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