DOI: 10.1002/chem.201200389

A Flexible Copper(I)-Complexed [4]Rotaxane Containing Two Face-to-FacePorphyrinic Plates that Behaves as a Distensible Receptor

C�cile Roche, Ang�lique Sour,* and Jean-Pierre Sauvage*[a]

Introduction

Porphyrin-containing catenanes and rotaxanes[1] representan important class of such molecules,[2] either in relation tomolecular machines,[3] a particularly active area of research,or as far as electron or energy-transfer processes are con-cerned.[4] Free-base porphyrins and variously metalated por-phyrins (mostly Zn-complexed systems) are important build-ing blocks in the elaboration of multi-component moleculesor assemblies able to undergo photoinduced electron andenergy transfer processes.[5] Multi-rotaxanes incorporatingporphyrinic components are not very common, althougha few examples are known in relation to intramolecularelectron and energy transfer.[6] Alternatively, such com-pounds are promising as adaptable or switchable receptorsas well as molecular presses.[7] Along these lines, our grouphas recently reported the synthesis and the host–guest prop-erties of porphyrinic rotaxanes whose key elements wereZn-complexed porphyrins attached to one or two coordinat-ing rings incorporating a 2,9-diphenyl-1,10-phenanthroline(dpp) chelate.[7–8] The linker between the porphyrin and the

chelate was very rigid since the porphyrin nucleus and thechelate were fused through a pyrazine-benzene-pyrazinemotif. In addition, the porphyrin was attached to the “back”of the 1,10-phenanthroline aromatic fragment, imposing anexo coordination mode of this chelate with respect to theporphyrin. The connection mode was such that the lateralring was rigid and could not rotate much with respect to theporphyrin plane.

In the present series of compounds, the situation is dra-matically different since 1) the porphyrin and the coordinat-ing rings are linked through single C�C bonds between theporphyrin meso positions and the macrocycle, making thelatter susceptible to rotate with respect to the plane of theporphyrin, and 2) the coordinating axis of the dpp fragmentis pointing towards the porphyrin. In other words, the dppunit will be located far away from the porphyrin and it willbe electronically decoupled from it, contrary to the firstseries of porphyrin-incorporating multi-rotaxanes made inour group. The structural difference between both of theseseries of compounds is illustrated in Scheme 1.

Results and Discussion

Design : The chemical structure of the target rotaxane de-scribed in this report is represented in Figure 1. The [4]ro-taxane consists of two dumbbells and of two bis-macrocyclesincorporating a central Zn-porphyrin. Each dumbbell isthreaded through one of the rings of each porphyrin ina cyclic structure. The dumbbells contain a central bis-3,8-(o-pyridyl)-4,7-phenanthroline fragment, which features tworigidly linked bidentate chelates pointing in the same direc-tion. The porphyrin bis-macrocycle is a meso tetra-substitut-

Abstract: A new cyclic [4]rotaxanecomposed of two flexible bis-macrocy-cles and two rigid axles is described.Each bis-macrocycle consists of tworings attached to antipodal meso posi-tions of a central Zn porphyrin throughsingle C�C bonds. Each ring incorpo-rates a 2,9-diphenyl-1,10-phenanthro-line chelation site. The axles containtwo coplanar bidentate sites derivedfrom the 2,2’-bipyridine motif. Thebuilding blocks were assembled by

using a one-pot threading-and-stopper-ing reaction, which afforded the [4]ro-taxane in 50 % yield. The “gathering-and-threading” effect of copper(I) wasutilised in the formation of a [4]pseu-dorotaxane, which was immediatelyconverted to the corresponding [4]ro-

taxane by a quadruple CuAAC stop-pering reaction. The rotaxane containstwo face-to-face zinc porphyrins, whichallowed the coordination of ditopicguest substrates. The rotaxane hostshowed remarkable flexibility and wasable to adjust its conformation to theguest size. It can be distended and ac-commodate rod-like guests of 2.6 to15.8 � in length.

Keywords: copper · host–guest sys-tems · porphyrinoids · rotaxanes ·template synthesis

[a] C. Roche, Dr. A. Sour, Prof. J.-P. SauvageLaboratoire de Chimie Organo-Min�raleInstitut de ChimieUniversit� de Strasbourg-CNRS/UMR71774 rue Blaise Pascal67070 Strasbourg Cedex (France)Fax: (+33) 368851637E-mail : a.sour@unistra.fr

jpsauvage@unistra.fra

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201200389.

