The First Heterotriquinone and Dicyanoheterotriquinone Methide That Undergo a Five-Stage Amphoteric Redox Reaction

The dicyanomethylene group and not the quinone oxygen atoms is the site of the first one-electron reduction for the dicyanohetereotriquinone methide 1 , although the dicyanomethylene group is substituted at a cyclopentadienyl-like five-membered ring! Compound 1 is amphoteric and undergoes a five-stage sequence of one-electron redox reactions.     

The First Structural Characterization of an Azoaromatic Radical Anion Stabilized by Dicopper(I) Coordination

Double chelate coordination of [Cu(Ph₃P)₂]⁺ stabilizes the radical anion of 2,2′-azobis(5-chloropyrimidine), which exhibits a N–N bond length of 1.345(7) Å in the complex (see picture). This is consistent with a one-electron reduced azo functionality.     

The Radical Anion of Cyclopentasilane‐Fused Hexasilabenzvalene

The radical anion of cyclopentasilane‐fused hexasilabenzvalene was synthesized by the reduction of the corresponding neutral compound. X‐ray crystallographic analysis showed a more trans‐bent structure of the disilene moiety than the neutral compound. Theoretical calculations showed that the highly trans‐bent structure is attributed to the hexasilabenzvalene structure. The EPR spectrum showed that an unpaired electron exists mainly at the disilene moiety. In the UV/Vis spectrum, a large bathochromic shift was observed compared with the neutral compound.     

The Features of Interaction of Bis(4, 6‐di‐tert‐butyl‐N‐(2, 6‐diisopropylphenyl)‐o‐amidophenolato)tin(IV) with Bromine and Iodine

The oxidation of tin(IV) bis‐amidophenolate (APiPr)₂Sn · THF ( I ) by bromine and iodine leads to the formation of monoradical mixed‐ligand complexes (APiPr)(ISQiPr)SnBr · THF ( II ) and (APiPr)(ISQiPr)SnI · THF ( III ) or diradical complexes (ISQiPr)₂SnBr₂ ( IV ) and (ISQiPr)₂SnI₂ ( V ), respectively [APiPr = dianion 4, 6‐di‐tert‐butyl‐N‐(2, 6‐diisopropylphenyl)‐o‐amidophenolate; ISQiPr = radical‐anion 4, 6‐di‐tert‐butyl‐N‐(2, 6‐diisopropylphenyl)‐o‐iminobenzosemiquinone], depending on the molar ratio of reagents (2:1 or 1:1). According to EPR data for compounds II and III , the unpaired electron is delocalized between both organic ligands. The EPR spectrum of IV in toluene matrix at 130 K is typical for diradical species with S = 1 with parameters D = 530 G, E = 105 G. The mixed‐ligand complexes II and III are unstable and undergo to symmetrization leading to formation of IV or V . The molecular structures of IV and V are determined by X‐ray analysis.     

Structure and Decay of Gaseous Organic Radical Cations Studied by Their Radiative Decay,Exemplified by the 1,3-Pentadiyne Cation

The radiative decay of over a hundred open-shell organic radical cations has now been established. As a result, the spectral structure of such cations in their ground and excited electronic states can be probed with resolutions of the order of ? 1 cm^?1. This is achieved by means of emission and laser-induced fluorescence techniques. The analysis of the emission and excitation spectra provides the vibrational frequencies of many of the totally symmetric fundamentals of the cations in the two electronic states. In order to study the relaxation behavior of these cations under “isolated conditions”, the lifetimes and fluorescence quantum fields can be obtained by means of photoelectron-photon coincidence measurements. These data yield the radiative and non-radiative rate constants as a function of the internal energy of the cations. The structural and decay information obtained from each of these techniques is illustrated using the 1,3-pentadiyne radical cation as example.     

The Radical Anion and Dianion of Tetraazapentacene

The mono‐ and bis‐reduction of 6,13‐bis((triisopropylsilyl)ethynyl)quinoxalino[2,3‐b]phenazine ( 1 ) with potassium anthracenide in THF is reported. Both the radical anion 1 ^.? and the dianion 1 ^2? were isolated and characterized by optical and structural (single‐crystal X‐ray diffraction) methods. Solutions of the radical anion 1 ^{. ?} were stable in air for several hours and characterized by EPR spectroscopy. Dianion 1 ^2? is highly fluorescent and photostable.     

Direct Synthesis of N-Arylquinone Imine Acetals and Quinol Imines from Acetals

In the absence of any added acidic catalyst quinone acetals as well as simple ketone and aldehyde acetals react with a variety of substituted anilines to form imines [Eq. (a)]. Contrary to what is expected from arguments based on their nucleophilicity, the most reactive anilines are those bearing electron-withdrawing groups.     

