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Cucurbit[7]uril Complexes of Crown-Ether Derived Styryl and (Bis)styryl Dyes Olga A. Fedorova,*,† Ekaterina Yu. Chernikova,† Yuri V. Fedorov,† Elena N. Gulakova,† Aleksander S. Peregudov,† Konstantin A. Lyssenko,† Gediminas Jonusauskas,‡ and Lyle Isaacs*,§ A. N. NesmeyanoV Institute of Organoelement Compounds of the Russian Academy of Sciences, 28 VaViloVa str., Moscow, 119991 Russia, Centre de Physique Moleculaire Optique et Hertzienne - UMR CNRS 5798, Bordeaux 1 UniVersity, 351 Cours de la Liberation, 33405 Talence, France, and Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742 ReceiVed: April 9, 2009; ReVised Manuscript ReceiVed: May 27, 2009
In this paper, we report the interaction of the CB[7] molecular container with crown ether styryl and (bis)styryl dyes 1-6. The interaction of monostyryl dyes (1 and 2) with CB[7] results in the formation of 1:1 complexes where the CB[7] molecule is located on the region of the guest encompassing the pyridinium ring, CdC double bond, and a portion of the aryl ring of benzocrown ethers 1 and 2. For (bis)styryl dyes (3-5), the formation of two types of complexes with composition dye · CB[7] · dye and CB[7] · dye · CB[7] was confirmed by a combination of optical and electrospray ionization mass spectroscopy (ESI-MS) methods. In the case of (bis)styryl dye (6), both 2:1 and 1:1 compositions 6 · CB[7] · 6 and CB[7] · 6 were formed. Complex formation is accompanied by substantial changes in the optical characteristics of the dyes and formation of long-lived excimer species. We tested the stimuli responsiveness of this system in response to metal ions. We find that the metal ions prefer to bind to the electrostatically negative ureidyl CdO portals of the CB[7] rather than with the crown ether moiety of the styryl dyes. Introduction The cucurbit[n]uril (CB[n], n ) 5, 6, 7, 8, 10) family of macrocycles are prepared from the acid catalyzed condensation reaction of glycoluril and formaldehyde.1,2 The rigid structure, good water solubility, and ability to form tight (Ka up to 1015 M-1)3-5 complexes with molecules and ions make CB[7] particularly attractive as a building block for the construction of supramolecular architectures and devices. For example, a number of research groups have studied the interactions between CB[6], CB[7], or CB[8] and cationic guests including those based on the alkanes, aromatics viologens, adamantanes, and ferrocenes.6-8 These studies have uncovered some of the basic recognition properties governing the recognition properties of the CB[n] family including tight and selective binding in water, high environmental responsiveness to changes in pH or metal ion concentrations,9-11 pKa shifts,12,13 host electrostatic potential,6,14 and constrictive binding processes.9,15 These outstanding recognition properties have been put to use toward a number of applications including the preparation of pH and electrochemically responsive molecular devices,16,17 drug delivery systems,12,18,19 (bio)chemical sensors,20 separations materials,21 electronic materials,22 and supramolecular polymers.23 Of particular relevance to the work described in this paper are the growing number of reports on the use of CB[n] molecular containers as hosts for dyes and fluorophores. For example, Wagner and co-workers reported an early example of the fluorescence enhancement upon inclusion of the environmentally * To whom correspondence should be addressed. E-mail: fedorova@ ineos.ac.ru (O.A.F.);
[email protected] (L.I.). † A. N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences. ‡ Centre de Physique Moleculaire Optique et Hertzienne - UMR CNRS 5798. § University of Maryland.
