Macrocycles redox properties

The consequences of polychlorination of porphyrins on redox properties of complexes has been investigated.1404 The highly chlorinated porphyrin 3-octachloro-/ /c.vo-tetrakis(3,5-dichloro-2,6-dimethoxyphenyl)porphyrin exhibits a substantial anodic shift for reduction of over 0.5 V and a smaller shift for oxidation versus the unchlorinated precursor. Contrastingly, small potential shifts for the octabromo-substituted 5,10,15,20-tetraphenylporphyrinate arise from the dominance of macrocycle ruffling over electronic effects. In the polychloro complex, distortion does not compensate fully for electron-withdrawing effects of the Cl substituents. 

The characterization, redox properties, and pulse radiolysis study of manganese(III) complexes of type [MnLCy (where L = cyclam, meso-, and rac-5,7,7,12,14,14-hexamethylcyclam (tet a and tet b, respectively)) have been reported." An X-ray crystal structure of the meso-5,l,l, 2, A, A-hexamethyl-l,4,8,ll-tetraazacyclotetradecane complex shows that the coordination geometry of the tet a complex is close to octahedral with the macrocycle coordinated equatorially and the chlorides occupying irons axial sites. 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

An extensive study of the redox properties of tetraaza macrocyclic complexes of nickel has been performed by Busch and co-workers.3056,3133,3134 Electrochemical data for selected Nim/Nin couples are reported in Table 117. From an analysis of the EPR spectra it has been found that acetonitrile, as well as other molecules or ions like Cl and S04, can coordinate in axial position to give six-coordinate complexes.3056,3141 The g values are indicative of a dj... 

The redox properties of the copper(II) ion in saturated polyaza macrocyclic complexes CuN4 and CuNj have been shown1020 to vary with the chelate ring size, and the very negative E° values (Figure 62) are consistent with a macrocyclic effect , from stability constant measurements.976... 

The co-ordination chemistry of macrocyclic ligands containing imine or hydrazone groups has been widely studied and, as expected, the presence of the imine functionality in the ring confers unusual redox properties to the complexes. 

Levason, W., Quirk, J. J., Reid, G., Smith, S. M., Synthesis, spectroscopic and redox properties of ruthenium complexes with selenoether macrocycles crystal structures of cis-[RuCl2([16]aneSe4)] and trans-[RuCl(PPh3)([16]aneSe4)]PF6 ([16]aneSe4 = 1,5,9,13-tetraselenacyclohexadecane). J. Chem. Soc., Dalton Trans. 1997, 3719-3724. 

Aprotic solvents mimic the hydrophobic protein interior. However, a functional artificial receptor for flavin binding under physiological conditions must be able to interact with the guest even in competitive solvents. As found by spectroscopic measurements with phenothiazene-labeled cyclene, the coordinative bond between flavin and Lewis-acidic macrocyclic zinc in methanol was strong enough for this function. Stiochiometry of the complex was proved by Job s plot analysis. Redox properties of the assemblies in methanol were studied by cyclic voltammetry which showed that the binding motif allowed interception of the ECE reduction mechanism and stabilisation of a flavosemiquinone radical anion in a polar solvent. As a consequence, the flavin chromophore switched from a two-electron-one-step process to a two-step-one-electron-each by coordination. 

The preparation and characterization of novel man-ganese(III) complexes of various porphyrin and porphyrin-likes macrocycles have continued to attract strong attention especially because of their importance in catalytical oxidation processes through the formation of a Mn(V)0 intermediate (see Section 6) and as model for metalloenzymes. In this line, an artificial enzyme formed through a directed assembly of a molecular square that encapsulated a Mn porphyrin has been prepared and investigated as a catalyst. In contrast to symmetrical binuclear bis(phenoxo) bridged macrocyclic Mn(III)Mn(III) complexes, unsymmetrical ones are rare. A new series of these kinds of carboxylate-free complexes has been described and their redox properties investigated. ... 

The insertion into porphyrins of central metals capable of easy oxidation or reduction can shift the site of redox chemistry from the macrocycle n-electron system to the metal. While this substitution allows large changes in the redox properties of a porphyrin, it may introduce changes in the photophysics if a transition metal is employed. For example, iron (III) porphyrins are particularly good electron acceptor moieties, and have been used as components of porphyrin dyad and more complex systems that show photoinduced electron transfer behavior [11, 18, 19, 26, 31, 32, 34, 43, 44]. McLendon and coworkers used this strategy with dyad 3, in... 

Macrocyclic polythioethers form coordination compounds with many transition metal ions [55] and, owing to their moderate rr-acidity (intermediate between that of amines and phosphines), can exert a stabilizing effect on lower oxidation states of the encircled metal [56]. Copper complexes of thiacrowns have been widely investigated, even in view of the fact that they can be considered convenient models in the study of redox properties of cuproproteins (systems whose active site is a copper center bound to the thioether groups of methionine residues [57]). 

Two or more macrocyclic entities can be linked in several different ways, as shown by (144) and (145).241 Linked macrocyclic systems can bind simultaneously two or more metal ions, giving rise to complexes with unusual electronic, catalytic, and/or redox properties.242 Macrocycles incorporating phenanthroline moieties (146) are useful sensing agents for ion-selective electrodes and fluorimetry, and suitable carriers for selective ion transport in liquid-membrane systems.243,244... 

Perhaps the most significant information regarding the redox properties ofpyrolyzed macrocycles has been obtained from in situ Fe K-edge XANES measurements for... 

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