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; 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. 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‚‚‚πinteractions 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
Noncovalent interactions between host and guest molecules are ubiquitous in biology and include the vital functions of immune responses, transcription, replication, and biochemical signaling.1-3 They also lie at the heart of several practical
chemical technologies, including chemical sensing, separa-tions, and catalysis.4-6Extensive efforts have been made to
design components that mimic natural systems by undergoing molecular self-organization through selective noncovalent interactions such as H-bonding, electrostatic, and
π-π-stacking interactions.7-9 In addition, these noncovalent
interactions have been actively used to strengthen structural and electronic communication between organic ligands in multifunctional metal complexes.10Among these, a CH‚‚‚π
interaction occurring between CH’s (soft acids) andπ groups
(soft bases) has been noted as an important weak
H-bond-* To whom correspondence should be addressed. E-mail: lu@chem.sinica.edu.tw. Fax: int. code +886-2-27831237.
†Academia Sinica.
‡National Taipei University of Technology. §National Taiwan University.
#Current address: Pondicherry University, India.
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like force by many chemists and biochemists.11Despite being
the weakest of the H bonds, it plays a significant role in tuning the physical, chemical, and biological properties of substances.12-17A number of calixarenes and cryptophans
have been employed as potent synthetic macrocycles to include various guests, whereby CH‚‚‚π interactions are
believed to be a crucial driving force in determining the stability of host-guest complexes and in assembling mo-lecular units into an organized supramomo-lecular structure.18-20
Hunter’s group used an amide macrocycle with a highly preorganized cavity containing both polar and nonpolar recognition sites to form stable complexes with cyclic peptides in water via CH‚‚‚π interactions.21 During the
investigation of organic conductive materials such as
tetra-thiafulvalene and related derivates, CH‚‚‚π interactions were
also reported to contribute significantly to the formation of two- and three-dimensional networks in addition to CH‚‚‚S,
π-π stacking, and close chalcogen contacts.22 CH‚‚‚π
interactions have also been inferred from structural studies of p-tert-butylcalix[4]arene‚guest compounds, although some of these are significantly disordered.23a,bKojima et al.23chave
recently shown that the reaction of a RuII complex with β-diketone gave β-diketonato complexes in which
hydro-phobicπ-π or CH‚‚‚π interactions were confirmed by NMR
spectroscopy and X-ray crystallography. However, CH‚‚‚π
interactions in metallocyclophanes have been examined to a much lesser extent.24Herein we report on the characteristics
associated with the recognition of thiolato- and alkoxy-bridged ReI molecular rectangles with respect to several
planar aromatic molecules and the Ag ion. Their recognition toward a highly conjugated aromatic guest, pyrene, via a perfect CH‚‚‚π interaction was observed and confirmed by
solid-state data. This is a fairly novel example of an interaction that is rarely designed into a host-guest pair. Furthermore, the interaction of the thiolato-bridged rectangle toward Ag ions results in the self-organization of an interesting supramolecular array.
Results and Discussion
Self-assembly and Characterization of Thiolato- and Alkoxy-Bridged Rectangles. The self-assembly of new thiolato-bridged ReI molecular rectangles [{(CO)
3Re(
µ-SR)2Re(CO)3}2(µ-bpy)2] (1a, R ) C4H9; 1b, R ) C8H17) is
achieved from Re2(CO)10, 4,4′-bipyridine (bpy), and
mer-captan (butanethiol or octanethiol) under solvothermal condi-tions (Scheme 1). The known alkoxy-bridged molecular rectangles [{(CO)3Re(µ-OR)2Re(CO)3}2(µ-bpy)2] (2a, R )
C8H17; 2b, R ) C12H25) were prepared via literature
procedures.25Preliminary studies of the thiolato- and alkoxy-(10) (a) Coronado, E.; Galan-Mascaros, J. R.; Gomez-Garcıa, C. J.; Laukhln,
V. Nature 2000, 408, 447. (b) Uji, S.; Shinagawa, H.; Terashima, T.; Yakabe, T.; Terai, Y.; Tokumoto, M.; Kobayashi, A.; Tanaka, H.; Kobayashi, H. Nature 2001, 410, 908. (c) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334 and references cited therein.
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Scheme 1. Molecular Rectangles 1 and 2
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bridged ReI-based rectangles have been reported by Hupp
et al.,26Sullivan et al.,27and Lu et al.25Compounds 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 devices.
