Subsecond Annealing of Advanced Materials. Wolfgang Skorupa. Denise Gabuzda. Lasers in Materials Science. Marta Castillejo. Encyclopedia of Nuclear Physics and its Applications. Reinhard Stock.
Amorphous Nanophotonics. Carsten Rockstuhl. In-situ Materials Characterization. Heinz Graafsma. Nanostructured Materials for Magnetoelectronics. Bekir Aktas. Springer Handbook of Crystal Growth. Govindhan Dhanaraj. Excitonic and Photonic Processes in Materials.
Designing Next-Generation Devices
Jai Singh. Studying Kinetics with Neutrons. Imaging and Manipulating Molecular Orbitals. Leonhard Grill. Emilio Scalise. Edoardo Baldini. Ellipsometry of Functional Organic Surfaces and Films. Karsten Hinrichs. Non-equilibrium Phenomena in Confined Soft Matter. Simone Napolitano. Joseph Woicik.
Alex Frano. Low-Dimensional Functional Materials. Reinhold Egger. Physical Properties of Nanorods. Roman Krahne. Progress in Nanophotonics 2. Motoichi Ohtsu. Clusters in Nuclei, Volume 3. Christian Beck. Complex Plasmas. Michael Bonitz. Inorganic Scintillators for Detector Systems. Paul Lecoq. Jeroen A. Zhiming M. Soft Matter at Aqueous Interfaces. Peter Lang. Novel Functional Magnetic Materials. Arcady Zhukov. Youichi Murakami. Adsorption and Diffusion in Nanoporous Materials. Rolando M. Shinichiro Seki. New Trends in Atomic and Molecular Physics. Man Mohan.
Local Structural Characterisation. Duncan W. Nanoparticulate Materials. Kathy Lu. Topological Structures in Ferroic Materials. Jan Seidel. Photoelectron Spectroscopy. Shigemasa Suga. Quantum Dot Devices. Electronic Structure of Metal Phthalocyanines on Ag Cornelius Krull.
Laser Processing and Chemistry. Non-Centrosymmetric Superconductors. Ernst Bauer. Progress in Ultrafast Intense Laser Science. Kaoru Yamanouchi. Guido Langouche. Advanced Transmission Electron Microscopy.
Jian Min Zuo. Length-Scale Dependent Phonon Interactions. Subhash L. Andrey V. Polymer Materials. Kwang-Sup Lee. Surface Science Techniques. Gianangelo Bracco. Magnetic Fields in Diffuse Media. Alexander Lazarian.
Polymer Nanoassemblies and Their Nanohybridization with Metallic Nanoparticles
Photorefractive Optoelectronic Tweezers and Their Applications. Michael Esseling. Chemical Looping Partial Oxidation. Liang-Shih Fan. Theory of Bilayer Graphene Spectroscopy. Active Plasmonic Devices. Nanoscience with Liquid Crystals. Quan Li. Organic Electro-Optics and Photonics. Larry R. Shi-Gang Sun. Nanomaterials and Nanoarchitectures.
Solid State Properties. Mildred Dresselhaus. Optical Binding Phenomena: Observations and Mechanisms. Jonathan M. Glass Nanocomposites. Recently, libraries of circular and double-stranded helicates have been provided in order to develop this subject In the same manner, complex 43 could be transformed into the corresponding circular helicate through heating in an acid medium These results are due to different thermodynamic and kinetic parameters of formation of the complexes from the metal ions and the bipyridine units.
In addition, formation of these circular structures can be considered sequential self-assembled processes , since the corresponding triple-stranded helicate is formed as an intermediate. Structural changes in the tris bipyridine 42 may produce different self-assembled circular structures. Thus, the modification of the CH 2 CH 2 bridges into CH 2 OCH 2 bridges yields a ligand able to form a tetranuclear circular helicate, under the same conditions described above, independent of the counteranion The features displayed by this self-assembled architecture can be related to the increased length of the ligand and its greater flexibility.
