Structural and electronic factors are crucial to rationalize
the
different N,O or N,N chelating coordination of pyrazolones containing
a pyridine ring. The reactivity of proligand 3-phenyl-1-(pyridin-2-yl)-5-pyrazolone
(HLpy,ph) with the (arene)Ru(II) fragment was explored.
Neutral and ionic (arene)Ru(II) complexes were obtained and fully
characterized, also by X-ray diffraction, revealing the ligand to
coordinate in an unusual N,O-chelating
fashion. Other ruthenium complexes were also synthesized with 3-methyl-1-(pyridin-2-yl)-5-pyrazolone
(HLpy,me) and 3-methyl-1-(pyridin-2-yl)-4-trifluoroacetyl-5-pyrazolone
(HQpy,CF3). In these complexes the ligands adopt the preferred N,N-chelating mode. Ligands and complexes
were theoretically analyzed by density functional theory (DFT). The
most stable tautomer of HLpy,ph matched well with the experimental
behavior of this proligand and the structures of Ru-complexes were
well described by calculations. The thermodynamic stability of the N,O- and N,N-coordination modes was analyzed and a proposal for the achievement
of the N,O-coordination mode in
complexes 1–4 was proposed. Cytotoxicity
tests were performed against human ovarian carcinoma (A2780 and Cisplatin-resistant
A2780cis) and nontumorigenic human embryonic kidney (HEK293T) cell
lines, showing the free ligands to be more cytotoxic that the ensuing
(arene)Ru(II) complexes.