High yields of piperylene in the transfer dehydrogenation of pentane catalyzed by pincer-ligated iridium complexes

Article abstract of DOI:10.1016/j.molcata.2016.10.019

Conjugated dienes are desirable reagents for several important applications. We report that sterically uncrowded PCP-pincer iridium complexes, including precursors of (^iPr4PCP)Ir and (^Me2tBu2PCP)Ir, catalyze the transfer d

Full text of DOI:10.1016/j.molcata.2016.10.019

Accepted Manuscript
Title: High Yields of Piperylene in the Transfer
Dehydrogenation of Pentane Catalyzed by Pincer-Ligated
Iridium Complexes
Author: Akshai Kumar Jason D. Hackenberg Gao Zhuo
Andrew M. Steffens Oleg Mironov Robert J. Saxton Alan S.
Goldman
PII:
S1381-1169(16)30441-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.10.019
MOLCAA 10081
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
6-6-2016
15-10-2016
17-10-2016
Please cite this article as: Akshai Kumar, Jason D.Hackenberg, Gao Zhuo,
Andrew M.Steffens, Oleg Mironov, Robert J.Saxton, Alan S.Goldman, High
Yields of Piperylene in the Transfer Dehydrogenation of Pentane Catalyzed by
Pincer-Ligated Iridium Complexes, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.10.019
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

High Yields of Piperylene in the Transfer Dehydrogenation of Pentane
Catalyzed by Pincer-Ligated Iridium Complexes
Akshai Kumar*,a,b Jason D. Hackenberg,a Gao Zhuo,a Andrew M. Steffens,a Oleg Mironov,c
Robert J. Saxton,c and Alan S. Goldman*a
a
Department of Chemistry and Chemical Biology, Rutgers, The State University of New
Jersey, New Brunswick, New Jersey 08903, United States
b
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati -781039,
Assam, India
c Chevron Energy Technology Company, 100 Chevron Way, Richmond, CA 94802
Email: alan.goldman@chem.rutgers.edu
akshaikumar@iitg.ernet.in
Dedicated to Prof. Georgiy B. Shul’pin on the occasion of his 70th birthday
1

Graphical abstract
Research Highlights:
• Pincer-iridium complexes catalyze dehydrogenation of n-pentane to give piperylene.
• Less-bulky pincer ligands are much more effective (e.g. iPr4PCP vs. tBu4PCP).
• Catalyst effectiveness correlates with effectiveness for alkane dehydroaromatization.
2

ABSTRACT:
Conjugated dienes are desirable reagents for several important applications. We report that
sterically uncrowded PCP-pincer iridium complexes, including precursors of (iPr4PCP)Ir and
(
Me2 Bu2PCP)Ir, catalyze the transfer dehydrogenation of pentane, using high concentrations
t
of t-butylethylene (TBE) as hydrogen acceptor, to give high yields of 1,3-pentadiene
(piperylene). Piperylene yields are ca. 100-fold greater than those obtained with the more
widely used di(t-butyl)phosphino substituted pincer iridium catalysts.
KEYWORDS: Pincer-Iridium complexes; Alkane dehydrogenation; C-H bond activation;
Catalytic alkane conversion
3

