Regeneration of an Iridium(III) complex active for alkane dehydrogenation using molecular oxygen

Article abstract of DOI:10.1021/om401241e

(^dmPhebox)Ir(OAc)₂(OH₂) (1a) has previously been found to promote stoichiometric alkane dehydrogenation, affording alkene, acetic acid, and the corresponding Ir hydride complex ( ^dmPhebox)Ir(OAc)(H) (2a) in high

Full text of DOI:10.1021/om401241e

Communication
pubs.acs.org/Organometallics
Regeneration of an Iridium(III) Complex Active for Alkane
Dehydrogenation Using Molecular Oxygen
Kate E. Allen,† D. Michael Heinekey,† Alan S. Goldman,‡ and Karen I. Goldberg*,†
†Department of Chemistry, University of Washington, Box 351700, Seattle Washington 98195-700
‡Department of Chemistry and Chemical Biology, Rutgers, 610 Taylor Road, Piscataway, New Jersey 08855
S
* Supporting Information
ABSTRACT: (dmPhebox)Ir(OAc)2(OH2) (1a) has previously been found to promote
stoichiometric alkane dehydrogenation, affording alkene, acetic acid, and the
corresponding Ir hydride complex (dmPhebox)Ir(OAc)(H) (2a) in high yield. In
this study, we describe the use of oxygen to quantitatively regenerate 1a from 2a and
HOAc. Distinct reaction intermediates are observed during the conversion of 2a to 1a,
depending upon the presence or absence of 1 equiv of acetic acid, highlighting
potential mechanistic implications regarding alkane dehydrogenation catalysis and the
use of oxygen as an oxidant in such systems.
agging behind the recent remarkable advances in C−H
L
bond functionalization has been the effective use of
molecular oxygen as an oxidant in such transformations. For
the development of commercially viable large-scale reactions,
including selective alkane oxidation, the use of dioxygen or air
holds tremendous potential on the basis of thermodynamic,
environmental, and economic considerations. Recently, very
successful catalysts have been reported for alkane dehydrogen-
ation to alkenes.1 The efficient production of alkenes is highly
attractive, as these molecules are important building blocks in
the chemical industry. However, alkane dehydrogenation
reactions are typically performed using a sacrificial olefin as
an oxidant (A, eq 1). The use of oxygen as the oxidant for
alkane dehydrogenation could have dramatic advantages in
terms of cost and environmental impact.
presence of acetic acid to regenerate the IrIII bis(acetate)
complex 1a (eq 3). Although we have not as yet been able to
Homogeneous alkane dehydrogenation catalysts have
primarily been based on low-valent Ir, Pt, and Rh species,2
with pincer-ligated iridium catalysts as the most successful
reported to date.3 The pincer Ir catalysts have been shown to
operate with various olefins as hydrogen acceptors at mode-
rate temperatures or in open systems (acceptorless) at high
temperatures.4 However, the proposed active catalyst in these
systems, a (pincer)IrI fragment, reacts with dinitrogen, water,
oxygen, or olefin, all inhibiting dehydrogenation.3a,b,5 Recently,
we reported stoichiometric n-octane dehydrogenation by the
IrIII complex (dmPhebox)Ir(OAc)2OH2 (1a; dmPhebox = 2,6-
bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) to generate
(dmPhebox)Ir(OAc)H (2a), HOAc, and octene (eq 2).6
This dehydrogenation reaction was not inhibited by the
presence of nitrogen, water, or alkene, an important advantage
in comparison with (pincer)IrI systems in terms of developing
practical systems for catalytic dehydrogenation.
run this system as a continuous catalytic cycle,7 the sum of eq 3
and the previously reported eq 2 represents a catalytic transfer
of hydrogen from alkane to oxygen to give alkene and water.
The chemistry reported herein thus offers a new paradigm
for alkane dehydrogenation catalysis wherein an IrIII species
activates and dehydrogenates alkane, and oxygen serves as the
hydrogen acceptor.
