Ru(II) Water Oxidation Catalysts with 2,3-bis(2-Pyridyl)Pyrazine and tris(Pyrazolyl)Methane Ligands: Assembly of Photo- and Catalytic-Active Subunits in a Dinuclear Structure
Two mononuclear Ru(II) complexes, i.e. [RuCl(κ3N-terpy)(κ2N-dpp)]PF6 (PF6; terpy = 2,2':6',2"-terpyridine; dpp = 2,3-bis(2'-pyridyl-pyrazine) and [RuCl(κ3N-tpm)(κ2N-dpp)]Cl (Cl; tpm = tris(1-pyrazolyl)methane), and one dinuclear complex, i.e. [Ru2Cl(κ3N-tpm)(κ2N:κ2N-dpp)Ru(κ2N-bpy)2][PF6]3 ([PF6]3; bpy = 2,2'-bipyridine), have been synthesized and their water oxidation catalytic properties have been investigated. A combined DFT and experimental (35Cl NMR, conductivity measurements) study aimed to elucidate the nature of + and + in aqueous solution has also been performed, indicating that one water molecule is allowed to enter the first coordination sphere of + at the ground state, replacing one tpm nitrogen. Conversely, in the case of +, water coordination, assumed to be needed for the water oxidation process, presumably occurs following oxidation of the metal. For all the complexes, a catalytic wave has been detected in acetonitrile/water 1:1 (v/v) solution in the range 1.4-1.7 V vs SCE. In all cases, water oxidation (investigated at pH < 8) takes place initially via a proton-coupled two-electron, two-proton process with formation of a Ru(IV)=O moiety, followed by one electron oxidation and water nucleophilic attack. The TON and TOF values (within the range 16-33 and 1.3-2.2 h-1, respectively) of the complexes are higher than those of the benchmark for [Ru(LLL)(LL)(OH2)]2+-type species (LLL and LL are tridentate and bidentate polypyridine ligands, respectively), that is [Ru(terpy)(bpm)(OH2)]2+.