Self-assembly of a nanoscopic prism via a new organometallic Pt3 acceptor and its fluorescent detection of nitroaromatics

Article abstract of DOI:10.1021/om701082y

A nanoscale-sized cage with a trigonal prismatic shape is prepared by coordination-driven self-assembly of a predesigned organometallic Pt₃ acceptor with an organic cliptype ligand. This trigonal prism is fluorescent and undergoes efficient fluorescence quenching by nitroaromatics, which are the chemical signatures of many explosives.

Full text of DOI:10.1021/om701082y

316
Organometallics 2008, 27, 316–319
Self-Assembly of a Nanoscopic Prism via a New Organometallic Pt3
Acceptor and Its Fluorescent Detection of Nitroaromatics
Sushobhan Ghosh and Partha Sarathi Mukherjee*
Inorganic and Physical Chemistry Department, Indian Institute of Sciences, Bangalore-560012, India
ReceiVed October 26, 2007
construction. A few 3D cages are known with the former
combination3 (where M ) bidentate acceptor and L ) tridentate
donor), while the latter combination (where M ) tridentate
acceptor and L ) bidentate donor) is very rare in the litera-
ture because of the difficulty in the synthesis of shape-selective
tridentate Pd/Pt acceptors.4 This prompted us to prepare a Pt3
organometallic planar acceptor 4,4′,4′′-tris[ethynyl-trans-
Pt(PEt3)2(NO3)]triphenylamine (M) as a shape-selective linker
toward the construction of a trigonal-prism (Scheme 1). Here,
we report the design and synthesis of a nanoscopic prism (1)
prepared by the self-assembly of M with a clip-type amide
containing the ligand 1,3-bis(3-pyridyl)isophthalic amide (L)
and the preliminary study of its fluorescent detection of
nitroaromatics, which are chemical signatures of many explo-
Summary: A nanoscale-sized cage with a trigonal prismatic
shape is prepared by coordination-driVen self-assembly of a
predesigned organometallic Pt3 acceptor with an organic clip-
type ligand. This trigonal prism is fluorescent and undergoes
efficient fluorescence quenching by nitroaromatics, which are
the chemical signatures of many explosiVes.
Self-assembly is the spontaneous association of either a few
or many chemical entities to form a larger aggregate under
thermodynamic control. With a careful and informed choice
of the starting materials, the resulting compounds can often
be predicted, and therefore, design strategies can be formu-
lated.1 Self-assembly via the directional bonding approach
is the most efficient way to the synthesis of designed
coordination assemblies. The major requirement for this
coordination driven self-assembly approach is the use of rigid
precursors of appropriate size and shape. Square planar Pt(II)
and Pd(II) have long been used as favorite metals in this area
because of their rigid coordination environment, and thus, it is
easy to control and predict the shapes and sizes of the final
assemblies.2 The most attractive feature of the metallasupramo-
lecular assemblies formed by the directional bonding approach
is the possibility of the introduction of functionality by
introducing required functional groups or by simply tuning the
structure. Of the various types of structures reported so far, 2D
assemblies dominated the literature. Three-dimensional cages2e
are relatively less compared to 2D assemblies because of the
difficulty in synthesis and isolation due to very high molecular
weight. M3L2 or M2L3 are the simple combinations for 3D cage
(3) (a) Mukherjee, P. S.; Das, N.; Stang, P. J. J. Org. Chem. 2004, 69,
3526. (b) Ghosh, S.; Mukherjee, P. S. Tetrahedron Lett. 2006, 47, 9297.
(c) Ghosh, S.; Mukherjee, P. S. Organometallics 2007, 26, 3362. (d) Ghosh,
S.; Turner, D.; Batten, S. R.; Mukherjee, P. S. Organometallics 2007, 26,
3252. (e) Radhakrishnan, U.; Schweiger, M.; Stang, P. J. Org. Lett. 2001,
3, 3141. (f) Kuehl, C. J.; Yamamoto, T.; Seidel, R.; Stang, P. J. Org. Lett.
2002, 4, 913. (g) Yang, H. B.; Ghosh, K.; Das, N.; Stang, P. J. Org. Lett.
2006, 8, 3991. (h) Yang, H. B.; Ghosh, K.; Northrop, B. H.; Stang, P. J.
Org. Lett. 2007, 9, 1561. (i) Yang, H. B.; Ghosh, K.; Arif, A. M.; Stang,
P. J. J. Org. Chem. 2006, 9, 9464.
(4) Chi, K. W.; Addicott, C.; Kryeschenko, Y. K.; Stang, P. J. J. Org.
