Macrocyclic ligands electronic effects

Electronic effects on the reactions of [Rh(Por)h dimers and hydrides were probed by varying the porphyrin macrocycle. OEP and TPP vary considerably in their properties, with OEP being one of the strongest and TPP one of the weakest (7-donors among porphyrin derivatives. However. Rh(Por)]2, Rh(Por)H, and Rh(Por)r showed the same reactivity in a variety of reactions for both OEP and TPP, indicating that electronic effects relating to the porphyrin ligand have... 

Effect of Pressure on Proton-Coupled Electron Transfer Reactions of Seven-Coordinate Iron Complexes in Aqueous Solution It has been shown that seven-coordinate Fe(III) diaqua complexes consisting of a pentaaza macrocyclic ligand possess superoxide dismutase (SOD) activity, and therefore could serve an imitative SOD function.360 Choosing appropriate chemical composition of a chelate system yielded suitable pKa values for the two coordinated water molecules so that the Fe(III) complexes of 2,6-diacetylpyridine-bis(semicarbazone) (dapsox) and 2,6-diacetylpyridine-bis(semioxamazide) (dapsc) (see Scheme 7.12) would be present principally in the highly active aqua-hydroxo form in solution at physiological pH.361... 

The peculiarities of classical localized coordination bonds (two-electron and two-center [1,4,5]) are displayed most clearly in MCC. Mostly, the elements of the first period of the Periodic Table (C, N, O) participate as electron donors in the formation of such bonds. In complexes of this type, the role of Ji-dative interactions is significant. These interactions are revealed in coordination compounds of ligands containing the elements of the next periods as donor centers (P, As, Sb S, Se, Te Cl, Br, I). We note that the examined complexes are the most successful objects to study the influence of ligand and metal nature on the character of the coordination bond, since, in this case, the factors which could distort this influence (chelate, macrocyclic, and other effects [117,135]) are absent. 

As demonstrated in this chapter, the binding of metal ions to maclocyclic ligands (e.g., porphyrins) results in the change in both the thermodynamic and dynamic properties of ET reactions of metalloporphyrins. Excellent models of the photosynthetic reaction center were developed by the appropriate choice of combination of metal ions and macrocyclic ligands. The lifetimes of the CS states in models of photosynthetic reaction center composed of electron donors and acceptors also were controlled by binding of metal ions to radical anions of electron acceptor moieties in the electron donor-acceptor hnked molecules. The control of ET processes by coordination of metal ions to the dyads led us to develop a unique fluorescence sensor for the ion. The binding of metal ions to radical anions of electron acceptors results in acceleration of thermal ET reactions, which would otherwise be impossible to occur. Such effects of metal ions to enhance the ET... 

It has been found that in two component systems zinc (II) porphyrin-ligand coordination properties of the metalloporphyrins increase in the following order ZnTPhPelectron effects of peripheral substitutes. In three component systems zinc (II) porphyrin-ligand-benzene the mentioned above properties change in the following order ZnPP>ZnDP>ZnHP>ZnTPhP and are inversely proportional to an ability of the macrocycle to 7r-7i-interactions with benzene. Thus, the new approach to molecular recognition based on specific salvation 71-71-interactions is demonstrated on example of znc (II) porphyrins. 

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