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.

Palladium-catalyzed B7-11 penta-arylation of the {CB11} monocarborane cluster.

Regioselective, five-fold B-H activation of the monocarborane cluster [CB11H12]- at the positions B7-11 has been accomplished. Selective substitution of these positions by B-H activation has not been reported before. Our protocol involves directing group assistance by the carboxylic acid functionality and is based on palladium catalysis using iodoarene coupling partners. Penta-arylated products are obtained in a single step with yields ranging from 42% to 89% and with good functional group tolerance. X-Ray crystal structures for five new compounds confirm the selective substitution of the lower belt of the monocarborane cage.

An Iridium-Stabilized Borenium Intermediate.

Exploration of new organometallic systems based on polyhedral boron clusters has the potential to solve challenging chemical problems such as the stabilization of reactive intermediates and transition-state-like species postulated for E-H (E = H, B, C, Si) bond activation reactions. We report on facile and clean B-H activation of a hydroborane by a new iridium boron cluster complex. The product of this reaction is an unprecedented and fully characterized transition metal-stabilized boron cation or borenium. Moreover, this intermediate bears an unusual intramolecular B···H interaction between the hydrogen originating from the activated hydroborane and the cyclometallated metal-bonded boron atom of the boron cluster. This B···H interaction is proposed to be an arrested insertion of hydrogen into the Bcage-metal bond and the initiation step for iridium "cage-walking" around the upper surface of the boron cluster. The "cage-walking" process is supported by the hydrogen-deuterium exchange observed at the boron cluster, and a mechanism is proposed on the basis of theoretical methods with a special focus on the role of noncovalent interactions. All new compounds were isolated and fully characterized by NMR spectroscopy and elemental analysis. Key compounds were studied by single crystal X-ray diffraction and X-ray photoelectron spectroscopy.

Sonogashira coupling of the ethynyl monocarborane [CB11H11-12-CCH].

The Sonogashira cross coupling between the monocarborane cluster 12-ethynylmonocarba-closo-dodecaborate [CB11H11-12-CCH]- and bromoarenes under Pd catalysis has been developed, providing access to aryl carboranyl alkynes in yields of 42-95%. The transformations proceed under mild conditions and with high functional group tolerance using commercially available coupling partners. X-ray structural analyses and the identification of an unusual phosphonium zwitterion byproduct are reported as well.

Natural-Product-Directed Catalytic Stereoselective Synthesis of Functionalized Fused Borane Cluster-Oxazoles for the Discovery of Bactericidal Agents.

The identification of an alternative chemical space in order to address the global challenge posed by emerging antimicrobial resistance is very much needed for the discovery of novel antimicrobial lead compounds. Boron clusters are currently being explored in drug discovery due to their unique steric and electronic properties. However, the challenges associated with the synthesis and derivatization techniques of these compounds have limited their utility in the rapid construction of a library of molecules for screening against various biological targets as an alternative molecular platform. Herein, we report a transition-metal-catalyzed regioselective direct B-H alkylation-annulation of the closo-dodecaborate anion with natural products such as menthol and camphor as the directing groups. This method allowed the rapid construction of a library of 1,2,3-trisubstituted clusters, which were evaluated in terms of their antibacterial activity against WHO priority pathogens. Several of the synthesized dodecaborate derivatives displayed medium- to high-level bactericidal activity against Gram-positive and Gram-negative bacteria.

Comprehensive Pore Tuning in an Ultrastable Fluorinated Anion Cross-Linked Cage-Like MOF for Simultaneous Benchmark Propyne Recovery and Propylene Purification.

