# Electron-Donating para-Substituent (X) Enhances the Water Oxidation Activity of the Catalyst Ru(4′-X-terpyridine)(phenanthroline-SO3)+

## TL;DR

Adding an electron-donating group to a ruthenium catalyst improves its water oxidation performance significantly.

## Contribution

Electron-donating substituents enhance water oxidation activity through altered redox potentials and reaction pathways.

## Key findings

- Electron-donating substituent OEt (1b) increases water oxidation rate 30-fold compared to the parent catalyst.
- Computations reveal differences in PCET steps and increased oxide relay pathway probability with substituent variation.
- Water oxidation by 1b is three times faster than 1a under electrocatalytic conditions.

## Abstract

Recently, our group has developed a Ru-based water oxidation
catalyst
(WOC) with pendant sulfonate (1, Ru(4′-X-terpyridine)(phenanthroline-SO3)OTf (X = H, 1a) that shows high activity under
both sacrificial oxidant (CAN, Ce(NH4)2(NO3)6, CeIV) and electrocatalytic conditions,
in both acidic and neutral media. Here, we demonstrate that the functionalization
of the 4′-X-terpyridine ligand with an electron-donating substituent
X = OEt (1b) makes potentials of RuII/RuIII redox catalysis more negative, whereas when X = NO2(1c) and CF3(1d), potentials
are more positive. For 1b, full conversion of the sacrificial
oxidant CeIV occurred in 0.4 h (7 h for 1a), with an initial rate of 2.07 μmol O2 s–1 and a turnover frequency of 7.6 s–1, which is
30-fold faster than that for 1a at [cat]0 =
20 μM. Under electrocatalytic conditions, water oxidation by 1b is three times faster than that by the parent catalyst 1a at close to the same potential. Extensive computations
have identified differences in the initial PCET steps of the water
oxidation by catalysts 1a, 1b, and 1d, and
demonstrated the increased probability of the O2 formation
via the oxide relay pathway in the order 1b< 1a < 1d.

The developed
Ru-based WOC with pendant sulfonate Ru(4′-X-terpyridine)(phenanthroline-SO3)OTf shows high activity under both sacrificial oxidant and
electrocatalytic conditions in both acidic and neutral media. The
functionalization of 4′-X-terpyridine ligand with an electron-donating
substituent X = OEt makes the water oxidation by this system several
times faster than that by the parent catalyst. Computations show that
two sequential ETPT’s, in the presence of CAN, transform RuII-aqua complexes into the associated RuIII-oxyl
and RuV-peroxo complexes.