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.

Isolated [B2(CN)6]2-: Small Yet Exceptionally Stable Nonmetal Dianion.

We report the observation of a small, yet remarkably stable, metal-free hexacyanodiborate dianion [B2(CN)6]2- in the gas phase. Negative ion photoelectron spectroscopy (NIPES) was employed to measure its spectra at multiple laser wavelengths, yielding a 1.9 eV electron binding energy (EBE) ─a remarkably high value of electronic stability and a ∼2.60 eV repulsive Coulomb barrier (RCB) for electron detachment. This rationalizes the observation of this dianion, although homolytic charge-separation dissociation into two [B(CN)3]•- is energetically favorable. Quantum chemical calculations demonstrate a D3d staggered conformation for both the dianion and radical monoanion, and the calculated EBE and RCB match the experimental values well. The simulated density of states spectrum reproduces all measured electronic transitions, while the simulated vibrational progressions for the ground state transition cover a much narrower EBE range compared to the experimental band, indicating appreciable auto-photodetachment via electronically excited dianion resonances.

Stable and Storable N(CF3 )2 Transfer Reagents.

Fluorinated groups are essential for drug design, agrochemicals, and materials science. The bis(trifluoromethyl)amino group is an example of a stable group that has a high potential. While the number of molecules containing perfluoroalkyl, perfluoroalkoxy, and other fluorinated groups is steadily increasing, examples with the N(CF3 )2 group are rare. One reason is that transfer reagents are scarce and metal-based storable reagents are unknown. Herein, a set of CuI and AgI bis(trifluoromethyl)amido complexes stabilized by N- and P-donor ligands with unprecedented stability are presented. The complexes are stable solids that can even be manipulated in air for a short time. They are bis(trifluoromethyl)amination reagents as shown by nucleophilic substitution and Sandmeyer reactions. In addition to a series of benzylbis(trifluoromethyl)amines, 2-bis(trifluoromethyl)amino acetate was obtained, which, upon hydrolysis, gives the fluorinated amino acid N,N-bis(trifluoromethyl)glycine.

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.

Cyanohydridoborate Anions: Synthesis, Salts, and Low-Viscosity Ionic Liquids.

High-yield syntheses up to molar scales for salts of [BH(CN)3 ]- (2) and [BH2 (CN)2 ]- (3) starting from commercially available Na[BH4 ] (Na5), Na[BH3 (CN)] (Na4), BCl3 , (CH3 )3 SiCN, and KCN were developed. Direct conversion of Na5 into K2 was accomplished with (CH3 )3 SiCN and (CH3 )3 SiCl as a catalyst in an autoclave. Alternatively, Na5 is converted into Na[BH{OC(O)R}3 ] (R=alkyl) that is more reactive towards (CH3 )3 SiCN and thus provides an easy access to salts of 2. Some reaction intermediates were identified, for example, Na[BH(CN){OC(O)Et}2 ] (Na7 b) and Na[BH(CN)2 {OC(O)Et}] (Na8 b). A third entry to 2 and 3 uses ether adducts of BHCl2 or BH2 Cl such as the commercial 1,4-dioxane adducts that react with KCN and (CH3 )3 SiCN. Alkali metal salts of 2 and 3 are convenient starting materials for organic salts, especially for low viscosity ionic liquids (ILs). [EMIm]3 has the lowest viscosity and highest conductivity with 10.2 mPa s and 32.6 mS cm-1 at 20 °C known for non-protic ILs. The ILs are thermally, chemically, and electrochemically robust. These properties are crucial for applications in electrochemical devices, for example, dye-sensitized solar cells (Grätzel cells).

Perfluoroalkyltricyanoborate and Perfluoroalkylcyanofluoroborate Anions: Building Blocks for Low-Viscosity Ionic Liquids.

