Equilibrium Acidities of Superacids

Who is the strongest? DOI: 10.1002/anie.200((will be filled in by the editorial staff))

Equilibrium Acidities of Superacids**

Agnes Kütt,* Toomas Rodima, Jaan Saame, Elin Raamat, Vahur Mäemets, Ivari Kaljurand, Ilmar A. Koppel,* Romute Yu. Garlyauskayte, Yurii L. Yagupolskii, Lev M. Yagupolskii, Eduard Bernhardt, Helge Willner, Ivo Leito*

((Dedication----optional))

Acids of very high strength – superacids1 – and their anions a(SH  )a(A ) pK  log (2) (usually weakly coordinating anions – WCAs) have found wide area a a(HA) of applications.2,3,4,5 WCAs are of key importance as counterions in catalysis and stabilizing of highly reactive species, as well as charge carriers in electrochemical power sources.6,7,8,9,10 23,24 Cyanocarbon acids,11,12,13 sulfonic acids,3 sulfonimides5,14 and other Using the previously published method based on relative types of strong acids have been known and used for a long time, but acidity measurements of acids HA1 and HA2 we can eliminate + solution acidity data of most of them range from scarce to non- the need to determine the activity of solvated proton a(HS ) inconsistent,15 and are often indirect.16 Although (besides their gas- and taking into account formation of ion-pairs of acid anion 17,18 and protonated base during the relative direct acidity phase acidities ) the pKa values of some of them have been published in acetic acid,19 most of the acidity data have been measurements: obtained in solvent mixtures of changing composition, such as 2 Kip aqueous H2SO4 at different concentrations, acetonitrile acidified to HA + [HB+A –]  HA + [HB+A –] (3) different extent,15 i.e. not in constant-composition liquid media. 1 2 2 1 Accurate self-consistent scale of superacids in a solvent of constant composition would be desirable, as this would enable rigorous a([HB A ])a(HA ) pK  log 1 2 (4) analysis of structure and solvent effects on acidity. ip a([HB A ])a(HA ) The choice of the medium for measurement of acid strength of 2 1 strong acids is not straightforward. A large part of the available 20 solvents (e.g. heptane, SO2) are either not very convenient for acid- Assuming that the ratio of activity coefficients of ion-paired and base measurements (because of low polarity, poor solvating ability neutral species of both acids are similar at each titration point: 20 + – + – of ions and polar solutes) (e.g. heptane, SO2) or are too basic (e.g., f([HB A1 ])/f(HA1) = f([HB A2 ])/f(HA2), concentrations can be used DMSO21) for studying strong acids. A large body of data has been instead of activities. Also, because of the large size and inertness of 2 + obtained using the H0 approach, whereby different acid strengths the counterion HB (mainly protonated phosphazene base t- + actually refer to different solvent compositions of the medium. The BuP1(pyrr) or sometimes Alk4N ) used in this work we can assume – – Reed group has reported an infrared spectroscopic scale of ion- that both anions A1 and A2 will be influenced by ion-pairing to the pairing ability of superacid anions with trioctylammonium same extent and thus pKa  pKip: counterion – the NH scale.16 Although being useful in characterizing anions of superacids and providing implications for [HBA  ][HA ] the acidities of the parent acids, the scale is not an equilibrium pK (HA ) - pK (HA )  pK  pK  log 1 2 (5) a 2 a 1 a ip [HBA  ] [HA ] acidity scale and characterizes the hydrogen-bond acceptor strength 2  1 of anions rather than the Brønsted acidity of their parent acids. In the present work, the acidities of many of previously The results of relative acidity-base measurements are presented in reported superacids have been measured using equilibrium acidity Table 1. pKa values of 62 acids are interconnected with 176 relative measurements in 1,2-dichloroethane (DCE). DCE is an appropriate acidity measurements to obtain a self-consistent acidity scale solvent for studying superacids because it combines negligible basic ranging from picric acid 1 (pK = 18.7) to 1,1,2,3,3- properties and inertness with the ability to dissolve many polar and a pentacyanopropene 62 (pKa = 3.4), i.e. altogether 15 pKip units. The ionic compounds. Although its dielectric constant ( = 10.60) is exact positions of the individual acids on the scale were found among the highest known for chloroalkanes it is still too low to 23,24 22 similarly to the previous works by minimizing the sum of efficiently support dissociation of ion pairs into free ions. For this squares of differences between the directly measured pK pK reason and because of low ion solvating ability of DCE relative ion- ip a values and differences of the assigned pKa values. pair acidities (pKip) are directly measured. This The low polarity Our measurement method allows to measure relative acidities also causes several side-processes in acidity measurements: ion-pair expressed as pKavalues. In order to obtain absolute pKa values, the formation, homo- and heteroconjugation are much more extensive in scale has to be anchored to an acid with known pK value. There are DCE than in e.g. ANacetonitrile (AN), also, measurements in DCE a currently no reliable absolute experimental pKa values available in are more influenced by traces of water and other polar or ionic DCE (see discussion in supporting information25). We thus keep the impurities. scale relative at this time, until a sound anchor point is found. That pKa value is the most common wayly used value to is, we arbitrarily anchor it to the pKa value of Picric acid 1 arbitrarily describe the equilibrium acidity of an acid HA in a solvent S: assigned (equal to 0.0 units). At present, we use only relative values

Ka in discussion of the data HA + S  A– + SH+ (1)

Table 1. Equilibrium acidity scale in 1,2-dichloroethane.

