EVIDENCE FOR DISSOCIATIVE PHOTOSUBSTITUTION REACTIONS OF [RU(TRPY)(BPY)(NCCH3)]2+ - CRYSTAL AND MOLECULAR-STRUCTURE OF [RU(TRPY)(BPY)(PY)](PF6)2.(CH3)2CO

In acetonitrile and in the presence of appropriate nucleophiles, the complex [Ru(trpy)(bpy)(NCCH3)]2+ undergoes a ligand substitution reaction when irradiated into the metal-to-ligand absorption band system. (The symbol trpy denotes 2,2':6',2"-terpyridine, and bpy denotes 2,2'-bipyridine.) At 25-degrees-C quantum yields for displacement of the acetonitrile ligand are 0.0013 (1), 0.0013 (1), and 0.0026 (5) for 1.0 M solutions of pyridine, 4-phenylpyridine, and 2-methylpyridine, respectively. No photochemistry is observed for the osmium(II) analogue, and all findings can be understood in terms of a dissociative reaction mechanism involving a reactive 3d-d state. The 3d-d state is accessed from a 3CT state, which is emissive below ca. 180 K. From measurements of the temperature dependence of the emission lifetime, the barrier to population of the 3d-d state is approximately 1500 cm-1. Finally, the crystal structure of [Ru(trpy)(bpy)(py)](PF6)2.(CH3)2CO has been determined with a diffractometer. The compound crystallizes with two independent formula units in a triclinic unit cell. The space group is P1BAR, and there are four formulas per unit cell (Z = 4) with a = 13.150 (1) angstrom, b = 14.927 (1) angstrom, c = 19.771 (2) angstrom, alpha = 102.840 (9)-degrees, beta = 91.655 (8)-degrees, and gamma = 103.371 (8)-degrees. The structure was solved by the full-matrix least-squares method, and after the final stage of refinement, R = 0.055 There is evidence of interligand steric strain within the coordination sphere, and the bpy ligand is coordinated asymmetrically with a difference in Ru-N bond lengths of about 0.04 angstrom. The internal strain probably explains why we have been unable to prepare the 2-methylpyridine analogue by standard methods.