Many interesting circular architectures can be constructed with the use of a well-designed ligand and a suitable metal ion For example, Jones et al. The crystal structure of the complex revealed an anion encapsulated in its central cavity, as observed in complex The self-assembly of helical systems from achiral ligands and metal ions yield racemates containing the right-handed P and the left-handed M helicates Figure Since these complexes display special features, the isolation of helical architectures having high optical purity is of considerable interest.
One strategy for yielding chiral helicates involves their spontaneous resolution, which has been observed in the process of crystallization of the triple-stranded helicate 43 This result represents a successful application of molecular programming to the spontaneous but directed formation of a given supramolecular structure.
Resolution techniques have also been developed in order to achieve optically pure helicates. The recent advances in resolution techniques of helical structures have been revised The enantiospecific synthesis of helicates is an interesting approach that has been used to achieve asymmetric induction. One method involves the synthesis of enantiomerically pure ligands with chiral links between the bipyridine units. In this way, the optically pure chiral ligand 46 with S , S -configuration was synthesized Space-filling models and molecular-mechanics calculations suggested that steric effects may be responsible for the preferential formation of the right-handed P double helicate, from the two possible helical diastereomers for helicate 47 The induced sense of helicity by the presence of chiral centers in the ligand strands has been previously reported 66, and this highlights the importance of the ligand design in the organization of the binding centers during the self-assembly process aimed towards the formation of supramolecular species.
Other approaches have been studied in order to accomplish asymmetric induction utilizing an auxiliary template 81,, and the incorporation of a chiral substituent at the extremity of the ligand 67, A very interesting example of the complete stereospecific self-assembly of a circular helicate has been recently reported The reaction of a , a '-bis pinene-2,2'-bipyridyl -p-xylene chiral ligand 48 with AgPF 6 in a mixture of acetonitrile and chloroform led to the spontaneous formation of the circular single-stranded helicate 49 Figure Its structure was confirmed by X-ray crystallography and NMR and circular dichroism data demonstrated that the self-assembled structure is maintained in solution This may illustrate the countless possibilities in this area attainable through the suitable design of chiral ligands responsible for predetermining the chirality in self-assembled architectures As shown in the preceding sections, considerable work has been done in the area concerning metallohelicates, and their applications have also been recently reviewed The first visible possibility offered by this class of self-assembled structure is that specially designed ligands can be created in order to generate helical systems possessing endoreceptor properties.
The proper choice of a bridge connecting the binding elements in the ligand may lead to an informed system capable of yielding a cavity after the metal reading, which can act as an endoreceptor. Although this approach has been used independently by Beer , Harding 65a,b , and Nabeshima and co-workers , considerable effort has to be made to achieve expressive interactions between the receptor and anionic, cationic and neutral species. These template-directed self-assembled architectures see Section 5. Furthermore, the design of metallohelicates with exoreceptor properties has been explored 45 and the potential of this field will be discussed later see Section 5.
The use of double-stranded metallohelicates represents an interesting synthetic pathway towards molecules with non-trivial topology. Sauvage and co-workers have employed this strategy in the synthesis of interlocked catenanes and trefoil knots , which cannot be obtained in another way. Electron-microscopy studies were performed on supramolecular liquid-crystalline polymers with right-handed helicity The use of different strategies to generate inorganic polymers and the study of their interesting properties have been successfully employed The design of helical architectures can be explored in the development of various supramolecular devices.
Shanzer and co-workers have prepared ligands 50 and 51 , containing in their structure one 2,2'-bipyridine and one hydroxamate Chart 7 Recently, lipid-like ligands 52 and 53 were used in the assembly of metallohelicates in order to promote the occurrence of a liquid crystalline phase at room temperature The development of fluorescent sensors and switches represents a very important challenge in the comprehension of many chemical, biochemical and material science events 6,7, This rationalization was used by Piguet and co-workers in the conception of light-converting devices based on triple-stranded helicates with lanthanide metal ions 84, 85, Along with the fact that the helical structures are very beautiful molecular sculptures aesthetically created by means of self-assembly, these complexes may be able to perform innumerable functions.
