Degenerate [2]rotaxanes with electrostatic barriers

Abstract

A synthetic approach to the preparation of [2]rotaxanes (1–5·6PF₆) incorporating bispyridinium derivatives and two 1,5-dioxynaphthalene (DNP) units situated in the rod portions of their dumbbell components that are encircled by a single cyclobis(paraquat-p-phenylene) tetracationic (CBPQT⁴⁺) ring has been developed. Since the π-electron-deficient bispyridinium units are introduced into the dumbbell components of the [2]rotaxanes 1–5·6PF₆, there are Coulombic charge–charge repulsions between these dicationic units and the CBPQT⁴⁺ ring in the [2]rotaxanes. Thus, the CBPQT⁴⁺ rings in the degenerate [2]rotaxanes exhibit slow shuttling between two DNP recognition sites on the ¹H NMR time-scale on account of the electrostatic barrier posed by the bispyridinium units, as demonstrated by variable-temperature ¹H NMR spectroscopy. Electrochemical experiments carried out on the [2]rotaxanes 1·6PF₆ and 2·6PF₆ indicate that the one-electron reduced bipyridinium radical cation in the dumbbell components of the [2]rotaxanes serves as an additional recognition site for the two-electron reduced CBPQT²⁽˙⁺⁾ diradical cationic ring. Under appropriate conditions, the ring components in the degenerate rotaxanes 1·6PF₆ and 2·6PF₆ can shuttle along the recognition sites – two DNP units and one-electron reduced bipyridinium radical cation – under redox control.

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Introduction

Molecular switches and machines^1–3 incorporating donor–acceptor [2]catenanes^1a–e and [2]rotaxanes^1a–e have been the focus of extensive experimental investigations in solution^1a,1c–e as well as in solid-state devices.^1b,3a,3e In particular, switchable [2]rotaxanes, containing π-electron-rich tetrathiafulvalene (TTF) and/or 1,5-dioxynaphthalene (DNP) recognition sites, located in the rod portions of their dumbbell components and encircled by a single cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) ring have been investigated^3a,3b,3e as leading candidates for expressing the relative intramolecular translational motion^1m between its dumbbell and ring components.

Fig. 1 (a) Structural formulae for the five degenerate [2]rotaxanes 1–5·6PF₆. (b) The two different kinds of dumbbell components present in these five degenerate [2]rotaxanes. In one set (1–3·6PF₆) of rotaxanes, the two positive charges are part of a conjugate aromatic system, while, in the other set (4·6PF₆ and 5·6PF₆), the charges are associated with separate aromatic systems.

The constitution of the spacers between the recognition sites on the dumbbell components of the degenerate [2]rotaxanes is an important factor governing the movement of the CBPQT⁴⁺ ring. Previously, we investigated the switching behaviour of the CBPQT⁴⁺ ring in degenerate [2]rotaxanes with spacers such as polyethylene glycol chains,⁴terphenyl units,⁵ rigid arylmethyl and butadiynyl rods,⁶ and azobenzene units.⁷ In the case of degenerate [2]rotaxanes^4c,6c,6d,7 containing two equivalent recognition sites – e.g., two DNP units – the CBPQT⁴⁺ ring shuttles back and forth between the two sites usually under the influence of heat. In a few instances,⁷ the rate of shuttling has been controlled by means of light when, for example, azobenzene units are the spacers. In the case of bistable [2]rotaxanes,^4,5 like those mentioned above, the CBPQT⁴⁺ ring can be induced to move between TTF and DNP units by means of chemical⁵ or electrochemical^4,5 stimuli. The bistability of the switchable [2]rotaxanes relies (Fig. 2) on the ability of the CBPQT⁴⁺ ring to encircle much more strongly the TTF unit in the so-called ground state co-conformation (GSCC) than it does the considerably less π-electron rich DNP unit in the metastable state co-conformation (MSCC) in the dumbbell components. Switching is achieved by the reversible oxidation of the TTF unit, firstly to the radical cation^4d,4e,8 (TTF⁺˙) and then to its dication (TTF²⁺), thus producing Coulombic charge–charge repulsion between the oxidised TTF⁺˙ or TTF²⁺ species and the CBPQT⁴⁺ ring, leading to the movement of the CBPQT⁴⁺ ring to the DNP unit. After reduction of the TTF⁺˙ radical cation or TTF²⁺ dication to their neutral form, the MSCC becomes populated, and the CBPQT⁴⁺ ring begins to migrate back onto the TTF recognition unit. The barrier to this relaxation process (ΔG^‡) is 3 kcal mol⁻¹ higher^10b in the presence of the bipyridinium unit (BIPY²⁺).

Fig. 2 Graphical representation of a rotaxane undergoing redox-stimulated switching employing TTF and DNP recognition units, with a bipyridinium unit (BIPY²⁺) in the middle of the dumbbell component as a “speed bump”. The GSCC and the MSCC represent the ground state co-conformation and the metastable state co-conformation, respectively.

In addition to the traditional TTF and/or DNP recognition sites for the CBPQT⁴⁺ ring in the [2]rotaxanes, of particular interest is the introduction of potential electrostatic barriers – that is, positively charged entities – which would potentially curtail the translational motion undergone by the CBPQT⁴⁺ ring. In order to investigate the influence of placing “speed bumps” in the shape of positively charged entities between two DNP units in degenerate molecular shuttles, the [2]rotaxanes 1–5·6PF₆ (Fig. 1) have been synthesised (Schemes 1, 2 and 3). In 1·6PF₆ and 2·6PF₆, the speed bumps are BIPY²⁺ units. In 3·6PF₆, the speed bump is a 1,4-bis(pyridinium)benzene⁹ unit. In the case of 4·6PF₆ and 5·6PF₆, the conjugation between the two pyridinium rings is broken by spacers containing saturated chains of atoms. All of the five molecular shuttles have been investigated by variable temperature (dynamic) ¹H NMR spectroscopy: they all exhibit slow shuttling on the ¹H NMR time-scale, at least up to +70 °C in CD₃CN solutions.

Scheme 1 Synthesis of the [2]rotaxane 1·6PF₆.

Scheme 2 Syntheses of the [2]rotaxanes 2·6PF₆, 4·6PF₆ and 5·6PF₆.

Scheme 3 Synthesis of the [2]rotaxane 3·6PF₆.

