PROJECT SUMMARY REPORTchem.chem.rochester.edu/~wdjgrp/doe/doe04.pdfExecutive Summary of FY 2001-4...
Transcript of PROJECT SUMMARY REPORTchem.chem.rochester.edu/~wdjgrp/doe/doe04.pdfExecutive Summary of FY 2001-4...
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PROJECT SUMMARY REPORT
submitted to
THE U.S. DEPARTMENT OF ENERGY
for
TRANSITION METAL ACTIVATION AND FUNCTIONALIZATION OF CARBON-HYDROGEN BONDS
William D. Jones, Principal Investigator University of Rochester
Department of Chemistry Rochester, New York 14627
Phone: 585-275-5493
Grant No. DE-FG02-86ER13569
Total Project Period: December 1, 2001 - November 30, 2004 Total Award Amount (3 years): $ 395,000
Unexpended Balance from Previous Year: $0
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Executive Summary of FY 2001-4 Research in the Chemical Sciences Transition Metal Activation and Functionalization of Carbon-Hydrogen Bonds Grant FG02-86ER13569 William D. Jones, Department of Chemistry, University of Rochester, Rochester, NY 14627 Total Grant Period: 12/1/01-11/30/04, $395,000 for three years
Overview of Research Goals and Accomplishments: This project has as its overall goal improvement in the intelligent use of our energy
resources, specifically pertroleum derived products, and is aimed at the development of new routes for the manipulation of C-H and C-C bonds. During the 3 year project period, our research has focussed on the following specific goals: (1) discovery of new carbon-carbon bond cleavage reactions, (2) fundamental studies of C-H bond cleavage reactions of trispyrazolyl-boraterhodium complexes, (3) catalytic C-H and C-C bond functionalization, and (4) carbon-fluorine bond activation. This year we have made progress in several of these areas, as described in the following report.
The specific accomplishments of the current grant period include:(1) we have successfully measured and qunatitatively modeled the processes available to metal alkyl hydride complexes in a trispyrazolylborate-rhodium complex; (2) we have for the first time determined the isotope effects on the specific fundamental steps involved in alkane oxidative addition and reductive elimination; (3) we have preliminary results that measure for the first time the selectivity for a metal fragment binding to methyl vs methylene groups in a linear hydrocarbon; (4) we have cleaved C-C bonds in biphenylene, aryl-nitriles, and aryl-acetylenes, expanding tremendously the breadth of C-C cleavage; (5) we have established 3 different mechanisms for C-F bond cleavage of fluorocarbons using Cp*2ZrH2; (6) we have synthesized and studied several hemi-labile P-N complexes of platinum and nickel, and demonstrated that the labile nitrogen give enhanced reactivity not observed with the well-known P-P chelate complexes; (7) we have discovered new C-H and C-C functionalizations that allow introduction of reactive boronate and olefin functional groups; (8) we have cleaved C-C bonds in allyl nitriles, leading to isomerization of the C-C skeleton. This reaction is critical to the DuPont synthesis of Nylon from butadiene; (9) we have measured C-H activation selectivities in chloroalkanes, showing a preference for activation of the terminal methyl groups and the facile β-chloride elimination reaction.
A wide variety of chemistry has been examined, resulting in publication of several manuscripts. The work has been communicated at both national and international meetings. DOE funds have been used for the partial support of 4 graduate students (Andrew Vetter, Brad Kraft, Steve Oster, Karlyn Skugrud) and 1 postdoc (Nicole Brunkan) during the current grant period, as well as several undergraduates (Jason Holt, Mike Evans, John Curley, Brian Warsop, Susan Golisz).
The continued success of this work will lead to the development of new techniques and processes for the manipulation of petroleum-based hydrocarbons. These new processes will be based upon the new methods for making and breaking strong bonds in organic molecules of the type studied here. The work has the potential to have a significant impact in science and in technologies of interest to DOE.
A detailed report follows, followed by a listing of the 20 DOE supported publications for the current grant cycle and recent special recognitions received by the PI.
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DOE Report, 2001-2004 3 William D. Jones
Detailed Progress Report for the Project Period Dec. 1, 2001- Nov. 30, 2004.
This report summarizes research that has been performed since during the current 3-year
grant, as well as work that will be completed and published by the end of the grant period.
1. Tris-pyrazolylborate Rhodium C-H Activation Studies.
Our rhodium-trispyrazolylborate studies on hydrocarbon activation make use of the
reactive 16-electron fragment [HB(3,5-dimethylpyrazolyl)3]Rh(CNCH2CMe3), abbreviated
herein as Tp'RhL. In the prior 3-year project period, we established that the Tp'RhL fragment
coordinates an alkane to give a σ-complex. A series of labelling studies allowed the
determination of the relative rates of the processes available to the alkane σ-complex,
specifically: (1) C-H activation (called oxidative cleavage), (2) migration down the alkane chain,
or (3) simple dissociation. These experiments involved modelling of the scrambling of
deuterium in complexes such as Tp'Rh(L)(CH2CH2CH3)D before loss of propane-d1. Figure 1
shows the relative rates of these processes for methyl, ethyl, n-propyl, and n-butyl derivatives.
For methane, C-H activation is strongly preferred over dissociation, whereas for ethane, the rates
of these two processes are closer. End-to-end migration in ethane is intermediate. For propane,
terminal C-H activation is favored over dissociation to a lessor extent than methane, but
comparable to ethane. Migration from the end to the middle of propane is slightly slower than
C-H activation. For the secondary propane complex, migration to the end and dissociation occur
at about the same rate. Interestingly, migration down a butane chain (secondary to secondary) is
the fastest process, accounting for the observed kinetic preference for terminal C-H activation.
0 5 10 15
methane
ethane
propane
butane
migration from 2°-2°migration from 2°-1°2° dissociation
migration from 1°1° dissociationactivation
relative rate (kd1=1)
fixed ratio
Figure 1. Relative rates of σ-alkane processes.
