A Dearomatized Anionic PNP Pincer Rhodium Complex: C-H and H-H Bond Activation by Metal-Ligand Cooperation and Inhibition by Dinitrogen

The anionic dearomatized complex [(PNP*)(RhCl)-Cl-I]K (2; PNP = 2,6-bis((di-tert-butylphosphino)methyl)pyridine, PNP* = deprotonated PNP) was prepared by reaction of the aromatic (PNP)(RhCl)-Cl-I complex 1 with KN(SiMe3)(2) in dry benzene. Spectroscopic characterization and DFT calculations confirm a nonaromatic square-planar structure of complex 2. Under an atmosphere of dry argon, 2 undergoes facile C H activation of benzene by cooperation between the metal center and the pincer ligand, with aromatization of the ligand, to form the complex (PNP)Rh-I(C6H5) (3a). This reaction is inhibited by dinitrogen, which reacts with 2 to form the complex (PNP*)Rh-I(N-2) (4), indicating higher stabilization of the 14-electron (PNP*)Rh-I species 5 by dinitrogen as compared with chloride. Similarly, treatment of 2 with CO results in KCl liberation to form the dearomatized (PNP*)(RhCO)-C-I (8). In a protic environment, the dearomatized complex 2 is quickly reprotonated to regenerate the aromatic starting complex 1. Upon treatment with MeI, 2 undergoes oxidative addition to form the nonaromatic (PNP*)Rh-III(CH3)Cl (10), while the dearomatized ligand remains intact. Complex 2 undergoes facile activation of H-2 to form the monohydride (PNP)Rh-I(H) (11a) and with D-2 to form (PNP)Rh-I(D) (11b) with benzylic-D incorporation, via metal ligand cooperation by aromatization of the ligand. The reactivity of 2 with H-2 is significantly higher than that of 4.