Unsymmetrical PNP-Pincer Type Phosphaalkene Ligands Protected by a Fused-Ring Bulky Eind Group: Synthesis and Applications to Rh(I) and Ir(I) Complexes

• Published in: Organometallics 2016-05, Vol.35 (10), p.1526-1533
• Main Authors: Taguchi, Hiro-omi, Sasaki, Daichi, Takeuchi, Katsuhiko, Tsujimoto, Shota, Matsuo, Tsukasa, Tanaka, Hiromasa, Yoshizawa, Kazunari, Ozawa, Fumiyuki
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.6b00113

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.