Selective Formation of S4-and T-Symmetric Supramolecular Tetrahedral Cages and Helicates in Polar Media Assembled via Cooperative Action of Coordination and Hydrogen-Bonds

: We report on the synthesis and self-assembly study of novel supramolecular monomers encompassing quadruple hydrogen-bonding motifs and metal coordinating 2,2’-bipyridine units. When mixed with metal ions such as Fe 2+ or Zn 2+ , the tetrahedron cage complexes are formed in quantitative yields and full diastereoselectivity, even in highly polar acetonitrile or methanol solvents. The symmetry of the complexes obtained has been shown to depend critically on the flexibility of the ligand. Restriction of the rotation of the hydrogen-bonding unit with respect to the metal coordinating site results in a T -symmetric cage, whereas by introducing flexibility either through a methylene linker or rotating benzene ring allows the formation of S 4 -symmetric cages with self-filled interior. In addition, the possibility to select between tetrahedral cages or helicates, and to control the dimensions of the aggregate, has been demonstrated with three component assembly using external hydrogen-bonding molecular inserts or by varying the radius of the metal ion (Hg 2+ vs Fe 2+ ). Self-sorting studies of individual Fe 2+ complexes of with ligands of different sizes revealed their inertness toward ligand scrambling.


INTRODUCTION
The introduction of the concept of coordination driven selfassembly of simple ligands and metal ions into non-covalent cage structures resulted in the enormous progress of the field of supramolecular chemistry. 1One-step reliable generation of sophisticated capsule structures from simple precursors enabled researchers to access nanoscopic molecular vessels to experimentally interrogate various molecular phenomena of fundamental importance.Beyond molecular recognition, 2 further developments include enzyme-like modulation of chemical reactivity, 3 stabilization of highly reactive elusive intermediates, 4 selective extraction, 5 sensing 6 and X-ray characterization of analytes by providing ordered solid-state environment for otherwise non-crystalline compounds. 7wo families of coordinative molecular cages are currently dominating the field: cavity architectures combining the square geometry of Pd 2+ with monodentate ligands and metal complexes built upon the octahedral arrangement of bidentate ligands.The latter class of cage complexes enjoys the diversity of molecular topologies (tetrahedron, cube, triple stranded helicate, prism), the use of non-toxic and inexpensive metals (Fe, Zn, Ga etc.), and well-established synthetic methods to decorate the metal binding unit (2,2'-bipyridine or related compounds). 8The selection of a particular shape has been shown to be possible by structural modification of ligands or by a templation effect of the guest. 9Some of these structures are inherently chiral with asymmetry stemming either from chiral ligands or the metal center itself, and thus provide a platform for enantioselective binding. 10,11In addition, the reversible nature of coordination bonds enables the building of more sophisticated molecular networks consisting of several ligands and metal ions to explore emerging complexity. 12n a classical approach, the coordination cage is built using covalent ligands, synthesized prior to the self-assembly step 2 (Fig. 1a).To simplify the synthesis and to incorporate dynamicity into the system, subcomponent self-assembly, combining both covalent (e.g.imine bonds) and coordinative bonds, has been introduced and extensively developed by Nitschke and coworkers as a powerful strategy to construct stimuli responsive dynamic libraries of various supramolecular constructs (Fig. 1b). 13We speculated that the further extension of subcomponent self-assembly would be possible by incorporating hydrogen-bonding (H-bonding) motifs within the ligand structure, in between the metal coordinating units (Fig. 1c).This would not only allow for an easier synthesis, but also add an orthogonal input channel for external structure modulation.The strong directionality, high sensitivity to polarity of the solvent and temperature, and the possibility for tautomerization 14 define other attractive features of this noncovalent interaction.Despite obvious advantages, the combination of metal and H-bonds as integral elements has not been successfully applied to obtain metal-organic polyhedra and to date, the known examples are limited to planar polygons in non-polar chlorinated solvents only. 15,16Here we provide the first experimental demonstration of a new class of tetrahedron molecular cages and helicates held together by the cooperative action of coordination and H-bonds in acetonitrile or even methanol and disclose some of their properties.RESULTS AND DISCUSSION First generation ligands: design, self-assembly and characterization.Our design of the monomer 1 is outlined in Figure 2a.This compound was synthesized