Synthesis,Spectroscopic Properties,and Crystal Structure of 2,2′‐Bipyridyldimesitylnickel(II)

The dimesitylnickel(II) complex [(bpy)NiMes₂] (Mes = mesityl = 2,4,6‐trimethylphenyl) was prepared and examined spectroscopically and electrochemically. The crystal and molecular structure was determined from single crystal X‐Ray diffraction experiments (monoclinic, P2₁/n, Z = 4, a = 8.3092(8) Å, b = 18.233(2) Å, c = 15.226(2) Å, β = 98.035(6)°). The nickel atom displays a distorted square planar environment. The axial positions of the square plane are shielded by each one of the methyl groups on the mesityl substituents. The complex shows electrochemical reduction processes that are mainly centered on the bpy ligand as inferred from spectroelectrochemical investigations (EPR and UV/Vis/NIR absorption) of the radical anion or dianion. The observed oxidation is assigned to a Ni^II/Ni^III couple. The title complex exhibits strongly solvatochromic longwavelength electronic absorptions.     

Tetrathiafulvalene‐Based Mixed‐Valence Acceptor–Donor–Acceptor Triads: A Joint Theoretical and Experimental Approach

This work presents a joint theoretical and experimental characterisation of the structural and electronic properties of two tetrathiafulvalene (TTF)‐based acceptor–donor–acceptor triads (BQ–TTF–BQ and BTCNQ–TTF—BTCNQ; BQ is naphthoquinone and BTCNQ is benzotetracyano‐p‐quinodimethane) in their neutral and reduced states. The study is performed with the use of electrochemical, electron paramagnetic resonance (EPR), and UV/Vis/NIR spectroelectrochemical techniques guided by quantum‐chemical calculations. Emphasis is placed on the mixed‐valence properties of both triads in their radical anion states. The electrochemical and EPR results reveal that both BQ–TTF–BQ and BTCNQ–TTF–BTCNQ triads in their radical anion states behave as class‐II mixed‐valence compounds with significant electronic communication between the acceptor moieties. Density functional theory calculations (BLYP35/cc‐pVTZ), taking into account the solvent effects, predict charge‐localised species (BQ ^{. ?}–TTF–BQ and BTCNQ ^{. ?}–TTF–BTCNQ) as the most stable structures for the radical anion states of both triads. A stronger localisation is found both experimentally and theoretically for the BTCNQ–TTF–BTCNQ anion, in accordance with the more electron‐withdrawing character of the BTCNQ acceptor. CASSCF/CASPT2 calculations suggest that the low‐energy, broad absorption bands observed experimentally for the BQ–TTF–BQ and BTCNQ–TTF–BTCNQ radical anions are associated with the intervalence charge transfer (IV‐CT) electronic transition and two nearby donor‐to‐acceptor CT excitations. The study highlights the molecular efficiency of the electron‐donor TTF unit as a molecular wire connecting two acceptor redox centres.     

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane

Dibenzo[a,e]pentalene (DBP) is a non-alternant conjugated hydrocarbon with antiaromatic character and ambipolar electrochemical behavior. Upon both reduction and oxidation, it becomes aromatic. We herein study the chemical oxidation and reduction of a planar DBP derivative and a bent DBP-phane. The molecular structures of its planar dication, cation radical and anion radical in the solid state demonstrate the gained aromaticity through bond length equalization, which is supported by nucleus independent chemical shift-calculations. EPR spectra on the cation radical confirm the spin delocalization over the DBP framework. A similar delocalization was not possible in the reduced bent DBP-phane, which stabilized itself by proton abstraction from a solvent molecule upon reduction. This is the first report on structures of a DBP cation radical and dication in the solid state and of a reduced bent DBP derivative. Our study provides valuable insight into the charged species of DBP for its application as semiconductor.     

