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Gas-Phase HostЦGuest Chemistry of Dendritic Viologens and Molecular Tweezers A Remarkably Strong Effect on Dication Stability.

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Angewandte
Chemie
that are absent in the gas phase. This advantage of the gas
phase is particularly beneficial for the analysis of weakly
bound complexes, in which the noncovalent bonds have bond
energies not much larger than the interactions with their
environment in the condensed phase.[1] Although mass
spectrometry has often been used as a powerful tool for the
analytical characterization of dendrimers,[2] their gas-phase
fragmentations have only rarely been studied,[3] and their
host?guest chemistry in the gas-phase is to date almost
completely unexplored.[4]
Viologens substituted with Frchet dendrons[5, 6]
(Scheme 1) have a highly electron-deficient, dicationic core.
Dendritic Effects
Gas-Phase Host?Guest Chemistry of
Dendritic Viologens and Molecular
Tweezers: A Remarkably Strong Effect on
Dication Stability**
Christoph A. Schalley,* Carla Verhaelen, FrankGerrit Klrner, Uwe Hahn, and Fritz Vgtle
Scheme 1. Viologens 22+?42+ and the molecular tweezers 1.
Gas-phase chemistry in the ultrahigh vacuum of a
mass spectrometer provides insight into intrinsic features of
the species under study. Comparison of gas-phase and
solution properties thus permits a detailed analysis of the
influence of the solvents and counterions in the solution phase
[*] Priv.-Doz. Dr. C. A. Schalley, Dr. U. Hahn, Prof. Dr. F. Vgtle
Kekul-Institut fr Organische Chemie und Biochemie
Universitt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Fax: (+ 49) 228-735-662
E-mail: c.schalley@uni-bonn.de
Dipl.-Chem. C. Verhaelen, Prof. Dr. F.-G. Klrner
Institut fr Organische Chemie
Universitt Duisburg-Essen
Campus Essen, Universittsstrasse 5, 45117 Essen (Germany)
[**] We are grateful to Dipl.-Chem. W. Reckien and Prof. Dr. S.
Peyerimhoff for support with density functional calculations and to
W. M. Mller and U. Mller for synthetic assistance. This work was
funded by the Deutsche Forschungsgemeinschaft (DFG) and the
Fonds der Chemischen Industrie (FCI). C.A.S. thanks the DFG for a
Heisenberg fellowship and the FCI for a Dozentenstipendium.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 477 ?480
They are excellent guests for the molecular tweezers 1[7] with
its extended aromatic systems.[8] A rather negative electrostatic potential is generated on the concave interior of 1 owing
to overlapping p-systems.[9] The positively charged viologens
form highly stable complexes with 1 (Ka 9000 m 1 in
dichloromethane).[10] The stability of the viologen?tweezers
complexes is, however, highly solvent-dependent.[11] Herein,
we report the intrinsic properties of the host?guest complexes
between tweezers 1 and the viologen derivatives 22+, 32+, and
42+. In view of the positively charged guests, insight into these
properties should be gained easily in the gas phase by
electrospray ionization Fourier-transform ion-cyclotron-resonance (ESI-FT-ICR) mass spectrometry.
