“Umpolung” von Bor im [B(CN)4]− Anion durch Reduktion zum Dianion [B(CN)3]2−

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.

Cyano(fluoro)borate and cyano(hydrido)borate ionic liquids: low-viscosity ionic media for electrochemical applications

The synthesis and detailed characterization of low-viscosity room-temperature ionic liquids (RTILs) and [BnPh₃P]⁺ salts with the cyano(fluoro)borate anions [BF(CN)₃]^- (MFB), [BF₂(CN)₂]^- (DFB), and [BF₃(CN)]^- as well as the new mixed-substituted anion [BFH(CN)₂]^- (FHB) is described. The RTILs with [EMIm]⁺ or [BMPL]⁺ as countercations were obtained in yields of up to 98% from readily available alkali metal salts and in high purities that allow application in electrochemical devices. Trends in thermal stability, melting and freezing behavior, density, electrochemical stability, dynamic viscosity, specific conductivity and ion diffusivity have been assessed and compared to those of the related tetracyanoborate- and cyano(hydrido)borate-RTILs. The crystal structure analysis of the [BnPh₃P]⁺ salts of [BF_n(CN)_4-n]^- (n = 0-4), [BH_n(CN)_4-n]^- (n = 1-3) and [BFH(CN)₂]^- provided experimental access to anion volumina that together with ion molecular mass, electrostatic potential, shape and chemical stability have been correlated to physicochemical properties. In addition, the cytotoxicity of the [EMIm]⁺-ILs and potassium or sodium salts was studied.

Tricyanoborane-Functionalized Anionic N-Heterocyclic Carbenes: Adjustment of Charge and Stereo-Electronic Properties

The 1-methyl-3-(tricyanoborane)imidazolin-2-ylidenate anion (2) was obtained in high yield by deprotonation of the B(CN)3 -methylimidazole adduct 1. Regarding charge and stereo-electronic properties, anion 2 closes the gap between well-known neutral NHCs and the ditopic dianionic NHC, the 1,3-bis(tricyanoborane)imidazolin-2-ylidenate dianion (IIb). The influence of the number of N-bonded tricyanoborane moieties on the σ-donating and π-accepting properties of NHCs was assessed by quantum chemical calculations and verified by experimental data on 2, IIb, and 1,3-dimethylimidazolin-2-ylidene (IMe, IIa). Therefore NHC 2, which acts as a ditopic ligand via the carbene center and the cyano groups, was reacted with alkyl iodides, selenium, and [Ni(CO)4 ] yielding alkylated imidazoles 3 and 4, the anionic selenium adduct 5, and the anionic nickel tricarbonyl complex 8, respectively. The results of this study prove that charge, number of coordination sites, buried volume (%Vbur ) and σ-donor and π-acceptor abilities of NHCs can be effectively fine-tuned via the number of tricyanoborane substituents.

Dismutation of Tricyanoboryllead Compounds: The Homoleptic Tetrakis(tricyanoboryl)plumbate Tetraanion

A series of unprecedently air-stable (tricyanoboryl)plumbate anions was obtained by the reaction of the boron-centered nucleophile B(CN)3 2- with triorganyllead halides. Salts of the anions [R3 PbB(CN)3 ]- (R=Ph, Et) were isolated and found to be stable in air at room temperature. In the case of Me3 PbHal (Hal=Cl, Br), a mixture of the anions [Me4-n Pb{B(CN)3 }n ]n- (n=1, 2) was obtained. The [Et3 PbB(CN)3 ]- ion undergoes stepwise dismutation in aqueous solution to yield the plumbate anions [Et4-n Pb{B(CN)3 }n ]n- (n=1-4) and PbEt4 as by-product. The reaction rate increases with decreasing pH value of the aqueous solution or by bubbling O2 through the reaction mixture. Adjustment of the conditions allowed the selective formation and isolation of salts of all anions of the series [Et4-n Pb{B(CN)3 }n ]n- (n=2-4) including the homoleptic tetraanion [Pb{B(CN)3 }4 ]4- .

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.

The 9H-9-Borafluorene Dianion: A Surrogate for Elusive Diarylboryl Anion Nucleophiles

Double reduction of the THF adduct of 9H-9-borafluorene (1⋅THF) with excess alkali metal affords the dianion salts M2 [1] in essentially quantitative yields (M=Li-K). Even though the added charge is stabilized through π delocalization, [1]2- acts as a formal boron nucleophile toward organoboron (1⋅THF) and tetrel halide electrophiles (MeCl, Et3 SiCl, Me3 SnCl) to form B-B/C/Si/Sn bonds. The substrate dependence of open-shell versus closed-shell pathways has been investigated.

