A Mechanistic Dichotomy in Two‐Electron Reduction of Dioxygen Catalyzed by N,N’‐Dimethylated Porphyrin Isomers

Abstract

Selective two‐electron reduction of dioxygen (O2) to hydrogen peroxide (H2O2) has been achieved by two saddle‐distorted N,N’‐dimethylated porphyrin isomers, an N21,N’22‐dimethylated porphyrin (anti‐Me2P) and an N21,N’23‐dimethylated porphyrin (syn‐Me2P) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved by anti‐Me2P with higher turnover number (TON=250 for 30 min) than that by syn‐Me2P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti‐Me2Iph or syn‐Me2Iph) by spectroscopic measurements. The rate‐determining step in the catalytic ORRs was concluded to be proton‐coupled electron‐transfer reduction of O2 with isophlorins based on kinetic analysis. The ORR rate by anti‐Me2Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn‐Me2Iph by external protons was observed. The different mechanisms in the O2 reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O2 with inner NH protons of the isophlorins.

Nonplanar porphyrins: synthesis, properties, and unique functionalities

“…On the other hand, H 2 253 was The dimethylated porphyrins H 2 252 and H 2 253 were employed as organocatalysts for a catalytic oxygen reduction reaction (ORR) to produce H 2 O 2 . 352 In the presence of Fc derivatives as electron donors, higher catalytic performance in ORR was achieved by N21,N22-dimethylated H 2 252 with higher turnover number (TON = 250 for 30 min) than that by N21,N23-dimethylated H 2 253 (TON = 218 for 60 min). Based on the kinetic analysis of catalytic ORR, a mechanistic dichotomy was proposed as shown in Fig.…”

“…Fig. 68 Proposed mechanisms of selective two-electron reduction of O 2 to form H 2 O 2 catalyzed by N,N 0 -dimethylated and diprotonated H 2 252 (a) and H 2 253 (b) 352. …”

Highly Efficient Catalytic Two-Electron Two-Proton Reduction of Dioxygen to Hydrogen Peroxide with a Cobalt Corrole Complex

“…Extensive efforts have so far been devoted on H 2 O 2 production by 2e – /2H + reduction of O 2 under chemical, electrochemical, and photochemical conditions. − Transition metal-based molecular catalysts, such as cobalt, iron, manganese, and copper complexes, have been widely used, as these are easy to handle for the investigation of reaction mechanisms, including the systematic structure–activity analysis and the characterization of catalytically reactive species. Such findings are of importance to design new generation of catalysts.…”

Shape-assisted self-assembly

“…Fivemembered rings, which would ordinarily foster bowl shapes, can be faced opposite to one another to devise 'non-classical' saddles 44 . In the absence of central polygons, this shape can also be achieved through distortions in normally planar architectures due to coordination or steric crowding [45][46][47][48] . Consideration of these design principles and the available synthetic methodologies convinced us that a contorted porphyrinoid previously reported by Müllen 44 could be tailored to become a versatile platform to explore shape-assisted linear stacking (Fig.…”