Inorg. Chem.

2006, 45, 8070−8077

CH‚‚‚π Interaction for Rhenium-Based Rectangles: An Interaction That

Is Rarely Designed into a Host−Guest Pair
Bala. Manimaran,†,# Liang-Jian Lai,‡ P. Thanasekaran,† Jing-Yun Wu,† Rong-Tang Liao,†
Tien-Wen Tseng,‡ Yen-Hsiang Liu,† Gene-Hsiang Lee,§ Shie-Ming Peng,§ and Kuang-Lieh Lu*,†
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, Department of Chemical
Engineering, National Taipei UniVersity of Technology, Taipei 106, Taiwan, and
Department of Chemistry, National Taiwan UniVersity, Taipei 107, Taiwan
Received March 20, 2006

Alkoxy- and thiolato-bridged ReI molecular rectangles [{(CO)3Re(µ-ER)2Re(CO)3}2(µ-bpy)2] (ER ) SC4H9, 1a;
Published on September 1, 2006 on http://pubs.acs.org | doi: 10.1021/ic0604720

SC8H17, 1b; OC4H9, 2a; OC12H25, 2b; bpy ) 4,4′-bipyridine) exhibit strong interactions with several planar aromatic
molecules. The nature of their binding was studied by spectral techniques and verified by X-ray diffraction analysis.
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Standard absorption and fluorescence titrations showed that a relatively strong 1:1 interaction occurs between
aromatic guests such as pyrene and these rectangles. The results of a single-crystal X-ray diffraction analysis
show that the recognition of 1 with a pyrene molecule is mainly due to CH‚‚‚π interactions and the face of the
guest pyrene is located over the edges of the bpy linkers of 1. This is a fairly novel example of an interaction that
is rarely designed into a host−guest pair. Furthermore, the interaction of 1 with Ag+ results in the self-organization
of supramolecular arrays, as revealed by solid-state data.

Introduction interactions have been actively used to strengthen structural

Noncovalent interactions between host and guest molecules and electronic communication between organic ligands in
are ubiquitous in biology and include the vital functions of multifunctional metal complexes.10 Among these, a CH‚‚‚π
immune responses, transcription, replication, and biochemical interaction occurring between CH’s (soft acids) and π groups
signaling.1-3 They also lie at the heart of several practical (soft bases) has been noted as an important weak H-bond-
chemical technologies, including chemical sensing, separa-
tions, and catalysis.4-6 Extensive efforts have been made to (4) (a) Atwood, J. L.; Steed, J. W. Supramolecular Chemistry; John Wiley
& Sons: Ltd.: Chichester, U.K., 2000. (b) Lehn, J. M. Proc. Natl.
design components that mimic natural systems by undergoing Acad. Sci. U.S.A. 2002, 99, 4763. (c) Hirschberg, J. H. K. K.;
molecular self-organization through selective noncovalent Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. P.;
Meijer, E. W. Nature 2000, 407, 167. (d) Prins, L. J.; Reinhoudt, D.
interactions such as H-bonding, electrostatic, and π-π- N.; Timmerman, P. Angew. Chem., Int. Ed. 2001, 40, 2382.
stacking interactions.7-9 In addition, these noncovalent (5) (a) Lehn, J. M. In The New Chemistry; Hall, N., Ed.; Cambridge
University Press: Cambridge, U.K., 2000; pp 200-251. (b) Davis,
* To whom correspondence should be addressed. E-mail: A. P.; Wareham, R. S. Angew. Chem., Int. Ed. 1999, 38, 2978. (c)
lu@chem.sinica.edu.tw. Fax: int. code +886-2-27831237. Lee, H. K.; Park, K. M.; Jeon, Y. J.; Kim, D.; Oh, D. H.; Kim, H. S.;
† Academia Sinica. Park, C. K.; Kim, K. J. Am. Chem. Soc. 2005, 127, 5006.
‡ National Taipei University of Technology. (6) (a) Schug, K. A.; Lindner, W. Chem. ReV. 2005, 105, 67. (b) Atwood,
§ National Taiwan University. J. L.; Barbour, L. J.; Dalgarno, S. J.; Hardie, M. J.; Raston, C. L.;
# Current address: Pondicherry University, India. Webb, H. R. J. Am. Chem. Soc. 2004, 126, 13170. (c) Meyer, E. A.;
(1) (a) Samaranayake, M.; Bujnicki, J. M.; Carpenter, M.; Bhagwat, A. Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed. 2003, 42, 1210.
S. Chem. ReV. 2006, 106, 700. (b) Keeble, A. H.; Kirkpatrick, N.; (d) Przybylski, M.; Glocker, M. O. Angew. Chem., Int. Ed. Engl. 1996,
Shimizu, S.; Kleanthous, C. Biochemistry 2006, 45, 3243. (c) Wang, 35, 806.
T.; Gu, S.; Ronni, T.; Du, Y. C.; Chen, X. J. Proteome Res. 2005, 4, (7) (a) Yang, J.; Fan, E.; Geib, S. J.; Hamilton, A. D. J. Am. Chem. Soc.
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8070 Inorganic Chemistry, Vol. 45, No. 20, 2006 10.1021/ic0604720 CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/01/2006
 CH‚‚‚π Interaction for Rhenium-Based Rectangles

Scheme 1. Molecular Rectangles 1 and 2

like force by many chemists and biochemists.11 Despite being thiafulvalene and related derivates, CH‚‚‚π interactions were
the weakest of the H bonds, it plays a significant role in also reported to contribute significantly to the formation of
tuning the physical, chemical, and biological properties of two- and three-dimensional networks in addition to CH‚‚‚S,
substances.12-17 A number of calixarenes and cryptophans π-π stacking, and close chalcogen contacts.22 CH‚‚‚π
have been employed as potent synthetic macrocycles to interactions have also been inferred from structural studies
include various guests, whereby CH‚‚‚π interactions are of p-tert-butylcalix[4]arene‚guest compounds, although some
believed to be a crucial driving force in determining the of these are significantly disordered.23a,b Kojima et al.23c have
Published on September 1, 2006 on http://pubs.acs.org | doi: 10.1021/ic0604720