Propyne/propylene (C3 H4 /C3 H6 ) separation is an important but challenging industrial process to produce polymer-grade C3 H6 and recover high-purity C3 H4 . Herein, we report an ultrastable TiF6 2- anion cross-linked metal-organic framework (ZNU-2) with precisely controlled pore size, shape and functionality for benchmark C3 H4 storage (3.9/7.7 mmol g-1 at 0.01/1.0 bar and 298 K) and record high C3 H4 /C3 H6 (10/90) separation potential (31.0 mol kg-1 ). The remarkable C3 H4 /C3 H6 (1/99, 10/90, 50/50) separation performance was fully demonstrated by simulated and experimental breakthroughs under various conditions with excellent recyclability and high productivity (42 mol kg-1 ) of polymer-grade C3 H6 from a 1/99 C3 H4 /C3 H6 mixture. A modelling study revealed that the symmetrical spatial distribution of six TiF6 2- on the icosahedral cage surface provides two distinct binding sites for C3 H4 adsorption: one serves as a tailored single C3 H4 molecule trap and the other boosts C3 H4 accommodation by cooperative host-guest and guest-guest interactions.

Gram-Scale Synthesis of an Ultrastable Microporous Metal-Organic Framework for Efficient Adsorptive Separation of C2H2/CO2 and C2H2/CH4.

A highly water and thermally stable metal-organic framework (MOF) Zn2(Pydc)(Ata)2 (1, H2Pydc = 3,5-pyridinedicarboxylic acid; HAta = 3-amino-1,2,4-triazole) was synthesized on a large scale using inexpensive commercially available ligands for efficient separation of C2H2 from CH4 and CO2. Compound 1 could take up 47.2 mL/g of C2H2 under ambient conditions but only 33.0 mL/g of CO2 and 19.1 mL/g of CH4. The calculated ideal absorbed solution theory (IAST) selectivities for equimolar C2H2/CO2 and C2H2/CH4 were 5.1 and 21.5, respectively, comparable to those many popular MOFs. The Qst values for C2H2, CO2, and CH4 at a near-zero loading in 1 were 43.1, 32.1, and 22.5 kJ mol-1, respectively. The practical separation performance for C2H2/CO2 mixtures was further confirmed by column breakthrough experiments.

Rational Design of Microporous MOFs with Anionic Boron Cluster Functionality and Cooperative Dihydrogen Binding Sites for Highly Selective Capture of Acetylene.

Separation of acetylene (C2 H2 ) from carbon dioxide (CO2 ) or ethylene (C2 H4 ) is important in industry but limited by the low capacity and selectivity owing to their similar molecular sizes and physical properties. Herein, we report two novel dodecaborate-hybrid metal-organic frameworks, MB12 H12 (dpb)2 (termed as BSF-3 and BSF-3-Co for M=Cu and Co), for highly selective capture of C2 H2 . The high C2 H2 capacity and remarkable C2 H2 /CO2 selectivity resulted from the unique anionic boron cluster functionality as well as the suitable pore size with cooperative proton-hydride dihydrogen bonding sites (B-Hδ- ⋅⋅⋅Hδ+ -C≡C-Hδ+ ⋅⋅⋅Hδ- -B). This new type of C2 H2 -specific functional sites represents a fresh paradigm distinct from those in previous leading materials based on open metal sites, strong electrostatics, or hydrogen bonding.

A 3D Analogue of Phenyllithium: Solution-Phase, Solid-State, and Computational Study of the Lithiacarborane [Li-CB11 H11 ].

The lithiacarborane [Li-CB11 H11 ]- plays a central role in carborane chemistry, as it is a key intermediate to achieve the selective functionalization of the monocarba-closo-dodecaborate [CB11 H12 ]- for applications in various fields. Also, it is an organometallic species of fundamental interest because it represents a 3D analogue of phenyllithium featuring an exo C-Li bond in addition to the delocalized negative endo charge of the spherical cluster. For the first time, the elusive and highly reactive endo/exo formal dianion [CB11 H11 ]2- has been isolated as its lithiate as well as zincate in pure form and fully characterized. DFT calculations corroborate the experimental findings and underscore the remarkably high reactivity of the lithiacarborane. Subsequent derivatizations demonstrate the relevance of its initial clean formation.

The 12-ethynylmonocarba-closo-dodecaborate anion as a versatile ligand for Cu(i) alkyne and heterobimetallic Cu(i)/M(ii) (M = Pd, Pt) alkynide complexes.

