Two Allogons of an O₂-activating Bis(disiloxido)ferrate(II) Accessible Selectively just by Variation of the Crystallization Temperature

Deprotonation of O(iPr2SiOH)2 (iPrLH2) with LiOtBu followed by reaction with FeCl2 in THF led to the complex [iPrL2Fe][Li(THF)2]2, 2, which represents a structural and spectroscopic model of the alpha-Fe sites of Fe/ZSM-5. Reaction with O2 in THF solution proceeds rather fast and is complete within 200 ms; an intermediate O2 adduct could not be identified by stopped-flow methods. Cooling blue solutions of 2 to -80 degrees C led to the growth of blue crystals of 2 & sdot;THF, the analysis of which by XRD revealed a FeO4 core that is somewhat distorted from planarity towards a tetrahedral structure. By contrast, cooling such solutions to -30 degrees C led to pink crystals of an allogon featuring a perfectly square planar FeO4 entity. Hence, 2 represents a unique case where two different structural isomers (allogons) can be crystallized from the same solvent selectively, controlled by the temperature. DFT calculations were performed to understand this finding.