structures and solution dynamics of pseudorotaxanes mediated by alkali-metal cations
TRANSCRIPT
ISSN 1477-9226
www.rsc.org/dalton Number 35 | 21 September 2007 | Pages 3849–3972
An international journal of inorganic chemistry
PAPERSofia I. Pascu and Thibaut Jarrosson et al.Structures and solution dynamics of pseudorotaxanes mediated by alkali-metal cations
COMMUNICATIONGeorge Christou et al.The highest nuclearity metal oxime clusters: Ni14 and Ni12Na2 complexes from the use of 2-pyridinealdoximate and azide ligands 1477-9226(2007)35;1-2
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PAPER www.rsc.org/dalton | Dalton Transactions
Structures and solution dynamics of pseudorotaxanes mediated byalkali-metal cations†
Sofia I. Pascu,*‡ Christoph Naumann, Guido Kaiser, Andrew D. Bond,§ Jeremy K. M. Sanders andThibaut Jarrosson*¶
Received 23rd April 2007, Accepted 2nd July 2007First published as an Advance Article on the web 2nd August 2007DOI: 10.1039/b706043b
Kinetic stability studies of a series of pseudorotaxanes formed from electron-rich crown ethers (hosts 1and 2) and naphthalene diimide (guest A) in the presence of alkali salt templates MX (where M+ = Li+
and Na+, and X− = Cl−, Br−, I−, NO3− and CF3SO3
−) were performed by 1H NMR. The switchingbetween the (bound) host 1 and its linkage isomer host 2 (free) was monitored in solution in thepresence and absence of alkali salts, to establish the relative thermodynamic stabilities in the series. Wealso report here six new crystal structures, for pseudorotaxanes of type: [1·A], [M2·1·A]2+ and[M2·2·A]2+. Their solution-phase structures are in good agreement with the solid-state structuresdetermined by X-ray crystallography.
Introduction
In recent years, much attention has been focused on the de-velopment of functional supramolecular architectures.1–4 Cationreceptors in particular have been studied since the discovery ofcrown ethers.5 The fact that simple crown ethers show moderateselectivity towards cations of differing sizes has prompted thedevelopment of more selective ion receptors and spectroscopicreporter groups that respond to the presence of an ionic guest.Donor–acceptor interactions6 have been employed extensively indesigning such systems.7–10 This type of binding has frequentlybeen referred to as charge-transfer (CT) interaction, a term thathas also been used often to describe temporary electron-transferprocesses in otherwise unrelated systems, e.g. inter- and intramole-cular photophysical processes,11 metal–ligand coordination,5 non-covalent supramolecular interaction between p-systems12–14 andmethods of crystal engineering.15–17
We have shown earlier that alkali-metal cations can influencethe association of electron-donor host 1 and electron-acceptorguests such as A or B (Fig. 1) and that these systems showselectivity towards small, singly charged cations with geometricallyundemanding s-orbitals, i.e. Li+ and Na+.18,19 We also reported therole of naphthalene diimide acceptor molecule A in amplifyinga mixed porphyrin–naphthylglycol macrocycle from a dynamiccovalent disulfide library.20 Prior to this, we had studied the donor–
Department of Chemistry, University of Cambridge, Lensfield Road, Cam-bridge, UK CB2 1EW† Electronic supplementary information (ESI) available: 1H NMR datafor A·1, A·2 and A·1·2 complexes; 13C NMR data for A·2 complexes;NOESY and COSY data for A·2 complexes; kinetic measurements for A·1complexes. See DOI: 10.1039/b706043b‡ Present address: Chemistry Research Laboratory, University of Oxford,Mansfield Rd., Oxford, UK OX1 3TA; Email: [email protected]§ Present address: Department of Physics and Chemistry, University ofSouthern Denmark, Campusvej 55, 5230 Odense M, Denmark¶ Present address: Universite de Geneve, Sciences II, 30, QuaiErnest-Ansermet 3 bis, 1211 Geneva 4, Switzerland; Email: [email protected]
acceptor interactions between an electron-rich macrocycle (crownether 1) and pyromellitic diimide (acceptor guest B). We exploitedthe chemistry of acceptor molecules A and B in the synthesis ofa range of symmetric and asymmetric donor–acceptor catenanes,prepared using reversible alkene metathesis or irreversible Glasercoupling reactions.21–25 Switching of the acceptor A with B couldalso be induced in solutions of pseudorotaxanes in the presenceof an excess of lithium cations.18,26 The presence of alkali-metalcations preferentially reinforced the donor–acceptor interactionsbetween the host molecule 1 and the threading acceptor moleculeB. Therefore, significant differences in the solution behaviour ofthe host–guest complexes of the two acceptors, and an output inthe visible region of the electromagnetic spectrum, could renderthis system suitable for incorporation into a neutral rotaxane-based molecular switch.
Fig. 1 Schematic representations and partial labelling scheme for hosts 1and 2 and guests A and B (Hex : n-hexyl).
