Two concomitant polymorphs and two isomorphous forms with different chemical compositions, which transform into the same substance upon thermal treatment

Abstract

The organometallic cationic acid [Co^III(η⁵-C₅H₄COOH)₂]⁺ (AC) is crystallised from hydrochloric acid solutions in two polymorphic modifications of the salt [Co^III(η⁵-C₅H₄COOH)₂]Cl·H₂O, ACCl·H₂O_cube and ACCl·H₂O_sword. On the other hand, ACCl·H₂O_sword is isomorphous with the triply-hydrated zwitterion [Co^III(η⁵-C₅H₄COOH)(η⁵-C₅H₄COO)], ZW·3H₂O. ACCl·H₂O_sword, ACCl·H₂O_cube and ZW·3H₂O are all converted into the anhydrous form [Co^III(η⁵-C₅H₄COOH)(η⁵-C₅H₄COO)], ZW, upon thermal treatment. The structural relationship between the ionic and neutral systems and their interconversion have been investigated with a combination of solid-state techniques (single crystal and powder X-ray diffraction, thermogravimetric analysis, mass spectrometry and solid state ¹³C CP/MAS NMR).