Radical Cation Salts Research Articles

Nickel Bis(dithiolene) Complexes and Tetrathiafulvalenes from Sterically Hindered Di‐ and Tetra‐Substituted Dithiine‐dithiolo‐thione Precursors

AbstractThree variants of 5,6‐dihydro‐1,4‐dithiin‐2,3‐(1’,3’‐dithiole‐2’‐thione) (dddt) bearing up to four substituents on the ethylene bridge have been obtained by hetero‐Diels‐Alder reaction. The substituted dddt derivatives have the advantage of providing modulation of the steric hindrance and stereogenic centres, and represent valuable intermediates for both tetrathiafulvalene (TTF) and metal‐dithiolene complexes. Within this work, disubstituted and tetrasubstituted dddt derivatives have been used for the synthesis of Ni(II) monoanionic and neutral complexes, as well as for tetra‐ and octa‐methylated EDT‐TTF and BEDT‐TTF (EDT=ethylenedithio, BEDT=bis(ethylenedithio)) derivatives. While the influence of the bulkiness and number of the substituents is observed in the solid state packing of both types of materials (complexes and TTF), the absence of the counterion in the case of the neutral complexes translated into increased intermolecular interactions between the donors and the formation of 1D and 2D layers. The two examples of tetrasubstituted TTF radical cation salts reported in here show semiconducting behaviour due to the formation of dimers and lateral interactions through the S atoms. Our findings highlight the importance of substitution on these type of dddt precursors and their potential for electroactive molecular materials.

Improved Charge‐Transfer Doping in Crystalline Polymer for Efficient and Stable Perovskite Solar Cells

AbstractPerovskite solar cells (PSCs) have significant potential for next‐generation photovoltaic technology applications. However, the instability of hole transport layers (HTLs) becomes the major obstacle to long‐term operational devices, which are affected by the intrinsic thermal instability and loose structure of hole transport materials, as well as the hygroscopicity and migration of dopants. Here, poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) is used as a model crystalline polymer to thoroughly investigate effective p‐type doping strategies and the underlying mechanism. According to Hard–Soft‐Acid–Base theory, the soft base P3HT is more likely to form a stable Lewis acid–base adduct with the reactive soft acid radical, resulting in a strong charge‐transfer interaction, thereby enhancing conductivity and regulating the energy band of the HTL. Meanwhile, the radical cation salt can promote pre‐nucleation to optimize the crystallization orientation of P3HT. The resulting PSCs exhibited the efficiency of 25.16%, which is the highest efficiency reported so far based on doped P3HT. In addition, the resulting devices demonstrated excellent stability, maintaining 96.5%, 96%, and 91% of their initial efficiency after aging under continuous illumination for 2028 h, at 85 °C for 1080 h, and at maximum power point (MPP) tracking under continuous 1 Sun illumination at 85 °C for 528 h, respectively.

Chiral and racemic BINOL spiroborate anions and radical-cation salt with BEDT-TTF

Sodium salts of the spiroborate anion bis[(1,1′-binaphthalene)-2,2′-diolato-O,O’]borate in enantiopure and racemic forms have been obtained through reaction of BORAX (sodium tetraborate hexahydrate) with the corresponding stereoisomers of BINOL [(1,1′-binaphthalene)-2,2′-diol]. Electrocrystallisationof these saltswith BEDT-TTF produces single crystals of a 1:1 radical-cation salt, (BEDT-TTF){B[1,1′-bis(BINOL)]2}•THF, with the racemic spiroborate anion whilst no crystals were obtained with the enantiopure forms.

