Multipoint molecular recognition withina calix[6]arene funnel complexDavid Coquierea, Aurelien de la Landeb,c, Sergio Martíc, Olivier Pariselb, Thierry Pranged, and Olivia Reinauda,1

aLaboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques, Unite Mixte de Recherche 8601, Centre National de la Recherche Scientifique,Universite Paris Descartes (Paris 5), 45 Rue des Saints-Peres, 75006 Paris, France; bLaboratoire de Chimie Theorique, Unite Mixte de Recherche 7616, CentreNational de la Recherche Scientifique, Universite Pierre et Marie Curie (Paris 6), 4 Place Jussieu, F-75252, Paris cedex 05, France; cDepartament de QuímicaFísica i Analítica, Universitat Jaume I, Box 224, SP-12080 Castello, Spain; and dLaboratoire de Cristallographie et Resonance Magnetique NucleaireBiologiques, Unite Mixte de Recherche 8015, Centre National de la Recherche Scientifique, Universite Paris Descartes (Paris 5), 4 Avenue de l’Observatoire,75006 Paris, France;

Edited by Julius Rebek, Jr., The Scripps Research Institute, La Jolla, CA, and accepted January 6, 2009 (received for review November 16, 2008)

A multipoint recognition system based on a calix[6]arene is de-scribed. The calixarene core is decorated on alternating aromaticsubunits by 3 imidazole arms at the small rim and 3 aniline groupsat the large rim. This substitution pattern projects the anilinenitrogens toward each other when Zn(II) binds at the Tris-imida-zole site or when a proton binds at an aniline. The XRD structureof the monoprotonated complex having an acetonitrile moleculebound to Zn(II) in the cavity revealed a constrained geometry at themetal center reminiscent of an entatic state. Computer modelingsuggests that the aniline groups behave as a tritopic monobasicsite in which only 1 aniline unit is protonated and interacts with theother 2 through strong hydrogen bonding. The metal complexselectively binds a monoprotonated diamine vs. a monoaminethrough multipoint recognition: coordination to the metal ion atthe small rim, hydrogen bonding to the calix-oxygen core, CH/�interaction within the cavity’s aromatic walls, and H-bonding tothe anilines at the large rim.

amines � biomimetic receptor � host–guest interactions � Zn complex

Molecular recognition is a fundamental process in biologyarising from weak, noncovalent interactions such as elec-

trostatics, hydrogen-bonding CH/� and cation/� interactions aswell as hydrophobic effects. Synthetic receptors (1–6) offermodels for understanding the driving forces that govern therecognition event and provide applications for developing tech-nologies such as sensor devices. Inspired by Nature (7, 8), wehave developed a calixarene-based system that mimics someaspects of mononuclear metalloenzyme active sites. The modelcompounds offer a hydrophobic cone-shaped cavity to a guestmolecule and present an additional binding site at the narrowend in the form of a fixed metal ion (9). The host–guest metalcoordination and other noncovalent interactions within the conedrive the selective recognition events (10). The flexible calix coreallows adjustment of its properties (11) for the recognition of arange of guests (12, 13). Among the 3 generations (14–16) ofthese so-called funnel complexes, the most flexible one featurescoordination of the metal ion to 3 appended imidazole arms atthe small rim (Fig. 1). This fixes 3 aromatic walls in a well-definedcone-shaped cavity required of a host structure.

The prototype of this family of biomimetic receptors is ligand1tBu, a calix[6]arene functionalized in alternate positions at thesmall rim by methyl and 2-methyl-N-methylimidazole groups(17). Recent developments related to the large rim functional-ization of this system have expanded its versatility, allowingtuning its properties such as water solubility (13, 18). It alsoopened a route to ditopic receptors (19–20). Specifically ligand1NH2 presents 3 amino functions at the large rim on aromaticunits that alternate with those bearing the imidazole arms.Besides a significant increase of the hydrophilicity, replacing 3tBu groups for NH2 at the large rim was shown to (i) widen thedoor for guest binding, (ii) enlarge the cavity, (iii) increase theflexibility of the calix core—allowing the induced-fit process

with a cavity that expands for large guests (such as dopamine ortryptamine) and shrinks for small ones (H2O) (13), and (iv) allowthe binding of a second metal ion at the large rim with a guestsandwiched between the 2 metal ions (19).