Exposure of a bright orange benzene-d6 solution of 2a and
1 equiv of acetic acid to ambient air at room temperature
caused the solution to turn yellow within several minutes.8
1
Monitoring the reaction by H NMR spectroscopy revealed
quantitative formation of 1a over the course of 2 h (eq 3). The
identity of 1a was confirmed by spiking the product solution
In this contribution, we report that the IrIII hydride product
of alkane dehydrogenation, 2a, reacts with oxygen in the
Received: December 31, 2013
Published: March 11, 2014
© 2014 American Chemical Society
1337
dx.doi.org/10.1021/om401241e | Organometallics 2014, 33, 1337−1340

Organometallics
Communication
with an authentic sample of 1a.9 When a benzene-d6 solution of
2a and 1 equiv of acetic acid was exposed to 3 atm of oxygen
(instead of air), 1a formed after 2 h in slightly lower yield
(83%). When the same reaction with 3 atm of oxygen was
performed with added water (12 equiv), the yield of 1a
increased to 96%; this result is consistent with the presence of
coordinated water in 1a.
The pivalate derivative 2c was similarly generated in quantitative
yield from the reaction of 2a and 1 equiv of pivalic acid (eq 4).
The tert-butyl substituent of the pivalate ligand was observed at
1.46 ppm and integrated to nine protons relative to the two
singlets, each six protons, assigned to the two unique oxazoline
methyl substituents of the dmPhebox ligand. The hydride signal in
1
the H NMR spectrum of 2c was observed at −33.8 ppm,
consistent with the data obtained for 2a,b.
During the reaction of 2a and acetic acid with oxygen (or air)
as described above, an intermediate species (3a) was observed
in the 1H NMR spectrum (Figure 1). After only 10 min, signals
characteristic of 3a are quite clear in the alkyl and oxazolinyl
regions of the spectrum. Two singlets indicative of inequivalent
methyl groups on the oxazoline rings of 3a were observed at
The addition of 1 equiv of benzoic acid to a benzene-d6
solution of 2b resulted in broadening of the benzoate aromatic
signals consistent with exchange between benzoic acid and the
coordinated benzoate. This exchange is similar to that observed
for 2a with acetic acid.8 When the benzene-d6 solution of 2b
and benzoic acid was exposed to air at room temperature,
within 10 min an intermediate species (3b) and the product 1b
1
1.24 and 1.48 ppm. In the oxazolinyl region of the H NMR
spectrum, two doublets assigned to the methylene protons
were observed for 3a at 3.95 and 4.20 ppm consistent with Cs
symmetry for the intermediate. During the course of the
reaction these doublets shifted downfield to 4.09 and 4.21 ppm
(Figure 1, 1 h 30 min). The shift in the signals may represent
an additional intermediate or merely a change in chemical shift
of the signals of 3a caused by changing reaction conditions.
Attempts to grow X-ray-quality crystals of the intermediate(s)
at various temperatures, as low as −80 °C, only afforded
crystals of the final product complex 1a.
To investigate the generality of the oxygen reaction,
analogous IrIII hydride complexes bearing different carboxylate
groups, (dmPhebox)Ir(X)H (X = OBz (2b), OPiv (2c)), were
prepared and their reactivity with oxygen was examined.
Addition of 1 equiv of benzoic acid to 2a in benzene-d6 resulted
in displacement of the acetate ligand with benzoate, generating
2b in quantitative yield by 1H NMR spectroscopy (eq 4). In the
1
were both observed in the H NMR spectrum. Similar to the
data observed for 3a, the spectral data for 3b were consistent
with a Cs structure. Complete disappearance of 2b and inter-
mediate 3b was observed after 2.5 h at room temperature,
generating complex 1b in 80% yield (by integration against
an internal standard). The identity of 1b was confirmed by
spiking the final solution with an authentic sample of 1b.10 In
addition, the structure of complex 1b was determined by X-ray
crystallography (Figure 2). The C2v symmetry noted in solution
is also evident in the solid state, as the two benzoate ligands are
Figure 2. ORTEP diagram of (dmPhebox)Ir(OBz)2(OH2) (1b)
(thermal ellipsoids at 50% probability, H atoms omitted for clarity).
