Wiadomości Chemiczne

Supramolecular Chemistry, 1997

Library of Wiadomości Chemiczne

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  1. From organizer of seminars of Polish network of supramolecular chemistry
    Marek Pietraszkiewicz

  2. Heteropolyacids based supramolecular systems
    Adam Bielański

  3. Heteropolyanions and their complexes with lanthanide(III) ions. Spectroscopic studies
    Stefan Lis

  4. II. Molecular recognition and self-organisation: supramolecular synthons in crystal engineering
    Veneta Videnova-Adrabińska

  5. Calixarenes as ligands for transition metals, actinides and lanthanides
    Marek Pietraszkiewicz

  6. Crown ethers and their analogs as catalysts, cocatalysts and substrates in the reactions involving ion pairs
    Krystyna Brandt, Iwona Porwolik, Mariola Siwy

  7. Some aspects of synthesis of multicyclic calixarenes
    Gabriel Rokicki, Wojciech Wąsikiewicz, Jędrzej Kiełkiewicz

  8. The calixarene synthesis from polyhydroxyphenols and their derivatives
    Waldemar Iwanek

  9. Selected topics in supramolecular self-assembly
    Marek Pietraszkiewicz

  10. Nomenclature of some macrocyclic compounds
    Grzegorz Schroeder, Bogusława Łęska

  11. Polyaza and polyoxaaza macrocyclic complexes of scandium group metal ions and lanthanides
    Wanda Radecka-Paryzek

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FROM THE ORGANIZER OF SEMINARS OF POLISH NETWORK OF SUPRAMOLECULAR CHEMISTRY

Marek Pietraszkiewicz

Instytut Chemii Fizycznej Polskiej Akademii Nauk, ul. Kasprzaka 44/52, 01-224 Warszawa


The beginning of supramolecular chemistry is usually associated with pioneering works of Charles Pedersen on macrocyclic polyethers, published in 1967. After 30 years, it turns out that this branch of supramolecular chemistry is the most dynamically developing interdisciplinary science, combining the elements of chemistry, physics and biology. Supramolecular chemistry is based on non-covalent interactions, such as electrostatic, hydrogen bonding, CT, Van der Waals, hydrophobic, etc. The great variety of synthetic possibilities and a vast number of molecular intractions places supramolecular chemistry on unique position, unpreceeded in the history of science. Three main contributions to the developnemt of supramolecular chemistry: those of C. Pedersen, D. J. Cram and J.-M. Lehn have been awarded with Nobel Prize in Chemistry in 1987. The evolution of this discipline was associated with the labels, such as "crown ether chemistry", "macrocyclic chemistry", "chemistry of inclusion phenomena", "host-guest chemistry", "lock-and-key chemistry", and "supramolecular chemistry". Actually more appropriate would be to use the term "supramolecular science". The development of supramolecular chemistry goes in two directions: biomimics and advanced materials. Biomimics is represented by research on enzyme models, inorganic helix models, solar energy convertors, whereas advanced materials are associated with nanostructures, molecular films, multilayers, magnetic, semiconducting, superconducting materials, sensor components, analytical materials, etc. One can distinguish separate branches of supramolecular chemistry, such as supramolecular photochemistry and supramolecular electrochemistry. The sope of these sub-disciplines is, respectively, as follows: nonlinear optics, solar energy conversion and storage, photochemistry and photophysics of metal ion complexes, photochromism, proton and electron transfer, photochemical reactions in receptor cavities, modification of redox properties of transition metal complexes, electrocatalysis, surface electrode modification, electroactive LB films and multilayers. Both biomimics and advanced materials possess common subsections: self-assembly and self-organization, preorganization, molecular recognition, proton and electron transfer, nonlinear optics, cooperative, allosteric and magnetic effects. These phenomena and processes can be applied to performing complicated functions on molecular level, called "integrated supramolecular devices". These functions can be utiilized in a number of practical applications in medicine, medical diagnostics, electronics, telecommunication, informatics, analytical chemistry, metal technology, environmental protection, and in alternative energy sources. The interest in supramolecular chemistry involves more and more R & D laboratories in big companies, and distinguised scientists of this field forcast that supramolecular chemistry will be the mainstream of chemistry of XXI century. Although the papers on supramolecular chemistry have been published in more than 150 journals, there are four specialist periodicals: "Journal of Inclusion Phenomena and Molecular Recognition in Chemistry", "Journal of Molecular Recognition", "Supramolecular Chemistry, "Supramolecular Science", as well as periodical editions of "Advances in Supramolecular Chemistry", and "Perspectives in Supramolecular Chemistry".


