A second generation dendrimer incorporating nine S2N2-donor macrocycles and its palladium(II) complex

Article abstract of DOI:10.1039/b205776j

A new second generation dendrimer incorporating nine S₂N₂-donor macrocyclic units that bind nine Pd(II) cations is reported.

Full text of DOI:10.1039/b205776j

A second generation dendrimer incorporating nine S2N2-donor
macrocycles and its palladium(II) complex
Ian M. Atkinson,a Jy D. Chartres,a Andrew M. Groth,a Leonard F. Lindoy,*b Mark P. Loweb and
George V. Meehan*a
a School of Pharmacy and Molecular Sciences, James Cook University, Townsville, Qld. 4811, Australia.
E-mail: george.meehan@jcu.edu.au
b Centre for Heavy Metals Research, School of Chemistry F11, University of Sydney, Sydney, N.S.W. 2006,
Australia. E-mail: lindoy@chem.usyd.edu.au
Received (in Cambridge, UK) 14th June 2002, Accepted 6th September 2002
First published as an Advance Article on the web 24th September 2002
A new second generation dendrimer incorporating nine
S2N2-donor macrocyclic units that bind nine Pd(II) cations is
reported.
In a separate reaction, the amide groups of 5 were reduced with
borane–dimethyl sulfide complex to give the substituted benzyl
alcohol 6 in 89% yield (Scheme 2). This two-step sequence was
much more efficient (overall yield 86%) than direct reduction of
4 with borane–dimethyl sulfide complex which produced 6 in
very low yield.
Conversion of the benzyl alcohol 6 to the corresponding
chloride 7 (in 95% yield) using triphenylphosphine in refluxing
carbon tetrachloride, provided the required dendron for the
peripheral section of the target second generation dendrimer.
Thus, the peripheral amino groups of macrocyclic trimer 2 (X =
OCH2CH2, R = H) were alkylated by 7 (3 equiv.) in refluxing
THF containing approximately 10 equiv. of hexamethyl-
phosphorous triamide (HMPA) and sodium carbonate as base.
The desired dendrimer 8 was obtained as a viscous oil in 69%
yield after purification by column chromatography (Scheme
3).
Interest in the potential applications of nanoscopic structures
has fuelled rapid progress in the chemistry of dendrimers and
hyperbranched polymers.1 While less studied than purely
organic systems, the inclusion of transition metals into such
structures is of particular interest as the resulting materials may,
for example, display novel magnetic, electronic and/or catalytic
properties.2,3 Dendritic structures incorporating macrocyclic
ligand units have received less attention.4–7
Recently, we have reported metal binding studies8,9 involv-
ing the tri-linked N2S2-donor macrocyclic ligands derived from
the common precursor 1.10 We now report an extension of these
studies in which the linking of nine such rings in a dendritic
architecture has been achieved.
Dendrimer 8 has a similar substitution pattern on all nine of
its constituent macrocyclic rings. In the 1H NMR spectrum
dendrimer 8 exhibits both the characteristic doublet–triplet
combination (d 6.46 and 6.34) for the protons of the un-
symmetrical aromatic ring linking the first and second genera-
tions, and the expected singlet (d 6.05) indicating the symmetry
of the central core.
Dendrimer 8 (Mr > 3600 amu), like its precursors, was found
to tenaciously retain solvent even after prolonged evacuation.
As a consequence, mass spectrometry was the method of choice
for determining molecular composition. The FTICR electro-
spray mass spectrum of 8 shows a prominent [M + 2H]2+ peak
at m/z 1813.988 (C195H314N18O9S18 requires [M + 2H]2+,
1813.982), with this composition being supported by a close
coincidence of the predicted and observed isotopic envelopes
for this ion (Fig. 1).
At the outset it was decided to employ the linked macrocyclic
AtrimerA 2 (X = OCH2CH2, R = H), as the core in a Adouble-
stage convergentA11 strategy for forming the dendrimer. In
principle, this should simplify the synthesis by reducing the
number of simultaneous reactions that need to occur at the
individual stages of the stepwise process. The commercial
availability of 3,5-dihydroxybenzaldehyde suggested a readily
accessible route to the preparation of an appropriate bifurcating
dendron for use in such an approach. This substituted
benzaldehyde possesses the necessary bifurcation capacity, and
by appropriate manipulations of the aldehyde functionality
should provide a dendron capable of alkylating the core 2 (X =
OCH2CH2, R = H). Through this approach, the macrocyclic
ring substitution pattern in the second generation dendrimer
would be similar to that in the previously prepared trimer 2 (X
= OCH2CH2, R = benzyl)10—a desirable feature in the present
context since it would facilitate a planned comparative
investigation of the difference in metal ion binding between the
dendritically linked macrocyclic product and its discrete ring
monomeric counterpart.9
Attempts to prepare Pd(II
)
complexes from either
Pd(CH3CN)2Cl2 [formed in situ from palladium(II) chloride and
acetonitrile] in the presence of excess hexafluorophosphate, or
from palladium(II) acetate followed by addition of excess
ammonium hexafluorophosphate, resulted in yellow solids
whose microanalysis suggested that they were highly (and
variously) solvated. Frustratingly, the Pd(II) complex of 8
resisted mass spectral characterisation using a variety of
Macrocyclic chloroamide 3 was prepared from 1 as pre-
viously described,10 and used to bis-alkylate 3,5-dihydrox-
ybenzaldehyde in 94% yield (Scheme 1). The aldehyde group of
the resulting bis-macrocycle 4 was then reduced with sodium
borohydride to give the corresponding alcohol 5 in 97% yield.
Scheme 1 Reagents and conditions: Cs2CO3, DMF, rt, 12 h.
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CHEM. COMMUN., 2002, 2428–2429
This journal is © The Royal Society of Chemistry 2002

Fig. 1 (a) Predicted isotopic envelope for the [M + 2H]2+ ion in the mass
spectrum of 8 and (b) observed mass spectral distribution.
point at 1+9—consistent with the formation of the expected
dendritic complex species.
Scheme 2 Reagents and conditions: i, NaBH4, CH2Cl2–MeOH, rt, 2 h; ii,
BH3·Me2S, THF, reflux, 12 h; iii, PPh3, CCl4, reflux, 12 h.
ionisation methods (EI, FAB, MALDI or electrospray). In each
case only uncharacterised low mass fragment ions were
observed, perhaps reflecting the 18+ charge that would be
associated with a nonanuclear cation of the present type.
Finally, definitive evidence for the complexation of nine
Pd(II) ions by 8 was obtained via spectrophotometric titration.
The titration was employed to follow complexation of Pd(II) by
the dendritic ligand 8. The addition of 8 in acetonitrile to PdCl2
in acetonitrile was characterised by the build up of a peak at 300
nm, with an associated decrease of the PdCl2 absorption at 245
nm. The increase in absorbance at 340 nm was used to derive the
plot shown in Fig. 2 which shows a sharp ligand to metal end
Fig. 2 Spectrophotometric titration curve for the addition of 8 to
palladium(II) chloride in acetonitrile.
We thank the Australian Research Council for support.
Notes and references
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Scheme 3 Reagents and conditions: 0.32 equiv. 2 (X = OCH2CH2, R = H),
Na2CO3, HMPA, THF, reflux, 72 h.
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