Macrocyclic ligands flexibility

Like their phosphorus(III) analogues, cycloarsoxanes (RAsO) undergo metal-mediated ring expansion to produce flexible macrocyclic ligands n = 5, 6, 8) that... 

The thermodynamic data of Eq. (11) for the various Ni(II) macrocyclic complexes are summarized in Table IV. Since the octahedral species should have longer Ni—N bond distances than the square-planar species, the complexes with flexible macrocyclic ligands have larger values. For example, the formation constants of octahedral species for... 

The size and shape of the binding cavity that they define and their rigidity or flexibility are determined by the nature of the structural subunits making up the branches of the graphical representations. Their overall shape and appearance has also led to the coining of a number of so-called trivial names that refer to a particular aspect of the structure. Proposals have been made to name and define different types of macrocyclic ligands and molecular receptors [2.12]. Without being ex-... 

Template reactions between malonaldehydes and diamines in the presence of copper(II), nickel(II) or cobalt(II) salts yield neutral macrocyclic complexes (equation 15).99-102 Both aliphatic102 and aromatic101 diamines can be used. In certain cases, non-macrocyclic intermediates can be isolated and subsequently converted into unsymmetrical macrocyclic complexes by reaction with a different diamine (Scheme ll).101 These methods are more versatile and more convenient than an earlier template reaction in which propynal replaces the malonaldehyde (equation 16).103 This latter method can also be used for the non-template synthesis of the macrocyclic ligand in relatively poor yield. A further variation on this reaction type allows the use of an enol ether (vinylogous ester), which provides more flexibility with respect to substituents (equation 17).104 The approach illustrated in equation (15), and Scheme 11 can be extended to include reactions of (3-diketones. The benzodiazepines, which result from reaction between 1,2-diaminobenzenes and (3-diketones, can also serve as precursors in the metal template reaction (Scheme 12).101 105 106 The macrocyclic complex product (46) in this sequence, being unsubstituted on the meso carbon atom, has been shown to undergo an electrochemical oxidative dimerization (equation 18).107... 

Interest in the size-match selectivity of macrocyclic ligands has grown in recent years. Size-match selectivity reflects the idea that a metal ion will form a stable complex with a macrocycle possessing a cavity size close to the size of the metal ion. Interest stems from promising applications of these systems for selective removal of toxic metal ions. However, it is well known that tetraaza macrocycles are too flexible to show size-match selectivity. >4 45 To improve this selectivity, Mali, Wade, and Hancock synthesized and characterized, via... 

M. G. B. Drew, S. Hollis, and P. C. Yates, / Chem. Soc., Dalton Trans., 1829 (1985). Studies in the Flexibility of Macrocycle Ligands. Calculation of Macrocycle Cavity Size by Force-Field Methods. Crystal and Molecular Structures of [CoLCl][CIO.,], and [CuLlfPFgJ jL = 2,13-Dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-l(18),i4,16-triene. ... 

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