Microsphere vs. microbelt morphology of ionic palladium(II) complexes

Article abstract of DOI:10.1246/bcsj.81.1461

A series of palladium(II) complexes containing bis[(4-isonicotinoyloxy) phenyl]sulfide (L), [Pd^II(L)(tmen)]₂X₄(tmen =N,N,N^'N^'-tetramethylethane-1,2-diamine; X = NO ₃ and CIO₄) were prepared and characterized. Reaction of [Pd(tmen)](NO₃)₂ with L in a mixture of water and acetone affords microspheres whereas reaction of [Pd(tmen)](ClO₄)₂ with L produces microbelts. The solubility of the products is a significant factor for the formation of morphology. The spherical sizes can be controlled by the evaporation rate of solvents. An array of ≈300 nm and ≈ 4μ bimodal spheres was carried out via sonication.

Full text of DOI:10.1246/bcsj.81.1461

ꢀ 2008 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 81, No. 11, 1461–1464 (2008) 1461
Microsphere vs. Microbelt Morphology
of Ionic Palladium(II) Complexes
Tae Hwan Noh,1 In Sung Chun,1 Young-A Lee,2 Sungho Ahn,3
ꢀ1
Jongki Hong,3 and Ok-Sang Jung
1Department of Chemistry, Pusan National University, Pusan 609-735, Korea
2Department of Chemistry, Chonbuk National University, Jeonju 561-756, Korea
3College of Pharmacy, Kyung Hee University, Seoul 130-701, Korea
Received April 15, 2008; E-mail: oksjung@pusan.ac.kr
A series of palladium(II) complexes containing bis[(4-isonicotinoyloxy)phenyl]sulfide (L), [PdII(L)(tmen)]2X4
(tmen = N,N,N0,N0-tetramethylethane-1,2-diamine; X ¼ NO3 and ClO4) were prepared and characterized. Reaction of
[Pd(tmen)](NO3)2 with L in a mixture of water and acetone affords microspheres whereas reaction of [Pd(tmen)](ClO4)2
with L produces microbelts. The solubility of the products is a significant factor for the formation of morphology. The
spherical sizes can be controlled by the evaporation rate of solvents. An array of ꢁ300 nm and ꢁ4 mm bimodal spheres
was carried out via sonication.
Elemental analysis (C, H, and N) was carried out at the Pusan
Center, KBSI, using a Perkin-Elmer 2400 CHNS analyzer. Infra-
red spectra were obtained in the 4000–400 cmꢂ1 range on a
Perkin-Elmer 16F PC FTIR spectrophotometer, the samples
One recent hot issues in supramolecular chemistry is the for-
mation and control of ordered morphology.1–4 The ability to
systematically manipulate micro-sized morphology remains
an important goal for functional micro materials such as cata-
lysts, electronic devices, drug-delivery systems, ceramics, pig-
ments, and cosmetics.5–12 The strategy for controlling such
task-specific shapes including spheres, rods, tetrapods, prisms,
and cubes is the modification of chemical structures or, alter-
natively, the change of external condition.13 Steric effects, sur-
face tension, capillary effects, electric and magnetic forces,
van der Waals interaction, hydrophilic interactions, pendent
functional groups, and the surfactant/precursor ratio have been
applied as driving forces in the formation of artificial morphol-
ogy.14–21 When the binding affinity of simple organic/inorgan-
ic molecules is sufficiently high, a particle-based morphology
is superior to single crystals. A variety of well-defined metal
chalcogenide shapes have been produced, but the morphogen-
esis of discrete metal complexes is rare. Thus, a new, easy
method for the morphogenesis of metal complexes, one that
does not require any organic additive is highly desired. More-
over, no attention has been focused on the counter anion ef-
fects on the formation of ionic metal complexes. To explore
the direct counter anion effects on morphology, we report
the relevant properties of a uniform morphology formed with-
out the addition of any additives and based on discrete ionic
palladium(II) complexes of bis[(4-isonicotinoyloxy)phenyl]-
sulfide (L). An array of bimodal spheres via sonication was at-
tempted. Such diaminepalladium(II) chemistry might be appli-
cable to various fields such as catalysis,22 the synthesis of
square planar corner building molecules,23 and the preparation
of a magic ring with a ‘‘dual character Pd–N bond.’’24
1
having been prepared as KBr pellets. The H NMR spectra were
recorded on a Varian Gemini 300 operating at 300.00 MHz,
and the chemical shifts were found to relative to internal SiMe4.
