Synthesis, physical and photophysical properties of Ru(II) pyridyl-pylimidine and pyridyl-pyrimidinone complexes: effect of substituents on the pyrimidine ring

[Ru(bpy)2((EtO)2pypmR)]2+ (R = H, Me and pypm = 2-(2-pyridyl)pyrimidine) forms by ethoxide substitution of 4,6-halo groups when Ru(bpy)2CO3·2H2O reacts with 4,6-dihalo-2-(2-pyridyl)pyrimidine in ethanol. When these ligands react with Ru(bpy)2Cl2·2H2O in acetone, [Ru(bpy)2(ClpypnR)]+ (R = H, Me, Ph and pypn = 2-(2-pyridyl)pyrimidinone) forms by a hydrolytic reaction. The complexes give rise to metal-to-ligand charge transfer (MLCT) absorptions in the 320-480 nm region and π → π* transitions below 300 nm. The lowest energy dπ → π* absorptions of [Ru(bpy)2(ClpypnR)]+ shift to the red region of the spectrum when compared with those of [Ru(bpy)2(pypmR)]2+ and [Ru(bpy)2((EtO)2pypmR)]2+. Emission spectra maxima are observed from 606 to 653 nm with the high energy absorptions associated with the pypm derivatives and the low energy absorptions associated with the pypn derivatives. The oxidation potentials for Ru3+/2+ couples vary from +1.33 to +0.95 V versus SSCE; ClpypnR ligands make Ru2+ more easily oxidizable. Reductions are ligand-centered and sequential for each ligand π* system; the order of decreasing ease of reduction in the complexes is pypmR ∼ (EtO)2pypmR > ClpypnR (R = H, Me) > ClpypnPh. Excited-state Ru2+*/+ couples range from +0.77 to +0.46 V; those of Ru3+/2+* range from -0.56 to -0.96 V. A variable-temperature emission lifetime study reveals the presence of a low-lying 3dd state with an energy gap between the 3MLCT and the 3dd (ΔE) ranging from 1800-3920 cm-1, the thermal deactivation rate constants from 3dd (k1) vary from 1.4 × 1010 to 5 × 1014 s-1. The ground state pKa values of the complexes determined by spectrometric titration range from -4.1 to -6.7. The excited state pKa*(app) values measured by fluorescence titration range from 0.5 to 3.03. © 1999 Elsevier Science S.A. All rights reserved.