Anion-driven self-assembly of tetrapyridyl ligand with a twist

Abstract

The unique three-dimensional tetrapyridyl ligand (tpb) with a twist is found to exhibit remarkable dependence on the counter anion. The treatment of tpb with Cu(OAc)₂ is found to lead to a kinetically driven tricopper-mediated metal organic framework 1, which dissociates under the conditions of its formation upon extended duration into a discrete supramolecular metal organic cube. Although tricopper complexes have largely been examined from the point of view of their importance in biology and magnetic properties, it is only the second instance, to the best of our knowledge, where such a species has been found to mediate the self-assembly into a three-dimensional coordination polymer. In contrast, an altogether distinct and unique three-dimensional helical structure 3 with voids for guest occupation is obtained when the same ligand tpb is treated with Cu(NO₃)₂. The reason for a dramatic change in the architectures of the coordination polymers for variation of an acetate anion in 1 to a nitrate in 3 for the same metal cation in both of the cases should be traceable to the stronger propensity of the acetate anion in 1 to be involved in direct coordination to the metal; the weak nitrate ligand in 2 remains as a counter anion, and facilitates, presumably, the formation of helical structure along with the solvent molecules.