Hexameric palladium(II) terpyridyl metallomacrocycles: Assembly with 4,4′-bipyridine and characterization by TWIM mass spectrometry

Article abstract of DOI:10.1002/anie.200906198

TWIM peaks: A macrocycle containing 12 PdII terpyridyl centers was assembled using 4,4′-bipyridyl ligands and characterized by NMR and traveling wave ion mobility mass spectrometry (TWIM-MS). The macrocyclic architecture was also examined by tandem mass s

Full text of DOI:10.1002/anie.200906198

Angewandte
Chemie
DOI: 10.1002/anie.200906198
Macrocycles
Hexameric Palladium(II) Terpyridyl Metallomacrocycles: Assembly
with 4,4’-Bipyridine and Characterization by TWIM Mass
Spectrometry**
Sujith Perera, Xiaopeng Li, Monica Soler, Anthony Schultz, Chrys Wesdemiotis,*
Charles N. Moorefield, and George R. Newkome*
The construction of 2D or 3D materials using supramolecular
chemistry principles has become an intriguing area of
research. In particular, terpyridine-based building blocks
have played a pivotal role in the construction of dimers,[1,2]
triangles,[3–5] trigonal prisms,[6] squares,[4,5,7–9] pentagons,[10]
and hexagons,[11–13] based on their planar tridentate coordi-
nation mode[4,5,12] and their facile potential for modifica-
tion.[14] Terpyridine (terpy) and its metal complexes are also
of interest because of their well-known photophysical and
electronic properties.[15] Thus, the use of terpyridine for the
construction of materials and molecular architectures with
increasing complexity will continue to mature.
However, the difficulty of obtaining single crystals
suitable for X-ray structure determination means that other
reliable techniques are essential for the characterization of
macromolecular structures. Electrospray ionization (ESI)
mass spectrometry has been applied in the identification
and characterization under mild ionization conditions.[16] In
particular, the cold-spray (CSI) technique reported by Fujita
and co-workers,[16d,e] and the Fourier transform mass spec-
trometry (FTMS) technique developed by Schalley and co-
workers[16f,g,h] are the most prominent ESI-based methods.
The work of Piguet and co-workers[16i] is also notable.
Unfortunately, the signals that correspond to different
charge states are superimposed and only a few isotope
patterns of different charge states can be deconvoluted.
Recently, traveling wave ion mobility mass spectrometry
(TWIM-MS),[17] a variant of ion mobility mass spectrometry
(IM-MS), has been successfully applied to the detection and
characterization of supramolecules.[18] Ion-mobility-based
separation enhances the resolving power of mass spectrom-
etry by adding shape- and charge-dependent dispersion,
which reduces isomer superposition and can deconvolute
the isotope patterns of different charge states.[18] Notably,
isomeric linear and cyclic structures have been separated
based on their different drift time in the ion mobility device.
Herein, we report the 4,4’-bipyridine (bpy) assisted
assembly of a hexagonal, dodeca PdII terpyridyl based
macrocycle, and its characterization by NMR and TWIM-
MS. A recent example of the use of terpyridine and its
Pd coordination for the formation of a metallocyclic rectangle
was reported by Bosnich and co-workers,[7] whereby two
cofacially oriented, PdII terpyridine MeCN adducts were
dimerized upon the addition of 4,4’-bipyridine. Our synthetic
efforts began with an improved preparation of
1,3-bis(2,2’:6’,2’’-terpyridin-4’-yl)-5-tert-butylbenzene[19] (1),
1
which was isolated in 60% yield and exhibited identical H
and 13C NMR spectra to that of the initially reported
ligand.[19]
Ligand 1 was prepared by the reduction of commercially
available 5-tert-butylbenzene-1,3-dicarboxylic acid with
BH3·THF, followed by selective oxidation with pyridinium
chlorochromate (PCC) and subsequent grinding with
2-acetylpyridine (4.05 equivalents) and NaOH to give an
orange solid, which was then added to NH4OH and EtOH and
heated at reflux for 24 hours. This ligand, which has coordi-
nation sites that are positioned 120 degrees apart, was treated
with [PdII(MeCN)4](BF4)2 in dry MeCN to give
[(1,3-bis(2,2’:6’,2’’-terpyridin-4’-yl)-5-tert-butylbenzene)Pd2-
(MeCN)2](BF4)4 2 in nearly quantitative yield (Scheme 1).
