Fluoro- and perfluoralkylsulfonylpentafluoroanilides: Synthesis and characterization of NH acids for weakly coordinating anions and their gas-phase and solution acidities

Article abstract of DOI:10.1002/chem.201405391

Fluoro- and perfluoralkylsulfonyl pentafluoroanilides [HN(C₆F₅)(SO₂X); X=F, CF₃, C₄F₉, C₈F₁₇] are a class of imides with two different strongly electron-withdrawing su

Full text of DOI:10.1002/chem.201405391

DOI: 10.1002/chem.201405391
Full Paper
&
NH Acids
Fluoro- and Perfluoralkylsulfonylpentafluoroanilides: Synthesis
and Characterization of NH Acids for Weakly Coordinating Anions
and Their Gas-Phase and Solution Acidities
Julius F. Kçgel,[a] Thomas Linder,[a] Fabian G. Schrçder,[a] Jçrg Sundermeyer,*[a]
Sascha K. Goll,[b] Daniel Himmel,[b] Ingo Krossing,*[b] Karl Kütt,[c] Jaan Saame,[c] and
Ivo Leito*[c]
Abstract: Fluoro- and perfluoralkylsulfonyl pentafluoroani-
lides [HN(C6F5)(SO2X); X=F, CF3, C4F9, C8F17] are a class of
imides with two different strongly electron-withdrawing sub-
stituents attached to a nitrogen atom. They are NH acids,
the unsymmetrical hybrids of the well-known symmetrical
bissulfonylimides and bispentafluorophenylamine. The syn-
theses, the structures of these perfluoroanilides, their sol-
vates, and some selected lithium salts give rise to a structural
variety beyond the symmetrical parent compounds. The
acidities of representative subsets of these novel NH acids
have been investigated experimentally and quantum-chemi-
cally and their gas-phase acidities (GAs) are reported, as well
as the pKa values of these compounds in acetonitrile (MeCN)
and DMSO solution. In quantum chemical investigations
with the vertical and relaxed COSMO cluster-continuum
models (vCCC/rCCC), the unusual situation is encountered
that the DMSO-solvated acid Me2SO–H-N(SO2CF3)2, optimized
in the gas phase (vCCC model), dissociates to Me2SO-H+
À
–N(SO2CF3)2 during structural relaxation and full optimiza-
tion with the solvation model turned on (rCCC model). This
proton transfer underlines the extremely high acidity of
HN(SO2CF3)2. The importance of this effect is studied compu-
tationally in DMSO and MeCN solution. Usually this effect
is less pronounced in MeCN and is of higher importance
in the more basic solvent DMSO. Nevertheless, the
neglect of the structural relaxation upon solvation causes
typical changes in the computational pKa values of 1 to 4
orders of magnitude (4–20 kJmolÀ1). The results provide evi-
dence that the published experimental DMSO pKa value of
HN(SO2CF3)2 should rather be interpreted as the pKa of
À
a Me2SO-H+–N(SO2CF3)2 contact ion pair.
1. Introduction
and has been examined in various theoretical and experimen-
tal works.[2] It has been applied in numerous Brønsted acid-cat-
alyzed reactions like Diels–Alder reactions,[3] hetero-Michael ad-
ditions,[4] cyclization of siloxyalkynes with arenes and alkenes,[5]
or imine amidation reactions.[6] Due to its weakly coordinating
nature, the corresponding anion [NTf2]À is widely used in imi-
dazolium-,[7] pyrrolidinium-,[8] guanidinium-[9] or phosphonium-
based[10] ionic liquids with low melting points, which can serve
as reaction media[11] or components of electrochemical devi-
ces.[12] Furthermore, [NTf2]À can act as a ligand, generating ex-
tremely electron-poor and therefore highly Lewis-acidic metal
centers that have been successfully tested in Lewis acid-cata-
lyzed reactions.[13] Other applications of its metal complexes in-
clude the field of materials for gas absorption[14] or lumines-
cent lanthanide complexes.[15] The related fluorosulfonylimide
HN(SO2F)2 has also been a prominent subject of theoretical
and electrochemical investigations.[16] Although less acidic by
more than ten orders of magnitude than both sulfonylimides,
decafluorodiphenylamine HN(C6F5)2 is another well-known rep-
resentative of the nitrogen acid family.[17] Research to date has
focused on its use as a weakly basic and sterically demanding
amido ligand for transitions metals,[18] lanthanides[19] and main
group metals.[20] The sterically demanding aryl substituents
Bis(trifluoromethanesulfonyl)imide (HN(SO2CF3)2 or HNTf2) was
first described by DesMarteau in 1984.[1] Since then, it has at-
tracted most scientific interest among the NH-acidic com-
pounds. Its extraordinary Brønsted acidity is caused by the two
strongly electron-withdrawing triflyl (CF3SO2À, TfÀ) moieties
[a] J. F. Kçgel, T. Linder, F. G. Schrçder, J. Sundermeyer
Fachbereich Chemie der Philipps-Universität
Wissenschaftliches Zentrum für Materialwissenschaften (WZMW) Marburg
Hans-Meerwein-Str., 35043 Marburg (Germany)
E-mail: jsu@staff.uni-marburg.de
[b] S. K. Goll, D. Himmel, I. Krossing
Institut für Anorganische und Analytische Chemie
Freiburger Materialforschungszentrum (FMF) and Freiburg Institute for Ad-
vanced Studies (FRIAS), Section Soft Matter Science
Universität Freiburg, Albertstr. 21, 79104 Freiburg (Germany)
E-mail: ingo.krossing@ac.uni-freiburg.de
[c] K. Kütt, J. Saame, I. Leito
University of Tartu, Institute of Chemistry
Ravila 14a, 50411 Tartu (Estonia)
E-mail: ivo.leito@ut.ee
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405391.
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provide a shielding of the Lewis acidic metal centers and che-
lating coordination of the ortho-fluorine atoms can lead to an
increase in the coordination number through secondary
metal–fluorine contacts. Additionally, ionic liquids with the
rather naked amide anion [N(C6F5)2]À have been prepared and
structurally characterized.[21]
hygroscopic and fuming in the air. The higher homologues
with nonafluorobutyl and heptadecafluorooctyl chains,
HN(Pf)(Nf) and HN(Pf)(Hf), are generated in analogous proce-
dures using the sulfonylfluorides C4F9SO2F and C8F17SO2F as S-
electrophiles. In case of HN(Pf)(Hf), protonation of the corre-
sponding sodium salt is preferably carried out with meta-phos-
phoric acid prepared from mixing H3PO4 (85%) and P4O10. The
less volatile the NH acid, the more difficult is its isolation by
distillation or sublimation free from traces of H2SO4, if 100%
sulfuric acid is used as protonating agent. In general, we can
recommend the use of meta-phosphoric acid as a nonvolatile
protonating reagent superior to sulfuric acid to yield the
purest NH acids, such as the known HNTf2, from the corre-
sponding lithium or sodium salts. Conversion of pentafluoro-
aniline with chlorosulfuric acid and subsequent reaction with
phosphorus pentachloride yields HN(C6F5)(SO2Cl).[23] The chlor-
ine–fluorine exchange is now achieved by potassium fluoride
in acetonitrile in the presence of 0.1 equivalents of 18-crown-6,
followed by protonation of the generated potassium salt
with hydrochloric acid in diethyl ether to yield HN(Pf)(Sf)
(Scheme 1).
This report concerns hybrids of both classes of NH acids,
namely HN(C6F5)(SO2Rf) (Rf=F, CF3, C4F9, or C8F17). For easier
notation, the following abbreviations are used: C6F5 =Pf,
SO2F=Sf, SO2CF3 =Tf, SO2C4F9 =Nf and SO2C8F17 =Hf (Figure 1).
Figure 1. Symmetric and asymmetric NH acids investigated in this work.
The syntheses of these molecules are reported, their molecular
structures as pure acid, as solvate, or as lithium salt are pre-
sented, and pKa values in MeCN are experimentally deter-
mined. Additionally, the Gibbs solvation energies and pKa
values in MeCN and DMSO are also calculated with the high-
level vertical and relaxed COSMO cluster-continuum (vCCC and
rCCC) models. Unusually, the DMSO-solvated HNTf2 acid disso-
ciated upon optimization in the solvent, and neglect of this
effect led to a pKa value erroneous by 10 orders of magnitude.
Therefore, the importance of accounting for possible dissocia-
tion of the acid upon solvation during computations has also
been studied for an exemplarily chosen set of systems in
DMSO and MeCN. It should be noted that these investigations
would have been impossible without the experimental input.
