Heterotopic Assemblage of Two Different Disk-Shaped Ligands through Trinuclear Silver(I) Complexation: Ligand Exchange-Driven Molecular Motion

Article abstract of DOI:10.1021/ja036388q

The sandwich-shaped heterotopic trinuclear Ag⁺ complex Ag ₃1·2 was exclusively formed from two different tris(thiazolyl) and hexa(thiazolyl) disk-shaped ligands, 1 and 2, with the aid of three Ag ⁺ ions. The variable-temperature ¹H NMR study on its complexation behavior revealed that metal-ligand exchanges between the two neighboring thiazolyl nitrogen donors of 2 take place at the three Ag ⁺ centers in concert. ΔH? and ΔS? for the exchange process were calculated to be 50.5 kJ mol^-1 and -26.7 J mol^-1 K^-1, respectively, and its energy barrier at 298 K was estimated to be 58.5 kJ mol^-1. Each concerted metal-ligand exchange leads to an intramolecular 60°-rotational motion ((P) ? (M) conversion) between the two disk-shaped ligands.

Full text of DOI:10.1021/ja036388q

Published on Web 01/13/2004
Heterotopic Assemblage of Two Different Disk-Shaped
Ligands through Trinuclear Silver(I) Complexation: Ligand
Exchange-Driven Molecular Motion
Shuichi Hiraoka,† Motoo Shiro,‡ and Mitsuhiko Shionoya*,†
Contribution from the Department of Chemistry, Graduate School of Science, The UniVersity of
Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and Rigaku Corporation,
3-9-12 Matsubaracho, Akishima, Tokyo 196-8666, Japan
Received May 28, 2003; E-mail: shionoya@chem.s.u-tokyo.ac.jp
Abstract: The sandwich-shaped heterotopic trinuclear Ag+ complex Ag31‚2 was exclusively formed from
two different tris(thiazolyl) and hexa(thiazolyl) disk-shaped ligands, 1 and 2, with the aid of three Ag+ ions.
The variable-temperature 1H NMR study on its complexation behavior revealed that metal-ligand exchanges
between the two neighboring thiazolyl nitrogen donors of 2 take place at the three Ag+ centers in concert.
∆Hq and ∆Sq for the exchange process were calculated to be 50.5 kJ mol-1 and -26.7 J mol-1 K-1
,
respectively, and its energy barrier at 298 K was estimated to be 58.5 kJ mol-1. Each concerted metal-
ligand exchange leads to an intramolecular 60°-rotational motion ((P) a (M) conversion) between the two
disk-shaped ligands.
Introduction
Self-assembly protocols with the aid of biomolecules or
predesigned artificial molecules are widely used to fabricate
well-defined higher-order structures with an increasing number
of components.1 For example, in biological systems, DNA
duplexes are formed from two polynucleotides whose nucleo-
base sequences are complementary to each other,2a and protein
homo- or heterodimers are assembled preferentially through
accurate molecular recognition between their amino acid
residues at the surfaces.2b As for nonnatural systems, consider-
able effort has been made toward the construction of multi-
component systems using hydrogen-bonding,3 metal coordina-
tion,4,5 and aromatic interactions.6
Herein, we describe the quantitative heterotopic formation
of a sandwich-shaped trinuclear Ag31‚2 complex from two
Figure 1. Schematic representation of the formation of the heterotopic
sandwich-shaped Ag31‚2 complex consisting of disk-shaped ligands, 1 and
2, and three Ag+ ions.
† The University of Tokyo.
‡ Rigaku Corporation.
different disk-shaped ligands, a tris(thiazolyl) ligand 1 and a
hexa(thiazolyl) ligand 2, and three Ag+ ions (Figure 1). A
trinuclear Ag31‚2 complex is exclusively formed from a mixture
of 1, 2, and Ag+ ions in a 1:1:3 ratio, whereas an Ag312 complex
is quantitatively obtained from 1 and Ag+ ions in a 2:3 ratio.7
Introduction of 2 with an increased number of ligand rings from
three to six thus alters the association pattern from “homo” to
(1) Lehn, J.-M. Science 2002, 295, 2400-2403.
