Silacalix-[n]-phosphaarenes:: Macrocyclic ligands based on dicoordinate phosphorus centers

The synthesis of polyphosphinine macrocycles, which consist of a 16-membered ring with four phosphorus atoms (P-4) and a 12-membered ring with three phosphorus atoms (P-3), is described. Their high coordination ability is demonstrated by the quantitative synthesis of the rhodium and iridium cation complexes of the P-4 macrocycle and by quantitative synthesis of the W(CO)(3) complex of the P-3 compound. Unlike the other available macrocyclic ligands bearing oxygen, sulfur, di- or tricoordinate nitrogen, and even tricoordinate phosphorus as ligating atoms, which are all essentially a donors, these dicoordinate phosphorus-based macrocycles have strong pi-acceptor properties. Their use can be envisaged for the stabilization of negative oxidation states of transition metals or in reductive catalysis.