Synthesis and Characterization of Two Polymorphic Crystalline Phases and an Amorphous Powder of Nickel(II) Bisimidazolate

Nickel(II) bisimidazolate is polymorphic. Depending on the synthetic strategy adopted, two crystalline phases (α-
and β-Ni(im)2) or an amorphous material of the same composition can be prepared. The thermodynamically stable
α-Ni(im)2 phase, which can be prepared in water at elevated temperatures, contains a two-dimensional polymer (of
nearly square meshes) with square-planar NiN4 chromophores and exo-bidentate imidazolate ligands bridging nickel
atoms that are ca. 5.73 Å apart. The β-Ni(im)2 phase can be kinetically stabilized at lower temperatures, but the
structural complexity and the lack of single crystals prevented its full structural characterization, even in the presence
of an indexed powder diffraction pattern. The spectroscopic features of these crystalline phases are compared with
those of the amorphous material.