Crystallographic studies into the role of exposed rare earth metal ion for guest sorption

Crystallographic studies into the role of exposed metal ions on pore surfaces of a porous gadolinium 1,3,5-bezenetricarboxylate have been performed. High-quality single crystals of [Gd(btc)] (1) were readily obtained by heating the dichloromethane-exchanged single crystals of [Gd(btc)(H2O)]·(DMF)(H2O)0.5 (1–H22O·DMF), since the latter always crack during direct heating. Crystal structure analyses revealed that the Gd3+ ions were exposed on the channel surface of 1, although the coordination environments were slightly changed from the as-synthesized phase. 1 is able to adsorb and exchange some small molecules reversibly, but the PXRD patterns give very limited information about the sorption mechanism. Single-crystal structures of the guest-loaded phases [Gd(btc)(H2O)]·(H2O)5 (1–H22O·H2O), [Gd(btc)(EtOH)]·(EtOH) (1a–EtOH·EtOH), and [Gd(btc)(EtOH)]·(MeCN) (1–EtOH·MeCN) have been successfully determined, which revealed that Gd3+ ions coordinate with the oxygen donors of guest molecules. After the metal sites are saturated, other adsorbed guests were stabilized by hydrogen-bonding interactions with the coordinated guests or carboxylate O atoms on the channel walls. The exposed Gd3+ ions are preferentially coordinated by ethanol molecules, even though ethanol has a much lower concentration than that of acetonitrile in the mixture. Loading pure EtOH distorts the framework to a low symmetry (P41), while all other phases have a higher symmetry (P4122). The stabilities against heating and humidity, and gas adsorption properties have also been studied for the related compounds.