Recently, we introduced¹⁰BIPY²⁺ units into the dumbbell components of the CBPQT⁴⁺ ring-containing [2]rotaxanes and demonstrated¹⁰ that the one-electron reduced radical cation (BIPY˙⁺) serves as an additional recognition site for the two-electron reduced CBPQT²⁽˙⁺⁾ ring. Electrochemical experiments carried out on 1·6PF₆ and 2·6PF₆ indicate that the BIPY˙⁺ radical cation does act as an additional recognition site for the CBPQT²⁽˙⁺⁾ ring. Thus, ring shuttling can be induced by reduction in the case of 1·6PF₆ and 2·6PF₆. This paper describes how speed bumps can be introduced into molecular shuttles and how, in some instances, they can be modified by replacing the spacers. Establishing this kind of control in the switching of bistable [2]rotaxanes could have important consequences for the development of molecular flash memory using molecules of this kind.

Results and discussion

The synthesis of the [2]rotaxane 1·6PF₆ is summarised in Scheme 1. Reaction of the tosylated half dumbbell¹⁰6 with 4,4′-bipyridine afforded the dumbbell 7·2PF₆ after counterion exchange. The [2]rotaxane 1·6PF₆ was obtained from 7·2PF₆, the 8·2PF₆ salt,¹¹ and 1,4-bis(bromomethylbenzene) using a template-directed protocol¹² in DMF under 10 kbar pressure at room temperature for 3 days. After the reaction was complete, the crude mixture was purified by preparative TLC on silica gel using Me₂CO/NH₄PF₆ (100/1 v/w) as the mobile phase to afford 1·6PF₆ in an overall yield of 37%. The syntheses of the [2]rotaxanes 2·6PF₆,4·6PF₆ and 5·6PF₆ are summarised in Scheme 2.

In a manner reminiscent of the synthesis of the [2]rotaxane 1·6PF₆, the dumbbell compounds 13·2PF₆–15·2PF₆ were obtained from the tosylated half dumbbell 12 by reactions, in turn, with 4,4′-bipyridine, 1,5-bis(pyridinyloxy)pentane, and 1,2-bis(2-(pyridinyloxy)ethoxy)ethane. The [2]rotaxanes 2·6PF₆, 4·6PF₆ and 5·6PF₆ were prepared from corresponding dumbbells by means of a templation protocol,¹² similar to that employed in the synthesis of 1·6PF₆, in yields ranging from 30% to 40%. The synthesis of the [2]rotaxane 3·6PF₆ is summarised in Scheme 3. In contrast with the synthetic strategies associated with the clipping methodology,¹³ a strategy of threading-followed-by-stoppering¹⁴ was utilised in the synthesis of the [2]rotaxane 3·6PF₆. Reaction (Scheme 3) of the azide derivative 17 with 1,4-bis(4-pyridyl)benzene afforded the dumbbell 18·2PF₆ after counterion exchange. The [2]rotaxane 3·6PF₆ was isolated in 27% yield, following the reaction of 17·2PF₆ with the alkyne derivative 19 in Me₂CO using the copper(I)-catalyzed azide-alkyne cycloaddition in the presence of CBPQT·4PF₆ salt. The [2]rotaxanes 1–5·6PF₆ were all fully characterized by ¹H and ¹³C NMR spectroscopies and by electrospray ionization mass spectrometry.

In the ¹H NMR spectra of these five [2]rotaxanes recorded in CD₃CN at 298 K, the protons of the DNP units and stoppers resonate as two sets of peaks, demonstrating that the CBPQT⁴⁺ rings in the [2]rotaxanes encircle one of the two equivalent DNP units in their dumbbell components, i.e., the CBPQT⁴⁺ ring is not shuttling rapidly between the two DNP units on the ¹H NMR time-scale. As a result of this slow shuttling on the ¹H NMR time-scale, two sets of the resonances are observed for the DNP protons for all five [2]rotaxanes – one DNP unit that is “free” and one that is encircled by the CBPQT⁴⁺ ring. This fact is further substantiated by the observation that the set of resonances corresponding to the protons of the free DNP units have very similar δ values to those recorded for the DNP protons in their corresponding dumbbell compounds. The resonances for the protons on the other DNP units encircled by the CBPQT⁴⁺ ring experience large upfield shifts that can be observed at ca. δ = 2.3, 5.9, and 6.2 ppm as a result of [π⋯π] stacking interactions with the encircling CBPQT⁴⁺ ring.

A series of variable-temperature (VT) ¹H NMR spectroscopic experiments have been performed in CD₃CN or d₇-DMF in order to investigate^6d,7 the shuttling behaviour of the CBPQT⁴⁺ rings in the degenerate [2]rotaxanes. We did not observe any substantial changes in the ¹H NMR spectra of 1–5·6PF₆ in the temperature range 233–343 K, indicating that the CBPQT⁴⁺ rings are not shuttling rapidly back and forth between the two DNP units on the ¹H NMR time-scale in this temperature range. Two examples are presented in Fig. 3 which illustrates the partial VT ¹H NMR spectra of the degenerate [2]rotaxane 1·6PF₆ and 3·6PF₆ recorded in CD₃CN in the temperature range 233–343 K. The methyl protons (H_Me) on the 2,6-diisopropylphenyl stoppers, which can be employed in order to probe the shuttling process, feature two doublets at 273 K. While the two doublets in each case undergo temperature-dependent shifts, they were not observed to coalesce. These observations indicate that the shuttling of the CBPQT⁴⁺ ring in the case of all five [2]rotaxanes is slow on the ¹H NMR time-scale, even at 343 K.

Fig. 3 Partial ¹H NMR spectra of the [2]rotaxane a) 1·6PF₆ and b) 3·6PF₆ recorded in CD₃CN at different temperatures showing the signals for the probe protons (H_Me) in the form of the methyl groups on the 2,6-diisopropylphenyl stoppers. No coalescence could be observed.

The location of the BIPY²⁺ unit in the [2]rotaxanes 1·6PF₆ and 2·6PF₆ allows for a reduction-based switching process to occur which also eradicates the electrostatic barrier to shuttling of the ring between the two DNP units in the degenerate [2]rotaxane 1·6PF₆ and 2·6PF₆. We have shown recently^10b that, upon a two-electron reduction of the CBPQT⁴⁺ ring to its diradical dication CBPQT²⁽˙⁺⁾ and a one-electron reduction of the BIPY²⁺ unit – located in either the dumbbell component of a rotaxane or in the thread component of a pseudorotaxane – to its radical cation BIPY˙⁺, encirclement of the ring around the dumbbell or thread occurs as a result of radical-pairing interactions (Fig. 4). These interactions have been characterized by cyclic voltammetry (CV) which provides evidence for the bistability of the [2]rotaxanes 1·6PF₆ and 2·6PF₆ under the influence of reductive potentials.

Fig. 4 Schematic of the switching of the CBPQT⁴⁺ ring in the [2]rotaxane 1·6PF₆ or 2·6PF₆ upon redox control.

