Radical Salts Research Articles

Tricoordinate Nontrigonal Pnictogen-Centered Radical Anions: Isolation, Characterization, and Reactivity.

The search for main-group element-based radicals is one of the main research topics in contemporary chemistry because of their fascinating chemical and physical properties. The Group 15 element-centered radicals mainly feature a V-shaped two coordinate structure, with a couple of radical cations featuring trigonal tricoordinated geometry. Now, nontrigonal compounds R3 E (E=P, As, Sb) were successfully synthesized by introducing a new rigid tris-amide ligand. The selective one-electron reduction of R3 E afforded the first stable tricoordinate pnictogen-centered radical anion salts; the pnictogen atoms retain planar T-shaped structures. EPR spectroscopy and calculations reveal that the spin density mainly resides at the p orbitals of the pnictogen atoms, which is perpendicular to the N3 E planes.

Cationic Polymerization Induced by Tris-(p-bromophenyl) Amine Cation-Radical Salts

Substituted triphenylamine cation radical salts having anions of the type SbF6-, PF6-, BF4- and SbCl6-, were prepared and used to initiate cationic polymerization of cyclohexene oxide (CHO), tetrahydrofuran (THF), and N-vinyl carbazole (NVC), thermally in dichloromethane at room temperature. Experimental results are presented to show the effects of salt counter ion, concentration, and polymerization conditions on the yield and the molecular weight of the obtained polymer. THF polymerization was enhanced by photolysis of the reaction mixture for short time. A general mechanism for the polymerization by cation-radical salts of substituted triphenylamine is proposed.

Bromide and Tribromide 4-Cyanobenzene-Ethylenedithio-Tetrathiafulvalene Radical Salts by Chemical and Electrochemical Routes

Two salts of the dissymmetric TTF-derivative 4-cyanobenzene-ethylenedithio-tetrathiafulvalene (4-CNB-EDT-TTF) with bromide and tribromide anions and with different stoichiometries, namely, (1:1) (4-CNB-EDT-TTF)Br3 (1) and (4:1) (4-CNB-EDT-TTF)4Br (2), were obtained by electrocrystallization and diffusion methods, respectively. The crystal structures of these compounds, as determined by single-crystal X-ray diffraction, are based on head-to-tail donor dimers with ring over ring overlap and donor stack arrangement, interleaved by anions depending on the (1:1) or (4:1) salt, respectively. The 4:1 salt behaves as a Mott insulator. In both salts, the donors are connected to adjacent donors through C–N···H–C interactions, which can be described as an modified R24 (10)* synthon for 1 and a combination of R22(10) and R24(10) synthons for 2.

Controlling the Conductivity of Oligomer Radical Cations by Tuning Stacking Structures of π-Dimers

Salts containing radical cations of dialkoxyterphenyls (DP) including 4,4′-dimethoxyterphenyl (1) and 4,4′-diethoxyterphenyl (2) have been isolated with weakly coordinating anions [Al(ORF)4]− (ORF = OC(CF3)3) or [Al(ORMe)4]− (ORMe = OC(CF3)2CH3) as the counterions. The radical cation salts have been characterized by single-crystal X-ray diffraction analysis, UV–vis absorption, and electron paramagnetic resonance spectroscopy, as well as conductivity measurements. Single-crystal structures indicate that the radical cations assemble into infinite stacks of π-dimer in these salts, while single-crystal conductivity measurements show that the conductivity of the salts is significantly affected by substituent groups and the interaction among the cation, anion, and solvent.