1. Introduction
Light alkanes (C3-C7) are undesirable as transportation fuel components but are highly
abundant in global reservoirs and from multiple sources such as Fischer-Tropsch plants,
natural gas refineries and shale oil. In contrast, the corresponding light alkenes, which are
much less abundant, are versatile precursors in the synthesis of commodity chemicals. In
addition, the corresponding light alkenes could potentially be dimerized (or cross-dimerized)
to give alkanes of molecular weight more suitable for fuel.1 While α-olefins (primary
dehydrogenation products) are extensively used as monomers for ethylene co-polymers, the
corresponding products of secondary dehydrogenation (dienes) find wide utility as
precursors to synthetic rubber and cycloalkanes. The catalytic dehydrogenation of alkanes
to give olefins thus has great potential value in this context, and has been a field of intense
research.
Heterogeneous dehydrogenation catalysts are normally unselective and operate under
severe conditions.2,3 Homogeneous organometallic catalysts for alkane dehydrogenation
were reported as early as in 1979.4-6 This area has been extensively explored since then
and a significant milestone in homogeneous alkane dehydrogenation was the report by
Kaska and Jensen using a pincer-ligated iridium catalyst based on (tBu4PCP)IrHn (1-Hn;
R4PCP = η3-C6H3-2,6-(CH2PR2)2; n = 2 or 4).7 Subsequently we showed that the less
crowded iPr4PCP analogue (2) exhibited greater catalytic activity.8 Both complexes 1 and 2
showed kinetic selectivity for dehydrogenation of n-alkanes at the terminal position to give
the highly desirable α -olefins. Catalysts 1 and 2 were also found to be effective for the
acceptorless dehydrogenation of alkanes.9,10 Over the years there have been numerous
reports on several catalytically active variants with the (PCP)Ir motif, including other bis-
phosphines,11-20
bis-phosphinites
(POCOP),21-24
hybrid
phosphine–phosphinites
(PCOP),25,26 arsines (AsOCOAs),27 hybrid phosphine-thiophosphinites (PSCOP)28 and
hybrid amine-phosphinites (NCOP).29 The scope of pincer-iridium-catalyzed alkane
dehydrogenation has been broadened by employing these reactions in tandem with
secondary catalytic systems resulting in valuable transformations of hydrocarbons, including
alkane metathesis,14,26,30-37 alkyl group metathesis,38 dehydroaromatization,25,39,40 alkyl-aryl
coupling reactions,41,42 alkane–alkene coupling reactions,43-45 Diels-Alder cyclization,46,47
borylation of alkanes,29 silylation of alkanes48 and the dehydrogenation of several non-
alkane substrates.28,49-53 This has triggered intensive research in pincer chemistry for
hydrocarbon functionalization and more recently, several new pincer motifs have been
explored, such as (PNBNP)Ir,54 (CCC)Ir,55-58 (PCP)Ru,59-62 (POCOP)Ru,62 (PCP)Os,63 and
(NCN)Ir.64,65 Though most of these new complexes show promise for alkane
dehydrogenation, none of them have proven as effective as the well-investigated PCP-type
iridium-based systems.
We have recently reported very high turnover rates and exceptional selectivity for the
formation of α-olefins in the transfer-dehydrogenation of light gas-phase alkanes by
molecular solid-phase pincer-iridium catalysts, using economical gaseous olefins such as
propene and ethylene.66-69 It should be noted that in addition to 1-pentene, a secondary
dehydrogenation product, piperylene (1,3-pentadiene) is also of great synthetic utility.47 In
4

