Finely Resolved Threshold for the Sharp M12L24/M24L48 Structural Switch in Multi-Component MnL2n Polyhedral Assemblies: X-ray, MS, NMR, and Ultracentrifugation Analyses

Article abstract of DOI:10.1002/asia.201500519

In the self-assembly of M_nL_2n polyhedra, the bend angle (θ) of the divalent ligand components determines the final structure. The threshold for the sharp structural switch between M₁₂L₂₄ and M₂₄L₄₈ was finely resolved to within just 4 by demonstrating the exclusive formation of M₁₂L₂₄ cuboctahedra or M₂₄L₄₈ rhombicuboctahedra from two similar ligands with θ values of 130 and 134. This sharp structural switch was fully confirmed by X-ray crystallography, mass spectrometry, NMR spectroscopy, and ultracentrifugation analyses.

Full text of DOI:10.1002/asia.201500519

DOI: 10.1002/asia.201500519
Full Paper
Supramolecular chemistry
Finely Resolved Threshold for the Sharp M12L24/M24L48 Structural
Switch in Multi-Component MnL2n Polyhedral Assemblies: X-ray,
MS, NMR, and Ultracentrifugation Analyses
Hiroyuki Yokoyama, Yoshihiro Ueda, Daishi Fujita, Sota Sato, and Makoto Fujita*[a]
Abstract: In the self-assembly of MnL2n polyhedra, the bend
angle (q) of the divalent ligand components determines the
final structure. The threshold for the sharp structural switch
between M12L24 and M24L48 was finely resolved to within just
48 by demonstrating the exclusive formation of M12L24 cu-
boctahedra or M24L48 rhombicuboctahedra from two similar
ligands with q values of 1308 and 1348. This sharp structural
switch was fully confirmed by X-ray crystallography, mass
spectrometry, NMR spectroscopy, and ultracentrifugation
analyses.
Introduction
The self-assembly of giant structures from a large number of
small components is currently one of the most exciting chal-
lenges in chemistry, and allows the bottom-up control of
chemical structures on the nanoscale.[1] Metal–ligand self-as-
sembly provides a highly efficient and powerful approach to
discrete giant structures, and several groups have been inten-
sively studying the self-assembly of coordination polyhedra
with framework topologies that are described by Platonic or
Archimedean solids.[2–4] From four-coordinate metals (M) and
divalent bridging ligands (L), a series of MnL2n regular/semi-reg-
ular polyhedra, in which four edges meet at every vertex, can
be formed with geometrically restricted n values of 6, 12, 24,
30, and 60.[5] The most important structural parameter that de-
termines the n value is the bend angle (q) of the ligand com-
ponent, and we have previously shown that q values below
1278 give M12L24 (n=12) structures, whereas those above 1358
result in M24L48 (n=24) structures. We have not observed any
mixtures of the two structures when using ligands with q
values over a range of roughly 120–1508.[6] To finely resolve
the threshold of the sharp structural “switch”,[7] we synthesized
ligands 1 and 2, which have q values that are very close to
each other (1308 and 1348 for 1 and 2, respectively). To our
surprise, despite such a small difference in the q values, 1 and
2 exclusively assembled into M12L24 and M24L48, respectively
(Figure 1). This indicates that there is a very distinct threshold
Figure 1. Self-assembly of bent ligands 1 and 2 into MnL2n polyhedra, 3
(n=12) and 4 (n=24), with cuboctahedral and rhombicuboctahedral sym-
metry, respectively.
for the q value, and this is reminiscent of the phase transition
of some bulk materials. The very sharp structural switch ob-
served in this study was fully confirmed by combining reliable
analysis methods: X-ray crystallography, NMR spectroscopy,
mass spectrometry, and ultracentrifugation analyses.
