Carborane Acids. New ‘‘Strong Yet Gentle’’ Acids for Organic and Inorganic Chemistry
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FEATURE ARTICLE www.rsc.org/chemcomm | ChemComm Carborane acids. New ‘‘strong yet gentle’’ acids for organic and inorganic chemistry Christopher A. Reed Received (in Cambridge, UK) 5th October 2004, Accepted 8th February 2005 First published as an Advance Article on the web 25th February 2005 DOI: 10.1039/b415425h 2 Icosahedral carborane anions such as CHB11Cl11 are amongst the least coordinating, most chemically inert anions known. They are also amongst the least basic, so their conjugate acids, H(carborane), are superacids (i.e. stronger than 100% H2SO4). Acidity scale measurements indicate that H(CHB11Cl11) is the strongest pure Brønsted acid presently known, surpassing triflic and fluorosulfuric acid. Nevertheless, it is also an extremely gentle acid—because its conjugate base engages in so little chemistry. Carborane acids separate protic acidity from anion nucleophilicity and destructive oxidative capacity in the conjugate base, to a degree not previously achieved. As a result, many long-sought, highly acidic, reactive cations such as protonated + + + benzene (C6H7 ), protonated C60 (HC60 ), tertiary carbocations (R3C ), vinyl cations + + + + (R2CLC –R), silylium ions (R3Si ) and discrete hydronium ions (H3O ,H5O2 etc.) can be readily isolated as carborane salts and characterized at room temperature by X-ray crystallography. Introduction crystallization of salts. They make it much easier to isolate long-sought, reactive cations and easier to stabilize the fleeting Acids and acid catalysis have been an important part of intermediates of acid catalyzed reactions. chemistry from its very beginnings. In the last half century, a Carborane acids are also leading to new conceptual thinking superacidic medium (i.e. acidity greater than 100% H2SO4) has about superacidity. Carborane acids protonate weakly basic become widely recognized as a useful if still somewhat exotic molecules more easily than expected, attaining superacidity + 1 2+ 2 environment to stabilize reactive cations (e.g. R3C , S8 , without the addition of Lewis acid. This has led to the + 3 + 4 2+ 5 6 HCO , N5 , AuXe4 , etc.). It would be easy to think that previously unrecognized concept of ‘‘basicity suppression’’ in acidity is a mature field, and that there is little left to be traditional mixed Lewis/Brønsted superacids and suggests that discovered. the basicities of many weakly basic molecules (e.g. alkenes, Xe) 7 However, the preparation of carborane acids shows that have been significantly underestimated. traditional strong acids and superacids have significant The title phrase ‘‘strong yet gentle’’ captures the essential limitations. Carborane acids differ from traditional superacids value of carborane acids. They separate Brønsted (protic) by bringing unexpectedly high superacidity to weakly basic acidity from oxidation and nucleophilicity in the conjugate organic solvents without the complicating necessity of adding a base, in a manner not previously achieved. Typical strong strong Lewis acid (e.g. SbF ). They surpass triflic acid in terms 2 2 5 acids and superacids have oxyanions (HSO4 ,CF3SO3 etc.) of acid strength, low anion nucleophilicity and ease of 2 2 or fluoroanions (BF4 , SbF6 etc.) as their conjugate bases and the oxidative capacity and nucleophilicity of these anions Christopher Reed is Distinguished Professor and Executive frequently conspire to decompose protonated molecules. The Director of the Center for S and P Block Chemistry at the corrosiveness of oxyacids and fluoroacids is often the result of University of California, Riverside. He obtained his PhD at The anion redox and nucleophile chemistry. The wise chemist University of Auckland in New Zealand with Warren R. Roper chooses hydrochloric acid not nitric acid to clean a copper FRS and did postdoctoral studies with James P. Collman at Stanford University. His areas of research interest include kettle, thereby saving the kettle from oxidative destruction by coordination chemistry and organometallic chemistry, the the nitrate anion. By contrast, icosahedral carborane anions of 2 bioinorganic chemistry of iron porphyrins, copper and dioxygen, the type, CHB11R5X6 (R 5 H, CH3, Cl; X 5 Cl, Br, I, see spin coupling phenomena, a magnetochemical alternative to the Fig. 1), are amongst the least nucleophilic, least-redox active, Spectrochemical Series, fullerene chemistry, metal–organic most chemically inert anions of modern chemistry.8 framework solids, weakly binding ligands, weakly coordinating As a result, their conjugate acids are very effective in anions, carboranes, acids, and reactive cations across the protonating sensitive molecules without subsequent decom- periodic table. His work has been recognized by Sloan, position chemistry. Dreyfus Teacher-Scholar, Guggenheim and Senior von An added bonus is the tendency of salts of carborane anions Humboldt Awards. He is a Fellow of the AAAS and the NZIC and serves on the Editorial Advisory Boards of Chemical to crystallize nicely. Thus, cations that had only been available Communications, Accounts