Alkane dehydrogenation by C-H activation at iridium(III)

Article abstract of DOI:10.1021/om301267c

Stoichiometric alkane dehydrogenation utilizing an Ir^III pincer complex, (^dmPhebox)Ir(OAc)₂(OH₂) (1a), has been described. The reaction between 1a and octane resulted in quantitative formation of (^dmPhebox)Ir(OAc)(H) (3a) and octene. At early reaction times 1-octene is the major product, indicative of terminal C-H activation by 1a. In contrast to prior reports of alkane dehydrogenation with Ir, C-H bond activation occurs at Ir^III and the dehydrogenation is not inhibited by nitrogen, olefin, or water.

Full text of DOI:10.1021/om301267c

Communication
pubs.acs.org/Organometallics
Alkane Dehydrogenation by C−H Activation at Iridium(III)
Kate E. Allen,† D. Michael Heinekey,† Alan S. Goldman,‡ and Karen I. Goldberg*,†
†Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
‡Department of Chemistry and Chemical Biology, Rutgers, 610 Taylor Rd, Piscataway, New Jersey 08855, United States
S
* Supporting Information
ABSTRACT: Stoichiometric alkane dehydrogenation utilizing
an IrIII pincer complex, (dmPhebox)Ir(OAc)2(OH2) (1a), has
been described. The reaction between 1a and octane resulted
in quantitative formation of (dmPhebox)Ir(OAc)(H) (3a) and
octene. At early reaction times 1-octene is the major product,
indicative of terminal C−H activation by 1a. In contrast to
prior reports of alkane dehydrogenation with Ir, C−H bond
activation occurs at IrIII and the dehydrogenation is not inhibited by nitrogen, olefin, or water.
ethods for the conversion of alkanes directly to alkenes,
alcohols, amines, and other valuable products would
Intermolecular benzene C−H bond activation resulted when
solutions of 1a or (iPrPhebox)Ir(OAc)2(OH2) (1b) were heated
at 100 °C for 1.5 h. The corresponding Ir−phenyl products,
(RPhebox)Ir(OAc)(Ph) (2a,b), were formed in quantitative
and 78% yields, respectively (eq 1).7 In comparison, Nishiyama
M
have far-reaching applications in the production of fuels and
chemicals. To this end, there has been intense study of
transition-metal complexes that can selectively activate and
functionalize alkane C−H bonds.1 A particular challenge is to
discover metal species that can remain active in the presence of
the functionalized products and water. The active site needed
for C−H bond activation is often inhibited by coordination of
the oxidized product or water.2 In this communication, we
report on an iridium complex that stoichiometrically activates
and dehydrogenates alkanes to produce alkenes. Not only is the
system unaffected by the olefin product and stable in the
presence of water but the reaction is also accelerated in the
presence of water.
Several years ago, Nishiyama reported that the RhIII complex
(dmPhebox)Rh(OAc)2(H2O) (dmPhebox = 2,6-bis(4,4-dimethy-
loxazolinyl)-3,5-dimethylphenyl) activated C−H bonds of
substituted arenes (C6H5X; X  H, CH3, CF3, OCH3,
COCH3, Cl) to form RhIII−aryl complexes and acetic acid.3
More recently, this same group reported that the IrIII analogue
(dmPhebox)Ir(OAc)2(OH2) (1a)4 activates arene and alkane
C−H bonds to form IrIII−aryl and −alkyl complexes.5 We have
also examined the reactivity of 1a toward arenes and alkanes.
