Rotaxanes and pseudorotaxanes with threads containing viologen units

In the present review rotaxanes and pseudorotaxanes with threads containing viologen units are described. First the rotaxanes and pseudorotaxanes in which the crown ether serves as a ring are presented, they are followed by rotaxanes and pseudorotaxanes containing the crown-based cryptand as a ring. For the above interlocked species the synthetic approaches and properties, especially those promising for their use in sensors and switches are shown


Introduction
Rotaxanes [1][2][3][4][5] and pseudorotaxanes [6][7][8][9] are examples of mechanically interlocked species, 10,11 besides them, to interlocked architectures belong also catenanes, [12][13][14][15] as well as more complex structures such as trefoil knots, 16 Solomon knots, 17 Borromean rings, 18 daisy chains 19 or ravels.Rotaxanes are a topic of enormous number of reports due to their valuable properties, interesting in the construction of molecular machines [21][22][23][24] and supramolecular polymers. 25,26seudorotaxanes are also intensively studied, they are prototypes of molecular machines [27][28][29] and serve as building blocks for interlocked supramolecular assemblies, 30 some of them are promising in preparation of molecular sensors 31 and in drug delivery. 32here should be mentioned here also rotaxanes and pseudorotaxanes containing cyclodextrins, [33][34][35] calixarenes 36,37 or cucurbiturils. 38,39][56][57][58][59] In the paper rotaxanes and pseudorotaxanes with threads containing viologen units are described.They are divided into two classes: those containing crown ether as a ring, presented in the first part, and those containing the crown-based cryptand as a ring, shown in the second part.The review is a continuation of our former papers dealing with supramolecular structures with quaternary azaaromatic character. 40,60aking into account the great number of reports concerning rotaxanes and pseudorotaxanes with threads containing viologen units, in the review only selected examples of such species are presented.

