Coordination metallacycles of an achiral dendron self-assemble via metal-metal interaction to form luminescent superhelical fibers [17]

Full text of DOI:10.1021/ja010426t

5608
J. Am. Chem. Soc. 2001, 123, 5608-5609
Coordination Metallacycles of an Achiral Dendron
Self-Assemble via Metal-Metal Interaction To Form
Luminescent Superhelical Fibers
Chart 1
Masashi Enomoto, Akihiro Kishimura, and Takuzo Aida*
Department of Chemistry and Biotechnology
Graduate School of Engineering, The UniVersity of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
ReceiVed February 19, 2001
Hierarchical self-assembly of dendritic macromolecules via
1
weak interactions such as H-bonding forces has attracted great
attention for the design of well-defined mesoscopic materials and
also in relation to self-organization events in natural systems. For
example, Percec and co-workers have reported that a carboxylic
acid-anchored poly(benzyl ether) dendron having long alkyl chains
at the periphery self-assembles via H-bonding and van der Waals
2
interactions to form a cylindrical columnar assembly, which can
be regarded as a primary synthetic model of icosahedral viruses.3
Recently, we have reported that a poly(benzyl ether) dendron
anchoring a H-bonding dipeptide core self-assembles to form
micrometer-scale fibers to induce efficient gelation of various
1
0
of acetylacetone derivatives of the corresponding dendrons with
hydrazine.11 Metal pyrazolate complexes of LnPZ with group 11
4
organic solvents.
7
a,9a
metal ions were prepared according to a literature method
with
Herein we report results of the first study on utilization of weak
1
1
metal-metal interactions among group 11 metal ions5 for
partial modification and unambiguously characterized. For
example, the IR spectrum of the Au(I) complex of L2PZ (KBr
hierarchical self-organization of dendritic macromolecules (Chart
-
1
6
pellet) showed a vibrational band at 1517 cm assignable to a
1
). For this purpose, we chose pyrazole as the metal-ligating
metal-coordinated NdC bond8
d,9a,12
without any N-H stretching
module, which is an exobidentate ligand capable of forming
-1
1
coordination metallacycles with Cu(I), Ag(I), and Au(I).7,8 X-ray
vibrational band at 3200-3000 cm . H NMR spectrum in CDCl
3
of L2PZ, upon coordination with Au(I), showed a slight downfield
diffraction studies have suggested that they form in the solid-
shift (∆δ ) 0.03 ppm) of the signal due to the pyrazole methyl
state long columnar assemblies involving a weak metal-metal
11
interaction.8d More recently, pyrazoles having long alkyl chains,
groups. In size-exclusion chromatography (SEC) of the complex,
a single, sharp elution peak was observed at a higher molecular
weight region than L2PZ.11 Although the complexation of
pyrazole with group 11 metal ions is known to form linear
coordination polymers in the solid state,13 the absence of any
polydisperse character in the SEC profile, together with the
in the presence of Au(I), have been reported to form discotic liquid
9
crystalline materials via an edge-to-edge zigzag stacking of the
metal pyrazolate coordination triangles.9b
A series of pyrazole-anchored poly(benzyl ether) dendrons
(LnPZ, Chart 1) with different generation numbers (n [number
1
simplicity of the H NMR spectrum, indicates that the Au(I)
of alkoxybenzyl layers] ) 2-4) were synthesized by the reaction
complex adopts a uniformly sized, highly symmetric cyclic
structure. MALDI-TOF-MS analysis showed a single peak
centered at 2144, which corresponds to a trimer of Au(L2PZ)
(
1) For recent reviews, see: (a) Zeng, F.; Zimmerman, S. C. Chem. ReV.
1
997, 97, 1681. (b) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem.
ReV. 1999, 99, 1665. (c) V o¨ gtle, F.; Gerstermann, S.; Hesse, R.; Schwierz,
+
11
H.; Windisch, B. Prog. Polym. Sci. 2000, 25, 987.