To explore the generality of this cation-induced effect we havenow carried out a parallel structural and kinetic study of thedonor–acceptor interactions between the naphthalene diimideguest A and two crown ethers of similar sizes, namely host 1and its linkage isomer 2 (Fig. 1); the latter has a slightly larger
3874 | Dalton Trans., 2007, 3874–3884 This journal is © The Royal Society of Chemistry 2007
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binding cavity. The alkali-metal salts used as templates are simpleLi and Na halides and triflates. A striking similarity is observedbetween the solid-state structures (studied by X-ray diffraction)and solution structures (determined by NOE experiments inCD2Cl2 or a 98 : 2 mixture of CHCl3 and MeOH), demonstratingthe robust nature of the pseudorotaxanes when mediated byalkali-metal cations. We also discuss the relative thermodynamicstabilities of complexes [M2·1·A]2+ and [M2·2·A]2+ based on aswitching experiment between the two hosts in solution mediatedby lithium salts.
Results and discussion
Pseudorotaxane formation
Mixing the host donor 1 and the guest acceptor A in CHCl3–MeOH (98 : 2) or CH2Cl2 induces the immediate formation ofthe deep red-coloured 1 : 1 complex [1·A] (Fig. 2). Thus, strongcomplex formation occurs even in the absence of alkali-metalcations, as confirmed by UV–vis spectroscopy (Table 1). This
Fig. 2 Alkali-metal cation induced colour changes of CHCl3–MeOH (98 :2) solutions of donor 1 and acceptors A or B (5 mM).
Table 1 UV–vis data for the series of pseudorotaxanes with guest A(2.5 mM solution in CH2Cl2)
MX kmax/nm DE/eV emax/L mol−1 cm−1
Host 1 No salt 428 2.90 1600LiBr 480 2.58 172LiI 492 2.52 287Li(CF3SO3) 500 2.48 734NaI 430 2.88 2400
Host 2 No salt — — —LiBr 526 2.36 304LiI 580 2.14 360Li(CF3SO3) 532 2.33 380NaI 522 2.37 634
behaviour is distinctly different from that of guest acceptor B,which displays only a very weak yellow colour under the sameconditions (Fig. 2), indicative of relatively inefficient complexationwith host 1. Addition of salts such as LiCl, LiBr, LiI, LiNO3,Li(CF3SO3), NaI or Na(CF3SO3) to the mixture of host 1 andguest A in solution leads to further colour changes, producingsolutions ranging from light orange to purple (Fig. 2). Thevariation of kmax (Table 1) in lithium complexes with host 1ranges from 480 nm (LiBr) to 500 nm (Li(CF3SO3)), comparedto 430 nm for NaI and 428 nm in the absence of any alkali-metalsalt. By analogy with our previously studied system [M2·1·B]X2,we propose general structures of the type [M2·1·A]X2 for thecomplexed species. The symmetric nature of the complexes isconfirmed by 1H NMR spectroscopy, and full assignments wereachieved for the most soluble pseudorotaxanes (those with X− =Br−, I− or CF3SO3
−; Table 2). The 1H NMR spectrum (CD2Cl2)of complex [Na2·1·A]X2 showed only small differences in thearomatic region with respect to those of the lithium-based series[Li2·1·A]X2: (i) for [Na2·1·A]X2 the proton resonances H4 and H5
of the bound guest are separated by Dd = 0.26 ppm, comparedto Dd = 0.14 ppm for [Li2·1·A]X2; (ii) the host’s aromatic protonresonances H1 and H2 are nearly overlapped in [Na2·1·A]X2, whilethey are ca. 0.2 ppm apart in [Li2·1·A]X2. When NaI was addedto mixtures of 1 and A in CD2Cl2, the 1H NMR spectra showedsignals assignable to the complex [Na2·1]I2, in addition to those of[Na2·1·A]I2 (Fig. 3). As before, we assign the exceptional behaviourof the NaI complex to the lower solubility of the salt in CH2Cl2
and to its greater tendency towards ion pairing compared to thatof LiX.
Fig. 3 1H NMR spectra showing formation of a guest free complex[Na2·1]I2 (*) together with pseudorotaxane formation: a) A:1 (2 : 1);b) B:1:LiX (2 : 1 : 10), X = Br−, I−, CF3SO3
−; c) A:1:NaI (2 : 1 : 10).
Kinetic studies
Rate constants for complex dissociation in CD2Cl2 solution (k−1)were estimated using 1D NOESY NMR experiments.27–30 Samplesof complexes were obtained using a two-fold excess of guest A anda ten-fold excess of the MX template respective to the host. Thereare two possible conformations of the guest inside the complex,yielding conformations I and II as shown in Fig. 4. The twoarrangements are related by ca. 60◦ rotation of guest A with respect
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Tab
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Fig. 4 Pseudorotaxane conformations I and II.
