Self-Assembly of molecular prisms via Pt3 organometallic acceptors and a Pt2 organometallic clip

Article abstract of DOI:10.1021/om900309x

The design and two-component [2 + 3] self-assembly of a series of new organometallic molecular prisms (3a-d) are described. Assemblies 3a,b incorporate 4,4',4'-tris[ethynyl-trans-Pt(PEt2)₂(N0 ₃)]triphenylamine (la) containing a Pt-et

Full text of DOI:10.1021/om900309x

4288 Organometallics 2009, 28, 4288–4296
DOI: 10.1021/om900309x
Self-Assembly of Molecular Prisms via Pt3 Organometallic Acceptors
and a Pt2 Organometallic Clip
Sushobhan Ghosh, Bappaditya Gole, Arun Kumar Bar, and Partha Sarathi Mukherjee*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560 012, India
Received April 23, 2009
The design and two-component [2 þ 3] self-assembly of a series of new organometallic molecular
prisms (3a-d) are described. Assemblies 3a,b incorporate 4,40,40-tris[ethynyl-trans-Pt(PEt3)2(NO3)]-
triphenylamine (1a) containing a Pt-ethynyl functionality as tritopic planar acceptor and organic
“clips” 2a and 2b, respectively [where 2a=1,3-bis(3-pyridyl)isophthalic amide; 2b = 1,3-bis(ethynyl-
3-pyridyl)benzene]. In a complementary approach an organic tritopic planar donor ligand 2c [2c=
4,40,40-tris(4-pyridylethynyl)triphenylamine] was assembled with an organometallic “clip”, 1,8-bis-
[{trans-Pt(PEt3)2(NO3)}ethynyl]anthracene (1b), to obtain prism 3c. A new organometallic carbon-
centered acceptor, 1,1,1-tris[4-{trans-Pt(PEt3)2(NO3)}ethynylphenyl]ethane (1c), has been prepared,
and its prism derivative (3d) using an organic “clip” is prepared. Assemblies (3a-d) were character-
ized by multinuclear NMR spectroscopy, electrospray ionization mass spectroscopy, and elemental
analysis. 3a-d showed fluorescence behavior in solution, and quenching of fluorescence intensity (3a,
3c-d) was noticed upon addition of TNT (2,4,6-trinitrotoluene), a common constituent of many
commercial explosives. A thin film of the assembly 3d made by spin coating of a solution of 3ꢀ10-5 M
in DMF on a 1 cm2 quartz plate showed fluorescence response to the vapor of TNT.
Introduction
containing proper functional groups.6 Among the several
kind of linkers used to design polynuclear transition metal
complexes, the ones that contain ethynyl spacers in the
Directional self-assembly of large discrete architectures
driven by metal-ligand coordination bonding interaction is
established as one of the most prevalent areas of modern
supramolecular chemistry.1-4 Symmetrical polypyridyl do-
nors are the most widely used class of linkers, though in a few
recent cases oxygen donor carboxylates have also been used
to design neutral Pt/Pd architectures.5 Since the properties of
a molecule are guided mainly by the functional groups
present, the important aspect of the coordination-driven
self-assembly is the introduction of functionality into the
final assemblies by choosing appropriate building units
€
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*Corresponding author. Fax: 91-80-2360-1552. Tel: 91-80-2293-3352.
E-mail: psm@ipc.iisc.ernet.in.
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pubs.acs.org/Organometallics
Published on Web 07/02/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 15, 2009 4289
backbones have been shown to be good members to design a
variety of assemblies with potential applications in optical
electronics.7 Detection of trace analyte is a central challenge
in the field of chemical sensors. For security reasons atten-
tion has been paid recently to discovering suitable sensors for
nitroaromatic explosives (DNT, TNT). However, polyace-
tylenes and other conjugated organic polymers are the
commonly studied materials as sensors for these explosives.
The possibility of conjugated metal-organic discrete
assemblies of finite shapes and sizes as sensors for nitroaro-
matics needs to be explored. Recently, we have used a new
Pt3 organometallic acceptor, 4,40,400-tris[ethynyl-trans-Pt-
(PEt3)2(NO3)]triphenylamine (1a), containing Pt-ethynyl
functionality to construct a fluorescent trigonal prism (3a)
in combination with an organic “clip”.8 Interestingly, the
solution fluorescence of 3a was quenched efficiently by
adding nitroaromatics, which are the chemical signatures
of many explosives. Here, we report the synthesis of a
new trigonal prism (3b) from 1a using an ethynyl-con-
taining donor clip, 1,3-bis(3-pyridylethynyl)benzene9 (2b)
(Scheme 1). A complementary approach has also been used
to prepare a prism (3c) from an organic tritopic planar
donor, 4,40,400-tris(4-pyridylethynyl)triphenylamine (2c), in
combination with an organometallic Pt2-clip (1b) containing
an ethynyl functionality (Scheme 1). In addition, we report
here the synthesis of a new tripodal Pt3-organometallic
acceptor, 4,40,400-tris[ethynyl-trans-Pt(PEt3)2(NO3)]triphe-
nylethane (1c), and its [2 þ 3] self-assembled prismatic
derivative (3d) by assembling with a conjugated organic clip,
[1,8-bis(4-pyridylethynyl)anthracene] (2d) (Scheme 2). All
four supramolecular prisms (3a-d) show luminescent beha-
vior due to the presence of a Pt-ethynyl functionality and
conjugated π-electrons. The solution state fluorescence of
molecular prisms 3a, 3c, and 3d is gradually quenched upon
addition of TNT. In the case of 3d, a spin-cast film prepared
by spin coating of a DMF solution of 3d over quartz shows
fluorescent quenching on exposure to TNT vapor.
