When the Ferrocene Analogy Breaks Down: Metallocene Transmetallation Chemistry.

Ferrocene 1 and its dianionic Fe(bis)(dicarbollide) analogue 2 are classical compounds that display unusual stability. These compounds are not known to undergo transmetallation chemistry of the Fe-center and have been used extensively as chemical building blocks with consistent integrity. In this manuscript we describe the preparation of a charge compensated Fe(bis)(dicarbollide) species 3 Fe and its unprecedented transmetallation chemistry to Ir. Such reactions are hitherto unknown for any transition metal metallocene or metallacarborane complex. Additionally, we show that 3 Fe can be deprotonated to afford the corresponding bis(NHC) Li-carbenoid 5 that also displays unique reactivity. When 5 is reacted with [Ir(COD)Cl]2 it also undergoes a rapid transmetallation of the ferrocene "like" core to afford 6 but with the added twist that the Li-carbenoid moiety stays intact and does not transmetalate. However, when 6 is subsequently treated with CuCl, the Li-carbenoid transmetalates to Cu, which allows the controlled formation of the corresponding heterobimetallic Ir/Cu aggregate. Lastly, when Li-carbenoid 5 is treated directly with CuCl, a double transmetallation occurs from both Fe to Cu and Li-carbenoid to Cu, resulting in the trimetallic Cu cluster 8. These novel reactions pave the way for new synthetic methods to build complicated polymetallic clusters in a controlled fashion.

A Carboranyl Electrolyte Enabling Highly Reversible Sodium Metal Anodes via a "Fluorine-Free" SEI.

Realization of practical sodium metal batteries (SMBs) is hindered due to lack of compatible electrolyte components, dendrite propagation, and poor understanding of anodic interphasial chemistries. Chemically robust liquid electrolytes that facilitate both favorable sodium metal deposition and a stable solid-electrolyte interphase (SEI) are ideal to enable sodium metal and anode-free cells. Herein we present advanced characterization of a novel fluorine-free electrolyte utilizing the [HCB11 H11 ]1- anion. Symmetrical Na cells operated with this electrolyte exhibit a remarkably low overpotential of 0.032 V at a current density of 2.0 mA cm-2 and a high coulombic efficiency of 99.5 % in half-cell configurations. Surface characterization of electrodes post-operation reveals the absence of dendritic sodium nucleation and a surprisingly stable fluorine-free SEI. Furthermore, weak ion-pairing is identified as key towards the successful development of fluorine-free sodium electrolytes.

Strongly Coordinating Ligands To Form Weakly Coordinating Yet Functional Organometallic Anions.

Weakly coordinating anions (WCAs) are generally tailored to act as spectators with little or no function. Here we describe the implementation of strongly coordinating dianionic carboranyl N-heterocyclic carbenes (NHCs) to create organometallic -ate complexes of Au(I) that serve both as WCAs and functional catalysts. These organometallic WCAs can be utilized to form both heterobimetallic (Au(I)-/Ag(I)+; Au(I)-/Ir(I)+) and organometallic/main group ion pairs (Au(I)-/(CPh3+ or SiEt3+). Because parent unfunctionalized dianionic carboranyl NHC complex 3 is unstable in most solvents when paired with CPh3+, novel synthesis methodology was devised to create polyhalogenated carboranyl NHCs, which show superior stability toward electrophilic substitution and cyclometalation chemistry. Additionally, the WCAs containing polyhalogenated carboranyl NHCs are among the most active catalysts reported for the hydroamination of alkynes. This investigation has also produced the first examples of a low-coordinate Au(III) center with two cis accessible coordination sites and the first true dianionic carbene. These studies pave the way for the design of functional ion pairs that have the potential to participate in tandem or cooperative small-molecule activation and catalysis.

Vinyl Carbocations Generated under Basic Conditions and Their Intramolecular C-H Insertion Reactions.

Here we report the surprising discovery that high-energy vinyl carbocations can be generated under strongly basic conditions, and that they engage in intramolecular sp3 C-H insertion reactions through the catalysis of weakly coordinating anion salts. This approach relies on the unconventional combination of lithium hexamethyldisilazide base and the commercially available catalyst, triphenylmethylium tetrakis(pentafluorophenyl)borate. These reagents form a catalytically active lithium species that enables the application of vinyl cation C-H insertion reactions to heteroatom-containing substrates.

Fusing Dicarbollide Ions with N-Heterocyclic Carbenes.