The [2]rotaxane 1·6PF₆ displays the characteristic CV behaviour observed previously¹⁰ for the reduction-induced switching of the CBPQT⁴⁺ ring in relation to the BIPY²⁺ unit in its dumbbell component. The first feature of significance in the CV (Fig. 5) is a three-electron reduction process at −0.29 V, the potential at which two electrons are gained by the CBPQT⁴⁺ ring component while the third electron is taken up by the BIPY²⁺ unit in the dumbbell component. The three-electron reduction results in the encirclement of the CBPQT²⁽˙⁺⁾ diradical cationic ring component around the BIPY˙⁺ radical cationic unit in the dumbbell component. In the radical pairing interactions which are driving this intramolecular reaction to occur, only one of the two BIPY˙⁺ radical cations on the CBPQT²⁽˙⁺⁾ ring is engaged intimately with BIPY˙⁺ present in the dumbbell component. This biased interaction results in the splitting of the second reduction cycle of the ring component into two one-electron processes at peak potentials of −0.73 and −1.03 V, the more negative of which is coupled to the second reduction of the BIPY²⁺ unit in the dumbbell component. The return scan shows (Fig. 5) that these two redox processes are completely reversible. Re-oxidation of the trisradical species shows one broad anodic process at −0.18 V (200 mV s⁻¹ scan rate) corresponding to a potential that is considerably more positive than would be expected for a totally reversible process with respect to the initial three-electron reduction, an observation which reflects the highly stabilizing interactions that result as a consequence of a BIPY˙⁺ radical cation when the dumbbell component is encircled by a CBPQT²⁽˙⁺⁾ ring. The [2]rotaxane 2·6PF₆ also exhibited similar bistability and switching behaviour when investigated by CV. It follows that the two [2]rotaxanes 1·6PF₆ and 2·6PF₆, where bipyridinium radical cations can be generated in conjugated systems such that electron-delocalization can occur, may also be regarded as the precursors to tristable [2]rotaxanes when one of the two DNP units is replaced, for example, by a TTF unit.

Fig. 5 The first (black) and second (red) scans arising from the CV of the [2]rotaxane 1·6PF₆ in the reduction region.

We have also investigated the reduction properties of the degenerate [2]rotaxanes 4·6PF₆ and 5·6PF₆ under the same conditions as for 1·6PF₆ and 2·6PF₆ and did not observe any interactions between the CBPQT²⁽˙⁺⁾ diradical cationic ring and the flexible spacers containing two pyridinium units in their dumbbell components. Consequently, the CBPQT⁴⁺ ring cannot be induced to shuttle electrochemically between the two degenerate DNP units in the dumbbell component of 4·6PF₆ and 5·6PF₆. The reason for the inability to undergo well-defined switching upon reduction no doubt resides in the fact that the two pyridinium units are not conjugated and so cannot generate radical cations for binding with the CBPQT²⁽˙⁺⁾ diradical cationic ring. In the case of the [2]rotaxane 3·6PF₆, we are currently investigating its electrochemical behaviour, which is revealing interesting preliminary results – likely as a consequence of the aromatic conjugation between the two pyridinium units.

Conclusions

In degenerate donor–acceptor rotaxanes, the insertion of positively charged entities which can be reduced readily leads to electrostatic barriers to the shuttling of a cyclobis(paraquat-p-phenylene) ring. When, however, the positive charge is associated with a conjugated bipyridinium ring, or its higher homologues, then reduction to form radical cations leads to a dramatic stabilisation when interacting with the ring also in a partially reduced form. This interaction, not only removes the electrostatic barrier to shuttling, but also renders the [2]rotaxanes bistable. Investigation of the potential for the 1,4-bis(pyridinium)benzene unit in the degenerate [2]rotaxane 3·6PF₆ to serve as a recognition site for a CBPQT⁴⁺ ring is still under investigation at this time. These redox-active bistable molecules involving direct and reversible control on oxidation, could come into their own as candidates for developing the next generation of molecular electronic devices.

Experimental

General

All reagents were purchased from Aldrich and were used without further purification. The starting materials 6,¹⁰8·2PF₆,¹¹9,¹⁵10,¹⁶16,¹⁵1,4-bis(4-pyridyl)benzene¹⁷ and CBQPT·4PF₆¹⁸ were prepared according to literature procedures. Deuterated solvents (Cambridge Isotope Laboratories) for nuclear magnetic resonance (NMR) spectroscopic analyses were used as received. NMR spectra were recorded on a Bruker Avance 500 MHz or 600 MHz NMR spectrometer. Chemical shifts were reported in parts per million (ppm) downfield from the Me₄Si resonance which was used as the internal standard when recording ¹H NMR spectra. High resolution mass spectra were measured on an Applied Biosystems Voyager DE-PRO MALDI TOF mass spectrometer or a Micromass Q-TOF Ultima electrospray ionization mass spectrometer. The reported molecular mass (m/z) values were the most abundant monoisotopic mass. UV-Vis spectra were recorded at room temperature on a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer. Electrochemical experiments were carried out at 298 K in Ar-purged MeCN, with a Gamry Multipurpose instrument (Reference 600) interfaced to a PC. Cyclic voltammetry (CV) experiments were performed using a glassy carbon working electrode (0.071 cm², Cypress system). Its surface was polished routinely with 0.05 μm alumina-water slurry on a felt surface immediately before use. The counter electrode was a Pt coil and the reference electrode was an Ag/AgCl electrode. The concentration of the sample and supporting electrolyte tetrabutylammonium hexafluorophosphate (TBA·PF₆) were 1.0 × 10⁻³ mol L⁻¹ and 0.1 mol L⁻¹, respectively.

Syntheses

7·2PF₆. A mixture of 6 (65.0 g, 0.10 mmol) and 4,4′-bipyridine (6.3 mg, 0.04 mmol) in anhydrous DMF (5.0 mL) was transferred to a teflon tube and subjected to 10 kbar pressure at room temperature for 4 d. The purple solution was subjected directly to column chromatography (SiO₂) and unreacted starting materials were eluted with Me₂CO, whereupon the eluent was changed to Me₂CO/NH₄PF₆ (100 : 1 v/w) and the purple band was collected. Most of the solvent was removed in vacuo, followed by addition of H₂O (15 mL). The resulting precipitate was collected by filtration, affording the dumbbell 7·2PF₆ (24.2 mg, 42%). ¹H NMR (500 MHz, (CD₃)₂CO, TMS): δ = 1.14–1.15 (d, 24H, J = 7.0 Hz), 3.35–3.37 (m, 4H), 3.66–3.79 (m, 20H), 4.47–4.51 (m, 12H), 6.89–6.91 (d, 4H, J = 7.6 Hz), 7.07–7.10 (d, 6H), 7.43–7.45 (d, 4H, J = 7.6 Hz), 7.89–7.90 (d, 4H, J = 7.6 Hz), 8.12–8.14 (d, 4H, J = 7.8 Hz), 9.09–9.11 ppm (d, 4H, J = 7.8 Hz). ¹³C NMR (125 MHz, (CD₃)₂CO, TMS): δ = 23.5, 28.4, 68.4, 69.9, 70.5, 108.1, 115.8, 121.7, 123.6, 124.3, 126.2, 128.6, 142.0, 144.8, 148.7, 150.8, 154.3 ppm. HRMS: calcd for C₇₀H₈₆F₆N₂O₁₀P [M − PF₆]⁺m/z = 1259.5924, found m/z = 1259.5914; calcd for C₇₀H₈₆N₂O₁₀ [M − 2PF₆]²⁺m/z = 557.3141, found m/z = 557.3179.

