The intramolecular carbene-carbonyl coupling has been investigated for the simple M(CH2)(CO)3 (M = Co, Rh, Ir) radical complexes at the DFT PBEPBE/TZVP level of theory. The coupling is predicted to be very fast for the cobalt-containing system, but it is still feasible for the systems based on the other two metals. The back-way reaction, that is, the conversion of the ketene complex into carbonyl-carbene complex, cannot be excluded from the Ir-containing system in CH2Cl2, and it is even favored in gas phase. The intermolecular ketene formation by the addition of external CO onto the CH2 moiety is the favored pathway for the Ir-complex. The Laplacian distribution, as well as the natural spin density distribution of all the species, being involved in the reaction, gives explanation for the significant difference between the nature of the Co-complex and the Rh- and Ir-systems.
Ketenes belong to the first generation of reactive intermediates [
The formation of ketenes from carbenes usually takes place via diamagnetic pathways; therefore less attention has been paid to the reaction of mononuclear radical carbene complexes. In particular, De Bruin and coworkers have reported various transition metal carbene radicals of the cobalt group, with catalytic applications, such as cyclopropanation [
The main goal of this study is to further explore computationally the reactivity of simple carbonyl carbene radicals, which have already proven their applicability in the carbonylation of ethyl diazoacetate [
For all the calculations the PBEPBE gradient-corrected functional by Perdew et al. [
For initial species the complexes M(CH2)(CO)3 were considered and designated as
Computed structures of the genuine minima and transition states involved in the carbene-carbonyl coupling. Bond lengths are given in Å.
The intramolecular carbene-carbonyl coupling takes place via the methylene group and with either of the CO groups in
The inspection of the geometries of the transition states connecting the carbenoids with the corresponding ketene complexes (i.e.,
The reaction free energies and free energy barriers for the three complexes are presented in Table
Reaction Gibbs free energies and free energy barriers for the carbene-carbonyl couplings.
Reaction |
|
|
---|---|---|
|
−13.7 | 3.2 |
|
−18.3 | 1.6 |
|
−9.1 | 5.6 |
|
−13.3 | 5.4 |
|
1.4 | 11.1 |
|
−3.9 | 10.8 |
Repeating the calculations in gas phase, however, provided interesting results. The reaction profile has not changed dramatically for the Co and Rh pathways, although the coupling is notably less exergonic and takes place with somewhat higher barrier in both cases. For the route
The unsaturated complexes can take up CO from the carbon-monoxide atmosphere affording coordinatively saturated ketene complexes. Flexible potential energy scan calculations revealed that this process takes place without barrier for all metals. Including solvent effects, the CO addition is exergonic for all cases with a reaction free energy of −19.3, −12.5, and −17.5 kcal/mol, leading to complexes
Reaction Gibbs free energies and free energy barriers for the formation of coordinatively saturated ketene complexes via CO uptake (top 6 rows) and via the concerted mechanism (bottom 6 rows).
Reaction |
|
|
---|---|---|
|
−22.0 | — |
|
−19.3 | — |
|
−15.8 | — |
|
−12.5 | — |
|
−22.2 | — |
|
−17.5 | — |
|
−35.7 | 8.0 |
|
−37.5 | 7.9 |
|
−24.9 | 9.0 |
|
−25.8 | 8.7 |
|
−20.8 | 10.2 |
|
−21.4 | 10.1 |
Computed structures of the genuine minima, involved in the formation of coordinatively saturated ketene complexes. Bond lengths are given in Å.
The ketene complexes
Reaction Gibbs free energies for the ketene elimination step.
Reaction |
|
|
---|---|---|
|
12.4 | — |
|
7.7 | — |
|
4.2 | — |
|
0.8 | — |
|
9.6 | — |
|
5.3 | — |
In CO atmosphere it is interesting to compare the ketene displacement by CO for the coordinatively saturated ketene complexes. The equilibrium is shifted to the tetracarbonyl side for all cases with free energies of −15.7, −11.0, and −9.6 kcal/mol in CH2Cl2 and −15.9, −9.7, and −7.8 kcal/mol for
For the formation of coordinatively saturated ketene complexes an alternative, intermolecular pathway was found, when the CH2 group is attacked by an external carbon monoxide. The related transition states (
Computed structures of the transition states involved in the formation of coordinatively saturated ketene complexes with external CO. Bond lengths are given in Å.
From the electron density distribution within a molecule, detailed information can be obtained by the Laplacian of electron density,
Contour-line diagram of the Laplacian distribution of all complexes and transition states discussed in this study. Solid lines indicate charge depletions [
The comparison of the carbenoid radicals
The spin density distribution between atoms has been computed within the framework of Natural Bond Orbital (NBO) analysis (see Table
Natural spin density for all complexes and transition states within this study.
Complexes | Natural spin density | |
---|---|---|
Metal | Cmethylene | |
|
0.224 | 0.434 |
|
0.003 | 0.829 |
|
−0.004 | 0.747 |
|
0.174 | 0.720 |
|
0.047 | 0.812 |
|
0.053 | 0.706 |
|
0.720 | 0.032 |
|
0.368 | 0.048 |
|
0.343 | 0.031 |
It is concluded that transition metal carbonyl radicals of the cobalt group, especially the cobalt-complex
The authors are thankful for the support of the Developing Competitiveness of Universities in the South Transdanubian Region (SROP-4.2.2/B-16-10/1-2010-0029) Project as well as the Supercomputer Center of the National Information Infrastructure Development (NIIF) Program.