Classically closo-carborane anions, particularly [HCB11H11]− and [HCB9H9]−, and their derivatives have primarily been used as weakly coordinating anions to isolate reactive intermediates, platforms for stoichiometric and catalytic functionalization, counteranions for simple Lewis acid catalysis, and components of materials like liquid crystals. The aim of this article is to educate the reader on the contemporary nonclassical applications of these anions. Specifically, this review will cover new directions in main group catalysis utilized to achieve some of the most challenging catalytic reactions such as C–F, C–H, and C–C functionalizations that are difficult or impossible to realize with transition metals. In addition, the review will cover the utilization of the clusters as dianionic C σ-bound ligands for coordination chemistry, ligand substituents for coordination chemistry and advanced catalyst design, and covalently bound spectator substituents to stabilize radicals. Furthermore, their applications as solution-based and solid-state electrolytes for Li, Na, and Mg batteries will be discussed.

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 ( M g ( C B 1 1 H 1 2 ) 2 ) a n d m a g n e s i u m b i s -(trifluoromethanesulfonyl)imide (Mg(TFSI) 2 ). Cyclic voltammetry and chronopotentiometry of Mg plating−stripping demonstrate significantly lower overpotential in the Mg-(CB 11 H 12 ) 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(CB 11 H 12 ) 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.

Solid-state ion conductors based on closo-polyborate anions combine high ionic conductivity with a rich array of tunable properties. Cation mobility in these systems is intimately related to the strength of the interaction with the neighboring anionic network and the energy for reorganizing the coordination polyhedra. Here, we explore such factors in solid electrolytes with two anions of the weakest coordinating ability, [HCB11H5Cl6]− and [HCB11H5Br6]−, and a total of 11 polymorphs are identified for their lithium and sodium salts. Our approach combines ab initio molecular dynamics, synchrotron X-ray powder diffraction, differential scanning calorimetry, and AC impedance measurements to investigate their structures, phase-transition behavior, anion orientational mobilities, and ionic conductivities. We find that M­(HCB11H5X6) (M = Li, Na, X = Cl, Br) compounds exhibit order–disorder polymorphic transitions between 203 and 305 °C and display Li and Na superionic conductivity in the disordered state. Through detailed analysis, we illustrate how cation disordering in these compounds originates from a competitive interplay among the lattice symmetry, the anion reorientational mobility, the geometric and electronic asymmetry of the anion, and the polarizability of the halogen atoms. These factors are compared to other closo-polyborate-based ion conductors to suggest guidelines for optimizing the cation–anion interaction for fast ion mobility. This study expands the known solid-state poly­(carba)­borate-based materials capable of liquid-like ionic conductivities, unravels the mechanisms responsible for fast ion transport, and provides insights into the development of practical superionic solid electrolytes.

In this feature article we cover new directions in the fundamental and applied chemistry of the closo-carborane anions [HCB11H11]−1 and [HCB9H9]−1, including energy storage applications, ionic liquids, anionic carborane fused heterocycles/radicals, ligand substituents, and ligands for catalysis and coordination chemistry.

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 [HCB11H11]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.