Self-assembly of giant supramolecular cubes with terpyridine ligands as vertices and metals on edges

Article abstract of DOI:10.1039/c3sc52965g

Self-assembly of three-dimensional (3-D) architecture using terpyridine (tpy)-based building blocks is challenging and seldom addressed due the fixed geometry (around 180°) of tpy-M(ii)-tpy (M = Ru, Fe, Zn, and Cd) connectivity. Here we describe the self-

Full text of DOI:10.1039/c3sc52965g

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Self-assembly of giant supramolecular cubes with
terpyridine ligands as vertices and metals on edges†
Cite this: Chem. Sci., 2014, 5, 1221
a
b
a
c
c
a
Chao Wang,‡ Xin-Qi Hao,‡ Ming Wang,* Cunlan Guo, Bingqian Xu, Eric N. Tan,
d
d
e
e
b
Yan-Yan Zhang, Yihua Yu, Zhong-Yu Li, Hai-Bo Yang, Mao-Ping Song
and Xiaopeng Li*
a
Self-assembly of three-dimensional (3-D) architecture using terpyridine (tpy)-based building blocks is
ꢀ
challenging and seldom addressed due the fixed geometry (around 180 ) of tpy-M(II)-tpy (M ¼ Ru, Fe,
Zn, and Cd) connectivity. Here we describe the self-assembly of 3-D giant metallo-supramolecular
cubes using three-armed terpyridine ligands constructed on adamantane with molecular weight up to
1
8 k and edge length at ꢁ4.9 nm, which is significantly larger than the sizes of previous metallo-
supramolecular cubes. Instead of using metal center as vertices in the commonly used synthetic strategy
of 3-D molecular coordination ensembles, these cages [M12 ] bear 8 ligands as vertices with 12 metal
Received 26th October 2013
Accepted 21st November 2013
L
8
ions on the edges. With a suitable edge length, the giant cubes appear to be the sole product after self-
assembly from a variety of possible architectures. The 3-D metallo-supramolecules were characterized
and supported by NMR, DOSY, ESI-MS, travelling wave ion mobility-MS and AFM.
DOI: 10.1039/c3sc52965g
www.rsc.org/chemicalscience
act as catalysts or molecular asks for reactions that t the
shape of the conned space. The microenvironment inside 3-D
Introduction
5
The spontaneous self-assembly of one or more subunits is structures can also enhance intermolecular interactions and
6,7
common in nature to create different biological systems with stabilize reactive intermediates. While the initial focus was on
1
well-dened architectures. Due to the highly directional and the encapsulation of small molecules or ions, recent research
predictable feature of the metal coordination, metal-mediated efforts have shied to designing larger 3-D architectures that
self-assembly acts as a powerful tool to mimic nature's activi- are capable of encapsulating inorganic catalysts, larger
2
8
ties. This has been used as a chemical approach for construc- substrates, and even small proteins.
0 0 00
tion of a series of 3-D structures with precise geometries and
2,2 :6 ,2 -Terpyridine (tpy)-based subunits attract consider-
3,4
sizes. Among these 3-D structures, in particular for Pt(II) and able attention in the coordination-driven approach for supra-
Pd(II)-mediated self-assembly with pyridyl ligands, the coordi- molecular self-assemblies, due to their inherent ability to bind
nation geometry of the metal ions and angle of ligands plays a different transition metal ions strongly and the relative ease for
9
vital role in guiding the topologies of the nal architectures. In constructing multi-nuclear complexes. However, the connec-
most circumstances, end-capped metal components or naked tivity of tpy-M(II)-tpy (M ¼ Ru, Fe, Zn, and Cd) is generally xed
ꢀ
metal ions were used as corner units and organic ligands were at 180 and thus, limits the use of metal ions as cornered
incorporated into the edges. Like enzymes, these 3-D structures directing units. Therefore, many previous studies of tpy-M(II)-
tpy only concentrated on linear and 2-D structures compared to
9,10
the numerous reports of 3-D structures based on pyridine. Up
to date, a few 3-D supramolecular cages and prisms were
reported using tpy-based building blocks to the best of our
a
Department of Chemistry and Biochemistry, Texas State University, San Marcos, TX
7
b
8666, USA. E-mail: X_L8@txstate.edu; M_W134@txstate.edu
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou
50052, P. R. China
11
4
c
knowledge. We herein, for the rst time, utilize tpy-based
Single Molecule Study Laboratory, College of Engineering and Nanoscale Science and organic ligand built on adamantane as directing unit in the
Engineering Center, University of Georgia, Athens, GA 30602, USA
d
vertices to construct giant 3-D supramolecular cubes possessing
highly symmetric structure with metal ions on edges, which
Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China
Normal University, Shanghai 200062, P. R. China
6
could be potentially useful in cavity-templated synthesis and
e
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of
7
guest encapsulation.
