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

Abstract

Modeling the chemistry of ketene complex {κ 2 -(t-Bu) 2 PCH 2 P(t-Bu) 2 }IrCl[η 2 -(C,C)-Ph 2 Cd CdO] (1), DFT studies have been carried out for (, and for their interconversion by intramolecular CdC double bond cleavage/formation. A qualitative MO analysis from extended Hu ¨ckel calculations shows the CdC 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 (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 CdC 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.

Computational Study on the Intramolecular Carbene-CO Coupling in M(CH2)(CO)3 Radicals (M = Co, Rh, Ir)

“…For the rhodium pathway Δ = −13.3 kcal/mol has been obtained as reaction free energy with a small barrier of 5.4 kcal/mol. Finally, the reaction via 2TSIr proceeds via a barrier of 10.8 kcal/mol, and the process is still exergonic by −3.9 kcal/mol in contrast to the results of Grotjahn and coworkers, who found that the equilibrium is on the carbene side and not on the ketene side for phosphine substituted complexes [7].…”

“…The second purpose is to scrutinize the vertical trends in the cobalt group of transition metals and to investigate the reaction profile for the rhodium and iridium analogues and to check whether the equilibrium can be on the carbene side in some cases, like it was reported by Urtel and coworkers [7].…”

“…The most common method to access ketenes is from acyl halides via dehalogenation promoted by bases. Therefore, carbonylation of metal carbene provides an alternative, straightforward approach to the generation of ketene species [7][8][9][10]. There are also many ketenes that cannot be synthetized by classical synthetic methods due to their high reactivity.…”

Organometallic chemistry

“…His research interests combined all three fields, with a focus on homogeneous catalysis with transition metal complexes. Bidentate donor ligands with a small bite angle such as bis(di- tert -butylphosphino)methane (dtbpm) were a unique feature of his work [61,67,70–72 80–81 84,86,93–94 97,100–101 103,107,124,128,148–149] that ultimately enabled C–Si activation of organosilanes and the C–C activation of oxiranes by coordinatively unsaturated platinum species [66,149]. In the mechanism of the Dötz reaction, he found that chromacyclobutene structures were unrealistic intermediates and instead proposed vinylcarbene complexes as much more stable isomers [64,68,88].…”

Organometallic Chemistry

“…His research interests combined all three fields, with a focus on homogeneous catalysis with transition metal complexes. Bidentate donor ligands with a small bite angle such as bis(di-tert-butylphosphino)methane (dtbpm) were a unique feature of his work [61,67,[70][71][72]80,81,84,86,93,94,97,100,101,103,107,124,128,148,149] that ultimately enabled C-Si activation of organosilanes and the C-C activation of oxiranes by coordinatively unsaturated platinum species [66,149]. In the mechanism of the Dötz reaction, he found that chromacyclobutene structures were unrealistic intermediates and instead proposed vinylcarbene complexes as much more stable isomers [64,68,88].…”