New Coordination Polymer Changing Its Color upon Reversible Deintercalation and Reintercalation of Water: Synthesis, Structure, and Properties of Poly[Diaqua-(μ₂-Squarato-<i>O</i>,<i>O</i>‘)-(μ₂-4,4‘-Bipyridine-<i>N</i>,<i>N</i>‘)- Manganese(II)] Trihydrate

The new coordination polymer poly[diaqua-(μ2-squarato-O,O‘)-(μ2-4,4‘-bipyridine-N,N‘)-manganese(II)] trihydrate was prepared by hydrothermal reaction of MnCl2·4 H2O with squaric acid and 4,4‘-bipyridine in water. In the crystal structure the manganese atoms are octahedrally coordinated by two squarate anions, two 4,4‘-bipyridine ligands, and two water molecules. The squarate anions and the 4,4‘-bipyridine ligands connect the metal atoms into interpenetrated sheets and are connected via O−H···O hydrogen bonding. From this arrangement channels are formed in which additional water molecules are intercalated. The channel water molecules can be reversibly de- and intercalated by heating or in a vacuum in a presumably topotactic reaction. The water molecules bound to the manganese atoms can also be removed on heating leading to the new intensely yellow compound Mn(C4O4)(4,4‘-bipyridine). Even this compound incorporates water leading to the formation of the starting material. The reaction is accompanied with a change of the color of the material from yellow to colorless. All reactions were investigated using simultaneous differential thermoanalysis, thermogravimetry and mass spectroscopy, ex-situ X-ray powder diffraction and in-situ temperature-resolved X-ray powder diffraction, as well as optical microscopy and UV−vis spectroscopy.