DOI: 10.1002/chem.201203376

Self-Assembled Tetragonal Prismatic Molecular Cage Highly Selective forAnionic p Guests

Cristina Garc�a-Sim�n,[a] Marc Garcia-Borr�s,[b] Laura G�mez,[a] Isaac Garcia-Bosch,[a]

S�lvia Osuna,[b] Marcel Swart,[b, c] Josep M. Luis,[b] Concepci� Rovira,[d]

Manuel Almeida,[e] Inhar Imaz,[f] Daniel Maspoch,[c, f] Miquel Costas,*[a] andXavi Ribas*[a]

Introduction

Supramolecular metal-driven self-assembly is a very power-ful method for the rational design of supramolecular entitiesexhibiting a variety of bi- (2D) and tridimensional (3D) geo-metric organizations,[1,2] with potential applications as chem-ical sensors and reaction nanovessels.[3,4] Along this strategy,a variety of structures that include molecular nanosizedsquares and rectangles,[5] trigonal[6] and tetragonal prismaticcapsules,[7] as well as tetrahedrons,[8] octahedrons,[9] andhigher order suprastructures[10–12] have been reported, whichuse mainly Rh-, Pd-, and Pt-based coordination compounds,and pyridine-based organic linkers. Discrete nanosized cagesand capsules are of particular interest because they providean inner cavity that can be filled with functional guest mole-cules when brought together in solution to perform a varietyof organic and organometallic transformations.[8,13–16] Impor-tantly, the capacity to exert control over the intrinsic physi-cal or chemical properties of the guest molecules is a re-markable target to design new molecules exhibiting chemi-cal properties distinct from those characteristic for free dif-fusion in solution, and also for the design of materials withimportant applications.[17,18]

We have recently reported an alternative strategy thatrelies on the use of hexaaza macrocyclic dinuclear CuII com-

[a] C. Garc�a-Sim�n, Dr. L. G�mez, Dr. I. Garcia-Bosch, Dr. M. Costas,Dr. X. RibasQBIS Group, Departament de Qu�micaInstitut de Qu�mica Computacional I Cat�lisis (IQCC)Universitat de Girona, Campus MontiliviE17071 Girona, Catalonia (Spain)Fax: (+34) 972-41-81-50E-mail : miquel.costas@udg.edu

xavi.ribas@udg.edu

[b] M. Garcia-Borr�s, Dr. S. Osuna, Prof. M. Swart, Dr. J. M. LuisInstitut de Qu�mica Computacional i Cat�lisis (IQCC) andDepartament de Qu�mica, Universitat de GironaCampus Montilivi, E17071 Girona, Catalonia (Spain)

[c] Prof. M. Swart, Prof. D. MaspochInstituci� Catalana de Recerca i Estudis AvanÅats (ICREA)Pg. Llu�s Companys 23, E08010 Barcelona, Catalonia (Spain)

[d] Prof. C. RoviraInstitut de Ci�ncia de Materials de Barcelona (ICMAB-CSIC)Campus UAB, 08193 Bellaterra, Catalonia (Spain)

[e] Prof. M. AlmeidaDepartamento de Qu�mica, IST/ITNInstituto Superior T�cnico / CFMCULP-2686-953 Sacav�m (Portugal)

[f] Dr. I. Imaz, Prof. D. MaspochCIN2 ACHTUNGTRENNUNG(ICN-CSIC), Catalan Institute of NanotechnologyEsfera UAB, 08193 Bellaterra, Catalonia (Spain)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201203376.

Abstract: The metal-directed supramo-lecular synthetic approach has pavedthe way for the development of func-tional nanosized molecules. In thiswork, we report the preparation of thenew nanocapsule 3· ACHTUNGTRENNUNG(CF3SO3)8 with aA4B2 tetragonal prismatic geometry,where A corresponds to the dipalladi-um hexaazamacrocyclic complex Pd-1,and B corresponds to the tetraanionicform of palladium 5,10,15,20-tetrakis ACHTUNGTRENNUNG(4-carboxyphenyl)porphyrin (2). Thelarge void space of the inner cavity andthe supramolecular affinity for guestmolecules towards porphyrin-basedhosts converts this nanoscale molecular

3D structure into a good candidate forhost–guest chemistry. The interactionbetween this nanocage and differentguest molecules has been studied bymeans of NMR, UV/Vis, ESI-MS, andDOSY experiments, from which highlyselective molecular recognition hasbeen found for anionic, planar-shapedp guests with association constants (Ka)higher than 109

m�1, in front of non-in-

teracting aromatic neutral or cationicsubstrates. DFT theoretical calculationsprovided insights to further understandthis strong interaction. NanocageACHTUNGTRENNUNG3· ACHTUNGTRENNUNG(CF3SO3)8 can not only strongly hostone single molecule of M(dithiolene)2

complexes (M= Au, Pt, Pd, and Ni),but also can finely tune their opticaland redox properties. The very simplesynthesis of both the supramolecularcage and the building blocks representsa step forward for the development ofpolyfunctional supramolecular nano-vessels, which offer multiple applica-tions as sensors or nanoreactors.

Keywords: cage compounds · den-sity functional calculations · goldanionic guests · host–guest systems ·self-assembly

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FULL PAPER

plexes (Cu-1) as metal–organic molecular clips capable ofself-assembling at room temperature with different dicar-boxylate linkers to yield molecular 2D rectangles[19] anddouble-stranded molecular helicates.[20] Moreover, we ex-panded this strategy on a 3D scale with the synthesis andcharacterization of 3D trigonal prismatic (and antiprismatic)A3B2-type cages (where A= [CuII-1]4+ ; B= BTB (1,3,5-tris-(4-carboxyphenyl)benzene), aH-PTMTC (tricarboxylatepolychloro triphenylmethane), or PTMTC (tricarboxylatepolychloro triphenylmethyl radical).[21] In the latter familyof compounds we showed that the introduction of moleculartectons bearing fluorescent and/or magnetic properties tothese structures was possible, conferring new sensor proper-ties to the capsule-like compounds. Based on our previousexperience, in this work we report our design and synthesisof the new 3D molecular cage 3·(X)8 (X=CF3SO3, ClO4),which is built from tetracarboxylate porphyrin based linkersand PdII-based macrocyclic synthons (Scheme 1). We envi-

sioned that the expected tetragonal prismatic structurewould be more robust than our previously reported trigonalprismatic structures,[21] and the inner cavity was expected tobe much larger and capable to show a rich host–guest chem-istry. In addition, the dinuclear PdII-based molecular clipswere especially designed to confer diamagnetic properties tothe molecular cage, thus allowing NMR characterizationand monitoring of the host–guest reactions. Here, we reportthe structural characterization of synthon Pd-1 and cage 3(Scheme 1), and we describe the host–guest properties ofcompound 3 in detail by means of different spectroscopictechniques with support of computational methods. Amongall molecular nanocages reported in the literature, a largenumber of them are prone to encapsulate cationic or neutral

guests,[2,8,22,23] although several examples of anionic encapsu-lation exist, especially with inorganic counteranions (NO3

�,BF4

�, PF6�, SO4

2�).[24–28] Here, we describe a high selectivityof our designed cage for several molecular anionic guests,most of them bearing a flat structure with a p-conjugatedsystem, including gold bisdithiolene compounds. The subtle-ties of the host–guest reactivity at the inner cavity are de-scribed in depth.

