Jonathan C. Axtell

Atomically precise organomimetic cluster nanomolecules assembled via perfluoroaryl-thiol SNAr chemistry

The majority of biomolecules are intrinsically atomically precise, an important characteristic that enables rational engineering of their recognition and binding properties. However, imparting similar precision to hybrid nanoparticles has been challenging due to inherent limitations of the existing chemical methods and availability of properly designed functional building blocks. Here we report a new approach to form atomically precise and highly tunable hybrid nanomolecules with well-defined three-dimensionality. Perfunctionalization of atomically precise clusters with pentafluoroaryl-terminated linkers produces size-tunable rigid cluster nanomolecules. These species are amenable to facile modification with a variety of thiol-containing molecules and macromolecules. Assembly proceeds at room temperature within hours under mild conditions, and the resulting nanomolecules exhibit high stabilities due to their full covalency. We further demonstrate how these nanomolecules grafted with saccharides can exhibit dramatically improved binding affinity toward a protein. Ultimately, the developed strategy allows the rapid generation of precise molecular assemblies for investigating multivalent interactions.

Luminescent metal complexes featuring photophysically innocent boron cluster ligands

We report the synthesis and characterization of a series of d8 metal complexes featuring robust and photophysically innocent strong-field chelating 1,1′-bis(o-carborane) (bc) ligand frameworks.

Visible-Light-Induced Olefin Activation Using 3D Aromatic Boron-Rich Cluster Photooxidants

We report a discovery that perfunctionalized icosahedral dodecaborate clusters of the type B12(OCH2Ar)12 (Ar = Ph or C6F5) can undergo photo-excitation with visible light, leading to a new class of metal-free photooxidants. Excitation in these species occurs as a result of the charge transfer between low-lying orbitals located on the benzyl substituents and an unoccupied orbital delocalized throughout the boron cluster core. Here we show how these species, photo-excited with a benchtop blue LED source, can exhibit excited-state reduction potentials as high as 3 V and can participate in electron-transfer processes with a broad range of styrene monomers, initiating their polymerization. Initiation is observed in cases of both electron-rich and electron-deficient styrene monomers at cluster loadings as low as 0.005 mol%. Furthermore, photo-excitation of B12(OCH2C6F5)12 in the presence of a less activated olefin such as isobutylene results in the production of highly branched poly(isobutylene). This work introduces a new class of air-stable, metal-free photo-redox reagents capable of mediating chemical transformations.

B-N, B-O, and B-CN Bond Formation via Palladium-Catalyzed Cross-Coupling of B-Bromo-Carboranes.

Carboranes are boron-rich molecules that can be functionalized through metal-catalyzed cross-coupling. Here, for the first time, we report the use of bromo-carboranes in palladium-catalyzed cross-coupling for efficient B-N, B-O, and unprecedented B-CN bond formation. In many cases bromo-carboranes outperform the traditionally utilized iodo-carborane species. This marked difference in reactivity is leveraged to circumvent multistep functionalization by directly coupling small nucleophiles (-OH, -NH2, and -CN) and multiple functional groups onto the boron-rich clusters.

Rapid synthesis of redox-active dodecaborane B12(OR)12 clusters under ambient conditions

We have developed a fast and efficient route to obtain perfunctionalized ether-linked alkyl and benzyl derivatives of the closo-[B12(OH)12]2- icosahedral dodecaborate cluster via microwave-assisted synthesis. These icosahedral boron clusters exhibit three-dimensional delocalization of the cage-bonding electrons, tunable photophysical properties, and a high degree of stability in air in both solid and solution states. A series of closo-[B12(OR)12]2-, hypocloso-[B12(OR)12]1- and hypercloso-[B12(OR)12]0 clusters have been prepared with reaction times ranging from hours to several minutes. This method is superior to previously reported protocols since it dramatically decreases the reaction times required and eliminates the need for inert atmosphere conditions. The generality of the new microwave-based method has been further demonstrated through the synthesis of several new derivatives, which feature redox potentials up to 0.6 V more positive than previously known B12(OR)12 cluster compounds. We further show how this method can be applied to a one-pot synthesis of hybrid, vertex-differentiated species B12(OR)11(OR) that was formerly accessible only via multi-step reaction sequence.

Synthesis and Applications of Perfunctionalized Boron Clusters

This Viewpoint describes major advances pertaining to perfunctionalized boron clusters in synthesis and their respective applications. The first portion of this work highlights key synthetic methods, allowing one to access a wide range of polyhedral boranes (B4 and B6-B12 cluster cores) that contain exhaustively functionalized vertices. The second portion of this Viewpoint showcases the historical developments in using these molecules for applications ranging from materials science to medicine. Last, we suggest potential new directions for these clusters as they apply to both synthetic methods and applications.