Thermal analyses were performed under a nitrogen atmosphere
at a scan rate of 10 ꢃC minꢂ1 with a Stanton Red Croft TG 100.
Mass spectrometric analysis via fast atom bombardment was
performed in chloroform, using a KMS-700 Mstation Mass
Spectrometer (Jeol, Japan) and an MS-MP9020D data system.
The SEM images were measured on a Hitachi S-4200.
Preparation of Bis[(4-isonicotinoyloxy)phenyl]sulfide (L).
A chloroform solution (30 mL) of isonicotinoyl chloride hydro-
chloride (0.06 mol, 10.56 g) was added to a chloroform solution
(30 mL) of 4,40-thiodiphenol (0.03 mol, 6.54 g) and pyridine at
0 ꢃC. The reaction mixture was stirred for 3 h at room temperature.
The mixture was quenched with a saturated aqueous K2CO3 solu-
tion (50 mL) and extracted with chloroform (3 ꢄ 50 mL). The or-
ganic layer was dried using MgSO4, and the solvent was removed
to afford pure L as colorless solid (yield: 92%). Anal. Calcd for
C24H16N2O4S: C, 67.28; H, 3.76; N, 6.54%. Found: C, 67.21;
H, 3.75; N, 6.41%. 1H NMR (300 MHz, CDCl3, SiMe4): ꢀ 7.20
(dd, 4H, J ¼ 4:8 Hz, J ¼ 1:8 Hz), 7.44 (dd, 4H, J ¼ 4:5 Hz, J ¼
1:8 Hz), 8.01 (dd, 4H, J ¼ 3:0 Hz, J ¼ 1:0 Hz), 8.87 (dd, 4H,
J ¼ 3:0 Hz, J ¼ 1:8 Hz). 13C NMR (125.76 MHz, CDCl3, SiMe4):
ꢀ 163.82, 151.10, 149.95, 136.72, 133.73, 132.58, 123.41, 122.65.
[Pd(L)(tmen)]2(NO3)4. L (0.1 mmol, 0.042 g) in 10 mL of
acetone was slowly added to an aqueous solution (10 mL) of
[Pd(tmen)](NO3)2 (0.1 mmol, 0.035 g). The mixture was stirred
for 2 h at room temperature. Evaporation of acetone afforded
white spherical product (yield: 70–83%). Slow evaporation for 3
days at ambient temperature produced ꢁ4 mm diameter micro-
spheres while fast evaporation of acetone for 1 min using a rotary
evaporator gave 300 nm diameter submicrospheres. After the
product was dried in a vacuum oven for 24 h, elemental analysis
Experimental
Materials and Measurements.
[Pd(tmen)](NO3)2 and
[Pd(tmen)](ClO4)2 were prepared according to established proce-
dures,25,26 as was bis[(4-isonicotinoyloxy)phenyl]sulfide (L).3
Published on the web November 12, 2008; doi:10.1246/bcsj.81.1461

1462 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008)
Microsphere vs. Microbelt Morphology
S
O
O
(tmen)PdX2 +
O
O
N
N
-
-
NO3
ClO4
Figure 1. SEM images of microspheres, [Pd(L)(tmen)]2-
(NO3)4 and microbelts, [Pd(L)(tmen)]2(ClO4)4. Scale
bar: 5 mm.
S
S
O
O
O
In the case of Xꢂ ¼ NO3ꢂ, the reaction yields amorphous mi-
crospheres whereas, for Xꢂ ¼ ClO4ꢂ, the reaction produces
crystalline microbelts. Microspheres of 6–8 mm were obtained.
The size of the monodisperse spheres could be controlled by
the evaporation rate of the solvents. For example, fast evapo-
ration of acetone using a rotary evaporator produces submicro-
spheres of ꢁ300 nm diameters. Thus, slow evaporation of ace-
tone produces larger spheres whereas fast evaporation of ace-
tone affords smaller spheres. The microspheres were collected
by filtration using a membrane (membrane filter, Advantec
O
O
O
O
N
N
N
N
N
Pd
Pd
N
N
N
4X-
O
Scheme 1.
was conducted. Anal. Calcd for C60H64N12O20Pd2S2 4H2O: C,
ꢅ
44.42; H, 4.47; N, 10.36%. Found: C, 44.11; H, 4.40; N,
10.21%. 1H NMR (300 MHz, Me2SO-d6, SiMe4): ꢀ 2.61 (s,
12H, CH3), 3.02 (s, 8H, CH2CH2), 7.29 (d, 8H, J ¼ 4:8 Hz,
Ar–H), 7.41 (d, 8H, J ¼ 1:8 Hz, Ar–H), 8.28 (d, 8H, J ¼
4:8 Hz, Ar–H), 9.47 (d, 8H, J ¼ 6:6 Hz, Ar–H). IR (KBr, cmꢂ1):
1748 (s, CO), 1381 (s, NO3ꢂ).