The anionic monocarborane alkyne [12-(HC[triple bond, length as m-dash]C)-CB11H11]- was employed as a ligand towards Cu(i) to form terminal alkyne complexes. Spectroscopic methods and X-ray crystallography allowed for a detailed structural analysis of complexes with nitrogen ligands, which are the first examples featuring carborane-C[triple bond, length as m-dash]CH → metal π coordination. Addition of phosphines to compound 2 afforded homoleptic Cu(i) complexes [Cu(PR3)n]+ (4), in which case the carborane becomes a non-coordinating anion. Polymeric acetylide 5 was obtained in almost quantitative yield from 2 and proved to be a suitable precursor to heterobimetallic Cu(i)/M(ii) (M = Pd, Pt) alkynide complexes with side-on and end-on coordination to the metal centers.

Highly selective palladium-catalyzed one-pot, five-fold B-H/C-H cross coupling of monocarboranes with alkenes.

Palladium-catalyzed dehydrogenative B-H/C-H cross coupling of monocarborane anions with alkenes is reported, allowing for the first time the isolation of selectively penta-alkenylated boron clusters. The reaction cascade is regioselective for the cage positions, leading directly to B2-6 functionalization. Under mild and convenient conditions, styrenes, benzylic alkenes and aliphatic alkenes are demonstrated to be viable coupling partners with exclusive vinyl-type B-C bond formation. Multiple subsequent transformations provide access to directing group-free products, chiral derivatives and penta-alkylated cages. The five-fold coupling, combined with the latter reactions, represents a powerful methodology for the straightforward synthesis of new classes of boron clusters.

A Microporous Metal-Organic Framework Supramolecularly Assembled from a CuII Dodecaborate Cluster Complex for Selective Gas Separation.

A novel 3D metal-organic framework BSF-1 based on the closo-dodecaborate cluster [B12 H12 ]2- was readily prepared at room temperature by supramolecular assembly of CuB12 H12 and 1,2-bis(4-pyridyl)acetylene. The permanent microporous structure was studied by X-ray crystallography, powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and gas sorption. The experimental and theoretical study of the gas sorption behavior of BSF-1 for N2 , C2 H2 , C2 H4 , CO2 , C3 H8 , C2 H6 , and CH4 indicated excellent separation selectivities for C3 H8 /CH4 , C2 H6 /CH4 , and C2 H2 /CH4 as well as moderately high separation selectivities for C2 H2 /C2 H4 , C2 H2 /CO2 , and CO2 /CH4 . Moreover, the practical separation performance of C3 H8 /CH4 and C2 H6 /CH4 was confirmed by dynamic breakthrough experiments. The good cyclability and high water/thermal stability render it suitable for real industrial applications.

Atomically Defined Monocarborane Copper(I) Acetylides with Structural and Luminescence Properties Tuned by Ligand Sterics.

The weakly coordinating cluster [CB11 H12 ]- is used as a versatile building block for the preparation of luminescent copper(I) complexes. Treatment of [CB11 H11 -12-C≡CH]- with CuI and ammonia affords {(CB11 H11 -12-C≡C)2 Cu4 (NH3 )3 }n (2). Addition of selected phosphine and pyridine ligands transforms 2 to a range of homometallic products 3-13. All compounds have been characterized by X-ray crystallography, which reveals a surprising variety of structural complexity featuring cores with two to ten Cu+ centers. In the solid state, products 3-13 exhibit room-temperature phosphorescence across the visible spectrum in colors from blue to deep red. Lifetimes at room temperature are in the microsecond regime, and quantum yields of up to Φ=0.99 are observed.

Synthesis and Structural Characterization of Amidine, Amide, Urea and Isocyanate Derivatives of the Amino-closo-dodecaborate Anion [B12H11NH3].