Selected bond distances (Å) and angles (deg): Ir(1)−C(1) =
1.935(3), Ir(1)−N(1) = 2.0597(17), Ir(1)−O(2) = 2.0418(15),
Ir(1)−O(5) = 2.218(3); N(1)−Ir(1)−N(1) = 158.52(9), C(1)−
Ir(1)−O(2) = 88.85(4), C(1)−Ir(1)−O(5) = 170.67(7), C(1)−
Ir(1)−N(1) = 79.26(5).
1H NMR spectrum of 2b, the hydride resonance was observed
at −33.7 ppm and three signals indicative of the benzoate
protons were observed at 7.18 (1H), 7.22 (2H), and 8.55 ppm
(2H) consistent with benzoate coordination at the IrIII center.
Figure 1. 1H NMR spectral data in the aromatic and oxazolinyl regions acquired over the course of the reaction of 2a with acetic acid and air at room
temperature in benzene-d6.
1338
dx.doi.org/10.1021/om401241e | Organometallics 2014, 33, 1337−1340

Organometallics
Communication
related by a mirror plane. The water molecule in 1b is located
trans to the aryl backbone of the dmPhebox ligand with an Ir−O
bond length of 2.218(3) Å. This is very close to the reported
Ir−O bond length of 2.206(3) Å for the Ir−OH2 moiety of
1a.11 The Ir(1)−O(2) bond length of 2.0418(15) Å is also
similar to the Ir−OAc bond lengths reported for 1a (2.037(3)
and 2.058(3) Å).
Scheme 1
Upon the addition of 1 equiv of pivalic acid to a benzene-d6
solution of 2c, exchange between the pivalate ligand and pivalic
acid, similar to that observed for 2a,b and their respective acids,
was indicated by the presence of a broad signal at 1.25 ppm
for the tert-butyl groups.8 When the solution of 2c and pivalic
acid was exposed to air, the reaction proceeded similarly to the
reactions of the carboxylate complexes 2a,b described above;
formation of an intermediate species 3c and the product 1c was
immediate reaction. When the reaction mixture was left at room
temperature for 72 h, complex 1a was generated in 83% yield
based on the amount of 4a. In the absence of added acetic acid,
decomposition of 4a led to the generation of 1a in 23% yield after
72 h. The lack of a rapid reaction when acetic acid was added to 4a
is not consistent with this species being an intermediate in the
reaction of 2a and acetic acid with oxygen. Moreover, we can
conclude that the differences in the 1H NMR spectra of 3a and 4a
are not simply due to differences in the reaction media; these
species are indeed different complexes.
1
observed by H NMR spectroscopy within 10 min at room
temperature (Figure S3). The presence of 3c was indicated by
the appearance of two new doublets, integrating in a 1:1 ratio at
3.94 and 4.18 ppm in the oxazolinyl region. Additionally, two
singlets at 1.49 and 1.20 ppm were assigned as the oxazoline
methyl groups, each integrating to six protons. After 3 h,
1
complex 1c had formed in quantitative yield by H NMR
In terms of proposing compositions of 3a and 4a, it seemed
plausible that at least one of these complexes could be an
Ir−OOH species formed by oxygen insertion into the Ir−H bond:
e.g., (dmPhebox)Ir(OAc)(OOH). Notably there is literature
precedent for oxygen insertion into metal−hydride bonds and for
the formation of Ir−OOH complexes.12−15 Nocera and co-workers
have reported the structural characterization of Ir2II,II(tfepma)2-
(CNtBu)2Cl3(OOH) (tfepma = MeN[P(OCH2CF3)2]2), which
was generated from the reaction of Ir2II,II(tfepma)2(CNtBu)2-
Cl3(H) with oxygen in a THF solution.13 Insertion of oxygen
into an IrIII−H bond to form a IrIII−OOH intermediate was
proposed in Rauchfuss’ catalytic hydrogenation of oxygen
by Cp*IrH(rac-TsDPEN) (TsDPEN = H2NCHPhCHPhN-
(SO2C6H4CH3)−).14 Ison and co-workers proposed a Cp*Ir−
OOH intermediate in the catalytic oxidation of alcohols in
toluene using [Cp*IrCl2]2 as a precatalyst.15 If an OOH ligand
was present in intermediate species 4a, protonation of 4a should
yield hydrogen peroxide. Thus, an assay was performed to
detect hydrogen peroxide released upon protonation of 4a.16
