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Mar 16, 2012 - The alkyl chains can either point away from the metallaprism or hide in the hydrophobic environments of the metallaprism. 2D 1H–13C H...
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Physical and Physicochemical Stimuli-Responsive Arene Ruthenium Metallaprism Mona A. Furrer,† Julien Furrer,‡ and Bruno Therrien*,† †

Institute of Chemistry, University of Neuchatel, Avenue de Bellevaux 51, CH-2000 Neuchâtel, Switzerland Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland

‡

S Supporting Information *

ABSTRACT: A stimuli-responsive metallaprism composed of six p-cymene ruthenium corners bridged by 3-undecyl-1,4benzoquinonato-2,5-diolato ligands and connected by triangular 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine panels has been synthesized and characterized. In solution, the nature of the solvent and the temperature dictate the conformation of the metallaprism. The alkyl chains can either point away from the metallaprism or hide in the hydrophobic environments of the metallaprism.

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INTRODUCTION Metalla-assemblies obtained from arene ruthenium building blocks are becoming extremely popular.1 Following the preparation 10 years ago by Severin and his co-workers of arene ruthenium metallacycles able to selectively complex Li+ in water,2 several other applications have been found in the meantime. Recently, Mukherjee, Chi, and Stang have designed a series of arene ruthenium metallaprisms with good affinity for nitroaromatic compounds,3 which confirms the potential of using arene ruthenium metalla-assemblies as sensors. Exploiting the large cavity of a conformationally flexible metallaprism, Severin has also shown that distinct structures can be obtained for an adaptable metalla-assembly by simply adding an appropriate guest molecule.4 The host−guest properties of analogous water-soluble arene ruthenium assemblies were also exploited to deliver guest molecules to cancer cells.5 Various guest molecules have been encapsulated in the hydrophobic cavity of arene ruthenium metalla-assemblies, and the host− guest systems were used as drug delivery vectors of bioconjugated derivatives,6 metal complexes,7 and photosensitizers.8 In addition, a structural reorganization in solution between tetra- and octanuclear arene ruthenium assemblies has been recently observed.9 The stimuli-responsive process was possible thanks to the introduction of solvent-responsive metallacrown recognition units within the structure of the arene ruthenium metalla-assemblies. In this note, a new cationic arene ruthenium metallaprism, [(η6-p-cymene)6Ru6(μ3-tpt-κN)2(μ-C6HRO4-κO)3]6+ ([1]6+) (tpt = 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine, R = −(CH2)10CH3), with amphiphilic character is presented. The metallaprism is composed of three dinuclear p-cymene ruthenium clips appended with an undecyl chain, {(η6-pcymene)2Ru2(μ-C6HRO4-κO)}2+, and connected by two tpt panels. The introduction of alkyl chains in the proximity of the hydrophobic cavity of the cationic metallaprism offers, when needed, a lipophilic environment for the alkyl chains to retract. © 2012 American Chemical Society

Consequently, the conformation of the metallaprism can adjust in solution according to the polarity of the solvent or by changing the temperature of the solvent, thus giving rise to stimuli-responsive arene ruthenium metalla-assembly.

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RESULTS AND DISCUSSION The synthesis of [(η6-p-cymene)6Ru6(μ3-tpt-κN)2(μ-C6HRO4κO)3]6+ is performed in two steps (see the Experimental Section). First, 2 equiv of silver triflate are used to remove the chlorine atoms of the novel dinuclear complex (η6-pcymene)2Ru2(μ-C6HRO4-κO)Cl2. Then, the coordinatively unsaturated intermediate obtained in the first step (not isolated) reacts with the tpt ligands to afford metallaprism [1]6+ in excellent yield (Scheme 1). The hexacationic metallaassembly is isolated as its triflate salt [1][CF3SO3]6 and characterized by IR, NMR, and ESI-MS as well as by elemental analysis. The presence of the undecyl chain on the hydroxybenzoquinonato bridging ligand allows the formation of two isomers for metallaprism [1]6+, a symmetrical isomer [1a]6+ and an unsymmetrical isomer [1b]6+ in which two undecyl chains are sharing the same quadrant (Figure 1). Because of steric hindrance, these two undecyl chains are forced to point away from each other, upward or downward from the plane of the metallaprism and consequently further reducing the symmetry of the system. On the other hand, the undecyl chains in the symmetrical isomer [1a]6+ are all perfectly equivalent. Despite the presence of two isomers, the 1H NMR spectrum of [1][CF3SO3]6 in CD2Cl2 shows a well-organized structure with a relatively simple set of signals (Figure 2A), exhibiting the expected resonances and chemical shifts for the metallaprism as well as for the undecyl chains. In particular, the resonances of the undecyl chains can be found between δ 0.87 and 2.38 (1H) Received: January 18, 2012 Published: March 16, 2012 3149

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Scheme 1.

