Pyridine-based dendritic wedges with a specific metal ion coordination site and their palladium(II) complexes

Article abstract of DOI:10.1039/a900783k

The 2,6-di(thiomethyl)pyridine sub-unit has been incorporated into pyridine-based dendritic structures providing a series of dendritic wedges, which readily form Pd(II) complexes.

Full text of DOI:10.1039/a900783k

Pyridine-based dendritic wedges with a specific metal ion coordination site and
their palladium(ii) complexes
Gavino Chessa,* Alberto Scrivanti, Luciano Canovese, Fabiano Visentin and Paolo Uguagliati
Dipartimento di Chimica, Università di Venezia, Calle Larga S. Marta, 2137, Venezia, Italy. E-mail: chessa@unive.it
Received (in Cambridge, UK) 28th January 1999, Accepted 19th April 1999
OBn
The 2,6-di(thiomethyl)pyridine sub-unit has been incorpo-
rated into pyridine-based dendritic structures providing a
series of dendritic wedges, which readily form PdII com-
plexes.
i or ii
N
2b
S
S
The incorporation of metal centres into a dendritic structure is a
challenging idea aimed at generating materials with potentially
useful features.1–3 Numerous dendrimers, which contain coor-
dination sites on the outer surface,4 within the macrostructure
throughout all layers,5 and at the inner core6 have been
prepared. In contrast, except for scant coordination studies,7
very little has been done about the construction of dendrimers
with specific coordination centres within the dendritic wedges
and their reaction with metal substrates. In particular, no report
has yet appeared on the synthesis of dendrimers with a pyridine-
based skeleton incorporating a specific coordination site for
platinum-group metals.
3 R = I
4 R = II
5 R = III
OR
OR
X
N
X
N
I
X
O
O
X
As a preliminary study of the assembly of such dendrimers
we report herein the synthesis of the pyridine-based dendrons
3–5 containing the 2,6-di(thiomethyl)pyridine sub-unit, which
has been shown to form very stable PdII complexes.8 The
complexation of PdII with 3–5 to give metallodendrons 6–8 is
also reported on.
X = CO2Et
N
N
II
X
X
Our approach to the synthesis of the dendrons 3-5 is based on
a convergent strategy which implies the preparation of the
dendritic fragments I–III as 4-hydroxo species9 followed by
attachment to the metal binding site 2b. The key building block
2b (Scheme 1) was prepared by coupling 2,6-bis(chlor-
omethyl)-4-benzyloxypyridine 19 with 4-mercaptobenzyl alco-
hol10 (DMF, K2CO3, 18-crown-6, 70 °C). This reaction
occurred with preferential formation of the thioether bond,
providing the hydroxymethyl derivative 2a. Subsequent chlor-
ination (SOCl2, 50 °C) followed by attachment of the diester I
to the resulting chloride 2b, gave 3 in 92% yield after
purification by column chromatography (SiO2, CH2Cl2–2%
MeOH) (Scheme 2). Similarly, the reactions of 2b with
tetraester II and octaester III yielded the dendrons 4 and 5
(Scheme 2) in 85 and 51% yield, respectively.
N
O
O
X
X
O
N
X
N
X
O
N
N
O
O
III
X
N
X
X
N
X
Scheme 2 Reagents and conditions: i, K2CO3, DMF, 70 °C, 24 h; ii, K2CO3,
DMF, 60 °C, 48 h.
The dendrons obtained were fully characterised by IR and
NMR techniques as well as by matrix assisted laser desorption
ionization mass spectrometry (MALDI-MS; dithranol 10 mg
cm23 acetone matrix). The reaction of the dendritic fragment 3
with Pd(PhCN)2Cl2 in CH2Cl2 at 25 °C (1:1 Pd/3 molar ratio)
gave a stable orange microcrystalline PdII complex 6 in 76%
yield (Fig. 1). Direct evidence of a successful complexation at
SH
OBn
OBn
OBn
N
HO
N
+
S
S
i
N
S
Pd
S
Cl
Cl
Cl
1
X–
O
O
R
R
X
X
X– = Cl–, ClO4
–
6 R = I
7 R = II
8 R = III
2a X = OH
2b X = Cl
ii
Scheme 1 Reagents and conditions: i, K2CO3, 18-crown-6, DMF, 70 °C,
Fig. 1 Schematic representation of metallodendrons 6–8 (dendritic
24 h; ii, SOCl2,50 °C, 4 h.
polypyridines of 0, 1, 2 generation).
Chem. Commun., 1999, 959–960
959

the 2,6-di(thiomethyl)pyridine site is provided by the sig-
nificant downfield shifts of the thiomethylene protons (1.2 ppm)
and carbons (11.2 ppm) in the 1H NMR and 13C NMR spectra,
respectively. The absence of the 3,5-pyridine protons peak at d
6.87 further supports the complexation hypothesis. Moreover,
the presence of the characteristic molecular peak at m/z 1074
([3PdCl]+) in the MALDI-TOF spectrum of 6 clearly indicates
the formation of a 1:1 complex between PdII and 3.
Notes and references
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The complexation of PdII with the dendron 4 did not proceed
smoothly under the same experimental conditions. The 1H
NMR spectrum of a CDCl3 solution of complex 7, isolated as its
chloride salt, suggests the presence of an undesired product. A
possible explanation for its formation is the competition of
chloride ion as a nucleophile with other donor centres in the
dendritic structure, namely the sulfur and nitrogen atoms of the
2,6-di(thiomethyl)pyridine sub-unit.11 However, 7 was success-
fully purified by conversion into its yellow ClO42 salt, the 1H
NMR spectrum of which exhibits a pattern similar to those
observed for 6, furthermore the MALDI-TOF mass spectrum
shows a parent ion at m/z 1792 coorresponding to [4PdCl]+.
Extension of this finding to the reaction of 5 with PdII allowed
us to obtain metallodendron 8. As in the foregoing spectra, the
resonances associated with the 2,6-di(thiomethyl)pyridine frag-
ment are affected by complexation and lie at lower field than
those of the uncoordinated domain. In particular, the CH2S
singlet, which is found at d 4.21 in a uncoordinated ligand, shifts
to d 5.26 (br s) in a coordinated unit. Once again, the MALDI-
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peak at m/z 3225 ([5PdCl]+) provides convincing evidence for
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synthetic approach (1:1 Pd/5 molar ratio and the ensuing 78%
yield) and the presence of one single signal for the carboxylate
ethyl protons (X = CO2Et) strongly support the formulation of
Fig. 1.
These results indicate that the presence of the 2,6-di(thiome-
thyl)pyridine sub-unit in dendritic wedges is particularly
attractive for developing dendrimers with specifically localised
metal centres. The physical properties of the metallodendrons
6–8 are now under extensive investigations. Of particular
interest are the selectivity towards other transition metals and
the influence of dendrimer framework on the metal-binding
process.
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Seraglia (C.N.R. of Padova) for mass spectra, and Mrs L.
Gemelli for technical assistance.
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Chem. Commun., 1999, 959–960