Discovered by Hawthorne in 1965, dicarbollide ions are an intriguing class of nido-carboranes that mimic the behavior of the cyclopentadienyl anion. Herein, we show that it is possible to directly link the dicarbollide ion to an N-heterocyclic carbene (NHC) to form an isolable N-dicarbollide-substituted NHC dianion. This molecule can be accessed by the sequential double deprotonation of a mono-nido-carboranyl imidazolium zwitterion. As revealed by a single-crystal X-ray diffraction study, the first deprotonation leads to a monoanionic dicarbollide ion that adopts a bis(dicarbollide) structure in the solid state. Subsequent deprotonation of this monoanion leads to the first N-dicarbollide NHC, which was fully characterized by multinuclear NMR spectroscopy as well as single-crystal X-ray diffraction.

Dehydrogenation of icosahedral carborane anions via gas-phase collisional activation.

RATIONALE: The icosahedral carborane anion HCB11 H11-1 is well known for its exceptional stability in solution and the solid state, but the gas-phase properties of this molecule have not previously been explored. METHODS: Electrospray ionization ion trap mass spectrometry, collisional activation, and isotopic labelling were used to examine the formation and reaction pathways of covalent and noncovalent carborane clusters. RESULTS: Covalent attachment of an amino group to the C-vertex of HCB11 H11-1 dramatically alters its stability in the gas phase. Interestingly, covalently bound carborane dimers are produced during electrospray ionization. Subsequent collisional activation of these dimers leads to facile hydrogen losses (7 H2 molecules). The loss of H2 does not follow a stochastic pattern, but occurs preferentially by loss of 3 H2 , followed by loss of 2 H2 and then another 2 H2 molecules. CONCLUSIONS: This study provides insight into new possible modes of B-H activation and functionalization in carborane cages. Copyright © 2016 John Wiley & Sons, Ltd.

Synthesis and reactivity of a zwitterionic palladium allyl complex supported by a perchlorinated carboranyl phosphine.

A zwitterionic palladium complex of a phosphine bearing a perchlorinated carba-closo-dodecaborate anion as a ligand substituent is reported. A single-crystal X-ray diffraction study reveals that, in the solid state, one of the chlorides of the carborane cage occupies a coordination site of the square-planar complex. However, in solution, the P-carborane bond of the ligand is rapidly rotating at temperatures as low as -90 °C, which demonstrates the carborane substituent's weak coordinative ability even though this anion is covalently linked to the phosphine ligand. The complex is thermally stable and catalyzes the vinyl addition polymerization of norbornene.

Synthesis of a hybrid m-terphenyl/o-carborane building block: applications in phosphine ligand design.

A hybrid terphenyl/o-carborane ligand building block is synthesized by the reaction of m-terphenylalkyne with B10H14. This sterically demanding substituent can be installed into ligands, as demonstrated by the preparation of carboranylphosphine. The bulky phosphine reacts with [ClRh(CO)2]2 to produce monophosphine complex ClRhL(CO)2, which subsequently extrudes CO under vacuum to afford the dimeric species [ClRhL(CO)]2. The latter complex does not react with excess phosphine and is resistant toward cyclometalation, which is in contrast to related o-carborane phosphine complexes. Data from a single-crystal X-ray diffraction study are utilized to quantify the steric impact of the ligand via the percent buried volume approach.

Comparative reactivity of different types of stable cyclic and acyclic mono- and diamino carbenes with simple organic substrates.

A series of stable carbenes, featuring a broad range of electronic properties, were reacted with simple organic substrates. The N,N-dimesityl imidazolylidene (NHC) does not react with isocyanides, whereas anti-Bredt di(amino)carbene (pyr-NHC), cyclic (alkyl)(amino)carbene (CAAC), acyclic di(amino)carbene (ADAC), and acyclic (alkyl)(amino)carbene (AAAC) give rise to the corresponding ketenimines. NHCs are known to promote the benzoin condensation, and we found that the CAAC, pyr-NHC, and ADAC react with benzaldehyde to give the ketone tautomer of the Breslow intermediate, whereas the AAAC first gives the corresponding epoxide and ultimately the Breslow intermediate, which can be isolated. Addition of excess benzaldehyde to the latter does not lead to benzoin but to a stable 1,3-dioxolane. Depending on the electronic properties of carbenes, different products are also obtained with methyl acrylate as a substrate. The critical role of the carbene electrophilicity on the outcome of reactions is discussed.

Fusing N-heterocyclic carbenes with carborane anions.

Here we describe the fusion of two families of unusual carbon-containing molecules that readily disregard the tendency of carbon to form four chemical bonds, namely N-heterocyclic carbenes (NHCs) and carborane anions. Deprotonation of an anionic imidazolium salt with lithium diisopropylamide at room temperature leads to a mixture of lithium complexes of C-2 and C-5 dianionic NHC constitutional isomers as well as a trianionic (C-2, C-5) adduct. Judicious choice of the base and reaction conditions allows the selective formation of all three stable polyanionic carbenes. In solution, the so-called abnormal C-5 NHC lithium complex slowly isomerizes to the normal C-2 NHC, and the process can be proton-catalyzed by the addition of the anionic imidazolium salt. These results indicate that the combination of two unusual forms of carbon atoms can lead to unexpected chemical behavior, and that this strategy paves the way for the development of a broad new generation of NHC ligands for catalysis.