Stable Diporphyrinylaminyl Radical and Nitrenium Ion

Nitrenium ions, isoelectronic nitrogen counterparts of carbenes, are important intermediates in various biological and chemical processes. Herein we describe the first synthesis and characterization of a stable nitrenium ion without resonance stabilization by adjoining amino groups. Namely, a stable salt of a diporphyrinylnitrenium ion was synthesized by stepwise oxidation of the corresponding diporphyrinylamine through a stable aminyl radical. The nitrenium ion exhibits characteristic features such as a singlet ground state, enhanced double‐bond character of the central C?N bonds, no reactivity toward water and methanol, and negative solvatochromic behavior.     

A Stable and Crystalline Triarylgermyl Radical: Structure and EPR Spectra

A radical thing : After being obtained unexpectedly in low yields, the synthesis of the first stable triarylgermyl radical ^.Ge[3,5‐tBu₂‐2,6‐(EtO)₂C₆H]₃ ( 1 ; C gray, O blue) was considerably optimized, and the product was investigated by X‐ray analysis and EPR spectroscopy. The results were compared with DFT‐MO studies for the model compound ^.Ge[2,6‐(MeO)₂C₆H₃].

     

Synthesis of Zwitterionic Cobaltocenium Borate and Borata‐alkene Derivatives from a Borole‐Radical Anion

Chemical single‐electron reduction of 1‐mesityl‐2,3,4,5‐tetraphenylborole ( 3 ) gave a stable radical anion [CoCp*₂][ 3 ] as shown in earlier investigations. Herein, we present the reaction of [CoCp*₂][ 3 ] with the 2,2,6,6‐tetramethylpiperidine‐N‐oxyl radical (TEMPO), a common radical trap. Instead of radical recombination, the reaction proceeds through a redox pathway involving oxidation of the borole radical anion combined with reduction of TEMPO. This electron‐transfer process is accompanied by a deprotonation reaction of the cobaltocenium counterion by the base TEMPO^? to give TEMPO‐H and a neutral cobalt(I) fulvene complex ( 7 ). The latter was not observed directly during the reaction, because it instantaneously reacts as a nucleophile attacking at the boron center of the in situ generated borole 3 to give the borate 6 . However, 7 was synthesized independently by deprotonation of [CoCp*₂][PF₆]. In addition, the obtained zwitterionic cobaltocenium borate 6 undergoes a photolytic rearrangement to form the borata‐alkene derivative 9 that thermally transforms to the chiral cobaltocenium borate 12 . Our investigations are based on spectroscopic evidence, X‐ray crystallography, elemental analysis, as well as DFT calculations.     

Molecular Structure and Magnetic and Optical Properties of Endometallonitridofullerene Sc3N@Ih-C80 in Neutral,Radical Anion,and Dimeric Anionic Forms

A series of compounds with Sc₃N@I_h-C₈₀ in the neutral, monomeric, and dimeric anion states have been prepared in the crystalline form and their molecular structures and optical and magnetic properties have been studied. The neutral Sc₃N@I_h-C₈₀ ⋅ 3 C₆H₄Cl₂ ( 1 ) and (Sc₃N@I_h-C₈₀)₃(TPC)₂ ⋅ 5 C₆H₄Cl₂ ( 2 , TPC=triptycene) compounds both crystallized in a high-symmetry trigonal structure. The reduction of Sc₃N@I_h-C₈₀ to the radical anion resulted in dimerization to form diamagnetic singly bonded (Sc₃N@I_h-C₈₀⁻)₂ dimers. In contrast to {[2.2.2]cryptand(Na⁺)}₂(Sc₃N@I_h-C₈₀⁻)₂ ⋅ 2.5 C₆H₄Cl₂ ( 3 ) with strongly disordered components, we synthesized new dimeric phases {[2.2.2]cryptand- (K⁺)}₂(Sc₃N@I_h-C₈₀⁻)₂ ⋅ 2 C₆H₄Cl₂ ( 4 ) and {[2.2.2]cryptand- (Cs⁺)}₂(Sc₃N@I_h-C₈₀⁻)₂ ⋅ 2 C₆H₄Cl₂ ( 5 ) in which only one major dimer orientation was found. The thermal stability of the (Sc₃N@I_h-C₈₀⁻)₂ dimers was studied by EPR analysis of 3 to show their dissociation in the 400–460 K range producing monomeric Sc₃N@I_h-C₈₀^.− radical anions. This species shows an EPR signal with a hyperfine splitting of 5.8 mT. The energy of the intercage C−C bond was estimated to be 234±7 kJ mol⁻¹, the highest value among negatively charged fullerene dimers. The EPR spectra of crystalline (Bu₃MeP⁺)₃(Sc₃N@I_h-C₈₀^.−)₃ ⋅ C₆H₄Cl₂ ( 6 ) are presented for the first time. The salt shows an asymmetric EPR signal, which could be fitted by three lines. Two lines were attributed to Sc₃N@I_h-C₈₀^.−. Hyperfine splitting is manifested above 180 K due to the hyperfine interaction of the electron spin with the three scandium atoms (a total of 22 lines with an average splitting of 5.32 mT are observed at 220 K). Furthermore, each of the 22 lines is additionally split into six lines with an average separation of 0.82 mT. The large splitting indicates intrinsic charge and spin density transfer from the fullerene cage to the Sc₃N cluster. Both the monomeric and dimeric Sc₃N@I_h-C₈₀⁻ anions show an intrinsic shift of the IR bands attributed to the Sc₃N cluster and new bands corresponding to these species appear in the NIR range of their UV/Vis/NIR spectra, which allows these anions to be distinguished from neutral species.     

