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-[RuII(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 pKa values were obtained:  pKa1(RuII) =
2.13, pKa2(RuII) = 6.88, and pKa3(RuII) = 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
pKa1(RuIII) = 0.01, pKa2(RuIII) = 2.78, and pKa3(RuIII) = 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 RuIVO species derived from 3a is
capable of electrocatalytically oxidizing benzyl alcohol with a second-order rate constant of kcat = 17.1 M-1 s-1.