Series of Hydrated Lanthanide (La, Ce, Pr, Dy, Yb) Pyromellitate 3D Coordination Polymer with Tunnels Encapsulating Dimethylammonium Species and Its Crystal-to-Crystal Transformation upon Dehydration

Synthesis and characterization of two distinct structural types of coordination polymers are reported with 1,2,4,5-benzenetetracarboxylate (btec) ligands associated with lanthanides (Ln) Ce(III), La(III), and Pr(III) for compounds <b>1-3</b>, and Dy(III) and Yb(III) for compounds <b>4-5</b>. The two structures are built from the connection of lanthanide centers with the linkers to construct a 3D framework composed of 1D tunnels encapsulating dimethylammonium and water in (CH₃)₂NH₂[Ln(btec)]00b72H₂O (<b>1</b>-<b>3</b>) or dimethylammonium only in (CH₃)₂NH₂[Ln(btec)(H₂O)] (<b>4</b>-<b>5</b>). These two atomic arrangements differ by the occurrence of infinite chains of edge-sharing {LnO₉} units and, the presence of trapped water species in <b>1</b>-<b>3</b>, instead of dinuclear carboxylate-bridged {2 × LnO₈} units and, bonded aquo species to Ln centers in <b>4</b>-<b>5</b>. The dehydration process was investigated for compound <b>1</b>. Removal of water induces a crystalline phase transition related to the shrinkage of the 3D network to form ([Ln(btec)]·(CH₃)₂NH₂, noted <b>1'</b>). Elucidation of the mechanism is provided by combining water adsorption measurements with GCMC calculations. This transformation shows a relative flexibility of the network (cell volume variation of -6%) reminiscent of the breathing effect reported in the MOF such as the MIL-53 series. The anhydrous phase <b>1'</b> is able to adsorb reversibly water as experienced with the measurement of water vapor isotherms.