclic voltammetry; this allowed us to determine the kinetics and equilibrium constant for this process by digital simulation. Mößbauer spectroscopy indicated that 1 a + and 1 b + retain VT even in the solid state. UV/Vis/NIR spectro‐electrochemistry revealed the poly‐electrochromic behaviour of these complexes by establishing the distinctly different electronic absorption profiles of the corresponding oxidized and reduced forms.

Identification of the Irreversible Redox Behavior of Highly Fluorescent Benzothiadiazoles

AbstractRedox switches are applied in various fields of research, including molecular lifts, electronic devices and sensors. Switching the absorbance between UV and Vis/NIR by redox processes is of interest for applications in light harvesting or biomedicine. Here, we present a series of push‐pull benzothiadiazole derivatives with high fluorescence quantum yields in solution and in the crystalline solid state. Spectroelectrochemical analysis reveals the switching of UV‐absorption in the neutral state to Vis/NIR absorption in the reduced state. We identify the partial irreversibility of the switching process, which appears to be reversible on the cyclic voltammetry timescale.

Electrochemistry of Tri‐substituted Porphyrins with β‐Appended Ethyl Acetoacetate and Acetylacetone in Neutral and Basic Nonaqueous Solvents

AbstractThe electrochemistry of β‐substituted tetraphenylporphyrins with a single ethyl acetoacetate (EAA), acetylacetone (acac), or ethyl acetate (EA) group and two antipodal H, Br or Ph groups was investigated in CH2Cl2 containing 0.1 M tetrabutylammonium perchlorate (TBAP) before and after addition of base to solution in the form of tetrabutylammonium hydroxide (TBAOH). The tri‐substituted porphyrins in their neutral form are represented as MTPP(EAA)X2, MTPP(acac)X2 and MTPP(EA)X2, where TPP is the dianion of tetraphenylporphyrin, M=H2, NiII, CuII or ZnII and X=H, Br or Ph. The singly reduced porphyrins are relatively stable on the cyclic voltammetry timescale, but this is not the case for the doubly reduced EAA and acac derivatives, which are highly reactive at room temperature, leading to a porphyrin product assigned as containing a deprotonated EAA− or acac− (enolate) substituent. Enolate‐appended porphyrins were also generated in‐situ by the addition of TBAOH, resulting in mono‐anionic porphyrins that are harder to reduce by 110–160 mV as compared to the neutral compounds with the same β‐substituents, suggesting a high degree of interaction between the deprotonated EAA− or acac− substituent and the porphyrin π‐ring system. The chemically or electrochemically generated enolate‐appended porphyrin could also be irreversibly oxidized by one electron at peak potentials between 0.10 and 0.30 V; the exact value depending upon solvent and type of β‐diketo substituent.

Fast responding hydrogen gas sensors using platinum nanoparticle modified microchannels and ionic liquids

From a safety perspective, it is vital to have fast responding gas sensors for toxic and explosive gases in the event of a gas leak. Amperometric gas sensors have been developed for such a purpose, but their response times are often relatively slow – on the order of 50 seconds or more. In this work, we have developed sensors for hydrogen gas that demonstrate ultra-fast response times. The sensor consists of an array of gold microchannel electrodes, electrodeposited with platinum nanoparticles (PtNPs) to enable hydrogen electroactivity. Very thin layers (∼9 μm) of room temperature ionic liquids (RTILs) result in an extremely fast response time of only 2 s, significantly faster than the other conventional electrodes examined (unmodified Pt electrode, and PtNP modified Au electrode). The RTIL layer in the microchannels is much thinner than the channel length, showing an interesting yet complex diffusion pattern and characteristic thin-layer behavior. At short times (e.g. on the timescale of cyclic voltammetry), the oxidation current is smaller and steady-state in nature, compared to macrodisk electrodes. At longer times (e.g. using long-term chronoamperometry), the diffusion layer is large for all surfaces and extends to the liquid/gas phase boundary, where the gas is continuously replenished from the flowing gas stream. Thus, the current response is the largest on the microchannel electrode, resulting in the highest sensitivity and lowest limit of detection for hydrogen. These microchannel electrodes appear to be highly promising surfaces for the ultrafast detection of hydrogen gas, particularly at relevant concentrations close to, or below, the lower explosive limit of 4 vol-% H2.

