Synthesis of platinum acetylide derivatives with different shapes and their gel formation behavior

Article abstract of DOI:10.1021/om2003317

The synthesis of a series of platinum acetylide derivatives, featuring linear, triangular, and rectangular shapes, respectively, is described. The structures of these complexes were characterized by multinuclear NMR ( ₁H, ₁₃C, and

Full text of DOI:10.1021/om2003317

ARTICLE
pubs.acs.org/Organometallics
Synthesis of Platinum Acetylide Derivatives with Different Shapes and
Their Gel Formation Behavior
Li-Jun Chen,† Jing Zhang,† Jiuming He,‡ Xing-Dong Xu,† Nai-Wei Wu,† De-Xian Wang,§ Zeper Abliz,‡ and
Hai-Bo Yang*,†
†Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University,
3663 N. Zhongshan Road, Shanghai 200062, People's Republic of China
‡Institute of Materia Medica, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100050,
People's Republic of China
§Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China
S Supporting Information
b
ABSTRACT: The synthesis of a series of platinum acetylide
derivatives, featuring linear, triangular, and rectangular shapes,
respectively, is described. The structures of these complexes were
characterized by multinuclear NMR (1H, 13C, and 31P), CSI-TOF
mass spectrometry, and elemental analysis. It was found that
these complexes exhibited unexpectedly different gel formation
properties in the most common organic solvents. Moreover, the
organometallic gels of C1 and C2 exhibited concentration- and
temperature-dependent emission properties.
upramolecular gels, as a family of classical soft materials, have
received a great deal of attention due to their wide applica-
noncovalent interaction from small- or medium-sized molecules.
With the aim to gain further insight into the structural require-
ments for organometallic gelators, a series of platinum acetylide
derivatives with different shapes (Scheme 1) have been designed
and synthesized. Moreover, the structural effects on their gela-
tion properties have been investigated.
S
tions in material sciences and biological fields. In particular, the
smart organogelators have been extensively explored in the areas
of organic light emission diodes (OLEDs), photovoltaic cells,
drug delivery, and light harvesting systems.1 Given the scientific
importance of understanding gel-phase behavior, there has been
considerable focus on the design and synthesis of artificial
architectures that can be employed to underpin nanostructured
gels. So far, a variety of organogelators based on hydrogen
bonding, πÀπ stacking, solvophobic interaction, and van der
Waals interactions have been reported.2
Recently there has been growing interest in the investigation
of organometallic gels, because the presence of metal centers
allows for the additional scope for tuning gel properties such as
catalytic activity, magnetism, and photo- and electrochemistry.3
For instance, a trinuclear Au(I) pyrazolate complex, which can
form red-luminescent organometallic gels via Au(I)ÀAu(I)
metallophilic interaction, has been reported by Aida’s group.3g
In addition, Yam and co-workers have reported a new family of
alkynylplatinum(II) bzimpy complexes, which exhibited unusual
luminescence enhancement during the gel-to-sol phase transi-
tion at elevated temperature.3h
’ RESULTS AND DISCUSSION
All platinum acetylide derivatives consist of a π-conjugated,
rigid core moiety with different shapes and peripheral alkyl chain
groups. The π-conjugated core skeleton was synthesized by
using multistep synthetic strategies, including palladium-
catalyzed cross-coupling reactions, in accordance with similar
procedures reported in the literature.3f,5 Subsequent coupling
reactions with the corresponding alkynes resulted in the final
target compounds in high yield. An exact description of the
synthesis and characterization of linear complexes A1 and A2,
triangular complexes B1 and B2, and rectangular complexes C1
and C2 can be found in the Supporting Information.
The presence of a π-conjugated aromatic core and peripheral
alkyl chains on the newly designed complexes stimulated us to
investigate their gelation abilities in various organic solvents by
the “stable to inversion in a test tube” method. Surprisingly, linear
molecules A1 and A2 could not form gels even at high concen-
tration at room temperature. Only unstable turbid gels were
Previously we have reported the construction of nanoscale
functionalized polygons and polyhedra via coordination-driven
self-assembly from platinum building blocks.4 Recently, our
research interests have turned to the fabrication of large-scale
supramolecular microstructures, such as organometallic gels, via
Received: April 19, 2011
Published: October 10, 2011
r
2011 American Chemical Society
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Scheme 1. Chemical Structures of Target Molecules AÀC
Figure 1. (A) Photograph of C1 gel formation in various organic solvents at their CGCs (from left to right): (a) cyclohexane; (b) n-hexane; (c) n-
heptane; (d) n-propyl alcohol; (e) isopropyl alcohol; (f) n-dodecane; (g) n-octane; (h) n-decane. (B) Photograph of C2 gel formation in various organic
solvents at their CGCs (from left to right): (a) n-hexane; (b) n-heptane; (c) n-dodecane; (d) n-octane; (e) n-decane.
