Synthesis of novel phenylazomethine dendrimers having a cyclam core and their zinc complex

Article abstract of DOI:10.1021/ol034817j

(Matrix presented) Novel phenylazomethine dendrimers having a cyclam core were synthesized by the convergent method. The dendrimers showed selective coordination with zinc chloride on the cyclam ring and with tin chloride on the imine groups. The metal cy

Full text of DOI:10.1021/ol034817j

ORGANIC
LETTERS
2003
Vol. 5, No. 14
2547-2549
Synthesis of Novel Phenylazomethine
Dendrimers Having a Cyclam Core and
Their Zinc Complex
Osamu Enoki, Takane Imaoka, and Kimihisa Yamamoto*
Department of Chemistry, Faculty of Science & Technology, Keio UniVersity,
Yokohama 223-8522, Japan
yamamoto@chem.keio.ac.jp
Received May 13, 2003
ABSTRACT
Novel phenylazomethine dendrimers having a cyclam core were synthesized by the convergent method. The dendrimers showed selective
coordination with zinc chloride on the cyclam ring and with tin chloride on the imine groups. The metal cyclam exhibits a metal-assembling
function to provide a multinuclear hetero metal complex.
The metallodendrimer is an attractive material for catalysts1
and optic/electronic/magnetic devices2 as a result of total
control of its properties through modification of the chemical
structure. We found that phenylazomethine dendrimers have
the special property of performing a stepwise radial com-
plexation,3 layer-selective complexation between a tin(II)
chloride and the imine group from the core to periphery.
Recently, we reported that these metal-coordinated dendrons
cause an interesting effect on the core molecule and
drastically change its function.3b
Cyclam (1,4,8,11-tetraazacyclotetradecane, 1) is a mac-
rocyclic polyamine ligand for various metal salts and forms
a stable 1:1 complex. The cyclam and its complex are
interesting materials because some of them exhibit extra-
ordinary properties for catalysis or medical use.4 In this
Letter, we report the synthesis of novel phenylazomethine
dendrimers having a cyclam core and their metal complexes,
which are expected to provide a new structure for metal-
organic hybrid nanomaterials.
The dendrimer core 2 was obtained in good yield from 1
by simple reduction of the nitro groups after reaction with
R-bromo-p-nitrotoluene (Scheme 1). The phenylazomethine
dendrons were synthesized by the convergent method,5
repeating the dehydration between the amino and ketone
group using titanium(IV) chloride as a dehydrating agent.
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(3) (a) Yamamoto, K.; Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba, H.
Nature 2002, 415, 509. (b) Imaoka, T.; Horiguchi, H.; Yamamoto, K. J.
Am. Chem. Soc. 2003, 125, 340.
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10.1021/ol034817j CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/13/2003

Scheme 1. Synthesis of Phenylazomethine Cyclam Dendrimers and Their Zinc Complexes
The phenylazomethine cyclam dendrimers 3-5 were
but those on the imine groups were entirely removed through
the isolation process with GPC. ZnCl2 on the cyclam ring
merely remained, which was stabilized by the chelating
effect.
The 13C NMR signals of the cyclam ring became split and
much less intense by complexation. This suggested that the
synthesized via the same reaction for the dendrons, the
dehydration between the amino groups of 2 and the ketone
groups of the dendrons. The raw product was purified by
silica gel column chromatography; however, the isolation
of the dendrimer was difficult because the Rf value of the
dendron and that of the dendrimer were so similar at the
higher generation. To employ preparative scale gel perme-
ation chromatography (GPC), the dendron and the dendrimer
were perfectly separated by the distinction in molecular size.
The 1H NMR spectra of the dendrimer showed two regions
of signals, around 1.5-3.5 ppm for the core and around 6-8
ppm for the dendron. The core signals were broader at higher
generations, because the molecular vibration of the cyclam
ring was prevented by the steric hindrance of the bulky
dendrons. The MALDI-TOF MS of each dendrimer showed
the molecular ion [M]+ and a fragment ion [dendron + 90]+,
which was generated by the cleavage of the benzylamine
C-N bond at the cyclam core. The fragment ion was more
intense at the higher generation compared with the molecular
ion, suggesting that the charge of the fragment cation was
stabilized by the electronic and/or steric effect of the dendron.
The 1H NMR signals of the cyclam ring were shifted and
very broad as a result of the complexation; meanwhile, those
of the dendron were not (Figure 1). This result indicates that
ZnCl2 selectively coordinates on the cyclam ring, never on
the imines group. ZnCl2 may coordinate on the imine groups,
Figure 1. 1H NMR spectra of (a) G1 cyclam dendrimer and (b)
its zinc complex in CDCl3.
(5) (a) Higuchi, M.; Shiki, S.; Ariga, K.; Yamamoto, K. J. Am. Chem.
Soc. 2001, 123, 4414. (b) Higuchi, M.; Shiki, S.; Yamamoto, K. Org. Lett.
2000, 2, 3079.
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Org. Lett., Vol. 5, No. 14, 2003

conformation of the cyclam ring was completely fixed in
two forms, trans(III) and trans(I), the form that the cyclam
complex usually takes6 (Scheme 2).
Scheme 2. Change of Conformation State of Cyclam Core by
Complexation
Figure 2. UV-vis spectra change of Zn(G3-cyclam)Cl2 on addition
Coordination of the dendrimer complex with a tin chloride
was confirmed by the titration method. With the addition of
a tin chloride to the dendrimer complex in solution, the UV-
vis spectrum gradually changed with the isosbestic point
(Figure 2). This change continued until the molecular amount
of the added tin chloride was equal to the stoichimetry of
the imine groups. This result indicated that the dendrimer
complex was capable of assembling the very same number
of tin chlorides as the imine groups.
In conclusion, we synthesized novel dendrimers having a
cyclam core. The dendrimer selectively coordinated a ZnCl2
on the cyclam ring, never on the dendron imines, and formed
a stable 1:1 complex that was soluble in an organic solvent.
The structure of the dendrimer was investigated by NMR
and showed that the steric property of the core was affected
of tin chloride.
by the generation of the dendrimer and coordinated metal.
The dendrimer complex was capable of assembling the very
same number of tin chlorides as the imine groups. As a
consequence, a dendrimer is able to possess two kinds of
metal salts in its structure, and these dendrimers are expected
to be used as novel organic-inorganic hybrid materials.
Acknowledgment. This work was partially supported by
the 21st COE program (Keio-LCC) and for Scientific
Research (15655019, 15350073, 15036262) from MEXT,
Japan, and KAST Research Grant (Project 23).
Supporting Information Available: Detailed experi-
mental procedures and characterization data of phenyl-
azomethine cyclam dendrimers and their zinc complexes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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