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

Published data suggest that sparingly soluble metal complexes of TCNQFn1-${{\rm{TCNQF}}_{\rm{n}}^{{\rm{1 - }}} }$ , where n=0, 1, 2, 4, can act as heterogeneous catalysts for the kinetically very slow [Fe(CN)6]​3-/4-${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - /4 - }}} }$ - S2O32-${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ / S4O62-${{\rm{S}}_{\rm{4}} {\rm{O}}_{\rm{6}}^{{\rm{2 - }}} }$ reaction in aqueous solution. This study shows that the coordination polymer CuTCNQF4${{\rm{CuTCNQF}}_{\rm{4}} }$ , participates as a homogeneous catalyst via an extremely small concentration of dissolved TCNQF41-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ . This finding suggests that the generally accepted mechanism of catalysis by TCNQF4${{\rm{TCNQF}}_{\rm{4}} }$ 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 [Fe(CN)6]​3-${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - }}} }$ (1.0 mM) with S2O32-${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ (100 mM) in the presence of (i) a precursor catalyst, TCNQF40${{\rm{TCNQF}}_{\rm{4}}^{\rm{0}} }$ ; (ii) the catalyst, TCNQF41-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ , as the water soluble Li+ salt; and (iii) CuTCNQF4${{\rm{CuTCNQF}}_{\rm{4}} }$ . A homogeneous reaction scheme that utilises the TCNQF41-/2-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - /2 - }}} }$ couple is provided. In the case of TCNQF41-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ derived from highly soluble LiTCNQF4${{\rm{LiTCNQF}}_{\rm{4}} }$ , quantitative conversion of 1.0 mM S2O32-${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ to 0.50 mM S4O62-${{\rm{S}}_{\rm{4}} {\rm{O}}_{\rm{6}}^{{\rm{2 - }}} }$ occurs with complete reduction of [Fe(CN)6]​3-${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - }}} }$ to [Fe(CN)6]​4-${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{4 - }}} }$ being rapidly accelerated by sub-micomolar concentrations of TCNQF41-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ . TCNQF42-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{2 - }}} }$ generated in the catalytic cycle, reacts with [Fe(CN)6]​3-${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - }}} }$ to reform TCNQF41-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ and produce [Fe(CN)6]​4-${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{4 - }}} }$ . Along with the rapid catalytic reaction, the sluggish competing reaction between TCNQF41-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ and S2O32-${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ occurs to give TCNQF42-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{2 - }}} }$ , which is protonated to HTCNQF41-${{\rm{\;HTCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ , along with a trace amount of S4O62-${{\rm{S}}_{\rm{4}} {\rm{O}}_{\rm{6}}^{{\rm{2 - }}} }$ . On addition of the precursor catalyst, TCNQF40${{\rm{TCNQF}}_{\rm{4}}^{\rm{0}} }$ , rapid reduction with S2O32-${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ occurs to form TCNQF41-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ - the active catalyst. CuTCNQF4${{\rm{CuTCNQF}}_{\rm{4}} }$ added to water is shown to be sufficiently soluble to provide adequate TCNQF41-${{\rm{TCNQF}}_{\rm{4}}^{{\rm{1 - }}} }$ to act as the catalyst for the [Fe(CN)6]​3-/4-${{\rm{[Fe(CN)}}_{\rm{6}} {\rm{]}}^{{\rm{3 - /4 - }}} }$ - S2O32-${{\rm{S}}_{\rm{2}} {\rm{O}}_{\rm{3}}^{{\rm{2 - }}} }$ / S4O62-${{\rm{S}}_{\rm{4}} {\rm{O}}_{\rm{6}}^{{\rm{2 - }}} }$ reaction.