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These conclusions can be expressed in a schematic fashion as shown in Scheme 1. Our
studies do not permit the determination of either the absolute or relative energies of the primary
and secondary alkane complexes, so we cannot establish the relative rates of processes between
these two intermediates. Relative rates are shown assuming the energies of the two alkane
complexes are equal.
Scheme 1: Relative rates of processes available in primary (1°) and secondary (2°) alkane
complexes.
*Assuming 1° alkane σ-complex is at the same energy as the 2° alkane σ-complex
(6x)
(3x)
(1x)
MH
M C C2H5H
HCH3
M +
M CH
HH
koxidative cleavage
kmigration
kdissociation
MH
primary
primary
butane σ-complex
secondarykoxidative cleavage
kmigration(back)(3x)*
(11x)*
(1.6x)*secondarykdissociation
CH
HH
M
CH
HCH3
Mkmigration(2°-2°)
(0.01x)*
With the relative rates of all of these processes now known for any linear alkane, we have
completed studies to determine which C-H bond of an alkane first binds to the Tp'RhL fragment.
The execution of this experiment is not completely obvious, as reaction of the fragment with any
linear hydrocarbon only gives a single product, the n-alkyl hydride (eq 1). One cannot determine
how the alkane initially bound if a single product is observed.
kprimary
ksecondary
MH
M C CH3H
HCH3
M +
MH
M CH
HH
(1)
In the current project period, we established that (1) the Tp'RhL fragment coordinates to a
linear alkane to give a σ-complex and that the coordination is favored at the methylene group
over the methyl group by a ratio of 1.5:1. (2) a methyl group in pentane coordinates 1.9 times
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faster than the more hindered methyl group of isobutane. (3) the oxidative cleavage of a methyl
C-H bond (primary C-H) occurs 65K12 times faster than the C-H bond in a methylene group
(secondary C-H), where K12 represents the equilibrium constant between primary and secondary
alkane complexes. These conclusions were made building upon our earlier studies of the relative
rates of oxidative cleavage, migration, and dissociation indicated in Scheme 1. The experimental
evidence for these conclusions is given below.
The first new result is determination of the rate at which a reactive metal complex with a
vacant site coordinates a methyl group C-H vs a methylene C-H. This was determined by
looking at the product distribution in a competition experiment between pentane and decane. In
this experiment, the metal intermediate would see an equal number of methyl groups in each
substrate but differing numbers of methylenes (see Scheme 2). Note that in either case, only the
terminal activation product would be seen since coordination to an internal methylene would
lead to migration of the metal along the chain to the end, where oxidative cleavage would occur.
If
Scheme 2: Pathways for C-H coordination/activation in pentane-decane competitition.
kd1kd2kd2 kd2
kd2
Tp'LRh
H3kOC
H
km12
km21
RhLTp'
C HH
H
RhLTp'
C HH
H3C
RhLTp'
C HH RhLTp'
C HH
RhLTp'
C HHkm22
km22
km22
km22
km22
km22
km22
km22
km22
km22
km22
km22Tp'LRh
CHH
Tp'LRh
CHH
Tp'LRh
CHH Tp'LRh
CHH
CH3 CHH
H
Tp'LRh
km12
3kOCH
RhLTp'
H
km22
km22
kCH3kCH2kCH2kCH2 kCH2kCH2 kCH2
kCH2kCH2kCH3 kd2kd2
kd2 kd2kd1
A
B C D E FG H
I J K L M N O P Q RS T
1 2 3 4 5 6
78 9 10 11
1213 14 15 16 17 18 19 20 21 22
2324
2526
27 28 29 3031
32
km21
L = CNCH2CMe3
kCH2kCH3
km21
km12
km22
km22
RhTp'L
H
Tp'LRh
C HH
HRhLTp'
C HH
H3CRhLTp'
C HH
pentane/decane
3kOCH
kd1 kd2kd2
RhLTp'
C HH
H3CRhLTp'
C HH
H
Tp'L Rh
H
km22
km22 km12
km21
3kOCH
kCH2 kd2 kd1kCH3kCH2
Tp'LRh(PhN=L)hν
-PhN=L[Tp'LRh]
kCH3 @ 7, 11, 23, 32
kCH2 @ 8, 9, 10, 24, 25, 26, 27, 28, 29, 30, 31
kRCH
kRCH
the reactive intermediate were to bind only to methyl groups C-H bonds, then a 1:1 product ratio
would be expected. If the reactive intermediate were to bind only to methylene C-H bonds, then
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a product ratio favoring decane activation would be expected (corresponding to the increased
number of methylene groups).
Scheme 2 shows all possible intermediates and pathways. It is rather complicated (!), but
since the earlier studies established many of these relative rates, only the rates of primary
coordination (kCH3) and secondary coordination (kCH2) need to be determined. In fact, we only
want to know the ratio of these rates so that the ratio can be adjusted in a simulation to match the
experimentally observed ratio of products. In the experiment, Tp*Rh(CNR)(carbodiimide) was
irradiated in a 1:1 mixture of pentane/decane. The alkyl hydride products were quenched with
CCl4 to give the stable chloro derivatives in a 1.1:1 ratio (Scheme 3). Consequently, the results
are consistent with a slight preference for methylene group coordination, and the simulation
indicates an actual preference of kCH2/kCH3 = 1.5:1.
Scheme 3. Competition between pentane and decane.
[Tp'LRh]-PhN=L
hνTp'LRh(PhN=L)
L = CNCH2CMe3
Tp'LRhH
Tp'LRhH
+
-20 °C
CCl4
+
Tp'LRhCl
Tp'LRhCl
1
:
1.1
Steric hinderence can interfere with the ability to coordinate to a methyl group C-H bond.
