Towards larger polygonal architectures: Synthesis and characterization of iron(II)- and ruthenium(II)-bis(terpyridine) metallomacrocycles

Article abstract of DOI:10.1002/chem.201100559

Five- through ten-membered homo- and heteronuclear metallomacrocycles possessing II (or Ru^II)-tpy> connectivity have been prepared and isolated. Characterization includes a detailed analysis using traveling wave ion-mobility m

Full text of DOI:10.1002/chem.201100559

DOI: 10.1002/chem.201100559
Towards Larger Polygonal Architectures: Synthesis and Characterization of
Iron(II)– and Ruthenium(II)–Bis(terpyridine) Metallomacrocycles
Yi-Tsu Chan, Xiaopeng Li, Charles N. Moorefield, Chrys Wesdemiotis,* and
George R. Newkome*[a]
Construction of supramolecular architectures employing
the self-assembly of predesigned building blocks has drawn
considerable attention in the past decades.[1] Based on the
combinatorial library proposed by Lehn,[2] self-assembling
constructs are often an equilibrium distribution of possible
assemblies possessing comparable stabilities. Assemblies
with similar components resulting in an equilibrating mix-
ture of binary[2a,c,3] or multiple[4] entities have been reported.
Under appropriate conditions, the thermodynamically most
stable species can be obtained in quantitative yield. Based
on these tenants, many successful strategies have been de-
veloped for the synthesis of metallocycles with triangular,[5]
rectangular,[6] pentagonal,[7] and hexagonal[8] shapes. In con-
trast, few examples of larger polygonal structures are docu-
mented.[9] Towards larger finite architectures, subtle varia-
tion of building blocks could result in a dramatic change to
the final products.[10] In the self-assembly of bis(terpyridine)
ligands possessing a 1208 angle between two ligating moiet-
ies, only hexameric metallomacrocycles were previously re-
ported due to the difficulty of separation and characteriza-
tion.[8b–d] More recently, Newkome et al. successfully isolated
unexpected pentameric[7a] and heptameric[11] macrocycles
from the FeII-mediated complexation of rigid bis(terpyri-
dine) ligands. Herein, we employ kinetically stable <tpy-
MII-tpy> (in which tpy=2,2’:6’,2’’-terpyridine, M=Fe and
Ru)[12] connectivity to afford a series of homo- and hetero-
nuclear metallomacrocycles by means of stepwise assembly
procedures. By using metals that form strong coordinative
bonds (FeII and RuII), the self-assembly process becomes ki-
netically controlled and irreversible, thus allowing for the
formation of macrocycles other than the thermodynamically
most stable hexameric ring.[12c] Further, the use of elongated
bis(terpyridine) dimers as precursors introduces steric con-
straints and increased rotational freedom, which obstruct
the formation of small metallocycles, while facilitating the
construction of novel, large metallocycles that would be dif-
ficult to realize otherwise. Traveling-wave ion-mobility mass
spectrometry (TWIM-MS)[8g,11,13] and molecular modeling
provide unique insight into the distinct ring sizes formed
and their conformational flexibility.
Reaction (Scheme 1a) of 3,5-bis(4’-terpyridinyl)anisole (1)
with 1.05 equivalents of FeCl2·4H2O in a mixed solvent of
MeOH/CHCl3 at reflux for 18 h gave the pentamer (2; 6%),
hexamer (3; 15%), and heptamer (4; 3%), which were iso-
lated by column chromatography (SiO2) eluting with H2O/
MeCN/sat.-KNO3(aq) (1:12.5:1; v/v/v). Subsequently, the
À
À
counterions were exchanged from NO3 to PF6 by adding a
slight excess of methanolic NH4PF6 (1m). The 1H NMR
spectrum (Figure S1a in the Supporting Information) of pen-
tamer 2 displayed four sharp singlets at d=9.41 (3’,5’-tpyH),
8.61 (4-ArH), 8.16 (2,6-ArH) and 4.34 ppm (OCH3), sup-
porting the presence of a single homogeneous environment
for complexed bis(terpyridine) ligands, in contrast to linear
oligomers that have more complicated patterns.[14] Other
supportive data included an expected upfield shift in the
1H NMR spectrum for the 6,6’’-tpyH protons (d=7.27 ppm,
Dd=À1.48 ppm) relative to the corresponding peaks of
ligand 1 and one sharp peak in the 13C NMR spectrum at
d=57.3 ppm (OCH3). Its structure was further confirmed by
the intense ESI-MS peaks (Figure S2a in the Supporting In-
formation) at m/z 1382.7, 1000.4, 771.3, 618.6, 509.5, and
427.7, corresponding to [MÀnPF6]n+ ions (n=3–8), respec-
1
tively. The H NMR spectra of macrocycles 3 and 4 revealed
similar patterns, but all aromatic peaks exhibited a slight
downfield shift relative to the corresponding peaks in 2, pre-
sumably due to conformational changes in the larger rings.
