Enantiomeric Polyoxometalates Based on Malate Chirality-Inducing Tetra-Zr^IV–Substituted Keggin Dimeric Clusters

Abstract

On the basis of the synergistic strategy of lacunary polyoxometalate structure-directing function and chiral ligand inducting role, two pairs of enantiomeric polyoxotungstates, (NH₄)₄(TMA)₄[Zr₄(μ₃-O)₂(l-/d-mal)₂(B-α-HSiW₁₀O₃₇)₂] (TMA = tetramethylammonium, mal = malate (C₄H₅O₅); l-mal for 1a, d-mal for 1b) and (NH₄)₄(TMA)₄[Zr₄(μ₃-O)₂(l-/d-mal)₂(B-α-PW₁₀O₃₇)₂] (l-mal for 2a, d-mal for 2b), and a mesomeric polyoxotungstate, (NH₄)₃Na₂K₅[Zr₄(μ₃-O)₂(l-mal)(d-mal)(B-α-SiW₁₀O₃₇)₂] (3), were hydrothermally synthesized. 1a, 2a and 1b, 2b respectively exhibit 1-D 2₁ right- and left-hand helical chains formed by hydrogen-bonding interactions, and 3 forms a 3-D (3,10)-connected framework by Na⁺/K⁺ ions with {4¹⁸.6²⁴.8³}{4³}₂ topology. These homochiral compounds represent the first examples of enantiomerically pure Zr^IV-substituted Keggin POMs. In this system, {Zr₄(μ₃-O)₂(l-/d-mal)₂} clusters transfer chirality from d- or l-mal to Keggin polyoxotungstate dimeric clusters, which was demonstrated by structural comparison between the homochiral architecture and mesomer as well as circular dichroism spectra of enantiomers. UV-vis diffuse reflectance spectra reveal that 1-3 are potential semiconductor materials. In addition, 1 and 2 exhibit second harmonic generation response with their response intensities of 0.8 times that of KDP.