Similarly to the Nishiyama group, we found that benzene
activation yields the Ir−phenyl complex, but in contrast to their
results we observed alkane functionalization rather than simply
alkane activation. In the absence of added potassium carbonate
and at higher temperatures alkane dehydrogenation occurred,
resulting in a novel IrIII−hydride complex and free alkene. The
differences between our system and that of Nishiyama have
been investigated and provide valuable insight into the
mechanism of C−H bond activation and functionalization by
(Phebox)Ir complexes. Alkane functionalization by 1a differs
significantly from the previous reports of alkane dehydrogen-
ation reactions at Ir and Pt centers, wherein metal centers in
low-valent oxidation states are proposed to activate the C−H
bonds.6
and co-workers reported an 84% yield of 2a as determined by
NMR spectroscopy after 1a was heated in neat benzene for 8 h
at 80 °C.5 When benzene-d6 was used as the substrate, we
observed the expected 1H NMR signals for the product 2a-d5 in
solution; however, the signal corresponding to the methyl
group of coordinated acetate (2.08 ppm) was quite broad. A
broad signal at 1.56 ppm, attributed to acetic acid, was also
present. In contrast, when 2a was isolated and then examined
1
by H NMR spectroscopy in benzene-d6 (with no acetic acid
present), the acetate signal at 2.07 ppm was very sharp. The
addition of 1 equiv of acetic acid to the benzene-d6 solution of
isolated 2a at room temperature resulted in broadening of the
acetate signal and the appearance of a second broad signal at
1
1.6 ppm for the added acetic acid. The H NMR spectrum of
2a was otherwise unchanged. At 40 °C the two broad acetate
signals coalesce, resulting in a single resonance at 1.79 ppm.
These observations indicate that the bound acetate ligand in
complex 2a exchanges with free acetic acid on the NMR time
scale. The signals for the methyl groups of the oxazoline rings
Received: December 31, 2012
Published: February 18, 2013
© 2013 American Chemical Society
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Communication
do not coalesce, indicating that the phenyl group remains at a
fixed coordination site at the IrIII center during the acetate/
acetic acid exchange.
Nishiyama and co-workers reported that thermolysis of 1a in
neat heptane and octane solutions at 160 °C, over 70 h, in the
presence of K2CO3 results in formation of (dmPhebox)Ir(OAc)-
(n-alkyl) in 80% and 78% isolated yields, respectively.5 We
found that when 1a was heated in neat octane at 150 °C (no
added K2CO3), no reaction was observed even after heating for
6 days. When the temperature was increased to 180 °C, a small
amount of a new Ir−hydride species was observed after 5 days.
Finally, when the reaction mixture was heated at 200 °C for 72
h, the new complex (dmPhebox)Ir(OAc)(H) (3a) was obtained
in 96% yield (eq 2). Removal of the volatiles from the reaction
Figure 1. ORTEP diagram of (dmPhebox)Ir(OAc)(H) (3a) (thermal
ellipsoids at 50% probability, H atoms omitted for clarity). Select bond
distances (Å) and angles (deg): Ir(1)−C(1) = 1.923(2), Ir(1)−N(1)
= 2.043(2), Ir(1)−N(2) = 2.055(2), Ir(1)−O(3) = 2.251(2), Ir(1)−
O(4) = 2.243(2), Ir(1)−H(1) = 1.50(3); N(1)−Ir(1)−N(2) =
158.30(8), C(1)−Ir(1)−O(4) = 167.92(9), C(1)−Ir(1)−O(3) =
109.66(8), C(1)−Ir(1)−H(1) = 85.2(12).
mixture of octenes, composed of trans-4-octene (20%), trans-3-
octene (28%), trans-2-octene (32%), cis-3-octene (8%), and cis-
2-octene (11%), was found (Scheme 1, Table 1). The product
that would be obtained from terminal C−H bond activation, 1-
octene, was not observed. The absence of this isomer was
somewhat surprising, given that Nishiyama and co-workers
reported that only the primary n-alkyl product was observed in
their reactions. In order to determine if 1-octene was a kinetic
product that was subsequently consumed under the conditions
for dehydrogenation, the distribution of octene was monitored
as a function of time (Table 1).
mixture afforded 3a as a light orange solid, which was
1
characterized by H and 13C NMR spectroscopy, single-crystal
X-ray diffraction, and elemental analysis. In the 1H NMR
spectrum (benzene-d6), the hydride resonance was clearly
observed as a singlet at −33.8 ppm. Hydride signals for five-
coordinate IrIII hydrides are often found in this high upfield
region,8 while six-coordinate hydrides typically appear farther
downfield.8a,9 The high upfield shift for the hydride in 3a may
indicate that the acetate is κ1 in solution or that the structure is
distorted significantly from an octahedral geometry. Two
doublets (3.79 and 3.84 ppm, JH−H = 8.3 Hz) are observed
for the inequivalent methylene protons of the dmPhebox
oxazoline rings and two singlets for the inequivalent methyl
groups (1.29 and 1.33 ppm). The acetate resonance appears as
a singlet at 2.06 ppm. Addition of 1 equiv of acetic acid to
isolated 3a resulted in loss of the acetate signal and the
When the reaction of 1a with octane was stopped after 3 h, a
20% yield of 1-octene (by far the major product considering ca.
