18
J.X. Mao et al. / Journal of Organometallic Chemistry 741-742 (2013) 15e19
Fig. 6. Energy diagram (in unit kJ/mol) of catalytic 1,4-CHD isomerization to show steric effect. The blue diagram is from B3LYP calculations with PH3 ligands; the cyan diagram is
from ONIOM(B3LYP:UFF) calculations with PPh3 ligands. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
as the energy barriers associated with them, undoubtedly play
critical roles in the isomerization process. They were confirmed as
the correct transition states first by intrinsic reaction coordinate
calculations carried out by Gaussian 09. The forward and reverse
paths lead to correct reactants and products. They were further
analyzed by Bader’s quantum theory of atoms in molecules
(QTAIM) method [19,32]. In QTAIM analysis, the presence of a bond
critical point (BCP) in the electron density field between atoms is
treated at quantum mechanics (QM) level by density functional
theory (DFT).
A two-step decomposition analysis [1,2] was carried out to
decompose steric and electronic effects induced by PPh3 ligands. To
accomplish this, three different calculations were performed:
1. QM-PPh3: full Quantum Mechanics (QM) calculation of the
whole system. In a full QM calculation, both steric and elec-
tronic effects from PPh3 ligands are included.
indicative of a bonding between them. Electron densities
r at
critical points are used to characterize the bonding feature. A strong
bonding is accompanied by high densities. The results of QTAIM
analysis are shown in Fig. 5 and Table 3. The BCPs between RueH
and CeH (Fig. 5), as well as the electron densities at these BCPs
(Table 3), clearly indicate that the hydrogen atom is bonded both to
ruthenium catalyst and CHD in these transition state structures.
The bulky phenyl groups of the catalyst could strongly influence
the reactivity of transition states by preventing the coordination
positions or by altering electronic structure of ruthenium center. To
distinguish steric and electronic factors of the phenyl groups,
ONIOM methodology [38] was employed to analyze the energy
diagram change of the catalytic isomerization reaction. ONIOM
methods have been widely used to study reaction mechanisms
involving transition metal complexes [7,8,13,14,16,23,45,47], but as
far as we know this is the first ONIOM investigation of the alkene
isomerization process.
In a two-layer ONIOM method, the full molecular system under
investigation is referred to as real system. A small but most
important part of the real system, which is normally the part where
the bond formation and breakage occur, is referred as model sys-
tem. The real system is treated at relatively low computational
levels, and the model system is treated at more accurate compu-
tational levels. The energy is calculated as:
2. QM-PH3: full Quantum Mechanics calculation of the whole
system, but the PPh3 ligands were modeled as PH3 ligands. In
this simplified model, neither steric nor electronic effects from
PPh3 ligands are included since all the Ph3 groups are replaced
by H atom.
3. ONIOM(QM:MM): the whole molecular system is treated with
QM except PPh3 ligands which are treated with Molecular
Mechanics (MM). In this ONIOM calculations, only steric effects
from PPh3 ligands are included. Electronic effects are not
included since PPh3 ligands are not treated in QM level.
All the calculations were carried out using optimized geometries
resulting from the DFT calculations. As a first step of the decom-
position analysis, evaluation of steric effects was performed by
comparing, QM-PH3 versus ONIOM(QM:MM). As a second step,
electronic effects were analyzed by comparing QM-PPh3 versus
ONIOM(QM:MM). The results are shown in Figs. 6 and 7.
It was found both the steric and electronic effects influence the
transition states as well as the initial and final metal complexes, and
they changed the reactivity in opposite manner: the steric effect
introduced by PPh3 ligands decreases activation barriers of rate
determining step (from 26 kJ/mol to 17 kJ/mol, Fig. 6), while the
electronic effect increased the activation barriers (from 17 kJ/mol to
23 kJ/mol, Fig. 7). The steric repulsions should destabilize all
complexes, including transition states, initial and final complexes.
The decrease of activation barrier suggests that initial and final
complexes are more destabilized than transition states. However,
electronic effects from PPh3 ligands increase the barrier about 6 kJ/
mol. These calculations provide insight for developing new cata-
lysts in future: different bulky ligands, which may provide same
EONIOM ¼ Ereal,low þ Emodel,high ꢀ Emodel,low
For our ONIOM investigations of 1,4-CHD isomerization by
RuHCl(CO)(PPh3)3, all atoms except those from phenyl groups are
treated as model system. The real system is treated at molecular
mechanics (MM) level by UFF method [34], and the model system is
Fig. 7. Energy diagram (in unit kJ/mol) of catalytic 1,4-CHD isomerization to show electronic effect. The red diagram is from B3LYP calculations with PPh3 ligands; the cyan diagram
is from ONIOM(B3LYP:UFF) calculations with PPh3 ligands. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)