Electrochemical Evidence of the Catalytic Activity of Meso-Tetraphenylporphyrin Diacid in the Proton Reduction in Benzonitrile

Our work shows the electrochemical behavior of meso-tetraphenylporphyrin diacid (H4TPP2+) in tetrabutylammonium perchlorate Bu4NClO4 0.1 M / benzonitrile. On a cathodic sweep and scan rates between 0.1 and 3.0 V/s, H4TPP2+ undergoes two closely spaced consecutive reversible one-electron transfers (Ic’/Ia’ and IIc’/IIa’; Figure 1), which can be assigned to formation of diprotonated porphyrin radical cation (H4TPP•+), and isophlorin H4TPP species, respectively1. In presence of an extra equivalent of HClO4, a new reduction peak, IIIc’, is seen, suggesting that isophlorin H4TPP is being protonated. It is believed that the protonation takes place at one of the meso-positions of the porphyn ring leading to phlorin cation (H4TPFH+). The phlorin cation is electroactive and can be either oxidized (peaks IVa’ and Va’) or reduced (IIIc’)2. As seen in Figure 1, the addition of extra equivalents of H+ to solutions of H4TPP2+ provokes an increase in the current and loss of reversibility of the two reduction processes, Ic’/Ia’ and IIc’/IIa’. This behavior was explained as due to the reaction between the free protons in the medium and the hydrogen atom of the meso-position of the H4TPFH+, in a reaction that produces molecular hydrogen and regenerates the porphyrin diacid. Digital simulation of the experimental voltammograms of H4TPP2+ indicates that the H4TPP•+ and H4TPP species are stable on the cyclic voltammetry time scale. However, coulometry experiments at the potential of the second reduction process show that at longer time scale, the isophlorin H4TPP undergoes coupled chemical reactions. Voltammograms of the electrolyzed solution exhibit oxidation and reduction signals that are characteristics of H4TPFH+ species, also the presence of the bands at 460 nm and 773 nm in the UV-visible spectrum is consistent with the presence of H4TPFH+ species3,4. It is also demonstrated that the H4TPFH+ species can be reoxidized back to the original H4TPP2+ in a two-electron process. References. 1. H. M. Castro-Cruz, thesis, UNAM (2017). http://132.248.9.195/ptd2017/junio/410088896/Index.html 2. Y. Cui, L. Zeng, Y. Fang, J. Zhu, C. H. Devillers, D. Lucas, N. Desvbois, C. P. Gros and K. M. Kadish, ChemElectroChem, 3, 228–241 (2016). 3. G. Peychal-Heiling and G. S. Wilson, Anal. Chem., 43, 550–556 (1971). 4. G. L. Closs and L. E. Closs, J. Am. Chem. Soc., 85, 818–819 (1963). Figure 1

Electrochemical switching fluorescence emission in rhodamine derivatives

Three rhodamine derivatives exhibiting electrofluorochromic properties were investigated by cyclic voltammetry and UV–Vis/fluorescence spectroelectrochemistry. Rhodamine 101 (Rh101, compound 1) was used as a reference model. In compound 2, the carboxylate anion of Rh101 was replaced by an alkyne moiety to allow further functionalization. The compound 3 was prepared from 2 by conversion of the alkyne to a triazole group bearing an alkyl chain with an alcohol function. These three rhodamine derivatives exhibited similar electrochemical behaviors. Their mono-electronic reductions produced the corresponding radical species which were stable on the time-scale of cyclic voltammetry. Additional reduction of electrogenerated radicals produced unstable anions which underwent subsequent chemical reaction, most likely protonation. Based on cyclic voltammetry investigations, absorption and fluorescence spectroelectrochemistry were then performed on compounds 1, 2, 3 and their parent reduced radicals 1a, 2a, 3a. UV–Vis spectroelectrochemistry, combined with TD-DFT calculation, confirmed the formation of radicals upon mono-electronic reduction of starting rhodamines. Fluorescence spectroelectrochemistry showed that, contrary to their parent molecules, electrogenerated radicals were non-fluorescent. Electrochemical fluorescence extinction was successfully achieved with all studied compounds. Moreover, compound 1 underwent on/off switching between fluorescent and non-fluorescent states repeatedly. Also, recovery of fluorescence in compound 3 was observed, which open interesting opportunities for the development of versatile rhodamine-based probes.