[a] [b] No Acid b pK a(DCE) Directly measured DpK ip values in DCE pK a(AN)

1 Picric acid b 0.0 11.0 2 HCl b -0.4 0.73 0.71 10.6 1.48 0.36 3 2,3,4,6-(CF3)5-C6H-CH(CN)2 -0.7 1.09 10.3 0.77 0.74 [c] 2.08 1.00 4 4-NO2-C6H4SO2NHTos -1.5 9.6 0.28 1.78 5 HNO3 -1.7 9.4 1.01 1.13 6 4-NO2-C6H4SO2NHSO2C6H4-4-Cl -2.4 8.8 0.88 0.08 0.24 7 H2SO4 -2.5 8.7 0.12 1.26 8 C6(CF3)5CH(CN)2 -2.6 8.6 1.02 1.02 1.05 1.48 9 (4-NO2-C6H4-SO2)2NH -3.7 7.7 0.47 0.80 10 3-NO2-4-Cl-C6H3SO2NHSO2C6H4-4-NO2 -4.1 7.3 0.36 1.35 11 (3-NO2-4-Cl-C6H3SO2)2NH -4.5 7.0 0.39 0.93 12 HBr -4.9 0.62 6.6 0.19 1.03 13 4-NO2-C6H4SO2CH(CN)2 b -5.1 6.4 b 0.80 1.33 14 2,4,6-(SO2F)3-Phenol b -5.9 5.7 [d] b 0.64 15 2,4,6-Tf3-Phenol -6.4 5.2 0.94 16 CH(CN)3 -6.5 5.1 0.42 1.14 1.12 0.33 0.65 17 4-Cl-C6H4SO(=NTf)NHTos b -6.8 4.9 0.51 0.05 b 18 NH -TCNP [e] -6.8 0.24 4.9 2 b 0.20 19 2,3,5-tricyanocyclopentadiene b -7.0 4.7 0.67 0.84 20 Pentacyanophenol b -7.6 4.2 1.77 21 4-Cl-C6H4SO(=NTf)NHSO2C6H4-4-Cl -7.6 4.1 1.56 22 HI -7.7 1.64 1.00 4.1 b 0.98 1.13 b 1.10 0.93 23 4-NO2-C6H4SO2NHTf -7.8 4.0 b 1.04 1.01 0.81 24 Me-TCNP -8.6 0.96 0.90 3.3 b 0.09 0.13 1.42 25 3,4-(MeO)2-C6H3-TCNP -8.7 3.2 b -0.02 b 0.12 26 4-MeO-C6H4-TCNP -8.7 3.2 0.12 0.46 0.24 27 C(CN)2=C(CN)OH -8.8 3.1 0.28 1.61 0.22 0.59 28 4-Cl-C6H4SO(=NTf)NHSO2C6H4-NO2 -8.9 3.0 b 0.74 0.67 0.06 29 2,4-(NO2)2-C6H3SO2OH -8.9 3.0 b 0.59 30 C6F5CH(Tf)2 -9.0 0.60 2.9 0.52 b 31 HB(CN)(CF3)3 -9.3 0.47 0.44 2.6 1.33 0.13 32 Ph-TCNP b -9.4 1.56 1.62 2.5 b 0.83 1.57 33 HBF4 -10.3 1.23 1.8 1.06 34 FSO2OH -10.5 0.26 1.34 1.5 0.01 b 35 3-CF3-C6H4-TCNP -10.5 0.21 1.5 0.60 0.22 0.58 36 H-TCNP b -10.7 1.3 0.73 0.78 0.46 37 [C6H5SO(=NTf)]2NH -11.1 0.84 1.0 b 0.84 38 [(C2F5)2PO]2NH b -11.3 0.89 0.91 0.8 0.29 0.93 39 2,4,6-(NO2)3-C6H2SO2OH -11.3 0.28 0.8

40 [C(CN)2=C(CN)]2CH2 -11.4 0.44 0.21 0.8 0.10 41 TfOH -11.4 0.12 0.47 0.7 0.04 0.07 0.09 0.49 b 42 C6H5SO(=NTf)NHTf -11.5 0.40 0.32 0.47 0.7