Isolation of a Neutral Boron‐Containing Radical Stabilized by a Cyclic (Alkyl)(Amino)Carbene

Utilizing a cyclic (alkyl)(amino)carbene (CAAC) as a ligand, neutral CAAC‐stabilized radicals containing a boryl functionality could be prepared by reduction of the corresponding haloborane adducts. The radical species with a duryl substituent was fully characterized by single‐crystal X‐ray structural analysis, EPR spectroscopy, and DFT calculations. Compared to known neutral boryl radicals, the isolated radical species showed larger spin density on the boron atom. Furthermore, the compound that was isolated is extraordinarily stable to high temperatures under inert conditions, both in solution and in the solid state. Electrochemical investigations of the radical suggest the possibility to generate a stable formal boryl anion species.     

FOURIER TRANSFORM INFRARED SPECTROELECTROCHEMISTRY OF THE BACTERIOCHLOROPHYLL a ANION RADICAL

Abstract— Fourier transform infrared (FTIR) difference spectroscopy of the electrochemically generated anion radical of bacteriochlorophyll a was used to follow the molecular changes upon one-electron reduction. An IR spectroelectrochemical cell was constructed, allowing in situ electrolysis in connection with spectroscopic investigations from 200 to 10 000 nm. FTIR difference spectra of the BChl a anion formation in THF d₈ at U =+0.9 V (as determined by ferrocene calibration) were obtained. After complete formation of the radical, the reverse process was followed. Comparison of visible and IR spectra of the reduction and re-oxidation processes indicates that more than 90% of the BChl a anion could be formed with 90% of it being reversible. The main IR absorbance changes are observed for the conjugated and even for the non-conjugated C=O groups of BChl a . These results demonstrate the use of the combination of FTIR spectroscopy and electrochemistry for the characterization of radicals of the isolated pigments and of their in vivo bonding to the protein environment.     

Charged states of α,ω-dicyano β,β'-dibutylquaterthiophene as studied by in situ ESR UV-vis NIR spectroelectrochemistry

The influence of the molecular structure on the stabilization of charged states was studied in detail by in situ ESR UV-vis NIR spectroelectrochemistry at a novel α,ω-dicyano substituted β,β'-dibutylquaterthiophene (DCNDBQT) and the electrochemically generated cation and anion radicals have been proved for the first time. The voltammetry of DCNDBQT results in two separate oxidation steps with the reversible first one. The experimental absorption maxima at 646 and 1052 nm together with the calculated ones (by DFT method) as well as an ESR signal at the first anodic step prove the presence of a radical cation. Three additional optical bands (554, 906, and 1294 nm for CT-transition) can be attributed to the formation of cation radical dimer. The dicationic structure formed in the second oxidation step is not stable. The stabilization proceeds via a dimer formation in two chemical follow-up reactions. The existence of the dimeric structures was proved by ex situ MALDI TOF mass spectrometry. As the substitution by cyano groups opens the route to cathodic reductions, DCNDBQT shows a single quasi-reversible reduction step. Here, the in situ ESR UV-vis NIR spectroelectrochemical measurements and theoretical calculations let us confirm the electrochemical generation of an anion radical. As we found a low number of anion radicals by quantitative ESR spectroelectrochemistry and an appearance of additional bands in the UV-vis NIR absorption spectra, the formation of dimeric structures must be considered and was corroborated by mass spectrometry. The role of dimerization in the reaction mechanism of the DCNDBQT oxidation and reduction are discussed in general. The experimental results were interpreted using the quantum chemical calculations based on density functional theory.     

Spectroelectrochemistry of a photochromic [2.2]paracyclophane-bridged imidazole dimer: clarification of the electrochemical behavior of HABI

The photochromic behavior of the imidazole dimers can be attributable to the photoinduced homolytic cleavage of the C-N bond between the two imidazole rings. On the other hand, although the simultaneous formation of the imidazolyl radical and imidazole anion by the one-electron reduction of an imidazole dimer was reported, no definitive evidence for this electrochemical reaction has been demonstrated. We report the first direct evidence for the electrochemical generation of the imidazolyl radical from the radical anion of the imidazole dimer by conducting the UV-vis-NIR spectroelectrochemical analysis of the [2.2]paracyclophane-bridged imidazole dimer.     