in two steps from reported simple starting materials and features Meijer's ureidopyrimidinone (UPy) 17 as quadruple H-bonding unit together with the metal binding 2,2'-bipyridyl (Bipy) unit, connected through single methylene bridge (for synthesis see the Supporting Information).UPy motif was chosen for its wellknown ability to form very stable dimers (up to K a = 10 8 M -1 in toluene) whereas Bipy is a ligand of choice for octahedral metal coordination ultimately leading to tetrahedral arrangement of the ligands in the complex.The use of semi-flexible -CH 2linker to bridge two orthogonal sites was essential for several reasons.First, it provides necessary conformational freedom for the Bipy units to adjust around the metal ion coordination center for optimal binding.Second, by providing limited flexibility and relatively short distances between metal centers, a competing aggregation mode to dinuclear triple-stranded helicate is inhibited due to the steric clash between three bulky UPy 2 dimeric units located close to a helicate axis (Fig. 2b).Considering the stereochemical outcome of the M 4 L 12 assembly process, there are three possible diastereomers for the most commonly occurring all-fac coordination mode, belonging to the symmetry point groups T, C 3 or S 4 .It is very common that either the pure homochiral (racemic mixture of  and  enantiomers) T-isomer or the mixture of all stereoisomers is obtained.However, it has also been shown that the extent of stereochemical communication between metal centers, and thus the ratio of the isomers, is sensitive to the structure of the ligand. 18With regard to isomerism, in the bifunctional ligand 1 containing UPy moieties, the tetrahedral assemblies might also form via dimerization of selfcomplementary enolic forms of the UPy unit (Fig. 2b).In this case, the isocytosine side chains will change the position with respect to the edges of the so-formed tetrahedron moving from equatorial in keto-dimer to parallel arrangement in enol-dimer.Although the formation of tetrahedral complexes is possible for both tautomers, larger substituents might favor the formation of the enol-cage as side chains experience less steric interaction in this aggregation mode.In order to prove the concept and probe the impact of the bulkiness of the UPy 2 dimer on the diastereoselectivity of the cage formation, five different substituents R (H, Me, n-Bu, n-C 11 H 23 and 3,4,5-tridodedecyloxyphenyl) were attached to UPy.The complex formation was then accomplished by simply mixing the monomer 1 and Fe(OTf) 2 in a 3:1 ratio in d 3acetonitrile under ambient atmosphere.The formation of the complexes was evident from the appearance of deep red color and the dissolution of otherwise insoluble ligands.To our delight, in case of monomers 1a (R = H) and 1b (R = Me), welldefined 1 H NMR spectra were obtained consisting of three sets of resonances of equal integral intensities (Fig. 2c).Based on symmetry arguments, it was assigned to the exclusive formation of the S 4 -symmetric M 4 L 12 cage. 19The downfield chemical shifts of the N-H resonances clearly indicate the establishment of quadruple H-bonds.In addition, DOSY NMR analysis was also in agreement with the presence of a single species.Namely, all resonances in the spectrum correlated to a single diffusion coefficient of D = 5.00  10 -10 m 2 s -1 (1a) and D = 4.83  10 -10 m 2 s -1 (1b), which translate into hydrodynamic radii R H = 1.10 nm and R H = 1.14 nm, respectively.The values obtained agree well with the radius of circumscribed sphere calculated for the molecular model of the S 4 -Fe 4 (1b) 12 tetrahedron (see Fig. S91).Electrospray ionization mass spectrometry (ESI-MS) also confirmed the formation of tetrahedron assemblies (Fig. 2e and Fig. S94-95, S97).In order to fully characterize the supramolecular aggregates in solution, detailed NMR studies were undertaken.As shown in Figure 2d, the keto and enol tautomeric forms and their dimers can be distinguished based on the expected specific through-bond and through-space interaction map as outlined in Figure 2d using dimer (1b) 2 as an example.Based on the ROESY correlation observed between the protons at 12.58 ppm, 12.56 ppm and 12.37 ppm (labeled in blue in Fig. 2c) with isocytosine methyl group protons d, the former was assigned as N-H protons a, and thus confirm the involvement of the keto form in quadruple H-bonding.Likewise, the COSY and ROESY crosspeaks of N-H proton resonances at 10.52 ppm, 10.32 ppm and 10.30 ppm (labeled in green in Fig. 2c) with CH 2 resonance e allows unambiguous assignment of these protons as c.Moreover, characteristic long-range W-type COSY coupling of N-H protons a to C(sp 2 )-H proton f of the isocytosine ring as well as ROESY interaction between protons b and c fully corroborates the above assignments (Fig. 2d).The same spectral features are also observed for complexes obtained using ligands 1a, 1c and 1d or by exchanging Fe 2+ to Zn 2+ (Fig. 