This can be seen in a competition between pentane and isobutane. In this experiment, two
methyl groups in a linear alkane compete with three methyl groups in a branched alkane. The
competitive reactions involved are shown in Scheme 4, with the two different rate constants for
methyl group binding shown as kCH3 and kCH3'.
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Scheme 4. Pathways for C-H coordination/activation in pentane-isobutane competitition.
3kCH3'
3kOC1°
Tp'LRhC
H
H
Tp'LRhC
H
HCH3
Tp'LRhC
H
HH
Tp'LRhH
3kOC1°
km22 km22
km12km212kCH3
2kCH2
kCH2
Tp'LRh
H
Tp'LRhC
H
HH
[Tp'LRh]
-PhN=Lhν
Tp'LRh(PhN=L)kd1
kd2
kd2
kd2
kCH3/kCH3' = 1.9
In the experiment, Tp*Rh(CNR)(carbodiimide) was irradiated in a 1:1 mixture of
pentane/isobutane. The alkyl hydride products were quenched with CCl4 to give the stable
chloro derivatives in a 1.35:1 ratio (Scheme 5). Once again, simulation of the experiment while
varying only the ratio kCH3/kCH3' allows determination of the ratio as 1.9:1. Note that in this
experiment, we assume that the rate of oxidative cleavage of the C-H bond in the two methyl
complexes is the same. It is possible to interpret this experiment in terms of an equal rate of
binding to the two types of methyl groups, but with a 1.9:1 ratio between the rate of the two
oxidative cleavage rates.
Scheme 5. Competition between pentane and isobutane.
[Tp'LRh]-PhN=L
hνTp'LRh(PhN=L)
L = CNCH2CMe3
Tp'LRhH
Tp'LRhH
+
-20 °C
CCl4+
Tp'LRhCl
Tp'LRhCl
1.35
:
1
In the pentane/decane competition experiment, we learned about the relative rates of
binding of unhindered CH3 vs CH2. It would be reasonable to expect this rate ratio to apply in
other competitions involving the binding of similar types of bonds. For example, the methylene
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C-H bonds in cyclohexane would be expected to bind to the [Tp*RhL] fragment with the same
ease as those in pentane, since cyclohexane is an unstrained, 'natural' conformation similar to
that in a linear alkane. Interestingly, cyclohexane does undergo activation of its C-H bonds by
the [Tp*RhL] fragment because secondary C-H bonds are the only ones available. In a
competition between pentane and cyclohexane, the metal fragment should bind to both alkanes
in a predictable fashion since kCH3/kCH2 is known. From the ratio of products obtained, one
should be able to determine the relative rates of oxidative cleavage of a secondary C-H bond
(kOC2°) vs a primary C-H bond (kOC1°), provided the alkane σ-complexes are at the same energy.
These pathways and the corresponding rate constants are shown in Scheme 6.
Scheme 6. Pathways for C-H coordination/activation in pentane-cyclohexane competitition.
kRC2
2kOC2°
1312kd2 kCH2
Tp'L Rh
H
RhLTp'
HH
kRCH
kRCH
[Tp'LRh]-PhN=L
hνTp'LRh(PhN=L)
kCH2 kCH3kd1
kd2kCH2
3kOCH
km21
km12km22
km22
Tp'L Rh
H
RhLTp'
C HH
H
RhLTp'
C HH
H3C
kd2kd2
kd1
3kOCH
pentane/cyclohexane
RhLTp'
C HH
RhLTp'
C HH
H3C
Tp'LRh
C HH
H RhTp'L
Hkm22
km22
km12
km21
kCH3kCH2
L = CNCH2CMe3
1110987
654321
HG FEDCB
A
kCH2 @ 8, 9, 10, 12
kCH3 @ 7, 11, 23, 32
kOC1°/kOC2° = 65 (with 1° and 2° alkane complexes at same E)
or otherwise:kOC
1°/kOC2° = 65 (km12/km21)= 65 (K12)
I J
In the competition experiment, Tp*Rh(L)(carbodiimide) was irradiated in a 1:1 mixture of
pentane/cyclohexane and quenched with CCl4. A 6.5:1 ratio of the n-pentyl and cyclohexyl
chloride products was seen (Scheme 7). From the simulation, this corresponds to a 65:1 ratio for
kOC1°/kOC2°, assuming the alkane σ-complexes are at the same energy. If they are at different
energies, then a 'correction' must be applied corresponding to Keq between the two σ-complexes.
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Scheme 7. Competition between pentane and cyclohexane.
[Tp'LRh]-PhN=L
hνTp'LRh(PhN=L)
L = CNCH2CMe3
Tp'LRhH
Tp'LRhH
+
-20 °C
+
Tp'LRhCl
6.5
:
1 Tp'LRhCl
CCl4
Finally, we have also looked at activation of cyclopentane vs pentane in a competition
experiment. Here, due to the strain in the cyclopentane ring, it is not reasonable to assume that
the rate of methylene coordination is the same as in a linear alkane. Consequently, the scheme
for this experiment shows different rates for the two types of methylene coordination (Scheme
8), but the same rate of oxidative cleavage in the methylene-alkane complex.
Scheme 8. Pathways for C-H coordination/activation in pentane-cyclopentane competitition.
5kCH2'
3kOC1°
Tp'LRhH
2kOC2°
km22 km22
km12km212kCH3
2kCH2
kCH2
Tp'LRh
H
kd1
kd2
kd2
kd2
kCH2 /kCH2 = 1.7'
Tp'LRhC
H
H
Tp'LRhC
H
HCH3
Tp'LRhC
H
HH
Tp'LRhC
H
H
[Tp'LRh]
-PhN=Lhν
Tp'LRh(PhN=L)
In the competition experiment, Tp*RhL(carbodiimide) was irradiated in a 1:1 mixture of
pentane and cyclopentane, and CCl4 added to quench the products. A 4.5:1 ratio of n-pentyl to
cyclopentyl products were seen (Scheme 9), showing that cyclopentane is slightly more reactive
than cyclohexane, as anticipated. From the kinetic simulation of the competition, a ratio for
kCH2'/kCH2 of 1.7 was obtained.
