The Role of Polar, Lamdba (Λ)-Shaped Building Units in Noncentrosymmetric Inorganic Structures

A methodology for the design of polar, inorganic structures
is
demonstrated here with the packing of lambda (Λ)-shaped basic
building units (BBUs). Noncentrosymmetric (NCS) solids with interesting
physical properties can be created with BBUs that lack an inversion
center and are likely to pack into a polar configuration; previous
methods to construct these solids have used NCS octahedra as BBUs.
Using this methodology to synthesize NCS solids, one must increase
the coordination of the NCS octahedra with maintenance of the noncentrosymmetry
of the bulk. The first step in this progression from an NCS octahedron
to an inorganic NCS solid is the formation of a bimetallic BBU. This
step is exemplified with the compound CuVOF4(H2O)7: this compound, presented here, crystallizes in an
NCS structure with ordered, isolated [Cu­(H2O)5]2+ cations and [VOF4(H2O)]2– anions into Λ-shaped, bimetallic BBUs to form
CuVOF4(H2O)6·H2O,
owing to the Jahn–Teller
distortion of Cu2+. Conversely, the centrosymmetric heterotypes
with the same formula MVOF4(H2O)7 (MII = Co, Ni, and Zn) exhibit ordered, isolated [VOF4(H2O)]2– and [M­(H2O)6]2+ ionic species in a hydrogen bond network.
CuVOF4(H2O)7 exhibits a net polar
moment while the heterotypes do not; this demonstrates that Λ-shaped
BBUs give a greater probability for and, in this case, lead to NCS
structures.