2c, bottom, see the Supporting Information).Interestingly, increasing the size of UPy substituents in 1c and 1d gives rise to an additional three sets of resonances of lower intensities (Fig. 2c).Initially thought to belong to the enolic form of the UPy, these minor resonances were shown to be part of the keto tautomer, giving the same characteristic 1 H-1 H crosspeaks as described above.Careful inspection of the ROESY spectra, however, revealed some subtle differences in the surroundings of proton c (see Fig. S35, S38).Most likely, the minor species observed are due to different arrangement of the UPy dimers caused by bulkier butyl and undecyl chains in 1c and 1d, respectively, resulting in slowly equilibrating conformers.The DOSY spectra indicate similar hydrodynamic radii of the minor and major species (Fig. S36, S39).Increasing the size of the isocytosine substituent even further as in monomer 1e, eventually prevents the formation of a tetrahedron cage leading to a broad undefined 1 H NMR spectrum, and only ions of smaller complexes Fe n (1e) m are detected in the ESI-MS spectrum (Fig. S96).Dilution of the solution of S 4 -Fe 4 (1b) 12 in acetonitrile with diethyl ether provided a red precipitate.The recorded 1 H NMR spectrum of the freshly dissolved crystals showed only the major conformer, which gradually returned to an equilibrium mixture.).The molecular models for S 4 -Fe 4 (1b) 12 obtained using semiempirical PM7R8 method show a highly deformed tetrahedron with negligible internal cavity (Fig. 3a-b).Compared to the structure of the hypothetical T-Fe 4 (1b) 12 complex, the former features a geometry where all UPy 2 dimers within the ligand (1b) 2 are forced to move into the cavity as a result of the connecting CH 2 groups, which dictates the ligands' bent shape.Although the computationally obtained model is perhaps not fully accurate, it provides some important clues regarding the data obtained from the 1 H and ROESY NMR spectra.The most downfield proton of the c subset is unique as it gives no cross-peak with the Bipy C-H Ar protons (Fig. 2d and 3d) indicating that it assumes a conformation with rather large distance between these protons.Moreover, in contrast to the downfield protons f, the most upfield proton f at 4.01 ppm gives a cross-peak with C-H Ar , indicating their proximity.As seen from the molecular model, proton f of the isocytosine ring is located inside the tetrahedron and nearby the Bipy ring, experiencing significant shielding (Fig 3c-e).In addition, a weak cross-peak between protons c and f is also in accord with the calculated structure (Fig. S26).The small cavity within the S 4 -symmetric cage suggests that the formation of the cage complex most likely requires no anion templation.The exchange of a triflate counterion with BF 4 -, BPh 4 -, SbF 6 -, Br -or C 12 H 25 OSO 3 -did not affect the symmetry and resulted in no change of the 1 H NMR spectrum (Fig. S29-S33).Second generation ligands: effect of the linker and conformational rigidity.In order to gain more insight into the importance of the flexibility and length of the ligand on the symmetry of the complexes, two new rigid monomers 2 and 3, lacking methylene linkers, were synthesized.In monomer 2, the Bipy unit is connected to UPy directly through the urea functionality, whereas monomer 3 features an additional benzene ring spacer (Fig. 4a-b).The 1 H NMR spectra of complexes obtained using Fe(OTf) 2 and Zn(OTf) 2 as metal ion sources in  were not fully resolved due to broadening, however, they were clearly visible in the spectrum of S 4 -Zn 4 3 12 .Vapor diffusion of chloroform into an acetonitrile solution of T-Fe 4 2 12 allowed the isolation of single crystals.However, the crystals were found to be very weakly diffracting, even with extremely prolonged exposure times using Cu radiation.This is routinely observed for supramolecular species, especially when they are comprised of predominantly light atoms, as is the case for the cages described herein.As expected for supramolecular complexes with a high void volume, the data collected for this structure suffered from rapid fall-off of reflection intensity at high diffraction angles, and this had a negative impact on the quality of the high angle reflections collected, with some not being observed.Fortunately, the positions of the heaviest atoms present (Fe) could be located with high certainty.The perfectly tetrahedral arrangement of four Fe 2+ ions, separated by 16.1 Å, was found, supporting the formation of the T-symmetric cage Fe 4 2 12 .Overlay of experimentally determined coordinates of Fe 2+ ions with the computationally obtained structure resulted in good agreement (<0.6 Å error) (Fig. 4c).The X-ray quality crystals of S 4 -Fe 4 3 12 could not be obtained and the molecular model of the cage is shown in Figure 4d.The hydrodynamic radii for T-Fe 4 2 12 and S 4 -Fe 4 3 12 extracted from DOSY spectra also agree well with the ones obtained from the above models (Fig. S91).
