Dimer Radical Research Articles

A High-Valent Iron–Oxo Corrolazine Activates C–H Bonds via Hydrogen-Atom Transfer

Oxidation of the Fe(III) complex (TBP(8)Cz)Fe(III) [TBP(8)Cz = octakis(4-tert-butylphenyl)corrolazinate] with O-atom transfer oxidants under a variety of conditions gives the reactive high-valent Fe(O) complex (TBP(8)Cz(+•))Fe(IV)(O) (2). The solution state structure of 2 was characterized by XAS [d(Fe-O) = 1.64 Å]. This complex is competent to oxidize a range of C-H substrates. Product analyses and kinetic data show that these reactions occur via rate-determining hydrogen-atom transfer (HAT), with a linear correlation for log k versus BDE(C-H), and the following activation parameters for xanthene (Xn) substrate: ΔH(++) = 12.7 ± 0.8 kcal mol(-1), ΔS(++) = -9 ± 3 cal K(-1) mol(-1), and KIE = 5.7. Rebound hydroxylation versus radical dimerization for Xn is favored by lowering the reaction temperature. These findings provide insights into the factors that control the intrinsic reactivity of Compound I heme analogues.

Electrooptical and Nonlinear Optical Properties of Lyotropic Liquid Crystals Doped with Electrochromic Viologen

This work presents the results of experimental studies of the electrooptical and nonlinear optical properties of lyotropic ionic liquid crystals (LILC) with soluted electrochromic admixtures of viologens. It is established that the Smectic A ordering of the LILC of potassium caprylate is not disrupted by the introduction of viologens. Moreover, LILC-viologen composites obtain electrochromic properties inherent to viologens, which produce colored radical cations and dimers, while reducing. The resence of radical cations and dimers is fixed by the optical absorption spectra. It is found that, under the action of an electric field, the LILC-viologen samples form a bilayer structure consisting of a liquid crystal layer and an absorptive layer of viologen redox products (radical cations and dimers). A dynamic grating recording is realized and studied in bilayer LILC-viologen cells. It is determined that the recording takes place in the colored layer of viologen redox products. A possible mechanism of grating recording in LILC-viologen cells is proposed.

The oxidative degradation of dibenzoazepine derivatives by cerium(iv) complexes in acidic sulfate media

The kinetics of the oxidation of imipramine and desipramine using cerium(IV) complexes were studied in the presence of a large excess of azepine derivative (TCA) in acidic sulfate media using UV-Vis spectroscopy. The reaction proceeds via dibenzoazepine radical formation, identified by EPR measurements. The kinetics of the first degradation step were studied independently of the further slower degradation reactions. Linear dependences, with zero intercept, of the pseudo-first-order rate constants (k(obs)) on [TCA] were established for both dibenzoazepine radical formation processes. Rates of reactions decreased with increasing concentration of the H(+) ion indicating that cerium(IV) as well as both reductants exist in an equilibrium with their protolytic forms. The activation parameters for the degradation of dibenzoazepine derivatives in the first oxidation stage were as follows: ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 8 J K(-1) mol(-1) for imipramine and ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 6 J K(-1) mol(-1) for desipramine, respectively. Imipramine and desipramine radicals dimerized leading to an intermediate radical dimer, which decayed in a first-order consecutive decay process. These two further reactions proceed with rates which are characterized by non-linear dependences of the pseudo-first-order rate constants (k(obs)) on [TCA]. The degradation reaction of the intermediate radical dimer leads to an uncharged dimer as a final product. Mechanistic consequences of all the results are discussed.

Bonds or not bonds? Pancake bonding in 1,2,3,5-dithiadiazolyl and 1,2,3,5-diselenadiazolyl radical dimers and their derivatives

Unusually long bonds or short intermolecular contacts occur in the title compounds reminiscent of pancake bonding. Pancake bonding interactions seem analogous to π-stacking interactions, but they display much shorter contact distances than normally seen in van der Waals (vdW) dimers. The interpretation of these SN and SeN containing structures has been an outstanding challenge for some time. The antibonding (π*) singly occupied molecular orbital (SOMO) of the radical is the source of two-electron multicenter bonding (2e/mc). Preferred conformations thus can be traced back to SOMO-SOMO overlap. We used several computational methods to understand the nature of pancake bonding in the title compounds including four wave function methods (WFT) and a dozen density functional theories (DFT) including empirical dispersion corrections. We used experimental data and high level CCSD(T)/6-311++G(d,p) and MRPT2/6-311++G(d,p) calculations for comparison. The analysis provided the interpretation a wealth of experimental data including conformational preferences of these SN and SeN containing radical dimers leading to a better overall understanding of pancake bonding. Analysis of the various components of the inter-radical interactions showed that SOMO-SOMO bonding interaction and dispersion interaction contribute to the binding energy and neither of these interactions alone is sufficient to bind the dimer. The dimer is predicted to show weak diradical character.