Ligand 1 was initially insoluble in MeCN, but was readily
solubilized when treated with [PdII(MeCN)4](BF4)2 in MeCN.
The 1H NMR data confirmed the formation of adduct 2, with
signals at d = 8.68 (s, 3’,5’-terpyH), 8.63-8.58 (m, 6,6’’-terpyH
and 3,3’’-terpyH), 8.49 (t, 4,4’’-terpyH), and 7.89 ppm (t, 5,5’’-
terpyH); see Figure 1. A downfield shift of the tert-butyl
singlet peak (from d = 1.51 to 1.57 ppm, Dd = 0.06 ppm) and
the IR absorptions observed at 2334 and 2304 cmÀ1 assigned
to the C ꢀ N stretch[20] also support the formation of a PdII
adduct 2.
[*] Dr. S. Perera,[+] Dr. X. Li,[+] Dr. M. Soler, A. Schultz,
Prof. Dr. C. Wesdemiotis, Dr. C. N. Moorefield,
Prof. Dr. G. R. Newkome
Department of Polymer Science, Department of Chemistry,
The University of Akron
302 Buchtel Common, Akron, OH 44325 (USA)
Fax: (+1)330-972-2368
E-mail: wesdemiotis@uakron.edu
newkome@uakron.edu
Homepage: http://www.dendrimers.com
[+] These authors contributed equally to this work.
[**] We thank the National Science Foundation for generous financial
support (grant nos. CHE-0517909 and 0833087 to C.W., no. DMR-
0705015 to G.R.N., and no. DMR-0821313 for the purchase of the
instrument for the TWIM-MS studies). We gratefully acknowledge
the expertise of Dr. Mingming Guo, Solid State NMR Manager at
The University of Akron for his help with the 2D-DOSY NMR
experiments. We are grateful to Dr. Thomas Wyttenbach and Prof.
Michael T. Bowers for helpful discussions on collision cross
sections in ion-mobility experiments. TWIM=traveling wave ion
mobility.
Supporting information (synthesis and characterization of the
ligands and complexes) for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906198.
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Scheme 1. Assembly of the palladium hexamer 3 represented by a
Figure 1. 1H NMR spectrum (CD3CN) of complexes 2, 3, and 4
showing the aromatic and tert-butyl regions. Orange spheres=PdII.
computer-generated, partial space-filling model; reagents and condi-
tions: a) MeCN, [Pd(MeCN)4](BF4)2, 4 h, 258C; b) MeCN, 4,4’-bipyr-
idine (1 equiv), 1 h, 258C. C gray, N blue, PdII orange.
Based on the lability of weak ligands such as NO3À, OTfÀ,
and MeCN,[7] the addition of one equivalent of 4,4’-bipyridine
to bis(PdII terpyridine) adduct 2 in MeCN leads to the self-
assembly of the dodeca PdII metallomacrocycle 3 (Scheme 1).
molecule (see page S8 in the Supporting Information[31]). The
COSY and 13C NMR spectra of the adduct 2, and macro-
cycles 3 and 4 also further verified the peak assignments and
support the structures (see pages S10–S11 in the Supporting
Information).
1
Evidence for structure 3 in the H NMR spectrum includes
the observation of a small downfield shift of the singlet
assigned to the tert-butyl group (from d = 1.57 to 1.61 ppm,
Dd = 0.04 ppm), and a sharp singlet (d = 8.68 ppm, 3’,5’-
terpyHs) indicative of the symmetric, cyclic structure. An
expected downfield shift of the 6,6’’-terpyHs (d = 8.61 to
8.95 ppm, Dd = 0.34 ppm) and the presence of the
4,4’-bipyridine proton absorptions (d = 8.46 and 7.71 ppm),
as well as a signal at m/z 3174 in the ESI mass spectrum,
further confirmed the structure of 3. Notably, the symmetry
observed in the 1H and 13C NMR spectra eliminates the
possibility of oligomeric products with uncomplexed terpyr-
idine moieties, which, if present, could easily be detected
above the sensitivity limits of the NMR.
