Uniform submicro spherical morphology of ionic palladium(II) complexes

Article abstract of DOI:10.1246/cl.2007.548

Spherical morphology with diameters of 300-500 nm is easily formed by a reaction of palladium(II) species in water with bis(nicotinoyl)-4,4- thiophenolate in acetone and followed by evaporation of acetone. The surface hydrophilicity of the spheres is stro

Full text of DOI:10.1246/cl.2007.548

548
Chemistry Letters Vol.36, No.4 (2007)
Uniform Submicro Spherical Morphology of Ionic Palladium(II) Complexes
In Sung Chun,1 Kil Sun Lee,1 Jongki Hong,2 Youngkyu Do,3 and Ok-Sang Jungꢀ1
1Department of Chemistry, Pusan National University, Pusan 609-735, Korea
2College of Pharmacy, Kyung Hee University, Seoul 130-701, Korea
3Department of Chemistry, Korea Advanced Institute of Science and Technology,
Daejon 305-701, Korea
(Received January 18, 2007; CL-070067; E-mail: oksjung@pusan.ac.kr)
Spherical morphology with diameters of 300–500 nm is
easily formed by a reaction of palladium(II) species in water
with bis(nicotinoyl)-4,40-thiophenolate in acetone and followed
by evaporation of acetone. The surface hydrophilicity of the
spheres is strongly dependent on the counter anions of the
complexes.
The ability to systematically manipulate the morphology
of materials remains an important goal since submicrometer or
colloidal spheres play important roles in many applications such
as catalysts, electronic devices, drug-delivery medicines, ceram-
ics, pigments, biosensor, colloidal lithography, and cosmetics.1,2
Thus, considerable attention has been paid to strategies for
fabricating monodisperse colloidal spheres.3,4 Steric effects,
surface tension, capillary effects, electric and magnetic forces,
permanent dipoles, van der Waals interaction, hydrophilic
interactions, surfactant/precursor ratio, and shape anisotropy
have been applied as driving forces in the formation of artificial
morphology.5–9 Spherical morphology of metal oxides has been
widely carried out, but the morphogenesis of metal complexes is
rare. In particular, no attention has been focused on the prepara-
tion of submicrometer spheres of ionic metal complexes.
To explore the formation of new conceptual submicro spherical
materials, in this communication, we report the first formation
of spherical colloids based on discrete ionic palladium(II)
complexes of bis(nicotinoyl)-4,40-thiophenolate (L) without
addition of any additives. The counter anion effects on surface
hydrophilicity of the spheres are investigated.
Figure 1. Synthetic scheme.
Figure 2. SEM image of [(en)Pd(L)]2(NO3)4. Inset indicates
high magnification.
The ionic palladium(II) complexes were prepared by the
reaction of an aqueous solution of (en)PdX2 (en: ethylenedi-
amine; Xꢁ = NO3ꢁ, ClO4ꢁ, BF4ꢁ, CF3SO3ꢁ, and PF6ꢁ) with
an acetone solution of L as shown in Figure 1. In a typical prep-
aration, L (0.1 mmol) in 10 mL of acetone was added slowly
to (en)Pd(NO3)2 (0.1 mmol) in 10 mL of distilled water. The
mixture was refluxed for 2 h, and evaporation of acetone at
ambient temperature gave a white spherical product of
[(en)Pd(L)]2(NO3)4 in 83% yield. The spheres were collected
by the filtration using a membrane (membrane filter, Advantec
MFS Inc.) for further characterization. The spherical product is
a stable solid even under air and moisture. In order to measure
the molecular weight of the product, the spheres were dissolved
in N,N-dimethylformamide, and then the solution was mixed
with 3-nitrobenzyl alcohol (Sigma, U.S.A.) on a FAB probe
tip. The molecular weight was identified by the presence of
the ion clusters at m=z ¼ 1374 [M – NO3]þ and 1314 [M –
NO3 – en]þ (Supporting Information).19 Thus, the skeletal
structure of [(en)Pd(L)]2(X)4 is a cyclodimeric molecule consist-
ing of two square planar palladium(II) units.
140
130
120
110
100
90
[(en)Pd(L)]2(PF6)4
[(en)Pd(L)]2(ClO4)4
[(en)Pd(L)]2(NO3)4
80
0
5
10
15
Time/s
20
25
30
Figure 3. Contact angles of a water-droplet on the monolayer