The potassium perfluoroalkyltricyanoborates K[Cn F2 n+1 B(CN)3 ] [n=1 (1 d), 2 (2 d)] and the potassium mono(perfluoroalkyl)cyanofluoroborates K[Cn F2 n+1 BF(CN)2 ] [n=1 (1 c), 2 (2 c)] and [Cn F2 n+1 BF2 (CN)]- [n=1 (1 b), 2 (2 b), 3 (3 b), 4 (4 b)] are accessible with perfect selectivities on multi-gram scales starting from K[Cn F2 n+1 BF3 ] and Me3 SiCN. The K+ salts are starting materials for the preparation of salts with organic cations, for example, [EMIm]+ (EMIm=1-ethyl-3-methylimidazolium). These [EMIm]+ salts are hydrophobic room-temperature ionic liquids (RTILs) that are thermally, chemically and electrochemically very robust, offering electrochemical windows up to 5.8 V. The RTILs described herein, exhibit very low viscosities with a minimum of 14.0 mPa s at 20 °C for [EMIm]1 c, low melting points down to -57 °C for [EMIm]2 b and extraordinary high conductivities up to 17.6 mS cm-1 at 20 °C for [EMIm]1 c. The combination of these properties makes these ILs promising materials for electrochemical devices as exemplified by the application of selected RTILs as component of electrolytes in dye-sensitised solar cells (DSSCs, Grätzel cells). The efficiency of the DSSCs was found to increase with a decreasing viscosity of the neat ionic liquid. In addition to the spectroscopic characterisation, single crystals of the potassium salts of the anions 1 b-d, 2 d, 3 b and 4 c as well as of [nBu4 N]2 c have been studied by X-ray diffraction.

Borylation of fluorinated arenes using the boron-centred nucleophile B(CN)32- - a unique entry to aryltricyanoborates.

The potassium salt of the boron-centred nucleophile B(CN)32- (1) readily reacts with perfluorinated arenes, such as hexafluorobenzene, decafluorobiphenyl, octafluoronaphthalene and pentafluoropyridine, which results in KF and the K+ salts of the respective borate anions with one {B(CN)3} unit bonded to the (hetero)arene. An excess of K21 leads to the successive reaction of two or, in the case of perfluoropyridine, even three C-F moieties and the formation of di- and trianions, respectively. Moreover, all of the 11 partially fluorinated benzene derivatives, C6F6-n H n (n = 1-5), generally react with K21 to give new tricyano(phenyl)borate anions with high chemo- and regioselectivity. A decreasing number of fluorine substituents on benzene results in a decrease in the reaction rate. In the cases of partially fluorinated benzenes, the addition of LiCl is advantageous or even necessary to facilitate the reaction. Also, pentafluorobenzenes R-C6F5 (R = -CN, -OMe, -Me, or -CF3) react via C-F/C-B exchange that mostly occurs in the para position and to a lesser extent in the meta or ortho positions. Most of the reactions proceed via an SNAr mechanism. The reaction of 1,4-F2C6H4 with K21 shows that an aryne mechanism has to be considered in some cases as well. In summary, a wealth of new stable tricyano(aryl)borates have been synthesised and fully characterized using multi-NMR spectroscopy and most of them were characterised using single-crystal X-ray diffraction.

Dynamic Disorder and Electronic Structures of Electron-Precise Dianionic Diboranes: Insights from Solid-State Multinuclear Magnetic Resonance Spectroscopy.

