Kinetics and Mechanism of Zn(II) Ions Electroreduction Catalyzed by Organic Compounds

The paper describes the results of the studies of organic substances catalytic activity on Zn(II) ions electroreduction on mercury in perchlorate solutions. Zn(II) ions electroreduction in the presence of catalyzing substance proceeds in two one‐electron stages. The first electron transfer is the stage determining the process rate. All catalyzing substances increase the rate of first electron transfer. The acceleration effect is connected with the stability of active complexes formed on the electrode surface. The rate of the second electron transfer depends mainly on the adsorption of the catalyzing substance on the electrode surface – of the surface excess and the structure of the adsorption layer. Hence the second electron transfer can be inhibited or catalyzed. The mechanism of the organic substance catalytic activity is also given.     

Effect of halides and alkali cations on the Zn(Hg)/Zn(II) electrode reactions

The title subject has been studied by galvanostatic single-pulse, chronopotentiometric and equilibrium measurements on the Zn(Hg)/Zn(II) electrode in x M KI+(1?x) M KCl (x from 0 to 1), 1 M KBr and 1 M MeCl (Me=Li, Na, K and Cs) solutions of pH 3 at 25°C. Quantitative information about the effect of specifically adsorbed halides on the rates of the Zn(II)/Zn(I) and the Zn(I)/Zn(Hg) steps is obtained separately (for the latter step mainly at potentials near ?1.0 V(SCE)), and the latter step seems to be more influenced than the former by the adsorption. An attempt is made to correlate the adsorption effect on the rate of the Zn(II)/Zn(I) step to double-layer parameters according to recent models for such effects. The extra current observed at potentials where the halides are adsorbed, seems to vary with the surface activity of the specifically adsorbed ion. The lack of any observed kinetic effect of Cs⁺, which is specifically adsorbed at these potentials, is possibly due to the Cs⁺ specific adsorption enhancing the Cl^? specific adsorption and vice versa, so that the decelerating and accelerating effects by these ions may cancel each other.     

Solvent effects on the geometry,electronic structure,and bonding style of Zn(N5)2: A theoretical study

Nitrogen-rich compounds involving the cyclo-pentazole anion (cyclo-N₅⁻) have attracted extensive attention due to higher energy release and environmental friendliness than traditional high energy density materials (HEDMs). However, the synthesis of stable HEDMs with cyclo-N₅⁻ is still a challenge. In this study, the effect of nine solvents on the geometrical and electronic structures and solvation energies of Zn(N₅)₂, one of the recently synthesized nitrogen-rich compounds, was studied using the density functional theory and the polarized continuum model. The results indicated an increase in the stability of Zn(N₅)₂ in the solution phase compared to the vacuum phase, and the stability of Zn(N₅)₂ increases with increasing dielectric constants. The energy gap of frontier molecular orbitals and the absolute value of total energy in water are the largest, revealing that Zn(N₅)₂ is more stable in water than in other solvents. To understand the stabilization mechanism of Zn(N₅)₂ by water, further studies were performed with the natural bond orbital (NBO) analysis and the quantum theory of atoms in molecules (QTAIM) analysis using the explicit solvent model. The charge transfer and the hydrogen bonds are observed between Zn(N₅)₂ and water, which are beneficial to improvement of the stability of Zn(N₅)₂. This may indicate the solvents that have strong interactions with the cyclo-N₅⁻ candidate may improve the possibility of success of synthesis.     

Inorganic Syntheses,Volume XXI : edited by J.P. Fackler,Jr., Wiley-Interscience,New York, 1982, xvii + 215 pages, £29.25.

The 1/1 reaction of Et₂Zn with N-t-butyliminopropanone (t-BuNC(H)C(Me)O) leads to quantitative formation of dinuclear [EtZ$\text{n(Et)(t-Bu)NC(H)=C(Me)O}$]₂ via ethyl transfer within the unstable Et₂Zn(t-BuNC(H)?C(Me)O) complex. An X-ray structure determination has shown the product to have a dinuclear structure involving a N,O-chelate bonded [Et(t-Bu)NC(H)C(Me)O]^? mono-anionic ligand and a central four membered Zn₂O₂ ring formed by intermolecular ZnO coordination. Hydrolysis of this zinc complex gives a quantitative yield of N-t-butyl-N-ethylamino propanone, which upon reaction with Et₂Zn reforms the dinuclear zinc complex.     