To further establish the ease of displacement of the MeCN
ligand from the coordination sphere, adduct 2 was treated
with two equivalents of 4-tert-butylpyridine to produce the
model biscomplex 4 (Scheme 2). As expected, this reaction
produced a compound with two different tert-butyl groups in a
1:2 ratio, which arises from the bis(terpyridine) and 4-tert-
butylpyridine moieties. This assignment was confirmed by
1H NMR spectroscopy, which showed C(CH3)3 signals at
d = 1.61 and 1.47 ppm, respectively. Furthermore, the chem-
ical shift (d = 1.61 ppm) of the tert-butyl groups on the
macrocycle 3 mirrors that of the bis(terpyridine) tert-butyl
moiety of 4. Notably, temperature-dependent 1H NMR
experiments exhibited sharpened signals and confirmed that
the broadened signals observed for 4 arise from a single
The full mass spectrum of the hexagonal dodeca PdII
metallomacrocycle 3 (Figure S1 in the Supporting Informa-
tion) is complicated for a multitude of reasons, which are
discussed in detail in the Supporting Information. Briefly,
1) the large number of Pd isotopes gives rise to broad isotope
patterns that extend over a wide m/z range; 2) the macro-
À
cycle 3 contains 24BF4 counterions, which can lead to
several different charge states in ESI, depending on the
À
À
number of BF4 ions lost; 3) the remaining BF4 ions may
undergo loss of BF3 during the mass spectrometry experiment
to form FÀ counterions; and 4) linear fragments with different
À
charges and BF4 /FÀ content can be generated in the mass
spectrometer. To address these problems, the ions at m/z 3174
were mass-selected and subjected to IM separation. Addi-
tionally, tandem mass spectrometry (MS/MS) was employed
after IM separation to further characterize the different
components of m/z 3174.
Based on the calculated m/z values, the ions at m/z 3174
can be either cyclic [6L1 + 12Pd + 6L2]2+ (L1 = 1 and L2 =
4,4’-bipyridine) with 20 FÀ and 2 BF4 counterions, or linear
À
[3L1 + 6Pd + 3L2]+ with 10 FÀ and 1 BF4À counterions. Three
species are detected after ion mobility separation (Figure 2).
Analysis of the corresponding isotope patterns reveals that
the signal with a drift time of 13.85 ms represents singly
charged ions (isotope spacing Dm = 1.0 Da) from the linear
[3L1 + 6Pd + 3L2]+ ion; whereas, the signal at 7.87 ms arises
from ions with an isotope spacing of Dm = 0.5 Da and
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Figure 2. 2D ESI-TWIM-MS/MS plot for m/z 3174, acquired using a
Waters Synapt quadrupole/time-of-flight (Q/TOF) mass spectrometer;
m/z 3174 was mass-selected by Q for IM separation at a traveling
wave height of 12 V and velocity of 350 msÀ1; trap and transfer
voltages were set at 6 V and 40 V, respectively (see the Supporting
Information for complete experimental details). After separation, m/z
3174 gives rise to signals at 5.90, 7.87, and 13.85 ms, corresponding
to triply charged aggregates (5.90 ms), cyclic [6L1 +12Pd+6L2]2+ with
20 FÀ and 2 BF4 counterions (7.87 ms) and linear species (13.85 ms).
À
CAD with Ar, occurring in the transfer cell, causes losses of BF3 and/or
HF, which are indicated by green or white arrows respectively; m/z
values of the fragments are marked on the y axis.
The stability of the macrocyclic architecture 3 was also
examined by MS/MS. For this purpose, the ions at m/z 3174
were mass-selected and subjected to collisionally activated
dissociation (CAD) experiments using argon in the trap cell,
which is a collision cell located before the IM region, at
collision energies ranging from 6 to 75 eV (Figure 3). The
cyclic [6L1 + 12Pd + 6L2]2+ species dissociate completely
when the trap voltage reaches 75 V, which corresponds to a
center-of-mass collision energy (Ecm) of 0.47 eV. Under
equivalent experimental conditions, the Ecm value needed
for complete fragmentation of [6L1 + 6Cd]2+, which is also
macrocyclic,[19] is 0.43 eV.
Scheme 2. Synthesis of the palladium model complex 4; reagents and
conditions a) MeCN, [Pd(MeCN)4](BF4)2, 4 h, 258C; b) MeCN, 4-tert-
butylpyridine (2 equiv), 1 h, 258C. Orange spheres=PdII.
corresponds to the doubly charged [6L1 + 12Pd + 6L2]2+ ion.
The isotope cluster of the signal at 5.90 ms was not well-
resolved; the pattern of BF3 losses upon MS/MS in the
transfer cell, which is a collision cell following the IM region,
indicates a + 3 charge state for this component, which could
result from aggregation of lower charge states.