to host 1, and they give rise to different NOEs (at 220 K) betweenNCH2 and the H1–H3 resonances. For example, conformer I givesa strong NOE between H2 and NCH2, whereas H3 lies close toNCH2 in conformer II. For [Li2·1·A]I2, the NOEs to and from theNCH2 protons are illustrative: the strongest NOE is to H2 (whichalso exchanges with the NCH2CH2 protons), followed by NOEsto H3 and H1. The 1D and 2D NOESY experiments indicate thatboth modes of binding I and II are present in solution for allcomplexes formed between host 1 and guest B, regardless of thepresence or absence of alkali-metal salts. This contrasts with thesolid state (vide infra), for which conformation I has been observedfor the pseudorotaxane complex in the absence of alkali-metalcations, while conformation II is found for complexes [Li2·1·A]I2
and [Na2·1·A]I2.The 1D NOESY experiments also show that the free and bound
guest A are in exchange for all of the different MX salts. Table 3shows a summary of the activation barriers for decomplexation of[M2·1·A]X2 complexes in CDCl3–MeOD (98 : 2) at 220 K. Similarresults were observed in CD2Cl2. The temperature for eachexperiment was chosen such that the exchange process was slowon the NMR chemical shift timescale. For each complex, theresonances corresponding to the methyl and the protons of thebound guest A were irradiated in turn, and the responses ofthe corresponding resonances of the free guest were measured.The rates of complex dissociation were then extracted using theinitial rate approximation method.28–30 This assumes that the freeenergy of activation does not change within that temperaturerange. Equilibrium constants K could not be calculated, dueto the absence of free host 1 from the system. The exchangebetween the bound and free acceptor (monitored in 1H NMRspectra using the H4 and H5 resonances) was used to estimate therates of dissociation, k−1, whilst the exchange between the boundspecies aromatic resonances H4 and H5 describe the site exchangeas shown in Fig. 4. Most of the pseudorotaxanes show similardecomplexation rate constants, and there are no observed effectsof metal binding on the kinetics. The exception is the [Na2·1·A]I2
complex, which shows an on–off decomplexation exchange ca.5 times faster than those found in the LiX series. This is likelydue to the large size of NaI that may destabilize the [Na2·1·A]I2
complex. In the absence of LiX, decomplexation of complex [1·A]takes place ca. 10 times faster than when LiX is present. The siteexchange between H4 and H5 is similar for all LiX complexes,and is ca. 10 times faster than decomplexation. The slowest siteexchange process was found with NaI.
These experiments indicate that the host (donor 1) and the guest(acceptor A) are preorganized for the formation of a pseudorotax-ane. This is consistent with the colour change observed in the 1:Asystem, prior to the addition of alkali-metal cations. Although
3876 | Dalton Trans., 2007, 3874–3884 This journal is © The Royal Society of Chemistry 2007
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Table 3 Summary of activation barriers for decomplexation of [A·1·MX] complexes in CDCl3–MeOD (98 : 2) obtained by 1D NOESY (EXSY)experiments at 220 K (500 MHz).1 The rate constants were obtained by initial rate approximation; errors for rate constants are ± 20%. Hbound representsany resonance in the complex, Hfree represents any uncomplexed resonance
A:1:MX complex k (Hbound–Hfree)a/s−1 DG‡/kcal mol−1 k (Hbound–Hbound)/s−1 k (Hbound–Hbound)/k(Hbound–Hfree)
A:1 1.0 : 1.0 3.6 12.2 7.0 2A:1 2.2 : 1.0 0.37 13.2 7.7 20A:1:LiBr 2.2 : 1.0 : 10 0.32 13.2 7 22A:1:Li(CF3SO3) 2.1 : 1.0 : 10 0.44 13.1 7.0 16A:1:LiI 2.1 : 1.0 : 10 0.32 13.2 3.5 11A:1:NaI 2.6 : 1.0 : 10 2.2 12.4 2.9 1.3
Rate constants were adjusted for unequal populations.a Also k (Hfree–Hbound).
cation binding can occur, it causes only small structural changes,illustrated by the small changes in chemical shifts and almostno changes in kinetic properties of the complexes. This contrastswith the system formed by donor 1 and the smaller, poorerelectron acceptor B, where the presence of Li+ and Na+ cationssignificantly reinforces pseudorotaxane formation. Based on ourprevious studies on this class of compounds we therefore proposethe overall kinetic stability order in the [M2·1·A/B]X2 series to bethat shown in Fig. 5.
Studies with host 2
To ascertain the effect of subtle conformational changes in thehost molecule on the kinetic stability of the pseudorotaxanesin the presence and absence of alkali-metal salts, we preparedcrown ether host 2, which is a linkage isomer of host 1 with aslightly larger cavity. Similar experiments to those described abovewere performed using the electron-donor host 2, the electron-acceptor guest A and some of the alkali-metal salts that provedsuccessful in the earlier studies, i.e. LiBr, LiI, Li(CF3SO3), NaIand Na(CF3SO3). In this case, mixing acceptor A and donor 2in CDCl3–MeOD (98 : 2) or CD2Cl2 yielded 1 : 1 complexesonly in the presence of metal cations. The colour of the resultingpseudorotaxanes was generally darker than those observed for the
analogous series of complexes based on host 1 (Table 1), implyinga more efficient electron transfer between the guest and the host.As was the case for host 1, the solution structures of [M2·2·A]X2
(analyzed at 220 K where decomplexation exchange stopped onthe NMR timescale) are closely comparable to their solid-statestructures (vide infra).
Fig. 6 shows the qualitative geometries of typical pseudoro-taxanes derived from the 1D and 2D NOESY experiments(partial spectra and full assignments are given in the SupportingInformation†).
The kinetic stability of [M2·2·A]X2 complexes depends on thenature of the cation, the nature of the anion and on the A:2:MXratio. In contrast to the complexes of host 1, addition of excessacceptor A to host 2 in the presence of an MX salt led to complexeswith a wide range of kinetic stabilities.
