Redox-active carborane clusters in bond activation chemistry and ligand design

Noncatalyzed Intramolecular B-N and B-O Cross-Coupling of "Inert" Carboranes Lead to the Formation of an Unusual Oxoborane, via Reversible Cluster C-B Bond Scission

Polyhalogenated closo-12-vertex carborane anions are thought to be inert species incapable of participating in direct B-X substitution reactions. Here, we show that this is not true and that such species can be easily coaxed into intramolecular cross-coupling cyclizations without the need for a catalyst. When cage C-tethered O and N-heteroallylic anions are generated, a variety of cyclized products can be formed in high yield under mild conditions. Additionally, we show that even C-tethered neutral nucleophiles, such as the pyridine moiety, undergo facile B-X substitution chemistry and these reactions are not dependent on the countercation. Serendipitously, we also found that when these cyclizations are attempted with acetamide derivatives, an unprecedented cluster C-B bond scission reaction occurs, producing an unprecedented oxoborane stabilized by multicentered bonding. Amazingly this molecule can be protonated, leading to reformation of the C-B bond and cluster reorganization, and this process is reversible.

Palladium-Catalyzed Regioselective B(3,4,5,6)-H Tetra-Arylation of o-Carboranes

A transition-metal-catalyzed iterative multiple cage B-H activation reaction for constructing multifunctionalization of o-carboranes in one pot is challenging. Herein, palladium-catalyzed regioselective tetra-arylation of a wide range of C(1)-N-aryl-o-carboranyl amides with aryl iodides has been developed. A variety of B(3,4,5,6)-tetra-arylated o-carboranes were synthesized in good-to-excellent yields. Moreover, the mono-, di-, and triarylated intermediate products were isolated, and the exact structures were determined by NMR, high-resolution mass spectrometry, and X-ray analysis, which provide a rationale that the order of introduction of aryl groups into o-carborane is B(4) > B(5) > B(3)/B(6). This protocol represents a powerful synthetic method for constructing polyfunctionalization of o-carborane derivatives under very mild and simple reaction conditions, which offers a valuable reference for the design and synthesis of molecular propellers based on carboranes.

The 12-Ethynylmonocarba-closo-dodecaborate Anion: A Ligand for Metal Alkynide Complexes (M = Li, Na, Ca, Zn)

We report the first isolation of metal alkynide complexes (2a-d) incorporating lithium, sodium, calcium and zinc based on the monocarborane endo/exo-dianionic framework [CB11H11-12-C≡C]2-. These complexes are synthesized via the deprotonation of terminal alkynes and optimized using lithium, sodium, calcium, and zinc hexamethyldisilazide bases to achieve enhanced yields and crystal stability. X-ray crystallography and NMR spectroscopy reveal unique coordination geometries, including a rare chair-shaped Li4C2O2 unit in 2a, multinuclear units in 2b-c, and a hexagonal Zn3C3 unit in 2d. Subsequent nucleophilic reactions with benzophenone afford propargylic alcohols (3) with high efficiency, demonstrating their utility in selective synthetic routes for complex molecular assembly. This work not only highlights the reactivity of alkali and alkaline-earth metals in monocarborane metal acetylides but also offers valuable insights into their structural characteristics and potential applications in advanced materials and catalytic systems.

Advances in the selective functionalization of B(3,6)-H of o-carboranes

This review summarizes the methodologies for the selective functionalization of the B(3,6) vertices of o-carboranes, including the deboration-capitation reaction, the coupling reaction of B-X (X = I, Br) bonds, reactions of 1,3-dehydro-o-carborane and [3-N2-o-C2B10H11][BF4] as well as transition-metal-catalyzed B-H activation. These works offer a versatile toolbox for synthesizing B(3,6)-substituted o-carborane derivatives and will promote their applications in material science, pharmaceutical chemistry, and related disciplines.