stability of host-guest complexes and in assembling mo- recently shown that the reaction of a RuII complex with
lecular units into an organized supramolecular structure.18-20 â-diketone gave â-diketonato complexes in which hydro-
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Hunter’s group used an amide macrocycle with a highly phobic π-π or CH‚‚‚π interactions were confirmed by NMR
preorganized cavity containing both polar and nonpolar spectroscopy and X-ray crystallography. However, CH‚‚‚π
recognition sites to form stable complexes with cyclic interactions in metallocyclophanes have been examined to
peptides in water via CH‚‚‚π interactions.21 During the a much lesser extent.24 Herein we report on the characteristics
investigation of organic conductive materials such as tetra- associated with the recognition of thiolato- and alkoxy-
bridged ReI molecular rectangles with respect to several
(10) (a) Coronado, E.; Galan-Mascaros, J. R.; Gomez-Garcıa, C. J.; Laukhln, planar aromatic molecules and the Ag ion. Their recognition
V. Nature 2000, 408, 447. (b) Uji, S.; Shinagawa, H.; Terashima, T.;
Yakabe, T.; Terai, Y.; Tokumoto, M.; Kobayashi, A.; Tanaka, H.; toward a highly conjugated aromatic guest, pyrene, via a
Kobayashi, H. Nature 2001, 410, 908. (c) Kitagawa, S.; Kitaura, R.; perfect CH‚‚‚π interaction was observed and confirmed by
Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334 and references cited
therein. solid-state data. This is a fairly novel example of an
(11) (a) Mobian, P.; Kern, J. M.; Sauvage, J. P. Angew. Chem., Int. Ed. interaction that is rarely designed into a host-guest pair.
2004, 43, 2392. (b) Schneider, H. J. Angew. Chem., Int. Ed. Engl.
1991, 30, 1417. (c) Su, C. Y.; Cai, Y. P.; Chen, C. L.; Smith, M. D.;
Furthermore, the interaction of the thiolato-bridged rectangle
Kaim, W.; zur Loye, H. C. J. Am. Chem. Soc. 2003, 125, 8595. toward Ag ions results in the self-organization of an
(12) (a) Nishio, M.; Hirota, M. Tetrahedron 1989, 45, 7201. (b) Nishio, interesting supramolecular array.
M.; Umezawa, Y.; Hirota, M.; Takeuchi, Y. Tetrahedron 1995, 51,
8665. (c) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π Interac-
tion: EVidence, Nature and Consequences; Wiley-VCH: Weinheim, Results and Discussion
Germany, 1998.
(13) (a) Miyake, Y.; Hosoda, A.; Takagaki, M.; Nomura, E.; Taniguchi, Self-assembly and Characterization of Thiolato- and
H. Chem. Commun. 2002, 132. (b) Matsumoto, A.; Tanaka, T.; Alkoxy-Bridged Rectangles. The self-assembly of new
Tsubouchi, T.; Tashiro, K.; Saragai, S.; Nakamoto, S. J. Am. Chem.
Soc. 2002, 124, 8891. thiolato-bridged ReI molecular rectangles [{(CO)3Re(µ-
(14) (a) Matsumoto, A.; Sada, K.; Tashiro, K.; Miyata, M.; Tsubouchi, T.; SR)2Re(CO)3}2(µ-bpy)2] (1a, R ) C4H9; 1b, R ) C8H17) is
Tanaka, T.; Odani, T.; Nagahama, S.; Tanaka, T.; Inoue, K.; Saragai, achieved from Re2(CO)10, 4,4′-bipyridine (bpy), and mer-
S.; Nakamoto, S. Angew. Chem., Int. Ed. 2002, 41, 2502. (b) Barreca,
M. L.; Carotti, A.; Carrieri, A.; Chirri, A.; Monforte, A. M.; Calace, captan (butanethiol or octanethiol) under solvothermal condi-
M. P.; Rao, A. Bioorg. Med. Chem. 1999, 7, 2283. tions (Scheme 1). The known alkoxy-bridged molecular
(15) (a) Amabilino, D. B.; Ashton, P. R.; Balzani, V.; Boyd, S. E.; Credi,
A.; Lee, J. Y.; Menzer, S.; Stoddat, J. F.; Venturi, M.; Williams, D. rectangles [{(CO)3Re(µ-OR)2Re(CO)3}2(µ-bpy)2] (2a, R )
J. J. Am. Chem. Soc. 1998, 120, 4295. (b) Vyas, N. K.; Vyas, M. N.; C8H17; 2b, R ) C12H25) were prepared via literature
Quiocho, F. A. Nature 1987, 327, 635.
(16) (a) Matsugi, M.; Nojima, M.; Hagimoto, Y.; Kita, Y. Tetrahedron
procedures.25 Preliminary studies of the thiolato- and alkoxy-
Lett. 2001, 42, 8039. (b) Kitamura, M.; Nakano, K.; Miki, T.; Okada,
M.; Noyori, R. J. Am. Chem. Soc. 2001, 123, 8939. (21) Allot, C.; Bernard, P. L.; Hunter, C. A.; Rotger, C.; Thomson, J. A.
(17) Quiocho, F. A.; Vyas, N. K. Nature 1984, 310, 381. Chem. Commun. 1998, 2449.
(18) (a) Notti, A.; Occhipinti, S.; Pappalardo, S.; Parisi, M. F.; Pisagatti, (22) (a) Potrzebowski, M. J.; Michalska, M.; Koziol, A. E.; Kazmierski,
I.; White, A. J. P.; Williams, D. J. J. Org. Chem. 2002, 67, 7569. (b) S.; Lis, T.; Pluskowski, J.; Ciesielki, W. J. Org. Chem. 1998, 63, 4209.
Darbost, U.; Rager, M. N.; Petit, S.; Jabin, I.; Reinaud, O. J. Am. (b) Nova, J. J.; Rovira, M. C.; Rovira, C.; Veciana, J.; Tarres, J. AdV.
Chem. Soc. 2005, 127, 8517. Mater. 1995, 7, 233.
(19) (a) Canceill, J.; Lacombe, L.; Collet, A. J. Am. Chem. Soc. 1986, 108, (23) (a) Ungaro, R.; Pochini, A.; Andreetti, G. D.; Domiano, P. J. Chem.
4230. (b) Canceill, J.; Cesario, M.; Collet, A.; Guilhem, J.; Lacombe, Soc., Perkin Trans. 2 1985, 197. (b) Andreetti, G. D.; Pochini, A.;
L.; Lozach, B.; Pascard, C. Angew. Chem., Int. Ed. Engl. 1989, 28, Ungaro, R. J. Chem. Soc., Perkin Trans. 2 1983, 1773. (c) Kojima,
1246. T.; Miyazaki, S.; Hayashi, K. i.; Shimazaki, Y.; Tani, F.; Naruta, Y.;
(20) (a) Piatnitski, E. L.; Flowers, R. A., II; Deshayes, K. Chem.sEur. J. Matsuda, Y. Chem.sEur. J. 2004, 10, 6402.
2000, 6, 999. (b) Arena, G.; Casnati, A.; Contino, A.; Lombardo, G. (24) (a) McNelis, B. J.; Nathan, L. C.; Clark, C. J. J. Chem. Soc., Dalton
G.; Sciotto, D.; Ungaro, R. Chem.sEur. J. 1999, 5, 738. (c) Oh, M.; Trans. 1999, 1831. (b) Boncella, J. M.; Cajigal, M. L.; Abboud, K.
Stern, C. L.; Mirkin, C. A. Inorg. Chem. 2005, 44, 2647. A. Organometallics 1996, 15, 1905.

