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This journal is ©the Owner Societies 2020 Phys. Chem. Chem. Phys., 2020, 22, 1923--1931 | 1923
Cite this: Phys.Chem. Chem. Phys.,
2020, 22,1923
Designing aromatic heterocyclic superacids in
terms of Brønsted and Lewis perspectives†
Rakesh Parida,
ab
Sachin Ramesh Nambiar,
b
G. Naaresh Reddy
ab
and
Santanab Giri *
a
The unexplored area of organic superacids was investigated in terms of both Brønsted and Lewis concepts of
acids and bases. The primary requirement of a superacid-high affinity for electron/fluoride ions was fulfilled
using two strategies: (i) using the superhalogen-type heterocyclic framework and (ii) selecting systems that have
an electron count one short of attaining (4n+2)Hu
¨ckel aromaticity. With these in mind, eleven systems were
considered throughout the study, expected to cross the target of 100% H
2
SO
4
acidity and/or the fluoride affinity
of SbF
5
.ToenhancethepK
a
and F
affinity values of the considered systems, electron-withdrawing ligands F
and CN were employed. The superhalogen and aromaticity properties were verified by vertical detachment
energy (VDE) and nucleus independent chemical shift (NICS) calculations, respectively. Finally, the collective
effect of the potential super Lewis acids was looked into using a BL
3
skeleton with them acting as ligands.
1. Introduction
The term ‘‘superacid’’
1–4
has gained a lot of momentum in
recent times, in both experimental
5–11
and computational
12–17
research fields. This is because of its immense relevance in
several chemical applications such as catalysis and activation of
inert bonds.
18,19
The classical definition of a Brønsted super-
acid states that it has acidity greater than 100% sulphuric acid,
with Hammett acidity function
20
(H
0
)=12.20. Many such
systems have been studied in the past with the aim of breaking the
barrier of the high acidity of H
2
SO
4
. When we say acidity, it mostly
means the ease of dissociation of a generic acid ‘HA’: HA -H
+
+A
.
From this, logic dictates that a stronger acid would be the one that
results in a more stable A
anion. This suggests that ‘A’ should
have a high electron affinity. Pursuing this logic, several findings
have been published using the tool of superhalogens,
21–33
which are characteristic of having extremely high electron
affinities. While the concept of superhalogens was first introduced
inthe80’sbyGutsevandBoldyrev,
22,23
the first experimental proof
came in 1999 from Wang et al.
34
By using the photoelectron
spectroscopy technique, they have shown the existence of super-
halogen anions such as MX
2
(M=Li,Na;X=Cl,Br,I)andMX
3
(M = Be, Mg, Ca; X = Cl, Br).
35
The high vertical detachment energy
(VDE) makes them superhalogens. Very high VDE was also experi-
mentally observed for anionic sodium chloride clusters.
36
These
studies enable VDE to be a criterion to define superhalogens. While
this line of thought conforms to the Brønsted–Lowry acid–base
theory,
37,38
we decided to additionally explore the various superacids
using the criterion for Lewis acids,
39
defined as electron-deficient
species that are capable of accepting a pair of non-bonding elec-
trons. SbF
5
is considered as one of the strongest Lewis acids that
exist. Hence, a superacid, as per the definition, should have a higher
F
ion affinity than SbF
5
. With such high expectations with respect
tothenegativechargeaffinity,theapplicationsofsuperLewis
acids
40–42
could be numerous, such as in the work of Czapla and
Skurski,
43
which looked over the possibility of the oxidation of the
weakly reactive CO
2
by the use of molecules belonging to the series,
Sb
n
F
5n+1
. It was reported that the binding energy values were
consistent with the oxidation of CO
2
to Sb
3
F
16
–CO
2+
. The synthesis
ofthefirstnoblegascompoundin1962byBartlett
44
was based on
the rationale that if a molecule such as PtF
6
has a high electron
affinity strong enough to oxidize O
2
, then it is possible to employ it
to oxidize Xe to Xe
+
[PtF
6
]
, thus leading to the deconstruction of the
myth that noble gases are always inert. With such interesting
applications for superacids, we were motivated to examine a
potential class of Lewis acids, wherein the collective effects of
some electronegative atoms/ligands were examined. The idea of
heterocyclic Lewis acids has been a relatively unexplored area,
a
School of Applied Sciences and Humanities, Haldia Institute of Technology,
Haldia, 721657, India. E-mail: santanab.giri@gmail.com
b
Department of Chemistry, National Institute of Technology Rourkela, Odisha,
769008, India
†Electronic supplementary information (ESI) available: Fig. S1: Optimized geome-
tries of normal heterocyclic compounds with H-atom at carbon centre. Fig. S2:
Optimized neutral geometries of F-substituted heterocyclic complex with H-atom at
carbon Centre. Fig. S4: Optimized neutral geometries of CN-Substituted heterocyclic
complexwithH-atomatcarboncentre.Fig.S9: Optimized geometries of the studied
trinuclear complexes. Table T1: Free energy changes due to deprotonation, NICS and
pK
a
of normal heterocyclic molecules (when H at carbon centre). Table T2: Free
energy changes due to deprotonation, NICS and pK
a
of F-substituted heterocyclic
molecules (when H is at the carbon centre). Table T3: Free energy changes due to
deprotonation, NICS and pK
a
of CN-substituted heterocyclic molecules (when H is at
the carbon centre). See DOI: 10.1039/c9cp06054e
Received 7th November 2019,
Accepted 13th December 2019
DOI: 10.1039/c9cp06054e
rsc.li/pccp
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due to which we have focused our studies on the acidity of
heterocyclic compounds in terms of both Brønsted and Lewis
acid perspectives.