In this article, we report that this Tris-aniline core can alsobehave as a tritopic monobasic site, whose unique propertiesstem from the supramolecular behavior of the calixarene-basedsystem. We show that this basic site can provide recognition ofpolyfunctional diamine guests. The funnel complex features aZn-binding site, an oxygen-rich small rim as second coordinationsphere, �-rich cone walls, and a basic ‘‘door.’’

Evidence for a Tritopic Basic Site at the Large Rimof Calixarene 1NH2

The Zn complex derived from calixarene 1NH2 binds 1 equiv ofsolvent in its cavity (13, 19), as is typically observed for this familyof funnel receptors. To probe the basic properties of the anilinodoor, The protonation of the complex as monitored by 1H NMRspectroscopy is shown in Fig. 2. Comparison of the spectrarecorded after the addition of aliquots of a strong acid (HClO4),reveals the formation of 3 species. All display an apparent C3vsymmetry in view of the number of proton signals, consistentwith a cone conformation. The process is fully reversible becausesubsequent addition of Et3N restored the starting mononuclearcomplex. The titration involves 3 different and successive acid-base equilibria. The coexistence of peaks belonging to thedifferent species shows that, in some cases, the equilibria areslow with respect to the NMR time scale. Based on all ourprevious experience with related Tris-imidazole-based systems,the key points that allow interpretation of the NMR spectra arethe following:

� A C3v symmetrical signature with well differentiated dou-blets for the calix-bridging methylene protons accounts for aflattened cone conformation where the large rim substituentsalternate in ‘‘in’’ and ‘‘out’’ positions.

Y For a tBu substituent, an in position is characterized by anup-field shift of �0.5 ppm relative to an out position. Therelated aromatic unit displays correspondingly an up-fieldshifted resonance for its protons.

Y Similarly, the methoxy resonance reports its position relative tothe cavity: A high-field shifted resonance (up to 2 ppm) denotes

Author contributions: O.R. designed research; D.C. performed research; D.C., A.d.l.L., S.M.,O.P., and T.P. analyzed data; A.d.l.L., S.M., and O.P. performed computational modeling;and O.R. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Data deposition: The atomic coordinates have been deposited in the Cambridge Crystal-lographic Data Centre (Cambridge, U.K.) (accession no. 623586).

1To whom correspondence should be addressed. E-mail: olivia.reinaud@parisdescartes.fr.

This article contains supporting information online at www.pnas.org/cgi/content/full/0811663106/DCSupplemental.

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an in position with the corollary that the corresponding parasubstituents at the large rim (i.e., the NH2 groups) are out.

� The binding of the metal ion enforces the 3 imidazole armsto adopt in positions and the methoxy groups adopt the outpositions.

Y When coordinated to a mono-Zn(II) center, the imidazolerings display 2 resonances that are separated by �0.6 ppm, andthe methylene protons to which they are connected aredown-field shifted (�0.5 ppm) compared with the free ligand.

Y The guest molecule undergoes up-field shifts of its protons ashigh as 3 ppm as a result of its inclusion into the aromatic coreof the calix cone.

Stepwise Analysis of the 1H NMR Titration of Complex[Zn(1NH2)(CD3CN)](ClO4)2First Step. The addition of the first equivalent of HClO4 to aCD3CN solution containing complex [Zn(1NH2)(CD3CN)]2� in-duces the modification of a single resonance, with no detectablechange for the others. This shift indicates that only the aniline units

Fig. 1. Shaping the calix[6]arene cone into a Zn(II) funnel complex. The high flexibility of the calixarene allows spectacular induced-fit processes: thecoordination of Zn(II) to three appended imidazolyl arms at the small rim fixes three aromatic units (in blue) into a well-defined cone-shaped cavity and projectsthe three others (green) in an in position. With small R substituents at the large rim (NH2), the cavity can open to large guests (L) and shrink for small ones; itis also spatially well preorganized for a second metal ion binding at the large rim.

