Supramolecularly controlled surface activity of an amphiphilic ligand. Application to aqueous biphasic hydroformylation of higher olefins

Abstract

A biphasic catalytic system has been elaborated in which the amphiphilic species concentrations at the aqueous/organic interface could be thermocontrolled by the supramolecular interaction between β-cyclodextrins and the 1-(4-tert-butyl)benzyl-1-azonia-3,5-diaza-7-phosphaadamantyl ligand (1). The system proved efficient in Rh-catalyzed hydroformylations of higher olefins. An increase in the catalytic activity was observed without alteration in either the chemo- or the regio-selectivity. The main advantage of the cyclodextrin/1 couple lies in the rapid decantation of the biphasic system at the end of the reaction. This study represents the first example of thermoregulation of the surface activity of an amphiphilic phosphane by a cyclodextrin.

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Introduction

During the past thirty years, aqueous organometallic catalysis has proven to be an effective eco-friendly process especially through the Ruhrchemie/Rhône-Poulenc rhodium-catalyzed hydroformylation of lower olefins.^1,2 This process allows for transforming small water-soluble alkenes such as propene or butene into linear aldehydes under CO/H₂ pressure with a rhodium catalyst stabilized in water by the sodium salt of the trisulfonated triphenylphosphane (TPPTS) (ESI†). However, extension of this concept to higher olefins suffers from their low solubility in water. To circumvent this problem, several strategies have been implemented using co-solvents,^3,4surfactants,^5–15 amphiphilic phosphanes,^16–22 molecular receptors,^23–25polymers²⁶ or dispersed particles.²⁷ The use of amphiphilic phosphanes appears to be especially promising as they combine at the same time the surface activity properties and the coordination ability to metals. However, although beneficial in terms of catalytic performances, their surface activity hampered the decantation process of the biphasic system at the end of the reaction due to emulsion formation.

For instance, we previously reported that the N-benzyl derivative of 1,3,5-triaza-7-phosphaadamantane (=N-Bz-PTA, Scheme 1)²⁸ significantly improved the catalytic activity in a rhodium-catalyzed hydroformylation of terminal alkenes due to its surface-active character.²⁹ However, stable emulsions were often obtained, giving N-Bz-PTA a limited interest.

Scheme 1 N-Benzyl PTA ammonium salt.

In this context, a new approach has been developed that solves the emulsion problem. We have imagined that the amphiphilic character of a phosphane could be masked or revealed depending on the temperature by addition in the medium of a water-soluble supramolecular receptor (Scheme 2). More precisely, we anticipated that total or partial inclusion of the hydrophobic part of an amphiphilic phosphane into the hydrophobic cavity of a supramolecular receptor could be thermocontrolled. Knowing that the proportion of phosphane included in the receptor should decrease when increasing the temperature, a higher concentration of an amphiphilic phosphane at the aqueous/organic interface is expected at high temperature with beneficial effects on the catalytic performances. In contrast, decreasing the temperature should lead to a lower proportion of phosphane at the interface resulting in an easier decantation between the aqueous and organic phases.

Scheme 2 The principle of thermocontrolled catalysis using a receptor/phosphane couple. P: amphiphilic phosphane. Cat.: organometallic catalyst.

The proof of concept was implemented using native β-cyclodextrin (β-CD) or randomly methylated β-cyclodextrin (RAME-β-CD) as a supramolecular receptor and the PTA-based amphiphilic phosphane 1 capable of interacting with the β-CD cavity (Scheme 3). The consequences of the CD/phosphane interaction have been evaluated in a rhodium-catalyzed hydroformylation of higher olefins.

Scheme 3 Cyclodextrins and the water-soluble ligand 1.

We show below that the thermocontrol of the CD/phosphane interaction can advantageously improve both the phase separation and the catalytic activity without affecting either the chemo- or regio-selectivity of the hydroformylation reaction.

Results and discussion

The synthesis of a CD-interacting phosphane based on the PTA structure involves the attachment on one of the PTA-nitrogens of a substituent capable of being included into the β-CD cavity. As our previous works clearly showed that a benzyl group was not appropriate as a nitrogen substituent to strongly interact with β-CDs,²⁹ we decided to graft a tert-butylbenzyl substituent on a nitrogen. Actually, tert-butylbenzyl groups are well recognized by the β-CD cavity due to the size adequacy between the guest and the CD inner walls.³⁰ The target molecule 1 (Scheme 1) was conveniently prepared in one step from PTA and 4-tert-butylbenzyl bromide following a method described in the literature with slight modification.³¹ The new phosphane 1 was isolated in good yield (88%) and gave analytical and spectroscopic data consistent with its structure.