30% conversion) was determined by GC analysis (Table 1).
While at early reaction times (3−6 h) 1-octene was present as
the major isomer, its concentration slowly decreased over time,
until at 72 h 1-octene was no longer observed. The decrease in
the concentration of 1-octene tracked with the increase in the
concentrations of the four internal isomers (Table 1). The
absence of the kinetic product of octane dehydrogenation at 72
h could be explained by isomerization of 1-octene by 3a.6j An
independent experiment confirmed that complex 3a was an
effective catalyst for the isomerization of 1-octene: heating a 0.1
M solution of 1-octene and 5.5 mol % 3a in octane at 200 °C
for 24 h resulted in isomerization of 1-octene to the same
distribution of octene observed after 72 h of heating 1a in
octane (Table 1). Together these experiments demonstrate that
octane is dehydrogenated regioselectively, to give 1-octene,
followed by 1-octene isomerization by 3a. Generation of 1-
octene is presumed to occur via formation of the Ir−n-alkyl
derivative (dmPhebox)Ir(OAc)(n-octyl) (4a), in accord with
Nishiyama’s observations.5 In support of this hypothesis, when
1
appearance of one broad resonance in the H NMR spectrum
at 1.75 ppm corresponding to an averaged signal for the acetate
ligand and acetic acid. These observations suggest an
interaction of acetic acid with the acetate ligand similar to
that observed with 2a.
The X-ray data obtained for complex 3a reveal a distorted-
octahedral structure in the solid state. The hydride ligand is
located approximately trans to one oxygen of the κ2-acetate
ligand; the H(1)−Ir(1)−O(3) angle is 164.6(2)° rather than
180° (Figure 1). The C(1)−Ir(1)−O(4) angle is comparable at
167.92(9)°, and the Ir−O bond distances are quite similar
(2.251(2) and 2.243(2) Å), suggesting that the trans effects of
the hydride and aryl carbon of the dmPhebox ligand are
comparable. The Ir−O bond distances are also comparable to
the Ir−O distances observed in 2a.5 The Ir−H bond length of
3a was determined to be 1.50(3) Å, which is within the typical
range for pincer IrIII−H bond lengths (1.5−1.6 Å)10 and is in
close agreement with the Ir−H bond distance reported for
(tBuPCP)Ir(H)(κ2O-ONCH2) (1.52(2) Å).10c
1
an octane solution of 4a was heated at 200 °C for 24 h, H
NMR spectroscopic analysis revealed conversion to the hydride
complex 3a.
An issue with the well-known (RPCP)Ir (R = alkyl)
dehydrogenation systems which activate C−H bonds by
oxidative addition to IrI is that the reactions are inhibited by
nitrogen, water, and/or the alkene product.6f,11 Nitrogen and
alkenes coordinate to the open site at the IrI center, blocking
the site needed for alkane C−H activation. In the case of water,
oxidative addition to the low-valent IrI center is observed.11
The observation of hydride species 3a suggested that β-
hydride elimination had occurred from an Ir−octyl complex.
Indeed, we were able to observe the expected organic product
of the reaction, octene, via GC analysis. After 72 h at 200 °C, a
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Scheme 1
Table 1. Octene Distribution as a Function of Time in the
Reaction of 1a with Octane
molecules nitrogen, water, and alkene that have been found to
adversely affect C−H activation at IrI centers.