Chemistry, East China Normal University, Shanghai 200062, P. R. China
A strategic design with coordination-driven self-assembly of
†
Electronic supplementary information (ESI) available: Synthetic procedure,
8
characterization data (NMR, ESI-MS, UV-vis, PL, AFM) for the cubic cages and the supramolecular cube [M12L ] (Fig. 1) is outlined in Scheme
details of CCS calculations and molecular simulations. See DOI: 1. The construction of the cubic cages only required 12 metal
1
0.1039/c3sc52965g
ions and 8 three-armed building blocks. The three-armed
‡
These authors contributed equally.
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demonstrated theoretically and experimentally that it is
possible for rigid aromatic rings to bend to a certain small
13
angle. Unpredicted structures oen appear because organic
ligands are much more exible than expected and metal centers
can also permit considerable deviation in their coordination
14
angles. Hence, instead of forming a large dodecahedron with
ꢀ
1
08 angle, we predicted that the cubic geometry as the smaller
ensemble can be formed because the linker between ada-
mantane and terpyridine may provide enough exibility to
accommodate the strain of the three-armed ligand. We
hypothesize that these symmetric cubic cages would be more
thermodynamically stable than other assembles (e.g., dodeca-
hedron) if an appropriate length of linker between adamantane
and terpyridine was applied.
8
Fig. 1 ESI mass spectrum for [Zn12L1 ]. The different charge states
ions were derived from losing different numbers of counter-ions.
Results and discussion
Synthetic procedures
Under this hypothesis, L1 was chosen in our initial efforts as the
candidate ligand, which has the same structure with L2 but
1
5
without bulky t-butyl groups. 1,3,5-Triphenyl-adamantane,
16
0
1
,3,5-tri(4-iodophenyl)-adamantane
2 and 4 -(4-boronato-
phenyl)-[2,2 :6 ,2 ']terpyridine B1 were prepared according to
literature procedure. Ligand L1 was synthesized from B1 and 2
0
0
0
10b
by Suzuki coupling reaction using Pd
2
(dba)
3
and P(t-Bu)
3
as
catalyst. Although the self-assembly with Zn12L1
8
composition
was observed by ESI-MS (see Fig. 1), the low solubility of both
ligand and complex obstructed further characterization by
common techniques such as NMR to unambiguously prove the
cubic structure. Therefore, the three-armed ligand L2 was
redesigned to bring bulky t-butyl moieties to increase the
solubility of the assemblies and simplify the NMR spectra. Note
that the bumpy baseline of the mass spectrum in Fig. 1 is
mainly because of the low intensity of signal due to the low
8
solubility of M12L1 and the encapsulation of solvent or salt
molecules inside the cube.
Boronic acid B2 was synthesized from 4-formyl-phenyl-
boronic acid and 2-acetyl-4-t-butylpyridine. Ligand L2 was
synthesized similarly as L1 by Suzuki coupling of B2 and 2. Due
8
Scheme 1 Synthetic route to the ligands and [M12L ].
to the existence of bulky t-butyl groups, not only the solubility of
1
ligand was designed by incorporating three tpy groups into rigid L2 had increased signicantly, but also the H NMR was easily
ꢀ
0
adamantane core, which provides 109 28 geometry based on discernible compared to the ligand without t-butyl groups (see
3
sp hybridization at its points of bisection. When the three- ESI†). Ligand L2 was fully characterized by NMR and ESI-MS. In
1
armed ligands and metal ions are mixed together with the the aromatic region of H NMR (Fig. 2), there was only one set of
correct ratio (2 : 3) in a non-coordinating solvent, these protons for tpy and a sharp singlet at 1.48 ppm that was
components will go through exchange processes of possible assigned to the bulky t-butyl groups.
intermediate states and nally reach the most thermodynami-
cally stable states. Based on the geometry of Platonic solids, we octahedral metal edges come together to simultaneously form a
In one simple step, 8 three-armed ligand ‘corners’ and 12
12
reasoned that both cube and dodecahedron could be formed giant cubic cage with large cavity. A stoichiometric ratio (2 : 3)
ꢀ
using this ligand with high possibility. Entropically, structures of L2 and Cd(NO
of smaller size are more favourable (i.e., cube). However, a for 2 h, followed by the addition of excess of NH
smaller self-assembly like cube structure would require the white precipitate. Aer a thorough washing with water,
3
)
2
$4H
2
O were mixed in MeOH–CHCl
3
at 60 C
4
PF
6
to give a
ꢀ
2+
arms of the ligand to bend to at least a mutual 90 angle as they [Cd L2 ] was isolated (90%). The related Zn -based structure
1
2
8
are bonded to metal ions. This seems unlikely to happen since was assembled following the same procedure but using
ꢀ
0
ꢀ
2+
2+
the twisting of those arms from 109 28 angle to 90 angle will Zn(NO
3 2 2
) $6H O instead. Both the Cd - and Zn -based struc-
produce a strained substructure. Nevertheless, it has been tures were soluble in MeCN.