In our study, we have taken some known heterocyclic molecular
systems, namely, CH
2
BN
2
OH, C
4
H
5
BNH, C
3
H
4
BN
2
H, C
2
H
3
BNSH,
C
2
H
3
BNOH, C
3
H
4
BSH, C
2
H
4
BN
2
HandC
3
H
5
BNH. Their Brønsted
acidities were checked by finding the change in energy upon
deprotonation. To support this change in energy, the concept of
Hu
¨ckel’s aromaticity rule
45–47
has been used. The basic idea is to
allow the dissociation of the proposed superacid system to a
proton and an anion that is resonance stabilized. In the case of
super Lewis acids, an extra F
electron would enhance the fluoride
ion affinity, which is the key parameter. In order to facilitate this,
the heterocyclic aromatic systems that we have chosen are just one
electronshortofachievingthe(4n+2)pelectron configuration;
consequently, the electron affinity of the system to form the A
anion is greatly increased due to the attainment of the (4n+2)
electron. In addition, the electron affinities of these heterocyclic
systems were tuned by introducing electronegative species such as
F or CN at the H– sites, in a bid to further increase the acidity of
the species. The increase in the observed Brønsted and Lewis
acidity has been discussed in the following sections.
2. Computational details
All the anions and neutral molecules considered in the scope of
this study were analyzed at various levels of theory: the Becke
three-parameter Lee–Yang–Parr (B3LYP)
48–50
with 6-31+G(d,p)
51
basis
set was used for all the calculations except the tri-substituted super
Lewis acids, which were optimized using the wB97xD functional
52
and def2-TZVPP basis set.
53
To compare our results, the super Lewis
acid molecules were optimized using both the B3LYP/6-31+G(d,p)
and wB97xD/def2-TZVPP levels of theory. All optimized structures
were ensured to be local minima by verifying the absence of
imaginary frequencies. To estimate the aromaticity of the systems,
nucleus-independent chemical shifts (NICS)
54
calculations were
performed. The vertical detachment energy (VDE) of molecules
was calculated to understand the superhalogen properties of the
molecule. Vibrational analysis yielded the free energy (DG)values
of various systems, which are inclusive of changes in electronic
energies, zero-point vibration energy and thermal corrections. The
net change in the Gibbs’ Free Energy in a reaction was calculated
from basic thermochemistry.
Given that G
H
+
,G
A
and G
HA
are the free energies of the
proton, anion and the neutral molecule, respectively, the DGfor
deprotonation in the gas phase was calculated
55,56
as follows:
DG
Gas
=G
H
+
+G
A
G
HA
(1)
Further, the gas phase optimized geometries were taken to
perform solvent phase calculations. We have used water as a
solvent in the PCM
57
(polarizable continuum model) model. To
determine the solvation free energies in an aqueous medium,
DG
Sol
can be defined as the difference in the energy in the
solution phase and gas phase:
DG
Sol
=DG
Gas
+DDG
Sol
(2)
DDG
Sol
represents (DG
H
+
+DG
A
)(DG
HA
) in the solution
phase. Once DG
Sol
is calculated, the pK
a
value can be evaluated
using the following expression:
pK
a
=DG
sol
/RT ln 10 (3)
The fluoride ion affinity was calculated using the same
principle: (E
A
+E
F
)E
AF
.
All calculations were performed using the Gaussian 09
58
program package.
3. Results and discussion
3.1. Superhalogen properties of heterocyclic compounds
The systems considered were either 5-membered or 6-membered
heterocyclic aromatic rings. However, since our target revolved
around the idea of stabilization of the A
anion, one approach for
designing superacids was to explore heterocyclic compounds
with superhalogen properties. With various heteroatoms such
as B, N, O and S being shuffled around the rings in various
permutations, the eleven Brønsted and Lewis superacid systems
had impressive VDE values. As mentioned in Table 1, of the
eleven systems, seven already had a VDE in the range of 3.63
(1.5) eV (the electron affinity of chlorine), making them good
contenders for being superhalogens. Furthermore, the importance
of substituting hydrogen with fluorine in the superhalogen moiety
can be realized by noting that the VDE increased by 4.45 eV
compared to the unsubstituted C
2
F
4
BN
2
(b), which reported a
VDE of only 1.54 eV. Similarly, in C
3
L
5
BN the VDE increased from
1.00 to 5.68 eV when L was changed from hydrogen to fluorine.
The five-membered ring of C
2
F
4
BN
2
showed excellent super-
halogen tendencies with a VDE of 6 eV.
Looking at Table 1, the CN-substituted molecules showed even
greater increases in VDE; in fact, all eleven molecules behaved as
superhalogens. The best superhalogen of all was observed to be the
six-membered C
3
(CN)
4
BN
2
with a VDE of 6.25 eV. To emphasize the
importance of the CN group, it can be stated that while none of the
initial heterocyclic molecules were superhalogens, substituting H
with CN converted all the rings into superhalogens.
3.2. Aromatic heterocyclic superacids
The benchmark for a species to be a superacid can be set
by a free energy of deprotonation of B300 kcal mol
1
or less
Table 1 VDE values of the heterocyclic complexes with substitution of F
and CN
Molecule VDE (eV) Molecule VDE (eV) Molecule VDE (eV)
CH
2
BN
2
O 3.30 CF
2
BN
2
O 4.04 C(CN)
2
BN
2
O 5.01
C
3
H
4
BN
2
(a) 2.90 C
3
F
4
BN
2
(a) 4.15 C
3
(CN)
4
BN
2
(a) 5.67
C
4
H
5
BN 2.55 C
4
F
5
BN 4.09 C
4
(CN)
5
BN 5.95
C
3
H
4
BN
2
(b) 3.25 C
3
F
4
BN
2
(b) 5.00 C
3
(CN)
4
BN
2
(b) 6.25
C
2
H
3
BNS 2.42 C
2
F
3
BNS 3.00 C
2
(CN)
3
BNS 4.64
C
2
H
3
BNO (a) 2.76 C
2
F
3
BNO (a) 3.88 C
2
(CN)
3
BNO (a) 5.12
C
2
H
3
BNO (b) 2.22 C
2
F
3
BNO (b) 3.07 C
2
(CN)
3
BNO (b) 4.74
C
2
H
4
BN
2
(a) 1.88 C
2
F
4
BN
2
(a) 3.61 C
2
(CN)
4
BN
2
(a) 5.15
C
3
H
4
BS 1.88 C
3
F
4
BS 2.73 C
3
(CN)
4
BS 4.74
C
2
H
4
BN
2
(b) 1.54 C
2
F
4
BN
2
(b) 6.00 C
2
(CN)
4
BN
2
(b) 4.92
C
3
H
5
BN 1.00 C
3
F
5
BN 5.68 C
3
(CN)
5
BN 4.96
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(in the gas phase), since it has been reported for 100% sulphuric
acid.
