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The Strongest Acid. Protonation of Carbon Dioxide[**]

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The Strongest Acid. Protonation of Carbon Dioxide[**] Acid Test DOI: 10.1002/anie.200((will be filled in by the editorial staff)) The Strongest Acid. Protonation of Carbon Dioxide[**] Steven Cummings, Hrant P. Hratchian† and Christopher A. Reed* + [22] Abstract: The strongest carborane acid, H(CHB11F11), protonates H(SO2)2 , but none of the aforementioned gases has sufficient [23] CO2 whereas traditional mixed Lewis/Brønsted superacids such as basicity or solubility to compete with SO2 for protonation. “magic acid” do not. The product is deduced from IR spectroscopy The issue of whether carborane acids can protonate weakly + and calculation to be the proton disolvate, H(CO2)2 . The carborane basic molecules more easily than traditional mixed Lewis/Brønsted acid H(CHB11F11) is therefore the strongest known acid. The failure acids goes to the heart of the question of which class of acids can of traditional mixed superacids to protonate weak bases such as claim title to the strongest acids.[24] Traditional mixed acids are CO2 can be traced to a competition between the proton and the liquids and therefore lend themselves to quantitative description in [16] Lewis acid for the added base. The high protic acidity promised by terms of the H0 Hammett acidity function. On the other hand, [25] large absolute values of the Hammett acidity function (H0) is not carborane acids are solids and rely on the νNH scale. We draw realized in practice because the basicity of an added base is attention to the poorly recognized phenomenon of “basicity suppressed by Lewis acid/base adduct formation. suppression”[26] whereby the Lewis acid in a mixed acid can form an adduct with an added base, thereby lowering the intrinsic basicity of The recent synthesis of the strongest pure acid, the carborane acid that base, and making it harder to protonate. An alternative way to [1] H(CHB11F11), which can protonate alkanes at room temperature, view this phenomenon is to recognize that in any mixed acid there opens up the possibility of protonating molecules that haven’t been will always be a competition between H+ and the Lewis acid for an protonated before. The most interesting targets are small gaseous added base. Thus, Brønsted acidity in a mixed acid, as indicated by [27] molecules such as H2, N2, O2, CO, CO2 as well as atomic Xe. Mass H0 (or pH) is, in practice, unavailable for protonation of an added spectral data show that these extremely weak bases can add a proton base. For this reason, we take a pragmatic approach to deciding [2–9] in the gas phase to form stable cations. Protonated CO2 has been which acid is the strongest. The strongest acid will simply be that observed in the gas phase via infrared spectroscopy,[10,11] solvated which successfully protonates the weakest base. As we shall see [12] [13,14] by a noble gas, and is a confirmed interstellar species. On below, the strongest carborane acid protonates CO2 whereas the other hand, demonstrating protonation in condensed phases traditional mixed Brønsted/Lewis superacids do not. Therefore, presents a much greater challenge. Gillespie and Pez[3] could find no carborane acids are, in practice, shown to be the strongest acids. [18] evidence for the protonation of these gases in a HSO3F/SbF5/SO3 Using sapphire NMR tube technology, CO2 was condensed [1] “magic acid” system, one of the strongest known mixed onto ground H(CHB11F11) at liq. N2 temp. Upon warming to room [15] Brønsted/Lewis acids. Drews and Seppelt could find no evidence temperature to obtain CO2 as a liquid, the crystalline form of the + + for the involvement of HXe in the formation of the Xe2 ion in the acid was observed to change its aggregation appearance. After gas [16] somewhat stronger HF/SbF5 mixed acid system. Olah and removal, evacuation at 50 mTorr to remove possible physisorbed [17] Shen found indirect evidence for the protonation of Xe and CO2 species, and back filling with dinitrogen, the IR spectrum of the -1 in HF/SbF5 by observing slowed rates of H exchange into D2 when solid showed the appearance of a sharp band at 2365 cm and a + + -1 these gases were present. The necessity of HXe and HCO2 ions as weaker band at 2343 cm (Fig. 1c), assigned to the in-phase and [18] reaction intermediates was postulated. Gladysz and Horváth et al. out-of-phase νasym stretches of protonated CO2. When the same 13 produced the first evidence for the existence of protonated carbon experiment was carried out with 99% C-labelled CO2, appropriate monoxide in condensed media by assigning a sharp peak in the 13C red shifts to 2302(s) and 2276(w) cm-1 (Fig. 1d) were observed. NMR spectrum at 139.5 ppm and a very broad IR band near 2110 These shifts of 63 and 67 cm-1 respectively, are entirely consistent -1 + -1 13 -1 cm to the formyl cation, HCO , when CO was dissolved in with the 65 cm C isotope shift obtained for νasym (2349 cm ) in [28,29] HF/SbF5 at high pressures. This evidence is considered “strong but matrix isolated CO2. IR bands of the starting acid (νFHF ca. not conclusive”.