Spontaneous Resolution of Evans-Showell-Type Polyoxometalates in Constructing Chiral Inorganic-Organic Hybrid Architectures.

Abstract

Six new hybrid compounds based on [Co2Mo10H4O38](6-) polyoxoanion, (4-H2pya)6Co2Mo10H4O38·10H2O 1 (4-Hpya =3-(4-pyridyl)acrylic acid) and (4-H2pya)4[M(H2O)6][Co2Mo10H4O38]·5H2O (M = Co 2, Ni 3, Cu 4, Zn 5, Cd 6), were synthesized and characterized by elemental analysis, IR spectroscopy, TG analysis, solid diffuse reflective spectrum, powder X-ray diffraction, and single crystal X-ray diffraction. Compound 1 crystallizes in the centrosymmetric space group P-1, which is built up of protonated 4-H2pya molecules and isolated [Co2Mo10H4O38](6-) polyoxoanions. Isostructural compounds 2-6 crystallize in the chiral space group P1 by spontaneous resolution, as a conglomerate of two enantiomerically pure crystals, which were obtained by adding different metal cations to the reaction system of compound 1. The structures of 2-6 comprise chiral [Co2Mo10H4O38](6-) anions, cationic [M(H2O)6](2+) units, and protonated 4-H2pya ligands. The chiral [Co2Mo10H4O38](6-) polyoxoanions can be connected up together by directional hydrogen-bonding interactions among terminal oxygen atoms of anions, coordinated water molecules and organic ligands to construct a 3D supramolecular chiral framework in 2-6. The absolute configuration of 2-6 was determined from the Flack parameter by X-ray crystallography and solid state circular dichroism spectroscopy. As far as we know, compounds 2-6 represent the first examples of chiral inroganic-organic hybrid species based on Evans-Showell-type polyoxometalates. Furthermore, the second harmonic generation (SHG) activities for 2 and 4 were measured, showing an SHG efficiency of approximately 0.9 × KH2PO4.