2.1.Rotaxanes
From among a great number of rotaxanes built from a thread containing viologen, and having crown ether as a ring, some examples will be presented.Design of molecular machines is today a great challenge on the scientific field, below selected works concerning this topic are described.
The operation mechanism of bistable rotaxane 1 6+ which acts as an molecular shuttle driven by visible light was studied by computational calculations.The shuttling free energy and shuttling relaxation time were calculated and the experimental estimations for the relative stability of the different molecular co-conformations were reproduced.
It was established that the rotaxane 1 6+ undergoes photoinduced ring shuttling in solution, its thread contains ruthenium (II) complex [Ru(bpy)3] 2+ (P 2+ ) serving as a photosensitizer and as a stopper, the second stopper is T. The electron-accepting stations are 4,4'-bipyridinium unit, i.e.V1 2+ and 3,3'-dimethyl-4,4'-bipyridinium unit, i.e.V2 2+ .The rigid spacer S consists of three pphenylene units, and the -electron-donating ring is bis-p-phenylene [34]crown-10.The V1 unit is a stronger electron-acceptor station than V2, therefore the ring encircles V1 unit, and it is the stable translational isomer of 1 in the ground state.Step 3 Step 4 Step 5 e Step 6 e + + Step 7 a = electron transfer from P to V The quantitative comparison of calculations with experimental findings can be achieved by calculating the MM (molecular mechanical) free-energy profile and then the shuttling relaxation time by solving the Fokker-Planck equation.
The role of counterions is very important and should be taken into account for the computational reproduction of the observed shuttling kinetics.Anions are strongly bound to the thread, it was shown that the shuttling is almost barrierless in the absence of counterions.If the interaction between the positively charged rotaxane and its counteranions could be weakened, either by changing the counterion or by changing the solvent, such system would be able to work as a fast-switching molecular machine.
Another structure interesting in building molecular machines is the bistable system 2 6+ based on a two-component [c2]daisy chain stoppered at both ends.Each component consists of the crown ether ring bound with the thread containing two stations, i.e. viologen and NH2 + units (Scheme 2). 62n this system, in CD3CN solution the two crown ether rings move between two recognition sites under acid-base control; during this process the length of the whole system (i.e. the distance between two stoppers) undergoes contraction by treatment with a base (A→B) and expansion by treatment with acid (B→A).Such behavior is similar to the action of a muscle.
In A the rings encircle the -NH2 + -units; the rings are close by, i.e. the system is expanded, the distance between two stoppers is a.Upon addition of phosphazene P1-tBu base the -NH2 + -units undergo deprotonation, resulting in shift of both rings to viologen units, this motion leads to formation of B, where rings are far away from each other, i.e. the system is contracted, the distance between two stoppers is b.Since a>b, the system behaves as a muscle.The addition of trifluoroacetic acid causes the reprotonation of -NH-groups to-NH2 + -and return to original expanded system A. The above reversible muscle-like structures are promising for design of molecular machines.
In daisy chains the molecules are linked together by mechanical bonds; the mechanically interlocked molecules (MIMs) attract today a growing attention. 10,63][66][67][68][69][70] Below the synthesis of polymer daisy chain 3 using "threading-followed-by-polymerization" method is described. 71It is noteworthy that in this procedure the tedious syntheses of monomers are avoided.The synthesis begins with the treatment of acid 4 containing crown ether unit with thionyl chloride affording acid chloride 5 which directly, without purification was mixed with viologen derivative 6 in acetone at -70 °C, this low temperature was employed to minimize the esterification and maximize the complexation of 6 with 5 affording the pseudorotaxane-type monomer 6•5.The temperature was allowed next to rise slowly to -50 °C and triethylamine was added to catalyze esterification, then the reaction mixture was warmed to room temperature.
The in situ generated 6•5 underwent polymerization to give the polymer daisy chain 3.After the exchange of Cl‾ into PF6‾ ions, the side products were removed by precipitation with DMSO.
It was established that 45 repeating units 7 exist in a single polymer chain of 3, the molecular weight of 3 is 64 kDa. 72he above results show that the dynamic supramolecular monomers, i.e. monomers with different parts linked by noncovalent bonds may serve for convenient construction of mechanically interlocked polymers (Scheme 3).
In order to perform a quantitative conformational study of redox-active [2]rotaxanes, at first the conformational analysis of [2]rotaxane 8 2+ serving as a model and of its radical cation 8 +• was made.Rotaxane 8 2+ consists of the viologen V 2+ , the ring 9 and stoppers A and B. The stopper A is rigidly connected with viologen, while the stopper B ia attached to viologen by a flexible polyether chain.In this investigation the combined electrochemical techniques and NMR spectroscopic methods such as COSY, NOESY and PASSY (paramagnetic line-broadening and suppression) were used (Scheme 4).It was observed that in the rotaxane 8 2+ in its dicationic V 2+ state in solution the thread is folded, the B stopper being directed back to the centre of the molecule, the crown ether 9 encircles the viologen site.Upon one-electron reduction of viologen, affording the radical cation V +• , the thread retains its folded conformation and 9 remains localized at the V +• , although V +• has lower affinity than V 2+ for 9.It was found that even in the case of the fully reduced viologen V 0 , the ring 9 still resides at the viologen unit. 73he above study of the simple [2]rotaxane 8 i.e. containing one viologen unit was followed by conformational study in solution of two bistable rotaxanes 10 and 11, i.e. containing two viologen units. 74The investigation of both rotaxanes 10 and 11 in their parent and reduced states was made using the same electrochemical and NMR spectroscopic methods as in the former experiment. 73

