Ligand interactions macrocyclic ligands

Complex formation between a metal ion and a macrocyclic ligand involves interaction between the ion, freed of its solvation shell, and dipoles inside the ligand cavity. The standard Gibbs energy for the formation of the complex, AGjv, is given by the difference between the standard Gibbs... 

Thermodynamic aspects of the interaction of metal ions with macrocyclic ligands have been well studied. In many instances such studies have involved a comparison of the behaviour of cyclic ligand systems with that of their open-chain analogues. In this manner, information concerning the thermodynamic consequences arising from the cyclic nature of the macrocyclic ligand has been obtained. Frequently these studies have been restricted to stability constant (log K) measurements and, for such studies, a variety of techniques has been employed (Izatt etal., 1985). 

Optical Properties. The double-decker complexes of porphyrazines have characteristic electronic absorption spectra (Table V). The intense Soret bands of the double-decker complexes are blue shifted with respect to the single pz ligand as a consequence of the strong n-n interactions. Another characteristic of sandwich compounds is the additional appearance of absorption bands shifted to the red (termed Q ) and to the blue (termed Q") of the normal g-band region. These new transitions are thought to result from orbitals delocalized over the two macrocyclic ligands (33, 82). 

Pentadentate macrocyclic ligands L13-L15 accommodate Ni(II) ion with simultaneous dissociation of the two amide protons to form the square-pyramidal high-spin complex of [NiH 2L]°, 27. These complexes are air-sensitive and show a very low Ni(II)/Ni(III) oxidation potential of +0.24 V vs SCE (72, 73). They form 1 1 dioxygen adducts, 27-02, at room temperature in aqueous solution, which are formulated as Nilu-02 based on the EPR spectral data. The magnetic moment of the 1 1 02 adduct is 2.83 BM, which is interpreted in terms of weak interactions of Ni(III) with the superoxide where the spin coupling is weak. The oxygen uptake reaction is first-order with respect to both [02] and [NiH 2L]° in aqueous solutions and yields a second-order rate constant... 

Receptor and substrate are terms describing the species involved in complex formation. Throughout the chapter the receptor will refer tp the macrocyclic ligand, the substrate to other interacting species. Substrates may be metal or molecular catibns, neutral molecules, or atomic or molecular anions. The terms receptor and substrate imply that the complex formed has the well-defined structural and chemical properties of a supermolecule, as in biological receptor-substrate associations. They exclude species formed only in the solid state (clathrates). They are also easily converted and understood in many languages. 

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