59–64
To stabilize the A
anion after dissociation from a
potential superacid HA, apart from looking at the superhalogen-
like tendencies, the strategy of resonance stabilization of the
anion has been used in this section (Table 2).
As stated earlier, if we consider systems that require one
electron to attain Hu
¨ckel aromaticity, they will be energetically
inclined to dissociate from the proton. In order to study this,
the optimized geometries of these heterocyclic systems, both
un-dissociated and anions, were analyzed. To determine the
dissociation energy, the formula mentioned in Section 2 was
used. DG
Gas
and DDG
Sol
were calculated using eqn (1) and (2),
respectively. Following this, the pK
a
was calculated using eqn (3).
From this, we were convinced that all 11 of the considered
rings have potential superacidic capabilities, subject to certain
modifications such as the introduction of electronegative
ligands. All the heterocyclic compounds are depicted in Fig. 1
and the hydrogen-substituted at the boron atom of the hetero-
cyclic compound in Fig. 2. The fluorine-substituted heterocyclic
compounds and their hydrogen-substituted analogues are
shown in Fig. 3 and 4, respectively.
The high proton dissociation was supplemented by the s
and paromaticity obtained after carrying out NICS calculations,
as depicted in Tables 5–7. NICS values for this molecule also
indicate an increase in aromaticity upon the attainment of the
electron. The most acidic CH
2
BN
2
O was found to have high
aromaticity of 10.497 (s) and 9.113 (p). Similarly, the NICS
values of the other molecules were observed to be negative,
indicating the aromatic nature of the anions.
To further stabilize the A
anion, electronegative –I groups
were incorporated into the molecule so that the electron cloud
is not localized. For this purpose, two specimen ligands, F
and
CN
, were used on all 11 species for relative comparison of the
acidic behavior. Upon replacing the hydrogens in the hetero-
cyclic ring with fluorine, the results improved significantly as
the proton dissociation energy of a six-membered ring with
heteroatoms boron and nitrogen decreased to 258.95 kcal mol
1
.
An impressive change of 60.15 kcal mol
1
was observed for
Table 2 Free energy changes due to the deprotonation (in the solvent
and gas phases) and pK
a
of normal heterocyclic molecules
Molecule
Free energy change
due to deprotonation
DG(kcal mol
1
)
(gas phase)
Free energy change
due to deprotonation
DG(kcal mol
1
)
(solvent phase) pK
a
CH
2
BN
2
O 289.34 132.81 21.55
C
3
H
4
BN
2
(a) 313.09 160.38 0.42
C
4
H
5
BN 292.41 139.28 16.60
C
3
H
4
BN
2
(b) 293.49 139.93 16.10
C
2
H
3
BNS 306.91 153.18 5.94
C
2
H
3
BNO (a) 300.68 143.50 13.36
C
2
H
3
BNO (b) 313.66 157.55 2.59
C
2
H
4
BN
2
(a) 320.35 162.55 1.24
C
3
H
4
BS 323.81 170.19 7.10
C
2
H
4
BN
2
(b) 327.57 171.92 8.43
C
3
H
5
BN 343.15 187.44 20.34
Fig. 1 Optimized anion geometries of all heterocyclic compounds.
Fig. 2 Optimized neutral geometries of heterocyclic compounds with
H-atoms at the boron centre.
Fig. 3 Optimized anion geometries of heterocyclic compounds with
F-substitution.
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another five-membered ring with one boron and two nitrogens
when its H atoms were substituted with F atoms. Table 3 shows
that upon fluorination of the heterocyclic rings, the number
of superacids (molecules having proton dissociation less than
300 kcal mol
1
) increased from 4 to 7. Amplified acidic behavior
was observed in C
2
H
4
BN
2
(b) (DpK
a
=42.72) upon fluorination.
Upon substituting H with F atoms in the heterocyclic molecules,
the pK
a
values ranged from 4.07 to 34.29, showing an expo-
nential increase in proton release. Only 3 molecules showed a
positive pK
a
value, implying that all other remaining molecules have
ahighH
+
release tendency. In the fluorinated molecules, it was also
generally observed that the sand paromaticities were considerably
improved as seen in the NICS (0) and NICS (1) values.
When the H atoms of the initial eleven molecules were
replaced by CN ligands, which are known to be superior electron-
withdrawing entities, all the systems showed super-acidic behavior,
as reported in Table 4.
All the optimized geometries of CN-substituted complexes
and their corresponding hydrogenated complexes are given in
Fig. 5 and 6, respectively. With all the recorded acid dissociation
energies being less than 300 kcal mol
1
, the most acidic super-
acid, C
4
(CN)
5
BN, showed an H
+
dissociation energy of only
208.41 kcal mol
1
. The remarkable effect of CN-substitution
can be noted in the fact that even the least acidic of all cyano-
substituted molecules (C
3
(CN)
4
BN
2
(b)) had a dissociation
energy of only 289.42 kcal mol
1
. A remarkable decrease in
the dissociation energy in the range of 80 kcal mol
1
was
observed in C
4
(CN)
5
BN and C
2
(CN)
4
BN
2
as compared to their
non-substituted counterparts. In terms of pK
a
values, the cyano-
substitution fared really well, with every single molecule exhibiting
apK
a
value of at least 32.96, which in itself is significantly
higher than the H
+
dissociation of 100% H
2
SO
4
. All the proton
dissociations of the superacids discussed in Section 3.1 had the
sample dissociating H atom attached to the boron atom. A batch
of all such reactions where HA -H
+
+A
took place was also
carried out with the H
+
dissociating from the carbon atom for
comparative studies. They have been tabulated in the supplementary
information for the reader’s reference. However, the boron-centred
proton dissociation has been discussed at length here due to its
relevance in super Lewis acids.
