?Noncoordinating? Anions, II

Alkylcyanoborate Anions: Building Blocks for Fluorine-Free Low-Viscosity, Electrochemically and Thermally Stable Ionic Liquids

A set of mixed-substituted potassium alkylcyano- and alkylcyanofluoroborates has been synthesized using easily accessible starting compounds and characterized by elemental analysis, NMR and vibrational spectroscopy, and mass spectrometry. In addition, single-crystal structures of salts of the cyanoborate anions have been derived from X-ray diffraction experiments. The 1-ethyl-3-methylimidazolium room temperature ionic liquids ([EMIm]+ -RTILs) with the new borate anions have been synthesized and their physicochemical properties, that is, high thermal and electrochemical stability, low viscosity, and high conductivity, have been compared to the properties of related [EMIm]+ -RTILs. The influence of the different alkyl substituents at boron has been assessed. The exemplary study on the properties with the [EMIm]+ -ILs with the mixed water stable alkylcyanoborate anions points towards the potential of these fluorine-free borate anions, in general.

The Interplay of Weakly Coordinating Anions and the Mechanical Bond: A Systematic Study of the Explicit Influence of Counterions on the Properties of (Pseudo)rotaxanes

Weakly coordinating anions (WCAs) have attracted much attention in recent years due to their ability to stabilise highly reactive cations. It may well be argued, however, that a profound understanding of what truly defines a WCA is still lacking, and systematic studies to unravel counterion effects are scarce. In this work, we investigate a supramolecular pseudorotaxane formation reaction, subject to a selection of anions, ranging from strongly to weakly coordinating, which not only aids in fostering our knowledge about anion coordination properties, but also provides valuable theoretical insight into the nature of the mechanical bond. We employ state-of-the-art DFT-based methods and tools, combined with isothermal calorimetry and 1H NMR experiments, to compute anion-dependent Gibbs free association energies ΔGa, as well as to evaluate intermolecular interactions. We find correlations between ΔGa and the anions’ solvation energies, which are exploited to calculate physico-chemical reaction parameters in the context of coordinating anions. Furthermore, we show that the binding situation within the (pseudo)rotaxanes can be mostly understood by straight-forward electrostatic considerations. However, quantum-chemical effects such as dispersion and charge-transfer interactions become more and more relevant when WCAs are employed.

Shedding light on the physical nature of ion pair interactions involving carba-closo-dodecaborate anions. Insights from computation

closo-Carboranes are structures that have been studied for many decades due to their vast applicability in medicine, catalysis, and energy storage. In order to go deeper into the physics behind the interactions of oppositely charged ions, which have potential applications in electrical energy storage and conversion devices, the present work aims to shed light on the physical nature of the interactions involving (R-CB₁₁H₁₁^-, R = H, F, CH₃, CF₃) and M^q+ = Li⁺, Na⁺, Mg²⁺, Zn²⁺ ions. The bonding situations are evaluated in light of both canonical Kohn-Sham energy decomposition, EDA-NOCV, and local energy decomposition, LED, analyses. Electron and hole transports are also evaluated through charge transfer integrals. The findings reveal that such complexes present not only more significant electrostatic, but also non-negligible orbital contributions. Both energy decomposition analyses EDA-NOCV and DLPNO-LED confirm that the strength of ionic pair interactions (R-CB₁₁H₁₁^- ↔ M^q+) is much more dependent on the nature of the cation employed than on the substituent R used. The results also reveal that charge transfers are not significant in such interacting systems.

The crystal structure of trimethylsulfonium tris(trifluoromethylsulfonyl)methanide, C7H9F9O6S4

C7H9F9O6S4, orthorhombic, P212121 (no. 19), a = 8.80180(10) Å, b = 10.96580(10) Å, c = 16.91360(10) Å, V = 1632.48(3) Å3, Z = 4, R gt (F) = 0.0222, wRref (F 2) = 0.0604, T = 100 K.

Solvation-Free Li+ Lewis Acid Enhancing Reaction: Kinetic Study of [5,6]-Li+@PCBM to [6,6]-Li+@PCBM

Kinetic parameters for the [5,6]- to [6,6]-[Li⁺@PCBM]TFSI^- transformation were determined experimentally, revealing a ca. 700-fold faster reaction rate at 423 K than empty PCBM and a 57.4 kJ mol^-1 lower activation energy. The encapsulated Li⁺ can be considered as solvation-free Li⁺, forming a 1:1 complex with the substrate.

