In Situ Insight into Reversible O2 Gas-Solid Reactions

Non-porous crystalline solids containing a series of cationic tetracobalt complexes reversibly, selectively and stoichiometrically chemisorb dioxygen in temperature/O2 partial pressure induced processes involving the oxidation of cobalt with concurrent reduction of two equivalents of sorbed O2 to form μ-η1, η2-peroxide ligands. The attenuation of O2 affinity by the introduction of electron withdrawing or electron donating substituents into the supporting ligand framework, otherwise dominant in solution is overridden in the crystalline state. Here O2 affinity is tuned predominantly by phase and a two-step gas sorption isotherm is apparent. By following in situ reversible single-crystal to single-crystal (SCSC) transformations using a gas-crystal cell and synchrotron X-ray radiation we can show that two distinctive channels through the crystalline solids are operative under sorption and desorption processes. The reaction proceeds via a semi-oxy form that we had been seeking for some time, and the results have enabled an updated hypothesis that the transient conduit for O2 escapes from an oxy crystal, albeit a counter anion obstacle race is lined by stacks of one end of the molecule. This is contrary to our original hypothesis that communication between the binding sites caused an allostery or anti-cooperative effect: Crystal structure overrides molecule tweaking.