Table 3 Free energy changes due to deprotonation (both solvent and gas
phase) and pK
a
of fluorinated heterocyclic molecules
Molecule
Free energy change
due to deprotonation
DG(gas phase)
(kcal mol
1
)
Free energy change
due to deprotonation
DG(solvent phase)
(kcal mol
1
)pK
a
CF
2
BN
2
O 277.60 124.89 27.64
C
3
F
4
BN
2
(a) 290.66 146.66 10.94
C
4
F
5
BN 258.95 153.81 5.45
C
3
F
4
BN
2
(b) 314.88 169.68 6.71
C
2
F
3
BNS 304.86 155.22 4.37
C
2
F
3
BNO (a) 277.72 127.22 25.85
C
2
F
3
BNO (b) 319.85 168.33 5.68
C
2
F
4
BN
2
(a) 294.02 144.78 12.38
C
3
F
4
BS 312.68 166.23 4.07
C
2
F
3
BN
2
(b) 267.42 116.21 34.29
C
3
F
4
BN 279.56 131.89 22.26
Table 4 Free energy changes due to deprotonation (both solvent and gas
phase) and pK
a
of CN-substituted heterocyclic molecules
Molecule
Free energy change
due to deprotonation
DG(kcal mol
1
)
(Gas Phase)
Free energy change
due to deprotonation
DG(kcal mol
1
)
(Solvent Phase) pK
a
C(CN)
2
BN
2
O 243.47 102.41 44.88
C
3
(CN)
4
BN
2
(a) 243.00 112.64 66.00
C
4
(CN)
5
BN 208.41 82.50 34.99
C
3
(CN)
4
BN
2
(b) 289.42 74.87 44.13
C
2
(CN)
3
BNS 249.08 115.30 36.39
C
2
(CN)
3
BNO (a) 239.50 103.38 41.26
C
2
(CN)
3
BNO (b) 248.80 113.48 32.96
C
2
(CN)
4
BN
2
(a) 239.17 107.12 37.92
C
3
(CN)
4
BS 248.64 117.95 34.98
C
2
(CN)
4
BN
2
(b) 243.42 111.48 37.03
C
3
(CN)
5
BN 243.99 115.31 60.15
Fig. 4 Optimized neutral geometries of F-substituted heterocyclic complexes
with H-atoms at the boron centre.
Fig. 5 Optimized anion geometries of heterocyclic complexes with
CN-substitution.
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3.3. Aromatic heterocyclic superacids
Along with the Brønsted acid properties, the heterocyclic sys-
tems were also treated as potential super Lewis acids and their
fluoride ion affinities were compared to SbF
5
. This affinity
should have a value of at least 493 kJ mol
1
(approximated as
500 kJ mol
1
in this study) for the molecule to be termed as a
super Lewis acid.
63,65,66
This was verified by calculating the
fluoride ion affinity of SbF
5
at the wB97xD/def2-TZVPP level of
theory. This calculation yielded a value of 528.24 kJ mol
1
,
which is considered to be a relatively accurate estimation for
this study.
When the F
affinities of normal heterocyclic molecules
were evaluated, none of the molecules exhibited super Lewis
acid characteristics; in fact, the one with the highest F
affinity
was still nearly 200 kJ mol
1
less than the required cut-off for
being a super Lewis acid (Table 5). Hence, even in this case,
there was the need to tweak the molecule in favour of obtaining
aF
affinity using electronegative substituents. As applied in
the case of superacids, the ligands used for attaining super
Lewis acidity were, again, F and CN (Fig. 7–11).
In the case of fluorine-substituted heterocyclic rings, all
eleven molecular systems reported an increase in the F
affinity,
as shown in Table 6. None of the molecules could breach the
500 kJ mol
1
mark to attain super Lewis acidity but several
molecules such as CF
2
BN
2
O, C
3
F
4
BN
2
,C
2
F
3
BNS, C
2
F
3
BNO and
C
2
F
4
BN
2
showed a significant increase in Lewis acidity with F
affinity in the range of 450–500 kJ mol
1
; however, they had
F
affinities slightly less than SbF
5
. Nevertheless, such high
fluoride ion affinities are indicative of strong Lewis acids, even
if it not a super Lewis acid.
A similar increase was measured in one of the isomers of the
molecule C
2
L
4
BN
2
. When L was hydrogen, it showed no super
Lewis acid behaviour, with a F
affinity of 276 kJ mol
1
.
Fig. 6 Optimized neutral geometries of CN-substituted heterocyclic sys-
tems with H-atom at the boron centre.
Table 5 Fluoride ion affinity, NICS (0,1) of anion and neutral geometries of
normal heterocyclic molecules
Molecule
F
affinity (kJ mol
1
)
NICS (0)
a
NICS (1)
a
B3LYP/
6-31+G(d,p)
wB97XD/
def2-TZVPP
CH
2
BN
2
O 301.38 359.04 1.2 [10.5] 5.3 [9.1]
C
3
H
4
BN
2
(a) 254.80 327.44 15.8 [3.1] 12.1 [7.6]
C
4
H
5
BN 243.11 307.75 14.6 [4.3] 10.3 [7.2]
C
3
H
4
BN
2
(b) 220.65 299.99 13.9 [6.7] 9.8 [16.1]
C
2
H
3
BNS 297.52 353.15 0.20 [7.9] 3.3 [5.9]
C
2
H
3
BNO (a) 265.80 324.35 2.1 [7.6] 2.7 [13.6]
C
2
H
3
BNO (b) 266.49 322.36 0.1 [6.3] 3.9 [2.6]
C
2
H
4
BN
2
(a) 219.39 276.00 4.2 [8.7] 0.6 [1.2]
C
3
H
4
BS 247.04 306.00 0.9 [7.9] 1.5 [7.1]
C
2
H
4
BN
2
(b) 211.59 266.32 0.4 [7.4] 2.9 [8.4]
C
3
H
5
BN 167.01 223.73 0.8 [7.3] 1.6 [20.3]
a
The values in [ ] represent the NICS (0) and NICS (1) of the anion
complex.
Fig. 7 Optimized anion geometries of normal heterocyclic systems with
fluoride ions at the boron centres.
Fig. 8 Optimized anion geometries of F-substituted heterocyclic systems
with Fluoride ions at the boron centre.
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However, when L was substituted by CN, the F
affinity
increased to 580.34 kJ mol
1
, making it a super Lewis acid.
From Table 7, it can be safely stated that all cyano-substituted
rings show super Lewis acidic character having fluoride ion
affinity values above 500 kJ mol
1
. The trend of NICS values in
cyano-substituted molecules also showed an increase in aromaticity
upon substitution of hydrogen with CN. In the search for even more
super Lewis acids, we used the collective effect of ligands that
showed high fluoride affinity in the above section.