Surface-tension measurements revealed the surface active character of 1. Indeed, a regular decrease in the interfacial tension γ was observed when increasing the concentration (Fig. 1), indicative of the adsorption of 1 at the air/water interface. Its behaviour was similar to that of N-Bz-PTA. No critical micellar concentration (cmc) could be detected in the studied concentration range. The insolubility of 1 beyond 25 mM at room temperature prevented getting information on its surface activity at higher concentration. Indeed, solid particles were observed in water above this concentration. However, the impact of 1 on γ at low concentrations clearly illustrated its ability to adsorb at the aqueous/air interface. Interestingly, addition of RAME-β-CD (1 equiv.) led to a higher water solubility of 1 (53 mM). Moreover, addition of RAME-β-CD onto an aqueous solution of 1 (50 mM) resulted in an increase in γ to an intermediate value (57.2 mN m⁻¹) between those of pure 1 (54.6 mN m⁻¹) and pure RAME-β-CD (58.0 mN m⁻¹). The effect was even more marked with addition of excess native β-CD onto an aqueous solution of 1 (20 mM) as the surface tension value of pure native β-CD was recovered (72 mN m⁻¹). These observations demonstrated that RAME-β-CD and the native β-CD can conceal the amphiphilic character of 1. Note that no effect on the surface tension was noticed when RAME-β-CD was added onto an aqueous solution of N-Bz-PTA, thus highlighting the essential role of the tert-Bu group. The decrease in the surface activity of 1 in the presence of β-CD or RAME-β-CD was attributed to the formation of inclusion complexes between 1 and CD.

Fig. 1 Surface tension of N-Bz-PTA and 1 in water at 22 °C.

The stoichiometry of these inclusion complexes has been determined by NMR at room temperature and 80 °C (temperature of the catalytic experiments) using the method of continuous variation (Job plot).³² For each complex and regardless of the temperature, the symmetrical curves (host/(host + guest) = 0.5) were indicative of a 1 : 1 stoichiometry. As an example, the Job plot derived from the tert-butyl protons variation for native β-CD/1 mixtures is depicted in Fig. 2. The association constant was determined by isothermal titration calorimetry (ITC, see Experimental) at 80 °C, the lowest temperature used in catalysis.

Fig. 2 Continuous variation plot (Job plot) derived from chemical shift variations of the tert-Bu signal on ¹H NMR spectra of β-CD/1 mixtures in D₂O at 25 °C. Δδ₍₁₎ = δ_{tBu obs.} − δ_{tBu init.}. The total concentration of species was 10 mM.

While no association constants could be measured between RAME-β-CD and N-Bz-PTA at 80 °C, the RAME-β-CD/1 and native β-CD/1 interactions remained strong (K_ass = 5440 and 4930 M⁻¹, respectively). Note that the RAME-β-CD/1 association constant determination at 100 °C and 120 °C (highest temperature used in catalysis) could not be performed for experimental reasons (ITC measurements require an aqueous phase under atmospheric pressure). 2D T-ROESY NMR experiments confirmed the recognition process between the CD “host” and the phosphane “guest”. Actually, at room temperature and 80 °C, correlations were detected in the 2D T-ROESY spectra of equimolar mixtures of RAME-β-CD or native β-CD and ligand 1. Cross-peaks between the tert-butyl protons and the H-3, H-5 and H-6 CD protons were indicative of a deep inclusion of the tert-butyl group in the CD cavity. As an example, the 2D T-ROESY spectrum of a native β-CD/1 equimolar mixture is depicted in Fig. 3.

Fig. 3 Partial 2D T-ROESY NMR spectrum (tBu protons) of an equimolar mixture of native β-CD and 1 at room temperature in D₂O (mixing time = 300 ms, species concentration = 10 mM).

The direction of inclusion of 1 (from the primary or secondary face of the CD) could be precisely determined as dipolar contacts could be detected between the aromatic protons and the H-3 CD protons, indicative of an inclusion from the secondary face (Fig. 4). From the T-ROESY experiment, it follows that the PTA group protruded beyond the CD cavities as no contact was detected between PTA protons and CD inner protons. The tert-Bu group appears to be essential for the recognition process to take place. Actually, no contact could be detected between native β-CD or RAME-β-CD and N-Bz-PTA. One can then easily understand why addition of these CDs did not affect the surface tension of an aqueous solution of N-Bz-PTA.