Scheme 5
Tripodal stopper A serves in rotaxanes 10 and 11 for attachment of the thread to the surface of semiconductors or metals; in experiments the attachment to the surface of TiO2 nanoparticles and nanostructured electrodes was made.Both rotaxanes 10 and 11 contain flexible polyether PE units connecting V2 with the stopper B while their links between stations V1 and V2 have different rigidity.In rotaxane 10 the viologen units V1 and V2 are bound by a flexible trimethylene chain L1, while in rotaxane 11 the viologen units V1 and V2 are linked by a rigid terphenylene chain L2 (Scheme 5). 74t should be pointed out that the flexibility of the chains L1 and L2 influences the shuttling of the ring.In 10 the trimethylene link L1 is flexible and may be folded, therefore the shuttling of the ring is limited, while in 11 the terphenylene link L2 is rigid and the shuttling proceeds without limitations.In the study the role of intramolecular folding in the shuttling of the ring 9 in rotaxanes 10 and 11 was examined.
In 10 and 11, in the parent state the ring resides at V1, since V1 is more electron deficient and less sterically hindered than V2.At the beginning the behavior of 10 and 11 was investigated when V1 was reduced to the first reduced state.It was possible to selectively reduce V1 (without the reduction of V2), since V1 and V2 have sufficiently different reduction potentials.The reduction of V1 in 10 and 11 may be performed by applying an appropriate negative potential or by using Zn metal, under these conditions the stations V2 do not undergo reduction; as a result 10•3PF6 and 11•3PF6 in singly reduced radical cation states are formed.
The given above NMR methods enable the monitoring of structural and conformational changes of [2]rotaxane molecule at different oxidation states.It was found that in radical cations of reduced viologens the unpaired electron density causes a measurable PRE (paramagnetic relaxation enhancement).The PRE values allow to characterize the localization of the ring and to estimate the conformation of the thread of the [2]rotaxane in the reduced states.
In 10•3PF6 the viologen V2 is folded back toward V1, so that methyl groups of V2 can approach V1.The analysis of PRE has shown that the stopper B starts to be in the vicinity of V1 due to the folding of L1 and of the PE chain.In 10•3PF6 the ring still predominantly resides at V1 +• , although the interaction of the ring with V1 +• is weakened since V1 +• has higher electron density than had V1 2+ .Even after a second reduction (i.e. for 10•2PF6) the ring still encircles V1.At last, after the third reduction (i.e. for 10•PF6), the ring shuttles to V2 In 11•3PF6 however, V2 cannot approach to V1; the ring mainly resides at V2 which is a weaker recognition site than V1.The folding of L2 in 11 is impossible in all reduction states.The shuttling occurs after the first reduction (i.e. for 11•3PF6), in this time the ring encircles V2, which is a weaker recognition site than V1, and also after the second reduction (i.e. for 11•2PF6) and after the third reduction (i.e. for 11•PF6 ) the ring always encircles V2 The above investigation of tripodal rotaxanes 10 and 11 shows that the nature of the link L between stations V1 and V2 is crucial for their switching ability. 74n order to explain how electrostatic attraction may help to control the shuttling process, [2]rotaxanes and related [2]pseudorotaxanes containing oppositely charged threads and rings have been investigated. 75In these systems, referred to as ion-paired [2]rotaxanes and ion-paired [2]pseudorotaxanes, the positive and negative charges are compensated and the mutual molecular recognition is considerably increased by electrostatic attraction between components, which are not only mechanically interlocked, but also strongly electrostatically attached.The studied rotaxanes and pseudorotaxanes contain doubly positively charged viologen threads which go through doubly negatively charged crown ether rings.
As examples served the ion-paired [2]rotaxanes and ion-paired [2]pseudorotaxanes consisting of the thread incorporating cationic viologen unit and of bis-p-phenylene-34-crown-10 rings with anionic groups ─COO ─ and ─SO3 ─ , 12 and 13, respectively, the neutral crown ethers 9 and 14 served for comparison purposes.It was observed that the association constants of viologens 15 and 16 with anionic crown ethers 12 and 13 are higher than those with neutral crown ethers 9 and 14; the affinity of the investigated crown ethers toward viologens decreases in the order: 13 > 12 ≥ 14 > 9 (Scheme 6). 75

Scheme 7
It was found that the reaction of 17 with the mixture of syn-and anti-isomers of the crown ether 13 (in form of the Bu4N + salt) and with 19 affords in 50%, two syn-and anti-isomers of ion-paired rotaxanes 22 and 23,respectively, due to the presence of equal amounts of syn-and anti-isomers in the starting material 13.One should point out that, contrary to the starting 13, the isomers 22 and 23 could be separated; it was observed that the anti-rotaxane 23 shows cycloenantiomerism (Scheme 8).The crystal structures of hydrated pseudorotaxanes 24 and 25 have been examined: In pseudorotaxane 24, the viologen cationic nitrogen atoms and the anionic carboxylate groups of the ring 12 are far from each other, and the carboxylates point away from the viologen unit into layers of hydrating water molecules.The hydration shell of water molecules (twelve water molecules per each ion-paired [2]rotaxane molecule) forms layers separating adjacent ion-paired [2]pseudorotaxane units.Such hydration prevents the direct contacts of positively charged nitrogen atoms with carboxylate groups.
In pseudorotaxane 25, however, the viologen cationic nitrogen atoms are more close to anionic sulfonate groups of the ring 13 than in the case of 24.Moreover, the number of hydrating water molecules is lower than for 24 (it is only two per each ion-paired [2]pseudorotaxane molecule), and as a result, in the solid 25 the viologen cationic nitrogen atoms and anionic sulfonate groups are not separated from each other by layers of water molecules, therefore Ka of 25 is higher than Ka of 24.The above observations show that the sulfonate groups provide a better charge delocalization than the carboxylate groups, and enhance the stability of ion-paired [2]pseudorotaxane.
One should point out that the behavior of ion-paired [2]rotaxanes and ion-paired [2]pseudorotaxanes is different from common [2]rotaxanes and [2]pseudorotaxanes since in the ion-paired systems the electrostatic attraction between components exists.It is noteworthy that carboxylate groups of ion-paired [2]rotaxanes can be reversibly protonated, in the contrary to sulfonate groups which cannot be protonated, due to this property rotaxanes containing sulfonated rings can maintain their charges in media of a wider acidity range.One should also note that ion-paired [2]pseudorotaxanes are considerably stable even in aqueous solutions.© ARKAT-USA, Inc.
The above described ion-paired [2]rotaxanes and ion-paired [2]pseudorotaxanes, consisting of oppositely charged interlocked components are promising for design of molecular electronic devices driven by electrostatic attraction forces. 75

