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.