Fig. 2. The spatially preorganized Tris-aniline core of the Zn(II) complex behaves as a tritopic mono-basic site and traps a single proton. (Left) From bottomto top: Evolution of the 1H NMR spectra (300 K, 250 MHz) of a CD3CN solution containing complex [Zn(1NH2)(CD3CN)](ClO4)2 upon the progressive addition ofHClO4. �, tBu; E, OCH3; Œ, NCH3 and HIm; �, CH2Ar; ƒ, NH2; F, CH2Im; �, HArNH2; and ■, HArtBu. (Right) Schematic representation of the corresponding speciespresent in solution.

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are affected, whereas the overall Zn structure is maintained. Hence,the basic site interacting with the first equivalent of proton is theTris-aniline core at the large rim, the acid–base equilibrium beingfast vs. the analysis time scale (Fig. 2, red spectra).

Second Step. Upon further addition of protons, another discretespecies starts to grow (Fig. 2, green resonances). The latterrepresents �90% of all species present in solution after theaddition of a total of 4 equiv of HClO4. The shifts of theimidazole resonances show that decomplexation of Zn(II) atthe small rim has occurred because of the protonation of thecoordinating arms. Only minor changes are observed for therest of the calixarene structure, indicating that the f lattenedcone conformation has not changed. A slight convergence of thebridging methylene doublets (�) denotes, however, an increasingflexibility of the macrocycle. All these observations attest to theformation of a quadruply protonated species [1NH2.4H]4� in whicha single proton situated at the aniline core constrains the structureinto a ‘‘complex-like’’ flattened conformation.

Third Step. Last, the per-protonated species (Fig. 2, blue species,[1NH2.6H]6�), as expected, appears after further addition of 2 equivof acid and remains unchanged upon increasing acidity. Thechanges for the tBu and methoxy resonances are characteristic ofthe inversion of the cone conformation into a ‘‘ligand-like’’ structure.

When carried out in protiated MeCN, the same experimentallowed the 1H NMR detection of CH3CN bound inside thecalixarene cavity. Indeed, the resonance of the bound MeCNinside the cavity of complex [Zn(1NH2)(CH3CN)]2� was detectedat 0.53 ppm. Upon the stepwise addition of the first equivalentof proton to this complex, the guest CH3CN peak underwent anup-field shift from 0.53 ppm to 0.35 ppm, which is best explainedby a change in its spatial position relative to the center of the calixcone. Further addition of H� led to the progressive vanishing ofthat resonance, consistently with zinc decomplexation.

Further Characterization of the Monoprotonated Complex[Zn(1NH2.H)(L)]3�

Reaction of the starting complex with 1 equiv of either picric ortrif lic acid led, beside some signal broadening, to a very similarNMR signature [see supporting information (SI) Figs. S1–S3 tobe compared with the red profile depicted in Fig. 2]. This atteststo significant interaction of the tricationic calix-core with thecounter anion. The progressive addition of water to a CH3CNsolution containing the monoprotonated complex[Zn(1NH2.H)(CH3CN)]3� led to the gradual disappearance of theCH3CN guest at 0.35 ppm. The guest was completely gone after5% (by volume at 10 mM Zn complex concentration) was added.All other resonances were barely affected, which attests to aprotonated Zn complex that has exchanged its MeCN guest fora water ligand {[Zn(1NH2.H)(H2O)]3�}. Further addition ofwater (�40% by volume) led to the displacement of the acid–base equilibrium [Zn(1NH2.H)(H2O)]3�/[Zn(1NH2)(H2O)]2� infavor of the latter species. Conversely, when HClO4 was stepwiseadded to a 1:1 MeCN/H2O (vol/vol) solution of the dicationiccomplex [Zn(1NH2)(H2O)]2�, the hexaprotonated ligand wasdirectly produced, and no intermediate monoprotonated com-plex could be observed. This shows that the Tris-anilino basic siteis very sensitive to the medium, and with a hydrogen-bondingsolvent such as water, the cooperative trapping of a single protonat the large rim competes unfavorably with the solvent.

