A Tripodal [2]Rotaxane on the Surface of Gold - Langmuir (ACS

Oct 27, 2007 - In the case of the tripodal axle 2, the surface coverage is 7 × 10-11 mol·cm-2, while for the tripodal [2]rotaxane 3 the surface cove...
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Langmuir 2007, 23, 12147-12153

12147

A Tripodal [2]Rotaxane on the Surface of Gold Kirill Nikitin,* Elena Lestini, Mariachiara Lazzari, Silvano Altobello, and Donald Fitzmaurice School of Chemistry and Chemical Biology, UniVersity College Dublin, Belfield, Dublin 4, Ireland. ReceiVed June 5, 2007. In Final Form: August 20, 2007 Tripodal [2]rotaxane, 3, and the structurally related axle, 2, incorporating a viologen moiety, a crown ether, and three thiol anchoring groups have been synthesized. Analogous monopodal derivatives, 1, have also been prepared. Self-assembled monolayers of the above tripodal and monopodal systems on gold have been studied by cyclic voltammetry. It has been shown that a thiol anchoring group is required to attach the monopodal viologen 1 to the surface of gold and that the maximum surface coverage of 1 corresponds to 2.7 × 10-10 mol‚cm-2. The adsorbed monopodal viologen 1 does not thread bis-p-phenylene-34-crown-10 ether, 6. However, the tripodal axle 2 adsorbed on the surface of gold threads the crown ether 6 to form a hetero [2]rotaxane. In the case of the tripodal axle 2, the surface coverage is 7 × 10-11 mol‚cm-2, while for the tripodal [2]rotaxane 3 the surface coverage reaches 1.1 × 10-10 mol‚cm-2.

Introduction Supramolecular assemblies on the surfaces of semiconductors and metals inspire modern synthetic, physical, and surface chemists.1a Recently, surface-targeted supramolecular synthetic chemistry has produced oriented functional molecular arrays incorporating interlocked molecules at the surface.1b-e Molecular devices prepared on conductive surfaces can be tested, controlled, and switched electrochemically or by light.1-3 Rapidly growing interest in conductive interfaces and breathtaking progress in this field relies on promising applications in new-generation electronics and photonics. [2]Rotaxanes are undoubtedly an important class of interlocked molecules. Their topology allows them to undergo a reversible internal translational motion (RITM), not losing their integrity and thus working as a molecular shuttle or a switch in solution or on the surface.1e,2-4 For example, redox active [2]rotaxanes attached to surfaces of metals can control binding and surface wettability properties.3 It is conceivable that the fabrication of future electronic devices will rely on the operation of switchable [2]rotaxane junction devices on the surface, since it has recently * To whom correspondence should be addressed. E-mail: kirill.nikitin@ ucd.ie. (1) (a) Forster, R. J; Keyes, T. E.; Vos, J. G. Interfacial Supramolecular Assemblies; Wiley: New York, 2003. (b) Wei, L.; Padmaja, K.; Youngblood, W. J.; Lysenko, A. B.; Lindsey, J. S.; Bocian, D. F. J. Org. Chem. 2004, 69, 14611469. (c) Galoppini, E.; Guo, W.; Zhang, W.; Hoertz, P. G.; Qu, P.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 7801-7811. (d) Long, B.; Nikitin, K.; Fitzmaurice, D. J. Am. Chem. Soc. 2003, 125, 5152-5160. (e) Long, B.; Nikitin, K.; Fitzmaurice, D. J. Am. Chem. Soc. 2003, 125, 15490-15498. (f) Nikitin, K.; Long, B.; Fitzmaurice, D. J. Chem. Soc., Chem. Commun. 2003, 282-283. (2) (a) Bryce, M. R.; Cooke, G.; Duclairoir, F. M. A.; John, P.; Perepichka, D. F.; Polwart, N.; Rotello, V. M.; Stoddart, J. F.; Tseng, H. R. J. Mater. Chem. 2003, 13, 2111-2117. (b) Mendes, P.; Flood, A. H.; Stoddart, J. F. Appl. Phys. A 2005, 80, 1197-1209. (c) Katz, E.; Lioubashevsky, O.; Willner, I. J. Am. Chem. Soc. 2004, 126, 15520-15532. (d) Sheehy-Hai-Ichia, L.; Willner, I. J. Phys. Chem. B 2002, 106, 13094-13097. (e) Kim, K.; Jeon, W. S.; Kang, J.-K.; Lee, J. W.; Jon, S. Y.; Kim, T.; Kim, K. Angew. Chem. 2003, 115, 2395-2398. (3) (a) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 16241626. (b) Berna, J.; Leigh, D. A.; Lubomska, M.; Mendoza, S. M.; Perez, E. M.; Rudolf, P.; Teobaldi, G.; Zerbetto, F. Nat. Mater. 2005, 4, 704-710. (c) Weber, N.; Hamann, C.; Kern, J.-M.; Sauvage, J.-P. Inorg. Chem. 2003, 42, 6780-6792. (d) Kay, E. R; Leigh, D. A; Zerbetto, F. Angew. Chem. 2007, 46, 72-191. (4) (a) Balzani, V.; Clemente-Leo´n, M.; Credi, A.; Ferrer, B.; Venturi, M.; Flood, A. H.; Stoddart, J. F. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 1178. (b) Nygaard, S.; Laursen, B. W.; Flood, A. H.; Hansen, C. N.; Jeppesen, J. O.; Stoddart, J. F. Chem. Commun. 2006, 144-146.

been demonstrated that a monolayer of bistable [2]rotaxane molecules can serve as ultrahigh density data storage media.5 In this context, we have earlier prepared and characterized tripodal supramolecular systems, tripodal [2]pseudorotaxanes, and [2]rotaxanes on the surfaces of semiconductor titania nanoparticles.1d,e We have successfully demonstrated that the tripodal geometry of the tether permits the virtually intact function of the viologen-crown ether supramolecular moieties due to their displacement from and orientation normal to the surface. As noted above, the tripodal thiol tethers have recently been prepared and used to attach diverse types of redox-active molecules to electroactive surfaces.1b,6 However, very little is known about the applicability of tripodal tethers to supramolecular systems at the surface, while the latest results using less complex anchoring groups are undoubtedly very stimulating.2a,c-e,3c,5a In this paper, we take our initial step to connect electroactive [2]rotaxanes to metal surfaces by means of a tripodal linker. Tripodal configuration of the linker in this case should increase the possibility of an oriented adsorption, improve the monolayer stability, and reduce the likelihood of unwanted staking and folding7 of the interlocked functional molecular moiety on the surface. In this study, few monopodal and tripodal thiolated viologen-based systems 1-3 (Chart 1) have been prepared and adsorbed on gold-coated glass. Cyclic voltammetry (CV) was used to characterize the adsorption density and the viologencrown ether interactions within these systems at the surface.

