Reversible gas–solid reactions between the organometallic zwitterion [Co III (η ⁵ -C ₅ H ₄ COOH)(η ⁵ -C ₅ H ₄ COO)] and vapours of difluoro- and chloro-acetic acids

Exposure of the solid zwitterion [CoIII(η5-C5H4COOH)(η5-C5H4COO)], 1, to vapours of CHF2COOH quantitatively produces the corresponding salt of the cation [CoIII(η5-C5H4COOH)2]+, 2, namely 2-[CHF2COO]. The reaction with hydrated vapours of CH2ClCOOH produces the hydrated salt 2-[CH2ClCOO]·H2O. Crystalline [CoIII(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.