Results and Discussion

Synthesis and characterization of the dinuclear PdII molecu-lar clip : The synthesis of the molecular clip [Pd2(1) ACHTUNGTRENNUNG(AcO)2]-ACHTUNGTRENNUNG(AcO)2 (Pd-1·ACHTUNGTRENNUNG(AcO)4; AcO =acetate anion) was carried outby following a synthetic procedure parallel to that previous-ly employed for dinuclear CuII molecular clips.[19] The palla-dium metallomacrocycle (Pd-1· ACHTUNGTRENNUNG(AcO)4) was prepared by thereaction of ligand 1 with Pd ACHTUNGTRENNUNG(AcO)2 salt in CH3CN, whichwas heated to reflux (see Scheme 1 and the Supporting In-formation), yielding the Pd-1· ACHTUNGTRENNUNG(AcO)4 complex as an intenseyellow crystalline solid in 87 % yield. Complex Pd-1· ACHTUNGTRENNUNG(AcO)4

has been characterized in solution and in the solid state byNMR spectroscopy, ESI-MS, and X-ray diffraction analysis.

The crystal structure of the cationic unit of this complex isshown in Figure 1 and the corresponding data are summar-

ized in Table 1. Its d8 electronic configuration enforces thepalladium ions to adopt a tetra-coordinated square-planargeometry formed by three N atoms of the macrocyclicligand 1 and one O atom from the carboxylate group, whichcoordinates in a monodentate mode. As expected from pre-vious studies with Cu-1 complexes,[19] the coordinationplanes of the two Pd atoms are nearly parallel, oriented inthe same direction. This arrangement creates a well-definedmolecular “clip”, which is very similar to the one previouslyobserved with copper (Cu-1· ACHTUNGTRENNUNG(ClO4)4), and which is, there-fore, susceptible to be used in rational building of supramo-lecular structures. Indeed, the crystal structure of Pd-1·-

Scheme 1. Schematic representation of a) the building blocks used inb) the synthesis of tetragonal-prismatic nanocapsule 3 (Porph= porphy-ACHTUNGTRENNUNGrin).

Figure 1. Crystal structure of macrocyclic compound Pd-1· ACHTUNGTRENNUNG(AcO)4 (hydro-gen atoms are omitted for clarity).

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ACHTUNGTRENNUNG(AcO)4, and analogous structures with different counteran-ions, have also been determined and display the same paral-lel oriented coordination of anions, showing a consistency ofthe structures in the solid state (see the Supporting Informa-tion).

Furthermore, the structure of Pd-1· ACHTUNGTRENNUNG(AcO)4 was obtainedcomputationally, including both solvent and London disper-sion effects by using the BLYP-D3/DZ(COSMO:MeCN) ap-proach (see computational details in the Supporting Infor-mation). The differences between the DFT geometry andthe crystal structure are very small, suggesting that the struc-ture determined in the solid state by X-ray diffraction analy-sis is retained in solution (see Figure S38 in the SupportingInformation). This theoretical evidence fully agrees with theresults obtained by the NMR characterization of Pd-1·-ACHTUNGTRENNUNG(AcO)4. Overall, the spectrum indicates that the complexcan be divided in four parts that are magnetically equiva-lent. One- and two-dimensional NMR analysis allows the as-signment of all protons of the molecule, as shown inFigure 2.

Aromatic protons appear as two singlets at d= 9.42 and7.33 ppm (H(a) and H(b)), and indicate equivalence of twogroups of four protons. Benzylic protons appear as two dou-blets (H(c)). In the NOESY spectra a NOE signal between�CH2� (H(c), d=4.01 ppm) and H(a) as well as between�CH2� (H(c), d=3.08 ppm) and H(b) can be observed.Central and lateral N-methyl protons appear at d= 3.38 and1.43 ppm as two singlets (�CH3, H(g), 6 H and �CH3, H(d),12 H). The NOESY spectrum shows a coupling between N�CH3, H(g) and H(a). Alkylic protons appear as AB systems

and double doublets (�CH2�, H(f) and �CH2�, H(e)). Acoupling between �CH2� (H(c), d=4.01 ppm), �CH3 (H(d),d= 3.38 ppm), and aAcO (d= 2.05 ppm) can be observed inthe NOE spectrum, which indicates that this signal corre-sponds to Pd-coordinated AcO� ions. The other two non-co-ordinating acetate groups appear at d=1.65 ppm (bAcO).

The integrity of the complex Pd-1· ACHTUNGTRENNUNG(AcO)4 in CH3CN solu-tion is further confirmed by ESI-MS analysis. Peaks corre-sponding to the expected species with loss of two counteran-ions have been found, showing the correct isotopic pattern(see the Supporting Information).

Synthesis and characterization of the tetragonal prismaticnanocage : The molecular cage 3· ACHTUNGTRENNUNG(CF3SO3)8 was easily pre-pared by reaction of two equivalents of 5,10,15,20-tetra-kis(4-carboxyphenyl)porphyrin–PdII (2), previously deproto-nated with NEt3, and four equivalents of Pd-1· ACHTUNGTRENNUNG(AcO)2-ACHTUNGTRENNUNG(CF3SO3)2 (see Scheme 1). The reaction was carried out inDMF at 378 K during 16 h. The obtained reaction mixturewas filtered and recrystallized by slow diffusion of diethylether. A dark red crystalline solid corresponding to 3·-ACHTUNGTRENNUNG(CF3SO3)8 was obtained in 93 % yield. The diamagneticnanocapsule has been characterized in solution by means ofNMR spectroscopy and ESI-MS (see below). Single crystalssuitable for X-ray diffraction were grown from slow diffu-sion of diethyl ether over a solution of 3· ACHTUNGTRENNUNG(ClO4)8 in DMF. X-

Table 1. Crystal data and structure refinement for compounds Pd-1·-ACHTUNGTRENNUNG(AcO)4 and 3· ACHTUNGTRENNUNG(ClO4)8.