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 pyrene is used as a probe and rectangles 1 and 2 are titrated in CH2Cl2, the absorbance of pyrene (guest) is enhanced with
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 formation between the rectangles and pyrene were evaluated using the Benesi-Hildebrand relationship (eq 1).28
Here∆A is the change in the absorbance of the guest upon
the addition of the host, ∆ denotes the difference in the
molar extinction coefficient between the bound and free guest molecules, and K is the binding constant, while [H] and [G] are the total concentrations of the host and guest molecules, respectively. A double-reciprocal plot of the change in the intensity of the absorption of the guest with a change in the concentration of the host yields a linear correlation, indicating 1:1 host-guest complex formation. The binding constants (K) for this study are given in Table 1.
Further, the fluorescence intensities of pyrene in CH2Cl2
are efficiently quenched by rectangle 2a in CH2Cl2. By an
increase in the amount of 2a added to the pyrene, the emission intensity of the latter decreases (Figure 3). The quenching is believed to be the result of an intermolecular transfer of energy from the emittingπ-π* state of the guest
to the low-lying CT excited state, which returns to the ground state via radiationless decay. It has been reported by the Yip group that CT interactions are responsible for the complex-ation of Au rectangles29and cyclobis(paraquat-p-phenylene)
ions,30with electron-rich aromatic guests. Another study by (25) (a) Manimaran, B.; Rajendran, T.; Lu, Y. L.; Lee, G. H.; Peng, S. M.;
Lu, K. L. J. Chem. Soc., Dalton Trans. 2001, 515. (b) Manimaran, 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, 5323.
(26) Benkstein, K. D.; Hupp, J. T.; Stern, C. L. Inorg. Chem. 1998, 37, 5404.
(27) Woessner, S. M.; Helms, J. B.; Shen, Y.; Sullivan, B. P. Inorg. Chem. 1998, 37, 5406.
(28) (a) Murakami, Y.; Kikuchi, J. I.; Suzuki, M.; Matsuura, T. J. Chem. Soc., Perkin Trans. 1 1988, 1289. (b) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.
(29) Lin, R.; Yip, J. H. K.; Zhang, K.; Koh, L. L.; Wong, K. Y.; Ho, K. P. J. Am. Chem. Soc. 2004, 126, 15852.
(30) (a) Nielsen, M. B.; Jeppesen, J. O.; Lau, J.; Lomholt, C.; Damgaard, D.; Jacobsen, J. P.; Becher, J.; Stoddart, J. F. J. Org. Chem. 2001, 66, 3559. (b) Ballardini, R.; Balzani, V.; Dehaen, W.; Dell’Erba, A. E.; Raymo, F. M.; Stoddart, J. F.; Venturi, M. Eur. J. Org. Chem. 2000, 591.
Figure 1. Crystallographic drawing of [1a‚pyrene] showing CH‚‚‚π interactions in the solid state.
Figure 2. Electronic absorption spectra of pyrene (2× 10-5M) increasing with an increase in the concentration of host 2a in dichloromethane: (a) 0
× 10-6, (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 (l) 22× 10-6M.
Table 1. Ground-State Binding Constants (K), Excited-State Dynamic (KD) and Static (KS) Stern-Volmer Constants, and Quenching Rate
Constants (kq) of Hosts 1 and 2 with Pyrene at 298 K
host K, M-1 KD, M-1 KS, M-1 kq, M-1s-1 1a 1.9× 104 7.4× 104 3.6× 104 2.3× 1012 1b 2.3× 104 1.2× 105 5.1× 104 3.2× 1012 2a 9.7× 103 3.5× 104 1.7× 104 1.1× 1012 2b 1.1× 104 4.8× 104 1.9× 104 1.5× 1012 1/∆A ) 1/∆[G] + (1 + ∆K[H][G]) (1)
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Hupp and co-workers reported that the N-heterocyclic-bridged ReIrectangles recognize guest molecules via
elec-trostatic interactions or exterior side or intermolecular cavities.31 Takagi and co-workers reported that the main
contributions to CH‚‚‚π interactions were the electrostatic
and CT terms, based on an energy decomposition analysis.32
Because both 2a and pyrene are neutral species, we conclude that CT was observed as a result of the CH‚‚‚π interaction.
The quenching rate constants, kq, calculated from the
Stern-Volmer equation are given in Table 1. The quadratic relationship between I0/I and [Q] predicted an upward
curvature in the Stern-Volmer plot.33 This indicates that
binding takes place along with efficient quenching. To explain the nonlinearity of the curve, the extended Stern-Volmer equation (eq 2) was used.
where KDand KSare the dynamic and static Stern-Volmer
constants, respectively. A nonlinear plot of eq 2 suggests the presence of a static component in the quenching mechanism along with dynamic quenching (Figure 4).
