Self-assembly of hyperbranched spheres

Article abstract of DOI:10.1039/a608577f

A new type of building block with two coordinatively unsaturated palladium centres has been described that self-assembles in nitromethane solution and disassembles when acetonitrile is added. The resulting hyperbranched, organopalladium spheres have a remarkably narrow size distribution as was evidenced by light-scattering, AFM and TEM measurements. Variation of the structure of the building blocks showed the possibility to vary the size of the self-assembled spheres between 100 and 400 nm.

Full text of DOI:10.1039/a608577f

Self-assembly of hyperbranched spheres
Wilhelm T. S. Huck, Frank C. J. M. van Veggel* and David N. Reinhoudt*
L aboratory of Supramolecular Chemistry and T echnology, T wente University, P.O. Box 217, 7500 AE Enschede,
T he Netherlands
A new type of building block with two coordinatively unsaturated palladium centres has been described that self-assembles in
nitromethane solution and disassembles when acetonitrile is added. The resulting hyperbranched, organopalladium spheres have a
remarkably narrow size distribution as was evidenced by light-scattering, AFM and TEM measurements. Variation of the
structure of the building blocks showed the possibility to vary the size of the self-assembled spheres between 100 and 400 nm.
There is an ongoing demand for the development of nanosize
materials. Especially, the electronics industry is searching for
nanosize architectures permitting increased processing speeds,
and higher levels of integration. Current nanophysical ‘engin-
eering down’ approaches can routinely fabricate structures in
the range of 0.1–0.2 mm. Alternatively, chemists exploit a
‘bottom up’ approach to synthesize nanosize devices with
molecular precision. In this respect, synthetic organic chemistry
oÂers a powerful and versatile tool for the synthesis of large
and complex molecules with a variety of functions and shapes.
However, the synthesis of very large molecules using only
covalent bonds is less practical. Non-covalent interactions can
be exploited to connect building blocks in the construction of
much larger architectures. The rational use of non-covalent
interactions requires the understanding of the principles of
self-assembly.1 Our current research aims at the development
of new methods of self-assembly that will lead to large struc-
tures.2 Dendrimers represent a class of polymers that combine
a high molecular mass with a well defined spherical shape of
nanometre dimensions. Sequential synthesis, either divergent3
or convergent,4 can be used to grow dendrimers stepwise,
generation after generation. Alternatively, instead of using this
laborious method a one-pot polymerization of suitable building
blocks yields less regular hyperbranched organic polymers.
tures have been synthesized and the resulting assemblies have
been studied by quasi-elastic light scattering (QELS), atomic
force microscopy (AFM) and transmission electron microscopy
(TEM). In this paper we present a general methodology to
synthesize spherical particles in the range 100–400 nm.
Experimental
Melting points were determined with a Reichert melting point
apparatus and are uncorrected. 1H NMR and 13C NMR
spectra were recorded in CDCl (unless indicated otherwise)
3
with Me Si as internal standard on a Bruker AC 250 spec-
4
trometer. Mass spectra were recorded with a Finnigan MAT
90 spectrometer using m-nitrobenzyl alcohol as a matrix. THF
was freshly distilled from Na/benzophenone, hexane (referring
to light petroleum with bp 60–80 °C) and CH Cl from K CO .
2
2
2
3
Nitromethane was washed with 1  HCl and water and
distilled from CaCl . NaH was a 50% dispersion in mineral
2
oil and was used after washing with hexane. Other chemicals
were of reagent grade and were used as received. Column
chromatography was performed with silica gel 60H
(0.005–0.040 mm) from Merck. [Pd(MeCN) ][BF ] ,8 5-
4
4 2
hydroxyisophthalic acid dimethyl ester,9 and a,a∞,a◊-tribromo-
mesitylene10 were prepared according to literature procedures.
QELS was performed with a Malvern PCS 100 goniometer
(Malvern Instruments, Malvern, England) at an angle of 90°,
with an Adlas Model DPY 305 II, 50 mW, continuous wave
Recently, Frechet et al. used branched monomers that contain
´
polymerizable groups at both ends, so-called AB -type mon-
2
omers, that yield hyperbranched polymers with a relatively
narrow size distribution and high molecular masses.5
Recently, several groups have used transition metals to build
small metallodendrimers.6 Our approach is to design AB -type
2
building blocks for the self-assembly of hyperbranched coordi-
nation polymers.7 These building blocks should contain all
information necessary to form well defined three-dimensional
hyperbranched structures. In each building block 1, two kin-
etically inert Pd centres and one kinetically labile CH CN
2
ligand, are present. In a coordinating solvent, like MeCN, a
solvent molecule is weakly coordinating to the fourth coordi-
nation site and hence the building blocks remain monomeric.
