Unusually fast electron and anion transport processes observed in the oxidation of "electrochemically open" microcrystalline [{M(bipy)2}{M′(bipy)2}(μ-L)](PF6)2 complexes (M, M′ = Ru, Os; bipy=2,2′-bipyridyl; L=1,4-dihydroxy-2,5-bis(pyrazol-1-yl)benzene dianion) at a solid-electrode-aqueous electrolyte interface

The oxidation of a series of bridged dinuclear metal complexes [{M(bipy)(2)}(M'(bipy)(2))}mu-L)(2+) (M, M' = Ru, Os; bipy = 2,2'-bipyridyl; L = 1,4-dihydroxy-2,5-bis(pyrazol- 1'-yl)benzene dianion) in microcrystalline solid form has been studied at the basal plane pyrolytic graphite electrode-aqueous electrolyte interface. The solid materials undergo unusually rapid and essentially exhaustive electrolysis even under fast scan rate conditions of cyclic voltammetry via two one-electron oxidation charge-transfer steps: [{M(bipy)(2)}{M'-(bipy)(2))(mu-L)]X(2) X2(solid) + X(-)(solution) reversible arrow [{M(bipy)2}{M'(bipy)(2)}(mu-L)]X(3)(solid) + e(-) (step 1); [{M(bipy)(2)}{M'(bipy)(2)}- (mu-L)]X(3)(solid) + X(-)(solution) = [{M(bipy)(2)}{ M'(bipy)(2)}(mu-L)]X(4)(solid) + e(-) (step 2). To maintain charge neutrality, the solid-state charge-transfer processes are coupled to rapid insertion/expulsion of anions from/to the aqueous electrolyte solution phase (X(-) = PF(6)(-), ClO(4)(-), SCN(-), or NO(3)(-)). The conclusion is reached that "electrochemically open" solid-state structural features are responsible for the uncommonly fast electron transfer and anion charge neutralization processes, which are of relevance to charge storage and photochromic devices and which proceed without nucleation and redistribution processes frequently identified in other systems. Thus, a description can be based on an interacting thin layer model with a "Donnan" type potential term for the anion dependence of the reversible potential. In situ spectroelectrochemical measurements (controlled potential Raman spectroscopy) and ex situ scanning electron microscopy studies yield detailed complementary information concerning the solid-state aspects of the redox transformations, which are localized on the dioxolene bridge and correspond to reversible hydroquinone, semiquinone, and quinone interconversions.