The J(11B,11B) coupling constants of various salts of the electron-precise hexacyanodiborane(6) dianion, [B2(CN)6]2-, were obtained using 11B double-quantum-filtered (DQF) J-resolved solid-state nuclear magnetic resonance (SSNMR) spectroscopy. Our results show that the magnitude of the DQF J splitting is influenced by both the crystallographic symmetry of the system and the presence of dynamics. The splittings are amplified by a factor of 3 as compared to the corresponding theoretical J coupling constants for cases where (1) there is an absence of dynamics but the boron pairs are crystallographically equivalent or (2) the boron pairs are crystallographically inequivalent but are rendered magnetically equivalent on the time scale of the experiment due to dynamic disorder, which was identified by 11B and 13C SSNMR experiments. Consequently, molecular motions need to be taken into consideration when interpreting the results of DQF J-resolved experiments, and conversely, these experiments may be used to identify dynamic disorder. Variable-temperature NMR data support the notion of three different motional processes with correlation times ranging from 102 to 106 s-1 over the temperature range of 248-306 K. When molecular motion and crystallographic symmetry are both accounted for, the J(11B,11B) coupling constants for various [B2(CN)6]2- salts were measured to range from 29.4 to 35.8 Hz, and their electronic origins were determined using natural localized molecular orbital and natural bond orbital analyses. The coupling constants were found to strongly correlate to the hybridization states of the boron orbitals that form the B-B bonds and to the strength of the B-B bonds. This study provides a novel tool to study dynamics in ordered and disordered solids and provides new perspectives on electron-precise dianionic diboranes featuring two-center-two-electron bonds in the context of related compounds featuring multiply and singly bonded boron spin pairs.

Anhydrous, Homoleptic Lanthanide Frameworks with the Pentafluoroethyltricyanoborate Anion.

Pentafluoroethyltricyanoborate frameworks of rare-earth metal ions of the general formula [Ln{C2F5B(CN)3}3(OH2)n] (Ln = La, Eu, Ho; n = 0, 3; [Ln13(OH2)n]) were synthesized using the oxonium salt (H3O)[C2F5B(CN)3] ((H3O)1) and lanthanide chlorides LnCl3·nH2O as starting compounds. Single-crystals of ∞3[La{C2F5B(CN)3}3] (∞3[La13]) are obtained from the room temperature ionic liquid (RTIL) [EMIm]1 using either a ionothermal approach or by recrystallization of anhydrous microcrystalline [La13] that is obtained from reactions in aqueous media after drying in a vacuum. Removal of water from [Ln13(OH2)3] (Ln = Eu, Ho) to give microcrystalline ∞3[Ln13] is achieved in a vacuum at elevated temperatures. All compounds were characterized by vibrational and NMR spectroscopy, thermogravimetry, and elemental analysis. The structures of the three-dimensional coordination polymers ∞3[Eu13(OH2)3] and ∞3[La13] were elucidated by single-crystal X-ray-diffraction. According to powder diffraction studies on anhydrous ∞3[Ln13] (Ln = La, Eu, Ho), the three compounds are isotypic. A study of the photoluminescence properties reveals that both Eu3+ compounds, [Eu13] and [Eu13(OH2)3], are strongly luminescent, the emission of the anhydrous framework being significantly more intense than the one of the hydrate. The Eu-compounds benefit from a sensitizer effect of the anion. In contrast, the Ho-containing framework ∞3[Ho13] exhibits separate chromophores and a strong reabsorption of the fluorescence by the Ho3+ ions.

Deprotonation of a Hydridoborate Anion.

The first deprotonation of a borohydride anion was achieved by treatment of [BH(CN)3 ]- with strong non-nucleophilic bases, which resulted in the formation of alkali-metal salts of the tricyanoborate dianion B(CN)32- in up to 97 % yield and 99.5 % purity. [BH(CN)3 ]- is less acidic than (Me3 Si)2 NH but a stronger acid than iPr2 NH. Less sterically hindered, more nucleophilic bases such as PhLi and MeLi mostly attack a CN group under formation of imine dianions [RC(N)B(CN)3 ]2- , which can be hydrolyzed to ketones of the [RC(O)B(CN)3 ]- type. The boron-centered nucleophile B(CN)32- reacts with CO2 and CN+ reagents to give salts of the [B(CN)3 CO2 ]2- dianion and the tetracyanoborate anion [B(CN)4 ]- , respectively, in excellent yields.

