A polynuclear coordination glutarate of lanthanum(III) with an uncommon cage feature

The title compound, tri­aqua­tris­(glutarato)­dilanthanum(III) dihydrate, {[La₂(C₅H₆O₄)₃(H₂O)₃]·2H₂O}_n, is the first re­ported glutarate coordination polymer of lanthanum(III) without a protonated ligand. The noteworthy features in the structure are, firstly, the unusual binuclear lanthanum cage formed by three bridging bonds through O atoms involved in different coordination modes and, secondly, the very rare `malonate' mode exhibited by a di­carboxyl­ate ligand with an alkyl chain of five C atoms. To our knowledge, this η⁷ chelation for the glutarate ligand has not been reported and was thought to be forbidden for steric reasons. The gauche–gauche conformation of the corresponding ligand favours cage formation, but trans geometries created along the ligating O atoms prevent cluster packing. The two independent La atoms are nine‐ and tenfold coordinated, leading to distorted one‐face‐sharing LaO₇(H₂O)₂ and LaO₉(H₂O) polyhedra, respectively. In the three‐dimensional framework, these asymmetric subunits are linked in a zigzag manner via one‐edge‐sharing LaO₉(H₂O) polyhedra and are connected by the carbon backbone chains of the ligands. The structure is very compact and, unlike many other reported di­carboxyl­ate lanthanides, connectivity between the two metal atoms and the three ligands yields a crystal packing with cavities accommodating two guest water mol­ecules but without an open framework.