Macrocycles as ligands

ABSTRACT. The application of tetraimine Schiff base macrocycles as ligands in the modelling of di- and trinuclear copper(I) and copper(II) biosites and dinuclear manganese(II) biosites is discussed. 

A metal ion M as part of a polymer chain is surrounded by a multivalent ligand. As a result, stacked arrangements 107 are realized with a face-to-face orientation of the ligands. Porphyrins, phthalocyanines, naphthalocyanines, etioporphyrin, tetraaza[14]annulene, hemiporphyrazines, and related macrocycles as ligands have been most intensively investigated. Because several reviews summarize preparations and properties (conductivity, photoconductivity, NLO, electroluminescent detectors) of these materials, only a short overview on these polymers is given [1,3,34,40,375-377]. Several years ago, unsubstituted macrocycles were employed as ligands. These polymers are much less soluble. Recently, the use of substituted macrocycles has led to soluble polymers which are analytically easier to characterize. 

Poly(phenylenepyrazoles), 5, 300 Polypyrazoles, 5, 300 N-substituted, 5, 300 Polypyrazolines, 5, 300 Poly(pyrazol-l-yl) borates as ligands, 5, 225, 235 Polypyrroles applications, 4, 376 Polypyrrole tetrafluoroborate conductors, 1, 355 Polysaccharides as pharmaceuticals, 1, 152 Poly-2,5-selenienylenes applications, 4, 971 Polysilacyclopentanes, 1, 609 Polysufides macrocyclic... 

The new macrocyclic hexaamine ligand X has the mixed protonation constants, log K of 10.10, 10.01, 8.96 and 8.02 at 25 °C and 1=0.2 M for the four most basic amines. The values for other weaker bases, including the six carboxylates, are all less than 5. From a comparison with logA values of the parent macrocycle [18]aneN6, it was deduced that the initial four protonations occur to the macrocyclic amine bases. Thus, the most abundant species of X at neutral pH is depicted as XI. 

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

Based on these observations a selective receptor for lithium 117 has been synthesized from a smaller diazadioxa macrocycle [160]. Ligands 110 and 117 may be regarded also as binucleating ligands capable of binding two cations at a time. 

The following is a comprehensive smwey of the chemistry of macrocycles comprised entirely of phenyl and acetylenic moieties. Although over fom" decades old, this area of research has come into its own just in the last few years. Widespread interest in the field has been spurred by recent discoveries utilizing these compoimds as ligands for organometallic chemistry, hosts for binding guest molecules, models of synthetic carbon allotropes, and precursors to fullerenes and other carbon-rich materials. This review will discuss the preparation of a tremendous variety of novel structm-es and detail the development of versatile synthetic methods for macro cycle construction. 

Using 2,6-diformyl-4-methylphenol dioxime, Tasker, Schroder, and their co-workers conducted a magnetostructural study on complex (374) (pseudo macrocyclic structure Cu-Cu 2.994 A 2J= — 904 cm-1).330 Using 2-hydroxy-5-nitro-benzaldehyde benzoylhydrazone as ligand, complex (375) (Cu-Cu 3.041 A 2J=—372cm 1) was reported.331 Using 3-formylsalicylic acid oxime Okawa and co-workers reported complex (376) (square-planar and SP geometry Cu-Cu 2.961 A).332... 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

Bis(ethylenediamine)gold(III) chloride reacts with /3-diketonates in aqueous base via Schiff base condensation to form complexes of gold(III) with a 14-membered macrocyclic tetraaza ligand such as (292).1711-1713 The X-ray structure showed the cation to be nearly planar. Delocalization of -electrons within the six-membered /3-diketonate rings was indicated by the observed pattern of C—C and C—N distances. Open-chain tetraaza ligand complexes in which condensation of only one /3-diketonate has occurred, can be isolated as intermediates in this reaction. They may be used for further condensation with a different /3-diketonate. Oxidation of this complex with trityl tetrafluoroborate introduces a double bond in position C2C3.1712,1714... 

Stable Cu(i) complexes of tetraaza macrocycles are able to be generated in oxygen-free aprotic solvents (Palmer, Papaconstantinou Endicott, 1969 Olson Vasilevskis, 1971). In aqueous solvents there is a tendency for such species to decompose via loss of ligand (Freiberg, Meyerstein Yamamoto, 1982). Indeed, aqueous Cu(i) is unstable with respect to disproportionation to Cu(n) and elemental copper. However, extensive N- and C-alkylation of the macrocycle, as occurs in (291), slows... 

The sulfimide group R2S = NR is isoelectronic with R2S = O, and recent studies have revealed the first examples of macrocyclic derivatives from reaction of the thia crown with MSFI.155,156 These compounds are of interest as ligands in their own right, and also because they provide a means of functionalizing the thiacrown at the S atom and hence the possibility of introducing pendant groups. 

With these features in mind, we envisioned a new family of macrocyclic ligands for olefin polymerization catalysis (Fig. 9) [131, 132], We utilized macrocycles as the ligand framework and installed the catalytic metal center in the core of the macrocycles. Appropriate intra-annular binding sites are introduced into cyclophane framework that not only match the coordination geometry of a chosen metal but also provide the appropriate electronic donation to metal center. The cyclophane framework would provide a microenvironment to shield the catalytic center from all angles, but leaving two cis coordination sites open in the front one for monomer coordination and the other for the growing polymer chain. This could potentially protect the catalytic center and prevent it from decomposition or vulnerable side reactions. 

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