Complex 4a was isolated as a dark green solid by removal of
benzene-d6 in vacuo. Isolated 4a was dissolved in acetonitrile and
subjected to an excess of H2SO4. The formation of hydrogen
peroxide was clearly evident when this mixture was added to an
aqueous solution of KMnO4. The reaction between H2O2 and
KMnO4 was monitored by UV−visible spectroscopy.16 Notably,
the Ir dicarboxylate product 1a was found to catalyze the
disproportionation of hydrogen peroxide; the addition of 50 equiv
of a 30% hydrogen peroxide solution to a benzene solution of 1a
resulted in immediate bubbling. After 20 h, gas evolution had
ceased and complex 1a was still present in solution without any
decomposition to metal black. This disproportionation would
complicate using the assay described above for 4a to determine
whether intermediate 3a contained a hydroperoxide ligand. If
hydrogen peroxide was produced upon protonation of 3a, 1a
(which appears even at early reaction times) would promote its
disproportionation to oxygen and water.
spectroscopy. The identity of 1c was confirmed by independent
synthesis of the complex.10
In an effort to identify the intermediate 3a, the reaction of 2a
with oxygen in the absence of acetic acid was performed.
Surprisingly, exposure of a benzene-d6 solution of 2a to 3 atm
of oxygen (or 1 atm of air) at room temperature did not result
1
in an immediate reaction by H NMR spectroscopy. Over the
course of 3.5 h at room temperature, the solution slowly
darkened from bright orange to jungle green. At this time
complete consumption of 2a was observed and a new complex
(4a), with signals different from those assigned to 3a, was
1
1
present in the H NMR spectrum (eq 5). The H NMR
spectral data for 4a were indicative of the formation of a species
with Cs symmetry akin to the starting material 2a; two doublets
were present in the oxazolinyl region at 3.84 and 3.91 ppm,
each integrating to two protons. Singlets at 1.25 (6H), 1.46
(6H), and 2.66 ppm (6H) were assigned to two different types
of methyl substituents on the oxazoline rings and the methyl
groups in the dmPhebox backbone, respectively. A signal
attributable to a single coordinated acetate ligand was observed
at 1.89 ppm (3H). Integration of the dmPhebox aryl proton at
6.42 ppm against an internal standard showed that 4a was
formed in 54% yield. No other products were observed in the
spectrum, and no precipitate was present in the NMR tube, which
suggests that additional paramagnetic Ir species were formed.
To investigate whether 4a was a potential intermediate in the
reaction of 2a and acetic acid with oxygen, acetic acid was added
to a benzene-d6 solution of 4a (Scheme 1). Upon the addition of
1 equiv of acetic acid, the mixture did not change color and there
The formation of different intermediate species in the
reactions of 2a with oxygen in the presence or absence of acetic
acid implies different mechanisms for the two reactions. In the
absence of acetic acid, the reaction with oxygen could form a
hydroperoxide intermediate (4a), as shown in Scheme 1. This
proposal is consistent with detection of hydrogen peroxide
1
was no evidence of reaction by H NMR spectroscopy. The
addition of a second equiv of acetic acid also did not result in an
1339
dx.doi.org/10.1021/om401241e | Organometallics 2014, 33, 1337−1340

Organometallics
Communication
(3) (a) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C.
M. Chem. Commun. 1996, 2083. (b) Liu, F.; Pak, E. B.; Singh, B.;
Jensen, C. M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086.
(c) Kuklin, S. A.; Sheloumov, A. M.; Dolgushin, F. M.; Ezernitskaya,
M. G.; Peregudov, A. S.; Petrovskii, P. V.; Koridze, A. A.
Organometallics 2006, 25, 5466. (d) Huang, Z.; Brookhart, M.;
Goldman, A. S.; Kundu, S.; Ray, A.; Scott, S. L.; Vicente, B. C. Adv.
