Evidence for a π Dimer in the Electrochemical Reduction of 1,3,5-Trinitrobenzene: A Reversible N2-Fixation System

Electrochemical Detection of Explosive Compounds in an Ionic Liquid in Mixed Environments: Influence of Oxygen, Moisture, and Other Nitroaromatics on the Sensing Response

From a security point of view, detecting and quantifying explosives in mixed environments is required to identify potentially concealed explosives. Electrochemistry offers a viable method to detect nitroaromatic explosive compounds owing to the presence of easily reducible nitro groups that give rise to a current signal. However, their reduction potentials can overlap with interfering species, making it difficult to distinguish particular compounds. We have therefore examined the effect of oxygen, moisture, and other nitroaromatic species on the cyclic voltammetry and square wave voltammetry of nitroaromatic compounds of a range of mixed environments, focussing on 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT) as model analytes, and using the hydrophobic room-temperature ionic liquid (RTIL) [P14,6,6,6][NTf2] as the solvent. Oxygen (0–20% vol.) minimally affected the current of the first reduction peak of TNT in [P14,6,6,6][NTf2], but significantly affects the current for DNT. The impact of water (0 to 86% relative humidity), however, was much more dramatic – even in the hydrophobic RTIL, water significantly affected the currents of the analyte peaks for TNT and DNT, and gave rise to additional reduction features, further contributing to the current. Additionally, the voltammetry of other related di- and tri-nitro compounds (2,6-dinitrotoluene, 1,3-dinitrobenzene, 2,4,6-trinitrotoluene, 1,3,5-trinitrobenzene, and musk xylene) was also studied to understand how different substituents on the aromatic ring may affect the reduction potentials. A 50:50 mixture of TNT and DNT revealed that both analytes could be separately identified and quantified using square wave voltammetry. Overall, this information is useful in determining the effect of other species on the current signals of electrochemical explosive sensors, and reveals that it may be necessary to dry the aprotic RTIL electrolyte when used in humid environments.

Electrochemical Reduction of 2,4-Dinitrotoluene in Room Temperature Ionic Liquids: A Mechanistic Investigation

The reduction mechanism of 2,4-dinitrotoluene (DNT) has been studied in eight room temperature ionic liquids (RTILs) using cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry, and digital simulation. Two distinctive peaks are observed in the voltammetry, corresponding to the stepwise reduction of the two nitro groups on the aromatic ring. Diffusion coefficients (D) and electron counts (n) were calculated from chronoamperometric transients, revealing an electron count of one in most RTILs, and a linear relationship between D and the inverse of viscosity. Focusing on the first reduction only, the peak appears to be chemically reversible at low concentrations. However, as the concentration increases, the current of the reverse peak diminishes, suggesting that one or more chemical steps occur after the electrochemical step. The results from digital simulation of the CVs in one of the RTILs reveal that the most likely mechanism involves a deprotonation of the methyl group of a parent DNT molecule by the electrogenerated radical anion and/or a dimerisation of two electrogenerated radical anions. Elucidation of the reduction mechanism of DNT (and other explosives) is vital if electrochemical techniques are to be employed to detect these types of compounds in the field.

Reversible Redox Switching of Chromophoric Phenylmethylenepyrans by Carbon-Carbon Bond Making/Breaking

Electrochromic organic systems that can undergo substantial variation of their optical properties upon electron stimulus are of high interest for the development of functional materials. In particular, devices based on radical dimerization are appropriate because of the effectiveness and speed of carbon-carbon bond making/breaking. Phenylmethylenepyrans are organic chromophores which are well suited for such purposes since their oxidation leads to the reversible formation of bispyrylium species by radical dimerization. In this paper, we show that the redox and spectroscopic properties of phenylmethylenepyrans can be modulated by adequate variation of the substituting group on the para position of the phenyl moiety, as supported by DFT calculations. This redox switching is reversible over several cycles and is accompanied by a significant modification of the UV-vis spectrum of the chromophore, as shown by time-resolved spectroelectrochemistry in thin-layer conditions.

