Reversible gas–solid reactions between the organometallic zwitterion [CoIII(η5-C5H4COOH)(η5-C5H4COO)] and vapours of difluoro- and chloro-acetic acids

Abstract

Exposure of the solid zwitterion [Co^III(η⁵-C₅H₄COOH)(η⁵-C₅H₄COO)], 1, to vapours of CHF₂COOH quantitatively produces the corresponding salt of the cation [Co^III(η⁵-C₅H₄COOH)₂]⁺, 2, namely 2-[CHF₂COO]. The reaction with hydrated vapours of CH₂ClCOOH produces the hydrated salt 2-[CH₂ClCOO]·H₂O. Crystalline [Co^III(η⁵-C₅H₄COOH)(η⁵-C₅H₄COO)] was also made to react in solution with CHF₂COOH and CH₂ClCOOH; single crystals of 2-[CHF₂COO] and anhydrous 2-[CH₂ClCOO] for X-ray diffraction were thus obtained, allowing comparison between computed and measured powder diffraction patterns. In the case of 2-[CH₂ClCOO] there is evidence of formation of pseudo-polymorphic forms depending on the reaction condition (heterophase or solution). The heterogeneous reactions are fully reversible; the acids can be quantitatively removed by mild thermal treatment, which regenerates the starting material.