Two pressure-induced molecular superconductors with different types of crystal and electronic structures derived from an unsymmetrical tetrachalcogenafulvalene

Abstract We newly synthesized an organic donor molecule ETTM-STF (ethylenedithio(trimethylene)diselenadithiafulvalene), a molecular hybrid of HMTTF (hexamethylenetetrathiafulvalene) and BETS (bis(ethylenedithio)tetraselenafulvalene). Electrochemical oxidation of ETTM-STF provided two cation radical salts, (ETTM-STF)2Au(CN)2 and (ETTM-STF)2BF4. Crystal structure of the Au(CN)2 salt is characterized by a monolayer structure based on dimerized donor molecules. On the other hand, the BF4 salt shows a bilayer structure that is composed of two crystallographically independent conducting layers with different types of electronic structures. At ambient pressure, the Au(CN)2 salt is a Mott insulator and the BF4 salt undergoes a metal–nonmetal transition where the two types of conducting layers exhibit different behavior. Application of pressure suppressed the nonconducting behavior and induced a superconducting state in the Au(CN)2 salt (Tc = 3.4 K at 9.5 kbar) and in the BF4 salt (Tc = 6.8 K at 8 kbar).

A 3D Printed Pumpless Nonaqueous Organic Redox Flow Cell

Redox flow batteries (RFBs) have considerable promise in addressing the ever-increasing demands for robust energy storage devices. The nature of these devices makes it possible to create storage with potential applicability at multiple size scales1. However, conventional RFBs require energy to pump the liquid electrolyte, reducing the overall system efficiency, and limiting its viability to mid- and large-scale operations only2. Eliminating the pumping load can improve overall efficiency and create opportunities for mobile applications. In this work, we demonstrate a 3D printed prototype of a pumpless non-aqueous organic RFB that induces flow in the electrolyte from sinusoidal rocking motion. The body of the flow cell was created using 3D printing, allowing us to modify and adapt geometry easily during testing.First, the compatibility of the 3D-printed material was studied by testing the chemical stability of widely available additive manufacturing materials under RFB conditions. Then, we evaluated the performance of this pumpless system using an easily scalable, soluble, and stable one electron donor phenothiazine derivative, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT) which has been widely studied for RFB applications3. We used 0.25 M MEEPT and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT-TFSI) as a redox-active couple in 0.5 M tetraethylammonium bis(trifluoromethanesulfonyl)imide (TEATFSI)/acetonitrile (MeCN). Our results suggest an inversely proportional empirical relationship between limiting current density and the frequency of sinusoidal motion due to varying mass transfer rates. After 100 cycles in this symmetric cell with an area specific resistance of 2.62 Ωcm2, we achieved a capacity retention of 63% and an average Coulombic efficiency of 96%, with minimal sign of decomposition of the redox couple when analyzed post-cell cycling.The results of this study can be used for 3D printing material selection in numerous electrochemical devices that use organic solvents, including flow cells, supercapacitors, and lithium-ion batteries. Using sinusoidal motion to drive electrolytes instead of a pump can increase the theoretical efficiency and decrease the overall size of a flow cell. This makes it attractive in wearable applications where biomechanical motion can be harnessed.1 Wang, W. et al. Recent Progress in Redox Flow Battery Research and Development. Advanced Functional Materials 23, 970-986 (2013). https://doi.org:10.1002/adfm.2012006942 Tian, C. H., Chein, R., Hsueh, K. L., Wu, C. H. & Tsau, F. H. Design and modeling of electrolyte pumping power reduction in redox flow cells. Rare Metals 30, 16-21 (2011). https://doi.org:10.1007/s12598-011-0229-13 Milshtein, J. D. et al. High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries. Energy & Environmental Science 9, 3531-3543 (2016). https://doi.org:10.1039/C6EE02027E

Magic Blue-Initiated SN2-Type Ring Opening of Activated Aziridines: Friedel-Crafts-Type Alkylation of Electron-Rich Arenes/Heteroarenes.

A transition metal-free, atom-economical, and highly stereospecific synthetic approach to Friedel-Crafts-type alkylation of arenes/heteroarenes has been developed. The protocol involves the catalytic aminium radical-cation salt (Magic Blue)-initiated SN2-type nucleophilic ring opening of activated aziridines with arenes/heteroarenes to give the corresponding 2,2-diarylethylamines up to 99% yield and 85% ee (for nonracemic aziridines) in a very short reaction time. Moreover, on reaction with 1,3-dimethylindole and benzofuran, aziridines undergo domino-ring-opening cyclization (DROC) to give the various biologically significant heterocyclic scaffolds in moderate to good yields.