From the drift time of [6L1 + 12Pd + 6L2]2+, an exper-
imental collision cross section can be obtained by calibrating
the drift time scale of the TWIM device with standards. Such a
calibration was performed using the procedure of Scrivens
and co-workers[22] (Figure S7 in the Supporting Information)
and led to a collision cross section of 601.7 ꢀ2 for macro-
cycle 3. However, the molecular architecture 3, optimized by
molecular mechanics/dynamics calculations (see Figure S4
and page S12 in the Supporting Information), leads to
substantially larger cross sections. Specifically, the
MOBCAL algorithm for cross sectional areas[23] leads to
cross sections of 1212.5 ꢀ2 and 1388 ꢀ2 with the projection
approximation and exact hard-sphere scattering methods,
respectively. Similarly, SIGMA[24] and Driftscope 2.1[25] algo-
rithms give rise to 1309 ꢀ2 and 1277.4 ꢀ2, respectively (both
employ the projection approximation model). Substantial
discrepancies between experimental and theoretical cross
sections have been reported previously for the cyclic serine
Considering that the cyclic [6L1 + 12Pd + 6L2] structure is
the major product according to the NMR data (see Figure 1),
and that the bis(terpyridine) 1 readily self-assembles with
divalent transition metals to form metallomacrocycles,[19] the
signal at 7.87 ms is consistent with the assignment of a cyclic
structure for the [6L1 + 12Pd + 6L2]2+ ion. Compact ions
have been shown to drift faster than ions with more extended
structures upon ion-mobility separation;[18] hence, the
shoulder signal at 9.70 ms, which, based on its isotope spacing,
also carries a + 2 charge, is assigned to the linear [6L1 +
12Pd + 6L2]2+ isomer. These findings and the fact that the
ESI mass spectrum (Figure S1 in the Supporting Information)
displays unique m/z values that correspond to hexameric
species (e.g., m/z 3174), but no signals for pentameric and/or
heptameric species, provide evidence that the macrocyclic
structure 3 ( Scheme 1) was indeed synthesized.
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Table 1: UV/Vis absorptions of the bis Pd adduct 2. Values in paren-
theses are for complex 3.
Complex
lmax [nm]
e [103 m-1 cm-1]
2, (3)
240 (240)
289 (289)
336 (336)
366 (366)
31 (207)
28 (157)
17 (98)
10 (64)
idine) ligand (1), 4,4’-bipyridine, and Pd(COD)Cl2 are also
reported (cod = 1,5-cyclooctadiene).
Complex 2 shows two quasi-reversible reductions at À2.37
and À2.53 V, which can be assigned to two consecutive one-
electron reduction processes of the bis(terpyridine) ligand;
this assignment is based on the reduction potentials of the free
bis(terpyridine) ligand 1 (Table 2). Complex 2 also shows an
irreversible reduction at À1.06 V, which can be attributed to
Figure 3. 2D ESI-TWIM-MS/MS plot of m/z 3174. The separated m/
z 3174 components are indicated by arrows next to the trap cell
voltages used for MS/MS. The signals for the [6L1 +12Pd+6L2]2+
macrocycles are surrounded by a green rectangle.
(Ser) octamer,[26] [(Ser)8 + H]+, the computationally predicted
structure of which contains a large cavity, as does doughnut-
shaped macrocycle 3. In analogy to our results, the collision
cross section of [(Ser)8 + H]+ obtained from ion-mobility
experiments[26a–d] was much smaller than the value derived
from modeling studies.[26e] The calculated cross-sectional
areas for cationic crown ethers were found to be smaller
than those measured by ion-mobility experiments.[27] The
algorithms used for modeling the geometries and cross
sectional areas could be inadequate for large cyclic species
that also contain sizable cavities, like our macrocycle or the
serine octamer. A TWIM-MS study of metallomacrocycles of
varying size (5-ring to 12-ring) is currently underway.
To further verify the structure of hexamer 3, DOSY NMR
experiments were conducted. The DOSY NMR shows only
one species with a slow diffusion coefficient of D = 3.07 ꢁ
10À10 m2 sÀ1, which gives
a
hydrodynamic radius of
rH = 2.34 nm (page S10 in the Supporting Information). This
value corresponds well with a modeled average diameter of
5.2 nm.
