A Ru(II) Bis-terpyridine-like Complex that Catalyzes Water Oxidation: The Influence of Steric Strain

The complexation of 2,9-dicarboxy-1,10-phenanthroline
(DPA) with
[Ru­(tpy)­Cl3] (tpy = 2,2′;6,2″-terpyridine)
provides a six-coordinate species in which one carboxyl group of DPA
is not bound to the Ru­(II) center. A more soluble tri-t-butyl tpy analogue is also prepared. Upon oxidation, neither species
shows evidence for intramolecular trapping of a seven-coordinate intermediate.
The role of the tpy ligand is revealed by the preparation of [Ru­(tpy)­(phenq)]2+ (phenq = 2-(quinol-8′-yl)-1,10-phenanthroline) that
behaves as an active water oxidation catalyst (TON = 334). This activity
is explained by the expanded coordination geometry of the phenq ligand
that can form a six-membered chelate ring that better accommodates
the linear arrangement of axial ligands required for optimal pentagonal
bipyramid geometry. When a 1,8-naphthyidine ring is substituted for
each of the two peripheral pyridine rings on tpy, increased crowding
in the vicinity of the metal center impedes acquisition of the prerequisite
reaction geometry.