Closed three-dimensional molecular cage-type structures that have the capability for encapsulating guest species have provided the chemical community with a continual source of fascination and study over the past three decades. A large variety of such molecules have currently been prepared, and which now encompass several classes of compounds. For example, cryptates which were developed for alkali metal ion and anion binding , multi-walled cyclophanes and carceplexes that imprison neutral molecular guests, sepulchrates and sarcophagines, into which are captured transition metal ions and inorganic clusters which enclose various cations and anions The importance of these compounds is mirrored by the diverse range of applications for which they have been used in materials science, medicine and chemical technology.
Additionally, the unusual properties of these systems provide interesting and challenging opportunities for the study of theoretical physicochemical issues However, the limited availability of many of these substances has in part posed a serious obstacle to their technological development and originated from the lengthy multistep reaction sequences and low overall yields often encountered during their synthesis Early investigations showed that it was possible to use metal ion-ligand interactions as a driving force for the generation of structural complexity at a molecular level.
This approach allowed direct access to topographically unusual metal ion containing entities such as helicates see preceding section , catenates , racks 7, , grids 7, and metallomacrocyclic receptors It was therefore decided to explore the possibility of using metal ion-mediated self-assembly as a design principle for the generation of molecular cages and cage-type receptors of controllable size and shape. In addition, inorganic cage architectures which incorporate metal ions as integral structure generating units would be expected to exhibit novel and interesting physicochemical properties such as optical, magnetic, electrochemical and catalytic functions.
As a first step we envisioned the self-assembly of the C 3 symmetric cage complex 54 Figure Most significantly, the resultant architecture would comprise two different ligand species and a single metal ion type and would therefore express a higher degree of structural informational complexity compared to the majority of preexisting self-assembled entities, which consist of a single ligand and metal ion species. Such multi-ligand type architectures must be generated via a multicomponent self-assembly pathway in which the recognition, growth and termination events involve selective discrimination of hetero-ligand containing species along the reaction coordinate.
Multicomponent self-assembly of cylindrical coordination architectures from five ligand units and six metal ions. The 13 C NMR spectrum also showed the expected seventeen bands. Addition of a small quantity of 55 or 56 to a nitromethane solution of the product complex resulted in complication of the 1 H NMR spectrum with sharp peaks due to 54a still present as a major component.
Thus 54a is undergoing slow exchange on the NMR timescale in nitromethane solution. The nature of cation 54a was confirmed by determination of its crystal structure a. The structure possesses a C 2 axis passing through the middle of the central C-C bond of one of the qpy units; this distortion from ternary symmetry may be due to crystal packing.
The overall twist of the whole structure results in a triple helical shape of the complex. Bond lengths and angles do not show any peculiarity. Complex 54a presents an internal cavity of cylindrical shape with a height of 7. Thus 54a represents a self-assembled molecular receptor for appropriately sized substrate species. The overall dimensions of the complex are The cation 54b possesses an internal cavity of 4. The guests are positioned in the same plane in such a way that almost all available space within the cavity is filled.
Each guest species also partly protrudes through each of the three cavity portals into the antechambers defined by the 56 ligand surfaces and the 55 phenyl rings. Interestingly, the 1 H NMR spectra of complexes 54a - b show the bands assignable to the ortho-phenyl ring protons of the 55 ligands to be considerably line broadened. This phenomenon appears to linked to the presence of anions in the cage cavity.
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. 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. 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.
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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.
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.
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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. 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. Finally, the advent of the metallodendrimers, as well as the possibilities of their use as exoreceptors, are also discussed.
Metallo-exoreceptors formed by template strategies. It was shown in Section 3. More recently, Ghadiri and co-workers have synthesized a peptide containing a 2,2'-bipyridine at the N -terminus The peptide units were organized through a similar strategy and the proprieties of the systems obtained were studied. The peptide chain, comprised by a sequence of five different amino acids, possesses a well-established helical structure. The formation of the three-helix structure was confirmed by circular dichroism and mass spectrometry.
