Acidity of Superacids

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NO Superacid derivatives 2 in your pocket?

Ivo Leito F C F F F [email protected] F F

F F F F University of Tartu F Estonia F

University of Shanghai for Science and Technology, March 2011

Overview • Acidity, basicity, acids, bases • In solutions • In the gas phase • Measurement of acid strength • Superacids • Measurement of their strength • Design of superacids

1 “Acid is a sour substance” • Acidshavebeeninusesincelongtime • Food preservation • Making paints and dyes • … • Obtaining: • Sour fruits (citric acid) HO H H • Fermentation (acetic acid) O H • Heating minerals (sulfuric acid, …)

“Alkali is caustic substance” • Alkalies (inorganic bases) have also been in use for a long time • Washing (NaOH, one of the starting materials of soap)

• Constrauction (Lime - CaO, Ca(OH)2 – lime mortar)

• Obtained: • Ashes • Limestone

2 Ancient landmarks

Soap-making, Babylon, 2800 BC

Lime processing, Since antiquity

Vineager, Since antiquity

Preparation of H2SO4 and HCl Jābir ibn Hayyān, 8. saj

In 18. Century all common acids and alkalies were known and in use Images: Wikipedia 5

The essence of acidity?

18. century: The role of acids is large

But the origin of acidity was still largely unclear

3 The Origin of Acidity?

Antoine Lavoisier, 1776: acids are compounds that contain oxygen (oxygen: producer of acid)

Humphrey Davy, 1810: HCl, HBr,

H2S etc do not contain oxygen

Justus von Liebig, 1838: acids are compounds, in which H atom can be substituted by a metal atom

Images: Wikipedia 7

Modern Understanding • Acids dissociate in solution giving H+ ion Svante Arrhenius (1859-1927) Nobel prize 1903

• Different acids dissociate to a different extent Wilhelm Ostwald (1853-1932) Nobel prize 1909

• Acidity of a solution: + pH = -log[H ] Søren Sørensen (1868-1939)

• Acid: proton donor, conjugate acids and bases Images: Wikipedia Johannes Nicolaus Brønsted (1879-1947) 8

4 Acidity of molecules and acidity of media

• Brønsted acidity of a molecule refers to its ability to donate proton to other molecules • Usually defined in terms of equilibrium constants

(Ka, pKa) or deprotonation energies (GA or ΔGacid)

• Brønsted acidity of a medium refers to the ability to donate proton to molecules in the medium • In aqueous solution: pH

• Strongly acidic solutions: H0 9

Acidity of molecules in solution

• Acidity (acid strength) of molecules in solution is usually defined in the framework of the Brønsted theory via the

pKa values

Ka HA + S A- + SH+

Acid Conjugate acid Base Conjugate base of acid HA of base S a(A− )⋅a(SH+ ) pKa = −log Ka = −log a(HA) 10

5 Acidity and molecular structure

OH O + S ' + SH+

H2O pKa =10

OH O- O2N NO2 O2N NO2 + S ' + SH+ NO2 NO2

H2O 11 pKa = ~ 0

Acid-base reaction in a non-aqueous solvent

K1 K2 K3 ¯ + ¯ + (AH)S + (:B)S ' (AH⋅⋅⋅:B)S ' (A :⋅⋅⋅HB )S or (A :HB )S ' HB complex HB complex contact ion pair

K3 K4 ¯ + ¯ + ' (A : // HB )S ' (A :)S + (HB )S solvent-separated ion pair free ions

6 Acidity of molecules in the gas phase

• Acidity of molecules in the gas phase is expressed via deprotnation Gibbs' free energy Ka

HA A- + H+ 0 GA = ΔGacid = −RT ln Ka

• ΔGºacid is called gas-phase acidity of HA • "º" is often omitted 13

Example of acidity differences in different media

Acid pKa pKa ΔGa (GP) pKa (water) (MeCN) kcal/mol (GP) HBr ca -9 5.5 318.3 233.4 2,4-Dinitro- 3.96 16.7 308.6 226.2 phenol

• In water HBr is 1013 times stronger than 2,4- DNP • In the gas phase HBr is 107 times weaker than

2,4-DNP 14

7 Gas phase Dissociation in the gas phase – Gas-phase acidiy

ΔG°a(HA) >>> 0 HA H+ + A-

+ −ΔG°a(SH ) << 0 S desolva- tion + - SH ΔG°s(A )<< 0 ΔG°ds(HA) > 0 + ΔG°s(SH ) << 0 Solution Dissociation S S S S S S S in solution S S S + S S HA + - ΔG°a(HA,s) S SH S A S S S S S

S S S S 15

Acidity of solutions: pH

• Simplified: pH = -log[H+]