of [(en)Pd(L)]2(X)4.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.4 (2007)
549
Scanning electron microscopy images show that the
colloidal spheres have diameters in the range of 300–500 nm
as shown in Figure 2. The spheres are hardly soluble in water
or acetone but are soluble in a mixture of water and acetone
(Supporting Information),19 indicating that the spheres are typi-
cal amphiphilic materials. Such an amphiphilic solubility may be
a driving force for the formation of spherical morphology. The
part around palladium(II) cation and NO3ꢁ counter anions seems
to be hydrophilic, and L moiety seems to be hydrophobic. Of
course, L itself does not show amphiphilic properties and forma-
tion of any spherical morphology. The ionic palladium(II) com-
plexes easily form the submicro spheres, presumably owing to
such amphiphilic properties. At this stage, the formation mech-
anism is not clear, the amphiphilic properties seem to play an im-
portant role in the formation of submicro spheres. The submicro
spheres from the reaction of H2PtCl4 with p-phenylenediamine2
may have both hydrophilic and hydrophobic moiety. According
to thermogravimetric analysis (TGA) result, the spherical prod-
uct contains about 5 wt % water and drastically decomposes at
285 ꢂC. Water molecules on the surface of the spheres easily
evaporate without a great change of the spherical shape. This
easy evaporation of the water molecules on the surface in the
wide range 45–100 ꢂC suggests that the water molecules act as
mediators in the formation of sphere rather than crystallization
solvates. The water molecules on the surface evaporate at low
temperature. Of course, water molecules are essential elements
for the formation of the submicro spheres. Elemental analysis,
properties such as wettability, adhesion, and biocompatibility
has important implications in both fundamental and technologi-
cal advances.16–18
In conclusion, assembly of ionic metal complexes in a
mixture of solvents was proved to be an effective strategy for
the preparation of monodisperse spherical colloids. This is the
first case of the formation of the spherical morphology using
the concept of ionic metal complexes. The surface wettability
of the spheres could be extrapolated to the properties of counter
anions. The structural modification via pliability of metal com-
plexes will contribute to the development of micro-based func-
tional morphology. Such colloidal spheres are easily soluble in
water by an alkaline reagent, which is promising as pH-sensitive
decomposable materials.
This work is supported by the KRF-2006-312-C00578 in
Korea.
References and Notes
1
2
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ꢁ
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als.10–14 Thus, in order to quantify the effects of counter anions
on the hydrophilicity of spherical surface, the contact angles
of a water-droplet on the submicro spheresꢁwere measured.
As shown in Figure 3, the spheres with PF6 shꢁowed highꢁer
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´
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contact angle than that of the spheres wiꢁth NO3 or ClO4
.
The contact angle of the sphere with ClO4 is higher than that
of the spheꢁres with NO3ꢁ. Thus, the results indicate that the or-
ꢁ
der of NO3 > ClO4 > PF6ꢁ favors water, which is consistent
with a modified Hofmeister series.15 The wettability of the
colloidal spheres is strongly dependent upon the hydrophilicity
ꢁ
of the counter anions. That is, NO3 anion has strong hydro-
gen-bonding interactions with water molecules compared to
ꢁ
ClO4 and PF6ꢁ. It is worth noting that this technique is useful
19 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
for the determination of quantitative counter anion effects on hy-
drophilicity of submicro spheres. Such a modulation of surface
Published on the web (Advance View) March 14, 2007; doi:10.1246/cl.2007.548