1·6PF₆. The dumbbell compound 7·2PF₆ (70.2 mg, 0.05 mmol), 8·2PF₆ (28.2 mg, 0.04 mmol), and 1,4-bis(bromomethylbenzene) (10.5 mg, 0.04 mmol) were dissolved in anhydrous DMF (8 mL). The reaction mixture was subjected to 10 kbar pressure at room temperature for 3 d. After the solvent had been removed in vacuo, the purple solid was dissolved in Me₂CO and the [2]rotaxane 1·6PF₆ was isolated by means of preparative TLC using Me₂CO/NH₄PF₆ (100 : 1 v/w) as the mobile phase. The product was recovered from the silica gel by washing with an excess of eluent. The solution was concentrated to a minimum volume and the product was precipitated from the solution by addition of H₂O (12 mL). The resulting precipitate was collected by filtration, affording the pure [2]rotaxane 1·6PF₆ (37.1 mg, 37%). ¹H NMR (500 MHz, CD₃CN, TMS): δ = 1.07–1.08 (d, 12H, J = 6.8 Hz), 1.16–1.17 (d, 12H, J = 6.8 Hz), 2.31–2.33 (m, 2H), 3.21–3.23 (m, 2H), 3.32–3.34 (m, 2H), 3.59–3.89 (m, 20H), 4.32–4.47 (m, 12H), 5.68–5.74 (m, 8H), 5.86–5.88 (m, 2H), 6.21–6.23 (m, 2H), 6.86–6.88 (d, 2H, J = 8.0 Hz), 7.03–7.16 (m, 14H), 7.44–7.46 (d, 2H, J = 8.0 Hz), 7.77–7.79 (d, 2H, J = 8.0 Hz), 7.88–7.92 (m, 8H), 8.08–8.14 (m, 4H), 8.89–8.91 (d, 2H, J = 7.0 Hz), 8.94–8.96 (d, 2H, J = 7.0 Hz), 9.06–9.11 ppm (m, 8H). ¹³C NMR (125 MHz, CD₃CN, TMS): δ = 23.4, 28.6, 62.4, 68.7, 69.5, 70.7, 108.2, 115.7, 121.2, 121.7, 123.4, 124.6, 126.2, 127.5, 128.8, 134.7, 142.1, 144.9, 148.6, 150.8, 154.4 ppm. HRMS: calcd for C₁₀₆H₁₁₈F₃₀N₆O₁₀P₅ [M − PF₆]⁺m/z = 2359.7118, found m/z = 2359.7467; calcd for C₁₀₆H₁₁₈F₂₄N₆O₁₀P₄ [M − 2PF₆]²⁺m/z = 1107.3739, found m/z = 1107. 4133.

1,2-Bis(2-(pyridinyloxy)ethoxy)ethane . A mixture of 4-chloropyridine (2.5 g, 22.0 mmol), methylene glycol (1.5 g, 10.0 mmol), and NaH (1.0 g, 40.0 mmol) in dry DMF (80 mL) was stirred for 2 d at 80 °C under an atmosphere of Ar. After the solvent had been removed under reduced pressure, the residue was triturated with CH₂Cl₂ and the precipitated salts were removed by filtration. The filtrate was concentrated under reduced pressure to yield a crude product, which was subjected to column chromatography (SiO₂, EtOAc) to give 1,2-bis(2-(pyridinyloxy)ethoxy)ethane (0.9 g, 31%). ¹H NMR (500 MHz, CD₂Cl₂, TMS): δ = 3.53–3.55 (m, 4H, OCH₂), 3.77–3.80 (m, 4H, OCH₂), 4.27–4.29 (m, 4H, OCH₂), 7.07–7.09 (d, 4H, Ar-H), 8.41–8.43 ppm (d, 4H, Ar-H). ¹³C NMR (125 MHz, CD₂Cl₂, TMS): δ = 69.1, 70.3, 110.2, 150.8, 161.7 ppm. MS: calcd for C₁₆H₂₁N₂O₄ [M + H]⁺m/z = 305.150, found m/z = 305.173.

1,5-Bis(pyridinyloxy)pentane . A mixture of 4-chloropyridine (2.5 g, 22.0 mmol), 1,5-pentanediol (1.0 g, 10.0 mmol), and NaH (1.0 g, 40.0 mmol) in dry DMF (80 mL) was stirred for 2 d at 80 °C under an atmosphere of Ar. After the solvent had been removed under reduced pressure, the residue was triturated with CH₂Cl₂ and the precipitated salts were removed by filtration. The filtrate was concentrated under reduced pressure to yield a crude product, which was subjected to column chromatography (SiO₂, CH₂Cl₂) to give 1,5-bis(pyridinyloxy)pentane (1.0 g, 39%). ¹H NMR (500 MHz, CD₂Cl₂, TMS): δ = 1.59–1.61 (m, 2H, CH₂), 1.76–1.78 (m, 4H, CH₂), 4.05–4.07 (m, 4H, CH₂), 7.08–7.10 (d, 4H, Ar-H), 8.42–8.44 ppm (d, 4H, Ar-H). ¹³C NMR (125 MHz, CD₂Cl₂, TMS): δ = 22.5, 30.2, 68.9, 110.1, 150.7, 161.5 ppm. MS: calcd for C₁₅H₁₉N₂O₂ [M + H]⁺m/z = 259.145, found m/z = 259.157.

