Kinetics and mechanism of the redox reactions of binuclear complexes [X nRu II–L–Ru II,III (NH 3) 5] (n+1)+ (X=polypyridines; L=cyanide and N-heterocyclic derivatives) with peroxydisulfate

Abstract

The fully reduced (R) and mixed-valence (M) complexes [X n Ru II–L–Ru II,III (NH 3) 5] n , (n+1)+, with X n =2,2′-bipyridine (bpy), 2,2′:6′,2″-terpyridine (terpy) and L=cyanide, pyrazine (pz), 4,4′-bipyridine (4,4′-bpy), 4-cyanopyridine (4-CNpy) and trans-1,2 (4-pyridyl)ethylene (bpe), were prepared in aqueous solution. The R complexes reacted with peroxydisulfate yielding the M complexes, with oxidation at the Ru II (NH 3) 5 fragment. Both R and M were characterized through UV–VIS spectroscopy and electrochemistry. The one-electron oxidation reactions were measured under stopped-flow conditions for L=pz, 4,4′-bpy and 4-CNpy ( T=25.0°C, I=0.1 mol dm −3, pH=4.8); the rate law was −d[R]/d t=k 2 [R] [S 2O fn2 fn2 By performing the corrections for the electrostatic interactions, some of the points fitted quite well in the plot, which includes a value for oxidation of Ru (bpy) 23 + [20], although some others showed lower values of k et than expected. Probably, slow decomposition reactions are operative and, besides, the calculations based on the ion-pair model are probably failing with such a broad range of charges afforded by each complex. The different k ex values for the Ru II,III (a) and Ru II,III (p) are also different (1×10 5 s −1 and 5×10 5 s −1, respectively) [3, 18] and, also, some influence of the specific interactions operating in a different way on each center should be considered [11, 22]. 8 −]. A plot of ln k et against E 0Ru (the redox potential at the pentaammine fragments) showed a good linear behavior, consistent with a Marcus LFE relationship for intramolecular electron-transfer in the ion pairs {R//S 2O 8}. The results fitted into a broader correlation including other binuclear and mononuclear [Ru II (NH 3) 5L] n complexes, after due account of electrostatic corrections. The mixed-valence complexes decayed slowly in excess of peroxydisulfate, suggesting a complete oxidation process through direct attack at the remaining Ru II-polypyridine center. These oxidation rates could also be included in the Marcus plot. A mechanistic analysis of both one-electron processes was made, considering evidences favoring direct attack of S 2O 28 − on each metal center; alternative paths with intramolecular electron-transfer assistance for oxidation of the mixed-valence complexes were discarded.