Fig. 4 Partial 2D T-ROESY NMR spectrum (aromatic protons) of an equimolar mixture of native β-CD and 1 at room temperature in D₂O (mixing time = 300 ms, species concentration = 10 mM).

Knowing that 1 has a strong affinity for the RAME-β-CD and native β-CD cavities, we then carried out catalytic experiments to evaluate the performances of 1 in hydroformylation of hydrophobic higher olefins. The reactions were performed at various temperatures under 50 bar CO/H₂ for 6 h. The results are gathered in Table 1. At 80 °C without CD, 1 appeared to be an effective ligand as the conversion was close to that obtained with N-Bz-PTA in hydroformylation of 1-decene (26 vs. 19%, respectively, entries 5 and 1). Note that 1 was much more effective than the TPPTS ligand for which a very low 3% conversion was measured under the same experimental conditions.²⁹

Table 1 Biphasic rhodium-catalyzed hydroformylation of higher olefins^a

Entry	Higher olefin	Phosphane	Cyclodextrin	T/°C	Conv.^b (%)	Chemoselectivity^c	l/b Ratio^d
a Experimental conditions: Rh(acac)(CO)2 (4.07 × 10^−2 mmol), water-soluble ligand (0.21 mmol), cyclodextrin (0.48 mmol), H2O (11.5 mL), 1-alkene (20.35 mmol), 1500 rpm, CO/H2 (1/1): 50 bar, 6 h. b Calculated with respect to the starting olefin. c (mol. of aldehydes)/(mol. of converted olefins) × 100. The side products were mainly isomeric olefins. d Ratio of the linear to branched aldehyde product. e Recycling of the aqueous phase of entry 9. f Recycling of the aqueous phase of entry 10. g This kinetic profile of this experiment is given in ESI.†
1	1-Decene	N-Bz-PTA	—	80	19	98	1.9
2	1-Decene	N-Bz-PTA	RAME-β-CD	80	24	98	1.9
3	1-Decene	N-Bz-PTA	—	100	77	99	1.8
4	1-Decene	N-Bz-PTA	RAME-β-CD	100	86	96	1.8
5^g	1-Decene	1	—	80	26	99	2.1
6^g	1-Decene	1	RAME-β-CD	80	10	97	1.8
7	1-Decene	1	Native β-CD	80	9	97	1.9
8^g	1-Decene	1	—	100	56	99	1.7
9^g	1-Decene	1	RAME-β-CD	100	72	99	2.0
10^e	1-Decene	1	RAME-β-CD	100	71	99	1.9
11^f	1-Decene	1	RAME-β-CD	100	66	97	1.9
12	1-Decene	1	Native β-CD	100	67	95	1.8
13	1-Decene	1	—	120	94	99	1.9
14	1-Decene	1	RAME-β-CD	120	98	99	1.8
15	1-Dodecene	1	—	80	39	96	1.5
16	1-Dodecene	1	RAME-β-CD	80	11	99	1.7
17	1-Dodecene	1	Native β-CD	80	8	98	1.7
18	1-Dodecene	1	—	100	49	98	1.5
19	1-Dodecene	1	RAME-β-CD	100	75	99	1.7
20	1-Dodecene	1	Native β-CD	100	71	98	1.6
21	1-Tetradecene	1	—	80	53	93	1.7
22	1-Tetradecene	1	RAME-β-CD	80	14	95	1.6
23	1-Tetradecene	1	Native β-CD	80	11	98	2.1
24	1-Tetradecene	1	—	100	57	97	1.5
25	1-Tetradecene	1	RAME-β-CD	100	88	97	1.9
26	1-Tetradecene	1	Native β-CD	100	66	98	1.7

