Self-assembly and traveling wave ion mobility mass spectrometry analysis of hexacadmium macrocycles

Article abstract of DOI:10.1021/ja907262c

(Graph Presented) Self-assembly of 1,3-di(4′-terpyridinyl)arenes by using the labile tpy-Cd(II)-tpy (where tpy = 2,2′:6′,2″- terpyridine) connectivity afforded access to hexacadmium macrocycles in high yield. These supramolecular assemblies were character

Full text of DOI:10.1021/ja907262c

Published on Web 10/15/2009
Self-Assembly and Traveling Wave Ion Mobility Mass Spectrometry Analysis
of Hexacadmium Macrocycles
Yi-Tsu Chan,† Xiaopeng Li,‡ Monica Soler,† Jin-Liang Wang,† Chrys Wesdemiotis,*,†,‡ and
George R. Newkome*,†,‡
Departments of Polymer Science and Chemistry, The UniVersity of Akron, Akron, Ohio 44325-3601
Received August 27, 2009; E-mail: wesdemiotis@uakron.edu; newkome@uakron.edu
Scheme 1. Self-Assembly of Hexameric Metallomacrocycles 4 and
5 and Linear Polymer 6a
The self-assembly of designed building blocks has been exten-
sively applied in the construction of supramolecular species.1 The
reversibility of weak bonding plays an important role in production
of thermodynamically favored structures. Lehn,2 Stang,3 Fujita,4
Mirkin,5 and others6 have demonstrated the construction of highly
ordered architectures via coordination interactions. We have also
isolated hexameric metallomacrocycles formed by a variety of tpy-
M(II)-tpy (where tpy ) 2,2′:6′,2′′-terpyridine, M ) Ru, Fe, and
Zn) bonds.7 Since it is often difficult to obtain a single crystal of
this type of metal complexes for X-ray crystallography, mass
spectrometry analysis becomes an essential tool. Many organome-
tallic assemblies have been detected and characterized by electro-
spray ionization (ESI) mass spectrometry under mild ionization
conditions.8 As a variety of ESI, cold-spray ionization (CSI) was
developed to improve the signal intensities of labile species,
especially those in which noncovalent interactions are prominent.9
With its high resolving power, Fourier transform mass spectrometry
(FTMS) allows one to investigate the isotope patterns in a broad
distribution of different charge states;8b,10 however, in all studies
so far, the peaks from different charge states and shapes were
superimposed. This problem can be resolved by traveling wave ion
mobility mass spectrometry (TWIM-MS),11 which enables two-
dimensional gas-phase separation and complete deconvolution of
the isotope patterns of ions with the same m/z value, thus
substantially facilitating the structural analysis for self-assembled
supramolecules. With TWIM-MS, ions with different charges and
ions with cyclic and linear shapes are separated based on their drift
time in the ion mobility device. The separated structures or charge
states are subsequently characterized by their mass spectra and their
fragmentation patterns in tandem mass spectra.
TWIM-MS is a variant of ion mobility mass spectrometry (IM-
MS),12 which within the past decade has developed into a
particularly useful method for differentiating isobars, isomers, and
conformers from biomolecules and biopolymers,12 and more
recently synthetic polymers, as well.13 Herein, we report the
preparation in high yield of hexagonal macrocycles and linear
polymers with the labile tpy-Cd(II)-tpy (L-Cd2+-L) connectivity14
and the first characterization of such compounds by TWIM-MS.
Scheme 1 shows the three building blocks that were used to
construct hexagonal macrocycles and linear polymers. Bisterpy-
ridines 1 and 2 were synthesized according to our previous
procedures.7a,d 2,3,5,6-Tetramethylterephthalaldehyde (Supporting
Information) was ground with 5.5 equiv of 2-acetylpyridine via
the mortar and pestle procedure15 under basic conditions, and the
mixture was then refluxed with NH4OH in EtOH to give the desired
rigid monomer 3 (30%). The structure 3 was established (1H NMR)
by the presence of peaks at 8.37 (3′,5′-tpyHs), 7.91 (4,4′′-tpyHs),
7.36 (5,5′′-tpyHs), and 2.04 ppm (CH3).
a Reagents and conditions: Cd(NO3)2 ·4H2O, CHCl3/MeOH (3:2), 25 °C,
1 h.
