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 {κ 2 -( t -Bu) 2 PCH 2 P( t -Bu) 2 }IrCl[η 2 -( C,C )-Ph 2 C C O] ( 1 ), DFT studies have been carried out for (κ 2 -H 2 PCH 2 PH 2 )IrCl[η 2 -( C,C )-H 2 C C O] ( A ), for cation (κ 2 -H 2 PCH 2 PH 2 )Ir[η 2 -( C,C )-CH 2 C O] + ( B ), for its carbene carbonyl isomer (κ 2 -H 2 PCH 2 PH 2 )Ir(CH 2 )(CO) + ( C ), and for their interconversion by intramolecular C C double bond cleavage/formation. A qualitative MO analysis from extended Hückel calculations shows the C C cleavage and formation to be symmetry allowed at a d 8 -ML 2 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 {κ 2 -( t -Bu) 2 PCH 2 P( t -Bu) 2 }Ir(CPh 2 )(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 (κ 2 -H 2 PCH 2 PH 2 )Rh[η 2 -( C,C )-CH 2 C O] + ( G ) is computed to be favored by 8.0 kcal mol - 1 compared to its carbene carbonyl isomer (κ 2 -H 2 PCH 2 PH 2 )Rh(CH 2 )(CO) + ( H ), with a barrier of 22.9 kcal mol - 1 for the endothermic C C cleavage step. Conceivable dynamic processes were treated theoretically for the temperature dependence of NMR line shapes of carbene complex {κ 2 -( t -Bu) 2 PCH 2 P( t -Bu) 2 }Ir(CPh 2 )(CO) + ( 3 ). A comparison with the experimental data suggests a plausible pathway for the observed exchange of the two P centers.