The values of KD and KS calculated from least-squares
fitting are given in Table 1. Both the static (KS) and dynamic
(KD) quenching rate constants were found, and a good
agreement between binding constants (K) (obtained from absorption measurement) and KS (obtained from emission
measurement) was found. Complexation studies reveal that there is no significant difference in binding constants when the alkyl chain lengths of 1 and 2 are varied. The close resemblance of the experimental data to the theoretical fits
using both UV-vis and an emission method is supportive of a 1:1 complexation model. Further, the high value of the quenching rate constant, kq, indicates efficient bimolecular
quenching between the ReIrectangles and pyrene, along with
binding.34Thus, we conclude that the unusual Stern-Volmer
plots obtained are caused by the formation of a ground-state complex between the probe and the host.
The host-guest interaction of rectangle 1a with aromatic hydrocarbons was investigated by monitoring the chemical shifts ofδHof 1a as a function of the different concentrations of the guest in acetone-d6. The1H NMR studies showed that
all of the guest protons and the H3and H2of the bpy of 1a
are shifted upfield. Similar upfield shifts have been observed in the binding of aromatic guests to the Pd cage35and a Au
rectangle.29The H2signal of bpy is affected more than that
of H3 in all cases (Table 2), indicating that H2 is more
shielded by theπ ring of the guests. Furthermore, there was
only one set of signals for the entire titration, and the chemical shift of these signals changed as a function of the amount of guest added, suggesting that the exchange of the guest with 1a is fast on the NMR time scale. Note that the alkyl protons of the alkoxy/thiolate bridges do not experience an appreciable change in the chemical shift upon the addition of the guests. These observations indicate that the added
(31) (a) Benkstein, K. D.; Hupp, J. T.; Stern, C. L. J. Am. Chem. Soc. 1998, 120, 12982. (b) Benkstein, K. D.; Stern, C. L.; Splan, K. E.; Johnson, R. C.; Walters, K. A.; Vanhelmont, F. W. M.; Hupp, J. T. 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, H.; Sasaki, Y. J. Chem. Soc., Perkin Trans. 2 1987, 1015. (33) (a) Wang, D.; Wang, J.; Moses, D.; Bazan, G. C.; Heeger, A. J.
Langmuir 2001, 17, 1262. (b) Harrison, B. S.; Ramey, M. B.; Reynolds, J. R.; Schanze, K. S. J. Am. Chem. Soc. 2000, 122, 8561.
(34) (a) Sun, S. S.; Anspach, J. A.; Lees, A. J.; Zavalij, P. Y. Organome-tallics 2002, 21, 685. (b) Flamigni, L.; Johnston, M. R. New J. Chem. 2001, 25, 1368.
(35) Yoshizawa, M.; Nakagawa, J.; Kumazawa, K.; Nagao, M.; Kawano, M.; Ozeki, T.; Fujita, M. Angew. Chem., Int. Ed. 2005, 44, 1810. Figure 3. Emission intensity of pyrene (2× 10-5M) decreasing with an
increase of the concentration of host 2a in dichloromethane: (a) 0× 10-6, (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 (l) 22× 10-6M.
I0/I ) (1 + KD[host])(1 + KS[host]) (2)
Figure 4. Stern-Volmer plot for the emission quenching of pyrene with an increase in the concentration of host 2a.
Table 2. Complexation-Induced Shift Values for H2and H3of bpy in
Host 1a by Interaction with Aromatic Guests and Inorganic Saltsa host + guest (δ, ppm) shift (∆δ, ppm)
guest H3 H2 H3 H2 diphenyl 9.073 7.795 -0.020 -0.060 pyrene 9.009 7.618 -0.084 -0.237 anthracene 9.072 7.801 -0.021 -0.054 triphenylene 9.042 7.741 -0.051 -0.114 benzopyrene 9.045 7.723 -0.048 -0.132 AgNO3 9.356 7.859 +0.263 +0.004
aFor free host 1a, δ 9.093 and 7.855 ppm for H3 and H2of bpy,
respectively; [H] ) [G] ) 4× 10-2M.
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pyrene interacts strongly with the pyridyl protons of the bpy ligand of rectangle 1a. This prompted us to further investigate the details of the interaction by a single-crystal X-ray diffraction analysis.