2. Results and Discussion
2.1. Synthesis and characterization of fluoro- and perfluoro-
alkylsulfonyl pentafluoroanilides
2.1.1. Synthesis
Herein, we provide a full account of an optimized procedure
to synthesize HN(Pf)(Tf), which, in preliminary communications,
has been used as an anion-generating acid for various highly
lipophilic ionic liquids[21] and thermally stable, as well as elec-
trochemically suitable, lithium electrolyte systems.[22] The reac-
tion of C6F5NHNa generated in situ from pentafluoroaniline
and NaN(SiMe3)2 with (CF3SO2)2O gives the sodium salt
NaN(Pf)(Tf) and one equivalent of sodium triflate. If only one
equivalent of the base NaN(SiMe3)2, instead of two, is applied
in this synthesis, the acidic product HN(Pf)(Tf) quenches the
nucleophile C6F5NHNa and, as a result, the yield does not
exceed the theoretical maximum of 50%. After protonation of
this salt mixture with sulfuric acid followed by sublimation, the
pure nitrogen acid is obtained as colorless solid. It is slightly
Scheme 1. Syntheses of pentafluorophenyl-substituted NH acids.
For the chemical shifts of the nitrogen acids’ acidic protons,
a strong solvent dependency is observed in the proton NMR
spectra. In the 13C NMR spectra carbon–fluorine coupling is
1
2
found with typical J and J coupling constants. The 19F NMR
spectra show characteristic chemical shifts for the C6F5 moiet-
ies and the aliphatic fluorine atoms. The sulfur-bound fluorine
atom in HN(Pf)(Sf) exhibits a considerable low-field shift of d=
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54.2 ppm (471 MHz, C6D6, 258C).
Furthermore, it is worth men-
tioning that the CF3 group in
Table 1. Selected bond lengths [pm] and angles [8] found for the molecular structures discussed herein.
CÀN
141.8(2)
NÀH
OÀH
NÀS
159.5(2)
SÀO
SÀO
C-N-S
HN(Pf)(Sf)
83(2)[a]
227(3)[b]
241(2)[b]
210(4)
217(2)
206(2)
205(5)
108(4)[d]
109(4)[d]
–
140.8(1)[c]
141.3(1)[c]
120.5(1)
HN(Pf)(Tf) shows
a
coupling
through space with the ortho-
HN(Pf)(Tf)
HN(Pf)(Nf)
142.1(4)
142.0(4)
142.2(4)
143.3(5)
142.0(4)
143.0(4)
139.5(4)
82(4)
82(2)[a]
84(2)[a]
86(5)
185(4)
195(4)
–
159.6(3)
160.5(3)
160.3(3)
161.5(6)
153.0(3)
154.8(3)
152.2(3)
141.6(3)
141.3(2)
140.9(2)
141.6(4)
142.7(3)[c]
144.6(3)[c]
143.7(3)
142.6(2)[c]
142.6(2)[c]
142.3(2)[c]
143.6(4)[c]
143.5(3)[c]
143.4(2)[c]
145.0(3)
123.2(2)
125.5(2)
123.3(2)
120.9(3)
120.3(2)
119.0(2)
121.0(2)
fluorine atoms of the C6F5 group
with
a coupling constant of
5.8 Hz.
HN(Pf)(Hf)·C7H8
HN(Pf)(Tf)·H2O
Huber et al. reported the
preparation of LiNPf2, LiN(Pf)(Tf)
and LiN(Pf)(Nf), which show in-
teresting electrochemical proper-
ties and are discussed as compo-
nents in lithium batteries.[24]
LiN(Pf)(Hf) was prepared in an
analogous procedure by the de-
LiN(Pf)(Nf)·2THF
[a] NÀH bond lengths set during refinement by DFIX command.; [b] the nitrogen atom acts as hydrogen bond
donor to the oxygen atoms of two neighboring molecules; [c] oxygen atom acting as hydrogen bond accept-
or; [d] O5ÀH1 and O6ÀH4.
protonation of the corresponding NH acid with LiN(SiMe3)2. In
the case of HN(Pf)(Sf), deprotonation led to a nucleophilic
attack of the nitrogen atom at the SO2F moiety of another
molecule under formation of lithium fluoride, which can be at-
tributed to the lability of the SÀF bond (Scheme 2). The result-
ing product was characterized via (À)-ESI HRMS and XRD anal-
ysis (Figure 2).
tances range from 141.8(2) pm (HN(Pf)(Sf)) to 143.3(5) pm
(HN(Pf)(Hf)) and are slightly longer than those in the symmetri-
cal HNPf2 (139.7(3) pm).[25] The NÀS distances lie between
159.5(2) pm (HN(Pf)(Sf)) and 161.5(6) pm (HN(Pf)(Hf)) and are
shorter than observed for the classical acid HNTf2 (NÀS
164.4(1) pm).[26] The SÀC bond lengths (184.0(3) pm) in HNTf2
are similar to those for HN(Pf)(Tf) (183.4(4) pm), HN(Pf)(Nf)
(184.8(3) pm), and HN(Pf)(Hf) (183.4(7) pm). The same is true
for the SÀO distances (140.1(2) pm and 141.7(2) pm in HNTf2).
As expected, oxygen atoms acting as hydrogen-bond accept-
ors show slightly longer SÀO bond lengths (e.g. 142.6(2) pm in
HN(Pf)(Tf)) than oxygen atoms that are not involved in hydro-
gen bonds (e.g. 141.6(3) pm in HN(Pf)(Tf)).
Scheme 2. Reaction of HN(Pf)(Sf) with LiN(SiMe3)2.
HN(Pf)(Sf): Single crystals of HN(Pf)(Sf) were obtained by
sublimation at 658C and 1.8·10À2 mbar. The low steric demand
of the SO2F moiety allows the nitrogen atom to act as a hydro-
gen bond donor for two oxygen atoms of two vicinal mole-
cules (Figures 3 and 4). Thus, double strands held together by
hydrogen bonds and p-interactions of the C6F5 moieties are
formed. The NÀS and the SÀF bonds (N1ÀS1 159.5(2) pm, S1À
F6 156.1(1) pm) exhibit similar lengths to those for
HN(C6H5)(Sf) (NÀS 158.7(3) pm, SÀF 156.14(19) pm).[27] The sym-
metrical HN(Sf)2 has a longer NÀS distance (162.7(3) pm) and
a shorter SÀF bond (153.3(2) pm).[16b]
Figure 2. Molecular structure of the dimeric product obtained from the reac-
tion between HN(Pf)(Sf) and LiN(SiMe3)2 (thermal ellipsoids set at 30% prob-
ability).
2.1.2. Structural characterization
In the following section, the molecular structures of the four
novel NH acids are described and light is shed on their inter-
molecular interactions, which are dominated by NÀHÀO hydro-
gen-bonds, as well as p-contacts and T-shaped interactions of
aromatic groups. Furthermore, molecular structures of the hy-
drate of HN(Pf)(Tf) and the lithium salt LiN(Pf)(Nf) are present-
ed. Selected bond lengths and angles are summarized in
Table 1.
Figure 3. Molecular structure of HN(Pf)(Sf) (thermal ellipsoids set at 30%
probability). Selected bond lengths [pm] and angles [8]: C6ÀN1 141.8(2), N1À
S1 159.5(2), S1ÀO1 140.8(1), S1ÀO2 141.3(1), S1ÀF6 156.1(1); C6-N1-S1
120.5(1), O1-S1-O2 123.4(7).
The nitrogen acids reported herein exhibit similar values
concerning corresponding bond lengths and angles. CÀN dis-
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Figure 6. Interactions of one-dimensional HN(Pf)(Tf) chains via T-shaped in-
teractions of C6F5 moieties.
Figure 4. Double strands observed in the solid state structure of HN(Pf)(Sf).
HN(Pf)(Tf): Single crystals of HN(Pf)(Tf) were obtained by
slow sublimation. In contrast to HN(Pf)(Sf) no double strands,
but only one-dimensional chains connected via NÀHÀO hydro-
gen bonds are observed (Figures 5 and 6). Within a chain, the
aromatic rings of two neighboring molecules always lie on op-
posite sides of the chain so that the C6F5 moieties are each vi-
cinal to two CF3 groups. Vicinal chains are linked via T-shaped
interactions of pentafluorophenyl groups in a staircase-like
manner.
HN(Pf)(Nf): Single crystals of HN(Pf)(Nf) were obtained by
sublimation at 508C and 5”10À2 mbar. The hydrogen-bonding
network is similar to that observed for HN(Pf)(Tf) (Figures 7
and 8). T-shaped interactions between the aromatic rings are
evident within the one-dimensional chains.
HN(Pf)(Hf)·C7H8: HN(Pf)(Hf)·C7H8 was crystallized as colorless
needles by cooling a saturated solution in toluene slowly to
À308C. Hydrogen bonds are formed between the nitrogen
atom and an oxygen atom of a neighboring HN(Pf)(Hf) mole-
cule (Figures 9 and 10). On all sides of the chain aromatic rings
and alkyl groups alternate, which allows for van der Waals in-
teractions between the C8F17 tails of every second molecule.