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Increasing the temperature to 100 or 120 °C logically led to an increase in the conversion (entries 8 and 13). Also note that, the conversions being of the same order of magnitude whatever the substrate, the catalytic system is truly an interfacial process. Very different conversions would have been observed for a reaction in the bulk water phase due to the difference in higher olefins solubility. Astonishingly, at 80 °C, addition of excess RAME-β-CD led to a drop in catalytic activity (from 26 to 10% conv., entries 5 and 6). Similar results were obtained with 1-dodecene (from 39 to 11% conv., entries 15 and 16) and 1-tetradecene (from 53 to 14% conv., entries 21 and 22). Conversely, at 100 and 120 °C, addition of excess RAME-β-CD resulted in a marked increase in catalytic activity. At 100 °C, the conversion increased from 56 to 72% for 1-decene (entries 8 and 9), from 49 to 75% for 1-dodecene (entries 18 and 19) and from 57 to 88% for 1-tetradecene (entries 24 and 25). Thus, RAME-β-CD impacted upon the conversion either positively or negatively depending on the reaction temperature. In contrast, catalytic tests relative to the CD non-interacting ligand N-Bz-PTA showed a different behaviour. Indeed, an increase in the conversion was observed upon addition of RAME-β-CD whatever the temperature (entries 1–4).

The explanation of the above results lies in the strength of the RAME-β-CD/1 interaction depending on the temperature. At 80 °C, the surface active character of 1 and its adsorption ability at the aqueous/organic interface were masked due to inclusion of the hydrophobic part of 1 in the RAME-β-CD cavity (K_ass = 5440 M⁻¹).

Hence, most of the catalytically active species were stuck in aqueous solution. Only a few of these catalytic species were present at the aqueous/organic interface resulting in an activity decrease. In contrast, at 100 or 120 °C, the markedly higher activities were indicative of a change in the behaviour of the catalytic system components. Though the RAME-β-CD/1 association constant determination could not be performed at 100 °C (see above), it is well known that the association constant values decrease with the temperature.³³ Hence, at 100 °C, we postulate that the concentrations of the “free” ligand (non-included 1) increased at the aqueous/organic interface. The surface activity of 1 being revealed, an increase in the catalytic activity at the aqueous/organic interface was then observed. Additionally, the RAME-β-CD/1 dissociation also contributed to increase the “free” RAME-β-CD concentration. More “free” RAME-β-CDs were then available to promote the mass transfer by inclusion of higher olefins in their cavity,²³ thus participating in the catalytic activity enhancement (Scheme 2).

The implication of both the ligand surface activity and RAME-β-CD properties has been indirectly confirmed when replacing RAME-β-CD by the native β-CD. At 80 °C, the native β-CD/1 association constant (K_ass = 4930 M⁻¹) was similar to that of RAME-β-CD. Hence, as described above for RAME-β-CD, the native β-CD could act as an inhibitor of the ligand surface activity. However, the native β-CD could not be considered as a mass transfer promoter as it did not adsorb at the aqueous/organic interface.³⁴ Consequently, both these features could be translated in terms of catalytic activity. While only a slight difference in conversion was noticed between the native β-CD and RAME-β-CD at 80 °C (11 vs. 14% conv. for 1-tetradecene for example), a significant variation was measured at 100 °C (66 vs. 88% conv.). The 22% conv. gap between both CDs at 100 °C was relative to the mass transfer contribution of RAME-β-CD.

Besides catalytic aspects, the interest of the RAME-β-CD/1 couple lies in the easy recovery of both the aqueous and the organic phases after 6 h reaction time. While an emulsion was observed for the RAME-β-CD/N-Bz-PTA couple due to the impossibility to conceal the surface activity of N-Bz-PTA by the formation of an inclusion complex, a clear phase separation was obtained for 1. More precisely, the decantation was about hours with N-Bz-PTA and about seconds with 1. In fact, decreasing the temperature at the end of the reaction led to inclusion of 1 in the RAME-β-CD cavity (Scheme 4). The subsequent loss in surface activity resulted in a rapid decantation. Thus, by supramolecularly controlling the surface activity, the products and the rhodium catalyst can be easily recovered in two distinct phases. Indeed, the aqueous phase has been recycled twice in hydroformylation of 1-decene in the presence of RAME-β-CD with no significant loss either in conversion or selectivities (entries 10 and 11).

Scheme 4 CD-based thermocontrolled hydroformylation of higher olefins.