In this and our previous studies involving pseudorotaxanesof host 1, we have observed that the Li+ complexes are morekinetically stable than the Na+ complexes. Surprisingly, whenhost 2 was used, the influence of the cation (Na+ or Li+) onthe chemical shifts was relatively small. For all alkali-metal salts,there are only small differences for the bound aromatic donorprotons H1–H3, although the chemical shifts of H4 and H5
seem to be affected by the nature of the anion. In particular,the chemical shift difference for bound acceptor protons H4
Fig. 5 Proposed kinetic stability order in the [M2·1·A/B]X2 series.
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Fig. 6 Spatial structures (from NOE data) of typical pseudorotaxanes.
and H5 are much larger for LiBr and LiI complexes than forthe Li(CF3SO3) complex (see Supporting Information†). Largervariations of chemical shifts with the nature of anions werefound for the OCH2 proton resonances that are closest to thenaphthalene aromatic ring. It appears therefore that the glycol‘loop’ points in different directions in solution when host 1 isreplaced with host 2: in [Li2·1·A]X2, this loop is oriented away fromthe complex, whereas in [Li2·2·A]X2, it faces towards the cavity ofthe complex. This could shield H4 and therefore shift its resonanceupfield. This suggestion is confirmed by the solid-state structures(vide infra).
NOE experiments allowed direct comparison between the solu-tion structures of [Li2·1·A]I2 and [Li2·1·A](CF3SO3)2. Surprisingly,large 1H NMR chemical shift differences are observed uponchanging the anion from triflate to halides, for both host 1 and 2.X-Ray structure determination of [Li2·2·A](CF3SO3)2 (vide infra)indicates that this complex forms extended 1D chains in the solidstate by incorporation of a further equivalent of Li(CF3SO3),which links the complexes through interactions between Li+ andthe O atoms of the triflate anions. The different behaviour ofthe Li(CF3SO3) complexes in solution compared to the lithiumhalide complexes could be explained if this aggregation is main-tained to a significant degree in solution, especially at low tem-peratures.
Thermodynamic stabilities and switching of hosts
The apparent association constants for the binding of host 1to guest A in the presence and absence of LiI in a mixture ofCHCl3–MeOH (98 : 2) was determined using isothermal titration
microcalorimetry (ITC‖). This technique permits direct determi-nation of the binding constant (and, hence, DG◦) and the enthalpyof binding, and therefore allows calculation of the entropy ofbinding. The association constant K is found to be ca. 1 ×102 M−1 in the absence of any alkali-metal cation (thermodynamicparameters: DG◦ = −11.5 kJ mol−1, DH◦ = −22.9 kJ mol−1,TDS◦ = −11.3 kJ mol−1). Although this association must be basedsolely on stacking interactions between the aromatic systems of thedonor and acceptor, the binding is five times stronger than whenthe pyromellitic diimide guest B is used as an acceptor. In thepresence of 50 mM LiI, (10 equivalents excess) the effective affinityfor the acceptor A remains essentially unchanged, with K = 1.3 ×102 M−1 (thermodynamic parameters: DG◦ = −0.12 kJ mol−1,DH◦ = −14.5 kJ mol−1, TDS◦ = −2.5 kJ mol−1). Inspection ofthese thermodynamic parameters indicates that the presence ofLiI destabilises the complex, thereby acting as a negative template.This is in marked contrast to the behaviour of guest B, forwhich Li+ and Na+ cations reinforce the formation of [M2·1·B]X2
complexes.
‖ We have chosen to titrate guest A into a solution of 1 containing alarge excess of LiI and under these conditions it is reasonable to assumethat binding of Li+ to 1 is essentially complete, since this could not beindependently confirmed. Furthermore, the use of the large excess of LiIensures that the concentration of I− counterions in the solution is relativelyconstant independent of the presence of the complex, thus eliminatingcomplications arising in the determination of the dissociation constantfor the ion pair.31 Data analysis is now simplified since we only need toconsider 1 : 1 binding of the acceptor A to the pre-formed [Li2·1]I2 complex.Titration of a 200 mM solution of acceptor into a solution containing5.0 mM crown ether and 50 mM LiI fitted a 1 : 1 binding model.
3878 | Dalton Trans., 2007, 3874–3884 This journal is © The Royal Society of Chemistry 2007
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Table 4 Summary of activation barriers for decomplexation of A : 2 : salt complexes in CDCl3–MeOD 98 : 2 obtained by 1D NOESY (EXSY)experiments at different temperatures at 500 MHz. The rate constants were obtained by initial rate approximation;29,30 errors for rate constants are ±20%. Hbound represents any resonance in the complex, Hfree represents the proton resonances of any uncomplexed species
Components ratio T/K k (Hbound–Hfree)a/s−1 DG‡/kcal mol−1 k (Hbound–Hbound)/s−1 k (Hbound–Hbound)/k (Hbound–Hfree)
A:2:LiBr 1 : 1 : 10 220 4–9 12.0 4 0.5–1A:2:LiBr 2 : 1 : 20 230 0.02–0.06 14.8 0.3–0.5 15A:2:LiCl 1 : 1 : 10 260 3.2 14.5 1.4 0.5A:2:LiCl 2 : 1 : 10 280 1.2–3.8 15.8 3.6–4.5 1–3A:2:LiI 1 : 1 : 10 280 1 16.3 2.9 3A:2:LiI 2 : 1 : 10 310 0.06 19.9 2.6–3.1 50A:2:Li(CF3SO3) 1 : 1 : 10 220 0.41 13.1 — —A:2:Li(CF3SO3) 2 : 1 : 10 230 0.15 14.2 — —
260 1.8 14.8 — —260 6.8 14.1 — —
A:2:Li(CF3SO3) 1 : 1 : 20 255 1.2–2 14.6 1.6–2 1A:2:NaI 2 : 1 : 10 240 0.9–1.5 13.8 — —A:2:Na(CF3SO3) 2 : 1 : 10 220 2.5 12.3 — —
When several resonances within one molecule were irradiated, the range of k is given.a This is also k (Hfree–Hbound).