Results and Discussion Design and Preparation of Monopodal and Tripodal Linkers. Our goal was to attach a viologen moiety to a rigid (5) (a) Tseng, H. R.; Wu, D.; Fang, N. X.; Zhang, X.; Stoddart, J. F. ChemPhysChem 2004, 5, 111-116. (b) Jang, S. S.; Jang, Y. H.; Kim, Y.-H.; Goddard, W. A.; Flood, A. H.; Laursen, B. W.; Tseng, H. R.; Stoddart, J. F.; Jeppesen, J. O.; Choi, J. W.; Steuerman, D. W.; DeIonno, E.; Heath, J. R. J. Am. Chem. Soc. 2005, 127, 1563-1575. (c) Green, J. E.; Choi, J. W.; Boukai, A.; Bunimovich, Y.; Johnston-Halperin, E.; DeIonno, E.; Luo, Y.; Sheriff, B. A.; Xu, K.; Shin, Y. S.; Tseng, H.-R.; Stoddart, J. F.; Heath, J. R. Nature 2007, 445, 414-417. (6) (a) Roth, K. M.; Gryko, D. T.; Clausen, C.; Li, J.; Lindsey, J. S.; Kuhr, W. G.; Bocian, D. F. J. Phys. Chem. B 2002, 106, 8639-8648. (b) Wei, L.; Tiznado, H.; Liu, G.; Padmaja, K.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Phys. Chem. B 2005, 109, 23963-23971. (c) Jian, H.; Tour, J. M. J. Org. Chem. 2003, 68, 5091-5103. (7) (a) Yamamoto, T.; Tseng, H.-R.; Stoddart, J. F.; Balzani, V.; Credi, A.; Marchioni, F.; Venturi, M. Collect. Czech. Chem. Commun. 2003, 68, 14881514. (b) Nikitin, K.; Fitzmaurice, D. J. Am. Chem. Soc. 2005, 127, 8067-8076.

10.1021/la701657r CCC: $37.00 © 2007 American Chemical Society Published on Web 10/27/2007

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Nikitin et al. Chart 1

Scheme 1. Synthesis of Monopodal Viologensa

a

Reagents and conditions: (i) KSAc, THF, 60 °C, 6 h; (ii) benzonitrile, 100 °C, 6 h; (iii) AcCl, MeOH, 60 °C, 6 h.

C3-symmetrical (“tripodal”) molecular scaffold bearing three terminal anchoring groups. Since the footprint area of the tripodal scaffold determines the respective surface coverage, the dimensions of the tripodal moiety were similar to those of the previously described tripods.1,6 The linkage between the terminal anchoring groups and the rigid tripodal scaffold has to be flexible to avoid possible registry limitations at the adsorption stage.1b Thiol and thioacetate terminal anchoring groups have been chosen, since they have been most extensively studied on the surface of gold.8 Moreover, these functional groups can be conveniently introduced and interconverted.6,8 Although it is well-known that thiol anchoring groups provide the formation of self-assembled monolayers (SAMs) on gold,8 the significance of SAMs on gold motivated the search for new linker groups suitable for gold surfaces.2,8b,9 In their extensive study of the adsorption of tripodal porphyrin derivatives on gold, Lindsey and co-workers1b,6 demonstrated that the thioacetate group may undergo deacylation-type cleavage on the surface. It was observed that thioacetates and free thiols afford products that bound identically to the surface of metal. In aiming to understand if thiol or thioacetate is the most appropriate terminal anchoring group suitable to tether electrochemically active tripodal [2]rotaxanes to gold, we synthesized and characterized model compounds, 1a and 1b, comprising respective monopodal linkers. The SAMs of viologens on gold have previously been prepared using unprotected thiol groups as linkers.11 The application of a more chemically inert thioacetate terminal group is particularly appealing in the case of electrochemically active viologens and related [2]rotaxanes, since these compounds can possibly facilitate the oxidation of free thiol functionalities, leading to the formation of disulfides.10,11a,b In the case of tripodal thiol linkers, a possible oxidation of the unprotected thiols would imply the coupling of

tripodal moieties in a “leg-to-leg” fashion which would make their upright orientation on the surface unfeasible. Consequently, care has to be taken to avoid possible oxidation of the thiols before and during SAM preparation.12 The electrochemical behavior and stability of the model monopodal thioaceatate 1a and thiol 1b were studied to compare thioacetate and thiol anchoring groups. The compounds 1a and 1b consist of a phenylviologen moiety and a benzylic thioacetate linker group (Chart 1). The linker, 4, was prepared by reacting dibromo-p-xylene with 1 equiv of potassium thioacetate (Scheme 1). The linker was subsequently treated with 4-phenyl-1,1′bipyridinuim chloride 5. Since the pyridine nitrogen in 5 is a relatively weak nucleophile, this reaction is very slow under ambient conditions. At 100 °C, however, it leads to the formation of an equilibrium amount of the viologen 1a. This product was separated by column chromatography on silica using gradient elution with aqueous ammonium chloride solution, after which the pure product 1a was separated from the excess ammonium chloride by extraction with nitromethane. The methanolysis of (8) (a) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (b) Poirier, G. E. Chem. ReV. 1997, 97, 1117-1127. (c) Yang, G.; Liu, G. J. Phys. Chem. B 2003, 107, 8746-8759. (d) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1-68. (e) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (9) Zhao, Y.; Perez-Segarra, W.; Shi, Q.; Wei, A. J. Am. Chem. Soc. 2005, 127, 7328-7329. (10) (a) Tang, X. Y.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 5921-5933. (b) Tang, X. Y.; Schneider, T.; Buttry, D. A. Langmuir 1994, 10, 2235-2240. (11) (a) Gittins, D. I.; Bethell, D.; Nichols, R. J.; Schiffrin, D. J. J. Mater. Chem. 2000, 10, 79-83. (b) Sagara, T.; Maeda, H.; Yuan, Y.; Nakashima, N. Langmuir 1999, 15, 3823-3830. (c) Widrig, C. A.; Majda, M. Langmuir 1989, 5, 689-695. (d) Tang, X.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 5921-5933. (12) (a) Raymo, F. M.; Alvarado, R. J.; Pacsial, E. J.; Alexander, D. J. Phys. Chem. B 2004, 108, 8622-8625. (b) Alvarado, R. J; Mukherjee, J.; Pacsial, E. J.; Alexander, D.; Raymo, F. M. J. Phys. Chem. B 2005, 109, 6164-6173.

