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The Strongest Isolable Acid This Work Was Supported by NSF Grant CHE-0095206 and NIH Grant GM 23851 UC Riverside UC Riverside Previously Published Works Title The strongest isolable acid. Permalink https://escholarship.org/uc/item/3g85n9gp Journal Angewandte Chemie (International ed. in English), 43(40) ISSN 1433-7851 Authors Juhasz, Mark Hoffmann, Stephan Stoyanov, Evgenii et al. Publication Date 2004-10-01 DOI 10.1002/anie.200460005 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Communications Brønsted Acids The Strongest Isolable Acid** Mark Juhasz, Stephan Hoffmann, Evgenii Stoyanov, Kee-Chan Kim, and Christopher A. Reed* Acids based on carborane anions as conjugate bases (Figure 1) are a new class of Brønsted (protic) acids, notable for their “strong yet gentle” qualities.[1] For example, whereas conventional strong acids (e.g. H2SO4) and superacids (e.g. HFSO3/SbF5) decompose fullerenes even at low temper- atures, the carborane acid H(CHB11H5Cl6) cleanly protonates C60 at room temperature to give the isolable salt [HC60] [CHB11H5Cl6].We now show that carborane acids are the [*] M. Juhasz, S. Hoffmann, Dr. K.-C. Kim, Prof. Dr. C. A. Reed Department of Chemistry Universityof California Riverside, CA 92521-0403 (USA) Fax : (+ 1)909-787-2027 E-mail: [email protected] Dr. E. Stoyanov Boreskov Institute of Catalysis Prospekt Lavrentieva 5, Novosibirsk 630090 (Russia) [**] This work was supported byNSF grant CHE-0095206 and NIH grant GM 23851. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. 5352 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200460005 Angew. Chem. Int. Ed. 2004, 43, 5352 –5355 Angewandte Chemie 13C NMR spectroscopy data for 0.15m solutions of various À acids and 0.10m mesityl oxide were obtained at room Figure 1. The [CHB11R5X6] carborane ions. temperature and are given in Table 1.The choice of oxyacids strongest isolable (Lewis-free) Brønsted acids presently known. Table 1: Acid-strength ranking from protonation of mesityl oxide in liquid Two reactivity considerations have led us to believe that SO2. 13 carborane acids must have uncommon strength.First is their Acid Dd ( C) [ppm] H0 stabilization of the benzenium ion in salts such as [C H ] [a] 6 7 H(CHB11Cl11) 84.0 Æ0.1 [a] [CHB11Me5Br6].The strongest known neat oxyacid, HFSO 3, H(CHB11H5Cl6) 83.8 Æ0.1 [a] (whose Hammett acidity function H0 = À15.1) is not strong H(CHB11H5Br6) 83.8 Æ0.1 [2] [a] enough to fully protonate benzene, yet carborane acids do H(CHB11H5I6) 83.3 Æ0.1 so readily at the one-equivalent level in dilute solution.[3] FSO3H 73.8 Æ0.5 À15.1 CF3SO3H 72.9 Æ0.4 À14.1 Second, we have recently shown that carborane anions [a] HN(SO2CF3)2 72.0 Æ0.4 stabilize tert-butyl cation in dichloromethane or liquid SO2 H SO 64.3 Æ3.1[b] À12.1 [4] 2 4 solution, as well as in isolable crystalline salts. This means Unprotonated mesityl oxide 32.4 Æ0.1 that carborane acids are strong enough to shift the equili- [a] H values unavailable because acids are solids, not liquids. [b] Incom- brium of Equation (1) completely to the right hand side.If 0 plete miscibilityof H 2SO4 in liquid SO2 leads to higher error limits and this were not the case, deprotonation of tert-butyl cation to possible underestimate of Dd. give iso-butene and subsequent decomposition by cationic oligomerization would occur rapidly. for comparison to carborane acids is limited by their avail- ability in pure form and the need for miscibility or dissolution in liquid SO2.Nevertheless, it is immediately evident from their high chemical shift values that, as a class, carborane acids are stronger than conventional oxyacids.They outrank fluorosulfuric acid, the strongest known pure Brønsted acid Since mixed Lewis/Brønsted superacid media such as on the H0 acidity scale (À15.1), as well as triflic acid (H0 = “Magic Acid” (HFSO3/SbF5) having H0 somewhere in the À14.1). A strict correlation between Dd values in liquid SO2 [5] range À17 to À27 are used to stabilize tert-butyl cation, it is and the H0 values of neat acids is not expected because acidity apparent that carborane acids have intrinsic Brønsted acid- is a solvation-dependent phenomenon that changes as the ities comparable to those only found previously in Lewis/ medium changes.Nevertheless, for the limited data set Brønsted acid mixtures. available, the ranking of acid strength by Dd values follows The acidities of carborane acids cannot be measured in the the H0 order (Table 1). conventional manner of an H0 Hammett acidity function It is also evident from the data of Table 1 that whereas because carborane acids are solids not liquids.They sublime oxyacids only partially protonate mesityl oxide, carborane under vacuum, or melt at atmospheric pressure, at temper- acids are strong enough to move Equation (2) completely to atures well above 1508C.Gas-phase acidities have been the right hand side, that is, their acidities are leveled.The calculated for carborane acids, which rank them the strongest Dd value maximizes at d = 84.0 ppm. Though close to the of any known isolable acid,[6] but it is presently not possible to limits of