When taken together, it can be noted that the supramolecular ligands 1-3 show a preference for S 4 -symmetry wherever the flexibility of the ligand allows it.Owing to the many available conformations of 1 2 and distances between Bipy units due to unrestricted rotation around the -CH 2 -linker, a deformed S 4symmetric cage is formed easily and is stabilized by multiple non-covalent inter-ligand interactions (internal solvation) inside the cavity.Ligand 2 2 is completely flat because of the conjugation of the urea nitrogen lone pair with the Bipy aromatic system.Although a syn ligand geometry is still possible, the capability of the system to adjust for optimum packing is limited, and therefore the most symmetric Tgeometry is adopted with all UPy 2 dimers located at the maximum distance from each other, tangential to the tetrahedron surface.Finally, in the case of ligand 3 2 , the Bipy and urea moieties are twisted almost perpendicular with respect to the benzene ring linker for the anti-configuration, whereas such a twist angle for the syn-configuration is much smaller. 18n contrast to the fully covalent rigid ligands reported in the literature, the distance between Bipy units, and thus between the metal ions, can also be modulated by distorting the quadruple H-bonding motifs from coplanarity.As demonstrated previously with other supramolecular systems, quadruple Hbonds can tolerate bending up to ca. 20 and 10 with respect to the center or the plane of the H-bonding interface, repectively.14e,f The molecular model shows that within the shortest edge comprising anti-conformer of ligand 3 2 , the UPy units are unidirectionally twisted along the line of quadruple Hbonding interface resulting in a distorted V-shaped geometry and reduced distance between metal centres.The preferential formation of the S 4 -symmetric cage is most likely governed by partial accommodation of the UPy 2 dimers inside the cavity of the cage owing to a significantly larger cavity size as compared to T-Fe 4 2 12 .Indeed, the void volume of S 4 -Fe 4 3 12 is not significant and the assembly resembles more a molecular bowl with open cavity rather than a cage (Fig. 4d).In the case of S 4 -Fe 4 (1b) 12 and T-Fe 4 2 12 , the H/D exchange rate of N-H protons with D 2 O correlates with their pK a values and inversely with the strength of the H-bonds they form.In both cases, the least acidic N-H proton c exchanged at the slowest rate.Surprisingly, these protons exchanged even slower than the N-H protons a, involved in intramolecular H-bonding.This observation cannot be explained by solvent accessibility alone since all N-H protons of the UPy 2 dimer in complex T-Fe 4 2 12 are residing on the surface, and thus are equally accessible to D 2 O. Contrarily, in cage S 4 -Fe 4 3 12 , one full set of protons exchanged slower suggesting either markedly different stability of one particular pair of quadruple H-bonds or their reduced solvent accessibility (Fig. 5f).Intrigued by the remarkable stability of these aggregates in polar solvents, we then characterized them in pure d 4 -MeOD.In contrast to water, methanol provided sufficient solubility of complexes S 4 -Fe 4 (1b) 12 and T-Fe 4 2 12 for 1 H NMR experiments (Fig. 6a-b).The evaluation of the size of the complexes was made by using DOSY together with control ligands 4 and 5 containing a Bipy unit and non-H-bonding 1,3-dimethyl benzene isostere as the isocytosine ring (Fig. 6c).These controls can form only monomeric octahedron complexes Fe(4-5) 3 2+ and thus provided a reliable reference for the size of a single vertex of the tetrahedral assembly.The data obtained shows the formation of S 4 -Fe 4 (1b) 12 in d 3 -MeOD based on both DOSY (single species), 1 H NMR (three sets of resonances) and ESI-MS experiments.DOSY measurements also indicate the formation of single species in the case of T-Fe 4 2 12 .Compared to diffusion coefficients obtained for controls Fe4 3 2+ and Fe5 3 2+ , the so formed aggregates are much larger, in agreement with the formation of cage complexes.Moreover, the hydrodynamic radii of cage complexes are very close to the ones obtained in a CD 3 CN solution.Unfortunately, the low solubility of the complex S 4 -Fe 4 3 12 in d 3 -MeOD impeded the analogous experiments; however, the titration of a CD 3 CN solution of the latter with an excess of d 3 -MeOD also confirms the formation of the tetrahedron complex (Fig. S65, S104).The stability of H-bonds in UPy 2 dimers in complexes S 4 -Fe 4 (1b) 12 , T-Fe 4 2 12 and S 4 -Fe 4 3 12 is exceptional and exceeds greatly the stability of cyclic tetrameric aggregates based on UPy aggregation, previously reported by us.To further evaluate the robustness of the complexes, we next performed experiments with external competitors for metal ion and H-bonds.First, a stoichiometric amount of phenanthroline (Phen) was added to the CD 3 CN solution of S 4 -Fe 4 (1b) 12 (Fig. 7a, reaction 1).Owing to much stronger complexing properties of Phen compared to Bipy, the cage was fully converted into homoleptic Fe(Phen) 3 2+ and the free ligand 1b.Addition of an equimolar amount of Zn(OTf) 2 to the above mixture resulted in a complex mixture with an unexpected speciation of species (Fig. 7a,b reaction 2).Instead of simple scavenging of the liberated 1b by Zn 2+ ions, the redistribution of metal ions between various species took place.In an equilibrated mixture, Zn(Phen) 3 2+ and Fe(Phen) 3 2+ were found in the molar ratio ~1.26, together with homoleptic cages S 4 -Fe 4 (1b) 12 /S 4 -Zn 4 (1b) 12 (~2.5:1)and mixed cages Fe n Zn 4-n (1b) 12 (Fig. S66-S67). 