α-Silyl Ethers as Hydroxymethyl Anion Equivalents in Photoinduced Radical Electron Transfer Additions

Nucleophilic α -hydroxymethyl radicals can be generated from α-silyl ethers by irradiation in the presence of 9,10-anthracenedicarbonitrile (ADC) and biphenyl (BP). Under these conditions the hydroxymethyl radicals are not oxidized further but add directly to electron-poor alkenes [Eq. (a); EWG=electron-withdrawing group]. The radical adduct undergoes back electron transfer to form the carbanion, which is protonated. R=PhCH₂, tBuMe₂Si, Me; TMS=Me₃Si.     

Generation and Characterization of the First Persistent Platinum(I)‐Centered Radical

The first persistent platinum(I)‐centered radical was generated by homolytic cleavage of a Pt?HgSiR₃ bond of a mercury‐substituted platinum(II) complex. The Pt^I radical was characterized by EPR spectroscopy, chemical trapping experiments, and density functional theory (DFT) calculations.     

Fulvalene‐Embedded Perylene Diimide and Its Stable Radical Anion

The first example of a two‐state (neutral and reduced), stable electron‐accepting material and its radical anion is presented. FV‐PDI, generated from cyclocarbonylation and then a carbonyl coupling reaction, shows a largely degenerate LUMO of ?4.38 eV based on the delocalization of π‐electrons across the whole molecular skeleton through a fulvalene bridge. The stability and electron affinity allow spontaneous electron transfer to afford a stable radical anion. Spectroscopic characterization and structural elucidation showed that the radical anion [FV‐PDI]^.? has remarkable stability and near‐infrared absorptions extending to 1200 nm. Single‐crystal X‐ray diffraction analyses revealed significant changes in the molecular shape and packing arrangement of the formed radical anion. The central C?C bond linking the two PDI halves is lengthened from approximately 1.33 to 1.43 Å, and the alternating arrangement of positively and negatively charged units favor the stable charge‐transfer complex.     

Electron‐Self‐Exchange Kinetics of the Cyclooctatetraene/Cyclooctatetraene Radical‐Anion Couple

Rate constants k_hom and k_het are reported for the homogeneous electron‐self‐exchange and the heterogeneous electrochemical electron‐transfer reactions, respectively, of the cyclooctatetraene/cyclooctatetraene^? (COT/COT^.?) redox couple. In acetonitrile, the values k_hom (298 K)=(5±3)×10⁵ M ^?1 s^?1 and k_het (295 K)=8×10^?3 cm s^?1 are found, whereas slightly faster rates are obtained in dimethylformamide, namely, k_hom (298 K)=(1.6±0.6)×10⁶ M ^?1 s^?1 and k_het (295 K)=2×10^?2 cm s^?1. The k_hom rates are obtained from electron spin resonance (ESR) line broadening whereas the k_het rates are measured at a mercurized Pt electrode by using Nicolson’s method. The slowness of both electron‐transfer reactions is caused by the high inner‐sphere reorganization energy that results from the inevitable conformational change that takes place upon going from the tub‐like COT molecule to the planar COT^.? anion. The rates are well‐understood in terms of Marcus theory, including an additional medium inner‐sphere mode which is responsible for the flattening of COT.     

Supramolecular Control of Spin Exchange in a Spin‐Labelled [2]Rotaxane Incorporating a Tetrathiafulvalene Unit

The EPR properties of a novel triradical obtained by single‐electron oxidation of a nitroxide‐spin‐labelled rotaxane containing a tetrathiafulvalene unit and cyclobis(paraquat‐p‐phenylene) ring is reported. Rotaxanation is proved to have a dramatic effect on through‐space magnetic interactions between radical fragments. Analysis of the EPR spectra by a three‐jump model, allowed us to obtain structural information on the interlocked structure.