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

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.