Control of the co-ordination mode of 1,8-naphthyridine ligated to ruthenium(II) bipyridine complexes

The complexes [Ru(bipy)(2)(napy-N)(MeCN)][PF6](2) 1 and [Ru(bipy)(2)(napy-N,N')][PF6](2) 2 (bipy = 2,2'-bipyridine, napy = 1,8-naphthyridine) were prepared, and their crystal structures determined by X-ray analysis. The crystal structure of 1 displays an octahedral co-ordination with monodentate napy, acetonitrile and two chelating; bipy. Despite the inequivalency of two nitrogens of napy in 1 in the solid state, the H-1 NMR spectra in the aromatic region resemble those of 2 over the range -90 to 60 degrees C, which implies dynamic behaviour of napy in 1 in solution. Both 1 and 2 were reduced irreversibly at -0.98 V (vs. Ag-AgCl) in dimethylformamide at -20 degrees C, and the process gradually becomes a reversible redox reaction on increasing the temperature to 30 degrees C. An EPR study revealed that one-electron reduction of 1 takes place in the napy-localized orbital without appreciable increase in electron density on one of the nitrogens of napy. The distinct inequivalence in the charge density between the two nitrogen atoms of singly reduced napy results in stabilization of the N rather than N,N' co-ordination mode.