It was shown that each isomer, after isolation, undergoes slow interconversion to an equilibrium mixture containing all diastereomeric forms within a few hours. Sakai and Sasaki have linked a sugar to a 2,2'-bipyridine and organized the resultant ligands around the transition metal ions, in order to position three sugar groups close together in the periphery of the complex tris 2,2'-bipyridine The self-organized trimeric carbohydrate showed stronger complexation to Vicia villosa B 4 lectin than the monomeric unity. This protein is able to recognize repeating units of GalNAc-modified serine or threonine amino acid residues The isomer ratio of the dynamic mixture could also be changed by the addition of lectins , For instance, the addition of Vicia villosa B 4 lectin led to the detection of the L -mer as the major isomer in the mixture Topological templates have been used as a built-in strategy for directing covalently appended peptide blocks to a predetermined packing arrangement in order to yield branched chain architectures The development of this area is fundamental in de novo protein design, and three-helix and four-helix protein mimics have been successfully produced through this approach , An additional attractive feature in the design of metallo-exoreceptors is that systems designed through this approach mimic an ubiquitous mechanism of molecular recognition in biological systems: the self-assembly of enzyme complexes leading to the formation of binding sites, which do not preexist in the individual subunits For instance, enzymatic systems such as aspartate transcarbamoylase and phospho-fructokinase display these features, and the assembly or dissociation steps are responsible for the execution of their biological functions.
The podands obtained could be employed to perform the efficient and selective extraction of alkali metal ions from aqueous solution. A similar strategy was used to yield metallopolyethers with the general structure of 69 Schepartz and co-workers have used with elegance the design of peptides linked to 2,2'',2"-terpyridine derivatives The addition of metal ions has been conducted towards the dimerization of functionalized peptides, making them able to perform the association and recognition of DNA. Metallo-exoreceptors in the anionic recognition and design of inorganic architectures.
Anions are ubiquitous in a great variety of functions in the organic and mineral worlds. In spite of the fact that they play an important role in the existence of living beings, anionic recognition by synthetic molecular receptors still remains a puzzle for the supramolecular chemistry world, perhaps due to the intrinsic complications associated with these systems The complexation of anions by synthetic receptor molecules has been recognized recently and is currently growing as a new area of coordination chemistry The design of 5,5'-disubstituted 2,2'-bipyridines in order to generate clefts by means of formation of complexes with an adequate metal ion has been explored This picture allows the visualization of three important aspects: firstly, the free ligand is not capable of executing functions such as recognizing anions.
When the metal ion performs an octahedral reading of the ligands, they are organized in such a way as to yield a self-assembled system with the concomitant generation of two clefts. These sites allow the association of the complex with different anions, such as phosphate, phosphonate and sulphate, to varying extents.
Formation of the complex also results in the lowering of pK a values for the bonded hydroxamic acids. For instance, the pK a values for the hydroxamic acid groups in the free ligand 70 are 8. This observation is interesting because it permits us to think of the possibility of employing this and other complexes with similar structures, under mild pH conditions, in studies involving catalysis by the hydroxamic acid group of the hydrolysis of compounds of biological interest, such as phosphate esters.
Finally, the positive charge of the central metal ion is important to bring the anions close together to be recognized via electrostatic interactions. Metal ions can be chemically or voltametrically oxidized, and the increase in the net charge of the complex will surely increase the interaction of the anions with the complex. Many related approaches have been employed in order to generate receptors able to recognize anionic and cationic substrates. Hamilton has recently reviewed some of the strategies that are being employed in this field 16e.
The self-assembly of organized molecular structures of great proportions, or molecular tectonics , was developed from the molecular recognition involving different building blocks called tectons from greek tektwn : builder. Hosseini and co-workers have prepared linear molecular arrays, called linear koilates , based on the self-assembly of rigid and compact exoreceptors having two diametrically opposing cavities, the linear koilands from Greek koilos : hollow , and well-designed connectors having two extremities complementary to the cavities of the exoreceptor Metallo-exoreceptors can also be used as building blocks in the construction of more complex inorganic self-assembled structures.