Synth. Catal. 2009, 351, 188. (e) Adams, J. J.; Arulsamy, N.; Roddick,
D. M. Organometallics 2012, 31, 1439. (f) Shi, Y.; Suguri, T.; Dohi, C.;
Yamada, H.; Kojima, S.; Yamamoto, Y. Chem. Eur. J. 2013, 19, 10672.
(4) Selected examples: (a) Fujii, T.; Saito, Y. J. Chem. Soc.; Chem.
Commun. 1990, 757. (b) Aoki, T.; Crabtree, R. H. Organometallics
1993, 12, 294. (c) Xu, W.; Rosini, G. P.; Gupta, M.; Jensen, C. M.;
Kaska, W. C.; Krogh-Jespersen, K.; Goldman, A. S. Chem. Commun.
1997, 2273. (d) Punji, B.; Emge, T. J.; Goldman, A. S. Organometallics
2010, 29, 2702.
upon the addition of sulfuric acid to 4a. However, we also have
evidence that 4a is not formed when the reaction is carried out
in the presence of acetic acid. The reaction of 4a and acetic acid
does lead to 1a, but the reaction is too slow for 4a to be
considered a kinetically competent intermediate in the
formation of 1a from the reaction of 2a with acetic acid and
oxygen. It appears that the interaction of 2a with acetic acid
allows a faster reaction with oxygen to generate 3a. The
intermediate 3a observed in the reaction of 2a and acetic acid
with oxygen may also be a hydroperoxide species, but it is not
4a. Several possibilities for 3a−c include protonated hydroperoxo,
hydroxo or peracetate complexes.17
We have previously reported that (dmPhebox)Ir(OAc)2(OH2)
(1a) reacts with octane to give octene, (dmPhebox)Ir(OAc)H
(2a), acetic acid, and water. In this work we have demonstrated
that 2a reacts with oxygen (in the presence of acetic acid
and water) to regenerate 1a in high yield. The derivatives
(dmPhebox)Ir(X)2(H) (2b,c; X = OBz, OPiv) show similar
behavior. These reactions occur at room temperature and
proceed through a (dmPhebox)Ir intermediate, possibly an
Ir−OOH species. Generation of 1a−c from 2a−c with oxygen
represents the key regeneration step needed to achieve aerobic
dehydrogenation of alkanes. In contrast, most of the well-known
homogeneous alkane dehydrogenation catalysts are incompat-
ible with oxygen. This work involving high-oxidation-state IrIII
complexes for alkane activation thus suggests a novel approach
toward developing alkane functionalization systems that can utilize
oxygen as a hydrogen acceptor. Current studies are focused on
determining the identities of the intermediate species and the
mechanism of this reaction, as well as applying these findings
toward the development of a catalytic system.
(5) (a) Lee, D. W.; Kaska, W. C.; Jensen, C. M. Organometallics 1998,
17, 1. (b) Morales-Morales, D.; Lee, D. W.; Wang, Z.; Jensen, C. M.
Organometallics 2001, 20, 1144. (c) Ghosh, R.; Kanzelberger, M.;
Emge, T. J.; Hall, G. S.; Goldman, A. S. Organometallics 2006, 25,
5668. (d) Williams, D. B.; Kaminsky, W.; Mayer, J. M.; Goldberg, K. I.
Chem. Commun. 2008, 4195.
(6) Allen, K. E.; Heinekey, D. M.; Goldman, A. S.; Goldberg, K. I.
Organometallics 2013, 32, 1579.
(7) Catalytic n-octane dehydrogenation using 2a was not observed at
200 °C with air (1 atm). While some oxidation of n-octane was
observed, similar results in the absence of Ir were noted. An oxygen
atmosphere at high temperature may be incompatible with these Ir
complexes and/or intermediates, but further studies are needed.
