Photoinduced Selective B-H Activation of nido-Carboranes

Post-coordination of Ru(ii) controlled regioselective B(4)-H acylmethylation of o-carboranes with sulfoxonium ylides

Despite significant progress in the B-H functionalization of carboranes, the development of cost-effective catalytic systems devoid of noble metals, coupled with mechanistic validation of regioselectivity control, remains a formidable challenge. Herein, we disclose an Ag salt-free, redox-neutral, and inexpensive ruthenium(ii)-catalyzed protocol that enables exclusive B(4)-H acylmethylation of o-carboranes through a novel post-coordination strategy. By exploiting weakly coordinating carboxylic acid as a traceless directing group, this method achieves excellent mono-site selectivity for B-C(sp3) bond formation using diverse sulfoxonium ylides, demonstrating both functional group tolerance and synthetic scalability. This work not only establishes a practical synthetic platform but also addresses critical mechanistic questions unresolved in prior analogous studies. Through deuterium labeling, in situ high-resolution mass spectrometry (HRMS) tracking, and single-crystal X-ray analysis of critical Ru intermediates, we unequivocally demonstrate that the mono-site selectivity originates from a unique post-coordination mode of Ru(ii). The Ru catalyst simultaneously engages both the carboxylic acid and the enolizable acylmethyl moiety in the mono-acylated intermediate, thereby dictating the B(4)-H activation trajectory. Our findings establish a generalizable platform for regiocontrolled carborane functionalization while defining mechanistic paradigms in transition metal-mediated B-H activation chemistry.

Three-fold aromatic boranes: spherical aromaticity in borane ortho-carboranes as useful trimer nodes for cluster-based architectures

The characterized tris(ortho-carboranyl)borane (BoCb3) structure enables further understanding of building blocks in three-fold architectures as useful nodes for envisaging cluster-based materials, extending the already known linear array. Our results show the formation of shielding cones enabled in adjacent cluster units that overlap in long-range regions by different orientations of the applied field, in contrast to planar aromatic triarylborane counterparts. Thus, three spherical aromatic circuits or states are retained in the resulting molecular unit, as indicated by the isotropic and anisotropic descriptors of the magnetic behavior. In addition, the superacidic Lewis characteristics of BoCb3, in comparison to triarylboranes, are enabled by the increase in the orbital interaction towards adduct formation, highlighting the relevance of the donor-acceptor charge transfer, where the control of steric repulsion may lead to further stabilization, suggesting plausible enhanced Lewis acidic performance. These results enhance the understanding of cluster-based architectures, paving the way for explorative synthesis efforts toward the achievement of novel superacidic Lewis species by using polyhedral standing molecules.

Taming Inert B-H Bond with Low Energy Light: A Near-Infrared Light-Induced Approach to Facile Carborane Cluster-Amino Acid Coupling

The selective functionalization of inert B-H bonds in carborane clusters has been a formidable challenge. Recent advances have witnessed such reactions through photoredox methods utilizing ultraviolet or visible light irradiation. However, high-energy light sources often suffer from poor energy efficiency, a limited substrate scope, undesired side reactions, and low scalability. Here, we present the first successful B-H bond functionalization under low-energy near-infrared (NIR) light using a carborane-based electron donor-acceptor complex. Both photophysical investigations and theoretical modeling reveal a facile single-electron transfer from the carborane cage to the electron-deficient photocatalyst, generating a carborane cage radical under NIR light irradiation. The follow-up radical pathway enables the direct coupling of carboranes with amino acids or oligopeptides, yielding a diverse array of carborane-functionalized amino acids or oligopeptides. Beyond expanding the known chemical space of boron cluster derivatives, we further demonstrate that carborane-based amino acids with imaging and targeting capabilities could serve as promising multifunctional boron carriers for boron neutron capture therapy. Thus, the selective B-H bond functionalization of the carboranes via NIR light not only provides a straightforward and practical strategy in boron cluster synthetic chemistry but also lays the foundation for the development of next-generation boron-containing biomolecules and advanced functional materials.

Palladium-Catalyzed Cross-Coupling Reactions of Carboranes with Alkenes via Selective B-H Bond Activation

A palladium-catalyzed Heck-type cross-coupling reaction of carboranes with alkenes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was realized. This reaction shows good B(9) selectivity for m-carboranes and is also suitable for o- and p-carborane. Meanwhile, a series of mono-, di-, and trisubstituted alkenes were compatible substrates to afford the alkenylated products in 16-89% yields. The 1,2-bis(carboranyl)ethylene was first synthesized by the reaction of vinyltrimethylsilane and m-carborane. Further transformations of the C═C bond in the product were examined by hydroboration oxidation, oxidation, hydroboration, and bromination reaction to generate corresponding B(9)-functionalized m-carboranes.

