Water-induced reversible structural phase transformation with chromotropism in metal supramolecular frameworks containing aminopyrazine and sulfate anions

Abstract

Three new supramolecular metal-coordination architectures [M(H₂O)₄(ampyz)₂][M(H₂O)₆](SO₄)₂(H₂O)₂ (M = Co (1), Fe (2), and mixed Co/Fe (3); ampyz = 2-aminopyrazine) and a Cd(II) coordination polymer [Cd(ampyz)(H₂O)₂(SO₄)]_n(H₂O)_n (4) were synthesized by layered diffusion and structurally characterized. Compounds 1–3 are isomorphous. Monomeric complex units [M(H₂O)₄(ampyz)₂]²⁺ are assembled by intermolecular hydrogen bonding and π–π stacking via ampyz moieties to build 2D sheets. These layers are linked by intermolecular hydrogen bonding via ampyz ligand, [M(H₂O)₆]²⁺ unit, sulfate anion and lattice and coordinated water molecules to stabilize 3D supramolecular structure. Compound 4 shows a 2D coordination network where the sulfato connector and the bridging ampyz link Cd(II) ions. Lattice water molecules are filled in between the layers via the hydrogen bonds stabilizing 3D supramolecular network of 4. Interestingly, compounds 1–3 exhibit water-induced reversible crystal-to-amorphous transformations with chromotropism and compound 4 also shows water-induced reversible structural phase transformation confirmed by spectroscopic techniques, elemental analyses, TGA, and XRPD. These results demonstrate the significant role between coordination bridges and weaker intermolecular interactions for the structural conversion even though 1–3 lack coordination bridges for the maintenance of their structures, however, the weak intermolecular interactions, especially hydrogen bonding, play a key role to the recovery of the crystalline phase.