Synthesis and Properties of the Weakly Coordinating Anion [Me3NB12Cl11]−

The weakly coordinating anion [Me₃NB₁₂Cl₁₁]^? has been prepared by a simple two‐step procedure. The anion [Me₃NB₁₂Cl₁₁]^? is easily obtained in batches of up to 20 g by chlorination of the known [H₃NB₁₂H₁₁]^? anion with SbCl₅ at about 190 °C and subsequent N‐methylation with methyl iodide. Starting from Na[Me₃NB₁₂Cl₁₁], several synthetically useful salts with reactive cations ([NO]⁺, [Ph₃C]⁺, and [(Et₃Si)₂H]⁺) were prepared. Full spectroscopic (NMR, IR, Raman, TGA, MS) characterization and single‐crystal X‐ray diffraction studies confirmed the identity and purity of the products. The thermal, chemical, and electrochemical stability as well as the basicity of the [Me₃NB₁₂Cl₁₁]^? anion is similar to that of the structurally related weakly coordinating 1‐carba‐closo‐dodecaborate and closo‐dodecaborate anions. The facile preparation of the [Me₃NB₁₂Cl₁₁]^? anion and its ideal chemical and physical properties make it a cheap alternative to other classes of weakly coordinating anions.     

Synthesis of Five‐ and Six‐Coordinate Tris(pentafluoroethyl)fluorosilicates

The research area of perfluoroalkylsilanes is still in its infancy. Although there are already many examples of difluorotriorganylsilicates, the first example of a completely characterized trifluorotriorganylsilicate is presented, the dianion [Si(C₂F₅)₃F₃]^2?. The strongly electron‐withdrawing influence of the pentafluoroethyl groups appears to be a fundamental cause of the stability of this compound. This dianion is also the first structurally characterized example of a tris(pentafluoroethyl)silicon compound. The synthesis and complete characterization of [PPh₄]₂[Si(C₂F₅)₃F₃] and [PPh₄][Si(C₂F₅)₃F₂] along with the precursor [H(OEt₂)₂][Si(C₂F₅)₃F₂] was achieved from SiCl₄ and LiC₂F₅.     

From Ion‐Like Ethylzinc Aluminates to [EtZn(arene)2]+[Al(ORF)4]− Salts

Attempts to prepare previously unknown simple and very Lewis acidic [RZn]⁺[Al(OR^F)₄]^? salts from ZnR₂, AlR₃, and HO?R^F delivered the ion‐like RZn(Al(OR^F)₄) (R=Me, Et; R^F=C(CF₃)₃) with a coordinated counterion, but never the ionic compound. Increasing the steric bulk in RZn⁺ to R=CH₂CMe₃, CH₂SiMe₃, or Cp*, thus attempting to induce ionization, failed and led only to reaction mixtures including anion decomposition. However, ionization of the ion‐like EtZn(Al(OR^F)₄) compound with arenes yielded the [EtZn(arene)₂]⁺[Al(OR^F)₄]^? salts with arene=toluene, mesitylene, or o‐difluorobenzene (o‐DFB)/toluene. In contrast to the ion‐like EtZn(η³‐C₆H₆)(CHB₁₁Cl₁₁), which co‐crystallizes with one benzene molecule, the less coordinating nature of the [Al(OR^F)₄]^? anion allowed the ionization and preparation of the purely organometallic [EtZn(arene)₂]⁺ cation. These stable materials have further applications as, for example, initiators of isobutene polymerization. DFT calculations to compare the Lewis acidities of the zinc cations to those of a large number of organometallic cations were performed on the basis of fluoride ion affinity. The complexation energetics of EtZn⁺ with arenes and THF was assessed and related to the experiments.     

Synthesis and Characterization of Tetrakis(pentafluoroethyl)aluminate

While perfluorinated aryl, aryloxy and alkoxy aluminum species are well-established as weakly coordinating anions (WCAs), corresponding perfluoroalkyl aluminum derivatives are virtually unknown. Reaction of Si(C₂F₅)₃CH₃ with Li[AlH₄] afforded the tetrakis(pentafluoroethyl)aluminate, [Al(C₂F₅)₄]⁻. Several salts of the [Al(C₂F₅)₄]⁻ ion were synthesized and characterized by NMR spectroscopic methods, mass spectrometry, X-ray diffraction studies and elemental analysis.     

A Copper‐Catalyzed Decarboxylative Amination/Hydroamination Sequence: Switchable Synthesis of Functionalized Indoles

A copper‐catalyzed decarboxylative amination/hydroamination sequence of propargylic carbamates with various nucleophiles is described for the first time. It features an earth‐abundant metal catalyst, mild reaction conditions, and high efficiency. Further treatments of the resultant key intermediates using an acid or a base in one pot enable the controllable and divergent synthesis of two types of functionalized indoles. Moreover, experiments to demonstrate the synthetic potential of this methodology are performed.     