Tripodal [2]Rotoxane on Gold

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Table 1. Conversion of the Thioacetate 1a into the Thiol 1ba run

T (°C)

HCl (M)

t1/2 (h)

t95% (h)

1 2 3

20 20 60

0.1 1.0 0.2

110 4.0 1.4

490 17 6.0

a Reactions were run in CD3OD using CH3COCl as the source of HCl.

Scheme 2. Synthesis of the Tripodal [2]Rotaxane 3b and Axle 2ba

Figure 1. Molecular structure of the tripodal alcohol 8.

a Reagents and conditions: (i) BuLi, Ether, -75 °C then ethyl 4-bromobenzoate, 0 °C: (ii) 4-(hydroxymethyl)-phenylboronic acid, palladium catalyst, ethanol, reflux; (iii) Lucas reagent, 60 °C, 20 min; (iv) KSAc, THF, 3 h; (v) PhNH3Cl, AcOH, 100 °C, 12 h; (vi) 1-[2,4-dinitrophenyl]-4,4′-dipyridyl chloride, ethanol, toluene, 85 °C; (vii) benzonitrile, room temperature, 1-4 d; (viii) AcCl (0.1 mol‚dm-3), MeOH/acetone, 55 °C, 1 d.

the thioacetate 1a leading to 1b under acidic conditions was monitored by 1H NMR spectroscopy (Table 1).11 It was found that the thiol 1b can be prepared in a quantitative yield (run 3) using acetyl chloride as the source of acid under a nitrogen atmosphere at 60 °C. In the presence of air, the hydrolysis is accompanied by the formation of byproducts due to the possible viologen-catalyzed oxidation of thiol groups. The synthesis of the more complex tripodal axle 2 and rotaxane 3 is shown in Scheme 2. The adopted synthetic strategy involves (i) construction of the tripodal moiety, (ii) introduction of the thioacetate groups, (iii) alkylation of the pyridine nitrogens, and (iv) deprotection of the thiol groups. Initially, the three aromatic “legs” of the tripodal linker were attached to the tertiary carbon atom by addition of p-bromolithiobenzene to ethyl p-bromobenzoate.1c All three legs of the resulting alcohol 7 were extended using a palladium-catalyzed coupling with p-hydroxymethylphenylboronic acid to furnish the tetraalcohol, 8. The structure of the key intermediate alcohol 8 was characterized by X-ray crystallography, and it appears in Figure 1. The dimensions and footprint area of the tripodal axle 2 and [2]rotaxane 3 prepared using the alcohol 8 can be estimated on the basis of this structure. Although, due to the crystal packing effects,

the interatomic distances between the three chemically equivalent oxygen atoms in 8 are 16.1, 17.0, and 18.5 Å, it can be reasonably assumed that on average in solution or on the surface the tripods can be approximated as equilateral triangles with each side being 17.2 Å. Having successfully prepared and characterized the alcohol 8, we attempted to introduce good leaving groups at the tripodal benzylic positions. It was found that 8 can be quantitatively converted into the benzylic trichloride 9 by treatment with Lucas reagent at 60 °C in the absence of solvent. Noteworthy, the more reactive tertiary alcohol function of 8 is preserved in this reaction due to the formation of a stable deeply colored product. This material, presumably a tertiary carbocation salt, had not been isolated but instead was converted into 9 by treatment with water. The hydrolysis stage readily regenerates the tertiary alcohol function and leaves all three peripheral chloromethyl functions intact. These three benzylic positions were further converted into thioacetates by treatment with potassium thioacetate. The resulting thioaceatate 10 was coupled with aniline to furnish amine 11, followed by an aromatic nucleophilic substitution reaction yielding pyridinium monocation salt 12. The remaining nucleophilic nitrogen of the salt 12 was alkylated with 4-(2(2-phenoxyethoxy)-ethoxy)-benzylbromide, 13, yielding the tripodal axle 2a. Alternatively, the salt 12 was alkylated under the high concentration conditions1f with the bulky stopper bromide 14 (see Experimental section) in the presence of the crown ether 6 to furnish the tripodal [2]rotaxane 3a. The corresponding thiols 2b and [2]rotaxane 3b were prepared by acid-catalyzed methanolysis using acetyl chloride as the source of acid. Preparation and Characterization of Monolayers. The SAMs of the monopodal thioacetate 1a and thiol 1b were formed by immersion of gold electrodes for up to 25 h into a methanol solution of the above compounds (2.0 × 10-3 mol‚dm-3) under nitrogen. The electrochemical characterization of the SAMs was carried out by using a cyclic voltammetry technique. The resulting peak reduction and oxidation potentials are listed in Table 2. The cyclic voltammograms of 1a/Au and 1b/Au are shown in Figure 2. The cyclic voltammogram of 1a/Au displays broad reduction and oxidation peaks due, possibly, to desorption of the thioaceatate 1a in this case. The cyclic voltammogram of the thiol 1b/Au shows a well-resolved reduction peak at -0.592 V related to the V2+/V+• process. The oxidation peak of 1b/Au, -0.528 V, is also well-resolved, but the current intensity is lower

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Table 2. Electrochemical Characterization of Viologen Monolayersa system

V2+/V+• (V)

V+•/V2+ (V)

1b/Au 1b•6/Aub 2b/Au 2b•6/Aub 3b/Au

-0.592 -0.596 -0.570 -0.620 -0.635

-0.528 -0.530 -0.536 -0.536 -0.605

a Acetonitrile containing supporting electrolyte Bu4NClO4 (0.10 mol‚dm-3); scan rate 100 mV‚s-1; 25 °C. b Crown ether 6 (1.0 × 10-3 mol‚dm-3) was added to the supporting electrolyte solution.

