Predictable electronic tuning of FeII and RuII complexes via choice of azine: correlation of ligand pKa with Epa(MIII/II) of complex

Abstract

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 (L^azine): [Ru(L^pyridine)](PF₆)₂ (1), [Ru(L^pyridazine)](PF₆)₂ (2), [Ru(L^4-pyrimidine)](PF₆)₂ (3), [Ru(L^pyrazine)](PF₆)₂ (4), [Ru(L^2-pyrimidine)](PF₆)₂ (5). Three of them, 1·2MeCN·Et₂O, 3·2MeCN·Et₂O and 4·2MeCN, 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 (R² = 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 L^azine family of ligands was developed. A strong linear relationship was found between the readily calculated pK_a of the L^azine ligand and the E_pa of the M^II complex, for all three families of complexes (R² = 0.98).