Synthesis of a dendrimer reactor for clusters with a magic number

Article abstract of DOI:10.1246/cl.2012.828

A new type of phenylazomethine dendrimer (DPA) that has one more coordination site compared to the tetraphenylmethane core DPA was synthesized for the magic number 13 metal cluster template that has cubic or hexagonal densely packed structures. The stepwise complexation property was revealed, and this property is expected to be the magic number 13 metal cluster template.

Full text of DOI:10.1246/cl.2012.828

doi:10.1246/cl.2012.828
828
Published on the web August 1, 2012
Synthesis of a Dendrimer Reactor for Clusters with a Magic Number
Hirokazu Kitazawa, Ken Albrecht, and Kimihisa Yamamoto*
Chemical Resources Laboratory, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503
(Received May 8, 2012; CL-120397; E-mail: yamamoto@res.titech.ac.jp)
A new type of phenylazomethine dendrimer (DPA) that has
one more coordination site compared to the tetraphenylmethane
core DPA was synthesized for the “magic number 13” metal
cluster template that has cubic or hexagonal densely packed
structures. The stepwise complexation property was revealed,
and this property is expected to be the “magic number 13” metal
cluster template.
It is well known that properties of metal clusters, which are
different from bulk materials, change with the number of atoms
or the size.1 In particular, metal clusters that consist of the
“magic number” 13, 55, 147, 309, etc., of metal atoms are
known as especially stable clusters that have cubic or hexagonal
densely packed structures with the complete, regular outer
geometry of a cuboctahedron.2 With the evolution of the field of
nanotechnology, the function of “magic number” clusters has
received much attention, and the synthesis of a finely size-
controlled cluster is strongly desired for advanced nanomate-
rials.3
Figure 1. Structure of the Py-DPA G4 dendrimer.
In particular, dendrimers, such as the poly(amidoamine)
dendrimer (PAMAM) incorporating metal ions, have received
much attention as the cluster template for synthesizing clusters
and nanocatalysts.4 However, metal clusters using traditional
dendrimers have a statistical cluster size distribution.5 On the
other hand, the dendritic poly(phenylazomethine) (DPA) shows
a stepwise radial complexation with metal ions from the core
imines to the terminal imines based on the gradients in the
basicity of the imine groups.6 Therefore, it is possible to control
the number and location of metal ions incorporated into the
dendrimers, and DPA can be used as the size-controlled cluster
template.7
We have already reported that dendrimer-encapsulated
platinum clusters consisting of 12 platinum atoms can be
synthesized using the tetraphenylmethane core DPA (TPM
dendrimer) which has a dense-shell structure and an internal
cavity.8 However, the finely controlled synthesis of the “magic
number 13” cluster cannot be achieved with this dendrimer.
Therefore, in this paper, we report a new type of phenyl-
azomethine dendrimer, Py-DPA G4, which has one more
coordination site at the core compared to the traditional DPA,
and revealed the stepwise complexation (Figure 1).
We designed a new kind of core molecule, (5-aminopyridin-
2-yl)tris(4-aminophenyl)methane (NH2TrPyNH2), which has
one position altered in the pyridine of tetrakis(p-aminophenyl)-
methane, as an additional coordination site for synthesis of the
“magic number 13” cluster template. The strategy for the
preparation of the core of Py-DPA, NH2TrPyNH2, is outlined in
Scheme 1 (Schemes S1-S5, see the Supporting Information for
details11). Similarly to the synthesis of tetrakis(p-aminophenyl)-
methane,9 the pyridine-substituted tetraphenylmethane backbone
Scheme 1. Synthesis of the core of Py-DPA (NH2TrPyNH2).
(TrPyNH2) was synthesized by heating 3-aminopyridine and
trityl chloride at 220 °C without a solvent, and the structure was
identified by an X-ray single-crystal analysis (Figure S111). The
other amine groups were added by the nitration reaction of the
phenyl group followed by a hydrogenation reaction.
Chem. Lett. 2012, 41, 828-830
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