Supramolecular Chemistry, 1997, 9.
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HETEROPOLYACIDS BASED SUPRAMOLECULAR SYSTEMS

Adam Bielański

Instytut Katalizy i Fizykochemii Powierzchni, Polska Akademia Nauk, ul. Niezapominajek, 30-239 Kraków,


The hydrates and alcoholates of heteropolyacids (HPA) represent supramolecular systems in which HPAs play the role of acceptors and the molecules of water or alcohol -the role of substrates. The paper deals with dodecaheteropolyacids of Mo and W exhibiting the structure of anions described by Keggin forming primary structure. In hydrated HPAs protons are solvated and the hydronium ions are bonded by the intermediation of water molecules (hydrogen bonded) to the Keggin units, thus forming the secondary structure. The thermal analysis and FTIR investigation of hydrated HPA are described. At room temperature anhydrous HPAs are sorbing alcohol vapours forming alcoholates in which depending on the pressure each proton in the bulk may bond one, two, three or more ethanol molecules. Above 150oC ethanol is transformed on tungsten containing HPA (H3PW12O40, H4SiW12O40) into the products of dehydration: ethylene and diethyl ether thus giving an example of a catalytic process occurring in supramolecular system. Three possible types of such reaction are presented.


Supramolecular Chemistry, 1997, 17.
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HETEROPOLYANIONS AND THEIR COMPLEXES WITH LANTHANIDE(III) IONS. SPECTROSCOPIC STUDIES

Stefan Lis

Wydział Chemii Uniwersytetu im. A. Mickiewicza, ul Grunwaldzka 6, 60-780 Poznań


The results of investigations related to synthesis, structure and physico-chemical properties of polyoxometatales (POM) and their lanthanide(III) complexes are briefly reviewed. POM structures categorized into three structural groups based on octahedrally, tetrahedrally or icosahedrally coordinated heteroelements are presented. The spherical-like (plenary) and their degradated derivatives (lacunary) structures of the POMs are discussed. Useful techniques (elemental analysis, various spectroscopic and electrochemical methods) for the verification of POM compositions and the determination of components are presented and compared. An important role of absorption and luminescence spectroscopic methods in complexation studies of metal ions, especially of lanthanide(III) ions, with POM's in solution and solid state is described. Absorption studies of certain Ln(III) ions, in the UV-visible and the near IR region, characterized by narrow bands corresponding to hypersensitive f®f transitions (in which the absorption maximum and intensity are sensitive to the ligand field) are used to evaluate the site symmetry of the lanthanide-polyoxometalate sandwiched and encapsulated complexes. Laser-induced europium(III) ion luminescence (lifetime measurements and 7F0 ¬ 5D0 selective excitation) spectroscopy is shown as a very useful tool in studying the primary coordination sphere of the cation, metal-ligand interactions and the number of Eu(III) environments present in solution.

A role Gd(III) complexes in EPR study and as potential magnetic resonance imiging agents is discussed. The significance of polyoxometalates in biological studies as antyviral agents is described.


Supramolecular Chemistry, 1997, 27.
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II. MOLECULAR RECOGNITION AND SELF-ORGANISATION: SUPRAMOLECULAR STNTHONS IN CRYSTAL ENGINEERING

Veneta Videnova-Adrabińska

Instytut Chemii Nieorganicznej i Metalurgii Pierwiastków Rzadkich, Politechnika Wrocławska, ul. Smoluchowskiego 23, 50-370 Wrocław


Crystal engineering is aimed into establishing connections between molecular and supramolecular structures via intermolecular interactions and ideally into identifying supramolecular synthons, which are the robust modules that can be carried over from one crystal structure to another. The complete crystal structure is nowhere pre-existing as such. Its design plan, however, already exists in its basic components. The balance and interplay between factors that are sometimes in consonance and sometimes in conflict binds molecules into supermolecules and the crystal can be considered as a supermolecule par excellence. Thus, crystal engineering is focused on finding an algorithm for a predictable and controllable molecular organisation in 3D networks. Overwhelming physical forces decide the structure of the matter, yet the extended periodicity supervened by the subtle balance between specific middle- and long-range intermolecular interactions and short-range packing regularities ultimately decide for the crystal architecture. The apparent difference between the classic organic chemistry and the supramolecular chemistry are only quantitatively as far as the universal physical forces are concerned. Phenomenologically the structural subunits and the coalescing forces gluing them are considered distinct by convention, however the problem related with intermolecular valence and connectivity patterns remains very similar on both molecular and supramolecular levels. Both crystal engineering and molecular recogniting depend on multiple matching of functionalities among molecular components, so as to optimise the number of intermolecular interactions that may be of different strengths, directionalities and distance dependence properties.