11 . A mixture of 9 (2.0 g, 3.0 mmol), 10 (1.5 g, 3.0 mmol), K₂CO₃ (0.8 g, 6.0 mmol), LiBr (17.2 mg, 0.2 mmol), and [18]crown-6 (26.4 mg, 0.1 mmol) in anhydrous MeCN (80 mL) was heated under reflux for 16 h. After cooling down to room temperature, the reaction mixture was filtered and the solid was washed with MeCN. The combined organic solution was concentrated and the residue was purified by column chromatography (SiO₂, EtOAc: MeOH = 98 : 2) to give compound 11 (2.6 g, 87%). ¹H NMR (500 MHz, CD₂Cl₂, TMS): δ = 1.22–1.24 (d, 6H, J = 8.0 Hz), 1.33 (s, 18H), 2.86–2.87 (m, 1H), 3.48–3.56 (m, 20H), 3.79–3.81 (m, 6H), 4.33–4.35 (m, 6H), 6.75–6.77 (d, 2H, J = 8.2 Hz), 6.82–6.83 (d, 2H, J = 7.8 Hz), 7.08–7.18 (m, 10H), 7.28–7.30 (d, 4H), 7.45–7.47 (d, 2H, J = 7.8 Hz), 7.88–7.90 ppm (d, 2H, J = 7.8 Hz). ¹³C NMR (125 MHz, CD₂Cl₂, TMS): δ = 23.6, 31.5, 33.9, 34.7, 61.7, 64.6, 69.4, 70.8, 108.2, 115.3, 116.1, 125.7, 126.8, 128.0, 128.9, 139.9, 145.2, 146.4, 149.5, 154.3, 156.5 ppm. MS: calcd for C₆₂H₈₁O₁₀ [M + H]⁺m/z = 985.583, found m/z = 985.598.

12 . A solution of TsCl (1.0 g, 5.0 mmol) in CH₂Cl₂ (20 mL) was added dropwise to a solution of 11 (3.9 g, 4.0 mmol), Et₃N (1 mL), and DMAP (12.5 mg, 0.1 mmol) in CH₂Cl₂ (100 mL) at 0 °C under an atmosphere of Ar. The mixture was warmed up to room temperature while stirring for 16 h. After the precipitated salts were filtered off and the solvent had been evaporated under reduced pressure, the residue was purified by column chromatography (SiO₂, EtOAc) to give compound 12 (3.7 g, 81%). ¹H NMR (500 MHz, CD₂Cl₂, TMS): δ = 1.21–1.23 (d, 6H, J = 7.8 Hz), 1.34 (s, 18H), 2.33 (s, 3H), 2.86–2.87 (m, 1H), 3.51–3.57 (m, 18H), 3.76–3.82 (m, 8H), 4.32–4.35 (m, 6H), 6.80–6.82 (d, 2H, J = 8.0 Hz), 6.83–6.84 (d, 2H, J = 8.0 Hz), 7.10–7.18 (m, 10H), 7.35–7.37 (d, 2H, J = 7.5 Hz), 7.46–7.47 (d, 2H, J = 8.0 Hz), 7.76–7.78 (d, 2H, J = 8.2 Hz), 7.88–7.90 (d, 2H, J = 8.0 Hz), 7.29–7.31 ppm (d, 4H). ¹³C NMR (125 MHz, CD₂Cl₂, TMS): δ = 21.5, 23.6, 31.4, 33.7, 34.3, 64.5, 68.1, 69.6, 70.5, 108.0, 115.4, 116.2, 125.6, 126.9, 128.2, 129.1, 130.7, 139.5, 144.2, 145.1, 146.2, 149.3, 154.5, 156.7 ppm. MS: calcd for C₆₉H₈₇O₁₂S [M + H]⁺m/z = 1139.592, found m/z = 1139.607.

13·2PF₆. A mixture of 12 (0.11 g, 0.10 mmol) and 4,4′-bipyridine (6.3 mg, 0.04 mmol) in anhydrous DMF (5.0 mL) was transferred to a teflon tube and subjected to 10 kbar pressure at room temperature for 4 d. The purple solution was directly subjected to column chromatography (SiO₂) and the unreacted starting materials were eluted with Me₂CO, whereupon the eluent was changed to Me₂CO/NH₄PF₆ (100 : 1 v/w) and the purple band was collected. Most of the solvent was removed in vacuo, followed by addition of H₂O (15 mL). The resulting precipitate was collected by filtration, affording the dumbbell 13·2PF₆ (39.1 mg, 41%). ¹H NMR (500 MHz, (CD₃)₂CO, TMS): δ = 1.18–1.20 (d, 12H, J = 6.9 Hz), 1.31 (s, 36H), 2.83–2.85 (m, 2H), 3.58–3.66 (m, 12H), 3.70–3.72 (m, 4H), 3.73–3.80 (m, 16H), 3.82–3.84 (m, 4H), 3.93–3.96 (m, 8H), 4.03–4.04 (m, 4H), 4.09–4.12 (m, 4H), 4.18–4.21 (m, 8H), 4.76–4.78 (m, 4H), 6.73–6.78 (m, 8H), 7.05–7.12 (m, 20H), 7.29–7.31 (m, 8H), 7.41–7.43 (d, 4H, J = 8.4 Hz), 7.94–7.95 (d, 4H, J = 6.8 Hz), 8.13–8.15 (d, 4H, J = 8.4 Hz), 9.03–9.04 ppm (d, 4H, J = 6.8 Hz). ¹³C NMR (500 MHz, (CD₃)₂CO, TMS): δ = 23.3, 30.6, 33.2, 33.8, 53.1, 63.9, 69.2, 70.0, 70.4, 106.6, 113.8, 115.0, 121.6, 125.4, 126.1, 127.9, 128.7, 139.4, 144.6, 145.5, 145.9, 148.4, 150.6, 153.7, 156.7 ppm. HRMS: calcd for C₁₃₄H₁₆₆N₂O₁₈ [M − 2PF₆]²⁺m/z = 1045.6068, found m/z = 1046.0909.

14·2PF₆. A mixture of 12 (0.11 g, 0.10 mmol) and 1,5-bis(pyridinyloxy)pentane (10.3 mg, 0.04 mmol) in anhydrous DMF (5.0 mL) was transferred to a teflon tube and subjected to 10 kbar pressure at room temperature for 4 d. The purple solution was subjected directly to column chromatography (SiO₂) and the unreacted starting materials were eluted with Me₂CO, whereupon the eluent was changed to Me₂CO/NH₄PF₆ (100 : 1 v/w) and the purple band was collected. Most of the solvent was removed in vacuo, followed by addition of H₂O (15 mL). The resulting precipitate was collected by filtration, affording the dumbbell 14·2PF₆ (37.7 mg, 38%). ¹H NMR (500 MHz, (CD₃)₂CO, TMS): δ = 1.18–1.21 (d, 12H), 1.65–1.73 (m, 6H), 1.32 (s, 36H), 2.82–2.84 (m, 2H), 3.58–3.65 (m, 12H), 3.70–3.73 (m, 4H), 3.74–3.81 (m, 16H), 3.82–3.85 (m, 4H), 3.92–3.96 (m, 8H), 4.03–4.07 (m, 8H), 4.10–4.13 (m, 4H), 4.18–4.22 (m, 8H), 4.75–4.78 (m, 4H), 6.74–6.78 (m, 8H), 7.06–7.14 (m, 24H), 7.28–7.32 (m, 8H), 7.42–7.44 (d, 4H, J = 8.4 Hz), 8.13–8.15 (d, 4H, J = 8.4 Hz), 8.85–8.87 ppm (d, 4H, J = 7.0 Hz). ¹³C NMR (500 MHz, (CD₃)₂CO, TMS): δ = 22.7, 23.4, 30.3, 33.0, 33.7, 53.2, 63.9, 68.6, 69.3, 70.1, 70.6, 100.4, 106.8, 113.8, 115.5, 125.1, 126.3, 127.9, 128.9, 139.5, 144.8, 145.7, 148.6, 150.3, 153.4, 156.5, 161.5 ppm. HRMS: calcd for C₁₃₉H₁₇₆N₂O₂₀ [M − 2PF₆]²⁺m/z = 1096.6408, found m/z = 1096.6402.