20Independently, we also showed that cage S 4 -Fe 4 (1b) 12 is much more stable than S 4 -Zn 4 (1b) 12 (Fig. 7a, Fig. S71).Interestingly, the distribution of Fe 2+ and Zn 2+ within M(Phen) 3 2+ (M = metal) complexes favors otherwise less stable Zn(Phen) 3 2+ .Based on lgK 3 values, the Fe(Phen) 3 2+ is more stable than Zn(Phen) 3 2+ by four orders of magnitude, 21 and the control experiment with Fe(Phen) 3 2+ and added Zn(OTf) 2 shows that Fe(Phen) 3 2+ remains in the mixture as the dominant species together with concomitant formation of Zn(Phen) 2+ and Zn(Phen) 2 2+ , and no Zn(Phen) 3 2+ (Fig. S70).Moreover, the same species as in reaction 2 are also formed in the control experiment in which equimolar amounts of S 4 -Fe 4 (1b) 12 and S 4 -Zn 4 (1b) 12 were mixed, thus ruling out the presence of mixed species of the type M n (1b) m (Phen) l (M = metal) (Fig. S68).These results show that the combination of Fe/Zn/Phen and Fe/Zn/1b subsets, where the resulting equilibrium manifold becomes overwhelmed by the equilibrium between cage complexes resulting in the formation of S 4 -Fe 4 (1b) 12 as the dominant complex.Surprisingly, the metal exchange between cages is very fast, observed shortly after mixing the components. 22,23The importance of the formation of cage complexes to the outcome of species distribution was also corroborated by another control experiment.Namely, repeating the same experiment (reactions 1 and 2) and replacing the cage forming ligand 1b with simple Bipy, the homoleptic complexes Fe(Phen) 3 2+ and Zn(Bipy) 3 2+ were found to be the major products, whereas scrambled complexes formed only to a small extent (Fig. S72).The driving force for metal scrambling within cages is most likely of both enthalpic and entropic origin.Borrowing Fe 2+ from S 4 -Fe 4 (1b) 12 to form the mixed cage Fe n Zn 4-n (1b) 12 results in an increase of cage stability compared to S 4 -Zn 4 (1b) 12 , at the same time preserving a large concentration of the most stable species, S 4 -Fe 4 (1b) 12 , and increasing the entropy of the system.The example described herein thus provides a simple system from which a complicated reaction network can be built that is relevant to systems chemistry.The stability of UPy 2 dimer within cage S 4 -Fe 4 (1b) 12 was also probed by using the powerful competing agent for ADDA Hbonding array -2,7-diamido-1,8-naphthyridine (DAN) (Fig. 7c). 24DAN is a non-self-complementary H-bonding motif that is only weakly aggregating in solution.On the other hand, it forms a very stable ADDA-DAAD heterocomplex with UPy (K a = 10 7 M -1 in CDCl 3 ), and even though the value of K a is comparable to those of the UPy dimerization (enol and keto forms), the heterocomplex is favored because of the increase of the total number of H-bonds in the system.Even highly stable cyclic supramolecular constructs assembled using cooperative UPy dimerization is fully disintegrated by the action of DAN.14e In our hands, treating the complex S 4 -Fe 4 (1b) 12 with an excess of DAN derivative 6 resulted in no changes in 1 H NMR spectrum.Moreover, the pre-formed heterodimer 1b-6 is also transformed into cage S 4 -Fe 4 (1b) 12 and free DAN upon addition of Fe(OTf) 2 , indicating the remarkable stability of the H-bonds of the UPy 2 dimers in S 4 -Fe 4 (1b) 12 .In fact, to our knowledge this is the first example of UPy 2 dimers that resists DAN.This can be explained by the cooperative action of the network of coordination and H-bonds within the cage structure.Finally, the assembly and disassembly of S 4 -Fe 4 (1b) 12 can also be controlled by the action of acid and base.The cage was fully destroyed by adding an excess of trifluoroacetic acid, and then regenerated to some extent by neutralizing with triethylamine (Fig. S74).Self-sorting studies: effect of ligand structure.Having complexes S 4 -Fe 4 (1) 12 , T-Fe 4 2 12 and S 4 -Fe 4 3 12 in hand we became interested in whether the length or rigidity of the ligands can have any influence on the fidelity of the assembly process.The precise control of the assembly process within a mixture with multiple species present is one of the main goals of systems chemistry towards creation of adaptive molecular networks. 