Hydrogen-Bonding Assembly of Coordination Polymers Showing Reversible Dynamic Solid-State Structural Transformations

We herein report the synthesis, single-crystal structures of coordination polymers, and structural transformations of complexes employing 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazinedicarbonitrile (tdpd(2-)) and pyrazine (pyz) as bridging ligands. {[M(H2O)(4)(pyz)][M(tdpd)(2)(pyz)]6(H2O)}(n), [1.10H(2)O and 2.10H(2)O where M = Co (1) and Zn (2)], consists of two types of crystallographically independent one-dimensional (1D) structures packed together. One motif, [M(tdpd)(2)(pyz)](2-) (A), is an anionic infinite pyz bridged 1D array with chelating tdpd(2-) ligands, and the other motif is a cationic chain, [M(H2O)(4)(pyz)](2+) (B), which is decorated with four terminal water molecules. The 1D arrays (A) and (B) are arranged in parallel by multi-point hydrogen-bonding interactions in an alternate (A)(B)(A)(B) sequence extending along the c-axis. Both compounds exhibit structural transformations driven by thermal dehydration processes around 350 K to give partially dehydrated forms, 1.2H(2)O and 2.2H(2)O. The structural determination of the partially dehydrated form, 2.2H(2)O, reveals a solid-state structural transformation from a 1D chain structure to a two-dimensional (2D) coordination sheet structure, [Zn-2(tdpd)(2)(H2O)(2)(pyz)](n) (2.2H(2)O). Further heating to 500 K yields the anhydrous form 2. While the virgin samples of 1.10H(2)O and 2.10H(2)O crystallize in different crystal systems, powder X-ray diffraction (PXRD) measurements of the dehydrated forms, 1.2H(2)O and 2.2H(2)O, are indicative of the same structure. The structural transformation is irreversible for 1.10H(2)O at ambient conditions. On the other hand, compound 2.10H(2)O shows a reversible structural change. The solid-state structural transformation for 1.10H(2)O was also confirmed by monitoring in-situ magnetic susceptibility, which is consistent with other thermally-induced measurements.