Utilizing the Perfluoronaphthalene Radical Cation as a Selective Deelectronator to Access a Variety of Strongly Oxidizing Reactive Cations.

A selective deelectronation reagent with very high potential of +2.00 (solution)/+2.41 V (solid-state) vs. Fc+/0 and based on a room temperature stable perfluoronaphthalene (naphthaleneF) radical cation salt was developed and applied. The solid-state deelectronation of commercial naphthaleneF with [NO]+[F{Al(ORF)3}2]- generates [naphthaleneF]+⋅[F{Al(ORF)3}2]- (ORF=OC(CF3)3) in gram scale. Thermochemical analysis unravels the solid-state deelectronation potential of the starting [NO]+-reagent to be +2.34 V vs. Fc+/0 with [F{Al(ORF)3}2]- counterion, but only +1.14 V vs. Fc+/0 with the small [SbF6]- ion. Selective reactions demonstrate the selectivity of [naphthaleneF]+⋅ for deelectronation of a multitude of organ(ometall)ic molecules and elements in solution: providing the molecular structures of the acene dications [tetracene]2+, [pentacene]2+ or spectroscopic evidence for the carbonyl complex of the ferrocene dication [Fc(CO)]2+, the [P9]+ cation from white phosphorus, the solvent-free copper(I) salt starting from copper metal and the dicationic Fe(IV)-scorpionate complex [Fe(sc)2]2+.

MT-pyrene-Based Conductive Radical Cation Salts: Crystal and Electronic Structure and Transport Properties

MT-pyrene-Based Conductive Radical Cation Salts: Crystal and Electronic Structure and Transport Properties

Radical Cation Salts of Hetera‐Buckybowls: Polar Crystals, Negative Thermal Expansion and Phase Transition

AbstractRadical cation salts of π‐conjugated polycycles are rich in physical properties. Herein, two kinds of hetera‐buckybowls, ethoxy‐substituted trithiasumanene (3SEt) and triselenasumanene (3SeEt), are synthesized as electron donors. Galvanostatic oxidation of them affords radical cation salts (3SEt)5(TTFMPB)3, (3SeEt)5(TTFMPB)3, (3SEt)4PMA, and (3SeEt)4PMA, where PMA is Keggin‐type phosphomolybdate and TTFMPB is tetrakis[3,5‐bis(trifluoromethyl)‐phenyl]borate. In these salts, 3SEt/3SeEt are partially charged and show distinct conformation change with the site charge and counter anions. In TTFMPB salts, (TTFMPB)− forms hexagonal channels that accommodate the packing columns of 3SEt/3SeEt. In particular, (3SEt)5(TTFMPB)3 adopts the R3c space group and is a polar crystal with the columns of 3SEt all in the up‐bowl direction. The PMA salts of 3SEt/3SeEt are polar crystals (C2 space group) with 3SEt/3SeEt being planar and forming columnar stacks. (3SeEt)4PMA shows a structural modulation below 200 K, namely, negative thermal expansion (NTE) of the unit cell volume and enlargement of the intermolecular distances between neighboring 3SeEt molecules. The four salts are semiconductors with an activation energy of 0.18–0.38 eV. The conductivity of (3SeEt)4PMA shows a reversible transition upon cooling and heating, in accordance to the NTE structural modulation. This work paves the way toward conducting materials based on hetera‐buckybowls.

Radical Cation Salts of Hetera-Buckybowls: Polar Crystals, Negative Thermal Expansion and Phase Transition.