Electrospray ionization of 50 mm methanol solutions of
viologen 22+(PF6 )2 gives the mass spectrum shown in
Figure 1 a. Irrespective of the ionization conditions, no
naked 22+ dications were detected in the spectrum. This
situation is in line with preliminary BHLYP/TZP density
functional (DFT) calculations for the model compound in
Scheme 2, which predict the viologen dication to be intrinsically unstable. Upon its transfer into the gas phase, 22+ tends
to stabilize itself by 1) formation of singly charged
(22+)n(PF6 )2n 1 (n = 1) and doubly charged (22+)n(PF6 )2n 2
(n > 1) clusters in which the anions counterbalance the
DOI: 10.1002/anie.200461872
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
477
Communications
Figure 1. ESI-FT-ICR mass spectra of 50 mm solutions of a) viologen
22+(PF6 )2 in methanol (asterisk: position of absent, naked 22+ dications) and b) 22+(PF6 )2 with one equivalent of tweezers 1. Inset:
c) high-resolution isotope pattern for monocationic [22+ H+] and
[22+ + e ] ions at m/z 561 and 562 and the isotope pattern of the
signal at m/z 1559 confirming the presence of a singly and a doubly
charged cluster with peak spacings of 1 and 0.5 Da, respectively. Inset:
d) isotope pattern (Dm = 0.5 Da) of the 22+�tweezers?guest complex
before and after isolation of the monoisotopic peak for MS/MS experiments. Inset: e) The isotope patterns corresponding to the dications
of dendritic substituted viologens 32+ and 42+ and the corresponding
tweezers complexes.
Scheme 2. Results of BHLYP/TPZ DFT calculations on the stability of a
model viologen cation.
repulsion of the positive charges, 2) by separation of the two
charges through dissociation into the two singly charged
fragment ions with m/z 359 and 203, 3) by proton loss giving
rise to singly charged [22+ H+] ions with m/z 561, and 4) by a
one-electron reduction during the ionization process leading
to monocationic [22+ + e ] with m/z 562. The processes (3)
and (4) lead to overlapping isotope patterns which can easily
be separated by the high resolving power of the FT-ICR
instrument (Figure 1 c, left). These results already point to the
478
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
importance of the environment: Owing to the presence of
counterions, 22+ is stable in solution, but is not observed as a
naked dication in the gas phase because of strong intramolecular charge-repulsion effects.
Upon addition of one equivalent of tweezers 1 to the
solution of 22+(PF6 )2, the spectrum changes dramatically
(Figure 1 b):[12] Instead of any cluster ions, a tweezers?
viologen complex is detected at m/z 1407 confirming the 1:1
stoichiometry in solution. Loss of a [C6H5CH2]+?[PF6] ion
pair gives rise to a signal at m/z 1059. In addition, a rather
intense signal for doubly charged 22+�with an isotope pattern
spacing of Dm = 0.5 Da is observed at m/z 631. Complex
formation with the tweezers significantly stabilizes the
dication through charge-transfer interactions that dissipate
the charge and diminish charge repulsion sufficiently to
prevent the fragmentation of 22+.
Significantly different behavior was encountered for the
dendritic viologens 32+(PF6 )2 and 42+(PF6 )2 : Under the
mildest possible ionization conditions the naked dication of
the first generation (G1) viologen 32+ could be generated with
some difficulties; the formation of naked 42+ was achieved
easily even under somewhat harder conditions (Figure 1 e,
left). Evidently, a trend towards higher dication stability with
increasing dendron size exists. Both compounds formed
abundant doubly charged complexes with tweezers 1 (Figure 1 e, right).
For an analysis of its fragmentation behavior, monoisotopic 22+�was isolated in the FT-ICR cell and subjected to
collisions with argon as the collision gas (Figure 2 a).[13] Owing
to the intrinsic instability of 22+, this dication is not observed
as fragmentation product. Instead, one zero generation (G0)benzyl ion (m/z 203) is expelled giving rise to a singly charged
tweezers?bipyridinium ion complex with m/z 1059 which
subsequently fragments through loss of the tweezers to yield
the bipyridinium ion with m/z 359 (for the structural formula
of this ion, see Figure 1 a). As a mechanistic alternative, the
two ions with m/z 203 and 359 could be formed by tweezers
loss, immediately followed by decomposition of the resulting
instable 22+ ion. However, a double-resonance experiment
(Figure 2 b), in which the intermediate with m/z 1059?and
with it, all the subsequent fragments?is constantly expelled
from the FT-ICR cell during the whole experiment, provides
evidence that the latter fragmentation pathway does not play
a major role. The intensity of the signal at m/z 359 almost
vanishes in this experiment, which indicates that the corresponding ion is thus formed predominantly from 22+�through the upper pathway in Scheme 3. The same behavior
is observed for G1-substituted 32+� The only difference is
that the G1-benzyl cation undergoes further consecutive
fragmentations giving rise to the 2-naphthylmethyl ion (m/
z 141). This type of fragmentation pattern is observed as well
for naked 32+.