sensitive probes 1-anilinonaphthalene-8-sulfonic acid and 2-anilino-naphthalene-6-sulfonic acid into CB[7].24 Nau and coworkers reported the enhanced polarizability inside CB[7] based on studies with 2,3-diazabicyclo-[2.2.2]oct-2-ene as a probe guest.25 Additionally, a number of recent reports have greatly expanded the range of dyes and fluorophores known to bind to CB[n] molecular containers.26 Beyond its use in basic science, CB[7]sdue to its strong affinity toward organic guests and dyessholds great potential in the treatment of effluents from dye industries.27,28 Fluorescent dyes have widespread technological, scientific, and medicinal applications. In the search for the ultimate fluorescent dyes with the highest photostability and brightness in water as environmentally benign and biologically relevant solvents, strategies involving stabilizing, solubilizing, deaggregating, and enhancing additives have become popular. For these purposes, the host-guest complexation between suitable dyes as guests and cyclodextrins (CDs)29 or watersoluble calixarenes as macrocyclic hosts have been reported.30 More recently, these strategies have begun to be applied with CB[n] molecular containers. For example, Nau and co-workers have studied the potential of CB[7] as a stabilizing additive and enhancement agent toward the following dyes in aqueous solution: rhodamine 6G, rhodamine 123, tetramethylrhodamine, cresylviolet, fluorescein, coumarin 102, pyronin B, pyronin Y, two cyanine 5 and one cyanine 3 derivative, and IR 140 as well as IR 144.28 For most of the cationic dyes, photostabilization was established, and a pronounced thermal stabilization due to deaggregation and solubilization was observed for the xanthene dyes. The advantageous effects are attributed to the formation of inclusion complexes with different photophysical and photochemical properties. The complexation is generally accompanied by spectral shifts characteristic for dye inclusion in a less polar environment which usually results in increased fluorescence quantum yields and brightness. Potential applica-
10.1021/jp903289q CCC: $40.75 2009 American Chemical Society Published on Web 07/06/2009
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SCHEME 1: Chemical Structures of Styryl and (Bis)styryl Dyes 1-6 Used in this Work
tions of this work include the development of new photostabilizing additives for dye lasers, for prolonged storage of dye solutions, in scanning confocal microscopy, and in fluorescence correlation spectroscopy. In this paper, we report the interaction between CB[7] and styryl dyes (1 and 2) as well as dimeric bis(styryl) dyes (3-6) derivatized with 15-crown-5 termini. Results and Discussion This results and discussion section is organized as follows. First, we discuss the design concepts of guests 1-6. Next, we describe the optical and 1H NMR changes that occur upon formation of complexes between 1-6 and CB[7] with a particular emphasis on the absolute stoichiometry and geometrical features of the resultant complexes. Lastly, we describe the potential stimuli responsiveness of this system with respect to addition of metal perchlorate salts. Design Aspects of Dyes 1-6. Scheme 1 shows the structures of styryl dyes 1-6 used in this study. Dyes 1 and 2 contain cationic pyridinium rings that are known to be good binding sites for CB[n] because the complexes benefit from: (1) cation-dipole interactions between the positively charged dyes and the ureidyl CdO dipoles which line the CB[n] portals and (2) hydrophobic interactions between the guest and the CB[n] host cavity.3,6,7 In dyes 3-6, two styryl dyes units are connected by short linking groups (propyl, o-xylylene, m-xylylene, pxylylene). These central linkerssflanked by two cationic N-atomssare also potential binding sites for CB[7]. Dye 2-6 all incorporate 15-crown-5 end-capping groups. Because both the ureidyl CdO portals of CB[n],31 and the 15-crown-5 endcapping groups of 2-6 are known to interact with metal ions we anticipated that interesting stimuli responsive behavior (e.g., shuttling, dissociation, molecularity change) might be observed. Types of Complexes Between CB[7] and Dye. CB[7] was chosen for these studies due to its appropriate size (cavity diameter 5.4 Å, cavity height 9.1 Å, cavity volume 272 Å3) and excellent water solubility (>20 mM) which allowed us to work in the less competitive neutral D2O solution. Scheme 2 shows the wide variety of complexes that are possible between CB[7] and the bis(styryl) dyes 3-6. Three geometrically distinct CB[7] · dye complexes of 1:1 stoichiometry are possible. For example, CB[7] may undergo a threading process over one of the 15-crown-5 end groups to yield the CB[7] · dye (pyridinium) complex. Depending on the nature of the linking group, this CB[7] · dye (pyridinium) complex may undergo a shuttling process to give the CB[7] · dye (linker) complex. CB[7] may also promote folding16,32 of dye to yield the CB[7] · dye (folded) complex. Depending on the absolute stoichiometry of CB[7]: dye, two further complexes are possible. When CB[7] is present