X-Ray Diffraction Structure of Complex [Zn(1NH2.H)(L)]3� (Fig. 3). Com-plex [Zn(1NH2.H)(CH3CN)](ClO4)3 was isolated from CH3CN,and single crystals were grown by slow diffusion of Et2O into aCH2Cl2/MeOH solution of the complex. The space group is P63,the crystal thus presenting homochiral C3 symmetrical com-plexes connected to each other along the crystallographic C3 axis

through a water molecule that shows a weak interaction with 1Zn(II) center (d[Zn-O1W] � 2.41 Å) and hydrogen bonds to 3perchlorate anions. Two counterions are also situated at hydro-gen bond distances from the Tris-aniline core of the neighboringcalix-complex {d[N24-O(ClO4)] � 3.06 and 3.22 Å}. In agree-ment with the solution studies, the Zn complex exhibits aflattened cone conformation with tBu and methoxy substituentsin out positions, projected away from the calix cavity. Thegeometry at the metal center is best described as a trigonalpyramidal N4 arrangement provided by the Tris-imidazole core(the base of the pyramide) and the nitrilo guest ligand bound ata short distance, [d(Zn1-NIm)av � 1.99 Å, d(Zn1-N1W) � 1.93 (4)Å] respectively. The presence of 3 perchlorate anions per calixcomplex attests to the tricationic nature of the latter, andconfirms the presence of an additional proton at the large rim,as anticipated by the [Zn(1NH2.H)(CH3CN)]3� formulation. Ac-cordingly, the relatively short distance separating the anilineunits [d(N,N) � 3.48 Å] suggests hydrogen bonding betweenthem. Also, the head-to-tail stacking, which does not appear intypical dicationic funnel complexes, resembles the dimetalliccomplex [Zn2(1NH2)(H3O2)](ClO4)3. This is probably becausethe funnel structure is positively charged at both extremities.

Compared with the nonprotonated complexes of the sameligand [Zn(1NH2)(L)]2� and the nitrilo hexa-tBu derivative[Zn(1tBu)(MeCN)]2� (21) that show an almost regular tetrahe-dron around the metal center, the cavity space for the guest hereappears to be reduced by the monoprotonated aniline door thatdraws the corresponding aromatic units close to each other. Asa result, the MeCN guest is forced to move toward the small rim,and distorts the geometry at the metal center. This compressionmay be regarded as an ‘‘entatic state.’’*

Computational Modeling of the Complex [Zn(1NH2.H)(MeCN)]3�. Be-cause the X-ray diffraction (XRD) analysis (see Table S1 andTable S2) did not allow fixing the location of the proton at thelarge rim, we turned to computer modeling for an alternative

*An entatic state refers to a state of an atom or group, which, due to its binding in a protein,has its geometric or electronic condition adapted for function (it is derived from the Greekentasis, meaning tension). De Bolster MWG (1997) Glossary of terms used in bioinorganicchemistry (IUPAC recommendations 1997) Pure Appl Chem 69:1251–1303.

Fig. 3. XRD structure of complex [Zn(1NH2.H)(MeCN).H2O](ClO4)3. Selectedinteratomic distances (Å) Zn1…Zn1 12.63; Zn…O1W 2.41 (3); O1W

…O(ClO4) 3.30,3.72, 3.72; N24-ClO4 3.06 and 3.22. Bond distances (Å) and angles (°) Zn-NIm 1.986(19);Zn1-N1W 1.93 (4);Nanilino

…Nanilino 3.48;NIm-Zn-NIm 118.3 (2);NIm-Zn-N1W 97.7 (2).

