New classification of cyclic P2N2-ligands predicting bridging and chelate coordination

The geometry and conformations of all published single crystal X-ray (SCXRD) structures of cyclic P₂N₂-ligands (1,5-diaza-3,7-diphosphacyclooctanes) were analyzed. It was found that the cyclic P₂N₂-ligands exist in two fundamentally different structural motifs with relatively high or low angles between phosphorus lone pairs. The interdependence of the angle between phosphorus lone pairs, distance between phosphorus atoms, distance between nitrogen atoms and endocyclic CNC angles was demonstrated via the correlation plots of these parameters. The detailed analysis of XRD structures accompanied with the DFT calculations allowed to establish that the endocyclic CNC angle, which is determined by the hybridization of N-atom, is the main driving force for the structural variations. These data are demonstrating that the P₂N₂-ligands are the “breathing” cycles, which are able to increase or decrease the P^…P-distances accompanied with rotation of phosphorus atom affecting the angle between phosphorus lone pairs. According to the obtained massive of data it was hypothesized that the N-alkyl substituted P₂N₂-cycles preferably forms the P,P-chelate complexes, whereas the N-aryl substituted ligands are able to form both P,P-bridged and P,P-chelate complexes. All of the foundations provide the explanations for the behavior of the P₂N₂-ligands during the coordination with transition metals.