Fedorova et al. in excess, the CB[7] · dye (pyridinium) complex can thread on a second CB[7] to yield the CB[7] · dye · CB[7] complex. Conversely, when dye is present in excess the dye · CB[7] · dye (folded and dimerized) complex can be populated. Complexation Between CB[7] and Dyes 1-6. Addition of CB[7] to solutions of the cationic dyes (1-6) in water results in the formation of inclusion complexes which was followed by the concomitant changes of the photophysical parameters of the dyes and the complexation induced upfield shifts in the 1 H NMR spectra of the guests which are well-known for CB[n] binding processes.3 The optical spectral changes can be conveniently employed to derive the binding stoichiometry and stability constants of the dyes with CB[7]. For example, the complexation between CB[7] and dyes 1 and 2, which contain only a single cationic binding site, gave UV-vis binding curves that fit well to a 1:1 binding model (Figure 1). The fluorescence spectra of free 2 and the CB[7] · 2 complex are shown in Figure 2. Table 1 summarizes the changes in the optical properties of CB[7] · 1 and CB[7] · 2 observed along with the 1:1 binding constants. Figure 3 shows the MMFF minimized structure of CB[7] · 1 which illustrates the geometry of this 1:1 complex. The ureidyl CdO groups are located near the positively charged pyridinium N-atom and the styryl fragment is encapsulated inside the cavity of CB[7] leaving the methoxy groups on the Ar-ring outside the opposite portal of CB[7]. Similar geometrical features were observed for the computed structures of complexes between 2 and CB[7]. In the case of dyes 3-5, the addition of CB[7] to the dye solution causes a small hypsochromic shift and then a substantial bathochromic shift (e.g., dye 3, Figure 4) in the UV-vis spectrum, which is diagnostic for inclusion of the dye in a less polar environment. The complexation also results in a reduced Stokes shift for all complexes relative to the free dyes (up to 2307 cm-1 for dye 2 and its complex), suggesting a smaller geometrical and solvent relaxation of the complexed dye, as expected for inclusion in the less polar and more confined environment provided by CB[7]. The titration data reveal 2:1 and a 1:2 complexes for dyes 3-5 (Table 1; Figures S3-S8 of the Supporting Information). The first type of complex contains one CB[7] and two dye molecules and assumes the dye · CB[7] · dye (folded and dimerized) geometry shown in Scheme 2. The second type of complex assumes the CB[7] · dye · CB[7] geometry shown in Scheme 2. The binding constants, as obtained by analysis of UV-vis spectrophotometric titration data, reveal a high thermodynamic stability (Table 1). These high binding constants allow the use of low CB[7] concentrations while ensuring a virtually quantitative (>90%) complexation at the 20 µM fluorescent dye concentration employed. Figure 5 shows the fluorescence spectra collected for dye 5 alone and as its 5 · CB[7] · 5 complex. Quite interesting is the presence of an excimer emission band at ≈700 nm. Additional evidence of excimer formation comes from the existence of a long-lived component (1200-1750 ps) found during the time-resolved analysis of complexes of dyes 3-5 with CB[7] (Table 1). The optical response in absorbance spectrum on binding of dye 6 with CB[7] is not pronounced showing only a small hypsochromic shift and a small change of intensity (Figure 6). In contrast, the fluorescence quantum yield (Figure 7, Table 1) increases to a similar extent as was found for dyes 1-5. In the case of dye 6, the spectrophotometric experiments showed the formation of two complexes of compositions 6 · CB[7] · 6 and 6 · CB[7] (see Table 1). The addition of small amounts of CB[7] causes an increase in fluorescence intensity (Figure 7, Supporting Information Figures S7 and S8). The more efficient fluorescence
CB[7] · Styryl Dye Complexes
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SCHEME 2: Schematic Representation of the Various Modes of Complexation Possible between (Bis)styryl Dyes 3-6 and CB[7]
emission inside CB[7] is presumably due to a combination of factors, in particular the inclusion in a rigid confined environment, which may slow down intramolecular deactivation pathways by preventing intramolecular rotation. X-ray Crystallographic Characterization of CB[7] · 6. Slow evaporation of an aqueous solution of CB[7] and dye 6 at room temperature produced colorless crystals of complex CB[7] · 6. The structure of the 1:1 inclusion complex of CB[7] · 6 along with 29 solvate water molecules was confirmed by single crystal X-ray diffraction measurements (Figure 8). The X-ray crystal structure of CB[7] · 6 shows the encapsulation of central p-
Figure 1. Electronic absorption spectra of dye 2 and complex CB[7] · 2 obtained by a global fit of the spectrophotometric titration data using the SpecFit/32 program. Concentration of dye 2 and complex CB[7] · 2 in the system (2 + CB[7]) as a function of [CB[7]] (inset).
