The [2+2] cycloaddition product of perhalogenated cyclopentadienyl cations: Structural characterization of salts of the [C10Cl10]2+and [C10Br10]2+dications

Article abstract of DOI:10.1039/d0cc04226a

Instead of monomeric cyclopentadienyl cations, the low-temperature reaction of hexachloro-and hexabromocyclo-pentadiene (C5Cl6 and C5Br6) with powerful Lewis acids SbF5 and AsF5 in SO2ClF yields salts of perhalogenated dications [C10Cl10][Sb3F16]2 and [C1

Full text of DOI:10.1039/d0cc04226a

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The [2+2] Cycloaddition Product of Perhalogenated
Cyclopentadienyl Cations: Structural Characterization of Salts of
the [C10Cl10]2+ and [C10Br10]2+ Dications
Susanne Margot Rupf,a Patrick Pröhma and Moritz Malischewski*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Instead of monomeric cyclopentadienyl cations, the low- this result turned out to be erroneous.15 The reduced species
temperature reaction of hexachloro- and hexabromocyclo- that had been isolated instead, turned out to be [C5Me5H2]+,16-
19
pentadiene (C5Cl6 and C5Br6) with powerful Lewis acids SbF5 and
being a rare example of an allylic cation without stabilizing
AsF5 in SO2ClF yields salts of perhalogenated dications heteroatoms or additional π-systems.20-22 Hydrogen-
[C10Cl10][Sb3F16]2 and [C10Br10][As2F11]2 which are characterized via abstraction from the organic solvent also seems to cause the
single crystal X-ray diffraction and NMR spectroscopy. Additionally, decomposition of [C5(i-Pr)5]+.5
this reactivity is rationalized by quantum-chemical calculations.
In order to block decomposition pathways via the hydrogen
atoms, the synthetic target of
a
perhalogenated
cyclopentadienyl cation is very appealing.23 Breslow et al.
Carbocations have been an equally important and fascinating
research field in organic chemistry for many decades.1 But even
lately, important landmarks have been reported, e.g. the
structural characterization of the 2-norbornyl cation and of the
pentagonal pyramidal hexamethylbenzene dication with a
hexacoordinated carbon atom.2, 3 One important carbocation
that so far has resisted all attempts at isolation and
crystallization due to its instability, is the cyclopentadienyl
cation. As the cyclopentadienyl cation has an antiaromatic 4-π
electron system the ground state of [C5H5]+ is a (diradical) triplet
state.4 This is also the case for [C5(iPr)5]+ 5 and [C5Cl5]+ 6, while
for [C5Ph5]+ a singlet ground state seems to be favored although
in general the singlet-triplet energy gap is usually very small in
observed the formation of a triplet state cyclopentadienyl
cation after reaction of C5Cl6 with neat antimony pentafluoride
24
by EPR spectroscopy.6,
However, the triplet signal of the
presumed [C5Cl5]+ in the SbF5 matrix vanishes rapidly above the
freezing point of SbF5. Quenching experiments with methanol
resulted only in < 5% C5Cl5OCH3 but >90
% dimeric
perchlorinated ketones.6 This can be explained by the tendency
of C5Cl6 to either dimerize or polymerize in presence of Lewis
acids (e.g. AlCl3).25-27
For this study, we decided to (re-)investigate the reaction of
C5Cl6 and C5Br6 with the powerful Lewis acids SbF5 and AsF5 in
the non-coordinating solvent SO2ClF at low temperatures
7
these systems.6, While [C5H5]+ dimerizes,8 the pentaphenyl
(Scheme 1).
Single
crystals
of
compounds
cyclopentadienyl cation decomposes via C-H cleavage and
formation of additional C-C bonds - some of the decomposition
products have been identified.9 Although permethylation is an
effective strategy to stabilize highly electrophilic cations3, 10-13
this is not the case for the cyclopentadienyl cation. Jutzi
reported that [C5Me5]+ decomposes via loss of a proton and
[C10Cl10][Sb3F16]2 ∙ SO2ClF (4a) and [C10Br10][As2F11]2 ∙ 2 SO2ClF
(4b) were obtained from the reaction mixtures at −75 °C. The
compound 4a crystallizes in monoclinic space group P21/n and
1
4b in triclinic space group P . The asymmetric units contain a
[C5X5] unit as well as one monoanion, respectively (Figure 1).