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description of this supramolecular system. A hybrid densityfunction theory (DFT)/molecular mechanics (MM) approachhas been used to ascertain the location of the proton within theTris-aniline core. This approach includes the entire calixareneligand, the metal cation, the host guest, and the solvent mole-cules in the model (see SI Text). A gas phase modeling of thesmall aggregate [(NH3)3H�] was initially performed and led tothe conclusion that geometries in which the proton is equallyshared by the 3 amine groups, even if possible, is not energet-ically favorable (see Table S3). In contrast, protonation of 1ammonia molecule, together with the formation of 2 hydrogenbonds with the remaining amine functions, appears to be themost favorable arrangement. This is effectively the geometrythereafter obtained at the DFT/MM level for the full system(Fig. 4). The average distance between the aniline nitrogenatoms is 3.49 Å, in remarkable agreement with the X-ray value(3.48 Å). Moreover, the calculated structure shows the com-pression of the guest molecule in the cavity. This is apparentfrom the geometry at the zinc center that is nearly a trigonalpyramid: The NIm-Zn-NIm angles are found to be close to 120°and the NIm-Zn-NCMe is 94° (Fig. 4), in full accord with theX-ray structure of Fig. 3.

The correspondence of the geometrical features in the X-ray andDFT/MM structures is reassuring and leads to the conclusion thatprotonation of only 1 of the aniline functions induces the geometricchanges. The Tris-aniline core is then stabilized by formation of 2hydrogen bonds to the anilinium cation. The pseudo C3v symmetryobserved in solution indicates rapid exchange of the proton amongthe 3 aniline nitrogens on the NMR timescale.

Acid–Base Titration of Ligand 1NH2: Evidence of Supramolecular Con-trol in a Polytopic Basic Site. We also carried out an acid titrationof the free ligand 1NH2 in MeCN solution.

Fig. 5 (and Fig. S4) shows that on the addition of HClO4,ligand 1NH2 undergoes conformational changes, all species beingin fast exchange on the NMR time scale. Three stages can bedistinguished clearly: The first equivalent of acid fixes thecalixarene in a ‘‘complex-like’’ f lattened cone conformation. Thearomatic aniline proton signals (in red) undergo a telling up-fieldshift consistent with their monoprotonation in an in position,whereas the signals of the imidazole ‘‘arms’’ remain barelyaffected. The second step consists in the triprotonation of theimidazole sites as the related signals (blue and green peaks) aredown-field shifted. After 4 equiv of added acid, the NMR profileis superimposable on that of Fig. 2 (green signature) andcorresponds to the quadruply protonated species [1NH2.4H]4�.The last step leads, as expected, to [1NH2.6H]4�, which has adifferent conformation. Finally, when the same acid–base titra-tion was carried out with ligand 1NO2 (presenting NO2 substitu-ents in place of NH2, see Fig. 1 and Fig. S5), a continuum of

NMR spectra were observed with no change in � shifts, exceptfor the imidazole arms. This shows that in this case, no structuralchange is induced upon protonation, because there is no Tris-anilino core at the large rim.

These experiments show that:

Y A proton is selectively trapped at the large rims of receptor[Zn(1NH2)(L)]2�and free ligand 1NH2.

Y The aniline sites at the large rim appear to act cooperativelyand behave as more basic than the imidazole groups situatedat the small rim, despite the differences in their pKas, 4.6 (22)and 6.9 (23), respectively. The cooperative effect arises fromthe specific location of the anilines and the flexibility of thecalixarene core.

Y The monoprotonated large rim site forces the calixarene into aflattened cone, even in the absence of a metal ion at the large rim.

Y Further addition of protons affects the 3 imidazole arms, withno characterizable discrete species, even in the presence ofZn(II). In these experiments, protonation of the first imida-zole arm causes the loss of metal then protonation of theremaining 2 imidazoles.

Evidence for Multisite Recognition of DiaminesThe receptor 1NH2 offers specific attractive interactions at its largerim, and we explored its possibilities for molecular recognition. Westudied the competitive binding of a monoamine and a diamine withcomparable size, namely N-butylamine and 1,3-propyldiamine.

When an equimolar mixture of these 2 amines was added toa CD3CN solution of complex [Zn(1NH2)(MeCN)](ClO4)2, 2 newspecies were identified by 1H NMR spectroscopy (Fig. 6 and Figs.S6–S8). The first displays a C3v symmetrical signature associat-edwith 2 up-field shifted resonances (red traces) correspondingto complex [Zn(1NH2)(nBuNH2)]2� having N-butylamine as aguest. The second species corresponds to free ligand 1NH2. Thepartial loss of Zn is apparently related to the presence of diaminethat competes as a ligand for Zn to form the more stable

Fig. 4. Computed structure of complex [Zn(1NH2.H)(MeCN)]3�. Selectedinteratomic distances (Å) and angles (°) Zn-NIm 2.045, 2.046, 2.051, Zn-Nnitrilo

2.00, Nanilino…Nanilino 3.023, 3.073, 4.374, NIm-Zn-NIm 118.9, 125.6, 113.9, NIm-

Zn-Nnitrilo 93.03, 93.80, 96.05.

