Journal of the American Chemical Society
COMMUNICATION
Scheme 8. Decarbonylation Pathway by Which Heating 2a*
Yields Acetate Complexes 3a* and 3a1*
The observed reaction ultimately involves two key steps
required for the selective conversion of CO to C2+ oxygenates:
CÀC bond formation and, observed here for the first time, CÀO
bond formation in the form of an acyl migration. Oxo ligands
usually stabilize high-valent transition-metal complexes and as a
result are typically inert. Thus, the apparent activation of the
RetO bond is novel. Current efforts in our laboratories are
aimed at developing these reactions for the design of homogeneous
catalysts for syngas activation. In addition, the mechanistic details of
both steps are currently being explored computationally.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data (CIF)
b
for 2a, 2b, 3a, and 3b; NMR spectra of 2a and 3a. This material
Scheme 9. Addition of CO to 2a* and 2b
’ AUTHOR INFORMATION
Corresponding Author
’ ACKNOWLEDGMENT
The authors acknowledge funding by North Carolina State
University and by the National Science Foundation via the
CAREER Award (CHE-0955636). Also, the Department of
Chemistry of North Carolina State University and the State of
North Carolina are acknowledged for funding the purchase of the
Apex2 diffractometer. The authors thank Dr. Hanna Gracz for
assistance with the 2D NMR experiments.
’ REFERENCES
The formation of 3 via intermolecular transfer of the acyl
ligands from two molecules of 2 (pathway D) was investigated by
the addition of CO to a 1:1 mixture of 2a* and 2b (Scheme 9).
After 16 h, only 3a* and 3b were observed, indicating that no
crossover had occurred. This implies that the intermolecular path-
way (pathway D) is not viable under these reaction conditions.
Pathway B appears to be the only pathway among those
considered that is supported by all of the available data. The
observed migration of the acyl ligand to the oxo ligand represents
an unprecedented mechanism for acetate complex formation and
is only the second thermal example of the migration of a ligand
from the metal center to a terminal oxo ligand.9 Addition of CO
to 2a depletes the metal center of electron density. As a result, the
oxo ligand becomes electrophilic and is subject to intramolecular
nucleophilic attack from the acyl σ bond. In the absence of CO,
migration of the acyl ligand to the oxo ligand is not observed.
To summarize, the activation of CO by 1 was investigated and
found to result in the synthesis of rhenium(III) acetate complex 3.
Four mechanisms for the formation of 3 were considered; an
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plexes that involves insertion of CO into the rheniumÀmethyl bond
to yield 2 followed by migration of the acyl ligand to the terminal oxo
to yield 3 is the only mechanism supported by all of the available data.
The major evidence for this mechanism is the following: (a) the
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under the reaction conditions; (c) when the time course of the
reaction was followed, it was observed that 2 accumulated and was
consumed as expected; and (d) labeling studies showed that the acyl
carbon in 2 is the acetate carbon in 3 and that scrambling does not
occur at room temperature.
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’ NOTE ADDED AFTER ASAP PUBLICATION
Scheme 2 was replaced and reposted August 10, 2011.
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dx.doi.org/10.1021/ja205477w |J. Am. Chem. Soc. 2011, 133, 13288–13291