Due to a center of inversion the overall formula are
[C10Cl10][Sb3F16]2 and [C10Br10][As2F11]2. Based on the variations
subsequent
polymerization
of
the
resulting
tetramethylfulvene.14 Surprisingly, in 2002 the structural
characterization of the stable [C5Me5]+ cation was reported but
X
X
X
X
a: SbF5, SO2ClF
b: AsF5, SO2ClF
X
X
-
X
a: [Sb3F16
]
2
X
X
-
X
b: [As2F11]
2
X
a S. M. Rupf, P. Pröhm, M. Malischewski
Freie Universität Berlin, Institut für Chemie und Biochemie
Institut für Anorganische Chemie
X
X
X
X
X
Fabeckstraße 34-36, 14195 Berlin (Germany)
E-mail: moritz.malischewski@fu-berlin.de
1a
1b
: X=Cl
: X=Br
4a
4b
: X=Cl
: X=Br
Scheme 1: Reaction of C5X6 (X = Cl, Br) with strong Lewis acids in SO2ClF to
[2+2] cycloaddition products [C10X10
2+ of two cyclopentadienyl cations [C5X5]+.
Electronic Supplementary Information (ESI) available: [Experimental, NMR and X-
Ray data, (CCDC 2009907-2009910), Calculations)] See DOI: 10.1039/x0xx00000x
]
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in C-C bond lengths, the bonding situation in these dications is ‧‧F: 2.843 to 3.212 Å; Br ‧‧‧F: 2.877 to 3.310VÅie;w CArltic‧le‧O‧nlOine:
DOI: 10.1039/D0CC04226A
best described as two allyl cations separated by a saturated 3.268 Å, Br‧‧‧O: 2.975 and 3.196 Å).
four-membered ring (see SI, Table S3).
13C and 19F NMR spectra of 4a and 4b were recorded in liquid SO2 at
–60°C while the spectra of 1a and 1b were measured in organic
solvents at room temperature. The difference in 13C chemical shifts
between 1a and 1b, as well as 4a and 4b is most pronounced for the
sp3 hybridized carbon atoms. The 13C NMR spectra of 4a and 4b
display three signals, respectively (see SI) which is consistent with the
symmetry of the compounds. The most downfield shifted chemical
shifts (4a: 218.08, 4b: 219.32 ppm) correspond to the carbon atoms
Figure 1: Ellipsoid (50% probability at 100 K) plot of compounds [C10Cl10][Sb3F16]2 ‧
C2 and C4 which are mostly affected by the positive charges. These
SO2ClF (left; 4a) and [C10Br10][As2F11]2 ‧ 2 SO2ClF (right, 4b). Counterions and solvent
signals are significantly deshielded compared to the starting
molecules are omitted for clarity. Color code: red – bromine, green – chlorine, grey –
materials (1a: 131.49, 1b: 130.21 ppm) which confirms carbocation
formation.44 The inner carbon of the allyl cation (C3) resonates at
158.23/155.23 ppm (4a/4b). The four-membered saturated ring is
characterized by one signal at 76.05/66.27 ppm (4a/4b). Very similar
chemical shifts are observed for [C10(cyclopropyl)10]2+ and the less-
carbon.
Interestingly, the C-Cl bond lengths vary considerably within the
dication. In comparison with the C-Cl bond lengths in C5Cl6
(Csp3-Cl: 1.780(2)-1.781(2) Å; Csp2-Cl: 1.696(1)-1.699(2) Å) the
C-Cl bonds C1-Cl1 and C5-Cl5 from the saturated four-
membered ring in 4a are slightly shortened (1.739(2)-
1.740(2) Å). However, the so-called α-chlorine effect is even
more evident in the allyl cation: Here, the C2-Cl2 and C4-Cl4
bonds are shortened to 1.643(2) and 1.634(2) Å while the effect
is less pronounced for C3-Cl3 (1.678(2) Å). This finding is in
accordance with calculated NBO charges (see SI) which are the
highest for Cl2 and Cl4. In general, the high electrophilicity of
the allyl cation is reduced by chlorine as a π-donor (+M-effect).
The same effects can be observed for the perbrominated
dication 4b (Table 1). The resulting bond-shortening of the C-
halogen bond in carbocations has been observed before.28-34
symmetric [C10H8Me2]2+ which both have not been structurally
46
characterized so far.45,
The 13C NMR spectrum of the former
displays three signals at δ= 222.6 , 148.3 and 83.2 ppm. The 19F NMR
spectrum of 4a (see SI) shows five signals consistent with the counter
ion [Sb3F16]−. The 19F NMR spectrum of 4b only shows a broad singlet
for [As2F11]− due to fast exchange processes.47, 48
X
X
X
X
158.23
155.23
X
76.05
66.27
X
88.66
56.96
X
128.01
122.97
X
X
X
X
Table 1: Selected bond lengths of the compounds 1a, 1b, 4a and 4b in [Å].
