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The formation of structures 54a-b represents a remarkable example of the spontaneous formation of a closed inorganic architecture through a process of multicomponent self-assembly from eleven particles belonging to two types of ligands and one type of metal ion.
These results therefore successfully demonstrated the use of metal ion-mediated multicomponent self-assembly as a method of access to structurally complex molecular architecture, and represent a further step in the control of the self-organization of large and complex supramolecular structures through molecular programming. The design and generation of elongated inorganic cylindrical cage architectures via metal ion-directed multicomponent self-assembly. Having established the success of the above design principle in constructing inorganic cages, an important further question concerned the possibility to engineer the size and shape of the internal void within cations such as 54a-b in a predictable and controllable way.
Such species would have the potential capacity for shape selective and multiple guest inclusion.
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Towards this goal, vertical elongation of the cage 54a was attempted by utilizing ligands structurally similar to 56 but incorporating bridging groups between the bipyridine subunits, and repeating the reaction conditions which were successful for the self-assembly of 54a-b. Attempted self-assembly of cylindrical cages with conformationally flexible ligand bridges.
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In all cases, no evidence for cage formation was observed even after prolonged reaction times and elevated temperatures. Interestingly, the cage complex 57 was isolated from the reaction of a 1. It could however be isolated pure in the solid state by slow diffusion of an excess of diisopropyl ether into the reaction mixture.
The identity of 57 was confirmed by X-ray crystallography The cation is shaped into a highly twisted helical cage in which two 55 ligands form respectively the top and bottom and three 58 ligands, the walls of the complex. Complex 57 is helically twisted to a greater degree than 54a. Inside the cage is a small cavity of dimensions 4. The contracted diameter of the cavity of 57 relative to 54a results directly from the steric volume imposed by the internally facing bridging ethylene groups of the three 58 ligands.
Self-assembly of elongated cylindrical cage architectures with rigid ligand bridges. Cages possessing cylindrical cavities were however successfully prepared by using ligands with rigidly preorganized bridges between the bipyridine subunits The 1 H and 13 C NMR spectra of 59 - 63 indicated the presence of a highly symmetrical species in solution with all ligands in a single magnetic and chemical environment. In the complexes elongated by means of acetylene containing bridges 59 , 62 - 63 , the band corresponding to the ortho phenyl ring protons of 55 appeared as a sharp doublet.
This is consistent with an unrestricted movement of anions through the larger portals in the walls of these complexes. The ES mass spectra showed bands assignable to the cylindrical complexes with successive loss of PF 6 - counterions. Thus, rigid preorganization of both reacting ligand species proved to be a successful design modification for generating cylindrical cages with a range of cavity sizes.
Complex 63 is therefore a truly nanoscopic cylinder which has been designed and generated through a multicomponent self-assembly strategy. Ligand selection in the self-assembly of hexacopper inorganic cages. In order to explore the degree to which recognition can operate within a complex mixture, an experiment was performed in which the correct stoichiometric combination of the ligand and metal ion components required to generate 54a , 60 and 61 , were allowed to stir in nitromethane for 72 hours. Analysis of the resulting solution by 1 H NMR and ES mass spectroscopy showed that only three species were present in solution, i.
Previous studies with helicate mixtures 44,63 showed that only products comprising identical ligand strands were formed through a process of self-recognition. In the above example many more particles are initially present within the reaction mixture and the products form only by recognition between ligands of different identity. This situation which is of a higher information content, may be termed as nonself-recognition , and bears analogy to biological phenomena as found for instance in the immune system. The designed self-assembly of multicomponent and multicompartmental cylindrical nanoarchitectures.
The successful generation of the hexanuclear cage complexes described above raised the question as to whether this process could give access to multicellular inorganic architectures that would present several internal cavities and might in addition incorporate selected substrates in the course of the assembly. The formation of supermolecular entities of this type would represent abiological analogues of numerous biological processes mediated by collective interactions and recognition events between large molecules.
In particular, it would amount to a self-compartmentalization process presenting analogies with that displayed by multicompartmental proteases Potential applications may also exist for example in materials science and nanotechnology, where the establishment of pathways for the controlled access to nano-sized chemical entities is of paramount interest.
Further experimental investigations successfully demonstrated that the generation of multicellular inorganic architectures was indeed possible. Thus, the bicompartmental 64 and tricompartmental 65 complex cations were generated in a single operation by self-assembly from the corresponding stoichiometric mixtures of ligand components of two different types and metal ions Figure In addition, X-ray structural determinations and 1 H NMR solution studies revealed that the multicellular architectures encapsulated anions in their cavities Self-assembly of multicompartmental nanoarchitectures. Evidence that the products from the above reactions possessed multicellular cage-type structures in solution came from inspection of their 1 H NMR spectra.
In all cases, the spectra were particularly simple and indicative of the presence of a highly symmetrical species in nitromethane solution. For example, in 64a - b the peaks due to the ortho- and meta- protons of the phenyl rings of 55 were divided into two groups in a ratio of corresponding to the two outer and single inner ligands 55 in 64a - b.
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In 65a - b , the above mentioned protons were divided into two groups in a ratio corresponding to the two outer and two inner ligands The H6' and H6" protons of ligand 66 in 64a - b were found to be shielded relative to the remaining protons H3', H4', H4", H3" of the four central pyridine rings of This shielding effect is exactly what would be expected for protons pointing towards the interior of the cage cavity.
One also observes that the signals of the ortho- protons of the phenyl rings on the inner and outer HAT ligands are broadened for both 64a and 65a. This indicates that a slow kinetic process is taking place, possibly linked to the presence of anions inside the cavities. The ES mass spectra of the reaction products were also supportive of structures 64a and 65a.