This reactivity pattern?first loss of a benzyl cation
followed by dissociation of the remaining complex?is
completely reversed for 42+�to a tweezers loss with
subsequent decomposition of the dicationic viologen. Upon
collisional activation of 42+� an intense signal for the naked
42+ dication is observed at m/z 1005, while the product of the
loss of a second generation (G2) benzyl cation (signal
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Angew. Chem. Int. Ed. 2005, 44, 477 ?480
Angewandte
Chemie
Scheme 3. Main fragmentation pathways for 22+?42+ in the gas phase.
Figure 2. a) Collision-induced decomposition (CID) of mass selected
22+�ions. b) Double resonance experiment with mass selected 22+�
in which the fragment at m/z 1059 was ejected from the reaction cell
during the whole experiment. c) CID experiment with 32+� d) CID
experiment with 42+�revealing a complete change in reactivity. Tweezers loss is the major fragmentation pathway giving rise to dicationic
42+ at m/z 1005. The structures on the right show the fragments by
which dendritic benzyl cations can give 2-naphthylmethyl ions at m/
z 141 without the formation of the benzyl cation itself rather than the
complete G1 and G2 dendritic wedges.
expected at m/z 1783) is not seen at all. Consecutive
fragmentations of 42+ can be identified at m/z 1083 and 141
and are in agreement with those found in the same experiment with independently generated naked 42+. This remarkable change in reactivity can be traced back to the stabilizing
effect of the larger dendrons on the dication. While the
dication becomes more stable with larger dendritic substituents, its binding energy to tweezers 1 is reduced. Consequently, these experiments nicely uncover a remarkable effect
of the dendritic substituents on the host?guest chemistry of
the viologen?tweezers complexes studied.
Angew. Chem. Int. Ed. 2005, 44, 477 ?480
In conclusion, we have shown 1) tweezers 1 forms stable
1:1 complexes with dendritic viologens 22+?42+ in the gas
phase. 2) Tweezers 1 significantly stabilizes dicationic guests,
which are unstable (22+) or hardly stable (32+) as isolated
dications in the gas phase, most likely through charge-transfer
interactions. 3) The dendritic substituents have a surprisingly
large influence on the fragmentation pattern of the tweezers
complexes of viologen dications 22+?42+. The fragmentation
pattern changes completely from the G0 and G1 to the G2
dendrimers. Evidently, dendritic substituents of increasing
size at the viologen core provide increasing stability of the
naked dication. This finding can be explained by force-field
calculations (Figure 3). The G2 dendron is calculated to
completely embrace the viologen dication by back folding[14]
of the dendritic ?arms? leading to an ?intramolecular
solvation? and hence to a stabilization of the viologen core.
The viologen core is, however, calculated to be less efficiently
or entirely not embraced by folding of the G1 and G0
dendrons, respectively, which results in the instability
observed for 22+ and 32+.
These studies not only provide insight into the host?guest
chemistry of dendrimers and the influences of dendron size as
Figure 3. Lowest energy conformations of the viologen derivatives substituted by two 3,5-di-tert-butylbenzyl groups (22+), by model complexes with methyl and G1 dendron (3 a2+), and by methyl and G2 dendron (4 a2+) calculated by Monte Carlo conformer search using the
MMFF force field implemented in SPARTAN 04.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
479
Communications
it increases with the generation number. They also underline
the importance of gas-phase studies of such systems which
significantly contribute to our understanding of the effects of
the environment on noncovalent compounds.
Received: September 2, 2004
.
Keywords: dendrimers � gas-phase chemistry � host?guest
chemistry � mass spectrometry � supramolecular chemistry
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480
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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