The asymmetric unit contains one disordered toluene molecule
Figure 7. Molecular structure of HN(Pf)(Nf) (thermal ellipsoids set at 30%
probability). Selected bond lengths [pm] and angles [8]: C1ÀN1 142.0(4), N1À
S1 160.5(3), S1ÀO1 141.3(2), S1ÀO2 142.6(2), S1ÀC7 184.8(3), C11ÀN2
142.2(4), N2ÀS2 160.3(3), S2ÀO3 140.9(2), S2ÀO4 142.3(2), S2ÀC17 184.8(3);
C6-N1-S1 125.5(2), O1-S1-O2 122.8(1), C11-N2-S2 123.3(2), O3-S2-O4 123.2(1).
Figure 5. Molecular structure of HN(Pf)(Tf) (thermal ellipsoids set at 30%
probability). Selected bond lengths [pm] and angles [8]: C6ÀN1 142.1(4), N1À
S1 159.6(3), S1ÀO1 142.6(2), S1ÀO2 141.6(3), S1ÀC7 183.4(4); C6-N1-S1
123.2(2), O1-S1-O2 122.9(1).
Figure 8. T-shaped interactions between vicinal C6F5 moieties within one-di-
mensional HN(Pf)(Nf) chains.
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Figure 9. Molecular structure of HN(Pf)(Hf)·C7H8 (thermal ellipsoids set at
30% probability; toluene omitted for clarity). Selected bond lengths [pm]
and angles [8]: C1-N1 143.3(5), N1-S1 161.5(6), S1-O1 141.6(4), S1-O2 143.6(4),
S1-C7 183.4(7), C6-N1-S1 120.9(3), O1-S1-O2 122.9(2).
Figure 11. Molecular structure of HN(Pf)(Tf)·H2O (thermal ellipsoids set at
30% probability). Selected bond lengths [pm] and angles [8]: C1ÀN1
142.0(4), N1ÀS1 153.0(3), S1ÀO1 142.7(3), S1ÀO2 144.6(3), S1ÀC7 183.7(4),
C8ÀN2 143.0(4), N2ÀS2 154.8(3), S2ÀO3 143.5(3), S2ÀO4 143.4(2), S2ÀC14
183.2(3); C6-N1-S1 120.3(2), O1-S1-O2 117.3(2), C8-N2-S2 119.0(2), O3-S2-O4
117.8(1).
Figure 10. One-dimensional chains in the solid-state structure of
HN(Pf)(Hf)·C7H8. p-stacking between the toluene molecule and a C6F5 ring is
observed.
forming p-interactions with a C6F5 moiety (angle between C6F5
plane and toluene plane: 6.1(2)8; distances between centroids:
368.54(4) pm and 373.46(4) pm).
Figure 12. Molecular structure of LiN(Pf)(Nf)·2THF (thermal ellipsoids set at
30% probability; hydrogen atoms omitted for clarity). Selected bond lengths
[pm] and angles [8]: C1ÀN1 139.5(4), N1ÀS1 152.2(3), S1ÀO1 143.7(3), S1ÀO2
145.0(3), S1ÀC7 185.9(4), Li1ÀO1 191.6(7), Li1ÀO2 195.5(7), Li1ÀO3 192.3(5),
Li1ÀO4 192.7(8), C6-N1-S1 121.0(2), O1-S1-O2 116.0(1), O1-Li1-O2 126.0(3),
O1-Li1-O3 105.6(3), O1-Li1-O4 105.8(3), O2-Li1-O3 107.4(3), O2-Li1-O4
106.8(3), O3-Li1-O4 103.1(3).
HN(Pf)(Tf)·H2O: The crystal structure suggests that the acidi-
ty of the reported compounds is sufficient to protonate water
(Figure 11). As observed for the lithium salt of HN(Pf)(Nf) (see
below), the NÀS distance in HN(Pf)(Tf)·H2O (153.0(3) pm and
154.8(3) pm) is significantly shortened in comparison to the
free acid (159.6(3) pm). This emphasizes the high negative par-
tial charge on the oxygen atoms. The water molecules are in-
volved in a complex network of hydrogen bonds. Furthermore,
p-contacts between C6F5 rings play a role in intermolecular in-
teraction (distance between centroids: 402.8(8) pm, angle be-
tween planes of the phenyl rings: 23.4(1)8).
LiN(Pf)(Nf)·2THF: Single crystals of LiN(Pf)(Nf)·2THF were
obtained from a solution of LiN(Pf)(Nf) in THF, revealing a lithi-
um atom coordinated in a distorted tetrahedral fashion by two
THF molecules and the oxygen atoms of two neighboring
N(Pf)(Nf) moieties (Figures 12 and 13). One-dimensional chains
with the sequence Li-O-S-O with bond angles around the lithi-
Figure 13. One-dimensional chains formed by LiN(Pf)(Nf)·2THF (including
the disorder of one THF molecule).
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um atom ranging from 103.1(3)8 to 126.0(3)8 and LiÀO distan-
ces between 191.6(7) pm and 195.5(7) pm are formed. Neigh-
boring molecules are oriented with C4F9 groups standing at
the same side of the chain, allowing for van der Waals interac-
tions. Coordination of the lithium atom by nitrogen atoms or
short contacts between the lithium atom and aromatic fluorine
atoms are not observed. The NÀS distance in the lithium salt
(152.2(3) pm) is significantly shorter than those in the free acid
(160.5(3) pm and 160.3(3) pm), emphasizing a stronger double-
bond character. In contrast to the linear chains observed for
LiN(Pf)(Nf)·2THF, the related lithium salt LiNPf2·THF forms
dimers exhibiting short contacts between the lithium atom
and two ortho-fluorine atoms of vicinal C6F5 groups.[20a]
2.2.2. Experimental pKa values in MeCN
The UV/Vis spectrophotometric titration method used herein
was described in detail in ref. [36]. For each investigated com-
pound, the relative acidity and basicity were measured against
at least two (mostly three) different reference compounds with
known pKa values in MeCN.[2c,36] To determine the relative acidi-
ty of two compounds, a mixture of two different acids was ti-
trated with UV/Vis radiation non-absorbing acidic (solution of
triflic acid in MeCN) and basic (solution of phosphazene tBuN=
P(pyrr)3 in MeCN) titrants to obtain several spectra of solutions
containing both neutral and anionic forms of the two acids in
different proportions. Both of the acids were also titrated sepa-
rately to obtain the spectra of the neutral and anionic forms of
the pure compounds. From the titration data, the relative acid-
ity of the acids—the difference between their pKa values
(DpKa)—was calculated.[36] All of the compounds investigated
herein have different spectra of protonated and deprotonated
forms in the UV region and calculation methods using only
spectral data obtained from the titration of pure compounds
and mixture were used to obtain DpKa values. From each titra-
tion experiment of the mixture of two compounds, the DpKa
value was determined as the mean of 8–20 values. Concentra-
tions of the measured compounds were mostly in the order of
n”10À5 m to n”10À4 m, whereas concentrations of acidic and
basic titrants were in the n”10À3 m range.
2.2. Investigation of the acidity of the NH acids
In the following, we analyze the acidity of the new acids in
comparison to the known symmetric HNTf2 and HNPf2 acids on
experimental and computational grounds.
2.2.1. Background to the acidity measurements
Gas-phase acidities: The gas-phase acidity (GA) of an acid HÀ
A is given by its Gibbs energy of dissociation (DrG8) or the gas-
phase basicity (GB) of its conjugated base [Eq. (1)]. Strong
acids have low GA values. A large collection of GA/GB values
that have been obtained by experimental techniques such as
FT-ICR-MS and/or quantum chemical procedures is available in
the literature.[2a,28]
From the DpKa values measured in this work and from the
absolute pKa values assigned to the reference compounds in
previous works,[2c,36] the absolute pKa values of the investigated
compounds were obtained and are presented in Table 2. Also
included in Table 2 is the pKa value of the recently reported
hybrid acid HN(Pf)C(CF3)3.[37]
DrGꢀ
H À A ¼ GAðHÀAÞ Hþ þ AÀ
ƒƒ !
ð1Þ
¼GAðAÀÞ
2.2.3. Computational Analysis:
Calculation of pKa values in MeCN and DMSO: Similar to the
scheme introduced to determine anchor points[29] for the
Gibbs solvation energies of the proton in DMSO and MeCN
(among others), the pKa values of an acid HÀA in DMSO or
MeCN solution may be calculated by using the following
Born–Fajans–Haber cycle (BFHC 1, Scheme 3; for energy and
pKa values, see Table 4 below):
With the known[29] anchor points DsolvG8(H+, g!S)
(À1119.6 kJmolÀ1 for DMSO and À1056.3 kJmolÀ1 for MeCN),
we only had to evaluate the GA values, as well as the Gibbs
We calculated the GA values of the acids HNPf2, HN(Pf)(Tf),
HN(Pf)(Nf), and HN(Pf)(Sf) using isodesmic proton-exchange re-
actions based on an initially at the high-level G3mod-calculat-
ed[29] GA value of HNTf2.