3.4. BL
3
-type super Lewis acid
For designing each BL
3
type super Lewis acid, the central atom
(boron) was tri-substituted by the species that showed the
highest fluoride ion affinity in Section 3.3. Two species that
showed the highest F
affinity in each case, normal hetero-
cyclic, F-substituted and cyano-substituted, were considered.
The use of such superior ligands as the building blocks of BL
3
resulted in impressive results. Among the normal heterocyclic
molecules, CH
2
BN
2
O and C
2
H
3
BNS showed the greatest fluoride
affinity in the range of 300 kJ mol
1
. When the boron central atom
was tri-substituted with these two species, the F
affinity increased
to 418.04 and 416.03 kJ mol
1
, respectively. Similarly, among
Fig. 9 Optimized anion geometries of CN-substituted heterocyclic sys-
tems with Fluoride ions at the boron centre.
Fig. 10 Optimized neutral geometries of tri-molecular-substituted B atoms.
Fig. 11 Optimized anion geometries of tri-molecular-substituted B atoms
with F
at the boron centre.
Table 7 Fluoride ion affinity, NICS (0,1) of the anion and neutral geometries of
CN-substituted heterocyclic molecules
Molecule
F
affinity (kJ mol
1
)
NICS (0)
a
NICS (1)
a
B3LYP
/6-31+G(d,p)
wB97XD/
def2-TZVPP
C(CN)
2
BN
2
O 443.07 513.01 1.8 [11.0] 5.3 [9.0]
C
3
(CN)
4
BN
2
(a) 495.39 572.73 1.9 [6.1] 4.0 [8.7]
C
4
(CN)
5
BN 506.24 586.52 1.1 [7.5] 2.8 [8.3]
C
3
(CN)
4
BN
2
(b) 452.44 538.46 14.6 [6.0] 9.7 [8.1]
C
2
(CN)
3
BNS 471.17 542.44 2.6 [11.1] 4.5 [8.7]
C
2
(CN)
3
BNO (a) 472.78 544.15 0.02 [10.6] 3.4 [8.1]
C
2
(CN)
3
BNO (b) 465.93 536.56 2.6 [10.0] 5.1 [7.7]
C
2
(CN)
4
BN
2
(a) 507.02 580.34 0.1 [12.7] 2.3 [8.6]
C
3
(CN)
4
BS 482.35 556.66 2.7 [12.4] 3.0 [8.5]
C
2
(CN)
4
BN
2
(b) 495.93 569.18 3.4 [11.6] 4.5 [7.9]
C
3
(CN)
5
BN 509.33 585.31 4.3 [13.5] 3.3 [7.9]
a
The values in [ ] represent the NICS (0) and NICS (1) of the anion
complex.
Table 6 Fluoride ion affinity, NICS (0,1) of anions and neutral geometries
of fluorinated heterocyclic molecules
Molecule
F
affinity (kJ mol
1
)
NICS (0)
a
NICS (1)
a
B3LYP/
6-31+G(d,p)
wB97XD/
def2-TZVPP
CF
2
BN
2
O 368.66 418.04 0.7 [9.2] 3.6 [6.4]
C
3
F
4
BN
2
(a) 363.46 408.52 1.4 [10.8] 3.9 [8.7]
C
4
F
5
BN 347.40 391.78 3.3 [14.2] 2.9 [8.9]
C
3
F
4
BN
2
(b) 363.74 405.18 1.3 [9.2] 2.8 [6.9]
C
2
F
3
BNS 384.03 416.03 4.2 [11.5] 3.0 [5.1]
C
2
F
3
BNO (a) 353.18 399.32 3.1 [14.4] 2.0 [6.0]
C
2
F
3
BNO (b) 355.25 399.74 4.2 [10.9] 3.1 [4.1]
C
2
F
4
BN
2
(a) 383.34 409.49 4.2 [18.9] 0.1 [7.2]
C
3
F
4
BS 346.53 395.38 8.3 [16.9] 3.0 [8.3]
C
2
F
4
BN
2
(b) 430.98 421.74 7.9 [2.8] 2.7 [2.0]
C
3
F
5
BN 425.52 485.92 9.5 [9.0] 3.3 [2.0]
a
The values in [ ] represent the NICS (0) and NICS (1) of the anion
complex.
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the L-substituted specimens, the highest F
affinity was shown
by molecules B[C
2
L
4
BN
2
]
3
and B[C
3
L
5
BN]
3
. When the ligand was
F, the increase in fluoride affinity was not very impressive;
however, in the case of CN-substitution, the value increased
beyond 650 kJ mol
1
(Table 8).
The fluoride affinity of B[C
3
(CN)
5
BN]
3
was 650.49 kJ mol
1
and that of B[C
2
(CN)
4
BN
2
]
3
was 694.64 kJ mol
1
, making them
the best super Lewis acids in the scope of this study. Hence the
collective effect of super Lewis acids as substituents in building
a superior super Lewis acid can be appreciated here. In all of
the super Lewis acid calculations, the F
affinity was checked
for carbon centres, but since boron centres show significant
Lewis acidity, the same has been discussed here. For the results
of the carbon-centred F
affinity, the supplementary information
may be consulted.
4. Conclusions
With the help of DFT, we investigated whether aromatic hetero-
cyclic molecular systems could have superacidity in both Brønsted
and Lewis acid–base theories. For this, we examined various
parameters of eleven chosen heterocyclic five-membered and
six-membered rings with various combinations of boron, nitrogen,
oxygen and sulphur heteroatoms. Some of these rings showed
potential superhalogen properties with VDE values approaching
that of chlorine. It was additionally observed that the VDE of the
specimen molecules increased with the collective effects of F and
CN-substitution. F substitution, in some cases, increased the VDE
by 4.68 eV, and CN substitution enabled all eleven molecules to
cross the limit of 3.63 eV to become superhalogens. Moreover, some
of the specimen molecules already showed super acidity by having
proton dissociation energies below 300 kcal mol
1
;thiswas
enhanced significantly by substituting H with fluorine in one case
and cyano (CN) in the other. Out of eleven, seven F-substituted
molecules and all CN-substituted molecules showed superacidic
behavior. The pK
a
values of the molecules also became increasingly
negative, further affirming superacidity. In the pursuit of achieving
super Lewis acidity, substitution with F and CN characteristically
increased the F
affinity, with all cyano-substituted molecules
clearly showing super Lewis acidity. The collective effect of
super Lewis acids was analyzed by taking a BL
3
framework
and substituting L with the best two Lewis acids in each class,
normal, F-substituted and CN-substituted systems. While there
was a visible increase in the F
affinity in most cases, the best
CN-substituted super Lewis acids led to a collective fluoride
affinity of more than 650 kJ mol
1
. From this, we can conclude
that collectively, good Lewis acids and super Lewis acids can be
used as building blocks to design enhanced super Lewis acids.