(2) (a) Seeman, N. C. Nature 2003, 421, 427-431. (b) Nowick, J. S.; Chung,
D. Angew. Chem., Int. Ed. 2003, 42, 1765-1768.
(3) (a) Archer, E. A.; Krische, M. J. J. Am. Chem. Soc. 2002, 124, 5074-
5083. (b) Huaqiang, Z.; Yang, X.; Flowers, R. A., II.; Gong, B. J. Am.
Chem. Soc. 2002, 124, 2903-2910. (c) Corbin, P. S.; Zimmerman, S. C.;
Thiessen, P. A.; Hawryluk, N. A.; Murray, T. J. J. Am. Chem. Soc. 2001,
123, 10475-10488. For a recent report on heterochiral dimer formation
through hydrogen bonding, see: (d) Wu, A.; Chakraborty, A.; Fettinger, J.
C.; Flowers, R., II; Isaacs, L. Angew. Chem., Int. Ed. 2002, 41, 4028-
4031. For construction of nanosized architectures using hydrogen bonding,
see: (e) Hof, F.; Rebek, J., Jr. Proc. Natl. Acad. Sci. U.S.A. 2002, 99,
4775-4777. (f) Atwood, J. L.; Barbour, L. J.; Jerga, A. Proc. Natl. Acad.
Sci. U.S.A. 2002, 99, 4837-4841. (g) Prins, L.; Reinhoudt, D.; Timmerman,
P. Angew. Chem., Int. Ed. 2001, 40, 2382-2426.
1
“hetero”. Moreover, variable-temperature H NMR measure-
(6) (a) Bisson, A. P.; Carver, F. J.; Eggleston, D. S.; Haltiwanger, R. C.; Hunter,
C. A.; Livingstone, D. L.; McCabe, J. F.; Rotger, C.; Rowan, A. E. J. Am.
Chem. Soc. 2000, 122, 8856-8868. (b) Berl, V.; Huc, I.; Khoury, R. G.;
Krische, M. J.; Lehn, J.-M. Nature 2000, 407, 720-723. For heterodimer
formation of oligomers through electron donor-acceptor interactions, see:
(c) Gabriel, G. J.; Iverson, B. L. J. Am. Chem. Soc. 2002, 124, 15174-
15175.
(4) (a) Albrecht, M.; Schneider, M. Eur. J. Inorg. Chem. 2002, 1301-1306.
(b) Hiraoka, S.; Kubota, Y.; Fujita, M. Chem. Commun. 2000, 1509-1510.
(c) Albrecht, M.; Schneider, M.; Rottele, H. Angew. Chem., Int. Ed. 1999,
38, 557-559. (d) Hasenknopf, B.; Lehn, J.-M.; Baum, G.; Fenske, D. Proc.
Natl. Acad. Sci. U.S.A. 1996, 93, 1397-1400.
(5) Hiraoka, S.; Yi, T.; Shiro, M.; Shionoya, M. J. Am. Chem. Soc. 2002, 124,
14510-14511.
(7) Hiraoka, S.; Harano, K.; Tanaka, T.; Shiro, M.; Shionoya, M. Angew. Chem.,
Int. Ed. 2003, 42, 5182-5185.
9
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J. AM. CHEM. SOC. 2004, 126, 1214-1218
10.1021/ja036388q CCC: $27.50 © 2004 American Chemical Society

Assemblage of Two Different Disk-Shaped Ligands
A R T I C L E S
Figure 2. Crystal structure of 2. Color labels: C (red), N (blue), S (green),
and H (white).
