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.