Polyoxometalates: Chiral rods from achiral building blocks

Three-unit-long chiral rods, potentially useful as chiral catalysts, have been built from achiral molecules.

Polyoxometalates (POMs) — clusters built from metal centers linked through oxygen atoms — have numerous interesting properties, including the ability to catalyze organic reactions. Chiral rods, made from stringing together POMs, which are naturally achiral (not occurring in paired mirror-image forms), are expected to find even more uses than the individual units.

Fig. 1: Three POMs held together by covalent linkers.© 2010 ACS

Various routes have previously been reported for building chiral POM rods, but in the vast majority of cases the POM clusters are held together by ionic interactions. Now, Yongge Wei and colleagues at Tsinghua University in Beijing, China,¹ have devised a new, simple route for linking three POM units through organic ligands via covalent bonds (Fig. 1). “The covalent linkage not only improves the stability of the system, but also enhances the interaction between the POMs,” Wei explains.

The molecular rods prepared by Wei’s team contain one Anderson-type anion (with the molecular structure XM₆O₂₄^n–, where X and M are two different metals) sandwiched between two Lindquist-type anions (M₆O₁₉^n–).

To make the rods, one amino group was attached on each side of the Anderson anion’s central metal. The functionalized anion was subsequently refluxed in dry solvent with the Lindquist anions, causing the nitrogen atoms in the amino groups to react to form imido (M=N–R) bonds with the metals in the Lindquist anions.

“Enantiopure crystals of these compounds were obtained by spontaneous resolution upon crystallization in the absence of any chiral source,” explains Wei. This means that when the ‘racemic’ solution — one in which both mirror-image forms (enantiomers) of a compound are present in equal quantity — crystallizes, two different crystals are obtained, each containing only one of the enantiomers. Typically, a chiral crystal would be needed as a seed to trigger such a spontaneous resolution, but in this approach it is not needed. An X-ray diffraction study revealed that the rods obtained were always three units long.

Potential applications for these enantiomerically pure crystals are now being investigated. “For future homogeneous catalysis uses, we are currently testing their stability to ensure they retain their chirality in solution,” says Wei. Heterogeneous catalytic applications are also currently being looked into, and other uses such as in ferroelectric and non-linear optic materials will be explored in the future, he adds.

Wei’s team has so far made two different types of POM nanorods using this route, but Wei expects the research to be widely applicable: “The work opens the road to assemble POM-based chiral nanostructures from achiral POMs in a controllable way.”