Kinetic and mechanistic study of the succinic anhydride reductive-elimination reaction from the six-coordinate Ir(H)[σ-CHCH2C(O)OC(O)](σ-carb)(CO)(PhCN)(PPh 3) complex

The hydridoalkyliridium(III) complex Ir(H)[σ-CHCH2C(O)OC(O)](σ-carb)(CO)(PhCN)(PPh 3), where carb = 7-C6H5-1,7-C2B10H10, undergoes reductive-elimination reaction of succinic anhydride under mild conditions. This reaction, which represents a crucial step in the homogeneous hydrogenation of maleic anhydride catalyzed by the four-coordinate iridium(I) complex Ir(σ-carb)(CO)(PhCN)(PPh3) at T = 50°C (PH2 = 1 atm), has been kinetically investigated in 1,2-dichloroethane solution by IR spectroscopy between 35 and 45°C. The elimination reaction implies preliminary PhCN dissociation to give a five-coordinate intermediate that then undergoes intramolecular reductive elimination of succinic anhydride. The obtained activation parameters indicate that the reductive elimination of succinic anhydride from the five-coordinate hydridoalkyliridium(III) intermediate is a highly concerted process whose drawing force is the incipient formation of the strong C-H bond of the product. © 1984 American Chemical Society.