Results and Discussion
Bent bridging ligands 1 and 2 were easily synthesized in a few
steps using Suzuki–Miyaura cross-coupling (for 1) or Ullmann
condensation (for 2) as key reactions (for details, see the Sup-
porting Information). The bend angles of 1 and 2 were deter-
mined from DFT (B3LYP/6-31G*) calculations.[8] When 1 (q=
1308) was treated with [Pd(CH3CN)4](BF4)2 in [D6]DMSO at 708C
for 3 days, the formation of a single product was indicated by
1H NMR spectroscopic analysis (Figure 2a). The downfield shifts
[a] H. Yokoyama, Dr. Y. Ueda, Dr. D. Fujita, Dr. S. Sato, Prof. Dr. M. Fujita
Department of Applied Chemistry
Graduate School of Engineering
The University of Tokyo, and ACCEL
Japan Science and Technology Agency (JST)
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: mfujita@appchem.t.u-tokyo.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500519.
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sharply switches from M12L24 to
M24L48 at a threshold that lies be-
tween 1308 and 1348.
The sharp structural switch
between 3 and 4 was more care-
fully confirmed by analytical ul-
tracentrifugation (AUC) measure-
ments,[12] which have recently
been applied to the estimation
of the dispersity and average
molecular weight of metal-
ligand
self-assembled
com-
plexes.[13] AUC sedimentation ve-
locity measurements clearly
showed the high monodispersity
of 3 and 4 in solution with dis-
tinguishable sedimentation coef-
ficients of 1.58 and 2.34 (Fig-
ure S22, Supporting Informa-
Figure 2. 1H NMR spectra (1D and DOSY) of (a) 3 and (b) 4 (500 MHz, [D6]DMSO, 300 K). All edges (ligands) are
equivalent in cuboctahedral complex 3, whereas there are two different edge (ligand) positions in rhombicubocta-
hedral complex 4.
of the PyHa and PyHb signals (DdPyHa=0.76 ppm, DdPyHb=
0.63 ppm) are characteristic of the coordination of the pyridine
rings to the PdII ion. Observation of only one set of four signals
indicates the equivalency of all the ligands in the complex; this
is consistent with a complex of cuboctahedral symmetry in
which all edges are equivalent. Diffusion-ordered NMR spec-
troscopy (DOSY) showed the formation of a single product
with a diffusion coefficient (D) of 5.0”10À11 [m2 sÀ1] (logD=
À10.30) at 300 K in [D6]DMSO. This is comparable to the D
values of previously reported cuboctahedral M12L24 com-
plexes.[6,9] The product was therefore deduced to be cubocta-
hedron 3.
tion). From these coefficients, the hydrodynamic diameters of
3 and 4 were calculated to be 3.1 and 5.7 nm, respectively.
These values are consistent with those estimated from molecu-
lar modeling of 3 and 4 (3.3 and 4.3 nm, respectively).
Finally, X-ray crystallographic analysis was carried out on 3
and 4 to unambiguously confirm the M12L24 and M24L48 frame-
works. Single crystals of 3 and 4 suitable for synchrotron X-ray
crystallography were obtained by the very slow vapor diffusion
À
of ethyl acetate into DMSO solutions of 3 (BF4 salt) and 4
(SbF6À salt). Structural refinements revealed the M12L24 cubocta-
Ligand 2 (q=1348) was treated with Pd(SbF6)2 in [D6]DMSO
at 708C for 3 h, and the product was analyzed by NMR spec-
troscopy. Despite having a very similar q value to 1, 1D NMR
and DOSY experiments showed that the self-assembled prod-
uct was not a cuboctahedron, but instead rhombicuboctahe-
dral complex 4. In the 1D NMR spectrum, two sets of signals
were observed in a ratio of 1:1, which is consistent with rhom-
bicuboctahedral symmetry (Figure 2b). All the signals were
fully assigned to Ha-Hf and Ha’-Hf’ of the two ligands based on
COSY, NOESY, and HSQC studies (see the Supporting Informa-
tion). A DOSY experiment confirmed the generation of only
one species with a logD value of À10.45; this is nearly identical
to those of other M24L48 complexes.[6,11]
The molecular weights and formulae of complexes 3 and 4,
as well as their exclusive formation, were clearly confirmed by
cold-spray ionization mass spectrometry (CSI-MS)[11] (Figure 3).
