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Superacid derivativesin your pocket?Ivo Leito

ivo.leito@ut.ee

University of TartuEstonia

University of Shanghai for Science and Technology, March 2011

C

FFF

F

F F

FF F F

FF

OHNO2

NO2

O2N

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Overview

• Acidity, basicity, acids, bases• In solutions• In the gas phase• Measurement of acid strength

• Superacids• Measurement of their strength• Design of superacids

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“Acid is a sour substance”• Acids have been in use since long time

• Food preservation• Making paints and dyes• …

• Obtaining:• Sour fruits (citric acid)• Fermentation (acetic acid)• Heating minerals (sulfuric acid, …)

H O

O

H

HH

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“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

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Ancient landmarks

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

Soap-making, Babylon, 2800 BC

Lime processing, Since antiquity

Vineager, Since antiquity

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

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The essence of acidity?

But the origin of acidity was stilllargely unclear

18. century: The role of acids islarge

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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, inwhich H atom can be substituted by a metal atom

Images: Wikipedia

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Modern Understanding• Acids dissociate in solution

giving H+ ion

• Different acids dissociate to a differentextent

• Acidity of a solution:pH = -log[H+]

• Acid: proton donor,conjugate acids and bases

Svante Arrhenius (1859-1927)Nobel prize 1903

Wilhelm Ostwald (1853-1932)Nobel prize 1909

Søren Sørensen (1868-1939)

Johannes Nicolaus Brønsted (1879-1947)Images: Wikipedia

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Acidity of molecules and acidity of media

• Brønsted acidity of a molecule refers to itsability to donate proton to othermolecules• Usually defined in terms of equilibrium constants

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

• Brønsted acidity of a medium refers to theability to donate proton to molecules inthe medium• In aqueous solution: pH• Strongly acidic solutions: H0

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• Acidity (acid strength) of molecules insolution is usually defined in theframework of the Brønsted theory via thepKa values

HA + S A- + SH+

Acidity of molecules in solution

Ka

HA)()SH()A(loglogp aa a

aaKK+− ⋅

−=−=

Acid Base Conjugate baseof acid HA

Conjugate acidof base S

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+ S + SH+

pKaH2O = 10

+ S + SH+

pKaH2O = ~ 0

Acidity and molecular structure

ONO2

NO2

O2N

-OHNO2

NO2

O2N

OH O

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Acid-base reaction in a non-aqueoussolvent

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

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• Acidity of molecules in the gas phase isexpressed via deprotnation Gibbs' freeenergy

HA A- + H+

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

Acidity of molecules in the gas phase

Ka

a0acid lnGA KRTG −=Δ=

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Example of acidity differences in differentmedia

• In water HBr is 1013 times stronger than 2,4-DNP

• In the gas phase HBr is 107 times weaker than2,4-DNP

16.7

5.5

pKa(MeCN)

226.2308.63.962,4-Dinitro-phenol

233.4318.3ca -9HBr

pKa (GP)

ΔGa (GP)kcal/mol

pKa(water)

Acid

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HAS

S

S

S

S

S

SSHA

desolva-tion

ΔG°ds(HA) > 0

A-H+

Dissociation in the gas phase – Gas-phase acidiyΔG°a(HA) >>> 0

Solution

Gas phase

+

SH+

SH+S

S S

S

S

S

S

SDissociationin solution

ΔG°a(HA,s) A-S

S S

S

S

S

S

+

−ΔG°a(SH+) << 0

ΔG°s(SH+) << 0

ΔG°s(A-) << 0

S

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Acidity of solutions: pH

• Simplified:pH = -log[H+]

• Expresses the abilityof te medium todonate proton

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Unified acidty scale

–800 –900 –1000 –1200–1100 –1300 –1400 –1500

140 160 180 200 220 240 260

H2SO4 143.1 H2O

HF

MeCN

DMSO

Et2O

CH2Cl2

C6H6

SO2

12.6 34

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86*

86*

120*

HSO3F 7

μabs(H+) in kJ mol–1

pHabs

superacidic medium acidic medium

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

140 160 180 200 220 240 260*approximate value, calculated autoprotolysis

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

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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

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

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What is a superacid?• Superacidic molecule:

Superacidic molecule in a given medium isone that is more acidic than H2SO4 in that

medium

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Why do we need to measure acidity?