Pd-1· ACHTUNGTRENNUNG(AcO)4 3· ACHTUNGTRENNUNG(ClO4)8

formula C34H56N6O4Pd2·2(C2H3O2) C216H212N32O16Pd10·8 (ClO4)MW 943.74 5317.8crystal system monoclinic hexagonalspace group C2/m P6̄2ma [�] 35.76(4) 37.421(5)b [�] 11.688(12) 37.421(5)c [�] 13.033(13) 27.418(6)a [8] 90.00 90.00b [8] 97.174(14) 90.00g [8] 90.00 120.00V [�3] 5404(9) 27.418(6)Z 4 31calcd [gcm�3] 1.160 0.805T [K] 180(2) 150(2)l ACHTUNGTRENNUNG(MoKa) [�] 0.71073 0.908m [mm�1] 0.709 0.4862qmax [8] 28.38 26.37reflns collected 21828 37018independentreflns

6793 11104

Rint 0.1394 0.0457parameters 265 674GOF on F2 0.996 1.56R/Rw

[a] R1=0.1082, wR2 =0.2670 R1 =0.1311, wR2= 0.432

[a] R=� jF0�Fc j /�F0 and Rw= {�[w ACHTUNGTRENNUNG(F02�Fc

2)2]/�[w ACHTUNGTRENNUNG(F02)2]}1/2 where w=

1/[s2 ACHTUNGTRENNUNG(F02+(aP)2 +bP), P = (F0

2 +2Fc2)/3, and a and b are constants given

in the Supporting Information (cif format).

Figure 2. a) Schematic representation (top) and 1H NMR spectrum(CD3CN, 298 K, 400 Hz) (bottom) of the Pd-1· ACHTUNGTRENNUNG(AcO)4 complex.

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FULL PAPERSelf-Assembled Tetragonal Prismatic Molecular Cages

ray diffraction analysis required the use of synchrotron radi-ation and a sealed capillary because the crystals are highlyunstable towards loss of solvent molecules, and also due toweak reflections. Nanocage 3 consists of two parallel tetra-carboxylated porphyrins (the Pd···Pd distance is 7.49 �)linked by four macrocyclic dinuclear Pd complexes (seeFigure 3 and Table 1). The four carboxylate residues of each

porphyrin are linked by means of h1-O monodentate coordi-nation to one Pd center. Altogether, the overall structure isdefined as a tetragonal prismatic cage taking the set of eightequivalent atoms of the four carboxyphenyl moieties as ver-tices of the polyhedron, thus bearing a D4h symmetry. Allperchlorate molecules could be identified on the outside ofthe cage, indicating that the latter is an octacationic mole-cule. Electron density maps of the inner cavity of the cageshow the presence of residual, poorly defined electron den-sity that was assigned to disordered solvent molecules, butthey could not be refined.

The nanocapsule 3· ACHTUNGTRENNUNG(CF3SO3)8 has been characterized byNMR experiments. Two-dimensional COSY and NOESY ex-periments were necessary to fully assign all proton signals.

All the 1H NMR signals assigned are detailed in Figure 4. Aclear parallelism can be observed with the 1H NMR spec-trum of Pd-1· ACHTUNGTRENNUNG(AcO)4. New signals have appeared, whichbelong to the porphyrin protons. Protons from the pyrrolering (Hpyrrole) are all equivalent due to the high symmetry ofthe resultant structure (D4h), and so they appear as a singletat d=8.36 ppm. Protons from the carboxyphenyl groupsappear at different shifts. Two of them remain inside thecavity, and they appear at d=8.74 (H(h)) and 8.15 ppm(H(j)). The other ones, that is, H(i) (d= 8.24 ppm) and H(k)(d=7.62 ppm), are located outside the cavity. The NOESYspectrum shows a NOE signal between H(a) and H(h), con-firming that the aromatic protons H(h) remain inside thecavity. Signals between H(h) and H(j), and H(i) and H(k)are also observed in the NOESY and COSY experiments.

Analytical evidence of the molecular composition ofnanocapsule 3· ACHTUNGTRENNUNG(CF3SO3)8 in solution was obtained by ESI-MS experiments in CH3CN. The observed peaks coincide inall cases with the calculated ones (Table 2), indicating thatthese architectures retain their integrity in solution. Peakscorresponding to the molecular clip Pd-1 were not observed,thus cage 3 remains intact even at low concentrations in sol-ution, which were employed in the ESI-MS analyses.

We have also calculated the structure of cage 3·(Cl)8

(where the eight triflate counterions have been replaced bychloride ions) by using the BLYP-D3/DZ (COSMO:MeCN)approach (acetonitrile solvent effect included, see the Sup-porting Information). The calculations reproduce very wellthe crystal structure of the octacationic moiety 3, showing aslightly shorter distance between the porphyrin planes (thePd···Pd distance is 6.84 � for computed 3·(Cl)8 and 7.49 � inthe crystal structure). Thus, the results obtained by NMRspectroscopy, ESI-MS, and DFT are all in agreement to indi-cate that the structures obtained in the solid state fromXRD analyses are retained in solution.

Host–guest chemistry of nanocapsule 3·ACHTUNGTRENNUNG(CF3SO3)8 : The tet-ragonal prismatic nanocapsule 3 has a large empty innercavity, which allows its study as a molecular receptor bymeans of host–guest supramolecular interactions. The analy-sis of the crystal structure and the DFT geometry promptedus to study the characteristics, properties, and dimensions ofthis nanovessel, in order to determine, which kind of sub-strates could be suitable guests. Chemical properties of cage3 can be predicted by carefully analyzing its structure. Itcontains two metallo-porphyrins that are p-electron donors,thus the capsule can establish p–p interactions with a givensubstrate. Lateral molecular clips bear aromatic rings and,like porphyrins, they can act as electron-donor units, andcan also establish hydrogen-bonding or p–p interactions.[29,30]

Moreover, square-planar PdII centers from porphyrins mightpresent weak interactions at their axial coordination vacan-cies. This tetragonal prismatic architecture is highly positive-ly charged (+8), so it is also susceptible to host anionic sub-strates.

Taking into account its dimensions and the different kindof interactions that nanocapsule 3 can potentially establish,

Figure 3. Crystal structure of 3· ACHTUNGTRENNUNG(ClO4)8. Lateral (top) and top view(bottom) of the structure (hydrogen atoms are omitted for clarity).

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M. Costas, X. Ribas et al.

we selected a first set of organic molecules and coordinationcomplexes as substrates to carry out different host–guest ex-periments (see Table 3). All selected substrates are flat mol-ecules, soluble in CH3CN, with suitable dimensions to dif-fuse into the inner cavity of the receptor. Because the inter-planar distance between the porphyrin moieties is found tobe in the 6.8–7.5 � range, flat substrates are ideally suited tointeract with both porphyrin moieties at an optimum dis-tance at the center of the cavity (�3.5 �). In general, allsubstrates shown in Table 3 should be able to interact with

this nanocapsule by van derWaals interactions and p–p

stacking. Besides, the evalua-tion of other characteristics hasbeen pursued. As mentionedabove, cage 3 could act as anelectron donor, so we selectedtetracyanoquinodimethane(TCNQ) as potential substratebecause of its electron-accept-or characteristics. For the samereason, an electron-deficientcationic species, that is, thePF�6 salt of 4,4-bipyridyl-N,N’-dimethyl (Paraquat), has alsobeen studied as potentialguest. On the other hand, dueto the highly positive charge ofthis nanocapsule an anionicspecies has been tested as apossible substrate: TBA[Au-ACHTUNGTRENNUNG(tdas)2] (I) (TBA = tetrabuty-lammonium, tdas2�=1,2,5-thia-diazole-3,4-dithiolate).[31]

Host–guest studies of allcandidates have been per-formed by means of UV/Vismonitoring of the changes ob-served at the Soret band of theporphyrin moiety. Thesechanges are indicative of theoccurrence of host–guest inter-actions (see the Supporting In-formation).[32] All the experi-ments were carried out atroom temperature and gradualUV/Vis changes were moni-tored upon addition of a

Figure 4. Schematic representation (top) and 1H NMR spectrum (CD3CN, 298 K, 400 Hz) (bottom) of nano-capsule 3· ACHTUNGTRENNUNG(CF3SO3)8.

