Cyclic Water Hexamers and Decamers in a Porous Lanthanide−Organic Framework:  Correlation between Some Physical Properties and Crystal Structure

Erbium fumarate [Er₂(fum)₃(H₂O)₄]·8H₂O, <b>1</b>, (fum = fumarate dianion) crystallizes at room temperature in a silica gel column. The structure is three-dimensional and consists of layers of erbium fumarate pillared by fumarate dianions acting in the bis-chelating mode. The layers contain arrays of erbium cations bridged by fumarate dianions acting in the bis-bidentate bridging mode. The relatively large channels formed in the direction of the <i>a</i> and <i>c</i> axes are filled by water molecules that form a hydrogen bonded layer, comprising cyclic hexamers with both boat and chair conformations and cyclic decamers with boat−chair−boat conformation. The water molecules are reversibly removed at 90 °C. The evacuated metal−organic framework, <b>2</b>, is stable up to 400 °C. Both compounds <b>1</b> and <b>2</b> exhibit weak antiferromagnetic interactions.