[Pd(L)(tmen)]2(ClO4)4. The same reaction as with L, but
with [Pd(tmen)](ClO4)2 in place of [Pd(tmen)](NO3)2, produced
[Pd(L)(tmen)]2(ClO4)4. Crystalline microbelts were easily ob-
tained after slow evaporation of acetone at room temperature
(yield: 90%). Anal. Calcd for C60H64Cl4N8O24Pd2S2: C, 42.39;
1
MFS Inc.) for further characterization. The H NMR and IR
spectra of both complexes exhibited a similar pattern. The
water-solubility of the complexes was not sufficient for
1
measurement of NMR spectra in water. Instead, the H NMR
spectrum of [Pd(L)(tmen)]2(NO3)4 in a mixture of water and
Me2SO (1:1) (2 mM) did not show any catenane phenomenon
in the solvent system. The carbonyl peaks of the metal
complexes (1749–1751 cmꢂ1) were blue-shifted relative to that
of L (1745 cmꢂ1). The characteristic anion peaks appeared at
1
H, 3.79; N, 6.59%. Found: C, 42.15; H, 3.70; N, 6.69%. H NMR
1382 cmꢂ1 for NO3 and 1092 cmꢂ1 for ClO4ꢂ. In order to
ꢂ
(300 MHz, Me2SO-d6, SiMe4): ꢀ 2.60 (s, 12H, CH3), 3.08 (s, 8H,
CH2CH2), 7.29 (d, 8H, J ¼ 4:8 Hz, Ar–H), 7.41 (d, 8H,
J ¼ 2:1 Hz, Ar–H), 8.28 (d, 8H, J ¼ 4:8 Hz, Ar–H), 9.47 (d,
8H, J ¼ 6:3 Hz, Ar–H). IR (KBr, cmꢂ1): 1748 (s, CO), 1086 (s,
ClO4ꢂ). FAB-Mass (matrix: nitrobenzyl alcohol): m=z ¼ 1075:0;
½Dimer ꢂ L ꢂ HClO4 ꢂ ClO4ꢆþ, 1173.7; ½Dimer ꢂ L ꢂ ClO4ꢆþ,
1490.5; ½Dimer ꢂ tmen ꢂ ClO4ꢆþ, 1600.5; ½Dimer ꢂ ClO4ꢆþ.
Samples for SEM. The spheres were collected by the filtra-
tion using a membrane (membrane filter, Advantec MFS Inc.)
for further characterization. The spheres were re-dispersed in dis-
tilled water, the suspension was added drop-wise on a glass plate
(5 ꢄ 5 mm2). The product was dried in a desicator, and then SEM
images of the spheres on the glass plate were taken. The SEM im-
age of the crystalline microbelts, [Pd(L)(tmen)]2(ClO4)4 was mea-
sured after the obtained microcrystalline materials were dried.
Array of Bimodal Spheres in H2O. Equimolar microspheres
and submicrospheres (10 mg) were sonicated in 10 mL of water at
room temperature for 3 min.
measure the molecular weight of the [Pd(L)(tmen)]2(ClO4)4,
the spheres were dissolved in N,N-dimethylformamide, and
then the solution was mixed with 3-nitrobenzyl alcohol
(Sigma, USA) on a FAB probe tip. The molecular weight
was determined by the presence of the main fragment peaks
and isotope ratios at m=z ¼ 1075:0; ½Dimer ꢂ L ꢂ HClO4 ꢂ
ClO4ꢆþ, 1173.7; ½Dimer ꢂ L ꢂ ClO4ꢆþ, 1490.5; ½Dimer ꢂ
tmen ꢂ ClO4ꢆþ, 1600.5; ½Dimer ꢂ ClO4ꢆþ (Supporting Infor-
mation), indicating that the skeletal structure was a cyclodimer
consisting of two square-planar palladium(II) units. Both prod-
ucts are stable solids, and soluble in N,N-dimethylformamide,
dimethylsulfoxide, or a mixture of water and acetone.