Table 2. ESI-MS of cage 3· ACHTUNGTRENNUNG(CF3SO3)8. Experimental conditions: 100 mm

in CH3CN.

m/z (calculated) m/z (obtained)

[3· ACHTUNGTRENNUNG(CF3SO3)5]3+ 1786.0 1785.9

[3· ACHTUNGTRENNUNG(CF3SO3)4]4+ 1302.2 1301.9

[3· ACHTUNGTRENNUNG(CF3SO3)3]5+ 1011.9 1011.9

[3· ACHTUNGTRENNUNG(CF3SO3)2]6+ 818.5 818.4

[3· ACHTUNGTRENNUNG(CF3SO3)]7+ 680.2 680.2[3]8+ 576.6 576.6

Table 3. Organic compounds and coordination complexes initially ex-plored as guests for nanocage 3· ACHTUNGTRENNUNG(CF3SO3)8.

Coronene Pyrene

Anthracene Xanthene

Pyrazine TCNQ

TBA[Au ACHTUNGTRENNUNG(tdas)2] (I) Paraquat

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FULL PAPERSelf-Assembled Tetragonal Prismatic Molecular Cages

known concentration of substrates. Only one of eight sub-strates showed a clear interaction with the capsule, whereasfor the rest, only dilution effects on the absorbance of thesubstrate and nanovessel were observed. In the case of com-plex I, significant changes on the Soret band were observedand were assigned to host–guest supramolecular interac-tions, as detailed below. On the contrary, neither cationicnor neutral molecules tested interact with cage 3.

Complex I as guest : The UV/Vis titration of 3· ACHTUNGTRENNUNG(CF3SO3)8

with complex I shows a decrease of the intensity of theSoret band (l=408 nm) during the initial additions of sub-strate to a solution of the nanocage. The l= 408 and 520 nmbands show bathochromic shifts (7 and 5 nm, respectively),and two isosbestic points are present during the whole titra-tion (Figure 5). These two facts are indicative for a host–

guest interaction taking place between cage 3· ACHTUNGTRENNUNG(CF3SO3)8 andcomplex I.

A Jobs plot allowed the determination of the 1:1 stoichi-ometry of the host–guest complex formed between complexI and 3· ACHTUNGTRENNUNG(CF3SO3)8, showing a maximum at x�0.5 (Figure 6).The data from Figure S8 in the Supporting Informationrender a Ka value of approximately 4 109

m�1 (this value is

at the experimental upper limit of accuracy for the spectro-scopic titration), which indicates a very strong host–guest as-sociation between the nanocapsule 3· ACHTUNGTRENNUNG(CF3SO3)8 and complexI. The corresponding free energy can be calculated at roomtemperature (DG8=�RTlnKa), affording a value of�54.3 kJ mol�1, illustrating the highly favorable host–guestinteraction.

Further support for the 1:1 host–guest interaction be-tween 3· ACHTUNGTRENNUNG(CF3SO3)8 and I is provided by 1H NMR and ESI-MS experiments, by comparing the data with that obtainedfor the free cage. Different signals in the 1H NMR spectrumof 3· ACHTUNGTRENNUNG(CF3SO3)8 shift upon addition of one equivalent of I

(Figure 7). The guest molecule does not bear any proton inits structure, but significant variations of the signals of thearomatic protons of the capsule are observed. The most no-table variation belongs to the peak of the tetrapyrrole ring(Hpyrrole), which shows a Dd=0.23 ppm upfield shift. Thissignal remains as a singlet, thereby strongly suggesting thatthe guest is inside the box, and there are no non-equivalentsignals that should have arisen from outside-box interac-

Figure 5. Top) UV/Vis monitoring of the titration of the 3· ACHTUNGTRENNUNG(CF3SO3)8

nanocapsule with complex I. Up to 20 equivalents of complex I wereadded to a solution of 3· ACHTUNGTRENNUNG(CF3SO3)8 in CH3CN with c =3.77 10�3 mm.Bottom) Absorbance variation at the Soret band with l =408 nm versusdifferent concentrations of the substrate.

Figure 6. Jobs plot for the host–guest reactivity between nanocapsuleACHTUNGTRENNUNG3· ACHTUNGTRENNUNG(CF3SO3)8 and substrate I (n= (1�xmax) x�1max, where x = [Host]/ ACHTUNGTRENNUNG([Host]-ACHTUNGTRENNUNG[Guest])�1 and n=host–guest complex stoichiometry).

Figure 7. 1H NMR spectra (selected aromatic region) of free nanocageACHTUNGTRENNUNG3· ACHTUNGTRENNUNG(CF3SO3)8 (top) and of the host–guest complex between the nanocageand guest I (bottom).

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M. Costas, X. Ribas et al.

tions. The signals from the aromatic rings of the porphyrinmoieties are also shifted, especially those closest to the por-phyrin ring H(j) (Dd= 0.12 ppm downfield shift) and H(k)(Dd=0.12 ppm upfield shift). The rest of the aromatic sig-nals suffer minor shifts. The aliphatic protons do not presentsignificant variations either. All shifts observed agree withthe occurrence of a host–guest interaction.

Moreover, ESI-MS experiments of the 1:1 host–guest mix-ture showed peaks corresponding to I@3· ACHTUNGTRENNUNG(CF3SO3)7

(Table 4). Minor signals of free nanocapsule 3· ACHTUNGTRENNUNG(CF3SO3)8

were observed probably due to the ionization conditionsduring the ESI experiment (see the Supporting Informa-tion).

Volume estimation of the host–guest adducts : DOSY NMRexperiments were used as the technique of choice to evalu-ate the dimensions of the different compounds in solu-tion.[33,34] For comparison purposes, the Pd-1 molecular clip,the 3· ACHTUNGTRENNUNG(CF3SO3)8 nanocage, and the host–guest compoundI@3· ACHTUNGTRENNUNG(CF3SO3)7 have been studied. The hydrodynamic radii(rH) obtained for each one corresponds to the estimation ofthe volume of the hypothetical sphere of the rotating mole-cule moving through the solution (Table 5), and the values

obtained from these analyses are in reasonable agreementwith the expected radii based on the X-ray or DFT struc-tures in solution.