Chem. 2004, 69, 964.
(5) (a) Synthesis of 4,4′,4′′-Tris[trans-Pt(PEt3)2I(ethynylphenyl)]amine.
First tri[p-ethynylphenyl]amine was prepared (see Supporting Information).
To a 100 mL round bottom shlenk flask were added trans-diiodobis(tri-
ethylphosphine)platinum (750 mg, 1.094 mmol) and tri(p-ethynylpheny-
l)amine (98.9 mg, 0.312 mmol). Then 25 mL of dry, degassed toluene and
10 mL of dry diethyl amine were added under nitrogen. The solution was
stirred for 10 min at room temperature before 25 mg of CuI was added in
one portion. After 16 hr at room temperature, a small amount of
diethylammonium iodide started precipitating out of the solution. The solvent
was removed under vacuum, the resulting yellow residue was separated by
column chromatography on silica gel with a solvent mixture of benzene/
hexane (2:1) to obtain the product in 64% yield. Anal. Calc. (%) C ) 36.23;
H ) 5.17; N ) 0.70. Found: C ) 36.38; H ) 5.37; N ) 1.01. 1H NMR
(CDCl3, 400 MHz): δ 7.26 (d, 6H, ArH), 6.93 (d, 6H, ArH), 2.2 (q, 36H,
* To whom correspondence should be addressed. Tel: 91-80-2293-3352.
Fax: 91-80-2360-1552. E-mail: psm@ipc.iisc.ernet.in.
(1) (a) Vickers, M. S.; Beer, P. D. Chem. Soc. ReV. 2007, 2, 211. (b)
Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853. (c)
Nehete, U. N.; Anantharaman, G.; Chandrasekhar, V.; Murugavel, R.;
Roesky, H. W.; Vidovic, D.; Magull, J.; Samwer, K.; Sass, B. J. Angew.
Chem., Int. Ed. 2004, 43, 3832. (d) Ghosh, A. K.; Ghoshal, D.; Ribas, J.;
Mostafa, G.; Ray Chaudhuri, N. Cryst. Growth Des. 2006, 6, 36. (e)
Maurizot, V.; Yoshizawa, M.; Kawano, M.; Fujita, M. J. Chem. Soc., Dalton
Trans. 2006, 2750. (f) Pathak, B.; Pandian, S.; Hosmane, N. S.; Jemmis,
E. D. J. Am. Chem. Soc. 2006, 128, 10915. (g) Chai, J.; Jancik, V.; Singh,
S.; Zhu, H.; He, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.;
Hosmane, N. S. J. Am. Chem. Soc. 2005, 127, 7521. (h) Oh, M.; Carpenter,
G. B.; Sweigart, D. A. Angew. Chem., Int. Ed. 2002, 41, 3650. (i) Oh, M.;
Carpenter, G. B.; Sweigart, D. A. Angew. Chem., Int. Ed. 2003, 42, 2025.
(j) Chandrasekhar, V.; Thirumoorthi, R. Organometallics 2007, 26, 4515.
(k) Shin, R. Y. C.; Tan, G. K.; Koh, L. L.; Vittal, J. J.; Goh, L. Y.; Webster,
R. D. Organometallics 2005, 24, 539. (l) Mondal, K. C.; Song, Y.;
Mukherjee, P. S. Inorg. Chem. 2007, 46, 9736. (m) Ghosh, S.; Mukherjee,
P. S J. Chem. Soc., Dalton Trans. 2007, 2542.
-CH2), 1.2 (t, 54H, -CH3). 31P NMR (CDCl3, 400 MHz): δ 8.57 (s, 1JPt-P
)
2874 Hz). The iodide derivative was treated with AgNO3 to obtain the nitrate
product (M). (Yield: 11.7 mg, 65%.) Anal. Calc. (%) C ) 40.16; H )
5.73; N ) 3.12. Found: C ) 40.36; H ) 5.51; N ) 3.32. 1H NMR (CDCl3,
400 MHz): δ 6.84 (d, 6H, ArH), 7.01 (d, 6H, ArH), 1.9 (q, 36H, ArH),
1
1.15 (t, 54H, ArH). 31P NMR (CDCl3, 400 MHz): δ 20.1 (s, JPt-P)3064
Hz). IR (Nujol): 2121, 1487, 1384, 1274, 1034 cm-1. Preparation of 1.