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ed porphyrin with a trans-A2B2 geometry. Two of the transsubstituents are 31-membered rings incorporating dpp che-lating sites, and the other meso substituents are mesitylgroups. In a preliminary study we showed that this porphy-rin bis-macrocycle could be used to form threaded assem-blies in quantitative yield.[9] In the present report the axleswere designed such that a [4]rotaxane would be formed bycopper(I)-templated threading followed by stoppering(Scheme 2). The rotaxane incorporates two central face-to-face Zn-porphyrins, which will allow the complexation oflinear ditopic guests through axial coordination between themetalated porphyrins. As already mentioned, the specificityof the present system is that it will be adaptable even in itscopper-complexed form, contrary to a previously describedsystem that was more rigid and therefore collapsed after de-metalation.[7d] Due to a higher flexibility of the connectorsbetween the porphyrinic plates and the axles, the receptorshould be “inflatable”, depending on the complexed sub-strate, in a manner reminiscent of an enzyme induced-fit.[10]

As shown in previous studies the 31-membered ring can bedistorted and a non-planar ring can be generated.[11] Anoth-er main difference with the previously published system re-sides in the orientation of the bidentate chelating group: thedpp unit is oriented towards the central porphyrin in the

present system, whereas thephenanthroline coordinationsite was pointing outward in thepreviously published [4]rotax-anes.

Threading and stoppering reac-tions—[4]rotaxane synthesis :The synthetic approach was toform [4]rotaxane 14 + ·4 ACHTUNGTRENNUNG(PF6

�)from bis-macrocycle 2, thread 3and stopper 4 in a two-step,one-pot experiment involvingboth the threading and stopper-ing reactions (Figure 2). Thebuilding blocks (porphyrin-bis-macrocycle 2,[9] acetylenicthread 3[13] and azide stopper4)[13] were synthesised as de-scribed in the literature. Thegathering-and-threading effectof copper(I)[12] was utilised inthe first step (threading), whichled to the formation of a cyclic[4]pseudorotaxane consisting oftwo porphyrin-bis-macrocyclesand two threads. First, an aceto-nitrile solution of [Cu-ACHTUNGTRENNUNG(CH3CN)4] ACHTUNGTRENNUNG(PF6) was added toa suspension of the poorly solu-ble porphyrin 2 in dichloro-methane to give a deep-red so-lution of bis-macrocycle 2 com-

Scheme 1. Schematic view of the structural difference between the por-phyrin-incorporating bis-macrocycles previously made in our group(a),[7c,d,8b] and those described in this paper (b). The main differencesreside in 1) the nature of the bridge between the central porphyrin andthe macrocycles (rigid pyrazine-benzene-pyrazine motif vs. single C�Cbond), 2) the position of the bridge on the porphyrin (b-pyrrolic vs. mesosubstitution), 3) the orientation of the coordinating dpp unit (exo vs.endo coordination mode with respect to the porphyrin) and 4) the elec-tronic coupling between the dpp chelate and the porphyrin (electronical-ly coupled vs. decoupled).

Figure 1. Chemical structure of the target [4]rotaxane 14 +·4 ACHTUNGTRENNUNG(PF6�) complexed to four copper(I) centres.

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FULL PAPER

plexed to two copper(I) ions at the dpp chelating sites. Thissolution was then added to a suspension of acetylenic thread3 (stoichiometric amount) in dichloromethane. This resultedin the formation of the [4]pseudorotaxane with four tetrahe-dral copper(I) complexes linking the dpp sites of the macro-cycles and the bipyridine-type sites of the threads. Ina second step, bulky azide-functionalised stopper precursorswere introduced at each acetylenic end of the threads byusing the Huisgen copper(I)-catalysed azide–alkyne cycload-dition (CuAAC).[14] Stopper 4, catalytic [Cu ACHTUNGTRENNUNG(CH3CN)4]ACHTUNGTRENNUNG(PF6)and base (Na2CO3) were added to the reaction mixture con-taining the [4]pseudorotaxane. The resulting stoppered[4]rotaxane 14 +·4 PF6 was obtained as a deep-purple solidafter purification by consecutive alumina, size-exclusion andsilica column chromatographies. The global yield over twosteps (threading and quadruple stoppering) was 50 %.

Characterisation of the rotaxane : The [4]rotaxane14 +·4(PF6

�) was characterised by high-resolution electro-spray mass spectrometry (HRES-MS), 1H and 13C NMR (1Dand 2D experiments) and UV/Vis absorption spectroscopy.The HRES-MS analysis resulted in a very clean spectrumwith a major peak at m/z=1760.169, the calculated valuefor rotaxane 14 + being 1760.202 (Figure 3). The isotope pro-file confirms that the [4]rotaxane obtained has four positivecharges and consists of two dumbbells and two bis-macrocy-cles (2:2 species), linked by copper(I) cations. A differentthreading scheme could have been expected where a singleaxle would be threaded through both rings of the same bis-macrocycle. This would have led to a doubly charged [2]ro-taxane consisting of one dumbbell and one bis-macrocycle(1:1 species). The 1:1 species would have had the same m/zvalue as the 2:2 species, but the isotope profile would havebeen different. The gap between two signals in the isotopeprofile would have been twice as large. We can thus rule outthe possibility that a [2]rotaxane was formed in the reaction.