Solid-State Evidence of a Host-Guest Pair. To obtain insight into the host-guest binding mode, we attempted to obtain solid-state evidence for a 1a‚pyrene complex. Single crystals of [1a‚pyrene] suitable for X-ray crystallographic analysis were obtained by the slow evaporation of solvent from an acetone solution of 1a in the presence of pyrene at 25°C. An ORTEP drawing of [1a‚pyrene] is shown in Figure 1. The crystallographic refinement data and selected bond distances and angles are listed in Tables 3 and 4. The distances between Re1‚‚‚Re2 and Re1‚‚‚Re2A are 3.787 and 11.560 Å, respectively, confirming the rectangular architec-ture of 1a. The two face-to-face bpy ligands in 1a exhibit weak π-π-stacking interactions (both centroid‚‚‚centroid
distances are 3.730 Å), which significantly stabilize the structure of 1a.
The crystallographic data unambiguously show that the pyrene binds to 1a in a 1:1 ratio, consistent with observations from the Benesi-Hildebrand approach. The bpy ligands in 1a interact with pyrene via CH‚‚‚π interactions. The face of
the pyrene guest sits over the edges of the bpy linkers, nearly orthogonal with a dihedral angle of approximately 95°. This is a fairly novel example of an interaction that is rarely designed into a host-guest pair. The central pyridyl H atoms (H10 and H13) interact with theπ cloud of the pyrene with
H(pyridyl)‚‚‚C(pyrene) distances of 2.769-3.295 Å. As shown in Figure 5, the host-guest pairs, 1a‚pyrene, are packed in a stairlike arrangement, in which the pyrene molecules are not located within the molecular cavity of 1a but are parallel and remain in the space between the two different rectangle belts, leading to the formation of a supramolecular array. The spacing for accommodating the guest pyrene between the two parallel bpy linkers is ca. 6.40 Å, where the distances between the bpy C atoms in adjacent rectangles are 7.23-7.27 Å. This type of arrangement for CH‚‚‚π interactions is perfect from the standpoint of the best
overlap of the CH‚‚‚π interactions (Figure 6).
Interaction of Thiolato-Bridged Rectangles with the Ag Ion. The AgI ion is regarded as an extremely soft acid,
favoring coordination to soft bases, such as ligands contain-ing S and unsaturated N.36 AgI complexes with these soft
ligands give rise to an interesting array of stereochemical and geometric configurations, with coordination numbers of 2-6 all occurring. In addition, AgI complexes with
S-containing ligands have a wide range of applications in medicine, analytical chemistry, and the polymer industry.37
The biomedical applications and uses of AgIcomplexes are
related to their antibacterial action,38which appears to involve
interactions with DNA.39 Thus, the molecular design and
structural characterization of AgIcomplexes are intriguing
aspects of bioinorganic chemistry and metal-based drugs.40
It has been established that, although thiolato groups coordinated to metal centers have the ability to bind to a second metal ion to form a S-bridged structure,41,42 their
binding ability toward higher oxidation state metal centers has recently been investigated.43We therefore examined the (36) Suenga, Y.; Kuroda-Sowa, T.; Maekawa, M.; Munakata, M. J. Chem.
Soc., Dalton Trans. 2000, 3620 and references cited therein. (37) (a) Krebs, B.; Hengel, G. Angew. Chem., Int. Ed. Engl. 1991, 30, 769.
(b) Blower, P. G.; Dilworth, J. R. Coord. Chem. ReV. 1987, 76, 121. (c) Raper, E. S. Coord. Chem. ReV. 1996, 153, 199.
(38) Wruble, M. J. Am. Pharm. Assoc. Sci. Ed. 1943, 32, 80.
(39) Rosenkranz, H. S.; Rosenkranz, S. Antimicrob. Agents Chemother. 1972, 2, 373.
(40) (a) Nomiya, K.; Kondoh, Y.; Nagano, H.; Oda, M. J. Chem. Soc., Chem. Commun. 1995, 1679. (b) Nomiya, K.; Takahashi, S.; Noguchi, R. J. Chem. Soc., Dalton Trans. 2000, 2091.
(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.; Yamaguchi, T.; Ito, T.; Hirotsu, M. Angew. Chem., Int. Ed. 2000, 39, 4098.
(42) (a) Blake, A. J.; Collison, D.; Gould, R. O.; Reid, G.; Schroder, M. J. Chem. Soc., Dalton Trans. 1993, 521. (b) Clarkson, J.; Yagbasan, R.; Blower, P. J.; Rawle, S. C.; Cooper, S. R. J. Chem. Soc., Chem. Commun. 1987, 950.
