Long-range electron transfer rate constants for complexes of the type [(bpy)2RuIIL-Pron-apyRuIII)(NH3)5]5++ proline residues (n) varying from 0 to 9 were determined by complementary electron pulse radiolysis and flash photolysis techniques from the picosecond to the millisecond time scales. The activationless kmax values from both techniques coalesce into one data set. The distance dependence of the reactions is consistent with a smooth transition from a superexchange mechanism with attenuation constant beta = 1.4 A-1 to a hopping mechanism with attenuation constant beta = 0.17 A-1. The transition occurs between n = 3 and 4 prolines, and the virtual hopping rate constant at the shortest distance is about 1 x 106 times slower than that observed for the superexchange value.
The binding of the host H1 (N,N′-bis(6-pivalamidopyrid-2-yl)-3,5-pyridinedicarboxamide) to three different ruthenium polypyridine complexes with an attached barbituric acid and barbital moieties (RuG1, RuG2, RuG3) (where G1 ) 5-[4-(4′-methyl)-2,2′-bipyridylidene]-2,4,6-(1H,3H,5H)-pyrimidinetrione, G2 ) 5-[4-(4′-methyl)-2,2′bipyridyl]methyl-2,4,6-(1H,3H,5H)-pyrimidinetrione, and G3 ) 5-ethyl, 5-[4-(4′-methyl)-2,2′-bipyridyl]methyl-2,4,6-(1H,3H,5H)-pyrimidinetrione) and Ru ) (4,4′-di-tert-butyl-bpy) 2 Ru (bpy ) 2,2′-bipyridine) has been studied in chlorinated solvents by NMR and fluorescence titrations. Significant binding was only observed between H1 and the RuG2 series, while steric hindrance significantly diminished binding between H1 and RuG1 or RuG3. The high binding constant for RuG2 was related to the presence of the enolate form of the barbituric acid guest which forms strong H-bonds with the complementary host H1. For the organic barbituric acid and barbital guests, the keto and enol bind only weakly to H1 (K ∼ 10 2 M -1 ); binding is further increased in the presence of base to generate the enolate. In contrast, formation of the RuG2 enolate occurs upon binding to H1 without any additional base. The ruthenium polypyridine cation (compared to the organic barbituric acid derivatives) facilitates ionization of the enol to enolate thus producing a better complementary H-bonding site between the guest and host. Molecular mechanics calculations confirmed the experimental observatons that the enolate has the highest binding constant to the Host H1, while the corresponding enol form has the weakest binding.
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