Its formation illustrates the concept of sequential complexation , inasmuch as the first metal ion added organizes the ligands disposed in the 5,5'-positions to accomodate the second metal ion. The synthesis of sequential oligodonor ligands constitutes a very interesting approach to heteronuclear coordination compounds and to the construction of switches Thus, the strategy depicted here seems very attractive to the design of novel supramolecular inorganic architectures, starting from carefully synthesized ligands Another possible strategy for the construction of species possessing exoreceptor properties involves the design of dendrimers from greek dendron : tree and meroz : part These globular, well-defined and structured oligomeric fractal-like macromolecules are synthesized by planned and controlled routes and are also known as starburst, cascade, arborols or cauliflower polymers They are designed conventionally by strategies that involve the formation of carbon-carbon or carbon-heteroatom bonds.
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These strategies comprise a convergent or a divergent synthetic route, similar to the methods utilized in the synthesis of common organic compounds. These symmetrical macromolecules possess great potentiality to act as endoreceptors, considering that they contain a variety of heteroatoms or functional groups. In addition, dendrimers can be produced in a given size, having an external shape suitable for the exploration of their proprieties as exoreceptors.
Newkome has coined the term suprasupermolecular chemistry to represent the relationships involving dendrimeric architectures and supramolecular chemistry More recently, strategies have been developed towards the construction of metallodendrimers, i. In the same manner in which the covalent approach can produce dendrimers with many generations, synthetic strategies can be envisioned in order to give metallodendrimers with several generations.
As an example, Constable and co-workers synthesized the heptanuclear metallocentric metallodendrimer 73 Chart 9 by a convergent synthetic strategy The growing interest in the chemistry of metallo-dendrimers has resulted in several classes of new compounds 22c, and although supramolecular aspects are currently being explored within and at the surface of the dendritic structures, studies involving metallodendrimers as metallo-exoreceptors constitute an as yet unexplored area.
This makes these compounds very attractive to the broadening of many current fields in science, as the development of molecular switches, anion, cation and small molecule receptors, fluorescent sensors, enzymatic models and compounds with antiviral and antimicrobial activities. The results reviewed herein demonstrate that it is possible to access supramolecular architectures of nanometric size possessing a high level of structural complexity by metal ion-mediated self-assembly. Their formation may comprise the operation of a programmed, informational process that involves three main stages: a initiation, b propagation, and finally c termination of the assembly leading to a discrete metallosupramolecular entity.
Entropic and solvation factors are also expected to play an important role in the generation of such species. Of particular interest is the simultaneous occurrence of several processes, such as in the case of cage-type species: the self-assembly of a multitopic receptor and the selection of substrate entities possessing features suitable for inclusion in the internal cavities. Extensions may be envisaged towards the preparation of larger multicomponent architectures which may be built on several substructures of different geometries as well as towards the exploration of functional systems incorporating components possessing various physical and chemical properties photoactivity, electroactivity, etc.
From another point of view, the ability to generate large complex architectures spontaneously through programmed self-organization represents a powerful alternative to nanofabrication and nanomanipulation, which may be expected to have a profound impact in nanoscience and nanotechnology 7.
In a broader perspective, the present results represent a further step in the progressive design of programmed systems undergoing spontaneous but controlled complexification, displaying for instance features such as multiple subprograms and adaptive behavior Lehn, J.
Pure Appl. Science , , Dugas, H. Balzani, V. Supramolecular Chemistry ; Horwood; Chichester, Comprehensive Supramolecular Chemistry , 11 vols. Whitesides, G. Fraenkel-Conrat, H. USA , 41 , Biochemistry ; John Wiley; New York, , p Voet, D. For artificial molecular machine concepts, see for example: Balzani, V. Boyer, P. For references containing discussion on self-assembly in natural systems, see, for instance: a Lindsay, J. New J. See, for instance: a Ringsdorf, H. For a view of the recent advances on metallosupra-molecular chemistry, see: Transition Metals in Supramolecular Chemistry ; Fabrizzi, L.
Baxter, P. Inclusion Phenom. Macrocyclic Chem. Werner, A. Cahn, R. For example, see: a Pauling, L. Watson, J. Nature London , , Saenger, W. Some examples of approaches to programmed molecular helical structures: a Judice, J.