Fourteen-Membered Macrocyclic Fe Complexes Inspired by FeN₄-Center-Embedded Graphene for Oxygen Reduction Catalysis

Abstract

For the broad application of polymer electrolyte fuel cells (PEFCs), the development of nonprecious-metal (NPM) catalysts for oxygen reduction is extremely important. To date, many NPM catalysts have been synthesized by pyrolyzing Fe-, N-, and C-containing precursors; however, they suffer from low density and uncertain chemical structure of their active sites. This study reports a novel 14-membered macrocyclic Fe complex, which was inspired by FeN4 centers in pyrolyzed catalysts, unlike typical macrocyclic MN4 complexes that have 16-membered rings. This 14-membered macrocycle possesses strong Fe–N bonds with an average bond distance of 1.90 Å, as evidenced by single-crystal X-ray diffraction (XRD), which are markedly shorter than those in porphyrin (2.0 Å). This complex demonstrates high electrocatalytic activity for oxygen reduction in both acidic and basic media.

High Durability of a 14-Membered Hexaaza Macrocyclic Fe Complex for an Acidic Oxygen Reduction Reaction Revealed by In Situ XAS Analysis

“…Fe-14MR was synthesized according to a previous report. 17 Fe-14MR/C was prepared by impregnating an aqueous solution of Fe-14MR with carbon black (Ketjen EC600JD) at 85 °C. The amount of Fe in Fe-14MR/C was determined to be 0.6 wt % by electron probe microanalysis (EPMA).…”

“…In fact, a previous study demonstrated that Fe-14MR has a shorter Fe–N bond length (1.9 Å) than Fe porphyrin (2.0 Å). 17 Furthermore, Fe-14MR exhibits better ORR activity than FePc under both acidic and basic electrolyte conditions. 17 It is assumed that the superior catalytic activity of Fe-14MR is attributed to the strong coordination of the compact macrocyclic ligand.…”

“… 17 Furthermore, Fe-14MR exhibits better ORR activity than FePc under both acidic and basic electrolyte conditions. 17 It is assumed that the superior catalytic activity of Fe-14MR is attributed to the strong coordination of the compact macrocyclic ligand. One may expect that the compact ligand also has the advantage of stability, as it strongly holds the Fe center; however, the stability of Fe-14MR has not yet been studied.…”

“… 16 Therefore, one may consider that FeN 4 structures with compact moieties, more specifically, short Fe–N bonds, are desirable for new Fe–N–C catalysts. 17 , 18 In fact, our research group has recently developed an Fe complex with a 14-membered hexaaza macrocyclic ligand (Fe(II) 1,14:7,8-ditethenotetrapyrido-[2,1,6- de :2′,1′6′- gh :2″,1″,6″- kl :2‴,1‴,6‴- na ][1,3,5,8,10,12]hexaazacyclotetradecine, referred to as Fe-14MR), as shown in Figure 1 . 17 The number of N atoms in Fe-14MR is the same as that of FePc, but the numbers of all atoms surrounding the Fe centers are smaller than FePc.…”

“… 17 , 18 In fact, our research group has recently developed an Fe complex with a 14-membered hexaaza macrocyclic ligand (Fe(II) 1,14:7,8-ditethenotetrapyrido-[2,1,6- de :2′,1′6′- gh :2″,1″,6″- kl :2‴,1‴,6‴- na ][1,3,5,8,10,12]hexaazacyclotetradecine, referred to as Fe-14MR), as shown in Figure 1 . 17 The number of N atoms in Fe-14MR is the same as that of FePc, but the numbers of all atoms surrounding the Fe centers are smaller than FePc. In fact, a previous study demonstrated that Fe-14MR has a shorter Fe–N bond length (1.9 Å) than Fe porphyrin (2.0 Å).…”