Acidity of Superacids (Superacid pKa values, measurements

Acidity of Superacids (Superacid pKa values, measurements

OH
NO2
O2N
Superacid derivatives
in your pocket?
Ivo Leito
[email protected]
University of Tartu
Estonia
NO2
F
F
F
C F
F
F
F
F
F
F
F
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
2
1
“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
H
H
3
“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
4
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
6
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
7
Images: Wikipedia
Modern Understanding
• Acids dissociate in solution
Svante Arrhenius (1859-1927)
giving H+ ion
Nobel prize 1903
• Different acids dissociate to a different
Wilhelm Ostwald (1853-1932)
extent
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
HA
Acid
+
Base
S
Ka
A-
Conjugate base
of acid HA
+
SH+
Conjugate acid
of base S
a (A − ) ⋅ a(SH + )
pK a = − log K a = − log
a(HA)
10
5
Acidity and molecular structure
OH
O
+ S '
+ SH+
pKaH2O = 10
OH
O-
NO2
O2N
NO2
NO2
O2 N
+ S '
+ SH+
NO2
pKaH2O = ~ 0
11
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
12
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 K a
• ΔGºacid is called gas-phase acidity of HA
• "º" is often omitted
13
Example of acidity differences in different
media
Acid
pKa
pKa
(water) (MeCN)
ΔGa (GP) pKa
kcal/mol (GP)
HBr
ca -9
5.5
318.3
233.4
2,4-Dinitrophenol
3.96
16.7
308.6
226.2
• In water HBr is 1013 times stronger than 2,4DNP
• 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
H+
+
HA
−ΔG°a(SH+) << 0
desolvation
ΔG°ds(HA) > 0
A-
S
ΔG°s(A-) << 0
SH+
ΔG°s(SH+) << 0
S
S
AS
S
S
S
S
S
S
SH+
S
+
S
S
S
ΔG°a(HA,s)
S
S
S
HA
S
S
S
S
Dissociation
in solution
S
S
S
Solution
15
Acidity of solutions: pH
• Simplified:
pH = -log[H+]
• Expresses the ability
of te medium to
donate proton
16
8
Unified acidty scale
μabs(H+)
in kJ mol–1
–800
–900
–1000
superacidic medium
H2SO4
–1100 –1200 –1300 –1400
acidic medium
3.1
14
HF 12.6
H2O
34
39
HSO3F 7
DMSO
MeCN
86*
Et2O
CH2Cl2
–1500
86*
120*
C6H6
SO2
1 bar
pHabs 140
140
160
160
10–20 bar
gaseous HCl (for comparison)
180
200
220
240
260
180
200
220
240
260
*approximate value, calculated autoprotolysis
Himmel, Goll, Leito, Krossing Chem. Eur. J. 2011, 17, 5808
17
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
18
9
What is a superacid?
• Superacidic molecule:
Superacidic molecule in a given medium is
one that is more acidic than H2SO4 in that
medium
19
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
20
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
Stoyanov, et al JACS, 2006, 128, 8500
• Indirect: characterizes
hydrogen bond rather than acidity
• Equilibrium measurement (pKa) in a low
basicity solvent
• Obvious, but almost unused appraoch!
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
23
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
24
12
Compound 1
UV-Vis ΔpKa measurement
2
1.8
OH
1.8
CN
anion
1.4
NC
1.6
Absorbance (AU)
1.6
Absorbance (AU)
Compound 2
2
CN
1.2
1
0.8
0.6
anion
CF2SO2
SO2CF3
1.4
1.2
SO2CF3
1
0.8
0.6
0.4
0.4
0.2
0.2
neutral
0
190
240
neutral
0
290
340
wavelength (nm)
190
390
240
290
340
wavelength (nm)
390
Mixture
2
OH
1.8
Absorbance (AU)
SO2CF3
CF2SO2
1.6
1.4
anion
1.2
CN
SO2CF3
1
NC
0.8
CN
0.6
0.4
0.2
ΔpK a = pK a (HA 2 ) − pK a (HA1 )
[A1- ] ⋅ [HA 2 ]
= − log K = log
[HA1 ] ⋅ [A -2 ]
= 0.87
neutral
0
190
240
290
340
390
wavelength (nm)
