Cold Crystallization in Schiff-Base Nickel(II) Complexes Derived from Three Toluidine Isomers

To test metal complexes as potential compounds for heat-storage materials, two Schiff-base Ni(II) complexes derived from two differently N-substituted toluidine isomers were examined. Comparing the behavior with that of a similar complex previously reported, it was found that the cold crystallization was realized in polymorphic complexes. The activation energy for crystallization estimated by using differential scanning calorimetry was well correlated to the ease of supercooling necessary for the cold crystallization. DFT calculations of molecular energy suggested that the cold crystallization phenomenon is closely related to a molecular flexibility that prevents normal crystallization during cooling. The molecular flexibility controls the ease in the supercooling of liquid and the nucleation–growth process of crystallization by altering the activation energy for crystallization.