Bis(heteroleptic) ruthenium(II) complexes with di-2-pyridylamine and methyl picolinate or 2-picolinato: Synthesis, crystal structure, DFT/time-dependent DFT calculations, and cytotoxicity

This is the authors' abstract. We don't add key points for this paper.

Ruthenium(II) polypyridyl complexes display redox, photochemical, and biochemical properties. Di-2-pyridylamine (Hdpa) facilitates hydrogen bonding, while picolinate (pic – ) and methyl picolinate (Mepic) enhance stability. In this study, two bis(Hdpa)Ru(II) complexes with Mepic or pic – as N,O -chelate ligands ( 1 2+ and 2 + , respectively) are synthesized and characterized. The Ru(II) complexes exhibit similar coordination environments around the Ru 2+ ions despite differences in the N,O -chelate ligands (neutral Mepic and anionic pic – ), resulting in distinct colors in the solid state: purple for 1 ·(OTf) 2 and red–orange for 2 ·(OTf)·H 2 O. Both complexes show intense absorption bands at 380–340 nm, with longer-wavelength absorptions at 510 nm for 1 2+ and 420 nm for 2 + . Density functional theory calculations reveal that the highest occupied molecular orbital (HOMO) levels of both complexes align at the same level, but the lowest unoccupied molecular orbital (LUMO) of 1 2+ is significantly lower than that of 2 + , with differing contributions from ligand orbitals. Time-dependent calculations show that absorption bands at 510 and 420 nm are due to HOMO-1 to LUMO and HOMO to LUMO transitions, respectively, with additional transitions at approximately 350 nm. Thermal experiments reveal that heating 1 ·(OTf) 2 in MeCN at 323 K causes Mepic release, forming a yellow complex, while heating 1 ·Cl 2 ·1.5H 2 O in water results in a color change from purple to orange due to hydrolysis, producing 2 + and MeOH. The 2 ·(OTf)·H 2 O complex is thermally stable in both solvents. These findings demonstrate how ligand selection affects the structural, electronic, and thermal properties of Ru(II) bis(heteroleptic) complexes, highlighting their potential in sensing, drug delivery, and functional materials due to their unique color, thermoresponsive behavior, and low cytotoxicity.