The design methodology is strongly based on the stereochemistry, internal symmetry, geometry and topology of the building blocks with a special attention to the directional dictates of the forces gluing them into structural platforms for the crystal lattices. The design strategy assumes a stepwise coalescing of structural units into 1D and 2D structural scaffolds, (Kitajgorodsky Aufbau Principle). The formal structural mesoforms inherit basic structural information (recognition) and develop new one (organisation). Analysis of possible combinations of chemically allowed symmetry operators governing the interactions leads to formation of supramolecular platforms with long-range periodicity.


Supramolecular Chemistry, 1997, 41.
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CALIXARENES AS LIGANDS FOR TRANSITION METALS, ACTINIDES AND LANTHANIDES

Marek Pietraszkiewicz

Instytut Chemii Fizycznej Polskiej Akademii Nauk, 01224 Warszawa, Kasprzaka 44/52


Calixarenes - cyclic oligomers of diverse phenols and aldehydes represent an unique class of macrocyclic molecular receptors capable of complexing practically all inorganic cations. They form a range of phenolate-type metallic compounds, as well as coordination complexes when appended with appropriate ligating groups. All calixarene complexes display a range of interesting features, and may serve as enzyme metallosite models, redox sensors, selective extractants for radioactive metal ions, luminescent probes and molecular magnetic materials. This rewiew is focused on transition, lanthanide and actinide complexes/derivatives of calixarenes. Calixarenes can bind metal ions by means of ionic interactions (phenolate-type), coordination, p-, and h-arene interactions. Their complexes with alkali and alkaline earth metals have been described [5]. Phenolate-type derivatives with transitions metals [7,8] (Fig. 1,2) and with lanthanides [10-14] (Fig. 3) have been published. The number of papers dealing with calixarene-metal derivatives based on p-tert-butylphenol was remarkably high when compared to calix[4] resorcinarenes, whose phenolate-type derivatives were hardly available. Tetranuclear complexes of calix[4]resorcinarenes of phenolate-type have been published by Floriani [22] (Fig. 7). A vast number of calixarene derivatives have been designed to form coordination complexes with transition [23, 24] (Fig. 9), lanthanide [27] (Fig. 11), and actinide ions. Ion transport and extraction properties of functionalized calixarenes have been described by Shinkai [35], Roundhill [36], and Arnaud [38]. By the way of design, these calixarenes were capable of interacting with transition metals, platinum, actinide and heavy metals. Calixarenes have been used also as the ligands for the second, and third coordiantion sphere [39,40] (Fig. 15, 16). Shinkai has described the synthesis of h-, and p-complexes between calixarenes and transitionmetal carbonyls [41]. Similar works appeared more recently [42, 43]. The combination of stereochemical and structural features of callixarens with coordination mode of binding of metal ions may result in a number of interesting molecular functions, manifested in a range of properties, such as redox, photoredox, magnetic, photophysical, catalytic, NLO, liquid-crystalline, etc.


Supramolecular Chemistry, 1997, 69.
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CROWN ETHERS AND THEIR ANALOGS AS CATALYSTS, COCATALYSTS AND SUBSTRATES IN THE REACTIONS INVOLVING ION PAIRS

