Evidence for Oxidative Decay of a Ru-Bound Ligand during Catalyzed Water Oxidation

In
the evaluation of systems designed for catalytic water oxidation,
ceric ammonium nitrate (CAN) is often used as a sacrificial electron
acceptor. One of the sources of failure for such systems is oxidative
decay of the catalyst in the presence of the strong oxidant CAN (Eox = +1.71 V). Little progress has been made
in understanding the circumstances behind this decay. In this study
we show that a 2-(2′-hydroxphenyl) derivative (LH) of 1,10-phenanthroline
(phen) in the complex [Ru­(L)­(tpy)]+ (tpy = 2,2′;6′,2″-terpyridine)
can be oxidized by CAN to a 2-carboxy-phen while still bound to the
metal. This complex is, in fact, a very active water oxidation catalyst.
The incorporation of a methyl substituent on the phenol ring of LH
slows down the oxidative decay and consequently slows down the catalytic
oxidation. An analogous system based on bpy (2,2′-bipyridine)
instead of phen shows much lower activity under the same conditions.
Water molecule association to the Ru center of [Ru­(L)­(tpy)]+ and carboxylate donor dissociation were proposed to occur at the
trivalent state. The resulting [RuIII–OH2] was further oxidized to [RuIVO] via a PCET process.