From the solution of 3, a series of prominent peaks for
n+
[Pd12(1)24(BF4)24-n
]
(n=10–13) were observed with very high
resolution (>25000), from which the molecular weight of 3
was determined to be 8982.46 Da. However, no peaks were
observed that could be assigned to an M24L48 structure. In con-
trast, CSI-MS of 4 revealed a series of peaks corresponding to
n+
[Pd24(2)48(SbF6)48-n
]
(n=16–19, resolving power: >22000),
À
Figure 3. CSI-MS of (a) 3 (BF4À salt) and (b) 4 (SbF6 salt). A series of promi-
from which the molecular weight was determined to be
24537.78 Da. These mass spectrometric data support the NMR
results, and show that the self-assembled polyhedral structure
nent peaks for M12L24 complex 3 and M24L48 complex 4 were observed. The
insets show the observed and simulated isotopic patterns of the
(a) [MÀ12(BF4À)]12+ and (b) [MÀ18(SbF6À)]18+ ions.
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lected at 300 K and the chemical shift values reported here are
with respect to an internal TMS standard for CDCl3 and to a residual
solvent signal for [D6]DMSO. Melting points were determined on
a Yanaco MP-500V melting-point apparatus. CSI-TOF mass spectra
were measured on a Bruker maXis instrument. The data analysis of
CSI-TOF mass spectra were processed using the Bruker Data Analy-
sis (Version 4.0 SP 2) software and the simulations were performed
using the Bruker IsotopePattern software. IR measurements were
carried out as KBr pellets using a DIGILAB FTS-7000 instrument.
Column chromatography was performed with silica gel 60N (spher-
ical, neutral) purchased from Kanto Chemical Co. Inc. All the chemi-
cals were of reagent grade and used without any further purifica-
tion.
Synthesis of 4,6-bis(4-pyridyl)pyrimidine 1
A mixture of 4,6-diiodopyrimidine (995 mg, 3.00 mmol), [1,1’-bis(di-
phenylphosphino)ferrocene]
dichloropalladium(II)
(439 mg,
0.600 mmol), and 4-pyridyl boronic acid (1.84 g, 15.0 mmol) was
dissolved in 1,4-dioxane (22.5 mL) and 2mK3PO4 solution (9 mL)
was added under argon atmosphere. After stirring at 808C for
2 days, 1,4-dioxane was evaporated in vacuo, and the residue was
extracted with chloroform. The organic layer was washed with
brine, dried over anhydrous Na2SO4, filtrated, and evaporated in
vacuo. The crude product was purified by column chromatography
on silica gel (ethyl actate/methanol=10:1) and GPC (gel permea-
tion chromatography) to give 4,6-bis(4-pyridyl)pyrimidine (149 mg,
0.634 mmol) as a brown solid in 21% yield.
Figure 4. X-ray crystal structures of (a) M12L24 complex 3 and (b) M24L48 com-
plex 4. Ligands and Pd ions are shown as stick and ball representations, re-
spectively. Counterions and solvents are omitted for clarity. The unsymmetri-
cal pyrazole core of ligand 2 is disordered over two positions.
m.p. 1758C; high-resolution ESI-TOF-MS m/z calcd for [C14H10N4
([M+H]+)]: 235.0979, found 235.0978 (error=0.4 ppm); 1H NMR
(500 MHz, CDCl3, 300 K): d=9.46 (s, 1H), 8.86 (d, J=6.0 Hz, 4H),
hedral and M24L48 rhombicuboctahedral structures of 3 and 4,
respectively (Figure 4). These structures are fully consistent
with the NMR, mass spectrometric, and AUC experiments.