(43) Konno, T.; Shimazaki, Y.; Yamaguchi, T.; Ito, T.; Hirotsu, M. Angew. Chem., Int. Ed. 2002, 41, 4711.
Table 3. Crystal Data and Structure Refinement for
[{1a‚pyrene}‚4C3H6O] and [{1a‚(Ag+)2(NO3-)2(C3H6O)2}‚C3H6O] formula C76H86N4O16Re4S4 C60H76Ag2N6O22Re4S4
Mr 2184.53 2322.05
cryst syst monoclinic monoclinic
space group P21/c P21/c a (Å) 10.4003(3) 11.7531(1) b (Å) 22.2940(6) 18.0464(2) c (Å) 17.3708(5) 18.7263(2) β (deg) 99.082(2) 106.5641(5) V (Å3) 3977.2(2) 3807.04(7) Z 2 2 Dcalc(g cm-3) 1.824 2.026 µ(Mo KR) (mm-1) 6.238 7.020 T (K) 120(1) 150(1) cryst dimens (mm) 0.10× 0.10 × 0.15 0.05× 0.12 × 0.20 θmin,θmax(deg) 1.50, 27.50 1.60, 27.50
F(000) 2124 2224
reflns collected 29715 29301
indep reflns 8926 (Rint) 0.054) 8747 (Rint) 0.063) observed data
[I > 2σ(I)]
7305 6903
R1a[I > 2σ(I)] 0.0610 0.0396
wR2a[all data] 0.1518 0.1176
largest diff. peak, hole (e Å-3)
-2.43, 2.01 -2.07, 2.38
GOF 1.198 1.123
aR1 )∑||F
o| - |Fc||/∑|Fo|; wR2 ) [∑w(Fo2- Fc2)2/∑w(Fo2)2]1/2.
Table 4. Selected Bond Lengths (Å) and Angles (deg) of [{1a‚pyrene}‚4C3H6O] Re1-S1 2.500(3) Re1-S2 2.507(3) Re1-N1 2.223(9) Re1-C1 1.944(14) Re1-C2 1.897(13) Re1-C3 1.924(13) Re2-S1 2.499(3) C15-C16 1.384(17) Re2-S2 2.497(3) C17-C18 1.488(19) Re2-C4 1.926(13) C18-C19 1.47(2) Re2-C5 1.859(14) C19-C20 1.54(2) Re2-C6 1.912(12) Re2-N2a 2.217(9) S1-C17 1.831(14) S2-C21 1.830(11) S1-Re1-S2 80.6(10) N2a-Re2-C5 92.3(4) S1-Re1-C1 173.2(5) Re1-S1-Re2 98.49(11) S1-Re1-C2 93.9(4) Re1-S1-C17 111.3(4) S1-Re1-C3 95.3(4) Re2-S1-C17 106.3(4) S2-Re1-C1 93.6(5) Re1-S2-C21 111.6(4) S2-Re1-C2 94.2(4) Re2-S2-C21 105.2(4) S2-Re1-C3 173.3(4) Re1-N1-C7 121.2(7) S1-Re2-C6 174.6(4) Re2-C5-O5 175.1(10) S2-Re2-C5 175.1(3) S2-Re2-C6 95.1(4)
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interaction of rectangles 1a and 1b with AgIbecause 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-centered transition, and the low-energy values are assigned to metal-to-ligand CT transitions. Upon the addition of AgI
to 1a, a shift in the ligand-centered transition from 226 to 230 nm occurs with an increase in the absorbance. Mean-while, during the addition of AgI, the shoulder at 243 nm
disappears, but there is no appreciable change in the metal-to-ligand CT transition, the spectrum of which is shown in Figure 7. This implies that AgI ions affect only theπ-π*
transition of the bpy ligand in 1a.