Characterisation of the [4]rotaxane by using NMR spec-troscopy was not as straightforward as the mass spectrome-try characterisation. 1H NMR spectra were recorded ina series of deuterated solvents (CDCl3, CD2Cl2, C2D2Cl4,

Scheme 2. Preparation of a [4]rotaxane containing two short rigid axlesand two bis-macrocycles, each of them consisting of two flexible rings at-tached to a central Zn-porphyrin. The synthetic strategy is based on the“gathering-and-threading” approach followed by stoppering.[12] The por-phyrin is represented as a square or a diamond, the bidentate chelatinggroups are indicated by U-shaped symbols and the gathering metalcentre (copper(I)) is represented as a dot. The stoppers are depicted aslarge spheres.

Figure 2. Two-step, one-pot synthesis of rotaxane 14 + by 1) the formationof a [4]pseudorotaxane and 2) a stoppering reaction.

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J.-P. Sauvage, A. Sour et al.

CD3CN, CD3NO2, C6D6 and [D6]DMSO). In most of thesesolvents very broad signals were observed at room tempera-ture. However, the signals sharpened and shifted upon heat-ing to higher temperatures as observed in particular with ni-tromethane as solvent (see the Supporting Information, S1).These results seem to indicate that several conformers ofthe rotaxane are in slow equilibrium in solution at low tem-perature with a solvent-dependent exchange rate. However,in the case of CD2Cl2 the proton NMR spectrum was wellresolved even at room temperature as shown in Figure 4.

Consequently, the 13C spectrumand 2D NMR experiments(COSY, NOESY, HSQC,HMBC and DOSY) were re-corded in this solvent and as-signment of the signals was pos-sible after close scrutiny of thedata.

The 1H NMR data confirmthe interlocked structure of therotaxane. This is evidenced bycharacteristic chemical shifts inthe rotaxane compared to thefree porphyrinic bis-macrocycle2, for example, the upfield shiftof the phenyl protons of thedpp units (protons o and m inFigure 4).[9] In addition, manyNOE interactions are observedbetween the ethyleneglycolchains of the macrocycles andthe central bis-3,8-(o-pyridyl)-

4,7-phenanthroline moiety of the axle on the NOESY spec-trum (Figure S2, Supporting Information). Considerable lossof symmetry was observed in the proton spectrum of rotax-ane 14+ compared to its components taken independently.The signals belonging to the protons of the bis-macrocycliccomponent 2 were split into two signals in the NMR spec-trum of the rotaxane. Yet the number of signals assigned tothe dumbbell component of the rotaxane reflects the samedegree of symmetry as its free precursors (axle 3 and stop-per 4): six different proton signals are observed for the cen-

Figure 3. HRES-MS spectrum of rotaxane 14+ . The isotope profiles are inserted on the right; the profile ob-tained experimentally for the main signal at 1760.169 (top) is in agreement with the calculated isotope profilefor [C440H416Cu4N36O28Zn2]

4+/4 (14+/4) (bottom).

Figure 4. 1H NMR spectrum of rotaxane 14+ in CD2Cl2 at room temperature: signals in the aromatic region are displayed (left), as well as proton assign-ments on the bis-macrocycle and dumbbell components of the rotaxane (right). Split signals are annotated with “a” and “b” subscripts.

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FULL PAPERFlexible Copper(I)-Complexed [4]Rotaxane

tral bis-3,8-(o-pyridyl)-4,7-phenanthroline moiety in axle 3and in the rotaxane. Based on the NMR data, we proposea potential geometry for the rotaxane in CD2Cl2 solution asschematically depicted in Scheme 3. If the rotaxane adoptsa twisted, pro-helical conformation as proposed, then both

halves of the dumbbells will have the same chemical envi-ronments, hence the number of signals observed in theproton NMR spectrum. On the contrary, the protons of thebis-macrocyclic components will have different environ-ments depending on their positions relative to the secondbis-macrocycle. Protons pointing towards the inside of therotaxane and located in closer proximity to the second bis-macrocycle will experience larger ring-current effects thanthose pointing outwards, hence the splitting observed. To il-lustrate this we can examine the signals observed for theCH3 groups ortho to the porphyrin on the mesityl substitu-ents. These methyl groups give a single signal at d=

1.75 ppm in porphyrin 2, whichsplits into two singlets at d=

1.47 and 0.64 ppm, respectivelyin rotaxane 14+ . The CH3 pro-tons are shielded due to thecloseness of the second porphy-rin moiety, with a strongershielding effect on the internalmethyl group than the externalone. The relative integrations ofsignals assigned to the dumb-bell and to the bis-macrocyclecomponents are consistent withthe above interpretations. Theratio of the bis-macrocycle sig-nals integrations (total integra-tion for both split signals) tothe integrations of the dumb-bell signals corroborates a 1:1dumbbell/bis-macrocycle stoi-chiometry in the rotaxane.