• Expresses the ability of te medium to donate proton

8 Unified acidty scale

+ μabs(H ) –800 –900 –1000–1100 –1200 –1300 –1400 –1500 in kJ mol–1

superacidic medium acidic medium H SO 14 H O 2 4 3.1 2 HF 12.6 34 DMSO

HSO3F 7 39 MeCN

Et2O 86*

CH2Cl2 86*

C6H6 120*

SO2 1 bar 10–20 bar gaseous HCl (for comparison)

pHabs 140 160 180180 200200 220220 240 260 *approximate value, calculated autoprotolysis 17 Himmel, Goll, Leito, Krossing Chem.Eur.J.2011, 17, 5808

What is a superacid? • Superacidic medium:

A Brønsted superacid is a medium, in which the chemical potential of the proton is higher than in pure sulfuric acid

18 Himmel, Goll, Leito, Krossing Chem.Eur.J.2011, 17, 5808

9 What is a superacid? • Superacidic molecule:

Superacidic molecule in a given medium is

one that is more acidic than H2SO4 in that medium

Why do we need to measure acidity?

• Acidity is a core parameter of an acidic molecule • Rationalization and prediction of mechanisms of chemical and industrial processes • Design of novel acids and bases • Development of theoretical calculation methods

10 How to measure solution acidities of highly acidic molecules?

• H0 scale • The classical approach

• Different H0 refer to different media

• H0 is rather a characteristic of a medium than of a molecule • X-H vibrational frequencies • Indirect: characterizes Stoyanov, et al JACS, 2006, 128, 8500 hydrogen bond rather than acidity

• Equilibrium measurement (pKa) in a low basicity solvent

• Obvious, butalmostunusedappraoch! 21

Solvent • As low basicity as possible • As high ε as possible

• As high pKauto as possible • Easy to purify, reasonably inert, electrochemically stable, transparent in UV, readily available, widely used

• There is no ideal solvent • Wth increasing ε as a rule basicity also increases

• Water cannot be used 22

11 Non-aqueous pKa values: not trivial

• Processes are often more complex than simple ionic dissociation • Ion-pairing, homoconjugation, … • Measurement of a(SH+) is not trivial • Traces of moisture can significantly affect

results K. Kaupmees et al, J. Phys. Chem. A 2010, 114, 11788

• Working in an inert gas atmosphere (glovebox) is necessary

Approach: Relative measurement

• Measurement of pKa differences

• ΔpKa values • No need to measure a(SH+) in solution • Many of the error sources cancel out, either partially or fully • Traces of moisture • Impurities in compounds • Baseline shifts and drifts • Technique: UV-Vis spectrometry

12 Compound 1 UV-Vis ΔpKa measurement Compound 2

2 2

1.8 1.8 OH CN CF SO SO CF 1.6 1.6 anion 2 2 2 3 anion 1.4 NC CN 1.4 1.2 1.2 SO2CF3 1 1 0.8 0.8 Absorbance (AU) Absorbance (AU) 0.6 0.6

0.4 0.4

0.2 0.2 neutral 0 0 neutral 190 240 290 340 390 190 240 290 340 390 wavelength (nm) wavelength (nm) Mixture

2 OH 1.8 CF SO SO CF 1.6 2 2 2 3 ΔpK = pK (HA ) − pK (HA ) 1.4 a a 2 a 1 anion 1.2 CN SO2CF3 - 1 [A ]⋅[HA ] NC CN 1 2 0.8 = −log K = log - Absorbance (AU) Absorbance 0.6 [HA1]⋅[A2 ] 0.4 0.2 neutral = 0.87 0 190 240 290 340 390 25 wavelength (nm)

1,2-Dichloroethane • Advantages of 1,2-DCE: • Very low basicity (B' = 40), low anion-solvating ability → strong superacids are measurable

• pKauto unmeasurably high • Reasonably inert and stable, readily available, widely used, dissolves also many ionic compounds well • Transparent in UV down to 230 nm