Inorganic Chemistry, Vol. 45, No. 20, 2006 8071

 Manimaran et al.
bridged ReI-based rectangles have been reported by Hupp
et al.,26 Sullivan et al.,27 and Lu et al.25 Compounds 1 and
2 are M4L2L′4 types of neutral molecular rectangles that
are self-assembled from 10 components. The solubility of
the rectangles can be greatly improved by increasing the
length of the alkyl chain present in the thiolato or alkoxy
group. IR, NMR, and fast atom bombardment mass spec-
trometry (FAB-MS) spectra of the compounds and elemental
analyses were all consistent with the proposed rectangular
structures. Their architectures are further supported by single-
crystal X-ray diffraction analyses (vide infra, Figures 1 and
8). Compounds 1 and 2 are neutral and air- and moisture-
stable. Their electroneutrality, extensive solubility, and high
stability make them potentially useful materials in sensor Figure 1. Crystallographic drawing of [1a‚pyrene] showing CH‚‚‚π
devices. interactions in the solid state.
Interactions of 1 and 2 with Aromatic Hydrocarbons.
Electronic absorption spectral measurements were carried out
to investigate the ability of the rectangles to bind the highly
conjugated aromatic guest pyrene. It is noteworthy that when
Published on September 1, 2006 on http://pubs.acs.org | doi: 10.1021/ic0604720

pyrene is used as a probe and rectangles 1 and 2 are titrated

in CH2Cl2, the absorbance of pyrene (guest) is enhanced with
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an increase in the concentration of the rectangular host,

revealing a strong host-guest interaction between the
rectangles and pyrene (Figure 2). The electron density of
the ReI-coordinated 4,4′-bpy ligand is reduced because of
the metal coordination at the two pyridyl sites. Therefore,
electron-rich pyrene is likely to form a charge-transfer (CT)
complex with 4,4′-bpy of 1 and 2, producing an adduct that
is stabilized by donor-acceptor complexation. A new
shoulder band observed at ∼360 nm is consistent with a CT
absorption band.
The binding constants for the donor-acceptor complex Figure 2. Electronic absorption spectra of pyrene (2 × 10-5 M) increasing
with an increase in the concentration of host 2a in dichloromethane: (a) 0
formation between the rectangles and pyrene were evaluated × 10-6, (b) 2 × 10-6, (c) 4 × 10-6, (d) 6 × 10-6, (e) 8 × 10-6, (f) 10 ×
using the Benesi-Hildebrand relationship (eq 1).28 10-6, (g) 12 × 10-6, (h) 14 × 10-6, (i) 16 × 10-6, (j) 18 × 10-6, (k) 20
× 10-6, and (l) 22 × 10-6 M.
1/∆A ) 1/∆[G] + (1 + ∆K[H][G]) (1) Table 1. Ground-State Binding Constants (K), Excited-State Dynamic
(KD) and Static (KS) Stern-Volmer Constants, and Quenching Rate
Here ∆A is the change in the absorbance of the guest upon Constants (kq) of Hosts 1 and 2 with Pyrene at 298 K
the addition of the host, ∆ denotes the difference in the host K, M-1 KD, M-1 KS, M-1 kq, M-1 s-1
molar extinction coefficient between the bound and free guest
1a 1.9 × 104 7.4 × 104 3.6 × 104 2.3 × 1012
molecules, and K is the binding constant, while [H] and [G] 1b 2.3 × 104 1.2 × 105 5.1 × 104 3.2 × 1012
are the total concentrations of the host and guest molecules, 2a 9.7 × 103 3.5 × 104 1.7 × 104 1.1 × 1012
respectively. A double-reciprocal plot of the change in the 2b 1.1 × 104 4.8 × 104 1.9 × 104 1.5 × 1012
intensity of the absorption of the guest with a change in the
increase in the amount of 2a added to the pyrene, the
concentration of the host yields a linear correlation, indicating
emission intensity of the latter decreases (Figure 3). The
1:1 host-guest complex formation. The binding constants
quenching is believed to be the result of an intermolecular
(K) for this study are given in Table 1.
transfer of energy from the emitting π-π* state of the guest
Further, the fluorescence intensities of pyrene in CH2Cl2
to the low-lying CT excited state, which returns to the ground
are efficiently quenched by rectangle 2a in CH2Cl2. By an
state via radiationless decay. It has been reported by the Yip
(25) (a) Manimaran, B.; Rajendran, T.; Lu, Y. L.; Lee, G. H.; Peng, S. M.; group that CT interactions are responsible for the complex-
Lu, K. L. J. Chem. Soc., Dalton Trans. 2001, 515. (b) Manimaran, ation of Au rectangles29 and cyclobis(paraquat-p-phenylene)
B.; Thanasekaran, P.; Rajendran, T.; Lin, R. J.; Chang, I. J.; Lee, G.
H.; Peng, S. M.; Rajagopal, S.; Lu, K. L. Inorg. Chem. 2002, 41, ions,30 with electron-rich aromatic guests. Another study by
5323.
(26) Benkstein, K. D.; Hupp, J. T.; Stern, C. L. Inorg. Chem. 1998, 37, (29) Lin, R.; Yip, J. H. K.; Zhang, K.; Koh, L. L.; Wong, K. Y.; Ho, K. P.
5404. J. Am. Chem. Soc. 2004, 126, 15852.
(27) Woessner, S. M.; Helms, J. B.; Shen, Y.; Sullivan, B. P. Inorg. Chem. (30) (a) Nielsen, M. B.; Jeppesen, J. O.; Lau, J.; Lomholt, C.; Damgaard,
1998, 37, 5406. D.; Jacobsen, J. P.; Becher, J.; Stoddart, J. F. J. Org. Chem. 2001, 66,
(28) (a) Murakami, Y.; Kikuchi, J. I.; Suzuki, M.; Matsuura, T. J. Chem. 3559. (b) Ballardini, R.; Balzani, V.; Dehaen, W.; Dell’Erba, A. E.;
Soc., Perkin Trans. 1 1988, 1289. (b) Benesi, H. A.; Hildebrand, J. Raymo, F. M.; Stoddart, J. F.; Venturi, M. Eur. J. Org. Chem. 2000,
H. J. Am. Chem. Soc. 1949, 71, 2703. 591.