Reactions of 1 and 2 with 1 equiv of Cd(NO3)2 ·4H2O in CHCl3/
MeOH (3:2) afforded the hexacadmium macrocycles 4 and 5 in
1
quantitative yields. Their H NMR and 13C NMR spectra agreed
with the structures given in Scheme 1 (see Supporting Information
and Figure S1). The hexameric composition of 4 was further
confirmed by ESI-MS peaks at m/z 2311.1, 1520.5, 1124.3, 888.0,
and 729.1, corresponding to +2 to +6 charge states, respectively.
Similarly, the ESI mass spectrum of 5 confirmed the presence of a
hexameric complex with signals at m/z 2437.9, 1604.6, 1187.9,
937.9, and 771.3 for the species in charge states +2 to +6,
respectively.
† Department of Polymer Science.
‡ Department of Chemistry.
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C O M M U N I C A T I O N S
Figure 2. Tandem mass spectra of m/z 1188. (A,B) Precursors of m/z 1188
from complex 5 (A) remained in high abundance at a trap collision energy
of 16.0 eV but (B) dissociated almost completely at a trap collision energy
of 22.5 eV. (C) Precursors of m/z 1188 from complex 6 dissociated almost
completely already at the collision energy of 16.0 eV.
Figure 1. Two-dimensional ESI-TWIM-MS plot for m/z 1188. (A)
Complex 5 analyzed on the Waters Synapt quadrupole/time-of-flight (Q/
ToF) mass spectrometer at a traveling wave height of 9.3 V and velocity
of 380 m/s. Ion mobility separation gave signals at 10.30, 6.45, and 5.13
ms, corresponding to linear [3L+3Cd]2+, linear [6L+6Cd]4+, and cyclic
[6L+6Cd]4+, respectively. The observed isotope patterns (shown) match
closely those calculated for the mentioned compositions. (B) Complex 6
analyzed under the same conditions. Ion mobility separation showed signals
at 11.55 and 6.59 ms, corresponding to linear [3L+3Cd]2+ and linear
[6L+6Cd]4+, respectively. As expected, no cyclic [6L+6Cd]4+ was observed
from complex 6.
To validate our assumption, monomer 3, which has an 180°-
angle geometry and is an isomer of ligand 2, was utilized to
synthesize complex 6 for comparison with complex 5. Ligand 3 is
constrained to form linear assemblies. In Figure 1B, signals for
two species from 6 are observed, corresponding to linear
[3L+3Cd]2+ (trace), and linear [6L+6Cd]4+. The isotope patterns
are identical to those observed from 5. The drift time of linear
[3L+3Cd]2+ from 6, 11.55 ms, is longer than that of [3L+3Cd]2+
from 5, which was synthesized using the 120°-angle building block
2. [3L+3Cd]2+ from 6 contains 180°-angle blocks instead and, thus,
is longer than [3L+3Cd]2+ from 5, explaining its longer drift time.
Similarly, linear [6L+6Cd]4+ from 6 is observed at a 6.59-ms drift
time, which is slightly longer than the 6.45 ms for linear
[6L+6Cd]4+ from 5. Most importantly, no signals are detected
around 5.13 ms for 6. This confirms that the signal at 5.13 ms in
Figure 1A arises from cyclic [6L+6Cd]4+, while that at 6.45 ms is
a linear [6L+6Cd]4+ isomer.
The tandem mass spectra of m/z 1188 provide further evidence
for the existence of linear and cyclic [6L+6Cd]4+ isomers in 5. In
the tandem mode of the Synapt mass spectrometer, m/z 1188 was
selected by the quadrupole and fragmented in the trap cell at
different collision energies. The m/z 1188 ion from 6 almost
disappears at 16.0 eV (Figure 2C). In contrast, m/z 1188 from 5
still remains largely intact at 16.0 eV (Figure 2A). Only when the
trap bias is increased to 22.5 eV, m/z 1188 from 5 almost disappears
by fragmentation (Figure 2B). This difference corroborates the
presence of macrocyclic isomers in [6L+6Cd]4+ from 5; these are
expected to be more stable than isomeric linear structures because
cyclics require cleavage of more bonds for fragmentation. Thus,
the tandem mass spectra confirm the existence of intact cyclic
[6L+6Cd]4+ ions in complex 5.