Correspondingly, throughout this study, the negative NICS values
for the reported superacids and super Lewis acids indicated the
increase in the aromaticity of the molecule. This is in the line with
our basic idea of using heterocyclic rings that would gain Hu
¨ckel
aromaticity through the addition of one electron by either F
affinity
or H
+
dissociation, the basic principles of Lewis acidity and
Brønsted acidity respectively.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is supported by Department of Science and Technology
INSPIRE award no. IFA14-CH-151 and SERB award no. SB/FT/
CS-002/2014, Government of India. Recourses and computational
facilities of National Institute of Technology Rourkela and Haldia
Institute of Technology are also acknowledged.
Notes and references
1 N. F. Hall and J. B. Conant, A Study of Superacid Solutions.
I. The Use of the Chloranil Electrode in Glacial Acetic Acid
and the Strength of Certain Weak Bases, J. Am. Chem. Soc.,
1927, 49, 3047.
2 D. Himmel, S. K. Goll, I. Leito and I. Krossing, A Unified pH
Scale for All Phases, Angew. Chem., Int. Ed., 2010, 49, 6885.
3 J. Axhausen, K. Lux and A. Kornath, The Existence of
Hexafluoroarsenic(v) Acid, Angew. Chem., Int. Ed., 2014,
53, 3720.
4 C. Bour, R. Guillot and V. Gandon, First Evidence for the
Existence of Hexafloroantimonic (V) Acid, Chem. – Eur. J.,
2015, 21, 6066.
5 G. A. Olah and R. H. Schlosberg, Chemistry in Super Acids.
I. Hydrogen Exchange and Polycondensation of Methane and
Alkanes in FSO
3
HSBF
5
(‘‘Magic Acid’’) Solution. Protonation of
Alkanes and the Intermediacy of CH
5+
and Related Hydrocarbon
Ions. The High Chemical Reactivity of ‘‘Paraffins’’ in Ionic
Solution Reactions, J. Am. Chem. Soc., 1968, 90, 2726.
6 K.Arata,H.Matsuhashi,M.HinoandH.Nakamura,Synthesis
of Solid Superacids and Their Activities for Reactions of
Alkanes, Catal. Today, 2003, 81,17.
7 C. A. Reed, K. Kim, R. D. Bolskar and L. J. Mueller, Taming
Superacids: Stabilization of the Fullerene Cations HC
60+
and
C
60+
,Science, 2000, 289, 101.
8 C. A. Reed, Carboranes: A New Class of Weakly Coordinating
Anions for Strong Electrophiles, Oxidants, and Superacids,
Acc. Chem. Res., 1998, 31, 133.
9 S. Furuta, H. Matsuhashi and K. Arata, Biodiesel Fuel
Production with Solid Superacid Catalysis in Fixed Bed
Reactor Under Atmospheric Pressure, Catal. Commun.,
2004, 5, 721.
Table 8 Fluoride ion affinity of BL
3
-type super Lewis acids calculated
using the wB97XD/def2-TZVPP level of theory
Complex F
affinity (kJ mol
1
)
B(CH
2
BN
2
O)
3
418.44
B(C
2
H
3
BNS)
3
372.14
B(C
2
F
4
BN
2
)
3
(b) 436.05
B(C
3
F
5
BN)
3
416.25
B[C
3
(CN)
5
BN]
3
650.49
B[C
2
(CN)
4
BN
2
]
3
(a) 694.64
PCCP Paper
Published on 16 December 2019. Downloaded by INDIAN INSTITUTE OF TECHNOLOGY BOMBAY on 2/2/2022 12:56:56 PM.
View Article Online
1930 |Phys. Chem. Chem. Phys., 2020, 22, 1923--1931 This journal is ©the Owner Societies 2 02 0
10 S. Nakamura, M. Tanaka, T. Taniguchi, M. Uchiyama and
T. Ohwada, Stereoselectivity of Superacid-catalyzed Pictet-
spengler Cyclization Reactions, Org. Lett., 2003, 5, 2087.
11 A. Kethe, A. F. Tracy and D. A. Klumpp, Protolytic Defluorination
of Triuoromethyl-Substituted Arenes, Org. Biomol. Chem., 2011,
9, 4545.
12 I. A. Koppel, P. Burk, I. Koppel, I. Leito, T. Sonoda and
M. Mishima, Gas-phase Acidities of Some Neutral Brønsted
Superacids: A DFT and Ab-initio Study, J. Am. Chem. Soc.,
2000, 122, 5114.
13 K. E. Gutowski and D. A. Dixon, Ab-initio Prediction of the
Gas and Solution-phase Acidities of Strong Brønsted Acids:
The Calculation of pK
a
Values Less Than 10, J. Phys. Chem.
A, 2006, 110, 12044.
14 A. K. Srivastava, A. Kumar, N. Misra and A. Path, to Design
Stronger Superacids by Using Superhalogens, J. Fluorine
Chem., 2017, 197, 59.
15 Y. Valadbeigi, Design of Neutral Organic Superacids Using
Fulvene Derivatives with Di-Enol Substituent, Int. J. Quantum
Chem., 2017, 117, 190.
16 P. Nava, Y. Carissan and S. Humbel, Labile Ligands on Some
Lewis Super Acids: A Computational Study, Phys. Chem.
Chem. Phys., 2009, 11, 7130.
17 R. Vianello, J. F. Liebman and Z. B. Maksic
´, In Search of
Ultrastrong Brønsted Neutral Organic Superacids: A DFT
Study on Some Cyclopentadiene Derivatives, Chem. – Eur. J.,
2004, 10, 5751.