Figure 1. Schematic representations of symmetrical isomer [1a]6+ (left) and unsymmetrical isomer [1b]6+ (right).

chemical shifts ranging between δ −0.30 and 0.78. This strong upfield shift can also be observed for the CH2 groups, whose resonances range from δ 0.54 and 2.50 (Figures 2E and S1E, Supporting Information). Interestingly, the CH2 group directly bonded to the hydroxybenzoquinonato moiety also split into four distinct resonances, but the upfield shift is much less pronounced compared to the other CH2 groups. Similar splitting and chemical shift of the proton signals are observed in CD 3 OD (Figure S2, Supporting Information) and in (CD3)2SO (Figure S3, Supporting Information). Taken together, these findings strongly suggest that in polar solvents, the chains retract inside or in the proximity of the lipophilic regions of the metallaprism. To further confirm the encapsulation of the undecyl chains within the hydrophobic cavity of the metallaprism in polar solvents, a 1D ROESY experiment in CD3CN with selective inversion of the Hα and Hβ protons of the tpt ligands has been measured (Figure S4, Supporting Information). In this spectrum, the spatial proximity of tpt and the alkyl chains is established. Indeed, noticeable cross-peaks are observed between the protons of the undecyl chains (CH3 and CH2)

as well as between δ 13.8 and 32.0 (13C), which are typical values for an alkyl chain bound to an aromatic moiety. Therefore, in CD2Cl2, the NMR spectrum suggests that the dangling undecyl chains are pointing away from the metallaprism. However, the three undecyl chains do not exhibit one single set of resonances, but at least three, as shown in Figure 3A for the methyl group. The occurrence of more than one set of resonances for the undecyl chains may be explained by the presence of various isomers. Similarly, the proton of the hydroxybenzoquinonato moieties shows four singlets centered at δ 5.76 (Figure 3B), which further confirms the presence of different isomers. On the other hand, the 1H NMR spectrum in CD3CN (Figure 2E) is much more complicated and displays numerous new resonances compared to the spectrum of [1][CF3SO3]6 recorded in CD2Cl2. A short analysis of this spectrum and of the 2D 1H−13C HSQC spectrum (Figure S1E, Supporting Information) reveals that the 1H resonances of the undecyl chains are strongly affected when the polarity of the solvent increases. In particular, the final methyl group centered at δ 0.87 in CD2Cl2 is now split into four distinct resonances, with 3150

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Figure 2. 500 MHz 1H NMR spectra of [1][CF3SO3]6 dissolved in CD2Cl2 (A), CD2Cl2/CD3CN (2:1) (B), CD2Cl2/CD3CN (1:1) (C), CD2Cl2/ CD3CN (1:2) (D), and CD3CN (E) recorded at 27 °C. The resonances at δ 0.00 in spectra B and D belong to TMS. Protons Hα and Hβ of the tpt ligands are indicated with ■, protons of p-cymene with Δ, the methyl group of the undecyl chain with ○, and the CH2 group directly bound to the dhbq moiety with *.

retract. Consequently, it is not surprising to observe several sets of signals. Accordingly, the undecyl chains retract gradually to the hydrophobic environment of [1][CF3SO3]6 when the polarity of the solvent is increased. As shown by Figures 2B−D and S1B−D (Supporting Information), the upfield shifts observed for the resonances of the undecyl chains increase gradually in concert with an increasing polarity of the solvent. The fact that an average chemical shift between the retracted undecyl chains and the free undecyl chains is observed indicates that at room temperature the exchange occurs at rates superior to those of the NMR time scale. As shown in Figure 5, the conformation of the metallaprism can also adjust in solution as a function of the temperature, thus responding to a physical stimulus. The undecyl chains spend more time in the lipophilic environment of [1][CF3SO3]6 when the temperature of the solvent is lowered. Indeed, the upfield shifts observed for the resonances of the undecyl chains decrease gradually in concert with a raise of the temperature. To gain further insight on the nature of [1][CF3SO3]6 in solution, we performed a series of diffusion ordered NMR spectroscopy (DOSY)10 measurements in CD2Cl2 and CD3CN and compared them with the DOSY measurements obtained with the hexanuclear metallaprism [(η6-p-cymene)6Ru6(μ3-tptκN)2(μ-C6H2O4-κO)3]6+ ([2]6+),7a in which the undecyl chain is replaced by a hydrogen. The DOSY spectra of [1][CF3SO3]6 and [2][CF3SO3]6 in CD2Cl2 and CD3CN are shown in Figure 6. Interestingly, in CD2Cl2 (Figure 6A), [1]6+ diffuses more rapidly than [2]6+, although [1]6+ is heavier than [2]6+. It seems therefore that the heavier metallaprism with the three undecyl chains pointing away from the core can diffuse more efficiently in CD2Cl2. In solution, substantial interactions between solute and solvent molecules occur; therefore, the Stokes’ radius may considerably vary from [1]6+ to [2]6+ in CD2Cl2. This phenomenon could to a certain extent explain why [1]6+ diffuses more rapidly than [2]6+ in CD2Cl2. In CD3CN (Figure 6B), on the other hand, [1]6+ diffuses slower than [2]6+, which