Seasonal Variation in Phenolic Compounds and Antioxidant Activity in Leaves of Cyclocarya paliurus (Batal.) Iljinskaja

Cyclocarya paliurus (Batal.) Iljinskaja is a plant with nutraceutical importance since its leaves have been used historically as folk medicines for hundreds of years. The content of 10 phenolic compounds was determined throughout the growing season by high-performance liquid chromatography (HPLC) with UV detector, while the antioxidant activities of C. paliurus leaf extracts were evaluated by 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt radical cation (ABTS), and ferric reducing antioxidant power (FRAP) methods. Seasonal variations in phenolic concentration and antioxidant activity as well as linkage between the phenolic composition and antioxidant activity were assessed. A significant seasonal variation of phenolic compounds was observed in the leaves and the highest content appeared in May, July, and November. Seventy percent ethanol extract of C. paliurus leaves possessed a good radical scavenging potency. Meanwhile, a significant correlation between antioxidant activities and contents of phenolics was detected. Results of the relationship between molecular structures and their antioxidant activities showed that both the number and configuration of H-donating hydroxyl groups are the main structural features influencing the antioxidant capacity of phenolics, while glycosylation may reduce the antioxidant capacity. The information provided by this study not only revealed the accumulative dynamics of phenolic compounds, but also established a basis for determining the optimal time for harvesting to improve the content of beneficial compounds in the leaves of C. paliurus in the future.

Stabilization of Pancake Bonding in (TCNQ)2 .- Dimers in the Radical-Anionic Salt (N-CH3-2-NH2-5Cl-Py)(TCNQ)(CH3CN) Solvate and Antiferromagnetism Induction.

We report a new antiferromagnetic radical‐anion salt (RAS) formed from 7,7,8,8‐tetracyanquinonedimethane (TCNQ) anion and 2‐amino‐5‐chloro‐pyridine cation with the composition of (N−CH3−2‐NH2−5Cl−Py)(TCNQ)(CH3CN). The crystallographic data indicates the formation of (TCNQ)2 .− radical‐anion π‐dimers in the synthesized RAS. Unrestricted density functional theory calculations show that the formed π‐dimers characterize with strong π‐stacking “pancake” interactions, resulting in high electronic coupling, enabling efficient charge transfer properties, but π‐dimers cannot be stable in the isolated conditions as a result of strong Coulomb repulsions. In a crystal, where (TCNQ)2 .− π‐dimers bound in the endless chainlets via supramolecular bonds with (N−CH3−2‐NH2−5‐Cl−Py)+ cations, the repulsion forces are screened, allowing for specific parallel π‐stacking interactions and stable radical‐anion dimers formation. Measurements of magnetic susceptibility and magnetization confirm antiferromagnetic properties of RAS, what is in line with the higher stability of ground singlet state of the radical‐anion pair, calculated by means of the DFT. Therefore, the reported radical‐anion (N−CH3−2‐NH2−5Cl−Py)(TCNQ)(CH3CN) solvate has promising applications in novel magnetics with supramolecular structures.

Highly robust cation radical salts: Aromatic oxidants from cycloannulated aromatic donors

A cost effective and easy Friedel-Craft Cycloannulation of simple aromatic compounds with readily available and cheap 2,5- dichloro-2,5-dimethylhexane in the presence of catalytic amount of anhydrous aluminum chloride produced various cycloannulated aromatic donors(I-XIII) in excellent yields and they were characterized by NMR spectroscopy as well as X-ray crystallography. All aromatic electron donors showed consistent reversibility upon elctrochemical oxidation at various scan rates (v = 50-1000 mVs−1). CRET, Magic Blue or NO+SbCl6- was used to produce cation radical salts. The cation radicals of V and VII were isolated in crystalline form and were analyzed by X – ray crystallography. The stability, different λmax values and the wide range of redox potential values (0.88–1.85 V vs SCE) of the cation radicals of (I-XIII) provided a wide window for the oxidation of various organic compounds. The cation radicals of II was successfully applied as the oxidant to evaluate the electron removal of tetraarylmethane and hexaarylbenzene and to evaluate a conformational switching behavior of cation radical of a biaryl compound. One electron oxidation of tetramethyl biphenylene, one electron oxidation of octamethoxychrysene and the formation of a new CC bond in octamethoxytetraphenylene were observed by applying the cation radicals of donors V, VII and VIII as the oxidant respectively.

