The first known examples of square-planar halide complexes containing a main group element have been characterised by chemists in Australia.

Bruce Wild and colleagues at the Australian National University, Canberra, used chloride and bromide ions to direct the spontaneous self-assembly of square-planar complexes. The central halide ion in the complex is surrounded by four phosphine-stabilised stibenium ions in a symmetrical arrangement.

 

 

'These air-stable stibenium complexes are remarkable in that they can be isolated from the organic phase of a dichloromethane-water mixture of the components, the electrospray mass spectra indicating supramolecular parent assembly,' said Wild.

The complexes show a near-linear binding of the halogen and phosphorus atoms to the stibenium cations. Unlike previously prepared Group 15-element halides, attractive edge-to-face interactions above and below the square plane are thought to help stabilise these supramolecular structures.

Ramon Vilar from Imperial College London, UK, an expert in anion templation, was quick to praise the work of Wild and colleagues. 'This [research] highlights the important directing role played by anions in the synthesis of stibenium compounds. The development of a wide range of p-block supramolecular assemblies could be achieved employing this interesting approach,' said Vilar.

According to Wild, halide ions are versatile templates for inorganic and organic syntheses because of their spherical symmetry and ability to interact with a variety of molecular components through Lewis acid-base interactions and hydrogen bonding. 

With this in mind, Wild plans to fully exploit the 'highly regular geometries' of his complexes for the self-assembly of supramolecular ladders and grids and for anion-templated syntheses of covalent main group macrocycles.

Stephen Wilkes