Pseudorotaxanes
Pseudorotaxanes are building blocks for interlocked supramolecular systems; due to their interesting properties they are widely investigated.Below selected examples of pseudorotaxanes with threads containing viologen units are presented.
Pseudorotaxanes consisting of the thread containing viologen and a terpyridine group with its transition metal complexes and of rings 24-crown-8(A), dibenzo-24-crown-8(B) or dinaphtho-24-crown-8(N) were studied in view of their ON/OFF behavior (Scheme 10). 762]pseudorotaxanes containing rings A and B increase with higher πstacking in order A<B, and no significant difference exists in the association between free 26 3+ , square planar complex or octahedral complex.However, when crown ether N is used as the ring, the above typical trend exists no more, and the observed differences depend on the coordination geometry of the complex (Scheme 11).

Scheme 11
As an explanation it was established that in the case of the pseudorotaxane [PtMe(26•N)] 4+ , i.e. 27, the square planar Pt complex provides a large electron-deficient surface for π-stacking with the naphtho unit of N, therefore the interaction between thread and ring is enhanced and Ka of pseudorotaxane 27 is higher than for the pseudorotaxane [Ru(terpy)(26•N)] 5+ , i.e. 28.This difference results from the fact that the octahedral Ru complex, present in 28 creates a steric hindrance between the ancillary terpy ligand and the naphtho unit, the πstacking is lower than in 27, therefore the Ka of pseudorotaxane 28 is lower (Scheme 12).
In order to investigate this behavior with more labile metals, as an analogue of [PtMe(26)] 4+ , the square planar complex [Ag(MeCN)(26)] 4+ was prepared, and as an analogue of [Ru(terpy)(26)] 5+ the bis-ligand octahedral complex [Cd(26)2] 8+ was used.It was found that the Ka values for formation of [2]pseudorotaxanes from square planar complexes of Pt(II) and Ag(I) with N (27 and 29), respectively) are higher than in the case of octahedral complexes of Ru(II) and Cd(II), (28 and 30, respectively).

