A Fluorescent Unnatural Mannosamine Derivative with Enhanced Emission Upon Complexation with Cucurbit[7]uril.

Metabolic incorporation of unnatural functionality on glycans has allowed chemical biologists to observe and affect cellular processes. Recent work has resulted in glycan-fluorophore structures that allow for direct visualization of glycan-mediated processes, shining light on their role in living systems. This work describes the serendipitous discovery of a small chemical reporter-fluorophore.

Expanding the Scope of Palladium-Catalyzed B - N Cross-Coupling Chemistry in Carboranes.

Over the past several years, a number of strategies for the functionalization of dicarba--dodecaboranes (carboranes) have emerged. Despite these developments, B - N bond formation on the carborane scaffold remains a challenge due to the propensity of strong nucleophiles to partially deboronate the parent -carborane cluster into the corresponding form. Here we show that azide, sulfonamide, cyanate, and phosphoramidate nucleophiles can be straightforwardly cross-coupled onto the B(9) vertices of the - and -carborane core from readily accessible precursors without significant deboronation by-products, laying the groundwork for further study into the utility and properties of these new B-aminated carborane species.

Carborane Guests for Cucurbit[7]uril Facilitate Strong Binding and On-Demand Removal.

High-affinity guests have been reported for the macrocyclic host cucurbit[7]uril (CB[7]), enabling widespread applications, but hindering CB[7] materials from being returned to their guest-free state for reuse. Here, we present polyhedral boron clusters (carboranes) as strongly binding, yet easily removable, guests for CB[7]. Aided by a Pd-catalyzed coupling of an azide anion, we prepared boron-functionalized 9-amino--carborane that binds to CB[7] with a ≈ 10 M.

Tunable Dopants with Intrinsic Counterion Separation Reveal the Effects of Electron Affinity on Dopant Intercalation and Free Carrier Production in Sequentially Doped Conjugated Polymer Films.

Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)-based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape.

A Super-Oxidized Radical Cationic Icosahedral Boron Cluster.

While the icosahedral -[BH] cluster does not display reversible electrochemical behavior, perfunctionalization of this species via substitution of all 12 B-H vertices with alkoxy or benzyloxy (OR) substituents engenders reversible redox chemistry, providing access to clusters in the dianionic, monoanionic, and neutral forms. Here, we evaluated the electrochemical behavior of the electron-rich B(O-3-methylbutyl) () cluster and discovered that a new reversible redox event that gives rise to a fourth electronic state is accessible through one-electron oxidation of the neutral species. Chemical oxidation of with [N(2,4-BrCH)] afforded the isolable [] cluster, which is the first example of an open-shell cationic B cluster in which the unpaired electron is proposed to be delocalized throughout the boron cluster core.

Sterically Unprotected Nucleophilic Boron Cluster Reagents.

A cornerstone of modern synthetic chemistry rests on the ability to manipulate the reactivity of a carbon center by rendering it either electrophilic or nucleophilic. However, accessing a similar reactivity spectrum with boron-based reagents has been significantly more challenging. While classical nucleophilic carbon-based reagents normally do not require steric protection, readily accessible, unprotected boron-based nucleophiles have not yet been realized.

Off-Cycle Processes in Pd-Catalyzed Cross-Coupling of Carboranes.

Off-cycle processes in catalytic reactions can dramatically influence the outcome of the chemical transformation and affect its yield, selectivity, rate, and product distribution. While the generation of off-cycle intermediates can complicate reaction coordinate analyses or hamper catalytic efficiency, the generation of such species may also open new routes to unique chemical products. Recently, we reported the Pd-mediated functionalization of carboranes with a range of O-, N-, and C-based nucleophiles.

Improved synthesis of icosahedral carboranes containing exopolyhedral B-C and C-C bonds.

Carboranes are boron-rich molecular clusters possessing electronic characteristics that allow for orthogonal approaches to vertex-selective modifications. We report improved functionalization methods utilizing orthogonal chemistry to achieve efficient substitution at electron-rich B-vertices and electron-poor C-vertices of carborane. Functionalization of B-vertices with alkyl and (hetero)aryl groups using the corresponding Grignard reagents has been improved through the use of a Pd-based precatalyst featuring an electron-rich biaryl phosphine ligand, resulting in reduced reaction times.

Photooxidative Generation of Dodecaborate-Based Weakly Coordinating Anions.

Redox-active proanions of the type B(OCHAr) [Ar = CF (), 4-CFCH (), 3,5-(CF)CH ()] are introduced in the context of an experimental and computational study of the visible-light-initiated polymerization of a family of styrenes. Neutral, air-stable proanions - were found to initiate styrene polymerization through single-electron oxidation under blue-light irradiation, resulting in polymers with number-average molecular weights () ranging from ∼6 to 100 kDa. Shorter polymer products were observed in the majority of experiments, except in the case of monomers containing 4-X (X = F, Cl, Br) substituents on the styrene monomer when polymerized in the presence of in CHCl.

Synthesis of 9-borafluorene analogues featuring a three-dimensional 1,1'-bis(o-carborane) backbone.

The synthesis of [1,1'-bis(o-carboranyl)]boranes was achieved through the deprotonation of 1,1'-bis(o-carborane) reagents followed by salt metathesis with (iPr)2NBCl2. X-ray crystallography confirms planar central BC4 rings and Gutmann-Beckett studies reveal an increase in Lewis acidity at the boron center in comparison to their biphenyl congener, 9-borafluorene.