Figure 3. Cyclic voltammograms of the tripodal axle 2b, tripodal hetero [2]pseudorotaxane 2b•6, and tripodal [2]rotaxane 3b on gold (100 mV‚s-1, 25 °C in acetonitrile containing 0.10 mol‚dm-3 n-Bu4NClO4).

Figure 2. Cyclic voltammograms of monopodal viologen thioaceatate 1a, thiol 1b, and 1b•6 on gold (100 mV‚s-1, 25 °C in acetonitrile containing 0.10 mol‚dm-3 n-Bu4NClO4).

than that of the oxidation peak. It possibly suggests that the redox process is quasi-reversible. Notably, the thioacetate group, -SAc, as a linker affords weakly adsorbed 1a/Au on the surface of gold, and the monolayer cannot be clearly detected by cyclic voltammetry. Under the same conditions, the thiol 1b forms a robust enough SAM and it can be detected by cyclic voltammetry at the scan rate of 100 mV‚s-1. An estimation of the surface coverage for 1b/Au was carried out using the charge involved in the reduction process of the viologen unit. Coulometric analysis indicates that the geometric area coverage is 1.1 × 10-10 mol‚cm-2 after 5 h of adsorption and reaches 2.7 × 10-10 mol‚cm-2 after 24 h. This latter value is substantially lower than the coverage observed earlier for alkyl- and mercaptoalkylviologens.11 The observed maximum coverage corresponds to the surface area of 0.74 nm2 (roughness 1.2) per molecule (1b), which is substantially larger than the area previously observed for the viologen monolayers11 (0.45 nm2), or than the area calculated on the basis of the structural dimensions of viologen moieties in the solid state (0.34 nm2). Consequently, the observed packing density of the SAMs on the gold surface is lower than can be expected for close packed brush-type SAMs with a normal orientation of the viologen moieties at the surface. This would imply that the adsorption of the aromatic viologen thiol 1b on gold involves possible multipoint binding, leading to a larger footprint of the adsorbed compound. Having established the formation of the 1b/Au SAM on gold, we investigated the possible formation of the hetero [2]pseudorotaxane 1b•6/Au on the surface.13 The formation of this [2]pseudorotaxane on the surface can be expected, since the viologens commonly have strong affinity to the crown ether 6 (13) Previously, the electrochemical studies of supramolecular systems incorporating viologen units and the crown ether 6 revealed that the first reduction process of the viologen is shifted to a more negative potential by 30 mV for a [2]pseudorotaxane and 80-100 mV for a [2]rotaxane system.1c

and form stable [2]pseudorotaxanes.14 Shown in Figure 2 is the cyclic voltammogram of 1b/Au in the presence of the crown ether 6 (1.0 × 10-3 mol‚dm-3). Since only a minor negative shift of the reduction and oxidation peaks is observed, it is suggested that the structure of the viologen monolayer does not significantly change upon the addition of the crown ether 6. The threading of 6 by the viologen moiety of 1b at the surface is hindered due to possible multipoint binding of 1b to gold as discussed above. It is expected that, in the case of the tripodal thiolated viologens, 2 and 3, an oriented SAM can be formed. It is noted that the “tripodal” effect has been previously observed at the surface of titanium oxide.1d,e The SAMs of the tripodal thiol 2b were formed by immersion of the gold electrodes for 25 h into a methanol solution of 2b (2 × 10-3 mol‚dm-3) under nitrogen. The cyclic voltammogram of 2b/Au is shown in Figure 3 (dotted line). It demonstrates a well-resolved reduction peak at -0.570 V and an oxidation peak at -0.536 V related to the V2+/V+• and V+•/ V2+ stages. Importantly, the thiol 2b forms robust SAMs and can be clearly detected by cyclic voltammetry at a scan rate of 100 mV‚s-1. Coulometric analysis indicates that the coverage is 0.7 × 10-10 mol‚cm-2 of the geometric area. Taking into account the typical roughness factor for vacuum-deposited gold films (1.2), this coverage corresponds to the surface area of ∼2.8 nm2 per molecule and is in reasonable agreement with the dimensions of the tripodal linker. This would imply that the adsorption of the viologen thiol 2b on gold involves, as expected, three-point binding of the tripod to the surface. The detailed mechanism of adsorption is, however, under further investigation. The formation of the hetero [2]pseudorotaxane 2b•6/Au was studied by cyclic voltammetry. Shown in Figure 3 (solid line) is the cyclic voltammogram of 2b/Au in the presence of the crown ether 6 (1.0 × 10-3 mol‚dm-3). Clearly, a decrease of the peak intensity and a negative shift of ∼40 mV can be observed, showing that, although the hetero [2]pseudorotaxane is formed, the monolayer is not sufficiently stable in the presence of the crown ether. A possible explanation is a concurrent adsorption of the crown ether 6 and replacement of the tripodal axle 2b.15 The cyclic voltammogram of the tripodal [2]rotaxane 3b/Au is shown in Figure 3 (dashed line). A well-resolved reduction (14) An equilibrium constant of 8.7 × 102 M-1 was determined in methanol at 25 °C for the threading of the crown ether 6 by the viologen 1a. (15) It has been reported that aliphatic ethers can desorb thiols attached to the surface of gold; see Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528-12536.

Tripodal [2]Rotoxane on Gold

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Experimental Section

Figure 4. Possible graphical representation of tripodal rotaxane packing on the surface.

peak at -0.635 V related to the V2+/V+• and an oxidation peak of 3b/Au at -0.605 V related to the V+/V2+• are observed. This shows that the tripodal [2]rotaxane 3b forms a SAM and can be clearly detected by cyclic voltammetry. An estimation of the surface coverage was carried out by the calculation of the charge involved in the first reduction process of the viologen unit. Coulometric analysis indicates that the coverage is 1.1 × 10-10 mol‚cm-2 of the geometric area. Taking into account the typical roughness factor for vacuum-deposited gold films (1.2), this coverage corresponds to the surface area of ∼2 nm2 per molecule. The coverage values observed for the rotaxane 3b suggest that the tripodal moieties are rather close-packed on the surface of gold. Figure 4 illustrates that a high-density packing of molecular tripods on the surface would imply apex-to-side orientation. Taking into account the average size of the tripod triangles, 17.2 Å, this type of packing leads to an average area per tripod of about 1.9 nm2, which is in reasonable agreement with the observed surface coverage density.