Electrosynthesis, spectral and structural studies of a semi-conducting oligomer deriving from a methoxy-substituted chalcone

The anodic oxidation of a substituted chalcone namely the (E)-1-(4-methoxyphenyl)-3-(3,4,5-trimethoxyphenyl) prop-2-enone (TMC) was investigated by different electrochemical techniques using a platinum working electrode in acetonitrile. On the cyclic voltammetry time scale, the TMC exhibited a single irreversible anodic peak around 1.4 V vs. Ag/AgCl and the coupling of the radical cations, issued from the first electron transfer, was the governing reaction near the electrode. Electrolysis at a constant potential revealed that an oligo o-phenylenevinylene was the main product of the anodic oxidation of TMC. The chemical structure of the isolated oligomer was elucidated by 1H, 13C NMR, and IR spectroscopy. Gel permeation chromatography indicated that the average chain length was about 5 units. In addition, the obtained oligomer was thermally stable up to 220 °C and exhibited a light emission in the indigo-blue region. Finally, a mechanism for the TMC electro-oligomerization was proposed on the basis of the electrochemical data and the theoretical calculation of the spin densities distribution for the TMC radical cation.

Oxidative ring cleavage of 2,3-dihydrophthalazine-1,4-dione in aqueous and non-aqueous solutions: Electrochemical and kinetic studies

Electrochemical oxidation of 2,3-dihydrophthalazine-1,4-dione (DHP) has been investigated in aqueous and some amphiprotic and aprotic non-aqueous solvents by cyclic voltammetric and controlled-potential coulometric techniques. Our data shows that electrochemically generated phthalazine-1,4-dione (PTD) in water and amphiprotic non-aqueous solvents participates in an oxidative ring cleavage (ORC) reaction to form phthalic acid. The rate of this reaction is dependent. On autoprotolysis constant (K SH) and basicity of the solvent. Therefore, in the aprotic non-aqueous solvents such as acetonitrile and DMSO, the rate of ORC is too slow to be observed on the time-scale of cyclic voltammetry. Electrochemical oxidation of 2,3-dihydrophthalazine-1,4-dione has been investigated in aqueous and a few amphiprotic and aprotic non-aqueous solvents. The electrochemically generated phthalazine-1,4-dione in water and amphiprotic non-aqueous solvents participates in a oxidative ring cleavage reaction to form phthalic acid.

Mechanistic insight into the electronic influences imposed by substituent variation in polyazine-bridged ruthenium(II)/rhodium(III) supramolecules.

Unusual and unprecedented multipathway electrochemical mechanisms for a new class of supramolecular Ru/Rh bimetallic photocatalysts have been uncovered. The near isoenergetic Rh(dσ*) and bridging ligand(π*) molecular orbitals and a rate of halide loss that occurs on the cyclic voltammetry timescale provide a series of closely related complexes which display four different electrochemical mechanisms. A single complex displays two concurrent electrochemical pathways in marked contrast to all previously studied cis-[Rh(NN)(2)X(2)] motifs.

