Synthesis, structure, and acid-base and redox properties of a family of new Ru(II) isomeric complexes containing the Trpy and the dinucleating Hbpp ligands

Three pairs of mononuclear geometrical isomers containing the ligand 3,5-bis(2-pyridyl)pyrazole (Hbpp) of general formula in- and out-[Ru-II(Hbpp)(trpy)X](n+) (trpy = 2,2':6',2-terpyridine; X = Cl, n = 1, 2a,b; X = H2O, n = 2, 3a,b; X = py (pyridine), n = 2, 4a,b) have been prepared through two different synthetic routes, isolated, and structurally characterized. The solid state structural characterization was performed by X-ray diffraction analysis of four complexes: 2a-4a and 4b. The structural characterization in solution was performed by means of 1D and 2D NMR spectroscopy for complexes 2a,b and 4a,b and coincides with the structures found in the solid state. All complexes were also spectroscopically characterized by UV-vis which also allowed us to carry out spectrophotometric acid-base titrations. Thus, a number of species were spectroscopically characterized with the same oxidation state but with a different degree of protonation. As an example, for 3a three pK(a) values were obtained: pK(a1)(Ru-II) = 2.13, pK(a2)(Ru-II) = 6.88, and pK(a3)(Ru-II) = 11.09. The redox properties were also studied, giving in all cases a number of electron transfers coupled to proton transfers. The pH dependency of the redox potentials allowed us to calculate the pKa of the complexes in the Ru(III) oxidation state. For complex 3a, these were found to be pK(a1)(Ru-II) = 0.01, pK(a2)(Ru-III) = 2.78, and pK(a3)(Ru-III) = 5.43. The oxidation state Ru(IV) was only reached from the Ru-OH2 type of complexes 3a or 3b. It has also been shown that the Ru-IV=O species derived from 3a is capable of electrocatalytically oxidizing benzyl alcohol with a second-order rate constant of k(cat) = 17.1 M-1 s(-1).