Ruthenium(ii) and osmium(ii) polypyridyl complexes of an asymmetric pyrazinyl- and pyridinyl-containing 1,2,4-triazole based ligand. Connectivity and physical properties of mononuclear complexes

Abstract

The synthesis, purification and characterisation of two coordination isomers of ruthenium(II) and osmium (II) complexes containing the ligand 3-(pyrazin-2′-yl)-5-(pyridin-2″-yl)-1,2,4-triazole (Hppt) are described. The X-ray and molecular structure of the complex [Ru(bipy)₂(ppt)]PF₆·CH₃OH (1a) is reported, where the Ru(bipy)₂-centre is bound to the ppt⁻ ligand via the pyridine nitrogen and the N1 atom of the triazole ring. ¹H NMR spectroscopic measurements confirm that in the second isomer (1b) the Ru(bipy)₂-moiety is bound via the N2 atom of the triazole ring and the pyrazine ring. Partially deuteriated metal complexes are utilised to facilitate interpretation of ¹H NMR spectra. The redox and electronic properties indicate that there are significant differences in the electronic properties of the two coordination isomers obtained. The acid–base properties of the compounds are also reported and show that the pK_a of the 1,2,4-triazole ring varies systematically depending on the nature of the non-coordinating substituent. Analysis of these data indicates a significant electronic interaction between the pyridyl/pyrazyl rings and the 1,2,4-triazole ring in the coordinated ppt⁻ ligand.