Scheme 1. Synthesis of Hexa(thiazolyl) Ligand 2
Figure 3. 1H NMR spectra (500 MHz, CD3OD, 293 K): (a) 2 and AgCH3-
SO3 in a 2:3 ratio, [2] ) 7.2 mM, [AgCH3SO3] ) 10.8 mM; (b) Ag312, [1]
) 7.2 mM, [AgCH3SO3] ) 10.8 mM; (c) Ag31‚2, [1] ) [2] ) 7.2 mM,
[AgCH3SO3] ) 21.6 mM.
different from both spectra of a mixture of 2 and AgCH3SO3 in
a 2:3 ratio (Figure 3a) and of the sandwich-shaped Ag312
complex7 (Figure 3b). These spectral changes suggest the
quantitative formation of an Ag+ complex comprising 1 and 2
in a 1:1 ratio through a heterotopic recognition process. In
addition, the p-tolyl protons He and Hf were observed upon
complexation as two inequivalent sets of signals due to the inner
and outer protons in the sandwich-shaped Ag+ complex, while
the p-tolyl methyl protons (Hg) and the thiazolyl protons of 1
and 2 showed a symmetrical pattern as shown with free ligands
1 and 2. The ESI-TOF mass spectrum of a mixture of 1, 2, and
AgCH3SO3 in a 1:1:3 ratio showed peaks at m/z 499.3, 796.4,
and 1687.4 which are assignable to [Ag31‚2]3+, [Ag31‚2‚
CH3SO3]2+, and [Ag31‚2‚(CH3SO3)2]+, respectively (Figure 4).
These results strongly suggest that the heterotopic Ag31‚2
complex is quantitatively formed in solution.
ments exhibited that, in Ag31‚2 complex, the intramolecular
rotational motion (i.e., (P) a (M) conversion) is driven by the
concerted ligand exchange in solution between the coordina-
tively paired ligands.
Results and Discussion
The formation of the Ag31‚2 complex is thermodynamically
favored; that is, the product does not depend on the mixing order
of the components. For example, the addition of a mixture of 2
([2] ) 16.3 mM) and AgCH3SO3 in a 2:3 ratio to a solution of
the Ag312 complex ([Ag312] ) 4.1 mM) rapidly generated the
heterotopic Ag31‚2 complex only.8 One of the reasons for the
exclusive heterotopic recognition observed here should be the
relatively low stability of the entities formed from hexa-
Synthesis of Disk-Shaped Ligands. The tris(thiazolyl) ligand
1 was synthesized according to our previous report.7 The hexa-
(thiazolyl) ligand 2 was prepared by trimerization of 1,2-bis-
(2-thiazolyl)ethylene 5 using 15 mol % of Co2(CO)8 as the
catalyst (Scheme 1). The alkyne 5 was prepared from 2-bro-
mothiazole and trimethylsilyl acetylene in three steps through
Sonogashira coupling and deprotection reactions. The hexa-
1
monodentate ligand 2 was fully characterized by H and 13C
1
monodentate ligand 2 and Ag+ ions. The H NMR titration
NMR, electrospray ionization-time-of-flight (ESI-TOF) mass
experiments using 2 and AgCH3SO3 showed that the signals
for Ha and Hb were gradually shifted in a complicated manner
to upfield and downfield, respectively, with an increasing
amount of Ag+ ions. Upon cooling the sample, prepared from
2 and AgCH3SO3 in a 2:3 ratio, down to 203 K, only broadening
of these signals was observed. In addition, unidentified pre-
cipitates appeared when more than 6 equiv of AgCH3SO3 was
added to a solution of 2, suggesting the formation of some
polymeric products. In the ESI-TOF mass study, the signals at
1
measurements, and elemental analysis. Its H NMR exhibited
two symmetrical signals for Ha and Hb, indicating the free
rotation of each thiazolyl ring. Furthermore, single crystals of
2 were obtained by slow evaporation of CHCl3 from a solution
of 2 in CHCl3-CH3OH. The X-ray diffraction of a single crystal
revealed that three nitrogen atoms of every other thiazolyl rings
are placed on the same side of the central benzene plane (Figure
2). Dihedral angles between the central benzene ring and three
thiazolyl rings are 53.29°, 54.79°, and 84.17°.
m/z 755.8, 539.5, and 684.9 assignable to [Ag322‚OH‚H2O]2+
,
[Ag422‚OH‚H2O]3+, and [Ag222]2+, respectively, were observed
Formation of Heterotopic Ag+ Complex. The quantitative
formation of the heterotopic Ag31‚2 complex in solution was
evidenced by 1H NMR and ESI-TOF mass measurements. The
1H NMR spectrum of a mixture of two ligands, 1 and 2, and
AgCH3SO3 in a 1:1:3 ratio in CD3OD (Figure 3c) was quite
(8) No higher-order structures such as a triple decker complex, 1‚Ag3‚2‚Ag3‚
1, were observed with mixtures of 1, 2, and AgCH3SO3 in various ratios.