8072 Inorganic Chemistry, Vol. 45, No. 20, 2006

 CH‚‚‚π Interaction for Rhenium-Based Rectangles

Figure 3. Emission intensity of pyrene (2 × 10-5 M) decreasing with an Figure 4. Stern-Volmer plot for the emission quenching of pyrene with
increase of the concentration of host 2a in dichloromethane: (a) 0 × 10-6, an increase in the concentration of host 2a.
(b) 2 × 10-6, (c) 4 × 10-6, (d) 6 × 10-6, (e) 8 × 10-6, (f) 10 × 10-6, (g)
12 × 10-6, (h) 14 × 10-6, (i) 16 × 10-6, (j) 18 × 10-6, (k) 20 × 10-6, and Table 2. Complexation-Induced Shift Values for H2 and H3 of bpy in
(l) 22 × 10-6 M. Host 1a by Interaction with Aromatic Guests and Inorganic Saltsa
Published on September 1, 2006 on http://pubs.acs.org | doi: 10.1021/ic0604720

Hupp and co-workers reported that the N-heterocyclic- host + guest (δ, ppm) shift (∆δ, ppm)
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bridged ReI rectangles recognize guest molecules via elec- guest H3 H2 H3 H2

trostatic interactions or exterior side or intermolecular diphenyl 9.073 7.795 -0.020 -0.060
cavities.31 Takagi and co-workers reported that the main pyrene 9.009 7.618 -0.084 -0.237
contributions to CH‚‚‚π interactions were the electrostatic anthracene 9.072 7.801 -0.021 -0.054
triphenylene 9.042 7.741 -0.051 -0.114
and CT terms, based on an energy decomposition analysis.32 benzopyrene 9.045 7.723 -0.048 -0.132
Because both 2a and pyrene are neutral species, we conclude AgNO3 9.356 7.859 +0.263 +0.004
that CT was observed as a result of the CH‚‚‚π interaction. a For free host 1a, δ 9.093 and 7.855 ppm for H3 and H2 of bpy,
The quenching rate constants, kq, calculated from the respectively; [H] ) [G] ) 4 × 10-2 M.
Stern-Volmer equation are given in Table 1. The quadratic
relationship between I0/I and [Q] predicted an upward using both UV-vis and an emission method is supportive
curvature in the Stern-Volmer plot.33 This indicates that of a 1:1 complexation model. Further, the high value of the
binding takes place along with efficient quenching. To quenching rate constant, kq, indicates efficient bimolecular
explain the nonlinearity of the curve, the extended Stern- quenching between the ReI rectangles and pyrene, along with
Volmer equation (eq 2) was used. binding.34 Thus, we conclude that the unusual Stern-Volmer
plots obtained are caused by the formation of a ground-state
I0/I ) (1 + KD[host])(1 + KS[host]) (2) complex between the probe and the host.
The host-guest interaction of rectangle 1a with aromatic
where KD and KS are the dynamic and static Stern-Volmer hydrocarbons was investigated by monitoring the chemical
constants, respectively. A nonlinear plot of eq 2 suggests shifts of δH of 1a as a function of the different concentrations
the presence of a static component in the quenching of the guest in acetone-d6. The 1H NMR studies showed that
mechanism along with dynamic quenching (Figure 4). all of the guest protons and the H3 and H2 of the bpy of 1a
The values of KD and KS calculated from least-squares are shifted upfield. Similar upfield shifts have been observed
fitting are given in Table 1. Both the static (KS) and dynamic in the binding of aromatic guests to the Pd cage35 and a Au
(KD) quenching rate constants were found, and a good rectangle.29 The H2 signal of bpy is affected more than that
agreement between binding constants (K) (obtained from of H3 in all cases (Table 2), indicating that H2 is more
absorption measurement) and KS (obtained from emission shielded by the π ring of the guests. Furthermore, there was
measurement) was found. Complexation studies reveal that only one set of signals for the entire titration, and the
there is no significant difference in binding constants when chemical shift of these signals changed as a function of the
the alkyl chain lengths of 1 and 2 are varied. The close amount of guest added, suggesting that the exchange of the
resemblance of the experimental data to the theoretical fits guest with 1a is fast on the NMR time scale. Note that the
(31) (a) Benkstein, K. D.; Hupp, J. T.; Stern, C. L. J. Am. Chem. Soc. alkyl protons of the alkoxy/thiolate bridges do not experience
1998, 120, 12982. (b) Benkstein, K. D.; Stern, C. L.; Splan, K. E.; an appreciable change in the chemical shift upon the addition
Johnson, R. C.; Walters, K. A.; Vanhelmont, F. W. M.; Hupp, J. T. of the guests. These observations indicate that the added
Eur. J. Inorg. Chem. 2002, 2818. (c) Benkstein, K. D.; Hupp, J. T.;
Stern, C. L. Angew. Chem., Int. Ed. 2000, 39, 2891.
(32) Takagi, T.; Tanaka, A.; Matsuo, S.; Maezaki, H.; Tani, M.; Fujiwara, (34) (a) Sun, S. S.; Anspach, J. A.; Lees, A. J.; Zavalij, P. Y. Organome-
H.; Sasaki, Y. J. Chem. Soc., Perkin Trans. 2 1987, 1015. tallics 2002, 21, 685. (b) Flamigni, L.; Johnston, M. R. New J. Chem.
(33) (a) Wang, D.; Wang, J.; Moses, D.; Bazan, G. C.; Heeger, A. J. 2001, 25, 1368.
Langmuir 2001, 17, 1262. (b) Harrison, B. S.; Ramey, M. B.; (35) Yoshizawa, M.; Nakagawa, J.; Kumazawa, K.; Nagao, M.; Kawano,
Reynolds, J. R.; Schanze, K. S. J. Am. Chem. Soc. 2000, 122, 8561. M.; Ozeki, T.; Fujita, M. Angew. Chem., Int. Ed. 2005, 44, 1810.