Results and Discussion
Synthesis of the Linkers. Halogenated aromatic com-
pounds are an adequate choice as starting materials in self-
assembly because a large number of acceptors/donors can
easily be synthesized starting from these compounds.10 The
acceptor 1a was prepared from tris(4-bromophenyl)amine as
a halogenated aromatic compound using the procedure
discussed in our recent communication.8 In a similar way,
1,8-dichloroanthracene was used as the starting material to
obtain 1,8-diethynylanthracene followed by treatment with
an excess of trans-Pt(PEt3)2I2 and subsequent nitration with
AgNO3 to synthesize 1b. The carbon-centered new tripodal
acceptor 1c was prepared from 1,1,1-tris(4-iodophenyl)-
ethane, which was prepared according to the litera-
ture procedure.11 Coupling of trimethylsilylacetylene
(Me3SiCCH) with 1,1,1-tris(4-iodophenyl)ethane followed
by desilylation gives the ethynyl-incorporated compound
1,1,1-tris(4-ethynylphenyl)ethane (Scheme 3). Treatment
of 1,1,1-tris(4-ethynylphenyl)ethane with 4 equiv of trans-
(PEt3)2PtI2 in the presence of CuI catalyst in dry Et2NH
medium at room temperature overnight produced the
iodo derivative 1,1,1-tris [4-(trans-Pt(PEt3)2I)ethynylphe-
nyl]ethane, which was separated by column chromatography
and isolated in yellow microcrystalline form. The trinitrate
derivative 1c was synthesized from the iodide derivative
upon treatment with 3 equiv of AgNO3 at room temperature
with an isolated yield of 88%.
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1
The H and 31P{1H} NMR spectra as shown in Figure 1
indicate the formation of a symmetric tritopic linker, 1c. The
appearance of a single peak in the 31P{1H} NMR along with
concomitant 195Pt satellites indicated the formation of a
single product as well. Finally, the actual composition of
1c was confirmed by the appearance of peaks at m/z=1807.5
and 1274 in the ESI mass spectrum, corresponding to the
molecular ion [M þ Hþ] and [M - 4PEt3 - NO-3 ]þ fragment,
respectively (Supporting Information).
All four donors 2a-d were synthesized using reported
procedures.8,9,11,12 1a and 1b were fully characterized by
multinuclear NMR, ESI mass spectrometry, and single-
crystal X-ray diffraction study.8,6c Several efforts to obtain
good-quality single crystals of 1c were unsuccessful, and only
a very tiny amount of crystalline material was obtained.
Although it was possible to determine the cell dimensions
˚
(crystal system, cubic; space group, Pa3; a = 25.4754(5) A;
R=90°) of 1c from a diffraction study, a full set of data could
€
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€
molecules 1998, 32, 722. (k) Chawdhury, N.; Kohler, A.; Friend, R. H.;
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Inorg. Chem. 2005, 45.

4290 Organometallics, Vol. 28, No. 15, 2009
Scheme 1. Self-Assembly of Trigonal Prisms 3a-c
Ghosh et al.
Scheme 2. Self-Assembly of the Cage 3d from the New Tripodal Pt3-Organometallic Acceptor 1c and the Organic Clip 2d
not be collected. Hence, the optimization of the tripodal
linker has been done by density functional theory (DFT)
calculations to obtain a view of the shape of 1,1,1-tris-
[4-(trans-Pt(PEt3)2I)ethynylphenyl]ethane (Figure 2). The
B3LYP functional was used for the DFT calculation, which
was performed with the Gaussian 03 program. Two basis sets
were employed, 6-31G for lighter elements (C, H, N, and O)
and LanL2DZ for heavier elements (Pt and P). The average
I-C(central)-I angle in the structure is 110.97°, which is
very close to a similar angle (110.53°) reported by Stang et al.
from X-ray structure analysis of an analogous linker con-
taining no ethynyl group.11 The angles around the Pt enviro-
nment vary between 86.44° and 93.11°, which is expected for
a square-planar metal center.

Article
Organometallics, Vol. 28, No. 15, 2009 4291
Scheme 3. Schematic Presentation of the Synthesis of 4,40,400-Tris[ethynyl-trans-Pt(PEt3)2(NO3)]triphenylethane (1c) from
1,1,1-Tris(4-iodophenyl)ethane
Self-Assembly of Molecular Prisms 3a-d. As per the
general concept of geometry, a trigonal prism can be de-
signed from two tritopic donors, six 90° corners, and three
linear linkers by a [2 þ 6 þ 3] self-assembly reaction as shown
in Scheme 4. The major disadvantage of three-component
self-assembly is the formation of a mixture of several possible
discrete assemblies (Scheme 4). Recently, Fujita’s group has
reported the formation of a prism including other expected
closed structures using a three-component self-assembly
reaction.13 Selective formation of the prism 4 of cis-(en)Pt-
(NO3)2 was achieved only in the presence of appropriate
guest, which favored the formation of a prism by host-guest
interaction.13a To avoid the formation of unnecessary by-
products, we have utilized a [2 þ 3] two-component self-
assembly strategy as shown in Schemes 1 and 2.