the dehydrogenation of pentane using propene as acceptor, Brookhart reported a maximum
yield of 50 mM of piperylene using (iPr4Anthraphos)Ir (9) as catalyst.47 We have
demonstrated that (iPr4PCP)Ir (2) gives up to 300 mM of piperylene using propene.68 In the
current study, we report remarkably high yields (ca. 1210 mM) of piperylene in the pincer-
iridium catalyzed pentane dehydrogenation using t-butylethylene (TBE) as acceptor (eq 1).70
It can be noted in this context that TBE can be regenerated from the hydrogenated product
(neohexane) using heterogeneous catalysts.71
catalyst
tBu
tBu
+ 2
+ 2
(1)
2. Experimental details
General. All manipulations were carried out under an inert atmosphere of dry argon either in
Bu4
a glove box or using a standard double manifold. (t PCP)IrH4,7 (tBu4POCOP)IrH4,21-23
Bu3MePCP)IrH4,14
(
Bu2Me2PCP)IrH4,14
(
Bu4PCOP)IrH4,26
(
Pr4PCOP)Ir(C2H4),25
t
t
t
i
(
(
Pr4PCP)Ir(C2H4),68 (p-OMe-iPr4PCP)Ir(C2H4)12 and
(
Pr4Anthraphos)Ir(C2H4)25 were
i
i
synthesized according to literature methods. For catalytic studies, anhydrous pentane
obtained from Sigma Aldrich was distilled over NaK alloy and stored over 4Å molecular
sieves.
General procedure for pincer-iridium catalyzed dehydrogenation of pentane. In a
typical experiment, n-pentane (700 µL, 6.07 mmol, 4.1 M) was mixed with TBE (780 µL,
6.05 mmol. 4.1 M) in a vial containing (iPr4PCP)Ir(C2H4) (0.8 mg, 1.4 µmol, 1.0 mM) and GC
internal standard mesitylene (20 µL, 0.14 mmol, 96 mM). About 100 µL of this stock solution
was transferred to each of 12 sealable custom-made thick-walled 200 µL ampoules inside
an argon-filled glove box. The ampoule was then connected to a Kontes adapter via Tygon
tubing and degassed on a high-vacuum line. The contents of the vials were frozen in liquid
nitrogen and the vials were flame sealed. Two vials were simultaneously placed in a pre-
heated aluminum block inside an oven maintained at 200 °C [note: extreme caution must be
exercised during this process, including the use of appropriate safety shields] and subjected
to interval free heating for 10 min. The oven was then cooled to room temperature, the
ampoules were removed, the contents were frozen in liquid nitrogen, the ampoules were
broken open, and the contents were analyzed by GC. The concentration of various products
were determined with reference to mesitylene as an internal standard. For the 10 min run,
two data points were obtained (from two separate ampoules) and the product concentrations
reported are an average of the two runs. Similarly, the procedure was repeated to get two
data points each for other reaction times (typically 40 min, 180 min and 360 min). The
pentene fragments were unambiguously identified by comparison with authentic samples.
Figure 1 provides an example of the C5 and C6 region of GC profiles obtained in the transfer
dehydrogenation of pentane (4.1 M)/TBE (4.1 M) at 200 °C catalyzed by the least active
Bu4
system (t PCP)Ir(H4) (1-H4) (1 mM) and the highly efficient system (iPr4PCP)Ir(C2H4) (2-
C2H4) (7 mM) after 180 minutes.
5

Gas chromatographic analysis. GC analyses (FID detection) were performed on a Varian
430– GC instrument fitted with Agilent J&W GS-GasPro column (60 m length x 0.32 mm ID)
using the following method: Varian 430–GC Detector: FID, Starting temperature: 40 °C,
Time at starting temp: 1.4 min, Ramp 1: 20 °C/min up to 200 °C with hold time = 3 min,
Ramp 2: 30 °C/min up to 260 °C with hold time = 70.6 min, Flow rate (carrier): 1.4 mL/min
(N2), Split ratio: 25, Inlet temperature: 250 °C, Detector temperature: 260 °C
3. Results and discussion
The transfer dehydrogenation of n-pentane (4.1 M) with TBE (4.1 M) was initially
investigated with nine different pincer-Ir complexes (Scheme 1 and Table 1). Sterically less
crowded complexes based on (p-OMe-iPr4PCP)IrH4 (7-H4)12 and (iPr4PCP)IrH4 (2-H4)8 have
been reported to be more active than 1 for transfer dehydrogenation of n-alkanes using
TBE. Similarly, we have shown that complexes (tBu3MePCP)IrH4 (3-H4) and (tBu2Me2PCP)IrH4
(4-H4) that are obtained by methyl-for-t-butyl substitution are catalytically more active than 1
for the transfer dehydrogenation of n-octane using either TBE or NBE as acceptor.14 In order
to obtain high yields of olefins we need to have significant amounts of acceptor; however in
most of these reports TBE has been used in the range of 0.2 M to 1.5 M in alkane, perhaps
in part because buildup of olefinic products can result in catalyst inhibition.72 However,
catalytic systems based on (iPr4PCOP)Ir(C2H4) (8-C2H4) were found to be quite stable to
olefinic products, and multiple equivalents of TBE have been used as acceptor in the
dehydroaromatization of alkanes.25,39,40 Hence the prospect of using sterically less crowded
pincer-iridium catalysts to obtain high yields of dienes by the sequential dehydrogenation of
light alkanes (which cannot cyclize/aromatize intramolecularly) appeared to be feasible.
It should be noted that under the dehydrogenation conditions, each of the pincer-iridium
terahydride, dihydride and the ethylene complexes have been shown to give rise to the
active 14e three-coordinate Ir(I) species.33-37 Hence all through this paper we would
represent the various pincer complexes as the 14e fragment for the sake of simplicity. For
Pr4
instance, (i PCOP)Ir(C2H4) (8-C2H4) would just be represented as (iPr4PCOP)Ir (8).
Following up on our recent studies using pentane as a model light alkane,68 we
investigated the pincer-iridium-catalyzed dehydrogenation of pentane (4.1 M) with TBE (4.1
M) as acceptor to give pentenes and 1,3-pentadiene (piperylene); the initial screening (Table
1) used 1 mM catalyst concentration in all cases. Use of (tBu4PCP)Ir (1) resulted in 55 TO
(catalytic turnovers) in 10 min (entry 1, Table 1). The reaction then leveled off, affording 75
TO after 360 minutes. No piperylene was observed at initial times and only 5 mM was
Bu4
obtained after 360 min. Complexes (t PCOP)Ir (5) and (tBu4POCOP)Ir (6), which are
sterically somewhat more open at the Ir center compared to 1, were not found to be any
more effective (entries 2 and 3, Table 1). This is in contrast with the excellent activity
demonstrated by (tBu4POCOP)Ir (6) towards dehydrogenation of cycloalkanes using
equimolar amounts of TBE.21-23 The lower catalytic activity towards dehydrogenation of
linear alkanes by (tBu4POCOP)Ir (6) has been attributed to its tendency to bind strongly to α-
olefins. 21-23,35
6

