Revisiting the TCNQF4 0/1-/2- Catalysis Mechanism for the [Fe(CN)6 ]3-/4- -S2 O3 2- /S4 O6 2- Redox Reaction

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