Morphology and Properties. Scanning electron micro-
scope (SEM) images show that [(Pd(L)(tmen)]2(NO3)4 formed
microspheres whereas [Pd(L)(tmen)]2(ClO4)4, under the same
conditions, produces microbelts (Figure 1). The microspheres
were of regular size (6–8 mm diameters) and had smooth sur-
faces. They contained 8% H2O based on thermogravimetric
analysis (TGA) data (Figure 2). Only half of the water mole-
cules could be evaporated, even in a vacuum desicator at
70 ꢃC for 24 h. The TGA showed that the spheres are thermally
stable up to 200 ꢃC. As the water component on the surface was
reduced, the surface became cracked. The cracked spheres
again absorbed water molecules, but the smooth surface did
not return. Such spheres are soluble in water or acetone but only
with difficulty, and are most effectively soluble in a mixture
(5:5) of water and acetone (Figure 3), indicating that the
Results and Discussion
Synthesis. The ionic palladium(II) complexes, [Pd(L)-
(tmen)]2(X)4, were prepared by the reaction of an aqueous so-
lution of [Pd(tmen)]X2 (tmen = N,N,N0,N0-tetramethylethane-
1,2-diamine; Xꢂ ¼ NO3 and ClO4ꢂ) with an acetone solu-
ꢂ
tion of L, as shown in Scheme 1. In a typical preparation, L
(0.1 mmol) in acetone was added slowly to [Pd(tmen)]X2
(0.1 mmol) in distilled water. The mixture was refluxed for
2 h, and evaporation of acetone at ambient temperature
afforded a white product of [Pd(L)(tmen)]2(X)4 in high yield.

T. H. Noh et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008) 1463
(a)
(b)
(c)
140
120
100
80
- H2O
+ H2O
X
+ H2O
(b)
(a)
100
80
60
60
40
0
10
20
30
40
50
60
Time(s)
40
Figure 4. Contact angles of a water-droplet on the surfaces
of spheres, [Pd(L)(tmen)]2(ClO4)4 (solid line) and mi-
crobelts, [Pd(L)(tmen)]2(NO3)4 (dashed line).
20
0
200
400
600
800
Temperature (°C)
Figure 2. Process of evaporation (a ꢀ b) and re-adsorp-
tion (b ꢀ c) of trace water on the microsphere (top) and
thermogravimetric analyses of (a) and (b) microspheres
(bottom).
10
8
6
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
Acetone
H2O
Figure 5. Array of bimodal spheres, [Pd(L)(tmen)]2(NO3)4
via sonication. Bar = 5 mm.
Figure 3. Solubility of microspheres, [Pd(L)(tmen)]2-
(NO3)4 (solid line) and microbelts, [Pd(L)(tmen)]2(ClO4)4
(dashed line) in mixture of water and acetone.
spheres (105ꢃ) was smaller than that for the microbelts
(129ꢃ), indicating that the microsphere surfaces are the more
hydrophilic. Thus, the wettability of the morphology is strongly
dependent on the hydrophilicity of the counter anions. It is
worth noting that measuring contact angles is a useful tech-
nique for determining quantitative counter anion effects on
the hydrophilicity of micro shapes. Such a modulation of sur-
face properties such as wettability, adhesion, and biocompati-
bility has important implications for both fundamental and
spheres are typical amphiphilic materials. The crystalline mi-
crobelts are best soluble in a mixture (8:2) of water and acetone.
The solubility seems to affect to the formation of desirable mor-
phology. In the case of spheres, such an amphiphilic solubility
might be a driving force behind the formation of microspherical
morphology. The part around the palladium(II) cations and the
NO3ꢂ counter anions seems to be hydrophilic, and the L moiety
seems to be hydrophobic. Of course, L itself does not show
amphiphilic properties or formation of any spherical morphol-
technological advances.27–29 Thus, the contact ꢂangles areꢂcon-
30
sistent with a modified Hofmeister series, NO3 > ClO4
.
ꢂ
ogy. The microbelt shapes consisting of ClO4 complex show
For [Pd(L)(tmen)]2(NO3)4, the size of uniform spheres (300
nm–4 mm) is significantly affected by the evaporaion rate of
acetone (Figure 5). Slow evaporation of acetone for 3 days
at room temperature produces ꢁ4 mm diameter microspheres.