The 4-ArH signal in pentamer 2 showed a significant upfield
shift (Dd=À0.13 ppm) in comparison with hexamer 3,
owing to the shielding by the adjacent pyridines in the more
crowded inner space (Figure S1b in the Supporting Informa-
tion). A similar phenomenon was observed in our previous
work.[7a] Heptamer 4 has more conformational flexibility,
minimizing this shielding effect; its 4-ArH signal was shifted
further downfield (Figure S1c in the Supporting Informa-
[a] Dr. Y.-T. Chan,+ Dr. X. Li,+ Dr. C. N. Moorefield,
Prof. Dr. C. Wesdemiotis, Prof. Dr. G. R. Newkome
Department of Polymer Science, Department of Chemistry
The University of Akron, 170 University Circle-RM501B
Akron, OH 44325-4717 (USA)
Fax : (+1)330-972-2368
E-mail: newkome@uakron.edu
wesdemiotis@uakron.edu
Homepage: http://www.dendrimers.com
tion). Macrocycles
3 (MW=5498.5 Da) and 4 (MW=
[+] Drs. Chan and Li contributed equally to this work.
6414.9 Da) were further verified by their ESI mass spectra
(Figure S2b and S2c in the Supporting Information). Hexa-
mer 3 gave rise to eight major peaks at m/z 1229.7, 954.7,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100559.
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COMMUNICATION
1
to 9+. The H NMR spectra of
heteronuclear metallomacrocy-
cles 7 and 8 showed similar pat-
terns to that of 6, but a signifi-
cant downfield shift (d=
8.71 ppm, Dd=+0.07 ppm) for
the inner 4-ArH protons rela-
tive to the corresponding peak
of 6 (Figures 1b and c). Lastly,
the structures of octamer 7 and
decamer 8 were established by
their distinctive ESI-MS spectra
(Figure 2b and c).
TWIM-MS and molecular
modeling were applied to vali-
date the structures of these dif-
ferent macrocycles. The ESI
and ion-mobility process were
optimized (see Supporting In-
formation) to reduce fragmen-
tation. Different charge states
of each macrocycle were sepa-
rated by ion mobility (Figure 3
and Figures S3–S7 in the Sup-
Scheme 1. a) FeII macrocycles 2–4 obtained by the macrocyclization of 1. b) RuII–FeII macrocycles 6–8 was sim-
porting Information), after
which mainly intact macrocy-
cles are observed. For this, the
ilarly obtained from 5. Reaction conditions: i) FeCl2·4H2O, MeOH, reflux, 18 h. (each macrocycle isolated as
the polyPF6À salt).
771.4, 640.5, 542.3, 465.9, 404.9, and 354.8, which correspond
to species with +4 to +11 charges, respectively; whereas,
heptamer 4 displayed nine intense peaks at m/z 1459.0,
1138.0, 924.1, 771.4, 656.8, 567.7, 496.5, 438.2, and 389.6,
which agree with multiply-charged entities having +4 to
+12 charges.
From a single-pot reaction of bis(terpyridine) ligand 1
with 0.5 equivalents of [RuCl2ACHTUNGTRENN(GU dmso)4], the [Ru(1)2ACHTUGNTREN(NGUN NO3)2]
dimer 5 was isolated in 29% yield after employing column
chromatography (basic Al2O3; eluent: H2O/MeCN/sat.-
KNO3(aq), 1:30:1 v/v/v). Reaction (Scheme 1b) of the
[Ru(1)2ACHTUNGTRENNUNG(NO3)2] dimer 5 with 1.05 equivalents of FeCl2·4H2O
in MeOH afforded hexamer (6; 36%), octamer (7; 9%),
and decamer (8; 2%), which were also isolated by column
chromatography (SiO2; eluent: H2O/MeCN/sat.KNO3(aq),
1:10:1 v/v/v). The alternating RuII–FeII sequence in the mac-
rocycles resulted in more complicated 1H NMR patterns
(Figure 1), exhibiting two sets (RuII and FeII complexes) of
1
Figure 1. 1H NMR spectra of a) hexamer 6, b) octamer 7, and c) decamer
8.
