Metal ions coordinating capacity

Recently, highly branched macromolecular polyamidoamine dendrimers have been prepared with Co11 bound where the metal ions have additional exchangeable coordination sites.450 These macromolecules show a capacity for catalyzing the hydrolysis of phosphate esters, presumably via intermediate bound phosphoester species. 

The varying participation of the alcohol functions in metal coordination in these complexes is undoubtedly a reflection of both the moderate donor capacity of alcohol oxygens and the different coordination preferences of the respective metal ions involved. 

The capacity of cyclic ligands to stabilize less-common oxidation states of a coordinated metal ion has been well-documented. For example, both the high-spin and low-spin Ni(n) complexes of cyclam are oxidized more readily to Ni(m) species than are corresponding open-chain complexes. Chemical, electrochemical, pulse radiolysis and flash photolysis techniques have all been used to effect redox changes in particular complexes (Haines McAuley, 1982) however the major emphasis has been given to electrochemical studies. 

The UV/Vis, Mossbauer, EXAFS, and EPR spectroscopic data suggest a rather complicated picture regarding the speciation of oxidized TAML species derived from 1 and various oxidants in aqueous solution (Scheme 5). Peroxides ROOH have the capacity to function as two-electron oxidants and usually do. In cases where prior coordination occurs, they can oxidize metal ions via one-electron processes where the 0-0 bond is cleaved homo-lytically or two-electron processes where it is cleaved hetero-lytically. The two-electron oxidation of 1 presumably would give the iron-oxo intermediate 6, two electrons oxidized above the iron(III) state (see below). Before 6 was actually isolated, there... 

The joint action of the reagents is especially pronounced in those cases where the coordination capacity of the metal ion is not fully achieved in the MR chelate, then the extractant S gives a mixed complex,... 

---