Krystyna Brandt, Iwona Porwolik, Mariola Siwy

Centrum Chemii Polimerów PAN, 41-800 Zabrze, ul. M. Curie-Skłodowskiej 34


The reactions involving ions and ion pairs are playing an important role in many organic processes. States of solvation or association affect the reactivity of ionic reagents in spectacular ways. The rates of the reactions of anionic nucleophiles with neutral molecules are greatly influenced by alkali and alkali-earth metal ions and either inhibition or catalysis is observed depending on whether the cation interacts preferentially with the anionic reactant (inhibition) or with the transition state (catalysis). The introduction to the such reaction systems cation complexing macrocyclic ligands affects the reactivity both by anion activation and cation deactivation, however, the latter phenomenon strongly depends on ligand topology. Whereas cryptation with macrobicyclic ligands (cryptands) practically excludes cation participation, producing very reactive "naked anions", weaker ligands of crown ether type enable so-called "electrophilic catalysis" or cation assistance by the involvement of the complexed cation (LigÉM+) into a transition state stabilization by its electrostatic interactions with the developing negative charge at the leaving group. A particular attention is given to the systems where the metal ion complexation by a functional side-armed ligand (lariat ether) is a major factor in controlling the reactivity of the ligand itself - both from the point of view of the kinetics and the regiochemistry of the occurring processes. It is emphasized that reactive crowns play part of both substrates and (co)catalysts in the nucleophilic reactions catalysed by the alkali or alkali earth metal cations, and that the catalytic function of crown macrocycles can be greatly enhanced with the introduction of suitable side-arms, capable to cooperate with the polyether backbone in the transition state stabilization.


Supramolecular Chemistry, 1997, 91.
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SOME ASPECTS OF SYNTHESIS OF MULTICYCLIC CALIXARENES

Gabriel Rokicki, Wojciech Wąsikiewicz, Jędrzej Kiełkiewicz

Wydział Chemiczny, Politechnika Warszawska, ul. Noakowskiego 3, 00-664 Warszawa


The review demonstrates the possibility of synthesis of calixarenes - macrocyclic compounds build up by phenolic units linked via methylene bridges. Many calixarenes with different number (4, 6 or 8) of phenol units can be easily prepared with very good yield starting from simple materials (p-tert-butylphenol and formaldehyde) [1]. The possible methods of synthesis of calixarenes, like one-pot synthesis, stepwise synthesis [8, 10] and fragment condensations [12] are presented. There are numerous compounds accessible by substitution at para positions (in some cases after prior elimination of the p-tert-butyl groups) and by reactions at phenolic OH groups [1]. Those macrocyclic compounds are excellent starting materials for the synthesis of various types of host molecules and may be used as building blocks for the construction of larger molecular systems with defined structure.

It is shown that when para-linked phenols, like a,w-bis(4-hydroxyphenyl)alkane are used in the fragment condensation method it is possible to obtain calixarenes in which two opposite p-positions are bridged by an aliphatic chain [17] (Fig. 9). Bridged calixarene with CH2CH2COCH2CH2 chain can be transformed into the bicyclocalixarene [18].

By fragment condensation it is also possible to prepare rigid structures like anneleated calixarenes [15]. Examples of such new macrobicyclic molecules with calixarene substructures are also presented (Fig. 8).

In the review double calixarenes - molecules containing two calixarene subunits combined together are more detaily described. The possibility of synthesis of double calixarenes in head-to-head [19, 20], tail-to-tail [21, 22] and head-to tail arrangements [23] are discussed. In the last case both calixarenes have the same orientation what may be important for the potential application in non-linear optics. The synthesis of such compounds consisting of two calixarene substructure connected by aliphatic chains of various length between two opposite p-positions and two distal phenolic oxygens with reasonable yields is described [26] (Fig. 11 and 12).

The alternative strategies of synthesis leading to double calixarenes are also discussed.


Supramolecular Chemistry, 1997, 119.
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THE CALIXARENE SYNTHESIS FROM POLYHYDROXYPHENOLS AND THEIR DERIVATIVES

Waldemar Iwanek

Wyższa Szkoła Pedagogiczna w Kielcach, Instytut Chemii, Chęcińska 5, 25-020 Kielce


A new problem emerged on the basis of the biological system analysis; it consists in construction and synthesis of organic compounds capable for imitation of some properties of biological systems. Among the molecules possesing the suitable properties, there is a class of polycyclic macromolecules - calixresorcarenes. They have the cavities, whose geometries depend on the molecular structure, and the walls of these cavities contain active sites, which serve for the substrate binding. Availability of many active sites makes such a molecule a suitable platform for the synthesis of many new types of calixresorcarenes.

