Reversible Carbon−Carbon Double Bond Cleavage of a Ketene Ligand at a Single Iridium(I) Center: A Theoretical Study

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Abstract

Modeling the chemistry of ketene complex {κ2-(t-Bu)2PCH2P(t-Bu)2}IrCl[η2-(C,C)-Ph2CCO] (1), DFT studies have been carried out for (κ2-H2PCH2PH2)IrCl[η2-(C,C)-H2CCO] (A), for cation (κ2-H2PCH2PH2)Ir[η2-(C,C)-CH2CO]+ (B), for its carbene carbonyl isomer (κ2-H2PCH2PH2)Ir(CH2)(CO)+ (C), and for their interconversion by intramolecular CC double bond cleavage/formation. A qualitative MO analysis from extended Hückel calculations shows the CC cleavage and formation to be symmetry allowed at a d8-ML2 late transition metal template. For the two iridium model complexes B and C the process is calculated by DFT to be reversible, with activation barriers of 17.3 kcal mol-1 toward the more stable 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 {κ2-(t-Bu)2PCH2P(t-Bu)2}Ir(CPh2)(CO)+ (3) upon chloride abstraction and regenerates 1 after the addition of chloride. QM/MM calculations 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 allow a validation of theoretical results by comparing computed and X-ray-determined structures. Contrasting the iridium case, the analogous rhodium ketene complex (κ2-H2PCH2PH2)Rh[η2-(C,C)-CH2CO]+ (G) is computed to be favored by 8.0 kcal mol-1 compared to its carbene carbonyl isomer (κ2-H2PCH2PH2)Rh(CH2)(CO)+ (H), with a barrier of 22.9 kcal mol-1 for the endothermic CC cleavage step. Conceivable dynamic processes were treated theoretically for the temperature dependence of NMR line shapes of carbene complex {κ2-(t-Bu)2PCH2P(t-Bu)2}Ir(CPh2)(CO)+ (3). A comparison with the experimental data suggests a plausible pathway for the observed exchange of the two P centers.

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