To a stirred solution of M (12 mg, 0.006 mmol) in acetone, the solution of
L (2.86 mg, 0.009 mmol) in methanol was added, and this resulting solution
was kept under stirring and heating in an oil bath at 50 °C overnight. The
product was obtained by ether addition (96% yield). Anal. Calc. (%) C )
45.97; H ) 5.41; N ) 6.16. Found: C ) 46.26; H ) 5.66; N ) 6.21. 1H
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,
(2) (a) Kryschenko, Y. K.; Seidel, S. R.; Arif, A. M.; Stang, P. J. J. Am.
Chem. Soc. 2003, 125, 5193. (b) Mukherjee, P. S.; Das, N.; Kryeschenko,
Y.; Arif, A. M.; Stang, P. J. J. Am. Chem. Soc. 2004, 126, 2464. (c) Fujita,
M.; Tominaga, M.; Hori, A.; Therrien, B. Acc. Chem. Res. 2005, 38, 371.
(d) Oin, Z.; Jennings, M. C.; Puddephatt, R. J. Chem. Commun. 2001, 2676.
(e) Seidel, S. R.; Stang, P. J. Acc. Chem. Soc. 2002, 35, 972; and references
therein. (f) Leininger, S.; Fan, J.; Schmitz, M.; Stang, P. J. Proc. Natl. Acad.
Sci. U.S.A. 2000, 97, 1380. (g) Stang, P. J.; Olenyuk, B. Acc. Chem. Res.
1997, 30, 502.
1
-CH3).31P NMR (CDCl3, 400 MHz): 15.60 (s, JPt-P ) 2872 Hz). (b)
Crystallographic data were collected at 100(K) K using a Bruker X8 Apex
II diffractometer equipped with monochromated Mo-KR radiation (λ )
0.71073 Å). Crystal data for linker M: C60 H102 I3 Fe3 N O26 P6 Pt3, MW
) 1989.22, monoclinic, P21/n, Z ) 4, a ) 16.944(5) Å, b ) 17.295(5) Å,
c ) 25.961(7) Å, ꢀ ) 101.622(7)°, V ) 7452.0(4) Å3, R1 ) 0.070 and
wR2 ) 0.1992.
10.1021/om701082y CCC: $40.75
2008 American Chemical Society
Publication on Web 01/11/2008

Communications
Organometallics, Vol. 27, No. 3, 2008 317
Scheme 1. Synthesis of the Organometallic Pt3 Acceptor M
sives. Cage 1 represents the first example of a trigonal-prism
obtained from a purely planar Pt3 acceptor.
been found that coordination complexes containing this func-
tional group frequently show fluorescent/luminescent behavior.
1
The acceptor M was characterized fully by H and 31P NMR.
From the general concept of geometry, one can design a
trigonal prism by linking two tritopic planar units with three
clip-type units. On the basis of this general principle, we have
prepared a new organometallic Pt3 tritopic acceptor (M)
following the steps shown in Scheme 1.5a
Finally, the iodide form of this linker was unambiguously
characterized by X-ray single crystal structure determination.5b
A view of the molecular structure is shown in Figure 1. The
crystal structure determination reveals the formation of a planar
triplatinum molecule with a central nitrogen atom. The Pt-N-Pt
angles lie between 117 and 121°, which is in the expected range.
The geometry around the Pt centers is distorted square planar
with I-Pt-P angles in the range of 89.3–92.1°, and the C-Pt-P
angles between 85.7 and 91.1°. The Pt-I bond lengths (2.64,
2.64, and 2.65 Å) are similar to the distances observed in the
literature for iodide trans to an acetylene group, such as
(Pt(PMe3)2I)CC(I(PMe3)2Pt.6
Ethynyl functionality was introduced to make the linker rigid
and planar, and particularly to make it fluorescent since it has
The cationic prism M2L3 (1) was prepared from the 2:3 self-
assembly of M and the amide based clip-type donor L (Scheme
2). When 4,4′,4′′-tris[ethynyl-trans-Pt(PEt3)2(NO3)]triphenyl-
amine (M)5a acceptor was treated with a 1.5 equivalent of the
donor ligand L7 in a mixture of methanol and dichloromethane
for 30 min, the self-assembly of the cationic prism (1) as a single
product occurred. An immediate visual change in color from
light yellow to intense yellow was also an indication of the
Figure 1. View of the molecular structure of the triiodide form of
M. Ethyl groups of PEt3 moieties are omitted for the sake of clarity.
Gray ) C; green ) Pt; blue ) N; yellow ) P; red ) I.
