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

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.