UV/Vis spectra recorded for Pd complexes 2 and 3
(Table 1) in a dilute MeCN solution exhibit the expected
absorption transitions; the signals that appeared below
300 nm were assigned to p–p* transitions that arise from
the heterocyclic ligands (terpy, 4,4’- bipy or 4-tert-butylpyr-
idine), while the absorptions between 300 and 370 nm were
assigned to metal-to-ligand charge-transfer (MLCT) transi-
tions.[28–30] The UV spectrum of 3 exhibited lmax values at 240,
289, 336, and 366 nm, which had extinction coefficients (e)
that were 6.5, 5.6, 5.6 and 6.1 times larger, respectively, than
the extinction coefficients of 2.
Figure 4. CV profiles of the bis Pd adduct 2 and hexamer 3. The
complete CV scans (a) and the expanded terpyridine–bipyridine
regions (b) are depicted. Fc/Fc+ =ferrocene/ferrocenium.
Table 2: Redox potentials for complexes 2 and 3 along with related
compounds.
Complex
Pd2+/Pd
E1/2 [V]
4,4’-bpy
E1/2[V]
Bis(terpy)
E1/2(1) E1/2(2)
bis(terpyridine) (1)
Pd bis(terpy)(MeCN) (2)
Pd hexamer (3)
[Pd(cod)Cl2]
4,4’-bipyridine
À2.42
À2.37
À2.38
À2.58
À1.06
À1.20
À1.05
À2.53
À2.59
Cyclic voltammograms (CV) of the precursor complex 2
and the Pd hexamer 3 in DMF are shown in Figure 4a and 4b,
respectively; the corresponding potentials are listed in
Table 2. For comparison, potentials for the free bis(terpyr-
À2.29
À2.32
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the bielectronic reduction of the palladium cation coordi-
nated to the bis(terpyridine) ligand; CV of the Pd(cod)Cl2
complex shows a reduction wave at a similar potential. The
broad irreversible wave at approximately À1.75 V has not
been assigned.
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The CV profiles for complex 2 and Pd hexamer 3 are
similar (Figure 4), thus indicating a similar electrochemical
behavior. Complex 3 also exhibits two quasi-reversible
reductions corresponding to the bis(terpyridine) ligand and
one irreversible reduction that is assigned to the two-electron
reduction of the PdII ion. Reduction of the 4,4’-bipyridine
ligand was also expected to occur in complex 3. The free
bipyridine ligand exhibits a reduction potential of À2.32 V. In
complex 3, this reduction is very close to the first reduction of
the bis(terpyridine) ligand. Specifically, the bipyridine reduc-
tion is detected at À2.29 V in the cathodic wave; however, in
the anodic wave, it overlaps with the first reduction of the
bis(terpyridine) ligand, and appears at a somewhat more
positive potential than what would be expected for only the
first bis(terpyridine) reduction.
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for the bis(terpyridine) ligands of complexes 2 and 3
(Figure 4b) reveals that the reduction processes for complex
3 lead to larger currents. Further, the anodic to cathodic peak
separation within each reduction process is almost twice as
large for complex 3 than complex 2, thus indicating that the
quasi-reversible electrochemical processes of complex 3 are
less reversible than that of complex 2 (see Table S3 in the
Supporting Information); notably, a less reversible redox
process is characterized by signals that are reduced in size and
more separated (compared to a more reversible process); this
effect is postulated[31] to arise from a larger structural
reorganization. The higher current intensity for complex 3
as compared to the Pd adduct 2 (see Table S3 in the
Supporting Information), can be explained by the larger
number of electrons exchanged in each reduction step; with
the hexameric complex 3, simultaneous reduction of all six
identical non-interacting Pd bis(terpyridine) units can occur.
In conclusion, we have demonstrated the assembly of a
dodeca PdII terpyridine based macrocycle (3) using the PdII
bis(terpyridine) adduct 2 and 4,4’-bipyridine. This hexameric
metallomacrocycle has been successfully characterized using
ESI-TWIM-MS, which enabled the detection of distinct
supramolecular isomers. Tandem mass spectrometry experi-
ments revealed that the PdII macrocycle dissociates less
readily than linear architectures, which is consistent with the
need to break more bonds in the fragmentation of a cyclic
structure compared with a linear species. The use of ESI-
TWIM-mass spectrometry for the study of higher order
supramolecular architectures is ongoing.
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Revised: June 16, 2010
Published online: July 28, 2010
Keywords: DOSY NMR spectroscopy · macrocycles ·
.
mass spectrometry · palladium · supramolecular chemistry
Angew. Chem. Int. Ed. 2010, 49, 6539 –6544
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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