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 = H2O, 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-OH2 complexes in chemical water oxidation at pH 1.0 points to poor stability (ligand oxidation), with subsequent evolution of CO2 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-beta-methylstyrene is employed as the substrate, no cis/trans isomerization takes place, thus indicating the existence of a stereospecific process.