Inorganic Chemistry, Vol. 45, No. 20, 2006 8073

 Manimaran et al.
Table 3. Crystal Data and Structure Refinement for distances are 3.730 Å), which significantly stabilize the
[{1a‚pyrene}‚4C3H6O] and [{1a‚(Ag+)2(NO3-)2(C3H6O)2}‚C3H6O] structure of 1a.
formula C76H86N4O16Re4S4 C60H76Ag2N6O22Re4S4 The crystallographic data unambiguously show that the
Mr 2184.53 2322.05
pyrene binds to 1a in a 1:1 ratio, consistent with observations
cryst syst monoclinic monoclinic
space group P21/c P21/c from the Benesi-Hildebrand approach. The bpy ligands in
a (Å) 10.4003(3) 11.7531(1) 1a interact with pyrene via CH‚‚‚π interactions. The face of
b (Å) 22.2940(6) 18.0464(2) the pyrene guest sits over the edges of the bpy linkers, nearly
c (Å) 17.3708(5) 18.7263(2)
â (deg) 99.082(2) 106.5641(5)
orthogonal with a dihedral angle of approximately 95°. This
V (Å3) 3977.2(2) 3807.04(7) is a fairly novel example of an interaction that is rarely
Z 2 2 designed into a host-guest pair. The central pyridyl H atoms
Dcalc (g cm-3) 1.824 2.026 (H10 and H13) interact with the π cloud of the pyrene with
µ(Mo KR) (mm-1) 6.238 7.020
T (K) 120(1) 150(1) H(pyridyl)‚‚‚C(pyrene) distances of 2.769-3.295 Å.
cryst dimens (mm) 0.10 × 0.10 × 0.15 0.05 × 0.12 × 0.20 As shown in Figure 5, the host-guest pairs, 1a‚pyrene,
θmin, θmax (deg) 1.50, 27.50 1.60, 27.50 are packed in a stairlike arrangement, in which the pyrene
F(000) 2124 2224
reflns collected 29715 29301 molecules are not located within the molecular cavity of 1a
indep reflns 8926 (Rint ) 0.054) 8747 (Rint ) 0.063) but are parallel and remain in the space between the two
observed data 7305 6903 different rectangle belts, leading to the formation of a
[I > 2σ(I)]
R1a [I > 2σ(I)] 0.0610 0.0396
supramolecular array. The spacing for accommodating the
wR2a [all data] guest pyrene between the two parallel bpy linkers is ca. 6.40
Published on September 1, 2006 on http://pubs.acs.org | doi: 10.1021/ic0604720

0.1518 0.1176
largest diff. peak, -2.43, 2.01 -2.07, 2.38 Å, where the distances between the bpy C atoms in adjacent
hole (e Å-3)
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rectangles are 7.23-7.27 Å. This type of arrangement for

GOF 1.198 1.123
CH‚‚‚π interactions is perfect from the standpoint of the best
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑w(Fo - Fc2)2/∑w(Fo2)2]1/2.
2
overlap of the CH‚‚‚π interactions (Figure 6).
Table 4. Selected Bond Lengths (Å) and Angles (deg) of Interaction of Thiolato-Bridged Rectangles with the Ag
[{1a‚pyrene}‚4C3H6O] Ion. The AgI ion is regarded as an extremely soft acid,
Re1-S1 2.500(3) Re1-S2 2.507(3) favoring coordination to soft bases, such as ligands contain-
Re1-N1 2.223(9) Re1-C1 1.944(14) ing S and unsaturated N.36 AgI complexes with these soft
Re1-C2 1.897(13) Re1-C3 1.924(13) ligands give rise to an interesting array of stereochemical
Re2-S1 2.499(3) C15-C16 1.384(17)
Re2-S2 2.497(3) C17-C18 1.488(19)
and geometric configurations, with coordination numbers of
Re2-C4 1.926(13) C18-C19 1.47(2) 2-6 all occurring. In addition, AgI complexes with S-
Re2-C5 1.859(14) C19-C20 1.54(2) containing ligands have a wide range of applications in
Re2-C6 1.912(12) Re2-N2a 2.217(9) medicine, analytical chemistry, and the polymer industry.37
S1-C17 1.831(14) S2-C21 1.830(11)
The biomedical applications and uses of AgI complexes are
S1-Re1-S2 80.6(10) N2a-Re2-C5 92.3(4) related to their antibacterial action,38 which appears to involve
S1-Re1-C1 173.2(5) Re1-S1-Re2 98.49(11) interactions with DNA.39 Thus, the molecular design and
S1-Re1-C2 93.9(4) Re1-S1-C17 111.3(4)
S1-Re1-C3 95.3(4) Re2-S1-C17 106.3(4)
structural characterization of AgI complexes are intriguing
S2-Re1-C1 93.6(5) Re1-S2-C21 111.6(4) aspects of bioinorganic chemistry and metal-based drugs.40
S2-Re1-C2 94.2(4) Re2-S2-C21 105.2(4) It has been established that, although thiolato groups
S2-Re1-C3 173.3(4) Re1-N1-C7 121.2(7) coordinated to metal centers have the ability to bind to a
S1-Re2-C6 174.6(4) Re2-C5-O5 175.1(10)
S2-Re2-C5 175.1(3) S2-Re2-C6 95.1(4) second metal ion to form a S-bridged structure,41,42 their
binding ability toward higher oxidation state metal centers
pyrene interacts strongly with the pyridyl protons of the bpy has recently been investigated.43 We therefore examined the
ligand of rectangle 1a. This prompted us to further investigate
the details of the interaction by a single-crystal X-ray (36) Suenga, Y.; Kuroda-Sowa, T.; Maekawa, M.; Munakata, M. J. Chem.
Soc., Dalton Trans. 2000, 3620 and references cited therein.
diffraction analysis. (37) (a) Krebs, B.; Hengel, G. Angew. Chem., Int. Ed. Engl. 1991, 30, 769.
Solid-State Evidence of a Host-Guest Pair. To obtain (b) Blower, P. G.; Dilworth, J. R. Coord. Chem. ReV. 1987, 76, 121.
(c) Raper, E. S. Coord. Chem. ReV. 1996, 153, 199.
insight into the host-guest binding mode, we attempted to (38) Wruble, M. J. Am. Pharm. Assoc. Sci. Ed. 1943, 32, 80.
obtain solid-state evidence for a 1a‚pyrene complex. Single (39) Rosenkranz, H. S.; Rosenkranz, S. Antimicrob. Agents Chemother.
crystals of [1a‚pyrene] suitable for X-ray crystallographic 1972, 2, 373.
(40) (a) Nomiya, K.; Kondoh, Y.; Nagano, H.; Oda, M. J. Chem. Soc.,
analysis were obtained by the slow evaporation of solvent Chem. Commun. 1995, 1679. (b) Nomiya, K.; Takahashi, S.; Noguchi,
from an acetone solution of 1a in the presence of pyrene at R. J. Chem. Soc., Dalton Trans. 2000, 2091.
25 °C. An ORTEP drawing of [1a‚pyrene] is shown in Figure (41) (a) Marr, A. C.; Spencer, D. J. E.; Schroder, M. Coord. Chem. ReV.
2001, 219-221, 1055. (b) Konno, T.; Chikamoto, Y.; Okamoto, K.;
1. The crystallographic refinement data and selected bond Yamaguchi, T.; Ito, T.; Hirotsu, M. Angew. Chem., Int. Ed. 2000, 39,
distances and angles are listed in Tables 3 and 4. The 4098.
(42) (a) Blake, A. J.; Collison, D.; Gould, R. O.; Reid, G.; Schroder, M. J.
distances between Re1‚‚‚Re2 and Re1‚‚‚Re2A are 3.787 and Chem. Soc., Dalton Trans. 1993, 521. (b) Clarkson, J.; Yagbasan, R.;
11.560 Å, respectively, confirming the rectangular architec- Blower, P. J.; Rawle, S. C.; Cooper, S. R. J. Chem. Soc., Chem.
Commun. 1987, 950.
ture of 1a. The two face-to-face bpy ligands in 1a exhibit (43) Konno, T.; Shimazaki, Y.; Yamaguchi, T.; Ito, T.; Hirotsu, M. Angew.
weak π-π-stacking interactions (both centroid‚‚‚centroid Chem., Int. Ed. 2002, 41, 4711.