Chiral Nickel(II) Binuclear Complexes: Targeted Diastereoselective Electrosynthesis

Ni(II) complexes containing (S)-o-[N-(N-benzylprolyl)amino]benzophenone as an auxiliary chiral moiety in the form of a Schiff base with α-amino acids (α-amino acid = glycine, alanine, dehydroalanine; Gly-Ni, Ala-Ni, and Δ-Ala-Ni) were subjected to various types of electrochemical activation (oxidation, reduction, and a treatment with electrogenerated base), affording regio- and diastereoselective synthesis of novel types of binuclear Ni(II) complexes via C–C coupling. New compounds were fully characterized by HRMS, MALDI-TOF, CD, and 1H and 13C NMR (including two-dimensional techniques) spectroscopy; two complexes were characterized by X-ray diffraction analysis. The structures of the novel complexes obtained via electrosynthesis completely match the predictions (made from preliminary voltammetric investigations of the starting complexes as well as from DFT estimations of the energy and symmetry of their frontier molecular orbitals) about the nature of chemical transformations which may follow the electron transfer steps. Electrochemical oxidation of Gly-Ni and Ala-Ni allows access to new dimeric complexes linked via benzophenone moieties in the Ni(II) coordination environment. These new binuclear Ni(II) complexes are of interest as chiral redox mediators for both oxidative and reductive transformations, since they exhibit quasi-reversible electrochemical behavior (their reduced and oxidized forms are stable, at least on the time scale of cyclic voltammetry). Three other binuclear Ni(II) complexes which were obtained via reductive dimerization of the Δ-Ala-Ni complex, via nucleophilic addition of electrochemically deprotonated Gly-Ni to Δ-Ala-Ni, and via oxidative electrochemical dimerization of deprotonated Gly-Ni are of interest as convenient precursors for the stereoselective preparation of diamino dicarboxylic acids HO(O)CCH(NH2)(CH2)n(NH2)CHC(O)OH (n = 2–0), since the obtained binuclear Ni(II)–Schiff base complexes can be easily disassembled using aqueous HCl in methanol.

The effect of solvents on the electrochemical behavior of homogentisic acid

The electrochemical oxidation of homogentisic acid (HGA) as an important biological molecule has been studied in aqueous and mixed acetonitrile–water solutions using cyclic voltammetry. The effects of pH, HGA concentration, time window of the chosen electrochemical method and different solvents were studied to investigate the proton and electron transfer processes. The oxidation product of HGA in aqueous solution is stable in acidic, neutral and weakly alkaline media at the time scale of cyclic voltammetry. The electrooxidation process of HGA in aqueous solution follows Eq mechanism. In mixed acetonitrile–water solution, the electrooxidation process of HGA follows an EqCiEq mechanism. The experimental results indicate that the subsequent chemical reaction of dimerization is more probable for the oxidized form of HGA. It is shown that the separation of the cathodic and anodic peak potentials, ΔEp, in the mixture of acetonitrile–water is greater than that in aqueous solution and it increases with the increase of acetonitrile content in the mixture.

The Observation of Dianions Generated by Electrochemical Reduction of trans-Stilbenes in Ionic Liquids at Room Temperature

Three highly aprotic bis(trifluoromethylsulfonyl)amide (NTf2(-)) based ionic liquids (ILs) containing the cations trihexyl(tetradecyl)phosphonium (P6,6,6,14(+)), N-butyl-N-methylpyrrolidinium (Pyrr4,1(+)), and (trimethylamine)(dimethylethylammine)dihydroborate ((N111)(N112)BH2(+)) have been examined as media for room temperature voltammetric detection of highly basic stilbene dianions electrochemically generated by the reduction of trans-stilbene (t-Stb) and its derivatives (4-methoxy-, 2-methoxy-, 4,4'-dimethyl-, and 4-chloromethyl-). Transient and steady-state data in the ILs were compared with results obtained in the molecular solvent acetonitrile. In all media examined, the t-Stb(0/•-) process is chemically and electrochemically reversible with a heterogeneous charge transfer rate constant in CH3CN of 1.5 cm s(-1), as determined by Fourier transformed AC voltammetry. However, further reduction to the dianion was always irreversible in this molecular but weakly acidic solvent. On the other hand, a substantial level of chemical reversibility for the reduction of t-Stb(•-) to t-Stb(2-) on the time scale of cyclic voltammetry is achieved when the concentration of trans-stilbene, [t-Stb], appreciably exceeds the concentration of adventitious water or other proton sources. In particular, these conditions are met when [t-Stb] ≥ 0.1 M in thoroughly dehydrated and purified ILs, while in the presence of CH3CN, t-Stb(2-) still suffers fast irreversible protonation under these stilbene concentration conditions. The E0/•-(0) values (vs Fc(0/+)) for substituted trans-stilbenes in acetonitrile and (N111)(N112)BH2-NTf2 do not differ substantially, nor do the E0/•-(0) and E•-/2-(0) differences or other aspects of the voltammetric behavior.

