Synthesis and characterization of two ruthenium(II) heteroleptic complexes of annelated derivatives of 2,2′-bipyridine

The synthesis and characterization of two annelated 2,2'-bipyridine molecules (L) and of the corresponding ruthenium(II) heteroleptic complexes, [Ru(bpy)(2)(L)][PF6](2) are described. The ligands L bear a terbutoxypropyl chain substituent at position 2. We discovered that under basic conditions condensation between an enolisable ketone and an alpha,beta-unsaturated ketone gives exclusively the 1,2-addition isomers with good yields. In contrast to 2,2'-bipyridine, the annelated ligands L show fluorescence emission, probably due to the rigidity of the molecules. Spectroscopic and photophysical results of the heteroleptic complexes at room temperature and at 77 K are similar to the reference complex, [Ru(bpy)(3)](2+), except for less intense emission quantum yields and lifetimes at room temperature. This behaviour is due to the presence of a substituent and its position adjacent to the nitrogen of the annelated ligands engaged in the coordination, rather than to the rigidity of these ligands. As shown by the crystallographic data, this substituent induces a lengthening of the adjacent Ru-N bond (2.16 Angstrom as compared to 2.05 Angstrom for the other Ru-N bonds). As a result, the octahedral structure of the heteroleptic complexes is distorted and the probability of nonradiative processes is thus favoured.