DFT calculations of the host–guest complex I@3·(Cl)7: Inorder to gain more insight in the specific interaction of thecage with guest molecules, a number of attempts to crystal-lize the host–guest adducts were performed without success.Several good quality crystals were obtained but extremesensitivity to solvent loss precluded their determination. Wethen resorted to DFT calculations of the cage and of thehost–guest complex I@3· ACHTUNGTRENNUNG(Cl7). Computational structural op-

timization studies (see Figure 8) were conducted by usingthe functional BLYP-D3/DZ-(COSMO:MeCN), which alsoincluded counterions for consistency (see the Supporting In-formation).

The computed host–guest complex I@3·(Cl)7 shows theperfect planar guest at the middle of the cage cavity(Figure 8), with a distance between the porphyrin planes of6.39 � and a Au···Pd ACHTUNGTRENNUNG(Porph) distance of 3.19 �. The lattersuggests a strong p–p interaction between the porphyrinplanes and the planar p-guest molecule. Most interestingly,the gold–palladium distance is shorter than the sum of thevan der Waals radii (3.29 �),[35] which suggests some degreeof metal–metal bonding. Interestingly, the shortened dis-tance between the porphyrin planes upon guest inclusion(from 6.84 to 6.39 �) indicates a substantial degree ofbreathing of the cage, which would be operative as an adap-tive mechanism towards guest inclusion. Moreover, theguest inclusion causes a twist on the porphyrin phenyl moi-eties due to significant contact interactions between the aro-matic C�H bonds and the N residues of the guest molecule(C�H ···N 2.49 �). Also, the interaction between the aro-matic rings of the porphyrin and the clip moieties results af-fected. These interactions match well with the changes inthe chemical shifts observed only for the aromatic 1H NMRsignals of I@3· ACHTUNGTRENNUNG(CF3SO3)7. A fast rotation of complex I inside

Table 4. ESI-MS peaks of complex I@3· ACHTUNGTRENNUNG(CF3SO3)7 from a 1:1 host–guestmixture. Experimental conditions: 100 mm in CH3CN.

m/z (calculated) m/z (obtained)ACHTUNGTRENNUNG[I@3· ACHTUNGTRENNUNG(CF3SO3)4]3+ 1900.7 1900.2ACHTUNGTRENNUNG[I@3· ACHTUNGTRENNUNG(CF3SO3)3]4+ 1388.3 1388.2ACHTUNGTRENNUNG[I@3· ACHTUNGTRENNUNG(CF3SO3)2]5+ 1080.7 1080.7ACHTUNGTRENNUNG[I@3· ACHTUNGTRENNUNG(CF3SO3)]6+ 875.8 875.7ACHTUNGTRENNUNG[I@3]7+ 729.4 729.2

Table 5. Different rH values calculated from DOSY NMR experiments inCD3CN as solvent.

D [m2 s�1] rH [�] XRD radii [�][a]

Pd-1 8.22 10�10 7.08 6.753· ACHTUNGTRENNUNG(CF3SO3)8 3.31 10�10 17.54 16.0I@3· ACHTUNGTRENNUNG(CF3SO3)7 3.34 10�10 17.42 16.0

[a] Approximate values of radii have been obtained from XRD and DFTstructures.

Figure 8. DFT-optimized structure of I@3·(Cl)7. Top) Lateral view. Bot-tom) Top view.

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FULL PAPERSelf-Assembled Tetragonal Prismatic Molecular Cages

the cage is proposed to account for the 4-fold symmetry ob-served by 1H NMR spectroscopy.

The possibility of triflate anion encapsulation was alsoscreened computationally. DFT calculations including a trif-ACHTUNGTRENNUNGlate anion as guest inside cage 3 show a very high energeticdestabilization (see Figures S41 and S42 in the SupportingInformation). Indeed, the inclusion of either a triflate orcompound I within cage 3 was also studied in a simplifiedmodel system consisting of only the two palladium porphy-ACHTUNGTRENNUNGrins sandwiching the guest. In this model system, compoundI sits nicely between the porphyrins in a triple-decker fash-ion with a Pd···Au distance of 3.28 �. On the contrary, thetrif ACHTUNGTRENNUNGlate completely distorts the planar model system (see theSupporting Information).

Expanding the scope of anionic p guests : At this point werationalized that other anionic species might also be encap-sulated in cage 3. Although the analysis of the crystal struc-ture of 3· ACHTUNGTRENNUNG(ClO4)8 showed that none of the eight perchloratecounterions was encapsulated in the cavity, we also testedlinear N3

� and SCN�, or planar NO3�, or AuCl4

� anions aspossible guests. Interestingly no interaction with the nano-cage was observed by means of UV/Vis or NMR titrations.These results indicate that not only the anionic nature of theguest seems to be necessary, but also other requisites needto be fulfilled, for instance, the existence of a planar struc-ture and a delocalized p system. We therefore screened aseries of transition-metal (M) bis-dithiolene complexes simi-lar to complex I (see compounds II–X, Table 6).[36–40] Follow-ing the same trend, significant changes occur in the UV/Visspectrum of cage 3 when it is titrated with several M(dithio-lene)2 guest candidates, showing a decrease on the intensityof the Soret bands and a bathochromic shift (�8 nm), withgeneration of two isosbestic points. Following the same pro-cedure of the previous section, Jobs plots for TBA[Au-ACHTUNGTRENNUNG(cdc)2] (II) and TBA[AuACHTUNGTRENNUNG(mnt)2] (V) afforded a maximumvalue of x�0.5, thus in agreement with a 1:1 host–guest stoi-chiometry. The titration of the nanocage with both sub-strates II and V shows a complete conversion towards theencapsulated complex, indicating an extremely high affinityand precluding the direct determination of the associationconstant.[41] UV/Vis experiments were carried out in orderto find out if it was possible to exchange encapsulatedguests II or V with substrate I, whose association constanthas already been calculated. We added one equivalent ofcompound II or V to a solution of 3· ACHTUNGTRENNUNG(CF3SO3)8, and thenfive equivalents of complex I were added gradually. Nochanges on the Soret band were observed after the additionof substrate I, indicating that the association with II and Vis stronger. On the contrary, a positive displacement of com-plex I by compounds II and V was observed. Thus the mag-nitude of Ka is well above the experimental upper limit ofaccuracy for the spectroscopic titration (>1010

m�1). The

host–guest interaction with guests II and V is further sup-ported by the changes observed in the aromatic region ofthe 1H NMR spectra, which resemble the one observed forsubstrate I, and by ESI-MS experiments in which the corre-

sponding 1:1 host–guest peaks with substrates II and V arefound (see the Supporting Information).

