Dinuclear Ru-II and/or Os-II complexes of bis-bipyridine bridging ligands containing adamantane spacers: Synthesis, luminescence properties, intercomponent energy and electron transfer processes

The (E,E)-1,3-bis[2-(2,2'-bipyridine-5-yl)ethenyl]adamantane (BAB) and (E,E)-3,3'-bis[2-(2,2'-bipyridine-5-yl)ethenyl]-1,1-biadamantane (BAAB) bridging ligands made of two 2,2'-bipyridine groups (B) separated by spacers containing one and two adamantane (A) units have been synthesized. Their dinuclear complexes [(bpy)(2)Ru(BAB)Ru(bpy)(2)](4+) (Ru-II.BAB.Ru-II), [(bpy)(2)Os-(BAB)Os(bpy)(2)](4+) (Os-II.BAB.Os-II), [(bpy)(2)Ru(BAB)Os(bpy)(2)](4+) (Ru-II.BAB.Os-II), [(bpy)(2)Ru(BAAB)-Ru(bpy)(2)](4+) (Ru-II.BAAB.Ru-II), and [(bpy)(2)Os(BAAB)Os(bpy)(2)](4+) (Os-II.BAAB.Os-II) have been prepared as PF6- salts. In these novel compounds, each Ru-based and Os-based unit displays its own absorption spectrum and electrochemical properties, regardless of the presence of a second metal-based unit. In the homodinuclear complexes also the luminescence properties of each unit are unaffected. In the mixed metal Ru-II.BAB.Os-II complex electronic energy transfer lakes place from the Ru-based to the Os-based unit with rate constant 5.8 x 10(8) s(-1) at room temperature, whereas at 77 K energy transfer takes place through two distinct processes with rate constants 1.4 X 10(8) pi and 2.0 x 10(7) s(-1), presumably because of the presence of two conformers (or families of conformers). Partial oxidation of the binuclear compounds Os-II.BAB.Os-II, Ru-II.BAB.Os-II, and Os-II.BAAB.Os-II by Ce-IV in acetonitrile/water solutions leads to the mixed-valence Os-II.BAB.Os-III, Ru-II.BAB.Os-III, and Os-II.BAAB.Os-III species where the oxidized metal-based unit quenches by electron transfer the luminescent excited state of the unit that is not oxidized. At room temperature the rate constants for the excited state *Os-III.BAB.Os-III --> Os-III.BAB.Os-II, *Os-II.BAAB.Os-III --> Os-III.BAAB.Os-II processes are 4.0 x 10(9) s(-1) and 8.8 x 10(8) s(-1), respectively. For Ru-II.BAB.Os-III, the rate constants for the excited state *Ru-II.BAB.Os-III --> Ru-III.BAB.Os-II process is 2.8 x 10(9) s(-1) and the rate constant for the back electron transfer process Ru-III.BAB.Os-II --> Ru-II.BAB.Os-III is 4.0 x 10(7) s(-1).