Complex (iPr4PCOP)Ir (8), which has shown excellent activity for dehydroaromatization of
cycloalkanes25 showed moderately good activity in this study, giving 270 TO in 10 min and
1140 TO in 360 min (total olefin), with 260 TO of piperylene produced in 360 min (entry 4,
Table 1). (iPr4PCP)Ir (2) (entry 5) was even more effective. The initial rate with 2, based on its
TON (turnover number) after 10 min, was the highest of all the catalysts investigated, 12-
fold greater than the tBu4PCP analogue, 1, while the total productivity (1540 TO) of 2 was ca.
20-fold that of 1. The final yield of piperylene was 485 mM (485 TO); this is much higher
than previously reported yields.47,68 The activity of (p-OMe-iPr4PCP)Ir (7) (the p-methoxy
analogue of 2) was somewhat less than that of 2 (entry 6). Complex (iPr4Anthraphos)Ir (9),
which was recently reported to be one of the most efficient catalysts for the synthesis of
piperylene and toluene47 exhibited good rates and conversions, but slightly lower than those
obtained with 2 (entry 7).
Complex (tBu3MePCP)Ir (3), in which one of the four t-Bu groups on the parent complex 1
is substituted by a methyl group, afforded a rate of dehydrogenation two-fold greater than
that of 1, and gave about 160 TO after 180 minutes (entry 8, Table 1). However no
pentadienes were formed in these reactions. A second Me-for-tBu substitution, to give
Bu2Me2PCP)Ir (4), resulted in an initial rate 7 times greater than that of 1 (entry 9).
t
(
Gratifyingly, piperylene was also formed with good initial rates (450 TO/h) and in high yield
(TON of 675). These results are in contrast with our earlier reports where we have
performed systematic studies to investigate the steric effects on catalyst efficiency by
comparing the activities of complexes 1, 3 and 4.14 While the substitution of the first Bu
t
group of the parent complex (1) was calculated to have a favorable effect in lowering the
overall energy barrier in the n-alkane/1-alkene transfer dehydrogenation cycle, additional
Me-for-tBu substitutions were predicted to have negligible effect.14 Experimentally, for the
transfer dehydrogenation of n-octane using TBE (0.2 M and 0.4M) or NBE (0.2 M, 0.4 M and
1.1 M) (NBE = norbornene) it was observed that 3 was indeed more active than 1 or 4.14 As
complex 4 tended to form dinuclear species, however, it was not clear whether the reduced
activity resulted from the intrinsic properties of the monomeric catalyst or from deactivation
via cluster formation. Our current studies with equimolar TBE affords a different trend in
reactivity: (tBu2Me2PCP)IrH4 (4-H4) > (tBu3MePCP)IrH4 (3-H4) > (tBu4PCP)IrH2 (1-H2). This trend
is very similar to that observed in our recent report on solid-gas phase dehydrogenation of
pentane using propene as acceptor.68,69 It appears likely that high concentrations of TBE or
the dehydrogenated product prevent (tBu2Me2PCP)Ir complexes from undergoing dimerization,
thus permitting high catalytic activity.
Thus, among the catalysts screened for pentane dehydrogenation in Table 1, catalysts 1,
t
t
i
5, 6, and 3, bearing Bu2P groups, gave relatively poor results, while BuMeP- and Pr2P-
substituted complexes 4 and 2 showed high activity and exceptional productivity for the
formation of pentenes and piperylene. After 360 minutes of reaction, TONs > 1000 were
obtained with the use of sterically less hindered catalysts 8 (1140 TO), 7 (1320 TO), 9 (1280
TO), 2 (1540 TO) and catalyst 4 (1710 TO). While we have previously reported greater
catalytic activity of 2 vs. 1,8,14 the difference was no more than a factor of 2.5 when TBE is
used in the range of 0.2 M to 1.45 M. In view of the much higher (12-fold) increase in
7