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observed in DOSY was close to [Cd12L2
8
] (Fig. S29†). Experi-
mental radii of 2.9 nm and 3.0 nm were determined for
Cd L2 ] and [Zn L2 ], respectively, in close agreement with
[
1
2
8
12
8
values obtained from molecular modelling (3.2 nm).
Characterization by mass spectrometry
8 8
[Cd12L2 ] and [Zn12L2 ] were further characterized by ESI-MS
coupled with travelling wave ion mobility-mass spectrometry
18
(
TWIM-MS) to investigate the proposed structures, which have
a molecular weight of 17813.6 Da and 17393.4 Da, respectively.
In ESI-MS, a series of peaks with charge states from 11+ to 21+
were detected (due to the loss of a different number of coun-
ꢂ
terion PF
6
) (Fig. 4A), and the isotope pattern of each peak with
1
the corresponding simulated isotope pattern is shown in
Fig. 2 H NMR spectra (400 MHz) of ligand L2 in CDCl
3
and [Cd12L2
8
]
in CD
3
CN.
Fig. S2.† Notably, the isotope pattern observed for the highly
2
1+
charged [Cd12L2
8
]
ion is in good accordance with the theo-
retical distribution (see Fig. S2†).
1
In order to separate any superimposed fragments and
examine the possible existence of overlapping isomers or
conformers, TWIM-MS was used as the advanced level of MS
analysis. Note that TWIM-MS provides information on the
molecules' shapes and sizes based on collision cross-section
The H NMR of [Cd12L2
8
] (Fig. 2) shows a simple pattern of
the expected peaks for a tpy–metal complex. In the aromatic
region, there were eight sets of aromatic protons from tpy units
and phenyl groups, which were consistent with the desired
0
0
structure. The protons at 6,6 position of tpy dramatically
18
(CCS), in addition to mass and compositional information. As
shied upeld (Dd ¼ 0.73 ppm) due to the electron shielding
1
shown in Fig. 4B, each charged state of the newly formed cube
effect aer complexation with metal ions. The broad H signals
were indicative of a very large complex due to the slow tumbling
4a
motion on the NMR time scale. Similarly, Fujita, Nitschke and
1
Schalley et al. observed broad H signals for 3-D complexes,
17
especially for assembles with more than 10 000 Da. Note that
1
the full assignments of H NMR shown in Fig. 1 are based on
1
3
2
D-COSY (see Fig. S22†). The C NMR of [Cd L2 ] showed only
12 8
one series of sharp peaks due to the high symmetric architec-
ture (see Fig. S20†), denoting the absence of any by-products
and uncomplexed tpy moieties. HSQC spectrum was consistent
with this giant cube as the sole product (see Fig. S23†). In DOSY
(
Fig. 3), the observation of a distinct band at log D¼ ꢂ9.708
1
conrmed the single product formation. H NMR pattern of
2
+
[Zn12L2
8
] was slightly different from Cd -based architectures
(
see Fig. S28†). Similarly, the log D ¼ ꢂ9.725 of [Zn L2 ]
1
2
8
Fig. 4 (A) ESI mass spectrum and (B) 2D ESI-TWIM-MS plot (m/z vs.
8 3
Fig. 3 2D-DOSY NMR (500 MHz) of [Cd12L2 ] in CD CN.
8
drift time) for [Cd12L2 ].