Besides the major set of sig-nals discussed above, a second,minor set of signals is observedin the 1H NMR spectrum of ro-taxane 14+ . These minor signals

were consistently observed in the same ratio to the majorsignals even after repeated chromatographic purification ofthe compound. In addition, diffusion-ordered spectroscopy(DOSY) NMR—an experiment that differentiates the NMRsignals of different species according to their diffusion coef-ficient[15]—shows that only one species is present in solution(Figure 5). Some of the minor peaks are too small and fallunder the detection limit for the DOSY experiment, but thesignals integrating for a larger number of protons among theminor peaks are a good probe for this experiment. For in-stance, the protons corresponding to the signals at d= 8.60,7.11 and 6.73 ppm (belonging to the minor set of signals)diffuse at the same rate as other protons. Consequently, wetend to believe that the minor signals observed do not corre-spond to an impurity but to a different conformer of the[4]rotaxane. This conformer does not seem to exchange fastwith the major conformer in CD2Cl2 solution. Unfortunately,we were not able to identify the structure of the minor con-formation.

The DOSY experiment can provide an estimation of thesize of the species in solution as well as sorting them out ac-cording to their diffusion coefficient. The Stokes-Einsteinequation links the diffusion coefficient to the hydrodynamicradius of molecules.[16] In the case of rotaxane 14+ , a diffu-sion coefficient of (228�5) mm2 s�1 was measured by DOSYNMR spectroscopy in CD2Cl2 at 298 K. By using theStokes–Einstein equation applied to a spherical model, wededuce a diameter of 48 � for the rotaxane. This is consis-tent with the formation of the [4]rotaxane 14+ , the size ofwhich was estimated using CPK models. According to theCPK model, the length of the porphyrinic bis-macrocycle

Scheme 3. Schematic view of the proposed geometry of rotaxane 14+ insolution deduced from the NMR data.

Figure 5. DOSY spectrum of rotaxane 14 + in CD2Cl2, displaying signals in the aromatic region. DOSY datashows the presence of a single chemical species in solution; arrows point at minor signals that belong toa second, minor conformer of the [4]rotaxane in very slow exchange with the major conformer. A good fit isobtained for the estimated size of the rotaxane by using CPK models or the diffusion coefficient measured byDOSY NMR spectroscopy. The rotaxane approximates to a 48 � sphere.

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approximates to 43 � and the dumbbell component in fullyextended conformation is approximately 54 � long. Giventhat the linker between the central part of the dumbbell andthe stopper is flexible, we can assume that the conformationof the dumbbell is not fully extended in solution and that itslength averages shorter values. In conclusion, the CPK-esti-mated size fits well with an experimentally measured ball-shaped model of 48 �.

Finally, the [4]rotaxane was characterised by UV/Vis ab-sorption spectroscopy. The spectrum shows a Soret band at420 nm and Q bands at 551 and 591 nm in dichloromethane.The absorption spectra of rotaxane 14+ and its precursor,porphyrin 2 were recorded in CH2Cl2 solutions at the sameeffective porphyrin concentration ([14 +]=0.94 � 10�6

m and[2]=1.9 � 10�6

m) for comparison (Figure 6). The rotaxane

has a slightly blueshifted and significantly broadened Soretband compared to the free porphyrin. A half-band width of13 nm is measured for the rotaxane, as opposed to 10 nm forporphyrin 2. These changes are typical of an excitonic cou-pling between two parallel porphyrins and suggest that thetwo porphyrins are in close proximity in the rotaxane.[17] An-other potential cause for a shift of the absorption bandcould be the impact of the nearby copper(I) complexes andthat of the dumbbell. However, this possibility can be ruledout by comparison with a similar copper-complexed rotax-ane built with the same dumbbell as rotaxane 14 + .[11c] We re-cently described a porphyrin similar to compound 2, substi-tuted with four macrocycles instead of two. Threading-and-stoppering of this porphyrinic tetra-macrocycle with axle 3and stopper 4 resulted in the formation of a [3]rotaxanecontaining the same dumbbell and the same copper(I) com-plexes as rotaxane 14+ . A 3 nm redshift was observed for theSoret band of the [3]rotaxane compared to its porphyrinic

precursor. Although the same axle and stopper were used inthe present system, the Soret band was shifted in the oppo-site direction. This is clear evidence that other factors areprevailing in the case of [4]rotaxane and are strong enoughto counterbalance the influence of complexation with cop-per(I) and threading of the dumbbells. A major differencebetween the [3]rotaxane described before and the present[4]rotaxane is the number of porphyrins: the [3]rotaxanecontains only one porphyrin whereas the [4]rotaxane con-tains two. This supports the idea that the changes in the ab-sorption spectrum are the result of an exciton coupling be-tween the porphyrins of the [4]rotaxane, as proposed above.Excitonic coupling between chromophores may lead tohypso- or bathochromic shifts depending on the relative ori-entation of the transition dipoles.[17] Thus, exciton couplingin a face-to-face bis-porphyrin leads to a hypsochromic shiftof the Soret band (parallel transition dipoles), whereasa bathochromic shift is observed in the case of linear, head-to-tail bis-porphyrins (in-line transition dipoles). As foroblique bis-porphyrin dimers, splitting of the Soret band istypically observed. The hypsochromic shift observed for thepresent bis-porphyrinic rotaxane is consistent with a face-to-face, parallel orientation of the porphyrins.

