Predictable electronic tuning of Fe II and Ru II complexes via choice of azine: correlation of ligand p K 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-4 -1,2,4-triazole ligands (Lazine): [Ru(Lpyridine)](PF ) (1), [Ru(Lpyridazine)](PF ) (2), [Ru(L4-pyrimidine)](PF ) (3), [Ru(Lpyrazine)](PF ) (4), [Ru(L2-pyrimidine)](PF ) (5). Three of them, 1·2MeCN·Et O, 3·2MeCN·Et O and 4·2MeCN, have been structurally characterised, confirming the presence of the isomer, as was previously reported for the Fe analogues. Cyclic voltammetry studies, in dry CH CN Ag/0.01 M AgNO , show that all five Ru complexes undergo a reversible Ru /Ru process, with the midpoint potential ( ) 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 ( = 0.98) is found between the Ru /Ru redox potential and the calculated HOMO orbital energies, which is consistent with the expectation that it is easier to oxidise (lower ) a metal ion with a higher HOMO orbital energy. The same trend was reported earlier for the family of analogous Fe complexes, albeit at lower values of in all cases. In addition, the ionisation potentials of the Ru complexes, as well as those of the other group 8 analogues (Fe and Os ), showed a linear relationship with . As the M redox potentials of a family of complexes has been previously reported to correlate with ligand p values, a computational protocol to calculate, , the p of the Lazine family of ligands was developed. A strong linear relationship was found between the readily calculated p of the Lazine ligand and the of the M complex, for all three families of complexes ( = 0.98).