Innovative Syntheses of Cyano(fluoro)borates: Catalytic Cyanation, Electrochemical and Electrophilic Fluorination

Different types of high-yield, easily scalable syntheses for cyano(fluoro)borates Kt[BFn (CN)4-n ] (n=0-2) (Kt=cation), which are versatile building blocks for materials applications and chemical synthesis, have been developed. Tetrafluoroborates react with trimethylsilyl cyanide in the presence of metal-free Brønsted or Lewis acid catalysts under unprecedentedly mild conditions to give tricyanofluoroborates or tetracyanoborates. Analogously, pentafluoroethyltrifluoroborates are converted into pentafluoroethyltricyanoborates. Boron trifluoride etherate, alkali metal salts, and trimethylsilyl cyanide selectively yield dicyanodifluoroborates or tricyanofluoroborates. Fluorination of cyanohydridoborates is the third reaction type that includes direct fluorination with, for example, elemental fluorine, stepwise halogenation/fluorination reactions, and electrochemical fluorination (ECF) according to the Simons process. In addition, fluorination of [BH(CN)2 {OC(O)Et}]- to result in [BF(CN)2 {OC(O)Et}]- is described.

Two heads are better than one: improving magnetic relaxation in the dysprosium metallocene upon dimerization by use of an exceptionally weakly-coordinating anion

Partial metathesis between two weakly-coordinating anions in the archetypical dysprosium metallocene results in the first example of [BPh4]- as a bridging ligand in 4f metals, with a unique η2,η2:η2,η2-bridge. Magnetic susceptibility and relaxation dynamics studies along with ab initio calculations reveal improved slow relaxation of the magnetization in over its mononuclear congener, resulting in an energy barrier of 490 K/340 cm-1 and waist-restricted hysteresis up to 6.5 K.

Mechanism of Olefin Metathesis with Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes

A series of neutral molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate and monotriflate monoalkoxide complexes as well as cationic molybdenum imido alkylidene triflate complexes have been subjected to NMR spectroscopic, X-ray crystallographic, and reaction kinetic measurements in order to gain a comprehensive understanding about the underlying mechanism in olefin metathesis of this new type of catalysts. On the basis of experimental evidence and on DFT calculations (BP86/def2-TZVP/D3/cosmo) for the entire mechanism, olefinic substrates coordinate trans to the NHC of neutral 16-electron complexes via an associative mechanism, followed by dissociation of an anionic ligand (e.g., triflate) and formation of an intermediary molybdacyclobutane trans to the NHC. Formation of a cationic complex is crucial in order to become olefin metathesis active. Variations in the NHC, the imido, the alkoxide, and the noncoordinating anion revealed their influence on reactivity. The reaction of neutral 16-electron complexes with 2-methoxystyrene is faster for catalysts bearing one triflate and one fluorinated alkoxide than for catalysts bearing two triflate ligands. This is also reflected by the Gibbs free energy values for the transition states, ΔG^‡₃₀₃, which are significantly lower for catalysts bearing only one triflate than for the corresponding bistriflate complexes. Reaction of a solvent-stabilized cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) monotriflate complex with 2-methoxystyrene proceeded via an associative mechanism too. Reaction rates of both solvent-free and solvent-stabilized cationic Mo imido alkylidene NHC catalysts with 2-methoxystyrene are controlled by the cross-metathesis step but not by adduct formation.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

Syntheses, characterisation and solid-state study of alkali and ammonium BArF salts

Syntheses, characterisation in solution and solid-state studies of alkali and ammonium BArF salts are reported.

Why can a gold salt react as a base?

A new mass-spectrometry method allows monitoring the key role of the gold–gold interaction in the transformation of gold salts to bases.

Potassium tris(triflyl)methide (KCTf3): a broadly applicable promoter for cationic metal catalysis

KCTf₃ enhanced the reaction rates and the chemical yields of a wide spectrum of cationic metal catalyzed reactions in consistent and significant manner. These reactions include traditional Lewis acid catalysis and transition metal catalysis. KCTf₃ is an easily handled neutral salt that is commercially available.

A systematic investigation of coinage metal carbonyl complexes stabilized by fluorinated alkoxy aluminates

The reaction of Cu(I), Ag(I), and Au(I) salts with carbon monoxide in the presence of weakly coordinating anions led to known and structurally unknown non-classical coinage metal carbonyl complexes [M(CO)n][A] (A = fluorinated alkoxy aluminates). The coinage metal carbonyl complexes [Cu(CO)n(CH2Cl2)m](+)[A](-) (n = 1, 3; m = 4-n), [Au2(CO)2Cl](+)[A](-), [(OC)nM(A)] (M = Cu: n = 2; Ag: n = 1, 2) as well as [(OC)3Cu⋅⋅⋅ClAl(OR(F))3] and [(OC)Au⋅⋅⋅ClAl(OR(F))3] were analyzed with X-ray diffraction and partially IR and Raman spectroscopy. In addition to these structures, crystallographic and spectroscopic evidence for the existence of the tetracarbonyl complex [Cu(CO)4](+)[Al(OR(F))4](-) (R(F) = C(CF3)3) is presented; its formation was analyzed with the help of theoretical investigations and Born-Fajans-Haber cycles. We discuss the limits of structure determinations by routine X-ray diffraction methods with respect to the C-O bond lengths and apply the experimental CO stretching frequencies for the prediction of bond lengths within the carbonyl ligand based on a correlation with calculated data. Moreover, we provide a simple explanation for the reported, partly confusing and scattered CO stretching frequencies of [Cu(I)(CO)n] units.

