Dinuclear RuII and/or OsII 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)2Ru(BAB)Ru(bpy)2]4+ (RuII.BAB.RuII), [(bpy)2Os-(BAB)Os(bpy)2]4+ (OsII.BAB.OsII), [(bpy)2Ru(BAB)Os(bpy)2]4+ (RuII.BAB.OsII), [(bpy)2Ru(BAAB)-Ru(bpy)2]4+ (RuII.BAAB.RuII), and [(bpy)2Os(BAAB)Os(bpy)2]4+ (OsII.BAAB.OsII) 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 RuII.BAB.OsII complex electronic energy transfer takes place from the Ru-based to the Os-based unit with rate constant 5.8 × 108 s-1 at room temperature, whereas at 77 K energy transfer takes place through two distinct processes with rate constants 1.4 × 108 s-1 and 2.0 × 107 s-1, presumably because of the presence of two conformers (or families of conformers). Partial oxidation of the binuclear compounds OsII.BAB.OsII, RuII.BAB.OsII, and OsII.BAAB.OsII by CeIV in acetonitrile/water solutions leads to the mixed-valence OsII.BAB.OsIII, RuII.BAB.OsIII, and OsII.BAAB.OsIII 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 *OsII.BAB.OsIII → OsIII.BAB.OsII, *OsII.BAAB.OsIII → OsIII.BAAB.OsII processes are 4.0 × 109 s-1 and 8.8 × 108 s-1, respectively. For RuII.BAB.OsIII, the rate constants for the excited state *RuII.BAB.OsIII → *RuIII.BAB.OsIII process is 2.8 × 109 s-1 and the rate constant for the back electron transfer process RuIII.BAB.OsII → RuII.BAB.OsIII is 4.0 × 107 s-1.