18 A. Martin-Mingot, G. Compain, F. Liu, M. Jouannetaud,
C. Bachmann, G. Frapper and S. Thibaudeau, Dications in
Superacid HF/SbF
5
: When Superelectrophilic Activation
Makes Possible Fluorination and/or C-H Bond Activation,
J. Fluorine Chem., 2012, 134, 56.
19 Q. Shen, Y. Huang, C. Liu, J. Xiao, Q. Chen and Y. Guo,
Review of Recent Advances in C-F Bond Activation of
Aliphatic Fluorides, J. Fluorine Chem., 2015, 179, 14.
20 R. J. Gillespie, T. E. Peel and E. A. Robinson, Hammett Acidity
Function for Some Super Acid Systems. I. Systems H
2
SO
4
-SO
3
,
H
2
SO
4
-HSO
3
F, H
2
SO
4
-HSO
3
Cl and H
2
SO
4
-HB(HSO
4
)
4
,J. Am.
Chem. Soc., 1971, 93, 5083.
21 G. L. Gutsev and A. I. Boldyrev, DVM-XaCalculations on the
Ionization Potentials of MX
(k+1)
Complex Anions and the Electron
Affinities of MX
(k+1)
Superhalogens, Chem. Phys., 1981, 56, 277.
22 G. L. Gutsev and A. I. Boldyrev, An Explanation of the High
Electron Affinities of the 5d-Metal Hexafluorides, Chem.
Phys. Lett., 1983, 101, 441.
23 G. L. Gutsev and A. I. Boldyrev, Theoretical Estimation of the
Maximal Value of the First, Second, and Higher Electron Affinity
of Chemical Compounds, J. Phys. Chem., 1990, 94, 2256.
24 C. Kçlmel, G. Palm, R. Ahlrichs, M. Bar and A. I. Boldyrev,
Ab initio Study of Structure, Stability and Ionization Potentials of
the Anions PF
6
and P
2
F
11
,Chem. Phys. Lett., 1990, 173, 151.
25 G. N. Reddy and S. Giri, Super/Hyperhalogen Aromatic
Heterocyclic Compounds, RSC Adv., 2016, 6, 47145.
26 G. N. Reddy, R. Parida and S. Giri, Li@Organic Superhalogens:
Possible Electrolytes in Li-ion Batteries, Chem. Commun., 2017,
53, 9942.
27 G. N. Reddy, R. Parida and S. Giri, Functionalized Deltahedral
Zintl Complexes Ge
9
R
3
(R = CF
3
,CNandNO
2
): A New Class of
Superhalogens, Chem. Commun., 2017, 53, 13229.
28 R. Parida, G. N. Reddy, A. Chakraborty, S. Giri and M. Jana,
A New Class of Superhalogen Based Anion Receptor in
Li-Ion Battery Electrolytes, J. Chem. Inf. Model., 2019, 59, 2159.
29 G. N. Reddy, R. Parida, P. Jena, M. Jana and S. Giri, Super-
halogens as Building Blocks of Super Lewis Acids, Chem-
PhysChem, 2019, 20, 1607.
30 R. Parida, G. N. Reddy, R. Inostroza-Rivera, A. Chakraborty
and S. Giri, Tailoring the Properties of Manganocene:
Formation of Magnetic Superalkali/Superhalogen, J. Mol.
Model., 2019, 25, 218.
31 D. Samanta, M. M. Wu and P. Jena, Au(CN)
n
Complexes:
Superhalogens with Pseudohalogen as Building Blocks,
Inorg. Chem., 2011, 50, 8918.
32 T.Zhao,S.Zhang,Q.Wang,Y.KawazoecandP.Jena,Tuning
Electronic and Magnetic Properties of Silicene with Magnetic
Superhalogens, Phys. Chem. Chem. Phys., 2014, 16, 22979.
33 B. Z. Child, S. Giri, S. Gronert and P. Jena, Aromatic Super-
halogens, Chem. – Eur. J., 2014, 20, 4736.
34 X.-B.Wang,C.-F.Ding,L.-S.Wang,A.I.BoldyrevandJ.Simons,
First Experimental Photoelectron Spectra of Superhalogens and
Their Theoretical Interpretations, J. Chem. Phys., 1999, 110, 4763.
35 B. M. Elliott, E. Koyle, A. I. Boldyrev, X.-B. Wang and
L.-S. Wang, MX
3
Superhalogens (M=Be, Mg, Ca; X=Cl,
Br): A Photoelectron Spectroscopic and ab Initio Theoretical
Study, J. Phys. Chem. A, 2005, 109, 11560.
36 A. N. Alexandrova, A. I. Boldyrev, Y.-J. Fu, X. Yang, X.-B.
Wang and L.-S. Wang, Structure of the Na
X
Cl
X+1
(X=1–4)
Clusters via ab initio Genetic Algorithm and Photoelectron
Spectroscopy, J. Chem. Phys., 2004, 121, 5709.
37 J. N. Brønsted, Einige Bemerkungen U
¨ber den Begri der
Sa
¨uren und Basen, Recl. Trav. Chim. Pays-Bas, 1923, 42, 718.
38 T. M. Lowry, The Uniqueness of Hydrogen, J. Chem. Technol.
Biotechnol., 1923, 42, 43.
39 N. F. Hall, Systems of Acids and Bases, J. Chem. Educ., 1940,
17, 124.
40 A. Hasegawa, K. Ishihara and H. Yamamoto, Trimethylsilyl
Pentafluorophenylbis (trifluoromethanesulfonyl) methide
as a Super Lewis Acid Catalyst for the Condensation of
Trimethylhydroquinone with Isophytol, Angew. Chem., Int.
Ed., 2003, 42, 5731.
41 E. J. Corey and T. W. Lee, Enantioselective Diels-Alder
reactions Between Cyclopentadiene and a,b-acetylenic Aldehydes
Catalyzed by a Chiral Super Lewis acid, Tetrahedron Lett., 1997,
38, 5755.
42 Y. Hayashi, J. J. Rohde and E. J. Corey, A Novel Chiral Super-
Lewis Acidic Catalyst for Enantioselective Synthesis, J. Am.
Chem. Soc., 1996, 118, 5502.