The chemoselectivity of the reaction, for its part, was independent of the length of the terminal alkene. Actually, increasing the length of the hydrocarbon chain from ten to fourteen carbons did not significantly modify the aldehydes proportions. A similar trend was observed for the regioselectivity as no significant variation in the l/b ratio was observed whatever the olefin. Interestingly, note that the chemo- and regio-selectivities were independent of the presence or absence of RAME-β-CD suggesting that the catalytically active species proved to be stable under these conditions. This observation is opposite to what was previously observed with TPPTS.³⁵

Conclusions

In this study, a new application for β-CD has been highlighted. Besides its well-known ability to transfer substrates from the organic to the aqueous phases, β-CD appeared to be an effective additive to thermoregulate the interfacial surface activity by varying the amphiphilic species concentration at the aqueous/organic interface. Extension of the concept to other biphasic reactions is currently on-going.

Experimental

General

RAME-β-CD was purchased from Wacker Chemie GmbH and was used as received. RAMEB was of pharmaceutical grade (Cavasol® W7M Pharma) and its degree of substitution was equal to 1.7. PTA was prepared as previously described.³⁶ Other chemicals were purchased from Acros and Aldrich Chemicals in their highest purity. All solvents were used as supplied without further purification. Distilled water was used in all experiments.

NMR measurements

NMR spectra were recorded at 25 °C on Bruker DRX300 spectrometers operating at 300.13 MHz for ¹H nuclei, 75.47 MHz for ¹³C nuclei and 121.50 MHz for ³¹P nuclei. ³¹P{¹H} NMR spectra were recorded with an external reference (85% H₃PO₄). D₂O (99.92% isotopic purity) was purchased from Euriso-Top. The NMR measurements for the Job plot were taken on 11 samples. The series of samples containing a variable ratio (from 0 to 1) of CD and 1 was prepared keeping the total concentration of species constant (10 mM in this present case). The differences of the chemical shift in ¹H NMR spectra were measured as a function of the molar ratio. The 2D T-ROESY experiments were run using the software supplied by Bruker. T-ROESY experiments were preferred to classical ROESY experiments as this sequence provides reliable dipolar cross-peaks with a minimal contribution of scalar transfer. Mixing times for T-ROESY experiments were set at 300 ms. Data matrix for the T-ROESY was made of 512 free induction decays, 1 K points each, resulting from the co-addition of 32 scans. The real resolution was 1.5–6.0 Hz per point in F2 and F1 dimensions, respectively. They were transformed in the nonphase-sensitive mode after QSINE window processing.

Synthesis of the water-soluble phosphane

1-(4-tert-Butyl)benzyl-1-azonia-3,5-diaza-7-phosphaadamantyl bromide) (1) was prepared as follows: PTA (860 mg, 5.5 mmol) was dissolved upon heating at 70 °C under nitrogen in 200 mL freshly distilled benzene. A solution of 4-(tert-butyl)benzyl bromide (1.25 g, 5.5 mmol) dissolved in benzene (1 mL) was then added dropwise. A white precipitate was instantaneously formed. After stirring for 30 min at 50 °C, the precipitate was filtered under nitrogen and washed with petroleum ether (50 mL). The resulting white solid was dried under vacuum. Yield: 942 mg (88%). ³¹P{¹H} NMR (D₂O, 121.50 MHz, 25 °C): δ −83.09 ppm. ¹H NMR (300.13 MHz, D₂O): δ 7.50 (d, ³J_HH = 8.1 Hz, 2H, Ar); 7.33 (d, ³J_HH = 8.1 Hz, 2H, Ar); 4.81 (d, AB system, ¹J_AB = 11.8 Hz, 4H, NCH₂N); 4.48 (d, AB system, ¹J_AB = 13.7 Hz, 1H, NCH₂N); 4.31 (d, AB system, ¹J_AB = 13.7 Hz, 1H, NCH₂N); 4.10 (d, ¹J_HP = 6.2 Hz, 2H, PCH₂N⁺); 4.01 (s, 2H, PhCH₂N⁺); 3.85–3.80 (m, 2H, PCH₂N); 3.65–3.57 (dd, ABX system, ¹J_AB = 15.3 Hz, ¹J_HP = 8.9 Hz, 2H, PCH₂N); 1.19 (s, 9H, tert-CH₃). ¹³C{¹H} NMR (75.47 MHz, D₂O): δ 154.69 (s, Ar); 132.70 (s, Ar); 128.44 (s, Ar); 126.27 (s, Ar); 78.43 (s, PhCH₂N⁺); 69.29 (s, 2 × NCH₂N); 66.37 (s, NCH₂N); 52.66 (d, ¹J_CP = 33.3 Hz, PCH₂N⁺); 45.50 (d, ¹J_CP = 21.1 Hz, 2 × PCH₂N); 34.19 (s, C–Me₃); 30.27 (s, 3 × CH₃). MS (nESI⁺), m/z: 304.17 (100) [M⁺]. Anal. calcd for C₁₇H₂₇N₃BrP (384.29 gmol⁻¹). Found (calc.): C, 53.58 (53.13); H, 7.16 (7.08); N, 10.82 (10.93)%.