We also evaluated the interaction of the cage with para-magnetic metal bis-dithiolene guests bearing maleonitriledi-thiolate ligands, that is, [Pt ACHTUNGTRENNUNG(mnt)2]

� (VIII), [Pd ACHTUNGTRENNUNG(mnt)2]� (IX),

and [NiACHTUNGTRENNUNG(mnt)2]�(X). UV/Vis titration also shows a 1:1 inter-

action for all of them with a complete conversion to the en-capsulated complex upon addition of one equivalent of thesubstrate. ESI-MS experiments also provide evidence forthe 1:1 host–guest stoichiometry by showing correspondingpeaks for each of the paramagnetic substrates (see the Sup-

Table 6. Planar bisdithiolene-type metal complexes tested as guests for3· ACHTUNGTRENNUNG(CF3SO3)8.

Compound Complex[a] Interaction occurs?

I yes

II yes

III no

IV no

V yes

VI decomposition

VII decomposition

VIII yes

IX yes

X yes

[a] All complexes were used as TBA salts. cdc2�=cyanodithioimidocar-bonate, pdt2�=pyrazine-2,3-dithiolate, pds2�=pyrazine-2,3-diselenolate,mnt=maleonitriledithiolate.

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M. Costas, X. Ribas et al.

porting Information). Very interestingly, in the 1H NMR ex-periments, only the aromatic protons of the cage suffersevere broadening, which is rationalized as a clear indicationthat the complex guest molecule is occupying the cavity ofthe host, because most of the aromatic protons are pointingtowards the inner cage space (see the Supporting Informa-tion). Indeed, a blank experiment by using a bulky non-flatparamagnetic (S= 1/2) substrate,[42] that is, a mononuclearCuII complex, does not cause any broadening or shift to theproton signals of the host, which is in agreement with a non-interacting substrate. Following the same procedure, wefound that substrates VIII, IX, and X displace guest I fromthe inner cavity, thus having a higher Ka value (approxi-mately above 1010

m�1).

It was intriguing to find that when bis-dithiolene com-plexes of CuIII or CuII were used as guests, that is, [Cu-ACHTUNGTRENNUNG(mnt)2]

� (VI) and [Cu ACHTUNGTRENNUNG(mnt)2]2� (VII), decomposition of the

cage was observed. The latter might be rationalized with theexistence of some degree of exchange with the Pd atoms ofthe clip moieties of the cage, although this is not clearly un-derstood.

Surprisingly, AuIII complexes, such as TBA[AuACHTUNGTRENNUNG(pdt)2] (III)or TBA[AuACHTUNGTRENNUNG(pds)2] (IV), bearing two pyrazine moieties, thatare structurally very similar to compounds I, II, and V, didnot show any interaction with the capsule. On one hand, theheteroatoms located on the extremes of the guest moleculesmay play a key role in the multiple contacts occurring be-tween the guest and the molecular clips of the capsule. Forinstance, the shift of proton H(a) observed in the 1H NMRspectra of I@3· ACHTUNGTRENNUNG(CF3SO3)7 (Figures 4 and 7) is an evidencefor these weak but additive interactions. In this line, thecharge distribution analysis obtained by DFT calculationson substrates I, II, and V indicates partial negative chargessitting on the heteroatoms at the far end of the guest,whereas this is absent in compounds III and IV. On theother hand, the main differences between the monoanionicgold complexes III and IV, and the other complexes is thehigher degree of aromaticity of the pyrazine moieties, as canbe deduced from a detailed study of the molecule DFTbond lengths (see the Supporting Information and Fig-ure S39 in the Supporting Information for further discus-sion). In this sense, because it has been described that aro-maticity might be a drawback for a proper p–p stacking in-teractions,[43] the aromatic pyrazine units of substrates IIIand IV might be a tentative justification for their exclusionfrom the inner cavity of the capsule. Furthermore, ESI-MSexperiments did not show any adduct of cage 3 with sub-strates III or IV, thus no interaction with the outer faces ofthe cage is found either. In general, these host–guest studiesshow that very subtle interactions are taking place, and astrong host–guest complexation process can occur if theseinteractions operate in a cooperative manner. Another pa-rameter that needs to be considered is the possible M···M in-teraction between the metal-based guest and the metallo-porphyrin moieties. As indicated before, the Pd···Au dis-tance is shorter than the van der Waals sum. In this respect,we are currently devoting our efforts to clarify the impor-

tance of the M···M interaction in directing the host–guestchemistry, and comparative host–guest studies with eitherthe demetallated porphyrin cage or the Zn–porphyrin cagewill be conducted.[44] Preliminary results with a Mn–porphy-ACHTUNGTRENNUNGrin cage show effective guest inclusion results of substrate I.

Tuning the optical and redox properties of the guest mole-cules : At this point we decided to study if the host–guest in-teraction between our molecular cage 3 and the bis-dithio-lene metal complexes was merely an inclusion phenomenonor if it was also affecting the physical properties of theguest. With respect to the electronic spectrum of complexX, we realized that the absorption band at l=860 nm,known to raise from ligand-to-metal charge transfer(LMCT) transitions,[45] was completely quenched when cage3 was treated with one equivalent of X, but the band recov-ered when more than one equivalent was added into the sol-ution, thus indicating that all excess of guest molecules arekept outside the cage (see Figure 9). The intensity of the

band corresponds to the excess of guest molecules added,and a good correlation is found with the absorbance varia-tion of the Soret band at l=408 nm (Figure 9, bottom). Werationalize this quenching effect due to the host–guest com-plex formation inside the cavity through p interactions be-tween the porphyrin planes and the guest molecule, thus af-fecting directly the LMCT band at l=860 nm. Analogousexperiments were carried out with substrates VIII and IX,and the same quenching effect of the low-energy LMCTbands were observed (see the Supporting Information).

On the other hand it is well known that most bis-dithio-lene metal complexes undergo multiple, reversible, one-elec-tron redox reactions. This led us to explore the electrochem-istry of our host–guest complexes, in order to explore if this

Figure 9. Top) UV/Vis monitoring of the titration of the nanocapsule 3·-ACHTUNGTRENNUNG(CF3SO3)8 with TBA[Ni ACHTUNGTRENNUNG(mnt)2] (X). Additions up to four equivalents ofX to a solution of 3·ACHTUNGTRENNUNG(CF3SO3)8 in CH3CN with c=3.77 10�3 mm. Bot-tom) Plot for comparison of the variation of the Soret band absorbanceat l =408 nm and the normalized variation of the absorbance of TBA[Ni-ACHTUNGTRENNUNG(mnt)2] (at l=860 nm) during different additions of substrate (^=Soretband (host), &=LMCT band (guest)).