Structure, stability and spectral signatures of monoprotic carborane acid-water clusters (CBW(n), where n = 1-6)

The gas phase structure, stability, spectra, and proton transfer properties of monoprotic carborane acid-water clusters [CB(11)F(m)H(11-m)(OH(2))(1)]-(H(2)O)(n) (where m = 0, 5, and 10; n = 1-6) have been calculated using density functional theory (DFT) with the Becke's three-parameter hybrid exchange functional and Lee-Yang-Parr correlation functional (B3LYP) using 6-31+G* basis set. Results reveal that Eigen cation defects are found in CBW(n) (where n = 2-6) clusters and these clusters are significantly more stable than the non-Eigen geometry. In addition to the conventional hydrogen bond (H-bond) the role of dihydrogen bond (DHB) and halogen bond (XB) in the stabilization of these clusters can be observed from the molecular graphs derived from the atoms in molecules (AIM) analysis. Spectral information shows the features of Eigen cation and proton oscillation involved in the proton transfer process. The dissociation of proton from the perfluoro derivatives with two water molecules is more favorable when compared to the other derivatives.

1-Bromo-1-lithioethene: a practical reagent in organic synthesis

[reaction: see text] A reliable preparative scale synthesis of 1-bromo-1-lithioethene is reported. This reagent undergoes clean 1,2-addition with a range of aldehydes and ketones at -110 degrees C to afford the corresponding 2-bromo-1-alken-3-ols in moderate to excellent yield. Trapping with other electrophiles (acylsilanes, chlorosilanes, tributyltin chloride, iodine) cleanly provides practically useful yields of various 1-substituted 1-bromoethene products. Unexpectedly high diastereoselectivities were observed during the addition of 1-bromo-1-lithioethene to alpha-siloxy aldehydes (typically 10:1, Felkin-Ahn control) and protected ketopyranose and ketofuranose sugars (> or = 10:1, addition from the less-hindered face). The title organolithium reagent possesses relatively low basicity at low temperature, and is compatible with a variety of common protecting groups. We believe that these unusual properties of 1-bromo-1-lithioethene may originate from the specific crystalline structure of the reagent in which lithium is coordinatively saturated and thus unavailable for chelation.

The role of halogenated carborane monoanions in olefin hydrogenation catalysed by cationic iridium phosphine complexes

Iridium hydridophosphine complexes of general formula [Ir(PR3)2H2(anion)](PR3= PPh3, PMe2Ph; anion =[1-closo-CB(11)H(6)Cl(6)]-, [1-closo-CB(11)H(6)I(6)]-, [BAr(F)4]-) have been prepared by hydrogenation of cyclooctadiene precursor complexes. Solid-state structures of selected examples of these complexes reveal intimate contacts between the carborane anion and cation, with the anion binding through two lower-hemisphere halogen ligands. In CD2Cl2 solution the very weakly coordinating anions [1-closo-CB(11)H(6)Cl(6)]- and [BAr(F)4]- are suggested to favour the formation of solvent complexes such as [Ir(PR3)2H2(solvent)n][anion], while the [1-closo-CB(11)H(6)I(6)]- anion forms a tightly bound complex with the cationic iridium fragment. Calculated DeltaG values for anion reorganisation in d8-toluene reflect this difference in interaction between the anions and cation. With the bulky anion [1-closo-CB(11)Me(5)I(6)]- different complexes are formed: Ir(PPh3)H2(1-closo-HCB(11)Me(5)I(6)) and [(PPh3)3Ir(H2)H2][1-closo-HCB(11)Me(5)I(6)] which have been characterised spectroscopically. Diffusion measurements in CD2Cl2 are also consistent with larger, solvent coordinated, complexes for the more weakly coordinating anions and a tighter interaction between anion and cation for [1-closo-CB(11)H(6)I(6)]-. All the complexes show some ion-paring in solution. Comparison with data previously reported for the [1-closo-CB(11)H(6)Br(6)]- anion shows that this anion--as expected--fits between [1-closo-CB(11)H(6)Cl(6)]- and [1-closo-CB(11)H(6)I(6)]- in terms of coordinating ability. Although not coordinating, the large [1-closo-CB(11)H(6)Cl(6)]- and [BAr(F)4)]- anions do provide some stabilisation towards the metal centre, as decomposition to the hydride bridged dimer [Ir2(PPh3)4H5]+ is retarded. This is in contrast to the [PF6]- salt where decomposition is immediate. As expected, complexes with the smaller phosphine PMe2Ph form tighter interactions with the carborane anions. These observations on the interaction between anion and cation in solution are reflected in benchmark hydrogenation studies that show a significant attenuation in rate of hydrogenation of cyclohexane on using the [1-closo-CB(11)H(6)I(6)]- anion or complexes with the PMe2Ph phosphine. We also comment on the reusability of the catalysts and their tolerance to water and oxygen impurities. Overall the catalyst with the [1-closo-CB(11)H(6)Br(6)]- anion shows the best combination of rate of hydrogenation, reusability and tolerance to impurities.