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

摘要

The fully reduced (R) and mixed-valence (M) complexes [X_nRu~(II)-L-Ru~(II,III)(NH_3)_5]~(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 deg C, I = 0.1 mol dm~(-3), pH = 4.8); the rate law was -d[R]/dt = k_2[R][S_2O_8~(2-)]. A plot of ln k_(et) against E_(Ru)~o (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_8~(2-) on each metal center; alternative paths with intramolecular electron-transfer assistance for oxidation of the mixed-valence complexes were discarded.