The lone pair electrons present on the S atom of the thiolato bridge of host 1 are proposed to coordinate to the AgIion of AgNO
3. The IR spectrum of 1a in the presence
of the AgI ion in acetone shows a marked change in νCO
absorption from 2018, 2005, and 1917 cm-1to 2027, 2015, and 1914 cm-1. Rectangles 1a and 1b that are capable of binding metal ions that undergo spectroscopic changes upon the formation metal ion complexes could be used as analytical reagents in the analysis of metal ions.1H NMR
spectral studies of 1a in the presence of the AgI ion in
acetone-d6shows a downfield shift of the H2and H3protons
of the pyridyl group (Table 2). This indicates the coordination of the Ag cation to the S atom, thus causing a reduction in the electron density on it and consequently on the metal. This, in turn, causes the Re atom to draw electrons from the bipyridyl moiety and, as a result, the H2and H3protons of
the pyridyl group are shifted downfield. These observations
indicate that the AgI‚‚‚S interactions are significant. This is
corroborated by the IR data. The marked change in νCO
absorption to a higher wavenumber showed that back-bonding to theπ* orbital of CO is less because of the lower
electron density at the Re center. The NO3-‚‚‚H(bpy)
interactions may occur but to a lesser extent than AgI‚‚‚S
interactions because nitrate anions (NO3-) may prefer to
interact with Ag cations in solution (vide infra).44
To further verify these observations, X-ray-quality crystals of compound [{1a‚(Ag+)2(NO3-)2(C3H6O)2}(C3H6O)] were
obtained by dissolving the host 1a with the guest AgNO3in
acetone, followed by slow evaporation at room temperature. A single-crystal X-ray analysis reveals that the species contains one rectangle [{(CO)3Re(µ-SC4H9)2Re(CO)3}2(
µ-bpy)2]2unit, two AgIatoms, two bridging nitrate ions, and
two coordinated acetone molecules. Complex [{1a‚(Ag+)2
-(NO3-)2(C3H6O)2}(C3H6O)] crystallizes in a monoclinic cell,
and the structure was solved in the space group P21/c. The
crystallographic refinement data and selected bond distances and angles are listed in Tables 3 and 5. As shown in Figure 8, the thiolato bridges of the ReIrectangles are coordinated (44) Vega, I. E. D.; Gale, P. A.; Light, M. E.; Loeb, S. J. Chem. Commun.
2005, 4913 and references cited therein.
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.
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.
Figure 7. Absorption spectra of 1a (1× 10-5M) upon the addition of the AgIion in a 80:20 (v/v) THF-H
2O mixture: (a) 0, (b) 2× 10-4, (c)
4× 10-4, (d) 6× 10-4, and (e) 8× 10-4M.
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to Ag atom [Ag1-S1, 2.4405(17) Å], which is further linked by two nitrate ions with the Ag-O distances of 2.308(5)-2.542(5) Å and coordinated by one acetone molecule [Ag1-O10, 2.432(5) Å; Supporting Information, Figure S6]. The intrinsic affinity of the S atom toward the AgIion together
with the Ag-ONO2interactions leads to the formation of a
one-dimensional supramolecular array through S‚‚‚Ag‚‚‚O connections. Additionally, two adjacent Ag salts are bridged together through nitrate ions with Ag‚‚‚Ag distances of about 4.05 Å. This reveals the existence of very weak argentophilic interactions in this system. Thus, the interaction of the rectangles toward the Ag ion results in the self-organization of interesting supramolecular arrays.
Conclusion
In summary, thiolato- and alkoxy-bridged molecular rectangles exhibit molecular recognition characteristics to-ward planar aromatic guest molecules and the Ag ion. As evidenced by UV-vis, fluorescence, and1H NMR
spectro-scopic data and single-crystal X-ray diffraction analyses, rectangles 1 and 2 strongly interact with the pyrene molecule in a 1:1 host-guest ratio. The results of a single-crystal X-ray diffraction analysis show that the recognition of 1 toward the pyrene molecule is mainly due to CH‚‚‚π interactions
and that the face of the guest pyrene sits over the edges of the bpy linkers of 1. In addition, the thiolato-bridged rectangle 1a recognizes AgI, generating an interesting
supramolecular array via Re-S‚‚‚Ag‚‚‚O interactions.
Experimental Section
Materials and General Methods. Reagents were used as
received without further purification. The solvents used in this study were of spectroscopic grade. Electronic absorption spectra were recorded on a Hewlett-Packard 8453 spectrophotometer. Fluorescence spectra were recorded on a Hitachi F4500 spec-trometer using a slit width of 2.5 nm for both excitation and emission measurements. The photomultiplier voltage was 700 V. 1H and 13C NMR spectra were recorded on Bruker AC 300 and AMX-400 FT-NMR spectrometers. Elemental analyses were per-formed using a Perkin-Elmer 2400 CHN elemental analyzer. FAB-MS data were obtained using a JFAB-MS-700 double-focusing mass spectrometer.