Conclusions The preparation and electrochemical behavior of thiolated monopodal and tripodal viologens and a thiolated tripodal [2]rotaxane have been described. It can be demonstrated that to form electrochemically detectable SAMs on gold the viologen substrate should be attached to the surface via an unprotected thiol group. However, even in this case, the monopodal thiol viologen 1b attached to the surface of gold does not thread the crown ether 6 to form [2]pseudorotaxane. This happens due, probably, to unfavorable orientation of the viologen moiety at the surface as a result of multipoint binding to the surface. An alternative interpretation is based on the relative nonavailability of the viologen moieties due to the close-packing on the surface. This alternative, however, is less likely, since the monopodal viologen 1b fails to form a hetero [2]pseudorotaxane even at a surface coverage of 1.1 × 10-10 mol‚cm-2, which is below the observed saturation limit of 2.7 ×10-10 mol‚cm-2. The tripodal viologens 2b and 3b form electrochemically detectable SAMs. The surface area occupied by tripodal viologen systems (2-3 nm2) is, as can be expected on the basis of their structural dimensions, substantially greater than that for monopodal viologens. Consequently, the viologen moieties are spatially separated from each other and are displaced from the surface. This effect can readily explain the formation of the [2]pseudorotaxane 2b•6/Au in the presence of the crown ether 6. The thiolated tripodal [2]rotaxane 3b can be electrochemically reversibly reduced at, as expected, more negative potentials than the corresponding tripodal axle 2b. However, the stability of the SAM formed by 3b is a subject of a more detailed study. The above findings open prospects of the design, preparation, and characterization of electrochemically controlled and potentially switchable [2]rotaxanes on the surfaces of gold and other metals.

Reagents and solvents were purchased from commercial sources and were used as received. All reactions were performed under nitrogen using glassware that was flame dried. Chromatographic separations were performed using silica (40-63 µm) in the specified solvent system. Melting points were estimated using a Gallenkamp melting point device and were not corrected. NMR spectra were recorded using a Varian Inova 300 or a Varian Inova 500 spectrometer at 25 °C. Mass spectra were run on a Micromass LCT spectrometer. Gold electrodes were prepared by metal vaporization under vacuum (2 × 10-5 Torr) using a LM 300 Coating system (VES). Indium tin oxide (ITO) glass (10 Ω, 0.5 µm thick, supplied by Glastron) was used as the conducting substrate to deposit a 150 nm thick layer. The geometrical area of the gold electrodes ranged from 2 to 3 cm2. Once such gold electrodes were prepared, they were immediately used. The SAMs were formed by immersion of the above gold electrodes for 25 h (unless otherwise specified) into methanolic solutions of the compounds 1-3 (2 × 10-3 mol‚dm-3). The electrodes were stored under nitrogen. All cyclic voltammograms were recorded on a Solartron SI 1287 potentiostat controlled by a LabView program running on a Macintosh Power PC at a scan rate of 100 mV‚s-1. The working electrode (WE) was one of the SAM modified electrodes whose preparation is reported above. The counter electrode (CE) was a platinum foil; the reference electrode (RE) was a nonaqueous Ag/Ag+ electrode with a fill solution consisting of 10 mM AgNO3 in the electrolyte solution (0.540 V vs NHE). The electrolyte solution consisted of 0.10 mol‚dm-3 tetrabutylammonium perchlorate, n-Bu4NClO4, in dry acetonitrile. The measurements were run under nitrogen. 2,4-Dinitrophenyl-4,4′-bipyridinium chloride; 1-phenyl-4,4′-bipyridinium chloride (5); bis-p-phenylene-34-crown-10 (6); 4-(2(2′-phenoxyethoxy)-ethoxy)-benzylbromide (13); and 4-(2-(2′-(4[tris-{4-t-butylphenyl}-methyl]-phenoxy)-ethoxy)-ethoxy)benzylbromide (14) were prepared as described else-where.1d-f,16 4: Thioacetic Acid S-(4-Bromomethylbenzyl) Ester. Tetrahydrofuran (THF) (60 cm3) was added to a mixture of dibromo-pxylene (0.93 g, 3.5 mmol) and potassium thiocetate (0.40 g, 3.5 mmol) under a nitrogen atmosphere, and the mixture was heated at 60 °C for 6 h. The solvent was removed under vacuum, and the residue was separated by column chromatography (cyclohexane 70%/ dichloromethane 30%). The product 4 (0.31 g, 34%) was isolated as a white solid. mp 78-79 °C. 1H NMR (CDCl3, 300 MHz) δ 7.32 (d, 2H, J ) 8.1 Hz), 7.26 (d, 2H, J ) 8.1 Hz), 4.47 (s, 2H), 4.10 (s, 2H), 2.35 (s, 3H). MS (ES) m/z 183 (M+ - SAc, 75%). Anal. Calcd for C10H11BrOS: C, 46.34; H, 4.28; Br, 30.83; S, 12.37. Found: C, 46.05; H, 4.26; Br, 30.97; S, 12.22. 1a: 1-Phenyl-1′-[4-(S-acetylthiomethyl)phenylmethyl]-4,4′-bipyridinium Chloride Bromide. 5 (0.22 g, 0.7 mmol) and 4 (0.26 g, 1.0 mmol) were dissolved in benzonitrile (1.7 cm3). The mixture was heated at 100 °C for 30 h, after which it was extracted with ethyl acetate/water to remove unreacted 4. The aqueous layer was retained and reduced to dryness under vacuum. The residue was taken up in methanol (2 cm3) and separated by column chromatography (aqueous 2 M NH4Cl 2%/acetone 20%/methanol 78% to elute 5; aqueous 2 M NH4Cl 4%/nitromethane 20%/methanol 76% to elute 1a). The product 1a was obtained as a dry mixture with NH4Cl; this mixture was extracted with nitromethane and filtered to remove NH4Cl. The product 1a (0.13 g, 29%) was isolated as a brown solid. 1H NMR (CD3OD, 300 MHz) δ 9.54 (d, 2H, J ) 6.8 Hz), 9.39 (d, 2H, J ) 6.7 Hz), 8.85 (d, 2H, J ) 6.8 Hz), 8.79 (d, 2H, J ) 6.6 Hz), 7.997.90 (m, 2H), 7.87-7.79 (m, 3H), 7.58 (d, 2H, J ) 8.2 Hz), 7.47 (d, 2H, J ) 8.2 Hz), 6.00 (s, 2H), 4.17 (s, 2H), 2.34 (s, 3H). MS (ES) m/z 411 (M+ - H - Br - Cl, 100%). Anal. Calcd for hydrate C26H26BrClN2O2S: C, 57.20; H, 4.80; Br, 14.64; Cl, 6.49; N, 5.13; O, 5.86; S, 5.87. Found: C, 57.80; H, 5.27; Br, 15.40; Cl, 6.80; N, 5.33; S, 6.48. 1b: 1-Phenyl-1′-[4-thiomethylphenylmethyl]-4,4′-bipyridinium Chloride Bromide. 1a (5.3 mg, 10 µmol) was dissolved in CD3OD (0.8 cm3), and the solution was degassed by repeated freezethaw cycles under vacuum. Acetyl chloride (12.3 mg) was added