Frontispiece: Hydrogen‐Bonded Polyrotaxane Cation Structure in Nickel Dithiolate Anion Radical Salts: Ferromagnetic and Semiconducting Behavior Associated with Structural Phase Transition

Frontispiece: Hydrogen‐Bonded Polyrotaxane Cation Structure in Nickel Dithiolate Anion Radical Salts: Ferromagnetic and Semiconducting Behavior Associated with Structural Phase Transition

Electronic Communication between S=1/2 Spins in Negatively-charged Double-caged Fullerene C60 Derivative Bonded by Two Single Bonds and Pyrrolizidine Bridge.

Radical anion salt {cryptand[2.2.2] (K+ )}2 (bispheroid)2- ⋅3.5C6 H4 Cl2 (1) of the double-caged fullerene C60 derivative, in which fullerene cages are linked by a cyclobutane bridging cycle and additionally by a pyrrolizidine moiety, was obtained. Each fullerene cage in this derivative accepts one electron on reduction, thus forming the (bispheroid)2- dianions with two interacting S=1/2 spins on the neighboring cages. Low-temperature magnetic measurements reveal a singlet ground state of the bispheroid dianions whereas triplet contributions prevail at increased temperature. An estimated exchange interaction between two spins J/kB =-78 K in 1 indicates strong magnetic coupling between them, nearly two times higher than that (J/kB =-44.7 K) in previously studied (C60 - )2 dimers linked via a cyclobutane bridge only. The enhancement of magnetic coupling in 1 can be explained by a shorter distance between the fullerene cages and, possibly, an additional channel for the magnetic exchange provided by a pyrrolizidine bridge. Quantum-chemical calculations of the lowest electronic state of the dianions by means of multi-configuration quasi-degenerate perturbation theory support the experimental findings.

Hydrogen-Bonded Polyrotaxane Cation Structure in Nickel Dithiolate Anion Radical Salts: Ferromagnetic and Semiconducting Behavior Associated with Structural Phase Transition.

The pseudo-polyrotaxane structure of [(H-bpy+ )- (DB-24-crown-8)]∞ (H-bpy+ = monoprotonated 4,4-bipyridinium; DB-24-crown-8 = dibenzo-24-crown-8) has been incorporated into the anion radical salt [Ni(dmit)2 ]- (dmit2- = 1,3-dithiole-2-thione-4,5-dithiolate). (H-bpy+ )(DB-24-crown-8)[Ni(dmit)2 ]- crystallized as two polymorphs, crystals 1 and 2. Crystal 1 was found to have a lower density and looser packing structure in which H-bpy+ forms a one-dimensional hydrogen-bonding chain that passes though the crown ether ring of DB-24-crown-8. DB-24-crown-8 adopts a U-shaped conformation in which two phenylene rings sandwich one of the pyridyl rings of H-bpy+ to stabilize the structure. The [Ni(dmit)2 ]- anions are arranged in a layer parallel to the (10) plane with uniform side-by-side interactions. A structural phase transition was observed at 235 K, accompanied by ordering of the polyrotaxane structure. In crystal 1, at 173 K, H-bpy+ is twisted around the central C-C bond, which perturbs the arrangement of [Ni(dmit)2 ]- through short C-H⋅⋅⋅S contacts. As a result, the semiconducting behavior, with an activation energy of 0.21 eV, becomes insulating below 235 K. The crystal exhibits ferromagnetic interactions because of the weak side-by-side interactions between [Ni(dmit)2 ]- anions. Crystal 2 has a similar pseudo-polyrotaxane structure but showed no phase transition. This suggests that the looser crystal packing in crystal 1 induces the structural change of the pseudo-polyrotaxane, perturbing the electron system of [Ni(dmit)2 ]- .

Ultrafast Pathways of the Photoinduced Insulator-Metal Transition in a Low-Dimensional Organic Conductor.