43 TfCH(CN)2 -11.6 0.6 0.36 0.20 0.25 44 Br-TCNP b -11.8 0.4 0.10 0.67 0.73 0.75 b 45 [C(CN)2=C(CN)]2NH -11.8 0.06 0.63 0.3 0.19 46 3,5-(CF3)5-C6H3-TCNP -11.8 0.21 0.45 0.4 0.19 47 Tf2NH -11.9 0.30 0.31 0.3 b 0.42 48 4-Cl-C6H4SO(=NTf)NHTf -12.1 0.15 0.46 0.1 b 0.01 0.36 49 Cl-TCNP -12.1 0.43 0.1 0.10 0.40 b 50 (C3F7SO2)2NH -12.2 0.13 0.21 0.1 0.29 51 (C4F9SO2)2NH b -12.2 0.19 0.27 1.29 0.0 0.69 0.10 b 52 CN-CH2-TCNP -12.3 0.93 -0.1 1.06 0.02 1.04 53 (C2F5SO2)2NH -12.3 1.05 1.06 0.47 0.44 -0.1 0.72 0.96 54 CF3-TCNP -12.7 0.77 -0.5 0.80 55 HClO4 -13.0 0.80 1.04 -0.7 0.40 56 CF2(CF2SO2)2NH -13.1 0.11 0.56 -0.8 0.89 0.86 0.07 57 4-NO2-C6H4SO(=NTf)NHTf -13.1 -0.8 0.19 58 HB(CN)4 -13.3 0.44 -1.0 1.76 59 (FSO2)3CH -13.6 1.92 -1.2 2.16 60 Tf2CH(CN) -14.9 1.46 -2.4 1.73 0.22 61 2,3,4,5-tetracyanocyclopentadiene -15.1 0.40 -2.6 0.21 0.23 62 CN-TCNP -15.3 -2.8 [f] 63 Tf3CH b -16.4 -3.7 [f] 64 CF3SO(=NTf)NHTf -18 -5

2 [a] Directly measured relative acidity values in DCE. [b] Predicted pKa values of AN (see supporting information for details). [c] Tos represents 4-Me-C6H4SO2– group. [d] Tf represents CF3SO2– group. [e] X-TCNP represents 2-X-1,1,3,3-tetracyanopropene. [f] Estimated DCE pKa values, see text.

Out of the acids not belonging to either of these groups the X O O N strongest is HB(CN)4 (58). Although composed according to a NC CN different principle, its anion displays charge delocalization H S CF3 S CH3 N P N CN CN O O N similar to anions of the cyanocarbon acids by forming 4 equally – weakly basic protonation centers. Ion B(CN)(CF3)3 is a good X-TCNP Tf Tos t-BuP1(pyrr) example of losing this symmetry: the CF3 groups with their weaker electron-acceptor power leave the single –CN – Scheme 1. Abbreviations for compounds and groups used. significantly more basic than in B(CN)4 and as a result 31 is by 4 orders of magnitude weaker acid than 58. HB(CN)4 is also – significantly stronger than HBF4: the small size of the BF4 anion The set of 64 acids measured includes all common strong mineral creates relatively high partial charges on the –F substituents so acids and a number of acids from different families. The critical that the very low intrinsic basicity of the –F center increases factor for the acidity of a superacid is the stability of its anion and considerably. 7 the lack of well-defined protonation centers in the anion. It is Except HClO4, all classical strong mineral acids are in the fair to say that designing of a superacid for the most part consists upper half of the scale. Their relative weakness compared to the in designing of its anion.26 acids discussed above is especially noticeable in the case 21 From Table 1 two rather different design concepts of hydrogen halides: HCl (pKa ca -8 in water ) is just slightly 27 23 superacid anions emerge as the most potent: cyanocarbon stronger than picric acid (pKa = 0.3 in water ), HI (pKa ca -10 in anions (in Table 1 denoted with green color) and anions of poly- water29) is by more than 5 orders of magnitude weaker than 29 SO2X-imides and methanes (in Table 1 denoted with blue color), HClO4 (pKa ca -10 in water ). The reason for this is obvious: especially those where in some sulfonyl groups the =O fragment most of the classical mineral acids have small anions and a large is replaced by =N-Tf. Both of these anion groups obtain their part of their strength in water comes from the efficient solvation stability from excellent charge delocalization but with different of the anions. mechanism. Cyanocarbon anions are an excellent beautiful CF3SO(=NTf)NHTf 64: – The the acidic titrant used in this example of delocalization of negative charge by efficient polar work to protonate the anions of the strongest acids – is a very resonance in a planar C3 (45, 49, 52, 54, 62, etc) or C5 (19, 61) strong neutral acid 64. It is highly interesting to know the acidity moiety. In the sulfo-anions the efficient combination of of this compound. The Yagupolskii substitution,28 i.e. polarizable sulfur atom and the high electron-acceptor power of replacement of =O by =NTf group in aromatic or aliphatic the =O atoms (or the even much higher electron-acceptor power sulphone imides leads to an acidity increase of 6.4 (6 and 28) to of =N-Tf28) is put to work. Both of these concepts lead to the 5.3 (23 and 57) units. Introducing Yagupolskii substitution to the formation of anions that have a number of protonation centers Tf2NH 47, compound 64 is obtained. Taking into account the with similar (and very low) basicity. average acidity increase (5.9 units), the pKa value of 64 can be Both of these families of superacids stretch across the estimated, giving the pKa value of as -18 units. whole scale and neither of the two have been yet taken to their