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Scheme 9. Competition between pentane and cyclopentane.
[Tp'LRh]-PhN=L
hνTp'LRh(PhN=L)
L = CNCH2CMe3
Tp'LRhH
+
-20 °C
+Tp'LRhCl
4.5 : 1Tp'LRhH
Tp'LRhCl
CCl4
As a check that this value represents the relative coordination abilities of cyclopentane vs
cyclohexane, a competition experiment was run by irradiation of Tp*RhL(carbodiimide) in a 1:1
mixture of cyclopentane/cyclohexane. A 1.9:1 ratio of cycloalkyl chloride products was
observed. The excellent agreement between the competition ratio from these two independent
experiments argues that the assumptions made in the simulations are reasonable.
One of the more interesting side-lights from these studies comes from the independent
determination of isotope effects for both the 'oxidative cleavage' and the 'reductive coupling'
steps of the C–H activation reaction indicated in equation 2. These isotope effects, both kinetic
isotope effects on a fundamental reaction step, were found to be normal isotope effects. The
overall effect on alkane reductive elimination, however, is to generate an inverse kinetic isotope.
The initial equilibrium isotope effect between the alkyl hydride complex and the alkane sigma-
complex is inverse, not because either of the individual rates are inverse, but because the ratio of
these isotope effects is inverse. As this was the first known system where these effects had been
completely sorted out, we published a more didactic article in Accounts of Chemical Research to
shed light on the analysis of this controversial subject.
slow+Lfast Ln+1M
LnMCH2R
D
[LnM] + R-CH2D
LnMCH2R
DLnM
CHDR
H kRCH
kOCH
kOCD
kRCD
(2)
2. C-C Bond Cleavage Studies
Our DOE supported work showed that several types of C-C bonds can be cleaved. We
have discovered 3 distinct classes of C-C bonds that can be cleaved: (1) strained rings such as
biphenylene undergo sp2-sp2 C-C cleavage with a number of metal complexes to give a variety
of products. (2) diphenylacetylene undergoes sp-sp2 cleavage photochemically when attached to
PtL2 complexes. (3) arylnitriles undergo sp2-sp C-CN cleavage when reacted with NiL2
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fragments, and η2-nitrile adducts can be observed as reaction intermediates. These results are
described in more detail below.
Earlier DOE supported work showed that the complexes Pt(PEt3)3, Pd(PEt3)3, and [Ni
(dippe)H]2 cleave the C-C bond of biphenylene to give (PEt3)2Pt(2,2'-biphenyl), (PEt3)2Pd(2,2'-
biphenyl), and (dippe)Ni(2,2'-biphenyl), respectively. These complexes underwent catalytic
chemistry to give functionalized products, such as tetraphenylene, biphenylene, fluorenone, or
phenanthrenes (Scheme 10).
Scheme 10:
ML
L
+ ML2
L2 = (PEt3)2, dippeM = Ni, Pd, Pt
H2
CO
O
RC CRR R
In the case of Ni(dippe), it was found that catalysis required the introduction of small
amounts of oxygen, just enough to oxidize the phosphine to phosphine oxide. The remaining
'naked' metal was efficient at catalyzing the insertion of alkynes into biphenylene to give
phenanthrenes. The requirement of a labile chelate led us to investigate the use of the hemi-
labile ligand Pri2PCH2CH2NMe2 in these same metal systems during the current grant period.
The platinum complexes proved to be the easiest to synthesize and study, since the adducts
are fairly stable. The strategy was to use a source of Pt(0) in the presence of the P-N chelate and
an alkyne, to isolate the complex, and then to look at reactions of the complex. With
diphenylacetylene, the preparation of the adduct was straightforward and allowed comparison of
the P-P chelate with the P-N chelate (eq 3).
Pt
PR2
ER2´
+ PtP
E
R2
R2´- CODCH2Cl2
E = N, R = iPr, R´= Me: 1E = P, R = R´= iPr: 2 (3)
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While the P-P chelated complex showed no reactivity with added diphenylacetylene even
after heating, the P-N chelated complex reacted at room temperature to give a
metallacyclopentadiene complex. Further studies of the system showed that the nitrogen of the
chelate is quite labile, and that once it dissociates the phosphine ligands can then redistribute
between metals to generate two observable intermediates (Scheme 11). Ultimately, however,
thermodynamics takes over and one winds up with quantitative production of the metallacycle.
Heating this sample results in the slow catalytic formation of hexaphenylbenzene.
Scheme 11:
+ 2 NMe2
Pt
(Pri)2 P C
C
Ph
Ph
NMe2
Pt
(Pri)2 P
Ph
Ph
Ph
Ph
Pt
Me2NPt
PPri2
PhPh
PhPh+ PtMe2N
R2P
Pt
PR2
Me2N
2 2
With trimethylsilylphenylacetylene, the preparation of the initial complex is similar but
now the alkyne complex undergoes insertion into the C-Si bond at room temperature (eq 4).
Once again, no such reaction is seen for the complex with a bis-phosphine chelate. Just as in the
case of diphenylacetylene, ligand dissociation and redistribution is observed at intermediate
times to give two observable intermediates (Scheme 12). Once again, thermodynamics takes
over and only a single product, the C-Si insertion complex, is observed at longer reaction times.