All spectra were recorded at a concentration of 10 -4 mol L -1 in nitromethane and no other peaks were seen, showing that 64a and 65a were the only species present in solution and were stable to dissociation down to at least 10 -4 mol L Crystal structures of the multicompartmental nanoarchitectures. Confirmation that the reaction products were indeed of multicellular type was obtained by determination of the X-ray crystal structures of 64a and 65a.
Both complexes are shaped into beautiful expanded triple helical cylindrical cages. The overall dimensions of 64a of In 64a the 55 ligands are not eclipsed as shown in Figure 20 but sequentially rotated with respect to each other by The complex 64a possesses two internal cavities of radius 5.
The larger compartment contains two PF 6 - anions and a water molecule with full occupancy such that almost all available internal space is filled. Cation 65a is also triple-helical and comprises four 55 ligands, two outer ones defining the ends of the cylinder, and two inner ones dividing the cylindrical cage into three compartments. The average distances between the mean planes through the 55 ligands are 7.
As in 64a , the cavities are occupied by guests i. In both 64a and 65a extensive p - p contacts within 3. The unprecedented structures of the complexes 64a and 65a might be described as mol ecular skyscrapers with occupants residing on each level! The slight difference in cavity size apparent in the crystal structure of 64a and 65a is not observed in their 1 H NMR spectra, which instead show single highly symmetric species with both halves of the molecule in a chemically and magnetically equivalent environment.
The complexes must therefore be undergoing intramolecular motions in solution, which are rapid on the NMR timescale and confer an average cylindrical symmetry to the species.
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As revealed by the crystal structures, anions are contained in the cavities of the complexes 64a and 65a. Their presence is also reflected in the spectral properties of these species. The chemical shifts of the latter differ by 0. The anions are therefore able to move into and out of the cavities at room temperature but cation 64a appears to have a distinct preference for the inclusion of CF 3 SO 3 - in the presence of PF 6 - Inspection of the crystal structure of 64a shows that the six portals in the walls of the cage are slightly smaller than a PF 6 - anion.
Intramolecular breathing of the complex by unwinding of the helix may result in opening up the windows thus facilitating anion exchange into and out of the cavities. The multicomponent approach represents a highly convergent type of self-assembly of greater information content than systems comprising metal ions and single ligand species, and which in principle should be capable of accessing the highest levels of structural complexity at the molecular level in the shortest number of steps.
The success of this strategy relies upon two main factors: i the utilization of metal ions of preferred tetrahedral coordination geometry and oligotopic ligands comprising bidentate binding sites which ensures that the metal ion-ligand bonding interactions are reversible and that the reaction proceeds under equilibrium thermodynamic control, and ii the ligands are designed in such a way as to destabilize the formation of polymers and stabilize the desired supramolecular heteroligand product In the latter case the use of rigidly preorganized ligands functionalised with sterically hindering groups was found to be of crucial importance to the success of the self-assembly.
All cage complexes described above possess an average C 3 symmetric architecture laterally expanded to provide an internal void. The superstructures of 64 and 65 display a unique combination of unusual properties: i they possess novel architectures of nanoscopic dimension, laterally expanded to provide an internal cavity; ii they have the features of multicompartmental or multicellular containers; iii their formation represents a self compartmentalisation process presenting biological analogies , iv they operate by way of multicomponent mixed-ligand self-assembly ; and v they behave as cryptands exhibiting multiple guest inclusion with four 64a and six 65a PF 6 - anions and solvent molecules encapsulated within a single receptor entity.
Multiple guest inclusion and compartmentalization are characteristic features of the organization of living organisms, ensuring that the correct chemical events take place within spatially confined, well-defined domains that may either be intracellular or belong to different cells in multicellular organisms. Most significantly, the ability to generate nanosized architectures spontaneously through programmed self-organization represents a powerful alternative to nanofabrication and nanomanipulation that may be expected to have a profound impact in nanoscience and nanotechnology.
The conception and the design of receptors were until recently founded in macrocyclic or macropolycyclic architectures and in rigid spacers or supports specially disposed to localize the fixation sites in the walls of the cavities 3,7. As a result, the fixation sites converge at the bound substrate. The macrocyclic receptor holds the metal ion, to form an inclusion complex. This principle of convergence, largely employed, defines a convergent or endosupramolecular chemistry , which deals with the design and use of endoreceptors 3,7.
An alternative strategy to the endoreceptors involves the use of an external surface having protuberances or cavities as receptor sites 3,7. This procedure is conceptually equivalent to moving from a convergent chemistry to a divergent or exosupramolecular chemistry , and from endoreceptors to exoreceptors 3,7. One of the various strategies for constructing exoreceptors includes the use of metallo-exoreceptors 3,45, The starting point for the assembly of these supramolecules involves the synthesis of specially designed ligands with appended substituents.
The information stored in the ligands can be read by an appropriate metal ion, which disposes tridimensionally the substituents in order to generate a cleft, where the recognition will take place Figure The metal ion has a double function, it disposes the units of recognition according to the coordination geometry of the ion, and it is also responsible for strong electrostatic interactions, due to the ion charge. This section starts with the discussion involving metallo-exoreceptors formed from specially designed 2,2'-bipyridine and 2,2'',2"- terpyridine based ligands with special emphasis on the template strategies leading to the construction of metallo-exoreceptors.
Later, the self-assembly of other types of metallo-exoreceptors and their use in anionic recognition and in the design of inorganic structures via reaction with proper metal ions are commented on.