Electrolyte Engineering with Carboranes for Next-Generation Mg Batteries.

To realize an energy storage transition beyond Li-ion competitive technologies, earth-abundant elements, such as Mg, are needed. Carborane anions are particularly well-suited to realizing magnesium-ion batteries (MIBs), as their inert and weakly coordinating properties beget excellent electrolyte performance. However, utilizing these materials in actual electrochemical cells has been hampered by the reliance on the Mg2+ salts of the commercially available [HCB11H11]- anion, which is not soluble in more weakly binding solvents apart from the higher glymes. Herein, we demonstrate it is possible to iteratively engineer the [HCB11H11]- anion surface synthetically to address previous solubility issues and yield a highly conductive (up to 7.33 mS cm-1) and electrochemically stable (up to +4.2 V vs Mg2+/0) magnesium electrolyte that surpasses the state of the art. This novel non-nucleophilic electrolyte exhibits highly dissociative behavior regardless of concentration and is tolerant of prolonged periods of cycling in symmetric cells at high current densities (up to 2.0 mA cm-2, 400 h). The hydrocarbon functionalized carborane electrolyte presented here demonstrates >96% Coulombic efficiency when paired with a Mo6S8 cathode. This approach realizes a needed candidate to discover next-generation cathode materials that can enable the design of practical and commercially viable Mg batteries.

High-Throughput Identification of Crystalline Natural Products from Crude Extracts Enabled by Microarray Technology and microED.

The structural determination of natural products (NPs) can be arduous because of sample heterogeneity. This often demands iterative purification processes and characterization of complex molecules that may be available only in miniscule quantities. Microcrystal electron diffraction (microED) has recently shown promise as a method to solve crystal structures of NPs from nanogram quantities of analyte. However, its implementation in NP discovery remains hampered by sample throughput and purity requirements, akin to traditional NP-discovery workflows. In the methods described herein, we leverage the resolving power of transmission electron microscopy (TEM) and the miniaturization capabilities of deoxyribonucleic acid (DNA) microarray technology to address these challenges through the establishment of an NP screening platform, array electron diffraction (ArrayED). In this workflow, an array of high-performance liquid chromatography (HPLC) fractions taken from crude extracts was deposited onto TEM grids in picoliter-sized droplets. This multiplexing of analytes on TEM grids enables 1200 or more unique samples to be simultaneously inserted into a TEM instrument equipped with an autoloader. Selected area electron diffraction analysis of these microarrayed grids allows for the rapid identification of crystalline metabolites. In this study, ArrayED enabled structural characterization of 14 natural products, including four novel crystal structures and two novel polymorphs, from 20 crude extracts. Moreover, we identify several chemical species that would not be detected by standard mass spectrometry (MS) or ultraviolet-visible (UV/vis) spectroscopy and crystal forms that would not be characterized using traditional methods.

Observation of room temperature B-Cl activation of the HCB11Cl11(-) anion and isolation of a stable anionic carboranyl phosphazide.

The perchlorinated carba-closo-dodecaborate anion is typically inert toward B-Cl functionalization. We present here the observation of two competing reactions that occur with this anion at ambient temperature. When this molecule is treated with n-BuLi and subsequently reacted with tosyl azide, a cycloaddition occurs and results in chloride substitution at a B-Cl vertex. The competing and dominant pathway is a substitution reaction to form the azide N3CB11Cl11(-). This rare anionic carboranyl azide reacts with PPh3 in FC6H5 to afford a stable anionic phosphazide. When dissolved in tetrahydrofuran, the phosphazide is in equilibrium with free PPh3 and N3CB11Cl11(-). Both the triazole and phosphazide are characterized by single-crystal X-ray diffraction, NMR and IR spectroscopy, and high-resolution mass spectrometry.

Click-like reactions with the inert HCB11Cl11(-) anion lead to carborane-fused heterocycles with unusual aromatic character.