Radical cation salts of π-conjugated polycycles are rich in physical properties. Herein, two kinds of hetera-buckybowls, ethoxy-substituted trithiasumanene (3SEt) and triselenasumanene (3SeEt), are synthesized as electron donors. Galvanostatic oxidation of them affords radical cation salts (3SEt)5 (TTFMPB)3 , (3SeEt)5 (TTFMPB)3 , (3SEt)4 PMA, and (3SeEt)4 PMA, where PMA is Keggin-type phosphomolybdate and TTFMPB is tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate. In these salts, 3SEt/3SeEt are partially charged and show distinct conformation change with the site charge and counter anions. In TTFMPB salts, (TTFMPB)- forms hexagonal channels that accommodate the packing columns of 3SEt/3SeEt. In particular, (3SEt)5 (TTFMPB)3 adopts the R3c space group and is a polar crystal with the columns of 3SEt all in the up-bowl direction. The PMA salts of 3SEt/3SeEt are polar crystals (C2 space group) with 3SEt/3SeEt being planar and forming columnar stacks. (3SeEt)4 PMA shows a structural modulation below 200 K, namely, negative thermal expansion (NTE) of the unit cell volume and enlargement of the intermolecular distances between neighboring 3SeEt molecules. The four salts are semiconductors with an activation energy of 0.18-0.38 eV. The conductivity of (3SeEt)4 PMA shows a reversible transition upon cooling and heating, in accordance to the NTE structural modulation. This work paves the way toward conducting materials based on hetera-buckybowls.

Catalytic Aminium Radical-Cation Salt (Magic Blue)-Initiated SN2-Type Nucleophilic Ring-Opening Transformations of Aziridines.

A simple and atom economic protocol for the construction of C-X/C-C bonds via catalytic aminium radical-cation salt (Magic Blue)-initiated SN2-type nucleophilic ring-opening transformations of racemic and nonracemic aziridines with different hetero and carbon nucleophiles to afford various amino ethers, thioethers, and amines in up to 99% yield, and with perfect enantiospecificity for some substrates but reduced ee with others (for nonracemic aziridines), is developed. This aminium radical-cation salt-initiated, SN2-type nucleophilic ring-opening strategy, along with various cyclization protocols, is employed to synthesize various biologically significant compounds.

BEDT-TTF radical-cation salts with tris(oxalato)chromate and guest additives.

The family of radical-cation salts β''-(BEDT-TTF)4[(A)M3+(C2O4)3]·guest (M = Fe, Cr, Ga, Al, Co, Mn, Rh, Ru; A = K+, H3O+, NH4 +) has produced superconductors, metals, semiconductors, and metal-insulators through introduction of different guest molecules into the structure. We present three new additions to the family β''-(BEDT-TTF)4[(A)Cr(C2O4)3]·guest with the guest molecules toluene, phenol, or salicylaldehyde. These new guests are liquid or solid additives within the electrocrystallisation medium. All three salts show metallic behaviour from room temperature down to <10 K and do not show a superconducting transition.

Introduction of new guest molecules into BEDT-TTF radical-cation salts with tris(oxalato)ferrate

We present the crystal structures of five new BEDT-TTF radical-cation salts with tris(oxalato)ferrate anions using solid or liquid additives toluene, phenol, benzaldehyde, 4-bromobenzaldehyde, and kojic acid.

Oxidants Controlled C-H Bond Functionalization of N-Aryltetrahydroisoquinolines: The Construction of the Quaternary Carbon Center and Cleavage of the C-N Bond.

Initiated by triarylamine radical cation salt (TBPA), the direct C-H bond functionalization of α-N-aryltetrahydroisoquinoline esters was smoothly realized, giving a series of α-hydroxylated derivatives with a quaternary carbon center in good yields. Differently, in the presence of tert-butyl nitrite (TBN), the C-N single bond was cleaved to keto esters. The mechanistic study revealed that these reactions were mediated by a similar mechanism, in which the N-nitrosation might provide a driving force to the C-N bond cleavage.

Homoleptic Transition Metal Carbonyl Cations: Synthetic Approaches, Characterization and Follow-Up Chemistry.

