Description of DOE
Project Managed by Robert Bergman
Alkanes, or saturated hydrocarbons, are made up only of carbon
and hydrogen, and therefore contain only carbon-hydrogen (C-H)
and carbon-carbon (C-C) bonds. Alkanes are primary constituents
in petroleum and natural gas. However, these molecules are normally
so inert toward conventional reagents that it has not been possible
to utilize them for synthetic chemistry on a laboratory or industrial
scale.
The key to making these molecules useful
is to "activate" the carbon-hydrogen bonds, converting
them into bonds between carbon and other, more reactive atoms
or groups. Several years ago the Bergman group discovered one
of the first "C-H activation" reactions, a process that
converts C-H bonds in alkanes into bonds between carbon and transition
metals (in this case, iridium and rhodium). Subsequent to this
discovery substantial work has taken place on understanding the
mechanism of this reaction. Mechanistic experiments carried out
in the Bergman laboratories, and fast and ultra-fast kinetics
experiments carried out in collaboration with C. B. Moore, C.
B. Harris and H. M. Frei and their coworkers, have provided detailed
information on the nature of a large majority of the steps in
the reaction mechanism, over an unprecedented range of time scales.
This has resulted in the most complete picture of the carbon-hydrogen
bond activation mechanism that has been obtained to date.
More recently a new type of C-H activation
reaction involving reaction of the C-H bond with a relatively
high oxidation state iridium center (+3), has been discovered.
In this process the cationic iridium complex [Cp*(PMe3)Ir-CH3]+
undergoes a stoichiometric reaction with alkanes (R-H), resulting
in replacement of the iridium-bound methyl group with the alkyl
group of the alkane, leading to the organometallic product [Cp*(PMe3)Ir-R]+.
Extensive mechanistic studies have been carried out on this class
of reactions. These have provided strong evidence that the reaction
proceeds via transient intermediates having the unusual +5 oxidation
state at the iridium center. Very recently the corresponding hydride
cation [Cp*(PMe3)Ir-H]+ has been successfully
prepared. In contrast to the behavior of the iridium methyl complex,
the hydride induces a very rapid catalytic C-H activation
reaction. This process allows the exchange of deuterium from relatively
inexpensive sources of the heavy isotope, such as benzene-d6,
into alkanes.
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Transition Metal Complexes for Catalytic Chemistry:
Understanding Carbon-Hydrogen Bond Activation
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| Transition Metal Complexes
for Catalytic Chemistry: Understanding Carbon-Hydrogen Bond
Activation |
Explanatory paragraph for the drawing, "Transition Metal
Complexes for Catalytic Chemistry: Understanding Carbon-Hydrogen
Bond Activation"
Alkanes, such as methane and its higher congeners, are potentially
important feedstocks for higher-value, functionalized organic
compounds. However, it is normally very difficult to break the
carbon-hydrogen bonds in alkanes, which is the first step required
to achieve such functionalization.
Earlier we found that the cationic iridium complex [Cp*(PMe3)Ir-CH3]+
undergoes a stoichiometric reaction with certain alkanes (R-H),
resulting in replacement of the iridium-bound methyl group with
the alkyl group of the alkane, leading to the organometallic product
[Cp*(PMe3)Ir-R]+. We have now prepared the
corresponding hydride cation [Cp*(PMe3)Ir-H]+.
In contrast to the behavior of the iridium methyl complex, the
hydride induces a very rapid catalytic C-H activation reaction.
This process allows the exchange of deuterium from relatively
inexpensive sources of the heavy isotope, such as benzene-d6,
into alkanes.
The illustration shows one important example of this process:
the early stages of the catalyzed exchange of deuterium from benzene-d6
into methane, in which one can observe methane (CH4) and its deuterium-exchanged
analogues CDH3, CH2D2 and CHD3.
This reaction takes place at a rapid rate at the unusually low
temperature of -50 oC. The transformation is conveniently observed
by in situ monitoring of the reaction by low-temperature
proton NMR spectrometry. Methane exhibits only a single line in
the spectrum. As the four methane hydrogens are sequentially replaced
by deuterium, the remaining hydrogen atoms of the molecule experience
an isotope shift, moving their resonance positions to higher field.
In addition, the spectrum shows the spin-spin coupling of these
hydrogen atoms (nuclear spin = ½) to the newly-introduced
deuterium atoms (nuclear spin = 1), resulting in multiple-line
spectra for CDH3, CH2D2 and CHD3.
Since it has no remaining hydrogens, the CD4 does not appear in
the proton NMR spectrum, but it can be detected by deuterium NMR
spectroscopy.
Our principle collaborations are with groups inside the catalysis
group (see other parts of this FWP). Outside of that group, but
within LBNL, we have worked with the C. B. Moore, C. B. Harris
and H. Frei groups, as an essential part of our flash kinetic
mechanistic studies.
External to LBNL:
In addition to local collaborations, our work has benefited from
collaboration and informal interaction with theoretical chemists
at other locations (John Nicholas at PNNL, Michael Hall at the
University of Texas, Per Siegbahn at the University of Stockholm,
Keiji Morokuma at Emory University). The collaboration with T.
D. Tilley also involves participation by chemists at the DuPont
Central Research and Development Dept. in Wilmington, Delaware,
and at the Zelinsky Institute in Moscow.
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