obtained from A2 in some nonpolar solvents under extremely
high concentrations (25À50 mg/mL) at low temperatures
(0 °C). This observation is obviously different from that pre-
viously reported, in which the gel formation of similar but shorter
platinum acetylide complexes was observed.3f This result de-
monstrates that a small modification of the gelator structure may
lead to a complete loss of gelation ability. Meanwhile, triangular
complexes B1 and B2 exhibited high solubility in the most tested
solvents and could not form gels. However, rectangular com-
plexes C1 and C2 displayed unexpectedly strong gelation ability
in the most nonpolar solvents and a few polar solvents (Figure 1).
For example, the critical gelator concentrations (CGCs) of
C1 in isopropyl alcohol and C2 in n-dodecane were 2.0 and
5.8 mg/mL, indicating that one rectangular C1 and C2 molecule
could entrap approximately 2.42 Â 104 and 0.35 Â 104 solvent
molecules, respectively. The organometallic gels obtained from
C1 and C2 are very stable and do not show any significant change
after several weeks. Upon heating, the organometallic gels melt to
a clear nonviscous solution that could reversibly turn back to the
gel state upon cooling. The gel-to-solution phase-transition
temperature (Tgel) values of the complexes C1 and C2 in several
different solvent systems were determined at CGCs given in
Table S2 (Supporting Information). It was found that the gel
formed by C2 turned into a viscous fluid more readily than that
formed by C1, suggesting that C1 has a better thermal stability
of gel.
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Figure 2. SEM images of the xerogels of C1 in isopropyl alcohol (a, scale bar 5 μm, 2.0 mg/mL) and C2 in n-octane (b, scale bar 5 μm, 5.8 mg/mL) and
TEM image of the xerogels of C2 in hexane (c, scale bar 0.2 μm, 5.8 mg/mL).
Figure 3. (a) Normalized emission spectra of C1 in hexane solution at different concentrations at 298 K. (b) Normalized emission spectra of C1 at
different temperatures at 5.5 Â 10À4 M concentration. Arrows indicates changes in spectra with increasing temperature from 298.2 to 323.2 K. The
excitation wavelength is 345 nm.
In order to investigate the morphologies of this family of
organometallic gels, scanning electron microscopy (SEM) was
performed with the corresponding xerogels (air-dried gels). The
typical fiberlike or ribbonlike morphologies for C1 and C2 in
different solvents were observed (Figure 2 and Figures S15 and
S16, Supporting Information). For example, the xerogels of C1 in
isopropyl alcohol showed a three-dimensional (3D) network
comprised of entangled fibers with diameter of 100À200 nm.
The 3D network consisting of interwoven fibers was further
confirmed by the transmission electron microscopy (TEM)
investigation of xerogels C2.
The absorption and emission properties of this series of
platinum acetylide derivatives were studied extensively (see the
Supporting Information). The electronic absorption spectra of
AÀC in dilute dichloromethane solution (ca. 10À5 M) showed
similar absorption patterns. The absorption spectra were domi-
nated by a strong and relatively broad band in the near-UV region
(ca. 345 nm) with weaker transitions at higher energy. The
dominant absorption band was described as an admixture of
πÀπ* and PtÀπ* transitions according to similar previous
reports.3f,hÀj,6 Each of the complexes exhibited a weak fluores-
cence band that is Stokes-shifted from the low-energy absorp-
tion. Upon being excited at 344 nm, A and B feature a band
with maximum at 464 nm in dilute dichloromethane solution
(ca. 10À5 M). Notably, with an exitation wavelength of 345 nm, C1
and C2 showed an emission band at 511 nm in dilute dichloro-
methane solution and gave a blue shift (39 nm) compared to that in
the gel state in hexane (ca. 472 nm). The photophysical data are
summarized in Table S3 (Supporting Information).