Improved Methods for the Synthesis of B10H14 and N-Heterocycle-Coordinated B9H13 (N-Het·B9H13)

Improved methods for the synthesis of B10H14 and a series of N-heterocycle-coordinated B9H13 complexes (N-Het·B9H13) have been developed with readily obtained KB11H14 as the starting material. Oxidation of KB11H14 could provide B10H14 in over 90% yield. Then, the N-Het·B9H13 complexes were prepared from the as-synthesized B10H14 through in situ multistep reactions by reacting with NaH, N-heterocycles, and dilute hydrochloric acid. Among these N-Het·B9H13 complexes, 4-(triphenylvinyl)pyridine-coordinated B9H13 (1k) exhibits a significant aggregation-induced emission (AIE) effect in a THF/H2O mixed solution, and 8-aminoisoquinoline-coordinated B9H13 (2p) exhibits a positive solvatochromism phenomenon. These improved methods provide new approaches to synthesizing B10H14 and N-Het·B9H13 complexes with potential applications in luminescent materials.

Regiocontrollable B(2/3)-H Alkenylation of nido-Carboranes

Anionic nido-carboranes, as open-cage analogues of closo-carboranes with strong hydrophilicity and higher potential in the development of biomedicines, have been notably more challenging because of their strong interaction with transition metals. While the exo-cage B-H activation reactions of nido-carboranes have been widely studied, there are few reports on the direct functionalization of B-H bonds located on a closed polyhedral sphere. Here, we report an efficient palladium-catalyzed regioselective B(2/3)-H alkenylation of nido-carboranes with various alkenes and alkyne coupling partners, enabled by 3-methylpyridine directing groups, to achieve a regiocontrollable functionalization of B(2/3)-H vertices over highly reactive exo-cage B11-H vertex in nido-carboranes.

Dative Bonding Activation Enables Precise Functionalization of the Remote B-H Bond of nido-Carborane Clusters

The innovation of synthetic strategies for selective B-H functionalization is a pivotal objective in the realm of boron cluster chemistry. However, the precise, efficient, and rapid functionalization of a B-H bond of carboranes that is distant from the existing functional groups remains intractable owing to the limited approaches for site-selective control from the established methods. Herein, we report a dative bonding activation strategy for the selective functionalization of a nonclassical remote B-H site of nido-carboranes. By leveraging the electronic effects brought by the exopolyhedral B(9)-dative bond, a cross-nucleophile B-H/S-H coupling protocol of the distal B(5)-H bond has been established. The dative bond not only amplifies the subtle reactivity difference among B-H bonds but also significantly changes the reactive sites, further infusing nido-carboranes with additional structural diversity. This reaction paradigm features mild conditions, rapid conversion, efficient production, broad scope, and excellent group tolerance, thus enabling the applicability to an array of complex bioactive molecules. The efficient and scalable reaction platform is amenable to the modular construction of photofunctional molecules and boron delivery agents for boron neutron capture therapy. This work not only provides an unprecedented solution for the selective diversification of distal B-H sites in nido-carboranes but also holds the potential for expediting the discovery of novel carborane-based functional molecules.

Intercluster B-H and B-B aggregation in iso- and trans-[B20H18]2-. Spherical aromaticity in borane dimers

The formation of molecular-based functional materials is a key step towards the development of technologies at the nanoscale. Recently, it has been shown that after oxidation of closo-[B10H10]2- anions, an induced aggregation of two cluster units is achieved, retaining their parent B10 backbones as persistent building blocks. Such characterization provides an interesting scenario to further understand relevant factors leading to aggregation in a minimal structure involving two units. Here, we explore the interaction between closo-[B10H10]2- units in two isomers, namely, iso- and trans-[B20H18]2-, involving different intercluster contacts based on B-B and B-H interactions, respectively. Our results show that the inherent spherical aromatic characteristics of the parent closo-[B10H10]2- cluster are persistent in both iso- and trans-[B20H18]2- isomers as an interplay between the spherical aromatic properties from both B10 motifs, leading to an overlap of the shielding regions from shielding cone properties, ascribed as a dual spherical-spherical aromatic cluster. From 11B-NMR features, it came out that trans-[B20H18]2- involves larger differences in comparison to closo-[B10H10]2-, owing to the variation of the B10-B10 backbone provided by the intercluster B-interaction, thus resulting in a more effective aggregation connecting such building units, towards boron-based cluster materials.

Selective B(3)-H Activation Affording Multinuclear Ir(III) Complexes with (o-Carboranyl)dithioester Ligands

A method was developed to link two or three o-carborane moieties to form a series of carboranyl dithioester bridging ligands via in situ substitution of haloalkanes by tetraphenylphosphonium carboranyldithiocarboxylates. Based on these ligands, direct B-H activation without the assistance of Ag(I) and alkali was successfully achieved with half-sandwich Ir(III) substrate [Cp*IrCl2]2 to yield corresponding bimetallic or trimetallic complexes. Single crystal structure analyses of the B-H activated complexes and corresponding SnCl2-inserted derivatives confirm the selective B(3)-H activation in these complexes.