RESUMEN

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-ortho-carborane that binds to CB[7] with a Ka ≈ 1010 M-1. Upon basic treatment, ortho-carborane readily undergoes deboronation to yield anionic nido-carborane, a poor guest for CB[7], facilitating recovery of guest-free CB[7]. We showcase the utility of the modified ortho-carborane guest by recycling a CB[7]-functionalized resin. With this report, we introduce stimuli-responsive decomplexation as an additional consideration in the design of high-affinity host-guest complexes.

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RESUMEN

Over the past several years, a number of strategies for the functionalization of dicarba-closo-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 closo-carborane cluster into the corresponding nido form. Here we show that azide, sulfonamide, cyanate, and phosphoramidate nucleophiles can be straightforwardly cross-coupled onto the B(9) vertices of the o- and m-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. We further showcase select reactivity of the installed functional groups highlighting some unique features stemming from the combination of the electron-donating B(9) position and the large steric profile of the B-connected carborane substituent.

RESUMEN

The chloride gradient plays an important role in regulating cell volume, membrane potential, pH, secretion, and the reversal potential of inhibitory glycine and GABAA receptors. Measurement of intracellular chloride activity, [Formula: see text], using liquid membrane ion-selective microelectrodes (ISM), however, has been limited by the physiochemical properties of Cl- ionophores which have caused poor stability, drift, sluggish response times, and interference from other biologically relevant anions. Most importantly, intracellular [Formula: see text] may be up to 4 times more abundant than Cl- (e.g. skeletal muscle) which places severe constraints on the required selectivity of a Cl- - sensing ISM. Previously, a sensitive and highly-selective Cl- sensor was developed in a polymeric membrane electrode using a trinuclear Hg(II) complex containing carborane-based ligands, [9]-mercuracarborand-3, or MC3 for short. Here, we have adapted the use of the MC3 anion carrier in a liquid membrane ion-selective microelectrode and show the MC3-ISM has a linear Nernstian response over a wide range of aCl (0.1 mM to 100 mM), is highly selective for Cl- over other biological anions or inhibitors of Cl- transport, and has a 10% to 90% settling  time of 3 sec. Importantly, over the physiological range of aCl (1 mM to 100 mM) the potentiometric response of the MC3-ISM is insensitive to [Formula: see text] or changes in pH. Finally, we demonstrate the biological application of an MC3-ISM by measuring intracellular aCl, and the response to an external Cl-free challenge, for an isolated skeletal muscle fiber.

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We report the discovery that a perhydroxylated dodecaborate cluster ([B12(OH)12]2-) can act as an inorganic polyol, serving as a molecular cross-linker in the synthesis of polyurethane-based materials. We further demonstrate how the inherent robustness of the utilized boron cluster can effectively enhance the thermal stability of the produced polyurethane materials incorporating [B12(OH)12]2- building blocks compared to analogous polymers made from carbon-based polyols. Ultimately, this approach provides a potential route to tune the chemical and physical properties of soft materials through incorporation of polyhedral boron-rich clusters into the polymer network.

RESUMEN

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. By utilizing a Pd-based catalytic system supported by a biaryl phosphine ligand developed by Buchwald and co-workers, we discovered an off-cycle isomerization process ("cage-walking") that generates four regioisomeric products from a single halogenated boron cluster isomer. Here we describe how several off-cycle processes affect the regioisomer yield and distribution during Pd-catalyzed tandem cage-walking/cross-coupling. In particular, tuning the transmetallation step in the catalytic cycle allowed us to incorporate the cage-walking process into Pd-catalyzed cross-coupling of sterically unencumbered substrates, including cyanide. This work demonstrates the feasibility of using tandem cage-walking/cross-coupling as a unique low-temperature method for producing regioisomers of mono-substituted carboranes.

RESUMEN

Over the past several decades, metal-catalyzed cross-coupling has emerged as a very powerful strategy to functionalize carbon-based molecules. More recently, some of the cross-coupling methodologies have been adapted to inorganic compounds including boron-rich clusters. The development of this chemistry relies on the ability to synthesize halogenated boron-rich clusters which can serve as electrophilic cross-coupling partners with nucleophilic substrates in the presence of a metal catalyst. While the cross-coupling chemistry with boron-clusters is conceptually reminiscent of that of its hydrocarbon counterparts, several key aspects including the spheroidal bulk of clusters and the distinct nature of boron-halogen/boron-heteroatom bonds make this chemistry unique. The utility of metal-catalyzed cross-coupling can be extended to several classes of polyhedral boranes including neutral and anionic carboranes, metallaboranes, and carbon-free boranes. Importantly, cross-coupling enables a suite of boron-heteroatom (C, N, O, P, S) couplings to prepare boron cluster-based systems that can be used for ligand design, medicinal chemistry, and materials applications.

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Electrochemical systems offer a versatile means for creating adaptive devices. However, the utility of electrochemical deposition is inherently limited by the properties of the electrolyte. The development of ionic liquids enables electrodeposition in high-vacuum environments and presents opportunities for creating electrochemically adaptive and regenerative spacecraft components. In this work, we developed a silver-rich, boron cluster ionic liquid (BCIL) for reversible electrodeposition of silver films. This air and moisture stable electrolyte was used to deposit metallic films in an electrochemical cell to tune the emissivity of the cell in situ, demonstrating a proof-of-concept design for spacecraft thermal control.

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Self-assembled monolayers (SAMs) of carborane isomers with different dipole moments passivate germanium to modulate surface work function while maintaining chemical environment and surface energy. To identify head groups capable of monolayer formation on germanium surfaces, we studied thiol-, hydroxyl-, and carboxyl-terminated carboranes. These films were successfully formed with carboxylic acid head groups instead of the archetypal thiol, suggesting that the carborane cluster significantly affects headgroup reactivity. Film characterization included X-ray and ultraviolet photoelectron spectroscopies as well as contact angle goniometry. Using these carboranes, the germanium surface work function was tailored over 0.4 eV without significant changes to wetting properties.

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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.

RESUMEN

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.

RESUMEN

In contrast to the plethora of metal-catalyzed cross-coupling methods available for the installation of functional groups on aromatic hydrocarbons, a comparable variety of methods are currently not available for icosahedral carboranes, which are boron-rich three-dimensional aromatic analogues of aryl groups. Part of this is due to the limited understanding of the elementary steps for cross-coupling involving carboranes. Here, we report our efforts in isolating metal-boryl complexes to further our understanding of one of these elementary steps, oxidative addition. Structurally characterized examples of group 10 M-B bonds featuring icosahedral carboranes are completely unknown. Use of mercurocarboranes as a reagent to deliver M-B bonds saw divergent reactivity for platinum and palladium, with a Pt-B bond being isolated for the former, and a rare Pd-Hg bond being formed for the latter.