Michael Y. Ogawa, James F. Wishart, Zuyung Young, John R. Miller, Stephan S. Isied
J. Phys. Chem. 97, 11456-11463 (1993)
Abstract:
A series of complexes of the type [(bpy)2RuIIL-(Pro) n-CoIII(NH3)5]4+, n = 1-6, where L = 4-carboxy-4'-methyl-2,2'-bipyridine, bpy = 4,4' bipyridine, have been synthesized from the corresponding [(bpy)2RuIIL] and [(NH3)5 CoIII(Pro)n] components. The compounds were characterized by metal analyses, electrochemical measurements, and absorption spectroscopy. For n = 4-6 prolines, the CD spectra of these complexes show a polyproline II helical structure. Intramolecular electron transfer within these complexes was studied by generating the [(bpy)2RuIIL.-(Pro) n-CoIII(NH3)5] intermediate from the reaction of eaq-, (generated by pulse radiolysis) with the [(bpy)2RuIIL-(Pro)n-CoIII (NH3)5] molecules. The driving force for these reactions is estimated to be -DG ° = 1.1 eV. The intramolecular electron transfer rates (k) and activation parameters (DHý (kcal/mol), DSý (eu) found for these studies were: 1.6 x 107 s-1, 6.0 ± 0.6, -5.6 ± 2.0 ; 2.3 x 105 s-1, 9.3 ± 0.9, -2.4 ± 2.9; 5.1 x 104 s-1 , 9.4 ± 0.3 , -5.4 ± 0.9 ; 1.8 x 104 s-1, 8.9 ± 1.0, -9.0 ± 3.2 ; 9.0 x 103 s-1, 9.1± 0.9, -9.4 ± 2.9 for n = 2-6, respectively. For n = 1 proline, k is > 5 x 10z8 s-1 and no temperature dependence could be determined. The rates of intramolecular electron transfer decrease rapidly with distance for n = 1-3 prolines but show a surprisingly weak decrease with distance for the n = 4, 5, and 6 prolines, which exhibit the polyproline II helical structure. The electron transfer pathways within these molecules and the relationship of the electron-transfer rates to the helical polyproline II structure are discussed.