a
It is significant that, during our study of alkane activation by
complex 1a, we did not observe the previously reported Ir−
octyl species 4a. Nishiyama and co-workers reported that they
isolated 4a from the reaction of 1a with octane in the presence
of 1 equiv of K2CO3 and suggested that the addition of base
stabilized 4a by reacting with the acetic acid generated from C−
H cleavage.12 Indeed, we observed that if 4a is treated with 1
equiv of acetic acid, octane and 1a are generated at 160 °C. In
contrast, the Ir−Ph complex 2a is generated in the presence of
1 equiv of acetic acid. This difference is consistent with a
thermodynamic advantage of aryl activation over alkane
activation.13 H/D exchange between benzene and various
deuterated solvents (e.g., CD3OD, TFA-d, and D2O) was also
not observed when 1a was heated in these mixtures for
prolonged periods at 100 °C; complex 2a was recovered from
these reactions and the benzene showed no deuterium
incorporation by GC/MS.14
If the role of the K2CO3 is to remove acetic acid product in
the octane experiments to provide the driving force for the
formation of 4a, then reversible C−H activation would be
expected in the absence of base. Indeed, when a solution of 1a
(1 mol %) in octane (1.4 mmol) with acetic acid-d4 (1.8 mmol)
was heated at 160 °C for 48 h, a small amount of deuterium
incorporation into the terminal position of octane was observed
(1.5(1)%; 1.5 TON (eq 3)).
yield (%)
b
octene
3 h
6 h
24 h
6(1)
48 h
1(1)
72 h
120 h
1-octene
20(2)
0
20(3)
0
0
0
trans-4-octene
trans-3-octene
trans-2-octene
cis-3-octene
cis-2-octene
total octene
5(1)
13(2)
25(1)
4(1)
16(1)
23(1)
26(1)
6(1)
20(1)
28(1)
32(1)
8(1)
20(1)
29(2)
29(2)
8(1)
0.6(1)
6(1)
3(1)
15(2)
1(1)
6(1)
45(3)
0.5(1)
2(1)
10(1)
63(4)
9(1)
11(1)
98(1)
11(1)
94(5)
29(3)
80(2)
a
All reactions were run under a nitrogen atmosphere at 200 °C.
Yields were determined by GC analysis using a mesitylene internal
b
standard.
Roddick utilized fluorinated alkyl substituents on the PCP
ligand to produce an Ir alkane dehydrogenation system that was
not affected by nitrogen and water and was more resistant to
alkene inhibition than the nonfluorinated analogue.6j Since 1a
activates C−H bonds by a different mechanism, involving an
IrIII center, we were interested in assessing the effect of
nitrogen, water, and alkenes on the octane dehydrogenation
reaction. When solutions of octane and 1a were degassed
thoroughly prior to heating, the yield of octene was determined
to be 30% (Table 2). In comparison, heating 1a in octane
Table 2. Octene Yield Obtained Using 1a and Octane in the
a
Presence of Various Additives
additive
octene yield (%)
N2
29(3)
30(3)
33(1)
none
1-hexene (10 equiv)
H2O (120 equiv)
The amount of H/D exchange that is observed is consistent
with the slow octane activation reported by Nishiyama, where
1a was heated for 70 h at 160 °C in the presence of K2CO3 in
order to produce good yields of 4a.5 We observed a slightly
faster reaction with complete consumption of 1a within 48 h
under these conditions. Interestingly, we found that with
weaker bases, KHCO3 and K2SO4, incomplete conversion
resulted with lower yields of the Ir−octyl product 4a within the
same 48 h reaction time at 160 °C. The results of a reaction
containing 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctade-
cane) and K2CO3 were similar to that without the crown
ether, indicating that the cation does not play a significant role.
The role of the base in Nishiyama’s system is to trap the
acetic acid released upon C−H activation to drive the
equilibrium toward the alkyl product, as shown in Scheme 2.
An equilibrium favoring 1a without base present was confirmed
by the addition of acetic acid to a sample of 4a in octane; as
noted above, regenerated 1a at 160 °C. The observation of H/
D scrambling with acetic acid-d4 at 160 °C supports the
proposal that activation of an octane C−H bond by 1a occurs
44(2)
a
b
All reactions were run in octane at 200 °C for 3 h. Yields were
determined by GC analysis using mesitylene internal standard.
under nitrogen resulted in a 29% yield of octene after 3 h. The
nearly identical yields of these reactions indicate that nitrogen
does not inhibit octane dehydrogenation by 1a. The addition of
1-hexene (73 mM, 10 equiv relative to 1a) to the reaction
mixture prior to heating resulted in a 33% octene yield. Thus,
olefin also does not inhibit the dehydrogenation reaction and
the long reaction time required to reach complete conversion is
not due to product inhibition. When alkane dehydrogenation
was performed with 1a using a water-saturated octane solution
(120 equiv of H2O relative to 1a) an increase in the yield of
octene to 44% after 3 h was observed. Determination of the
role of water in this system is currently under investigation.
