Trialkylammoniododecaborates: Anions for Ionic Liquids with Potassium, Lithium and Protons as Cations

Derivatization of Undecahalogenated closo-Dodecaborates [B12X11NH3]- (X = F-I): Attaching Isocyanate, Amidinium, and Formamide Functionalities

Halogenated closo-dodecaborates are very robust and versatile weakly coordinating anions for numerous applications. The introduction of additional substituents, e.g., pseudohalides, allows the tuning of their chemical and physical properties. In this report, the synthesis of the isocyanate-substituted closo-dodecaborates [B12X11(NCO)]2- (X = H, F-I) was investigated. In an attempt to synthesize the undecahalogenated derivatives, a selective and halogen-dependent reaction yielding boron clusters carrying the functional groups amidinium (-NHCHNMe2) and formamide (-NHC(O)H) was discovered. The halogenated anions were fully characterized by vibrational and NMR spectroscopy, mass spectrometry, and X-ray diffraction. Salts of the formamide-substituted anion [B12X11(NHC(O)H)]2- are surprisingly thermally stable in the condensed phase. In contrast, collision-induced dissociation in the gas phase reveals that the isolated dianion [B12X11(NHC(O)H)]2- in the gas phase preferentially loses water, while the protonated form, which was generated from decomposition of the tetraalkylammonium counterion [B12X11(NHC(O)H)H]-, tends to lose carbon monoxide. Possible reaction mechanisms are discussed.

Vertex Differentiation Strategy for Tuning the Physical Properties of closo-Dodecaborate Weakly Coordinating Anions

We report the synthesis and characterization of various compounds containing the 1,7,9-hydroxylated closo-dodecahydrododecaborate (B12H9(OH)32-) cluster motif. Specifically, we show how the parent compound can be synthesized on the multigram scale and further perhalogenated, leading to a new class of vertex-differentiated weakly coordinating anions. We show that a postmodification of the hydroxyl groups by alkylation affords further opportunities for tailoring these anions' stability, steric bulk, and solubility properties. The resulting dodecaborate-based salts were subjected to a full thermal and electrochemical stability evaluation, showing that many of these anions maintain thermal stability up to 500 °C and feature no redox activity below ∼1 V vs Fc/Fc+. Mixed hydroxylated/halogenated clusters show enhanced solubility compared to their purely halogenated analogs and retain weakly coordinating properties in the solid state, as demonstrated by ionic conductivity measurements of their Li+ salts.

Facile Attachment of Halides and Pseudohalides to Dodecaborate(2-) via Pd-catalyzed Cross-Coupling

Cross-coupling reactions with [B12H11I]2− as one partner have been used successfully for Kumada and Buchwald Hartwig couplings with Pd catalysis. Here, we found that the iodide could be substituted easily, and unexpectedly, with other halides such as Br and Cl, and with pseudohalides such as cyanide, azide, and isocyanate. We found that for Cl, Br, N3, and NCO, tetrabutylammonium salts—or sodium salts—were successful halide sources, whereas for cyanide, CuCN was the only halide source that allowed a successful exchange. The azide could be reacted further in a click reaction with triazoles. While no substitution with fluoride occurred, tetrabutylammonium fluoride in the presence of water led to [B12H11OH]2−. Yields were high to very high, and reaction times were short when using a microwave oven as a heating source.

Non-Covalent Interactions in the Crystal Structures of Perbrominated Sulfonium Derivatives of the closo-Decaborate Anion

A new series of compounds based on perbrominated disubstituted sulfonium derivatives of the closo-decaborate anion (n-Bu₄N)[2-B₁₀Br₉SR₂] (R = n-Pr, i-Pr, n-Bu, n-C₈H₁₇, n-C₁₂H₂₅, n-C₁₈H₃₇) was obtained, characterised by modern physicochemical methods of analysis. According to the results of an X-ray diffraction study, some of the anions and solvate molecules were disordered. The cations (n-Bu₄N)⁺ and anions [2-B₁₀Br₉SR₂]^- were associated via C-H…Br and H…H contacts. In addition, Br…Br interactions between anions were revealed. The role of these contacts was analysed in terms of Hirshfeld surface analysis, QTAIM theory and the NCI method using quantum chemical calculations. An increase in the size of the alkyl R moiety led to significant strengthening of the total energy of H…H interactions. In the case of R = -n-C₁₈H₃₇, a parallel mutual orientation of alkyl moieties was established that was similar to the packing of salts of fatty acids. The nature of C-H…Br and Br…Br interionic interactions was found to be attractive, in contrast to the repulsive nature of intermolecular Br…Br interactions.

Zinc Ammonio-dodecaborates: Synthesis, Lewis Acid Strength, and Reactivity

Two series of zinc salts, [EtZn][A] and Zn[A]₂, with weakly coordinating anions [A]^- as counterions have been prepared, and their activities as catalysts for hydrosilylation reactions of 1-hexene, benzophenone, and acetophenone have been investigated. The counterions and per- and partially chlorinated 1-ammonio-closo-dodecaborate anions [Me₃NB₁₂Cl₁₁]^- [1]^-, [Pr₃NB₁₂H₅Cl₆]^- [2]^-, [Bu₃NB₁₂H₄Cl₇]^- [3]^-, and [Hex₃NB₁₂H₅Cl₆]^- [4]^- were chosen as potential and more readily available alternatives to carborate anions such as [CHB₁₁Cl₁₁]^- and [HexCB₁₁Cl₁₁]^-. The basicity of anion [4]^- was determined as being close to that of the triflimide anion [N(SO₂CF₃)₂]^-, and the fluoride ion affinities (FIAs) of compounds [EtZn][2] and Zn[2]₂ are lower than those of the Lewis acids B(C₆F₅)₃ and Zn[HexCB₁₁Cl₁₁]₂. The higher anion basicity and the resulting lower Lewis acidity of the zinc centers result in low activity in 1-hexene hydrosilylation catalysis and only moderate activity in the hydrosilylation catalysis of benzophenone and acetophenone.

