Dinuclear RuII and/or OsII complexes of bis-bipyridine bridging ligands containing adamantane spacers: synthesis, luminescence properties, intercomponent energy and electron transfer processes

Abstract

Abstract 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)O9(bPY) 2 ] 4+ (ROSAB.Osn), [(bPY)2Ru(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 PF 6 − 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 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 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 .SAB.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 II .BAB.Os III → Os III .BAB.Os II , ∗ Os II .BAAB.Os III → Os III .BAAB.Os II processes are 4.0 × 109 s −1 and 8.8 × 108 si, 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 × 109 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 × 107 s −1 .