Figure 2. Fluorescence spectra of 2.0 × 10-5 M dye 2 and its complex CB[7] · 2 in H2O.
xylylene ring inside the cavity of CB[7]. The dye 6 is characterized by Z-type conformation with pyridinium cycles tilted with respect to the central benzene ring. Analysis of the interatomic contacts reveals that the dicationic dye 6 is held in place by C-H · · · O contacts formed by the pyridinium hydrogens and the ureidyl CdO groups of CB[7] with H · · · O distances in the range of 2.2-2.5 Å with C-H · · · O angles in the 134-168° range. It should be noted that the energy of short C-H · · · O contacts can be as large as 3-4 kcal/mol,33 and therefore, we believe the contacts observed here play a significant role in stabilization of inclusion complex. Both crown groups of dye 6 form weak H-bonds with two water molecules with O · · · O distances in the range of 2.772-3.180(6) Å. The geometry of these H-bonded waters assume an intriguing crisscross shape across the cavity of the benzo-15-crown rings. The remaining water molecules assemble the inclusion cationic complex with the halogen anions leading to the formation of an extended 3D H-bonded framework. Electrospray Ionization Mass Spectrometry. Additional support of the composition of the complexes was obtained from ESI-MS experiments. All CB[7] complexes were detected as multiply charged ions (see Table 2). When the ratio of dye: CB[7] is 1:3 the main observed complex is dye · CB[7]. Increasing the amount of CB[7] up to a ratio of dye:CB[7] (1: 30) causes the appearance of signals in the mass spectrum that correspond to complexes with the composition dye · (CB[7])2. These data agree with and provide additional support for the stoichiometries derived from the optical results. NMR Characterization of the Complexes. The 1H NMR spectra recorded for dyes 2-5 alone and in the presence of CB[7] in D2O reveal a combination of upfield and downfield shifting of the resonances for the pyridinium residues and the spacers. The direction and magnitude of the shifts provides confirmation of the proposed geometries of these complexes in accord with the well-known shielding region inside the CB[n] cavity and the deshielding region nearby to the portals. For example, Figure 9 shows the 1H NMR spectra recorded for 3 and for the 3 · CB[7] · 3 complex. The protons for the central N-(CH2)3-N spacer undergo substantial upfield shifts indicative of cavity binding whereas protons 8-16 undergo slight downfield shifts indicative of their location outside the CB[7] cavity. Similar experiments were performed for the complexes between CB[7] and 2, 4, and 5 (Table 3, Supporting Information). For example, in the 4 · CB[7] · 4 complex the resonances
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TABLE 1: Steady-State Absorption and Fluorescence Data and Equilibrium Constants for Dyes 1-6 and their Complexes with CB[7] in H2O compound
λabs (nm)
