Reversible carbon-carbon double bond cleavage of a ketene ligand at a single iridium(I) center: A theoretical study

Modeling the chemistry of ketene complex {kappa (2)-(t-Bu)(2)PCH2P(t-Bu)(2) }IrCl[eta (2)-(C,C)-Ph2C=C=O] (1), DFT studies have been carried out for (k appa (2)-H2PCH2PH2)IrCl[eta (2)-(C,C)-H2C=C=O] (A), for cation (kappa (2)-H 2PCH2PH2)Ir[eta (2)-(C,C)-CH2=C=O](+) (B), for its carbene carbonyl isomer (kappa (2)-H2PCH2PH2)Ir(CH2)(CO)(+) (C), and for their interconversion by i ntramolecular C=C double bond cleavage/formation. A qualitative MO analysis from extended Huckel calculations shows the C=C cleavage and formation to be symmetry allowed at a d(8)-ML2 late transition metal template. For the t wo iridium model complexes B and C the process is calculated by DFT to be r eversible, with activation barriers of 17.3 kcal mol(-1) toward the more st able carbonyl carbene system and 25.1 kcal mol(-1) for the reverse reaction , respectively. This is in line with experimental observations for 1, which generates {kappa (2)-(t-Bu)(2)PCH2P(t-Bu)(2)}Ir(CPh2)(CO)(+) (3) upon chlo ride abstraction and regenerates I after the addition of chloride. QM/MM ca lculations of the ONIOM type have been employed for the real systems 1 and 3, to take into account and to evaluate the role of steric effects and to a llow a validation of theoretical results by comparing computed and X-ray-de termined structures. Contrasting the iridium case, the analogous rhodium ke tene complex (kappa (2)-H2PCH2PH2)Rh[eta (2)-(C,C)-CH2=C=O](+) (G) is compu ted to be favored by 8.0 kcal mol(-1) compared to its carbene carbonyl isom er (kappa (2)-H2PCH2PH2)Rh(CH2)(CO)(+) (H), with a barrier of 22.9 kcal mol (-1) for the endothermic C=C cleavage step. Conceivable dynamic processes w ere treated theoretically for the temperature dependence of NMR line shapes of carbene complex {kappa (2)-(t-Bu)(2)PCH2P(t-Bu)(2)}Ir(CPh2)(CO)(+) (3). A comparison with the experimental data suggests a plausible pathway for t he observed exchange of the two P centers.