Mononuclear ruthenium compounds bearing N-donor and N-heterocyclic carbene ligands: structure and oxidative catalysis.

A new CNNC carbene-phthalazine tetradentate ligand has been synthesised, which in the reaction with [Ru(T)Cl ₃ ] (T = trpy, tpm, bpea; trpy = 2,2';6',2''-terpyridine; tpm = tris(pyrazol-1-yl)methane; bpea = N,N-bis(pyridin-2-ylmethyl)ethanamine) in MeOH or iPrOH undergoes a C-N bond scission due to the nucleophilic attack of a solvent molecule, with the subsequent formation of the mononuclear complexes cis-[Ru(PhthaPz-OR)(trpy)X] ⁿ⁺ , [Ru(PhthaPz-OMe)(tpm)X] ⁿ⁺ and trans,fac-[Ru(PhthaPz-OMe)(bpea)X] ⁿ⁺ (X = Cl, n = 1; X = H ₂ O, n = 2; PhthaPz-OR = 1-(4-alkoxyphthalazin-1-yl)-3-methyl-1H-imidazol-3-ium), named 1a ⁺ /2a ²⁺ (R = Me), 1b ⁺ /2b ²⁺ (R = iPr), 3 ⁺ /4 ²⁺ and 5 ⁺ /6 ²⁺ , respectively. Interestingly, regulation of the stability regions of different Ru oxidation states is obtained by different ligand combinations, going from 6 ²⁺ , where Ru(iii) is clearly stable and mono-electronic transfers are favoured, to 2a ²⁺ /2b ²⁺ , where Ru(iii) is almost unstable with regard to its disproportionation. The catalytic performance of the Ru-OH ₂ complexes in chemical water oxidation at pH 1.0 points to poor stability (ligand oxidation), with subsequent evolution of CO ₂ together with O ₂ , especially for 4 ²⁺ and 6 ²⁺ . In electrochemically driven water oxidation, the highest TOF values are obtained for 2a ²⁺ at pH 1.0. In alkene epoxidation, complexes favouring bi-electronic transfer processes show better performances and selectivities than those favouring mono-electronic transfers, while alkenes containing electron-donor groups show better performances than those bearing electron-withdrawing groups. Finally, when cis-β-methylstyrene is employed as the substrate, no cis/trans isomerization takes place, thus indicating the existence of a stereospecific process.