12152 Langmuir, Vol. 23, No. 24, 2007 to the above solution under a nitrogen atmosphere, after which the mixture was heated at 60 °C for 6 h. The product 1b was obtained in a quantitative yield. 1H NMR (CD3OD, 300 MHz) δ 9.42 (d, 2H, J ) 7.0 Hz), 9.30 (d, 2H, J ) 7.0 Hz), 8.75 (d, 2H, J ) 7.0 Hz), 8.70 (d, 2H, J ) 6.8 Hz), 7.86-7.83 (m, 2H), 7.71-7.69 (m, 3H), 7.49 (d, 2H, J ) 8.1 Hz), 7.39 (d, 2H, J ) 8.3 Hz), 5.91 (s, 2H), 3.67 (s, 2H). 7: Tris-(4-bromophenyl)-methanol. To a solution of 1,4dibromobenzene (11.79 g, 50 mmol) in THF (70 cm3) diethyl ether (70 cm3) was added. The mixture was cooled to -75 °C, and n-butyllithium (20 cm3 of 2.5 M in hexane, 50 mmol) was added dropwise during 90 min. The mixture was stirred for 1 h, after which ethyl 4-bromobenzoate (3.47 g, 15 mmol) in THF (5 cm3) was added slowly. The mixture was allowed to warm to 0 °C (ice bath) and stirred for 2 h. Water (350 cm3) and 1 M HCl (150 cm3) were added, the mixture was extracted with diethyl ether, and the organic layer was washed with water, dried (MgSO4), and concentrated. The oily residue was cooled in a refrigerator (4 °C), and the product 7 (4.0 g, 53%) was filtered as a white solid. mp 126 °C. 1H NMR (CDCl3, 300 MHz) δ 7.45 (d, 6H, J ) 8.5), 7.12 (d, 6H, J ) 8.5), 2.69 (s, 1H). Anal. Calcd for C19H13Br3O: C, 45.91; H, 2.64; Br, 48.23. Found: C, 46.0; H, 2.71; Br, 48.06. 8: Tris-(4-(4-hydroxymethylphenyl)-phenyl)-methanol. A flask was charged with the bromide 7 (773 mg, 1.55 mmol), 4-(hydroxymethyl)phenyl-boronic acid (0.789 g, 5.2 mmol), K2CO3 (1.58 g, 11.5 mmol), and dichlorobis(tri-o-tolylphosphine)-palladium(II) (36 mg, 3 mol %), after which ethanol (18 cm3) and toluene (26 cm3) were added. The mixture was stirred at 85 °C for 18 h and then filtered, and the solvents were evaporated under vacuum. The residue was extracted with ethyl acetate, and the organic layer was washed with 5% aqueous Na2CO3. The extract was dried over MgSO4, and the solvent was evaporated under vacuum. The residue was taken up in a minimum volume of hot acetone and crystallized to give 0.26 g of 8. The mother liquor was purified by chromatography (acetone/ cyclohexane) to give 0.34 g of 8 (total yield 0.60 g, 61%) as a white solid. mp 170 °C. 1H NMR ((CD3)2CO, hemiacetal with (CD3)2CO, 300 MHz) δ 7.66 (d, 12H, J ) 8.3 Hz), 7.49 (d, 6H, J ) 8.6 Hz), 7.45 (d, 6H, J ) 8.1 Hz), 5.47 (s, 1H), 4.68 (d, 4H, J ) 5.3 Hz), 4.67 (s, 2H), 4.22 (t, 2H, J ) 5.7 Hz), 2.79 (s, 1H). Anal. Calcd for hemiacetal C43H40O5: C, 81.11; H, 6.33. Found: C, 80.84; H, 6.28. The X-ray quality crystals of the tetraalcohol 8 as the solvate with cyclohexane were grown from an acetonitrile solution by using a vapor diffusion technique using cyclohexane as the cosolvent. 9: Tris-(4-(4′-chloromethylphenyl)-phenyl)-methanol. To the alcohol 8 (230 mg, 0.36 mmol) Lucas reagent (25 cm3) was added, and the dark-purple mixture was stirred at 60 °C for 20 min, after which the mixture was diluted with dichloromethane (300 cm3). The solution was washed with water (two times, 100 cm3) and saturated aqueous sodium bicarbonate (50 cm3), dried (Na2SO4), and concentrated to give 9 (0.23 g, 100%) as a white solid. mp 158 °C. 1H NMR (300 MHz, CDCl ) δ 7.59 (d, 6H, J ) 7.0 Hz), 7.56 (d, 3 6H, J ) 7.0 Hz), 7.46 (d, 6H, J ) 7.0 Hz), 7.45 (d, 6H, J ) 7.0 Hz), 4.64 (s, 6H), 2.85 (s, 1H). Anal. Calcd for C40H31Cl3O: C, 75.77; H, 4.93; Cl, 16.77. Found: C, 75.64; H, 5.08; Cl, 16.40. 10: Tris-(4-(4′-acetylthiomethylphenyl)-phenyl)-methanol. The trichloride 9 (0.24 g, 0.38 mmol) was taken up into toluene (4 cm3), and then acetonitrile (6 cm3) and potassium thioacetate (0.32 g, 2.8 mmol) were added and the mixture was stirred for 4 h at ambient temperature. The reaction mixture was filtered and evaporated under reduced pressure to afford 10 (0.28 g, 99%) as a white solid. mp 80 °C. 1H NMR (300 MHz, CDCl3) δ 7.5 (m, 12H), 7.41 (d, 6H, J ) 9.0 Hz), 7.3441 (d, 6H, J ) 9.0 Hz), 4.15 (s, 6H), 2.36 (s, 9H). Anal. Calcd for C46H40S3O4: C, 73.37; H, 5.35; S, 12.78. Found: C, 73.03; H, 5.49; S, 12.79. 11: 4-[Tris-(4-(4′-acetylthiomethylphenyl)-phenyl)-methyl]aniline.The thioacetate 10 (0.040 g, 0.052 mmol) and aniline hydrochloride (0.080 g, 0.62 mmol) were heated in acetic acid (5 cm3) for 12 h. The mixture was concentrated under vacuum and extracted with ethyl acetate, and then the extract was washed with aqueous HCl (5 cm3, 0.01 M) and aqueous 5% Na2CO3 (5 cm3). The solvent was removed, and the residue was purified by chromatography

