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Oxidation catalysts stable in aqueous solution under both harsh pH's and at high temperature would be environmentally friendly alternatives to current technologies. Transition metal complexes of tetraazamacrocycles reinforced with additional ethylene bridges have produced such oxidation catalysts. A controlling aspect of the usefulness of any metal catalyst is its set of oxidation and reduction potentials. Reversible redox processes that bracket a potential window within which useful oxidation of substrate molecules can occur are desirable. Though quite robust, and exhibiting reversible electrochemistry, some reinforced macrocycle complexes are not useful catalysts because their redox potentials are not in a desired potential range. An established method of modifying the electrochemical properties of a transition metal complex is to modify the ligand, which subsequently modifies the properties of its complexed metal ion. We wished to determine if the addition of pyridine pendant arms to the known reinforced macrocycle ligands would result in beneficial shifts in the redox potentials of their transition metal complexes. The resulting ligands must allow at least one open coordination site on the bound metal ion for oxidant and/or substrate binding. We have synthesized and characterized both cross-bridged and side-bridged cyclen and cyclam tetraazamacrocycles with pyridine pendant arms. Cobalt, nickel, copper, and zinc complexes were made. The synthesis and characterization of the ligands and the synthesis and characterization of their complexes will be presented.


Source: 2014 Spring National Meeting of the American Chemical Society