Scheme 2. Self-Assembly of the Trigonal Prismatic Cage 1

318 Organometallics, Vol. 27, No. 3, 2008
Communications
1
Figure 2. H NMR of L (top left), M (top right), and 1 (bottom) in CDCl3.
reaction. [2 + 3] self-assembly reaction of M and L may give
either a polymeric product or a discrete prism. Solution 31P{1H}
NMR analysis of the reaction solution showed the quantitative
formation of a single, highly symmetrical species (1) by the
appearance of a sharp singlet with concomitant 195Pt satellites,
shifted 5 ppm upfield (-∆δ) relative to M. The upfield shift of
the phosphorus signal in the 31P NMR spectrum is a clear
indication of the ligand to metal coordination (Supporting
Information).
The 1H NMR spectrum of 1 was also indicative of the
symmetrical arrangement of the components and displayed
significant spectroscopic differences from the precursor building
blocks (Figure 2). The downfield shift of the proton signals of
the protons adjacent to pyridyl nitrogen was associated with
the loss of electron density on coordination by the nitrogen lone
pair to the platinum metal center. NH protons of the clip L
also showed a strong downfield shift of 0.6 ppm upon complex
formation. In the IR spectrum of the prism, absorptions arising
from L were observed.
Figure 3. ESI-MS spectrum of 1 in MeOH. Inset: theoretical and
experimental isotopic distribution patterns of the [M1-3NO3]3+
peak.
The carbonyl stretching frequency of L appeared at 1672
cm-1, whereas the corresponding prism 1 showed carbonyl
(6) Ogawa, H.; Joh, T.; Takahashi, S.; Yamamoto, Y.; Yamazaki, Y.
Organometallics 1988, 7, 2257.
absorption at 1679 cm-1. These shifts to a higher wavenumber
are presumably due to the coordination of L to Pt. NMR
spectroscopy provides insight about the metal–ligand coordina-
tion and the formation of a single product; however, it does
not provide any information about the shape of this kind of
product. ESI mass spectrometry has proven to be a useful tool
in the corroboration of structural assignments for this kind of
self-assemblies.8 Mass-spectrometric study of 1 was performed
by ESI technique, which allowed us to keep the assembly intact
during the ionization process while obtaining the high resolution
(7) Nancy, L. S.; Dana, E. J.; Jennings, M. C.; Puddephatt, R. J. Inorg.
Chem. 2004, 43, 7671.
(8) Ghosh, S.; Mukherjee, P. S. J. Org. Chem. 2006, 71, 8412; and
references therein.
(9) (a) Naddo, T.; Che, Y.; Zhang, W.; Balakrishnan, K.; Yang, X.;
Yen, M.; Zhao, J.; Moore, J. S.; Zang, L. J. Am. Chem. Soc. 2007, 129,
6978. (b) Germain, M. E.; Vargo, T. R.; Khalifah, P. G.; Knapp, M. J.
Inorg. Chem. 2007, 46, 4422. (c) Splan, K. E.; Massari, A. M.; Morris,
G. A.; Sun, S. S.; Reina, E.; Nguyen, S. T.; Hupp, J. T. Eur. J. Inorg.
Chem. 2003, 2348. (d) Lee, S. J.; Lin, W. J. Am. Chem. Soc. 2002, 124,
4554. (e) Splan, K. E.; Keefe, M. H.; Aaron, M.; Massari, K. A.; Hupp,
J. T. Inorg. Chem. 2002, 41, 619. (f) Dinolfo, P. H.; Hupp, J. T. Chem.
Mater. 2001, 13, 3113; and references therein. (g) Jiang, H.; Lin, W. J. Am.
Chem. Soc. 2003, 125, 8084. (h) Jiang, H.; Lin, W. J. Am. Chem. Soc.
2004, 126, 7426.
(10) Rose, A.; Zhu, Z.; Madigan, C. F.; Swager, T. W.; Bulovic, V.
Nature 2005, 434, 876.

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Organometallics, Vol. 27, No. 3, 2008 319
Figure 4. Fluorescent quenching of 1 by TNT (left) and Stern-Volmer plot (right).
required for unambiguous determination of the charge states.
Electrospray mass spectroscopy study confirmed the formation
of the cage 1 assembling two Pt3 tritopic acceptors (M) and
three organic clip L with the molecular weight of 4540.33 Da.