12,25s shown in the previous mixing experiments (Fig. 7a), two cages of the same type built from two different metals of similar size (Fe 2+/ Zn 2+ ) resulted in scrambling of the metal ions to produce a significant amount of mixed-metal cages.We next probed the possibility for self-sorting based on the variation of the ligand.First, two 1 st generation ligands 1b and 1d having CH 3 -and C 11 H 23 -substituents in the isocytosine ring were mixed together in 1:1 ratio followed by addition of a stoichiometric amount of Fe(OTf) 2 .The formation of a complex mixture with no dominant species was noted (Fig. 8a).To avoid kinetic trapping of intermediates along the assembly pathway, a control experiment was performed with separately prepared individual cages S 4 -Fe 4 (1b) 12 and S 4 -Fe 4 (1d) 12 .Exchange of the ligands was observed shortly after mixing, resulting in the same complex spectral pattern (Fig. S75).DOSY experiments showed that all equilibrating species are of similar size and therefore represent isomeric cage complexes (Fig. S76).The mixing process is most likely driven by the increase of the entropy, and also by the release of steric crowding in S 4 -Fe 4 (1d) 12 .Unfortunately, it was not possible to prove whether the mixing process involved exchange of free ligands (or their dimers) via monomer dissociation or exchange of intact vertices via selective H-bond cleavage.Self-sorting experiments with pairs of preformed complexes Fe 4 (1b) 12 /Fe 4 2 12 , Fe 4 (1b) 12 /Fe 4 3 12 and Fe 4 2 12 /Fe 4 3 12 comprised of different ligands revealed full narcissistic self-sorting even after sonicating the solution at 50ºC overnight (Fig. 8b).Mixing of the free ligands with Fe(OTf) 2 , however, resulted in a kinetically trapped state consisting of various aggregates (Fig. S80-S81).In stark contrast, the more labile Zn cages afforded a mixture of scrambled aggregates even at room temperature reaching equilibrium after 15 minutes (Fig. 8b, Fig. S82-S84).Influence of the radius of metal ion.So far, using Fe 2+ (r (2+) = 61 pm) or Zn 2+ (r (2+) = 74 pm) ions as metal nodes resulted in formation of tetrahedral cages as the sole products.We next posed the question whether larger metal ions would provide enough space for UPy 2 to deliver different molecular architectures such as helicates or cubic cages.For this purpose, we decided to explore Hg 2+ as a suitable candidate having one of the largest ionic radii (r (2+) = 116 pm) among the transition metals.Surprisingly, supramolecular chemistry of homoleptic mercury complexes HgL 3 2+ with Bipy or Phen ligands is almost unexplored and only few examples of Hg(Bipy) 3 2+ or Hg(Phen) 3 2+ have been reported. 26These complexes can only be obtained from mercury salts with weakly coordinating counterions, such as triflates, perchlorates or hexafluorosilicates; otherwise, formation of complexes Hg(Phen/Bipy) n=1,2 X 2 (X = halogen, rodanide, etc.) is preferred as a result of the soft character of Hg 2+ and the hard nature of the Phen/Bipy ligands. 27Because of the lack of ligand field stabilization energy (5d 10 electronic configuration) in their coordination compounds, there is no particular coordination geometry preferred by Hg 2+ ions.Intrigued by the possible outcome, we prepared the corresponding complex by mixing 1b and Hg(OTf) 2 in 3:1 ratio in CD 3 CN.The 1 H NMR spectrum indicated the formation of the complex by displaying one set of rather broadened resonances (Fig. 9b).ESI-MS data supported the binuclear species, i.e. complexes Hg 2 (1b) 6 (Fig. 9a, Fig. S86).To our delight, the presence of heavy atoms allowed us to unambiguously confirm its structure by using X-ray diffraction of the crystals grown by using vapor diffusion of chloroform into an acetonitrile solution.The data revealed the unprecedented molecular structure of the helicate (Fig. 9c, Page S57).Namely, three UPy 2 dimers in their keto tautomeric form are perfectly - stacked along the helicate axis with distance between aromatic planes of isocytosines d = 3.4 Å.The - stacking of the UPy 2 dimers provides an energetically favorable way to avoid steric clash between them, however, this requires larger separation of the Bipy units at the metal nodes.For this reason, the helicate of this type is obtained only with the large Hg 2+ ions, and not with Fe 2+ .The broad N-H resonances in the 1 H NMR might result from the overlap of the resonances of three non-equivalent UPy 2 dimers having similar chemical shifts.Another possibility is the fast (on the NMR timescale) interconversion of H-bonding networks between three - stacked dimers giving rise to averaged signals.The comparison of the relative stabilities of Hg 2 (1b) 6 and S 4 -Fe 4 (1b) 12 was made by adding a stoichiometric amount of Fe(OTf) 2 into the solution of the helicate.Formation of S 4 -Fe 4 (1b) 12 was noted together with new species having one set of resonances (Fig. 9b).The latter was shown by ESI-MS and independent synthesis to be rod-shaped Hg 2 (1b) 2 (OTf) 4 (Fig. S107-S108).Surprisingly, the equilibrium was not affected even in the presence of a large excess of Fe 2+ .The exceptionally stable Hg 2 (1b) 2 (OTf) 4 might thus be useful in more complicated systems chemistry setups providing a reservoir for ligand 1b other than Fe 2+ or by building heterometallic architectures.