8074 Inorganic Chemistry, Vol. 45, No. 20, 2006

 CH‚‚‚π Interaction for Rhenium-Based Rectangles

Figure 5. Crystal packing drawing of [{1a‚pyrene}‚4(acetone)] showing one-to-one host-guest interactions (left) and a stairlike arrangement (right) in the
solid state.
Published on September 1, 2006 on http://pubs.acs.org | doi: 10.1021/ic0604720
Downloaded by NATIONAL TAIWAN UNIV on August 19, 2009

Figure 6. View of the CH‚‚‚π interactions between the bpy ligands of

host 1a and the pyrene guest, showing the offset orientation between the
two bpy ligands, which are placed above and below the pyrene molecule,
respectively.

interaction of rectangles 1a and 1b with AgI because these

rectangles contain thiolato bridges.
The electronic absorption spectrum of 1a in 80:20 (v/v)
THF-H2O shows absorptions at 226, 243(sh), 322, and 384
nm. The high-energy absorption is assigned to a ligand- Figure 7. Absorption spectra of 1a (1 × 10-5 M) upon the addition of
the AgI ion in a 80:20 (v/v) THF-H2O mixture: (a) 0, (b) 2 × 10-4, (c)
centered transition, and the low-energy values are assigned 4 × 10-4, (d) 6 × 10-4, and (e) 8 × 10-4 M.
to metal-to-ligand CT transitions. Upon the addition of AgI
to 1a, a shift in the ligand-centered transition from 226 to indicate that the AgI‚‚‚S interactions are significant. This is
230 nm occurs with an increase in the absorbance. Mean- corroborated by the IR data. The marked change in νCO
while, during the addition of AgI, the shoulder at 243 nm absorption to a higher wavenumber showed that back-
disappears, but there is no appreciable change in the metal- bonding to the π* orbital of CO is less because of the lower
to-ligand CT transition, the spectrum of which is shown in electron density at the Re center. The NO3-‚‚‚H(bpy)
Figure 7. This implies that AgI ions affect only the π-π* interactions may occur but to a lesser extent than AgI‚‚‚S
transition of the bpy ligand in 1a. interactions because nitrate anions (NO3-) may prefer to
The lone pair electrons present on the S atom of the interact with Ag cations in solution (vide infra).44
thiolato bridge of host 1 are proposed to coordinate to the To further verify these observations, X-ray-quality crystals
AgI ion of AgNO3. The IR spectrum of 1a in the presence of compound [{1a‚(Ag+)2(NO3-)2(C3H6O)2}(C3H6O)] were
of the AgI ion in acetone shows a marked change in νCO obtained by dissolving the host 1a with the guest AgNO3 in
absorption from 2018, 2005, and 1917 cm-1 to 2027, 2015, acetone, followed by slow evaporation at room temperature.
and 1914 cm-1. Rectangles 1a and 1b that are capable of A single-crystal X-ray analysis reveals that the species
binding metal ions that undergo spectroscopic changes upon contains one rectangle [{(CO)3Re(µ-SC4H9)2Re(CO)3}2(µ-
the formation metal ion complexes could be used as bpy)2]2 unit, two AgI atoms, two bridging nitrate ions, and
analytical reagents in the analysis of metal ions. 1H NMR two coordinated acetone molecules. Complex [{1a‚(Ag+)2-
spectral studies of 1a in the presence of the AgI ion in (NO3-)2(C3H6O)2}(C3H6O)] crystallizes in a monoclinic cell,
acetone-d6 shows a downfield shift of the H2 and H3 protons and the structure was solved in the space group P21/c. The
of the pyridyl group (Table 2). This indicates the coordination crystallographic refinement data and selected bond distances
of the Ag cation to the S atom, thus causing a reduction in and angles are listed in Tables 3 and 5. As shown in Figure
the electron density on it and consequently on the metal. 8, the thiolato bridges of the ReI rectangles are coordinated
This, in turn, causes the Re atom to draw electrons from the
bipyridyl moiety and, as a result, the H2 and H3 protons of (44) Vega, I. E. D.; Gale, P. A.; Light, M. E.; Loeb, S. J. Chem. Commun.
the pyridyl group are shifted downfield. These observations 2005, 4913 and references cited therein.