Synthesis of [{(CO)3Re(µ-SC4H9)2Re(CO)3}2(µ-bpy)2] (1a). A
suspension containing a mixture of Re2(CO)10(131 mg, 0.20 mmol) and bpy (65 mg, 0.40 mmol) in 10 mL of a 3:7 mixture of 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 orange crystals were separated by filtration, and the solvent from the filtrate was removed by applying a vacuum. The residue was redissolved in a minimum quantity of CH2Cl2and passed through a short silica gel column to give the pure product. Yield: 71%. IR (CH3COCH3): νCO2018 (s), 2005 (s), 1917 (vs), 1899 (vs) cm-1. 1H NMR (300 MHz, acetone-d6):δ 9.09 (d,3J ) 5.3 Hz, 8H, H3), 7.86 (d, 8H, H2), 3.32 (t,3J ) 7.4 Hz, 8H, CH2), 1.71 (m, 8H, CH2), 1.57 (m, 8H, CH2), 1.03 (t,3J ) 7.2 Hz, 12H, CH3).13C NMR (75 MHz, acetone-d6): δ 201.0, 196.1 (1:2 CO), 157.0 (C3), 145.5 (C1), 123.6 (C2), 39.5 (CH2), 35.6 (CH2), 22.3 (CH2), 14.0 (CH3). UV-vis (CH3CN): λmax[nm] 365 (MLCT), 244, 316 (LIG). FAB-MS: m/z 1750.2 (M+). Anal. Calcd for C48H52N4O12S4Re4: C, 32.94; H, 2.99; N, 3.20. Found: C, 32.94; H, 2.82; N, 2.92.
Synthesis of [{(CO)3Re(µ-SC8H17)2Re(CO)3}2(µ-bpy)2] (1b).
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 1-octanethiol and toluene in a 30-mL Teflon flask was placed in a steel bomb. The bomb was placed in an oven maintained at 140 °C for 48 h and then cooled to 25°C. Good-quality orange single crystals of 1b were obtained. The solvent from the reaction mixture was removed by vacuum distillation, and the residue was redis-solved in CH2Cl2and passed through a short silica gel column to get pure 1b. Yield: 53%. IR (CH2Cl2): νCO2019 (s), 2006 (s), 1919 (vs), 1898 (vs) cm-1. 1H NMR (300 MHz, acetone-d6): δ 9.09 (d, 3J ) 6.7 Hz, 8H, H3), 7.84 (d, 3J ) 6.7 Hz, 8H, H2), 3.31 (t,3J ) 7.3 Hz, 8H, CH2), 1.76 (m, 8H, CH2), 1.56 (m, 8H, CH2), 1.41 (m, 16H, CH2), 1.35 (m, 16H, CH2), 0.91 (t, 3J ) 6.6 Hz, 12H, CH3). 13C NMR (75 MHz, acetone-d6): δ 200.9, 196.1 (1:2 CO), 156.9 (C3), 145.5 (C1) 123.5 (C2), 39.8 (CH2), 33.6 (CH2), 32.6 (CH2), 29.9 (2CH2), 23.3 (2CH2), 14.4 (CH3).
Figure 8. (a) Crystallographic drawing indicating the inclusion of AgNO3moieties 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 AgNO3moieties 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 [{1a‚(Ag+)2(NO3-)2(C3H6O)2}(C3H6O)]
Re1-S1 2.5356(17) Re1-S2 2.4964(16) Re1-N1 2.213(5) Re2-S1 2.5253(16) Re2-S2 2.5135(17) Re2-N2 2.230(5) Re2-C4 1.926(7) Re2-C5 1.924(7) Re2-C6 1.919(7) Ag1-S1 2.4405(17) Ag1-O7 2.308(5) Ag1-O10 2.432(5) S1-C17 1.847(8) S2-C21 1.837(7) S1-Re1-S2 79.99(5) S1-Re1-N1 88.82(14) S1-Re1-C1 171.6(2) S1-Ag1-O7 157.94(15) S1-Re1-C2 90.8(2) S1-Ag1-O10 119.49(13) S1-Re1-C3 96.4(2) S2-Re1-N1 85.37(14) S2-Re1-C1 91.7(2) S2-Re1-C2 94.8(2) S2-Re1-C3 174.7(2) Re1-S1-Re2 99.09(6) Re1-S1-Ag1 126.79(7) Re2-S1-Ag1 115.32(6) Ag1-S1-C17 99.5(2) Re1-S2-Re2 100.46(6) S1-Re2-S2 79.87(5) S1-Re2-N2 85.24(14) S2-Re2-N2 88.00(14)
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UV-vis (CH3CN): λmax [nm] 368 (MLCT), 244, 316 (LIG). FAB-MS: m/z 1974.2 (M+). Anal. Calcd for C64H84N4O12S4Re4: C, 38.86; H, 4.28; N, 2.83. Found: C, 39.03; H, 3.74; N, 2.57.
