Importance of Solution Equilibria
A R T I C L E S
Pd2+).20–36 Among these metals, Pt2+ has yielded materials of
high pore order presumably because of its slower ligand
substitution kinetics during the linking reaction that allows the
structure more time to organize. Although the adamantane
[Ge4Se10]4- cluster and various linking metals resulted in high
quality mesostructured materials,24 the corresponding dimeric
[Ge2Se6]4- and monomeric [GeSe4]4- species have not been
studied. We will show here that unlike the adamantane these
two species undergo transformation equilibria that complicate
the assembly process.
(denoted as CnTMEDABr; n ) 20, 22) were synthesized by
refluxing the corresponding alkyl-bromide with an excess of
pyridine, N,N,N′,N′-tetramethyl-1,3-propanediamine (denoted as
TMPDA) and N,N,N′,N′-tetramethyl-1,2-ethanediamine (denoted as
TMEDA), respectively, in either ethanol or acetone. The solvent
was removed using a rotary evaporator and pure compounds were
obtained in high yield (>90%) after a single recrystallization from
1
CHCl3-ethyl acetate. Surfactant purity was checked by H NMR
after dissolving the solid sample in CDCl3. The materials we
describe here are generically formulated as Surf_Pt_Ge2Se6 and
Surf_Pt_GeSe4 and more specifically as CnPyPt_Ge2Se6 or
CnTMPDAPt_Ge2Se6 or CnTMEDAPt_Ge2Se6 or CnPyPt_GeSe4
based on the type of surfactants and starting building units used.
Mesostructured Platinum Germanium Selenides. All reactions
were carried out inside a glovebox under nitrogen. In a typical
preparation, 0.4 g of surfactant (for pyridinium based) was dissolved
in 8 mL of FM under stirring. In another vial, 0.078 g (0.1 mmol)
of K4Ge2Se6 was dissolved in 1 mL of hot FM forming a clear
deep yellow solution. An amount of 0.042 g (0.1 mmol) of K2PtCl4
was dissolved in another 1 mL of FM to make a deep orange-red
solution. These two solutions were added dropwise at the same
time to the surfactant solution over a period of 10 min using pipets.
A dark brown-red solid formed within a few minutes and the
mixture was stirred overnight at 75 °C. The dark pinkish brown
product was isolated by filtration, washed with copious amount of
warm (75 °C) FM and H2O, and dried under vacuum (overall yield
∼ 90% based on K4Ge2Se6). For the other surfactants, 0.25 g was
dissolved in 4 mL of hot FM and 0.15 mmol platinum salt was
added instead of 0.1 mmol.
Synthesis of mesostructured C16PyPt_Ge2Se6 in water was carried
out using the same procedure as above, except 0.4 g of surfactant
C16PyBr.H2O was dissolved in 4 mL of water (overall yield ∼90%
based on K4Ge2Se6). However, slight variation of surfactant
concentration did not affect the overall pore order of the final
mesostructures.
In the case of [GeSe4]4-, 0.05 g of K4GeSe4 (0.1 mmol) and
0.4 g of surfactant was dissolved in 1 and 4 mL of hot FM
respectively. An amount of 0.04 g (0.1 mmol) of K2PtCl4 dissolved
in 1 mL of FM was added at the same time with K4GeSe4 solution
to the surfactant solution. A dark brown precipitate was collected
after aging overnight at 75 °C followed by washing with 30 mL
each of hot FM and water with overall yield of ∼75% based on
K4GeSe4. The CnPyPt_Ge2Se6 materials are stable in air for a couple
of days, while those prepared from [GeSe4]4- are stable only for
couple of hours after which the pore structure collapsed.
We describe the synthesis of platinum-based hexagonal
mesostructured systems from [Ge2Se6]4- and the [GeSe4]4-
clusters and explore the role of solution equilibria in controlling
mesophase composition and pore organization. We find that
systematic variation of surfactants changes the pore size and
pore-pore spacing, the overall symmetry and even the energy
band gaps of the mesostructured materials. Surprisingly, com-
j
pared to the materials with cubic pore Ia3d symmetry obtained
with the corresponding [Sn2Se6]4-/Pt2+ combination,26 the
analogous [Ge2Se6]4- did not produce cubic pore symmetry.
