Enantiomeric Polyoxometalates Based on Malate Chirality-Inducing Tetra-Zr-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, (NH)(TMA)[Zr(μ-O)(l-/d-mal)(B-α-HSiWO)] (TMA = tetramethylammonium, mal = malate (CHO); l-mal for 1a, d-mal for 1b) and (NH)(TMA)[Zr(μ-O)(l-/d-mal)(B-α-PWO)] (l-mal for 2a, d-mal for 2b), and a mesomeric polyoxotungstate, (NH)NaK[Zr(μ-O)(l-mal)(d-mal)(B-α-SiWO)] (3), were hydrothermally synthesized. 1a, 2a and 1b, 2b respectively exhibit 1-D 2 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 {4.6.8}{4} topology. These homochiral compounds represent the first examples of enantiomerically pure Zr-substituted Keggin POMs. In this system, {Zr(μ-O)(l-/d-mal)} 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.