Removal of this solvent and introduction of a non-coordinating
solvent, such as MeNO , initiates intermolecular coordination
2
of the labile ligands. This means that a new building block is
added to the starting nucleus generating two new sites for
further growth. Schematically the self-assembly process is
shown in Fig. 1.
1
In a dendritic model, the centre of the self-assembled spheres
will be less dense than the surface. Therefore, the size of the
building blocks should be related to the size of the assemblies
formed. For this reason, building blocks with diÂerent struc-
Fig. 1 Schematic representation of the self-assembly of hyper-
branched polymers
J. Mater. Chem., 1997, 7(7), 1213–1219
1213

diode pumped YAG laser, wavelength 532 nm (Adlas, Lubeck,
¨
layer was concentrated under reduced pressure. The TBDMS
groups were then removed by dissolving the thioether in THF
(100 ml) and adding 1 equiv. of CsF (2.0 g, 0.013 mol). After
stirring overnight at 50°C, THF was evaporated and the
residue dissolved in dichloromethane followed by washing with
Germany), and an ALV 5000 multiple tau digital correlator
(ALV, Langen, Germany). The correlation function was trans-
formed into a diameter distribution with the CONTIN pro-
gram. The instrument used for atomic force microscopy (AFM)
was a Nanoscope III operating in constant force (ca. 50 nN)
mode with cantilever force constants of 0.58 N m−1 and a
home-made supertip. Samples for transmission electron
microscopy (TEM) were prepared by slow evaporation of a
nitromethane solution on a carbon-coated copper grid.
brine, drying over MgSO and concentration in vacuo. Column
4
chromatography (silica gel, eluent CH Cl ) gave pincer ligand
2
2
6a as a colourless oil. Yield 3.38 g (77%); 1H NMR d, 7.29–7.16
(m, 10H, SPh), 6.79 (s, 1H, ArH), 6.64 (s, 2H, ArH), 4.89 (br
s, 1H, OH), 4.00 (s, 4H, CH S); 13C NMR d, 155.7, 139.5,
2
131.2, 129.9, 128.9, 128.6, 126.4, 121.8, 114.6, 38.7; EIMS m/z,
5-tert-Butyldimethylsilyloxyisophthalic acid dimethyl ester 3
338.078 (M+, calc. for C H OS : 338.080).
20 18
2
TBDMSiCl (16.0 g, 0.106 mol) was dissolved in CH Cl (50 ml)
2
2
3,5-Bis(-1-naphthylthiamethyl)phenol 6b. White solid. Yield
76%, mp 75–77 °C; 1H NMR d, 7.88–7.68 (m, 8H, Snaphthyl),
7.48–7.30 (m, 6H, Snaphthyl), 6.90 (s, 2H, ArH), 6.72 (s, 1H,
and was slowly added to a solution of 5-hydroxyisophthalic
acid dimethyl ester 2 (11.18 g, 0.053 mol), Et N (6.99 g,
3
0.069 mol), and DMAP (catalytic amount) in CH Cl (150 ml)
2
2
ArH), 4.09 (s, 4H, CH S) 13C NMR d, 155.8, 139.4, 133.7,
at 0°C. After stirring overnight at room temperature, the
mixture was washed with 1  HCl, a saturated aqueous
solution of NaHCO , and brine. After drying (MgSO ), the
2
131.9, 128.4–125.8, 122.0, 114.8, 38.5; EIMS m/z, 438.402 (M+,
calc. for C H OS : 438.601).
28 22
2
3
4
solvent was evaporated and 3 was obtained as a white solid
upon drying under high vacuum. Yield 15.3 g (89%), mp
68–70 °C; 1H NMR d, 8.29 (s, 1H, ArH), 7.68 (s, 2H, ArH),
3.93 (s, 6H, OCH ), 1.00 (s, 9H, But), 0.23 (s, 6H, SiCH ); 13C
3,5-Bis(tert-butylthiamethyl)phenol 6c. Colourless oil, solidi-
fied slowly upon standing. Yield 47%, mp 80–82 °C; 1H NMR
d, 6.89 (s, 1H, ArH), 6.70 (s, 2H, ArH), 4.68 (bs, 1H, OH), 3.67
(s, 4H, CH S), 1.30 (s, 18H, CH ); 13C NMR d, 155.0, 140.5,
3
3
NMR d, 166.1, 156.0, 131.8, 125.4, 123.7, 52.6, 25.6, 18.2, −4.5;
FAB MS m/z, 324.1 (M+, calc. 324.1); Anal. Calc. for
C H O Si: C, 59.23; H, 7.46. Found: C, 59.55; H, 7.60%.