Fast evaporation of acetone for 1 min using a rotary evaporator
affords ꢁ300 nm diameter submicrospheres. As the evapora-
tion rate of acetone increases, the size of spheres decreases.
Thus, spheres with medium sizes could be smoothly produced.
The packing of bimodal spheres was accomplished in water
a crystalline XRD powder pattern (Supporting Information).
Actually, the formation of crystalline microbelts is slower than
that of microspheres. According to the TGA data, the mi-
crobeꢂlts contained almost no water molecules, indicating that
ClO4 is less hydrophilic than NO3ꢂ. In order to quantify the
information on surface wettability, the contact angles3 of a
water droplet on the surface of the microspheres and microbelts
were measured (Figure 4). The contact angle for the micro-

1464 Bull. Chem. Soc. Jpn. Vol. 81, No. 11 (2008)
Microsphere vs. Microbelt Morphology
images of [(tmeda)Pd(L)]2(CF3SO3)4 and [(tmeda)Pd(L)]2(PF6)4.
1H NMR spectra of [(tmeda)Pd(L)]2(NO3)4 and [(tmeda)Pd(L)]2-
(ClO4)4. These materials are available free of charge on the web
at http://www.csj.jp/journals/bcsj/.
suspension by sonication as shown in Figure 5, and indicated
the presence of interspherical interactions. Packed bimodal
spheres are not easily dissociated in water, presumably owing
to the presence of electrostatic interactions between anions and
cations31 on the surfaces of spheres consisting of ionic palla-
dium(II) complexes. This process is an advanced method for
reproducibly forming bimodal spheres. As expected, the for-
mation of spheres is fast, whereas the formation of crystalline
microbelts is slow. For [Pd(L)(tmen)]2(NO3)4, intermolecular
interactions including water molecules as a mediator can be
a significant factor in the formation of amorphous spheres. Wa-
ter molecules on spherical surfaces easily evaporate without
imparting any great change to the spherical shape. This easy
surface evaporation of water molecules suggests that the water
molecules act as mediators, rather than crystallization solvates,
in the formation of spheres. Of course, water molecules are es-
sential elements in the formation of spheres. Elemental analy-
sis before and after evaporation of water molecules and ꢁ(OH)
(3300–3400 cmꢂ1) indicate the presence and role of water
molecules in the formation of the spherical morphology. The
complex of an oxidized ligand (SO2(C6H4OOC-4-Py)2) pro-
duces hydrogels instead of microspheres. Such a fact indicates
that the appropriate hydrophobic properties are another impor-
tant factor in the formation of spheres. When ethylenediamine
was used as a coligand in place of tmen, similar shapes were
formed. However, the spheres are easily to aggregate, presum-
ably owing to the presence of interspherical hydrogen bonds.
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ꢂ
When other counter anions such as BF4ꢂ, CF3SO3ꢂ, and PF6
were used in place of NO3ꢂ, similar spheres were obtained.
We did not observe any significant counter anion effects on
the size and shape of the spheres. The exact mechanism is
not clear for the formation of the morphologies at this stage.
For the formation of the amorphous spheres vs. the crystalline
microbelts, the most important factor is solubility in the sol-
vent, that is, the precipitation rate of the solute in the media.
Fast precipitation affords amorphous solid. In particular, the
formation of sphere is correlated with the amphiphilic proper-
ꢂ
ties of the compound. The ClO4 analogue seems to have
a good crystallinity as well as appropriate solubility in the
solvent system to form the crystalline microbelts.
Conclusion
Assembly of ionic metal complexes in an aqueous mixture
of solvents was proved to be an effective strategy for the
formation of anion-dependent monodisperse morphology.
Furthermore, bimodal spheres could be arrayed in aqueous sus-
pension via sonicaiton. This is a rare case of the formation of
spherical morphology by means of the concept of ionic metal
complexes. The surface wettability of spheres could be ex-
trapolated to the properties of counter anions. The structural
modification of metal complexes via their pliability will con-
tribute to the development of micro-based functional materials.
This work was supported by the KOSEF R01-2007-000-
20245-0 in Korea.
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Supporting Information
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Pd(L)]2(ClO4)4. FAB mass of [(tmeda)Pd(L)]2(ClO4)4. SEM
31 H. J. Yoon, I. S. Chun, J. P. Kim, Y. S. Lee, O.-S. Jung,
Mater. Lett. 2008, 62, 2883.