Fig. 7 Selected 1H NMR and 1D ROESY spectra (acceptor guest A and crown ether protons, 9–5.5 ppm) of A:1:2:MX mixtures in CDCl3–MeOD 98 :2 (220 K). The arrow marks the irradiated resonance. ‘Ex’ or ‘NOE’ labels mean that an above signal is due to exchange or NOE, respectively. Colourcode: red for crown host 1, blue for crown host 2, black for free acceptor molecule A.
We have previously demonstrated switching of the acceptors Aand B in solution for the system based on host 1, and showed inthat case that LiBr can be used to trigger the acceptor exchange inCDCl3–MeOD (98 : 2) or CD2Cl2.18,26 The switching experimentbetween hosts 1 and 2 in solution allows us to test whethercomplexes such as [Li2·2·A]X2 can be prepared selectively in thepresence of host 1. This experiment provides a useful insight intothe thermodynamic stability of this series of complexes, sincelimited solubility prohibits direct measure of the thermodynamicparameters by ITC. Competition experiments using mixturescontaining host 1, 2 and guest A were monitored in CD2Cl2
solutions in the absence or presence of LiI and Li(CF3SO3)
between 260 and 220 K.†† This range of temperatures was chosenbecause of the differing kinetic stabilities of the species involved,as expected from Table 4. Fig. 7 shows the aromatic region ofthe resulting 1H NMR spectra. In the absence of LiX, only
†† The temperature for each experiment was chosen such that the exchangeprocess was slow on the NMR chemical shift timescale. The chemicalshifts of the exchanging resonances were not significantly temperaturedependent. For each complex, the resonances corresponding to the methyland the H4 protons of the bound guest were irradiated in turn and theresponses of the corresponding resonances of the free guest were measured.The rates of complex dissociation were then extracted using the initial rateapproximation method.
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complex [1·A] is formed, while host 2 remains mainly unboundin solution. Addition of Li(CF3SO3) induces the formation of[Li2·2·A](CF3SO3)2 at the expense of [1·A]. Under these conditions,most of host 2 is found in the bound state, leaving a significantamount of host 1 free in solution. Irradiation of H3 of bound host1 showed exchange with the H3 resonance of free host 1. Therefore,free and bound crown ether host 1 interchange at 220 K, whereasfree and bound host 2 interchange only at 240 K. At 240 K, threespecies can be observed in the equilibrated solution, namely freeguest A, complex [1·A] and [Li2·2·A](CF3SO3)2. Thus, in presenceof excess Li(CF3SO3), [Li2·2·A](CF3SO3)2 is clearly kinetically andthermodynamically more stable than [Li2·1·A](CF3SO3)2, [1·A] or[2·A]. When LiI was used instead of Li(CF3SO3), the ratio ofbound host 2 to bound host 1 increased slightly, but the sameoverall behaviour was observed for the switching experiment (seeTable 5).
Solid-state structures
Single crystals suitable for X-ray diffraction analysis were obtainedfor [1·A], [Li2·1·A]I2, [Na2·1·A]I2, [Li2·2·A]Br2, [Li2·2·A](CF3SO3)2
and [Na2·2·A]I2 from a mixture of CHCl3–MeOH (98 : 2).Crystallographic details are summarised in Table 6, and selectedgeometric parameters are listed in Table 7. The X-ray analysesconfirm that the solid-state structures of all pseudorotaxanesare closely comparable to the solution structures determined byNMR spectroscopy (Fig. 8). In each case, the complex lies on acrystallographic centre of inversion. The interplanar separationbetween the donor and acceptor rings lies in the range 3.21–3.62 A, consistent with the interplanar separations of otherpseudorotaxanes19 and catenanes.23,25 The mean planes throughthe central unit of A and the naphthalene rings of the host areessentially parallel in each case, with a maximum deviation of 4.2◦