For example, the 1H NMR spectrum of a mixture of 1, 2, and AgCH3SO3
in a 2:1:6 ratio showed the separated two sets of signals assignable to the
Ag31‚2 and Ag312 complexes.
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A R T I C L E S
Hiraoka et al.
Figure 4. ESI-TOF mass spectrum of the Ag31‚2 complex.
in the presence of 1.5 equiv of AgCH3SO3, whereas the former
peaks changed to a single peak at m/z 539.5 for [Ag422‚OH‚
H2O]3+ with an increasing amount of Ag+ ions within the range
of [Ag+]:[2] ratios from 3:2 to 6:1 (see Supporting Information,
Figure S2). These results indicate that a mixture of 2 and Ag+
ions in a 2:3 ratio does not afford any detectable, stable complex.
1
Variable-Temperature H NMR Studies for Heterotopic
1
Ag+ Complex. As described above, a H NMR spectrum of
the heterotopic Ag31‚2 complex showed one set of thiazolyl
proton signals for Ha and Hb at 293 K (Figure 3c). There are
two possible coordination structures around Ag+ ions for Ag31‚
2 complex. One is a trigonal coordination geometry in which
the neighboring two thiazolyl nitrogen atoms of 2 and one
thiazolyl nitrogen atom of 1, respectively, coordinate to the Ag+
center. The other one is a linear coordination geometry in which
every other thiazolyl nitrogen atom of 2 would possibly
coordinate to the Ag+ centers and the other three nitrogen donors
be uncoordinating. In this structure, metal-ligand exchanges
between the two neighboring thiazolyl rings of 2 should take
place at three Ag+ ions faster than the NMR time scale. In the
expectation that this metal-ligand exchange with a relative
rotational motion between the two disk-shaped ligands should
be temperature-dependent, we conducted variable-temperature
1H NMR measurements of the Ag31‚2 complex. As shown in
Figure 5, the dynamic behavior of the Ag31‚2 complex caused
by concerted metal-ligand exchange was observed in the
1
Figure 5. Variable-temperature H NMR spectra of the Ag31‚2 complex
(500 MHz, CD3OD, [Ag31‚2] ) 7.2 mM): (a) 263 K, (b) 253 K, (c) 243
K, (d) 233 K, (e) 223 K, (f) 213 K, and (g) 203 K.
1
spectra. The H NMR signals above 263 K for one set of the
6) but is fast enough above 253 K through (P) a (M)
intraconversion. The energy barrier of the (P) a (M) equilibrium
in the Ag31‚2 complex was evaluated by the simulation of the
line shapes of the Ha and Hb signals. Based on the Eyring
equation,9 ∆Hq and ∆Sq for the exchange process were
Ha and Hb protons of ligand 2 (Figure 5a) were separated into
two sets, (Ha1, Hb1) and (Ha2, Hb2), at 203 K (Figure 5g). In
contrast, the signals for ligand 1 were not divided into two sets
ranging from 293 to 203 K, although the Hc and Hd signals
shifted downfield and upfield, respectively, as the temperatures
went down. These results overall suggest that each Ag+ ion is
linearly coordinated by two nitrogen atoms (one from 1 and
the other from 2) in the ground state, where the ligand exchange
between the two types of thiazolyl groups of 2, corresponding
to coordinating (Ha1, Hb1) and uncoordinating (Ha2, Hb2) ones,
is not fast enough to be separately observed at 203 K (Figure
calculated to be 50.5 kJ mol-1 and -26.7 J mol-1 K-1
,
respectively, and finally the energy barrier of the exchange at
298 K (∆Gq298) was estimated to be 58.5 kJ mol-1
The metal-ligand exchange would take place by way of a
plausible transition state in which three thiazolyl nitrogen atoms
.
