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 PC 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 π-accepting properties of the PC
bond. However, PPEP had a stability problem that provokes the loss
of the PC 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 PC 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.