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

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(NO ₃ ) 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-Zn ₃ (PO ₄ ) 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 PO (symmetric), -POO ^- , and PO (asymmetric) stretching vibrations of the PO ₄ ^3- groups. According to theoretical simulations, the observed VCD activity can be rationalised in terms of vibrational coupling between two PO ₄ ^3- groups in a generated chiral zinc(II) orthophosphate cluster.