Nikitin et al. (ethyl acetate) to afford the amine 11 as a solid. mp 119 °C. 1H NMR (300 MHz, CDCl3) δ 7.6 (m, 12 H), 7.40 (d, 6H, J ) 8.0 Hz), 7.35 (d, 6H, J ) 8.0 Hz), 4.16 (s, 6H), 2.36 (s, 9H). Anal. Calcd for C52H45S3O3N: C, 75.42; H, 5.48; N, 1.69; S, 11.62. Found: C, 75.75; H, 5.46; N, 1.76; S, 12.00. 12: 1-{4-[Tris-(4′-(4′′-acetylthiomethylphenyl)-phenyl)-methyl]-phenyl}-4,4′-bipyridinium Hexafluorophospate. The amine 11 (0.102 g, 0.123 mmol) and 2,4-dinitrophenyl-4,4′-bipyridinium chloride (0.056 g, 0.18 mmol) were dissolved in a mixture of toluene (8 cm3) and ethanol (8 cm3). The mixture was heated at 85 °C for 24 h and then concentrated under vacuum, and the residue was extracted with dichloromethane (100 cm3) and washed with water. The organic layer was concentrated and purified by chromatography (2.5% saturated aqueous potassium hexafluorophosphate, 87.5% methanol, and 10% nitromethane). The respective fractions were concentrated, extracted with nitromethane, and washed with water. On evaporation, 12 (0.078 g, 57%) was obtained as a yellow solid. mp 145 °C (dec). 1H NMR (300 MHz, CDCl3) δ 9.56 (d, 2H, J ) 7.0 Hz), 8.91 (d, 2H, J ) 7.0 Hz), 8.84 (d, 2H, J ) 7.0 Hz), 8.09 (d, 2H, J ) 7.0 Hz), 8.02 (d, 2H, J ) 8.0 Hz), 7.81 (d, 2H, J ) 8.0 Hz), 7.68 (d, 2H, J ) 8.0 Hz), 7.64 (d, 6H, J ) 8.0 Hz), 7.48 (d, 2H, J ) 8.0 Hz), 7.42 (d, 2H, J ) 8.0 Hz), 4.18 (s, 6H), 2.35 (s, 9H). MS (m/z) 967 (M - PF6, 26%). Anal. Calcd for C62H51S3O3N2PF6: C, 66.89; H, 4.62; N, 2.52; S, 8.64. Found: C, 66.20; H, 4.89; N, 2.33; S, 8.08. 2a: 1-{4-[Tris-(4′-(4′′-acetylthiomethylphenyl)-phenyl)-methyl]-phenyl}-1′-[4-(2-(2′-phenoxyethoxy)-ethoxy)-benzyl]-4,4′bipyridinium Dichloride. The monocation 12 (22 mg, 20 µmol), the bromide 13 (15 mg, 50 µmol), and benzonitrile (0.2 cm3) were mixed and left at ambient temperature for 4 days. The mixture was purified by chromatography (methanol/nitromethane/2 M aqueous ammonium chloride, 80/18/2 by volume). The respective fractions were concentrated, extracted with nitromethane, and washed with water. On evaporation, 2a (30 mg, 100%) was obtained as a yellow solid. 1H NMR (300 MHz, CDCl3) δ 9.65 (d, 2H, J ) 7.0 Hz), 9.46 (d, 2H, J ) 7.0 Hz), 8.95 (d, 2H, J ) 7.0 Hz), 8.85 (d, 2H, J ) 7.0 Hz), 8.00 (d, 2H, J ) 8.0 Hz), 7.81 (d, 2H, J ) 8.0 Hz), 7.7 (m, 14H), 7.48 (d, 6H, J ) 8.0 Hz), 7.42 (d, 6H, J ) 8.0 Hz), 7.27 (m, 3H), 7.10 (d, 2H, J ) 8.0 Hz), 6.92 (d, 2H, J ) 8.0 Hz), 6.03 (s, 2H), 4.17 (s, 6H), 4.2 (m, 4H), 3.9 (m, 4H), 2.35 (s, 9H). MS (m/z) 1239 (M - 2Cl, 1%), 967 (7%), 271 (100%). 2b: 1-{4-[Tris-(4′-(4′′-thiomethylphenyl)-phenyl)-methyl]phenyl}-1′-[4-(2-(2′-phenoxyethoxy)-ethoxy)-benzyl]-4,4′-bipyridinium Dichloride. 2a (13.3 mg, 10 µmol) was dissolved in CD3OD (1 cm3), and the solution was degassed by triple repeated freezethaw cycles under vacuum. Acetyl chloride (7.7 mg) was added to the above solution under a nitrogen atmosphere, after which the mixture was heated at 60 °C for 25 h. The product 2b was obtained in a quantitative yield. 1H NMR (300 MHz, CDCl3) δ 9.65 (d, 2H, J ) 7.0 Hz), 9.46 (d, 2H, J ) 7.0 Hz), 8.95 (d, 2H, J ) 7.0 Hz), 8.85 (d, 2H, J ) 7.0 Hz), 8.00 (d, 2H, J ) 8.0 Hz), 7.81 (d, 2H, J ) 8.0 Hz), 7.7 (m, 14H), 7.48 (d, 2H, J ) 8.0 Hz), 7.42 (d, 2H, J ) 8.0 Hz), 7.27 (t, 1H, J ) 7.0 Hz), 7.10 (d, 2H, J ) 8.0 Hz), 6.92 (d, 2H, J ) 8.0 Hz), 6.03 (s, 2H), 4.2 (m, 4H), 3.9 (m, 4H), 3.8 (s, 6H). MS (m/z) 1169 ([M - Cl - H + Na]+, 7%). 3a: 1-{4-[Tris-(4′-(4′′-acetylthiomethylphenyl)-phenyl)-methyl]-phenyl}-1′-[4-(2-(2′-(4′-[tris-{4′′-t-butylphenyl}-methyl]-phenoxy)-ethoxy)-ethoxy)-benzyl]-4,4′-dipyridinium Bis-p-phenylene34-crown-10 [2]Rotaxane Dichloride. The monocation 12 (11.1 mg, 10 µmol), 14 (10 mg, 15 µmol), the crown ether 6 (10 mg, 18 µmol), and benzonitrile (0.06 cm3) were mixed, and the mixture was kept at ambient conditions for 1 day. The product was purified by chromatography (see 2a). The respective fractions were concentrated, extracted with nitromethane, and washed with water. On evaporation, 3a (9 mg, 36%) was obtained as a red solid. 1H NMR (300 MHz, CD3COCD3) δ 9.58 (d, 2H, J ) 7.0 Hz), 9.26 (d, 2H, J ) 7.0 Hz), 8.73 (d, 2H, J ) 7.0 Hz), 8.60 (d, 2H, J ) 7.0 Hz), 7.93 (m, 4H), 7.6 (m, 20H), 7.40 (d, 6H, J ) 8.0 Hz), 7.3 (m, 8H), 7.1 (m, 8H), 6.80 (d, 2H, J ) 8.0 Hz), 6.27 (s, 8H), 6.15 (s, 2H), 4.18 (s, 6H), 4.2 (m, 4H), 4.1 (m, 4H), 3.9 (m, 8H), 3.6 (m, 32H), 2.35 (s, 9H), 1.29 (s, 27H). MS (m/z) 1093.1 ([M - 2Cl]2+, 100%).

