Dendrimers based on multiple 1,4,7-triazacyclononane derivatives

Article abstract of DOI:10.1039/a909535g

New first and second generation dendritic poly-1,4,7-triazacyclononane molecules have been prepared and shown to coordinate multiple CU(II) and Ni(II) metal cations.

Full text of DOI:10.1039/a909535g

Dendrimers based on multiple 1,4,7-triazacyclononane derivatives
Paul D. Beer* and De Gao
Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK
OX1 3QR. E-mail: paul.beer@chem.ox.ac.uk
Received (in Cambridge, UK) 2nd December 1999, Accepted 9th February 2000,
Published on the Web, 29th February 2000
New first and second generation dendritic poly-1,4,7-triaza-
cyclononane molecules have been prepared and shown to
coordinate multiple Cu(II) and Ni(II) metal cations.
reaction of 8 with 3 equiv. of 5 in DMF in the presence of
1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride
(EDC) and 1-hydroxybenzotriazole hydrate (HOBt) (Scheme
3). After column chromatography using alumina and CH2Cl2, 9
was isolated as a yellow solid in 90% yield. The convergent
preparative approach is illustrated with the synthesis of 10 from
6 and 2 equiv. of 5 using a similar condensation reaction
(Scheme 4).
The last decade has witnessed an enormous growth in the field
of dendrimer chemistry with current interest being stimulated
by the discovery of specific functions and novel properties that
are a direct consequence of the dendritic architecture.1 Metal-
lodendrimers2 in particular have been shown to exhibit novel
electrochemical and photophysical properties. With a view to
creating new efficient extraction and membrane transporting
reagents for transition metals3 and for the preparation of metal
nanoclusters4,7 we report here, the first examples of dendrimers
containing the well established transition metal coordinating
ligand, 1,4,7-triazacyclononane.5
The new key asymmetric functionalised triazacyclononane
branching synthons 5 and 6 required for convergent and
divergent syntheses of the target dendritic macrocyclic mole-
cules were prepared as shown in Scheme 1. The reaction of 15
with 2 equiv. of 3-nitrobenzyl chloride in the presence of
K2CO3 in MeCN solution gave 2 in 90% yield. Detosylation of
2 using sulfuric acid produced 3 in 50% yield. Refluxing 3 with
1 equiv. of butyl bromoacetate in the presence of K2CO3
afforded 4 as a pale yellow solid in 82% yield. Hydrolysis of 4
with formic acid gave 5 in 80% yield and reduction of 4 using
hydrogen and Raney Ni produced the diamine 6 quantitatively
(Scheme 1).
Scheme 3 Reagents and conditions: EDC, HOBt, DMF, room temp.
Scheme 4 Reagents and conditions: EDC, HOBt, DMF, room temp.
Raney Ni/H2 reduction of 9 and reaction of the corresponding
amine 11 with 6 equiv. of 5 in DMF gave the second generation
dendrimer 12 in 60% yield (Scheme 5).†
Preliminary transition metal coordination investigations of
dendrimers 9 and 12 with Cu(II) and Ni(II) metal cations have
been undertaken using UV–VIS spectrophotometric titration
experiments. In the absence of dendrimer, MeCN and DMF
solutions of copper(II) acetate exhibit a broad d–d absorption at
710 nm. The addition of dendrimers 9 or 12 causes lmax to
Scheme 1 Reagents and conditions: i, 2 equiv. 3-nitrobenzyl chloride,
K2CO3, MeCN, reflux; ii, conc. H2SO4, 150 °C; iii, butyl bromoacetate,
K2CO3, MeCN, reflux; iv, formic acid, reflux; v, H2, Raney Ni, MeOH–
THF, 55 °C.
The dendritic core triazacyclononane derivative 8 was
synthesised via reaction of 1,4,7-triazacyclononane with 3
equiv. of 3-nitrobenzyl chloride to give 7 followed by reduction
with Raney Ni and H2 (Scheme 2).
Using a divergent synthetic procedure the first generation
dendrimer compound 9 was synthesised via a condensation
undergo a blue shift to 646 nm which is characteristic of Cu(II
being bound by amine ligands.5 The spectrophotometric
titration curves (Fig. 1) suggest 9 binds four, and 12 ten Cu(II
)
)
metal cations, one Cu is coordinates to each triazacyclononane
macrocyclic ligand of the respective dendrimer, suggesting the
transition metal is complexed at the core, branches and surface
dendritic recognition sites.‡ This result contrasts with PAMAM
dendrimers which are thought to bind most Cu cations at the
outermost tertiary amine donor sites of the dendrimer structure.7
With Ni(II) preliminary spectrophotometric titration results with
the first generation dendrimer 9 indicate as with Cu(II), each
Scheme 2 Reagents and conditions: i, 3 equiv. 3-nitrobenzyl chloride,
K2CO3, MeCN, reflux; ii, H2, Raney Ni, MeOH–THF, 55 °C.
DOI: 10.1039/a909535g
Chem. Commun., 2000, 443–444
This journal is © The Royal Society of Chemistry 2000
443