• Limitations: • ~Low polarity (ε = 10) • Ion-pair acidities

13 a No Acid pK a(DCE) Directly measured ΔpK ip values in DCE pK 1,2-DCE acidity 1 Picric acid 0.0 2 HCl -0.4 0.73 0.71 1.48 0.36 3 2,3,4,6-(CF3)5-C6H-CH(CN)2 -0.7 1.09 0.77 0.74 c 2.08 1.00 scale 4 4-NO2-C6H4SO2NHTos -1.5 0.28 1.78 5 HNO3 -1.7 1.01 1.13 6 4-NO2-C6H4SO2NHSO2C6H4-4-Cl -2.4 0.88 0.08 • The most acidic 0.24 7 H2SO4 -2.5 0.12 1.26 8 C6(CF3)5CH(CN)2 -2.6 equilibrium acidity 1.02 1.02 1.05 1.48 9 (4-NO2-C6H4-SO2)2NH -3.7 0.47 0.80 10 3-NO2-4-Cl-C6H3SO2NHSO2C6H4-4-NO2 -4.1 scale in a constant- 0.36 1.35 11 (3-NO2-4-Cl-C6H3SO2)2NH -4.5 0.39 0.93 composition 12 HBr -4.9 0.62 0.19 1.03 13 4-NO2-C6H4SO2CH(CN)2 -5.1 0.80 medium 1.33 14 2,4,6-(SO2F)3-Phenol -5.9 d 0.64 15 2,4,6-Tf3-Phenol -6.4 0.94 16 CH(CN)3 -6.5 0.42 1.14 1.12 17 4-Cl-C H SO(=NTf)NHTos -6.8 0.33 0.65 6 4 0.51 0.05 18 NH -TCNP e -6.8 0.24 • Relative acidities 2 0.20 19 2,3,5-tricyanocyclopentadiene -7.0 20 Pentacyanophenol -7.6 0.67 0.84 1.77 21 4-Cl-C6H4SO(=NTf)NHSO2C6H4-4-Cl -7.6 1.56 22 HI -7.7 1.64 1.00 • Acidity increases 0.98 1.13 1.10 0.93 23 4-NO2-C6H4SO2NHTf -7.8 1.04 1.01 0.81 24 Me-TCNP -8.6 0.96 0.90 downwards 0.09 0.13 1.42 25 3,4-(MeO)2-C6H3-TCNP -8.7 -0.02 0.12 26 4-MeO-C6H4-TCNP -8.7 0.12 0.46 0.24 27 C(CN)2=C(CN)OH -8.8 0.28 1.61 0.22 0.59 28 4-Cl-C6H4SO(=NTf)NHSO2C6H4-NO2 -8.9 0.74 0.67 0.06 29 2,4-(NO2)2-C6H3SO2OH -8.9 0.59 0.60 30 C6F5CH(Tf)2 -9.0 0.52 31 HB(CN)(CF3)3 -9.3 0.47 0.44 A. Kütt et al J. Org. Chem. 1.33 0.13 32 Ph-TCNP -9.4 1.56 1.62 0.83 1.57 27 33 HBF4 -10.3 1.23 2011, 76, 391 1.06 34 FSO2OH -10.5 0.26 1.34 001

0.67 0.06 29 2,4-(NO2)2-C6H3SO2OH -8.9 0.59 0.60 30 C6F5CH(Tf)2 -9.0 0.52 31 HB(CN)(CF3)3 -9.3 0.47 0.44 1.33 0.13 1,2-DCE acidity 32 Ph-TCNP -9.4 1.56 1.62 0.83 1.57 33 HBF4 -10.3 1.23 1.06 34 FSO2OH -10.5 0.26 1.34 scale 0.01 35 3-CF3-C6H4-TCNP -10.5 0.21 0.60 0.22 36 H-TCNP -10.7 0.58 0.73 0.78 0.46 37 [C6H5SO(=NTf)]2NH -11.1 0.84 0.84 38 [(C2F5)2PO]2NH -11.3 0.89 0.91 0.29 0.93 39 2,4,6-(NO2)3-C6H2SO2OH -11.3 0.28

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

43 TfCH(CN)2 -11.6 0.36 0.20 0.25 44 Br-TCNP -11.8 0.10 0.67 0.73 0.75 45 [C(CN)2=C(CN)]2NH -11.8 0.06 0.63 0.19 46 3,5-(CF3)5-C6H3-TCNP -11.8 0.21 0.45 0.19 47 Tf2NH -11.9 0.30 0.31 0.42 48 4-Cl-C6H4SO(=NTf)NHTf -12.1 0.15 0.46 0.01 0.36 49 Cl-TCNP -12.1 0.43 0.10 0.40 50 (C3F7SO2)2NH -12.2 0.13 0.21 0.29 51 (C4F9SO2)2NH -12.2 0.19 0.27 1.29 0.69 0.10 52 CN-CH2-TCNP -12.3 0.93 1.06 0.02 1.04 53 (C2F5SO2)2NH -12.3 1.05 1.06 0.47 0.44 • Aqueous pK (H ) 0.72 0.96 a 0 54 CF3-TCNP -12.7 0.77 0.80 55 HClO4 -13.0 0.80 1.04 0.40 values down to 0.11 0.56 56 CF2(CF2SO2)2NH -13.1 0.89 0.86 0.07 57 4-NO2-C6H4SO(=NTf)NHTf -13.1 0.19 -10 .. -15 58 HB(CN)4 -13.3 0.44 1.76 59 (FSO2)3CH -13.6 1.92 2.16 60 Tf2CH(CN) -14.9 1.46 1.73 0.22 61 2,3,4,5-tetracyanocyclopentadiene -15.1 0.40 0.21 0.23 62 CN-TCNP -15.3 f 63 Tf3CH -16.4 f 64 CF3SO(=NTf)NHTf -18 28

14 Important aspects for superacids • Brønsted acidity • As strong as possible • "Clean" protonation desirable • Lewis acidity is usually not desirable • Stability under superacidic conditions • Weak coordinating properties of the anions

Superacid derivatives in your pocket!