1
8.21 (s, 1H), 8.03 (d, J=6.0 Hz, 4H); H NMR (500 MHz, [D6]DMSO,
300 K): d=9.50 (s, 1H), 8.91 (s, 1H), 8.85 (d, J=8.0 Hz, 4H),
8.32 ppm (d, J=8.0 Hz, 4H); 13C NMR (125 MHz, CDCl3, 300 K): d=
163.2 (C), 159.8 (CH), 151.0 (CH), 143.7 (C), 121.1 (CH), 113.5 ppm
(CH); IR (KBr): n˜ =3084, 3037, 1956, 1576, 1507, 1458, 1411, 1370,
1309, 1256, 1213, 1174, 1061, 989, 839, 773, 704, 626, 548, 516,
Conclusions
In conclusion, we finely resolved the sharp structural switch
between M12L24 cuboctahedral and M24L48 rhombicuboctahe-
dral complexes formed from bent ditopic ligands to within just
48. We observed the exclusive formation of an M12L24 cubocta-
hedron from the self-assembly of 1 (q=1308) and an M24L48
rhombicuboctahedron from the self-assembly of structurally
very similar 2 (q=1348). The small initial difference in the
ligand structure (Dq=48) is amplified during the multicompo-
nent self-assembly and results in a large energy difference in
the final self-assembled structures. Therefore, a unique final
structure is formed even though that structure is only slightly
favored in the initial self-assembly steps. The observed distinct
M12L24/M24L48 structural switch can be compared to the phase
transition of some bulk materials.[14] Our observation may thus
demonstrate that the phase transition behavior of bulk materi-
als can be modeled with as few as about 50 components.
421 cmÀ1
.
Synthesis of 1,3-bis(4-pyridyl)pyrazole 2
A mixture of 3-(4-pyridyl)pyrazole (436 mg, 3.0 mmol), 4-iodopyri-
dine (616 mg, 3.0 mmol), Cs2CO3 (1.96 g, 6.0 mmol), salycylaldoxime
(164 mg, 1.2 mmol), and Cu2O (50.6 mg, 0.35 mmol) was dissolved
in dry DMF (5 mL) under argon atmosphere. After stirring at 110 8C
for 1 day, DMF was evaporated in vacuo, and the residue was ex-
tracted with ethyl acetate. The organic layer was washed with
brine, dried over anhydrous Na2SO4, filtrated, and evaporated in
vacuo. The crude product was purified by column chromatography
on silica gel (ethyl actate/methanol=20:1 to ethyl actate/metha-
nol=20:1 and 2% dimethylamine) and GPC to give 1,3-bis(4-pyri-
dyl)pyrazole (270 mg, 1.2 mmol) as a white solid in 41% yield.
m.p. 1828C, high-resolution ESI-TOF-MS m/z calcd for [C13H10N4
([M]+)]: 223.0977, found 223.0978 (error=0.5 ppm); 1H NMR
(500 MHz, CDCl3, 300 K): d=8.72 (d, J=6.0 Hz, 2H), 8.70 (d, J=
6.0 Hz, 2H), 8.13 (d, J=2.5 Hz, 1H), 7.80 (d, J=6.0 Hz, 2H), 7.73 (d,
J=6.0 Hz, 2H), 6.94 ppm (d, J=2.5 Hz, 1H); 1H NMR (500 MHz,
[D6]DMSO, 300 K): d=8.87 (d„ J=3.0 Hz, 1H), 8.70 (d, J=6.0 Hz,
2H), 8.68 (d, J=6.0 Hz, 4H), 7.98 (d, J=6.0 Hz, 2H), 7.93 (d, J=
6.0 Hz, 2H), 7.42 ppm (d, J=3.0 Hz, 1H); 13C NMR (125 MHz, CDCl3,
300 K): d=151.8 (C), 151.5 (CH), 150.5 (CH), 145.8 (C), 139.8 (C),
128.5 (CH), d=120.5 (CH), 112.7 (CH), 107.2 ppm (CH); IR (KBr): n˜ =
3084, 1589, 1508, 1420, 1371, 1308, 1209, 1051, 988, 957, 819, 762,
Experimental Section
General
1H NMR and 13C NMR spectra were recorded on a Bruker DRX-500
spectrometer equipped with
a Bruker AV-500 spectrometer equipped with a 5 mm TCI gradient
cryo probe, and a JEOL ECA-600 spectrometer equipped with a gra-
dient cold probe (53040HCNVC). All NMR spectral data were col-
a 5 mm BBO-Z-gradient probe,
704, 515 cmÀ1
.
