A R T I C L E S
Song et al.
The geometry optimizations and energy calculations of the
intermediates (IM) and products (RP) were also performed with
B3LYP/LANL2DZ in Gaussian 03W. To simplify the computa-
tion, the polymer chain and the 1-carbonyl amide group were
3
modeled as a methyl group and a CONHCH substituent,
respectively (Figure S9). To prevent ring-opening and to find
the local minimum of the intermediate structures, only one bond
in the metallocyclobutane ring was optimized while the other
three bond lengths were kept constant; the partial optimization
was rotated to optimize the other three bonds, until the optimized
-
4
structure changed little in energy (∆E < 6.3 × 10 kcal/mol).
Vibrational frequency calculations using B3LYP/LANL2DZ
were performed for all model compounds (Figure S9); there
were no imaginary vibrational frequencies indicating that local
minima had been found for each structure. Free energies
computed in Gaussian 03W for structures (Figure 8) in solvent
(
CH
energy from the conductor-like polarizable continuum model
CPCM) based on the United Atom Kohn-Sham (UAKS) radii.
. NBO Charge Calculations. For the ester and amide
2 2
Cl ) include the electronic energy plus the solvation free
(
2
cyclobutene monomers, the calculations showed that the electron
density is higher on C-1 than on C-2 as expected for olefins
bearing conjugated electron withdrawing groups (monomers 2a,
3
a, 4a). In contrast to the carbonyl substituents, the C-1
methanol ester substituent (monomer 5b) is electron donating,
making C-1 more electropositive than C-2. In all of the
ruthenium carbenes, the metal atom is more electropositive than
the adjacent carbon atom.
Figure 7. (a) NBO charge populations of 1-substituted cyclobutene
derivatives 2a, 3a, 4a, and 5b, the corresponding ring-opened ruthenium
carbenes of 2a and 3a, and ruthenium benzylidene. Hartree-Fock calcula-
tions were performed with the 6-311G++* basis set (for cyclobutene
monomers) and the LANL2DZ basis set (for ruthenium carbenes) in
Gaussian 03W. (b) AIM electron densities (atomic units) for the olefin bonds
in 1-substituted cyclobutene derivatives 2a and 4a were calculated with
AIMPAC.
3
. Relative Energies of Intermediates and Products. The
NBO calculations allowed us to rationalize the regiochemical
results of the ROM and ROMP processes. In the case of
monomers 2-4, alignment of the cyclobutene and the electron-
deficient Ru to pair C-1 and the Ru to form the π-complex (π-
SM) is favored by the electronic factors; however, in the case
of monomers 5, electronic factors favor a cycloaddition in which
C-2 becomes bonded to the ruthenium (Figure 7). Assuming
then that steric effects between the coordinating cyclobutene
and the carbene are relatively unimportant (because of the long
distance between the cyclobutene and Ru in the π-complex),
we ordered the energies of the regioisomeric cyclobutene
π-complexes (π-SM) according to the charge distribution of
the cyclobutenes.
1
. Calculation Methods. For optimization of the geometries
of the cyclobutene monomers (2a, 3a, 4a, and 5b), we used
B3LYP/6-31G*. This method is frequently employed for simple
20-23
organic compounds
because it provides accurate structural
predictions with economical calculations. To limit the number
of atoms included in the propagating ruthenium carbenes and
thereby facilitate the calculations, we modeled the substituents
as (CH
[
[
2
CH
Ru]-16. For these carbenes, and for the ruthenium benzylidene
Ru]-17 (Figure 7a), we used B3LYP/LANL2DZ. This method
2
CHdCHPh); that is, we used structures [Ru]-15 and
Noting energy values from the literature for the ring strain
of cyclobutenes and for π-stabilization of ruthenium carbene
has been used extensively for the geometry optimization of
ruthenium carbenes. The LANL2DZ basis set has superior
properties with respect to effective core potentials (ECPs) and
2
4-37
(
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and calculated
results agree very well with experimental data, for example,
(
27) Hoffmann, M.; Marciniec, B. J. Mol. Model. 2007, 13, 477–483.
3
2,33
36
X-ray crystal structures,
parameters,
IR and NMR spectra, thermal
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5,26,37
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58–461.
natural bond orbital (NBO) charge populations (Figure 7a), we
used Hartree-Fock with the 6-31G++* basis set (for cy-
clobutene monomers) and the LANL2DZ basis set (for ruthe-
nium carbenes) in Gaussian 03W.
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0518 J. AM. CHEM. SOC. 9 VOL. 132, NO. 30, 2010