Spontaneous Resolution of Chiral Polyoxometalate-Based Compounds Consisting of 3D Chiral Inorganic Skeletons Assembled from Different Helical Units

Four enantiomerically pure 3D chiral POM-based compounds, [Ni-2(bbi)(2)(H2O)(4)V4O12]center dot 2H(2)O (1a and 1b) and [Co(bbi)(H2O)V2O6] (2a and 2b) (bbi = 1,1'-(1,4-butanediyl)bisimidazole) based on the achiral ligand, different vanadate chains, and different metal centers have been synthesized by hydrothermal methods. Single-crystal X-ray diffraction analyses revealed that 1a and 1b, and 2a and 2b, respectively, are enantiomers. In 1a and 1b two kinds of vanadate chains with different screw axes link Ni cations to generate 3D chiral inorganic skeletons, which are connected by the achiral bbi ligands to form complicated 3D 3,4-connected chiral self-penetrating frameworks with (7(2).8)(7(2).8(2).9(2)) (7(3).8(2).10) topology. They represent the first examples of chiral self-penetrating frameworks known for polyoxometalate (POM) systems. Contrary to la and 1b, in 2a and 2b the vanadate chains link Coll cations to generate 3D chiral inorganic skeletons, which are assembled from two kinds of heterometallic helical units of opposite chirality along the c axes. The chiral inorganic skeletons are connected by bbi to form 3D 3,4-connected chiral POM-based frameworks with (6(2).8)(2)(6(2).8(2).10(2)) topology. It is believed that the asymmetrical coordination modes of the metal cations in 1a-2b generate the initial chiral centers, and that the formation of the various helical units and the hydrogen bond interactions are responsible for preservation of the chirality and spontaneous resolution when the chirality is extended into the homochiral 3D-networks. This is the first known report of chiral POM-based compounds consisting of 3D chiral inorganic skeletons being obtained by spontaneous resolution upon crystallization in the absence of any chiral source, which may provide a rational strategy for synthesis of chiral POM-based compounds by using achiral ligands and POM helical units.