| Summary: | Reaction of M(OAc)<sub>2</sub>·<i>x</i>H<sub>2</sub>O (M, <i>x</i> = Zn, 2 and Co, 4), 1,4-dihydro-5,6-dicyano-2,3-pyrazinedione (H<sub>2</sub>CN<sub>2</sub>pyzdione), and pyrazine (pyz) affords two compounds of the same molecular formula {[M(H<sub>2</sub>O)<sub>6</sub>][M(CN<sub>2</sub>pyzdione)<sub>2</sub>(pyz)]·6H<sub>2</sub>O}<sub>n</sub> (M = Zn for <b>1</b> and Co for <b>2</b>) in which discrete units of [M(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> are linked to one-dimensional chains of [M(CN<sub>2</sub>pyzdione)<sub>2</sub>(pyz)]<sup>2–</sup> via multiple O–H···O hydrogen-bonding interactions and M<sup>2+</sup>-bound H<sub>2</sub>O molecules in [M(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> also serve as linkers of hydrogen-bonded interstitial H<sub>2</sub>O molecules. Remarkably, <b>1</b> crystallizes in the monoclinic crystal system, the similar crystal system and unit cell parameters as <b>2</b>, but with a space group distinct from <b>1</b> and <b>2</b>, i.e., <b>1</b> is the noncentrosymmetric space group <i>C</i>2, whereas <b>2</b> is the centrosymmetric space group <i>C</i>2/<i>m</i>. This polar structure for <b>1</b> is induced by the presence of alternating arrangements of distinguishable two axial Zn–N bonds within [Zn(CN<sub>2</sub>pyzdione)<sub>2</sub>(pyz)]<sup>2–</sup> chains. Indeed, solid-state circular dichroism spectra of <b>1</b> exhibit significant Cotton effects, as evidenced by the polar space group <i>C</i>2. Moreover, these Cotton effects show clear temperature-dependence depending on contents of H<sub>2</sub>O molecules of <b>1</b>.
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