([Au(L2PZ)]
3
3
; [M + H ] calcd 2144). [Au(L2PZ)] was highly
(
2) (a) Percec, V.; Ahn, C.-H.; Ungar, G.; Yearday, D. J. P.; M o¨ ller, M.;
robust toward protonolysis with water and inert to autoxidation
in air. Although other metal complexes of LnPZ (n ) 2-4) with
Cu(I), Ag(I), and Au(I) all showed similar spectral profiles to
Sheiko, S. S. Nature 1998, 391, 161. (b) Hudson, S. D.; Jung, H.-T.; Percec,
V.; Cho, W.-D.: Johansson, G.; Ungar, G.; Balagurusamy, V. S. K. Science
1
997, 278, 449. (c) Percec, V.; Schlueter, D.; Ungar, G.; Cheng, S. Z. D.;
11
Zhang, A. Macromolecules 1998, 31, 1745. (d) Percec, V.; Cho, W.-D.;
Mosier, P. E.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998, 120,
3
those of [Au(L2PZ)] , the complexes of Cu(I) and Ag(I) were
rather susceptible to air and decomposed under SEC and MALDI-
TOF-MS conditions.
1
1061.
(
(
(
3) Casper, D. L. D. Biophys. J. 1980, 32, 103.
4) Jang, W.-D.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 2000, 122, 3232.
5) ∆G values for Au(I)-Au(I) interactions are almost identical to those
3
When a paraffin suspension of [Au(L2PZ)] was once heated
at 200 °C and allowed to cool to room temperature, it gradually
became turbid to give a white fibrous precipitate, as observed by
optical microscopy. Polarized microscopy of the precipitate
showed a strong birefringence, indicating that the fibers are
crystalline with a regular arrangement of the building blocks.
Quite unexpectedly, the fibers at a closer look were helical,
although precursor [Au(L2PZ)] is devoid of any chiral centers.
3
Figure 1b shows a SEM picture of the fiber, which is not single-
stranded but a bundle of several loosely twisted elementary fibrils
of H-bonding interactions (see ref 14).
(
6) Selected examples of metal ion-assisted assemblies of dendrimers: (a)
Newkome, G. M.; Guther, R.; Moorefield, C. N.; Cardullo, F.; Echegoyen,
L.; Perezcordero, E.; Luftmann, H. Angew. Chem., Int. Ed. Engl. 1995, 34,
2
023. (b) Tomoyose, Y.; Jiang, D.-L.; Jin, R.-H.; Aida, T.; Yamashita, T.;
Horie, K.; Yashima, E.; Okamoto, Y. Macromolecules 1996, 29, 5236. (c)
Issberner, J.; V o¨ gtle, F.; Cola, L. D.; Balzani, V. Chem. Eur. J. 1997, 3, 706.
(
d) Kawa, M.; Fr e´ chet, J. M. J. Chem. Mater. 1998, 10, 286. (e) Enomoto,
M.; Aida, T. J. Am. Chem. Soc. 1999, 121, 874.
(7) For reviews on coordination chemistry of pyrazole derivatives, see: (a)
Monica, G. L.; Ardizzoia, G. A. Prog. Inorg. Chem. 1997, 46, 151. (b)
Trofimenko, S. Prog. Inorg. Chem. 1986, 34, 115.
(
∼0.2 µm in diameter) with a helical pitch of approximately 2.5
µm. According to NMR and SEC profiles, the fibers were soluble
in CDCl via dissociation into the individual metallacycles,
(
8) (a) Minghetti, G.; Banditelli, G.; Bonatio, F. Inorg. Chem. 1979, 18,
6
2
58. (b) Murray, H. H.; Raptis, R. G.; Fackler, J. P., Jr. Inorg. Chem. 1988,
7, 26. (c) Raptis, R. G.; Fackler, J. P., Jr. Inorg. Chem. 1988, 27, 4179. (d)
3
Ehlert, M. K.; Rettig, S. J.; Storr, A.; Thompson, R. C.; Trotter, J. Can. J.
Chem. 1990, 68, 1444.
(10) Hawker, C. J.; Fr e´ chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638.