Targeted syntheses of homo- and heterotrinuclear complexes involving MII–NiII–MII (M = Ni, Cu, and Pd) nonlinear core: Structure, spectroscopy, magnetic and redox studies

Homo- and heterotrinuclear complexes [LNi{M(Ln)}2](ClO4)2·H2O involving NiIIMII2 nonlinear cores (M=Ni, Cu, and Pd) (1–6) have been synthesized by a single-pot reaction when the oximato metal complexes [MLn(H2O)]ClO4 (HLn are tridentate oxime ligands), prepared in situ in methanol are allowed to react with the precursor nickel(II) complex [LNi(H2O)2] (H2L=N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine). Single crystal X-ray diffraction analysis, and ESI-MS spectroscopy have been used to establish their identities which involve an octahedral Ni(II) site flunked by two metal-oximate moieties, each in a square planar environment. The electronic and molecular structures of these compounds are interesting due to a synergistic bonding mechanism operative through the deprotonated oxime and the phenolate oxygen atoms via the metal centers. Thus the weak spin-forbidden 3A2→1E transition (ε, 4–8mol−1cm2), characteristic of the octahedral Ni(II) center in Ni(II)–Ni(II)2 (1, 2) and Ni(II)–Pd(II)2 complexes (5, 6), gains intensity by ca. 25-fold in the spin-coupled Ni(II)–Cu(II)2 complexes (3, 4) in which the metal centers are connected antiferromagnetically through a moderately strong coupling (JCu-Ni/kB=−119.6(6)K). In the presence of excess pyridine (5×10−2M), the trinickel compound (1) in acetonitrile displays a pair of quasi-reversible oxidations at E1/2=0.48 and 0.69V (vs. Ag/AgCl reference at a high scan speed, 1000–3000mVs−1) due to Ni(II/III) oxidations at the square planar nickel sites. Coordinated oxime helps in stabilizing these Ni centers in +3 oxidation state in the time-scale of cyclic voltammetry possibly through the formation of octahedral geometry with the added pyridine as axial ligands. In the cathodic range, all the three Ni centers undergo Ni(II/I) reductions at E1/2=−1.17, −1.35 and −1.62V, the most cathodic one being due to the octahedral site.

An Electron‐Rich {RuNO} 6 Complex: trans ‐[Ru(DMAP) 4 (NO)(OH)] 2+ – Structure and Reactivity

The pseudo-octahedral nitrosyl species trans-[Ru(DMAP)4(NO)(OH)]2+ [12+, DMAP = 4-(dimethylamino)pyridine] was prepared by the reaction between [Ru(DMAP)5(H2O)]2+ and NaNO2 under mild conditions (room temperature, pH = 6–8), and precipitated with NaBF4 or NaPF6. Single-crystal X-ray diffraction data of 1(BF4)2·2H2O point to a {RuNO}6 electronic configuration of the cation, although there is a significant deviation from the expected linear arrangement of the RuNO moiety (Ru–N–O angle: 169.3°). The remarkably low wavenumber of the NO stretching band (νNO) of 1832 cm–1 in the solid state corresponds to an electron-rich ligand environment with strong backbonding interactions between dπ metal orbitals and π*NO orbitals of the (formal) NO+ (nitrosonium) ligand. Consistently, the complex acts as a poor electrophile: it is extremely unreactive toward OH– and toward the stronger nucleophile cysteine. Cyclic voltammetry (CV) experiments show three reduction processes in acetonitrile. The first one at –0.50 V [vs. Ag/AgCl, KCl(s)] is reversible on the CV time scale. Spectroelectrochemical experiments (IR and UV/Vis) suggest that the reduced complex has a {RuNO}7 configuration. The νNO wavenumber of 1603 cm–1 agrees with a reduction mostly centered at the nitrosyl ligand. This species appears to be inert toward substitution in the coordination sphere, and the original {RuNO}6 ion can be quantitatively recovered upon oxidation. In contrast, further reduction of this species is completely irreversible, even on the CV timescale, which is probably due to the lability of the presumably formed nitroxyl (NO–) ligand.

