The influence of bite angle in bisphosphine complexes has been modeled by DFT calculations employing the simple model compound HCo(CO)(PP) (PP = Xantphos or two monophosphine ligands). The increase of the bite angle increases the strength of the H–Co bond, whereas the C–O bond in the carbonyl ligand is weakened revealing an increase also in the donor character. The model compound
Catalytic hydroformylation of alkenes is one of the largest volume applications of homogeneous catalysts. In the recent decades the continued development of new P-donor ligands has resulted in significant advances in the selectivity of hydroformylation catalysts. An especially effective improvement in the control of catalyst regioselectivity involves the application of bulky diphosphines and diphosphites with wide bite angles [
Diphosphines BISBI, and Xantphos, possessing wide bite angles.
The understanding of fundamental aspects of transition metal-catalyzed reactions is profound for the discovery of new and more efficient catalysts. Computations play an increasingly important role in getting new insights in reaction mechanism by their ability to provide equilibrium geometries for transition states and high energy intermediates, as well as by the determination of electronic structure parameters of structures of importance.
Potential energy surfaces (PESs) are indispensable for studying reaction profiles and rates computationally [
The goal of this paper is to characterize the ligand Xantphos coordinated to the HCo(CO) moiety. These kinds of complexes can serve as catalysts for the cobalt-catalyzed hydroformylation reaction, containing diphosphine ligands. Besides the discussion of the corresponding Co-Xantphos complex, the bite angle effect is estimated via the complex
For all the calculations the PBEPBE gradient-corrected functional by Perdew et al. [
The computed structure of HCo(CO)(Xantphos) (
Computed structure of HCo(CO)(Xantphos) (
The electronic effect in wide bite angle ligands can be divided in two parts: one is the influence of the substituents on phosphorus, whereas the secondary electronic effect is determined by steric effects, that is, the bulk of the substituents, and the phosphorus-metal-phosphorus bite angle. For the interpretation of the secondary effect, a flexible potential energy surface scan has been completed using complex
The potential energy against the P-Co-P angle in complex
The PES scan was done in two directions starting from the equilibrium geometry of
At 113.5° a local genuine minimum appears, which is less stable than
The structural parameters of all the structures involved in the PES scan are compiled in Table
Selected bond lengths in HCo(CO)(phosphine) complexes as well as those of the structures involved in the PES scan changing the P-CO-P angle. The
Structure |
|
|
|
|
|
---|---|---|---|---|---|
HCo(CO) |
1.505 | 2.159 | 2.154 | 1.721 | 1.175 |
HCo(CO)(Xantphos) ( |
1.490 | 2.219 | 2.227 | 1.708 | 1.181 |
HCo(CO |
1.503 | 2.203 | 2.191 | 1.716 | 1.180 |
|
1.504 | 2.197 | 2.227 | 1.715 | 1.180 |
|
1.503 | 2.194 | 2.210 | 1.716 | 1.180 |
|
1.498 | 2.210 | 2.195 | 1.717 | 1.180 |
|
1.496 | 2.184 | 2.184 | 1.718 | 1.180 |
|
1.496 | 2.200 | 2.181 | 1.717 | 1.181 |
|
1.486 | 2.180 | 2.166 | 1.720 | 1.181 |
|
1.485 | 2.180 | 2.169 | 1.718 | 1.181 |
|
1.484 | 2.175 | 2.171 | 1.717 | 1.181 |
|
1.481 | 2.173 | 2.170 | 1.178 | 1.180 |
The electronic structure around the cobalt central atom has been elucidated within the framework of the Quantum Theory of Atoms in Molecules developed by Bader. One of the two QTAIM descriptors taken into account is the delocalization index
Delocalization indices
Structure |
|
|
|
|
|
---|---|---|---|---|---|
HCo(CO)(Xantphos) ( |
0.767 | 0.829 | 0.798 | 1.411 | 1.497 |
HCo(CO) |
0.802 | 0.942 | 0.866 | 1.352 | 1.533 |
HCo(CO) |
0.748 | 0.876 | 0.826 | 1.383 | 1.506 |
|
0.764 | 0.871 | 0.789 | 1.392 | 1.509 |
|
0.755 | 0.874 | 0.816 | 1.386 | 1.508 |
|
0.771 | 0.847 | 0.844 | 1.374 | 1.508 |
|
0.768 | 0.848 | 0.870 | 1.374 | 1.506 |
|
0.754 | 0.855 | 0.871 | 1.372 | 1.503 |
|
0.796 | 0.804 | 0.925 | 1.370 | 1.502 |
|
0.790 | 0.808 | 0.920 | 1.371 | 1.499 |
|
0.785 | 0.811 | 0.918 | 1.367 | 1.496 |
|
0.789 | 0.812 | 0.920 | 1.355 | 1.500 |
Electron density at bond critical points
Structure |
|
|
|
|
|
---|---|---|---|---|---|
HCo(CO) |
0.129 | 0.093 | 0.093 | 0.172 | 0.429 |
HCo(CO)(Xantphos) ( |
0.129 | 0.088 | 0.088 | 0.177 | 0.423 |
HCo(CO) |
0.126 | 0.092 | 0.092 | 0.173 | 0.424 |
|
0.126 | 0.091 | 0.088 | 0.174 | 0.424 |
|
0.126 | 0.092 | 0.091 | 0.173 | 0.424 |
|
0.132 | 0.095 | 0.098 | 0.174 | 0.423 |
|
0.128 | 0.090 | 0.096 | 0.173 | 0.423 |
|
0.128 | 0.092 | 0.096 | 0.174 | 0.423 |
|
0.132 | 0.094 | 0.098 | 0.172 | 0.423 |
|
0.132 | 0.094 | 0.098 | 0.173 | 0.423 |
|
0.132 | 0.095 | 0.098 | 0.174 | 0.423 |
|
0.133 | 0.095 | 0.098 | 0.173 | 0.424 |
The relationship between the delocalization index for the H–Co bond and the bite angle is not so obvious, than the relationship between the H–Co bond lengths and the bite angle. It reaches its minimum at 106.5° (hence in the case of
The
The electron density at bond critical points is, however, less indicative for the description of bite angle effect. For the Co–C and C–O bonds only a very subtle change can be observed. The value of
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 complex
The increase of the bite angle results in an enhancement of the strength of the H–Co bond, whereas the C–O bond in the carbonyl ligand becomes somewhat weaker revealing an increase also in the
The authors declare that there is no conflict of interests regarding the publication of this paper.