Calculated pKa values: In solution, the pKa value serves as
a measure for the acidity of an acid in a given medium. As
specified in some recent reviews,[30] tremendous effort has
been made to predict pKa values computationally, mainly in
aqueous solution. Some exemplarily selected examples are
given in ref. [31]. In our work, we have focused on calculating
pKa values in DMSO and MeCN solutions.[28s,32] As a standard
for the ab initio pKa calculations, combinations of high quality
GA calculations or experimental values with continuum solvent
models[33] were applied. To improve the obtained results, sol-
vent molecules have to be taken into account explicitly for the
dissociation of the acid, for example, in the cluster-continuum
model[31b,34] or the implicit–explicit solvent approach.[31c] In this
study, we employed the recently introduced rCCC model,[29]
coupled with the anchor points[29] of the newly established
unified pH scale,[35] to predict the pKa values of the five investi-
gated acids in DMSO and MeCN solutions.
Scheme 3. Born–Fajans–Haber cycle (BFHC) 1 for the calculation of the pKa
value of an acid HÀA in a solvent S.
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of the here-investigated acids:
We barely managed to calculate
the GA of HNTf2 with this
method. Therefore, after an ex-
tensive method validation with
Table 2. Results of pKa measurements in MeCN.
Acid (A)
Reference acid (Ra)[a]
Identity
DpKa
pKa
[A]
Assigned pKa
[A]
pKa (Ra)
(4-Me-C6F4)2CHCN
9-COOMe-fluorene
fluoradene
22.80
23.53
23.90
À1.17
À0.44
À0.06
23.97
23.97
23.96
HNPf2
23.97
11.43
11.22
11.22
24.01
experimental
literature
GA
for
values
(1198.7 kJmolÀ1
[28l]
HNTf2
and 1324.2 kJmolÀ1 for
picric acid
4-H-C6F4CH(CN)2
(4NC5F4)2CHCN
11.00
12.98
13.46
À0.41
1.52
1.89
11.41
11.46
11.57
HNPf2)[2a] as a benchmark (chap-
ter 1 of the theoretical section of
the Supporting Information), we
applied isodesmic reactions as
a cheaper alternative to obtain
reliable GAs for the acids
HN(Pf)(Tf), HN(Pf)(Sf), and HNPf2
HN(Pf)(Tf)
4-H-C6F4CH(CN)2
picric acid
C6(CF3)4CH(CN)2
12.98
11.00
10.45
1.81
À0.21
À0.78
11.17
11.21
11.23
HN(Pf)(Nf)
HN(Pf)(Hf)
HN(Pf)C(CF3)3
picric acid
C6(CF3)4CH(CN)2
4-H-C6F5CH(CN)2
11.00
10.45
12.98
À0.21
À0.79
1.65
11.21
11.24
11.33
(G3MP2mod!G3mod
,
see S-Fig-
ure 1 in the Supporting Informa-
tion). For the largest system
HN(Pf)(Nf) we had to use the
isodesmic approximation twice
9-COOMe-fluorene
fluoradene
C6H5CH(CN)C6F5
23.53
23.90
26.14
À0.48
À0.10
2.11
24.01
24.00
24.03
[a] pKa values of the reference acids are from refs. [35] and [38].
(M1!G3MP2mod!G3mod
with
M1=MP2(FC)/aug-cc-pVTZ//
B3LYP/aug-cc-pVTZ, see the Sup-
solvation energies of the anions AÀ and the corresponding
acids HÀA (Table 4; BFHCs in chapter 4 of the theoretical sec-
tion of the Supporting Information). We also had to take into
account a possible further coordination of solvent molecules
to the acids HÀA, as well as the corresponding anions AÀ. For
HÀA or for AÀ this can be done according to BFHC 2
(Scheme 4) and a subsequent evaluation of their Gibbs solva-
tion energies with the Gibbs solvation energies calculated for
the isolated HÀA or AÀ monomers.
porting Information, S-Figure 1), an approach we call isodesmic
squared (isodesmic2).
From the results collected in S-Table 1 (see the Supporting
Information), we estimate an error bar of around 5–8 kJmolÀ1
for the GA values obtained through the isodesmic and isodes-
mic2 approaches and calculated best estimates of the GA from
the most acidic HNTf2 (1197.7 kJmolÀ1
) over HN(Pf)(Nf)
(1251.3 kJmolÀ1), HN(Pf)(Tf) (1260.3 kJmolÀ1), and HN(Pf)(Sf)
(1273.5 kJmolÀ1) to the least acidic HNPf2 (1328.7 kJmolÀ1
with our selected reference methods (Table 4).
)
Gibbs energies of the gas-phase reaction of HÀA/AÀ with
MeCN or DMSO: For the determination of the Gibbs gas-
phase reaction energies of the coordination of the solvent S to
HÀA/AÀ, we employed the isodesmic2 approach (for HÀA=
HNPf2, HN(Pf)(Tf), HN(Pf)(Nf), HN(Pf)(Sf) and AÀ =N(Pf)(Tf)À,
N(Pf)(Nf)À) or the isodesmic method (for HÀA=HNTf2 and AÀ =
NTf2À,NPf2À). In the special case of AÀ =N(Pf)(Sf)À we used a di-
rectly calculated G3MP2mod value for the coordination of MeCN
and an isodesmic2 value for the coordination of DMSO (see the
Supporting Information; S-Tables 2 and 3, explanations and
BFHCs in chapters 2–4 of the theoretical section).
Scheme 4. Born–Fajans–Haber cycle (BFHC) 2 for the calculation of Gibbs
solvation energy of an anion AÀ or an acid HÀA, each coordinated to a sol-
vent molecule S.
Gibbs solvation energies of acid–solvent (HÀA·S) and
anion–solvent (AÀ·S) aggregates: We calculated Gibbs solva-
tion energies with COSMO@BP86/def-TZVP for the solvents[41]
MeCN (er =36.64) and DMSO (er =46.7) and compared the
values of the recently established vertical or relaxed COSMO
cluster-continuum models (vCCC/rCCC).[29] Vertical and relaxed
CCC differ in the inclusion of solvation; with the vCCC model
only a single point is done on the gas phase structure, whereas
in the rCCC, the gas-phase structure is optimized with COS-
MO@BP86/def-TZVP in C1 symmetry.
For BFHC 2 (Scheme 4), we extracted the Gibbs vaporization
energies DvapG8(S) (17.7 kJmolÀ1 for DMSO and 5.3 kJmolÀ1 for
MeCN) from experimental vapor pressures (8”10À4 bar for
DMSO[39] and 0.118 bar for MeCN).[40] The Gibbs gas-phase reac-
tion energies for the coordination of the solvent S to HÀA/AÀ
and the Gibbs solvation energies of the aggregates HÀA·S/AÀ·S
were calculated (see below and the Supporting Information, S-
Tables 2 and 3 and BFHCs in chapter 4 of the theoretical sec-
tion).
Assessment of the GA values: To calculate GAs with good
accuracy, we recently established the modified G3 method
(G3mod),[29] which unfortunately is too expensive for the larger
Ionization of HNTf2·DMSO: We discovered an amazingly
large decrease of the Gibbs solvation energy of HNTf2·DMSO in
DMSO when changing from the vCCC to the rCCC model
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(DsolvG8(rCCC)ÀDsolvG8(vCCC)=
À54.5 kJmolÀ1). This would be
commensurate to an increase of
Table 3. Bond lengths of the hydrogen bond of the acid-solvent adducts optimized in the gas phase (vCCC
model) and in solution (rCCC model). Changes of the Gibbs solvation energies from vCCC to rCCC.
Investigated species
MeCN adducts
HNTf2·MeCN
(Acid)NÀH bond
HÀNCMe bond
vCCC!rCCC
À54.5/À5.71=9.5 pKa
units
vCCC [pm] rCCC [pm]
vCCC [pm] rCCC [pm]
D(DsolvG8) [kJmolÀ1
]
upon changing from vertical to
105.2
103.1
104.0
103.8
104.2
107.2
103.5
105.0
105.0
105.5
181.9
198.8
190.3
191.2
188.6
169.8
194.2
181.3
181.7
178.8
À4.6
À0.6
À4.6
À4.5
À5.6
relaxed structures (DpKa =1 cor-
HNPf2·MeCN
responds
to
RT ln0.1=
HN(Pf)(Tf)·MeCN
HN(Pf)(Nf)·MeCN
HN(Pf)(Sf)·MeCN
À5.71 kJmolÀ1 at 298.15 K),[29,35]
neglecting the small energy dif-
ferences between the vCCC and
the rCCC model at other parts of
the BFHCs. Looking at the re-
spective structures reveals what
to our knowledge is a hitherto-
(Acid)NÀH bond
HÀO(DMSO) bond
vCCC!rCCC
D(DsolvG8) [kJmolÀ1
À54.5
DMSO adducts
HNTf2·DMSO
HNPf2·DMSO
HN(Pf)(Tf)·DMSO
HN(Pf)(Nf)·DMSO
HN(Pf)(Sf)·DMSO
vCCC [pm] rCCC [pm]
vCCC [pm] rCCC [pm]
]
110.0
104.4
106.7
106.9
106.9
157.9
105.4
109.9
110.5
111.3
151.7
176.3
163.1
162.8
162.8
104.9
169.9
151.3
149.3
147.3
À4.2
À12.9
À15.0
unencountered
The DMSO adduct of HNTf2
ionizes during optimization
with COSMO@BP86/def-TZVP by
phenomenon:
À20.0
transfer of the proton from N to O, yielding a contact ion pair.