Tripodal [2]Rotoxane on Gold 3b: 1-{4-[Tris-(4′-(4′′-thiomethylphenyl)-phenyl)-methyl]phenyl}-1′-[4-(2-(2′-(4′-[tris-{4′′-t-butylphenyl}-methyl]-phenoxy)ethoxy)-ethoxy)-benzyl]-4,4′-bipyridinium Bis-p-phenylene-34crown-10 [2]Rotaxane Dichloride. 3a (24.8 mg, 10 µmol) was dissolved in a mixture of CD3OD (0.5 cm3) and CD3COCD3 (0.5 cm3), and the solution was degassed by triple repeated freeze-thaw cycles under vacuum. Acetyl chloride (7.7 mg) was added to the above solution under a nitrogen atmosphere, after which the mixture was heated at 60 °C for 25 h. The product 3b was obtained in a quantitative yield. 1H NMR (300 MHz, CD3CN) δ 9.50 (d, 2H, J ) 7.0 Hz), 9.18 (d, 2H, J ) 7.0 Hz), 8.5 (d, 2H, J ) 7.0 Hz), 8.35 (d, 2H, J ) 7.0 Hz), 7.9 (m, 4H), 7.7-7.5 (m, 20H), 7.30 (d, 6H), 7.15 (d, 2H), 7.1 (m, 12H), 7.05 (d, 2H, J ) 8.0 Hz), 6.76 (d, 2H, J ) 8.0 Hz), 6.24 (s, 8H), 6.05 (s, 2H), 4.2 (m, 4H), 4.1 (m, 4H), 3.9 (s, 6H), 3.7 (m, 8H), 3.6 (m, 24 H), 1.29 (s, 18H). MS (m/z) 1028 ([M - 2Cl]2+). Crystal data were collected using a Bruker SMART APEX CCD area detector diffractometer. A full sphere of the reciprocal space was scanned by phi-omega scans. Pseudo-empirical absorption correction based on redundant reflections was performed by using the program SADABS.17a The structure was solved by direct methods using SHELXS-97 and refined by full matrix least-squares on F2 for all data using SHELXL-97.17b,c Friedel opposites were merged.

Langmuir, Vol. 23, No. 24, 2007 12153 Hydrogen atoms attached to oxygen were located in the difference Fourier map and allowed to refine freely. All other hydrogen atoms were added at calculated positions and refined using a riding model. Their isotropic temperature factors were fixed to 1.2 times (1.5 times for methyl groups) the equivalent isotropic displacement parameters of the carbon atom that the H-atom is attached to. Anisotropic temperature factors were used for all non-hydrogen atoms. The Cambridge Crystallographic Data Centre contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from CCDC 648422 via www.ccdc. cam.ac.uk/data_request/cif.

Acknowledgment. This research was funded by Science Foundation Ireland and EU Marie Curie Host Development grant. The authors thank Dr. Helge Mu¨ller-Bunz of the Chemical Services Unit at University College Dublin for technical support. LA701657R (16) Coe, B. J.; Harris, J. A.; Harrington, L. J.; Jeffery, J. C.; Rees, L. H.; Houbrechts, S.; Persoons, A. Inorg. Chem. 1998, 37, 3391-3399. (17) (a) Sheldrick, G. M. SADABS; Bruker AXS Inc.: Madison, WI, 2000. (b) Sheldrick, G. M. SHELXS-97; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (c) Sheldrick, G. M. SHELXL-97-2; University of Go¨ttingen: Go¨ttingen, Germany, 1997.