Modulation of self-assembly by H-bonding molecular inserts.
The H-bonded nature of the edges in the cages described herein render them amenable for structural modifications and stimuli-responsiveness using external Hbonding molecular blocks that have a high affinity for UPy.As discussed above (Fig. 7c), the addition of DAN to ligand 1b indeed provided the corresponding heterodimer in quantitative yield.Although mono-DAN derivative 6 (Fig. 7c) is readily expelled from the heterodimer in the presence of Fe 2+ , we speculated that the bis-DAN derivative could be in principle inserted into the tetrahedron edge leading to either a change in topology or dimensions of the aggregate.For the proof-ofprinciple, bis-DAN insert 7 containing a flexible adipic acid linker was synthesized.The attempt to insert 7 directly into preformed cage S 4 -Fe 4 (1b) 12 in CD 3 CN, however, was not successful (Fig. 10a, reaction 2).Mixing 1b, 7 and Fe(OTf) 2 components together gave the same result, most likely due to solubility issues (Fig. 10a, reaction 1).Compound 7 is completely insoluble in acetonitrile, whereas  the also poorly soluble 1b is gradually and selectively brought into solution during complexation.Hence, the formation of heterocomplex 1b-7-1b is not possible.Finally, after switching to a mixed solvent CD 3 CN:CDCl 3 -1:3 system, the 1:2 mixture of 7 and 1b afforded a clean 1 H NMR spectrum assigned to heterocomplex 1b-7-1b (Fig. 10a, reaction 3).After addition of Fe(OTf) 2 , a new spectrum consisting of a set of 10 resonances was obtained (Fig. 10b).The resonances corresponding to two protons at 12.07 ppm, 11.46 ppm and 9.99 ppm were not fully resolved due to similar values of chemical shifts.The spectrum is thus suggestive of two non-equivalent UPy-DAN heterodimers within the assembly.ESI-MS analysis indicated the formation of dinuclear species Fe 2 [7(1b) 2 ] 3 , and not the extended cage Fe 4 [7(1b) 2 ] 6 (Fig. S109).The obtained results can be rationalized by assuming the formation of a triplestranded structure with opposite  and  chirality at each sixcoordinated metal center, i.e. a mesocate. 28Based on the fact that the most stable conformation for alkyl chain is a zigzag, Albrecht et al. has proposed that a carbon chain linker comprised of an even number would favour a helicate, while a chain with an odd number of carbon atoms would result in a mesocate. 29Our results contradict this rule.In fact, Raymond et al. and others 30,9b have shown that formulating any general selection rule might be impossible due to stabilization of one of the aggregates by the formation of a host-guest complex with solvent molecules or counter anions.We assume that the selectivity towards a mesocate observed in our case stems from the bulkiness of the UPy-DAN unit.The formation of a helicate aggregate using S-shaped conformation of 1b-7-1b would result in steric clash between UPy-DAN heterodimers, which on the other hand can be relieved in a mesocate formed from the pseudo-C conformation of 1b-7-1b, as argued previously by Wu. 9b Host-guest chemistry.Tetrahedron cages are well documented to display rich host-guest chemistry having implications to catalysis, separation, or modulation of chemical reactivity of the complexed chemical entities. 2To demonstrate the encapsulat ion ability of the hybrid cages described herein, we have chosen S 4 -Fe 4 3 12 as a model system because of its largest metal-metal distance.Fullerene C 60 was selected as representative guest due to its high symmetry, its importance to material science and growing interest in the application of supramolecular cages for site selective functionalization. 31Although the calculated cavity in S 4 -Fe 4 3 12 is too small for encapsulation of C 60 , molecular modelling studies suggested that S 4 -Fe 4 3 12 can accommodate C 60 after rotation of the C Ar -N urea bond without a change in the overall symmetry.Heating the mixture of an excess of C 60 and S 4 -Fe 4 3 12 in CD 3 CN resulted in the formation of the corresponding inclusion complex C 60 @S 4 -Fe 4 3 12 , which was of the same symmetry as the free host based on 1 H NMR results (Fig. 11).Full conversion to the inclusion complex was confirmed by a control experiment using only 0.5 equivalent of C 60 .The spectrum consisted of two sets of resonances corresponding to free and complexed host in slow equilibrium (Fig. S89-S90).Similar diffusion coefficients were found for both species indicating that the host undergoes no significant structural changes upon complexation.The ESI-MS spectrum shows the ions derived from C 60 @S 4 -Fe 4 3 12 , and no signals of the cubic Fe 8 3 24 or C 60 @Fe 8 3 24 impurities were detected.Although we were not able to accurately determine the association constant K a using UV-vis spectroscopy, the lower limit of K a  10 6 M -1 can be estimated from 1 H NMR experiments (see Page S61).