Ab initio study of the mechanisms of reactions of NF3 with aliphatic and benzylic substrates

At temperatures around 400°C, nitrogen trifluoride (NF3) readily reacts with alkanes and benzene as well as ethers to produce stable N,N-difluoroamines. Difluoroamination of benzylic substrates results in initial N,N-difluoroamines that undergo eliminations or rearrangements. Toluene and ethylbenzene produce benzonitrile. Cumene produces α-methylstyrene. Diphenylmethane produces benzanilide. Little or no direct fluorination or radical dimerization is observed. This study uses ab initio calculations to help understand the reasons for such diverse reaction pathways.

Photocrosslinking between Peptide–Peptide or Peptide–Oligonucleotide by RuII–TAP Complexes

Ru(II)-TAP complexes have been shown to be very attractive compounds in the frame of developments of new anticancer drugs targeting the genetic material. This increasing interest originates from observations of covalent bond formations, triggered by photo-induced electron transfer (PET) between Ru(II)-TAP complexes and guanine bases of DNA. This photoreaction has recently been extended to the tryptophan (Trp) amino acid for future applications involving peptides. Thus, a double photo-addition of Trp residues of peptides on Ru(II) complexes is demonstrated by mass spectrometry with some structural issues. Such bi-adduct formations offer the possibility of photocrosslinking two Trp-containing biomolecules, which is investigated in this study. Thus, photocrosslinking between two complementary oligonucleotides (ODNs) derivatized by Trp-containing tripeptides is demonstrated by polyacrylamide gel electrophoresis (PAGE) in the presence of Ru(II)-TAP complexes. Both PAGE and MS indicate that such photocrosslinkings arise from two reaction pathways: either via the double addition of Trp residues on the Ru complex or from dimerization of Trp radicals. The competition between these two pathways depends on the experimental conditions. Heterobridgings between guanine bases and tryptophan residues mediated by Ru(II)-TAP complexes is also examined, opening the way to ODN-peptide photocrosslinkings.

Charge Shift Bonding Concept in Radical π-Dimers

We show that pancake bonding in radical π-dimers display features of charge shift (CS) bonding. While the CS bonding concept has been developed to interpret the unusual aspects of σ-bonds around centers with a large number of lone pairs, such as F(2) and HOOH, we find a similar role played by the nonbonding or slightly bonding π-electron pairs in π-stacking radical dimers. Arguments and computational evidence indicate that the CS bonding concept developed by Shaik and Hiberty et al. captures essential features of the intermolecular bonding in radical π-dimers in which the overlap of the two radical centered singly occupied molecular orbitals (SOMOs) play a crucial role. By using the tetracyanoethylene anion dimer, [TCNE](2)(2-), as a model, we show that compared to CAS(2,2) calculations, significant binding contributions are recovered in the calculations simply by including selected intrapair excitations of the SOMO-SOMO bonding orbitals and the nonbonding π-orbitals. This observation is the basis for the analogy of chemical bonding between pancake bonded radical π-dimers and other charge shift bonded molecules, such as F(2). By extending the CS bonding concept to a new class of molecules, we find a novel application of the lone pair bond weakening effect (LPBWE) in which the doubly occupied π-orbitals play the role of lone pairs.