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FULL PAPERSelf-Assembled Tetragonal Prismatic Molecular Cages

interaction could modify the redox properties of the guestmolecules.[46, 47] We therefore analyzed the cyclic voltamy-ACHTUNGTRENNUNGmet ACHTUNGTRENNUNGry (CV) experiments of the individual substrates VIII,IX, and X, of the free cage 3, and of the host–guest com-plexes VIII@3, IX@3, and X@3 (acetylferrocene (AcFc) wasused as internal reference). The CV spectrum of an equimo-lar mixture of complex X and acetylferrocene (see Fig-ure 10 a) shows two reversible redox couples, one belonging

to [Ni ACHTUNGTRENNUNG(mnt)2]�/[Ni ACHTUNGTRENNUNG(mnt)2]

2� (E1/2 =0.23 V vs. saturated calo-mel electrode (SCE)) and the other to AcFc/AcFc+ (E1/2 =

0.64 V vs. SCE). In a second experiment we obtained theCV spectrum of 3 with 0.5 equivalents of X and acetylferro-cene (a substoichiometric amount of guest was used toavoid interferences with no encapsulated guests). In thiscase the couple [NiACHTUNGTRENNUNG(mnt)2]

�/[Ni ACHTUNGTRENNUNG(mnt)2]2� is shifted to lower

potentials (E1/2 =0.18 V vs. SCE), suggesting that the mono-anionic form of the substrate is slightly stabilized upon in-clusion inside the cavity (see Figure 10 a). Moreover, if wecompare both experiments we can observe that the intensityof the redox wave belonging to [NiACHTUNGTRENNUNG(mnt)2]

�/[Ni ACHTUNGTRENNUNG(mnt)2]2� is

clearly quenched with respect to the intensity of the AcFc/AcFc+ pair, suggesting that the [NiACHTUNGTRENNUNG(mnt)2]

�/[Ni ACHTUNGTRENNUNG(mnt)2]2�

redox readout is significantly altered upon forming thehost–guest complex ACHTUNGTRENNUNG[X@3]7+ , whereas the acetylferroceneredox couple remains unaltered (see Figure 10 b). We also

discarded any interaction between acetylferrocene and cage3 by UV/Vis monitoring.

Finally, CV experiments of one equivalent of cage 3 and0.5 equivalents of substrates VIII, IX, or X, and acetylferro-cene at different scan rates, allowed us to obtain the diffu-sion coefficient (D) of the host–guest complexes formed(Randles–Sevick equation, see the Supporting Information),affording values of D in the range 3.3–6.3 10�10 m2s�1,which are in strong agreement with the value obtained fromthe DOSY NMR experiments carried out with substrate I(see above).

Conclusion

Summarizing, we report here a novel tetragonal prismaticnanocage using Pd-based macrocyclic and porphyrinic build-ing blocks, 3· ACHTUNGTRENNUNG(CF3SO3)8, along with an exhaustive study of itshost–guest properties. We have shown that this is a rare ex-ample of a supramolecular nanocage capable of hostinganionic substrates, and specifically, those bearing a planarstructure with a non aromatic p-delocalized system. It hasbeen shown that several bis-dithiolene-type transition-metalcomplexes exhibit a very strong interaction, although subtlechanges in the guest structure are critical for a proper recog-nition. Furthermore we have shown that this host–guest in-teraction is non-innocent, and the redox and optical proper-ties of guest molecules can be tuned. In addition, it has re-cently been highlighted by Nitschke et al. the importance ofselective recognition of gold compounds with supramolecu-lar cages,[48] therefore, our system adds to this importantclass of compounds and allows for the selection among Auand Pt or Pd transition-metal complexes. Furthermore, theability of these compounds to host planar transition-metalcomplexes is envisioned to find utility in tuning the chemicalreactivity of the inner guest. For example, we are currentlyinvestigating the capability of the host to also show controlon the reactivity properties of other bis-dithiolene metalcomplexes upon encapsulation, as for example, the reversi-ble trapping of ethylene by Ni ACHTUNGTRENNUNG[S2C2ACHTUNGTRENNUNG(CF3)2]2.

[47] The verysimple synthesis, not only of the supramolecular cage butalso of the building blocks, represents a step forward for thedevelopment of polyfunctional supramolecular nanovessels,which offer multiple applications as sensors, nanoreactors,use in catalysis, or selective recognition of interesting sub-strates such as anionic species. Indeed, the macrocyclic clipcan be size-tuned and the metallo-porphyrin linker possessesdifferent reactivity and sensing properties depending on themetal used. A large number of combinations of these pa-rameters open up a rich host–guest chemistry that is current-ly under study in our laboratories.

Experimental Section

Reagents and solvents were purchased from commercial sources andused as received, unless indicated otherwise. NMR data concerning prod-

Figure 10. Cyclic voltammetry spectrum of a) a 3.4 mm solution of X andb) a 0.25 mm solution of nanocapsule 3 with 0.5 equivalents of X. Condi-tions: scan rate=0.1 Vs�1, [TBAP]=0.1 m, CH3CN, by using a saturatedcalomel electrode (SCE) and AcFc/AcFc+ as internal reference (all E1/2

vs. SCE reference electrode).

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M. Costas, X. Ribas et al.

uct identity were collected in CD3CN at 298 K, and were calibrated rela-tive to the residual protons of the solvent. ESI-MS experiments were col-lected by using acetonitrile or DMF as the mobile phase. UV/Vis spec-ACHTUNGTRENNUNGtros ACHTUNGTRENNUNGcopy allowed the study of the Host–Guest interactions. The titrationswere carried out in CH3CN by using a 1 cm quartz cell at 297 K. A solu-tion of nanocapsule 3 (2 ml of a 3.75 10-6

m) was added to the cuvetteand, while stirring, different ratios of a 1 10-4

m substrate solution wereadded and the absorbance was measured after each addition. Finally, thestoichiometry of the complexes was studied by using the methods of con-tinuous variations. All cyclic voltamperometries (CV) were carried outby using a standard three-electrode configuration with a one-compart-ment cell, and were conducted at room temperature (297 K), under a ni-trogen atmosphere, by using anhydrous acetonitrile as the solvent, acetyl-ferrocene as the internal reference, and Bu4NPF6 (0.1 M) as the support-ing electrolyte. We focused our studies on the [M ACHTUNGTRENNUNG(mnt)2]

-/[M ACHTUNGTRENNUNG(mnt)2]2-

couple, thus we measured at the �0.2–1 V range. Different equivalents ofXIII, IX, or X substrates were added to a solution of 3·CF3SO3 (0.25 mm)

by using a 3.4 mM solution that contained a 1:1 equimolar fraction ofsubstrate and acetylferrocene.

Synthesis of Pd-1

Pd-1· ACHTUNGTRENNUNG(AcO)4 : Ligand 1 (0.06 g, 0.12 mmol; ligand 1 was synthesized byfollowing publication procedures[19]), Pd ACHTUNGTRENNUNG(COOCH3)2 (0.054 g, 0.24 mmol),and anhydrous CH3CN (20 mL) were mixed in a round bottom flask. Themixture was heated to reflux under a nitrogen atmosphere for 18 h. Thesolvent of the obtained solution was concentrated to approximately2 mL, filtered through Celite, and recrystallized under slow diffusion ofdiethyl ether. A yellow crystalline solid corresponding to Pd-1· ACHTUNGTRENNUNG(AcO)4

was obtained in good yield (88.6 %). 1H NMR (400 MHz, CD3CN): d=

9.42 (s, 4 H; arom), 7.38 (s, 4 H; arom), 4.01 (d, J =12.5 Hz, 4H; -CH2-),3.73 (m, 4 H; -CH2-), 3.38 (s, 12 H; N-CH3), 3.32 (m, 4H; -CH2-), 3.08 (d,J =12.5 Hz, 4 H; -CH2-), 2.45 (dd, J=4.0, 14 Hz, 4H; -CH2-), 2.29 (dd,J =4.0, 14 Hz, 4H; -CH2-), 2.05 (s, 6 H; COOCH3), 1.65 (s, 6 H; AcO),1.44 ppm (s, 6H; N-CH3); MS (ESI): m/z : 885.2 [1-Pd· ACHTUNGTRENNUNG(COOCH3)3]

1+ ,412.6 [1-Pd· ACHTUNGTRENNUNG(COOCH3)2]

2+ .