Attempts to demetalate the copper(I)-complexed rotax-ane 14+ with potassium cyanide gave mixtures of products.A very small amount (�1 mg) of copper-free rotaxane wasisolated, yet its characterisation proved difficult. MALDI-TOF mass spectrometry gave a signal at m/z=6788.327 (cal-culated for [M+H]+ : m/z= 6788.198), but proton NMRspectroscopy gave broad signals that were difficult to inter-pret.

The [4]rotaxane 14+ used as a receptor for guest moleculesof different lengths : The coordination mode and associationconstant of rotaxane 14+ with four guest substrates were in-vestigated by UV/Vis absorption titration in toluene. Theguests are depicted in Figure 7. Guest G1 is the aliphatic di-amine 1,4-diazabicycloACHTUNGTRENNUNG[2.2.2]octane (DABCO), whereasguests G2–G4 are bifunctional pyridine-type ligands with orwithout phenylene spacers between the pyridine units (0,1 and 2 phenylene spacers, respectively). All four guests arerigid, with a N�N distance varying from 2.6 to 15.8 � ac-

Figure 6. UV/Vis absorption spectra (Soret band) of [4]rotaxane 14+

(lmax =420 nm) and its precursor, porphyrinic bis-macrocycle 2 (lmax =

421 nm) in CH2Cl2 with [porphyrin]= 1.9� 10�6m. The blueshift and

broadening of the Soret band of the rotaxane is attributed to exciton cou-pling between the two central porphyrins.

Figure 7. Chemical structures of guests G1–G4.

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FULL PAPERFlexible Copper(I)-Complexed [4]Rotaxane

cording to crystallographic data.[18] They are known to formaxial complexes with zinc porphyrins.[7b,c,19] We performedhost–guest titrations with these rigid substrates of increasinglength to measure their association constants with the hostrotaxane. The Soret band of Zn porphyrins has been shownto undergo a bathochromic shift upon axial coordination ofnitrogen ligands.[19c] This makes UV/Vis titration an excel-lent method for assessing the host–guest interactions. In thetitration experiment, 0.1 equivalent aliquots of the guestwere added to a solution of the [4]rotaxane (10�6

m) in tolu-ene and the absorbance of the Soret band (400–450 nm) wasrecorded after stirring for 2 min. Several coordinationmodes between the rotaxane host 14+ and the ditopic guestscan potentially be considered with internal and/or externalbinding of the guest(s) (Scheme 4).

The data obtained in the case of the short DABCO guestG1 indicated the coexistence of several host–guest species.A bathochromic shift of the Soret band of the host was ob-

served upon addition of the guest. Superimposition of theabsorption spectra obtained upon addition of 0 to 0.7 equiv-alents of DABCO brought up an isosbestic point (Figure S3,see the Supporting Information). Mathematical treatment ofthe data with the Specfit/32 software corroborated a 1:1host–guest stoichiometry with an association constant of logKa1 =8.1�0.5 (see [Eq. (1)]). The binding constant is consis-tent with an inside coordination mode in which the guest iscomplexed between the porphyrins, inside the cavity of therotaxane. Indeed, DABCO typically binds simple zinc por-phyrins with an association constant of 105 mol L�1 in tolu-ene.[19c] The high affinity observed here reflects the coopera-tive effect of the coordination of the ditopic ligand betweentwo zinc porphyrins. The association constant of the veryshort DABCO guest with rotaxane host 14+ is slightly higherthan that observed with the more rigid [4]rotaxane de-scribed previously,[7c] which could be explained by the great-er adaptability of the present host compared to the previoussystem. Due to the flexible anchoring points between theporphyrinic component and the axles of the rotaxane, thetwo porphyrins can come to relatively close proximity so asto adjust the Zn···Zn distance to the small DABCO guest.However, beyond one equivalent of DABCO, a different co-ordination behaviour arises with disappearance of the isos-bestic point. The data indicate that several equilibria are tobe taken into account with several species coexisting in solu-tion in significant proportions. Given that Zn porphyrinspreferentially bind a single axial ligand, the most significantspecies are likely to be the 1:1 internal complex formedinitially (Scheme 4 a) and 1:2 coordination species(Scheme 4 c–e). The following equilibria account for the for-mation of the host (H)–guest (G) species.

HþGÐ H �GKa1 ¼H �G½ �H½ �* G½ � ð1Þ

Hþ 2 GÐ G �H �GKa2 ¼G �H �G½ �H½ �* G½ �2 ð2Þ

Both equilibria were considered simultaneously in themathematical treatment with log Ka1 =8.1�0.5 for the for-mation of the 1:1 host–guest complex. The data fitted withan association constant of log Ka2 =14.5�0.2 for the forma-tion of the 1:2 complex. We can conclude that the 1:1 guestinclusion complex is predominantly formed with a high asso-ciation constant in the presence of substoichiometricamounts of DABCO, whereas additional binding of theligand is observed in the presence of excess guest.