[19] 1605 cm-1 and δFHF 900-1000 cm-1, Fig. 1a) remain present, The question of whether carborane acids are better than indicating only partial reaction. This is presumably the result of traditional mixed Lewis/Brønsted superacids at protonating small restricted penetration of CO2 through the reacting surface layer of gaseous molecules boils down to two practical issues: acid strength the solid acid. The spectrum also shows increased intensity of bands -1 and reaction conditions. Carborane acids are solids with proton- from [H3O][CHB11F11] (broad νOH ca. 3300 and 3175 cm and [20] -1 bridged polymeric structures so their reactions with gases are δH3O at ca. 1625 cm , Fig. 1b), an inevitable contaminant arising hampered by slow kinetics at the solid/gas interface. High gas from the presence of trace water. There is no observable reaction of pressures may ameliorate this problem by converting the gas into a CO2 with pure hydrated acid, [H3O][CHB11F11]. Allowing longer liquid or a supercritical fluid, and the observation by IR that reaction times and heating the reaction to higher temperatures, H(CHB11F11)(s) appears to partially convert liquefied methane into t- where liquid CO2 goes supercritical, gave similar results but with butyl cation-like salts is encouraging in this regard.[21] The increased contamination by water. Similar experiments exposing the alternative approach of dissolving a carborane acid in a solvent undeca-chloro carborane acid, H(CHB11Cl11), to CO2 showed barely necessarily, and undesirably, levels acidity down to that of the detectable bands in the 2370-2340 cm-1 region, consistent with [25] protonated solvent. Carborane acids are quite soluble in liquid SO2, lower reactivity of this somewhat weaker carborane acid. -1 presumably because of the formation of the proton disolvate, The Raman active νsym vibration of free CO2 at 1384 cm is expected to become IR active in a protonated product. Although this 1 region is masked by strong νBB bands from the anion, computer νCH bands near 2800 cm-1 and the ν(CC)+δ(CCH) band at 1605 cm- subtraction of remnant acid revealed two possible bands centered at 1, were observed (Fig. S6). -1 -1 ca. 1325 cm and ca. 1295 cm (Fig. S2) which are attributed to the What is the structure of protonated CO2? It is most unlikely to in-phase and out-of-phase symmetric C=O stretches, respectively. be the result of simple protonation, i.e., a monoprotonated salt of Computer subtraction of the acid from the resulting spectrum of composition [HCO2][CHB11F11]. A major lesson learned from recent 13 -1 + protonated CO2 exposes a peak at 1293 cm with a shoulder at studies on the nature of H in condensed media is the prevalence of -1 [31] 1324 cm (Fig. S3). The small observed isotopic shift in νsym of 1-3 proton di-solvation. This must be connected to the observation in -1 -1 [30] cm is consistent with free CO2 (Δ18 cm ) and calculated values the gas phase, that the energetics of disolvation are remarkably of ca. 1 cm-1 (see below). Similarly, bending vibrations expected in constant whereas those of monosolvation (i.e., proton affinity) are -1[28] [32] + the region of the degenerate δ bands of free CO2 at 662 cm are highly variable. The monosolvated HCO2 ion should show a masked by strong νBF bands of the anion but computer subtraction single νasymCO band but two are observed (Fig. 1c, 1d inserts). In the -1 + -1 [13,33] of remnant acid revealed absorptions near 690 cm . gas phase, νOH for the HCO2 ion is observed at 3375 cm . In - a solid phase salt with CHB11F11 as counterion, H-bonding to the anion would be expected to broaden and lower this frequency into the 3200-2900 cm-1 region. The spectrum of the product (Fig. 1c) shows no obvious candidate for this band, even with computer A: H(CHB F ) 11 11 subtraction of bands from [H3O][CHB11F11] (Fig. S7). So there is no + evidence for the presence of the HCO2 ion. The anticipated product of CO2 protonation is a disolvate + containing the H(CO2)2 ion, particularly considering that CO2 is present in large excess (Eq. 1). B: [H O][CHB F ] 3 11 11 2CO2(l) + H(CHB11F11)(s) → [H(CO2)2][CHB11F11](s) (1) Such a proton disolvate is expected to have linear two-coordination at H+ and show the characteristics of short, strong, low-barrier (SSLB), symmetrical (or nearly symmetrical) H-bonding.[31,34–36] However, confirming the presence of the disolvated ion is not as simple as disproving the presence of the monosolvated ion.
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