NMe2
Pt
(Pri)2 P
SiMe3
C6D6, 5 days
22 °C
NMe2
Pt
(Pri)2 P SiMe3
(4)
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Scheme 12:
+
2 2
SiMe3
+ 2 NMe2
Pt
(Pri)2 P C
C
SiMe3
Ph
PtMe2N
R2P
Pt
PR2
Me2N
SiMe3
NMe2
Pt
(Pri)2 P SiMe3
NMe2
Pt
(Pri)2 P SiMe3
Me2 N
Pt P(Pri)2
SiMe3
Pt
Me3Si
While the dippe P-P chelate complex is unreactive thermally, it does react photochemically
to give a C-Si insertion product (eq 5). Remarkably, however, this complex reverts to the Pt(0)
alkyne complex thermally! This observation leads to the important conclusion that oxidative
addition is favored thermodynamically by the presence of the P-N ligand, whereas reductive
elimination is thermodynamically favored by the P-P ligand. This conclusion implies that the
study of a chelating, sterically hindered, bis-(dialkylamino)ethane ligand should give a metal
fragment that will strongly favor C-C cleavage. This hypothesis remains to be tested.
C6D6
PtP
P
SiMe3Pt
P
P
SiMe3Cy2
Cy2 Cy2
Cy2hν
∆
3 4 (5)
In addition to the above work with platinum, we have also prepared the analogous nickel
complex (eq 6). These complexes have proven to be efficient catalysts for the selective coupling
of biphenylene and alkynes to give phenanthrenes (eq 7).
Ni + +
P(iPr)2
NMe2
NiP
N- 2 COD(1)
(iPr)2
Me2
1 (6)
PhPhPhPh
+
T = 70°C
NiP
NPh
Ph(iPr)2
Me2
(7)
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We have also discovered that the nickel complex [Ni(dippe)H]2 reacts with benzonitrile to
give first an η2-nitrile complex, which then undergoes C-C cleavage of the carbon-CN bond (eq
8). Furthermore, the reaction does not go to completion but forms and equilibrium mixture of
the η2-nitrile and C-CN oxidative addition product. We know of no such example of reversible
C-C cleavage in the literature. Other examples of aryl C-CN cleavage are under investigation.
N
C
Ph
Ni
Pri2 P
PPri2
Ni
Pri2 P
PPri2
HNi
Pri2 P
PPri2
H
+ 2 Ph-C N
2-H2
KeqNi
Pri2 P
PPri2
Ph
CN
(8)
We have also looked at the effects of electron withdrawing and electron donating groups
on the aryl cyanide. The results show that both the rate and equilibrium for C-CN cleavage is
affected, as shown in the plots in Figure 2. For the equilibration, Keq is found to have a ρ value
of +6.1. This large and positive value for ρ indicates that negative charge is being stabilized on
the ipso carbon of the substituted aryl group, consistent with the organometallic nature of this
bond as possessing substantial Niδ+ —Cδ– character. From the rate of approach to equilibrium,
the rate of the forward reaction k1 can be determined. Similarly, a plot of ln k1 vs. σ, also shown
in Figure 2b, gives a ρ value of +1.3. While there is somewhat more scatter in this plot, the
positive slope correlation is unmistakable and is consistent with the localization of charge
density on the ipso carbon in the transition state for C-CN bond cleavage, although not so much
as the Ni(II) product.
-5
-3
-1
1
3
5
-1 -0.5 0 0.5 1
σp
ln( K
eq/ K
H)
(a)
-1
-0.5
0
0.5
1
1.5
-0.75 -0.25 0.25 0.75σp
ln( k
1/ kH)
(b)
Figure 2. (a) Hammett plot for the equilibrium constants (Keq) from equation 8 vs. σP at 54 °C. (b) Hammett plot for the forward rate constants (k1) vs. σP at 54 °C.
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We have also discovered an important new type of C-C bond oxidative addition, cleavage
of sp-sp2 C-C bonds in aryl acetylenes. This is a new class of C-C bond cleavage, and offers
many exciting possibilities. We have found that irradiation of either P-P or P-N chelate
complexes of Pt-(diphenylacetylene) leads to the clean and quantititive formation of the
oxidative addition product (eq 9). The reaction works for all complexes we have examined to
date. Furthermore, the oxidative cleavage reaction is reversible, in that heating the Pt(II)
complexes results in their reversion to the Pt(0)-alkyne complexes. Consequently, we now have
methodology to break and make C-C———C bonds, and further development of this reaction will be
the subject of future studies.
ER2'
R2P
Pthν
C6D6 ER2'
R2P
Pt
E = N, R = iPr, R'= MeE = P, R = R'= iPrE = P, R = R'= Cy
(9)
Recently, we have initiated investigations of a new class of C-C cleavage, that of allyl-
nitriles. At a metal center this cleavage reaction generates both a strong metal-cyanide bond and
a π-allyl ligand, and hence has been found to be both facile and reversible. Indeed, at nickel(0),
this reaction forms the basis of DuPont's synthesis of adiponitrile for the production of nylon via
addition of HCN to butadiene, to the tune of over 400 thousand metric tons per year!
We have discovered that the reactive hydride [(dippe)NiH]2, which serves as a room
temperature source of [Ni(dippe)], reacts with allylcyanide to give initially a π-olefin complex
(dippe = bis-(diisopropylphosphino)ethane). This species can be seen at low temperature by
NMR spectroscopy, and upon warming to RT competitive C-H and C-CN cleavage takes place.
C-H activation gives a π-allyl hydride complex that is not observed, because the hydride is
transferred back to the opposite end of the allyl group to give a very stable crotononitrile
complex (both cis and trans are formed). C-CN activation, however, leads to a metastable π-
allyl cyanide complex that can be isolated and was structurally characterized. C-CN cleavage is
reversible, so that ultimately, all nickel winds up as the crotononitrile complexes (Scheme 13).
