Jiří Urban

Comproportionation in the reduction of pyridinium derivatives: a combined ESR and electrochemical study

Combination of voltammetry at various types of solid electrodes (mostly Pt) with electrochemical generation of radical intermediates and their ESR investigation enabled a detailed elucidation of the reduction mechanism of 1,2,4,6-tetrasubstituted pyridinium cations and of analogous 1,1′-X-hexasubstituted bipyridinium dications. The experiments were carried out in aprotic solvents such as eg dimethylformamide. The measurements confirmed the reversible stepwise one-electron reduction of the quaternary pyridinium site leading to the formation of the ESR-visible radical (or radical cation) and followed by the uptake of a further electron. These reactions result in the formation of the corresponding anion and its protonation (possibly by traces of water present in the solution). The measurement of the change in the ESR intensity with time of the radical without applied voltage revealed that the redox mechanisms of the compounds are complicated by synproportionation between the starting substance and theanion formed by the uptake of two electrons per a single pyridinium group.

The influence of substitution on the reduction mechanism and stability of reduction intermediates in 1,1′-phenylene-, 1,1′-diphenylene- and 1,1′-polymethylene-bispyridinium dications

Abstract Electrochemical investigations in aprotic solvents showed a great influence of substituents in the pyridine nuclei on the reduction of the title compounds and also a sensitivity of the mechanism towards the groups forming a linkage between the two pyridine nuclei: whether the 1,1′-linkage is formed by a polymethylene chain or by phenylene groups is important. It was found that a full delocalization of the doubly positive charge only occurs over the whole particle with 1,1′-bispyridinium dications in which the separation of the two nigrogen atoms is effected by means of phenylene groups. If compared with the polymethylene-linked derivatives the uptake of the first electron takes place at considerably more positive potentials ( ie at about −0.3 V vs. nce ). The first reduction step is extremely sensitive towards even traces of hydroxyl ions (and water). An addition of N( n C 4 H 9 ) 4 OH leads to lowering or to disappearance of this wave. Spectral measurements in the uv-vis range proved that this is due not to a change in the electrochemical behaviour but, owing to the media, to a chemical change of the substance. On the other hand, in the 1,1′-bispyridinium dications where both nitrogen atoms are linked by several methylene groups (these compounds have two separated π-electron systems) the doubly positive charge is not delocalized over the whole particle. The electrochemical behaviour supports the assumption of two independent monovalent electroactive centres which are equivalent before the uptake of the primary electron. The electrochemical reduction of studied substances is strongly affected by oxygen and the mechanism of this effect is discussed.

ELUCIDATION OF MECHANISMS OF ORGANIC ELECTRODE PROCESSES BY SPIN TRAPPING.ELECTROREDUCTION OF SUBSTITUTED PYRYLIUM CATIONS

Abstract Pyrylium cations are reduced in aprotic solvents (acetonitrile) in two one-electron steps leading first to a neutral radical and to an anion after the uptake of the second electron. The neutral primary radical is only stable if the pyrane nucleus is substituted by electron-attracting substituents, in particular in position 4: in such a case the first reduction step is reversible. Otherwise the radical rapidly dimerizes and its existence cannot be proved by direct ESR spectroscopy. In this case three aromatic nitroso compounds were successfully used as spin traps for ESR spectroscopy. Their polarographic behaviour is described in this paper. These compounds are more convenient in such measurements than PBN since the ESR spectra of their adducts with the pyrylium radical better reflect the radical structure. The electrochemical generation of the radicals, their reaction with the spin trap and the measurement of the ESR spectra proper were carried out in a special cell.

Electrochemical reduction of 1,2,4,6-substituted pyridinium cations

In aprotic solvents such as dimethylformamide, acetonitrile or dimethylsulphoxide, the mechanism of electrochemical reduction of 1,2,4,6-substituted pyridinium cations was investigated. The stability of the key intermediate in the overall two-electron reduction, ie of the neutral radical, was increased by substituting the positions 2, 4 and 6 by phenyl groups. The measurements were carried out by dc polarography and cyclic voltammetry at mercury, platinum and carbon electrodes.

High- and low-temperature phases in isostructural 4-chloro-3-nitroaniline and 4-iodo-3-nitroaniline.

The structures of 4-chloro-3-nitroaniline, C6H5ClN2O2, (I), and 4-iodo-3-nitroaniline, C6H5IN2O2, (II), are isomorphs and both undergo continuous (second order) phase transitions at 237 and 200 K, respectively. The structures, as well as their phase transitions, have been studied by single-crystal X-ray diffraction, Raman spectroscopy and difference scanning calorimetry experiments. Both high-temperature phases (293 K) show disorder of the nitro substituents, which are inclined towards the benzene-ring planes at two different orientations. In the low-temperature phases (120 K), both inclination angles are well maintained, while the disorder is removed. Concomitantly, the b axis doubles with respect to the room-temperature cell. Each of the low-temperature phases of (I) and (II) contains two pairs of independent molecules, where the molecules in each pair are related by noncrystallographic inversion centres. The molecules within each pair have the same absolute value of the inclination angle. The Flack parameter of the low-temperature phases is very close to 0.5, indicating inversion twinning. This can be envisaged as stacking faults in the low-temperature phases. It seems that competition between the primary amine-nitro N-H···O hydrogen bonds which form three-centred hydrogen bonds is the reason for the disorder of the nitro groups, as well as for the phase transition in both (I) and (II). The backbones of the structures are formed by N-H···N hydrogen bonding of moderate strength which results in the graph-set motif C(3). This graph-set motif forms a zigzag chain parallel to the monoclinic b axis and is maintained in both the high- and the low-temperature structures. The primary amine groups are pyramidal, with similar geometric values in all four determinations. The high-temperature phase of (II) has been described previously [Garden et al. (2004). Acta Cryst. C60, o328-o330].

4-Amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one (metamitron) and 4-amino-6-methyl-3-phenyl-1,2,4-triazin-5(4H)-one (isometamitron).

The title structures, both C(10)H(10)N(4)O, are substitutional isomers. The N-N bond lengths are longer and the C=N bond lengths are shorter by ca 0.025 A than the respective average values in the C=N-N=C group of asymmetric triazines; the assessed respective bond orders are 1.3 and 1.7. There are N-H...O and N-H...N hydrogen bonds in both structures, with 4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one containing a rare bifurcated N-H...N,N hydrogen bond. The structures differ in their molecular stacking and the hydrogen-bonding patterns.

2-(2-Hydroxy­ethyl)-3-[(2-hydroxy­ethyl)imino]isoindolin-1-one

In the crystal structure of the title compound, C12H14N2O3, molecules are packed into layers parallel to (100). Each layer contains centrosymmetric dimers formed by a pair of strong O—H⋯N hydrogen bonds with an R 2 2(10) motif, while strong O—H⋯O hydrogen bonds forming C(10) chains connect molecules into a two-dimensional network. Additional stabilization is supplied by weak C—H⋯O hydrogen bonds and weak π–π stacking interactions with centroid–centroid distances in the range 3.4220 (7)–3.9616 (7) Å.