Supramolecular prisms 3a and 3b were synthesized from
the tritopic organometallic planar acceptor 1a by [2 þ 3]
self-assembly using flexible and rigid donor clips 2a and
2b, respectively. When 4,40,400-tris[ethynyl-trans-Pt(PEt3)2-
(NO3)]triphenylamine (1a)8 acceptor was treated with 1.5
equiv of the donor ligand 2a14 in a mixture of methanol and
dichloromethane for 30 min, the self-assembly of the cationic
prism 3a as a single product occurred. The completion of the
reaction was monitored by TLC. 31P{1H} NMR analysis of
the reaction solution showed the quantitative formation of a
single, highly symmetrical species (3a) by the appearance of a
sharp singlet with concomitant 195Pt satellites, shifted 5 ppm
upfield (-Δδ) relative to 1a. Finally, mass spectrometric
analysis confirmed the formation of the (1a)2(2a)3 cage
(Supporting Information).8 In a similar way 3b was prepared
by reacting 1a with 1.5 equiv of 2b in acetone medium for 12 h
at 50 °C. The product (3b) was formed as a yellow precipitate,
which was washed with acetone several times. 31P NMR of
the product in CDCl3 showed a sharp singlet at 15.6 with a
change in 1JP-Pt (230 Hz) for the Pt satellites. An upfield shift
of 4.5 ppm of the 31P{1H} NMR signal of 1a upon reaction
with 2b and complete disappearance of the peak due to
starting acceptor clearly indicate the formation of a single
product. 1H NMR shows the expected downfield shifts of the
pyridyl protons and the symmetric nature (Supporting In-
formation) of the product by the appearance of other
expected signals. Supramolecular prism 3c is an example of
the scope and versatility of the directional bonding metho-
dology, where in a complementary approach the clip type of
acceptor linker 1b with a tripodal donor 2c forms 3c in
almost quantitative yield, as shown in Scheme 1. To a stirred
suspension of the clip-type acceptor 1b in acetone was added
ligand 2c. The suspension became transparent yellow in a few
minutes after heating at 50 °C followed by the formation of
the final product (3c) as yellow precipitate after stirring for
12 h. A sharp singlet appeared at 14 ppm in 31P{1H} NMR
spectrum, which is 5 ppm upfield shifted from the 31P{1H}
NMR of the starting linker 1b (Figure S5, Supporting
Information). The downfield shift of the pyridine HR proton
by 0.3 ppm and the 0.1 ppm upfield shift of the H10 of the clip
were in strong support of the coordination of ligand 2c to the
(13) (a) Maurizot, V.; Yoshizawa, M.; Kawano, M.; Fujita, M.
Dalton Trans. 2006, 2750. (b) Yoshizawa, M.; Nakagawa, J.; Kumazawa,
J.; Nagao, M.; Kawano, M.; Ozeki, T.; Fujita, M. Angew Chem. Int. Ed.
2005, 44, 1810. (c) Kumazawa, K.; Biradha, K.; Kusukawa, T.; Okano, T.;
Fujita, M. Angew. Chem., Int. Ed. 2003, 42, 3909. (d) Yamauchi, Y.;
Yoshizawa, Y.; Fujita, M. J. Am. Chem. Soc. 2008, 130, 5832.
(14) Qin, Z.; Jennings, M. C.; Puddephatt, R. J. Inorg. Chem. 2003,
42, 1956.

4292 Organometallics, Vol. 28, No. 15, 2009
Ghosh et al.
product. However, it does not provide much information
about the shape of this kind of product and the actual
composition. ESI mass spectrometry has proven to be a
potential tool in the corroboration of structural assignments
for this kind of large supramolecular assembly.15 Mass
spectrometric characterizations of 3a-d were performed by
ESI technique, which allowed keeping the assembly intact
during the ionization process while obtaining the high re-
solution required for unambiguous determination of the
charge states. Electrospray mass spectroscopic study con-
firmed a (1a)2(2a)3 composition for cage 3a. The mass
spectrum of 3a (Supporting Information)8 showed signals
corresponding to the consecutive loss of nitrate counterions,
[M3a-3NO3]3þ, [M3a- 4NO3]4þ, and [M3a- 5NO3]5þ. For
3a: [M3a- 3NO3]3þ [m/z=1452.00 (calcd 1451.66); M3a
=
molecular mass of 3a]; [M3a- 4NO3]4þ [m/z=1073.49 (calcd
1073.18)]; [M3a -5NO3]5þ [m/z = 846.55 (calcd 845.67)].8
The isotopic distribution pattern of the peak at m/z corre-
sponding to [M3a- 3NO3]3þ matched well with the theore-
tical isotopic distribution pattern. In the case of 3b the peaks
at m/z corresponding to the fragments [M3b- 6NO-3 ]6þ, [M3b
- 5NO-3 ]5þ, and [M3b - 3NO-3 ]3þ were observed and the
Figure 1. 1H and 31P{1H} (inset) NMR spectra of 1c in
CDCl3.
isotropic distribution pattern of the peak due to [M3b
-
3NO-3 ]3þ matched well with the theoretically expected pat-
tern (Supporting Information). The ESI-MS spectrum of
supramolecular prism 3c gave a peak at m/z corresponding to
[M3c - 4NO-3 ]4þ along with that of [M3c - 4PEt3 -4NO-3 ]4þ
and [M3c-3PEt3 - 3NO-3 ]3þ fragments. The isotropic dis-
tribution pattern of the latter established the charge state
(Supporting Information). Similarly, 3d showed signals cor-
responding to the consecutive loss of nitrate counterions,
[M3d-3NO3]3þ and [M3d-5NO3]5þ. For 3d: [M3d- 3NO3]3þ
[m/z=1523.50 (calcd 1523.33); M3d= molecular mass of 3d];
[M3d - 5NO3]5þ [m/z =890.1 (calcd 889.3)]. Along with
these, peaks corresponding to the fragments [M- 7PEt3 -
4NO-3 ]4þ, [M-8PEt3-3NO3-]3þ, and [M-12PEt3-2NO-3 ]2þ
were also obtained. The isotopic distribution pattern of the
peak corresponding to [M - 3NO3]3þ matched well with
the theoretical pattern and thus established the formation
of a discrete assembly of composition (1c)2(2d)3. Several
attempts to obtain single crystals for X-ray diffraction
have failed. The proposed structure of the supramole-
cule 3d obtained by energy minimization is presented in
Figure 4.