of Chemical Research and for study in solution by spectroscopic methods at sub-ambient Heteroatom Chemistry. This article is based on a plenary temperatures can now be crystallized, put in a bottle, and lecture given at the 7th International Conference on Heteroatom mounted on an X-ray diffractometer for detailed structural Chemistry (ICHAC-7) in Shanghai, August 21–25, 2004. characterization. This journal is ß The Royal Society of Chemistry 2005 Chem. Commun., 2005, 1669–1677 | 1669 chloro substituent (from the 7 position on the carborane) indicating that the halide substituents are the most basic sites on the anion. The long Si????Cl bond, ca. 0.3 A˚ longer than a typical Si–Cl bond, has led to their description as ‘‘ion-like’’.16 Although clearly not ionic, these silylium species react as though they were silylium ions, because the carborane anion is the leaving group par excellence. Synthesis and characterization of carborane acids A strong Brønsted acid is typically prepared via the action of a strong Lewis acid on a weaker Brønsted acid. In the case of carborane acids, the extreme electrophilicity and halophilicity 16 of silylium species, R3Si(carborane), (see Fig. 2) is exploited (eqn. (1)). Fig. 1 The anions used as conjugate bases to carborane acids. Et3Si(carborane)(s) + HCl(l) A H(carborane)(s) + Et3SiCl(g) (1) Carborane anions The driving force for this reaction is the greater strength of the The parent icosahedral carborane anion, CB H 2, was first 11 12 Si–Cl bond (ca. 113 kcal mol21) relative to that of HCl synthesized by Knoth at Du Pont in 1967, at a time when (103 kcal mol21). The acid is produced in essentially industrial chemists were freer to pursue their curiosity.9 The 2 quantitative yield by stirring a slurry of the silylium starting chemistry of CB11H12 lay fallow for a couple of decades while research on the neutral and dianionic isolectronic analogues, material in liquefied anhydrous HCl at low temperatures. The 22 volatile byproduct and excess HCl are simply removed by C2B10H12 and B12H12 , took precedence. In the mid-1980s, the dedicated Czech boron group of Plesˇek, Sˇtibr and warming to room temperature under vacuum. Herˇma´nek reported an improved synthesis from decaborane10 Carborane acids are moisture-sensitive, white solids with and showed that halogenation proceeded quite selectively to extremely high thermal stability. They sublime under vacuum 2 at temperatures in the range 150–200 uC. The IR spectrum give 7,8,9,10,11,12-hexahalogenated anions, CHB11H5X6 (X 5 Cl, Br; see Fig. 1).11 Upon reading that anions of this shows very little perturbation of the CB11 cluster bands relative type had an ‘‘…oral toxicity to rats roughly comparable to to the silylium starting material so protonation of the anion sodium chloride…’’12 we began to comprehend their extra- must occur on the halogen substituents (red atoms X in Fig. 1) + ordinary inertness and their potential as weakly coordinating at the same positions (7–12) that coordination of R3Si and + anions. Ag occurs. No band ascribable to a simple nX–H stretch is While there have been periods of commercial availability, observed and, given the marked tendency of protons to be di- 17 + and there is a new synthesis available starting with sodium coordinate, a polymeric structure with H bridges between 13 2 halogen substituents of the anions is proposed, similar to that borohydride, the same basic synthesis of the CB11H12 is still + 18 used today. It is not difficult to produce 7 g of the cesium salt observed in a number of Ag salts of carboranes. from 10 g of decaborane starting material in about a week and Carborane acids are so strong that they protonate (or react 14 the synthetic details are freely available at a website. with) almost every solvent. In liquid SO2, one of the least basic The very weakly coordinating (i.e. weakly Lewis basic) solvents where good solubility is achieved (ca. 0.1 M), the 1 nature of B-halogenated carboranes is illustrated by extremely downfield-shifted H NMR resonance of the acidic 19 20 i-Pr3Si(CHB11H5Cl6) which, structurally, is the closest proton (20.4 ppm), along with IR data, suggest that approach to date to the long sought trialkylsilylium ion, complete ionization of H(CHB11Cl11) takes place. The nature + 15 + R3Si (Fig. 2). The anion is coordinated to silicon via a of the proton is most likely the di-solvated cation, H(SO2)2 . Acid strength of carborane acids The acidities of carborane acids cannot be measured in the conventional manner of an H0 Hammett acidity function because carborane acids are solids not liquids. The comparison of the strengths of solid and liquid phase acids is a classic problem of physical chemistry. Gas phase acidities have been calculated for carborane acids, ranking them the strongest of any known isolable acid,21 but it is presently not possible to translate gas phase DG data into measures of condensed phase acidity. In liquid SO2 solution, a fairly good solvent for carborane Fig. 2 X-ray structure of the i-Pr3Si(CHB11H5Cl6), approaching an acids, the strength of carborane acids has been compared to ‘ion-like’’ state.15 (H atoms have been omitted for clarity).