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2
+
[
8
Cd12L2 ], for Cd -based complexes has been detected with Simulation by molecular modelling agreed well with structural
narrow dri time distribution, indicating the absence of other information calculated from TWIM-MS. Theoretical CCSs for 70
isomers or structural conformers. From the TWIM-MS data, the candidate structures of the [Cd L2 ] cube (obtained from
1
2
8
CCSs, which correspond to the sizes of analytes, can be calcu- molecular mechanics/dynamics simulations) were calculated
19
lated to provide further evidence. Currently, there is no suit- using the trajectory (TJ), projection approximation (PA) and
22
able metallo-supramolecular calibrants spanning the size range exact hard sphere scattering (EHSS) methods. There were only
of the cubes under study. Therefore, we used denatured slight uctuations of theoretical CCSs among these 70 candi-
proteins as calibrants for calibration, which is based on the date structures, indicating a highly rigid structure. Among
15–17
correlation between dri time and collision cross section.
them, TJ method considered both long-range interactions and
Since the collision cross sections of denatured proteins were momentum transfer between the ions and the gas in the ion
well documented, the calibration should not introduce signi- mobility region, hence giving the most reliable CCS predic-
2
2
˚
2
cant deviation to experimental collision cross section calcula- tion. TJ result showing theoretical CCS at 2797.4 ꢃ 50.0 A
tions. The CCSs for the cubic ions in different charge states are agreed best with the average experimental CCS (i.e., 2809.5 ꢃ
2
˚
shown in Table 1. The observed values of charges changed from 26.7 A ) for charge states from 13+ to 21+ (Table 1), and an
1
3+ to 21+ for [Cd12L2
8
] are consistent with a small standard underestimated value (by 1%) is neglectable from the
2
˚
deviation, viz., 2809.5 ꢃ 26.7 A , suggesting these 3-D giant measurement errors.
cubic cages are shape-persistent. In contrast, the previous study
Absorption and emission properties of complexes were also
of supramolecular macrocycles shows signicant differences in tested (see Fig. S7–S10†). The ligand L2 in dilute CHCl3
CCSs for different charge states, indicating the existence of exhibited ligand-centered p–p* transitions at 296 nm. In
different conformers due to the repulsion between char- contrast, both [Cd L2 ] and [Zn L2 ] in dilute MeCN showed
1
2
8
12
8
18b,c,20
ges.
shape-persistent than macrocycles, because their structures are that were assigned to intra-ligand charge transfer (ILCT).
not twisted and their CCSs are unaffected by the charges. [Cd12L2 ] and [Zn12L2 ] showed photoluminescence (PL) emis-
8 8
Thus, [Cd12L2 ] and [Zn12L2 ] must be more rigid and two characteristic absorption peaks around 285 and 332 nm
23
8
8
Similarly, Bowers and co-workers have also reported a series of sion around 450 nm, with an excitation wavelength of 285 nm.
rigid prism structures whose CCSs are insensitive to charge
21
states. These results clearly demonstrate TWIM-MS, as an
advanced MS technique, not only probing different ions based
on mass, charge and shape, but also quantitatively and accu-
rately measuring the rigidity of these species through the
dependence of the CCS on the charge state.
Atomic force microscopy (AFM)
As additional evidence, the images from AFM show the
morphology of the giant cubic complex [Cd12L2 ] as dots on the
8
mica surface (Fig. 5A). Some of the dots displayed cube-like
shape (Fig. S29†). The measured height of these dots exhibited
two different values: 5.22 ꢃ 0.51 nm and 7.30 ꢃ 0.41 nm
Simulation and optical physical properties
(Fig. 5B), which are close to edge length (4.9 nm) and corner–
8
All efforts to grow single crystals of [M12L ] for an X-ray
corner length (6.4 nm) shown in molecular modelling (Fig. 5C),
diffraction study were unsuccessful, likely due to the large size
and cavity with highly disordered solvent molecules and coun-
terions. Therefore, to get a rough picture of the tetrahedron,
molecular modelling was performed to correlate the shape and
size information obtained from IM-MS. Representative energy-
minimized architectures are shown in Fig. 4C and S6.†
respectively. Due to the unavoidable tip broadening effect, the
Table 1 Experimental and theoretical Collision Cross Sections (CCSs)
of giant cubic cage [Cd12L2 ]
8
2
˚
CCS (A )
Z
Exptl
Exptl avg
Calcd avg
a
1
1
1
1
1
1
1
2
2
3+
2853.8
2804.3
2801.5
2799.0
2849.0
2785.3
2786.9
2823.0
2782.7
2809.5 (26.7)
2523.9 (5.5),
b
4+
5+
6+
7+
8+
9+
0+
1+
2797.4 (50.0),
c
2919.8 (8.2)
8
Fig. 5 (A) AFM image of [Cd12L2 ] on mica surface. (B) Statistical
PA value obtained using MOBCAL. TJ value obtained using MOBCAL. histogram of AFM for 100 particles. (C) Representative energy-mini-
a
c
b
EHSS value obtained using MOBCAL.
mized architecture of [Cd12L28].
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measured width of the dots in AFM image displayed large
D. Fenske, Angew. Chem., Int. Ed., 1996, 35, 1838; (c)
J.-M. Lehn, Chem. Soc. Rev., 2007, 36, 151.