Electroreduction of some pyridine carboxamides on carbon electrodes in aqueous solutions

The electroreductions of the NAD⁺ model compounds nicotinamide (I), N₁-methyl nicotinamide (II), N′-methyl nicotinamide (III) and isonicotinamide (IV) on carbon electrodes have been studied in aqueous media in the pH range 0–12 by linear-sweep cyclic voltammetry (Scheme 1, I-IV). Logarithmic analyses of the reduction peaks were performed by computing the convolution of the current with time as a function of the potential. On the basis of the experimental results it was concluded that the irreversibility of the electron transfers increased when a glassy carbon electrode was used, and this irreversibility being more marked when a plastic formed carbon electrode was employed. The reduction processes occurred with more difficulty on carbon electrodes than on mercury electrodes. Both the reduction and the reoxidation (when occurred) processes changed with respect to those observed on mercury electrodes, being irreversible electron transfers the rate-determining steps in most cases. Thus, for compounds I, II and III at pH < 2 the reductions occurred by the uptake of two electrons and two H⁺ ions, and the rate determining step was found to be the first one-electron transfer, for I and III, and the irreversible second electron transfer, preceded by the uptake of an H+ ion, for II. At pH>3 the processes consisted of electrodimerization reactions, preceded by the protonation of the heterocyclic nitrogen in cases I and III. The second electron transfer of the electroreduction of IV always appeared irreversible, in contrast with that found for mercury electrodes.     

Electrical transport properties of Zn doped ZnO

Electrical resistivity and Hall effect measurements at 77–373°K are presented for Zn doped ZnO crystals. The crystals have been doped systematically at 600–1100°C in controlled pressures of Zn. The concentration of electrons at room temperature is in the range n_RT = 2.5 × 10¹⁶, to 3.6 × 10¹⁸, cm^?3. The donor level E_D and the concentrations of donors N^D and acceptors N_A have been calculated from a best fit to the experimental relationships log n versus $\text{1}\text{T}$ and log μ_H versus log T. At dilute concentrations of donors, two donor levels have been observed, E_D^I = 0.043–0.045 eV and a deeper level E^II_D greater than 0.165 eV. The ZnO was found to behave as a metal at N_D ～ 6 × 10¹⁸, cm^?3.At least two different donors have to be assumed in order to explain the experimental results. It is suggested that interstitial Zn is the electrical active donor at higher doping levels. The nature of the other donor is not clear. Neither 1s¹ H-type nor 1s² He-type donors seem to explain all the observations consistently.     

On the structure of Zn(II) and Cu(II) cyanin complexes in aqueous solution

B3LYP/6-31++G(d,p) optimizations on models for the metal cyanin, Cy, complexes [MCy(H₂O) _n ]⁺, (M = Zn(II), Cu(II); n = 2, 3, 4) in aqueous solution indicate that 4 is the most favoured coordination number in both cases. SP -4 and T -4 geometries are nearly isoenergetic for the former, while SP -4 is the only one obtained for the latter. Anionic cyanin displays higher affinity for Cu(II) than for Zn(II) or Mg(II). The electron density reorganization of cyanin model accompanying the complexation process was analyzed by means of the quantum theory of atoms in molecules. This analysis reveals that: (1) the O4′–M bond is stronger than O3′–M; (2) anionic cyanin displays a dual character between 4′-keto-quinoidal and 3′,4′-dienolate resonance forms; (3) Cu(II) takes more electron density than Zn(II) from Cy^? and water ligands; (4) when the coordination number increases, each ligand (Cy^? or water) transfers less electron density; (5) complex formation modifies the electron density in all the atoms of the ligands, but the largest modifications are displayed within the AC bicycle of Cy^?; and (6) a third part of density lost by the Cy^? ligand is removed from hydrogens.     

Effect of ammonia on the Zn(Hg)/Zn(II) electrode reactions in aqueous chloride solutions

The title subject has been studied using single pulse and chronopotentiometric polarization measurements on the Zn(Hg)/Zn(II) electrode and equilibrium measurements on the same and the Zn/Zn(II) electrode, mainly in 2 M NH₄Cl with 0–0.3 M NH₃. At low ammonia concentrations, the Zn(Hg)/Zn(II) reactions are found to occur in two consecutive charge-transfer steps with Zn(I) as intermediate, and with little or no participation of ammonia. At higher ammonia concentrations, however, nearly symmetric transfer by divalent zinc ion (α=0.5 and n=2) to and from diammine species appears to be the predominant charge-transfer step.     