• Acidity is a core parameter of an acidicmolecule

• Rationalization and prediction of mechanisms of chemical and industrialprocesses

• Design of novel acids and bases• Development of theoretical calculation

methods

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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

hydrogen bond rather than acidity

• Equilibrium measurement (pKa) in a lowbasicity solvent• Obvious, but almost unused appraoch!

Stoyanov, et al JACS, 2006, 128, 8500

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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, widelyused

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

• Water cannot be used

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Non-aqueous pKa values: not trivial

• Processes are often more complex thansimple ionic dissociation• Ion-pairing, homoconjugation, …

• Measurement of a(SH+) is not trivial• Traces of moisture can significantly affect

results

• Working in an inert gas atmosphere (glovebox) isnecessary

K. Kaupmees et al, J. Phys. Chem. A2010, 114, 11788

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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

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0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

190 240 290 340 390wavelength (nm)

Abs

orba

nce

(AU

)

anion

neutral0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

190 240 290 340 390wavelength (nm)

Abs

orba

nce

(AU

)

neutral

anion

UV-Vis ΔpKa measurementCompound 1 Compound 2

Mixture

87.0]A[]HA[]HA[]A[loglog

)HA(p)HA(pp

-21

2-1

1a2aa

=⋅

⋅=−=

−=Δ

K

KKK

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

190 240 290 340 390

wavelength (nm)

Abs

orba

nce

(AU

)

anion

neutral

CN

CN

NC

CF2SO2

SO2CF3

SO2CF3

OH

CN

CN

NC

CF2SO2

SO2CF3

SO2CF3

OH

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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

