Acid–base chemistry of polypyridyl ruthenium compounds of (pyridin-2′-yl)-1,2,4-triazoles. X-Ray crystal structure of bis(2,2′-bipyridine)[3-methyl-5-(pyridin-2′-yl)-1,2,4-triazolato-N1]ruthenium hexafluorophosphate tetrahydrate

Abstract

The acid–base chemistry of the two co-ordination isomers of the complex of bis(2,2′-bipyridyl)-ruthenium(II)[Ru(bipy)₂]²⁺ with 3-(pyridin-2′-yl)-1,2,4-triazole (HL¹) and of the analogous compound with 3-methyl-5-(pyridin-2′-yl)-1,2,4-triazole (HL²) has been investigated. As expected the ligands act as stronger acids after binding to the Ru(bipy)₂ moiety. The two co-ordination isomers of the HL¹ compound show distinctly different pK_a values of 4.07 ± 0.05 and 5.95 ± 0.05. Luminescence titrations and lifetime measurements indicate that both ligands are stronger acids in the excited state than in the ground state. This suggests that they act as spectator ligands and are not directly involved in the photophysical processes. The crystal structure of [Ru(bipy)₂L²]PF₆·4H₂O has been determined by single-crystal X-ray diffraction. Crystal data: trigonal, space group P3₁ with a=b= 1 3.760(6), c= 30.503(9)Å, α=β= 90.00, γ= 120.00°, and Z= 6. The refinement based on 4 288 unique reflections [I > 2σ(I)] yielded a R(R′) value of 0.036(0.049). The pyridyltriazole ligand is co-ordinated to ruthenium via the N¹ atom of the triazolato ring. The Ru–N distances are between 2.050(5) and 2.086(4)Å. Four water molecules have been located near the pyridyltriazole ligand, giving rise to an extended hydrogen-bridged, structure.