Among functional organic materials, low-dimensional molecular crystals represent an intriguing class of solids due to their tunable electronic, magnetic, and structural ground states. This work investigates Cu(Me,Br-dicyanoquinonediimine)2 single crystals, a charge transfer radical ion salt which exhibits a Peierls insulator-to-metal transition at low temperatures. The ultrafast electron diffraction experiments observe collective atomic motions at the photoinduced phase transition with a temporal resolution of 1 ps. These measurements reveal the photoinduced lifting of the insulating phase to happen within 2 ps in the entire crystal volume with an external quantum efficiency of conduction band electrons per absorbed photon of larger than 20. This huge cooperativity of the system, directly monitored during the phase transition, is accompanied by specific intramolecular motions. However, only an additional internal volume expansion, corresponding to a pressure relief, allows the metallic state for long times to be optically locked. The identification of the microscopic molecular pathways that optically drive the structural Peierls transition in Cu(DCNQI)2 highlights the tailored response to external stimuli available in these complex functional materials, a feature enabling high-speed optical sensing and switching with outstanding signal responsivity.

A Main‐Group Element Radical Based One‐Dimensional Magnetic Chain

AbstractThe first main‐group element radical based one‐dimensional magnetic chain (1K)n was realized by one‐electron reduction of the pyridinyl functionalized borane 1 with elemental potassium in THF in the absence of 18‐crown‐6 (18‐c‐6). The electron spin density of (1K)n mainly resides at the boron centers with a considerable contribution from central benzene and pyridine moieties. The spin centers exhibit an antiferromagnetic interaction as demonstrated by magnetic measurements and theoretical calculations. In contrast, the reduction in the presence of 18‐c‐6 afforded the separated radical anion salt 1K(Crown), in which the potassium cation was trapped by THF and 18‐c‐6 molecules. Further one‐electron reduction of 1K(Crown) and (1K)n led to the diamagnetic monomer and polymer, respectively.

A Main-Group Element Radical Based One-Dimensional Magnetic Chain.

The first main-group element radical based one-dimensional magnetic chain (1K)n was realized by one-electron reduction of the pyridinyl functionalized borane 1 with elemental potassium in THF in the absence of 18-crown-6 (18-c-6). The electron spin density of (1K)n mainly resides at the boron centers with a considerable contribution from central benzene and pyridine moieties. The spin centers exhibit an antiferromagnetic interaction as demonstrated by magnetic measurements and theoretical calculations. In contrast, the reduction in the presence of 18-c-6 afforded the separated radical anion salt 1K(Crown), in which the potassium cation was trapped by THF and 18-c-6 molecules. Further one-electron reduction of 1K(Crown) and (1K)n led to the diamagnetic monomer and polymer, respectively.

Radical Cation Salts of Cyanobenzene‐Ethylenedithio‐TTF Electron Donors with Halide (Cu and Hg) Binuclear Anions

Three salts of the two isomers of the dissymmetric TTF‐derivative cyanobenzene‐ethylenedithio‐tetrathiafulvalene (CNB‐EDT‐TTF) with binuclear polyhalide complex dianions, namely [4‐CNB‐EDT‐TTF]2 [Cu2Cl6] (1); [5‐CNB‐EDT‐TTF]2 [Cu2Cl6] 2CH2Cl2 (2) and [5‐CNB‐EDT‐TTF]2 [Hg2Br6] (3) were prepared and characterized by single‐crystal X‐ray diffraction and magnetic susceptibility measurements. In spite of the different structural types of these compounds, the three compounds present donors in dimerized stacks. The crystal structures of 1 and 3 consist of segregated head‐to‐tail donor dimer stacks and dianion columns. The structural motif of 2 is mixed columns of alternating head‐to‐tail donor dimers and dianions. The three salts present C–N···H–C interactions, which in 2 and 3 can be described as a R22(10) synthon. The magnetic susceptibility of the salts with the paramagnetic (Cu2Cl6)2– dianion (1 and 2) show that they behave as paramagnets where the Cu2+ atoms of the dianion are antiferromagnetically coupled with magnetic exchange values J/kB of –58.8 and –155.6 K, respectively. The different J values are associated with differences in dianion geometry.