In the case of the 4,4’-bipyridine guest G2, a clear isosbes-tic point was observed (Figure 8). This indicates that onlyone equilibrium is to be considered and that a single com-plex is formed. Mathematical treatment of the spectra fitswith the formation of a 1:1 host–guest complex with an asso-ciation constant of log Ka1 =6.2�0.1. Other stoichiometriesbetween host and guest (e.g., 1:2 or 2:1) were not corrobo-rated by the mathematical treatment. The high binding con-stant is consistent with an inside coordination mode in

Scheme 4. Possible coordination modes of guests G1–G4 with rotaxanehost 14+ . a) 1:1 host–guest stoichiometry, internal complex (guest inclu-sion); b) 1:1 stoichiometry, external complex; c) 1:2 host–guest stoichiom-etry, internal complexes; d) 1:2 stoichiometry, external complexes, e) 1:2stoichiometry with one internal and one external guest.

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J.-P. Sauvage, A. Sour et al.

which the guest is complexed between the porphyrins (a pyr-idyl ligand typically binds a zinc porphyrin with an associa-tion constant of 103 mol L�1).[19c] The association constantobserved here is significantly lower than that obtained witha related bis-porphyrin system, with log Ka1 = 7.3, whichcould be explained by a lower Lewis acidity of the ZnII cen-tres. In the present system, the porphyrin is a classical tetra-aryl tetra-pyrrolic compound, whereas in the previoussystem investigated, electron-withdrawing groups were at-tached on two pyrrole rings of the porphyrin. The Lewisacidity of the zinc(II) centre is thus less pronounced in thepresent system than in the previous case reported by ourgroup.[7c,d]

The absorbance change at lmax =423 nm upon titrationwith guests G1 and G2 is depicted in Figure 9. The graphhighlights the difference in behaviour of the two guests to-wards complexation with rotaxane 14+ . At the start of the ti-tration 1:1 host–guest complexes are formed in both cases.However, a gradient change is then observed in the case ofthe DABCO guest G1, which is a clear indication that otherspecies are formed with this substrate.

Similar results were obtained with guests G3 and G4upon addition of 0 to 2 equivalents of guest. In both casesa bathochromic shift of the Soret band was observed as ex-pected and superimposition of the spectra brought up isos-bestic points (Figure S4 and S5, Supporting Information).The data fit with the formation of 1:1 complexes with an as-sociation constant of log Ka1 = 6.1�0.1 and log Ka1 =5.6�0.1 for G3 and G4, respectively. These values are two tothree orders of magnitude higher than the expected bindingconstant for the mono-complexation of a bipyridine-typeligand on a single porphyrin, which evidences a cooperativeeffect and hence internal binding of the guest inside thecyclic rotaxane. Very close binding constants were obtained

for guests G2 and G3 ; this attests for the flexibility of therotaxane that can equally accommodate these two guests ofincreasing lengths (7.1 and 11.4 �, respectively). The excep-tional flexibility of the rotaxane is confirmed by the bindingconstant obtained for the extended guest G4. The constantis slightly lower—presumably due to steric constraints—yetconsistent with guest inclusion into the central cavity of therotaxane. The results are summarised in Table 1. The titra-

tions point out the remarkable adaptability of the rotaxanehost, which “inflates” or “deflates” depending on the guestsubstrate, as illustrated on Scheme 5. This differentiates thepresent rotaxane 14+ from the previously reported [4]rotax-ane host.[7c] In the previous system the guest G4 was toolong to fit between the porphyrins. Only 1:2 host–guest spe-cies were observed where the guests were bound externally.An intermediate situation was observed in the case of G3,which suggested that guest G3 had a critical size and thatseveral complexes coexisted in solution. The previous rotax-ane was unable to adjust its conformation to the size of theguest because of the rigidity of the porphyrinic bis-macrocy-

Figure 8. UV/Vis titration spectra in toluene (Soret band) of rotaxane 14+

(0.92 � 10�6m) with 4,4’-bipyridine guest G2 (2.8 � 10�5

m) from 0.0 to 2.0equivalents. Arrows show changes in the host spectrum with increasingguest concentration; a 5 nm redshift from 423 to 428 nm is observed.

Figure 9. UV/Vis titration spectra of rotaxane 14 + with guests G1 and G2in toluene. Absorbance plotted against the added equivalents of guest atlmax =423 nm.

Table 1. Association constants of guests G1–G4 with rotaxane host 14+

and lengths of the ligands.

Guest log K[a] N�Ndistance [�][b]

G1 8.1�0.5 2.6G2 6.2�0.1 7.1G3 6.1�0.1 11.4G4 5.6�0.1 15.8

[a] For the 1:1 host–guest complexes. In the case of DABCO (G1) 1:2host–guest complexes were also formed with an association constant oflog Ka2 = 14.5�0.2. [b] From crystallographic data, see reference [18].