The results obtained highlight the potential of this type of cages in various applications relying on the host-guest chemistry.

CONCLUSIONS
In conclusion, we have demonstrated the fully diastereoselective assembly of supramolecular tetrahedral cages by using the cooperative action of coordination and quadruple H-bonds.The dynamic H-bonds embedded into the edges of a tetrahedron scaffold open up new ways for structural and functional modulation of these assemblies and might also facilitate the ingress of relevant species into the cavity.In addition, the enhanced stability of H-bonds allowed the formation of tetrahedral complexes even in highly competitive protic media which renders them prospective cavitands for nonpolar or anion (in)organic guests.We have realized for the first time the selection of a mesocate over a tetrahedral cage motif by modifying the rigidity of the ligand via insertion of flexible non-covalent wedge.The use of the large Hg 2+ ion provided yet another approach to direct the assembly process towards dinuclear triple-stranded architectures.The Fe 2+ tetrahedron cages obtained from ligands of different lengths showed no ligand scrambling when mixed together.These findings pave the way for simultaneous use of several cages for host-guest applications or synthetic cascades.Moreover, the possibility to exchange metal ions within the cage was demonstrated.The herein introduced molecular tetrahedron cages are potentially useful as receptors for guests as large as C 60 .Further studies aiming to employ the host-guest chemistry of these cages in catalysis are undergoing.

Figure 1 .
Figure 1.Coordination driven self-assembly exemplified with tetrahedron cages.(a) classical approach with covalent ligands; (b) subcomponent self-assembly combining dynamic covalent and coordinative bonds; (c) newly proposed subcomponent self-assembly employing H-bonded dimers as ligands.

Figure 2 .
Figure 2. (a) Design of the first-generation ligand 1; (b) Two possible Fe 4 1 12 tetrahedral complexes based on keto and enol tautomers of UPy; (c) 1 H NMR spectra of S 4 -symmetric tetrahedral complexes with DOSY trace.The resonances of the minor species and their quantities are indicated; (d) Characteristic through-bond and through-space interaction maps for keto and enol UPy dimers for ligand 1b (left) together with obtained ROESY and COSY spectra (right); (e) An example of experimental and simulated ESI-HRMS isotopic patterns for S 4 -Fe 4 (1b) 12 .

The 1 H
NMR spectra of the tetrahedral complexes S 4 -Fe 4 (1ac) 12 show that the chemical shifts of some proton resonances greatly differ from those of the other two of the same type, indicating their unique chemical environment.For instance, the resonances of vinyl proton f at 4.01 ppm in S 4 -Fe 4 (1b) 12 is shifted upfield ( = 1.40 ppm), whereas resonance c at 10.52 ppm is shifted downfield compared to the other two c resonances (Fig. 2c, Fig. 3c and Fig. S20, S25, S34, S37
Stability studies: effect of solvent polarity and competing agents.The quantitative formation of tetrahedron cages in acetonitrile indicates the high stability of quadruple H-bonds even in such polar solvents.To further probe the strength of Hbonding in these assemblies, we conducted more detailed titration experiments by gradually increasing the polarity of the media with addition of DMSO or water into an acetonitrile solution of S 4 -Fe 4 (1b) 12 , T-Fe 4 2 12 and S 4 -Fe 4 3 12 .As shown in Figure 5a, the cage S 4 -Fe 4 (1b) 12 is exceptionally stable; traces of the cage were still detected in solvent mixtures containing up to 31% (v/v) d 6 -DMSO and 64% D 2 O (v/v).Full H to D exchange of N-H protons was achieved at ca. 16% (v/v) content of D 2 O. Cage T-Fe 4 2 12 displays similar stability, surviving in as much as 22% (v/v) d 6 -DMSO in CD 3 CN (Fig. 5c).The addition of D 2 O resulted in full H to D exchange at ca.

Figure 8 .
Figure 8. Self-sorting of molecular cages composed of the same type of ligands 1b and 1d having substituents of different size (a) and selfsorting of preformed cages having different types of ligands (b).