Synthesis of the fluorohydridoborate anions [BHF3]- and [1-HF2B-9,12-X2-closo-1,2-C2B10H9]- (X = H, I): deboronation of 1,2- and 1,7-dicarba-closo-dodecaboranes with anhydrous [Me4N]F.

Deboronation of closo-1,7-C2B10H12 and 1-R1-closo-1,2-C2B10H12 (R1 = Me, Ph) with anhydrous [Me4N]F yields [Me4N][BHF3] that is the first structurally characterized anion of the series [BHxF4-x]- (x = 1-3). Deboronation of 9,12-R22-closo-1,7-C2B10H12 (R2 = H, I) gives a mixture of [Me4N][BHF3] and [Me4N][1-HF2B-9,12-R22-closo-1,2-C2B10H9].

The hexacyanodiborane(6) dianion [B2(CN)6](2-).

Diborane(6) dianions with substituents that are bonded to boron via carbon are very reactive and therefore only a few examples are known. Diborane(6) derivatives are the simplest catenated boron compounds with an electron-precise B-B σ-bond that are of fundamental interest and of relevance for material applications. The homoleptic hexacyanodiborane(6) dianion [B2 (CN)6 ](2-) that is chemically very robust is reported. The dianion is air-stable and resistant against boiling water and anhydrous hydrogen fluoride. Its salts are thermally highly stable, for example, decomposition of (H3 O)2 [B2 (CN)6 ] starts at 200 °C. The [B2 (CN)6 ](2-) dianion is readily accessible starting from 1) B(CN)3 (2-) and an oxidant, 2) [BF(CN)3 ](-) and a reductant, or 3) by the reaction of B(CN)3 (2-) with [BHal(CN)3 ](-) (Hal=F, Br). The latter reaction was found to proceed via a triply negatively charged transition state according to an SN 2 mechanism.

Syntheses of tricyanofluoroborates M[BF(CN)3] (M = Na, K): (CH3)3SiCl catalysis, countercation effect, and reaction intermediates.

Potassium tricyanofluoroborate, K[BF(CN)3], which is the starting material for tricyanofluoroborate room-temperature ionic liquids [N. Ignat'ev et al. J. Fluorine Chem., submitted] was obtained on a molar scale (140 g) from Na[BF4] and (CH3)3SiCN with a purity of up to 99.9%. The initial product of the reaction that was catalyzed by (CH3)3SiCl was Na[BF(CN)3]·(CH3)3SiCN that was characterized by multinuclear NMR and vibrational spectroscopy, elemental analysis, differential scanning calorimetry, and single-crystal X-ray diffraction. Na[BF(CN)3]·(CH3)3SiCN was converted to K[BF(CN)3] via a simple extraction protocol. The catalytic effect of (CH3)3SiCl was evaluated and some intermediates of the reaction, including the isocyanoborate anion [BF(NC)(CN)2](-), were identified using multinuclear NMR and vibrational spectroscopy. K[BF2(CN)2] also reacted with (CH3)3SiCN in the presence of (CH3)3SiCl, to result in K[BF(CN)3]. The interpretation of the experimental observations was supported by data derived from density functional theory (DFT) calculations. In addition, the influence of selected countercations of the tetrafluoroborate anion on the progress of the (CH3)3SiCl-catalyzed reaction was studied. The fastest reaction was observed for Na[BF4], while the conversion of [BF4](-) to [BF(CN)3](-) was slower with the countercation K(+). Li[BF4] and [Et4N][BF4] were converted under the reaction conditions applied to Li[BF2(CN)2] and [Et4N][BF2(CN)2] only.

Convenient access to the tricyanoborate dianion B(CN)3²â» and selected reactions as a boron-centred nucleophile.

Alkali metal tricyanoborates M2B(CN)3 (M = Na, K) are accessible by the reaction of tricyanofluoroborates with alkali metals (i) in liquid NH3 or (ii) in THF-naphthalene. The M2B(CN)3 are versatile starting materials for the synthesis of K[RB(CN)3] (R = Et, C6F5, CH2=CHCH2).