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FULL PAPERFlexible Copper(I)-Complexed [4]Rotaxane

cle component. Moreover, this rotaxane collapsed upon de-metalation of the copper(I) centres and did not exhibit anyhost–guest recognition properties after demetalation.[7d] Onthe contrary, the spacer between the porphyrin units and thecopper(I) anchoring points in rotaxane 14+ is very flexible,which allows for inclusion of a large range of guests, even inthe copper-complexed rotaxane. An analogy can be drawnbetween the rotaxane host 14 + and the induced fit of sub-strates in enzymes: both systems are flexible and can adjusttheir conformation to the size of the substrate to be recog-nised.

Conclusion

The now well-established “gathering-and-threading” princi-ple based on copper(I) and bidentate ligand-containingcyclic and acyclic fragments turned out to be remarkably ef-ficient for preparing a new [4]rotaxane consisting of two bis-macrocycles and two rigid axles. Each bis-ring fragment con-tains a central zinc-porphyrin. Two flexible coordinatingrings are attached through single C�C bonds on two antipo-dal meso carbon atoms of the porphyrin. The final rotaxanehas been characterised by high-resolution electrospray massspectrometry, 1H and 13C NMR and UV/Vis absorption spec-troscopy.

The new [4]rotaxane described in the present report in-corporates two face-to-face zinc-complexed porphyrins,making it an interesting receptor for a variety of substrates.It shows structural similarities with previously describedcompounds[7c,d] in the sense that it also belongs to the familyof [4]rotaxanes consisting of two axles and two bis-macrocy-cles. However, the greater flexibility of the system, originat-ing from the totally different connections between the por-phyrins and the rings, leads to very different host–guestproperties. Interestingly, rod-like guests of extremely differ-ent lengths could be accommodated between the two por-phyrins, affording complexes of similar stability constants,thus demonstrating that the present [4]rotaxane behaves asa distensible receptor, which is able to inflate or deflate atwill to adjust to the guest size. This property is well illustrat-ed by the fact that exotopic guests of the 4,4’-bipyridine

family, with overall lengths varying from 7.1 to 15.8 �, formcomplexes of comparable stability.

Experimental Section

General : Dry CH2Cl2 and CH3CN were distilled from CaH2 as dryingagent. Commercial chemicals were at the best available grade and usedwithout further purification. Porphyrinic bis-macrocycle 2,[9] [Cu-ACHTUNGTRENNUNG(CH3CN)4] ACHTUNGTRENNUNG(PF6),[20] acetylenic rod 3,[13] and azide stopper 4[13] were syn-thesised according to literature procedures. The guests G3 (1,4-bis(4-pyri-dyl)benzene) and G4 (4,4’-bis(4-pyridyl)biphenyl) were prepared bySuzuki coupling reactions between pyridine-4-boronic acid and the corre-sponding p-dibromobenzene and 4,4’-dibromobiphenyl, respectively, ac-cording to published procedures.[21] Column chromatographies were car-ried out using neutral alumina (Merck aluminium oxide 90, 0.063–0.200 mm), silica gel (Merck Kieselgel, silica gel 60, 0.063–0.200 mm) orBiobeads S-X1 support (Bio-Rad). NMR spectra were acquired onBruker AVANCE 600, 500 or 300 spectrometers. In the assignments, thechemical shift (in ppm) is given, followed by the multiplicity of the signal(s, singlet; d, doublet; m, multiplet), the value of the coupling constantsin Hz if applicable, the number of protons implied and finally the assign-ment. Mass spectra (ES-MS) were recorded on a Bruker MicroTOF spec-trometer by the Service de Spectrom�trie de Masse (Universit� de Stras-bourg). UV/Vis spectra were recorded with a Kontron InstrumentsUVIKON 860 spectrometer at 21 8C with 1 cm path cell. Titration spectrawere measured in 0.92 � 10�6

m toluene solutions of rotaxane 14+. Guestsolutions (2.8 � 10�5

m) were added to the rotaxane sample in 10 mL(0.1 equivalent) aliquots using a 10 mL Hamilton syringe. UV/Vis spectro-photometric titrations were analysed by fitting the series of spectra at1 nm intervals by using the SPECFIT/32 3.0 software (Spectrum SoftwareAssociates), which takes into account the changes in volume during thetitration.[22]

