Systematic centricity control using a chiral template: novel noncentrosymmetric polar niobium oxyfluorides and tantalum fluorides directed by chiral histidinium cations, [(l-hisH2)NbOF5], [(d-hisH2)NbOF5], [(l-hisH2)TaF7], and [(d-hisH2)TaF7]

Abstract

Enantiomorphic histidinium cation-driven niobium and tantalum (oxy)fluoride compounds have been systematically synthesized through a slow evaporation method. Single-crystal X-ray diffraction indicates that all four reported chiral materials crystallize in the noncentrosymmetric polar space group, P2₁. While [NbOF₅]²⁻ distorted octahedra are templated by enantiopure histidinium cations in [(L-hisH₂)NbOF₅] (L-Nb) and [(D-hisH₂)NbOF₅] (D-Nb), monocapped trigonal prismatic [TaF₇]²⁻ units are aligned by the acidified amino acid cations in [(L-hisH₂)TaF₇] (L-Ta) and [(D-hisH₂)TaF₇] (D-Ta). The frameworks of the title compounds exhibit thermal stability up to ca. 180 °C. While the band gaps of L-Nb and D-Nb were found to be ca. 4.45 and 4.47 eV, respectively, those of L-Ta and D-Ta turned out to be higher, with values of 5.07 and 5.06 eV, respectively. Clearly reversed signals observed in the solid-state circular dichroism spectra confirm the chirality of the reported materials. Powder second-harmonic generation (SHG) measurements indicate that the title compounds are type-I phase-matchable and reveal an SHG efficiency that is ca. 0.4–0.6 times that of KH₂PO₄.