Mechanism of the reaction of cuboidal [Mo3FeS4(H2O)10]4+ with dioxygen

Abstract

Solutions of the grey-purple mixed-metal cuboidal aqua ion [Mo₃FeS₄(H₂O)₁₀]⁴⁺ react with O₂ to give the now well characterised (green) incomplete cuboidal [Mo₃S₄(H₂O)₉]⁴⁺ and Fe²⁺ in a two-electron (per cube) redox process. With the cube in excess, spectrophotometric changes are consistent with the stoicheiometric equation: 2Mo₃ FeS₄⁴⁺+ O₂+ 4H⁺→ 2Mo₃S₄⁴⁺+ 2Fe²⁺+ 2H₂O. First-order rate constants k_obs. monitored at the [Mo₃S₄(H₂O)₉]⁴⁺ peak at 367 nm (Δε= 4 570 M^–1cm^–1) with [Mo₃FeS₄(H₂O)₁₀]⁴⁺ in > 10- fold excess can be expressed as 2k[Mo₃FeS₄⁴⁺]. On varying [H⁺], k= a +b[H ⁺], where at 25 °C a= 0.43 M^–1 s^–1 and b= 0.26 M^–2 s^–1,I= 2.0 M(LiClO₄). With O₂ in large excess spectrophotometric evidence (245 nm) has been obtained for an intermediate believed to be Mo₃FeS₄(O₂)⁴⁺, and a reaction sequence Mo₃FeS₄⁴⁺+ O₂ [graphic omitted] Mo₃FeS₄(O₂)⁴⁺, followed by Mo₃FeS₄(O₂)⁴⁺ [graphic omitted] Mo₃S₄⁴⁺+(FeO₂), is proposed. With [Mo₃FeS₄(H₂O)₁₀]⁴⁺ in excess the reaction (FeO₂)+ Mo₃FeS₄⁴⁺+ 4H⁺→ Mo₃S₄⁴⁺+ 2Fe ²⁺+ 2H₂O occurs, but is relatively fast and k remains equal to K₁k₂. With O₂ in excess the slower Fe²⁺/Fe³⁺-catalysed decomposition of the peroxide released is observed, and Fe³⁺(up to 30% of Fe) is obtained as a product. The stoicheiometry is therefore less than the 2:1 observed with [Mo₃FeS₄(H₂O)₁₀]⁴⁺ in excess. An alternative mechanism involving formation of the adduct [Mo₃FeS₄O₂Mo₃FeS₄]⁸⁺ does not explain these features, but may make some contribution with the reactant [Mo₃FeS₄(H₂O)₁₀]⁴⁺ in excess.