The chlorinated carba-closo-dodecaborate anion HCB11Cl11(-) is an exceptionally stable molecule and has previously been reported to be substitutionally inert at the B-Cl vertices. We present here the discovery of base induced cycloaddition reactions between this carborane anion and organic azides that leads to selective C and B functionalization of the cluster. A single crystal X-ray diffraction study reveals bond lengths in the heterocyclic portion of the ring that are shortened, which suggests electronic delocalization. Molecular orbital analysis of the ensuing heterocycles reveals that two of the bonding orbitals of these systems resemble two of the doubly occupied π-MOs of a simple 5-membered Hückel-aromatic, even though they are entangled in the carborane skeleton. Nucleus independent chemical shift analysis indicates that both the carborane cluster portion of the molecule and the carborane fused heterocyclic region display aromatic character. Computational methods indicate that the reaction likely follows a stepwise addition cyclization pathway.

Isolation of a carborane-fused triazole radical anion.

Outside the cage: A change in the redox properties of a triazole fused to a carborane anion through methylation to form a zwitterion enabled facile chemical reduction of the compound to an isolable triazole radical anion (see structure: C gray, H white, N blue, B brown, Cl green). The radical anion is stabilized by kinetic protection by the chlorinated carborane and the delocalization of spin density throughout the exo-cluster π system.

Correction: Cesium carbonate mediated C-H functionalization of perhalogenated 12-vertex carborane anions.

Correction for 'Cesium carbonate mediated C-H functionalization of perhalogenated 12-vertex carborane anions' by Sergio O. Lovera et al., Chem. Commun., 2022, 58, 4060-4062, DOI: https://doi.org/10.1039/D2CC00173J.

Cesium carbonate mediated C-H functionalization of perhalogenated 12-vertex carborane anions.

C-H functionalization of undecahalogenated carborane anions, [HCB11X11-] (X = Cl, Br, I), is performed with Cs2CO3 in acetonitrile. We show that the requisite Cl, Br and I carborane dianions can all be efficiently accessed with Cs2CO3. The utilization of Cs2CO3 eliminates the complications associated with competing E2 elimination reactions providing an efficient, more functional group tolerant, and broader scope than previously reported. The ensuing functionalized cages provide potential synthons for constructing advanced materials and other molecular architectures for various applications.

Fusing 10-vertex closo-carborane anions with N-heterocyclic carbenes.

Discovered by Knöth in 1964, the 10-vertex closo-carborane anion [HCB9H91-] is a classical bicapped square antiprism that contains an unusual pentacoordinate carbon center. Compared to its larger icosahedral cousin [HCB11H111-], few investigations have been made into its use as a weakly coordinating anion or as a ligand substituent. Here we show that it is possible to prepare both a dianionic N-heterocyclic carbene (NHC) Li+ adduct as well as a trianionic C-2, C-5 dilithio species featuring two 10-vertex carborane anion substituents. All compounds were characterized via multinuclear NMR spectroscopy, single crystal X-ray diffraction, and HRMS when possible.

Teaching an old dog new tricks: new directions in fundamental and applied closo-carborane anion chemistry.

This feature article covers new directions in the fundamental and applied chemistry of the closo-carborane anions [HCB11H11]- and [HCB9H9]-, as well as some related chemistry with the dicarbolide ion [H2C2B9]2-. Specifically the manuscript will focus on summarizing the authors' as well as related novel contributions to the field. The application of such clusters as solution based electolytes for Mg batteries and related materials for ionic liquids will be discussed. In addition, the preparation of heterocycles and radicals fused to carborane anions will be discussed as well as various novel chemical transformations. Furthermore, new developments in anionic carboranyl phosphines and N-heterocyclic carbenes in the context of catalysis will be summarized.

Comparative Study of Mg(CB11H12)2 and Mg(TFSI)2 at the Magnesium/Electrolyte Interface.

An essential requirement for electrolytes in rechargeable magnesium-ion (Mg-ion) batteries is to enable Mg plating-stripping with low overpotential and high Coulombic efficiency. To date, the influence of the Mg/electrolyte interphase on plating and stripping behaviors is still not well understood. In this study, we investigate the Mg/electrolyte interphase from electrolytes based on two Mg salts with weakly coordinating anions: magnesium monocarborane (Mg(CB11H12)2) and magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2). Cyclic voltammetry and chronopotentiometry of Mg plating-stripping demonstrate significantly lower overpotential in the Mg(CB11H12)2 electrolyte than in Mg(TFSI)2 under the same condition. Surface characterizations including X-ray photoelectron spectroscopy and scanning electron microscopy clearly demonstrate the superior chemical and electrochemical stability of the Mg(CB11H12)2 electrolyte at the Mg surface without noticeable interphase formation. On the other hand, characterizations of the Mg/electrolyte interface in the Mg(TFSI)2 electrolyte indicate the formation of magnesium oxide, magnesium sulfide, and magnesium fluoride as the interfacial compounds resulting from the decomposition of TFSI- anions because of both chemical reduction by Mg and cathodic reduction during Mg deposition.