Targeted Proton Delivery in the Catalyzed Reduction of Oxygen to Water by Bimetallic Pacman Porphyrins

Abstract

A combined experimental and theoretical investigation of the role of proton delivery in determining O2 reduction pathways catalyzed by cofacial bisporphyrins is presented. A homologous family of dicobalt(II) Pacman porphyrins anchored by xanthene [Co2(DPX) (1) and Co2(DPXM) (3)] and dibenzofuran [Co2(DPD) (2) and Co2(DPDM) (4)] have been synthesized, characterized, and evaluated as catalysts for the direct four-proton, four-electron reduction of O2 to H2O. Structural analysis of the intramolecular diiron(III) mu-oxo complex Fe2O(DPXM) (5) and electrochemical measurements of 1-4 establish that Pacman derivatives bearing an aryl group trans to the spacer possess structural flexibilities and redox properties similar to those of their parent counterparts; however, these trans-aryl catalysts exhibit markedly reduced selectivities for the direct reduction of O2 to H2O over the two-proton, two-electron pathway to H2O2. Density functional theory calculations reveal that trans-aryl substitution results in inefficient proton delivery to O2-bound catalysts compared to unsubstituted congeners. In particular, the HOMO of [Co2(DPXM)(O2)]+ disfavors proton transfer to the bound oxygen species, funneling the O-O activation pathway to single-electron chemistry and the production of H2O2, whereas the HOMO of [Co2(DPX)(O2)]+ directs protonation to the [Co2O2] core to facilitate subsequent multielectron O-O bond activation to generate two molecules of H2O. Our findings highlight the importance of controlling both proton and electron inventories for specific O-O bond activation and offer a unified model for O-O bond activation within the clefts of bimetallic porphyrins.

Role of Proton‐Coupled Electron Transfer in O—O Bond Activation

“…With the knowledge that O 2 reduction requires both proton and electron equivalents, the role of proton delivery in determining O 2 reduction pathways catalyzed by bimetallic systems was interrogated. 34 DFT calculations show an inversion in the nature of the HOMOs for the O 2 adducts of methoxyarylmodified and unmodified DPX and DPD Pacman porphyrins. Figure 6 shows the results for the oxygen adducts of the Co 2 (II,II) DPX/DPXM congeners.…”

“…The mixed-valence behavior of Co 2 (DPX) is displayed by Co 2 (DPXM) at comparable potentials (E°) +0.14 and +0.33 V vs Ag/AgCl). 34 Moreover, cyclic voltammograms of Co 2 (DPDM) in nitrobenzene ( Figure 3) give a single, reversible two-electron oxidative wave at a potential identical to that of the splayed derivative Co 2 -(DPD) (E°) +0.33 V vs Ag/AgCl).…”

“…Dicobalt(II) Pacman complexes of both DPX and DPD are selective catalysts for the direct reduction of O 2 to H 2 O over H 2 O 2 with 72% and 80% selectivities, respectively. 33,34 This selectivity is markedly attenuated by the installation of a methoxyaryl group onto the Pacman motif. Co 2 (DPXM) catalyzes the reduction of oxygen at a positive potential of +0.24 V (vs Ag/AgCl); however, only 52% of the reaction proceeds along the 4e -/4H + pathway to produce water, while DPDM catalyzes oxygen reduction at a potential of +0.25 V (vs Ag/AgCl) with 46% going directly to water.…”

“…Co 2 (DPXM) catalyzes the reduction of oxygen at a positive potential of +0.24 V (vs Ag/AgCl); however, only 52% of the reaction proceeds along the 4e -/4H + pathway to produce water, while DPDM catalyzes oxygen reduction at a potential of +0.25 V (vs Ag/AgCl) with 46% going directly to water. 34 Chemical Reduction. Oxygen reduction catalysis may be independently examined by chemical means using ferrocene as the chemical reductant and proton sources of varying acidities.…”

Promotion of oxygen reduction by a bio-inspired tethered iron phthalocyanine carbon nanotube-based catalyst

“…Alongside the pyrolysis strategy, state-of-the-art synthetic techniques have been used to modify the geometric and electronic structure of metal macrocycles and its derivatives to improve its performance toward ORR 16,28,29 . However, most of the synthetic strategies focus on four-coordinated systems in an effort to change the accommodating space for oxygen molecules between cofacial porphyrins in its dyads and little attention has been paid to tuning the coordination environment of the Fe ion 30,31 .…”