found in the cation-free complex [1·A].The crystallographic characterisation of complex [1·A] and its
salts [Li2·1·A]I2 and [Na2·1·A]I2 permits a precise description ofthe structural changes that take place on addition of alkali-metal cations, which was not possible previously for the [1·B]system.19 For the complexes of host 1, NMR indicates twopossible conformations in solution, as shown in Fig. 4. In thesolid state, conformation I is observed for complex [1·A] in theabsence of any alkali-metal cations, whereas conformation IIis observed for the complexes [Li2·1·A]I2 and [Na2·1·A]I2. Thelarger host–guest torsion angle in [Na2·1·A]I2 compared to theother two complexes (bringing the host and guest much closerto the perpendicular arrangement observed for [Na2·1·B][B{3,5-(CF3)2C6H3}4]2)19 clearly reflects the relatively larger size of Na+.In [Li2·1·A]I2, the Li+ cations are five-coordinate, with threecoordination sites occupied by O atoms of the host 1, onecoordination site occupied by one O atom of the guest A, andthe fifth coordination site occupied by a methanol molecule.The coordination geometry resembles a square pyramid with theMeOH molecule in the apical position. The s parameter32 of 0.07indicates only a minor distortion from tetragonal geometry. TheLi+ cation lies ca. 0.5 A above the basal plane defined by the Oatoms of host 1 and guest A. In [Na2·1·A]I2, relatively short Na–Idistances of 3.332(2) A exist, so that Na+ is better described as 6-coordinate. Nonetheless, the coordination geometry is comparableto that in [Li2·1·A]I2, except that the Li–O(H)Me bond in that T
able
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16:1
3880 | Dalton Trans., 2007, 3874–3884 This journal is © The Royal Society of Chemistry 2007
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Tab
le6
Sele
cted
crys
tallo
grap
hic
data
a
[1·A
]·2C
HC
l 3[L
i 2·1·
A]I
2·2M
eOH
·4CH
Cl 3
[Na 2
·1·A
]I2·2H
2O
·2CH
Cl 3
[Li 2
·2·A
]Br 2
·2H2O
·8CH
Cl 3
[Li 2
·2·A
](C
F3SO
3) 2
·2{
Li(
CF
3SO
3)}
·2CH
Cl 3
[Na 2
·2·A
]I2·8C
HC
l 3
For
mul
aC
64H
76C
l 6N
2O
14C
68H
86C
l 12I 2
Li 2
N2O
16C
64H
80C
l 6I 2
N2N
a 2O
16C
70H
86B
r 2C
l 24L
i 2N
2O
16C
68H
76C
l 6F
12L
i 4N
2O
26S 4
C70
H82
Cl 2
4I 2
N2N
a 2O
14M
1309
.97
1880
.47
1645
.78
2235
.91
1934
.01
2325
.96
k/A
0.71
070.
7107
0.71
070.
6923
b0.
6904
b0.
7107
T/K
180(
2)18
0(2)
180(
2)15
0(2)
150(
2)18
0(2)
Cry
stal
syst
emT
ricl
inic
Mon
oclin
icM
onoc
linic
Tri
clin
icT
ricl
inic
Tri
clin
icSp
ace
grou
pP
–1P
2 1/c
P2 1
/nP
–1P
–1P
–1a/
A10
.923
7(3)
12.1
350(
5)10
.760
4(5)
10.0
570(
5)11
.413
(6)
11.0
338(
3)b/
A12
.307
8(3)
10.6
602(
2)23
.624
5(6)
15.2
801(
7)12
.236
(5)
13.4
294(
3)c/
A13
.887
7(4)
32.3
295(
12)
15.4
728(
6)17
.222
3(8)
16.2
74(5
)17
.651
3(4)
a/◦
67.9
87(1
)90
9070
.219
(2)
98.1
9(3)
68.0
66(1
)b/
◦86
.014
(1)
93.6
92(1
)10
7.28
1(1)
78.4
84(2
)10
5.66
(2)
81.7
61(1
)c
/◦69
.563
(2)
9090
83.2
67(2
)99
.55(
4)86
.115
(1)
U/A
316
18.0
3(8)
4173
.5(2
)37
55.8
(2)
2436
.7(2
)21
15.6
(16)
2400
.87(
10)
Z1
22
11
1q c
alc/
gcm
−31.
344
1.49
61.
455
1.52
41.
518
1.60
9l/
mm
−10.
330
1.19
81.
124
1.54
70.
404
1.38
8To
tald
ata
1773
119
389
2195
824
659
1481
226
869
Uni
que
data
7299
6987
6586
1305
573
5010
852
Rin
t0.
049
0.05
70.
052
0.02
90.
029
0.04
7R
[I>
2r(I
)]0.
059
0.07
20.
067
0.06
20.
102
0.07
4w
R2
(all
data
)0.
138
0.20
70.
181
0.17
30.
290
0.20
4G
oodn
ess
offit
1.04
1.06
1.02
1.08
1.08
1.07
aT
hecr
ysta
lstr
uctu
reof
2·2C
HC
l 3is
give
nin
the
Supp
orti
ngIn
form
atio
n.b
Sync
hrot
ron
radi
atio
n,St
atio
n9.
8,D
ares
bury
SRS.
case bisects the I–Na–OH2 angle in [Na2·1·A]I2. The Na+ cationlies ca. 1.0 A above the basal plane defined by the O atomsof host 1 and guest A, demonstrating the far less effectivefit of Na+ compared to Li+ in the binding site of the [1·A]complex.