A σ,π-type monocarborane copper acetylide for regioselective 1,4-disubstituted 1,2,3-triazoles

A σ,π-bis(copper) acetylide complex, (CB11H11-12-CC)Cu2(PiPr3)3 (2), enables the formation of regioselective 1,4-disubstituted 1,2,3-triazoles containing monocarborane anion [CB11H12]-via CuAAC with organic azides under mild conditions. This method achieves 1,2,3-triazolylated monocarboranes (3) in up to 97% yield, including a luminescent compound with optoelectronic potential.

Beyond the Hawthorne Reaction: Li+ Induced Thermal Dehydrocoupling of closo-10-vertex Carborane Anions

In the 1970s Hawthorne reported an electrochemical dehydrocoupling reaction of the closo-carborane anion [HCB9H91-] 1 to form the biscarborane [C2B18H182-] 2. In this Communication we show that the said "Hawthorne Reaction" can be achieved thermally and that it tolerates C-butylation. The new compound 2butyl was fully characterized by 11B, 1H, and 13C NMR spectroscopies, high-resolution mass spectrometry, and single-crystal X-ray diffraction. One interesting caveat is that 2 or 2butyl only form thermally when they are salts of Li+ and not NEt4+, Na+, K+, or Cs+. This observation means that Li+ in some way facilitates this process, introducing a new kind of Li+ effect.

N-H Bond Activation of Ammonia by a Redox-Active Carboranyl Diphosphine

In this work, we report the room-temperature N-H bond activation of ammonia by the carboranyl diphosphine 1-PtBu2-2-PiPr2-closo-C2B10H10 (1) resulting in the formation of zwitterionic 7-P(NH2)tBu2-10-P(H)iPr2-nido-C2B10H10 (2). Unlike the other phosphorus-based ambiphiles that require geometric constraints to enhance electrophilicity, the new mode of bond activation in this main-group system is based on the cooperation between electron-rich trigonal phosphine centers and the electron-accepting carborane cluster. As an exception among many other metal-based and metal-free systems, the N-H bond activation of gaseous ammonia or aqueous ammonium hydroxide by carboranyl diphosphine 1 proceeds with tolerance of air and water. Mechanistic details of ammonia activation were explored computationally by DFT methods, demonstrating an electrophilic activation of ammonia by the phosphine center. This process is driven by the reduction of the boron cluster followed by an ammonia-assisted deprotonation and proton transfer. A subsequent reaction of 2 and TEMPO results in the cleavage of all N-H and P-H bonds with the formation of a cyclic phosphazenium cation supported by an anionic cluster N(7-PtBu2-8-PiPr2)-nido-C2B9H10 (3). Transformations reported herein represent the first example of ammonia oxidation via triple hydrogen atom abstraction facilitated by a metal-free system.

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.

Redox-Active Carboranyl Diphosphine as an Electron and Proton Transfer Agent

In this work, we report the first example of the PCET reactivity for a boron cluster compound, the zwitterionic nido-carboranyl diphosphonium derivative 7-P(H)tBu2-10-P(H)iPr2-nido-C2B10H10. This main-group reagent efficiently transfers two electrons and two protons to quinones to yield hydroquinones and regenerate a neutral closo-carboranyl diphosphine, 1-PtBu2-2-PiPr2-closo-C2B10H10. As we have previously reported the conversion of this closo-carboranyl diphosphine into the zwitterionic nido- derivative upon reaction with main group hydrides, the transformation reported herein represents a complete synthetic cycle for the metal-free reduction of quinones, with the redox-active carboranyl diphosphine scaffold acting as a mediator. The proposed mechanism of this reduction, based on pKa determination, electrochemical studies, and kinetic isotope effect determination, involves the electron transfer from the nido- cluster to the quinone coupled with the delivery of protons.