dehydrogenation rates that we observe with 2 vs. 1 under the current conditions, we
conducted several additional experiments.
The dehydrogenation of pentane (4.1 M) using TBE (4.1 M) as acceptor, with 1 mM
Pr4PCP)Ir(C2H4) (2-C2H4), initially proceeds at a good rate, but gradually slows down and
i
(
productivity levels off (entry 5, Table 1, and entry 1, Table 2). Increasing the catalyst
concentration to 3.5 mM increased the initial rate by ca. 1.5 fold and the final conversion by
ca. 1.2 fold. A maximum of 650 mM of piperylene was obtained after 180 min, which is much
higher than yields reported previously.47,68 The amount of piperylene then decreased,
apparently due to the formation of Diels-Alder cyclized products.46,47 Further increasing the
concentration of catalyst (iPr4PCP)Ir(C2H4) (2-C2H4) to 7.0 mM results in roughly 50%
conversion after 10 min and ca. 95% conversion after 360 min (entry 3, Table 2, 180 min). In
this case the maximum yield of piperylene obtained was about 880 mM, after 180 min.
The experiment with 7 mM (iPr4PCP)Ir(C2H4) (2-C2H4) was repeated in a sealed NMR
tube and the reaction was heated at 200 °C for about 100 min as this was the time when the
31
reaction had begun to level off (see entry 3, Table 2). Comparison of the P NMR spectra
(with PMe3 as external standard) of the reaction mixture at t = 0 and at t = 100 min is shown
in Fig. 2. About 55% of the catalyst had apparently decomposed, 22% had formed a 2-
complex with pentadiene, and 8% was bound to pentenes. The remaining 15% had formed
the inactive iridium carbonyl complex, probably through a reaction with adventitious
moisture. Removal of volatiles from the NMR tube and addition of fresh pentane (4.1 M) and
TBE (4.1 M) followed by heating at 200 °C and GC analysis indicated negligible
dehydrogenation activity. This appears to be because the pentadiene and pentene
complexes of (iPr4PCP)Ir (2) cannot be isolated in the solid state; loss of olefin during
evacuation may give the highly reactive 14-electron species which leads to decomposition.
In the case of (tBu4PCP)Ir (1) (7.0 mM), though the yields increase with higher catalyst
loading (compare Entry 4 and 5, Table 2), the yields are still poor and level off after 40 min.
In the corresponding 31P NMR studies using (7.0 mM) of (tBu4PCP)Ir (1), the reaction mixture
was heated only up to 30 minutes so as to determine the speciation under the conditions
where reactivity decreases. However, these studies revealed complete loss of the catalyst
under the reaction conditions (Fig. 3).
The very high yield of piperylene (880 mM) in the pentane transfer dehydrogenation
reactions catalyzed by 7.0 mM (iPr4PCP)Ir (2) prompted us to investigate a greater ratio of
TBE:n-pentane. When the ratio was changed from 1:1 to 2:1, the reaction proceeded at a
slightly lower rate (Table 3). However, this indeed afforded an even higher yield of
piperylene (1210 mM after 180 min). This is by far the highest yield of piperylene reported to
date from pentane dehydrogenation.47,68
Finally, we note that the pentane transfer dehydrogenation studies may be extended to
the transfer dehydrogenation of butane,68,69 and in this context the current results offer great
promise in the synthesis of butadiene which is a valuable precursor for synthetic rubber.
8