2
3
122.0, 114.5, 33.2, 30.9, 23.6; EIMS m/z, 298.142 (M+, calc. for
C H OS : 298.143).
16 26
2
16 24
5
3,5-Bis(ethylthiamethyl)phenol 6d. Pale yellow oil. Yield
23%; 1H NMR d, 6.83 (s, 1H, ArH), 6.69 (s, 2H, ArH), 5.21
3,5-Bis(hydroxymethyl )phenol tert-butyldimethylsilyl ether 4
LiAlH (4.8 g, 0.12 mol) was suspended in dry THF (200 ml)
(br s, 1H, OH), 3.62 (s, 4H, CH S), 2.44 (q, 4H, J 8.4 Hz,
4
2
and a solution of diester 3 (20.0 g, 0.062 mol) in THF (100 ml)
SCH CH ), 1.23 (t, 6H, J 8.4 Hz, CH ); 13C NMR d, 155.8,
2
3
3
was slowly added. The mixture was stirred overnight at room
temperature after which THF was evaporated. The resulting
paste was dissolved in dichloromethane (200 ml) and cooled
to 0 °C and 2  HCl (100 ml) was added, after which the layers
were separated. Extraction of the aqueous layer with dichloro-
methane (3×100 ml) and drying of the combined organic
layers gave, after removal of the solvent, pure 4 as a white
solid. Yield 15.3 g (93%), mp 99–100 °C; 1H NMR d, 6.94 (s,
140.5, 121.8, 114.4, 35.6, 25.6, 14.4; EIMS m/z, 242.080 (M+,
calc. for C H OS : 242.080).
12 18
2
a,a∞-Dibromo-a◊-cyanomesitylene 7. To a solution of a,a∞,a◊-
tribromomesitylene (5.0 g, 0.014 mol) in MeCN (200 ml) were
added powdered KCN (0.91 g, 0.014 mol), 18-crown-6 (0.25 g,
0.95 mmol), and water (0.5 ml). The mixture was refluxed for
48 h after which the solvent was evaporated and the residue
was taken up in CH Cl (100 ml). After washing with brine
1H, ArH), 6.77 (s, 2H, ArH), 4.62 (s, 4H, CH O), 0.99 (s, 9H,
2
But), 0.20 (s, 6H, SiCH ); 13C NMR d, 156.0, 142.8, 118.1, 64.9,
3
2
2
and drying over MgSO the crude reaction mixture was
4
25.7, 18.2, −4.4; FAB MS m/z, 268.0 (M+, calc. 268.1); Anal.
concentrated in vacuo and purified by column chromatography
Calc. for C H O Si: C, 62.64; H, 9.01. Found: C, 62.59;
(SiO , eluent CH Cl ). This yielded pure 7 as a colourless oil
14 24
3
2
2
2
H, 9.19%.
which solidified upon standing. Yield 0.85 g (20%), mp
67–69 °C; 1H NMR d, 7.40 (s, 1H, ArH), 7.32 (s, 2H, ArH),
4.47 (s, 4H, CH Br), 3.72 (s, 2H, CH CN); 13C NMR d, 139.6,
3,5-Bis(chloromethyl)phenol tert-butyldimethylsilyl ether 5
2
2
131.3, 129.3, 128.5, 117.3, 32.0, 23.4; EIMS m/z, 302.9 (M+,
calc. 303.0); IR (KBr) 2252 cm−1 (CON); Anal. Calc. for
C H NBr : C, 39.64; H, 2.99; N, 4.62. Found: C, 39.69; H,
Diol 4 (4.0 g, 0.015 mol) and Et N (6.2 g, 0.045 mol) were
3
dissolved in dry CHCl and cooled to 0 °C. Mesityl chloride
3
(MsCl) (3.44 g, 0.045 mol) was slowly added at 0 °C after
10
9
2
2.94; N, 4.50%.
