Unusually fast electron and anion transport processes observed in the oxidation of "electrochemically open" microcrystalline [{M(bipy)(2)}{M '(bipy)(2)}(mu-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-di hydroxy-2,5-bis(pyrazol- 1'-yl)benzene dianion) in microcrystalline solid f orm has been studied at the basal plane pyrolytic graphite electrode-aqueou s electrolyte interface. The solid materials undergo unusually rapid and es sentially exhaustive electrolysis even under fast scan rate conditions of c yclic voltammetry via two one-electron oxidation charge-transfer steps: [{M (bipy)(2)}{M'-(bipy)(2))(mu-L)]X-2 X2(solid) + X-(solution) reversible arro w [{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 soli d-state charge-transfer processes are coupled to rapid insertion/expulsion of anions from/to the aqueous electrolyte solution phase (X- = PF6-, ClO4-, SCN-, or NO3-). The conclusion is reached that "electrochemically open" so lid-state structural features are responsible for the uncommonly fast elect ron transfer and anion charge neutralization processes, which are of releva nce to charge storage and photochromic devices and which proceed without nu cleation and redistribution processes frequently identified in other system s. 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 potentia l Raman spectroscopy) and ex situ scanning electron microscopy studies yiel d 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 interconver sions.