43 M. Czapla and P. Skurski, Oxidizing CO
2
with Superhalogens,
Phys. Chem. Chem. Phys., 2017, 19, 5435.
44 N. Bartlett, Xenon Hexafluoroplatinate (V), Proc. Chem. Soc.,
1962, 6, 197.
45 E. Hu
¨ckel, Quantentheoretische Beitra
¨ge Zum Benzolproblem,
Z. Phys., 1931, 70, 204.
Paper PCCP
Published on 16 December 2019. Downloaded by INDIAN INSTITUTE OF TECHNOLOGY BOMBAY on 2/2/2022 12:56:56 PM.
View Article Online
This journal is ©the Owner Societies 2020 Phys. Chem. Chem. Phys., 2020, 22, 1923--1931 | 1931
46 E. Hu
¨ckel, Quantentheoretische beitra
¨ge zum problem der
aromatischen und ungesa
¨ttigten verbindungen. III, Z. Phys.,
1932, 76, 628.
47 I. Gutman, Total p-electron Energy of Benzenoid Hydro-
carbons, Springer, Berlin Heidelberg, 1992, vol. 162, p. 29.
48 A. D. Becke, Density-functional Exchange-energy Approximation
with Correct Asymptotic Behavior, Phys. Rev. A: At., Mol., Opt.
Phys., 1988, 38, 3098.
49 A. D. Becke, A new mixing of Hartree-Fock and Local
Density-Functional Theories, J. Chem. Phys., 1993, 98,
1372.
50 C. Lee, W. Yang and R. G. Parr, Development of the
Collesalvetti Correlation-Energy Formula into a Functional
of the Electron Density, Phys. Rev. B: Condens. Matter Mater.
Phys., 1988, 37, 785.
51 R. Ditchfield, W. J. Hehre and J. A. Pople, Self-Consistent
Molecular Orbital Methods. IX. An Extended Gaussian-Type
Basis for Molecular-Orbital Studies of Organic Molecules,
J. Chem. Phys., 1971, 54, 724.
52 J. Chai and M. Head-Gordon, Long-range Corrected Hybrid
Density Functionals with Damped Atom-Atom Dispersion
Corrections, Phys. Chem. Chem. Phys., 2008, 10, 6615.
53 F. Weigend and R. Ahlrichs, Balanced basis sets of Split
Valence, Triple Zeta Valence and Quadruple Zeta Valence
Quality for H to Rn: Design and Assessment of Accuracy,
Phys. Chem. Chem. Phys., 2005, 7, 3297.
54 Z. Chen, C. S. Wannere, C. Corminboeuf, R. Puchta and
P. Schleyer, Nucleus-Independent Chemical Shifts (NICS) as
an Aromaticity Criterion, Chem. Rev., 2005, 105, 3842.
55 R. Zhao, F. Zhou, W. Xu, J. Li, C. Li, J. Li and B. Yin,
Superhalogen-based Composite With Strong Acidity – A
Crossing Point Between Two Topics, Inorg. Chem. Front.,
2018, 5, 2934.
56 R. Parida and S. Giri, Negative Influence of pK
a
on Activation
Energy Barrier: A Case Study for Double Proton Transfer
Reaction in Inorganic Acid Dimers, J. Comput. Chem., 2018,
39, 993.
57 B. Mennucci, Polarizable Continuum Model, Wiley Interdiscip.
Rev.:Comput.Mol.Sci., 2012, 2, 386.
58 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A.
Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich,
J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P.
Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg,
D.Williams-Young,F.Ding,F.Lipparini,F.Egidi,J.Goings,
B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G.
Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro,
M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin,
V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand,
K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo,
R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma,
O. Farkas, J. B. Foresman and D. J. Fox, Gaussian 09 Revision
A.02, Gaussian Inc., Wallingford CT, 2009.
59 F.-Q. Zhou, W.-H. Xu, J.-F. Li, R.-F. Zhao and B. Yin, The
Combination of Superhalogens and Brønsted Acids HX
(X = F, Cl, Br): An Effective Strategy for Designing Strong
Superacids, Inorg. Chem., 2017, 56, 11787.
60 M. Czapla and P. Skurski, The existence and gas phase
acidity of the HAl
n
F
3n+1
superacids (n= 1–4), Chem. Phys.
Lett., 2015, 630,1.
61 M. Czapla, I. Anusiewicz and P. Skurski, Does The Protona-
tion of Superhalogen Anions Always Lead to Superacids?,
Chem. Phys., 2016, 465, 46.
62 I. A. Koppel, P. Burk, I. Koppel, I. Leito, T. B. Sonoda and
M. C. Mishima, Gas-Phase Acidities of some Neutral Brønsted
Superacids: A DFT and Ab Initio Study, J. Am. Chem. Soc., 2000,
122, 5114.
63 R.-F. Zhao, F. Q. Zhou, W.-H. Xu, J.-F. Li, C.-C. Li, J.-L. Li and
B. Yin, Superhalogen-Based Composite with Strong Acidity-
A Crossing Point Between Two Topics, Inorg. Chem. Front.,
2018, 5, 2934.
64 M. Czapla and P. Skurski, Strength of the Lewis–Brønsted
Superacids Containing In, Sn, and Sb and the Electron
Binding Energies of Their Corresponding Superhalogen
Anions, J. Phys. Chem. A, 2015, 119, 12868.
65 J. F. Ko
¨gel, D. A. Sorokin, A. Khvorost, M. Scott, K. Harms,
D. Himmel, I. Krossing and J. Sundermeyer, The Lewis Super-
acid Al[N(C
6
F
5
)
2
]
3
and its Higher Homolog Ga[N(C
6
F
5
)
2
]
3
Struc-
tural Features, Theoretical Investigation and Reactions of a
Metal Amide with Higher Fluoride ion Affinity than SbF
5
,Chem.
Sci., 2018, 9, 245.
66 H. Bo
¨hrer, N. Trapp, D. Himmel, M. Schleep and I. Krossing,
From Unsuccessful H
2
Activation with FLPs containing
B(Ohfip)
3
to a Systematic evaluation of the Lewis Acidity of
33 Lewis acids based on Fluoride, Chloride, Hydride and
Methyl Ion Affinities, Dalton Trans., 2015, 44, 7489.
PCCP Paper
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