Unsymmetrical Dinuclear RuII Complexes with Bridging Polydentate Nitrogen Ligands as Potential Water Oxidation Catalysts

Mononuclear RuII complex [RuCl(κ3N-tpm)(κ2N-bptz)]Cl, [1]Cl [tpm=tris(1- pyrazolyl)methane; bptz=3,6-di(2-pyridyl)-1,2,4,5-tetrazine], and dinuclear complexes [RuCl(κ3N-tpm)(μ-κ2N:κ2N-bptz)Ru(κ2N-bipy)2][PF6]3, [3][PF6]3, [RuCl(η6-p-cymene)(μ-κ2N:κ2N-dpp)Ru(κ2N-bipy)2][PF6]2, [4][PF6]2, and [RuCl(η6-p-cymene)(μ-κ2N:κ2N-dpp)Ru(κ2N-biqn)2][PF6]3, [5][PF6]3 [dpp=2,3-bis(2′-pyridyl)-pyrazine; bipy=2,2′-bipyridine; biqn=2,2′-quinoline], incorporating both potentially catalytic and photosensitive subunits, were synthesized and characterized by means of elemental analysis, mass spectrometry, and spectroscopic methods. The molecular structures of the new compounds were also investigated and compared by means of DFT calculations. The absorption spectra of all the compounds are dominated by metal-to-ligand charge-transfer bands in the visible (which in most cases largely extend over the red portion of the spectrum) and ligand-centered bands in the UV region. The oxidation behavior is based on metal-centered RuII to RuIII oxidation processes, which in phosphate buffer solution are followed by a catalytic water oxidation wave for [1]Cl and [3][PF6]3. For these compounds, the mechanism of water oxidation is proposed to consist in water nucleophilic attack, according to chemical experiments with Ce(IV) salts, so demonstrating for the first time that the bptz ligand can be profitably used to build ruthenium(II) complexes with catalytic properties. On the contrary, no catalytic process is observed for 4 and 5, most likely due to the high positive potential for RuII oxidation induced by the presence of the p-cymene moiety.