ConspectusCarbon monoxide, CO, is one of the most important ligands in organometallic chemistry. It is an excellent π-acceptor and a moderate σ-donor. Therefore, most of the known transition metal carbonyls (TMCs) exhibit a zerovalent or even negative metal oxidation state (OS) of up to -4. However, given the right conditions, the carbonyl ligand also forms homoleptic cationic complexes with one or more transition metal atoms, the transition metal carbonyl cations (TMCCs), known with an OS of up to +3. Despite their long-standing history upon discovery of the first [M(CO)6]+ examples (M = Mn, Tc, Re) by E. O. Fischer in 1962 as well as their very fundamental nature, it took until the 1990s for the scope to be widened by Aubke, Strauss and Willner. Yet, many potential TMCC entries known from gas-phase mass spectrometry work remained unknown on preparative grounds. This is due to their high reactivity, which puts scientists to new challenges and encourages the development of suitable solvents, anions and oxidants, to cope with the demands of these fundamental salts─later referred to as pseudo-gas-phase conditions and innocent deelectronators and solvents.Hence, the utilization of extremely weakly coordinating perfluorinated alkoxyaluminates [Al(ORF)4]- and [F{Al(ORF)3}2]- (ORF = -OC(CF3)3) in combination with the polar but non- or weakly coordinating innocent solvents 1,2-difluorobenzene (oDFB) and 1,2,3,4-tetrafluorobenzene (TFB) yielded the first TMCC salts containing heptacoordinate [M(CO)7]+ (M = Nb, Ta) as well as paramagnetic [M(CO)6]+· (M = Cr, Mo, W) or [Ni(CO)4]+·. However, the use of typical inorganic oxidants Ag+, [NO]+ and Ag+/0.5 I2 regularly led to unwanted side reactions. For example, the Lewis acidic silver(I) cations form Lewis pairs with various Lewis basic TMCs yielding partly clustered [Agx{TMC}y]x+ complex salts, while nitrosonium cations may substitute for carbonyl ligands, forming [M(CO)x-1(NO)]+ complexes. The synergistic oxidizing reagent Ag+/0.5 X2 can add halonium ions X+ to the TMCs (X = Cl, Br, I). This prevented the synthesis of univalent group 8 TMCC salts. Yet, the application of radical cation salts of perfluorinated arenes as innocent deelectronators finally yielded salts of [Fe(CO)5]+· and [M3(CO)14]2+ (M = Ru, Os).TMCC salts are excellent starting materials, and the reaction of [Co(CO)5]+ and [Ni(CO)4]+· with benzene led to the previously unknown bis(benzene) sandwich complexes [Co(benzene)2]+ and [Ni(benzene)2]+·. Under the right conditions, even the very weakly bound oDFB-complex salts with [M(oDFB)2]+ (M = Co, Ni) cations form, useful as naked metal(I) synthons and for small-molecule activation.

Isolation, Structural and Physical Characterization as well as Reactivity of Persistent Acenium Radical Cation Salts.

The unsubstituted acenium radical cations (ARCs) are extremely sensitive and were hitherto only studied in situ, i. e. in the gas phase, as dilute solutions in strong acids or by matrix isolation spectroscopy at about 10 K. In this study, room temperature stable ARC salts with the weakly coordinating anion [F{Al(ORF )3 }2 ]- (ORF =-OC(CF3 )3 ) supported by the weakly coordinating solvent 1,2,3,4-tetrafluorobenzene (TFB) were prepared and structurally, electrochemically and spectroscopically characterized. Reaction of the neutral acenes with Ag+ [F{Al(ORF )3 }2 ]- led, non-innocent,[54] to intermediate [Ag2 (acene)2 ]2+ complexes, which decompose over time to Ag0 and the corresponding (impure) ARC salts. By contrast, direct deelectronation with the recently developed innocent[54] deelectronator radical cation salt [anthraceneHal ]+⋅ [F{Al(ORF )3 }2 ]- led to phase-pure products [acene]+⋅ [F{Al(ORF )3 }2 ]- (anthraceneHal =9,10-dichlorooctafluoroanthracene; acene=anthra-, tetra-, pentacene). For the first time, a homogenous set of spectroscopic data on analytically pure ARC salts was obtained. In addition, cyclovoltammetric measurements of the acenes connected the potentials in solution with those in the gas-phase. Hence, the data complement the existing isolated gas-phase, strong acid or matrix isolation studies. A first entry to follow-up chemistry of the acenium radical cations as ligand forming oxidizers was demonstrated by reaction with Co2 (CO)8 giving [Co(anthracene)(CO)2 ]+ .

