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*)RhICl]K (2; PNP = 2,6-bis((di-tert-butylphosphino)methyl)pyridine, PNP* = deprotonated PNP) was prepared by reaction of the aromatic (PNP)RhICl 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)RhI(C6H5) (3a). This reaction is inhibited by dinitrogen, which reacts with 2 to form the complex (PNP*)RhI(N2) (4), indicating higher stabilization of the 14-electron (PNP*)RhI species 5 by dinitrogen as compared with chloride. Similarly, treatment of 2 with CO results in KCl liberation to form the dearomatized (PNP*)RhICO (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*)RhIII(CH3)Cl (10), while the dearomatized ligand remains intact. Complex 2 undergoes facile activation of H2 to form the monohydride (PNP)RhI(H) (11a) and with D2 to form (PNP)RhI(D) (11b) with benzylic-D incorporation, via metal–ligand cooperation by aromatization of the ligand. The reactivity of 2 with H2 is significantly higher than that of 4.