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

Exposure of the solid zwitterion [Co-III(eta(5)-C5H4COOH)(eta(5)-C5H4COO)], 1, to vapours of CHF2COOH quantitatively produces the corresponding salt of the cation [Co-III(eta(5)-C5H4COOH)(2)](+), 2, namely 2-[CHF2COO]. The reaction with hydrated vapours of CH2ClCOOH produces the hydrated salt 2-[CH2ClCOO].H2O. Crystalline [Co-III(eta(5)-C5H4COOH)(eta(5)-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.