1,5-Naphthyridine as a new linker for the construction of bridging ligands and their corresponding Ru(II) complexes

The 1,5-naphthyridine (1,5-nap) molecule has been elaborated into a series of new bidentate and tridentate ligands using Stille coupling or Friedlander condensation methodologies. Thus 2-(tri-n-butylstannyl)pyridine was coupled with 2-chloro, 4-chloro, or 2,6-dichloro 1,5-nap to prepare the analogous bidentate ligands. The condensation of 2-aminonicotinaldehyde or 8-amino-7-quinolinecarbaldehyde with a variety of acetyl derivatives of 1,5-nap produced ligands incorporating 1,8-naphthyrid-2-yl or 1,10-phenanthrolin-2-yl groups, respectively. These ligands were treated with either [Ru(bpy-d(8))(2)Cl(2)] or [Ru(tpy)Cl(3)] to prepare the corresponding heteroleptic mono- and dinuclear Ru(II) complexes. The NMR spectra of these complexes were simplified by the use of bpy-d(8) as an auxiliary ligand, allowing straightforward product identification. The long wavelength absorption of both the ligands and the complexes are shifted to lower energy with increasing delocalization or the incorporation of a second metal. Protonation of a remote uncomplexed nitrogen leads to the red-shifting of the absorption band. The degree of communication between the metal centers in dinuclear complexes can be evaluated from the comproportionation constant measured by electrochemistry. When compared to the pyrazine linker, communication through the 1,5-nap linker appears to be somewhat less efficient.