15·2PF₆. A mixture of 12 (0.11 g, 0.10 mmol) and 1,2-bis(2-(pyridinyloxy)ethoxy)ethane (12.2 mg, 0.04 mmol) in anhydrous DMF (5.0 mL) was transferred to a teflon tube and subjected to 10 kbar pressure at room temperature for 4 d. The purple solution was subjected directly to column chromatography (SiO₂) and the unreacted starting materials were eluted with Me₂CO, whereupon the eluent was changed to Me₂CO/NH₄PF₆ (100 : 1 v/w) and the purple band was collected. Most of the solvent was removed in vacuo, followed by addition of H₂O (15 mL). The resulting precipitate was collected by filtration, affording the dumbbell 15·2PF₆ (36.4 mg, 36%). ¹H NMR (500 MHz, (CD₃)₂CO, TMS): δ = 1.19–1.21 (d, 12H, J = 6.8 Hz), 1.30 (s, 36H), 2.84–2.85 (m, 2H), 3.58–3.65 (m, 16H), 3.71–3.74 (m, 4H), 3.72–3.81 (m, 20H), 3.81–3.84 (m, 4H), 3.92–3.96 (m, 8H), 4.03–4.05 (m, 4H), 4.08–4.13 (m, 4H), 4.21–4.32 (m, 12H), 4.73–4.75 (m, 4H), 6.74–6.80 (m, 8H), 7.08–7.13 (m, 24H), 7.29–7.32 (m, 8H), 7.41–7.43 (d, 4H, J = 8.4 Hz), 8.15–8.17 (d, 4H, J = 8.4 Hz), 8.89–8.91 ppm (d, 4H, J = 7.0 Hz). ¹³C NMR (500 MHz, (CD₃)₂CO, TMS): δ = 23.0, 30.5, 33.7, 34.2, 53.4, 64.1, 69.5, 70.1, 70.8, 100.2, 106.4, 113.8, 115.3, 125.7, 126.2, 127.9, 128.8, 140.3, 144.4, 145.8, 148.7, 150.2, 153.8, 156.5, 161.6 ppm. HRMS: calcd for C₁₄₀H₁₇₈N₂O₂₂ [M − 2PF₆]²⁺m/z = 1119.6436, found m/z = 1119.6588.

2·6PF₆. The dumbbell compound 13·2PF₆ (119.1 mg, 0.05 mmol), 8·2PF₆ (28.2 mg, 0.04 mmol), and 1,4-bis(bromomethylbenzene) (10.5 mg, 0.04 mmol) were dissolved in anhydrous DMF (8 mL). The reaction mixture was subjected to 10 kbar pressure at room temperature for 3 d. After the solvent had been removed in vacuo, the purple solid was dissolved in Me₂CO and the [2]rotaxane 2·6PF₆ was isolated by means of preparative TLC using Me₂CO/NH₄PF₆ (100 : 1 v/w) as the mobile phase. The product was recovered from the silica gel by washing with an excess of eluent. The solution was concentrated to a minimum volume and the product was precipitated from the solution by addition of H₂O (12 mL). The resulting precipitate was collected by filtration, affording the pure [2]rotaxane 2·6PF₆ (50.1 mg, 36%). ¹H NMR (500 MHz, CD₃CN, TMS): δ = 1.10–1.11 (d, 6H, J = 7.0 Hz), 1.18–1.20 (d, 6H, J = 7.0 Hz), 1.28 (s, 18H), 1.30 (s, 18H), 2.57–2.59 (m, 2H), 2.86–2.89 (m, 2H), 3.60–3.67 (m, 12H), 3.71–3.78 (m, 20H), 3.89–3.95 (m, 12H), 4.04–4.06 (m, 4H), 4.20–4.25 (m, 8H), 4.36–4.38 (m, 4H), 4.73–4.76 (m, 4H), 5.69–5.74 (q, 8H, J = 8.0 Hz), 5.95–5.97 (m, 2H), 6.31–6.34 (m, 2H), 6.69–6.72 (d, 2H, J = 8.6 Hz), 6.83–6.86 (m, 4H), 7.05–7.19 (m, 28H), 7.32–7.36 (m, 10H), 7.73–7.75 (d, 2H, J = 8.6 Hz), 7.99–8.02 (m, 8H), 8.11–8.15 (m, 4H), 8.89–9.01 ppm (m, 12H). ¹³C NMR (500 MHz, CD₃CN, TMS): δ = 23.6, 30.4, 33.3, 33.9, 53.2, 62.2, 64.0, 69.2, 70.1, 70.5, 106.6, 113.7, 115.3, 120.9, 121.6, 125.5, 126.3, 127.3, 128.0, 128.8, 134.6, 139.6, 144.1, 144.7, 145.3, 145.8, 148.6, 150.1, 150.7, 153.5, 156.8 ppm. HRMS: calcd for C₁₇₀H₁₉₈F₂₄N₆O₁₈P₄ [M − 2PF₆]²⁺m/z = 1595.6665, found m/z = 1596.2242; calcd for C₁₇₀H₁₉₈F₁₈N₆O₁₈P₃ [M − 3PF₆]³⁺m/z = 1015.4563, found m/z = 1015.9544.