Hydroxytricyanoborate Anion: Synthetic Aspects and Structural, Chemical, and Spectroscopic Properties

In recent years, salts of the hydridotricyanoborate anion [BH(CN)₃]^- (MHB) have become readily available. In spite of the unusually high stability of the MHB anion, it can be used as a valuable starting material for the preparation of selected tricyanoborates, for example, the boron-centered nucleophile B(CN)₃^2-. A further unprecedented example is the hydroxytricyanoborate anion [B(OH)(CN)₃]^- that is accessible by oxidation of (H₃O)MHB with elemental bromine in water. The Brønsted acid (H₃O)[B(OH)(CN)₃] was isolated as a crystalline solid. It serves as a versatile starting material for the synthesis of coordination compounds, metal salts, and ionic liquids. The [B(OH)(CN)₃]^- anion shows a rich coordination chemistry and a high tendency to form hydrogen-bonded motifs as demonstrated by a series of salts with different types of cations. Furthermore, the [B(OH)(CN)₃]^- anion itself serves as starting material for new tricyanoborates such as the unusual trianion [B{OB(CN)₃}₃]^3- and the silylated anions [B(OSiR₃)(CN)₃]^- (R = Me, Et, Ph). Some of these follow-up products have been characterized by single-crystal X-ray diffraction, e.g., [nBu₄N]₃[B{OB(CN)₃}₃] and [nBu₄N][B(OSiPh₃)(CN)₃].

Realization of Lewis Basic Sodium Anion in the NaBH3 - Cluster

We report a Na:^- →B dative bond in the NaBH₃ ^- cluster, which was designed on the principle of minimum-energy rupture, prepared by laser vaporization, and characterized by a synergy of anion photoelectron spectroscopy and electronic structure calculations. The global minimum of NaBH₃ ^- features a Na-B bond. Its preferred heterolytic dissociation conforms with the IUPAC definition of dative bond. The lone electron pair revealed on Na and the negative Laplacian of electron density at the bond critical point further confirm the dative nature of the Na-B bond. This study represents the first example of a Lewis adduct with an alkalide as the Lewis base.

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

The potassium perfluoroalkyltricyanoborates K[C_n F_2 n+1 B(CN)₃ ] [n=1 (1 d), 2 (2 d)] and the potassium mono(perfluoroalkyl)cyanofluoroborates K[C_n F_2 n+1 BF(CN)₂ ] [n=1 (1 c), 2 (2 c)] and [C_n F_2 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[C_n F_2 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.

Borylenes: An Emerging Class of Compounds

Free borylenes (R-B:) have only been spectroscopically characterized in the gas phase or in matrices at very low temperatures. However, in recent years, a few mono- and bis(Lewis base)-stabilized borylenes have been isolated. In both of these compounds the boron atom is in the formal oxidation state +I which contrasts with classical organoboron derivatives wherein the element is in the +III oxidation state. Mono(Lewis base)-stabilized borylenes are isoelectronic with singlet carbenes, and their reactivity mimics to some extent that of transition metals. They can activate small molecules, such as H₂ , and coordinate an additional ligand; in other words, they are boron metallomimics. Bis(Lewis base)borylene adducts are isoelectronic with amines and phosphines. In contrast to boranes, which act as electron acceptors and thus Lewis acids, they are electron-rich and act as ligands for transition metals.

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)₃^2- 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)₃^2- 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.

Dative Bonding between Group 13 Elements Using a Boron-Centered Lewis Base

An electron-rich monovalent boron compound is used as a Lewis base to prepare adducts with Group 13 Lewis acids using both its boron and nitrogen sites. The hard Lewis acid AlCl3 binds through a nitrogen atom of the Lewis base, while softer Lewis acids GaX3 (Cl, Br, I) bind at the boron atom. The latter are the first noncluster Lewis adducts between a boron-centered Lewis base and a main-group Lewis acid.

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

Tricyanoborates M₂B(CN)₃ (M = Na, K) are accessible by reduction of tricyanofluoroborates with alkali metals in liquid ammonia or in THF–naphthalene.

Deprotonation of a borohydride: synthesis of a carbene-stabilized boryl anion

An acidic hydride! Thanks to the presence of a π-acceptor cyclic alkyl amino carbene and of two electron-withdrawing nitrile groups, a borohydride reacts with a base to give a carbene-stabilized boryl anion, which reacts with carbon and metal electrophiles at the boron center. Dipp = 2,6-diisopropylphenyl, KHMDS = potassium bis(trimethylsilyl)amide.