Inorganic Chemistry, Vol. 45, No. 20, 2006 8075

 Manimaran et al.

Figure 8. (a) Crystallographic drawing indicating the inclusion of AgNO3 moieties by host 1a through silver-thiolate side-arm interactions and formation
of a linear supramolecular array. The coordinated acetone molecules are omitted for the sake of clarity. The AgNO3 moieties are enlarged for clarity. (b)
Details of the thiolate-silver-nitrate interactions. Key: orange, Re; yellow, S; pink, Ag; blue, N; red, O; gray, C.

Table 5. Selected Bond Lengths (Å) and Angles (deg) for Experimental Section
[{1a‚(Ag+)2(NO3-)2(C3H6O)2}(C3H6O)]
Materials and General Methods. Reagents were used as
Re1-S1 2.5356(17) Re1-S2 2.4964(16) received without further purification. The solvents used in this
Re1-N1 2.213(5) Re2-S1 2.5253(16)
Re2-S2 2.5135(17) Re2-N2 2.230(5) study were of spectroscopic grade. Electronic absorption spectra
Re2-C4 1.926(7) Re2-C5 1.924(7) were recorded on a Hewlett-Packard 8453 spectrophotometer.
Published on September 1, 2006 on http://pubs.acs.org | doi: 10.1021/ic0604720

Re2-C6 1.919(7) Ag1-S1 2.4405(17) Fluorescence spectra were recorded on a Hitachi F4500 spec-
Ag1-O7 2.308(5) Ag1-O10 2.432(5) trometer using a slit width of 2.5 nm for both excitation and
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S1-C17 1.847(8) S2-C21 1.837(7)

emission measurements. The photomultiplier voltage was 700 V.
1H and 13C NMR spectra were recorded on Bruker AC 300 and
S1-Re1-S2 79.99(5) S1-Re1-N1 88.82(14)
S1-Re1-C1 171.6(2) S1-Ag1-O7 157.94(15) AMX-400 FT-NMR spectrometers. Elemental analyses were per-
S1-Re1-C2 90.8(2) S1-Ag1-O10 119.49(13) formed using a Perkin-Elmer 2400 CHN elemental analyzer. FAB-
S1-Re1-C3 96.4(2) S2-Re1-N1 85.37(14) MS data were obtained using a JMS-700 double-focusing mass
S2-Re1-C1 91.7(2) S2-Re1-C2 94.8(2)
S2-Re1-C3 174.7(2) Re1-S1-Re2 99.09(6) spectrometer.
Re1-S1-Ag1 126.79(7) Re2-S1-Ag1 115.32(6) Synthesis of [{(CO)3Re(µ-SC4H9)2Re(CO)3}2(µ-bpy)2] (1a). A
Ag1-S1-C17 99.5(2) Re1-S2-Re2 100.46(6) suspension containing a mixture of Re2(CO)10 (131 mg, 0.20 mmol)
S1-Re2-S2 79.87(5) S1-Re2-N2 85.24(14) and bpy (65 mg, 0.40 mmol) in 10 mL of a 3:7 mixture of
S2-Re2-N2 88.00(14)
1-butanethiol and toluene in a 30-mL Teflon flask was placed in
an oven at 140 °C for 48 h and then cooled to 25 °C. The resulting
to Ag atom [Ag1-S1, 2.4405(17) Å], which is further linked
orange crystals were separated by filtration, and the solvent from
by two nitrate ions with the Ag-O distances of 2.308(5)- the filtrate was removed by applying a vacuum. The residue was
2.542(5) Å and coordinated by one acetone molecule [Ag1- redissolved in a minimum quantity of CH2Cl2 and passed through
O10, 2.432(5) Å; Supporting Information, Figure S6]. The a short silica gel column to give the pure product. Yield: 71%. IR
intrinsic affinity of the S atom toward the AgI ion together (CH3COCH3): νCO 2018 (s), 2005 (s), 1917 (vs), 1899 (vs) cm-1.
with the Ag-ONO2 interactions leads to the formation of a 1H NMR (300 MHz, acetone-d ): δ 9.09 (d, 3J ) 5.3 Hz, 8H, H3),
6
one-dimensional supramolecular array through S‚‚‚Ag‚‚‚O 7.86 (d, 8H, H2), 3.32 (t, 3J ) 7.4 Hz, 8H, CH2), 1.71 (m, 8H,
connections. Additionally, two adjacent Ag salts are bridged CH2), 1.57 (m, 8H, CH2), 1.03 (t, 3J ) 7.2 Hz, 12H, CH3). 13C
together through nitrate ions with Ag‚‚‚Ag distances of about NMR (75 MHz, acetone-d6): δ 201.0, 196.1 (1:2 CO), 157.0 (C3),
4.05 Å. This reveals the existence of very weak argentophilic 145.5 (C1), 123.6 (C2), 39.5 (CH2), 35.6 (CH2), 22.3 (CH2), 14.0
interactions in this system. Thus, the interaction of the (CH3). UV-vis (CH3CN): λmax [nm] 365 (MLCT), 244, 316 (LIG).
FAB-MS: m/z 1750.2 (M+). Anal. Calcd for C48H52N4O12S4Re4:
rectangles toward the Ag ion results in the self-organization
C, 32.94; H, 2.99; N, 3.20. Found: C, 32.94; H, 2.82; N, 2.92.
of interesting supramolecular arrays.
Synthesis of [{(CO)3Re(µ-SC8H17)2Re(CO)3}2(µ-bpy)2] (1b).
Conclusion A suspension consisting of a mixture of Re2(CO)10 (98 mg, 0.15
mmol) and bpy (32 mg, 0.10 mmol) in 10 mL of a 3:7 mixture of
In summary, thiolato- and alkoxy-bridged molecular 1-octanethiol and toluene in a 30-mL Teflon flask was placed in a
rectangles exhibit molecular recognition characteristics to- steel bomb. The bomb was placed in an oven maintained at 140
ward planar aromatic guest molecules and the Ag ion. As °C for 48 h and then cooled to 25 °C. Good-quality orange single
evidenced by UV-vis, fluorescence, and 1H NMR spectro- crystals of 1b were obtained. The solvent from the reaction mixture
scopic data and single-crystal X-ray diffraction analyses, was removed by vacuum distillation, and the residue was redis-
rectangles 1 and 2 strongly interact with the pyrene molecule solved in CH2Cl2 and passed through a short silica gel column to
get pure 1b. Yield: 53%. IR (CH2Cl2): νCO 2019 (s), 2006 (s),
in a 1:1 host-guest ratio. The results of a single-crystal X-ray
1919 (vs), 1898 (vs) cm-1. 1H NMR (300 MHz, acetone-d6): δ
diffraction analysis show that the recognition of 1 toward
9.09 (d, 3J ) 6.7 Hz, 8H, H3), 7.84 (d, 3J ) 6.7 Hz, 8H, H2),
the pyrene molecule is mainly due to CH‚‚‚π interactions 3.31 (t, 3J ) 7.3 Hz, 8H, CH2), 1.76 (m, 8H, CH2), 1.56 (m, 8H,
and that the face of the guest pyrene sits over the edges of CH2), 1.41 (m, 16H, CH2), 1.35 (m, 16H, CH2), 0.91 (t, 3J )
the bpy linkers of 1. In addition, the thiolato-bridged 6.6 Hz, 12H, CH3). 13C NMR (75 MHz, acetone-d6): δ 200.9,
rectangle 1a recognizes AgI, generating an interesting 196.1 (1:2 CO), 156.9 (C3), 145.5 (C1) 123.5 (C2), 39.8 (CH2),
supramolecular array via Re-S‚‚‚Ag‚‚‚O interactions. 33.6 (CH2), 32.6 (CH2), 29.9 (2CH2), 23.3 (2CH2), 14.4 (CH3).

