N. V. Nastapova

Mediated electrochemical synthesis of Pd0 nanoparticles in solution

Efficient electrosynthesis of palladium(0) nanoparticles is carried out in solution by mediated electrochemical reduction of a complex dianion [PdCl4]2– in the DMSO/0.1 M Bu4NCl medium. The mediator function was performed by methylviologen and tetraviologen calix[4]resorcines MVCA-Cn8+ with methyl (n = 1), n-pentyl (n = 5), and n-decyl (n = 10) substituents in resorcinol rings at the potentials of redox pairs MV2+/MV•+, MVCA-Cn8+/MVCA-Cn4•+. The resulting metal nanoparticles gradually aggregate to yield coarser particles. The smallest size (255 nm) have metal particles obtained in the presence of MVCA-C58+ for the rest mediators, metal particles of the micron size are formed. Sonication results in disintegration of aggregates to nanoparticles.

Methylviologen mediated electrosynthesis of gold nanoparticles in the solution bulk

Electrosynthesis of gold nanoparticles (AuNp) was carried out by methylviologen mediated reduction of Au(I) at potentials of the MV2+/MV˙+ redox couple in water/0.1 M NaCl medium, in the absence and in the presence of stabilizers. In all the cases, AuNp are formed in the solution bulk and are not deposited on the cathode. In the absence of stabilizers, AuNp (14–100 nm) coalesce to give aggregates of various shapes that eventually form a deposit. Sonication reversibly destructs the deposit into nanoparticles. In the presence of alkylamino-modified silicate nanoparticles (SiO2–NHR, 120–160 nm), spherical AuNp (≤20 nm) are bound as inclusions in the SiO2–NHR surface layer. Polyvinylpyrrolidone (40 000 D) stabilizes spherical AuNp with a mean diameter of 5–14 nm. All the particles were characterized by electron microscopy methods (SEM, STEM) and X-ray powder diffraction (XRPD).

Redox-switchable binding of the Mg2+ ions by 21,31-diphenyl-12,42-dioxo-7,10,13-trioxa-1,4(3,1)-diquinoxaline-2(2,3),3(3,2)-diindolysine-cyclopentadecaphane

The binding of the Li+, Na+, K+, Mg2+, and Co2+ ions by 21,31-diphenyl-12,42-dioxo-7,10,13-trioxa-1,4(3,1)-diquinoxaline-2(2,3),3(3,2)-diindolysine-cyclopentadecaphane containing two indolysine fragments, two quinoxaline fragments, and 3,6,9-trioxyundecane spacer in the acetonitrile/0.1 M Bu4NBF4 environment is studied by the method of cyclic voltammetry. It is demonstrated that the Li+, Na+, K+, and Co2+ ions are not bound by this macrocycle, whereas selective redox-switchable binding is observed for the Mg2+ ions. The macrocycle binds the Mg2+ ions way more efficiently as compared with its radical cation and dication. The indolysinequinoxaline fragments play the determining role in the binding.

3-Indolizin-2-ylquinoxalines and the derived monopodands

The reactions of 3-acetylquinoxalin-2-one with methyl-and benzylpyridines in the presence of iodine produce the corresponding 3-(2-alkylpyridinioacetyl)quinoxalin-2(1H)-one iodides. Treatment of the latter with triethylamine affords the corresponding 3-indolizin-2-ylquinoxalin-2-ones. Due to the presence of the endocyclic carbamoyl group, the reactions of these compounds with bisalkylating reagents give quinoxaline-containing monopodands and monoalkylation products containing spacers with different lengths and of different nature.

Voltammetric study of metal ions binding by biindolizine heterocyclophanes and their acyclic analogues

The binding of ions Li+, Na+, K+, (group I), Mg2+, Al3+, Ga3+ (group II), Ca2+, Pb2+ (group III) ions, Ba2+ and paraquat by heterocyclophanes containing biindolizine and quinoxaline fragments connected by 3,6,9-trioxaundecane and 5,8,11,14,17-pentaoxageneicosane spacers, and also their acyclic analogues, in the acetonitrile-0.1 M Bu4NBF4 is studied by cyclic voltammetry. A conclusion is drawn that the ions of the group I are not bound by these compounds; the paraquat is not bound by heterocyclophane with the 5,8,11,14,17-pentaoxageneicosan spacers. For ions of the group II, reversible redox-switchable binding by the macrocycles with the 3,6,9-trioxaundecane and 5,8,11,14,17-pentaoxageneicosan spacers is observed: the initial compounds show the binding; their radical cations and dications do not. The binding of the ions of the group III and Ba2+ is determined by the macrocycles’ size. In particular, these ions are bound not only by the heterocyclophane with 3,6,9-trioxaundecane spacers but also by its radical cation or dication. The binding results in the corresponding dication stabilization. The heterocyclophane with the 5,8,11,14,17-pentaoxageneicosan spacers demonstrates the redox-switchable binding of Ca2+ and Pb2+ ions; no effect of Ba2+ ions on the cyclic voltammograms of this heterocyclophane was observed. In the ternary system “heterocyclophane with 3,6,9-trioxaundecane spacers + ions of the group II (Al3+, Ga3+) + ions of the group III (Ca2+, Pb2+)” either primary binding of the group III ion Pb2+ or concurrent binding of the ions of the group II and the group III, with the system’s reversible redox-switching from one metal complex to another, was observed.