Ion mobility analysis was also performed on the ions at m/z 1604,
which are the highest m/z products observed with sizable abundance
(Figure 3). From complex 5, four species are clearly separated by
their ion mobilities. Analysis of the corresponding isotope patterns
reveals the presence of four charge states in m/z 1604 with isotope
spacings of ∆m ) 1.0, 0.50, 0.33, and 0.25 amu. From high to
low charge state, the signals observed are [8L+8Cd]4+ appearing
at 2.71 ms; [6L+6Cd]3+ appearing at 3.61 ms; [4L+4Cd]2+
appearing at 4.78 ms; and [2L+2Cd]1+ appearing at 10.11 ms
(Figure 3A). The ions of [4L+4Cd]2+ and [2L+2Cd]1+ are assigned
to linear fragments produced during the ionization process. Due to
the high abundance of [4L+4Cd]2+, some [8L+8Cd]4+ ions are
also detected in complex 5. Because of the 120°-angle in the
A stirred solution of monomer 3 and Cd(NO3)2 ·4H2O in a 1:1
1
ratio at 25 °C gave the linear polymer 6. The H NMR spectrum
of 6 exhibited unique signals, all of which were much broader than
those in the spectra of macrocycles 4 and 5, consistent with a
polymeric linear, not macrocyclic, structure.
The corresponding ESI spectra mostly consist of a series of
differently charged complexes, including the hexameric Cd(II)
macrocycle and linear fragments. Full spectra of complex 4, 5, and
6 are given in Figures S2, S3, and S4, respectively; the m/z values
of the observed peaks agree with the assemblies carrying a varying
number of charges, as summarized in Tables S1 and S2. For
example, m/z 1188 corresponds to linear [3L+3Cd]2+ (this complex
-
also contains four NO3 counterions which are omitted from the
acronym for brevity), linear [6L+6Cd]4+, or cyclic [6L+6Cd]4+
.
Note that combinations equal to or larger than [9L+9Cd]6+ have
not been considered, because the formation of such large complexes
is entropically disfavored.
The ions at m/z 1188 from 5 were selected for ion mobility
separation due to the low number of L/Cd2+ combinations possible
at this mass-to-charge ratio (Figure S3 and Table S2). Three species
are detected after ion mobility separation (Figure 1A). Analysis of
the corresponding isotope patterns reveals the presence of two
charge states at m/z 1188, having spacings of ∆m ) 0.5 and 0.25
amu, respectively. The weak signal with the longer drift time, 10.30
ms, has an isotope spacing of ∆m ) 0.5 amu and corresponds to
[3L+3Cd]2+. The signals at shorter drift times, 5.13 and 6.45 ms,
have an isotope spacing of ∆m ) 0.25 amu and correspond to
[6L+6Cd]4+. The two [6L+6Cd]4+ ions should originate from
cyclic and linear isomers. The question now is how to assign the
proper structures to these two species. Inside the gas-filled ion
mobility chamber, ions are separated not only on the basis of their
m/z ratios but also according to their shapes. Compact molecular
ions drift faster, while more extended structures drift more slowly
during the separation.12 Hence, cyclic [6L+6Cd]4+ is assumed to
have the 5.13-ms drift time, and linear [6L+6Cd]4+ is assumed to
have the 6.45-ms drift time.
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C O M M U N I C A T I O N S
the structural information obtained from ion mobility separation.
The recently introduced second generation Synapt G2 platform
provides enhanced ion mobility and mass resolution. Combination
of ESI with ion mobility separation and tandem MS fragmentation
are therefore expected to have a significant impact on supramo-
lecular characterizations.
Acknowledgment. We thank the NSF for generous financial
support (Grants CHE-0517909 and 0833087 to C.W., DMR-
0705015 to G.R.N., and DMR-0821313 for the purchase of the
TWIM-MS instrument).
Supporting Information Available: Synthetic procedures, NMR
data, complete ESI mass spectra, and tables of the possible charge states
and mass-to-charge ratios.This material is available free of charge via
the Internet at http://pubs.acs.org.
Figure 3. Two-dimensional ESI-TWIM-MS plot for m/z 1604. (A)
Complex 5 analyzed at a traveling wave height of 10.5 V and velocity of
380 m/s. Ion mobility separation gave signals at 10.11, 4.78, 3.61, and 2.71
ms, corresponding to linear [2L+2Cd]1+, linear [4L+4Cd]2+, cyclic
[6L+6Cd]3+, and linear [8L+8Cd]4+, respectively. The observed isotope
patterns (shown) match closely those calculated for the given compositions.
(B) Complex 6 analyzed under the same conditions. Ion mobility separation
showed signals at 15.43, 11.73, 6.50, and 4.24 ms, corresponding to linear
[2L+2Cd]1+, [2L+2Cd]1+, [4L+4Cd]2+, and [6L+6Cd]3+, respectively.
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