Thus, d(NÀH) is elongated from 110.0 pm (vCCC) to 157.9 pm
(rCCC) and d(HÀO) is shortened from 151.7 pm (vCCC) to
104.9 pm (rCCC) (Figure 14).
Investigation of other HÀA·S solvates: We then decided to
systematically examine the behavior of the MeCN and DMSO
adducts of all the investigated acids upon optimization with
COSMO (Table 3). For the adducts of the more basic DMSO sol-
vent we found a more pronounced trend towards “ionization”
during optimization that led to a larger decrease of the Gibbs
solvation energies from the vCCC to the rCCC model, and from
HNPf2 over HN(Pf)(Tf), HN(Pf)(Nf) and HN(Pf)(Sf) to HNTf2. This
order also corresponds to increased acidity according to the
GA values of the respective acids with HN(Pf)(Sf) as an anom-
aly.
Some words of caution have to be mentioned. Apart from
HNTf2·DMSO the developing ionization does not lead to real
contact ion pairs. However, there are significant energetic and
structural changes. For HN(Pf)(Sf)·DMSO the change of the NÀ
H/HÀO bonds from vCCC to rCCC amounts to +4.4/À15.5 pm
and the change of the Gibbs solvation energy value to
Figure 14. Calculated structures of the DMSO solvates of the HNTf2 acid op-
timized in the gas phase (a) and in DMSO solution (b). Distances in pm.
Note the transfer of the proton from the N to the O (DMSO) atom upon in-
clusion of solvation.
À20.0 kJmolÀ1.
Similar
values
were
calculated
for
HN(Pf)(Nf)·DMSO (NÀH/HÀO change: +3.6/À13.5 pm; energy
change: À15.0 kJmolÀ1) and HN(Pf)(Tf)·DMSO (+3.2/À11.8 pm
and À12.9 kJmolÀ1). The smallest alterations were for the
DMSO adduct of the weakest acid HNPf2 (+1.0/À6.4 pm and
À4.2 kJmolÀ1). For the adducts of the acids and the much less
basic solvent MeCN, the “ionization” effect was only marginal,
with overall small elongations of the (acid) NÀH bond and
a small decrease of the Gibbs solvation energy values, but a rel-
atively large shortening of the H–NCMe bond (from À4.6 pm
at HNPf2·MeCN to À12.1 pm at HNTf2·MeCN). The structural
and energetic changes are collected in Table 3.
Investigation of the AÀ·S solvates: When assessing the
Gibbs solvation energies of the anion–solvent aggregates AÀ·S,
switching from the vCCC to the rCCC model, a slight structural
relaxation in the rCCC model is accompanied by changes of
the Gibbs solvation energies that were in this case larger for
the MeCN adducts than for the DMSO adducts (up to
À17.2 kJmolÀ1 for N(Pf)(Tf)À·MeCN; see BFHCs in chapter 4 of
the theoretical section of the Supporting Information).
To rule out attribution of the phenomenon solely to an influ-
ence of the functional or the basis set, we also employed COS-
MO@B3LYP/aug-cc-pVTZ. The observed effect is even larger
here with an energy decrease of À68.1 kJmolÀ1 from vCCC to
rCCC, a corresponding elongation of d(NÀH) from 106.9 pm
(vCCC) to 170.0 pm (rCCC) and a shortening of d(HÀO) from
158.5 pm (vCCC) to 100.9 pm (rCCC). To eliminate also the pos-
sibility of a COSMO artifact, we transferred the vCCC and rCCC
methodology to the polarizable continuum model (PCM) by
performing a PCM@BP86/TZVP single point in DMSO based on
the BP86/def-TZVP-optimized gas-phase structure of
HNTf2·DMSO and
a PCM@BP86/TZVP full optimization in
DMSO. The obtained results confirm our previous studies ac-
companied by an energy decrease during optimization of
À41.9 kJmolÀ1 and an optimized structure with 154.7 pm for
d(NÀH) and 106.1 pm for d(HÀO). Thus, the magnitude of the
effect is similar to that for COSMO.
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2.3. Discussion of the gas-phase and solution acidity of the
investigated acids
acidity (overestimated pKa value), because our initial state is
not the neutral HNTf2 molecule any more but the ion pair, in
which HNTf2 is in fact already deprotonated. In other words,
the rCCC pKa value in DMSO is rather the dissociation constant
of the contact ion pair than the acidity constant of HNTf2.
This is also supported by observing the rCCC acidity change
in the hypothetical row of transformations HNPf2!
HN(Pf)(Tf)!HNTf2. In MeCN the pKa changes corresponding to
the two steps are around À14
Our measured and calculated (rCCC) pKa values are collected in
Table 4. The calculated values differ by at most 3.1 pKa units
from the experimental values. This is within our estimated
error bar of 20 kJmolÀ1 (20/5.71=3.5 pKa units), and mainly
due to the residual error of the rCCC Gibbs solvation energies.
and À12 pKa units, respectively;
Table 4. pKa values based on the rCCC model (bold type, vCCC values in brackets for comparison) of the five
investigated acids in MeCN and DMSO as well as GA values [in kJmolÀ1] and all rCCC Gibbs solvation energies
[kJmolÀ1] (known anchor points for H+, taken from Scheme 4 for HÀA, AÀ) needed to calculate these pKa
values using Scheme 3. Experimental pKa and GA values, if available.
in DMSO, however, the changes
are À13 and À2.4. Given the
generally very good correlation
between pKa values in MeCN
and DMSO, it is difficult to see
why the trends in this row of
changes should be so different
between the two solvents.
Acidity
HNTf2
HNPf2
HN(Pf)(Tf)
1260.3
HN(Pf)(Nf)
1251.3
HN(Pf)(Sf)
1273.5
GA (exp.)
GA (best estimate)
1198.7[28l]
1197.7
1324.2[2a]
1328.7
Solvation with MeCN
DsolvG8(H+, g) (rCCC)
DsolvG8(HÀA, g) (rCCC)
DsolvG8(AÀ, g) (rCCC)
pKaMeCN (rCCC)
Interestingly, the rCCC pKa of
HNTf2 in DMSO is in agreement
with the experimentally deter-
mined acidity. There is, however,
strong reason to believe that the
experimental pKa value of HNTf2
in DMSO of 2.0 is several orders
of magnitude too high for the
pKa of “molecular” HNTf2. This
value has been obtained using
potentiometric titration and the
actual presence of the molecule
HNTf2 in solution was not moni-
tored in any way. The dangers of
À1056.3
À60.2
À216.9
À2.7
À1056.3
À35.6
À174.0
23.5
23.97
[26.1]
À1056.3
À63.3
À211.8
9.7
11.43
[13.2]
À1056.3
À63.6
À202.7
9.8
11.22
[13.1]
À1056.3
À56.6
À223.9
8.7
pKaMeCN (exp.)
[pKaMeCN (vCCC)]
0.3[2c]
–
[0.5]
[11.9]
Solvation with DMSO
DsolvG8(H+, g) (rCCC)
DsolvG8(HÀA, g) (rCCC)
DsolvG8(AÀ, g) (rCCC)
pKa DMSO (rCCC)
À1119.6
À130.0
À214.3
À1.1
À1119.6
À54.2
À180.6
14.5
12.6[43]
[15.7]
À1119.6
À76.8
À209.8
1.3
–
[1.0]
À1119.6
À78.6
À201.1
1.6
–
[0.9]
À1119.6
À80.1
À224.9
1.6
–
[0.3]
pKa DMSO (exp.)