Absorption and Fluorescence Studies. Absorption spectra
of complexes 3b-d were recorded in DMF. The spectrum of
a 1.5ꢀ10-5 M solution of 3b in DMF shows peaks at 283
and 365 nm (dash-dot lines in Figure 5). The peak at 365 nm
is due to the metal-to-ligand (2b) charge transfer, whereas the
peak at 283 nm is due to π-π* transition. In the case of
complex 3c the absorption spectrum of a 3 ꢀ 10-6 M solution
in DMF shows peaks at 430 and 383 nm, which are originat-
ing from the anthracene moiety of 1b, whereas the peak at
270 nm corresponds to the metal-to-ligand (2c) charge
transfer (Figure 5, dashed line).
Figure 2. Calculated optimized structure of 1,1,1-tris[4-(trans-
Pt(PEt3)2I)ethynylphenyl]ethane obtained using DFT.
acceptor and formation of the symmetric supramolecular
prism 3c (Supporting Information).
To expand the methodology of designing a trigonal
prism by [2 þ 3] self-assembly of two components, we
modified the tritopic planar acceptor, replacing the central
nitrogen of 1a by -C(CH3), and prepared a new organo-
metallic tripodal acceptor (1c) containing Pt-ethynyl func-
tionality. Reaction of this new 1c with 1.5 equiv of the
rigid organic clip 2d7f in acetone at 50 °C overnight yielded
3d as a fluorescent yellow precipitate. The expected upfield
shift of the phosphorus signal of 1c upon reaction with 2d
was observed. Similarly, the HR, Hβ, and H9 protons of
the donor ligand showed the expected downfield shifts
(Figure 3).
For complex 3d peaks at 421 and 398 nm corresponding to
the anthracene part are observed, whereas the peak at 309 nm
is attributed to platinum-to-ligand (2d) charge transfer
(Figure 5, solid line). When excited at 400 nm all complexes
(3b-d) show luminescent behavior and emit between 400
and 500 nm. Fluorescence spectra of the complexes 3b-d
recorded in DMF solutions are assembled in Figure S10
(Supporting Information), and corresponding photophysi-
cal data are given in Table S1 (Supporting Information).
Multinuclear NMR spectroscopy provides an indication
of the metal-ligand bonding and the formation of a single
(15) (a) Jude, H.; Disteldorf, H.; Fischer, S.; Wedge, T.; Hawkridge,
A. M.; Arif, A. M.; Hawthorne, M. F.; Muddiman, D. C.; Stang, P. J.
J. Am. Chem. Soc. 2005, 127, 12131. (b) Jude, H.; Sinclair, D. J.; Das, N.;
Sherburn, M. S.; Stang, P. J. J. Org. Chem. 2006, 71, 4155, and references
therein.

Article
Organometallics, Vol. 28, No. 15, 2009 4293
Scheme 4. Three-Component Self-Assembly of 90°, Linear, and Tritopic Planar Linkers and the Formation of the Cage 4 Using This
Methodology in the Presence of Appropriate Guest
Quantum yields (ΦF) of 3c and 3d are high compared to
complex 3b. Lifetimes of 3a and 3d were measured to be 8
and 6.2 ns.
factors in mind, the cages 3a-d were designed for the
following reasons: (a) a Pt-ethynyl functionality was intro-
duced to make the assemblies fluorescent and to make them
π-electron rich; (b) open spaces in the large cages are suitable
to accommodate electron-deficient small nitroaromatics; (c)
bulky PEt3 groups help to avoid intermolecular stacking and
thus prevent the chance of self-quenching of fluorescence.
The solution fluorescence of 3a was quenched efficiently by
adding nitroaromatics such as DNT and TNT, which are the
chemical signatures of many explosives.8 The fluorescence
intensity in the solution state of both 3c and 3d undergoes
quenching upon gradual addition of a TNT (2,4,6-trinitro-
Fluorescence Quenching Studies. Fluorescence-quenching-
based detection of chemical explosives is very important
because of huge applications in defense and security.16
Conjugated polymers are proven to be efficient redox sensors
for explosives, as they are electron donors.17,16b Porous
polyethynyl compounds have been investigated as potential
fluorescent sensors for nitroaromatics, which are the chemi-
cal signatures of many explosives. Porous polymers are
better sensors than simple nonporous analogues since the
porous nature of the polymers produces a high free volume in
the structure, which imparts permeability and allows small
nitroaromatic vapor to penetrate quickly into the poly-
mer.17,18 Nitroaromatics are electron deficient in nature,
and substitution of electron-withdrawing nitro groups on
the aromatic ring lowers the energy of the empty π* orbitals,
thereby making them good electron acceptors. Keeping these
toluene) solution. In the case of prism 3c, a 3 ꢀ 10-6
M
solution in DMF was titrated with a TNT solution (Figure 6)
and showed quenching of the fluorescence intensity. The
Stern-Volmer plot (Figure 6) gives a quenching constant of
KSV=13.04 ( 0.62.
When a 1.5 ꢀ 10-6 M solution of 3d in DMF was titrated
with a TNT solution in methanol, the fluorescence intensity
gradually decayed, as shown in Figure 7. The mechanism of
fluorescence quenching may be due to the formation of a
charge transfer complex between the excited state of
(16) (a) Rouhi, A. M. Chem. Eng. News 1997, 75, 14. (b) Rose, A.; Zhu,
Z.; Madigan, C. F.; Swager, T. M.; Bulovic, V. Nature 2005, 434, 876. (b)
McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537.