discrimination, the hexa-iodo carborane acid, translate gas-phase DG data into measures of condensed- [H(CHB11H5I6)] with a value of d = 83.3 ppm, appears to be phase acidity. perceptibly weaker than its chloro and bromo counterparts. The need to compare the strengths of solid acids with To discriminate between the acidities of the different liquid acids therefore led us to obtain a measure of acidity in carborane acids, we have developed a new, qualitative ranking dilute solution, conditions where most acid catalysis is carried of acidity based on the nNÀH frequencies of the ammonium out.The chosen solvent is liquid SO 2 because of its low salts of their conjugate base anions.The ranking is based on basicity, relatively high dielectric constant for solvation of the influence of the anion AÀ on the NÀH bond in a contact ions, and ease of drying.The chosen method is adapted from ion pair (Figure 2). that developed by Faˇrcas¸iu and Ghenciu.[7] It is based on the 13C NMR spectroscopy chemical shift difference (Dd) between the Ca and Cb carbon atoms of mesityl oxide which increases with increasing protonation as Equation (2) is shifted to the right hand side. Figure 2. Ammonium salt contact ion pair. Angew. Chem. Int. Ed. 2004, 43, 5352 –5355 www.angewandte.org 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5353 Communications The weaker the interaction of the base AÀ, the stronger and the scale should be useful for a much wider range of the NÀH bonding, and the higher the nNÀH frequency.[8] weakly coordinating anions.b) The ranking of the hexahalo À1 Since nNÀH frequencies (> 3000 cm ) are energetically far carborane acids, H(CHB11H5Cl6) > H(CHB11H5Br6) > À1 removed from nHÀA frequencies (< 300 cm ), vibrational H(CHB11H5I6), makes sense on the basis of electronegativity coupling or mass differences in AÀ will have an insignificant and polarizability considerations for the halogen substituents. effect on nNÀH, that is, nNÀH frequencies should be a good It is consistent with conceptually related deductions drawn measure of NÀH bond strength.Since hydrogen bonding is a recently from nCÀH frequencies in benzenium ion salts predominantly electrostatic phenomenon,[9] NÀH bond having the same series of carborane counterions.[3] c) The top strength should correlate strongly with AÀ basicity, that is, ranking (i.e. least basic) carborane anion measured to date is À [19] with HA acidity. nNÀH frequencies will decrease with the undecachlorinated ion, [CHB11Cl11] , which leads to increasing AÀ basicity, correlating with HA acidity.In a the prediction that its conjugate acid will be the strongest.As constant solvation environment within a series of isostructural detailed in the Supporting Information, H(CHB11Cl11) can be anions, the correlation should be quite direct.Although the prepared in a similar manner to the hexahalo carborane acids direct correlation of acidity with IR stretching frequency has as a sublimable, analytically-pure, colorless solid by the [10–12] been questioned for solid surfaces, acidity rankings based reaction of Et3Si(CHB11Cl11) with anhydrous HCl.Thus, on IR data have been shown to have excellent validity within H(CHB11Cl11) can lay claim to be the strongest isolable acid a number of classes of discrete compounds, particularly when presently known. structurally related.[13–17] Carborane acids are strong acids because their anions are Tri-n-octylammonium salts of carborane anions were exceptionally weak bases.This arises from the large size of the prepared by silver-salt metathesis reactions and characterized anions, the delocalized nature of the negative charge in the À as detailed in the Supporting Information.Octyl groups in the [CB11] cluster, and the shield of halide substituents.How- cation were chosen to provide good solubility of [n-Oct3NH] ever, there is an important additional necessity for preparing [carborane] ion pairs in CCl4 solution.To avoid possible such a strong acid, namely, the anion must be exceptionally effects from aggregation, nNÀH frequencies were measured inert from a chemical point of view.The inertness of in dilute solution where there was no concentration depend- carborane anions arises from s aromaticity within the À ence (Table 2).The strong dependence of nNÀH on the [CB11] icosahedral cage, a notable feature of boron cluster À chemistry.The [CB 11] framework resists chemical disruption to a truly exceptional degree.There are anions that probably Table 2: n˜NÀH frequencies of tri-n-octylammonium salts in CCl4. À1 À1 have lower basicity than carboranes, for example, organo- Anion nNÀH[Æ1cm ] Dn [cm ] À [20] fluoro anions, such as [B(C6F5)4] , but their conjugate acids À[a] [CHB11Cl11] 3163 0 cannot be prepared because the anion decomposes at high À [CHB11Me11] 3156 7 acidity.[21] This situation is mirrored in that traditional À[a] [CHB11H5Cl6] 3148 15 III À fluoroacids, commonly written as HBF4, HPF6, HSbF6 etc., [Co (C2B9H8Cl3)2] 3145 18 À are also nonexistent compounds.They are all unstable with [CHB11Me5Cl6] 3143 20 À respect to formation of HF and the corresponding Lewis acid.
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