A New Family of Lanthanide Borate Halides with Unusual Coordination and a New Neodymium-Containing Cationic Framework

The reactions of Ln2O3/CeO2/Pr6O11 (Ln = La–Nd, Sm), molten boric acid, and concentrated HBr or HI result in the formation of La­[B7O10(OH)3(H2O)­Br], Ln­[B6O9(OH)2(H2O)2Br]·0.5H2O (Ln = Ce, Pr), Nd2[B12O17.5(OH)5(H2O)4Br1.5]­Br0.5·H2O (NdBOBr), Sm4[B18O25(OH)13Br3], and Ln­[B7O11(OH)­(H2O)3I] (Ln = La–Nd, Sm). The lanthanide­(III) centers in these compounds are found with 9-coordinate hula hoop or 10-coordinate capped triangular cupola geometries, where there are six approximately coplanar oxygen donors provided by the polyborate sheet. The sheets are formed into three-dimensional frameworks via BO3 triangles that are roughly perpendicular to the layers. Additionally, a new cationic framework, NdBOBr, has been isolated. NdBOBr is unusual in that not only is it a cationic framework, but it is also the first trivalent f-element borate to have terminal halides bound exclusively to the base site of the hula hoop. The Ln­[B7O11(OH)­(H2O)3I] (Ln = La–Nd, Sm) structures require two corner-shared BO3 units in order to tether the layers together because of the large size of the capping iodine atom.