(c) Yinon, J. Anal. Chem. 2003, 75, 99A. (d) Content, S.; Trogler, W. C.;
Sailor, M. J. Chem.;Eur. J. 2000, 6, 2205. (e) Balakrishnan, K.; Datar, A.;
Zhang, W.; Yang, X.; Naddo, T.; Huang, J.; Zuo, J.; Yen, M.; Moore, J. S.;
Zang, L. J. Am. Chem. Soc. 2006, 128, 6576. (f) Chemosensors of Ion and
Molecule Recognition; Desvergne, J. P., Czarnik, A. W., Eds.; Kluwer
Academic Publishers: Boston, 1997. (g) de Silva, A. P.; Gunaratne, H. Q.
N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.;
Rice, T. E. Chem. Rev. 1997, 97, 1515. (h) Thomas, S. W.; Amara, J. P.;
Bjork, R. E.; Swager, T. M. Chem. Commun. 2005, 4572. (i) Albert, K. J.;
Myrick, M. L.; Brown, S. B.; James, D. L.; Milanovich, F. P.; Walt, D. R.
Environ. Sci. Technol. 2001, 35, 3193. (j) Moore, D. S. Rev. Sci. Instrum.
2004, 75, 2499. (k) Czarnik, A. W. Nature 1998, 394, 417.
(17) (a) Swager, T. M. Acc. Chem. Res. 1998, 31, 201. (b) Liu, Y.; Mills,
R.; Boncella, J.; Schanze, K. Langmuir 2001, 17, 7452.
(18) Tsuchihara, K.; Masuda, T.; Higashima, T. J. Am. Chem. Soc.
1991, 113, 8548.
Figure 3. 1H NMR spectra of 3d in CDCl3.

4294 Organometallics, Vol. 28, No. 15, 2009
Ghosh et al.
π-electron rich cage 3d and electron-deficient oxidizing 2,4,6-
trinitrotoluene (TNT). The Stern-Volmer plot (Figure 7)
gives a quenching constant of KSV = 173.3 ( 5.46. The high
quenching constant in the case of 3d is probably due to the
presence of electron-donating -CH3, which makes the
cage a better host for electron-deficient TNT. TNT absorbs
irradiation below 300 nm,6g and emissions of the supra-
molecules 3a-d are far above 350 nm (Figure S10 and
ref 8). The lack of overlap between the emission of supra-
molecular cages and the absorption of TNT ruled out the
possibility of energy transfer from the excited cages to the
explosives quencher. Hence the observed fluorescence
quenching in the present study is presumably due to the
photoinduced electron transfer from the excited state of the
cages to the ground state of the TNT. This facile electron
transfer is expected due to the strong oxidation potential of
Figure 4. Proposed view of the optimized structure of 3d. Ethyl
groups are omitted for the sake of clarity. Color codes: green =
Pt; pink = P; blue = N.
Figure 5. UV-vis spectra of 3b-d in DMF solution.
Figure 6. Quenching of fluorescence intensity of 3c on gradual addition of TNT (left) and Stern-Volmer plot (right).
Figure 7. Quenching of fluorescence intensity of 3d on gradual addition of TNT (left) and Stern-Volmer plot (right).

Article
Organometallics, Vol. 28, No. 15, 2009 4295
TNT and the presence of conjugated labile π-electrons in the
fluorophor.
benzene/hexane (1:1) mixture to obtain 4,40,400-tris[ethynyl-
trans-Pt(PEt3)2(I)]triphenylamine (MW 1989.25) (400 mg) as a
pale yellow powder in 64% isolated yield. 1H NMR (CDCl3, 400
MHz): δ 7.1 (d, 6H, Ph-H), 6.9 (d, 6H, Ph-H), 2.2 (q, 36H, PEt3),
1.2 (t, 72H, PEt3). 31P{1H} NMR (CDCl3, 120 MHz): δ 8.6
(s, 1JP-Pt=2874 Hz).
To a methanolic solution of AgNO3 [5.1 mg, 0.03 mmol] was
added a solution of 4,40,400-tris[ethynyl-trans-Pt(PEt3)2(I)]tri-
phenylamine [19.9 mg, 0.01 mmol] in CHCl3. After stirring for
1 h the yellowish precipitate of AgI was filtered through Celite.
After removal of the solvent 1a (MW 1794.5) [16.9 mg] was
obtained in 94% isolated yield. Anal. Calcd (%): C 40.16; H
5.73; N 3.12. Found: C 40.29; H 5.60; N 3.23. 1H NMR (CDCl3,
400 MHz): δ 7.0 (d, 6H, Ph-H), 6.9 (d, 6H, Ph-H), 1.9 (q, 36H,
PEt3), 1.2 (t, 72H, PEt3). 31P{1H} NMR (CDCl3, 120 MHz): δ
20.0 (s, 1JP-Pt=3052 Hz).
To check whether 3d can detect the presence of trace
amounts of TNT, a 1.5 ꢀ 10-5 M solution of 3d was titrated
with a ∼10-5 M solution of TNT (Supporting Information).
Quenching of fluorescence intensity of 3d was noticed upon
addition of TNT. However, the percentage of quenching was
less in this case. Interestingly, when the starting Pt3 acceptor
1c was titrated with TNT solution, no quenching of fluores-
cence intensity of 1c was noticed. In the case of 2d very weak
quenching was noticed. These observations suggest that
incorporation of the electron-deficient TNT inside the cavity
of electron-rich cage 3d and host-guest interactions among
them are possible reasons for this quenching.