Theoretical study on the polymerization mechanism of substituted maleimides by using a chiral catalyst with Zn

It is possible to synthesize poly(N-substituted maleimide) by using a chiral complex consisting of a zinc and N-diphenylmethyl-1-benzyl-2-pyrrolidinoethanamine (DPhBP). The optical specific rotations [α]₄₃₅²⁵ in obtained polymers depend on the chirality of ligands in the catalysts. In the present study, density functional theory (DFT) calculations were adopted to investigate the polymerization mechanism in detail. The bulky diphenylmethyl group in the chiral ligand is effective to enhance the formation of the product in the initiation reaction. The geometry related to the pyrrolidine ring of the chiral ligand in the Zn catalyst is responsible for determining the configuration of polymers. It was also confirmed that the bulky substituent on the N atom of the N-substituted maleimide is another factor for obtaining polymers with high [α]₄₃₅²⁵.     

First generation poly(propyleneimine) dendrimers functionalised with 1,8-naphthalimide units as fluorescence sensors for metal cations and protons

The synthesis of two new green fluorescent poly(propyleneimine) dendrimers from first generation has been described. The new materials are comprised of a 1,8-naphthalimide fluorophore having a substituent at C-4 position. The substituent in the first case is a N,N-dimethylaminoethylamino group while in the second one it is N-methylpiperazine. The spectroscopic and photophysical characteristics of the new dendrimers determined in organic solvent of different polarity have been presented. Both dendrimers show substantial increases in their fluorescence intensity in the presence of metal cations (Zn²⁺, Co²⁺, Ni²⁺, Pb²⁺, Mn²⁺, Cu²⁺, Fe³⁺ and Ag⁺) and protons. The influence of the photoinduced electron transfer on their sensing properties has been discussed.     

Synthesis,crystal structures and spectroscopic properties of two Zn(II) Schiff’s base complexes of pyridoxal

Two mononuclear zinc(II) complexes, [ZnL¹H₂Cl]Cl·2H₂O (1·Cl·2H₂O) and [ZnL²HCl]·H₂O (2·H₂O) (L¹H₂ and L²H₂ are N,N′-bis(pyridoxylidene)ethylenediamine and N,N′-bis(pyridoxylidene)1,3-propanediamine, respectively) have been synthesized and characterized by elemental analysis, FT-IR, fluorescence spectroscopy, TG–DTA and single crystal X-ray diffraction studies. The Zn(II) ion in complex 1 have a square pyramidal geometry with appreciable distortion towards trigonal bipyramid, whereas in 2 it has a geometry which is near the midpoint of square pyramid and trigonal bipyramid. The zinc atom is coordinated by two imine nitrogens, two phenolic oxygens and one chloride ion. The Zn(II) complexes show emission at 462 nm when excited at their lowest energy absorption at 372 nm.     

Correlation of reactivity with structural factors in a series of Fe(II) substituted cobalt ferrites

A series of powdered cobalt ferrites, Co_xFe_3−xO₄ with 0.66?x<1.00 containing different amounts of Fe^II, were synthesized by a mild procedure, and their Fe and Co site occupancies and structural characteristics were explored using X-ray anomalous scattering and the Rietveld refinement method. The dissolution kinetics, measured in 0.1 M oxalic acid aqueous solution at 70 °C, indicate in all cases the operation of a contracting volume rate law. The specific rates increased with the Fe^II content following approximately a second-order polynomial expression. This result suggests that the transfer of Fe^III controls the dissolution rate, and that the leaching of a first layer of ions Co^II and Fe^II leaves exposed a surface enriched in slower dissolving octahedral Fe^III ions. Within this model, inner vicinal lattice Fe^II accelerates the rate of Fe^III transfer via internal electron hopping. A chain mechanism, involving successive electron transfers, fits the data very well.     

