Nanoscale chirality generated in zinc(ii) orthophosphate clusters: evidence by vibrational circular dichroism

Abstract

Layered zinc(II) hydroxides (LZH) intercalating the deprotonated forms of R-(−) or S-(+)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphate (denoted as R- or S-BNDHPH, respectively) were prepared from Zn(NO₃)₂ at pH 5 and 60 °C by the mixing method. The obtained hybrid compounds (denoted as R- or S-BNDHP⁻/LZH, respectively) were heated from room temperature up to 800 °C under nitrogen atmosphere. According to the thermal gravimetric/differential thermal analysis measurements, hydroxyl groups were dehydrated at 270–400 °C, followed by the decomposition of organic components at 420–600 °C. X-ray diffraction patterns and scanning electron microscopy images indicated that the final products were a mixture of α-Zn₃(PO₄)₂, ZnO crystals and non-crystalline zinc(II) orthophosphates. Vibrational circular dichroism (VCD) spectra were recorded before and after calcination. Before calcination, R- or S-BNDHP⁻/LZH exhibited VCD peaks assigned to intercalated R- or S-BNDHP⁻. The calcined products exhibited several VCD peaks in the range of 900–1200 cm⁻¹, maintaining the mirror-image relationship between R-BNDHP⁻/LZH and S-BNDHP⁻/LZH used as starting materials. The observed peaks were assigned to the PO (symmetric), –POO⁻, and PO (asymmetric) stretching vibrations of the PO₄³⁻ groups. According to theoretical simulations, the observed VCD activity can be rationalised in terms of vibrational coupling between two PO₄³⁻ groups in a generated chiral zinc(II) orthophosphate cluster.