Revisiting the TCNQF40/1−/2− Catalysis Mechanism for the [Fe(CN)6]3−/4−‐S2O32−/S4O62− Redox Reaction

is shown to act as a homogeneous catalysis for the kinetically very slow - / reaction in aqueous solution. This contradicts the existing paradigm in which heterogeneous mechanisms have been proposed for materials with low solubility.

Abstract

Published data suggest that sparingly soluble metal complexes of , where n=0, 1, 2, 4, can act as heterogeneous catalysts for the kinetically very slow - / reaction in aqueous solution. This study shows that the coordination polymer , participates as a homogeneous catalyst via an extremely small concentration of dissolved . This finding suggests that the generally accepted mechanism of catalysis by based solids needs to be revisited to ascertain the role of homogeneous pathways. In the present study, UV-visible spectrophotometry was used to examine the catalysis of the aqueous redox reaction of (1.0 mM) with (100 mM) in the presence of (i) a precursor catalyst, ; (ii) the catalyst, , as the water soluble Li⁺ salt; and (iii) . A homogeneous reaction scheme that utilises the couple is provided. In the case of derived from highly soluble , quantitative conversion of 1.0 mM to 0.50 mM occurs with complete reduction of to being rapidly accelerated by sub-micomolar concentrations of . generated in the catalytic cycle, reacts with to reform and produce . Along with the rapid catalytic reaction, the sluggish competing reaction between and occurs to give , which is protonated to , along with a trace amount of . On addition of the precursor catalyst, , rapid reduction with occurs to form – the active catalyst. added to water is shown to be sufficiently soluble to provide adequate to act as the catalyst for the - / reaction.

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