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

Abstract

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(η⁵-C₅H₅)₂]⁺[Fe(η⁵-C₅H₄CO₂H)(η⁵-C₅H₄CO₂)]⁻1 by grinding the powder material and subsequently nucleating and crystallising the hydrated form [Co^III(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 [Co^III(η⁵-C₅H₄CO₂H)(η⁵-C₅H₄CO₂)] 4 by thermal dehydration of the hydrated species [Co^III(η⁵-C₅H₄CO₂H)(η⁵-C₅H₄CO₂)]·3H₂O 3, and (iii) the formation of the salt [Ru^II(η⁶-C₆H₆)₂][BF₄]₂5 either by direct crystallisation from water or by desolvation of the pseudo-polymorph [Ru^II(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.