(11) See Supporting Information.
(12) Ardizzoia, G. A.; Monica, G. L. Inorg. Synth. 1997, 31, 299.
(13) Masciocchi, N.; Moret, M.; Cairati, P.; Sironi, A.; Ardizzoia, G. A.;
Monica, G. L. J. Am. Chem. Soc. 1994, 116, 7668.
(
9) (a) Barber a´ , J.; Elduque, A.; Gim e´ nez, R.; Lahoz, F. J.; L o´ pez, J. A.;
Oro, L. A.; Serrano, J. L. Inorg. Chem. 1998, 37, 2960. (b) Kim, S. J.; Kang,
S. H.; Park, K.-M.; Kim, H.; Zin, W.-C.; Choi, M.-G.; Kim, K. Chem. Mater.
1
998, 10, 1889.
1
0.1021/ja010426t CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/18/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 23, 2001 5609
excitation spectrum of the fibers, monitored at 605 nm (ii), was
similar to the absorption spectrum, suggesting an excitation energy
transfer from the dendritic wedge as an antenna16 to the interior
metal ion cluster. Interestingly, when the fibers were dissolved
2 2
in CH Cl to allow dissociation into the individual metallacycles,
such a characteristic luminescence at the visible region dis-
appeared completely, while the fluorescence from the dendritic
wedge became pronounced. Although the superhelical fibers
containing Ag(I) (λem ) 521, 605 nm) and Au(I) (λem ) 594 nm)
exhibited similar luminescence profiles, they involved a relatively
strong fluorescence from the dendritic wedge.11
Higher generation L3PZ and L4PZ gave stable coordination
triangles with Au(I), which however gave only nonfibrous,
Figure 1. Scanning electron micrographs (SEM) of self-organized
structures formed upon heating paraffin suspensions of pyrazolate
complexes of (a) 4-benzyl-3,5-dimethylpyrazole (BnPZ) and (b) L2PZ
11
glassy aggregates upon thermal treatment in paraffin. Use of Cu(I)
or Ag(I) in place of Au(I) also resulted in the formation of glassy
aggregates. All these aggregates were luminescent, although they
were much less emissive than the superhelical fibers with lower
generation L2PZ. Together with the fact that the glassy aggregates
2
with [Au(SMe )]Cl at 200 °C, followed by cooling to room temperature.
m
in DSC showed T though at much lower temperatures than the
fibers with L2PZ,11 they are not random aggregates but involve
metal-metal interactions. In contrast, the metal complexes of a
nondendritic 4-benzyl-3,5-dimethylpyrazole (BnPZ) formed highly
luminescent but nonhelical, needle-like fibers (Figure 1a). There-
fore, the formation of superhelical fibers likely requires a delicate
balance between the steric and van der Waals interactions among
the dendritic wedges. Formation of helical assemblies from achiral
building blocks is very rare.1 Although the origin of the helicity
is not clear at present, a possibility for generating a chiral element
in the self-organization event is a helically twisted edge-to-edge
stacking, rather than a zigzag stacking, of the coordination
triangles, which can reduce the steric repulsion among the
dendritic wedges.
In conclusion, we demonstrated that medium-sized group 11
dendritic metallacycles without any chiral elements form micro-
meter-scale, luminescent superhelical fibers. The morphology of
the aggregate was totally dependent on the size of the dendritic
wedge. In view of materials science, the dendritic fiber containing
Cu(I) is particularly interesting, since it involves a long-range
interaction among the highly redox-active transition metal ions
surrounded by the dendritic coat. Thus, use of metal-metal
interactions as driving forces for self-organization of dendritic
macromolecules provides a novel strategy for the fabrication of
mesoscopic functional materials.
7,18
Figure 2. Differential scanning calorimetry (DSC) profiles of self-
organized pyrazolate complexes of L2PZ with (a) [Cu(MeCN)]PF , (b)
AgPF , and (c) [Au(SMe )]Cl.
