Pincer ligands with an all-phosphorus donor set: subtle differences between rhodium and palladium

Abstract

The synthesis of a new, all-phosphorus pincer PP^NEt2P ligand L3^NEt2, which is derived from 2-indolylphosphine and features a central N₂P(NEt₂) core, is described. This ‘PPP’ species shows coordination toward Rh as a neutral trisphosphine ligand. Tridentate diphenylphosphine-derived PP^HP ligands L1^H and L2^H, featuring a secondary phosphine core, show ‘ambivalent’ coordination, acting as persistent neutral triphosphine ligands with Rh, and as easily-formed monoanionic phosphido(bisphosphine) pincer ligands toward Pd. These subtle differences, which might be more general for group 9 and 10 metal complexes with this ligand set, are explained by comparative DFT calculations (BP86; def2-TZVP level of theory) for the Rh and Pd species involved, including those with the structurally related PN^HP ligands. The optimized structure for complex PdCl(L2) indicates minimal overlap of available Pd d-orbitals with the lone pair of the central, deprotonated phosphorus atom (formally a phosphido fragment), suggesting that it behaves predominantly like a bulky phosphine instead of a phosphido fragment.