Stable five-membered heteroallene rings containing phosphorus are shown to be strong ligands
The wide range of homogeneous catalysts in the toolkit of today's organic chemist is only available through the diversity of ligands that can tune the behaviour of the metals they surround. Those with strong donor properties, such as N-heterocyclic carbenes (NHCs), are particularly powerful. Similar compounds including phosphorus (vinylidenephosphoranes) have been proposed, but the sole example prepared so far was only fleetingly stable at low temperatures. Now, Guy Bertrand at the University of California, Riverside, and Antoine Baceiredo from the Université Paul Sabatier, France, and their team have made1 a cyclic vinylidenephosphorane that is stable at room temperature.
The five-membered ring contains an allenic carbon atom doubly bonded to both a phosphorus atom and a carbon atom, and was prepared from a cyclic phospholium salt. The resulting compound was indefinitely stable both in solution and the solid state. The bond distance between the phosphorus atom and the allenic carbon atom is exceptionally long compared with similar previously reported compounds. This leaves a lone pair of electrons on the carbon atom fully available to bond to metal atoms.
The ligand is able to displace a cycooctadienyl ligand from a rhodium complex to form a thermally stable orange solid. By studying the stretching frequency of a carbonyl group also coordinated to the rhodium atom, the team showed that their vinylidenephosphorane is a stronger donor than five-membered NHCs, but weaker than rings containing two phosphorus atoms.
Asay, A. et al. Synthesis and ligand properties of a stable five-membered-ring vinylidenephosphorane. Angew. Chem. Int. Ed. 10.1002/anie.200802741 (2008).
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Withers, N. Five-star phosphorus. Nature Chem (2008). https://doi.org/10.1038/nchem.57