Figure 3. Excerpts of the 500 MHz 1H NMR spectrum of [1][CF3SO3]6 dissolved in CD2Cl2 recorded at 27 °C showing the methyl resonance of the undecyl chains (A) and the CH aromatic proton of the hydroxybenzoquinonato moieties, highlighted with an oval (B).

and the protons Hα and Hβ of the cage molecule (Figure S4, Supporting Information). Therefore, together with the strong upfield shift observed in the 1H NMR spectra (up to 2 ppm), the ROESY experiment confirms the encapsulation of the undecyl chains within the hydrophobic cavity of the metallaprism. The occurrence of four distinct resonances for the undecyl chains, which is nicely illustrated by the four methyl resonances in the 1H and 2D 1H−13C edited HSQC NMR spectra, suggests the existence of four species in solution. These four isomers are not observed with a 1/1/1/1 ratio, but with a ratio of 0.6/0.7/1/0.7 (from the highest to the lowest δ value). As previously mentioned, two structural isomers of [1]6+ are observed (Figures 1 and 4), and in polar solvent the undecyl chains tend to retract within the hydrophobic environment of the metallaprism. However, for the unsymmetrical isomer [1b]6+, two chains are competing for the same lipophilic region (Figure 4), thus giving rise to different species in which two (isomer [1b]6+) or three (isomer [1a]6+) undecyl chains 3151

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Figure 4. Chem3D models of [1a]6+ (left) and [1b]6+ (right), with the undecyl chains within the hydrophobic environment of the metallaprism (top) or pointing away (bottom). Synthesis of (η6-p-Cymene)2Ru2(μ-C6HRO4-κO)Cl2. A mixture of (η6p-cymene)2Ru2(μ-Cl)2Cl2 (200 mg, 0.326 mmol) and 3-undecyl2,5-dihydroxy-1,4-benzoquinone (96 mg, 0.326 mmol) in MeOH (30 mL) was stirred for 15 h at room temperature. The brown precipitate was filtered, washed with diethyl ether, and dried in vacuo. Yield: 230 mg (85%); IR ν (cm−1) = 3435 (w, C−Har), 2919 (s, C−H), 2849 (s, C−H), 1512 (s, CO); 1H NMR (500 MHz, CD2Cl2) δ (ppm) = 5.62−5.59 (m, 4H, CHar‑cym), 5.57 (s, 1H, CHq), 5.36−5.34 (m, 4H, CHar‑cym), 2.87 (sept, 2H, 3JH−H = 6.90 Hz, CH(CH3)2), 2.34 (m, 2H, CH2Cq), 2.25 (s, 6H, CH3Car‑cym), 1.35−1.29 (m, 12H, CH(CH3)2 + m, 18H, CH2(CH2)9CH3), 0.89 (t, 3H, 3JH−H = 6.9 Hz, CH2CH3); 13 C{1H} NMR (125 MHz, CD2Cl2) δ (ppm) = 184.53 (CO), 181.89 (CO), 115.46 (Cq), 101.61 (CHq), 99.92 (Car‑cym), 96.59 (Car‑cym), 82.53 (CHar‑cym), 81.63 (CHar‑cym), 80.07 (CHar‑cym), 79.31 (CHar‑cym), 32.39 (CH2), 31.80 (CH(CH3)2), 30.25 (CH2), 30.19 (CH2), 30.15 (CH2), 29.99 (CH2), 29.85 (CH2), 28.65 (CH2), 23.14 (CH2), 22.61 (CH2), 22.55 (CH(CH3)2), 22.40 (CH2), 18.73 (CH3Car‑cym), 14.31 (CH2CH3); ESI-MS 798.8 [M − Cl]+ and 857.7 [M + Na]+; UV− visible (1.0 × 10−5 M, CH2Cl2, 298 K) λmax 230 nm (ε = 40 550 M1 cm−1), λmax 317 nm (ε = 18 993 M1 cm−1), λmax 502 nm (ε = 24 092 M1 cm−1). Elemental analysis (%) Calc. for C37H52Ru2O4Cl2: C, 53.30; H, 6.29. Found: C, 53.40; H, 6.27. Synthesis of [(η6-p-Cymene)6Ru6(μ3-tpt-κN)2(μ-C6HRO4-κO)3][CF3SO3]6 ([1][CF3SO3]6). A mixture of AgCF3SO3 (82 mg, 0.32 mmol) and (η6-p-cymene)2Ru2(μ-C6HRO4-κO)Cl2 (133 mg, 0.16 mmol) in MeOH (30 mL) was stirred at room temperature for 3 h and then filtered. To the red filtrate, tpt (35 mg, 0.11 mmol) was added. The solution was stirred for 15 h at 60 °C, and the solvent was then removed in vacuo. The residue was dissolved in dichloromethane (3 mL), and diethyl ether was added to precipitate a red solid. [1][CF3SO3]6: yield 175 mg (87%); IR ν (cm−1) = 3499 (w, C−Har), 2924 (s, C−H), 2852 (s, C−H), 1575 (s, CCtpt), 1519 (s, CO), 1259 (s, CF3); 1H NMR (400 MHz, CD2Cl2) δ (ppm) = 8.69−8.67 (m, 4 × 12H, Hpyr), 8.50−8.48 (m, 4 × 12H, Hpyr), 5.87−5.85 (m, 4 ×

