Novel noncentrosymmetric polar coordination compounds derived from chiral histidine ligands

Abstract

Crystals of chiral coordination compounds, [L-C₆H₁₀N₃O₂ZnBr₃] (L-ZnBr), [D-C₆H₁₀N₃O₂ZnBr₃] (D-ZnBr), [L-C₁₂H₂₀N₆O₄Cd₂Cl₅] (L-CdCl), and [D-C₁₂H₂₀N₆O₄Cd₂Cl₅] (D-CdCl), have been grown in high yields by using MX₂ (M = Zn or Cd; X = Br or Cl) and L- or D-histidine by a slow evaporation method in acidic media. Single crystal X-ray diffraction reveals that while the structures of L-ZnBr and D-ZnBr crystallizing in the polar space group, P2₁, consist of slightly distorted ZnOBr₃ tetrahedra and the respective chiral histidine ligands, those of L-CdCl and D-CdCl with the triclinic noncentrosymmetric (NCS) space group, P1, are composed of distorted CdOCl₄ trigonal bipyramids and the corresponding histidine ligands. Whereas the structures of L-ZnBr and D-ZnBr can be extended to pseudo-1D chains via hydrogen bonds, those of L-CdCl and D-CdCl can be assembled to pseudo-3D frameworks through hydrogen bonds and π–π interactions. UV-vis diffuse reflectance spectra suggest that the title compounds have large optical band gaps of ca. 5 eV. The powder second-harmonic generation (SHG) measurements indicate that the new materials with NCS structures have mild SHG efficiencies of ca. 0.2 times that of KH₂PO₄, and type-I phase-matching behaviors. Circular dichroism spectra have confirmed the absolute configurations of the title compounds further. The synthesis approach suggests a systematic method toward the discovery of novel coordination compounds with NCS structures.