829
1equiv
3equiv
1equiv
2equiv
6equiv
4equiv
12equiv
8equiv
24equiv
Scheme 2. Stepwise radial complexation of Py-DPA G4 with
GaCl3.
0-1equiv
5-7equiv
1-2equiv
7-13equiv
29-37equiv
2-5equiv
13-17equiv
37-61equiv
1-2equiv
2-5equiv
0-1equiv
17-29equiv
1equiv
3equiv
1equiv
Figure 2. UV-vis spectra of Py-DPA G4 (2.5 © 10¹6 M, in
acetonitrile/chloroform = 1:1) upon the addition of GaCl3
¹3
(2.1 © 10 in acetonitrile) at 293 K.
Figure 3. (a) UV-vis spectra of Py-DPA G1 (2.1 © 10¹5 M,
The phenylazomethine (DPA) dendrons, G1-G4, were
synthesized according to a literature method.10 The Py-DPA
dendrimers were obtained by the same dehydration reaction
between the core and the dendrons (Py-DPA G1 and G4,
Scheme S611). All products were identified by 1H NMR,
13C NMR, MALDI-TOF MS, and elemental analysis, and the
purity of Py-DPA G1 and G4 was verified by an HPLC (SEC)
analysis (Figure S211).
The coordination chemistry of the dendrimer was confirmed
by UV-vis titration with GaCl3. Upon the addition of GaCl3 to
the Py-DPA dendrimer solution, the spectra gradually changed
until the stoichiometry with the added GaCl3 was equal to the
number of coordination sites in the dendrimer. This means that
the GaCl3 and the coordination site of the Py-DPA dendrimer
quantitatively form a 1:1 complex. When the Py-DPA G4 was
used, nine isosbestic points were observed during the titration.
Each isosbestic point shifted stepwise, as 342 (0-1 equiv), 366
(1-2 equiv), 366 (2-5 equiv), 367 (5-7 equiv), 367.5 (7-13
equiv), 365.5 (13-17 equiv), 363.5 (17-29 equiv), 361.5 (29-37
equiv), and 360.5 nm (37-61 equiv) (Figure 2).
in acetonitrile/chloroform = 1:1) upon the addition of GaCl3
¹3
(6.5 © 10
in acetonitrile) at 293 K. (b) Stepwise radial
complexation of Py-DPA G4 with GaCl3.
The number of added equivalents of GaCl3 that induced the
shifts in the isosbestic points is in good agreement with the
number of coodination sites in the Py-DPA G4. This means that
the complexing process proceeds in a stepwise fashion from the
coordination site of the core to the terminal coordination site of
the Py-DPA G4, and this indicated that the order of coordination
in the same layer is first to the coordination site of the dendrons,
which was attached to the pyridine site of the core, then to the
other coordination sites of the same layer dendron (Scheme 2).
When a UV-vis titration was similarly performed by the
addition of GaCl3 to the Py-DPA G1, three isosbestic points
were observed during the titration. Each isosbestic point shifted
stepwise, as 264.5 (0-1 equiv), 280 (1-2 equiv), 282 nm (2-5
equiv) (Figure 3a). As
a
result,
a
similar stepwise
radial complexation was also observed for the Py-DPA G1
(Figure 3b).
Chem. Lett. 2012, 41, 828-830
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

