Theoretical studies on the chiral polyoxoanions [P2Mo18O62]6− and [PMo9O31(OH2)3]3− with histidine: chiral inversion and chiral induction

Abstract

Herein, the electronic structures and electronic circular dichroism (ECD) spectra of the chiral Wells–Dawson (W–D) polyoxoanion [(L,D-C₆H₁₀N₃O₂)P₂Mo₁₈O₆₂]⁵⁻ (1a/1b) and chiral polyoxoanion [(L,D-C₆H₁₀N₃O₂)PMo₉O₃₁(OH₂)₃]²⁻ (2a/2b) are investigated using density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Compared to the fully oxidized [P₂Mo₁₈O₆₂]⁶⁻ structure, the Mo–O_b bond lengths in the one-electron reduced state, [P₂Mo₁₈O₆₂]⁷⁻, tends to be average. It is reasonable that organic ligands can transfer electrons to POMs and affect their chirality. The energy difference between the chiral L-[P₂Mo₁₈O₆₂]⁶⁻ with D₃ symmetry and ideal [P₂Mo₁₈O₆₂]⁶⁻ with D_3h symmetry is 5.88 kcal mol⁻¹, which suggests that chirality inversion may occur from the L-isomer to the D-isomer through the ideal [P₂Mo₁₈O₆₂]⁶⁻ by crossing a small energy barrier. Meanwhile, the interaction energy between the L-isomer and L-histidine ligand is larger than that of L-isomer and D-histidine, which indicates that L,D-[P₂Mo₁₈O₆₂]⁶⁻ is induced and further separated by the chiral histidine. The origins of the chiral polyoxoanion are mainly ascribed to charge-transfer (CT) transitions from the O atoms to Mo atoms or organic ligands to Mo atoms. These results confirm that organic cations have an induced effect on chiral POMs.