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 [Co-III(eta (5)-C5H5)(2)](+)[Fe(eta (5)-C5H4CO2H)(eta (5)-C5H4CO2)](-) 1 by grinding the powder material and subsequently nucleating and crystallising the hydrated form [Co-III(eta (5)-C5H5)(2)](+)[Fe(eta (5)-C5H4CO2H)(eta (5)-C5H4CO2)](-).H2O 2, (ii) the quantitative preparation, nucleation and crystallisation of the elusive anhydrous form of the neutral zwitterion [Co-III(eta (5)-C5H4CO2H)(eta (5)-C5H4CO2)] 4 by thermal dehydration of the hydrated species [Co-III(eta (5)-C5H4CO2H)(eta (5)-C5H4CO2)]. 3H(2)O 3, and (iii) the formation of the salt [Ru-II(eta (6)-C6H6)(2)][BF4](2) 5 either by direct crystallisation from water or by desolvation of the pseudo-polymorph [Ru-II(eta (6)-C6H6)(2)][BF4](2). MeNO2 6 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.