Chemical and electrochemical dimerization of BODIPY compounds: electrogenerated chemiluminescent detection of dimer formation

The electrochemistry of several difluoroboradiaza-s-indacene (BODIPY) compounds lacking substituent groups in the meso (8)- and/or 3 (α)-positions was investigated. Chemical and electrochemical dimerization was demonstrated, and the dimerization depended on the character of substitution. The chemical dimerization was achieved by oxidative coupling using FeCl(3) in CH(2)Cl(2) at 0 °C. The electrochemical dimerization proceeded via anodic oxidation to the radical cation and monitored by both cyclic voltammetry (CV) and electrogenerated chemiluminescence (ECL). An available open 3-position was important for the formation of the dimer. The resulting 3,3'-dimer produced a second peak in the CV oxidation and also the appearance of a longer wavelength ECL peak at 656 nm, which is considerably shifted from the parent peak at 532 nm. No dimerization was seen for BODIPY molecules in which only the meso 8-position was unsubstituted, either by chemical or electrochemical means, demonstrating that dimerization occurs at position 3.

Integrated Study of the Dinitrobenzene Electroreduction Mechanism by Electroanalytical and Computational Methods

Electroreduction of 1,2-, 1,3-, and 1,4-dinitrobenzenes in DMF has been investigated by a set of experimental (cyclic voltammetry, chronoamperometry, and controlled potential electrolysis) and theoretical methods (digital simulation and quantum chemical calculations). The transformation of only one nitro group is observed in the presence of proton donors. The process selectivity is provided by reactions of radical anions' intermediate products. The key reactions here are protonation of radical anions of nitrosonitrobenzenes and N-O bond cleavage in radical anions of N-(nitrophenyl)-hydroxylamines.

Chemical exchange in novel spirobicyclic zwitterionic Janovsky complexes using dynamic 1H NMR spectroscopy

Highly coloured Janovsky complexes have been known for over 120 years, being used in many colourimetric analytical procedures. In this present study, two novel and stable nitrocyclohexadienyl spirobicyclic, zwitterionic Janovsky anionic hydantoin sigma-complexes, rac-1,3-diisopropyl-6-nitro-2,4-dioxo-1,3-diazaspiro[4.5]deca-6,9-dien-8-ylideneazinate, ammonium internal salt (1) and 1,3-diisopropyl-2,4-dioxo-1,3-diazaspiro[4.5]deca-6,9-dien-8-ylideneazinate, ammonium internal salt (2) have been prepared and characterised by NMR, electrospray ionization mass spectrometry (ESI-MS) and UV/visible methods. For the p-mononitro-substituted complex (2), we discovered chemical exchange behaviour using 1D saturation transfer and 2D exchange spectroscopy (EXSY) (1)H NMR techniques. The coalescence temperature was determined to be 62 degrees C in d(3)-acetonitrile. Analysis of these data provided a Gibbs free energy of activation, DeltaG double dagger, of + 67 kJ mole(-1), a rate constant, k, coalescence of 220 Hz and an equilibrium constant, K(eqm), of 0.98 as estimates of the exchange process in this solvent. Of the two mechanisms proposed for this fluxional behaviour, ring opening to a substituted benzene or proton exchange, a further theoretical modelling study of 1D (1)H NMR spectra was able to confirm that simple proton exchange between the two nitrogen sites of the hydantoin ring provided an accurate simulation of the observed experimental evidence. Interestingly, the o,p-dinitro-substituted complex (1) did not show any chemical exchange behaviour up to 150 degrees C in d(3)-acetonitrile (to 75 degrees C) and d(6)-dimethyl sulfoxide (DMSO). Molecular modelling at the MM2 level suggests that steric collisions of an N-acyl isopropyl substituent of the hydantoin ring with the ortho-nitro group of the spirofused cyclohexadienyl ring prevents the proposed proton exchange mechanism occurring in this case.

Oxygen carriers based on electrochemically reduced trinitroarenes

1,3,5-Trinitrobenzene, 2,4,6-trinitrotoluene and 1-methoxy-2,4,6-trinitrobenzene undergoes reversible dimerization, after electrochemical reduction of two units of corresponding anion radical. The pi-dimer formed captures molecular oxygen (O(2)) leading to a sigma(o-o)(H)-adduct, in an atmosphere of oxygen or even air. The sigma(o-o)(H)-adduct is able to reversibly bind and remove dioxygen after an external redox stimulus.