Redox-Active Boron Clusters

ConspectusIn this Account, we discuss our group's research over the past decade on a class of functionalized boron clusters with tunable chemical and physical properties, with an emphasis on accessing and controlling their redox behavior. These clusters can be thought of as three-dimensional aromatic systems that have distinct redox behavior and photophysical properties compared to their two-dimensional organic counterparts. Specifically, our lab has studied the highly tunable, multielectron redox behavior of B12(OR)12 clusters and applied these molecules in various settings. We first discuss the spectroscopic and electrochemical characterization of B12(OR)12 clusters in various oxidation states, followed by their use as catholytes and/or anolytes in redox flow batteries and chemical dopants in conjugated polymers. Additionally, the high oxidizing potential and visible light-absorbing nature of fluoroaryl-functionalized B12(OR)12 clusters have been leveraged by our group to generate weakly coordinating, photoexcitable species that can promote photooxidation chemistry.We have further translated these solution-phase studies of B12(OR)12 clusters to the solid state by using the precursor [B12(OH)12]2- cluster as a robust building block for hybrid metal oxide materials. Specifically, we have shown that the boron cluster can act as a thermally stable cross-linking material, which enhances electron transport between metal oxide nanoparticles. We applied this structural motif to create TiO2- and WO3-containing materials that showed promising properties as photocatalysts and electroactive materials for supercapacitors. Building on this concept, we later discovered that B12(OCH3)12, the smallest of the B12(OR)12 family, could retain its redox behavior in the solid state, a previously unseen phenomenon. We successfully harnessed this unique behavior for solid-state energy storage by implementing this boron cluster as a cathode-active material in a Li-ion prototype cell device. Recently, our group has also explored how to tune the redox properties of clusters other than B12(OR)12 species by synthesizing a library of vertex-differentiated clusters containing both B-OR and B-halogen groups. Due to the additive qualities of different functional groups on the cluster, these species allow access to a region of electrochemical potentials previously inaccessible by fully substituted closo-dodecaborate alkoxy-based derivatives.Lastly, we discuss our research into smaller-sized redox-active polyhedral boranes (B6- and B10-based cluster cores). Interestingly, these clusters show significantly less redox stability and reversibility than their dodecaborate-based counterparts. While exploring the functionalization of closo-hexaborate to create fully substituted derivates (i.e., [B6R6Hfac]-), we observed unique oxidative decomposition pathways for this cluster system. Consequently, we leveraged this oxidative instability to generate useful alkyl boronate esters via selective chemical oxidation. We further explored a closo-decaborate cluster as a platform to access electrophilic [B10H13]+ species capable of directly borylating arene compounds with unique regioselectivity. Upon chemical oxidation of the arylated decaborate clusters, we successfully synthesized various aryl boronate esters, establishing the generality of the oxidative cluster deconstruction concept.Overall, our work shows that boron clusters are an appealing class of redox-active molecules, and this fundamental and understudied property can be leveraged for constructing novel materials with tunable physical and electrochemical properties, as well as producing unique chemical reagents for small molecule synthesis.

Synthesis and cluster structure distortions of biscarborane dithiol, thioether, and disulfide

The synthesis and structural characterization of the first sulfur-containing derivatives of the C,C-biscarborane {ortho-C2B10}2 cluster - thiol, thioether, and disulfide - are reported. The biscarboranyl dithiol (1-HS-C2B10H10)2 exhibits an exceedingly long intracluster carbon-carbon bond length of 1.858(3) Å, which is attributed to the extensive interaction between the lone pairs of the thiol groups and the unoccupied molecular orbital of the carborane cluster. The structures of the doubly deprotonated biscarboranyl dithiolate anion (1-S-C2B10H10)22- with various counter cations feature an even longer carbon-carbon bond length of 2.062(10) Å within the cluster along with a short carbon-sulfur bond of 1.660(7) Å, both indicative of significant delocalization of electron density from the sulfur atoms into the cluster.

Iridium-catalyzed selective arylation of B(6)-H of 3-aryl-o-carboranes with arylboronic acid via direct B-H activation

This work discloses an iridium-catalyzed selective arylation of B(6)-H of 3-Ar-o-carboranes with arylboronic acid via direct B-H activation for the first time. A series of unsymmetric and symmetric 3,6-diaryl-o-carboranes decorated with diverse active groups have been synthesized with moderate to excellent yields under mild conditions. This work offers an efficient approach for selective arylation of B(6)-H with arylboronic acid and has important value for selective functionalization of o-carboranes.