Thermal and Photophysical Properties of Highly Quadrupolar Liquid-Crystalline Derivatives of the [closo-B12 H12 ]2- Anion

Crystal Structure of the Monocarborane Magnesium(II) Acetylide and Its Use in the Synthesis of α,β-Unsaturated Ketones

We report the first crystal structure of heteroleptic Grignard reagent 2 based on the carborane endo/exo dianion [CB11H11-12-C≡C]2-. Full characterization reveals a rare coordination pattern and affirms the bimetallic nature. Navigating the reactivity landscape, we unlock the potential of 2 in nucleophilic addition with ketones to afford propargylic alcohols 3, renowned for their synthetic versatility and potential biological activities, and unveil the Meyer-Schuster rearrangement, yielding α,β-unsaturated carbonyl compounds 4. This narrative of synthesis, characterization, and reactivity opens new horizons for carborane chemistry, offering avenues for innovation and facile functionalization of carborane scaffolds.

Regioselective B2-6 penta-iodination of the [CB11H12]- monocarborane cluster by palladium catalysis

Penta-iodination of the B2-6 positions of the {CB11} monocarborane cluster is reported. Products of the structure [2,3,4,5,6-I5-CB11H6-12-X]- (X = H, Me, Et, Ph, Br, I) were obtained and fully characterized. X-ray crystal structures of three new compounds confirm this particular substitution pattern. The synthetic method relies on palladium catalysis/B-H activation, assisted by the C1-COOH directing group. The one-pot procedure enables penta-iodination and subsequent decarboxylation under convenient conditions. The B2-6 regioselectivity is complementary to the commonly observed reactivity of {CB11} clusters, which follows the trend B12 > B7-11 > B2-6 for electrophilic substitution. Thus, for the first time upper-belt halogenation is achieved without prior modification of the lower-belt positions.

Icosahedral m-Carboranes Containing Exopolyhedral B-Se and B-Te Bonds

Chalcogen-containing carboranes have been known for several decades and possess stable exopolyhedral B(9)-Se and B(9)-Te σ bonds despite the electron-donating ability of the B(9) vertex. While these molecules are known, little has been done to thoroughly evaluate their electrophilic and nucleophilic behavior. Herein, we report an assessment of the electrophilic reactivity of m-carboranylselenyl(II), -tellurenyl(II), and -tellurenyl(IV) chlorides and establish their reactivity pattern with Grignard reagents, alkenes, alkynes, enolates, and electron-rich arenes. These electrophilic reactions afford unique electron-rich B-Y-C (Y = Se, Te) bonding motifs not commonly found before. Furthermore, we show that m-carboranylselenolate, and even m-carboranyltellurolate, can be competent nucleophiles and participate in nucleophilic aromatic substitution reactions. Arene substitution chemistry is shown to be further extended to electron-rich species via palladium-mediated cross-coupling chemistry.

Synthesis of Derivatives of closo-Dodecaborate Anion Based on Amino Acid Esters

Abstract

This work proposes a new method for the synthesis of N-borylated amino acids based on nucleophilic substitution reactions in the [B12H11IPh]– anion. Esters of glycine and L-phenylalanine were used as nucleophiles. The structure of the products has been determined by multinuclear NMR spectroscopy, IR spectroscopy, and ESI mass spectrometry.

Polar derivatives of [closo-1-CB9H10]- and [closo-1-CB11H12]- anions as high Δε additives to a nematic host: a comparison of the CH2CH2 and COO linking groups

A series of liquid crystalline pyridinium and sulfonium derivatives of the [closo-1-CB9H10]- and [closo-1-CB11H12]- anions containing the CH2CH2 linking group was prepared and their molecular and crystal structures were determined by single crystal XRD methods. Thermal and dielectric properties of the series were evaluated in a weakly polar nematic host. The highest extrapolated dielectric anisotropy, Δε, was observed for pyridinium zwitterions (up to 56.0). The dielectric data were analyzed with the Maier-Meier formalism augmented with density functional theory calculations, and the results were compared to those obtained for the analogous ester derivatives (COO linking group). The effect of the linking group on thermal and electrooptical properties is discussed.

Tuning the Liquid Crystallinity of Cholesteryl-o-Carborane Dyads: Synthesis, Structure, Photoluminescence, and Mesomorphic Properties

A set of mesomorphic materials in which the o-carborane cluster is covalently bonded to a cholesteryl benzoate moiety (mesogen group) through a suitably designed linker is described. The olefin cross-metathesis between appropriately functionalized styrenyl-o-carborane derivatives and a terminal alkenyl cholesteryl benzoate mesogen (all type I terminal olefins) leads to the desired trans-regioisomer, which is the best-suited configuration to obtain mesomorphic properties in the final materials. The introduction of different substituents (R = H (M2), Me (M3), or Ph (M4)) to one of the carbon atoms of the o-carborane cluster (Ccluster) enables the tailoring of liquid crystalline properties. Compounds M2 and M3 show the chiral nematic (N*) phase, whereas M4 do not show liquid crystal behavior. Weaker intermolecular interactions in the solid M3 with respect to those in M2 may allow the liquid crystallinity in M3 to be expressed as enantiotropic behavior, whereas breaking the stronger intermolecular interaction in the solid state of M2 leads directly to the isotropic state, resulting in monotropic behavior. Remarkably, M3 also displays the blue phase, which was observed neither in the chiral nematic precursor nor in the styrenyl-cholesterol model (M5) without an o-carborane cluster, which suggests that the presence of the cluster plays a role in stabilizing this highly twisted chiral phase. In the carborane-containing mesogens (M2 and M3), the o-carborane cluster can be incorporated without destroying the helical organization of the mesophase.

Photonic materials derived from the [closo-B10H10]2− anion: tuning photophysical properties in [closo-B10H8-1-X-10-(4-Y-NC5H5)]−

Substitution of the parent [closo-B₁₀H₉-1-NC₅H₅]⁻ either at the B(10) position or at the pyridine ring tunes absorption and emission energy.

C(1)-Phenethyl Derivatives of [closo-1-CB11 H12 ]- and [closo-1-CB9 H10 ]- Anions: Difunctional Building Blocks for Molecular Materials

C(1)-vinylation of [closo-1-CB₉ H₁₀ ]^- (A) and [closo-1-CB₁₁ H₁₂ ]^- (B) with 4-benzyloxystyryl iodide followed by hydrogenation of the double bond and reductive deprotection of the phenol functionality led to C(1)-(4-hydroxyphenethyl) derivatives. The phenol functionality was protected as the acetate. The esters were then treated with PhI(OAc)₂ and the resulting isomers were separated kinetically (for derivatives of anion A) or by chromatography (for derivatives of anion B) giving the difunctionalized building blocks in overall yields of 9 % and 50 %, respectively. A similar series of reactions was performed starting with anions A and B and 4-methoxystyryl bromide and iodide. Significant differences in the reactivity of derivatives of the two carborane anions were rationalized with DFT computational results. Application of the difunctionalized carboranes as building blocks was demonstrated through preparation of two ionic liquid crystals. The extensive synthetic work is accompanied by single crystal XRD analysis of six derivatives.

Directed B–H functionalization of the closo-dodecaborate cluster via concerted iodination–deprotonation: reaction mechanism and origins of regioselectivity

A concerted iodination–deprotonation process was discovered for the B–H bond iodination of closo-dodecaborate cluster, which is responsible for the regioselectivity of ortho-B–H functionalization.