[pKa DMSO (vCCC)]
2.0[42]
[À8.8]
vCCC values lack structural relaxation in their solvation Gibbs
energy and are only given for comparison in Table 4, but
should not be used. The trends of the pKa values in MeCN and
DMSO show a similar sequence to the GA values, but for
HN(Pf)(Tf), HN(Pf)(Nf), and HN(Pf)(Sf)—although their GA values
pKa determination of strong acids in DMSO, whereby the pro-
tonated form of the acid is actually not monitored in the solu-
tion, were discussed in a recent paper.[45] Thus, we conclude it
À
was actually the Me2SO-H+···NTf2 contact ion pair that was ti-
trated.
scatter over
a
range of 22.2 kJmolÀ1 (22.2/5.71=3.9 pKa
Such contact ion pair formation in DMSO solution may also
explain observations reported by Kütt et al.[36] When they plot-
ted pKa values in MeCN against pKa values in DMSO, there
were two significant outliers, namely the strong acids 4-Me-
C6H4SO3H and C6H5CH(SO2CF3)2, with deviations of À4.6 and
À6.4 pKa units in MeCN, respectively.
units)—nearly the same rCCC pKa values in DMSO occur due to
variations in Gibbs solvation energies of HN(Pf)(Tf)/(Pf)(Tf)À and
of the corresponding solvent aggregates (Scheme 4), com-
pared to HN(Pf)(Nf)/(Pf)(Nf)À and/or HN(Pf)(Sf)/(Pf)(Sf)À. Thus,
the weakest asymmetric acid in the gas phase—HN(Pf)(Sf)—is
in MeCN the most acidic of the three mixed acids HN(Pf)(Tf),
HN(Pf)(Nf), and HN(Pf)(Sf) according to the calculations. HNPf2
is expectedly the weakest acid in the series and is almost ex-
actly as acidic as HN(Pf)C(CF3)3. Thus, in the context of acidify-
ing effect, C6F5 and C(CF3)3 behave in a very similar way, al-
though they have quite different steric demands.
The vCCC pKa value of HNTf2, however, fits very well with
the above-discussed trend of acidity change from HNPf2 to
HNTf2. Indeed this vCCC pKa can be interpreted as the pKa of
a “HNTf2 molecule in DMSO”.[46] However, as this pKa value
exists only in silico, and has not been experimentally observed
in real DMSO, this pKa value is a hypothetical one.
For HNTf2, a significant anomaly appeared, mainly due to
the effect of contact ion pair formation in DMSO: The rCCC pKa
value in MeCN is lower than in DMSO due to the increase of
pKa value by 7.7 pKa units[44] from the vCCC to the rCCC value
in DMSO. The reason is that in the course of rCCC geometry
optimization, proton transfer occurs within the species
Me2SO···HNTf2, to give Me2SO-H+···NTf2À. Using the latter spe-
cies as the neutral acidic form leads to grossly underestimated
Overall, we found a much lower range of acidities in DMSO
than in MeCN. Similar to the difference of the absolute acidi-
ties at equal pH in MeCN and DMSO of 11.1 pHabs units
(D(DsolvG8(H+ in S): À1056.3+1119.6=63.3 kJmolÀ1; in loga-
rithmic units: 63.3/5.71=11.1 pHabs units), the difference be-
tween the pKa values in MeCN and DMSO at the rCCC level is
9.0 for HNPf2 and decreases through HN(Pf)(Tf) (8.4), HN(Pf)(Nf)
(8.2), and HN(Pf)(Sf) (7.1), to HNTf2 (À1.6, with full formation of
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ether (100 mL) at À788C. The resulting solution was brought to
08C and stirred at this temperature for 2 h. The reaction mixture
was cooled again to À788C and a precooled solution of triflic acid
anhydride (8.14 mL, 48.34 mmol) in diethyl ether (50 mL) was
added slowly to the reaction mixture via a cannula. The reaction
mixture was slowly brought to room temperature and stirred for
12 h. All volatiles were removed in vacuo at 508C. The colorless
residue was suspended in CFCl3 (100 mL) and the suspension was
cooled to 08C. Concentrated sulfuric acid (15 mL) was then added
and the mixture was heated at reflux for two additional hours. The
phases were separated by decantation and the fluorous phase was
stored at À308C, which lead to the formation of colorless crystals.
The precipitate was separated from the solution, dried in vacuo,
and finally sublimed (908C, 1.0”10À2 mbar). This gave HN(Pf)(Tf)
(10.45 g, 69%) as a colorless solid. M.p. 568C; 1H NMR (300 MHz,
[D6]DMSO, 258C, TMS): d=12.33 ppm (s, 1H, NH); 13C NMR
a contact ion pair in DMSO). This is mainly attributed to the in-
crease of contact ion pair formation in DMSO. Our computa-
tions and the above reasoning provide strong evidence that
the anomalously high experimental DMSO pKa value of Tf2NH
(and possibly other strong acids, such as TfOH) is in fact the
dissociation constant of a contact ion pair.
3. Conclusion/Outlook
We described a family of perfluorinated sulfonylanilides with
a tunable acidity and lipophilicity depending on the length of
the perfluoroalkyl chain. Due to the asymmetric nature of their
anions, their anion polarizabilities and ligand properties differ
from those of the parent symmetric NH acids HNTf2 and HNPf2.
This opens new perspectives in the design of thermally stable
Lewis acids with weakly coordinating anions or thermally
stable lithium electrolytes and ionic liquids with tunable anion
properties.
1
(101 MHz, CDCl3, 258C, TMS): d=146.6–143.8 (m, J(C,F)=252.3 Hz,
CFAr), 144.1–141.1 (m, 1J(C,F)=265.4 Hz, CFAr), 139.3–136.5 (m,
1
1J(C,F)=254.4 Hz, CFAr), 119.3 (q, J(C,F)=321.3 Hz, CF3), 108.2 ppm
2
(t, J(C,F)=17.1 Hz, ipso-C); 19F NMR (346 MHz, CDCl3, 258C, CFCl3):
d=À77.0 (t, tsJ(F,F)=5.8 Hz, 3F, CF3), À142.9 (d, 3J(F,F)=22.6 Hz, 2F,
o-F), À149.7 (t, 3J(F,F)=21.4 Hz, 1F, p-F), À160.1 ppm (t, 3J(F,F)=
18.6 Hz 2F, m-F); IR (Nujol): n˜ =631 (m), 681 (w), 895 (s), 986 (s),
1047 (s), 1119 (s), 1140 (w), 1159 (w), 1180 (s), 1205 (s), 1287 (s),
1317 (m), 1502 (m), 1520 (m), 1649 (w), 3219 cmÀ1 (br, NH); MS
(70 eV): m/z (%): 315 (8) [M+], 182 (92) [HNC6F5+], 155 (51) [C5F5+],
69 (100) [CF3+]; elemental analysis calcd (%) for C7HF8NO2S: C
26.68, H 0.32, N 4.44; found: C 26.21, H 0.57, N 4.37.
The gas-phase and solution acidities of these NH acids were
determined experimentally in MeCN, and computationally in
the gas phase, as well as in MeCN and DMSO solution. Experi-
mental and calculated values generally agreed well, with the
exception of the interesting HNTf2/DMSO case: In this case we
encountered the transition from molecular to medium acidity
when changing from the vertical (vCCC) to the relaxed (rCCC)
model. That means with the vertical model, including the gas-
phase structure of the DMSO adduct of HNTf2, we still describe
the—attenuated—acidity of the molecular acid HNTf2. If we
allow structural relaxation during the optimization of the sol-
vated adduct, a proton transfer occurs and now, in the rCCC
model, we describe the molecular acidity of protonated DMSO
HN(Pf)(Nf):
A solution of 2,3,4,5,6-pentafluoroaniline (8.56 g,
46.77 mmol) in diethyl ether (20 mL) was added to a solution of
sodium bis(trimethylsilyl)amide (17.15 g, 93.53 mmol) in diethyl
ether (100 mL) at À788C. The resulting solution was brought to
08C and stirred at this temperature for 2 h. The reaction mixture
was cooled again to À788C and a precooled solution of nonafluor-
obutylsulfonylfluoride (8.40 mL, 46.77 mmol) in diethyl ether
(50 mL) was added slowly to the reaction mixture via a cannula.
The reaction mixture was slowly brought to room temperature and
stirred for 12 h. All volatiles were removed in vacuo at 508C. The
colorless residue was suspended in CFCl3 (100 mL) and the suspen-
sion was cooled to 08C. Then concentrated sulfuric acid (15 mL)
was added and the mixture was heated at reflux for two additional
hours. The phases were separated by decantation and the fluorous
phase was evaporated to dryness in vacuo. The residue was sub-
limed (1008C, 1.0”10À2 mbar) to yield HN(Pf)(Nf) (18.18 g, 84%) as
a colorless solid. M.p. 738C; 1H NMR (300 MHz, [D6]DMSO, 258C,
TMS): d=13.24 ppm (s, 1H, NH); 13C NMR (101 MHz, CDCl3, 258C,
TMS): d=146.5–143.7 (m, 1J(C,F)=255.2 Hz, CFAr), 144.1–141.3 (m,
1J(C,F)=261.1 Hz, CFAr), 139.4–136.5 (m, 1J(C,F)=254.3 Hz, CFAr),
À
solvated by the NTf2 anion. This proton transfer signals that
HNTf2 in DMSO medium is a strong acid that allows solvent
protonation. However, it implies that our rCCC-calculated pKa
value of HNTf2 in DMSO is not the pKa of HNTf2 but that of HÀ
DMSO+[NTf2]À in DMSO.