is in agreement with the molecular weight difference between [1]6+ and [2]6+ and the retraction of the undecyl chains within the metallaprism. In conclusion, an amphiphilic arene ruthenium metallaprism responding to temperature and to solvent polarity has been described. The presence of undecyl chains, together with the nature of the metallaprism, allow the alkyl chains to point away from or retract into the core of the metalla-assembly. This stimuli-responsive system is paving the way to the design of host metallaprisms with the controlled release of guest molecules, which is particularly interesting for drug delivery applications. Overall, these results confirm the versatility and utility of arene ruthenium building blocks in the design of novel materials.1

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EXPERIMENTAL SECTION

3-Undecyl-2,5-dihydroxy-1,4-benzoquinone was purchased from ABCR GmbH, while tpt11 and (η6-p-cymene)2Ru2(μ-Cl)2Cl212 were prepared according to published methods. The 1H and 13C{1H} NMR spectra were recorded on Bruker Avance II 500 or Bruker Avance II 400 spectrometers using the residual protonated solvent or TMS as internal standard. The selective 1D 1H ROESY experiment has been recorded using the 1D GROESY pulse sequence,13 which has shown its effectiveness for confirming encapsulation of aromatic guests within metallaprisms.14 The mixing time of the experiment has been kept short (100 ms) to avoid spin diffusion. Infrared spectra were recorded as KBr pellets on a Perkin-Elmer FTIR 1720 X spectrometer. Microanalyses were performed by the Laboratory of Organic Chemistry, ETH Zürich (Switzerland). Electro-spray mass spectra were obtained in positive-ion mode with an LCQ Finnigan mass spectrometer. UV−visible absorption spectra were recorded on an Uvikon 930 spectrophotometer (10−5 M in CH2Cl2). 3152

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CF3SO3]3+, 1754.8 [1 + 4 CF3SO3]2+; UV−visible (1.0 × 10−5 M, CH2Cl2, 298 K) λmax 231 nm (ε = 130 763 M1 cm−1), λmax 308 nm (ε = 68 954 M1 cm−1), λmax 493 nm (ε = 48 181 M1 cm−1). Elemental analysis (%) Calc. for C153H180F18Ru6N12O30S6·6H2O: C, 46.93; H, 4.94; N, 4.29. Found: C, 46.50; H, 4.90; N, 4.46.

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ASSOCIATED CONTENT

S Supporting Information *

2D 1H−13C HSQC spectrum (500 MHz, CD3CN) of [1][CF3SO3]6; 500 MHz 1H spectra of [1][CF3SO3]6 in CD3OD and in (CD3)2SO; ROESY 1H spectrum of [1][CF3SO3]6 in CD3CN; and ESI-MS spectrum of [1][CF3SO3]6. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +41-32-7182511. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A generous loan of ruthenium chloride hydrate from the Johnson Matthey Technology Centre is gratefully acknowledged, and we would like to thank the Swiss National Science Foundation for financial support (Grant 200020-129518).