π-dimerization of pleiadiene radical cations at low temperatures revealed by UV–vis spectroelectrochemistry and quantum theory

One-electron oxidation of the non-alternant polycyclic aromatic hydrocarbon pleiadiene and related cyclohepta[c,d]pyrene and cyclohepta[c,d]fluoranthene in THF produces corresponding radical cations detectable in the temperature range of 293–263 K only on the subsecond time scale of cyclic voltammetry. Although the EPR-active red-coloured pleiadiene radical cation is stable according to the literature in concentrated sulfuric acid, spectroelectrochemical measurements reported in this study provide convincing evidence for its facile conversion into the green-coloured, formally closed shell and, hence, EPR-silent π-bound dimer dication stable in THF at 253 K. The unexpected formation of the thermally unstable dimeric product featuring a characteristic intense low-energy absorption band at 673 nm (1.84 eV; logemax = 4.0) is substantiated by ab initio calculations on the parent pleiadiene molecule and the PF6− salts of the corresponding radical cation and dimer dication. The latter is stabilized with respect to the radical cation by 14.40 kcal mol−1 (DFT B3LYP) [37.64 kcal mol−1 (CASPT2/DFT B3LYP)]. An excellent match has been obtained between the experimental and TD-DFT-calculated UV–vis spectra of the PF6− salt of the pleiadiene dimer dication, considering solvent (THF) effects.

ChemInform Abstract: Molecular Recognition and Switching via Radical Dimerization

AbstractReview: 67 refs.

Photochemical Two-Electron Reduction of a Dinuclear Ruthenium Complex Containing a Bent Tetraazatetrapyridopentacene Bridging Ligand: Pushing Up the LUMO for Storing More Energy

The synthesis and characterization of a ditopic bridging ligand, 9,12,21,22-tetraazatetrapyrido[3,2-a:2',3'-c:3″2″-m:2''',3'''-o]pentaphene (tatppα) and its dinuclear ruthenium complex, [(phen)(2)Ru(tatppα)Ru(phen)(2)][PF(6)](4) (1(4+)), are described. The tatppα ligand is structurally very similar to 9,10,20,33-tetraazatetrapyrido[3,2-a:2',3'-c:3″,2″-l:2''',3'''-n]pentacene (tatppβ), except that, instead of a linear tetraazapentacene backbone, tatppα has an ortho (or α) substitution pattern about the central benzene ring, leading to a 120° bend. Complex 1(4+) shows tatppα-based reductions at -0.73 and -1.14 V vs Ag/AgCl/saturated KCl and has an absorption spectrum showing the typical Ru(II) dπ → phen-like π* metal-to-ligand charge-transfer transition centered at ∼450 nm. In acetonitrile, visible-light irradiation of 1(4+) in the presence of triethylamine leads to two sequential changes in the absorption spectra, which are assigned to the formation of the one- and two-electron-reduced species, with the electrons stored on the tatppα ligand. These assignments were made by comparison of the spectral changes observed in 1(4+) upon stoichiometric chemical reduction with cobaltocene and by spectroelectrochemical analysis. Significantly, DFT calculations are very predictive of the optical and reductive behavior of the tatppα complex relative to the tatppβ complexes and show that modeling is a useful tool for ligand design. The chemical reactivity and differential reflectance spectroelectrochemical data reveal that the reductions are accompanied by radical dimerization of the tatppα ligand to species such as σ-{1}(2)(6+), which is only slowly reversible upon exposure to air and may limit the complexe's 1(4+) utility for driving photochemical H(2) production.

Palladium Induced Macrocyclic Preorganization for Stabilization of a Tetrathiafulvalene Mixed-Valence Dimer

Ditopic metalation of a flexible "Pacman"-like tetrathiafulvalene (TTF) modified Schiff-base-calixpyrrole results in the stabilization upon oxidation of an otherwise difficult-to-access mixed-valence TTF radical dimer. EPR and optical spectroscopies were used to characterize the mixed-valence species.

Isolated Supramolecular [Ru(bpy)3]–Viologen–[Ru(bpy)3] Complexes with Trapped CB[7,8] and Photoinduced Electron‐Transfer Study in Nonaqueous Solution

The synthesis of two supramolecular diruthenium complexes, 1⊂CB[7] and 1⊂CB[8] (CB[n]=cucurbit[n]uril), which contain the respective host CB[7] and CB[8], were synthesized and isolated. In the case of host CB[8], the desired supramolecular complex was obtained by utilizing dihydroxynapthalene as a template during the synthesis. The (1)H NMR spectra, electrochemistry, and photochemistry of these supramolecular complexes were performed in nonaqueous solution. The results show that both CB[7,8] hosts mainly bind to the linker part in solution in acetonitrile. This binding also lowers the oxidation potential of the ruthenium metal center and hinders the quenching effect by the viologen moiety. It has also been shown that external methylviologen can be included into 1⊂CB[8]. Analysis with NMR spectroscopy, electrochemistry, and photochemistry clearly shows a viologen radical dimer formation between the bound viologen and free methylviologen, thereby showing that the unique abilities of the CB[8] host can be utilized even in nonaqueous solution.

