From Racemic Units to Polar Materials

A new route is described that enables the design of polar materials using racemic basic building units (BBUs). Λ- and Δ-[Cu­(H2O)­(bpy)2]2+ complexes in noncentrosymmetric [Cu­(H2O)­(bpy)2]2[HfF6]2·3H2O and centrosymmetric [Cu­(H2O)­(bpy)2]­[BF4]2 reveal that racemic BBUs in the solid state can lead directly to noncentrosymmetry. The structure is polar if only mirror or glide planes relate the left- and right-handed enantiomers, whereas nonpolar, achiral structures result if rotoinversion relates the left- and right-handed enantiomers. This structural analysis also provides an alternative route in the design of polar materials that had always been engineered from polar BBUs.