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REFERENCES

(1) (a) Severin, K. Chem. Commun. 2006, 3859−3867. (b) Han, Y.F.; Jia, W.-G.; Yu, W.-B.; Jin, G.-X. Chem. Soc. Rev. 2009, 38, 3419− 3434. (c) Therrien, B. Eur. J. Inorg. Chem. 2009, 2445−2453. (d) Chakrabarty, R.; Mukherjee, P. S.; Stang, P. J. Chem. Rev. 2011, 111, 6810−6918. (2) (a) Piotrowski, H.; Polborn, K.; Hilt, G.; Severin, K. J. Am. Chem. Soc. 2001, 123, 2699−2700. (b) Grote, Z.; Lehaire, M.-L.; Scopelliti, R.; Severin, K. J. Am. Chem. Soc. 2003, 125, 13638−13639. (c) Grote, Z.; Scopelliti, R.; Severin, K. J. Am. Chem. Soc. 2004, 126, 16959− 16972. (3) Wang, M.; Vajpayee, V.; Shanmugaraju, S.; Zheng, Y.-R.; Zhao, Z.; Kim, H.; Mukherjee, P. S.; Chi, K. W.; Stang, P. J. Inorg. Chem. 2011, 50, 1506−1512. (4) Mirtschin, S.; Slabon-Turski, A.; Scopelliti, R.; Velders, A. H.; Severin, K. J. Am. Chem. Soc. 2010, 132, 14004−14005. (5) Therrien, B. Top. Curr. Chem. 2012, 319, 35−56. (6) (a) Zava, O.; Mattsson, J.; Therrien, B.; Dyson, P. J. Chem.Eur. J. 2010, 16, 1428−1431. (b) Mattson, J.; Govindaswamy, P.; Furrer, J.; Sei, Y.; Yamaguchi, K.; Süss-Fink, G.; Therrien, B. Organometallics 2008, 27, 4346−4356. (c) Mattsson, J.; Zava, O.; Renfrew, A. K.; Sei, Yamaguchi, K.; Dyson, P. J.; Therrien, B. Dalton Trans. 2010, 39,

Figure 5. Excerpts of the 500 MHz 1H NMR spectra showing the aliphatic part of [1][CF3SO3]6 dissolved in CD3CN recorded at 27 °C (A), 40 °C (B), 60 °C (C), and 80 °C (D). 6H, CHar‑cym), 5.83−5.81 (m, 4 × 6H, CHar‑cym), 5.77 (s, 3H, CHq), 5.76 (s, 3H, CHq) 5.75 (s, 3H, CHq) 5.74 (s, 3H, CHq), 5.74−5.71 (m, 4 × 6H, CHar‑cym), 5.67−5.66 (m, 4 × 6H, CHar‑cym), 2.87 (sept, 4 × 6H, 3JH−H = 6.90 Hz, CH(CH3)2), 2.36 (m, 4 × 6H, CH2Cq), 2.18 (s, 4 × 18H, CH3Car‑cym), 1.39−1.34 (m, 4 × 36H, CH(CH3)2), 1.49− 1.08 (m, 4 × 54H, CH2(CH2)9CH3), 0.88−0.81 (m, 4 × 9H, CH2CH3); 13C{1H} NMR (100 MHz, CD2Cl2) δ (ppm) = 184.78 (CO), 182.39 (CO), 170.21 (Ctpt), 154.42 (CHtpt), 144.89 (Ctpt), 125.42 (CHtpt), 122.90 (Cq), 119.81 (Cq), 104.92 (CCH(CH3)2), 104.89 (CCH(CH3)2), 101.36 (CHq), 101.31 (CHq), 99.33 (CCH3), 99.28 (CCH3), 84.21 (br, CHar‑cym), 83.58 (br, CHar‑cym), 82.72 (CHar‑cym), 82.66 (br, CHar‑cym), 82.59 (CHar‑cym), 82.49 (br, CHar‑cym), 32.40 (CH2), 31.81 (CH(CH3)2), 30.25 (CH2), 30.17 (CH2), 30.04 (CH2), 30.00 (CH2), 29.88 (CH2), 28.87 (CH2), 23.11 (CH2), 22.48 (CH(CH3)2), 22.39 (CH(CH3)2), 22.08 (CH2), 18.31 (CH3Car‑cym), 14.33 (CH2CH3), 14.32 (CH2CH3); ESI-MS 1120.2 [1 + 3

Figure 6. 500 MHz 2D 1H DOSY NMR spectra recorded at 27 °C of [1][CF3SO3]6 (blue cross peaks) and [2][CF3SO3]6 (black cross peaks) dissolved in CD2Cl2 (A) and CD3CN (B). 3153

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