131.49
130.21
218.08
219.32
X
X
X
C10X102+ (X=Cl) 4a
1.740(2)
1.643(2)
1.678(2)
1.634(2)
C10X102+ (X=Br) 4b
1.908(4)
1.803(6)
1.833(5)
1.797(4)
1.912(5)
C5X6 (X=Br) 1b
1.950(6)
X
X
C1-X1
C2-X2
C3-X3
C4-X4
C5-X5
1a
: X=Cl
1b
: X=Br
4a
: X=Cl
4b
: X=Br
2+
Figure 2: 13C NMR chemical shifts of compounds (1) and (4) in [ppm]. C10Cl10 and
C10Br102+: 100 MHz, SO2, ext. acetone-d6, -60 °C. C5Cl6: 100 MHz, CH2Cl2, ext. acetone-d6,
room temperature. C5Br6: 100 MHz, CDCl3, room temperature
1.739(2)
C5X6 (X=Cl) 1a
1.780(2)-1.781(2)
1.696(1)-1.699(2)
Csp3-X
Csp2-X
The isolation of the [C10X10]2+ dications instead of [C5X5]+ poses the
question how energetically favorable this dimerization is. Although
dimerization of the 4π-electron systems and diradicals [C5X5]+
appears reasonable, Coulomb-repulsion should destabilize the
dication. Structure optimizations and frequency calculations were
performed on the B3LYP-GD3BJ/def2-TZVP and MP2/cc-PVTZ level
of theory. The optimized structures are in accordance with the crystal
structures and the variations in C-C and C-X bond lengths are well
reproduced. As expected, the shortest C-X bond lengths are those
where the halogen atom (X) carries the highest positive charge (see
SI). Calculation of Gibbs free energies for the dimerization of (triplet)
[C5Cl5]+ (2a) to [C10Cl10]2+ and its isomers (4a, 5a and 5b) were
performed for a temperature of 213.15 K. While the dimerization of
2a to 4a is endergonic by 225 kJ/mol (using B3LYP-GD3BJ/def2-
TZVP), formation of the [4+2] Diels-Alder-products 5a and 5b is even
1.838(11)-1.875(11)
Only few examples of crystal structures containing [Sb3F16]− ions
are known, in which it can exist as either cis or trans isomer.34-
38
In 4a the SbF5 groups are attached to neighbouring cis-
fluorine atoms of a central [SbF6]− unit. The number of
structures containing [As2F11]- is similarly scarce.3, 12, 29, 34, 39-43
The structures of 4a and 4b show only weak contacts between
anion [Sb3F16]− and [As2F11]− to the allylic carbon centres. The
present C‧‧‧F contact distances of 3.031 and 3.110 Å in the 4a
and 3.096 and 3.158 Å in the 4b structure are comparable to
those observed in [CCl3]+ 17 (2.962(9) Å), [CBr3]+ 17 (3.09(2) Å),
[C6Cl6]+ 19 (3.003(1) Å) and [C6Br6]+ (2.994(1) Å).34 The chlorine
and bromine atoms also interact with the fluorine atoms of the
anion as well as the oxygen atoms of the SO2ClF molecules (Cl‧
2 | J. Name., 2012, 00, 1-3
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less favorable (317/316 kJ/mol). However, these values contradict against isolation of the monomeric cation in the solid VstieawteA,rtiscilnecOenlainne
+
−
2+
−
DOI: 10.1039/D0CC04226A
common chemical experience, especially since the dimerization A X salt has a significantly lower lattice energy than an A
X 2 salt.
already takes place at very low temperatures. It is important to note So even in case of a mildly endergonic dimerization, the gain of the
that these calculated values refer to isolated molecules in vacuum. higher lattice energy could lead to the isolation of the dimerization
In order to obtain a better description for the experimental products. Only for extremely large weakly-coordinating anions the
parameters, the calculations were repeated with a solvation model, energy differences between these different stoichiometries would
using dielectric constants (relative permittivity) of εr = 2 and εr = 10 become relatively small and could in principal be overcome by a
to simulate the influence of solvents of very low or medium polarity. highly endergonic dimerization reaction. However, the numbers of
Additionally, εr = 23 was used to model the dielectric constant of our MP2 calculations suggest that this is definitely not the case here.
liquid SO2 at 213.15 K.49 Taking solvation into account the dications
Table 2: Calculated energy differences ∆G in kJ/mol at 213.15 K. εr is the dielectric
constant, used for calculations with the Polarizable Continuum Model (PCM) for solvent
effects.
become significantly more stable, reducing the overall Gibbs free
energy of the formation of [C10Cl10]2+ (4a) to +108, +35 and +5 kJ/mol,
respectively (for εr = 2; 10; 23). The Diels-Alder-products exo-5a and
endo-5a remain about 100 kJ/mol less stable than 4a. If MP2/cc-PVTZ
instead of DFT is used for the calculations, all Gibbs free energies are
roughly shifted by 100-150 kJ/mol in favor of the dications.