Li-ion battery contains ultrapure lithium salt

(LiBF4, LiClO4, etc) and ultrapure solvent.

Purity must be analysed!

These are salts of superacids!

15 Design of superacidic molecules • "Acid-based" approach: • Pick a parent acid • Introduce substituents • electronegative, strong electron acceptors, highly polarizable O CF3 S O NO CN CN O O H F3C S OH NC P CN ON O S O NC CN ΔΔGacid = ΔΔGacid = 47 kcal/mol O CF3 40 kcal/mol

ΔGacid = 256303 kcal/mol ΔΔGGacidacid== 299.5 260 kcal/mol kcal/mol (DFT B3LYP/6-311+G**) (DFT B3LYP/6-311+G**)

Leito et al J. Mol. Stru. Koppel, Yagupolskii et al Theochem, 2007, 815, 43 to be submitted 31

Basicity Centers on Substituents

. . : :O CF3 S CNCN:: CNCN:: H S O: :N . . H . . F C SOHSO ::NCNC PP CN: H 3 . . N :N . . . . : ::NCNC CNCN:: S O: CF :.O . 3

It is not only about the acceptor power but also about basicity! 32

16 Design: Anion-based approach • Design an anion, which • Has delocalized charge • Is as stable as possible • Has as few as possible protonation sites and those are of low basicity

Not looking at any particular acidity center

Superacids: breaking the “limits of growth” • We have designed superacidic molecules that are

more acidic than H2SO4 by up to 100 orders of magnitude (!) F CF3 C F C NC CN F C CF F CF3 3 3 F F3C CF F F F C CF 3 - 3 3 + - NC - B CN H F3C CH(CN)2 F F F F C CF F F C 3 3 CF F + 3 3 F C CF H F3C H+ NC CN 3 3 F CF3 - Leito et al, J Mol Stru Theochem. 2007, 815, 41 -Kütt et alChem Phys Chem, 2009, 10, 499 - Kütt et al, J. Org. Chem. 2008, 73, 2607 - Kütt et al, J. Org. Chem. 2006, 71, 2829 - Kütt et al, J. Org. Chem. 2011, 76, 391

- Lipping et al, J. Phys. Chem. A, 2009, 11334, 12972

17 ΔGacid all values in kcal/mol Superacidity in the gas phase H2SO4 302 300 299.5 CF3SO2OH

CF3SO2 HBF4 NH 288 CF3SO2 286.5

284.0 C2F5SO2 NH 280 C2F5SO2 HPF6 276.6 CN NC CN H C H H H H NC CN H H - H H SO CF 267.2 H H N 2 3 266.5 H H+ H F3C S OH 260 260 N SO2CF3 35

256 CN CN + 255.5 H HSbF6 NC CN - NC P CN 250.8 HAuF + 6 NC - B CN H 250 250 NC CN

NC H CN C H H H Acidity in the H H 243 - 240 Cl Cl Cl Cl gas phase Cl + HB(CF3)4 H Cl Cl C Cl Cl Cl Cl Cl

Cl Cl Cl Cl Cl + H Cl GA = 241 ± 29 kcal/mol 225 225 CN C CN NC CN NC CN - NC CN -Kütt et alChem Phys Chem, F NC CN NC + C F H 2008 F F CN F F 213 - - Kütt et al, JOC, 2008, 73, 2607 F F F F CF F + 3 - Leito et al, J Mol Stru Theochem. H C CF F F C 3 3 CF3 2007, 815, 41 F3C CF3 - - Leito et al, JPC A, 2009, 113, 8421 F3C CF3 CF F3C 3 F3C H+ < 175 CF - Koppel et al, JACS, 2000, 3 36 200 122, 5114

18 Thanks to all these people! 37

Collaboration • Y.L. Yagupolskii (IOC, Ukrainian Academy of Sciences, Kiev) • I. Krossing, D. Himmel (Freiburg University) • A.A. Kolomeitsev, G.-V. Röschenthaler (IIPC, University of Bremen) • M. Rueping (Aachen University) • V.M. Vlasov (IOC, Russian Academy o Sciences, Novosibirsk) • M. Mishima (IMCE, Kyushu University)

19 Overview: http://tera.chem.ut.ee/~ivo/HA_UT/ 39

Thank you!