4·6PF₆. The dumbbell compound 14·2PF₆ (124.2 mg, 0.05 mmol), 8·2PF₆ (28.2 mg, 0.04 mmol), and 1,4-bis(bromomethylbenzene) (10.5 mg, 0.04 mmol) were dissolved in anhydrous DMF (8 mL). The reaction mixture was subjected to 10 kbar pressure at room temperature for 3 d. After the solvent had been removed in vacuo, the purple solid was dissolved in Me₂CO and the [2]rotaxane 4·6PF₆ was isolated by means of preparative TLC using Me₂CO/NH₄PF₆ (100 : 1 v/w) as the mobile phase. The product was recovered from the silica gel by washing with an excess of eluent. The solution was concentrated to a minimum volume and the product was precipitated from the solution by addition of H₂O (12 mL). The resulting precipitate was collected by filtration, affording the pure [2]rotaxane 4·6PF₆ (50.2 mg, 35%). ¹H NMR (500 MHz, CD₃CN, TMS): δ = 1.12–1.13 (d, 6H, J = 4.6 Hz), 1.19–1.21 (d, 6H, J = 4.6 Hz), 1.27 (s, 18H), 1.31 (s, 18H), 1.64–1.76 (m, 6H), 2.58–2.60 (m, 2H), 2.85–2.88 (m, 2H), 3.61–3.67 (m, 12H), 3.72–3.79 (m, 20H), 3.88–3.95 (m, 12H), 4.04–4.08 (m, 8H), 4.21–4.25 (m, 8H), 4.36–4.39 (m, 4H), 4.74–4.76 (m, 4H), 5.69–5.75 (q, 8H, J = 8.6 Hz), 5.95–5.97 (m, 2H), 6.32–6.34 (m, 2H), 6.70–6.73 (d, 2H, J = 8.2 Hz), 6.83–6.86 (m, 4H), 7.03–7.10 (m, 12H), 7.12–7.19 (m, 20H), 7.33–7.37 (m, 10H), 7.74–7.76 (d, 2H, J = 8.2 Hz), 7.98–8.02 (m, 8H), 8.89–9.01 ppm (m, 12H). ¹³C NMR (500 MHz, CD₃CN, TMS): δ = 22.5, 23.7, 30.1, 33.4, 33.8, 53.1, 62.3, 63.8, 68.8, 69.5, 70.4, 70.9, 100.3, 106.6, 113.7, 115.3, 121.7, 125.4, 126.6, 127.4, 127.9, 128.7, 134.7, 139.2, 144.2, 144.9, 145.5, 148.3, 150.7, 153.6, 156.3, 161.8 ppm. HRMS: calcd for C₁₇₅H₂₀₈F₂₄N₆O₂₀P₄ [M − 2PF₆]²⁺m/z = 1646.7006, found m/z = 1646.7394; calcd for C₁₇₅H₂₀₈F₁₈N₆O₂₀P₃ [M − 3PF₆]³⁺m/z = 1049.4790, found m/z = 1049.5258.

5·6PF₆. The dumbbell compound 15·2PF₆ (126.5 mg, 0.05 mmol), 8·2PF₆ (28.2 mg, 0.04 mmol), and 1,4-bis(bromomethylbenzene) (10.5 mg, 0.04 mmol) were dissolved in anhydrous DMF (8 mL). The reaction mixture was subjected to 10 kbar pressure at room temperature for 3 d. After the solvent had been removed in vacuo, the purple solid was dissolved in Me₂CO and the [2]rotaxane 5·6PF₆ was isolated by means of preparative TLC using Me₂CO/NH₄PF₆ (100 : 1 v/w) as the mobile phase. The product was recovered from the silica gel by washing with an excess of eluent. The solution was concentrated to a minimum volume and the product was precipitated from the solution by addition of H₂O (12 mL). The resulting precipitate was collected by filtration, affording the pure [2]rotaxane 5·6PF₆ (45.0 mg, 31%). ¹H NMR (500 MHz, CD₃CN, TMS): δ = 1.12–1.14 (d, 6H, J = 4.2 Hz), 1.18–1.20 (d, 6H, J = 4.2 Hz), 1.27 (s, 18H), 1.32 (s, 18H), 2.58–2.60 (m, 2H), 2.86–2.88 (m, 2H), 3.58–3.66 (m, 16H), 3.74–3.81 (m, 24H), 3.87–3.95 (m, 12H), 4.04–4.08 (m, 4H), 4.22–4.25 (m, 8H), 4.30–4.38 (m, 8H), 4.74–4.77 (m, 4H), 5.70–5.75 (q, 8H, J = 8.4 Hz), 5.96–5.98 (m, 2H), 6.32–6.34 (m, 2H), 6.71–6.73 (d, 2H, J = 8.6 Hz), 6.84–6.86 (m, 4H), 7.03–7.11 (m, 12H), 7.13–7.20 (m, 20H), 7.34–7.38 (m, 10H), 7.74–7.76 (d, 2H, J = 8.6 Hz), 7.97–8.02 (m, 8H), 8.89–9.02 ppm (m, 12H). ¹³C NMR (500 MHz, CD₃CN, TMS): δ = 23.1, 30.6, 33.5, 34.7, 53.3, 61.8, 64.4, 69.9, 70.3, 71.1, 100.5, 106.1, 113.6, 115.7, 121.6, 125.4, 126.6, 127.1, 127.8, 128.4, 134.7, 140.6, 144.5, 145.3, 148.2, 150.4, 153.6, 156.8, 161.2 ppm. HRMS: calcd for C₁₇₆H₂₁₀F₂₄N₆O₂₂P₄ [M − 2PF₆]²⁺m/z = 1669.7033, found m/z = 1669.7159; calcd for C₁₇₆H₂₁₀F₁₈N₆O₂₂P₃ [M − 3PF₆]³⁺m/z = 1064.8141, found m/z = 1064.8356.

17 . A 50% aqueous NaOH solution (8 mL) was added to a solution of compound 16 (360 mg, 1 mmol) in THF (50 mL) at 0 °C. After stirring the mixture for 30 min, p-toluene-sulfonylchloride (TsCl) (210 mg, 1.1 mmol) in tetrahydrofuran (THF) (50 mL) was added slowly to the mixture. The solution was stirred for 2 h, and then poured into H₂O. The resulting mixture was extracted with CHCl₃ (3 × 20 mL) and the combined organic phases were washed with a saturated aqueous NaCl solution (3 × 100 mL). After drying (MgSO₄), the solvent was removed in vacuo to afford the desired product 17 (510 mg, 99%) as a colorless oil, which was used immediately in the next step without further purification.