(8) In the absence of oxygen, only degenerate exchange of the acetate
ligand and the hydride moiety of 2a with the added acetic acid was
observed. Exchange of the acetate was evidenced by a single broad
resonance at 1.87 ppm in the 1H NMR spectrum that corresponded to
the methyl groups of both acetate and acetic acid. In addition, a slower
H/D exchange between the hydride ligand and acetic acid-d4 (3 equiv)
occurs under these conditions; 50% deuterium incorporation into the
hydride position was observed after 2 h at room temperature.
(9) Ito, J.; Shiomi, T.; Nishiyama, H. Adv. Synth. Catal. 2006, 348,
1235.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental details including X-ray crystallographic data
for 1b. This material is available free of charge via the Internet
at http://pubs.acs.org.
(10) See Supporting Information for synthesis of 1b and 1c.
(11) Ito, J.; Kaneda, T.; Nishiyama, H. Organometallics 2012, 31,
4442.
(12) (a) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1999, 121,
11900. (b) Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.;
Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 2508. (c) Konnick, M. M.;
Gandhi, B. A.; Guzei, I. A.; Stahl, S. S. Angew. Chem. Int. Ed. 2006, 45,
2904. (d) Look, J. L.; Wick, D. D.; Mayer, J. M.; Goldberg, K. I. Inorg.
Chem. 2009, 48, 1356. (e) Szajha-Fuller, E.; Bakac, A. Inorg. Chem.
2010, 49, 781. (f) Boisvert, L.; Goldberg, K. I. Acc. Chem. Res. 2012,
45, 899.
(13) (a) Teets, T. S.; Nocera, D. G. J. Am. Chem. Soc. 2011, 133,
17796. (b) Keith, J. M.; Teets, T. S; Nocera, D. G. Inorg. Chem. 2012,
51, 9499.
(14) (a) Heiden, Z. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 2007, 129,
14303. (b) Chowdhury, S.; Himo, F.; Russo, N.; Sicilia, E. J. Am. Chem.
Soc. 2010, 132, 4178.
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.I.G.: goldberg@chem.washington.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSF through the Center for
Enabling New Technologies through Catalysis (CENTC),
CHE-1205189. We thank Dr. Werner Kaminsky for X-ray crystallo-
graphy and Prof. Tom R. Cundari, Mr. Dale Pahls, Dr. Ash Wright,
and Prof. Alexander J. M. Miller for helpful discussions.
(15) Jiang, B.; Feng, Y.; Ison, E. A. J. Am. Chem. Soc. 2009, 130,
14462.
REFERENCES
■
(16) For examples of hydrogen peroxide detection see: (a) Coggins,
M. K.; Sun, X.; Kwak, Y.; Solomon, E. I.; Rybak-Akimova, E.; Kovacs,
J. A. J. Am. Chem. Soc. 2013, 135, 5631. (b) Kim, S.; Saracini, C.;
Siegler, M. A.; Drichko, N.; Karlin, K. D. Inorg. Chem. 2012, 51, 12603.
(17) For an example of a proposed Fe peracetate species see: Wang,
Y.; Janardanan, D.; Usharani, D.; Han, K.; Que, L., Jr.; Sharik, S. ACS
Catal. 2013, 3, 1334.
(1) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S.
Chem. Rev. 2011, 111, 1761.
(2) (a) Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc.
1979, 101, 7738. (b) Crabtree, R. H.; Mellea, M. F.; Mihelcic, J. M.;
Quirk, J. M. J. Am. Chem. Soc. 1982, 104, 107. (c) Burk, M. J.;
Crabtree, R. H. J. Am. Chem. Soc. 1987, 109, 8025. (d) Maguire, J. A.;
Boese, W. T.; Goldman, A. S. J. Am. Chem. Soc. 1989, 111, 7088.
(e) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003,
125, 15286. (f) Kostelansky, C. N.; MacDonald, M. G.; White, P. S.;
Templeton, J. L. Organometallics 2006, 25, 2993. (g) Khaskin, E.;
Zavalij, P. Y.; Vedernikov, A. N. J. Am. Chem. Soc. 2006, 128, 13054.
1340
dx.doi.org/10.1021/om401241e | Organometallics 2014, 33, 1337−1340