Immobilization, Trapping, and Anion Exchange of Perrhenate Ion Using Copper-Based Tripodal Complexes

We describe a multidentate tripodal ligand in which three pendant arms carrying di(2-picolyl)amine units are linked to the ortho positions of a tris(o-xylyl) scaffold, providing N(CH2-o-C6H4CH2N(CH2py)2)3 (L). Reaction of L with CuCl2 in the presence of hexafluorophosphate anion afforded blue cubes of [(CuCl)3L](PF6)3·5H2O (1). Crystallographic studies of 1 revealed that the three symmetry-related arms each coordinate a {CuIICl} unit, and two molecules of 1 are connected to one another through a Cu(μ-Cl)2Cu bridge, extending the molecular structure to form a two-dimensional (2-D) layer. These 2-D layers pack in an ABCABC... fashion with PF6– anions located in between. Reaction of 1 with a stoichiometric amount of perrhenate ion afforded blue plates of [(CuCl)3L](PF6)(ReO4)2·3H2O (2). Compound 2 has the same lattice structure as 1, but the tricopper unit backbone now traps one ReO4– anion through Coulombic interactions. In addition, three molecules of 2 are bridged by a perrhenate ion, forming a Cu3(μ3-ReO4) cluster, to give a different 2-D structure displaying a rare tridentate bridging ReO4– mode. Thus, in addition to classic perrhenate trapping through weak Coulombic interactions, 2 represents an exceptional example in which the ReO4– anion is immobilized in an extended framework through tight covalent interactions. The interlamellar PF6– anions in 1 can be exchanged with other anions including perrhenate, perchlorate, or periodate. The structural similarity between perrhenate and pertechnetate makes these materials of potential interest for pertechnetate trapping.