Electrochemical transformations of 4-(nitroaryl)-1,2- and -1,4-dihydropyridines in acetonitrile

The potentials of the electrolytic oxidation and reduction of 3,5-diethoxycarbonyl- 1,2- and -1,4-dihydropyridines with a nitroaryl substituent in the 4 position and their oxidized forms were determined by the method of a rotating disk electrode with a ring. The mechanism of the electrochemical oxidation of the starting dihydropyridines in acetonitrile on a graphite electrode was ascertained. The first step in the electrolytic reduction of the starting dihydropyridines is the addition of an electron to the nitro group to give anion radicals, the fine electronic structures and the hyperfine structure (hfs) constants of which were determined by EPR spectroscopy. The pyridinium fragment in the electrolytic oxidation products is reduced more readily than the nitro group in the aryl substituent.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1099–1105, August, 1985.     

Pyrazino-tetracyanonaphthoquinodimethanes: sterically deformed electron acceptors affording zwitterionic radicals

The X-ray analyses of the title electron acceptors (1) revealed their butterfly-shaped deformed geometry, which is not affected by the pyridyl group attached at 2-position of the pyrazino-TCNNQ skeleton. Small differences between the first and second reduction potentials (ca. 0.1 V) in pyrazino-TCNNQs show that their anion radicals (1⁻) are prone to disproportionate into the neutral (1) and dianionic (1²⁻) species. The thermodynamically unstable anion radical species based on the pyrazino-TCNNQ skeleton could be isolated as inner salts upon electrochemical reduction of the derivatives having an N-methylpyridinium moiety at 2-position (2⁺). The zwitterionic open-shell species (2) constitute a novel class of radicals that exhibit semiconducting behavior as a single component thanks to the high electrochemical amphotericity.     

Radical anions of nitrobenzothiazoles: EPR study of conjugative properties of benzothiazolyl systems

The electrochemical reduction in DMSO of the five isomers of nitrobenzothiazole (NBTZ) gave quite persistent radical anions that could be easily characterised by EPR spectroscopy. By contrast, the chemical reduction in alkaline solution, that is by t-BuOK in DMSO or by glucose and MeOK in MeOH, presented some problems with 6- and 4-NBTZ, and in the case of 2-NBTZ did not provide any detectable paramagnetic species. The internal consistency of coupling constants of the nitrobenzothiazole radical anions is in good agreement with the conjugative properties of the various benzothiazolyl systems and allows rectifying a recent EPR characterisation of 6-NBTZ radical anion.     

1. EPR Spectra of anion radicals formed in the electrochemical reduction of some quinoxaline 1,4-dioxides

The anion radicals formed in the electrochemical reduction of 2,3-dimethyl-quinoxoline and its N,N′-dioxide, the 6-methoxy and 6-chloro derivatives of the N,N′-dioxide, and the preparation quinoxidine in DMF were studied. Polarographic analysis showed that the first step in the reduction is a one-electron step and is reversible for all of the compounds except quinoxidine. However, the EPR spectra of the primary anion radicals were recorded only for 2,3-dimethylquinoxaline and its 1,4-dioxide. For the remaining compounds we were able to obtain anion radicals of only the reduction products, the structures of which in a number of cases were established from the EPR spectra. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No 3, pp. 376–380, March, 1980.     

Design,Synthesis and Electrochromic Properties of Anthraquinone Imide-based Donor-acceptor Type of Near Infrared Chromophores

A new series of near infrared(NIR)electrochromic anthraquinone imides containing a triphenylamine moiety as an electron donor was synthesized and their electrochemical and spectroelectrochemical behavi...     