Mechanistic and Adsorption Studies of Relevance to Photocatalysts on Titanium Grafted Mesoporous Silicalites

Ti-SBA-15 and Ti-MCM-41 were synthesized and evaluated as possible photocatalysts for the reduction of CO2, and for the photo-Kolbe decomposition of acetic acid. UV-Raman was used to study the adsorption of car- bon dioxide, water, formic acid, and acetic acid over Ti-MCM-41 by monitoring the UV enhanced resonance peak of the totally symmetric stretching band of the grafted Ti species at 1,085 cm -1 . Acetic and formic acid dissociate on Ti-SBA-15 and Ti-MCM-41 to form acetate and for- mate, respectively. The conjugate bases subsequently interact strongly with Ti sites. Water interacts with the Ti sites, while no change in the amplitude of the 1,085 cm -1 band is observed in the presence of CO2. Photocatalysis experiments indicate that these mesoporous silicalites are active in the photo-Kolbe decomposition of acetic acid. CO2 is formed by reaction of a hole with the acetate car- boxylate groups. The methyl radical co-products react with a surface proton and an electron to form methane. No products resulting from the dimerization of methyl radicals are observed, presumably because of the highly dispersed active sites.

Reversible dimerization of the anion radicals of some dicyanonaphthalenes

The electrochemical reduction of 7 of the 10 isomeric dicyanonaphthalenes has been studied in N,N-dimethylformamide. The studied compounds were 1,2-, 1,3-, 1,4-, 2,3-, 1,8-, 2,6- and 2,7-dicyanonaphthalene. For most of the isomers, the one-electron reduction to the anion radical occurred as a simple, reversible reduction without complications from coupled chemical reactions. However, for 1,3-, 2,3- and 2,7-dicyanonaphthalene, the reduction was affected by a reversible dimerization of the anion radicals. Dimerization equilibrium constants were determined at various temperatures. DFT calculations of the anion radicals of all 10 isomers provided spin densities at ring carbons 1–8. The highest spin density for carbons 1–8 not bearing a cyano group was largest for the three isomers with detectable dimerization. Not only do these calculations suggest the site of dimerization, it was found that the free energy change for dimerization varies in an approximately linear fashion with spin density.

Nanometer‐Sized Reactor—A Porphyrin‐Based Model System for Anion Species

Although many cagelike molecules can create a catalytic environment for promoting chemical reactions, the construction of receptors that can host anionic species and sensitize their reaction is novel. Here we report the photochemically induced dimerization of the anion radicals of TCNQ (7,7,8,8-tetracyano-para-quinodimethane) in organic solvent under aerobic conditions when they are encapsulated inside a cationic photoactive receptor. There is clear evidence for a rate increase of over two orders of magnitude, photosensitization of the host, and demonstrated turnover of the catalyst.

High temperature vapor phase reactions of nitrogen trifluoride with benzylic substrates

At temperatures around 400 °C, nitrogen trifluoride (NF 3) readily reacts with benzylic substrates. Products vary with the substrate, but are all the result of difluoroamination at the benzylic position. Toluene and ethylbenzene produce benzonitrile. Cumene produces α-methylstyrene. Diphenylmethane produces benzanilide. Little or no direct fluorination or radical dimerization is observed.

Effect of H2S in methane/air flames on sulfur chemistry and products speciation

Reaction behavior of H2S/O2 under different equivalence ratios in methane/air flames is examined. Three equivalence ratios extending from fuel-lean (Φ=0.5), stoichiometric (Φ=1.0), to fuel-rich (Claus condition, Φ=3.0) are examined. The results revealed that the presence of H2S prevents hydrogen oxidation in the primary reaction zone, while in the secondary reaction zone oxidation competition occurs between H2 and H2S. In presence of oxygen, oxidation of hydrogen sulfide forms sulfur dioxide. However, under Claus conditions, the depletion of oxidant causes the direction of hydrogen sulfide reaction to shifts towards the formation of elemental sulfur. Higher hydrocarbons are formed in trace amounts under Claus conditions wherein sulfur dioxide acts as a coupling catalyst which enhances the dimerization of CH3 radical to form higher series of hydrocarbons. Under Claus conditions, sulfur deposits are formed in low temperature regions of the reactor including the sampling line. The deposits are analyzed using X-ray powder diffractometer and were found to be cyclo-S8 (α-sulfur) with orthorhombic crystal structure. The formation of α-sulfur is mainly due to the agglomeration of elemental sulfur (S2) during its condensation at low temperatures.