Consequently, dimerization is slightly favorable even for extremely
non-polar solvents (εr = 2) but becomes significantly exergonic for 4a
and 5a in medium-polar solvents (εr = 10 and 23). For [C10Br10]2+ the
general trend is clearly the same for the DFT calculations, although
the calculations using MP2/cc-PVTZ were not performed for
[C10Br10]2+ due to the enormous demand of computational resources
(see SI).
B3LYP-GD3BJ/def2- MP2/cc-pVTZ
TZVP
2a / 3a
0 / +32
0/ +15
2a + 2a  4a / exo- +225/+317/+316
5a / endo-5a
+102/+156/+165
2a + 2a  4a / exo- +108/+195/+201
5a / endo-5a ; εr=2
2a + 2a  4a / exo- +35/+103/+101
5a / endo-5a ; εr=10
2a + 2a  4a / exo- +5/+89/+90
5a / endo-5a ; εr=23
-13/+37/+41
-109/-61/-61
-123/-75/-76
X
X
X
X
These results suggest that in order to isolate a monomeric
cyclopentadienyl cation the most promising strategy is to design
sterically bulky Cp rings, where sterics destabilize the dication.
Additionally, the larger molecular volume of the cation also
beneficially reduces the lattice energies. However, as stated in the
introduction, this is very difficult from a synthetic point of view since
C-H containing systems seem to be very prone to all sorts of
decomposition reactions.
X
X
X
X
X
X
X
X
X
X
X
X
1a
1b
: X=Cl
: X=Br
2a
2b
: X=Cl
: X=Br
3a
3b
: X=Cl
: X=Br
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
In summary, [C10Cl10][Sb3F16]2 and [C10Br10][As2F11]2 are the first
structurally characterized dications which are derived from the
dimerization of a substituted cyclopentadienyl cation. To our
knowledge, only one perhalogenated dication had been structurally
characterized so far ([C14F10]2+).42 The bonding situation in C10X102+ is
best described as two allyl cations that are separated by a four-
membered saturated ring. The chlorine and bromine atoms act as
efficient π-donors into the allyl-cation, which is reflected by
significant shortening of C-Cl and C-Br bonds.
X
X
X
X
X
X
X
X
exo-5a
exo-5b
: X=Cl
: X=Br
4a
4b
endo-5a
endo-5b
: X=Cl
: X=Br
: X=Cl
: X=Br
Figure 3: Top: perhalogenated cyclopentadiene derivatives (1) as well as triplet state (2)
and singlet state (3) cyclopentadienyl cations. Bottom: [2+2] cycloaddition product (4)
and [2+4] cycloaddition products of two cyclopentadienyl cations.
Dutton proposed a series of possible substitution patterns (typically,
CH3, CF3, C6H5, C6F5 substituents) which could give rise to a stable
cyclopentadienyl cation.50 Possible side-reactions (proton loss,
addition of fluoride etc.) were taken into account. By using M06-
2X/def2-TZVP, B3LYP/def2-TZVP and the polarizable continuum
model to account for solvent effects of dichloromethane (εr ≈ 9), he
calculated that the dimerization to dicationic (exo/endo)-Diels-Alder-
products similar to 5a is endergonic by about 195-212 kJ/mol which
is similar to our calculations. Since we can experimentally prove that
dimerization occurs (however to products that are even
about 50-100 kJ/mol more stable), we have to assume that the
probability is low to achieve a stable, monomeric cyclopentadienyl
cation with this approach. Even worse, also lattice energies work
Gefördert durch die Deutsche Forschungsgemeinschaft (DFG) –
Projektnummer 387284271 – SFB 1349. Computing time was
made available by HPC Service of ZEDAT, FU Berlin.
Conflicts of interest
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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29.
30.
K. O. Christe, X. Zhang, R. Bau, J. Hegge, G. AV.ieOwlaAhrt,icGle.OKn.linSe.
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DOI: 10.1039/D0CC04226A
The [2+2] Cycloaddition Product of Perhalogenated Cyclopentadienyl Cations: Structural
Characterization of Salts of the [C10Cl10]2+ and [C10Br10]2+ Dications
Susanne Margot Rupf, Patrick Pröhm and Moritz Malischewski
Perchlorinated and perbrominated organic dications [C10Cl10]2+ and [C10Br10]2+ have been obtained
+
+
instead of monomeric cyclopentadienyl cations C5Cl5 and C5Br5 .