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16
Scheme 13. C-C and C-H Bond Activation in allylcyanide.
k2, k3
[Ni(dippe)H2]2
CN+
PPri2
Ni
Pri2 P
CN
PPri2
Ni
Pri2 P
H
CN
PPri2
Ni
Pri2 P CN
PPri2
Ni
Pri2 P
NC
k1, C-C activation
C-H activation
C-C formation, k-1 ∆H‡ (kcal/mol) ∆S‡ (e.u.) ∆G‡ at 40 °C (kcal/mol)k1 (C-C cleavage) 18.0(0.3) -14.8(0.8) 22.71k-1 (C-C formation) 27.5(0.5) 11.7(1.6) 23.75k2 (C-H cleavage) 13.4(0.2) -29.9(0.6) 22.95k3 (C-H cleavage) 13.2(0.2) -31.2(0.7) 23.14
By monitoring the distribution of species over time, we have been able to extract the rate
constants for all of these species by kinetic simulation. In addition, by measuring the
distribution of species as a function of temperature, we can obtain activation parameters for the
various steps. The results are quite interesting, in that we find that while C-H activation and C-C
activation have small temperature dependences, C-C cleavage has a large temperature
dependence. The result is that by raising the temperature, one can selectively drive the reaction
in the direction of the less-favorable π-allyl cyanide complex. This is good news, since the
DuPont catalysis requires that the C-C cleavage dominate the C-H cleavage. The activation
parameters support the mechanism for C-H anc C-C cleavage shown in Scheme 14.
Scheme 14:
Ni
CN r.d.s.CN
Ni
H
crotononitrilesfast
‡
Ni
CN H
H
r.d.s.Ni
CNNi
CN
Ni
NC
‡
N
CNi
NiN
C
HH
Ni
CN
We have also investigated the effect of the Lewis acid BPh3 upon the isomerization
reaction. Addition of BPh3 to a cold solution of (dippe)Ni(η2-allylCN) leads to the immediate
and quantitative formation of the BPh3 adduct of the π-allyl cyanide adduct. Addition of a
second equivalent of the Lewis acid leads to the removal of the cyanide ligand as the
-
17
[Ph3B-CN-BPh3]- anion, leaving behind the [(dippe)Ni(π-allyl)]+ cation (Scheme 15). With only
one equivalent of BPh3, there is evidence of linkage isomerism of the initial Ni-C-N-BPh3 adduct
to the Ni-N-C-BPh3 adduct.
Scheme 15. Effect of Lewis acid on the C-C and C-H Bond Activation in allylcyanide.
PR2
Ni
R2P
HNi
H
PR2
R2P CN
BPh3
slow
PiPr2
Ni
iPr2 P
RT PiPr2
Ni
iPr2 P
BPh3NC
PiPr2
Ni
iPr2 P
BPh3CN
slow
[Ph3B-NC] [Ph3B-CN]
3. C-H and C-C Bond Functionalization Studies
We have also initiated investigations of the above systems for their ability to serve in
further functionalization reactions of hydrocarbons. For example, we have found that C-C bonds
can not only be cleaved, but functionalized using 'standard' organic reagents. A nice example of
the application of this chemistry appears in our work with palladium catalyzed reactions of
biphenylene. Using a Pd(0) precursor, we can activate the C-C bond of biphenylene and then
protonate one of the Pd-C bonds using a weak acid of the appropriate pH. Next, one can perform
Heck or Suzuki-type couplings using olefins or boronic acids to give functionalized products (eq
10, 11). Furthermore, other acidic C-H bonds can be added across the activated C-C bond using
pH control. For example, α-keto C-H bonds or α-nitrile C-H bonds can add across biphenylene
to give functionalized biaryls (eq 12, 13).
+R
(1.3 equiv)
Pd(PPh3)4 (5 mol%)
p-cresol (1 equiv)
C6D6, 120 °C, 6 dR
+ R
(10)
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18
C6D6, 120 °C
p-cresol (1 equiv)
Pd(PPh3)4 (5 mol%)
(1 equiv)
+
R
B(OH)2
R
(11)
C6D6, 120 °C
p-cresol (10 mol%)
Pd(PPh3)4 (5 mol%)+ R
OR
O (12)
C6D6, 120 °C, 1 d
p-cresol (10 mol%)
Pd(PPh3)4 (5 mol%)
(1 equiv)
+
Me
CN
CN
Me (13)
We have conducted an extensive investigation of C-H activation in chloroalkanes using the
reactive precursor Tp*Rh(CNR)(carbodiimide). Remarkably, the C-Cl bond does not undergo
oxidative addition. Rather, we find a strong selectivity for exclusive terminal methyl group C-H
bond activation. Thus, 1-chloro alkane gives the 5-chloropentyl hydride as the only product. 3-
chloropentane gives the 3-chloropentyl hydride product. In this case, however, two
diastereomers are formed in a 1:1 ratio, since the metal is chiral and the 3-chloro substituent
renders the ligand chiral. If a chlorine is present in a β-position, then β-chloride elimination
occurs to give an olefin and the metal chloride. Therefore, 2-chloropropane gives only propene
and the hydrido chloride Tp*Rh(CNR)HCl. 2-chloropentane gives a mixture of the C-H
activation product 4-chloropentyl hydride, pentene, and the hydrido chloride. These reactions
are summarized in Scheme 16.
We have also begun to investigate aliphatic nitrile complexes. It appears that there is once
again a selectivity for terminal methyl groups. With acetonitrile, the adduct Tp*LRh(CH2CN)H
is formed, and is found to be stable for days at 60 °C! This is the most stable alkyl hydride in
this series yet, and we may be able to do new functionalization chemistry with this derivative.
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19
Scheme 16. Reactions of Chloroalkanes with [Tp*Rh(CNR)].
Tp'LRh
+ PhNCNR
hν
L = CNCH2CMe3
-20 °C
Cl
ClTp'LRh
HCl
Tp'LRh
Cl
H
Cl
+
Tp'LRhClH
+ Tp'Rh(L)HCl
+ Tp'Rh(L)HCl
ClTp'LRhN
C
R
NPh
4. C-F Bond Cleavage Studies
We have now begun studies with the soluble, more reactive Cp*2ZrH2 and found that this
molecule cleaves a wide variety of aromatic and aliphatic C-F bonds. Systematic studies have
shown that primary, secondary, and tertiary C-F bonds can all be cleaved with progressively
greater difficulty (Scheme 17). In addition, di-fluorosubstituted carbons can be made to react
with even more forcing conditions. Trifluoromethyl groups scarcely react at all even under
extreme conditions.