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Self-assembly of sphere 3
To a solution of 4,6-bis(4-pyridyl) pyrimidine (0.80 mg, 3.42 mmol)
in [D6]DMSO (1.05 mL) was added a solution of Pd(NO3)2 or
[Pd(CH3CN)4](BF4)2 in [D6]DMSO (93.9 mL, 20.0 mm, 1.88 mmol), and
the resulting mixture was stirred at 708C for 3 d. The quantitative
1
formation of complex was confirmed by H NMR spectroscopy.
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1H NMR (500 MHz, [D6]DMSO, 300 K): d=9.61 (br, 96H), 9.48 (br,
48H), 8.95 (br, 48H), 8.66 ppm (br, 96H); diffusion coefficient
([D6]DMSO, 300 K, BF4Àsalt): D=5.0”10À11 m2 sÀ1
;
13C NMR
(125 MHz, [D6]DMSO, 300 K): d=160.1 (CH), 160.0 (CH), 151.9 (C),
À
146.2 (C), 125.7 (C), 124.5 (CH), 115.4 ppm (CH); CSI-TOF-MS (BF4
salt, CH3CN): The following picked signals are those of monoiso-
tropic ion signals (Figure S16 in the Supporting Information). m/z
calcd for [MÀ10(BF4À)]10+ 811.9085, found 811.9078; calcd for
[MÀ11(BF4À)]11 +
[MÀ12(BF4À)]12+
[MÀ13(BF4À)]13+
730.1893,
662.0899,
604.4674,
found
found
found
730.1889;
662.0898;
604.4669;
calcd
calcd
calcd
for
for
for
[MÀ14(BF4À)]14+ 555.0767, found 555.0740.
Self-assembly of sphere 4
To a solution of 1,3-bis(4-pyridyl)pyrazole (4.42 mg, 20.2 mmol) in
[D6]DMSO (146 mL) was added a solution of Pd(SbF6)2 in [D6]DMSO
(517 mL, 20.0 mm, 10.5 mmol) and the resulting mixture was stirred
at 70C8 for 3 h. The quantitative formation of complex was con-
1
firmed by H NMR spectroscopy.
1H NMR (500 MHz, [D6]DMSO, 300 K): d=9.32(br, 192H), 8.77(br,
48H), 8.18 (br, 192H), 7.33 ppm (br, 48H). Diffusion coefficient
([D6]DMSO,
300 K,
SbF6Àsalt):
D=3.5”10À11 m2 sÀ1.13C NMR
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(125 MHz, [D6]DMSO, 300 K): d=151.9 (CH), 151.4 (CH), 150.0 (C),
146.9 (C), 141.9 (C), 13À1.8 (CH), 123.0 (CH), 115.2 (CH), 109.2 ppm
(CH). CSI-TOF-MS (SbF6 salt, DMSO): The following picked signals
are those of monoisotropic ion signals (Figure S18 in the Support-
ing Information). m/z calcd for [MÀ16(SbF6À)]16+ 1296.4185, found
1296.4191; calcd for [MÀ17(SbF6À)]17+ 1206.3412, found 1206.3472;
calcd for [MÀ18(SbF6À)]18+ 1126.2725, found 1126.2628; calcd for
[MÀ19(SbF6À)]19+ 1054.6322, found 1054.6305; calcd for
[MÀ20(SbF6À)]20+ 990.1588, found 990.1498; [MÀ21(SbF6À)]21+
931.8201, found 931.8078.
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Acknowledgements
We thank Prof. Fumio Arisaka and Prof. Shuichi Hiraoka for ex-
perimental support and helpful discussion on AUC. This re-
search was in part supported by KAKENHI (MEXT) (24685010
and 25102007). The synchrotron X-ray crystallography was per-
formed at the NE3A beamline at the KEK Photon Factory
(2011G522, 2013G640) and at the BL38B1 beamline at SPring-8
(2012B0039, 2013A0039).
Keywords: cage compounds · nanostructures · polyhedera ·
self-assembly · supramolecular chemistry
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Manuscript received: May 19, 2015
Accepted Article published: June 10, 2015
Final Article published: July 14, 2015
Chem. Asian J. 2015, 10, 2292 – 2295
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