Scheme 12
This observation concerning the influence of coordination environment of the metal on Ka of formed pseudorotaxanes gave an idea for the design of an ON/OFF system in which the conversion between ON and OFF states occurs by changing one type of metal coordination environment for another 77  in a similar way as other systems are controlled by pH.78  For this purpose the Ag(I) (square planar) and Cd(II) (octahedral) complexes were used to construct a system of OFF/ON switching by exchanging metal ions, i.e. by changing coordination environment of the terpy site.The arbitrary thresholds of Ka >1300 M -1 and Ka < 600 M -1 represent the requirements for the threaded state (ON) and unthreaded state (OFF), respectively.In this experiment the equilibrium mixture containing the free [2]pseudorotaxane ligand [26 which by adding AgBF4 could restore the ON state.This procedure leads to the ON/OFF system, controllable by the coordination environment of the metal ion.
The above results have shown that the geometry of the metal coordination site can be used to monitor the association constant Ka for formation of [2]pseudorotaxanes; when the metal ions are exchangeable, and the ring is sufficiently large (as N), the control of ON/OFF threading is possible.It should be pointed out that such control involving coordination of metal ions is promising for design of molecular switches (Scheme 13).76 urea units in the crown ether; this improvement is controlled by the nature of the viologen anions. 79ue to the ion-pair recognition, the host can increase its binding affinity, and in some cases may even change its binding selectivity.One should point out the crucial role of ion-pair recognition in biological and analytical applications. 80,81It is noteworthy that the improvement of the binding affinity of viologens is very important for preparation of large supramolecular systems (Scheme 14).
In the experiments the heteroditopic receptor 34 was used as a ring, it contains the crown ether unit for binding the dicationic bipyridinium part of viologens 31-33 and two urea units for binding the two anions of these viologens.The complexation of 31-33 which have different anions with the crown ether 34 was investigated with the aim to improve the binding affinity of viologens.Crown ether 35 served here for comparative purposes.
It was found that complexes of 31-33 with 34 have the 1:1 stoichiometry in solution and they have higher Ka values than corresponding complexes with 35; this behavior results from the ionpair recognition.One should note that the Ka values for complexes of 31-33 with 34 are different, this fact indicates the different binding strengths of different anions of 31-33 with the urea groups of 34. 79he experiments show that the complexes of 31 and 32 with 34 are in the solid state the taco complexes, in which the bipyridinium part of 31 or 32 is sandwiched between two aromatic rings of the crown ether moiety in 34.However, 33•34 is not a taco complex in the solid state, but is a pseudorotaxane, in which the bipyridinium part of 33 is encircled by the crown ether 34.This result was unexpected, since 31•34 and 32•34, as well as previously reported host-guest complexes consisting of viologens and BMP32C10 82 have the taco structure in the solid state.
The above described viologen/crown ether complexation, easily controlled by changing viologen anions is promising in design of molecular machines.
The crown-based lariat ethers are widely used in preparation of membranes and cationconducting channels, [83][84][85][86][87][88] however their complexation with viologens has been only rarely reported.It was found that viologens 31 and 36 form pseudorotaxanes with lariat ether 37 in the solid state.One should note that viologen 31 affords with lariat ether 37 more stable pseudorotaxane than with the corresponding crown ether 38, i.e.BMP26C8; the Ka value for 31•37 i.e.A is ca 13 times higher than Ka for 31•38 in solution, due to the presence of two benzyloxy groups in 37.
On the other side, comparing the association constant values of different viologens 31 and 36 with crown ethers, it was observed that 31 forms with 37 stronger pseudorotaxane 31•37=A than 36 does, forming 36•37, i.e. pseudorotaxane B (Scheme 15). 89 . The solid-state structures of A and B are stabilized by face-to-face π-stacking and charge transfer interactions between the four phenylene rings of 37 and the two pyridinium rings of 31 or 36.It was established that the benzyloxy groups in 37 enable an efficient face-to-face interactions with the pyridinium rings of viologens 31 and 36.
In the packing structure of A two pseudorotaxanes A are linked by a bridge consisting of two PF6 ˉ ions and one water molecule.In the packing structure of B, however, four pseudorotaxanes B are linked by a PF6 ‾ anion, each pair of pseudorotaxanes B being connected by hydrogen bonds of hydroxyethyl groups between two molecules of viologen 36, each included in the cavity of 37.
It was observed that lariat ether 37 can include the K + ions.The addition of KPF6 to A resulted in its dissociation, since K + replaced the viologen 31 as the guest of 37.However, the subsequent addition of crown ether DB18C6 caused the inclusion of K + into DB18C6, followed by restoration of pseudorotaxane A. Due to this fact the K + ions and DB18C6 may be used as a switch to control the association of viologen 31 with lariat ether 37, it is noteworthy that the dissociation percentage depends on the concentration of added K + ions.The same process with the use of pseudorotaxane B has shown that for dissociation of B a smaller amount of KPF6 was required than for dissociation of A, because Ka of B is lower than Ka © ARKAT-USA, Inc.
of A. The obtained experimental results reveal that the N-substitution of viologens strongly influences the association strength between viologens 31 or 36 with lariat ether 37.

Rotaxanes
It is known that compounds serving as threads, containing viologen units form with crown-based cryptands the rotaxanes of higher association constants than rotaxanes formed with crown ethers.It should be pointed out that rotaxanes with crown-based cryptands as rings, are not so thoroughly studied as those containing crown ether rings.Below rotaxanes with crown-based cryptands as rings will be described.

