Search

The electrochemical and chemical oxidation of extended TTF 4 and 5 are analysed by cyclic voltammetry, Visible/NIR and ESR spectroscopies, and the X-ray structures of the new salts 5 x BF(4)(CH(2)Cl(2)) and 4 x ClO(4)(THF)(1/2) are presented. The effects of structural factors on the pi-dimerization or the disproportionation reaction of the cation radical are shown. The oxidation of compound 4 presents the successive formation of stable cation radical and dication species both in dichloromethane (DCM) and in a CH(3)CN/THF mixture. In contrast, for compound 5, the stability of the oxidation states strongly depends on the nature of the solvent. In DCM, the oxidation of 5 proceeds by two close one-electron transfers while in CH(3)CN/THF the dication is directly formed via a two-electron process. The X-ray structures of the two salts reveal the formation of pi-dimers of cation radical. While the dimer (5(2))(2+) is due mainly to pi-pi interactions between the conjugating spacer, the multiplication of the sulfur atoms in compound 4 contributes to stabilize the dimer by the combined effects of S-S and pi-pi interactions. Visible/NIR and ESR experiments confirm the higher tendency of 4(+)(.) to dimerize with the occurrence of dimer and monomer in solution, while for 5(+)(.) only the monomer is detected in DCM. On the other hand, by dissolution of 5 x BF(4)(CH(2)Cl(2)) in CH(3)CN, only the neutral and the dicationic states of compounds 5 are observed owing to the disproportionation reaction.

The efficiency and versatility of time-resolved FTIR spectroscopy has been used to follow concentration profiles of species produced during a cyclic voltammetric scan. It has been tested in situ and in resolved time, by probing the reduction of tetracyanoquinodimethane (TCNQ) on its first and second electrochemical wave. Besides the establishment of the method, the individual concentrations of TCNQ, of the monoanion and of the dianion were monitored at distinct infrared frequencies and the time derivatives of the concentration profiles were compared to the voltammograms. Keywords: Electrochemistry, IR spectroscopy, Spectroelectrochemistry, TCNQ

A first study by in situ FTIR spectroelectrochemistry of TTF and TTM-TTF has been undertaken. The oxidation, in this case, is caused only by electrochemistry, which constitutes a clear advantage over chemical oxidation since no side products are present in the solution. In this context, we obtained the signals of neutral, radical cation, and dication species of TTF and TTM-TTF. The experimental conditions were chosen in order to avoid the possible formation of π-dimer species and to obtain a satisfactory signal-to-noise ratio. A weak signal was detected for TTF and a stronger one for TTM-TTF. The changes induced by the oxidation process in the IR spectra of TTF and TTM-TTF have been analyzed with the aid of B3P86/6-31G** density functional theory (DFT) calculations, which allows for a comprehensive assignment of the bands observed. DFT calculations show that the IR signal associated to the asymmetric stretching of the lateral CC bonds can be used as a structural signature to identify the different oxidation states of TTF and TTM-TTF. The frequency downshift and the intensity increase undergone by the νasym(CC) vibration upon oxidation are shown to be the key factors to understand the evolution of the IR spectra.

A new series of linearly extended tetrathiafulvalene analogues with thienothiophene and dithienothiophene π-conjugating spacers has been synthesized. Electronic absorption spectra present a vibronic fine structure typical for rigid conjugated systems. Investigation of the electrochemical behaviour of the new donors by cyclic voltammetry reveals the successive generation of stable radical cation and dication species. The crystallographic structure of a single crystal of a dication salt of TT-TTF(ClO4)2 has been analysed by X-ray diffraction. The dication presents a syn conformation stabilised by S⋯S intramolecular interactions. The quinoid structure expected for the spacer for the +2 oxidation state is clearly revealed by the bond lengths.

Summary: Purpose: To evaluate the effect on visual function of a concomitant antiepileptic drug (AED) in patients treated with vigabatrin (VGB). Methods: Sixty-four consecutive patients with a history of partial seizures currently treated with VGB with either carbamazepine (CBZ) or valproate (VPA) were examined with automated kinetic perimetry, static perimetry, electrooculogram (EOG), and electroretinogram (ERG). An original device based on kinetic perimetry was developed to quantify the area of perception for each isopter. Results: Fifty-two patients were finally included. The results showed a significant difference in patients treated with VGB-VPA compared with patients treated with VGB-CBZ concerning the mean defect of static perimetry and the peripheral and midperipheral isopter (III 4e and III 1a Goldmann equivalent, respectively) in kinetic perimetry. EOG and ERG results did not differ significantly between the two groups. Conclusions: The visual impairment due to visual field constriction was more important in patients treated with VGB and VPA compared with patients treated with VGB and CBZ. The origin of this difference between the two associations could not be related to any particular retinal electrophysiologic abnormality.