Binding affinity of aniline-substituted dodecaborates to cyclodextrins

A new set of hybrid guest molecules bearing organic and inorganic residues have been studied for their recognition by cyclodextrins in aqueous solution. The guest molecules consist of nitroanilines linked through their amino group to the dodecahydrido-closo-dodecaborate cluster B₁₂H₁₂^2-, which serves as an anchor group. They show sizable affinity to cyclodextrins, and unexpected photophysical properties, with a very strong and low-energy charge-transfer band. The dodecaborate cluster increases the pK_a of the anilines by 5.0 to 5.7 pH units, and the deprotonated forms of the o- and p-nitroaniline derivatives show strong charge transfer absorption bands in the visible part of the spectrum.

B-N bond formation through palladium-catalyzed, microwave-assisted cross-coupling of nitrogen compounds with iodo-dodecaborate

Substituted undecahydrido-closo-dodecaborates [B₁₂H₁₁NR₂]^2- have potential use in materials and drugs, but have presented a synthetic challenge. Microwave-assisted palladium-catalyzed amination of iodo-dodecaborate [B₁₂H₁₁I]^2- allows mild and reproducible formation of B-N bonds with aromatic amines, HN-containing heteroaromatics, and amides. The reaction allows general access to amides, reproducible reactions to dodecaborate-substituted anilines, and, for the first time, the substitution of dodecaborate with HN-containing heterocycles.

Strong Binding of Noble Gases to [B12X11]-: A Theoretical Study

We systematically explore the stability and properties of [B₁₂X₁₁NG]^- adducts resulting from the binding of noble gas atoms to anionic [B₁₂X₁₁]^- clusters in the gas phase of mass spectrometers. [B₁₂X₁₁]^- can be obtained by stripping one X^- off the icosahedral closo-dodecaborate dianion [B₁₂X₁₂]^2-. We study the binding of the noble gas atoms He, Ne, Ar, Kr, and Xe to [B₁₂X₁₁]^- with substituents X = F, Cl, Br, I, and CN. While He cannot be captured by these clusters and Ne only binds at low temperatures, the complexes with the heavier noble gas atoms Ar, Kr, and Xe show appreciable complexation energies and exceed 1 eV at room temperature in the case of [B₁₂(CN)₁₁Xe]^-. The predicted B-NG equilibrium distance in the complexes with Ar, Kr, and Xe is only 0.10-0.25 Å longer than the sum of the covalent radii of the two corresponding atoms, and a significant charge transfer from the noble gas atom to the icosahedral B₁₂ cage is observed.

Versatile, one-pot introduction of nonahalogenated 2-ammonio-decaborate ions as boron cluster scaffolds into organic molecules; host-guest complexation with γ-cyclodextrin

We report the modification of the 2-ammonio group at halogenated decaborate ions with 2,3-epoxypropane, the product of which reacts readily with nucleophiles to form previously inaccessible coupling of polyhedra with organic molecules and materials. We demonstrate that these ions present a good binding motif in supramolecular chemistry.

The closo-Dodecaborate Dianion Fused with Oxazoles Provides 3D Diboraheterocycles with Selective Antimicrobial Activity

The synthesis and application of icosahedral boron cluster compounds has been studied extensively since their discovery several decades ago; however, two aspects of their chemistry have received little attention: The possibility to form inorganic/organic fused boraheterocycles and their potential to act as antimicrobial agents. This work comprises the preparation of a class of 3D diborabenzoxazole analogues with the closo-dodecaborate in place of the benzene moiety. The presented synthetic procedures provide access to a wide range of diboraheterocycles under mild conditions. These 3D heterocycles exhibit strong and selective antimicrobial activity against Neisseria gonorrhoeae, a widespread bacterial pathogen that has shown increasing incidences of multidrug resistance and for which the development of new antimicrobial compounds is urgently needed.

Alkoxy substituted halogenated closo-dodecaborates as anions for ionic liquids

Halogenated and alkoxylated closo-dodecaborates [B12X11OR](2-) (X = Cl, Br; R = propyl, octyl, dodecyl) have been synthesized by halogenation of the known [B12H11OH](2-) anion followed by alkylation in the superbasic medium DMSO/KOH. The obtained sodium salts were transformed by simple metathesis reactions in aqueous solution to the tetrabutylammonium ([NBu4](+)) and 1-hexyl-3-methylimidazolium ([C6mim](+)) salts. All compounds were fully characterized by heteronuclear NMR, IR and Raman spectroscopy, ESI mass spectrometry, and thermal analytical measurements. Selected anions were also structurally characterized as their [Ph4P](+) salts by single crystal X-ray diffraction. The [C6mim](+) salts are thermally stable up to more than 300 °C and show clear melting points. Surprisingly, the compound [C6mim]2[B12Cl11O-propyl] having the short propyl group bound to the boron cluster shows the lowest melting point (96 °C) of all the investigated compounds. Thus this compound is a rare member of the class of ionic liquids consisting of dianions.