Making and converting organometallic pseudo-polymorphs via non-solution methods †

The organometallic pseudo-polymorphism that arises from cocrystallisation of organometallic molecules or ions with solvent molecules is discussed, together with the possibility of interconversion between polymorphs and pseudo-polymorphs. Three cases have been investigated: (i) the quantitative preparation of a monohydrated pseudo-polymorph of the supramolecular salt [CoIII(η5-C5H5)2]+[Fe(η5-C5H4CO2H)(η5-C5H4CO2)]−1 by grinding the powder material and subsequently nucleating and crystallising the hydrated form [CoIII(η5-C5H5)2]+[Fe(η5-C5H4CO2H)(η5-C5H4CO2)]−·H2O 2, (ii) the quantitative preparation, nucleation and crystallisation of the elusive anhydrous form of the neutral zwitterion [CoIII(η5-C5H4CO2H)(η5-C5H4CO2)] 4 by thermal dehydration of the hydrated species [CoIII(η5-C5H4CO2H)(η5-C5H4CO2)]·3H2O 3, and (iii) the formation of the salt [RuII(η6-C6H6)2][BF4]25 either by direct crystallisation from water or by desolvation of the pseudo-polymorph [RuII(η6-C6H6)2][BF4]2·MeNO26 obtained from nitromethane. In order to investigate the processes, single crystal and powder X-ray diffraction measurements as well as thermogravimetry and differential scanning calorimetry have been carried out. These results show, inter alia, how elusive polymorphic or pseudo-polymorphic modifications of a given substance can be obtained by non-solution methods.