Silacalix-[ n ]-phosphaarenes: Macrocyclic Ligands Based on Dicoordinate Phosphorus Centers

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Abstract
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 σ donors, these dicoordinate phosphorus-based macrocycles have strong π-acceptor properties. Their use can be envisaged for the stabilization of negative oxidation states of transition metals or in reductive catalysis.