[4]Rotaxane 14+ : A degassed solution of [Cu ACHTUNGTRENNUNG(CH3CN)4] ACHTUNGTRENNUNG(PF6) (10 mg,27 mmol) in CH3CN (3 mL) was added by cannula to a degassed suspen-sion of Zn-porphyrin 2 (5.2 mg, 14 mmol) in CH2Cl2 (3 mL). The mixturewas stirred under argon for 30 min, resulting in a dark-red solution. Thissolution was then added to a degassed suspension of acetylenic thread 3(5.2 mg, 21 mmol) in CH2Cl2 (3 mL). The mixture was stirred for 24 hunder argon. Azide stopper 4 (24 mg, 41 mmol), [CuACHTUNGTRENNUNG(CH3CN)4] ACHTUNGTRENNUNG(PF6)(5.1 mg, 14 mmol), Na2CO3 (0.6 mg, 5.4 mmol) and sodium ascorbate(1.1 mg, 5.4 mmol) were then added to the solution. The mixture wasstirred under argon during 5 days. The solvents were then evaporatedand the resulting deep-red solid was purified by column chromatography.Consecutive alumina chromatography (using a gradient of elution fromCH2Cl2/MeOH 100:0 to 92:8), size-exclusion Biobeads S-X1 chromatog-raphy in distilled THF and silica chromatography (using a gradient ofelution from CH2Cl2/MeOH 98:2 to 94:6) gave 26 mg of rotaxane 14 +

·4PF6 (50 %) as a dark-red solid. 1H NMR (600 MHz, CD2Cl2, 298 K):d=9.08 (d, 3J=8.4 Hz, 4 H, H1’), 8.68 (d, 3J =7.8 Hz, 4H, H4,7a), 8.56 (d,3J=4.8 Hz, 4 H, HpyBa), 8.55 (d, 3J=7.8 Hz, 4 H, H4,7b), 8.47 (d, 3J =7.8 Hz,4H, H4’), 8.34 (d, 3J=4.2 Hz, 4H, HpyAa), 8.29 (s, 4H, H5’), 8.24 (d, 3J =

8.4 Hz, 8H, H2� + H3’), 8.15 (d, 3J =7.8 Hz, 4H, H3,8a), 8.14 (s, 8H, H5,6),8.12 (d, 3J=4.8 Hz, 4H, HpyBb), 7.99 (s, 4 H, H6’), 7.98 (m, 4 H, HpyAb),7.77 (s, 4 H, H7’), 7.62 (d, 3J =7.8 Hz, 4H, H3,8b), 7.55 (d, 3J= 8.4 Hz, 8H,Hoa), 7.44 (s, 4H, Ho“a), 7.36 (s, 4H, Hp”), 7.19 (d, 3J =8.4 Hz, 24H, H14’),7.06 (d, 3J= 8.4 Hz, 24H, H13’), 7.00 (d, 3J =8.4 Hz, 8 H, H12’), 6.97 (s, 4 H,Ho“b), 6.94 (s, 4H, Hm”a), 6.58 (d, 3J =7.8 Hz, 8 H, Hob), 6.51 (s, 4H, Hm“b),6.29 (d, 3J=8.4 Hz, 8 H, H11’), 6.12 (d, 3J= 8.4 Hz, 8H, Hma), 5.20 (d, 3J=

7.8 Hz, 8 H, Hmb), 4.60–4.19 (m, 24 H, OCH2), 4.24 (m, 8 H, H8’), 3.95–3.89(m, 8 H, OCH2), 3.77–3.59 (m, 24H, OCH2), 3.24 (m, 8H, H10’), 2.77–2.60(m, 8 H, OCH2), 2.00 (s, 12H, CH3p), 1.85 (m, 8H, H9’), 1.47 (s, 12H,CH3oa), 1.26 (s, 108 H, H15’), 0.64 ppm (s, 12H, CH3ob); 13C NMR(75 MHz, CD2Cl2, 298 K): d=159.85, 158.98, 158.63, 158.46, 158.34,157.65, 155.93, 152.99, 150.22, 149.95, 149.83, 149.65, 149.36, 148.81,146.14, 145.52, 144.77, 144.70, 144.28, 144.08, 143.36, 140.74, 139.02,138.97, 138.52, 138.32, 138.20, 137.68, 134.89, 133.06, 132.46, 132.39,132.17, 131.65, 131.23, 130.72, 130.35, 129.90, 129.74, 128.60, 128.40,

Scheme 5. a) “inflated” and b) “deflated” conformations of the host 14+ .The rotaxane can adjust its geometry to accommodate either extendedguests (e.g., G4) or very short guests (e.g., DABCO).

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J.-P. Sauvage, A. Sour et al.

127.77, 127.25, 126.92, 126.83, 125.73, 125.36, 124.67, 124.54, 122.32,121.69, 119.62, 119.32, 114.29, 113.77, 113.66, 113.55, 112.35, 101.97, 70.29,69.66, 69.35, 68.85, 68.51, 68.42, 67.80, 67.20, 64.16, 63.42, 34.57, 31.47,29.66, 25.21, 21.53, 21.09, 21.06 ppm; UV/Vis (CH2Cl2): (log e) lmax =280(5.31), 339 (5.34), 420 (6.00), 551 (4.68), 591 nm (4.09); HRES-MS: calcdfor [C440H416Cu4N36O28Zn2]

4 +/4 (14 +/4) m/z=1760.202; found 1760.169.

Acknowledgements

We would like to thank Lionel Allouche for assistance with the NMRdata collection and analysis. We acknowledge the Ecole Normale Sup�r-ieure (Paris) and the French Ministry of Education for a Ph.D. scholar-ship for C.R..

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