signals for the terpyridine units. The H NMR spectrum of
heteronuclear hexamer 6 exhibited four sharp singlets at d=
9.42 (FeII; 3’,5’-tpyH), 9.28 (RuII; 3’,5’-tpyH), 8.64 (4-ArH),
and 4.31 ppm (OCH3), supporting the symmetric macrocy-
clic architectures. Other proof included a diagnostic upfield
shift for the 6,6’’-tpyH proton signals (d=7.29 ppm, Dd=
À1.47 ppm) of FeII complexes and two sharp singlets at d=
8.14 and 8.08 ppm assigned to the 2,6-ArH protons resulting
from the neighboring RuII and FeII coordination. The hexag-
onal motif was further verified by the ESI-MS signals (Fig-
ure 2a) for the multiply-charged species in charge states 4+
potentials across the ESI source and ion entrance into the
vacuum system must be kept low (see Supporting Informa-
tion). Elevated potentials can cause fragmentation and ring-
opening.[8g,11,13a,b,15] In the present study, different cyclic con-
formers, instead of cyclic and ring-opened isomers,[8g,11,13a,b,15]
were observed for ions in specific charge states. For instance,
the 5+ ions of heptamer 4 contained two structures, drifting
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G. R. Newkome, C. Wesdemiotis et al.
contrast, multiple conformers
were observed for the 5+, 6+,
7+, and 8+ ions derived from
decamer 8. For the smallest
macrocycle, namely, 2, only one
conformer was observed due to
the low flexibility of this ring
size.
Experimental collision cross-
sections were obtained by cali-
brating the drift timescale[16] of
our TWIM device (Figure S8 in
the Supporting Information)
and are summarized in Table 1.
For the collision cross-sections
of the smallest macrocycle,
namely, the pentamer, no signif-
icant charge dependence was
found.
A similar observation
was recently reported by
Bowers et al. for small rigid tri-
angular and rectangular struc-
Figure 2. ESI-MS spectra of a) hexamer 6, b) octamer 7, and c) decamer 8.
Table 1. Experimental and calculated collision cross-sections (CCS) of
metallocycles.
Exptl CCS
[ꢁ2][a]
Average
Calcd CCS
ACHTUNGTRENNUNG
(STDEV) [ꢁ2][b]
exptl CCS [ꢁ2]
2
3
692.3 (3+)
679.4 (4+)
698.7 (3+)
725.9 (4+)
848.4 (4+)
813.4 (5+)
782.4 (4+)
846.3 (5+)
991.1 (5+)
930.9 (6+)
701.6 (3+)
821.5 (4+)
852.5 (4+)
806.9 (5+)
695.2 (5+)
873.2 (5+)
997.1 (5+)
964.9 (6+)
1040.2 (7+)
837.7 (5+)
1055.8 (5+)
1162.9 (5+)
993.7 (6+)
1199.3 (6+)
1131.6 (7+)
1457.6 (7+)
1236.8 (8+)
1407.3 (8+)
1309.8 (9+)
685.9
640.0 (4.8)
771.6
767.5 (6.8)
4
6
7
887.7
795.6
914.1
901.0 (10.6)
773.7 (6.0)
Figure 3. Two-dimensional ESI-TWIM-MS plot of heptamer 4, covering
charge states 4+ to 7+ and acquired at a traveling wave velocity of
350 msÀ1 and a traveling-wave height of 11 V. The insets show the drift
time distributions of 4+, 5+, and 6+ ions; the distribution of the 7+
ions is similar to those observed for the 4+ and 6+ charge states (unim-
odal).
1028.2 (13.2)
1195.0 (114.5)
8
1179.2
out at 2.05 and 2.82 ms; whereas, only a single structure was
observed for the ions with 4+, 6+, and 7+ charges
(Figure 3). If ring-opened isomers existed, distinct drift time
distributions for cyclic and linear species should have been
observed at all charge states, with the corresponding peaks
becoming increasingly resolved with the number of char-
ges;[8g,11] however, only ions at a few specific charge states
exhibited multiple peaks after ion-mobility separation. Thus,
it was concluded that cyclic conformers were being separat-
ed at these charge states. Two different conformers were ob-
served for 5+ ions from heptamer 4, as noted above, as
well as for 4+ ions from hexamers 3 and 6; whereas, three
conformers were detected for 5+ ions from octamer 7. In
[a] Collision cross-section (CCS) obtained by calibrating the drift time
scale of the TWIM device with standards of known cross-sectional data
(calibration plot shown in Figure S8). [b] Obtained from the energy-mini-
mized structures deduced computationally using the DriftScope 2.1 soft-
ware with the projection approximation; the corresponding standard de-
viations (STDEV) follow in parentheses.
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Polygonal Architectures
COMMUNICATION
tures.[15] The average collision cross-sections gradually in-
creased from pentamer to heptamer (Table 1). One charge
state of the hexamer (4+) and heptamer (5+) showed two
conformers with distinct cross-sections. When the macrocy-
cle size increased further to octamer or decamer, additional
conformers were detected. For example, the 5+ ions from
octamer 7 were dispersed into at least three conformers
with drift times of 1.62, 2.17, and 2.89 ms (Figure S6 in the
Supporting Information), leading to collision cross-sections
of 695, 873, and 997 ꢁ2, respectively.