which the reaction mixture was slowly heated to 50 °C. Stirring
overnight and subsequent washing with 1  NaOH and 1 
HCl, drying over MgSO and evaporation of the solvent gave
General procedure for the synthesis of thioether ligands 8a–d
4
5 as a colourless oil. Yield 4.2 g (92%); 1H NMR d, 6.80 (s,
a,a∞-Bis[3,5-bis(phenylthiamethyl)phenyloxy]-a◊-cyano-
mesitylene 8a. Compound 6a (0.3 g, 0.75 mmol) was stirred
with K CO (0.13 g, 0.94 mmol) in MeCN (50 ml) for 1 h at
1H, ArH), 6.62 (s, 2H, ArH), 4.32 (s, 4H, CH Cl), 0.78 (s, 9H,
2
But), 0.00 (s, 6H, SiCH ); 13C NMR d, 156.1, 139.2, 121.4,
3
118.9, 45.7, 26.6, 24.7, −3.5; EIMS m/z, 304.081 (M+, calc. for
2
3
room temperature (r.t.). Spacer 7 (0.13 g, 0.38 mmol) was added
and the reaction mixture was stirred for 4 days at r.t. The
reaction was concentrated under reduced pressure and
dichloromethane (50 ml) was added to the residue. The organic
C H SiCl O: 304.081).
14 22
2
General procedure for the synthesis of thioethers 6a–d
3,5-Bis(phenylthiamethyl)phenol 6a. Thiophenol (5.77 g,
0.052 mol) was added to a stirred suspension of NaH (2.52 g,
0.104 mol) in THF (200 ml) and the mixture was stirred for
1 h to allow formation of the sodium thiophenolate salt. To
the resulting milky solution dichloride 5 (4.0 g, 0.013 mol) was
added and the reaction mixture was stirred overnight at 50 °C.
After removal of the solvent the crude reaction mixture was
dissolved in dichloromethane (200 ml) and washed with brine
(100 ml). To remove excess of thiol the organic layer was
washed with 2  NaOH (100 ml). After drying, the organic
layer was washed with brine, dried over MgSO and concen-
4
trated. Column chromatography (silica gel, CH Cl –hexanes
2
2
80520) gave pure 8a as a colourless oil. Yield 0.26 g (72%);
1H NMR d, 7.31 (s, 1H, Ar H), 7.29–7.16 (m, 22H,
CN
SPh+Ar H), 6.85 (s, 2H, Ar H), 6.79 (s, 4H, Ar H), 4.94 (s,
CN
2
O
2
O
4H, CH O), 4.03 (s, 8H, CH S), 3.73 (s, 2H, CH CN); 13C
2
NMR d, 158.6, 139.3, 138.4, 136.1, 130.7, 129.9, 128.9, 126.4,
126.0, 122.2, 117.6, 114.0, 69.2, 38.9, 23.5; EIMS m/z, 817.216
(M+, calc. for C H NO S : 817.218); IR (KBr) 2251 cm−1
50 43
2 4
(CON).
1214
J. Mater. Chem., 1997, 7(7), 1213–1219

a,a∞-Bis[3,5-bis(1-naphthylthiamethyl)phenyloxy]-a◊-cyano-
mesitylene 8b. Colourless oil. Yield 70%; 1H NMR d, 7.80–7.59
(m, 16H, Snaphthyl), 7.47–7.32 (m, 12H, Snaphthyl), 7.30 (s,
1H, Ar H), 7.21 (s, 2H, Ar H), 6.95 (s, 2H, Ar H), 6.75 (s,
0.14 g (20%), mp 131–132 °C; 1H NMR d, 7.38 (s, 1H, Ar H),
CN
2
7.32 (s, 2H, Ar H), 6.68 (s, 4H, Ar H), 4.97 (s, 4H, CH O),
CN
Pd
4.10 (br s, 8H, CH S), 3.81 (s, 2H, CH CN), 1.69 (s, 36H, But);
2
2
13C NMR d, 156.2, 150.6, 138.5, 130.8, 126.5, 117.6, 108.4, 69.5,
52.0, 42.7, 30.6, 23.6; FAB MS m/z 984.3 [(M−Cl)+, calc.
983.6]; Anal. Calc. for C H NO S Pd Cl ·H O: C, 46.97; H,
CN
O
CN
2
O
2
4H, Ar H), 4.74 (s, 4H, CH O), 4.12 (s, 8H, CH S), 3.62 (s,
2H, CH CN); 13C NMR d, 158.7, 139.2, 138.3, 133.6, 131.9,
2
50 41
2 4
2
2
2
130.5, 128.8, 128.0, 127.8, 127.7, 126.5, 126.3, 125.9, 125.8, 122.3,
5.36; N, 1.38. Found: C, 47.58; H, 5.36; N, 1.38%.