1 CB[7] · 1 2 CB[7] · 2 3 3 · B[7] · 3 CB[7] · 3 · B[7] 4 4 · B[7] · 4 CB[7] · 4 · CB[7] 5 5 · B[7] · 5 CB[7] · 5 · B[7] 6 6 · CB[7] · 6 CB[7] · 6
382 407 382 407 395 385 425 395 385 425 395 400 425 395 400 385
a
∆λ
abs
ε × 104 (L · mol-1 · cm-1)
∆λabs (nm)a
1.00 0.87 2.70 2.35 5.31 9.51 5.38 5.38 11.19 5.47 4.66 8.33 5.10 4.42 9.46 4.53
25 25 –10 30 –10 30 5 30 5 –10
log K (M-1 or M-2) 5.98 ( 0.14 5.36 ( 0.02 8.72 ( 0.06 7.45 ( 0.03 9.92 ( 0.08 7.81 ( 0.04 8.5 ( 0.1 7.34 ( 0.06 8.6 ( 0.3 4.09 ( 0.07
λfl (nm) 537 522 545 525 555
∆λfl (nm)b
φfl, 10-2
τfl, (ps)
–15 –20
2.56 3.19 0.50
122, 355 470, 2100 ( 2σ(I)). All calculations were performed using the SHELXTL software (SHELXTL, version 6.1. Bruker AXS Inc., Madison, WI, 2005). Electrospray Ionization Mass Spectrometry. Investigations of inclusion complexes of dyes 1-6 with cucurbit[7]uril in a water/acetonitrile ) 100/1 mixture were performed with different ratios of CB[7]/dye using an Agilent 1100 series LC/ MSD trap ESI interface operated in the positive-ion mode. Direct infusion of the sample solution was used. The optimum flow rate was 400 µL h-1. The capillary and the capillary exit were maintained at potentials of 3.5 kV and 10 V, respectively. The temperature of the drying gas was 150 °C. Calculation of the isotope patterns were performed using the Molecular Weight Calculator, version 6.37 [Matthew Monroe]. Acknowledgment. This work was supported by the RAS program on Supramolecular Chemistry and the RFBR 09-03-
J. Phys. Chem. B, Vol. 113, No. 30, 2009 10157 00241. L.I. thanks the U.S. National Science Foundation (CHE0615049) for financial support. Supporting Information Available: Absorption and fluorescence spectra, NMR spectroscopic data, 2D COESY, ROESY spectra, X-ray data, and a cif file. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Freeman, W. A.; Mock, W. L.; Shih, N.-Y. J. Am. Chem. Soc. 1981, 103, 7367–7368. (b) Kim, J.; Jung, I.-S.; Kim, S.-Y.; Lee, E.; Kang, J.-K.; Sakamoto, S.; Yamaguchi, K.; Kim, K. J. Am. Chem. Soc. 2000, 122, 540–541. (c) Day, A. I.; Blanch, R. J.; Arnold, A. P.; Lorenzo, S.; Lewis, G. R.; Dance, I. Angew. Chem., Int. Ed. 2002, 41, 275–277. (d) Liu, S.; Zavalij, P. Y.; Isaacs, L. J. Am. Chem. Soc. 2005, 127, 16798– 16799. (2) Day, A.; Arnold, A. P.; Blanch, R. J.; Snushall, B. J. Org. Chem. 2001, 66, 8094–8100. (3) Mock, W. L.; Shih, N.-Y. J. Org. Chem. 1986, 51, 4440–4446. (4) Rekharsky, M. V.; Mori, T.; Yang, C.; Ko, Y. H.; Selvapalam, N.; Kim, H.; Sobransingh, D.; Kaifer, A. E.; Liu, S.; Isaacs, L.; Chen, W.; Moghaddam, S.; Gilson, M. K.; Kim, K.; Inoue, Y. Proc. Natl. Acad. Sci. USA. 2007, 104, 20737–20742. (5) Liu, S.; Ruspic, C.; Mukhopadhyay, P.; Chakrabarti, S.; Zavalij, P. Y.; Isaacs, L. J. Am. Chem. Soc. 2005, 127, 15959–15967. (6) Lee, J. W.; Samal, S.; Selvapalam, N.; Kim, H.-J.; Kim, K. Acc. Chem. Res. 2003, 36, 621–630. (7) Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L. Angew. Chem., Int. Ed. 2005, 44, 4844–4870. (8) Isaacs, L. Chem. Commun. 