Determination of association constants by isothermal titration calorimetry (ITC)

An isothermal calorimeter (ITC₂₀₀, MicroCal Inc., USA) was used for determining simultaneously the formation constant and the inclusion enthalpy and entropy of the studied complexes. The titration protocol was used. 204.5 μL of degassed aqueous solution of 1 (phosphate buffer, pH 6.2) was titrated with the CD solution (same buffer) in a 40 μL syringe. Concentrations of CD and 1 in stock solutions were respectively equal to 2.5 mM and 0.125 mM. Each titration was realised at 343 K in order to be as close as possible to the catalytic conditions. Such a temperature also implies a perfect homogeneous solution of 1. After a prior injection of 0.4 μL, 10 aliquots of 3.7 μL of CD solution were delivered over 7.4 s and the corresponding heat flow was recorded as a function of time. The time interval between two consecutive injections was 100 s and the agitation speed was 1000 rpm for all experiments. In addition, the heat effects due to dilution of CD were corrected for by performing blank titrations. The areas under the peak following each injection (obtained by integration of the raw signal) were then expressed as the heat effect per mole of added CD. Stoichiometries, binding constants and inclusion enthalpies were finally determined by nonlinear regression analysis of the binding isotherms, using built-in binding models within MicroCal Origin 7.0 software package (MicroCal, Northampton, MA). Each titration experiment was performed three times to ensure reproducibility of the results. Formation constants were then evaluated at different temperatures by means of the van't Hoff equation, with the assumption that inclusion enthalpy and entropy are constant over the temperature scale.

Surface-tension measurements

The interfacial tension measurements were performed using a KSV Instruments digital tensiometer (Sigma 70) with a platinum plate. The precision of the force transducer of the surface tension apparatus was 0.1 mN m⁻¹. The experiments were performed at 25 °C ± 0.5 °C controlled by a thermostated bath Lauda (RC6 CS). The samples were freshly prepared by dissolving the desired amount of the ligand in ultra pure water (Fresenius Kabi, France).

Catalytic experiments

All catalytic reactions were performed under nitrogen using standard Schlenk techniques. In a typical experiment, Rh(acac)(CO)₂ (0.04 mmol), the water-soluble ligand (0.21 mmol), and RAME-β-CD (0.48 mmol) were dissolved in 11.5 mL of water. The resulting aqueous phase and an organic phase composed of olefin (20.35 mmol) were charged under an atmosphere of N₂ into the 50 mL reactor, which was heated at the desired temperature. Mechanical stirring equipped with a multipaddle unit was then started (1500 rpm), and the autoclave was pressurized with 50 atm of CO/H₂ (1 : 1) from a gas reservoir connected to the reactor through a high-pressure regulator valve allowing to keep constant the pressure in the reactor throughout the whole reaction. The reaction medium was sampled once the reaction was complete for GC analyses of the organic phase after decantation. Note that RAME-β-CD has been preferred to the native β-CD (Scheme 1) for its better adsorption ability at the organic/aqueous interface.³⁷

Acknowledgements

This work was supported by the Centre National de la Recherche Scientifique (CNRS). NS is grateful to the Ministère de l'Enseignement Supérieur et de la Recherche for financial support (2008–2011). FH and LG thank CNRS and CNR for the bilateral CNR-CNRS agreement 2010–2011. We thank Grégory Crowyn for NMR analyses. Roquette Frères (Lestrem, France) is gratefully acknowledged for generous gifts of cyclodextrins. Ente Cassa di Risparmio di Firenze (ECRF) is thanked for support through FIRENZE HYDROLAB project. COST Action CM0802 “PhoSciNet” is also thanked for additional support.

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Footnote

† Electronic supplementary information (ESI) available: Job plots and 2D T-ROESY spectra of β-CD/1 and RAME-β-CD/1 inclusion complexes at various temperatures. See DOI: 10.1039/c1cy00156f

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