Article
Inorganic Chemistry, Vol. 49, No. 7, 2010 3363
the molecular mechanism of the calcium-mediated hydro-
genation of conjugated alkenes described above is still desir-
able. Understanding the mechanism of this type of reaction at
the molecular level will provide important information for
the design of more effective main-group metal-based cata-
lysts for alkene hydrogenation. In this work, we have per-
formed detailed density functional theory (DFT) calculations
to investigate the molecular mechanism for two typical
hydrogenation reactions catalyzed by ({CaH(dipp-nacnac)-
(thf)}2), the hydrogenation reactions of DPE and myrcene,
respectively. These two substrates can be considered as two
typical examples of conjugated alkenes. In addition, we have
also investigated the catalytic behaviors of the analogous
organomagnesium and organostrontium catalysts for the
hydrogenation reaction of DPE.
to obtain its gas-phase Gibbs free energy. In addition, all the
transition states were verified by intrinsic reaction coordinate
(IRC)47 calculations. The polarizable-continuum model
(PCM)48 was employed to compute the solvation Gibbs free
energy for each species at its gas-phase optimized structure.
For each species in the solvent (benzene), its free energy is
evaluated as the sum of its gas-phase free energy (obtained at a
given temperature and pressure) and the solvation free energy.
3. Results and Discussions
In this section, the generation of the actual catalyst is
discussed first. Then, results on the hydrogenation reactions
of DPE and myrcene catalyzed by the calcium hydride
complex will be discussed, respectively. In the last subsection,
we explore the free energy profiles of the hydrogenation of
DPE catalyzed by the analogous magnesium-hydride and
strontium-hydride catalysts.
3.1. Active Form of the Calcium Catalyst. The calcium
catalyst, {CaH(dipp-nacnac)(thf)}2, is a dimeric species,
as shown by its crystal structure.49 However, the crystal
structure of the hydride transfer intermediate suggests
that only its monomer is involved in the reaction. Thus, it
is worthwhile to investigate the energetics of the dissocia-
tion process of the dimeric species into two monomers.
We have optimized the structures of the dimer and its
corresponding monomer, the calcium hydride complex
(CaH(dipp-nacnac)(thf), species 1 in Figure 1). The opti-
mized structural parameters of the dimer (as shown in
Figure S1, Supporting Information) are in very good
agreement with those of the crystal structure. The dis-
sociation free energy from the dimer to two monomers is
calculated to be 13.3 kcal/mol in the gas phase (60 ꢀC and
20 bar) and -7.7 kcal/mol in the solvent (benzene). Thus,
it is very likely for the calcium catalyst, {CaH(dipp-
nacnac)(thf)}2, to decompose into two calcium hydride
complexes. On the basis of the experimental results and
calculated results described above, we assume that the
active form of the calcium catalyst would be the calcium
hydride complex.
2. Computational Details
All the species were fully optimized using the Gaussian03
program41 with the B3LYP hybrid functional.42,43 For Ca
and Mg, the 6-311þþG(d,p) basis set was employed, and for
Sr, the effective core potential LANL2DZ44 and a modified
LANL2DZ basis set with optimized 5p functions45 and a set
of f-type functions was used.46 For the reaction of DPE, the
6-31G basis set was used for atoms outside the active center,
which include two methyl groups and two 2,6-diisopropyl-
phenyl groups, the tetrahydrofuran (thf) ligand (except the
oxygenatom), and onephenyl group (except the carbon atom
bonded to the double bond). For all the other atoms, the
6-311þþG(d,p) basis set was used. For the substrate myr-
cene, the 6-311þþG(d,p) basis set was employed for the
butadiene component and the 6-31G basis set was used for
other atoms. For each stationary point on the potential
energy surface, a frequency calculation was carried out to
make sure whether it is a minimum or a transition state, and
3.2. The Calcium-Mediated Hydrogenation Reaction of
DPE. For this reaction, we have located all possible
intermediates and transition states which may be in-
volved. The optimized structures of these stationary
points are displayed in Figure 1. The Gibbs free energy
profiles calculated in gas phase (60 ꢀC and 20 bar) and in
the solvent are presented in Figure 2. In the following
discussion, free energies obtained in the solvent will be
used exclusively in discussing the reaction pathway.
(41) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A.
D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;
Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. A. Gaussian 03, revision
B.04; Gaussian Inc.: Wallingford, CT, 2004.
˚
In the active catalyst 1, the Ca-H1 distance is 2.073 A.
The natural atomic charges on Ca and H1 are 1.71e and
-0.82e, respectively, an indication that H1 is hydridic.
For the substrate 2 (DPE), the C1dC2 bond length is
˚
˚
1.343 A. The C2-C3 distance is 1.490 A, being close to a
typical single C-C bond rather than a typical CdC bond.
First, the hydride H1 migrates from the calcium center
in 1 to the terminal carbon (C1) of the double bond in the
substrate 2 via the transition state TS1. In TS1, the
˚
distances of Ca-H1 and H1-C1 are 2.169 and 1.647 A,
respectively. The activation free energy of this step is 29.2
kcal/mol. About the ion-pair product from this hydride
(42) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(43) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(44) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310.
(45) Couty, M.; Hall, M. B. J. Comput. Chem. 1996, 17, 1359–1370.
€
€
(46) Ehler, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth, A.;
(47) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154–2161.
(48) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027–2094.
(49) Spielmann, J.; Harder, S. Chem.;Eur. J. 2008, 1480–1486.
€
Jonas, V.; Kohler, K. F.; Segmann, R.; Velkamp, A.; Frenking, G. Chem.
Phys. Lett. 1993, 208, 111–114.