Controlling the Polymorph of Ln^III(NO₃)₃_-<i>_x</i>(OH)<i>_x</i>[15-MC_Cu^_II_(N)S-pheHA-5] Complexes through Solvent Type and Ln^III Ion Choice

Two general polymorphs of LnIII(NO3)3-x(OH)x[15-MCCuII(N)S-pheHA-5] (where LnIII = LaIII, SmIII, GdIII, TbIII, DyIII, HoIII, ErIII, LuIII, or YIII and x = 0, 0.5, or 1) have been previously reported; however, no general hypothesis had been proposed for a method to control the resulting polymorph. We now demonstrate that both solvent and LnIII ion choice play a pivotal role in the resulting three-dimensional structure. By using only water as the solvent, a dimer structure can be achieved; however, if a 5:1 methanol/water solvent is used, a helical chain is produced. Of interest, even in the 5:1 methanol/water solvent, LaIII 15-MC-5 complexes will only produce dimer structures in the solid state, and the helical chain has never been produced. We suggest that this restriction is due to the difference in size and preferred coordination number for LaIII, which does not allow for the proper positioning of nitrate anions to form the helical structure.