Tetradihydrobenzoquinonate and Tetrachloranilate Zr(IV) Complexes: Single-Crystal-to-Single-Crystal Phase Transition and Open-Framework Behavior for K<sub>4</sub>Zr(DBQ)<sub>4</sub>

The molecular complexes K<sub>4</sub>[Zr­(DBQ)<sub>4</sub>] and K<sub>4</sub>[Zr­(CA)<sub>4</sub>], where DBQ<sup>2–</sup> and CA<sup>2–</sup> stand respectively for deprotonated dihydroxybenzoquinone and chloranilic acid, are reported. The anionic metal complexes consist of Zr­(IV) surrounded by four O,O-chelating ligands. Besides the preparation and crystal structures for the two complexes, we show that in the solid state the DBQ complex forms a 3-D open framework (with 22% accessible volume) that undergoes a crystal-to-crystal phase transition to a compact structure upon guest molecule release. This process is reversible. In the presence of H<sub>2</sub>O, CO<sub>2</sub>, and other small molecules, the framework opens and accommodates guest molecules. CO<sub>2</sub> adsorption isotherms show that the framework breathing occurs only when a slight gas pressure is applied. Crystal structures for both the hydrated and guest free phases of K<sub>4</sub>[Zr­(DBQ)<sub>4</sub>] have been investigated.