“Dumbbell”‐ and “Clackers”‐Shaped Dimeric Derivatives of Monocarba‐closo‐dodecaborate

We designed, synthesized, and characterized two types of dimeric forms of monocarba‐closo‐dodecaborate, namely, a “dumbbell”‐shaped dianion having a C?C bond and a “clackers”‐shaped monoanion having an iodonium linker. The unique architectures of these anionic molecules were established by X‐ray analysis. Spectroscopic analysis, DFT calculations, and reactivity experiments revealed high anionic and chemical stability of both anions, which are crucial properties for weakly coordinating anions.     

Silver‐Free Activation of Ligated Gold(I) Chlorides: The Use of [Me3NB12Cl11]− as a Weakly Coordinating Anion in Homogeneous Gold Catalysis

Phosphane and N‐heterocyclic carbene ligated gold(I) chlorides can be effectively activated by Na[Me₃NB₁₂Cl₁₁] ( 1 ) under silver‐free conditions. This activation method with a weakly coordinating closo‐dodecaborate anion was shown to be suitable for a large variety of reactions known to be catalyzed by homogeneous gold species, ranging from carbocyclizations to heterocyclizations. Additionally, the capability of 1 in a previously unknown conversion of 5‐silyloxy‐1,6‐allenynes was demonstrated.     

The First Step of the Oxidation of Elemental Sulfur: Crystal Structure of the Homopolyatomic Sulfur Radical Cation [S8].+

The oxidation of elemental sulfur in superacidic solutions and melts is one of the oldest topics in inorganic main group chemistry. Thus far, only three homopolyatomic sulfur cations ([S₄]²⁺, [S₈]²⁺, and [S₁₉]²⁺) have been characterized crystallographically although ESR investigations have given evidence for the presence of at least two additional homopolyatomic sulfur radical cations in solution. Herein, the crystal structure of the hitherto unknown homopolyatomic sulfur radical cation [S₈]^.+ is presented. The radical cation [S₈]^.+ represents the first step of the oxidation of the S₈ molecule present in elemental sulfur. It has a structure similar to the known structure of [S₈]²⁺, but the transannular sulfur⋅⋅⋅sulfur contact is significantly elongated. Quantum-chemical calculations help in understanding its structure and support its presence in solution as a stable compound. The existence of [S₈]^.+ is also in accord with previous ESR investigations.     

Synthesis,Properties, and Application of Tetrakis(pentafluoroethyl)gallate, [Ga(C2F5)4]−

Weakly coordinating anions (WCAs) are important for academic reasons as well as for technical applications. Tetrakis(pentafluoroethyl)gallate, [Ga(C₂F₅)₄]^?, a new WCA, is accessible by treatment of [GaCl₃(dmap)] (dmap=4‐dimethylaminopyridine) with LiC₂F₅. The anion [Ga(C₂F₅)₄]^? proved to be reluctant towards deterioration by aqueous hydrochloric acid or lithium hydroxide. Various salts of [Ga(C₂F₅)₄]^? were synthesized with cations such as [PPh₄]⁺, [CPh₃]⁺, [(O₂H₅)₂(OH₂)₂]²⁺, and [Li(dec)₂]⁺ (dec=diethyl carbonate). Thermolysis of [(O₂H₅)₂(OH₂)₂][Ga(C₂F₅)₄]₂ gives rise to a dihydrate of tris(pentafluoroethyl)gallane, [Ga(C₂F₅)₃(OH₂)₂]. All products were characterized by NMR and IR spectroscopy, mass spectrometry, X‐ray diffraction, and elemental analysis. Furthermore, an outlook for the application of [Li(dec)₂][Ga(C₂F₅)₄] as a conducting salt in lithium‐ion batteries is presented.     

Phosphoryl‐Rich Flame‐Retardant Ions (FRIONs): Towards Safer Lithium‐Ion Batteries

The functionalized catecholate, tetraethyl (2,3‐dihydroxy‐1,4‐phenylene)bis(phosphonate) (H₂‐DPC), has been used to prepare a series of lithium salts Li[B(DPC)(oxalato)], Li[B(DPC)₂], Li[B(DPC)F₂], and Li[P(DPC)₃]. The phosphoryl‐rich character of these anions was designed to impart flame‐retardant properties for their use as potential flame‐retardant ions (FRIONs), additives, or replacements for other lithium salts for safer lithium‐ion batteries. The new materials were fully characterized, and the single‐crystal structures of Li[B(DPC)(oxalato)] and Li[P(DPC)₃] have been determined. Thermogravimetric analysis of the four lithium salts show that they are thermally stable up to around 200 °C. Pyrolysis combustion flow calorimetry reveals that these salts produce high char yields upon combustion.     

