Macrocycles metal complexation ability

The presence of the cyclic backbone in ligands of this type makes a substantial contribution to their metal-ion complexing ability even though coordination involves donors which are not directly incorporated in the ring fragment. The origins of the enhanced stability of the metal-containing species may be considered to reflect the operation of an indirect macrocyclic effect (see Chapter 6) in these systems. 

ATP4-.242 Results indicated associations to be influenced by macrocyclic and size effects, electrostatic interactions and structural complementarity. Of particular interest in this series is the observed very strong binding of anionic metal ion complexes. The electrochemistry of M(CN)6 /M(CN)4 (M = Fe, Ru) indicates the complexed complex to be nonlabile and the system to undergo a reversible one-electron exchange. The ability of the macrocycle to protect the reactivity of the complex is evidenced by the hindered photoaquation observed for Co(CN)j e [32]NgH8.247 The results of competition studies suggest predominantly coulombic interactions between the metal complex and macrocycle for these 1 1 complexes of complexes . 

HPNPP (101) was also used to probe the catalytic ability of zinc- and copper-containing calix[4]arenes that carried two or three [12]ane-N3 macrocycles on their upper rim. Cooperativity was found between the catalytically active metal complexes during phosphodiester transesterification provided that they were adjacent to each other, i.e. on proximal positions of the calixarene rim, whereas those on opposite... 

The ability to bind to DNA was studied by ethidium bromide displacement titrations. The phosphodiester backbone of DNA was fully stable in the presence of the coordination compounds. The apparent binding constants were dependent on the charge of the macrocycles. Apparent binding constants of the metal complexes were even stronger and co-operative effect could be observed for the dinuclear ligands. 

Thus, it is the polyazamacrocyclic ligands developed by Lehn which have proved to be the most successful and versatile cryptands, rather than those carbon-bridgehead cryptands sought initially by Stoddart (25-27) which define a spheroidal cavity (e.g., 6). On the other hand, the bridged macrocyclic polyethers developed by Parsons (28), such as 7, show high complexing ability with alkali metal cations (28-30)... 

Crown ethers1 are perhaps the most widely used family of host compounds in supramolecular chemistry2—the chemistry of the non-covalent bond. The fortuitous discovery3 of the macrocyclic polyethers by Pedersen in 1967 laid the foundation for an exhaustive study of their preparation and complexing abilities, primarily with metal cations4 but also with neutral and even anionic species.5 Moreover, the metal ions which are complexed by crown ethers can also be utilised as templates6 for their formation. 

Tanaka and co-workers have also prepared a variety of mixed donor macrocycles, albeit by more traditional methods <2001JOC7008>. They surveyed the complexation ability of the macrocycles obtained with a range of alkali and transition metal cations. 

The podand 52a as well as the macrocycle 54 show the ability to complex sodium ions with their methoxy functions. The macrocycle exceeds the open-chained compound with regard to the complexation constant by a factor of six. While 52a complexes only Na , 54 shows increasing complex constants for K and Cs with a distinct selKtivity for the larger cesium ion. The order of magnitude of the constants of the alkali-metal complexes corr ponds to those of the crown ethers [51]. 

The binding of cyclic thioethers to metal centers has also led to the isolation of complexes in which the coordinative properties of the ligand do not lit the stereochemical preferences of the metal ion(s) (188), Thus, a series of macrocyclic thioether complexes incorporating unusual stereochemistries and/or oxidation states has been generated (188). This is linked to the biological activity of the blue copper proteins and model systems in which the coordination geometry about Cu(II) is strained [in an entatic state (.212,221)] such that the Cu(II)/(I) couple occurs at a particularly positive potential that is, the Cud) state is stabilized. The ability of cyclic thioethers to modify their coordination properties is inherent in this approach (76,108,111). 

The ability of [22]porphyrin-(3.1.3.1) 4.68a to form metal complexes was investigated in the context of the original LeGoff-Berger report. Specifically, these workers found, based on spectral evidence, that this macrocycle did form complexes with Ni(II) and Cu(II). However, these complexes proved to be not only highly insoluble but also rather unstable. Indeed, rapid demetalation occurred on treatment with weak acid. Perhaps as a consequence of these initial findings, no further metala-tion-related reports have appeared involving this molecule. 

Ag(III) corroles, synthesized by the treatment of free-base corroles with three equivalents of Ag(I) acetate [68], exemplify the ability of these macrocycles to promote the formation of unusual, high-valent metal complexes. As Bruckner pointed out in a 2004 educational commentary, Ag(II) porphyrins [195] are themselves representative of relatively high oxidation state Ag [196], Ag(I) salts being far more common. The evidence for the Ag(III) oxidation state is quite strong the Ag corroles reported by Bruckner are diamagnetic and XPS comparison of Ag(tpc) to the Ag(II) and Ag(III) complexes of tpp shows that the Ag 3d3/2 and 3d5/2 binding energies of the Ag correlate closely resemble those of the Ag(III) porphyrinate. 

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