Understanding the role of helical chains in the formation of noncentrosymmetric solids.

A general, new principle for the design of noncentrosymmetric solids is evident in the packing of helical metal-oxyfluoride chains in Zn(C4H4N2)(H2O)2MoO2F4 (C4H4N2 ) pyrazine, pyz). Each inorganic helical chain comprises alternating Znand Mocentered octahedra and interleaves with six more helices of the same handedness through Zn-pyrazine-Zn bridges. From comparisons with similar structures which are centrosymmetric, the small organic ligand, pyrazine, plays a key role in the acentric packing of inorganic helices, while the cis-MoO2F4 and transZn(pyz)2(H2O)2F2 metal coordination geometries are important for helix formation. Major efforts in solid-state chemistry have as their goal the ability to engineer new inorganic frameworks that will yield desired physical properties. For instance, the designed synthesis of noncentrosymmetric solids is a challenging problem for the future fabrication of materials with ferroelectric, pyroand piezoelectric, and other nonlinear optical (NLO) properties.1 Because they lack an inversion center, helical chains have been found in many acentric solids, including KTiOPO4 (KTP),2 SiO2 (quartz),3 Cu(pyridine)4Cr2O7, and others.5-8 However, an equally large group of materials that contain helical chains do have inversion centers. In general, how to design or make a noncentrosymmetric solid from inorganic helices relies on our understanding of ligand effects and crystal packing. Here we explain a rational way to understand the combined effects of ligand length and helical symmetry in noncentrosymmetric compounds, as illustrated by Zn(pyz)(H2O)2MoO2F4. Zn(pyz)(H2O)2MoO2F4 was synthesized via typical hydrothermal procedures9 and was characterized by single-crystal X-ray diffraction methods to have two enantiomorphic forms, space groups P3121 and P3221 (used for the Figures and discussion).10-12 The helical chain repeat unit, alongside a longer space-filling version, is shown in Figure 1 down the [011] projection. The 31 screw axis passes down the center of the inorganic spiral, while a two-fold rotation bisects each O-Mo-O angle of 102.0(2)o. The local Mo environment, identified before as the MoO2F4 anion,13,14 includes two cis-O positions at 1.75 Å, and four F at 1.93 Å (×2) and 2.06 Å (×2). The Mo atom is displaced from the octahedral center toward the cis-O atoms, with distances that are (0.05 Å to that for the previously characterized MoO2F4 anion.14 The two longer Mo-F distances are cis to each other, notably trans to the cis-O atoms, and are shared with Zn at 2.06 Å (×2) again. The coordination around each Zn is completed by two pyrazine molecules, Zn-N at 2.21 Å (×2), and two water molecules, Zn-O at 2.05 Å (×2), to give a Zn(pyz)2(H2O)2 cation. The division of the helical chain into anionic and cationic components, although arbitrary here, is helpful for structural comparisons (below) where these ions are distinguishable.