Targeted proton delivery in the catalyzed reduction of oxygen to water by bimetallic pacman porphyrins.
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.
Chang
Christopher J
CJ
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
Loh
Zhi-Heng
ZH
Shi
Chunnian
C
Anson
Fred C
FC
Nocera
Daniel G
DG
D002384:Catalysis; D003035:Cobalt / Q000737:chemistry; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D005290:Ferric Compounds / Q000138:chemical synthesis / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011166:Porphyrins / Q000138:chemical synthesis / Q000737:chemistry*; D011522:Protons; D014867:Water / Q000737:chemistry*
D005290:Ferric Compounds; D009942:Organometallic Compounds; D011166:Porphyrins; D011522:Protons; D014867:Water; D003035:Cobalt; D010100:Oxygen