Scheme 17:
F
F
F
H
H
H
++
C6D12, H2 2 d, 25oC
C6D12, H2
C6D12, H2
4 d, 120oC
1 d, 150oC
primary C-F:
secondary C-F:
tertiary C-F:
ZrH
H
Me5
Me5
ZrF
H
Me5
Me5
Most remarkable, however, even trifluoromethyl C-F bonds can be easily cleaved if they
are adjacent to a double bond. 3,3,3-trifluoropropene is completely defluorinated in 5 min at
room temperature to give the zirconium-n-propyl hydride complex (Scheme 18).
Perfluoropropene undergoes a similar reaction to give the same product. Details of the
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20
Scheme 18:
F2C CF3
F
d12
Cp*2ZrHF +
CF3
F2C CH3
H2
CH3CH2CH3 + Cp*2ZrH2
22°C
+ Cp*2ZrHF
Cp*2ZrH2
22°C
d12
excess
+ Cp*2ZrHF
H2
ZrH
H
Me5
Me5
ZrH
CH2CH2CH3
Me5
Me5
ZrH
CH2CH2CH3
Me5
Me5
mechanism are under further study. Defluorination reactions are also seen with
nonafluorohexene, perfluorocyclobutene, perfluorocyclopentene, perfluorobenzene,
trifluorotoluene, and related substrates. Chlorofluorocarbons (CFCs) react rapidly to give first
fluorocarbons (HFCs), which then are converted to hydrocarbons (HCs) in accord with the above
established reactivities (Scheme 19). Mechanistic investigations into the aliphatic fluorocarbons
has revealed evidence for a radical chain mechanism. Further mechanistic work with the
fluoroolefins is underway suggesting an insertion/β-fluoride elimination pathway.
Scheme 19. x Cp*2ZrH2
CFCl2H
CF2Cl2
CF2ClH
CH3F
CF2H2
CF2H2
CH4
CH4
CH4
25oC, 5 min.
+
25oC, 5 min.
25oC, 5 min.
RT1 day
120oC
120oCx = 3, 4
slow
slow
We have now continued our investigation of the C-F cleavage in perfluoroolefins. These
appear to be a special class of substrate, in that the mechanism of C-F cleavage may be different
than that seen in our earlier studies with Cp*2ZrH2. Reaction with perfluoropropene gives first
the selective formation of E-CHF=CFCF3. Further reaction with zirconium hydride leads to
complete defluorination with no further intermediates being seen (Scheme 20).
-
21
Scheme 20. Reaction of Cp*2ZrH2 with perfluoropropene.
+CF3
F
H
F
6 Cp*2ZrH2
- 5 Cp*2ZrHFCF3
F
F
F+
ZrF
H
ZrH
H-40 C
ZrH
The mechanism for the reaction could involve hydridic attack on the olefin with H/F
metathesis, or an insertion/elimination mechanism. The olefin could approach centrally,
between the two hydrides, or laterally, with the two hydrides remaining cis to each other. In
order to investigate these possibilities, we have initiated a collaboration with a theory group in
Montpellier. Odile Eisenstein and Eric Clot have provided high level calculations investigating
these systems, and the work is providing guidance for the mechanism of reaction. Initial studies
were done on the insertion reaction of ethylene with Cp2ZrH2. We then progressed to
trifluoropropene, which has been shown experimentally to undergo an insertion/β-fluoride
elimination pathway. The calculations done thus far are able to reproduce the selective internal
insertion product, and further work is underway to look at the β-fluoride elimination step
(Scheme 21).
Scheme 21. Calculations on the reaction of Cp*2ZrH2 with trifluoropropene.
externalcoordination
internalcoordination
internalcoordination
0.0
-22.8
-36.8
C2
D2
C1
Cp
Zr
CpH
H
CF3Cp
Zr
CpH
H
CF3
Cp
Zr
CpH
H
CF3
-22.6
Cp2ZrH2 + C2H3CF3
E2
-31.6
Cp
Zr
CpH
H
CF3D1E1
-32.1
Cp
Zr
CpH
CF3Cp
Zr
CpH
CF3
-32.0
Further calculations have been recently completed with Cp2ZrH2 reacting with
perfluoropropene. The results show that interaction of the olefin internally with Cp2ZrH2 leads
to olefin insertion followed by an external β-fluoride elimination. Other pathways were
investigated but all were found to lie at higher energies (Scheme 22).
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22
Scheme 22. Calculations on the reaction of Cp*2ZrH2 with perfluoropropene.
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23
Publications appearing during the current grant cycle acknowledging DOE support, December 1, 2001 - November 30, 2004:
Manuscripts in print:
1. “Perspectives: Synthetic Chemistry. The Key to Successful Organic Synthesis is…,” William D. Jones, Science, 2002, 295, 289-290.
2. “Formation of Tetrafluorobenzyne by β-Fluoride Elimination In Zirconium-Perfluorophenyl Complexes,” Bradley M. Kraft, Rene J. Lachicotte, and William D. Jones, Organometallics, 2002, 21, 727-731.
3. “Cleavage of the Carbon-Carbon Bond in Biphenylene using Transition Metals,” Christophe Perthuisot, Brian L. Edelbach, Deanna L. Zubris, Nira Simhai, Carl N. Iverson, Christian Müller, Tetsuya Satoh, and William D. Jones, J. Mol. Catal. A, Chemical, 2002, 189, 157-168.