830
Aid for Encouragement of Young Scientists (B) (No. 24750099)
from the Japan Society for the Promotion of Science (JSPS).
References and Notes
1
a) A. P. Alivisatos, Science 1996, 271, 933. b) M. Valden,
X. Lai, D. W. Goodman, Science 1998, 281, 1647. c) H.
Tsunoyama, H. Sakurai, Y. Negishi, T. Tsukuda, J. Am.
Chem. Soc. 2005, 127, 9374. d) J. A. Creighton, D. G.
Eadon, J. Chem. Soc., Faraday Trans. 1991, 87, 3881.
a) G. Schmid, Endeavour 1990, 14, 172. b) G. Schmid, N.
Klein, B. Morun, A. Lehnert, J.-O. Malm, Pure Appl. Chem.
1990, 62, 1175. c) M. A. Watzky, R. G. Finke, J. Am. Chem.
Soc. 1997, 119, 10382.
2
3
a) H.-G. Boyen, G. Kästle, F. Weigl, B. Koslowski, C.
Dietrich, P. Ziemann, J. P. Spatz, S. Riethmüller, C.
Hartmann, M. Möller, G. Schmid, M. G. Garnier, P.
Oelhafen, Science 2002, 297, 1533. b) R. Jin, Y. Zhu, H.
Qian, Chem.®Eur. J. 2011, 17, 6584.
Figure 4. Differential spectra during the UV-vis titration of
Py-DPA G1 with GaCl3.
4
a) R. M. Crooks, M. Zhao, L. Sun, V. Chechik, L. K. Yeung,
Acc. Chem. Res. 2001, 34, 181. b) R. W. J. Scott, O. M.
Wilson, R. M. Crooks, J. Phys. Chem. B 2005, 109, 692. c) D.
Astruc, F. Chardac, Chem. Rev. 2001, 101, 2991. d) C.
Ornelas, J. R. Aranzaes, L. Salmon, D. Astruc, Chem.®Eur.
J. 2007, 14, 50. e) L. M. Bronstein, Z. B. Shifrina, Chem. Rev.
2011, 111, 5301. f) N. Satoh, T. Nakashima, K. Kamikura, K.
Yamamoto, Nat. Nanotechnol. 2008, 3, 106. g) B. I. Lemon,
R. M. Crooks, J. Am. Chem. Soc. 2000, 122, 12886.
a) P. J. Pellechia, J. Gao, Y. Gu, H. J. Ploehn, C. J. Murphy,
Inorg. Chem. 2004, 43, 1421. b) K. Torigoe, A. Suzuki, K.
Esumi, J. Colloid Interface Sci. 2001, 241, 346.
Furthermore, to confirm the complexation of the core
(SITE0) and the first layer dendron, which is attached to the
pyridine site of the core (SITE1), the differential spectra during
the UV-vis titration of the Py-DPA G1 with GaCl3 were
analyzed. These spectra show distinctly different between
SITE0, SITE1, and the other coordination sites (Figure 4). This
result indicated that GaCl3 is first coordinating to SITE0, then
GaCl3 coordinates to SITE1 between 0 and 2 equiv of GaCl3,
and then GaCl3 coordinates to the other coordination sites of the
same layer dendron between 2 and 5 equiv of GaCl3.
5
6
Therefore, these results suggested that GaCl3 first coordi-
nates to SITE0, and then GaCl3 coordinates to SITE1 between
0 and 2 equiv of GaCl3. After that, the order of coordination in
the same layer is first to the coordination site of the dendrons
which were attached to the pyridine site of the core, then the
other coordination sites of the same layer dendron in the case of
the Py-DPA dendrimers. In addition, these results obviously
suggested that the core of the Py-DPA G4 formed a 1:1 complex
and the Py-DPA G4 become the “magic number 13” metal
cluster template. Furthermore, the Py-DPA G4 showed a
stepwise radial complexation with metal ions and quantitatively
formed a 1:1 complex.
In conclusion, a new type of the poly(phenylazomethine)
dendrimer, Py-DPA, with a new core that has one more
coordination site compared to the core of the TPM dendrimer
was synthesized and clearly revealed the stepwise complexation.
Therefore, by using Py-DPA G4 as the metal cluster template,
it would be possible to synthesize “magic number 13” metal
cluster in the near future.
a) K. Yamamoto, M. Higuchi, S. Shiki, M. Tsuruta, H.
Chiba, Nature 2002, 415, 509. b) M. Higuchi, S. Shiki, K.
Ariga, K. Yamamoto, J. Am. Chem. Soc. 2001, 123, 4414. c)
M. Higuchi, M. Tsuruta, H. Chiba, S. Shiki, K. Yamamoto,
J. Am. Chem. Soc. 2003, 125, 9988. d) R. Nakajima, M.
Tsuruta, M. Higuchi, K. Yamamoto, J. Am. Chem. Soc. 2004,
126, 1630.
7
a) I. Nakamula, Y. Yamanoi, T. Imaoka, K. Yamamoto, H.
Nishihara, Angew. Chem., Int. Ed. 2011, 50, 5830. b) T.
Imaoka, Y. Kawana, K. Yamamoto, Polym. Adv. Technol.
2011, 22, 1261. c) K. Takanashi, A. Fujii, R. Nakajima, H.
Chiba, M. Higuchi, Y. Einaga, K. Yamamoto, Bull. Chem.
Soc. Jpn. 2007, 80, 1563.
a) K. Yamamoto, T. Imaoka, W.-J. Chun, O. Enoki, H.
Katoh, M. Takenaga, A. Sonoi, Nat. Chem. 2009, 1, 397. b)
O. Enoki, H. Katoh, K. Yamamoto, Org. Lett. 2006, 8, 569.
D. Laliberté, T. Maris, J. D. Wuest, Can. J. Chem. 2004, 82,
386.
8
9
10 K. Takanashi, H. Chiba, M. Higuchi, K. Yamamoto, Org.
Lett. 2004, 6, 1709.
11 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
This work was supported in part by CREST program of
the Japan Science and Technology (JST) Agency, Grant-in-Aids
for Scientific Research on Innovative Areas “Coordination
Programming” (area 2107, No. 21108009) and by a Grant-in-
Chem. Lett. 2012, 41, 828-830
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/