Unlike the pyrrolic macrocycles, (phen 2N 2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. Electrochemical studies reveal that (phen 2N 2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. N 1s XPS and XAS signatures for (phen 2N 2)Fe are remarkably similar to those of Fe-N-C. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen 2N 2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N 4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells however, their active site structures remain poorly understood.
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