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

J Am Chem Soc. 2004 Aug 18;126(32):10013-20. doi: 10.1021/ja049115j.

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Catalysis
  • Cobalt / chemistry
  • Crystallography, X-Ray
  • Electrochemistry
  • Ferric Compounds / chemical synthesis
  • Ferric Compounds / chemistry
  • Models, Molecular
  • Molecular Structure
  • Organometallic Compounds / chemical synthesis
  • Organometallic Compounds / chemistry
  • Oxidation-Reduction
  • Oxygen / chemistry*
  • Porphyrins / chemical synthesis
  • Porphyrins / chemistry*
  • Protons
  • Water / chemistry*

Substances

  • Ferric Compounds
  • Organometallic Compounds
  • Porphyrins
  • Protons
  • Water
  • Cobalt
  • Oxygen