Dehydrotropylium-Co2(CO)6 Ion: Generation, Reactivity and Evaluation of Cation Stability

The dehydrotropylium-Co(2)(CO)(6) ion was generated by the action of HBF(4) or BF(3)⋅OEt(2) on the corresponding cycloheptadienynol complex, which in turn has been prepared in four steps from a known diacetoxycycloheptenyne complex. The reaction of the cycloheptadienynol complex via the dehydrotropylium-Co(2)(CO)(6) ion with several nucleophiles results in substitution reactions with reactive nucleophiles (N>1) under normal conditions, and a radical dimerisation reaction in the presence of less reactive nucleophiles. Competitive reactions of the cycloheptadienynol complex with an acyclic trienynol complex show no preference for generation of the dehydrotropylium-Co(2)(CO)(6) ion over an acyclic cation. DFT studies on the dehydrotropylium-Co(2)(CO)(6) ion, specifically evaluation of its harmonic oscillator model of aromaticity (HOMA) value (+0.95), its homodesmotic-reaction-based stabilisation energy (≈2.8 kcal mol(-1)) and its NICS(1) value (-2.9), taken together with the experimental studies suggest that the dehydrotropylium-Co(2)(CO)(6) ion is weakly aromatic.

Mechanically Stabilized Tetrathiafulvalene Radical Dimers

Two donor-acceptor [3]catenanes-composed of a tetracationic molecular square, cyclobis(paraquat-4,4'-biphenylene), as the π-electron deficient ring and either two tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) containing macrocycles or two TTF-butadiyne-containing macrocycles as the π-electron rich components-have been investigated in order to study their ability to form TTF radical dimers. It has been proven that the mechanically interlocked nature of the [3]catenanes facilitates the formation of the TTF radical dimers under redox control, allowing an investigation to be performed on these intermolecular interactions in a so-called "molecular flask" under ambient conditions in considerable detail. In addition, it has also been shown that the stability of the TTF radical-cation dimers can be tuned by varying the secondary binding motifs in the [3]catenanes. By replacing the DNP station with a butadiyne group, the distribution of the TTF radical-cation dimer can be changed from 60% to 100%. These findings have been established by several techniques including cyclic voltammetry, spectroelectrochemistry and UV-vis-NIR and EPR spectroscopies, as well as with X-ray diffraction analysis which has provided a range of solid-state crystal structures. The experimental data are also supported by high-level DFT calculations. The results contribute significantly to our fundamental understanding of the interactions within the TTF radical dimers.

Simulations of photoemission and equilibrium redox processes of ionic liquids: the role of ion pairing and long-range polarization

We have studied oxidation and reduction of ionic liquids using methods of theoretical chemistry. In particular, we have modeled photoelectron spectra of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf(2)N]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf(2)N]) ionic liquids and their components. We have considered ion pairs in the gas phase and solvated in the water represented by a dielectric continuum. We have also characterized the isolated cation and anion of the ionic liquids. The calculated quantities have been compared with available experiments. The photoelectron spectra were modeled within linearized reflection approximation, using a composite ab initio approach based on combination of the MP2 method for the ground state, the PMP2 method for the first ionized electronic state and the TDDFT method to estimate the higher ionization energies. We have also briefly explored energetics of processes following the vertical electron attachment/detachment. Our calculations were able to reproduce the measured photoelectron spectra. We have shown that the valence photoelectron spectrum results from simultaneous ionization from many molecular orbitals. In the threshold region, the electron is ejected from the anion in the gas phase ion pair while the cationic moiety is ionized when the ion pair is solvated. We have investigated the effect of both short-range and long-range interactions on the ionization characteristics, showing that the long-range polarization of the solvent effectively screens the specific short-range interactions. The large difference between the HOMO-LUMO gap and the electrochemical window has been rationalized in terms of relaxation following the redox processes, i.e. solute and solvent geometric relaxation and reactions, e.g. dimerization of the imidazolium radical formed during the reduction.