8076 Inorganic Chemistry, Vol. 45, No. 20, 2006

 CH‚‚‚π Interaction for Rhenium-Based Rectangles

UV-vis (CH3CN): λmax [nm] 368 (MLCT), 244, 316 (LIG). X-ray Crystallographic Studies. Suitable single crystals with
FAB-MS: m/z 1974.2 (M+). Anal. Calcd for C64H84N4O12S4Re4: dimensions of 0.10 × 0.10 × 0.15 and 0.05 × 0.12 × 0.20 mm
C, 38.86; H, 4.28; N, 2.83. Found: C, 39.03; H, 3.74; N, 2.57. for [{1a‚pyrene}‚4C3H6O] and [{1a‚(Ag+)2(NO3-)2(C3H6O)2}-
Rectangles 2a and 2b were obtained following a similar (C3H6O)], respectively, were selected for indexing and intensity
procedure using 10 mL of aliphatic alcohol instead of a mixture of data collection. A total of 1420 frames constituting a hemisphere
thiol and toluene.25 Yield: 2a, 86%; 2b, 87%. of X-ray intensity data were collected with a frame width of 0.3°
Fluorescence Quenching Studies. Quenching experiments of in ω and a counting time of 10 s/frame, using a Bruker SMART
the fluorescence of pyrene were carried out under aerated conditions. CCD diffractometer. The first 50 frames were re-collected at the
The solvent used in this study was of spectroscopic grade. The end of data collection to monitor crystal decay. No significant decay
excitation wavelength was 336 nm in CH2Cl2 as the solvent. The was observed. The raw data frames were integrated into SHELX-
monitoring wavelength corresponded to the maximum of the format reflection files and corrected for Lorentz and polarization
emission band at 393 nm. Relative fluorescence intensities were effects using the SAINT program, and absorption correction was
measured for solutions of pyrene in CH2Cl2 and rectangles used as performed using the SADABS program.45 The space groups were
quenchers. There was no change in shape, but a change in the determined to be P21/c. Direct methods were used to solve the
intensity of the fluorescence peak was found, when these rectangles structure using the SHELX-TL46 program packages. All non-H atoms
were added. The Stern-Volmer (SV) relationship, I0/I ) 1 + KSV- were refined anisotropically by full-matrix least squares based on
[Q], was obtained for the ratio of the emission intensities (I0 and I F 2 values. The largest residual density peak is close to that of
are the emission intensities in the absence and presence of quencher) the Re atom. Basic information pertaining to crystal param-
and quencher concentration, [Q]. The quenching rate constants were eters and structure refinement for [{1a‚pyrene}‚4C3H6O] and
obtained from the Stern-Volmer constant, KSV, and the fluorescence [{1a‚(Ag+)2(NO3-)2(C3H6O)2}(C3H6O)] is summarized in Table 3,
lifetime, τ, of pyrene (32 ns). Excited-state lifetime studies were and selected bond distances and angles are provided in Tables 4
Published on September 1, 2006 on http://pubs.acs.org | doi: 10.1021/ic0604720

performed using an Edinburgh FL 920 single photon-counting and 5, respectively.

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system with a H2-filled or N2 lamp as the excitation source. The

emission decays were analyzed by the sum of exponential functions, Acknowledgment. We thank Academia Sinica and the
which allows the partial elimination of instrument time broadening, National Science Council, Taiwan, for financial support.
thus rendering a temporal resolution.
Binding Constant Measurements. The binding abilities of the Supporting Information Available: Crystallographic details
rectangles with pyrene were examined by both absorption and in CIF format, UV-vis absorption spectra of 2b and pyrene, Stern-
emission spectroscopic methods. The concentration of pyrene was Volmer plot for 1a with pyrene, and coordination environments
2 × 10-5 M, and those of the rectangles were 2 × 10-6-3 × 10-5 around the Ag ions. This material is available free of charge via
M. The binding constants from the electronic absorption experiment the Internet at http://pubs.acs.org.
were measured by changing the concentration of the rectangles with
pyrene and calculated on the basis of the Benesi-Hildebrand IC0604720
relationship for a 1:1 molar ratio. A good linear correlation of 1/∆A
vs 1/[H] was obtained for all of the measurements. The binding (45) SMART/SAINT/ASTRO, release 4.03; Siemens Energy & Automation,
Inc.: Madison, WI, 1995.
constants in the excited state were determined using the modified (46) Sheldrick, G. M. SHELX-TL; University of Gottingen: Gottingen,
Stern-Volmer equation. Germany, 1998.

Inorganic Chemistry, Vol. 45, No. 20, 2006 8077