18·2PF₆. A mixture of 17 (503 mg, 0.98 mmol) and 1,4-bis(4-pyridyl)benzene (57 mg, 0.25 mmol) in anhydrous DMF (5.0 mL) was transferred to a teflon tube and subjected to 10 kbar pressure at room temperature for 4 d. The purple solution was subjected directly to column chromatography (SiO₂) and the unreacted starting materials were eluted with Me₂CO, whereupon the eluent was changed to Me₂CO/NH₄PF₆ (100 : 1 v/w) and the purple fractions were collected. The solvent was removed in vacuo to a minimal volume, followed by the addition of H₂O (15 mL). The resulting precipitate was collected by filtration, affording the dumbbell 18·2PF₆ (180 mg, 61%). ¹H NMR (500 MHz, CD₃CN): δ = 3.35 (t, 4H, J = 4.5 Hz), 3.69 (t, 4H, J = 5.0 Hz), 3.79 (t, 4H, J = 4.5 Hz), 3.97 (t, 4H, J = 4.0 Hz), 4.05 (t, 4H, J = 5.0 Hz), 4.13 (t, 4H, J = 5.0 Hz), 4.16 (t, 4H, J = 4.0 Hz), 4.75 (t, 4H, J = 5.0 Hz), 6.71 (d, 2H, J = 8.0 Hz), 6.82 (d, 2H, J = 7.5 Hz), 7.26 (t, 2H, J = 8.0 Hz), 7.30 (t, 2H, J = 7.5 Hz), 7.52 (d, 2H, J = 8.5 Hz), 7.54 (s, 4H), 7.66 (d, 2H, J = 8.5 Hz), 7.93 (d, 4H, J = 7.0 Hz), 8.73 ppm(d, 4H, J = 7.0 Hz). ¹³C NMR (125 MHz, CD₃CN): δ = 50.1, 60.5, 67.2, 67.5, 68.6, 68.9, 69.0, 69.5, 105.2, 105.4, 113.7, 113.8, 124.2, 125.0, 125.1, 125.7, 126.1, 128.3, 135.8, 144.7, 153.6, 153.8, 154.0 ppm. HRMS: calcd for C₅₂H₅₆F₆N₈O₈P [M − PF₆]⁺m/z = 1065.3862, found m/z = 1065.3870.

19 . A mixture of 2,6-diisopropylphenol (178 mg, 1 mmol), propargyl bromide (130 mg, 1.1 mmol) and potassium carbonate (1.39 g, 10 mmol) was suspended in anhydrous DMF (25.0 mL). The mixture was stirred at 80 °C for 16 h. After cooling, the solution was poured into H₂O (200 mL). The resulting mixture was extracted with EtOAc (3 × 20 mL) and the combined organic phases were washed three times with saturated aqueous NaCl solution (3 × 100 mL). After drying (MgSO₄), the solvent was removed in vacuo to afford the desired product 19 (210 mg, 99%) as a colorless oil, which was used immediately in the next step without further purification.

3·6PF₆. A solution of 18·2PF₆ (32 mg, 0.026 mmol), 19 (80 mg, 0.37 mmol), CBPQT·4PF₆ (30 mg, 0.027 mmol), TBTA (9 mg,0.017 mmol), and tetrakis(acetonitrile)copper(I) hexafluorophosphate (6 mg, 0.017 mmol) in anhydrous Me₂CO (5 mL) were stirred for 24 h at room temperature. The solvent was then evaporated and the resulting purple solid was purified by column chromatography [SiO₂: 2 M NH₄Cl/MeOH/MeNO₂ (12 : 7 : 1)], then MeOH, Me2CO and 2% NH₄PF₆/Me₂CO, respectively]. The purple fraction in Me₂CO were collected, and concentrated to a minimum volume before the crude product was precipitated by the addition of H₂O. The resulting solid was collected by filtration to afford 3·6PF₆ (20 mg, 27%) as a purple powder. ¹H NMR (500 MHz, CD₃CN): δ = 1.17 (d, 12H, J = 5.5 Hz), 1.18 (d, 12H, J = 5.5 Hz), 2.55 (d, 1H, J = 6.0 Hz), 2.59 (d, 1H, J = 6.0 Hz), 3.35–3.40 (m, 4H), 3.90 (t, 2H, J = 3.5 Hz), 3.98 (t, 2H, J = 3.5 Hz), 4.03 (t, 2H, J = 4.5 Hz), 4.14 (t, 2H, J = 4.0 Hz), 4.20–4.23 (m, 4H), 4.27 (t, 2H, J = 3.0 Hz), 4.30 (t, 2H, J = 4.0 Hz), 4.36–4.41 (m, 6H), 4.49 (t, 2H, J = 4.3 Hz), 4.58 (t, 2H, J = 5.0 Hz), 4.73 (t, 2H, J = 4.0 Hz), 4.81 (s, 2H), 4.87 (t, 2H, J = 4.5 Hz), 4.88 (s, 2H), 5.05 (t, 2H, J = 4.3 Hz), 5.78 (d, 4H, J = 11.5 Hz), 5.86 (d, 4H, J = 11.5 Hz), 6.01 (t, 1H, J = 7.0 Hz), 6.05 (t, 1H, J = 7.0 Hz), 6.31 (d, 1H, J = 6.5 Hz), 6.38 (d, 1H, J = 6.5 Hz), 6.86 (t, 2H, J = 6.5 Hz), 7.09–7.15 (m, 6H), 7.31 (t, 2H, J = 7.0 Hz), 7.37 (d, 8H, J = 5.0 Hz), 7.64 (d, 1H, J = 6.5 Hz), 7.70 (d, 1H, J = 7.0 Hz), 7.84 (d, 2H, J = 7.5 Hz), 7.87 (s, 2H), 7.99 (s, 8H), 8.01 (d, 1H, J = 6.5 Hz), 8.11 (d, 2H, J = 6.5 Hz), 8.12 (s, 1H), 8.49 (d, 2H, J = 6.0 Hz), 8.68 (d, 2H, J = 5.5 Hz), 8.74 (b, 8H), 8.99 (d, 2H, J = 6.0 Hz). ¹³C NMR (125 MHz, CD₃CN): δ = 23.3, 23.3, 26.5, 26.5, 28.7, 29.9, 50.1, 54.2, 61.0, 67.2, 67.5, 67.7, 67.9, 69.0, 69.3, 69.3, 69.4, 69.5, 69.8, 70.0, 104.5, 105.6, 105.9, 114.1, 124.1, 124.2, 124.3, 124.4, 124.7, 125.0, 125.2, 125.4, 125.6, 126.5, 129.1, 129.1, 131.3, 136.6, 141.6, 141.9, 143.7, 144.6, 145.0, 145.3, 150.9, 152.7, 152.8, 154.0. HRMS: calcd for C₁₁₈H₁₂₈F₂₄N₁₂O₁₀P₄ [M − 2PF₆]²⁺m/z = 1226.9238, found m/z = 1226.9281.

Acknowledgements

The research was supported by the US National Science Foundation (NSF) under grant number CHE-0924620. We thank the NSF for the award of a graduate research fellowship to A. C. F.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c0ob00937g

‡ These authors contributed equally to this work.

§ Current Address: Division of Semiconductor Business, Samsung Electronics Co. Ltd., San #16 Banwol-Dong, Hwasung-City, Gyeonggi-Do 445-701, Korea.

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