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1,2-DCE acidityscale

A. Kütt et al J. Org. Chem.2011, 76, 391

• The most acidicequilibrium acidityscale in a constant-compositionmedium

• Relative acidities

• Acidity increasesdownwards

No Acid pK a(DCE) Directly measured ΔpK ip values in DCE a pK

1 Picric acid 0.0

2 HCl -0.4

3 2,3,4,6-(CF3)5-C6H-CH(CN)2 -0.7

4 4-NO2-C6H4SO2NHTos c -1.5

5 HNO3 -1.7

6 4-NO2-C6H4SO2NHSO2C6H4-4-Cl -2.4

7 H2SO4 -2.5

8 C6(CF3)5CH(CN)2 -2.6

9 (4-NO2-C6H4-SO2)2NH -3.7

10 3-NO2-4-Cl-C6H3SO2NHSO2C6H4-4-NO2 -4.1

11 (3-NO2-4-Cl-C6H3SO2)2NH -4.5

12 HBr -4.9

13 4-NO2-C6H4SO2CH(CN)2 -5.1

14 2,4,6-(SO2F)3-Phenol -5.9

15 2,4,6-Tf3-Phenol d -6.4

16 CH(CN)3 -6.5

17 4-Cl-C6H4SO(=NTf)NHTos -6.8

18 NH2-TCNP e -6.8

19 2,3,5-tricyanocyclopentadiene -7.0

20 Pentacyanophenol -7.6

21 4-Cl-C6H4SO(=NTf)NHSO2C6H4-4-Cl -7.6

22 HI -7.7

23 4-NO2-C6H4SO2NHTf -7.8

24 Me-TCNP -8.6

25 3,4-(MeO)2-C6H3-TCNP -8.7

26 4-MeO-C6H4-TCNP -8.7

27 C(CN)2=C(CN)OH -8.8

28 4-Cl-C6H4SO(=NTf)NHSO2C6H4-NO2 -8.9

29 2,4-(NO2)2-C6H3SO2OH -8.9

30 C6F5CH(Tf)2 -9.0

31 HB(CN)(CF3)3 -9.3

32 Ph-TCNP -9.4

33 HBF4 -10.3

34 FSO2OH -10.5

0.20 0.24

0.51

0.42 0.65

1.14 1.12 0.33

0.94

0.64

0.80 1.03

1.33

0.19 0.62

0.39 0.93 1.35

0.36 0.80

0.47 1.48

1.02 1.02 1.05

1.26 0.24

0.12

1.13 1.01

0.08

1.78 2.08

0.28 1.00

0.74 0.77 1.09

1.48 0.73 0.71

0.36

0.05

0.88

0 01

1.06 1.23

0.26 1.34

1.57 1.56 1.62

0.83

0.47 0.44 1.33 0.13

0.52 0.60

0.59 0.67

0.74

1.61 0.59 0.22

0.06

0.28 0.46 0.24

0.12 0.12

0.09

-0.02 0.13 1.42

0.96 1.04

1.13

1.01

1.64

1.10 0.98

0.93

0.90 0.81

1.00 1.56

1.77

0.67 0.84

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1,2-DCE acidityscale

• Aqueous pKa (H0)values down to-10 .. -15

29 2,4-(NO2)2-C6H3SO2OH -8.9

30 C6F5CH(Tf)2 -9.0

31 HB(CN)(CF3)3 -9.3

32 Ph-TCNP -9.4

33 HBF4 -10.3

34 FSO2OH -10.5

35 3-CF3-C6H4-TCNP -10.5

36 H-TCNP -10.7

37 [C6H5SO(=NTf)]2NH -11.1

38 [(C2F5)2PO]2NH -11.3

39 2,4,6-(NO2)3-C6H2SO2OH -11.3

40 [C(CN)2=C(CN)]2CH2 -11.4

41 TfOH -11.4

42 C6H5SO(=NTf)NHTf -11.5

43 TfCH(CN)2 -11.6

44 Br-TCNP -11.8

45 [C(CN)2=C(CN)]2NH -11.8

46 3,5-(CF3)5-C6H3-TCNP -11.8

47 Tf2NH -11.9

48 4-Cl-C6H4SO(=NTf)NHTf -12.1

49 Cl-TCNP -12.1

50 (C3F7SO2)2NH -12.2

51 (C4F9SO2)2NH -12.2

52 CN-CH2-TCNP -12.3

53 (C2F5SO2)2NH -12.3

54 CF3-TCNP -12.7

55 HClO4 -13.0

56 CF2(CF2SO2)2NH -13.1

57 4-NO2-C6H4SO(=NTf)NHTf -13.1

58 HB(CN)4 -13.3

59 (FSO2)3CH -13.6

60 Tf2CH(CN) -14.9

61 2,3,4,5-tetracyanocyclopentadiene -15.1

62 CN-TCNP -15.3

63 Tf3CH f -16.4

64 CF3SO(=NTf)NHTf f -18

0.23 0.21 0.40

1.73

2.16

0.22 1.46

1.76 1.92

0.44 0.19

0.89 0.86

0.36

0.19

0.12

1.06

1.29

1.05

0.01

0.31

0.21

0.73 0.75 0.06

0.40

0.45

0.10

0.36 0.46

0.96

0.07 0.11

0.80

0.56

0.80

0.40

0.19 0.10

0.02

0.77

0.15

0.13 0.10

1.04

0.27

0.21

1.04

0.69

0.72

0.93

0.30

0.44

0.42

1.06

0.40

0.47

0.29

0.43

0.19

0.20 0.25

0.32

0.10

0.09 0.07 0.04

0.63 0.67

0.44 0.21

0.49 0.47

0.47

0.84 0.73 0.78

0.58 0.60

0.01

0.22

0.46

0.21

1.06 1.23

0.26 1.34

1.57 1.56 1.62

0.83

0.47 0.44 1.33 0.13

0.52 0.60

0.59 0.67 0.06

0.29

0.84 0.89 0.91

0.93 0.28

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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

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Superacid derivatives in your pocket!

