Synthesis, crystal structures and investigations on the dehydration reaction of the new coordination polymers poly[diaqua-(μ2-squarato-O,O′)-(μ2-4,4′-bipyridine-N,N′)Me(II)] hydrate (Me=Co, Ni, Fe)

The three new isostructural coordination polymers polydiaqua-(μ₂-squarato-O,O′)-(μ₂-4,4′-bipyridine-N,N′)Me(II)] hydrate (Me=Fe, Co, Ni) were prepared by hydrothermal reaction. All compounds are isostructural and crystallize in the monoclinic space group P2₁/c with 4 formula units in the unit cell (a=18.893 (1) Å, b=11.450 (1) Å, c=8.0985 (4) Å, β=93.032 (5)°, V=1749.5 (2) ų, Fe(C₄O₄)(C₁₀H₈)(H₂O)₂]·(H₂O)₃; a=18.937 (1) Å, b=11.342 (1) Å, c=8.0545 (5) Å, β=91.83 (1)°, V=1725.3 (2) ų, Co(C₄O₄)(C₁₀H₈)(H₂O)₂] · (H₂O)₃; a=18.271 (1) Å, b=11.340 (1) Å, c=7.8946 (4) Å, β=90.69 (5)°, V=1633.1 (2) ų, Ni(C₄O₄)(C₁₀H₈)(H₂O)₂](H₂O)_1.7). In the crystal structures the metal atoms are coordinated by two squarate dianions, two 4,4′-bipyridine ligands and two water molecules. The metal atoms are connected via the squarate dianions and the 4,4′-bipyridine ligands into layers, which interpenetrate forming a three-dimensional coordination network. This arrangement yields channels in which additional water molecules are embedded. Thermoanalytic investigations show that upon heating the channel water is removed in the first step and that the water coordinated to the metal atoms is emitted in the second step. Both steps are fully reversible with the former reaction proceeding via a topotactic reaction. The hydration and dehydration of the compounds are accompanied with a continuous change of the color of the materials. The de- and reintercalation processes were investigated using single crystal structure analysis, X-ray powder diffraction, temperature-dependent X-ray powder diffraction, simultaneous differential thermoanalysis and thermogravimetry coupled to mass spectroscopy, differential scanning calorimetry and time-dependent UV-Vis spectroscopy. The results of the investigations are discussed and compared with those for the previously reported manganese compound.