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

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 ZrIV-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