Mechanism of CO sub(2) Fixation by Ir super(I)-X Bonds (X = OH, OR, N, C)

Density functional theory calculations have been used to investigate the CO sub(2) fixation mechanism proposed by Nolan et al. for the Ir super(I) complex [Ir(cod)(IiPr)(OH)] (1; cod = 1,5-cyclooctadiene; IiPr = 1,3-diisopropylimidazol-2-ylidene) and its derivatives. For 1, our results suggest that CO sub(2) insertion is the rate-limiting step rather than the dimerization step. Additionally, in agreement with the experimental results, our results show that CO sub(2) insertion into the Ir-OR super(1) (R super(1) = H, methyl, and phenyl) and Ir-N bonds is kinetically facile, and the calculated activation energies span a range of only 12.0-23.0 kcal/mol. Substantially higher values (35.0-50.0 kcal/mol) are reported for analogous Ir-C bonds. We report the DFT-based investigation of the mechanism of CO sub(2) insertion into Ir super(I)-C and Ir super(I)-heteroatom bonds (such as Ir-N and Ir-O). The calculated barriers show that CO sub(2) insertion is the rate-limiting step, in accordance with experimental results.