NMR and fluorescence spectral studies on bisdithiocarbamates of divalent Zn,Cd and their nitrogenous adducts: Single crystal X-ray structure of (1,10-phenanthroline)bis(4-methylpiperazinecarbodithioato) zinc(II)

The current paper describes the synthesis and characterization of the following adducts: [Zn(4-mpzdtc)₂(1,10-phen)] · H₂O (1), [Zn(4-mpzdtc)₂(2,2′-bipy)] (2), [Cd(4-mpzdtc)₂(1,10-phen)] (3), [Cd(4-mpzdtc)₂(2,2′-bipy)] (4), [Zn(padtc)₂(1,10-phen)] (5) and [Cd(padtc)₂(1,10-phen)] (6) (where, 4-mpzdtc = 4-methylpiperazinecarbodithioate anion, padtc = N,N′-(iminodiethylene)bisphthalimide dithiocarbamate anion, 1,10-phen = 1,10-phenanthroline and 2,2′-bipy = 2,2′-bipyridine). All the synthesized complexes were characterized by UV–Vis, IR, NMR, (¹H and ¹³C) and fluorescence spectra. A single crystal X-ray structural analysis was carried out for complex 1. IR spectra of the complexes show the contribution of the thioureide form to the structures. The observed deshielding of the α-protons for 1–6 in the ¹H NMR spectra is attributed to the drift of electrons from the nitrogen of the NR₂ groups, forcing a high electron density towards sulfur via the thioureide π-system. In the ¹³C NMR spectra, the most important thioureide (N¹³CS₂) carbon signals are observed in the region 206–208 ppm. Fluorescence spectra of complexes 5 and 6 show intense fluorescence due to the presence of rigid conjugated systems such as phthalimide and 1,10-phenanthroline. The observed fluorescence maxima for complexes with a MS₄N₂ chromophore in the visible region are assigned to the metal-to-ligand charge transfer (MLCT) processes. Single crystal X-ray structural analysis of 1 showed that the zinc atom is in a distorted octahedral environment with a MS₄N₂ chromophore. VBS equivalent to 1.81 supports the correctness of the determined structure. The piperazine ring in the dithiocarbamate fragment is in the normal chair conformation.     

Lithium zincopyrophosphate,Li2Zn3(P2O7)2

The title compound, dilithium(I) trizinc(II) bis[diphosphate(4−)], is the first quaternary lithium zincopyrophosphate in the Li–Zn–P–O system. It features zigzag chains running along c, which are built up from edge‐sharing [ZnO₅] trigonal bipyramids. One of the two independent Zn sites is fully occupied, whereas the other is statistically disordered by Zn²⁺ and Li⁺ cations, although the two Zn sites have similar coordination environments. Li⁺ cations occupy a four‐coordinated independent site with an occupancy factor of 0.5, as well as being disordered on the partially occupied five‐coordinated Zn site with a Zn²⁺/Li⁺ ratio of 1:1.     

Determination of electrochemical kinetic parameters by square-wave polarography: Polarographic reduction of Zn(II) in 1 M KCl,KBr, and KNCS solutions

A new method of determining electrochemical kinetic parameters by square-wave polarography was presented, in which the faradaic current at θ/2, θ being the half-period of superimposed square-wave voltage, was used for the analysis. The method gave the following kinetic parameters for the electrode reaction, Zn(II) + 2e(Hg), in aqueous solutions at 25° C: k_c^θ=0.0052 cm s^?1 and α_c=0.36 in 1 M KCl, k_c^θ=0.011 cm s^?1 and α_c=0.30 in 1 M KBr, and k_c^θ=0.020 cm s^?1 and α_c=0.52 in 1 M KNCS. Induced adsorption of Zn(II) on the dropping mercury electrode was suggested in solutions containing thiocyanate ions.     

Oxidation of phenothiazine dyes by manganese(III) in sulfuric acid solution

Manganese(III) sulfato complexes cause the oxidative degradation of methylene blue and its partially and fully N-demethylated derivatives, azure B and thionine dyes, respectively, in sulfuric acid media. The reaction proceeds through a colored reactive organic radical generated in the first stage via one-electron oxidation of the starting material, leading to a mixture of N-demethylated and/or deaminated species. The rates of formation of the methylene blue and azure B radicals are much higher than those of their further decomposition, whereas the generation of the thionine radical is much slower than its immeasurably fast decay. The kinetics of decomposition of all three dyes and the methylene blue and azure B radicals were studied spectrophotometrically under isolation conditions at 298 K. The first stage of each reaction proceeds according to a second-order rate expression, being first order in the dyes and in the manganese(III) concentrations. Dependence of the pseudo-first-order rate constants on the oxidant concentration for the second stage exhibits a saturation effect under the applied conditions. It is postulated that electron transfer takes place between the [Mn(SO₄)₃]³⁻ complex and the protonated form of the dye. The reactivity order of the dyes as determined from the second-order rate constants for the first reaction stage corresponds to the order of their HOMO energies.     

