Inorganic Chemistry
Forum Article
molecular and silica-supported molybdenum oxo alkylidenes
1−3 and 1−3@SiO2 (see Table 2 and Figure S17 for details);
calculations for the silica-supported systems were carried out
by using (MeO)3SiO as a simple model to interrogate the
electronic effects associated with a surface siloxy ligand. From
these data, it appears that the SP metallacycle, recognized as
the resting state intermediate of olefin metathesis, is always the
lower-energy metallacycle for all calculated complexes.
Regarding the TBP intermediates, their free energies are also
significantly lower than the energies of the separated reactants
for all calculated complexes with the exception of the
molecular complexes 2 and 3, where formation of the TBP
metallacycle is endergonic by ca. 2 or 3.5 kcal/mol,
respectively.
ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
Details on the reagents used, experimental setups,
synthetic procedures, and corresponding characteriza-
tions, computational details including atomic coordi-
nates for all calculated structures, experimental proce-
dures for the metathesis tests and corresponding
catalytic data, and crystallographic data (PDF)
xyz files for all computed structures (ZIP)
Accession Codes
An ideal metathesis catalyst would have a close-to-neutral
free energy of formation for the TBP isomer (ΔGTBP ≈ 0 kcal/
mol) and a destabilized SP isomer, favoring metathesis over
formation of the off-cycle intermediate. The data presented in
Table 2 clearly indicate that compound 1 allows the formation
of not too stable TBP and SP metallacyclobutanes, while 2 and
3 are associated with the positive free energies of formation of
the key TBP intermediate, likely associated with a higher
barrier for [2 + 2] cycloaddition. In addition, while not
calculated, olefin coordination/cycloaddition in complexes 2
and 3 is likely not favorable because of the bulky aryloxide
ligands. In contrast, the silica-supported systems with the
significantly smaller and weak σ-donating surface siloxy
ligand13a,16 are associated with the formation of slightly more
stable TBP and SP metallacycles.
CCDC 2053831 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
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Corresponding Authors
́
Christophe Coperet − Department of Chemistry and Applied
Biosciences, ETH Zürich (ETHZ), CH-8093 Zürich,
Richard R. Schrock − Department of Chemistry,
Massachusetts Institute of Technology, Cambridge,
This could explain their higher activity with respect to the
corresponding molecular complexes 2 and 3. These calcu-
lations also highlight the specificity of compound 1, containing
two very weak σ-donating ORF9 ligands that do not stabilize
the metallacycles too much. For 1@SiO2, two TBP isomers can
be formed with the siloxy ligand in either the axial or equatorial
position. The free energy of the siloxy equatorial isomer is 2
kcal/mol lower compared to the siloxy axial isomer, high-
lighting the weak σ-donor ability of ORF9 that ends up trans to
the very strong σ-donor oxo ligand. This may explain the lower
activity of 1@SiO2 compared to 1. To better understand the
high activity of 1, we also evaluated the metathesis pathway.
While the olefin complex and associated transition state for
coordination could not be located, the [2 + 2] cycloaddition
and turnstile isomerization transition states have very similar
Authors
Jordan De Jesus Silva − Department of Chemistry and
Applied Biosciences, ETH Zürich (ETHZ), CH-8093 Zürich,
Switzerland
Margherita Pucino − Department of Chemistry and Applied
Biosciences, ETH Zürich (ETHZ), CH-8093 Zürich,
Switzerland
Feng Zhai − Department of Chemistry, Massachusetts Institute
of Technology, Cambridge, Massachusetts 02139, United
Deni Mance − Department of Chemistry and Applied
Biosciences, ETH Zürich (ETHZ), CH-8093 Zürich,
Switzerland
Zachariah J. Berkson − Department of Chemistry and Applied
Biosciences, ETH Zürich (ETHZ), CH-8093 Zürich,
Switzerland
and quite low energies: ΔG⧧
(ΔH⧧[2+2]) = 8.12 (−6.25)
[2+2]
kcal/mol and ΔG⧧
(ΔH⧧turnstile) = 8.38 (−6.45) kcal/
turnstile
mol. The combination of facile [2 + 2] cycloaddition/
cycloreversion and formation of not too stable metal-
lacyclobutanes is consistent with the high catalytic activity of
compound 1.
Darryl F. Nater − Department of Chemistry and Applied
Biosciences, ETH Zürich (ETHZ), CH-8093 Zürich,
Switzerland
Amir H. Hoveyda − Department of Chemistry, Merkert
Chemistry Center, Boston College, Chestnut Hill,
Massachusetts 02467, United States
CONCLUSIONS
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Overall, this study further illustrates how subtle differences in
ligand substitution in Schrock alkylidenes can greatly influence
their metathesis activity, pointing out to the need to explore
this family of complexes in a more systematic fashion toward
the development of detailed structure−activity relationships.
This study also illustrates the uniqueness of silica, that enables
generated isolated metal sites bound to a weak σ-donating and
rather small siloxy ligand.
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Author Contributions
†These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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Inorg. Chem. XXXX, XXX, XXX−XXX