Computational Scaling Relationships Predict Experimental Activity and Rate-Limiting Behavior in Homogeneous Water Oxidation

While
computational screening with first-principles density functional
theory (DFT) is essential for evaluating candidate catalysts, limitations
in accuracy typically prevent the prediction of experimentally relevant
activities. Exemplary of these challenges are homogeneous water oxidation
catalysts (WOCs) where differences in experimental conditions or small
changes in ligand structure can alter rate constants by over an order
of magnitude. Here, we compute mechanistically relevant electronic
and energetic properties for 19 mononuclear Ru transition-metal complexes
(TMCs) from three experimental water oxidation catalysis studies.
We discover that 15 of these TMCs have experimental activities that
correlate with a single property, the ionization potential of the
Ru­(II)–O2 catalytic intermediate. This scaling parameter
allows the quantitative understanding of activity trends and provides
insight into the rate-limiting behavior. We use this approach to rationalize
differences in activity with different experimental conditions, and
we qualitatively analyze the source of distinct behavior for different
electronic states in the other four catalysts. Comparison to closely
related single-atom catalysts and modified WOCs enables rationalization
of the source of rate enhancement in these WOCs.