Rectangles 2a and 2b were obtained following a similar procedure using 10 mL of aliphatic alcohol instead of a mixture of thiol and toluene.25Yield: 2a, 86%; 2b, 87%.
Fluorescence Quenching Studies. Quenching experiments of
the fluorescence of pyrene were carried out under aerated conditions. The solvent used in this study was of spectroscopic grade. The excitation wavelength was 336 nm in CH2Cl2as the solvent. The monitoring wavelength corresponded to the maximum of the emission band at 393 nm. Relative fluorescence intensities were measured for solutions of pyrene in CH2Cl2and rectangles used as quenchers. There was no change in shape, but a change in the intensity of the fluorescence peak was found, when these rectangles were added. The Stern-Volmer (SV) relationship, I0/I ) 1 + KSV-[Q], was obtained for the ratio of the emission intensities (I0and I are the emission intensities in the absence and presence of quencher) and quencher concentration, [Q]. The quenching rate constants were obtained from the Stern-Volmer constant, KSV, and the fluorescence lifetime,τ, of pyrene (32 ns). Excited-state lifetime studies were performed using an Edinburgh FL 920 single photon-counting system with a H2-filled or N2lamp as the excitation source. The emission decays were analyzed by the sum of exponential functions, which allows the partial elimination of instrument time broadening, thus rendering a temporal resolution.
Binding Constant Measurements. The binding abilities of the
rectangles with pyrene were examined by both absorption and emission spectroscopic methods. The concentration of pyrene was 2× 10-5M, and those of the rectangles were 2× 10-6-3 × 10-5 M. The binding constants from the electronic absorption experiment were measured by changing the concentration of the rectangles with pyrene and calculated on the basis of the Benesi-Hildebrand 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 constants in the excited state were determined using the modified Stern-Volmer equation.
X-ray Crystallographic Studies. Suitable single crystals with
dimensions of 0.10× 0.10 × 0.15 and 0.05 × 0.12 × 0.20 mm for [{1a‚pyrene}‚4C3H6O] and [{1a‚(Ag+)2(NO3-)2(C3H6O)2} -(C3H6O)], respectively, were selected for indexing and intensity data collection. A total of 1420 frames constituting a hemisphere of X-ray intensity data were collected with a frame width of 0.3° inω and a counting time of 10 s/frame, using a Bruker SMART CCD diffractometer. The first 50 frames were re-collected at the end of data collection to monitor crystal decay. No significant decay was observed. The raw data frames were integrated into SHELX-format reflection files and corrected for Lorentz and polarization effects using the SAINT program, and absorption correction was performed using the SADABS program.45The space groups were determined to be P21/c. Direct methods were used to solve the structure using the SHELX-TL46program packages. All non-H atoms were refined anisotropically by full-matrix least squares based on F2 values. The largest residual density peak is close to that of the Re atom. Basic information pertaining to crystal param-eters and structure refinement for [{1a‚pyrene}‚4C3H6O] and [{1a‚(Ag+)2(NO3-)2(C3H6O)2}(C3H6O)] is summarized in Table 3, and selected bond distances and angles are provided in Tables 4 and 5, respectively.
Acknowledgment. We thank Academia Sinica and the National Science Council, Taiwan, for financial support.
Supporting Information Available: Crystallographic details
in CIF format, UV-vis absorption spectra of 2b and pyrene, Stern-Volmer plot for 1a with pyrene, and coordination environments around the Ag ions. This material is available free of charge via the Internet at http://pubs.acs.org.
IC0604720
(45) SMART/SAINT/ASTRO, release 4.03; Siemens Energy & Automation, Inc.: Madison, WI, 1995.
(46) Sheldrick, G. M. SHELX-TL; University of Gottingen: Gottingen, Germany, 1998.
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![Figure 1. Crystallographic drawing of [1a‚pyrene] showing CH‚‚‚π interactions in the solid state.](https://thumb-ap.123doks.com/thumbv2/9libinfo/8874987.249841/3.918.475.836.71.302/figure-crystallographic-drawing-pyrene-showing-interactions-solid-state.webp)