limits in this scalework. For example, obviously HC5(CN)5 is a significantly stronger acid than 61 and both 63 and 64 can still be Experimental Section

The spectrophotometric titration method used in this work is 23,24 [] Dr. A. Kütt, Dr. T. Rodima, J. Saame, E. Raamat, Dr. V. mostly the same as described earlier. The method is based on Mäemets, Dr. I. Kaljurand, Prof. I. A. Koppel, Prof. I. Leito UV-Vis spectrophotometric titration of a DCE solution of two Institute of Chemistry acids with the DCE solution of a non-absorbing base t- 30 University of Tartu BuP1(pyrr) to obtain neutral and anionic forms of the solution of Ravila 14a, Tartu, 50114, Estonia mixture. Both acids were also titrated separately to obtain spectra E-mail: [email protected], [email protected], [email protected] of their neutral and ionized forms. From the titration data, the relative acidity of the two compounds – the difference of their Dr. R. Y. Garlyauskayte, Prof. Y. L. Yagupolskii, Prof. L. M. pKa values (pKa) – is obtained. The reversibility of protonation- Yagupolskii deprotonation process was tested for all compounds. Compounds Institute of Organic Chemistry that did not behave reversibly (HPF6, HAl[OCH(CF3)2]4) were + – National Academy of Sciences of Ukraine, Murmanskaya 5, not included to the scale. Titration of Bu4N BF4 with acid 64 Kiev, 02094, Ukraine and afterwards with base t-BuP1(pyrr) demonstrated reversibility of the protonation-deprotonation process. The formed complex Dr. E. Bernhardt, Prof. H. Willner HF∙∙∙BF3 seems to be stable in solution because of its very low Anorganische Chemie, Bergische Universität Wuppertal, concentration and the extremely low Lewis basicity of DCE. Gaussstrasse 20, D-42097 Wuppertal, Germany During this experiment, it is impossible, that the acidity of pure HF was measured by mistake (based on the moles of titrants [] This work was supported by the Grants No. 7374 and 6701 consumed). HF is also so weak acid (considerably weaker than from the Estonian Science Foundation and by the target HCl) that it could not be included into the present scale. financing projects SF0180061s08 and SF0180089s08 from the Usually sharp isosbestic points were obtained during Ministry of Education and Science of Estonia. We are indebted titrations (see spectra of all UV-vis active compounds in to Prof. Takaaki Sonoda and JEMCO Inc., for compounds 47, supporting information), indicating that the measured compounds 50, 51, 53, 56. We are indebted to Prof. O. W. Webster for contain insignificant amounts of impuritieswere reasonably pure compounds 19 and 61. and possible side-processes were negligible. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author Received: ((will be filled in by the editorial staff)) strengthened by additional =O → =N-Tf modifications. Published online on ((will be filled in by the editorial staff))

3 Keywords: superacid · acidity measurements · acid strength · 1,2- dichloroethane · relative acidity

[1] Here and below the term „superacid“ refers to a superacidic molecule not to a superacidic medium. Superacidic molecule can be defined as a molecule having in a particular medium higher acidity

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4 Who is the strongest? We report the most comprehensive superacidity scale in a medium of constant Agnes Kütt,* Toomas Rodima, Jaan composition – 1,2-dichloroethane (DCE). Saame, Elin Raamat, Vahur Mäemets, DCE has very weak basic properties and it is Ivari Kaljurand, Ilmar A. Koppel,* an appropriate solvent for measuring Romute Yu. Garlyauskayte, Yurii L. acidities of very strong acids. DCE acidities Yagupolskii, Lev M. Yagupolskii, Eduard of well-known superacids – TfOH, Tf2NH, Bernhardt, Helge Willner, Ivo Leito* cyanocarbon acids, etc. – as well as common mineral acids are reported. Acidities of altogether 62 acids have been ______Page – Page determined, the scale spans for 15 orders of magnitude and is expected to be a useful Equilibrium Acidities of Superacids tool in design, studies and use of superacidic molecules.