The synthesis of compounds of type 1, derived from resorcinol and aldehydes, has already been described very early [1-3], but the structure of such a compound was corfirmed by Erdtman et al. in 1968 [5] by crystallographic analysis. The non-planar structure of calixresorcarenes makes it possible to them to exist in several different conformations (Fig. 2, 3). Calixresorcarenes can be prepared in high yield in a one-step procedure with neither template nor high-dilution effects. In most cases, the mineral acids are applied as the catalysts for cyclocondensation of polyhydroxyphenols (i.e. pyrogallol or resorcinol) or the derivatives thereof with aldehydes [9-25]. However, the preparations of these macrocycles making use of the Lewis acids [26, 27] or bases [28] as catalysts are known also. The effective method of controlling the spatial structure and physico-chemical properties of calixresorcarenes consists in: modification of the aldehyde type used, introduction of the substituent ortho to the hydroxy groups, and functionalization of the -OH groups. The electrophilic substituents such as bromo or diazo group can be readily introduced at the ortho position [10, 29, 30]. However, the most frequently employed electrophilic substitution in calixresorcarenes is the Mannich reaction. Depending on the type and amounts of the reactants, one can obtain the aminomethyl [31], oxazine [32], or oxazolidine derivatuves [33] (Fig. 14). In turn, the hydroxy groups of calixresorcarenes can be functionalized in several ways (Fig. 18), by:

  1. Modification of all -OH groups, e.g. by formation of the ester [35], ether [36, 37], alcohol [36, 37], silicon [38], phosphorus [39], and bipyridyl [40] derivatives;
  2. Selective modification of four -OH groups by formation of the acetyl derivatives [41];
  3. Covalent linking of the hydroxy groups of the neighbring phenyl rings with the bridges containing the appropriate number of atoms (Fig. 21).

The last of the above approaches leads to the synthesis of cavitands [29, 43-52], which are in turn employed in the synthesis of the double calixresorcarenes, named carcerande and carcaplexes (Fig. 22).

The presence of eight hydroxy groups in the calixresorcarene molecule enables it to complex the organic molecules containing polar substituents. The possibilities of comlexing are not limited to the neutral molecules [58-61], but they include also metal and ammonium cations. Many of these complexea can be obtained in high yield as solids.


Supramolecular Chemistry, 1997, 143.
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SYNTHESIS OF C-FUNCTIONALISED AZAMACROCYCLIC COMPOUNDS

Bohdan Korybut-Daszkiewicz

Instytut Chemii Organicznej PAN, 01-224 Warszawa, ul. Kasprzaka 44/52


The synthesis and properties of polyaza macrocycles bearing pendant arms have attracted considerable attention in recent years for various reasons [2-4]. Compounds of this class combine properties of a rigid macrocyclic unit with those of a flexible side chain to which the functional group is bound. The complexes of N-functionalised tetraaza macrocycles, propably due to the relative simplicity of their synthesis, have been studied more extensively than their C-functionalised analogues. However, tertiary amino groups, for steric reasons, are poorer donors than secondary in the parent macrocycles. This disadvantage can be avoided by a synthesis of C-functionalised ligands.

Several examples of synthetic strategies that lead to C-substituted azamacrocycles are described below. The main methods are as follows: (a) incorporation of the pendant group as part of the synthesis of macrocycle, which may be a metal directed ( Schemes 1, 2, 8, 9), [18, 19, 27, 28] or an organic condensation (Schemes 5-7), [24-26]; (b) introduction of a pendant group into already formed macrocyclic ring, usually proceeding with metal ion coordinated ligands (Schemes 4,11,12), [21-23,40, 44]; (c) transformation of substituents already incorporated in macrocycle structure (Schemes 3, 14), [20, 52].

Functionalised sarcophagine (hexaazabimacrocyclic cage) derivatives are usually obtained by template condensation of [Co(en)3]3+ or [Co(sen)]3+ cations with formaldehyde and nitromethane or another appropriate C-acid (Schemes 15, 18), [53, 61, 62]. Reactivity of the capping reagent may be enhanced by coordination of transition metal ion (Scheme 19), [63]. Several organic functional groups have been attached to sarcophagine moiety by transformation of easily accessible mono- and dinitr-substituted cages (Schemes 15-17), [9].