Fig. 5. Due to their specific location on the flexible calixarene core, theanilines act cooperatively and behave as more basic than the imidazole groupssituated at the small rim. (Left): From bottom to top: Evolution of the 1H NMRspectra (300 K, 250 MHz) of a CD3CN solution containing ligand 1NH2 upon theprogressive addition of HClO4. Œ, HIm; F, CH2Im; �, HArNH2; and ■, HArtBu. (Right)Schematic representation of the corresponding species present in solution.

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bis-chelate Zn adduct. Accordingly, no diamine can be detectedin the high-field region. However, after the addition of an aliquotof a strong acid such as picric or perchloric acid to the mixture,a new NMR signature appeared (blue trace) at the expense of theformer calix species. The top spectrum of Fig. 6 shows thepresence of a guest that could be identified as the monoproto-nated diamine. Hence, the addition of acid led to the partialprotonation of the diamine (the strongest base), release of theZn2� for recoordination to 1NH2 and final inclusion of themonoprotonated 1,3-propyldiamine to yield complex[Zn(1NH2)(NH2C3H6NH3)]3�. Integration of the resonances ofthe residual complex [Zn(1NH2)(nBuNH2)]2� shows that theselectivity for guest binding is �90%. Because these 2 compet-itive guests (NH2C3H6NH3

� and nBuNH2) present the samecoordinating function (RNH2) and similar steric bulk, the highselectivity of the recognition process is attributable to theinteraction of residue R with the calixarene cavity. Hydrogenbonding between the ammonium group and the Tris-anilino core(NH�. . . NAniline) seemed likely, and we tested this possibilitythrough computer modeling.

Computer Modeling. The optimized structure of the[Zn(1NH2)(NH2C3H6NH3)]3� at the DFT/MM level of compu-tation confirms this hypothesis. Two hydrogen bonds betweenthe ammonium group of the guest and 2 aniline groups of thehost are readily formed (see Fig. 6 and Figs. S9 and S10). Theseinteractions at the large rim level provide a supplementarystabilization and account for the difference in binding affinityobserved in NMR. When protonated, the 1,3-propyldiamine isstabilized both by its interaction with both the Zn(II) center andhydrogen bonds to the Tris-aniline core. The latter interactionis not available to butylamine.

ConclusionsThis study highlights an unusually basic behavior of anilines. Thesubstitution pattern at the small and the large rim of thecalix[6]arene confers a high propensity for the system to adopta cone conformation that can be flattened with the aromatic

units being in alternate in and out positions relative to the cavity.Upon metal binding, the imidazole arms on one side areassembled around the C3 axis of the cone, which places the 3anilines close to each other on the other side of the cavity. Theresulting basic lone pairs at the large rim can converge to behaveas a tritopic ‘‘mono’’ basic site. Such preorganization increasesthe basicity of the aniline core by �3 pKa units, and it binds aproton in preference to the free imidazole functions despite theintrinsically higher basicity of the latter. Computer modelingsuggests that only 1 aniline unit is protonated at a given time, butit interacts strongly with the other 2 through hydrogen bonding.In solution, the proton undergoes a rapid exchange between the3 N-donors. The high affinity of the Tris-aniline site for a singleproton is quite remarkable in view of the poor basicity of typicalaniline and has to be related to the spatial arrangement of the3 amino groups held at a short distance. In some ways, thisbehavior is reminiscent of proton sponges, in which fast protonmotion from 1 nitrogen to the other has been demonstrated (24).However, the case at hand differs in 2 ways: (i) in the free base,the aniline groups are not necessarily near each other (in theabsence of a metal ion), but the addition of a single protoninduces the conformational rearrangement required for thecooperative participation of the aniline sites; and (ii) the stabi-lization of the monoprotonated form relies on the convergenceof 3 nitrogen centers, not only 2. The X-ray structure of themonoprotonated complex having an acetonitrile moleculebound to Zn(II) in the cavity reveals a strained geometry at themetal center reminiscent of an entatic state.* For the metalcomplex, the large rim can also act as a recognition site for aprotic guest. This is illustrated by the selective binding of amonoprotonated diamine vs. a monoamine to the Zn complex.It is proposed that hydrogen bonding occurs at the large rim site,and computer modeling supports this surmise.

