Ru(II) water oxidation catalysts with 2,3-bis(2-pyridyl)pyrazine and tris(pyrazolyl)methane ligands: assembly of photo-active and catalytically active subunits in a dinuclear structure

Two mononuclear Ru(II) complexes, i.e. [RuCl(kappa N-3-terpy)(kappa N-2-dpp)]PF6 ([1]PF6; terpy = 2,2':6',2''-terpyridine; dpp = 2,3-bis(2'-pyridyl-pyrazine) and [RuCl(kappa N-3-tpm)(kappa N-2-dpp)]Cl ([2]Cl; tpm = tris(1-pyrazolyl)methane), and one dinuclear complex, i.e. [Ru2Cl(kappa N-3-tpm)(mu-kappa N-2:kappa N-2-dpp)Ru(kappa N-2-bpy)(2)][PF6](3) ([3][PF6](3); bpy = 2,2'-bipyridine), have been synthesized and their water oxidation catalytic properties have been investigated. A combined DFT and experimental (Cl-35 NMR and conductivity measurements) study aimed to elucidate the nature of [1](+) and [2](+) in aqueous solution has also been performed, indicating that one water molecule is allowed to enter the first coordination sphere of [2](+) in the ground state, replacing one tpm nitrogen. Conversely, in the case of [1](+), water coordination, assumed to be needed for the water oxidation process, presumably occurs following the oxidation of the metal. For all 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 the formation of an Ru(IV)=O moiety, followed by one electron oxidation and water nucleophilic attack. The TON and TOF values (within the range of 16-33 and 1.3-2.2 h(-1), respectively) of the complexes are higher than those of the benchmark [Ru(LLL)(LL)(OH2)](2+)-type species (LLL and LL are tridentate and bidentate polypyridine ligands, respectively), which is [Ru(terpy)(bpm)(OH2)](2+).