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 H -1,2,4-triazole ligands (Lazine): [Ru(Lpyridine)](PF 6 ) 2 (1), [Ru(Lpyridazine)](PF 6 ) 2 (2), [Ru(L4-pyrimidine)](PF 6 ) 2 (3), [Ru(Lpyrazine)](PF 6 ) 2 (4), [Ru(L2-pyrimidine)](PF 6 ) 2 (5). Three of them, 1·2MeCN·Et 2 O, 3·2MeCN·Et 2 O 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 CH 3 CN vs. Ag/0.01 M AgNO 3 , 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 p K a values, a computational protocol to calculate, in silico , the p K a of the Lazine family of ligands was developed. A strong linear relationship was found between the readily calculated p K a of the Lazine ligand and the E pa of the M II complex, for all three families of complexes ( R 2 = 0.98).