Fluorine Substitution of TCNQ Alters the Redox-Driven Catalytic Pathway for the Ferricyanide-Thiosulfate Reaction

Mechanistic variation in catalysis through substituent-based redox tuning is well established. Fluorination of TCNQ (TCNQ=tetracyanoquinodimethane) provides ~850 mV variation in the redox potentials of the $$ and $$ (n=0, 2, 4) processes. With $$ , catalysis of the kinetically very slow ferrocyanide-thiosulfate redox reaction in aqueous solution occurs via a mechanism in which the catalyst $$ is reduced to $$ when reacting with $$ which is oxidised to $$ . Subsequently, $$ reacts with $$ to form $$ and reform the $$ catalyst, in another thermodynamically favoured process. An analogous mechanism applies with $$ as a catalyst. In contrast, since the reaction of $$ with $$ is thermodynamically unfavourable, an alternative mechanism is required to explain the catalytic activity observed in this non-fluorinated system. Here, upon addition of $$ , reduction of $$ to $$ occurs with concomitant oxidation of $$ to $$ , which then acts as the catalyst for $$ oxidation. Thermodynamic data explain the observed differences in the catalytic mechanisms. $$ (n=0, 4) also act as catalysts for the ferricyanide-thiosulfate reaction in aqueous solution. The present study shows that homogeneous pathways are available following addition of these dissolved materials. Previously, these $$ (n=0, 4) coordination polymers have been regarded as insoluble in water and proposed as heterogeneous catalysts for the ferricyanide-thiosulfate reaction. Details and mechanistic differences were established using UV-visible spectrophotometry and cyclic voltammetry.