Supramolecular Chemistry, 1997, 171.
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SELECTED TOPICS IN SUPRAMOLECULAR SELF-ASSEMBLY

Marek Pietraszkiewicz

Instytut Chemii Fizycznej Polskiej Akademii Nauk, ul. Kasprzaka 44/52, 01-224 Warszawa


Supramolecular self-assembly can be perceived as an extended version of molecular recognition. The processes of self-assembly are governed by the same non-covalent interactions as in molecular recognition, but they are not limited to molecular receptors capable of binding small molecules. The scope of self-assembly is practically unlimited. This short review will emphasize the following interactions, leading to self-assembled structures: ionic/coordinative forces, charge-transfer and p-donor-p-acceptor interactions, and hydrogen bonding. This contribution begins with a classification of self-assembly systems [1], and several examples of a high-yield of synthetic procedures based on simple substrates that led to elaborated structures in an one-step reaction [3-10], (Fig.1-6). Noncovalent self-assembly is based on various above-mentioned molecular interactions, and their superpositions. In this respect the formation of polymetallic arrays [11,12, 17] (Fig 7, 8, 9), helicates [20] (Fig. 10), grids [27] (Fig. 12) has been described. Particularly efficient self-assembly involving barbituric acid derivatives and pyrimidine has been described by Lehn [33] (Fig. 15), Hamilton and Whitesides have used a similar systems in the formation of hydrogen-bonded networks [37, 43-46) (Fig. 16, 19). Nanostructured materials based on cyclic polypeptides have been developed by Ghadiri [41] (Fig. 18). Less-directed CT, Van der Waals and (-stacking interactions have been used in the formation of organic networks by Stoddart [49, 8] (Fig. 22). Small inorganic "bricks" - molybdates, vanadates and tungstates are capable of forming very elaborated inorganic networks of great interest in catalysis, photochromism, inorganic drugs, NLO advanced materials, etc. Several exaples have been given [54, 59] (Fig. 23,24). Inorganic hybrid materials involving transition metal ions and organic ligands can form infinite 1D, 2D and 3D networks with regular structures [59-66] (Fig. 25-27). Another type of self-assembly based on p-stacking interaction initiated by sodium ions led to columnar mesophases [67] (Fig. 28).


Supramolecular Chemistry, 1997, 189.
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NOMENCLATURE OF SOME MACROCYCLIC COMPOUNDS

Grzegorz Schroeder, Bogusława Łęska

Wydział Chemii Uniwersytetu im. A. Mickiewicza, ul. Grunwaldzka 6, 60-780 Poznań


In 1987 Charles Pedersen, Donald Cram and Jean-Marie Lehn were awarded the Nobel prize in chemistry for syntheses, studies on properties and application of macrocyclic compunds. Among other things Nobe prize winners explained the mechanism of selective formation of: host-guest complex, sandwich complex, cage complex, and cryptato-cavitate clathrate complex [1-6]. The development of knowledge relating to macrocyclic compounds has generated issue of many papers and monographs pertaining to syntheses, properties and application of these compounds [7-10]. IUPAC nomenclature of supramolecular compounds gives some troubles because of its complication. In this paper we would like to present nomenclature of some macrocyclic compounds: coronand - any medium-sized or macrocyclic system having only one ring and containing any heteroatoms; crown ethers - coronands containing only oxygen heteroatoms in the ring; cryptands - bi- or polycyclic compounds containing any heteroatoms; podants - open-chained analogs of either coronands or cryptands [11-17]; lariat crown ethers - compounds with single macrorings-like crowns [19-27]; catenanes - compounds formed by interlocked ring [28-36]; knots - compounds made of a single-knotted closed ring [28-36]; cyclodextrins - large ring molecules consisting of a minimum of six glucopyranose units [37-45]; calixarenes - cyclic oligomers consisting of a minimum of four aryl groups [47] and its complexes [11-15,35, 47-49].


Supramolecular Chemistry, 1997, 227.
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POLYAZA AND POLYOXAAZA MACROCYCLIC COMPLEXES OF SCANDIUM GROUP METAL IONS AND LANTHANIDES

Wanda Radecka-Paryzek

Wydział Chemii, Uniwersytet im. A. Mickiewicza, ul. Grunwaldzka 6, 60-780 Poznań


Our major research effort in recent years has been concerned with the design and template synthesis of new polyaza and polyoxaaza macrocyclic complexes of scandium and rare earth metal ions. These complexes are currently attracting considerable attention since they can be used as contrast agents in magnetic resonance imaging, potential radiopharmaceuticals, as possible bioinorganic models for the active sites in metallobiomolecules and as synthetic nucleases for in vivo application. This lecture will focus on the effectiveness of metal ions of varying radii as templates in the synthesis of macrocyclic compounds with different ring size and on factors which prove to be of importance in directing the synthetic pathway in these systems.


Supramolecular Chemistry, 1997, 251.
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