77Se NMR spectroscopy of [Ge2Se6]4-, in water and formamide
(FM) as well as fast atom bombardment mass spectroscopy
(FABS) show multiple species present in equilibrium all of
which can be incorporated into the Pt-Ge-Se framework. This
is in contrast to the [Sn2Se6]4- system which shows only a single
species. The implications of multiple building unit participation
to assemble hexagonally ordered materials is discussed. The
new materials described here exhibit reversible ion-exchange
properties and flexible inorganic networks. Small angle X-ray
scattering (SAXS) experiments on selected systems suggest a
very high mesoporous surface for the inorganic framework.
Experimental Section
Starting Materials. K4Ge2Se6 and K4GeSe4 were prepared
according to a literature procedure.37,38 K2PtCl4 was purchased from
Strem Chemical Inc. Cetylpyridinium bromide monohydrate
(C16PyBr.H2O) and formamide (FM) were purchased from Aldrich.
The surfactants CnH2n+1N(C5H5)Br (denoted as CnPyBr; n ) 14,
18, and 20), CnH2n+1N(CH3)2(CH2)3N(CH3)2Br (denoted as CnT-
MPDABr; n ) 20, 22), and CnH2n+1N(CH3)2(CH2)2N(CH3)2Br
(23) Rangan, K. K.; Trikalitis, P. N.; Bakas, T.; Kanatzidis, M. G. Chem.
Commun. 2001, 9, 809–810.
Ion-Exchange Experiments. Ion exchange reactions were
carried out in nitrogen atmosphere at 75 °C in FM. Typically, 1.2 g
of CnPyBr (e.g., n ) 12) surfactant was dissolved in 6 mL of hot
FM and then 0.15 g of mesostructured C18PyPt_Ge2Se6 was added
to it. The mixture was stirred for 22 h, filtered hot, washed
subsequently with hot FM and water and then dried under vacuum.
(24) Trikalitis, P. N.; Rangan, K. K.; Kanatzidis, M. G. J. Am. Chem. Soc.
2002, 124, 2604–2613.
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Kanatzidis, M. G. Nano Lett. 2002, 2, 513–517.
(26) Trikalitis, P. N.; Rangan, K. K.; Bakas, T.; Kanatzidis, M. G. J. Am.
Chem. Soc. 2002, 124, 12255–12260.
+
Ion exchange with smaller cationic species such as NH4 was
(27) Trikalitis, P. N.; Ding, N.; Malliakas, C.; Billinge, S. J. L.; Kanatzidis,
M. G. J. Am. Chem. Soc. 2004, 126, 15326–15327.
(28) Trikalitis, P. N.; Bakas, T.; Kanatzidis, M. G. J. Am. Chem. Soc. 2005,
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performed in a similar fashion as above using 0.12 g of
CH3COONH4 dissolved in 12 mL of ethanol and adding 0.208 g
of mesostructured C18PyPt_Ge2Se6 material at room temperature.
+
(29) Trikalitis, P. N.; Kerr, T. A.; Kanatzidis, M. G. Microporous
Mesoporous Mater. 2006, 88, 187–190.
The NH4 exchanged material is denoted as (NH4)Pt_Ge2Se6.
Synthesis of Hexagonal Mesostructured Pt/Ge/Se
Materials Using a Mixture of Selenogermanate Anions. An
amount of 0.4 g of surfactant (CnPyBr; n ) 16, 18) was dissolved
in 8 mL of FM under stirring. In another vial, 0.025 g (0.05 mmol)
of K4GeSe4 and 0.07 g (0.05 mmol) of (TMA)4Ge4Se10 were
dissolved together in 1 mL of hot formamide. An amount of 0.042
g (0.1 mmol) of K2PtCl4 was dissolved in another 1 mL of FM to
make a deep orange-red solution. These two solutions were added
dropwise at the same time to the surfactant solution over a period
of 10 min using pipets. A dark brown-red solid formed within a
few minutes and the mixture was stirred overnight at 75 °C. The
dark pinkish brown product was isolated by filtration, washed with
copious amount of warm (75 °C) FM and H2O, and dried under
(30) Ding, N.; Takabayashi, Y.; Solari, P. L.; Prassides, K.; Pcionek, R. J.;
Kanatzidis, M. G. Chem. Mater. 2006, 18, 4690–4699.
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