For the complexes of crown ether host 2 with guest A, thehost and guest lie closer to a perpendicular arrangement (Fig. 9),reflecting principally the substitution pattern on the naphthalenerings of host 2, i.e. bringing the O atoms of guest A into the positionrequired to bind M+ causes the long axis of guest A to lie approxi-mately perpendicular to the central C–C bond of the naphthalenerings in 2. The Li+ cations in [Li2·2·A]Br2 and [Li2·2·A](CF3SO3)2
are 5-coordinate (with the fifth coordination site occupied byH2O and CF3SO3
− in the two complexes, respectively), althoughthe coordination geometry differs significantly from that in thecomplexes with host 1. Specifically, the coordination environmentis distorted more significantly towards trigonal-bipyramidal (s ≈0.29), with the equatorial plane approximately coincident with theplane of the guest A, and the axial positions defined by two Oatoms from host 2. The absence of ion pairing in [Li2·2·A]Br2 isconsistent with the NMR observations that the solution structuresof the lithium complexes are not significantly influenced by thenature of the anion when X = Br− or I−. These observationsare also consistent with those made by Huang et al. regardingthe direct correlation between ion-pairing in solid state and insolution.33
The orientation of the guest A with respect to the host 2 isessentially indistinguishable in [Li2·2·A]Br2, [Li2·2·A](CF3SO3)2
and [Na2·2·A]I2, consistent with the observation that the identityof the cation (Li+ or Na+) has little influence on the chemicalshifts in solution 1H NMR spectra. Indeed, the coordinationgeometry of the Na+ site in [Na2·2·A]I2 is essentially identicalto that of Li+ in [Li2·2·A]Br2 and [Li2·2·A](CF3SO3)2. This resultsin surprisingly short Na–O and Na–I distances (Table 7). Thedisplacement ellipsoid refined for Na+ is rather large compared toits neighbours, suggesting that it may display fractional occupancy.The possibility that the site is actually occupied by Li+ is notsupported by the X-ray data (the displacement ellipsoid reducesto zero if the atom is refined as Li+), and there was no indicationof this in the NMR spectra.
Compared to the complexes with host 1, the cation bindingsites adopt significantly different orientations with respect to theguest A. In the [M2·1·A]X2 complexes, the vector from M+ to theapical position of the approximate square-pyramidal coordinationgeometry (or to the mid-point of the two atoms that straddle thatposition in the 6-coordinate Na+ complexes) lies close to parallel tothe n-hexyl chains of guest A (Fig. 8). In the [M2·2·A]X2 complexes,the comparable vector lies close to perpendicular to the n-hexylchains. The former arrangement has the glycol chains of the host1 wrapped around the edge of the guest A, introducing relativelyshort contacts between the glycol chains and H4 of the guest. In thelatter arrangement, the glycol chains of the host 2 point away fromthe guest A, and there are no such short contacts. This structuraldifference confirms the NMR observations regarding the differentdegrees of shielding for H4 in the complexes with hosts 1 and 2.The perpendicular arrangement is also observed in the previously-reported complexes [Li2·1·B]Br2, [Li2·1·B]I2, [Li2·1·B][B(C6F5)4]2
and [Na2·1·B]I2 (but not [Na2·1·B][B{3,5-(CF3)2C6H3}4]2), so thatthe distinctly different behaviour of the [1:A] and [1:B] systems
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Table 7 Relevant molecular parameters from X-ray diffraction studies for the complexes studied
[1·A] [Li2·1·A]I2 [Na2·1·A]I2 [Li2·2·A]Br2 [Li2·2·A](CF3SO3)2 [Na2·2·A]I2
M–O (guest)/A — 1.951(12) 2.276(5) 1.934(5) 1.955(9) 2.033(6)M–O (host)/A — 1.987(13) 2.333(5) 2.036(4) 2.010(9) 2.164(6)
2.282(15) 2.615(5) 2.215(5) 2.195(10) 2.291(6)2.290(15) 2.629(5) 2.250(5) 2.227(10) 2.334(6)
M–Ligand/A — 1.863(14) (MeOH) 3.332(2) (I−) 1.918(5) (H2O) 1.899(9) (CF3SO3−) 2.881(4) (I−)
2.350(6) (H2O)Host–guest separation/Aa 3.53 3.48 3.43 3.38 3.44 3.49Host–guest torsion angle/◦b 59.6 70.7 74.9 77.5 77.6 79.8Deviation from parallel forhost–guest planes/◦c
3.5 1.9 2.0 3.5 2.5 2.1
a Defined as half the separation between the centroids of the two naphthalene rings in the host molecule. b Defined as theN(guest)/centroid(guest)/centroid(naphthalene)/C(naphthalene) torsion angle (see Fig. 9). c Dihedral angle between least-squares plane through 20atoms of the central group in A and all 10 atoms of one naphthalene ring in the host.
Fig. 8 Molecular structures of (a) [1·A], (b) [Li2·1·A]I2, (c) [Na2·1·A]I2, (d) [Li2·2·A]Br2, (e) [Na2·2·A]I2 and (f) [Li·2·A](OTf)2. (g) shows the extendednetwork in the crystal structure of [Li·2·A](OTf)2.
3882 | Dalton Trans., 2007, 3874–3884 This journal is © The Royal Society of Chemistry 2007
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Fig. 9 The quasi-perpendicular arrangement between donor and accep-tor groups in the structures of [Li2·1·A]I2 and [Li2·2·A]Br2 showing thetorsion angles between the host and guests molecules.
with respect to the destabilising or stabilising influence of LiI iscorrelated with the observed parallel or perpendicular conforma-tions.
The complex [Li2·2·A](OTf)2 co-crystallises with two addi-tional molecules of lithium triflate, which link the pseudorotax-ane units together to form extended 1D chains (Fig. 8f,g). Thepseudorotaxane-bound Li-coordinated counterion CF3SO3
− isalso coordinated through one of its O atoms to a ‘pseudorotaxane-free’ Li+ cation, which is in turn ion-paired with a CF3SO3
− anion.This aggregation most likely accounts for the different behaviourof the Li(CF3SO3) complexes in solution compared to the lithiumhalide complexes.