Pd-1· ACHTUNGTRENNUNG(AcO)2 ACHTUNGTRENNUNG(CF3SO3)2 : Pd-1· ACHTUNGTRENNUNG(AcO)4 (0.1 g, 0.11 mmol) was dissolved inCH3CN (30 mL). An excess of NaCF3SO3 salt was added (1–4.2 equiv)and the mixture was stirred vigorously during 4 h. The reaction mixturewas concentrated to approximately 2 mL under reduced pressure, filteredthrough Celite, and recrystallized under slow diffusion of diethyl ether.Pd-1· ACHTUNGTRENNUNG(AcO)2 ACHTUNGTRENNUNG(CF3SO3)2 was obtained as a yellow crystalline solid(90.5 %).1H NMR (400 MHz, CD3CN): d =9.42 (s, 4H; arom), 7.38 (s,4H; arom), 4.00 (d, J =12.5 Hz, 4H; -CH2-), 3.62 (m, 4 H; -CH2-), 3.36 (s,12H; N-CH3), 3.35 (m, 4H; -CH2-), 3.09 (d, J =12.5 Hz, 4 H; -CH2-), 2.45(dd, J =4.0, 14 Hz, 4H; -CH2-), 2.30 (dd, J =4.0, 14 Hz, 4 H; -CH2-), 2.05(s, 6 H; AcO), 1.44 ppm (s, 6 H; N-CH3); MS (ESI): m/z : 975.2 [Pd-1·-ACHTUNGTRENNUNG(COOCH3)2 ACHTUNGTRENNUNG(CF3SO3)]1+ , 414.0 [Pd-1· ACHTUNGTRENNUNG(COOCH3)2]

2+ .

Synthesis of the molecular cage 3· ACHTUNGTRENNUNG(CF3SO3)8 : 5,10,15,20-Tetrakis(4-car-boxyphenyl)porphyrin-PdII (2) (7.96 mg, 0.01 mmol) was weighed in a10 mL flask, then DMF (1 mL) was added. Triethylenetriamine (5 mL)dissolved in DMF (0.5 mL) was added to the porphyrin solution. FinallyPd-1· ACHTUNGTRENNUNG(AcO)2 ACHTUNGTRENNUNG(CF3SO3)2 (20 mg, 0.02 mmol) dissolved in DMF (2.5 mL)was added to the mixture. The solution obtained was heated to 105 8Cunder reflux, during 16 h. After the reaction time, the mixture is cooledto room temperature, filtered through Celite, and recrystallized by slowdiffusion of diethyl ether, yielding the target compound (93.2 %).1H NMR (400 MHz, CD3CN): d=9.49 (s, 16 H; arom-clip), 8.74 (dd, J=

1.6, 8 Hz, 8H; arom-porph), 8.36 (s, 16H; pyrrole ring), 8.24 (dd, J =1.6,8 Hz, arom-porph), 8.14 (dd, J= 1.6, 8 Hz, arom-porph),7.62 (s, 16H;arom-clip), 4.08 (d, J=13 Hz, 16H; -CH2-), 3.76 (m, 16H; -CH2-), 3.63 (s,48H; N-CH3), 3.53 (m, 16H; -CH2-), 3.22 (d, J= 13 Hz, 16 H; -CH2-),2.61 (d, J=13.5 Hz, 16H; -CH2-), 2.46 (d, J=13.5 Hz, 16H; -CH2-),1.73 ppm (s, 24H; N-CH3); MS (ESI) m/z : 2750.8 [3· ACHTUNGTRENNUNG(CF3SO3)6]

2+ , 1785.9[3· ACHTUNGTRENNUNG(CF3SO3)5]

3+ , 1301.9 [3· ACHTUNGTRENNUNG(CF3SO3)4]4+ , 1011.9 [3·ACHTUNGTRENNUNG(CF3SO3)3]

5+ , 818.4 [3·-ACHTUNGTRENNUNG(CF3SO3)2]6+ , 680.2 [3· ACHTUNGTRENNUNG(CF3SO3)]7+ , 576.6[3]8+ .

X-ray diffraction studies : All details for the crystal structure determina-tions can be found in the Supporting Information. CCDC-900502 (Pd-1·-ACHTUNGTRENNUNG(AcO)4), 900512 (Pd-1· ACHTUNGTRENNUNG(CF3CO2)4), and 900503 (3· ACHTUNGTRENNUNG(ClO4)8) contain thesupplementary crystallographic data for this paper. These data can be ob-

tained free of charge from The Cambridge Crystallographic Data Centrevia www.ccdc.cam.ac.uk/data_request/cif.

Computational details : All calculations have been performed by usingthe Amsterdam Density Functional program (ADF, version 2010.01)[49, 50]

and the QUILD program.[51] The BLYP functional[5,53] was used includingdispersion corrections[54] with double-z (DZ) and triple-z (TZP) basissets. Solvent effects were included through the use of COSMO.[55–57]

Acknowledgements

We are grateful for financial support from the MICINN of Spain(CTQ2009-08464/BQU to M.C., CTQ2011-25086/BQU, CTQ2011-23156/BQU, PhD grant AP2010-2517 to M.G.B, INNPLANTA- INP-2011-0059-PCT-420000-ACT1 and Consolider-Ingenio CSD2010-00065), theFEDER fund (European Fund for Regional Development) for the grantUNGI08-4E-003, the European Research Council for Project ERC-2011-StG-277801 to X.R. and ERC-2008-StG-29910 to M.C., and the Generali-tat de Catalunya (2009SGR637, 2009SGR528, and a Ph.D. grant toC.G.S.). X.R. and M.C. thank ICREA-Academia awards. I.I. thanksMINECO for a Ram�n y Cajal contract. We thank Dr. Teodor Parellafor fruitful discussions. We thank STRs from UdG for technical support,and we also acknowledge the computer resources, technical expertise,and assistance provided by the Barcelona Supercomputing Center—Centro Nacional de Supercomputaci�n, and Centre de Serveis Cient�ficsi Acad�mics de Catalunya (CESCA) for partial funding of computertime.

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Received: September 20, 2012Revised: October 26, 2012

Published online: December 4, 2012

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M. Costas, X. Ribas et al.