Adding complexity to aromatics

Chemists have a few choices when they want to transform aromatic C–H bonds into something more complicated. Brominating and borylating these bonds are popular options, but those reactions usually produce a mixture of isomers. While seeking a more selective strategy, Tobias Ritter and colleagues at the Max Planck Institute for Kohlenforschung discovered that the tetrafluorothianthrene radical cation will react with aromatics almost exclusively at the para position (Nature 2019, DOI:10.1038/s41586-019-0982-0). “We had to develop new chemistry to quantify the selectivity, as it was so high,” Ritter says. Previously, chemists generated related persistent sulfur-based radical cations using moisture-sensitive, explosive thianthrenium perchlorate radical salts. But Ritter’s group developed a new method using a bench-stable, fluorinated, sulfoxide-based thianthrene reagent. They can use this reagent to functionalize aromatic C–H bonds in the presence of many functional groups, including amines, alcohols, esters, ...

Synthesis of ion-radical salts of indolizidine- and 1-amidoalkylfullerene derivatives

Indolizidinofullerenes containing trimethoxyarene substituents in the indolizidine cycle, 1-amidoalkylfullerenes, and their ion-radical salts with bis(arene)chromium complexes were synthesized. The compounds obtained were characterized by a number of spectral methods.

Photophysical properties of 1,3,6,8-tetraarylpyrenes and their cation radicals

The synthesis of 1,3,6,8-tetraaryl substituted pyrenes is described. The substituents on the aryl groups influence the optoelectronic properties of the pyrene core indicating a strong coupling between the aryl groups and the pyrene core. Electronic absroption spectra of the cation radicals generated in solution by the single electron oxidation of pyrenes reaffirmed the coupling of the aryl rings with the pyrene. The aryl groups at 1,3,6,8-positions completely inhibit the π-stacking of pyrene core in solution. Crystal structure of the cation radical salt of tetraphenyl shows charge delocalization between neutral pyrene unit and two cationic pyrene cores. Complete inhibition of the pyrene core stacking is observed in the solid state of cation radical when the dendritic pentaphenyl groups surround the pyrene core.

Charge localization in 1D tetramerized organic conductors: the special case of (tTTF)2ClO4

We report a detailed structural and spectroscopic study of the 1D 2:1 cation radical salt (tTTF)2ClO4, where tTTF = trimethylenetetrathiafulvalene, which exhibits a semiconductor-semiconductor phase transition at ca. T = 137 K. Crystal structures are determined above and below the transition; the tTTF molecules in stacks are grouped into weakly interacting tetramers. The reorganization of tTTF stacks is accompanied with an order-disorder transition in anion sublattice. Polarized infrared and Raman spectra of (tTTF)2ClO4 are measured in the broad frequency range as a function of the temperature (10–293 K). The structural and vibrational features are investigated to elucidate the origin of the semiconductor-semiconductor phase transition. We discuss the electron-intramolecular vibration coupling effects in the vibrational spectra of (tTTF)2ClO4 and identify signatures of high- and low-temperature states of charge localization in the tetramerized system. Both the C=C and C–S stretching modes of tTTF give evidence of strong charge distribution fluctuations in conducting stacks for T > 137 K, which are responsible for the appearance of molecules with charge +1e, and charge localization in tTTF tetramers for T < 137 K. The uniqueness of the salt (tTTF)2ClO4 in comparison with other tetramerized 1D systems is discussed.

Deprotonation of Benzoxazolium Salt: Trapping of a Radical-Cation Intermediate.

The deprotonation of N-2,6-diisopropylphenyl-substituted benzoxazolium tetrafluoroborate 1 with NaH results in the formation of electron-rich diaminodioxaethylene 2. The radical cation salt 2·+·BF4- is found to be an intermediate product in the redox reaction leading from 1 to 2.

Structural relations in (1 : 1) and (2 : 1) cyanobenzene-ethylenedithio-TTF radical salts; the role of CN⋯H interactions

Radical cation salts of 4-CNB-EDT-TTF presenting CN⋯H pairing interactions, semiconducting properties and chains of antiferromagnetically coupled donor dimers.