Solid State Fluorescence Quenching of 3d by TNT Vapor. A
thin film of the prism 3d was made by spin coating of a
solution of 3 ꢀ 10-5 M of 3d in DMF on 1 cm2 quartz plate.
The fluorescence response of this film to the vapor of TNT
was ascertained by inserting the film in a sealed vial contain-
ing solid TNT and cotton gauze separating the film from
direct contact with the TNT. The film was exposed to the
vapor of TNT, and the fluorescence was measured between a
particular time interval; it was found to be quenched gradu-
ally as shown in Figure S14 in the Supporting Information.
Synthesis of 1,8-Bis[{trans-Pt(PEt3)2(NO3)}ethynyl]anthra-
cene (1b). The synthesis was carried out using a standard
Schlenk technique under a N2 atmosphere. To a 100 mL
round-bottom Schlenk flask were added trans-Pt(PEt3)2I2
(1198.7 mg, 1.75 mmol) and 1,8-diethynylanthracene (113.0
mg, 0.5 mmol). Dry toluene (25 mL) and 10 mL of dry
diethylamine were added to the mixture. The solution was
stirred for 10 min at ambient temperature before 20 mg of CuI
was added in one portion under nitrogen. After 2 h of stirring at
room temperature a greenish-yellow fluorescent precipitate
started to appear. The solvent was removed under vacuum after
16 h to yield the crude coupled product as a greenish-yellow
residue. 1,8-Bis[{trans-Pt(PEt3)2(I)}ethynyl]anthracene (MW
1340.85) was separated from the mixture by column chroma-
tography on silica gel with a solvent mixture (benzene/hexane,
2:1). Yield: 60%. 1H NMR (CDCl3, 400 MHz): δ 9.43 (s, 1H),
8.37 (s, 1H), 7.83 (d, 2H), 7.49 (d, 2H), 7.34 (d, 2H), 2.26
(q, 24H), 1.18 (t, 36H); 31P{1H} NMR (CDCl3, 120 MHz): δ
8.35 (s, 1JPt-P=2756 Hz).
To a stirred solution of silver nitrate (25.1 mg, 0.147 mmol) in
methanol was added a solution of 1,8-bis[(trans-Pt(PEt3)2I)-
ethynyl]anthracene (100.0 mg, 0.074 mmol) in chloroform.
After stirring for 3 h the light yellow precipitate of AgI was
filtered through Celite and the clear filtrate was evaporated to
get 1b (MW 1211.05) in 90% yield. Anal. Calcd (%): C 41.65; H
5.66; N 2.31. Found: C 41.44; H 5.82; N 2.01. 1H NMR (CDCl3,
400 MHz): δ 9.27 (s, 1H), 8.35 (s, 1H), 7.82 (d, 2H), 7.42 (d, 2H),
7.35 (d, m), 2.00 (q, 24H), 1.27 (t, 36H). 31P{1H} NMR (CDCl3,
120 MHz): 19.06 (s, 1JPt-P=3072 Hz).
1,1,1-Tris(4-ethynylphenyl)ethane. In a flame-dried 100 mL
Schlenk flask 1,1,1-tris(4-iodophenyl)ethane14 (635.8 mg,
1.0 mmol), PPh3 (10.0 mg, 0.038 mmol), and PdCl2(PPh3)2
(60.0 mg, 0.085 mmol) were added, and the flask was degassed
three times followed by filling with N2. Then 40 mL of dried and
degassed triethylamine was added to the flask followed by the
addition of trimethylsilylacetylene (392.8 mg, 4 mmol). This
mixture was stirred under N2 for 30 min before the addition of
CuI (7.0 mg, 0.036 mmol). The reaction mixture was refluxed for
12 h followed by the removal of the solvent under vacuum to
yield a residue, which was dissolved in benzene and passed
through a short silica gel column. Removal of the solvent gave
MeC(4-C6H4-CCSiMe3)3 (MW 546.25). 1H NMR (CDCl3, 400
MHz): δ 7.3 (d, 6H, Ph-H), 7.0 (d, 6H,Ph-H), 2.1 (s, 3H, CH3),
0.26 (s, 27H, SiMe3). MeC(4-C6H4-CCSiMe3)3 (550 mg, 0.99
mmol) was stirred overnight with K2CO3 (409.8 mg, 3 mmol) in
methanol. Removal of methanol gave a brownish solid, and the
hydrolyzed product 1,1,1-tris(4-ethynylphenyl)ethane (MW
330.14) was extracted with toluene (250 mg, 0.75 mmol) in
75% yield. Anal. Calcd (%): C 94.51; H 5.49. Found: C 94.36;
H 5.77. 1H NMR (CDCl3, 400 MHz): δ 7.4 (d, 6H, Ph-H), 7.0 (d,
6H, Ph-H), 3.0 (s, 3H, ethynyl-H), 2.1 (s, 3H, CH3).
Conclusion
In conclusion, we herein report the synthesis and char-
acterization of a new Pt3 organometallic acceptor (1c) and a
series of nanoscopic conjugated prisms (3a-d) containing a
Pt-ethynyl functionality. A complementary approach has
been applied using a tritopic planar donor (2c) and Pt2-
organometallic clip (1b) to establish the versatility of direc-
tional bonding to obtain a similar kind of architecture using
a reverse approach. Incorporation of a Pt-ethynyl function-
ality makes the assemblies fluorescent in nature. Moreover,
the conjugated ethynyl functionality makes the assemblies
π-electron rich and possible hosts for electron-deficient
nitroaromatics. All four assemblies showed fluorescent be-
havior in solution, and the fluorescent intensity of the cages
3a, 3c, and 3d in solution was quenched upon addition of
electron-deficient TNT, which is the chemical signature of
many commercial explosives. Conjugated organic polyethy-
nyl compounds have been used as potential sensors for
chemical explosives.19 However, the discrete organometallic
cages 3a-d presented in this article represent an interesting
class of materials, which can detect the presence of TNT by
fluorescence quenching study.