The corresponding metal complex thus behaves as a multi-point recognition host: (i) a coordination link to the metal ionembedded at the small rim within the 3 imidazole arms, (ii) a firstset of hydrogen bond acceptors of the calix–oxygen crown thatfavors the binding of protic donors such as primary amines to themetal, (iii) CH/� interactions within the cavity space defined byaromatic walls, and finally (iv) a second set of hydrogen bondacceptors at the large rim that allows selective binding of guestswith complementary functions. We are now exploring thismultipoint recognition scaffold with the introduction of variousnew substituents at the large rim for the selective recognition ofincreasingly complex guest molecules.

Materials and MethodsComplex [Zn(1NH2.H)(CH3CN)](ClO4)3. Complex [Zn(1NH2)(H2O)](ClO4)2 (19 mg, 13�mol) was dissolved in CD3CN (1 mL). HClO4 diluted in CD3CN was thenprogressively added until complete formation of the monoprotonated com-pound (monitored by 1H NMR spectroscopy). Slow diffusion of Et2O led to thecrystallization of the expected tricationic complex as an orange/yellow solid.1H NMR (250 MHz; CD3CN; 300 K) � � 1.42 (s, 27 H, CH3), 3.48 (d, 6 H, J � 15 Hz,ArCH2), 3.63 (s, 18 H, OCH3, NCH3), 4.04 (d, 6 H, J � 15 Hz, ArCH2), 5.01 (s, 6 H,ImCH2), 5.86 (s, 6 H, HArNH2), 6.85 (s, 3 H, HIm), 7.41 (s, 6 H, HArtBu), 7.44 (s, 3 H,HIm); Anal. Calcd for [Zn(1NH2.H)(CD3CN)](ClO4)3 � 3H2O; C74H94D3Cl3N10O21Zn:C, 54.28; H, 6.16; N, 8.55. Found: C, 54.11; H, 6.41; N, 8.65. MS (ESI�, MeCN):calculated m/z for [Zn(1NH2.H)]3� � 412.9; found m/z � 413.1.

The cif file of the XRD structure was deposited with the Cambridge Crystallo-graphic Data Centre (Cambridge, U.K.) with the reference number 623586.

Additional Details. Details for X-ray analysis, computational modeling, titrationexperiments,andmaterials forspectroscopiccharacterizationareprovidedinSIText.

ACKNOWLEDGMENTS. We thank Beatriz Guimaraes and Andrew Thomsonfor access to the PX2a beamline at the synchrotron SOLEIL, France. This workwas supported by the Centre National de la Recherche Scientifique, Ministerede la Recherche (D.C.), and Agence National pour la Recherche (CalixzymeProject ANR-05-BLAN-0003).

Fig. 6. Thanks to multipoint recognition, the calix-receptor can discriminatebetween mono- and ditopic guests: it selectively binds propyldiamine in thepresence of butylamine, provided the former is mono-protonated. (Left, frombottom to top) Evolution of the up-field area of the 1H NMR spectra (300 K, 250MHz) of a CD3CN solution of complex [Zn(1NH2)(H2O)](ClO4)2 before (A) andafter (B) the addition of a 1:1 mixture of N-butylamine/1,3-propyldiamine (3,5equiv diluted in CD3CN). (C–E) after subsequent addition of aliquots (0.5 equiv) ofpicric acid (diluted in CD3CN). (Right) Computed model of the correspondingZn-funnel complex hosting monoprotonated 1,3-propyldiamine (blue trace).

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