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)]center dot(DMF) (H2O)(0.5) (1-H2O center dot 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)]center dot(H2O)(5) (1-H2O center dot(HO)-O-2), [Gd(btc)(EtOH)]center dot(EtOH) (1a-EtOH center dot EtOH), and [Gd(btc)(EtOH)]center dot(MeCN) (1-EtOH center dot 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 (P4(1)), while all other phases have a higher symmetry (P4(1)22). The stabilities against heating and humidity, and gas adsorption properties have also been studied for the related compounds.