(9) Eyring, H. J. Chem. Phys. 1935, 3, 107-115.
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1216 J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004

Assemblage of Two Different Disk-Shaped Ligands
A R T I C L E S
Figure 6. Plausible mechanism for the relative rotational motion mediated by intramolecular metal-ligand exchange within the Ag31‚2 complex.
then heated at 90 °C for 3 h. After cooling, the colorless solution was
filtered to remove unreacted Ag2O and then evaporated. The resulting
colorless solid was washed with acetone to remove CH3SO3H and then
dried in vacuo to obtain AgCH3SO3 (3.18 g, 91%) as a colorless solid.
1H NMR (500 MHz, CD3OD) δ 2.71 (s); 13C NMR (125 MHz, CD3-
OD) δ 39.5.
(thiazolyl ring (1) from ligand 1 and (A) and (B) from ligand
2; see Figure 6) coordinate to an Ag+ ion with a trigonal
coordination geometry. The negative value of ∆Sq should be
due to the changes in the environment, i.e., solvent ordering
through hydrogen bonding to the complex, association of
counterions with the complex, and additional coordination of a
thiazolyl nitrogen donor to the neighboring Ag+ ion in the
transition state.10 Moreover, it is worth describing that the
metal-ligand exchange should give rise to the (P) a (M)
racemization accompanying a reversible 60°-rotational motion
between the coordinatively paired ligands. In the (P) isomer,
for instance, a thiazolyl ring (1) in ligand 1 and a thiazolyl ring
(B) in ligand 2 coordinate to an Ag+ ion and, after the (P) f
(M) conversion, the thiazolyl ring (A) exchanged by another
thiazolyl ring (B) through the 60° rotation around the helix axis
leading to the (M) isomer (Figure 6).
1,2-Bis(2-thiazolyl)ethylene (5). To a solution of CuI (21 mg, 0.11
mmol, 3 mol %), PdCl2(PPh3)2 (78 mg, 0.11 mmol, 3 mol %), and
2-bromothiazole (0.40 mL, 4.4 mmol) in Et3N (10 mL) was added
2-ethynylthiazole 4 (0.40 g, 3.7 mmol). The mixture was degassed and
heated at 70 °C for 12 h under a nitrogen atmosphere. The resulting
dark brown mixture was filtered, and the solvent was then removed in
vacuo. Purification by silica gel chromatography was performed
(n-hexane/AcOEt (10:1-4:1)) to obtain the desired coupling product
1
5 (226 mg, 32%) as a pale yellow solid: mp 143 °C; H NMR (500
MHz, CDCl3) δ 7.93 (d, J ) 3.5 Hz, 2H), 7.50 (d, J ) 3.5 Hz, 2H);
13C NMR (125 MHz, CDCl3) δ 146.9, 144.1, 122.1, 85.9; IR (KBr) ν
3130, 3100, 3060, 1460, 1360, 1320, 1250, 1130, 1080, 730 cm-1. MS
(ESI-TOF) m/z exact mass [M + Na]+ 214.9697, C8H4N2S2Na requires
214.9714.
Conclusion
In conclusion, the exclusive heterotopic assemblage was
accomplished by the quantitative formation of a heterotopic
Ag31‚2 complex. The reversible rotational motion between the
two ligands 1 and 2 placed in parallel takes place concertedly
through metal-ligand exchange at the three Ag+ ions. Such
intramolecular rotational motion between two different coun-
terparts would be applicable to the development of novel metal-
mediated molecular devices particularly for multicomponent-
based molecular machinery.