4. Conclusions
We report the first high-yield synthesis of dienes via the dehydrogenation of an n-alkane
using molecular catalysts, specifically pincer-ligated iridium complexes. Sterically
uncrowded pincer iridium complexes, such as (iPr4PCP)Ir have been reported to be much
more active for n-alkane dehydrogenation than the more common, sterically hindered
Bu4
analogues such as (t PCP)Ir and (tBu4POCOP)Ir. The difference in reactivity has previously
been found to be ca. 2 – 5 fold under typical conditions, which comprise modest
concentrations of hydrogen acceptor and a large excess of alkane substrate. For the
purpose of obtaining high yields of multiply unsaturated products, however, very high
concentrations of acceptor are required. We have previously reported that with high
concentrations of the archetypal hydrogen acceptor, TBE, uncrowded catalysts (iPr4PCP)Ir
and (iPr4PCOP)Ir are orders of magnitude more effective than the more crowded analogues
for the dehydroaromatization of long chain (Cn, n ≥ 6) n-alkanes. Here we report that with n-
pentane, which cannot undergo dehydroaromatization, sterically uncrowded pincer
complexes with high concentrations of TBE afford piperylene in very high yields, as much as
100-fold greater than obtained with more crowded pincer-Ir catalysts.
Acknowledgements
This work was supported by NSF under the CCI Center for Enabling New Technologies
through Catalysis (CENTC) Phase II Renewal, CHE-1205189, and by Chevron Energy
Technology Company.
9