114.2, 69.2, 38.8; EIMS m/z, 1017.280 (M+, calc. for
C H NO S : 1017.280).
Bis(PdMCl) complex 9d. Brownish solid. Yield 16%, mp
66 51
2 4
106–107 °C; 1H NMR d, 7.39 (s, 1H, Ar H), 7.36 (s, 2H,
CN
a,a∞-Bis[3,5-bis(tert-butylthiamethyl)phenyloxy]-a◊-cyano-
mesitylene 8c. Slightly yellow oil. Yield 25%. 1H NMR d, 7.46
(s, 1H, Ar H), 7.37 (s, 2H, Ar H), 6.96 (s, 2H, Ar H), 6.85
Ar H), 6.68 (s, 4H, Ar H), 4.98 (s, 4H, CH O), 4.2 (br s, 8H,
CN
2
Pd
2
2
CH S), 3.81 (s, 2H, CH CN), 3.19 (q, 8H, J 8.4 Hz, SCH CH ),
2
3
1.67 (t, 12H, J 8.4 Hz, CH ); FAB MS m/z 871.8 [(M−Cl)+,
CN
CN
O
3
(s, 4H, Ar H), 5.08 (s, 4H, CH O), 3.80 (s, 2H, CH CN), 3.71
calc. 871.3]. Anal. Calc. for C H NO S Pd Cl ·0.5C H : C,
O
2
2
34 41
2 4
2
2
6 14
(s, 4H, CH S), 1.33 (s, 36H, CH ); 13C NMR d, 158.7, 140.4,
46.74; H, 5.09; N, 1.47. Found: C, 46.33; H, 4.96; N, 1.66%.
2
3
138.6, 130.6, 126.4, 122.5, 113.9, 69.3, 42.9, 33.4, 30.9, 23.6;
EIMS m/z, 737.339 (M+, calc. for C H NO S : 737.343).
42 59
2 4
Results and Discussion
a,a∞-Bis[3,5-bis(ethylthiamethyl)phenyloxy]-a◊-cyano-
mesitylene 8d. Colourless oil. Yield 48%; 1H NMR d, 7.47 (s,
1H, Ar H), 7.37 (s, 2H, Ar H), 6.88 (s, 2H, Ar H), 6.85 (s,
Synthesis of building blocks
A convergent synthesis route has been exploited to connect
two pincer ligands to a spacer containing the kinetically labile
(cyano) ligand. Key intermediates 6 were synthesized in six
steps from 5-hydroxyisophthalic acid as shown in Scheme 1.
The first step is the esterification of 5-hydroxyisophthalic acid
according to literature procedures.8 The phenolic group was
protected with TBDMSiCl in 95% yield and subsequently the
CN
O
CN
2
O
4H, Ar H), 5.06 (s, 4H, CH O), 3.77 (s, 2H, CH CN), 3.67 (s,
2
8H, CH S), 2.44 (q, 8H, J 8.4 Hz, SCH CH ), 1.22 (t, 12H, J
2
2
3
8.4 Hz, CH ); 13C NMR d, 158.7, 141.3, 138.5, 130.7, 127.8,
3
126.1, 122.2, 120.7, 117.6, 113.7, 69.2, 53.5, 37.5, 25.4, 23.5, 14.4;
FAB MS m/z, 625.620 (M+, calc. for C H NO S : 625.959).
34 43
2 4
esters were reduced with LiAlH to the diol 4 in 70% yield.
General procedure for the cyclopalladation of the pincer ligands
8a–d and conversion into the chloride complexes 9a,b and d
4
The diol was stirred overnight with MsCl–Et N at 50 °C which
3
gave complete conversion into the dichloride 5. The thioether
Bis(PdMCl) complex 9a. Ligand 8a (0.50 g, 0.62 mmol) was
dissolved in acetonitrile (150 ml) and placed under an Ar
atmosphere. Solid [Pd(MeCN) ][BF ] 7 (0.54 g, 1.22 mmol)
functions were introduced by stirring the appropriate thiol
with NaH in THF to generate the sodium thiolate, and
subsequent addition of the dichloride 5. This reaction yielded
the SCS pincer ligands with the p-hydroxy functions protected.