Binding of 1,5-bis(p-sulfonatophenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane with tetra(methyl viologen) calix[4]resorcinol

Binding of an amphiphilic dianion 1,5-bis(p-sulfonatophenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane (APCO2−) with an amphiphilic octacation tetra(methyl viologen) calix[4]resorcinol (MVCA8+) in media containing different amounts of water and DMSO using NaClO4 or NaCl as supporting electrolytes was shown for the first time by cyclic voltammetry and spectrophotometry. The stoichiometry of the complex depends on the MVCA8+: APCO2− ratio, medium, and supporting electrolyte. A 1: 1 charge-transfer complex is mainly formed (λmax = 480 nm) in 30% DMSO at a ratio of the compounds of 1: 1. A similar 1: 1 complex of APCO2− with a model compound methyl viologen MV2+ (λmax = 482 nm) is formed under these conditions. A donor-acceptor interaction occurs between the acceptor viologen units and nitrogen-centered electron-donating fragments of the APCO2− dianion. An increase in the content of APCO2− in the solution leads to an additional binding of one (30 vol.% DMSO, water, NaClO4) or two (30 vol.% DMSO, water, NaCl) particles of APCO2− with the hydrophobic fragments of MVCA8+. The complexes aggregate to form insoluble precipitates in aqueous and water-DMSO media. A selective reversible electroswitching from the bound to free state of one of the three bound APCO2− particles was performed when reducing MVCA8+ to MVCA4·+ in a 30 vol.% DMSO/NaCl medium.

Electrochemical reduction and oxidation of fullerenopyrrolidines* and the ESR spectra of paramagnetic intermediates

Electroreduction and electrooxidation of monosubstituted N-methyl[60]fullerenopyrrolidines were studied by cyclic voltammetry and potentiostatic microelectrolysis in the cavity of an ESR spectrometer. Stepwise reversible transfer of three electrons to the fullerenopyrrolidine molecule results in the formation of stable radical anions (according to ESR, g = 2.0000, ΔH = 0.8 G), dianions, and radical trianions (according to ESR, g = 2.0015, ΔH = 1.5 G). The reduction potentials vary over narrow limits depending on the nature of the substituents in the pyrrolidine fragment of the compounds. Electrooxidation is irreversible and occurs in either one or two steps. For compounds containing the aniline, indole, or phenol fragment, the first step is associated with oxidation of these fragments and only after that, is the fullerenopyrrolidine core oxidized. Oxidation of the pyrrolidine fragment is substantially more difficult than that of tertiary amines.

Binding of 1,5-bis(p-sulfonatophenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane with tetramethylviologen calix[4]resorcin with a methyl radical in the resorcinol ring

The binding of the amphiphilic 1,5-bis(para-sulfonatophenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane dianion (APCO2−) with the octacation of the amphiphilic tetramethylviologen calix[4]resorcin having a methyl radical in the resorcinol ring (MVCA-C18+) in 30% aqueous DMSO in a 0.1M NaCl supporting solution was studied by various methods. The dianion was bound at the upper rim of calixresorcin due to the donor-acceptor interactions between the viologen acceptor units and the nitrogen- and phosphorus-centered electron-donor fragments of APCO2− and at the lower rim due to hydrophobic interactions. The composition of the complex depended on the MVCA-C18+: APCO2− ratio. The complexes aggregated, forming insoluble precipitates. MVCA-C18+-APCO2− is a system with reversibly electro-switchable aggregation. The starting MVCA-18+ octacation partially bound APCO2−, a certain quantity of both substrates remaining in a free state in solution. The MVCA-C14+· tetraradical tetracation formed as a result of the reduction of MVCA-C18+ completely bound APCO2− due to hydrophobic interactions when the MVCA-C18+: APCO2− ratio was 1: 3. The reversible oxidation of MVCA-C14+· to the starting MVCA-C18+ octacation brought the system back to its starting state

Electricoswitchable bonding of metal ions and complexes by calixarenes

The results of authors on designing electroswitchable supramolecular systems based on calix[4]arenes, calix[4]resorcinols, and transition metal ions and complexes are summarized. We consider systems in which the switching is performed owing to electrochemical reactions both of groups grafted to the macrocycle and bound substrates. Examples of electrochemically switchable luminescence are given.

Electrochemical properties of n-sulfonatothiacalyx[4]arene complexes with Fe3+ and [Co(dipy)3]3+ ions

The electrochemical properties of n-sulfonatothiacalyx[4]arene (H4XNa4) complexes with [Co(dipy)3]3+ and Fe3+ ions were studied by means of cyclic voltammetry in aqueous solution at pH 2.5. The observed single-electron reduction of [Co(dipy)3]3+ bound extraspherically to the upper rim and Fe3+ ion bound intraspherically to the lower rim of n-sulfonatothiacalyx[4]arene in binary [Co(dipy)3]3+ · H3X5−, H3X5− · Fe3+, and ternary [Co(dipy)3]3+ · H3X5− · Fe3+ heterometal complexes was more difficult than in the free state. The reversible single-electron transfer to the metal ion results in lower binding energy ([Co(dipy)3]3+, ΔΔG0 = 3.9 kJ/mol) or in full fast dissociation of the complex (Fe3+). The ternary complex in the solution forms the aggregates, in which inner encapsulated Fe(III) and Co(III) ions are not reduced on the electrode. Their quantitative reduction takes place by the relay mechanism of intra- and intermolecular electron transfer through electrochemically generated [Co(dipy)3]2+ outer ions.