Molecular modeling was employed to gain more informa-
tion on the precise geometries of the different macrocycles
and their cross-sectional areas. For each macrocycle, 150 or
300 structures were energy-minimized by annealing simula-
tions; the resulting collision cross-section versus relative
energy plots are shown in Figure 4 and Figures S9–S13 in
the mass of the counterions does not significantly affect the
cross-sections of the macrocycles.
Notably, three distinct areas, corresponding to three
major families of conformers, can be identified in the plot of
collision cross-sections versus relative energies for decamer
8 (Figure 4). Figure 4 includes the average collision cross-
section within each area (i.e., for each family of conform-
ers). The computational prediction for the existence of dif-
ferent conformers spanning a wide range of collision cross-
sections reconciles the observation of several conformers
upon ion-mobility separation. Even though the macrocycles
consist of rigid ligands, the flexibility of the whole complex
increases with size, leading to a wider range of possible geo-
metries and a higher number of experimentally detected
structures for the larger macrocycles. Expectedly, the flexi-
bility is largest for decamer 8, the size of which permits the
formation of substantially dif-
ferent conformers (circular,
twisted
stretched,
twisted
folded; Figure 4). For the other
macrocycles, the various con-
formers possible mainly differ
in compactness, all having cir-
cular shapes (Figures S9–S13 in
the Supporting Information).
Also, the ions in low charge
states display small collision
cross-sections. For instance, 3+
of 3, 4+ of 4, 3+ of 6, 5+ of
7, and 5+ of 8 gave the small-
est collision cross-section com-
pared to the other, higher
charge states of the same com-
plex (Table 1). This can be at-
tributed to the lower number of
charges resulting in lower
charge repulsion and thereby
leading to the formation of
more compact conformers. The
sizes of conformers should thus
become larger as the charge
states increase, as confirmed by
the cross-sectional data of
Table 1. In comparison with
crystal structures provided by
X-ray crystallography and hy-
Figure 4. Plot of collision cross-section versus relative energy for 300 structures of complex 8 generated by an-
nealing simulations. The average collision cross-section of all 300 structures is listed in Table 1. The average
collision cross-sections of the conformers within the encircled areas are given underneath the corresponding
structures.
the Supporting Information. The average collision cross-sec-
tion of all structures calculated for each macrocycle are in-
cluded in Table 1, along with the corresponding standard de-
viations, which are a measure of the range of conformations
possible. Note that the counterions were omitted to simplify
the modeling,[3d,17] since previous studies revealed that they
are highly disordered and difficult to locate accurately in
even small self-assembled constructs.[3c,h,10,18] Moreover, frag-
ments generated from a specific charge state by losses of
PF5 maintain almost the same drift time as their parent ions
(Figure S6b in the Supporting Information), indicating that
drodynamic radii obtained by diffusion-ordered NMR spec-
troscopy, TWIM-MS captures each conformer and provides
size information based on its charge state and drift time.
Even though NMR and ESI-MS confirm the composition
and connectivity of the different metallocyclic species syn-
thesized, the TWIM-MS technique affords unique and un-
ambiguous insight into the sizes, geometries, and conforma-
tional flexibilities of the corresponding cyclic architectures,
which would not otherwise be evident.
In summary, a series of homo- and heteronuclear metallo-
macrocycles with <tpy-FeII (or RuII)-tpy> connectivity has
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G. R. Newkome, C. Wesdemiotis et al.
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been successfully synthesized and characterized by means of
1H and 13C NMR spectroscopy as well as ESI mass spec-
trometry. Larger macrocycles (octamer and decamer) have
been achieved by macrocyclization of an extended bis(ter-
pyridine) dimer, thereby eliminating the potential to form
smaller cyclic structures. For each self-assembly process,
using monomers possessing 1208 opposed binding sites, the
hexagonal structures always predominate over the other
species, which we believe to be derived from the initially
formed less stable kinetic products. The combination of
TWIM-MS and molecular modeling unambiguously pro-
vides size and structure data, as well as monitors the gradual
change of flexibility from small to large macrocycles.
Experimental Section
Full experimental details are provided in the Supporting Information.
Acknowledgement
The authors gratefully thank the National Science Foundation (DMR-
0812337 and DMR-0705015 to G.R.N.; CHE-1012636 and DMR-0821313
to C.W.) and the Ohio Board of Regents for financial support.
Keywords: conformer separation
· macrocycles · mass
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Received: February 19, 2011
Published online: May 30, 2011
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