Rhodium Complexes with Pincer Diphosphite Ligands. Unusual Olefin In-Plane Coordination in Square-Planar Compounds

Abstract

A family of rhodium complexes bearing pincer diphosphite (PCP) and neutral L (L = PPh₃, CO, CNXy, C₂H₄) ligands has been prepared and characterized. Reactions between Rh(Cl)(PPh₃)₃ and the diphosphites 2 lead to the chloro hydrides Rh(H)(Cl)(PCP)(PPh₃) (4), which can be deprotonated to yield the complexes Rh(PCP)(PPh₃) (5). In the latter, the PPh₃ ligand is labile and can be exchanged by CO, CNXy, and C₂H₄ to give derivatives 6−8. Most noteworthy is the fact that ethylene complexes 8a,b show a remarkable in-plane (ip) conformation in the solid state. Solution NMR studies for these complexes show fast olefin rotation in all the temperature ranges, while solution and solid-state ¹³C{¹H} NMR experiments give practically superimposable spectra, also indicating the prevalence of the ip conformer in solution. Complementary detailed DFT calculations performed with models of compounds 8 indicate that the olefin conformational preference is due to a dual combination of steric effects arising from the reduction of the P−Rh−P angle from 180°, caused by pincer ligand chelation and by the nature of the phosphite substituents, which in the case of t-Bu groups perfectly outline a cavity for an ip coordination of the olefin.