A new dinuclear Ru-Hbpp based water oxidation catalyst with a trans-disposition of the Ru-OH

The bis(2-pyridyl) ethylamine (bpea) ligand has been used as a starting material for the synthesis of dinuclear Ru complexes of general formula trans, fac-{[(RuX)-X-n(bpea)](2)(mu-bpp)}(m+) (for X = Cl, n = II, m = 1, trans-Ru-II-Cl, 1(+); for X = OH, n = III, m = 3, trans-Ru-III-OH, 2(3+)) where the 3,5-bis(2-pyridyl) pyrazolate anionic ligand (bpp) acts as bridging dinucleating ligand, the bpea ligand coordinates in a facial manner and the monodentate ligands X are situated in a trans fashion with regard to one another. These complexes have been characterized in solution by 1D and 2D NMR spectroscopy, UV-vis and electrochemical techniques and in the solid state by X-ray diffraction analysis. The reaction of 1(PF6) with Ag+ generates the corresponding solvated complex where the Cl ligand has been removed as insoluble AgCl, followed by the oxidation of Ru(II) to Ru(III) to generate the corresponding dinuclear complex trans-Ru-III-OH, 2(PF6)(3). The latter has been shown to catalytically oxidize water to molecular dioxygen using Ce(IV) as oxidant. Quantitative gas evolution as a function of time has been monitored on line by both manometry and mass spectroscopy (MS) techniques. Relative initial velocities of oxygen formation together with structural considerations rule out an intramolecular O-O bond formation pathway.