Turning the Inert Element Zinc into an Active Single-Atom Catalyst for Efficient Fenton-Like Chemistry

Amongst various Fenton-like single-atom catalysts (SACs), the zinc (Zn)-related SACs have been barely reported due to the fully occupied 3d¹⁰ configuration of Zn²⁺ being inactive for the Fenton-like reaction. Herein, the inert element Zn is turned into an active single-atom catalyst (SA−Zn−NC) for Fenton-like chemistry by forming an atomic Zn−N₄ coordination structure. The SA−Zn−NC shows admirable Fenton-like activity in organic pollutant remediation, including self-oxidation and catalytic degradation by superoxide radical (O₂⋅⁻) and singlet oxygen (¹O₂). Experimental and theoretical results unveiled that the single-atomic Zn−N₄ site with electron acquisition can transfer electrons donated by electron-rich pollutants and low-concentration PMS toward dissolved oxygen (DO) to actuate DO reduction into O₂⋅⁻ and successive conversion into ¹O₂. This work inspires an exploration of efficient and stable Fenton-like SACs for sustainable and resource-saving environmental applications.     

Tuning the reactivity of mononuclear nonheme manganese(iv)-oxo complexes by triflic acid

Triflic acid (HOTf)-bound nonheme Mn(iv)-oxo complexes, [(L)Mn^IV(O)]²⁺–(HOTf)₂ (L = N4Py and Bn-TPEN; N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine and Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine), were synthesized by adding HOTf to the solutions of the [(L)Mn^IV(O)]²⁺ complexes and were characterized by various spectroscopies. The one-electron reduction potentials of the Mn^IV(O) complexes exhibited a significant positive shift upon binding of HOTf. The driving force dependences of electron transfer (ET) from electron donors to the Mn^IV(O) and Mn^IV(O)–(HOTf)₂ complexes were examined and evaluated in light of the Marcus theory of ET to determine the reorganization energies of ET. The smaller reorganization energies and much more positive reduction potentials of the [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes resulted in greatly enhanced oxidation capacity towards one-electron reductants and para-X-substituted-thioanisoles. The reactivities of the Mn(iv)-oxo complexes were markedly enhanced by binding of HOTf, such as a 6.4 × 10⁵-fold increase in the oxygen atom transfer (OAT) reaction (i.e., sulfoxidation). Such a remarkable acceleration in the OAT reaction results from the enhancement of ET from para-X-substituted-thioanisoles to the Mn^IV(O) complexes as revealed by the unified ET driving force dependence of the rate constants of OAT and ET reactions of [(L)Mn^IV(O)]²⁺–(HOTf)₂. In contrast, deceleration was observed in the rate of H-atom transfer (HAT) reaction of [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes with 1,4-cyclohexadiene as compared with those of the [(L)Mn^IV(O)]²⁺ complexes. Thus, the binding of two HOTf molecules to the Mn^IV(O) moiety resulted in remarkable acceleration of the ET rate when the ET is thermodynamically feasible. When the ET reaction is highly endergonic, the rate of the HAT reaction is decelerated due to the steric effect of the counter anion of HOTf.     

A Zn(II) ion selective fluorescence sensor that is not affected by Cd(II)

Comparison of sensors sodium-2,6-diamino-(N,N,N′,N′-tetraacetate)-4-methylanisole 1 and sodium-2,6-diamino-(N,N,N′-triacetate)-4-methylanisole 2 reveal that the loss of an acetyl group in 2 leads to a more selective Zn(II) induced fluorescence enhancement and shows no response to any other metal ions including Cd(II). Structural modifications and AM1 calculations indicate that the sensor uses the three acetyl groups and the 3° amino nitrogen for binding the metal ion. AM1 calculations imply a trigonal bipyramidal coordination for Zn(II) with the solvent molecule occupying one of the axial positions.     

Water-soluble supramolecular porphyrin dimer: self-organization of mono(imidazolyl)-substituted Zn porphyrin to a special-pair type dimer in water

Tris(4-carboxylphenyl)-mono(N-methylimidazolyl)-substituted Zn porphyrin was synthesized as a precursor for a water-soluble supramolecular porphyrin dimer. The dimer formation was performed in a NaHCO₃ aq solution (pH 8.4) and phosphate buffer solutions (pH 7.4-9.0). The split Soret bands of Zn porphyrin observed in the absorption spectra clearly showed self-organization to a special-pair type slipped cofacial dimer via metal coordination of imidazole even in water.