Hexa(2-thiazolyl)benzene (2). Co2(CO)8 (27 mg, 78 µmol, 15 mol
%) and 5 (100 mg, 0.52 mmol) in 1,4-dioxane (6.0 mL) were placed
in a sealed tube flask. The mixture was degassed and heated at 100 °C
for 10 h. The solvent was removed in vacuo. Purification by silica gel
column chromatography (CHCl3/CH3OH (50:1)) afforded the desired
2 (56 mg, 56%) as a colorless solid: mp 430 °C (dec); 1H NMR (500
MHz, DMSO-d6) δ 7.67 (d, J ) 3.3 Hz, 6H), 7.58 (d, J ) 3.3 Hz,
6H); 13C NMR (125 MHz, DMSO-d6) δ 161.7, 142.2, 136.8, 123.5;
IR (KBr) ν 3120, 3100, 3060, 1500, 1480, 1360, 1300, 1160, 1090,
1070, 1060, 960, 770, 740 cm-1. Anal. Calcd for C24H12N6S6: C, 49.98;
H, 2.10; N, 14.57. Found: C, 49.77; H, 2.25; N, 14.40.
Experimental Section
General. 2-Ethynylthiazole 4 was prepared according to the
literature.10 All ambient and variable-temperature 1H NMR spectra
were recorded on a Bruker DRX 500 (500 MHz) spectrometer using
TMS as the internal reference. Electrospray ionization-time-of-flight
(ESI-TOF) mass spectra were recorded on a Micromass LCT mass
spectrometer KB 201. IR spectra were recorded on a Jasco IR-Report
100. Melting points were measured on a Yanaco MP-500D.
Formation of Ag31‚2‚(CH3SO3)3 Complex. To a solution of
AgCH3SO3 (1.8 mg, 8.6 µmol) and 1 (3.4 mg, 5.8 µmol) in CD3OD
(0.4 mL) was added a solution of AgCH3SO3 (1.8 mg, 8.6 µmol)
and 2 (3.3 mg, 5.8 µmol) in CD3OD (0.4 mL), and the mixture
1
stood at room temperature for 5 min. Its H NMR spectrum showed
the quantitative formation of the Ag31‚2‚(CH3SO3)3 complex. 1H
NMR (500 MHz, CD3OD, 293 K) δ 7.90 (d, J ) 3.3 Hz, 6H), 7.80
(d, J ) 3.5 Hz, 3H), 7.66 (d, J ) 3.5 Hz, 3H), 7.64 (d, J ) 3.3 Hz,
6H), 7.50 (dd, J ) 1.7, 7.8 Hz, 3H), 7.00 (d, J ) 8.0 Hz, 3H),
6.83 (dd, J ) 1.7, 8.0 Hz, 3H), 6.67 (d, J ) 7.8 Hz, 3H), 2.68
(s, 9H), 2.23 (s, 9H); 13C NMR (125 MHz, CD3OD) δ 170.1,
165.2, 146.8, 144.5, 143.3, 140.6, 138.6, 135.2, 133.7, 131.9, 131.1,
130.8, 130.4, 126.6, 125.6, 39.6, 21.3; MS (ESI-TOF) (CH3OH)
AgCH3SO3. Ag2O (2.0 g, 8.6 mmol) was added to a solution of
CH3SO3H (1.1 mL, 17.3 mmol) in H2O (40 mL), and the solution was
(10) For a similar discussion on the negative entropy change of metal-ligand
exchange was recently reported, see: Kajiwara, T.; Yokozawa, S.; Ito, T.;
Iki, N.; Morohashi, N.; Miyano, S. Angew. Chem., Int. Ed. 2002, 41, 2076-
2078.
(11) Neenan, T. X.; Whitesides, G. M. J. Org. Chem. 1988, 53, 2489-2496.
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A R T I C L E S
499.3 [Ag31‚2]3+ 796.4 [Ag31‚2‚(CH3SO3)]2+
, , 1687.4
Hiraoka et al.
m/z
)
Complexes” to S.H. (No. 420/15036216) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
[Ag31‚2‚(CH3SO3)2]+.
Supporting Information Available: X-ray crystallographic
data of 2 (CIF), Eyring plot, and ESI-TOF mass spectra of Ag+
complexes of 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
Acknowledgment. This study was supported by a Grant-in-
Aid for The 21st Century COE Program for Frontiers in
Fundamental Chemistry, a Grant-in-Aid for Encouragement of
Young Scientists (B) to S.H. (No. 14740361), and a Grant-in
Aid for Scientific Research on Priority Areas, “Dynamic
JA036388Q
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