In this, and previous work we showed that [1·A] can formwithout M+ present, and that the addition of MX destabilisesit. The complex [1·B] requires M+ to form and MX stabilises it.Also, the complex [2·A] requires M+ to form and MX stabilisesit. In [1·A], where no metal ions are present, the glycol chains“wrap around” the edge of the guest A. The C–H · · · O interactionscontribute to the enthalpic stabilisation. In the [1·B] complex (nometal ion), the smaller pyromellitic diimide guest must interact lessstrongly with the glycol chains. In [2·A], the relative positions ofthe glycol chains do not allow the glycol chains to “wrap around”the guest. Again the interaction between the guest and the glycolchains must be less significant. Therefore, the interactions betweenthe guest and the glycol chains could account for the stability of[1·A] (no metal ion) compared to [1·B] and [2·A].
When the metal ions bind to [1·A], the interactions of the guestwith the glycol chains are disrupted. The disruption of the guest–glycol chain interactions is most significant for [1·A], while theenthalpy gain on complexation of the metal is probably about thesame for each case.
Thus, [1·A] is the only complex studied that is overall desta-bilised by metal complexation.
Conclusions
Lithium and sodium cations destabilise the p–p donor 1–acceptorA interaction, but reinforce the formation of donor 1–acceptorB complexes. Small structural modifications in the host or guestgeometries affect the relative kinetic stabilities in the series. NaItends to remain associated within all complexes in solid stateand solution, and the triflate complexes tend to give polymericstructures.
In absence of salt only the acceptor A–donor 1 system iscapable of forming a kinetically stable complex, whereas acceptorA–donor 2 does not. In the presence of salt, the acceptor A–donor 2 system forms the most stable (thermodynamically andkinetically) complexes, followed by the acceptor A–donor 1 systemwhich is essentially unaffected by the presence of salt. Switchingbetween the acceptor A–donor 1 system to acceptor A–donor2 pseudorotaxanes is possible by addition (or removal) of Li+
cations.Significant differences in behaviour in solution between the two
donor molecules, coupled with output in the visible, render thissystem suitable to forming molecular switches.
Experimental
NMR spectroscopy
1H and 13C NMR spectra were recorded on a Bruker Avance500 MHz spectrometer using the residual 1H of the deuteratedsolvent as a reference. The pulse sequence used for 1D NOESYexperiments was selnogp.2.
UV–vis spectroscopy
UV–vis spectra were recorded on Hewlett-Packard 8452A diodearray spectrometer. All UV–vis samples were prepared in freshlydistilled CH2Cl2 and recorded at 25 ◦C.
Isothermal titration calorimetry
ITC experiments were performed using an MCS IsothermalTitration Microcalorimeter (Microcal Inc. Northampton, MA,USA). Host–guest titrations were corrected for heat of dilution ofthe syringe contents by subtracting blank titrations into solvent(CHCl3 : MeOH 98 : 2). Initially, a 250 mM solution of crown 1 wastitrated into a 25 mM solution of acceptor A. Then, a 200 mMsolution of acceptor A was titrated into a solution containing50 mM LiI and 5 mM crown ether host 1.
Preparation of crown ether 1 and 2
Crown ethers 1 and 2 were prepared following the procedure ofStoddart et al.34 The molecular structure of 2 was determined(Supplementary Information).†
General procedure for the formation of the cation templatedpseudorotaxanes
To the 5.0 mM solutions of crown ether 1 (3.2 mg, 5.5 lmol, inCH2Cl2 or CHCl3 : MeOH 98 : 2) the solid aromatic diimide 2(1.92 mg, 5.5 lmol) was added. In each case, alkali metal halides(anhydrous) were added in excess (as ca. 20 mM solutions inCH2Cl2 or CHCl3 : MeOH 98 : 2). The suspensions were sonicatedfor 30–60 seconds. Excess salts were separated by filtration, andproducts were isolated in quantitative yield after removal ofsolvent under reduced pressure.
Crystal structure determination
Crystals were isolated by filtration, and a specimen crystal wasselected under an inert atmosphere, covered with polyfluoroether
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oil, and mounted on the end of a nylon loop. Crystallographic dataare summarised in Table 6. Data for complexes [1·A], [Li2·1·A]I2,[Na2·1·A]I2 and [Na2·2·A]I2 were collected at 180 K on a NoniusKappaCCD instrument equipped with graphite monochromatedMoKa radiation (k = 0.7107 A). The images were processedwith the DENZO and SCALEPACK programs.35 Crystals of2, [Li2·2·A]Br2 and [Li2·2·A](CF3SO3)2 were small and weaklydiffracting, and data for these compounds were collected (at 150K) at Station 9.8, Daresbury SRS, UK, using a Bruker SMARTCCD diffractometer (k = 0.6923 A). The structures were solved bydirect methods using the program SIR9236 and refined against allF 2 data using the SHELXTL software package.37 CCDC referencenumbers 644501–644507. For crystallographic data in CIF orother electronic format see DOI: 10.1039/b706043b
Acknowledgements
We thank Dr Sijbren Otto for ITC training and helpful discussions,the Royal Society for a University Research Fellowship (SIP) andBBSRC, EPSRC, AstraZeneca and GSK for financial support.
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3884 | Dalton Trans., 2007, 3874–3884 This journal is © The Royal Society of Chemistry 2007
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