Concentration-dependent Cycling of Phenothiazine-based Electrolytes in Nonaqueous Redox Flow Cells.

Increasing redox-active species concentrations can improve viability for organic redox flow batteries by enabling higher energy densities, but the required concentrated solutions can become viscous and less conductive, leading to inefficient electrochemical cycling and low material utilization at higher current densities. To better understand these tradeoffs in a model system, we study a highly soluble and stable redox-active couple, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT-TFSI). We measure the physicochemical properties of electrolytes containing 0.2-1 M active species and connect these to symmetric cell cycling behavior, achieving robust cycling performance. Specifically, for a 1 M electrolyte concentration, we demonstrate 94% materials utilization, 89% capacity retention, and 99.8% average coulombic efficiency over 435 h (100 full cycles). This demonstration helps to establish potential for high-performing, concentrated nonaqueous electrolytes and highlights possible failure modes in such systems.

Crystal Engineering of Cation-Radical Salts with Weakly Coordinating Carbadodecaborate Anions

An X-ray structural analysis revealed that the salts of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), N-methylphenothiazine (MPTZ), and octamethylbiphenylene (OMB), with dodecamethyl- or hexabromo-substituted carbadodecaborate anions, comprise layers of the (partially) oxidized organic donors separated by sheets of the bulky counter-ions. The cationic layers comprise either well-separated TMPD+ or MPTZ+ cation radicals or π-stacks of partially oxidized OMB moieties consisting of more or less distinct (OMB)2+ units. Quantum mechanical calculations revealed that the formation of essentially isolated cation-radical or π-bonded associations in the salts with these weakly coordinating anions is correlated with the strength of the multicenter π-bonding between cation radicals. This pancake bonding is determined by the balance of the electrostatic repulsion between cationic counterparts and attractive dispersion and weakly covalent interactions.

Chiral Bis(tetrathiafulvalene)-1,2-cyclohexane-diamides.

Chiral bis(TTF) diamides have been obtained in good yields (54–74%) from 1,2-cyclohexane-diamine and the corresponding TTF acyl chlorides. The (R,R)-1 and (S,S)-1 enantiomers have been characterized by circular dichroism and the racemic form by single-crystal X-ray diffraction. The neutral racemic bis(TTF)-diamide shows the formation of a pincer-like framework in the solid state, thanks to the intramolecular S···S interactions. The chemical oxidation in a solution using FeCl3 provides stable oxidized species, while the electrocrystallization experiments provided radical cation salts. In particular, single-crystal resistivity measurements on the racemic donor with AsF6− as a counterion demonstrate semiconductor behavior in this material. The DFT and TD-DFT calculations support the structural and chiroptical features of these new chiral TTF donors.

Design, Synthesis, Electronic Properties, and X-ray Structural Characterization of Various Modified Electron-Rich Calixarene Derivatives and Their Conversion to Stable Cation Radical Salts.

We have designed and synthesized electron-rich calixarene derivatives, which undergo reversible electrochemical oxidation in a well-accessible potential range that allows the ready preparation and isolation of the corresponding cation radicals. Preparation of mono- or tetra-radical cation can be achieved by using stable aromatic cation-radical salts such as MA+•, MB+•, and NAP+• as selective organic oxidants. The cation radicals of calixarenes are stable indefinitely at ambient temperatures and can be readily characterized by UV-vis-NIR spectroscopy. These cation radicals bind a single molecule of nitric oxide within its cavity with remarkable efficiency.