Detailed concentration-dependent and variable-temperature
UVÀvis and emission spectroscopy of C1 and C2 was performed
in hexane solutions. Fluorescence spectra are frequently sensitive
to the microenvironment around the fluorescent probe, thus
providing important information on the molecular organization
of chromophores. Although no significant changes were ob-
served in UVÀvis absorption spectra, C1 and C2 both exhibited
concentration- and temperature-dependent emission properties
in hexane solution. As shown in Figure 3a, in very dilute solutions
(5.5 Â 10À6 and 1.1 Â 10À5 M), the spectrum of C1 showed an
emission band at 511 nm, similar to that observed in dilute
dichloromethane solutions. As concentrations increased from
1.1 Â 10À5 to 5.5 Â 10À4 M at 298 K, the low-energy emission
band at 511 nm was found to drop in intensity with a con-
comitant growth of a new emission band at 472 nm. It is also
significant that there is an isoemissive point maintained during
the concentration altering, suggesting that the spectra are
dominated by two principal forms of the species. Meanwhile,
as shown in Figure 3b, upon warming a gelled solution of C1
(5.5 Â 10À4 M), the high-energy broad emission band at 472 nm
decreased in intensity while the low-energy band at 511 nm
increased, with a well-defined isoemissive point at 495 nm. Over
the same temperature range the sample undergoes a gelÀsol
transition; therefore, the thermally induced red shift in the
emission spectra is clearly associated with the phase transition.
It is well-known that an increase in concentration or a decrease in
temperature would favor aggregate formation.7 We attribute the
“blue” emission band (λmax 472 nm) to the C1 aggregate state
present in the gel and the “red” emission band (λmax 511 nm) to
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the monomeric form in the molecularly dissolved state. Similar
concentration- and temperature-dependent behavior was also
observed in the emission spectrum of C2 in hexane (Figures S24
and S25, Supporting Information).
Anal. Calcd for C176H290O12P8Pt4: C, 58.29; H, 8.06. Found: C, 58.30;
H, 8.05.
Synthesis of Compound C2. Following the procedure for C1, 15
(100.0 mg, 0.04 mmol), 6b (203.2 mg, 0.32 mmol), cuprous iodide
(3.0 mg, 10 mol %), Et2NH (4 mL), and dried THF (4 mL) yielded
C2 as a wheat-colored solid (180 mg, 98.4%) after purification by
column chromatography on silica gel (acetone/petroleum ether 1/7).
Mp: 117 °C. 1H NMR (CDCl3, 400 MHz): δ 0.88 (t, 36H, J = 6.4 Hz),
1.17À1.37 (m, 264H), 1.43À1.44 (m, 24H), 1.70À1.79 (m, 24H),
2.15À2.16 (m, 48H), 3.90 À3.94 (m, 24H), 6.47 (s, 8H), 7.17 (s, 6H).
13C NMR (CDCl3, 100 MHz): δ 8.35, 14.10, 16.10, 16.27, 16.45, 22.67,
26.10, 29.35, 29.37, 29.37, 29.42, 29.64, 29.68, 29.71, 29.73, 30.27, 31.90,
69.01, 73.45, 105.28, 105.43, 105.59, 108.24, 108.67, 108.83, 108.98,
109.68, 121.03, 123.48, 131.39, 136.96, 152.57. 31P NMR (CDCl3, 161.9
MHz): δ 11.61 (JPtÀP = 2365.4 Hz). HR CSI-TOF-MS of C2: m/z
2319.1 [M + 2H]2+. Anal. Calcd for C248H434O12P8Pt4: C, 64.25; H,
9.44. Found: C, 64.53; H, 9.34.
’ CONCLUSION
In summary, a series of platinum acetylide derivatives featuring
linear, triangular, and rectangular shapes have been successfully
synthesized. It was found that they displayed unexpectedly
different gelation behavior. For example, linear molecules A
and triangular molecules B formed gels only with difficulty, while
rectangular molecules C1 and C2 exhibited very high gel forma-
tion efficiency in most organic solvents, even at concentrations as
low as 2.0 and 5.8 mg/mL, respectively. Again, all these results
indicate that the structural factors, including the shape of the
gelator molecules and the number of the side alkyl chains, play an
essential role during the formation of the supramolecualr gels,
although it has not been well understood up to date. In con-
clusion, we have reported the synthesis of a series of platinum
acetylide derivatives, which presented different gel formation
properties. This research obviously enriches the library of orga-
nometallic gels and might be helpful in gaining further insight
into the structural requirements for organometallic gelators.