4. Experimental Section
All reactions were carried out under an inert atmosphere using
standard Schlenk techniques. Moisture and air sensitive substances
were stored in a conventional nitrogen-flushed glovebox. Solvents
were purified according to literature procedures and kept under an
inert atmosphere. Pentafluoroaniline (Chempur), triflic acid anhy-
dride (Chempur), nonafluorobutylsulfonylfluoride (Aldrich), and
heptadecaoctylsulfonylfluoride (Chempur) were purchased from
commercial sources. Sodium bis(trimethylsilyl)amide,[47] lithium bis-
(trimethylsilyl)amide[47] and HN(C6F5)(SO2Cl)[23] were prepared ac-
cording to literature procedures.
1
2
1
117.1 (tq, J(C,F)=288.4 Hz, J(C,F)=32.9 Hz, CF3), 114.9 (tt, J(C,F)=
300.4 Hz, 2J(C,F)=35.5 Hz, -CF2-), 110.2 (tt, 1J(C,F)=270.0 Hz,
2J(C,F)=31.6 Hz, -CF2-), 108.5 (t, J(C,F)=14.9 Hz, ipso-C), 108.3 ppm
2
(tt, 1J(C,F)=270.5 Hz, 2J(C,F)=33.1 Hz, -CF2-); 19F NMR (376 MHz,
CDCl3, 258C, CFCl3): d=À81.1 (t, 3J(F,F)=9.8 Hz, 3F, CF3), À111.6–
À111.4 (m, 2F, -CF2-CF3), À121.1 (br s, 2F, -CF2-), À126.2–À126.4
(m, 2F, -SO2-CF2), À142.7–À142.9 (m, 2F, o-F), À150.1 (t, 3J(F,F)=
21.4 Hz, 1F, p-F), À160.5–À160.7 ppm (m, 2F, m-F); IR (Nujol): n˜ =
625 (m), 650 (m), 681 (w), 700 (w), 735 (m), 833 (w), 893 (s), 991 (s),
1053 (m), 1136 (s), 1209 (s), 1518 (m), 1629 (w), 3270 cmÀ1 (br, NH);
MS (70 eV): m/z (%): 465 (8) [M+], 232 (8) [M2+], 182 (100)
[HNC6F5+], 155 (30) [C5F5+], 69 (52) [CF3+]; elemental analysis calcd
(%) for C11HF14NO2S: C 25.82, H 0.22, N 3.01; found: C 25.62, H 0.28,
N 3.17.
Spectra were recorded on the following spectrometers: NMR:
Bruker ARX300, Bruker DRX400, Bruker DRX500; IR: ATR-FT-IR; MS:
LTQ-FT or QStarPulsari (Finnigan); elemental analysis: CHN-Rapid
(Heraeus).
Synthesis and characterization
HN(Pf)(Tf):
A solution of 2,3,4,5,6-pentafluoroaniline (8.86 g,
48.34 mmol) in diethyl ether (20 mL) was added to a solution of
sodium bis(trimethylsilyl)amide (17.75 g, 96.80 mmol) in diethyl
Chem. Eur. J. 2015, 21, 5769 – 5782
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Full Paper
HN(Pf)(Hf):
A
solution of 2,3,4,5,6-pentafluoroaniline (4.47 g,
for five minutes resulting in a color change to red. Precipitated po-
tassium chloride was separated by filtration and diethyl ether was
removed from the filtrate in vacuo. The residue was sublimed
(658C, 1.8”10À2 mbar) to yield HN(Pf)(Sf) (0.901 g, 72%) as a color-
less crystalline solid. M.p. 358C; 1H NMR (300 MHz, C6D6, 258C,
TMS): d=4.92 ppm (s, 1H, NH); 13C NMR (126 MHz, C6D6, 258C,
TMS): d=144.7 (d, 1J(C,F)=254.2 Hz, CFAr), 141.8 (d, 1J(C,F)=
256.8 Hz, CFAr), 137.8 (d, 1J(C,F)=252.1 Hz, CFAr), 109.2 ppm (d,
2J(C,F)=14.9 Hz, ipso-C); 19F NMR (471 MHz, C6D6, 258C, CFCl3): d=
24.4 mmol) in diethyl ether (20 mL) was added to a solution of
sodium bis(trimethylsilyl)amide (4.47 g, 24.4 mmol) in diethyl ether
(50 mL) at À788C. The resulting solution was brought to 08C and
stirred at this temperature for 2 h. The reaction mixture was cooled
again to À788C and a precooled solution of heptadecaoctylsulfo-
nylfluoride (6.6 mL, 24.4 mmol) in diethyl ether (25 mL) was added
slowly. The reaction mixture was brought to room temperature
overnight and all volatiles were removed in vacuo. The brown resi-
due was treated with a mixture of metaphosphoric acid and phos-
phorous pentoxide (1:1 m/m; 6 mL) and sublimed from the viscous
mixture (1408C, 1.0”10À2 mbar) to yield HN(Pf)(Hf) (7.17 g, 43%) as
an off-white solid. M.p. 908C; 1H NMR(400 MHz, [D6]DMSO, 258C,
TMS): d=14.16 ppm (s, 1H, NH); 13C NMR (101 MHz, C6D6, 258C,
TMS): d=145.0 (pd, 1J(C,F)=254.9 Hz, CFAr), 142.2 (pd, 1J(C,F)=
251.5 Hz, p-CAr), 137.7 (pd, 1J(C,F)=251.2 Hz, CFAr), 117.5 (qt,
1J(C,F)=288.5 Hz, J(C,F)=32.9 Hz, CF3), 115.4 (tt, J(C,F)=300.4 Hz,
2J(C,F)=35.2 Hz, SO2CF2), 114.5–107.9 ppm (m, CF2 +ipso-CAr);
19F NMR (376 MHz, [D6]DMSO, 258C, CFCl3): d=À79.6–À79.4 (m,
3F, CF3), À113.2–À112.9 (m, 2F, CF2), À119.1 (br s, 2F, CF2), À121.1–
À120.6 (m, 6F, CF2), À121.7 (br s, 2F, CF2), À125.1–À124.9 (m, 2F,
CF2), À148.8–À148.6 (m, 2F, o-FAr), À165.9–À165.6 (m, 2F, m-FAr),
À166.6–À166.2 ppm (m, 1F, p-F); IR: n˜ =444 (m), 481 (m), 516 (m),
549 (m), 601 (m), 618 (m), 657 (w), 694 (w), 738 (w), 747 (w), 852
(w), 888 (w), 992 (s), 1038 (m), 1125 (m), 1149 (s), 1187 (m), 1215
(m), 1370 (m), 1434 (m), 1516 (s), 3233 cmÀ1 (w, NH); (À)-ESI-MS
(CH2Cl2): m/z (%): 664 (100) [MÀH+], 181 (23) [M-SO2C8F17]; HRMS
((À)-ESI (CH2Cl2)): m/z calcd for C14F22NO2SÀ 663.9304 [MÀH+];
found: 663.9293; elemental analysis calcd (%) for C14HF22NO2S: C
25.28, H 0.15, N 2.11, S 4.82; found: C 25.12, H 0.22, N 2.35, S 4.97.
3
54.2 (m, 1F, -SO2F), À145.0 (m, 2F, o-F), À150.7 (t, J(F,F)=22.9 Hz,
3
1F, p-F), À160.9 ppm (t, J(F,F)=21.4 Hz, 2F, m-F); IR: n˜ =411 (m),
456 (m), 495 (m), 533 (m), 562 (s), 584 (w), 621 (m), 768 (s), 804 (s),
929 (m), 986 (s), 1037 (m), 1074 (m), 1164 (w), 1214 (s), 1319 (m),
1355 (w), 1462 (s), 1514 (s), 1652 (w), 3305 cmÀ1 (m, NH); (À)-ESI-
MS (MeCN): m/z (%): 264 (100) [MÀH+], 249 (25) [MÀH+ÀO];
HRMS ((À)-ESI (MeCN)): m/z calcd. for C6F6NO2SÀ 263.9559 [MÀH+];
found: 263.9559; elemental analysis calcd. (%) for C6HF6NO2S: C
27.18, H 0.38, N 5.28, S 12.09; found: C 27.25, H 0.40, N 5.47, S
12.49.