Experimental Section
Synthesis of 4,40,400-Tris[ethynyl-trans-Pt(PEt3)2(NO3)]tri-
phenylamine (1a). In a flame-dried Schlenk flask N,N,N-tris-
(4-ethynylphenyl)amine [98.9 mg, 0.312 mmol] and trans-Pt-
(PEt3)2I2 [1.0 g, 1.45 mmol] were taken, and the flask was
degassed and purged with nitrogen. Dry, deoxygenated toluene
(25 mL) and 10 mL of dry diethylamine were added successively.
After stirring for 30 min under N2, CuI [25 mg, 0.132 mmol] was
added and the mixture was stirred for 20 h under N2. The
precipitate of diethylammonium chloride (Et2NH2Cl) was se-
parated, and the solvent was removed under vacuum. The
residue was chromatographed using a silica gel column by a
1,1,1-Tris[4-(trans-Pt(PEt3)2I)ethynylphenyl]ethane. In
flame-dried Schlenk flask 1,1,1-tris(4-ethynylphenyl)ethane
a
(19) Toal, S. J.; Trogler, W. C. J. Mater. Chem. 2001, 16, 2871, and
references therein.
(120 mg, 0.363 mmol) and trans-Pt(PEt3)2I2 (994.6 mg, 1.45 mmol)

4296 Organometallics, Vol. 28, No. 15, 2009
Ghosh et al.
were taken and degassed three times followed by purging with
N2. Dried and degassed toluene (30 mL) and 10 mL of dry
diethylamine were added, and the mixture was stirred for 30 min
before addition of CuI (20.0 mg, 0.105 mmol). Then this was
stirred for 20 h under N2. Solvent was removed, and the residue
was chromatographed using silica gel column chromatography
(1:1, benzene/hexane). After removal of solvent, 1,1,1-tris-
[4-(trans-Pt(PEt3)2I)ethynylphenyl]ethane (MW 2002.29) (450
mg) was obtained as pale yellow powder in 61% isolated yield.
Anal. Calcd (%): C 37.19; H 5.29. Found: C 37.47; H 5.56. 1H
NMR (CDCl3, 400 MHz): δ 7.1 (d, 6H, Ph-H), 6.9 (d, 6H,
Ph-H), 2.2 (q, 36H, PEt3), 2.0 (s, 3H, CH3), 1.1 (t, 54H, PEt3).
31P{1H} NMR (CDCl3, 120 MHz): δ 8.5 (s, 1JP-Pt=2880 Hz).
1,1,1-Tris[4-{trans-Pt(PEt3)2(NO3)}ethynylphenyl]ethane (1c).
To a solution of AgNO3 (28.0 mg, 0.16 mmol) in methanol was
added a solution of 1,1,1-tris[4-(trans-Pt(PEt3)2I) ethynylphenyl]-
ethane (110 mg, 0.05 mmol) in CHCl3. After stirring for 1 h the
yellowish-white AgI was filtered through Celite, and removal of
solvent gave 1c (MW 1807.5) (90.2 mg) in 90% yield. Anal. Calcd
(%): C 41.20; H 5.85; N 2.32. Found: C 40.97; H 5.67; N 2.21. 1H
NMR (CDCl3, 400 MHz): δ 7.1 (d, 6H, Ph-H), 6.9 (d, 6H, Ph-H),
2.0 (s, 3H, CH3), 1.9 (q, 36H, PEt3), 1.2 (t, 54H, PEt3). 31P{1H}
NMR (CDCl3, 120 MHz): δ 20.1 (s, 1J PPt= 3072 Hz). MS (ESI):
1807.5 [M1c þ H]þ, 1274.7 [M1c -4PEt3 -NO-3 ]þ. Melting point:
>270 °C, dec.
Synthesis of the Prism 3c. To a suspension of clip-type metal
acceptor 1b (18.1 mg, 0.015 mmol) in acetone was added a
solution of the ligand 2c (5.4 mg, 0.01 mmol) in acetone. After
heating for 5 min at 50 °C a fluorescent yellow solution formed,
which after stirring for more than 12 h gave a yellow precipitate.
The precipitate after washing with acetone gave 3c (MW
4730.42) (23 mg) in 93% yield. Anal. Calcd (%): C 51.80; H
5.37; N 4.15. Found: C 51.42; H 5.51; N 4.33. 1H NMR (CDCl3,
400 MHz): δ 9.2 (s, 3H, anthra-H), 9.2 (d, 12H, Py-HR), 8.4 (s,
3H, anthra-H), 7.8 (m, 18H, Ph-H and anthra-H), 7.5 (m, 18H,
Py-Hβ and anthra-H), 7.3 (m, 6H, anthra-H), 7.1 (d, 12H, Ph-
H), 1.9 (q, 72H, PEt3), 1.2 (t, 108H, PEt3). 31P{1H} NMR
1
(CDCl3, 120 MHz): δ 14.34 (s, JPPt = 2880 Hz). 13C NMR
(CDCl3, 100 MHz): δ 153.0, 148.0, 144.0, 136.0, 134.0, 132.0,
131.8, 131.0, 125.0, 122.0, 112.0, 96.0, 87.0, 63.1, 57.6, 14.8, 8.5.
MS (ESI) m/z: 1396.27 [M3c - 3PEt3 - 3NO-3 ]3þ, 1122.27
[M3c - 4NO-3 ]4þ, 1003.33 [M3c - 4PEt3 - 4NO-3 ]4þ. Melting
point: >180 °C, dec.