6
6
2
Figure 3. Luminescence profiles of a self-organized pyrazolate complex
of L2PZ with [Cu(MeCN)]PF . (a) Fluorescence microscope image upon
6
Acknowledgment. We are grateful to Mr. T. Negishi of JEOL Co.,
excitation at 336 nm. (b) (i) Emission (λext ) 280 nm) and (ii) excitation
spectra (λobsd ) 605 nm) of its Nujiol sample.
Ltd. for FE-SEM measurements.
Supporting Information Available: Synthesis and analytical data
of LnPZ (n ) 2-4) and their metal complexes with Cu(I)-Au(I), SEC
profiles of BnPZ and LnPZ (n ) 2-4) and their metal complexes with
Au(I), MALDI-TOF-MS spectra of metal complexes of BnPZ and LnPZ
indicating that they are formed by the physical assembly of [Au-
3
(L2PZ)] without ring opening to give linear polymeric structures.
Likewise, superhelical fibers were formed from dendritic pyra-
zolate complexes of L2PZ with Cu(I) and Ag(I).
Figure 2 shows DSC profiles of the pyrazolate complexes of
L2PZ with (a) Cu(I), (b) Ag(I), and (c) Au(I). In every case, a
clear endothermic peak was observed, which is assignable to the
(n ) 2-4) with Au(I), DSC profiles of self-organized metal complexes
of BnPZ and LnPZ (n ) 2-4) with Cu(I), and selected luminescence
spectra of self-organized metal complexes of BnPZ and LnPZ (n ) 2-4)
with Cu(I)-Au(I) (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
m
melting temperature of the self-organized structure (T ), as a
JA010426T
possible consequence of the breakdown of the weak metal-metal
interaction. In principle, orbital mixing is more likely to occur
for heavier elements in the same group of the periodic table.
Accordingly, the assembly of the metal complex of L2PZ with
(
15) Similar luminescence profiles have been reported for polynuclear group
1
1 metal complexes: (a) Sabin, F.; Ryu, C. K.; Ford, P. C.; Vogler, A. Inorg.
Chem. 1992, 31, 1941. (b) Knotter, M.; Blasse, G.; van Vliet, J. P. M.; van
Koten, G. Inorg. Chem. 1992, 31, 2196. (c) Vikery, J. C.; Olmstead, M. M.;
Fung, E. Y.; Balch, A. L. Angew. Chem., Int. Ed. Engl. 1997, 36, 1179.
at 197.3 °C,14 while those with Ag(I)
at 193.7 and 179.6 °C, respectively.
Au(I) showed the highest T
and Cu(I) displayed lower T
m
(
16) (a) Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1998, 120, 10895. (b)
m
Sato, T.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 12658.
(17) (a) van Esch, J.; De Feyter, S.; Kellogg, R. M.; De Schrijver, F.;
Feringa, B. L. Chem. Eur. J. 1997, 3, 1238. (b) Yang, W.; Chai, X.; Chi, L.;
Liu, X.; Cao, Y.; Lu, R.; Jiang, Y.; Tang, X.; Fuchs, H.; Li, T. Chem. Eur. J.
Fluorescence microscopy (Figure 3a) showed that the super-
helical fibers from the metal complex of L2PZ with Cu(I) are
highly luminescent. Upon excitation of the dendritic wedge at
1
999, 5, 1144. (c) Feringa, B. L.; van Derden, R. A. Angew. Chem., Int. Ed.
1999, 38, 3418.
18) Examples of helicity induction in self-assembly of achiral molecules
280 nm, the fibers emitted an intense orange luminescence
(
centered at 605 nm (Figure 3b (i)) with a negligibly weak
by chiral triggers: (a) Palmans, A. R. A.; Vekemans, J. A. J. M.; Havinga, E.
E. Meijer, E. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 2648. (b) Brunsveld,
L.; Lohmeijer, B. G. G.; Vekemans, J. A. J. M.; Meijer, E. W. Chem. Commun.
2000, 2305.
fluorescence from the dendritic wedge at 305 nm.11,15 The
(14) Schmidbaur, H. Chem. Soc. ReV. 1995, 391.