Structures and single crystal to single crystal transformations of cadmium frameworks using a flexible tripodal ligand

Abstract

A highly flexible tris-imidazole ligand N(CH₂-m-C₆H₄-CH₂-imidazole)₃ (L) constructed using a tris(xylyl) backbone was synthesized. Its reactions with three cadmium(II) salts gave various Cd(II) coordination polymers, namely [CdL₂](ClO₄)₂ (1), [CdLCl₂] (2), and [CdL(OAc)₂] (3). X-ray studies revealed that 1 crystallized in the trigonal space group R with high symmetry, while 2 and 3 crystallized in lower symmetrical space groups due to Cd–anion coordination. The flexibility of ligand L was evidenced in these structures, showing different conformations of L upon coordination and crystallization. In addition, complex [CdL₂]Cl₂ (4) was obtained by the anion exchange of 1 with sodium chloride via a single crystal to single crystal transformation. Similar to 1, complex 4 also crystallized in the trigonal space group R with a = 13.0945(3), b = 37.4353(10), and V = 5559.2(2). Importantly, this single crystal to single crystal transformation is crucial to afford this Cd(II) coordination polymer with unbound Cl⁻ anions, as the direct reaction of L and CdCl₂ gives complex 2 with Cd-bound Cl⁻. This work shows the importance of flexible ligands in constructing various coordination polymer structures through the regulation of anions.