PPX/PXP-type ligands (X = O and S) and their transition metal complexes: synthesis, properties and applications

Short-bite diphosphines of the form R$_2$P–X–PR$_2$ (PXP; X = O, S; R = aryl, alkyl), incorporating an oxygen or sulphur atom as bridging unit X, are widely underexplored compared to their N- and C-containing PNP- and PCP-type counterparts. However, these PXP ligands undergo an interesting phosphorotropic equilibrium with the PPX (R$_2$P(vX)–PR$_2$) tautomer, which opens up a very versatile coordination chemistry. This article covers the impact of the ligand backbone in short-bite ligands on their coordination chemistry, reactivity and applications. Especially in PXP-type complexes, metallophillic interactions can be induced in the case of coinage metals, which lead to fascinating photo-optical properties. Furthermore, PPX/PXP-type complexes are believed to exhibit a promising behavior in catalysis, due to the potential hemilability of the ligand and the therewith involved availability of free active sites for substrate binding.