Overall, it is notable that C−H activation and functionalization
by the IrIII center of 1a is not inhibited by the common small
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(e) Activation and Functionalization of C-H Bonds; Goldberg, K. I.,
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Scheme 2
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Am. Chem. Soc. 2006, 128, 2005.
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reversibly at this temperature. In the absence of base, higher
temperature can drive the C−H activation by promoting β-
hydride elimination and formation of octene and 3a (Scheme
2). Notably, it was demonstrated in an independent experiment
that heating 4a in octane at 200 °C results in the formation of
3a and octene.
In summary, the IrIII complex (dmPhebox)Ir(OAc)2(OH2)
(1a) was shown to dehydrogenate octane to yield octenes and
the IrIII−H complex 3a in quantitative yield. This reaction
occurs at a higher temperature (200 °C) than the previously
reported formation of the alkyl complex 4a, which is observed
in the presence of the base K2CO3 at 160 °C. The added base
acts to remove the acetic acid, allowing observation of the alkyl
complex 4a. Higher temperatures promote β-hydride elimi-
nation from 4a and generation of the IrIII−hydride 3a. While
catalytic alkane dehydrogenation was not observed, it is notable
that this IrIII C−H activation system was not inhibited by
nitrogen, water, or alkene, all significant issues for the (RPCP)Ir
systems that activate C−H bonds by oxidative addition to IrI.
Current efforts are focused on promoting catalytic alkane
dehydrogenation with the (dmPhebox)Ir system.
(7) The product of benzene activation, (iPrPhebox)Ir(OAc)(Ph)
(2b), has been characterized by 1H and 13C NMR spectroscopy, X-ray
crystallography, and elemental analysis. See the Supporting Informa-
tion for details.
(8) (a) Moulton, C. J.; Shaw, B. L. Dalton Trans. 1976, 1020.
(b) Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jespersen, K.;
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P. S.; Swartz, R.; Goldberg, K. I.; Brookhart, M. J. Am. Chem. Soc.
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(9) (a) Kanzelberger, M.; Zhang, X.; Emge, T. J.; Goldman, A. S.;
Zhao, J.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125,
13644. (b) Frech, C. M.; Shimon, L. J. W.; Milstein, D. Organometallics
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(10) (a) Zhang, X.; Kanzelberger, M.; Emge, T. J.; Goldman, A. S. J.
Am. Chem. Soc. 2004, 126, 13192. (b) Zhang, X.; Emge, T. J.; Ghosh,
R.; Goldman, A. S. J. Am. Chem. Soc. 2005, 127, 8250. (c) Zhang, X.;
Emge, T. J.; Ghosh, R.; Krogh-Jespersen, K.; Goldman, A. S.
Organometallics 2006, 25, 1303. (d) Segawa, Y.; Yamashita, M.;
Nozaki, K. J. Am. Chem. Soc. 2009, 131, 9201. (e) Huang, Z.; Zhou, J.;
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, and a CIF file giving experimental details for arene
C−H activation and octane dehydrogenation using 1a and
characterization data for 3a, including X-ray crystallographic
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: goldberg@chem.washington.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(11) Morales-Morales, D.; Lee, D. W.; Wang, Z.; Jensen, C. M.
Organometallics 2001, 20, 1144.
(12) When we synthesized 4a using the published procedure, a
mixture of 4a and 3a was always obtained (3:1 ratio determined by 1H
NMR spectroscopy in C6D6 against hexamethyldisiloxane internal
standard).
This work was supported by the NSF through the CCI Center
for Enabling New Technologies through Catalysis (CENTC;
CHE-1205189 and CHE-0650456). We thank Dr. Werner
Kaminsky for solving the X-ray structure of 3a. Prof. Tom R.
Cundari, Mr. Dale Pahls, Prof. Melanie S. Sanford, Prof. Elon A.
Ison, and Prof. William D. Jones are thanked for helpful
discussions.
(13) Hartwig, J. Organotransition Metal Chemistry; University Science
Books: Sausalito, CA, 2011; pp 275−276.
(14) The aromatic signals of the Ir−phenyl protons integrated to the
expected values against the dmPhebox ligand by 1H NMR spectroscopy.
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