Two Polymorphs of the Magnetic <i>Catena</i> ‐Orthoperiodato‐Cuprate(II) K ₃ [CuIO ₆ ]·4H ₂ O from Ultra‐Alkaline Media

The blue α ‐polymorph of the orthoperiodato‐cuprate(II) K 3 [CuIO 6 ]·4H 2 O is synthesized in a polytetrafluoroethylene‐lined autoclave at 150 °C by reacting copper(II) nitrate and potassium periodate in aqueous 30 m potassium hydroxide solution. The monoclinic crystal structure (space group P 2 1 / c ) consists of CuIO 6 ] 3– chains, which are separated by K + cations and water molecules. The copper(II) cation is in square planer coordination by oxygen atoms of two fully deprotonated periodate octahedra. The (CuO 4/2 )(IO 4/2 O 2/1 )] 3– chain contains both the Δ and the Λ enantiomers. At the higher temperature of 200 °C and with an excess of potassium periodate, violet to purple crystals of β ‐K 3 [CuIO 6 ]·4H 2 O precipitate. The monoclinic structure (space group P 2 1 / m ) comprises chains (CuO 2/3 O 2/2 )(IO 1/3 O 2/2 O 3/1 )] 3– , which have the same composition as those of the α ‐form but a different internal connectivity. K 3 [CuIO 6 ]·4H 2 O is dehydrated above 100 °C. The α ‐polymorph is paramagnetic down to 1.8 K, and the magnetic moment aligns with the d 9 configuration of copper(II). The β ‐form, which allows for direct as well as super‐exchange between the moments in the linear spin‐½ chain, shows a highly complex magnetism that indicates antiferromagnetic coupling and magnetic frustration.