Electroreduction of the ruthenium complex [(bpy)2Ru(tatpp)]Cl2 in water: insights on the mechanism of multielectron reduction and protonation of the Tatpp acceptor ligand as a function of pH

The mononuclear ruthenium complex [(bpy) 2Ru(tatpp)] (2+) ( 1 (2+); bpy is 2,2'-bipyridine and tatpp is 9,11,20,22-tetra-aza-tetrapyrido[3,2-a:2'3'-c:3',2'-l:2',3']-pentacene) undergoes up to four reversible tatpp ligand-based reductions as determined by electrochemistry in aqueous solution. Specific redox and protonation states of this complex were generated by stoichiometric chemical reduction with cobaltocene and protonation with trifluoroacetic acid in acetonitrile. These species exhibit unique UV-visible absorption spectra, which are used to determine the speciation in aqueous media as a function of the potential during the electrochemical reduction. A combination of cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemistry showed that the voltammetric reduction peaks are associated with two-electron/two-proton processes in which the details of stepwise electron transfer and protonation steps vary as a function of the pH. Spectroelectrochemistry performed during potential scans with and without a small superimposed sinusoidal potential waveform was used to examine the mechanistic details of this proton-coupled multielectron reduction process. Under basic conditions, the radical [(bpy) 2Ru(tatpp (*-))] (+)( 1 (*+)) is the first electrogenerated species that converts to doubly reduced, single-protonated [(bpy) 2Ru(Htatpp-)] (+) (H 1 (+)) and doubly protonated [(bpy) 2Ru(H 2tatpp)] (2+)(H 2 1 (2+)) by subsequent electron-transfer (ET) and proton-transfer (PT) reactions. Partial dimerization of radical 1 (*+) is also observed in basic media. Neutral or acidic conditions favor an initial ET-PT reaction leading to the protonated, radical species [(bpy) 2Ru(Htatpp (*))] (2+) (H 1 (*2+)), which rapidly disproportionates to give 1 (2+) and H 2 1 (2+). This intermediate, H 1 (*2+), is only observed when potential modulation is used in the spectroelectrochemical experiment. At more negative potentials, the doubly reduced complexes (e.g., H 2 1 (2+), H 1 (+)) undergo a two-electron/two-proton reductions to give the quadruply reduced and protonated species H 4 1 (2+) and/or H 3 1 (+) throughout the pH range investigated. These species are also only detectable when potential modulation is used in the spectroelectrochemical experiment, as they rapidly comproportionate with 1 (2+) in the bulk, leading to the regeneration of intermediate double-reduced species, H 2 1 (2+).     

Electrochemical behaviour of tris (2,2′-bipyridine) osmium(II) complex in dimethylformamide

The electrochemical behaviour of Os(bpy)₃²⁺ (bpy=2,2′-bipyridine) has been investigated in N,N-dimethylformamide by utilizing predominantly the techniques of polarography and cyclic voltammetry. The study has been carried out at different temperatures in the range ?20 to +30° C. The number of reduction waves observed depends markedly on temperature. For intermediate temperatures, the complex exhibits six reduction waves, the maximum number of waves observed as a function of temperature.The first three reduction processes, corresponding to the first three reduction waves, are one-electron, diffusion-controlled reversible processes in all conditions. Conversely, process four is consistent with one-electron reversible transfer only at the lowest temperature. In fact, for higher temperatures the liberation of bpy, preferentially as a radical anion rather than a neutral molecule, occurs. In the latter case, the liberated neutral bpy molecule can be reduced by one-electron transfer. Process five is due to the reduction of species formed by chemical reaction in the preceding electrode process, i.e. the bpy radical anion and Os(bpy)₂^1?. Process six is consistent with the addition of five electrons to the starting complex, followed by the liberation and successive reduction of the bpy radical anion.     

Chromium tricarbonyl complex of phosphaalkene: Crystal structure and electrochemistry of the Cr(CO)3 complex of PhC(H)PMes*, EPR and DFT studies of its radical anion

We report the synthesis, crystal structure and electrochemical behaviour of a complex in which the Ph group of the phosphaalkene PhC(H)PMes* (Mes*: 2,4,6-tri-tert-butylphenyl) is coordinated to a chromium tricarbonyl group. The EPR spectra resulting from electrochemical and chemical reductions are described and the experimental g and hyperfine tensors (³¹P)T, as determined from the EPR data, are compared with those predicted by DFT calculations for the radical anion (Cr(CO)₃, PhC(H)PMes)⁻. The structural changes caused by the addition of an electron to the neutral complex are described, together with an estimation of the contribution of Cr(CO)₃ to the stabilization of the radical anion.     