The TBDMSi ether was deprotected with CsF to give the SCS
pincer type ligand 6 in 10% overall yield from 5-hydroxyisoph-
thalic acid. The thioethers 6 were coupled to spacer 7 which
introduces the weakly coordinating cyano group. The spacer
7 was prepared from a,a∞,a◊-tribromomesitylene by refluxing
in acetonitrile for two days with powdered KCN. From the
resulting mixture 7 could be isolated in 20% yield. The
formation of the benzylic ethers proceeded in rather poor
yields because 7 slowly decomposed. Ligands 8a–d were cyclo-
palladated with [Pd(MeCN) ][BF ] in acetonitrile in high
4
4 2
was added in one portion. The orange solution was warmed
to 40 °C and stirred until the colour changed to pale yellow.
After cooling to r.t. and evaporation of the solvent the yellow
cyclopalladated product was obtained in quantitative yield.
This product was dissolved in CH Cl –MeCN (351, 100 ml)
2
2
and the reaction mixture was stirred vigorously for 30 min
with brine (100 ml). The layers were separated and the organic
layer was washed with water (100 ml) and evaporated to
dryness. Purification by column chromatography (silica gel,
CH Cl –MeOH, 9555) gave 9a as a yellow solid (1.18 g, 50%),
2
2
mp 132–133 °C; 1H NMR d, 7.81–7.74 (m, 8H, SPh), 7.39 (s,
1H, Ar H), 7.35–7.29 (m, 14H, SPh+Ar H), 6.64 (s, 4H,
4
4 2
yields. Prior to self-assembly studies, the cationic solvento
complexes were converted into chloropalladium complexes by
stirring a solution in CH Cl and MeCN with brine. This
CN
CN
Ar H), 4.95 (s, 4H, CH O), 4.5 (bs, 8H, CH S), 3.76 (s, 2H,
Pd
2
2
CH CN); 13C NMR d, 156.4, 152.3, 150.2, 138.4, 132.3, 131.4,
2
2
2
130.9, 129.8, 129.7, 126.6, 126.0, 117.7, 109.2, 69.5, 51.7, 23.6;
made purification easier and allowed the simple introduction
of various non-coordinating anions (vide infra). The tert-butyl
derivative 9c was prepared by stirring the cationic palladium
FAB MS m/z, 1064.3 [(M−Cl)+, calc. 1064.3]; IR (KBr),
2252 cm−1 (CON). Anal. Calc. for C H NO S Pd Cl ·H O:
50 41
2 4
2
2
2
C, 53.72; H, 3.88; N, 1.25. Found: C, 53.65; H, 3.73; N, 1.64%.
complex from 8c with NMe Cl, as reaction with brine only
4
gave intractable polymeric products.
Bis(PdMCl) complex 9b. Orange solid. Yield 56%, mp
160–162 °C; 1H NMR d, 8.30 (s, 4H, Snaphthyl), 7.94–7.76 (m,
16H, Snaphthyl), 7.52–7.49 (m, 8H, Snaphthyl), 7.40 (s, 1H,
Ar H), 7.34 (s, 2H, Ar H), 6.67 (s, 4H, Ar H), 4.99 (s, 4H,
The 1H NMR spectra of 9a–d in CD CN show a broad
3
singlet for the CH SR protons at d 4.6 because of the slow
2
conformational interconversion of the palladium()-containing
five-membered rings.11 The signal for the protons ortho to
both donor atoms in 9a–d at d 6.85 is absent, indicating
complete cyclopalladation. The 1H NMR spectrum clearly
showed that no cyclopalladation had occurred at other
aromatic positions.
CN
CN
Pd
CH O), 4.63 (br s, 8H, CH S), 3.76 (s, 2H, CH CN); 13C
2
2
2
NMR d, 157.2, 150.3, 138.4, 133.4, 133.2, 131.2, 129.7, 129.3,
128.2, 127.8, 127.6, 127.1, 109.2, 69.5, 52.4, 23.1; FAB MS
m/z 1264.3 ([M−Cl]+, calc. 1264.6). Anal. Calc. for
C H NO S Pd Cl ·2H O: C, 59.33; H, 3.99; N, 1.05; S, 9.60.
Self-assembly takes place when the CH CN group of one
66 49
2 4
2
2
2
2
Found: C, 58.93; H, 3.73; N, 1.18; S, 9.46%.
building block coordinates intermolecularly to the Pd centre
of another. Therefore the Pd centres were first activated by
the replacement of the chloride by diÂerent non-coordinating
anions. Addition of one equivalent of the appropriate silver
salt activates the Pd centre by precipitation of AgCl in a fast
and quantitative reaction.† In acetonitrile solution a bis-
Bis(PdMCl) complex 9c. The cyclopalladation of ligand 8c
(0.50 g, 0.68 mmol) was performed as described for 9a. The
crude palladium complex was dissolved in CH Cl –MeCN
2
2
(151, 50 ml) and excess NMe Cl (0.50 g, 4.57 mmol) was added
4
in one portion. The reaction mixture was stirred overnight.