Li-ion battery containsultrapure lithium salt(LiBF4, LiClO4, etc)and ultrapure solvent.

Purity must be analysed!

These are salts ofsuperacids!

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POO

O

H SO

OOHF3C

Design of superacidic molecules• "Acid-based" approach:

• Pick a parent acid• Introduce substituents

• electronegative, strong electronacceptors, highly polarizable

ΔGacid = 303 kcal/mol(DFT B3LYP/6-311+G**)

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

Leito et al J. Mol. Stru.Theochem, 2007, 815, 43

P

CN

CN

CNCN

NC

NC

H

Koppel, Yagupolskii et alto be submitted

SN

NOHF3C

S

S

CF3

CF3

OO

OO

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

ΔGacid = 256 kcal/mol(DFT B3LYP/6-311+G**)

ΔΔGacid =47 kcal/mol

ΔΔGacid =40 kcal/mol

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S OHN

NF3C

S

S

OOCF3

CF3OO

:

:

:

:

:

:

. .

. .

. .

. .S ON

NF3C

S

S

OOCF3

CF3OO

H

:

:

:

:

:

. .

. .

. .

. .

. .

. .P

CN

CN

CNCN

NC

NC

H

::

:

: :

:P

CN

CNCN

NC

NCN H

::

:

: :

. .

Basicity Centers on Substituents

It is not only about the acceptorpower but also about basicity!

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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 aciditycenter

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

more acidic than H2SO4 by up to 100 orders of magnitude (!)

C

FFF

F

F F

FF F F

FF

H+

-

C

F3CCF3F3C

CF3

F3C CF3

F3CF3C CF3CF3

CF3

CF3

H+

- B CN

CNNC

NC

NC CN

H+-

- Leito et al, J Mol Stru Theochem.2007, 815, 41

- Kütt et al Chem 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, 113,12972

CH(CN)2F3C

F3C CF3

F3C CF3

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ΔGacid all values in kcal/mol

302

286.5

260

300 299.5

CF3SO2

NHCF3SO2

266.5

CF3SO2OH

267.2

H

NC

NC

CNCN

CN

276.6HPF6

C

HHH

H

H H

HH H H

HH

H+

-

280

H2SO4

288HBF4

SN

NOHF3C

SO2CF3

SO2CF3

260

284.0

Superacidity in thegas phase

C2F5SO2

NHC2F5SO2

36

250

200

225

C

FFF

F

F F

FF F F

FF

H+

-

C

F3CCF3F3C

CF3

F3C CF3

F3CF3C CF3CF3

CF3

CF3

H+

-

C

ClClCl

Cl

Cl Cl

HH H H

HH

H+

-243

213

< 175

240HB(CF3)4

225CCN

CNCN

CN

CNNC

NC NC

NCNC CN

CN

H+

-

255.5 HSbF6

250.8 HAuF6250 B CN

CNNC

NC

NC CN

H+-

- Kütt et al Chem Phys Chem,2008

- Kütt et al, JOC, 2008, 73, 2607

- Leito et al, J Mol Stru Theochem.2007, 815, 41

- Leito et al, JPC A, 2009, 113, 8421

- Koppel et al, JACS, 2000,122, 5114

P

CN

CN

CNCN

NC

NC

H+-

256

Acidity in thegas phase

C

ClClCl

Cl

Cl Cl

ClCl Cl Cl

ClCl

H+

GA = 241 ± 29 kcal/mol

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ThanksThankstoto all all thesethese

peoplepeople!!

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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)

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

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Thank you!