Spectroscopic, Electrochemical, and Kinetic Characterization of New Ruthenium(II) Tris‐chelates Containing Five‐Membered Heterocyclic Moieties

Abstract Synthesis, redox, photophysical, and photochemical properties of Ru(NN)   2+ 3complexes NN = 2‐((2′‐pyridyl)thiazole (pyth), 2‐(2′‐pyrazyl)thiazole (pzth), 2,2′‐bithiazole (bth), 5‐(2′‐pyridyl)‐1,2,4‐thiadiazole (pytda), 2‐(2′‐pyridyl)imidazole (pyim), 1‐methyl‐2‐(2′‐pyridyl)imidazole (Mepyim), and 2‐(2′‐pyridyl)oxazole (pyox)) are described. Oxidation potentials for the Ru 3+/2+ couples in MeCN varied from about 0.80 V to 1.60 V vs. NHE. Three reduction waves were observed in all the cases except for Ru(pyim)   2+ 3and Ru(Mepyim)   2+ 3complexes and asigned to the one‐electron reduction of each bidentate ligand. Absorption spectra contained bands in the UV (280–325 nm) and VIS (437–481 nm) regions which have been assigned to ligand‐centered π‐π* and metal‐to‐ligand charge‐transfer dπ‐π* transitions, respectively. Emission spectra at 77 K were determined for all the complexes presenting maxima in the 580–650‐nm region, with vibrational progression in some of them. Only pyth, pzth, bth, and pytda tris‐chelates showed luminescence at room temperature in aqueous solution, with quantum yields ranging from 0.0013 to 0.0095 and excited‐state lifetimes from 55 to 390 ns, as determined from pulsed laser techniques. Their E 0–0 spetroscopic energies have been estimated from emission wavelength maxima at 77 K which, in turn, have allowed calculation of excited‐state redox potentials. A plot of E 0–0 vs. Δ E 1/2 , where Δ E 1/2 = E 1/2 (3+/2+) − E 1/2 (2+/+), was linear with a slope of ca. 1.1 and a correlation coefficient of 0.999, demonstrating an identical nature of the orbital involved in spectroscopic and electrochemical processes. Photochemical properties of Ru(NN)   2+ 3complexes have been tested using methyl viologen (MV 2+ ) in Ar‐purged aqueous solution at pH 5. Stern‐Volmer treatment has led to the determination of bimolecular quenching constants (0.5 to 2 × 10 9 m −1 ·s −1 ) which parallel electron‐transfer free‐energy changes. Homogeneous back‐reaction of primarily produced MV +· and Ru(NN)   3+ 3has been measured resulting to be slightly higher than diffusion control and independent of ligand nature. Rate constants for the scavenging of Ru(NN)   3+ 3by added edta have been also determined (1.7 to 8.2 × 10 8 M −1 · S −1 ). Under such conditions, net production of MV +· is attained with quantum yields varying from 0.003 to 0.038 (single‐shot laser results).