Enantiodivergent Synthesis of (+)‐ and (−)‐Pyrrolidine 197B: Synthesis of trans‐2,5‐Disubstituted Pyrrolidines by Intramolecular Hydroamination

A highly efficient, diastereoselective, iron(III)‐catalyzed intramolecular hydroamination/cyclization reaction involving α‐substituted amino alkenes is described. Thus, enantiopure trans‐2,5‐disubstituted pyrrolidines and trans‐5‐substituted proline derivatives were synthesized by means of a combination of enantiopure starting materials, easily available from l ‐α‐amino acids, with sustainable metal catalysts such as iron(III) salts. The scope of this methodology is highlighted in an enantiodivergent approach to the synthesis of both (+)‐ and (?)‐pyrrolidine 197B alkaloids from l ‐glutamic acid. In addition, a computational study was carried out to gain insight into the complete diastereoselectivity of the transformation.     

Amine‐bis(phenolate) Iron(III)‐Catalyzed Formal Hydroamination of Olefins

A practical synthesis of highly functionalized amines by the formal hydroamination reaction of alkenes with nitroarenes catalyzed by an air stable amine‐bis(phenolate) iron(III) complex is reported. The reaction uses an easily handled silane, low catalyst loadings, and mild reaction conditions. A wide range of substrates are transformed with synthetically useful yields (21 examples).     

The Parent Cyclopentadienyltin Cation,Its Toluene Adduct,and the Quadruple‐Decker [Sn3Cp4]2+

Truly cationic metallocenes with the parent cyclopentadienyl ligand are so far unknown for the Group 14 elements. Herein we report on an almost “naked” [SnCp]⁺ cation with the weakly coordinating [Al{OC(CF₃)₃}₄]⁻ and [{(F₃C)₃CO}₃Al−F−Al{OC(CF₃)₃}₃]⁻ anions. [SnCp][Al{OC(CF₃)₃}₄] was used to prepare the first main‐group quadruple‐decker cation [Sn₃Cp₄]²⁺ again as the [Al{OC(CF₃)₃}₄]⁻ salt. Additionally, the toluene adduct [CpSn(C₇H₈)][Al{OC(CF₃)₃}₄] was obtained.     

Synthesis and Reactivity of the First Isolated Hydrogen‐Bridged Silanol–Silanolate Anions

We report on the first examples of isolated silanol–silanolate anions, obtained by utilizing weakly coordinating phosphazenium counterions. The silanolate anions were synthesized from the recently published phosphazenium hydroxide hydrate salt with siloxanes. The silanol–silanolate anions are postulated intermediates in the hydroxide‐mediated polymerization of aryl and alkyl siloxanes. The silanolate anions are strong nucleophiles because of the weakly coordinating character of the phosphazenium cation, which is perceptible in their activity in polysiloxane depolymerization.     

H(OEt2)2[P(1,2‐O2C6Cl4)3]: Synthesis,Characterization, and Application as a Single‐Component Initiator for the Carbocationic Polymerization of Olefins

The development of novel Brønsted acids featuring the hexacoordinate phosphorus(V) anion [TRISPHAT]^? {[ 1 ]^?=[P(1,2‐O₂C₆Cl₄)₃]^?} are reported. The title compound, H(OEt₂)₂[ 1 ], was synthesized from 1,2‐(HO)₂C₆Cl₄ (3 equiv) and PCl₅ in the presence of diethyl ether. This compound was fully characterized by ¹H, ³¹P and ¹³C NMR spectroscopy, X‐ray crystallography and elemental microanalysis. Dissolution of H(OEt₂)₂[ 1 ] in acetonitrile results in the slow precipitation of crystalline H(OEt₂)(NCMe)[ 1 ], which was characterized by X‐ray diffraction; however, in CD₂Cl₂ solution the [TRISPHAT]^? anion protonated and ring‐opened. The weighable, solid H(OEt₂)₂ [ 1 ] was found to be a competent initiator for the polymerization of n‐butyl vinyl ether, α‐methylstyrene, styrene and isoprene at a variety of temperatures and monomer‐to‐initiator ratios. At low temperatures, polymers with M_n>10⁵ were obtained for n‐butyl vinyl ether and α‐methylstyrene whereas slightly lower molecular weights were obtained with styrene and isoprene (10⁴<M_n<10⁵). The poly(α‐methylstyrene) synthesized at ?78 °C is syndiotactic‐rich (ca. 87 % rr) whereas the polystyrene obtained at ?50 °C is atactic. The polyisoprene obtained possessed all possible modes of enchainment as well as branched and/or cyclic structures that are often observed in polyisoprene.