N,N'-dimethyl-N,N'-diarylurea anion radicals: an intramolecular reductive elimination

The one-electron reduction of tertiary N,N'-dimethyl-N,N'-diarylureas (aryl = phenyl, beta-naphthyl, alpha-naphthyl), in HMPA, results in anion radicals that undergo novel intramolecular reductive elimination reactions leading to the formation of the anion radicals of the corresponding biaryls. These results are due to face to face pi-pi stacking interactions involving the two aromatic rings in the urea systems. The overlapping p(pi)() orbitals on the ipso carbons of opposing aryl groups evolve into a sigma bond leading to the formation of the biaryl anion radical. In the case of the N,N'-dimethyl-N,N'-di-2-pyrenylurea system, there is a node in the LUMO of the number 2 carbon, and the parent anion radical remains intact.     

Radical anions from some photochromic nitrocompounds. An electron paramagnetic resonance and electrochemical study

Five 6′-nitrospiro[indole-benzopyrans] have been reduced electrochemically in ACN to the corresponding radical anions. Their reduction potentials were in the range -1.32 to -1.40 V versus SCE, that is in the range expected for substituted nitrobenzenes. The EPR spectra of the species resulting from chemical reduction of the same compounds were also recorded, and suggested that the structure of the observed radicals varies with the reduction media: thus, while the spectra in DMSO could be attributed to the radical anions structurally similar to the starting compounds, those observed in THF under UV irradiation were assigned to the anions of the open merocyanines. It appears that also the species observed by reduction in DMF ought to be assigned the open structure.     

苝红化合物氧化还原反应的研究

苝红化合物作为一种新型功能性染料,由于吸收光能在可见范围,且性能稳定,在光导液晶及太阳能电池等方面已得到了应用^[1].     

Detection of nitrosofuran anion radicals in the electrochemical reduction of 5-nitrofuran. EPR investigations and quantum-chemical calculations

Previously unknown radicals that are associated with the 5-N-furylhydroxylamine-5-nitrosofuran redox system were detected by EPR spectroscopy in the electrochemical reduction of 5-nitrofuran. The electron structures of the free-radical products of the reduction of 5-nitrofuran were calculated by the INDO method. On the basis of the hyperfine structures (hfs) of the EPR spectra and the results of quantum-chemical calculations it was concluded that 5-nitrosofuran anion radicals and the nitrosofuran dimer were recorded.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1178–1183, September, 1989.     

Analysis of the substituent effect on the reactivity modulation during self-protonation processes in 2-nitrophenols

A voltammetric and spectroelectrochemical ESR study of the reduction processes of five substituted 4-R-2-nitrophenols (R = -H, -OCH(3), -CH(3), -CN, -CF(3)) in acetonitrile was performed. In the potential range considered here (-0.2 to -2.5 V vs Fc+/Fc), two reduction signals (Ic and IIc) were detected; the first one was associated with the formation of the corresponding hydroxylamine via a self-protonation pathway. The voltammetric analysis at the first reduction signal showed that there are differences in the reduction pathway for each substituted 4-R-2-nitrophenol, being the E1/2 values determined by the inductive effect of the substituent in the meta position with respect to the nitro group, while the electron-transfer kinetics was determined by the protonation rate (k(1)+ ) of the anion radical electrogenerated. However, at potential values near the first reduction peak, no ESR signal was recorded from stable radical species, indicating the instability of the radical species in solution. Nevertheless, an intense ESR spectrum generated at the second reduction peak was detected for all compounds, indicating the monoelectronic reduction of the corresponding deprotonated 4-R-2-nitrophenols. The spin-coupling hyperfine structures revealed differences in the chemical nature of the electrogenerated radical; meanwhile, the -CF(3) and -CN substituents induced the formation of a dianion radical structure, and the -H, -CH(3), and -OCH(3) substituents provoked the formation of an anion radical structure due to protonation by acetonitrile molecules of the initially electrogenerated dianion radical. This behavior was confirmed by analyzing the ESR spectra in deuterated acetonitrile and by performing quantum chemical calculations of the spin densities at each site of the electrogenerated anionic radicals.