The synthesis and structural characterization of new derivatives of [B12H12]2- is of fundamental interest and is expected to allow for extended applications. Herein we report on the synthesis of a series of amidine, amide, urea and isocyanate derivatives based on the amino-closo-dodecaborate anion [B12H11NH3]-. Their structures have been confirmed by spectroscopic methods, and nine crystal structures are presented.

Rhodium(iii)-catalyzed dehydrogenative dialkenylation of the monocarba-closo-decaborate cluster by regioselective B-H activation.

closo-Boranes and closo-carboranes, clusters with a closed-cage structure, exhibit remarkable inertness and are unique components of products with applications in areas such as medicinal chemistry, fluorescence and materials science. Herein, we report the first transition metal-catalyzed functionalization of the mono-closo-decaborate cluster [1-CB9H10]- by regioselective B-H activation. Using terminal alkenes bearing a wide range of functional groups, dehydrogenative coupling under mild conditions leads to B2/3-dialkenylated products, as verified by X-ray crystallography and NMR studies. This substitution pattern has not been accessible by other methods and enables the straightforward synthesis of new derivatives of the {CB9} cluster.

Palladium-Catalyzed Selective Five-Fold Cascade Arylation of the 12-Vertex Monocarborane Anion by B-H Activation.

A series of cage penta-arylated carboranes have been synthesized by palladium-catalyzed intermolecular coupling of the C-carboxylic acid of the monocarba- closo-dodecaborate anion [CB11H12]- with iodoarenes by direct cage B-H bond functionalization. These transformations set a record in terms of one-pot directing group-mediated activation of inert bonds in a single molecule. The methodology is characterized by high yields, good functional group tolerance, and complete cage regioselectivity. The directing group COOH can be easily removed during or after the intermolecular coupling reaction. The mechanistic pathways were probed using density functional theory calculations. A Pd(II)-Pd(IV)-Pd(II) catalytic cycle is proposed, in which initial coupling is followed by preferred B-H activation of the adjacent boron vertex, and continuation of this selectivity results in a continuous walking process of the palladium center. The methodology opens a new avenue toward building blocks with 5-fold symmetry.

RhIII -Catalyzed Functionalization of closo-Dodecaborates by Selective B-H Activation: Bypassing Competitive C-H Activation.

The closo-dodecaborate dianion is a fundamental icosahedral boron cage with 12 identical B-H vertices. The chemistry and applications of boron clusters have inspired researchers ever since their discovery several decades ago, and the selective modification of the cage positions has remained a major synthetic challenge. A rhodium(III)-catalyzed B-H functionalization-cyclization cascade of closo-dodecaborate amides is reported. The transformations occur chemoselectively at B-H positions in the presence of C-H bonds prone to competitive cyclometalation. Previously inaccessible cage derivatives with B-C(sp2 ) and B-C(sp3 ) bonds as well as a fused diboraoxazole ring are obtained in a one-pot process. The reactions proceed under mild conditions and exhibit complete cage regioselectivity with broad functional group tolerance. These cluster derivatives enable a largely extended investigation of the application of anionic boron clusters in research areas such as photoluminescent materials and medicinal 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.

Crystal structure of a carborane endo/exo-dianion and its use in the synthesis of ditopic ligands for supramolecular frameworks.

The crystal structure of the deprotonated monocarba-closo-dodecaborate [12-CN-CB11H11]- is reported. This dianion, featuring a delocalized endohedral negative charge and an exo-cluster C-[Li] moiety, was used for the synthesis of ditopic ligands with the potential to form novel supramolecular frameworks.

Fusing Carborane Carboxylic Acids with Alkynes: 3D Analogues of Isocoumarins via Regioselective B-H Activation.

An iridium-catalyzed alkenylation/annulation sequence between monocarba-closo-dodecaborate carboxylic acids and diarylacetylenes is reported. Regioselective activation of the B2 position, followed by B-C bond formation and ring closure, affords 3D bora-analogues of isocoumarins. The reaction tolerates a variety of functional groups on the aromatic rings and can be extended to B12-substituted derivatives. Furthermore, subsequent alkenylation of the B4 vertex has been achieved in high yields.