Synthesis and Reactivity of an Iridium(I) Acetonyl PNP Complex. Experimental and Computational Study of Metal-Ligand Cooperation in H-H and C-H Bond Activation via Reversible Ligand Dearomatization

The complex (PNP)Ir-I(CH2COCH3) 2 (PNP = 2,6-bis((di-tert-butylphosphino)methyl)pyridine) was prepared by reaction of the dearomatized, electron-rich complex (PNP*)Ir-I(COE) (1; PNP* = deprotonated PNP, CUE = cyclooctene) with acetone. Upon treatment with CO, complex 2 undergoes a surprising elimination of acetone to form the dearomatized species (PNP*)Ir-I(CO) (4), involving proton migration from the ligand arm to the acetonyl moiety. DFT studies reveal that this process occurs via the square-pyramidal intermediate 2+CO, formed upon CO coordination to 2, in which the acetonyl moiety is located at the apical position prior to proton migration. Reaction of 2 with H-2 (D-2) indicates an equilibrium between complex 2 and the nonaromatic (PNP*)Ir-III(H)(CH2COCH3) complex 2b, which is the species that actually activates H-2 to exclusively form the trans-dihydride (PNP)Ir-III(H)(2)(CH2COCH3) (5a) and activates D-2 to form the trans-hydride-deuteride 5b with benzylic-D incorporation, as also corroborated by DFT studies. Interestingly, benzene C-H activation by complex 2 results in formation of the complex (PNP)Ir-I(C6H5) (6a) and elimination of acetone. DFT studies show that the benzene C-H bond is actually activated by the dearomatized bare (PNP*)Ir-I intermediate 2c, formed upon acetone elimination from 2.