Scheme 16b
Two [2]rotaxanes 44 and 45 have been synthesized using the immediate solvent evaporation method (ISEM). 93It was observed that this procedure considerably accelerates organic reactions as compared to the conventional solution conditions, since the acceleration is due to enhancement of molecule-to-molecule contacts between reactants.One should point out that the yields of reactions performed by ISEM method are not lower than those obtained in conventional processes.
Syntheses of rotaxanes 44 and 45 are based on acylation reactions, i.e. the urethane formation and the esterification, respectively. 94For this purpose viologen 36 was mixed in acetonitrile with cryptand 46 and with 3,5-dimethylphenyl isocyanate used to form stoppers, and a catalytic amount of dibutyltin dilaureate was added, the subsequent evaporation yielded a film, which after purification by flash column chromatography afforded rotaxane 44.By the same procedure, using 3,5-dimethylbenzoic anhydride to form stoppers, and a catalytic amount of trimethylphosphine as a base, the rotaxane 45 was prepared (Scheme 17).

Scheme 17
Two [3]rotaxanes, 47 and 48 consisting of the thread 49 containing two viologen units and of bis(m-phenylene)-32-crown-10-based cryptands 42 and 43, respectively, have been obtained using the threading-followed-by-stoppering procedure. 95The reaction of bisviologen derivative One should mention that the synthesis of [3]rotaxanes is more difficult than synthesis of [2]rotaxanes.It is known that in the case of cooperative complexation between components, existing in formation of [3]rotaxanes or higher order rotaxanes, the yields of [2]rotaxanes are very low, this fact is advantageous for the synthesis of higher order rotaxanes.In the preparation of [3]rotaxanes 47 and 48 even in dilute solutions, the strong association and the cooperative complexation between the thread 49 and two cryptands exist, therefore no [2]rotaxanes are formed, and as a result, the [3]  were confirmed by theoretical calculations.96  Describing pseudorotaxanes in which the cryptand serves as a ring, one should also pay an attention to those containing pseudocryptands, 30-97,98  an example of such species is presented below.99  In this study there was performed the synthesis of pseudocryptand-type [2]pseudorotaxane involving self-assembly of viologen 31 with crown ether 52 bearing two pyridyl units.In the product, 52 is folded and the two pyridyl units interact via off-set facetoface πstacking.The two nitrogen atoms of pyridyl units are pointed to the central cavity

Scheme 20
This pseudorotaxane is stabilized by hydrogen bonds of hydrogen atoms of 31 with oxygen and nitrogen atoms of 52 and by offset facetoface πstacking between pyridinium rings of 31 and phenylene rings of 52.
It was found that the opening and closing of the pseudocryptand noncovalent linkage (i.e. the third pseudo-bridge) in 31•52 can be controlled owing to the basicity of pyridine units.The addition of TFA results in the opening of the pseudocryptand and formation of the taco complex 31•52 (TFA)2, whereas the addition of triethylamine causes the recovery of the pseudocryptand-type [2]pseudorotaxane 31•52.These experiments indicate that the opening and closing of the linkage of pyridyl units acts under acid-base control (Scheme 21). 99t should be pointed out that the formation of the third pseudo-bridge of pseudocryptand via the H-bonding of 31 with pyridyl units of 52 and the πstacking increases the association constant of the pseudocryptand-type [2]  .

Scheme 21
Studying the interaction of viologens 53a,f with the large triptycene-based macrotricyclic host 54 it was established that the kind of obtained stable pseudorotaxane-type complexes depends on the structure of viologens.
Among investigated species, the viologens 53 a-d bearing alkyl groups are threading both lateral crown units of 54 to give 1:1 pseudorotaxane-type complexes A; here the complex 53c•54 is shown.However viologens 53e,f bearing β-hydroxyethyl and γ-hydroxypropyl groups, respectively, thread (as two molecules 53e or 53f) the central cavity of 54 to give 2:1 pseudorotaxane-type complexes B, here the complex (53e)2•54 is shown.It is noteworthy that complexes A and B are formed in solution and in solid state and are stable (Scheme 22).
pseudorotaxane 31•52 nearly threefold as compared to association constant of complexation of 31 with unsubstitutd BMP32C10.An interesting feature of the pseudocryptand-type [2]pseudorotaxane 31•52 is the fact that it has the acid-base © ARKAT-USA, Inc.
adjustable association constant; this for the first time observed behavior is possible due to the basicity of pyridyl groups, which form the third pseudo-bridge.
The subsequent treatment of the OFF state with cyclam resulted in the removal of the Cd 2+ ion and regeneration of the free ligand [26•N] 3+ rotaxanes 47 and 48 are obtained in high yields (Scheme 19).95 ©ARKAT-USA, Inc. 100 ©ARKAT-USA, Inc.