Dodecaborane-Based Dopants Designed to Shield Anion Electrostatics Lead to Increased Carrier Mobility in a Doped Conjugated Polymer.

One of the most effective ways to tune the electronic properties of conjugated polymers is to dope them with small-molecule oxidizing agents, creating holes on the polymer and molecular anions. Undesirably, strong electrostatic attraction from the anions of most dopants localizes the holes created on the polymer, reducing their mobility. Here, a new strategy utilizing a substituted boron cluster as a molecular dopant for conjugated polymers is employed.

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 (B and B-B 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.

Cage-Walking: Vertex Differentiation by Palladium-Catalyzed Isomerization of B(9)-Bromo-meta-Carborane.

We report the first observed Pd-catalyzed isomerization ("cage-walking") of B(9)-bromo-meta-carborane during Pd-catalyzed cross-coupling, which enables the formation of B-O and B-N bonds at all boron vertices (B(2), B(4), B(5), and B(9)) of meta-carborane. Experimental and theoretical studies suggest this isomerization mechanism is strongly influenced by the steric crowding at the Pd catalyst by either a biaryl phosphine ligand and/or substrate. Ultimately, this "cage-walking" process provides a unique pathway to preferentially introduce functional groups at the B(2) vertex using B(9)-bromo-meta-carborane as the sole starting material through substrate control.

Atomically precise organomimetic cluster nanomolecules assembled via perfluoroaryl-thiol SAr chemistry.

The majority of biomolecules are intrinsically atomically precise, an important characteristic that enables rational engineering of their recognition and binding properties. However, imparting a similar precision to hybrid nanoparticles has been challenging because of the inherent limitations of existing chemical methods and building blocks. Here we report a new approach to form atomically precise and highly tunable hybrid nanomolecules with well-defined three-dimensionality.

Luminescent Metal Complexes Featuring Photophysically Innocent Boron Cluster Ligands.

We report the synthesis and characterization of a series of d metal complexes featuring robust and photophysically innocent strong-field chelating 1,1'-bis(-carborane) () ligand frameworks. A combination of UV-Vis spectroscopy, single crystal X-ray structural analysis, and DFT calculations of these species suggest that the dianionic ligand does not contribute to any visible metal-to-ligand charge transfer (MLCT) transitions, yet it provides a strong ligand field in these complexes. Furthermore, a -based Pt(II) complex containing a 4,4'-di--butyl-2,2'-bipyridine ligand (dtb-bpy) has been prepared and was found to display blue phosphorescent emission dominated by MLCT from the Pt(II) center to the dtb-bpy ligand.

Blue Phosphorescent Zwitterionic Iridium(III) Complexes Featuring Weakly Coordinating nido-Carborane-Based Ligands.

We report the development of a new class of phosphorescent zwitterionic bis(heteroleptic) Ir(III) compounds containing pyridyl ligands with weakly coordinating nido-carboranyl substituents. Treatment of phenylpyridine-based Ir(III) precursors with C-substituted ortho-carboranylpyridines in 2-ethoxyethanol results in a facile carborane deboronation and the formation of robust and highly luminescent metal complexes. The resulting nido-carboranyl fragments associate with the cationic Ir(III) center through primarily electrostatic interactions.

Rapid Synthesis of Redox-Active Dodecaborane B(OR) Clusters Under Ambient Conditions.

We have developed a fast and efficient route to obtain perfunctionalized ether-linked alkyl and benzyl derivatives of the -[B(OH)] 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 -[B(OR)], -[B(OR)] and -[B(OR)] clusters have been prepared with reaction times ranging from hours to several minutes.

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.

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.

Synthesis of Tungsten Imido Alkylidene Complexes that Contain an Electron-Withdrawing Imido Ligand.

Tungsten NAr alkylidene complexes have been prepared that contain the electron-withdrawing Ar groups 2,4,6-XCH (Ar, X = Cl, Br), 2,6-Cl-4-CFCH (Ar), and 3,5-(CF)CH (Ar). Reported complexes include W(NAr)Cl(dme) (dme = 1,2-dimethoxyethane), W(NAr)(CHCMe), W(NAr)(CHCMe)(OTf)(dme), and W(NAr)(CHCMe)(ODBMP) (DBMP = 4-Me-2,6-(CHPh)CH). The W(NAr)(CHCMe)(ODBMP) complexes were explored as initiators for the polymerization of 2,3-dicarbomethoxynorbornadiene (DCMNBD).

Stereospecific ring-opening metathesis polymerization of norbornadienes employing tungsten oxo alkylidene initiators.

We report here the polymerization of several 7-isopropylidene-2,3-disubstituted norbornadienes, 7-oxa-2,3-dicarboalkoxynorbornadienes, and 11-oxa-benzonorbornadienes with a single tungsten oxo alkylidene catalyst, W(O)(CH-t-Bu)(OHMT)(Me2Pyr) (OHMT = 2,6-dimesitylphenoxide; Me2Pyr = 2,5-dimethylpyrrolide) to give cis, stereoregular polymers. The tacticities of the menthyl ester derivatives of two polymers were determined for two types. For poly(7-isopropylidene-2,3-dicarbomenthoxynorbornadiene) the structure was shown to be cis,isotactic, while for poly(7-oxa-2,3-dicarbomenthoxynorbornadiene) the structure was shown to be cis,syndiotactic.