Scheme 5 Reagents and conditions: i, H2, Raney Ni, MeOH–THF, 55 °C; ii, 6 equiv. 5, EDC, HOBt, DMF, room temp.
CH2), 7.06–8.18 (36H, m, aromatics), 9.96 (3H, s, amide). C99H111N21O15
requires C, 63.36; H, 6.35; N, 16.68. Found: C, 62.95; H, 6.41; N, 16.37%.
FAB MS: m/z 1764 (M + H).
10: 1H NMR (CDCl3) d 1.43 (9H, s, CH3), 1.59–2.86 (36H, m, CH2CH2,
ring), 3.27 (2H, s, CH2), 3.52 (4H, s, CH2), 3.60 (4H, s, CH2), 3.78 (8H, s,
CH2), 6.46–8.20 (24H, m, aromatics), 9.96 (2H, s, amide). C70H89N15O12
.
FAB MS: m/z 1332 (M + H).
12: 1H NMR (CDCl3) d 2.60–2.86 (120H, m, CH2CH2, ring), 3.20 (6H,
s, CH2), 3.32 (6H, s, CH2), 3.50 (12H, s, CH2), 3.64 (12H, s, CH2), 3.76
(24H, s, CH2), 6.95–8.20 (84H, m, aromatics), 9.96 (9H, s, amide).
Fig. 1 Spectrophotometric titration of 8 (-) (3 mM) with Cu(OAc)2 in
acetonitrile, absorbance at 646 nm and of 12 (/) (0.5 mM) with Cu(OAc)2
in DMF, absorbance at 600 nm.
C225H273N51O33. ES MS: m/z (2109.87)2+, (1406.9)3+
.
‡ This assumes the absorption coefficients are additive. Also the error in
estimating the stoichiometry of metal uptake from the titration data is ±1 or
2.
triazacyclononane ligand complexes one Ni(II) cation resulting
in 4 Ni2+: 9 solution stoichiometry.
Further transition metal coordination chemistry investiga-
tions with these novel dendritic poly-triazacyclononane-mole-
cules are in progress.
We thank the Royal Society for a K. C. Wong Fellowship
(D. G.) and the EPSRC mass spectrometry service at the
University of Wales, Swansea.
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Notes and references
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6 J. L. Sessler, J. W. Sibert and V. Lynch, Inorg. Chem., 1990, 29, 4143.
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L. Balogh and D. A. Tomalia, J. Am. Chem. Soc., 1998, 120, 7355.
† All new compounds were characterised by 1H NMR, mass spectrometry
and elemental analysis.
Selected data: 9: 1H NMR (CDCl3) d 2.76 (16H, s, CH2CH2, ring), 2.85
(32H, s, CH2CH2, ring), 3.27 (6H, s, CH2), 3.54 (6H, s, CH2), 3.78 (12H, s,
Communication a909535g
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Chem. Commun., 2000, 443–444