25
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
26
13
1,2-DCE acidity
scale
• The most acidic
equilibrium acidity
scale in a constantcomposition
medium
• Relative acidities
• Acidity increases
downwards
A. Kütt et al J. Org. Chem.
2011, 76, 391
1,2-DCE acidity
scale
No
Directly measured ΔpK ip values in DCE a
pK a(DCE)
Acid
1
Picric acid
2
HCl
3
2,3,4,6-(CF3)5-C6H-CH(CN)2
-0.4
c
-0.7
4
4-NO2-C6H4SO2NHTos
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
-1.5
(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
0.73
0.71
1.48
0.36
1.09
0.77
0.74
0.28
2.08
1.78
1.00
1.01
1.13
0.88
0.08
0.24
0.12
1.26
1.02
1.02
1.48
0.47
1.05
0.80
0.36
0.39
1.35
0.93
0.62
0.19
1.03
0.80
1.33
0.64
0.94
0.42
1.14
0.33
0.65
1.12
0.51
0.24
0.05
0.20
0.67
0.84
1.77
1.56
1.64
1.00
0.98
1.10
1.13
0.93
1.04
1.01
0.96
0.09
0.13
-0.02
0.81
0.90
1.42
0.12
0.12
0.46
0.28
0.24
1.61
0.59
0.22
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
1.06
0.26
0 01
29
2,4-(NO2)2-C6H3SO2OH
-8.9
0.67
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 0.89
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
0.74
0.06
0.67
0.59
0.60
0.52
0.44
0.47
1.33
1.56
0.83
1.57
0.13
1.62
27
1.23
1.34
0.06
0.59
0.60
0.52
0.44
0.47
1.33
1.56
0.83
1.57
0.13
1.62
1.23
1.06
0.26
0.01
1.34
0.21
0.60
0.58
0.73
0.78
0.22
0.46
0.84
0.84
0.91
0.29
0.93
0.28
0.44
0.10
0.12
0.04
0.40
0.36
0.21
0.07
0.47
0.32
0.20
0.47
0.09
0.49
0.25
0.10
0.67
0.73
0.06
0.75
0.63
0.19
0.21
0.45
0.19
0.30
0.31
0.42
0.15
0.01
0.46
0.36
0.43
0.10
• Aqueous pKa (H0)
values down to
-10 .. -15
pK
0.0
f
63
Tf3CH
64
CF3SO(=NTf)NHTf f
0.40
0.21
0.13
0.29
0.19
0.69
0.27
0.10
1.29
0.93
1.06
0.02
1.05
1.06
0.72
1.04
0.47
0.44
0.96
0.77
0.80
0.80
0.40
0.11
0.89
1.04
0.56
0.86
0.07
0.19
0.44
1.76
-16.4
-18
1.92
2.16
1.46
1.73
0.22
0.40
0.21
0.23
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
29
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!
30
15
Design of superacidic molecules
• "Acid-based" approach:
• Pick a parent acid
• Introduce substituents
• electronegative, strong electron
acceptors, highly polarizable
O
CN CN
O
O H
P
NC
CN
O
NC
CN
ΔΔGacid =
47 kcal/mol
ΔGacid = 303
256 kcal/mol
(DFT B3LYP/6-311+G**)
Leito et al J. Mol. Stru.
Theochem, 2007, 815, 43
CF3
S O
O
N
F3C S OH
N
O
S O
O CF3
ΔΔGacid =
40 kcal/mol
ΔG
ΔGacid
260 kcal/mol
kcal/mol
acid==299.5
(DFT B3LYP/6-311+G**)
Koppel, Yagupolskii et al
to be submitted
31
Basicity Centers on Substituents
..
:O
CN: CN:
H
:NC
P
CN: H
.N.
:NC
CN:
CF3
H S .O. :
:N
..
F3C S .OH
O.
..
:N
S O:
:.O. CF3
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
33
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
F
C F
F
F
F
F
F
F
F
H+
F
F
F3C
F3C
F 3C
F 3C
CF3
C CF
3
CF3
CF3
-
NC
NC
CF3 CF
3
F 3C
H+
CF3
NC
CN
F3C
+
- B CN H F3C
CN
F3C
CF3
CH(CN)2
CF3
- 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,
34
12972
17
ΔGacid
Superacidity in the
gas phase
all values in kcal/mol
H2SO4
302
299.5
300
CF3SO2OH
CF3SO2
HBF4
288
286.5
CF3SO2
C2F5SO2
284.0
280
C2F5SO2
HPF6
276.6
NH
NH
CN
NC
H
CN
H
CN
NC
CH
H
H
H
H
-
267.2
266.5
H
H
H
H
H
H+
H
N
256
255.5
250
250.8
250
CN
HSbF6
HAuF6
NC
H
NC
H
243
240
CH
H
P
NC
+
- B CN H
NC
CN
NC
H
Cl
HB(CF3)4
Cl
Cl
Cl
Cl
H+
Cl
Cl
CCl
Cl
Cl
Cl
Cl
Acidity in the
gas phase
Cl Cl
H
Cl
+
GA = 241 ± 29 kcal/mol
225
CN
C CN
NC
NC
CN
CN
NC
F
F
213
C F
F
F
NC
CN CN
NC
H+
F
F
F
- Kütt et al, JOC, 2008, 73, 2607
F
H+
F
F 3C
F 3C
F3C
< 175
- Kütt et al Chem Phys Chem,
2008
CN
F
F
200
CN
Cl
Cl
Cl
225
35
CN
CN
H
SO2CF3
+
H
-
CN
SO2CF3
F3C S OH
260
260
N
CF3
C CF
3
CF3
CF3
-
F3C
F3C
CF3
CF3 CF
3
H+
- 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
36
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)
38
19
Overview:
http://tera.chem.ut.ee/~ivo/HA_UT/
39
Thank you!
40
20