Predictable electronic tuning of Fe^II and Ru^II complexes via choice of azine: correlation of ligand pK_a with E_pa(M^III/II) of complex

Five new mononuclear ruthenium(ii) tris-ligated complexes have been synthesised, varying through the choice of azine in the family of 3-azinyl-4-(4-methylphenyl)-5-phenyl-4H-1,2,4-triazole ligands (Lazine): [Ru(Lpyridine)](PF6)(2) (1), [Ru(Lpyridazine)](PF6)(2) (2), [Ru(L4-pyrimidine)](PF6)(2) (3), [Ru(Lpyrazine)](PF6)(2) (4), [Ru(L2-pyrimidine)](PF6)(2) (5). Three of them, 12MeCNEt2O, 32MeCNEt2O and 42MeCN, have been structurally characterised, confirming the presence of the meridional isomer, as was previously reported for the Fe-II analogues. Cyclic voltammetry studies, in dry CH3CN vs. Ag/0.01 M AgNO3, show that all five Ru-II complexes undergo a reversible Ru-III/Ru-II process, with the midpoint potential (E-m) increasing from 0.87 to 1.18 V as the azine is changed: pyridine < pyridazine < 2-pyrimidine < 4-pyrimidine < pyrazine. A strong inverse linear correlation (R-2 = 0.98) is found between the Ru-III/Ru-II redox potential and the calculated HOMO orbital energies, which is consistent with the expectation that it is easier to oxidise (lower E-m) a metal ion with a higher HOMO orbital energy. The same trend was reported earlier for the family of analogous Fe-II complexes, albeit at lower values of E-m in all cases. In addition, the ionisation potentials of the Ru-II complexes, as well as those of the other group 8 analogues (Fe-II and Os-II), showed a linear relationship with E-pa. As the M-III/II redox potentials of a family of complexes has been previously reported to correlate with ligand pK(a) values, a computational protocol to calculate, in silico, the pK(a) of the Lazine family of ligands was developed. A strong linear relationship was found between the readily calculated pK(a) of the Lazine ligand and the E-pa of the M-II complex, for all three families of complexes (R-2 = 0.98).