’ ASSOCIATED CONTENT
S
Supporting Information. Text, tables, and figures giving
b
details of synthesis and characterization of the compounds and
supplementary experimental data. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
’ EXPERIMENTAL SECTION
Corresponding Author
*E-mail: hbyang@chem.ecnu.edu.cn.
The target molecules C1 and C2 were prepared by a six-step synthetic
scheme as shown in Scheme S3 (see Supporting Information). Com-
pounds 10À14 and 6 were prepared according to previously reported
synthetic procedures3f,5cÀ5e,8 and showed spectroscopic properties
identical with those reported therein.
’ ACKNOWLEDGMENT
H.-B.Y. thanks the NSFC (Nos. 91027005 and 20902027),
Shanghai Pujiang Program (No. 09PJ1404100), Shanghai
Shuguang Program (No. 09SG25), Innovation Program of
SMEC (No. 10ZZ32), RFDP (No. 20100076110004) of Higher
Education of China, and “the Fundamental Research Funds
for the Central Universities” for financial support. We thank
professor Jun-Li Hou (Fudan University) for his help with SEM
and TEM measurements.
Synthesis of Compound 15. A solution of trans-diiodobis-
(triethylphosphine)platinum (1.32 g, 1.93 mmol) and cuprous iodide
(14.7 mg, 8 mol %) in a THF/Et2NH mixture (20 mL/10 mL) was
stirred at room temperature, and 14 (72.0 mg, 0.24 mmol) dissolved in
THF (10 mL) was added dropwise for 0.5 h. After another 1 h a small
amount of diethylammonium iodide started precipitating out of solu-
tion. The solvent was removed in vacuo. The yellow-red residue was
separated by column chromatography on silica gel (dichloromethane/
petroleum ether 3/2) to give the desired product 15 as a wheat-colored
solid. Yield: 390 mg, 64%. Mp: 222 °C. 1H NMR (300 MHz, CDCl3): δ
1.11À1.22 (m, 72H), 2.19 À2.23 (m, 48H), 7.17 (s, 2H), 7.21 (s, 4H).
13C NMR (75 MHz, CDCl3): δ 8.10, 8.24, 8.39, 16.12, 16.34, 16.57,
16.81, 17.03, 73.42, 81.44, 91.14, 91.33, 91.52, 98.95, 121.36, 128.47,
128.71, 128.93, 131.38, 133.72. 31P NMR (121 MHz, CDCl3): δ 9.10
(JPtÀP = 2309.9 Hz). Anal. Calcd for C72H126I4P8Pt4: C, 34.21; H, 5.02.
Found: C, 34.60; H, 5.12.
Synthesis of Compound C1. A 100 mL Schlenk flask was charged
with 15 (100.0 mg, 0.04 mmol), 6a (125.7 mg, 0.32 mmol), and cuprous
iodide (3.0 mg, 10 mol %), degassed, and back-filled three times with N2.
Et2NH (4 mL) and dried THF (4 mL) were introduced into the reaction
flask by syringe. The reaction mixture was stirred under an inert
atmosphere at room temperature for about 4 h. The solvent was
removed by evaporation on a rotary evaporator. The residue was purified
by column chromatography on silica gel (acetone/petroleum ether 1/7)
to give C1 (124.2 mg, 86.6%) as a wheat-colored solid. Mp: 110 °C. 1H
NMR (CDCl3, 400 MHz): δ 0.89 (br, 36H), 1.17À1.28 (m, 72H), 1.32
(br, 48H), 1.45 (br, 24H), 1.70À1.79 (m, 24H), 2.15À2.17 (m, 48H),
3.90 À3.95 (m, 24H), 6.48 (s, 8H), 7.18 (s, 6H). 13C NMR (CDCl3, 100
MHz): δ 8.35, 14.00, 16.13, 16.30, 16.48, 22.59, 29.32, 30.19, 31.55,
31.73, 69.00, 73.42, 105.38, 108.20, 109.66, 121.06, 123.48, 128.74,
131.38, 134.31, 136.99, 152.59. 31P NMR (CDCl3, 161.9 MHz): δ 11.64
(JPtÀP = 2365.4 Hz). HR CSI-TOF-MS of C1: m/z 1814.0 [M + 2H]2+.
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