Formation of CoIV=O intermediate at the Boundary of the “Oxo-wall” Induces Water Oxidation

Abstract

Development of economically viable artificial photosynthesis requires use of 3d metal-based catalysts. Water oxidation by [Co4O4]n+ cubane mimics water splitting by CaMn4O5 cluster in Nature but the exact mechanism of O-O bond formation is presently unknown. We demonstrate first in situ detection CoIV=O (~ 1.67 Å) moiety formed upon activation of [Co4O4Py4Ac4]0 (Py = pyridine and Ac = CH3COO−) towards O-O bond formation. Combined spectroscopic and DFT analyses show that the intermediate active in O-O bond formation has two CoIV centers and at least one CoIV=O unit of strong radicaloid character that participates in O-O bond formation via water nucleophilic attack. The multimetallic structure of the cubane provides unique stabilization for CoIV=O + H2O = Co-OOH + H+ transition with the carboxyl accepting the proton and the bridging oxygen stabilizing the peroxide via hydrogen bonding. Results are important for development of oxygen evolution catalysts based on Earth-abundant 3d elements.

Computational Analysis of Structure–Activity Relationships in Highly Active Homogeneous Ruthenium−Based Water Oxidation Catalysts

“…Craig and coworkers considered seventeen molecular OER catalysts based on a variety of transition metals (Ru, Ni, Mn, Co, Cu, and Fe); ruthenium acts at the metal center in nine of these seventeen catalysts. They found that unmodified use of conventional scaling relations for heterogeneous systems did not accurately predict the catalytic activity of a set of homogenous catalysts; typical OER−scaling relations addressing the OER for heterogenous systems neglects the ET reaction from Ru IV [72][73][74]. As such, that our work focuses more on high−valent redox and WNA reactions is appropriate given our exclusive focus on the Ru metal center.…”