4. “Chelating P,N versus P,P Ligands: Differing Reactivity of Donor Stabilized Pt-(η2-PhC———CPh) Complexes Towards Diphenylacetylene,” Christian Müller, Rene J. Lachicotte, and William D. Jones, Organometallics, 2002, 21, 1118-1123.
5. “Thermal and Photolytical Silicon-Carbon Bond Activation in Donor Stabilized Pt(0)-Alkyne Complexes,” Christian Müller, Rene J. Lachicotte, and William D. Jones, Organometallics, 2002, 21, 1190-1196.
6. “Catalytic C-C Bond Activation in Biphenylene and Cyclotrimerization of Alkynes: Reactivity of P,N versus P,P Substituted Nickel Complexes,” Christian Müller, Rene J. Lachicotte, and William D. Jones, Organometallics, 2002, 21, 1975-1981.
7. “Mechanism of Vinylic and Allylic Carbon-Fluorine Bond Activation of Non-Perfluorinated Olefins using Cp*2ZrH2,” Bradley M. Kraft, Rene J. Lachicotte, and William D. Jones, J. Am. Chem. Soc., 2002, 124, 8681-8689.
8. “Cleavage of Carbon-Carbon Bonds in Aromatic Nitriles using Nickel(0),” Juventino J. Garcia, Nicole M. Brunkan, and William D. Jones, J. Am. Chem. Soc., 2002, 124, 9547-9555.
9. “Carbon-Fluorine Bond Activation of Perfluorinated Arenes with Cp*2ZrH2,” Bradley M. Kraft and William D. Jones, J. Organomet. Chem., 2002, 658, 132-140.
10. “η2- Coordination and C–H Activation of Electron-poor Arenes,” Carl N. Iverson, Rene J. Lachicotte, Christian Müller and William D. Jones, Organometallics, 2002, 21, 5320-5333.
11. “Isotope Effects in C-H Bond Activation Reactions by Transition Metals,” William D. Jones, Acc. Chem. Res. 2003, 36, 140-146.
12. “Preparation, Structure, and Dynamics of a Nickel π-Allyl Cyanide Complex,” Nicole M. Brunkan and William D. Jones, J. Organomet. Chem. 2003, 683, 77-82.
13. “Activation of C-F Bonds using Cp*2ZrH2: A Diversity of Mechanisms,” William D. Jones, J. Chem. Soc., Dalton Trans. 2003, 3991-3995.
14. “Synthesis, Characterization, and Reactivity of a Rhenium Complex with a Corannulene Based Ligand,” Robert M. Chin, Benjamin Baird, Michael Jarosh, Shane Rassman, Brian Barry, and William D. Jones, Organometallics 2003, 22, 4829-4832.
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24
15. “Structural Properties and Inversion Mechanisms of [Rh(dippe)(µ−SR)]2 Complexes,” Steven S. Oster and William D. Jones, Inorg. Chim. Acta, 2004, 357, 1836-1846.
16. “Carbon-hydrogen bond activation of chloroalkanes by a rhodium trispyrazolylborate complex,” Andy J. Vetter and William D. Jones, Polyhedron, 2004, 23, 413-417.
17. “Kinetics, Thermodynamics and Effect of BPh3 on Competitive C-C and C-H Bond Activation Reactions in the Interconversion of Allyl Cyanide by [Ni(dippe)],” Nicole M. Brunkan, Donna M. Brestensky, and William D. Jones, J. Am. Chem. Soc., 2004, 126, 3627-3636.
18. “Defluorination of Perfluoropropene using Cp*2ZrH2 and Cp*2ZrHF: A Mechanistic Investigation from a Joint Experimental-Theoretical Perspective,” Eric Clot, Claire Megret, Bradley M. Kraft, Odile Eisenstein, and William D. Jones, J. Am. Chem. Soc., 2004, 126, 5647-5653.
Manuscripts Submitted or in press:
19. “Alkane Complexes as Intermediates in C-H Bond Activation Reactions,” William D. Jones, Andrew J. Vetter, Douglas D. Wick, Todd O. Northcutt, ACS Symposium Series, in press.
20. “Cleavage of Carbon-Carbon bonds in Alkyl Nitriles Using Nickel(0),”.Juventino J. García*, Alma Arévalo, Nicole M. Brunkan, and William D. Jones, Organometallics, submitted.
Manuscripts in Preparation:
21. “Alkane Coordination Selectivity in Hydrocarbon Activation by [Tp'Rh(CNneopentyl)]: The Role of Alkane Complexes,” Andrew J. Vetter, and William D. Jones, J. Am. Chem. Soc. 2004, to be submitted.
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25
Recent Special Recognitions Received by the PI:
ACS Award in Organometallic Chemistry, 2003
Associate Editor, J. Am. Chem. Soc., January 2003-
Chair of Organometallic Subdivision, Inorganic Division of the American Chemical Society, 2001.
Charles F. Houghton Professor of Chemistry, 2000-present
July 2000- 2003: Chairman, Department of Chemistry
Organometallic Gordon Conference, Chairman, Newport, RI, 2000.
Opening speaker at Inorganic Reaction Mechanisms Gordon Conference, Ventura, 2003.
Opening speaker at Organometallic Gordon Conference, Newport, 2003.
Speaker at Inorganic Gordon Conference, Newport, 2003.
Speaker at Isotope Effects Gordon Conference, Ventura, 2004.
Additional Comments: Our renewal budget for 2001-2004 was less than our budget for the prior grant period. On
the basis of our success and productivity, as delineated by the DOE request for the specific information included in this progress report, it is becoming increasingly difficult to accomplish such a large body of work with so little funding. It is imperative that the grant be increased so that 2.5 people can work on the projects. Our current stipend for a good student is $22K/year, and the current budget is supporting only about 80% of these costs. We usually obtain several undergraduate workers for 'free' (i.e., paid by the University), but supply and instrument usage costs still need to be paid.
We apologize for the length of this report.