Synthesis of the Prism 3d. To a solution of 1c (9.0 mg, 0.005
mmol) in acetone was added a suspension of ligand 2d (2.8 mg,
0.0075 mmol) in acetone. The fluorescent yellow solution after
stirring for 12 h at 50 °C gave a yellow precipitate. The
precipitate was centrifuged and washed with acetone to obtain
3d (MW 4756.48) (11.0 mg) in 92% yield. Anal. Calcd (%): C
52.52; H 5.47; N 3.53. Found: C 52.90; H 5.61; N 3.27. 1H NMR
(CDCl3, 400 MHz): δ 9.5 (s, 3H, anthra-H), 8.5 (d, 12H, Py-HR),
8.1 (d, 6H, anthra-H), 8.0 (s, 3H, anthra-H), 7.9 (d, 6H, anthra-
H), 7.5 (m, 6H, anthra-H), 7.4 (d, 12H, Py-Hβ), 7.1 (d, 12H, Ph-
H), 6.9 (d, 12H, Ph-H), 1.8 (s, 6H, CH3), 1.7 (q, 72H, PEt3), 1.2
(t, 108H, PEt3). 31P{1H} NMR (CDCl3, 120 MHz): δ 14.9 (s,
1JPPt = 2800 Hz). 13C NMR (CDCl3, 100 MHz): δ 157.0, 154.0,
147.0, 148.0, 135.0, 132.7, 131.6, 130.6, 129.6, 127.0, 119.8, 97.5,
Synthesis of the Prism 3a. To a stirred solution of 1a (12 mg,
0.006 mmol) in acetone was added a solution of 2a (2.86 mg,
0.009 mmol) in methanol, and the resulting solution was kept
under stirring at 50 °C for 12 h. The product was obtained as a
light yellow powder by ether addition (MW 4542) (96% yield).
Anal. Calcd (%): C 45.97; H 5.41; N 6.16. Found: C 46.26; H
1
5.66; N 6.21. H NMR (CDCl3, 400 MHz): δ 10.81 (s, 6H, -
NH), 9.23 (s, 3H, ArH), 9.19 (d, 6H, Py-H), 8.79 (d, 6H, Py-H),
8.3 (m, 12H, ArH), 7.61 (m, 9H, ArH), 7.15 (d, 12H, ArH), 6.91
(d, 12H, ArH), 1.9 (q, 72H, -CH2), 1.1 (t, 108H, -CH3). 31P
92.5, 61.3, 56.1, 20.9, 15.1, 8.6. MS (ESI) m/z: 1606.7 [M3d
12PEt3 -2NO-3 ]2þ, 1523.5 [ M3d - 3NO3-]3þ, 1209.6 [M3d
-
-
8PEt3 - 3NO-3 ]3þ, 921.67 [M3d- 7PEt3 - 4NO-3 ]4þ, 890.1 [M3d
- 5NO-3 ]5þ. Melting point: >200 °C, dec.
1
{1H} NMR (CDCl3, 120 MHz): δ 15.60 (s, JPt-P=2872 Hz).
MS (ESI) m/z: 1452 [M3a - 3NO-3 ]3þ, 1073.49 [M3a - 4NO-3 ]4þ
.
Acknowledgment. We are grateful to the Department
of Science and Technology, New Delhi, India (SR/S1/
IC-18/2008), and the Council of Scientific and Industrial
Research (CSIR), New Delhi, for financial support to
P.S.M. A research fellowship to S.G. from UGC is
gratefully acknowledged. The authors wish to thank
Matthew M. Lyndon for kind help in mass spectrometric
analysis of 3c. The authors also wish to thank Johnson
Mathey Ltd, UK for a generous supply of K2PtCl4 as a
loan.
Synthesis of the Prism 3b. To a solution of 1a (12.0 mg, 0.006
mmol) in acetone was added a solution of 2b (2.5 mg, 0.009
mmol). After stirring for 12 h at 50 °C the product was formed as
a yellow precipitate. The final product was centrifuged and
washed with acetone to obtain the supramolecular prism 3b
(MW 4430.069) (13.1 mg) in 91% yield. Anal. Calcd (%): C
48.80; H 5.46; N 4.42. Found: C 48.66; H 5.59; N 4.22. 1H NMR
(50% CDCl3/ MeOH, 400 MHz): δ 9.0 (d, 6H, Py-H), 8.8 (d, 6H,
Py-H), 8.2 (m, 6H, Py-H), 8.0 (d, 6H, Py-H), 7.8 (s, 3H, Ph-H),
7.7-7.5 (m, 9H, Ph-H), 7.1 (d, 12H, Ph-H), 6.9 (d, 12H, Ph-H),
1.9 (q, 72H, PEt3), 1.2 (t, 108H, PEt3). 31P{1H} NMR (50%
CDCl3/MeOH, 120 MHz): δ 15.6 (s, 1JPPt=2840 Hz). 13C NMR
(50% CDCl3/MeOH, 100 MHz): δ 155.8, 153.9, 146.5, 135.5,
134.0, 133.2, 130.8, 129.0, 124.6, 123.3, 94.4, 85.9, 61.2, 59.1,
Supporting Information Available: 1H and 31P NMR spectra
M
of linkers and cages, fluorescence quenching of 3d using 10-5
15.0, 8.4. MS (ESI) m/z: 1413 [M3b - 3NO-3 ]3þ, 823 [M3b
5NO3-]5þ, 675 [M3b- 6NO3-]6þ. Melting point: >200 °C, dec.
-
TNT, and ESI mass spectra are available free of charge via the
Internet at http://pubs.acs.org.