Reversible Single-Crystal to Single-Crystal Transformation and Associated Magnetism of a Cyanide-Bridged Chiral-Structured Magnet

A chiral molecule-based magnet [Mn^II (<i>S</i>-pnH) (H₂O)][Mn^III(CN)₆]·H₂O (<i>S</i>-pn = <i>S</i>-1,2-diaminopropane), <b>1<i>S</i></b>·2H₂O (<i>P</i>2₁2₁2₁), has been obtained, which has a two-dimensional (2D) square network of cyanide-bridged Mn^II-Mn^III ions. The crystallographic research on this coordination polymer indicates that it is robust enough to transform the single-crystal structure upon dehydration as well as rehydration. Meanwhile, the magnetic property changes are reversibly associated with the structural phase transitions. The complete reversibility upon dehydration/rehydration was demonstrated using in situ X-ray diffraction from the as-prepared <b>1<i>S</i></b>·2H₂O to the dehydrated [Mn^II (<i>S</i>-pnH)][Mn^III(CN)₆], <b>1<i>S</i></b>, then rehydrated to [Mn^II (<i>S</i>-pnH) (H₂O)][Mn^III(CN)₆]·H₂O, <b>1<i>S</i>-HP</b>, and dehydrated again to [Mn^II (<i>S</i>-pnH)][Mn^III(CN)₆], <b>1<i>S</i>-DeHP</b>. Neither the space groups nor the crystal axes changed during the dehydration/rehydration process. The dehydration is considered to be associated with a change in the coordination of the <i>S</i>-pnH from one 2D layer to the neighboring one involving a proton transfer and is accompanied by new cyanide bridging of Mn^II and Mn^III ions formed between the 2D sheet. Corresponding to the 2D to three-dimensional (3D) structural transition, the study of magnetic properties reveals that the Curie temperature of long-range magnetic ordering for <b>1</b><i><b>S</b></i> (<i>T</i>_C = 45 K) is more than double that for <b>1</b><i><b>S</b></i>·2H₂O (<i>T</i>_C = 21 K).