2009, 619–629. (9) Marquez, C.; Nau, W. M. Angew. Chem., Int. Ed. 2001, 40, 3155– 3160. (10) Ong, W.; Kaifer, A. E. J. Org. Chem. 2004, 69, 1383–1385. (11) Sindelar, V.; Silvi, S.; Kaifer, A. E. Chem. Commun. 2006, 2185– 2187. (12) Saleh, N.; Koner, A. L.; Nau, W. M. Angew. Chem., Int. Ed. 2008, 47, 5398–5401. (13) Mohanty, J.; Bhasikuttan, A. C.; Nau, W. M.; Pal, H. J. Phys. Chem. B 2006, 110, 5132–5138. (14) Ong, W.; Kaifer, A. E. Organometallics 2003, 22, 4181–4183. (15) Mukhopadhyay, P.; Zavalij, P. Y.; Isaacs, L. J. Am. Chem. Soc. 2006, 128, 14093–14102. (16) Ko, Y. H.; Kim, E.; Hwang, I.; Kim, K. Chem. Commun. 2007, 1305–1315. (17) (a) Mock, W. L.; Pierpont, J. J. Chem. Soc., Chem. Commun. 1990, 1509–1511. (b) Chakrabarti, S.; Mukhopadhyay, P.; Lin, S.; Isaacs, L. Org. Lett. 2007, 9, 2349–2352. (c) Wang, W.; Kaifer, A. E. Angew. Chem., Int. Ed. 2006, 45, 7042–7046. (18) Angelos, S.; Yang, Y.-W.; Patel, K.; Stoddart, J. F.; Zink, J. I. Angew. Chem., Int. Ed. 2008, 47, 2222–2226. (19) (a) Bali, M. S.; Buck, D. P.; Coe, A. J.; Day, A. I.; Collins, J. G. Dalton Trans. 2006, 5337–5344. (b) Lee, H.-K.; Park, K. M.; Jeon, Y. J.; Kim, D.; Oh, D. H.; Kim, H. S.; Park, C. K.; Kim, K. J. Am. Chem. Soc. 2005, 127, 5006–5007. (c) Park, K. M.; Suh, K.; Jung, H.; Lee, D.-W.; Ahn, Y.; Kim, J.; Baek, K.; Kim, K. Chem. Commun. 2009, 71–73. (d) Wang, R.; Macartney, D. H. Org. Biomol. Chem. 2008, 6, 1955–1960. (20) (a) Hennig, A.; Bakirci, H.; Nau, W. M. Nat. Methods 2007, 4, 629–632. (b) Bush, M. E.; Bouley, N. D.; Urbach, A. R. J. Am. Chem. Soc. 2005, 127, 14511–14517. (21) (a) Miyahara, Y.; Abe, K.; Inazu, T. Angew. Chem., Int. Ed. 2002, 41, 3020–3023. (b) Nagarajan, E. R.; Oh, D. H.; Selvapalam, N.; Ko, Y. H.; Park, K. M.; Kim, K. Tetrahedron Lett. 2006, 47, 2073–2075. (c) Liu, S.M.; Xu, L.; Wu, C.-T.; Feng, Y.-Q. Talanta 2004, 64, 929–934. (22) (a) Gadde, S.; Batchelor, E. K.; Weiss, J. P.; Ling, Y.; Kaifer, A. E. J. Am. Chem. Soc. 2008, 130, 17114–17119. (b) Liu, Y.; Shi, J.; Chen, Y.; Ke, C.-F. Angew. Chem., Int. Ed. 2008, 47, 7293–7296. (c) Eelkema, R.; Maeda, K.; Odell, B.; Anderson, H. L. J. Am. Chem. Soc. 2007, 129, 12384– 12385. (23) (a) Lee, J. W.; Ko, Y. H.; Park, S.-H.; Yamaguchi, K.; Kim, K. Angew. Chem., Int. Ed. 2001, 40, 746–749. (b) Choi, S.; Lee, J. W.; Ko, Y. H.; Kim, K. Macromolecules 2002, 35, 3526–3531. (c) Rauwald, U.; Scherman, O. A. Angew. Chem., Int. Ed. 2008, 47, 3950–3953. (24) Wagner, B. D.; Stojanovic, N.; Day, A. I.; Blanch, R. J. J. Phys. Chem. B 2003, 107, 10741–10746. (25) Marquez, C.; Nau, W. M. Angew. Chem., Int. Ed. 2001, 40, 4387– 4390. (26) (a) Bhasikuttan, A. C.; Mohanty, J.; Nau, W. M.; Pal, H. Angew. Chem., Int. Ed. 2007, 46, 4120–4122. (b) Wang, R.; Yuan, L.; Macartney, D. H. Chem. Commun. 2005, 5867–5869. (c) Montes-Navajas, P.; Corma,
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