Unsymmetrical pnp-pincer type phosphaalkene ligands protected by a fused-ring bulky eind group: Synthesis and applications to rh(i) and ir(i) complexes

We recently reported that 2-(phospholanylmethyl)-6-(2-phosphaethenyl)pyridine (PPEP) with a 2,4,6-tri-tert-butylphenyl group (Mes∗) as steric protection of the PC bond serves as a noninnocent ligand on Ir(I), leading to extremely high reactivity toward metal-ligand cooperative activation of ammonia and acetonitrile. The high reactivity is largely due to the strong accepting properties of the PC bond. However, PPEP had a stability problem that provokes the loss of the PC bond on other transition metals, including Rh(I), and restricts its utilization. This paper describes the synthesis of Eind-PPEP protected by an octaethyl-s-hydrindacen-4-yl group (Eind) instead of Mes∗. The fused-ring bulky Eind group successfully prevents the loss of the PC bond and enables us to compare the reactivity of Rh(I) and Ir(I) complexes toward ammonia. The complex K[RhCl(Eind-PPEP∗)], bearing a dearomatized Eind-PPEP∗ ligand, undergoes simple ligand displacement to give [Rh(NH₃)(Eind-PPEP∗)], whereas the iridium analogue K[IrCl(Eind-PPEP∗)] causes N-H bond cleavage to form [Ir(NH₂)(Eind-PPEP)]. DFT calculations indicate a thermodynamic cause of the metal-dependent product change.