2
1
X-ray structure analysis
IPDS I and IPDS II (Stoe) diffractometers were used for data collec-
tion by the x-ray department at the Philipps-Universität Marburg
(Dr. K. Harms, G. Geiseler, M. Marsch, and R. Riedel). Data collection,
reduction, and cell refinement were performed with Stoe IPDS
Software. Structures were solved with SIR92,[48] SIR97[49] or
SIR2004[50] and refined with SHELXL-97[51] Absorption correction
was performed with semi-empirical methods within WinGX (multi-
scan[52] or Gaussian).[53]
Carbon-bound hydrogen atoms were calculated in their idealized
positions and refined with fixed isotropic thermal parameters. Hy-
drogen atoms connected to heteroatoms were located on the
Fourier map and refined isotropically. In case of HN(Pf)(Nf),
HN(Pf)(Sf), and HN(Pf)(SO2N(Pf)(Sf)) NH distances were restrained at
87 pm using the DFIX command. All molecular structures were il-
lustrated with Diamond 3[54] using thermal ellipsoids at the 30%
probability level for all non-hydrogen atoms and fixed radii for het-
eroatom-bonded hydrogen atoms. Carbon-bonded hydrogen
atoms are omitted for clarity.
LiN(Pf)(Hf): A solution of lithium bis(trimethylsilyl)amide (75 mg,
0.451 mmol) in toluene (5 mL) was added dropwise to a solution
of HN(C6F5)(SO2C8F17) (300 mg, 0.451 mmol) in toluene (10 mL). Im-
mediately a white solid precipitated. After stirring the suspension
for 2 h at room temperature the precipitate was separated by cen-
trifugation, washed with hexane (2”15 mL) and dried in vacuo.
LiN(Pf)(Hf) (258 mg, 85%) was obtained as an off-white solid.
13C NMR (101 MHz, [D6]DMSO, 258C, TMS): d=142.9 (pd, 1J(C,F)=
240.1 Hz, CFAr), 136.9 (pd, 1J(C,F)=244.1 Hz, CFAr), 134.8 (pd,
1J(C,F)=241.6 Hz, p-CAr), 123.4 (t, 2J(C,F)=15.2 Hz, ipso-CAr), 116.5
(qt, 1J(C,F)=288.4 Hz, 2J(C,F)=33.3 Hz, CF3), 115.0 (tt, 1J(C,F)=
293.6 Hz, 2J(C,F)=32.8 Hz, SO2CF2), 109.6 (tt, 1J(C,F)=239.9 Hz,
2J(C,F)=31.4 Hz, CF2) 113.9-106.9 ppm (m, CF2); 19F NMR (376 MHz,
[D6]DMSO, 258C, CFCl3): d=À80.0 (t, 3J(F,F)=9.7 Hz, 3F, -CF3),
À113.5 (s, 2F, -CF2-), À119.4 (s, -CF2-), À121.2–À121.4 (m, 6F, -CF2-),
À122.1 (s, 2F, -CF2-) À125.4 (br s, 2F, -CF2-), À149.4 (d, 3J(F,F)=
Crystal data and experimental conditions are listed in S-Table 0
(see the Supporting Information). Selected bonding and nonbond-
ing distances and angles with standard deviations in parentheses
are collected in Table 1. The corresponding CIF files providing full
information concerning the molecular structures and experimental
conditions are deposited at the Cambridge Crystallographic Data
Center.
3
24.7 Hz, 2F, o-F), À166.6 (t, J(F,F)=21.6 Hz, 2F, m-F), À168.1 ppm
3
(t, J(F,F)=22.7 Hz, 1F, p-F); IR: n˜ =415 (w), 527 (m), 552 (m), 602
(w), 631 (m), 660 (w), 680 (w), 708 (w), 745 (w), 815 (w), 850 (w),
898 (m), 985 (s), 1061 (m), 1140 (s), 1199 (s), 1274 (m), 1320 (w),
1369 (w), 1465 (w), 1502 (m), 1521 (m), 1654 (w), 2979 cmÀ1 (w);
(À)-ESI-MS (MeCN): m/z (%): 664 (100) [M]; HRMS ((À)-ESI (MeCN)):
m/z calcd for C14F22NO2SÀ 663.9304 [MÀ]; found: 663.9295; elemen-
tal analysis calcd (%) for C14F22LiNO2S: C 25.05, H 0.00, N 2.09, S
4.78; found: C 25.81, H 0.48, N 2.56, S 4.32.
5. Computational Details
The program TURBOMOLE[55] was used for DFT and MP2 calcula-
tions. By default, vibrational, rotational, and translational, as well as
entropic corrections to thermochemistry were calculated with
BP86[56]/def-TZVP[57] using the FREEH tool (unscaled).[58] For this, the
vibrational frequencies were determined analytically with the AO-
FORCE[59] module. We ensured that all structures are true minima
and show no imaginary frequencies. For obtaining absolute ener-
gies, we applied the optimized structures gained from the
B3LYP[56a,60] hybrid functional in combination with augmented cor-
relation-consistent triple-zeta basis sets of Dunning et al.[61] (aug-
cc-pVTZ). We then calculated a MP2(FC)[62]/aug-cc-pVTZ single
point on the B3LYP/aug-cc-pVTZ geometry and called the method
“M1”. When available, we used the RI approximation for DFT[63] and
HN(Pf)(Sf): A mixture of H-N-(2,3,4,5,6-pentafluorophenyl)-sulfa-
moylchloride (1.34 g, 4.75 mmol), dried potassium fluoride (1.38 g,
23.7 mmol) and 18-crown-6 (0.126 g, 0.475 mmol) were suspended
in acetonitrile (40 mL). After 30 min the reaction mixture changed
color to orange and it was stirred for 3 d at room temperature. The
green suspension was filtered and all volatiles were removed from
the filtrate in vacuo. The green residue was dissolved in diethyl
ether (30 mL). Hydrogen chloride was passed through the solution
Chem. Eur. J. 2015, 21, 5769 – 5782
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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MP2.[64] As an alternative[65] we applied the RIJK[66] approach for
B3LYP/aug-cc-pVTZ. For our extensive model validation (see the
Supporting Information) we applied various combinations of densi-
ty functionals and MP2(FC) with different basis sets, in addition to
the methods mentioned above including the hybrid-functional
PBE0[67] and the basis sets def2-TZVPP,[68] def2-QZVPP[68] and aug-
cc-pVQZ.[61]
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We also used the Gaussian packages[69] to calculate the compound
approaches G3[70] and G3MP2.[71] Our recently introduced modified
[29]
G3 (G3mod
)
served as reference method for model validation (be-
sides the experimental values) by means of the GA values of the
investigated acids and for the isodesmic calculations. Thus, we im-
proved G3MP2 analogously to G3mod by using thermochemical cor-
rections at the BP86/def-TZVP level. The definition of the
G3MP2mod Gibbs energies at standard conditions follows Equa-
tion (2):
GꢀðG3MP2modÞ ¼ G3MP2ð0KÞÀZPEðG3MP2Þ þ Free H energy
þRTÀT Á Free H entropy
ð2Þ
As described in section 2.2, Gibbs solvation energies were calculat-
ed with COSMO[72] as implemented in TURBOMOLE at the BP86/
def-TZVP level of theory. We employed default COSMO options as
well as default optimized COSMO radii. For control purposes we
also applied COSMO at the B3LYP/aug-cc-pVTZ level and PCM[73] as
implemented in Gaussian 03 at the BP86/TZVP level. By default all
Gibbs solvation energy calculations were performed in C1 symme-
try and Gaussian 03 reference values were adopted for the dielec-
tric constants. Note: The value yielded from COSMO respectively
PCM reflects a transfer from a 1m ideal gas (24.79 bar at 298.15 K)
to a 1m ideal solution. To obtain the Gibbs standard solvation
energy DsolvG8 (transfer from an ideal gas at 1 bar to a 1m ideal so-
lution) we therefore had to add RT ln24.79=7.96 kJmolÀ1 [29]
.
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This work was supported by the Universities of Marburg and
Freiburg, the DFG and the ERC in the Advanced Grant Uni-
Chem. The work of I.L., K.K. and J.S. was supported by the In-
stitutional Funding Project IUT20-14 from the Estonian Ministry
of Education and Science and the Estonian Centre of Excel-
lence HIGHTECHMAT (SLOKT117T). The financial support of the
Fonds der Chemischen Industrie (doctoral scholarship for J.F.K)
is gratefully acknowledged. We thank Martin Scott for his syn-
thetic contribution and Lars Hendrik Finger (both University of
Marburg) for valuable support with crystal structure refine-
ment. The results and discussion of the synthetic and crystallo-
graphic part are, with some modifications, part of the doctoral
thesis of Julius F. Kçgel, University of Marburg, 2014: Bisphos-
phazene Proton Sponges and Metal Perfluoroanilides: Ligand
Design for Brønsted Superbases and Lewis Superacids. The re-
sults and discussion of the computations are, with some modi-
fications, part of the doctoral thesis of Sascha K. Goll, University
of Freiburg, 2013: Quantum Chemical Calculations Towards
a Unified pH Scale for All Phases.
Keywords: acidity · analytical methods · anions · density
functional calculations · sulfonamides
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Received: September 24, 2014
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Published online on February 26, 2015
Chem. Eur. J. 2015, 21, 5769 – 5782
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