After removal of the salts, the filtrate was evaporated to
dryness. Purification by column chromatography (silica gel,
CH Cl –MeOH, 9555) gave 9c as a brownish solid. Yield
† This has been demonstrated using 31P NMR spectroscopy in an
analogous PCP pincer complex; unpublished results.
2
2
J. Mater. Chem., 1997, 7(7), 1213–1219
1215

Scheme 1
Scheme 2 X−=BF −, ClO −, PF −, triflate, tosylate, BPh −
4
4
6
4
acetonitrile complex·2X− (X−=BF −, ClO −, PF −, triflate,
indicating large structures. When small amounts of acetonitrile
were added to these solutions all signals became sharp. This
proves that the self-assembly of 1a–d and disassembly of the
hyperbranched polymer is a reversible process. In order to
obtain information on the size and shape the resulting assembl-
ies were further characterized by QELS, TEM and AFM.‡
QELS measurements of nitromethane solutions of assemblies
4
4
6
tosylate or BPh −) is formed (Scheme 2).
4
Self-assembly and characterization
The self-assembly process of 1a–d was initiated by elimination
of the acetonitrile ligands. After removal of the acetonitrile
(the 1H NMR spectrum in CD NO showed no acetonitrile),
3
2
of 1a (X−=BF −) showed particles with an average hydrody-
4
the CH CN groups of the bis-palladium complexes occupy the
2
namic diameter of 180 nm using the CONTIN curve-fitting
fourth coordination site. In the IR spectrum the coordination
of the cyano group was confirmed by the characteristic shift
of the CON stretch vibration from 2250 cm−1 for the mon-
omeric building blocks to 2290 cm−1 upon coordination.12
The 1H NMR spectra of 1a–d showed broad peaks in CD NO ,
‡ Gel permeation chromatography (GPC) was not successful. Probably
the highly charged assemblies have a strong interaction with the
column material and are therefore diÃcult to characterize using GPC.
3
2
1216
J. Mater. Chem., 1997, 7(7), 1213–1219

aggregates than expected, but this might be explained by
solvent or water coordination to the anion even inside the
spheres, making the apparent size of this anion larger.
Increasing the thioether bulk from ethyl to phenyl groups
(X−=tosylate) gave smaller aggregates. The naphthyl thioether
building blocks showed a similar trend as the phenyl thioethers.
The larger naphthyl groups gave smaller spheres in all cases.
The combination of naphthyl thioethers with triflate anions
formed rather small aggregates. The general conclusion from
these data is that larger anions and/or thioether groups give
smaller assemblies.
–
400
300
200
–
–
100
0
125
225
anion volume/Å3
320
25
The size of the aggregates was also measured by contact
mode AFM. Samples were prepared by evaporation of a
nitromethane solution of self-assembled spheres 1a–d on a
clean gold surface. A representative part of these surfaces is
shown in Fig. 3(a)–( f ). In all cases, large, spherical objects
Fig. 2 Relationship between the sphere size and monomer or anion
volume as measured by light-scattering (+ , Et; *, Ph;
$ , naphthyl)
l
, But;
Fig. 3 AFM pictures of self-assembled spheres of (a) 9a (X−=BF −); (b) 9b (X−=BF −); (c) 9b (X−=OTf−); (d) 9d (X−=BF −); (e) 9a
4
4
4
(X−=Tos); ( f ) 9a (X−=PF −)
6
program. This size is independent of the sample concentration
indicating that these are single particles and no clusters. From
the peak width a standard deviation of approximately 30 nm
was estimated. By changing the anion from ClO − to BPh −
were observed that correspond quite well with the dimensions
determined by light-scattering. The average diameter for
assemblies of 1a (X−=BF −), as found by the grain size
4
analysis routine of the instrument software of these aggregates
4
4
the size of the spheres as measured by QELS decreased from
400 to 180 nm in diameter (Fig. 2).
is 205 nm, with a standard deviation of 30 nm. The aggregates
seem to have a disc-like shape when studied with AFM. The
flattening might be caused by spreading of the spheres on the
surface or by interaction of the sample with the AFM tip.
Repetitive scans of one spot ‘wiped’ the surface clean, which
indicates that the organic material has a soft constitution.