Synthesis, structure, properties and immobilization on a gold surface of the monoribbed-functionalized tris-dioximate cobalt(ii) clathrochelates and an electrocatalytic hydrogen production from H+ ions

The cycloaddition of the mono- and dichloroglyoximes to the cobalt(II) bis-α-benzyldioximate afforded the cobalt(II) mono- and dichloroclathrochelates in moderate yields (40-60%). These complexes undergo nucleophilic substitution of their reactive chlorine atoms with aliphatic amines, alcohols and thiolate anions. In the case of ethylenediamine and 1,2-ethanedithiol, only the macrobicyclic products with α,α'-N(2)- and α,α'-S(2)-alicyclic six-numbered ribbed fragments were obtained. The cobalt(II) cage complexes with terminal mercapto groups were synthesized using aliphatic dithiols. The crystal and molecular structures of the six cobalt(II) clathrochelates were obtained by X-ray diffraction. Their CoN(6)-coordination polyhedra possess a geometry intermediate between a trigonal prism and a trigonal antiprism, and the encapsulated cobalt(II) ions are shifted from their centres due to the structural Jahn-Teller effect with the Co-N distances varying significantly (by 0.10-0.26 Å). The electrochemistry of the complexes obtained was studied by cyclic voltammetry (CV). The anodic waves correspond to the quasi-reversible Co(2+/3+) oxidations, whereas the cathodic ranges contain the quasi-reversibile waves assigned to the Co(2+/+) reductions; all the cobalt(i)-containing clathrochelate anions formed are stable in the CV time scale. The electrocatalytic properties of the cobalt complexes obtained were studied in the production of hydrogen from H(+) ions: the addition of HClO(4) resulted in the formation of the same catalytic cathodic reduction Co(2+/+) waves. The controlled-potential electrolysis with gas chromatography analysis confirmed the production of H(2) in high Faraday yields. The efficiency of this electrocatalytic process was enhanced by an immobilization of the complexes with terminal mercapto groups on a surface of the working gold electrode.

π-dimerization of pleiadiene radical cations at low temperatures revealed by UV–vis spectroelectrochemistry and quantum theory

One-electron oxidation of the non-alternant polycyclic aromatic hydrocarbon pleiadiene and related cyclohepta[c,d]pyrene and cyclohepta[c,d]fluoranthene in THF produces corresponding radical cations detectable in the temperature range of 293–263 K only on the subsecond time scale of cyclic voltammetry. Although the EPR-active red-coloured pleiadiene radical cation is stable according to the literature in concentrated sulfuric acid, spectroelectrochemical measurements reported in this study provide convincing evidence for its facile conversion into the green-coloured, formally closed shell and, hence, EPR-silent π-bound dimer dication stable in THF at 253 K. The unexpected formation of the thermally unstable dimeric product featuring a characteristic intense low-energy absorption band at 673 nm (1.84 eV; logemax = 4.0) is substantiated by ab initio calculations on the parent pleiadiene molecule and the PF6− salts of the corresponding radical cation and dimer dication. The latter is stabilized with respect to the radical cation by 14.40 kcal mol−1 (DFT B3LYP) [37.64 kcal mol−1 (CASPT2/DFT B3LYP)]. An excellent match has been obtained between the experimental and TD-DFT-calculated UV–vis spectra of the PF6− salt of the pleiadiene dimer dication, considering solvent (THF) effects.