24
values. Furthermore, molecular modelling shows the structure
distortion of the cube is mostly neutralized by bending the two
phenyl groups between adamantane and terpyridine. Therefore,
the ligand bending is the major contribution to accommodate
the strain. This is consistent with the bended ligand in distorted
3 (a) R. Chakrabarty, P. S. Mukherjee and P. J. Stang, Chem.
Rev., 2011, 111, 6810; (b) M. Fujita, M. Tominaga, A. Hori
and B. Therrien, Acc. Chem. Res., 2005, 38, 369; (c)
S. J. Dalgarno, N. P. Power and J. L. Atwood, Coord. Chem.
Rev., 2008, 252, 825.
13
triangles as reported by other groups. Finally, this giant cube
with 4.9 nm edge length is signicantly larger than the previ-
ously reported metallo-supramolecular cubes, which all have
end-capped metal components as vertices and ligands as
4 For recent examples, see: (a) Q.-F. Sun, J. Iwasa, D. Ogawa,
Y. Ishido, S. Sato, T. Ozeki, Y. Sei, K. Yamaguchi and
M. Fujita, Science, 2010, 328, 1144; (b) Y.-R. Zheng, Z. Zhao,
M. Wang, K. Ghosh, J. B. Pollock, T. R. Cook and
P. J. Stang, J. Am. Chem. Soc., 2010, 132, 16873; (c)
C. J. Hastings, M. D. Pluth, R. G. Bergman and
K. N. Raymond, J. Am. Chem. Soc., 2010, 132, 6938; (d)
O. Chepelin, J. Ujma, X. Wu, A. M. Z. Slawin, M. B. Pitak,
S. J. Coles, J. Michel, A. C. Jones, P. E. Barran and
P. J. Lusby, J. Am. Chem. Soc., 2012, 134, 19334; (e) S. Freye,
J. Hey, A. Torras-Gal ´a n, D. Stalke, R. Herbst-Irmer, M. John
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W. Meng, T. K. Ronson and J. R. Nitschke, Proc. Natl. Acad.
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B. Grimm, R. M. Krick Calderon, C. G. Claessens,
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G. H. Clever, S. Tashiro and M. Shionoya, J. Am. Chem.
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and J. R. Nitschke, Angew. Chem., Int. Ed., 2013, 52, 9027.
25
edges.
Conclusions
In summary, we were able to employ, for the rst time, terpyr-
idine ligands as corner directing units and naked metal ions as
edge gluing elements to produce giant supramolecular cubic
architectures. By careful design of the arm length versus its
mutual angle, the structurally controlled cubic cages have been
0
0
00
prepared stoichiometrically from complementary 2,2 :6 ,2 -ter-
pyridine ligands and metal ions. To the best of our knowledge,
these are hitherto the largest cubic shape self-assemblies. DOSY
NMR and AFM prove the remarkable size of cube. TWIM-MS
unambiguously measures the molecular weight and elemental
compositions, and provides profound insight into the particles'
shapes, sizes, and the rigidity of the complexes. Through
changing the length, angle and rigidity of adamantane-based
terpyridine ligand, we may obtain discrete trigonal bipyramidal
like dimer (M L ), tetrahedron (M L ) and dodecahedron
3
2
6 4
30
(M L20) from Platonic solids in the future study. More impor-
tantly, this series of 3-D architectures with different sizes of
internal cavities and face windows may become an ideal system
for host–guest chemistry study.
Acknowledgements
5
(a) M. Yoshizawa, M. Tamura and M. Fujita, Science, 2006,
312, 251; (b) C. J. Hastings, M. D. Pluth, R. G. Bergman
and K. N. Raymond, J. Am. Chem. Soc., 2010, 132, 6938.
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K. Takao, K. Suzuki, T. Ichijo, S. Sato, H. Asakura,
K. Teramura, K. Kato, T. Ohba, T. Morita and M. Fujita,
Angew. Chem., Int. Ed., 2012, 51, 5893.
X. L. gratefully acknowledges the support from the Research
Enhancement Program of Texas State University, the Welch
Foundation (AI-0045), and the REU support from NSF (CHE-
1
2
156579) to Eric N. Tan. X.-Q. H. thanks the NSFC/China (no.
1102135) for nancial support. H.-B. Y. thanks the NSFC/
China (no. 21132005 and 91027005) and the SMSTC (no.
3JC1402200) for nancial support.
1
7
(a) A. Sørensen, A. M. Castilla, T. K. Ronson, M. Pittelkow
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1226 | Chem. Sci., 2014, 5, 1221–1226
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