International audience; The electrochemical and chemical oxidation of extended TTF 4 and 5 are analysed by cyclic voltammetry, Visible/NIR and ESR spectroscopies, and the X-ray structures of the new salts 5·BF4(CH2Cl2) and 4·ClO4(THF)1/2 are presented. The effects of structural factors on the π-dimerization or the disproportionation reaction of the cation radical are shown. The oxidation of compound 4 presents the successive formation of stable cation radical and dication species both in dichloromethane (DCM) and in a CH3CN/THF mixture. In contrast, for compound 5, the stability of the oxidation states strongly depends on the nature of the solvent. In DCM, the oxidation of 5 proceeds by two close one-electron transfers while in CH3CN/THF the dication is directly formed via a two-electron process. The X-ray structures of the two salts reveal the formation of pi-dimers of cation radical. While the dimer (5(2))2+ is due mainly to π–π interactions between the conjugating spacer, the multiplication of the sulfur atoms in compound 4 contributes to stabilize the dimer by the combined effects of S–S and π–π interactions. Visible/NIR and ESR experiments confirm the higher tendency of 4+· to dimerize with the occurrence of dimer and monomer in solution, while for 5+· only the monomer is detected in DCM. On the other hand, by dissolution of 5·BF4(CH2Cl2) in CH3CN, only the neutral and the dicationic states of compounds 5 are observed owing to the disproportionation reaction.; L'oxydation électrochimique ou chimique des analogues étendus du TTF 4 et 5 a été étudiée par voltamétrie cyclique et par spectroscopies Visible/proche-IR et RPE. La corrélation de la structure des radicaux cations et de leur aptitude à dimériser ou à se dismuter est discutée. En solution dans le dichlorométhane (DCM) ou dans un mélange acétonitrile/tétrahydrofuranne (CH3CN/THF), le composé 4 s'oxyde réversiblement en cation radical puis en dication. La présence d'un système conjugué plus étendu pour le composé 5 facilite l'accès au dication. Ainsi l'oxydation de 5 s'effectue en deux étapes monoélectroniques très proches dans le DCM, tandis qu'une seule étape à deux électrons est observée dans le mélange CH3CN/THF. Des monocristaux des sels 5⋅BF4(CH2Cl2) et 4⋅ClO4(THF)1/2 ont été analysés par diffraction des rayons X. La structure des sels révèle la formation de dimères de radicaux cations. Pour le composé 4, enrichi en atome de soufre, les dimères (4(2))2+ sont stabilisés à la fois par des interactions π–π et des interactions soufre–soufre. En solution dans le dichlorométhane ou l'acétonitrile, la présence de dimère est observée. Pour le composé 5 le processus de dimérisation est principalement dû à des interactions π entre les systèmes conjugués. En solution dans le dichlorométhane, seule la présence de radicaux cations est détectée, tandis que dans l'acétonitrile, ces derniers se dismutent en dications et donneurs neutres.

In order to put a final point to this discussion ([1 and 2] and preceding paper), it is meaningless to debate on minor points (ECE or EEC mechanism, nature of the working electrode or choice of the supporting electrolyte). The major issue is that the work of Pletcher and coworkers [1] ignores the role of the electroactivity of S3− in sulphur and polysulphide solutions, which is now admitted for many years. The electrochemical behaviour of S3− is unquestionable (Li+ electrolyte or not; Pt or vitreous carbon electrode): in Li2S6+DMF solutions, only S3− and its dimer S62− are present at equilibrium [3] and the experimental voltammograms have clearly shown that S3− is the first reducible species (a “nice” reversible one-electron process) observed in negative direction in these solutions [4]. This reduction corresponds precisely (same redox potential) to the second step of the electroreduction of S8 and, obviously, to those observed by Pletcher et al. (peak III/IV) in S8+DMF solutions. Now, these authors assign this reduction to S4−, originating on the dissociation of S82−, without direct proofs and they conclude that S4− is a transient species. First, assigning a reversible reduction, observed by cyclic voltammetry between 248 and 323 K at 0.1 V/s, to a transient species (here S4−) is, in our opinion, unsatisfactory. This interpretation has not been justified by a scan rate dependence: only a temperature dependence (at 0.1 V/s) has been observed [1] in order to provide conclusive evidence (already shown in the past [4]) of a homogeneous chemical process. Second, the mechanism proposed by Pletcher et al. is simple but it does not contribute to the interpretation of the electrochemical behaviour of polysulphide solutions. In fact, the crucial question is: what is the chemical behaviour of S82− on the timescale of cyclic voltammetry? A fast disproportionation or a fast dissociation? A dissociation of S82− seems, at first sight, to be reasonable, but S4− has never been observed by time-resolved spectroelectrochemistry (at several scan rates (between 1 mV/s and 1 V/s) and at several temperatures (between 223 and 313 K) under semi-infinite or finite diffusion conditions) and, unfortunately, such a reaction does not account for the fast variations of S3− and S32− during the second reduction step of S8 [4]. Consequently, a disproportionation of S82− appears to be more attractive in order to understand the key role of S3− but, its kinetics is fast on the timescale of cyclic voltammetry. A possible reaction (widely discussed in the past [5]) could be: (1)S82− S2+S62− (acleavagereaction) Therefore, the electroactivity of S3− is simply described by these two reactions: (2)S62− 2S3− (adissociationreaction) (3)S3−+e− S32− (reductionofS3−) Finally, the transient species should be S2 (rapidly converted into S8), rather than S4−.

