Synthesis, Structure, and Photophysical Properties of Novel Ruthenium(II) Carboxypyridine Type Complexes

A series of Ru(II) compounds and salts have been synthesized: [Ru(6-carboxylato-bpy)2] (5), [Ru(6-carboxylato-bpy)(tpy)]PF6 (9), [Ru(tpy)2](PF6)2 (8), and [Ru(bpy)2(Pic)]PF6 (11), where 6-carboxy-bpy (1) = 6-carboxy-2,2‘-bipyridine, tpy (2) = 2,2‘:6‘,2‘‘-terpyridine, and Pic = 2-carboxylatopyridine. The compounds have been characterized by NMR, electrospray mass spectrometry (ESI-MS), cyclic voltammetry, absorption and emission spectroscopy (at 100, 140, and 298 K), and single-crystal X-ray diffraction (complex 5). Complex 5 crystallizes in the monoclinic system, space group P21/n, formula RuC22H14N4O4·C2H5OH, with a = 11.088(3) Å, b = 11.226(3) Å, c = 35.283(9) Å, β = 91.41(2)°, and Z = 8. A linear dependence on the number of coordinated carboxylato groups and the electrochemical redox potentials was found, ca. 0.4 V lower reduction potential for the oxidation step (Ru(II/III)) per carboxylate group. Also, to the best of our knowledge, these are the first examples (9, 11) of mononuclear Ru(II) complexes containing a carboxypyridine−ruthenium moiety displaying any luminescence emission.