Ruthenium(II) Complexes of Carboxylated Terpyridines and Dipyrazinylpyridines

One-pot preparations of carboxylated 2,2';6',2 ''-terpyridine and 2,6-dipyrazin-2-ylpyridine ligands are reported in free acid and ester forms, as well as their transformations to homoleptic Ru-II complexes in very good yields. Density functional theory calculations of their geometry-optimized structures enabled their comparison with crystal structures; although the gross features of the crystal structures were reproduced, they showed much variability and distortion. The spectroscopic and electrochemical properties were analyzed from the point of view of their geometry-optimized structures, molecular orbital characteristics and electronic transitions, specifically to assess the effects of carboxylation and of inserting a phenylene spacer between the tridentate portion and the carboxyl group. The dipyrazinylpyridine complexes had HOMO levels stabilized by approximately 1 eV relative to the terpyridine analogues, and showed positive-shifted electrochemical potentials and redshifted electronic absorptions. Phenylene spacers were found to act as electron-donating groups, and the lowest-energy UV/Vis transitions showed intraligand character.