This paper presents a clarification of the electrochemical behaviour of sulphur and polysulphide ions in non-aqueous solvents following the recent publication of a paper [Electrochem. Commun. 3 (2001) 514] devoted to the mechanism of the electrochemical reduction of sulphur in dimethylformamide. Keywords: Sulphur, Polysulphides, Electrochemistry, Spectroelectrochemistry

The electrochemical reduction of a bicyclic hexaimino Schiff base cryptand 1 (N[(CH2)2N=CH---meta---C6H4---CH=N(CH2)2]3N) and that of one of its strands 2 ((CH3)2CH---N=CH---meta---C6H4---CH=N---CH(CH3)2) has been studied by visible and near infrared in-situ spectroelectrochemical techniques. These results are in good agreement with those obtained using alkali metals, but in this case the effect of the formation of ion pairs is minimized through the use of tetrabutylammonium cations. It is confirmed that 1--- and 1= have the same visible and near IR spectrum. The spectrum of the products of the electrochemical reduction of 2 is similar to those of 1--- or 1=.

International audience; Electrooxidation of tetrathiafulvalene (TTF) carried out in the presence of (Bu4N)2TTF(CO2H)2(CO2)2 as supporting electrolyte affords wholly TTF organic materials in which TTF cations are associated with TTF(CO2H)2(CO2−)2 as counteranions.

International audience; Comparison of crystal structures of chromates: M(I)M(III)(CrO4)2; M(l) = Li+, Na+,K+, Rb+, Cs+, NH4+, Ag+ and TI+; M(III) = Al3+, Fe3+, Cr3+, Tl3+ and Bi3+ . Among compounds of formula type M(I)M(III)(CrO4)2 , the structures of trivalent bismuth compounds are always different from the others, because the surrounding of bismuth is never well-defined. Besides, nearly all these compounds have two-dimensional structures with the exception of Tl(I)Tl(III)(CrO4)2 , which is three-dimensional. This structural type can exist only when the ionic radius of the monovalent cation has a value near that of O2- ion (1.40 Å ), as for Rb+, NH4+ and Tl+.structure / crystal / cation + / cation 3+ / chromate; Parmi les nombreux composés de formule type M(I)M(III)(CrO 4) 2 , le bismuth trivalent se distingue des autres cations à cause de son environnement particulier peu organisé. En outre, la quasi-totalité de ces composés possèdent des structures en feuillets. Tl(I)Tl(III)(CrO 4) 2 , fait exception à la règle en ayant une structure tridimensionnelle. Ce type structural n'existe que si le rayon ionique du cation monovalent M(I) est proche du rayon ionique de l'oxygène O 2- voisin de 1,40 Å, ce qui est le cas pour Rb + , NH 4 + et Tl +.

International audience; Crystals of Cu(NO2)2(NH3)4 and Cu(NO2)2(NH3)2 have been prepared and studied. Two allotropic species exist for each compound, alpha-Cu(NO2)2(NH3)4 is orthorhombic, space group Fmmm or Fmm2; a(Ǻ) = 20.671(15), b(Ǻ) = 6.796(5), c(Ǻ) = 11.414(8), Z = 8; beta-Cu(NO2)2(NH3)4 is orthorhombic, space group Cccm or Ccc2; a(Ǻ) = 10.467(7), b(Ǻ) = 17.766(9), c(Ǻ) = 13.700(9), Z = 12; alpha-Cu(NO2)2(NH3)2 is triclinic, space group P-1, a(Ǻ) = 4.4165(12), b(Ǻ) = 5.6104(14), c(Ǻ) = 6.088(2), alpha° = 78.45(3), beta° = 103.95(3), gamma° = 100.16(3),Z = 1; beta-Cu(NO2)2(NH3)2 is monoclinic space group Cm or C2; a(Ǻ) = 9.226(6), b(Ǻ) = 7.556(3), c(Ǻ) = 4.486(3), beta° = 104.84(6), Z=2.