The mass spectrum of 1 (Figure 3) showed signals corresponding
which was also gradually quenched by TNT with a concentration
order of 0.1 mM. The high values of quenching efficiencies
separates the quenching mechanism from that of SOPs and
amplifying fluorescent polymers (AFPs),10 which are quenched
by static quenching mechanisms. The proposed quenching
reaction is:
to the consecutive loss of nitrate counterions, [M1-3NO3]3+
,
[M1-4NO3]4+, and [M1-5NO3]5+. For 1, [M1-3NO3]3+ [m/z )
1452.00 (calcd 1451.66)]; [M1-4NO3]4+ [m/z ) 1073.49 (calcd
1073.18)]; [M1-5NO3]5+ [m/z ) 846.55 (calcd 845.67)]. The
inset in Figure 3 displays an expansion of the [M1-3NO3]3+
peak along with the theoretical isotopic distribution that matched
well with the experimental expansion (M1 ) molecular weight
of 1). A similar result was also obtained for the [M1-4NO3]4+
peak. Hence, the peaks at 1452.00 and 1073.49 (m/z) are due
to the +3 and +4 charge states, respectively, of cage 1. Several
attempts using different solvents and counterions to obtain single
crystals yielded only powder product. However, the presence
of expected peaks along with the isotopic distribution results
clearly confirmed M2L3 composition with a molecular weight
of 4540.33 Da for 1. The only possible shape of this composition
of a clip and a rigid tritopic linear unit is trigonal prism, as
shown in Scheme 2.
Fluorescence-quenching-based detection of explosives rep-
resents one of the most convenient methods that have been used
in defense and security. Conjugated aromatic compounds and
polymers with high π-electron density are proven as efficient
fluorescent sensing materials.9 However, finite 3D cages pre-
pared via the directional bonding approach have not been used
for this purpose. We have prepared a Pt3 organometallic ligand
to introduce conjugation and aromatic functionality for the
synthesis of a functional 3D supramolecule (1). Complex 1
shows absorptions at 275 and 368 nm in CH2Cl2-DMF (4:1).
The quenching study was performed by picric acid and
trinitrotoluene (TNT). Picric acid solution quenched the fluo-
rescence at 413 nm (Supporting Information), whereas TNT
quenched fluorescence at 546 nm (Figure 4). Analysis of the
normalized fluorescence intensity (I0/I) as a function of increas-
ing quencher concentration ([Q]) was well described by the
Stern-Volmer equation I0/I ) 1 + KSV[Q], for both cases.
Linear Stern-Volmer behavior is consistent with the quenching
that is dominated by a dynamic process. The Stern-Volmer
constants (KSV) in both the cases were determined from the plots.
The quenching efficiencies were 48.1 × 103 M-1 and 19.6 ×
103 M-1 in CH2Cl2-DMF (4:1) in the case of picric acid and
TNT, respectively. The complex was also excited at 380 nm,
and in that case, the emission peak was obtained at 440 nm,
1 + RNO2 f 1+ + RNO-2
where the nitro compound accepts an electron from the excited-
state of the complex. However, the dominant interaction of the
complex with the nitroaromatics is proposed to be π-stacking
with partial charge transfer.
In conclusion, we herein report the synthesis and character-
ization of a trigonal prism (1) prepared via directional self-
assembly of a new organometallic Pt3 planar acceptor and an
organic clip-type donor. Cage 1 represents the first example of
a trigonal prism-shaped derivative of a metal-based tritopic
planar acceptor. Preliminary results suggest that 1 is fluorescent
and that its solution fluorescence is quenched by nitroaromatics,
which are chemical signatures of explosives. The formation of
the cage using both the conjugated acceptor and donor enhances
the electron density and donating power of the molecule and
thus increases the efficiency of fluorescence quenching by
oxidative explosives. While there is enough room for improve-
ment in the response of 1 toward other potential explosives,
the Pt backbone with aromatic and ethynyl functionalities in
conjugation provides excellent starting material for new explo-
sive sensors. The use of a specially designed shape-selective
metal-based linker and the preparation of functional assemblies
have the potential to considerably expand the range of the
directional bonding paradigm in self-assembly. Detail fluorescent
quenching studies of this cage and related assemblies are under
investigation.
Supporting Information Available: Crystallographic informa-
tion of M in CIF format, synthesis of M, 31P and 13C NMR of M
and 1.
Acknowledgment. We are grateful for the financial
support provided to P.S.M. by Department of Science and
Technology, New Delhi, India (Under Fast Track Scheme
for Young Scientists) and Council of Scientific and Industrial
Research (CSIR), New Delhi, India. S. G. thanks the UGC
for research fellowship.
OM701082Y