The sizes of the anions were calculated using the Connoly
Surfaces option implemented in the Cerius2 program pack-
age.13 This gave arbitrary volumes which should be used only
in a relative sense. The BF − anion gave much smaller
4
J. Mater. Chem., 1997, 7(7), 1213–1219
1217

When a glass substrate was used instead of gold, the same
spheres were observed of roughly the same size. This indicates
that the possible interaction of sulfur atoms in the building
blocks and the gold surface did not significantly alter the
morphology of the spheres. Grazing-angle FTIR spectroscopy
on a gold surface covered with the spheres showed the charac-
teristic CON signal of coordinated cyano groups at 2289 cm−1
in agreement with the bulk spectra. From AFM data sizes of
A linear, non-branched palladium() complex containing
one pincer complex and one cyanomethyl group was treated
according to the same self-assembly procedures but AFM and
TEM measurements did not show globular structures. This
supports our concept in which branching is essential.
Indications for a dendritic structure, which results in a dense
outer sphere, came from additional disassembly experiments.
Light-scattering experiments show that when ca. 20 equiv. of
acetonitrile per building block are added to a nitromethane
solution of the spheres, they slowly disassemble in the course
of 10–15 min. The slow rate of disassembly indicates that
acetonitrile cannot easily penetrate the outer shell of building
blocks. When a larger nitrile like benzonitrile is used,
disassembly is hardly observed, even after heating to 70 °C
for 15 min. Dendrimers with a similar dense shell with a
solid-phase character have been reported by Meijer and
co-workers.15
160 (±20) nm for 1b (X−=BF −) and 110(±20) nm for 1b
4
(X−=OTf−) were measured respectively [Fig. 3(b) and (c)].
This is in good agreement with the QELS data (140 and
104 nm, respectively). In the case of 1a (X−=Tos) and 1a
(X−=PF −) extensive clustering made size determination
6
unreliable [Fig. 3(e)–( f )].
The samples for TEM were used without additional shading
or staining and the contrast results from the Pd centres that
are distributed throughout the spherical assembly. In all pic-
tures, the only structures that could be observed had spherical
shapes in the expected size range. Fig. 4(a) and (b) clearly show
globular aggregates in the range 150–200 nm for 1a and 1c
The explanation for the formation of rather well defined
spheres and the relationship between the size of the spheres
and the structure of the monomers and the non-coordinating
anions, remains speculative. In our dendritic model the anions
will occupy the voids created by the branching of the mon-
omers. When these cavities become too small the anions are
forced out of the sphere and will occupy the surface, thereby
blocking the assembly process. The size at which this occurs,
is apparently dependent on both the bulk of the bis-palladium
complexes as well as the size of the anion.
(both with BF − counter anions). This is in good agreement
4
with the diameter measured with QELS. Fig. 4(a) shows globu-
lar structures with a light shell and a little darker nucleus.
This indicates that the structures are a little thicker in the
middle than at the edges, as expected for spherical assemblies.
Energy dispersive X-ray spectrometry (EDX) revealed the
presence of the elements Pd and S in these aggregates. The
TEM image for 1a (X−=Tos) shows assemblies of ca. 225 nm
[Fig. 4(c)], which is also in good agreement with QELS data
(220 nm). As shown in Fig. 4(d), strong clustering is observed
Conclusions
for 1d (X−=BF −). By dividing the radius of the spheres by
Building blocks that contain all information necessary for self-
assembly give regular assemblies with a (relatively) small
polydispersity. The self-assembled structures are held together
via coordinative bonds. Introduction of bulky groups in the
building blocks and/or diÂerent sizes of non-coordinating
counter anions influences the outcome of the self-assembly
process. In this way spherical assemblies are obtained ranging
from 100 to 400 nm in diameter.
4
the size of the building blocks we estimate a number of roughly
50 ‘generations’. The outer layer of Pd complexes is not
occupied by MeCN ligands, as these are not observed in the
1H NMR spectrum. Probably, water present in nitromethane
is coordinating to these Pd centres.14
We are grateful to Dr. E. G. Keim (Center for Materials
Research, University of Twente) for TEM measurements and
Dr. J. W. Th. Lichtenbelt (Akzo Nobel Central Research) for
assistance with QELS measurements. We thank the Dutch
Foundation for Chemical Research (SON) for financial
support.
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Fig. 4 TEM pictures of (a) 9a (X−=BF −); (b) 9c (X−=BF −);
4
4
(c) 9a (X−=Tos); (d) 9d (X−=BF −) (bar represents 100 nm)
4
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