(R)- and (S)-[C8H10NO3]2[NbOF5]: noncentrosymmetric niobium oxyfluorides with large optical anisotropy

Abstract

Huge crystals of noncentrosymmetric (NCS) organic–inorganic hybrid niobium oxyfluorides, (R)-[C₈H₁₀NO₃]₂[NbOF₅] [(R)-Nb] and (S)-[C₈H₁₀NO₃]₂[NbOF₅] [(S)-Nb], have been easily grown via a slow evaporation method in high yields through the systematic driving of chiral structure-directing agents, 4-hydroxy-D-(−)-2-phenylglycine [(R)-HPG] and 4-hydroxy-L-(+)-2-phenylglycine [(S)-HPG], respectively. Both (R)-Nb and (S)-Nb crystallizing in the NCS nonpolar space group, P2₁2₁2, exhibit pseudo-three-dimensional structures assembled via multiple hydrogen bonding interactions between distorted [Nb(O/F)₂F₄]²⁻ octahedra and the respective protonated HPG cations. Interestingly, crystals of (R)-Nb and (S)-Nb exhibit large birefringence (Δn_exp) of 0.19–0.199@589.3 nm attributable to the strong π–π interactions arising from the stacked benzene rings in HPG cations. The reported materials exhibit strong stability in most solvents in a wide range of polarities. Detailed characterization studies, including thermal properties, various spectroscopic analyses, second-harmonic generation measurements, and electronic structure calculations on the reported materials, are also described.