References
(1) Kulkarni, A.; Kumar, A.; Goldman, A. S.; Celik, F. E. Catal. Commun. 2016, 75, 98.
(2) Zhang, Y.; Yao, W.; Fang, H.; Hu, A.; Huang, Z. Sci. Bull. 2015, 60, 1316.
(3) Nawaz, Z. Rev. Chem. Eng. 2015, 31, 413.
(4) Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc. 1979, 101, 7738.
(5) Baudry, D.; Ephritikhine, M.; Felkin, H.; Zakrzewski, J. J. Chem. Soc., Chem. Comm. 1982, 1235.
(6) Crabtree, R. H.; Mellea, M. F.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. Soc. 1982, 104, 107.
(7) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. Chem. Commun. 1996, 2083.
(8) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C. M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086.
(9) Xu, W.-w.; P. Rosini, G.; Krogh-Jespersen, K.; S. Goldman, A.; Gupta, M.; M. Jensen, C.; C. Kaska,
W. Chem. Commun. 1997, 2273.
(10) Liu, F.; S. Goldman, A. Chem. Commun. 1999, 655.
(11) Haenel, M. W.; Oevers, S.; Angermund, K.; Kaska, W. C.; Fan, H.-J.; Hall, M. B. Angew. Chem., Intl.
Ed. 2001, 40, 3596.
(12) Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2004, 126,
13044.
(13) 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.
(14) Kundu, S.; Choliy, Y.; Zhuo, G.; Ahuja, R.; Emge, T. J.; Warmuth, R.; Brookhart, M.; Krogh-Jespersen,
K.; Goldman, A. S. Organometallics 2009, 28, 5432.
(15) Punji, B.; Emge, T. J.; Goldman, A. S. Organometallics 2010, 29, 2702.
(16) Shi, Y.; Suguri, T.; Dohi, C.; Yamada, H.; Kojima, S.; Yamamoto, Y. Chem.-Eur. J. 2013, 19, 10672.
(17) Bezier, D.; Brookhart, M. ACS Catal. 2014, 4, 3411.
(18) Adams, J. J.; Arulsamy, N.; Roddick, D. M. Organometallics 2011, 30, 697.
(19) Adams, J. J.; Lau, A.; Arulsamy, N.; Roddick, D. M. Organometallics 2011, 30, 689.
(20) Adams, J. J.; Arulsamy, N.; Roddick, D. M. Organometallics 2012, 31, 1439.
(21) Göttker-Schnetmann, I.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 9330.
(22) Göttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804.
(23) Göttker-Schnetmann, I.; White, P. S.; Brookhart, M. Organometallics 2004, 23, 1766.
(24) Morales-Morales, D.; Redón, R. o.; Yung, C.; Jensen, C. M. Inorg. Chim. Acta 2004, 357, 2953.
(25) Ahuja, R.; Punji, B.; Findlater, M.; Supplee, C.; Schinski, W.; Brookhart, M.; Goldman, A. S. Nat Chem
2011, 3, 167.
(26) Nawara-Hultzsch, A. J.; Hackenberg, J. D.; Punji, B.; Supplee, C.; Emge, T. J.; Bailey, B. C.; Schrock,
R. R.; Brookhart, M.; Goldman, A. S. ACS Catalysis 2013, 3, 2505.
(27) Brayton, D. F.; Beaumont, P. R.; Fukushima, E. Y.; Sartain, H. T.; Morales-Morales, D.; Jensen, C. M.
Organometallics 2014, 33, 5198.
(28) Yao, W.; Zhang, Y.; Jia, X.; Huang, Z. Angew. Chem. 2014, 126, 1414.
(29) Jia, X.; Zhang, L.; Qin, C.; Leng, X.; Huang, Z. Chem. Commun. 2014, 50, 11056.
(30) Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.; Brookhart, M. Science 2006, 312, 257.
(31) Ahuja, R.; Kundu, S.; Goldman, A. S.; Brookhart, M.; Vicente, B. C.; Scott, S. L. Chem. Commun.
2008, 253.
(32) Biswas, S.; Brookhart, M.; Choliy, Y.; Goldman, A.; Huang, Z.; Krogh-Jespersen, K. In Abstracts of
Papers, 239th ACS National Meeting, San Francisco, CA, United States, March 21-25, 2010,
INOR-670.
(33) Choi, J.; Goldman, A. Top. Organomet. Chem. 2011, 34, 139.
(34) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011, 111, 1761.
(35) Findlater, M.; Choi, J.; Goldman, A. S.; Brookhart, M. Catal. Met. Complexes 2012, 38, 113.
(36) Haibach, M. C.; Kundu, S.; Brookhart, M.; Goldman, A. S. Acc. Chem. Res. 2012, 45, 947.
(37) Kumar, A.; Goldman, A. In The Privileged Pincer-Metal Platform: Coordination Chemistry &
Applications; van Koten, G., Gossage, R. A., Eds.; Springer International Publishing: 2016; Vol. 54, p
307.
(38) Dobereiner, G. E.; Yuan, J.; Schrock, R. R.; Goldman, A. S.; Hackenberg, J. D. J Am Chem Soc 2013,
135, 12572.
(39) Goldman, A.; Ahuja, R.; Schinski, W.; Rutgers, The State University of New Jersey, USA . 2013, p 9
pp.
(40) Steffens, A. M.; Goldman, A. S. In Abstracts of Papers, 245th ACS National Meeting & Exposition,
New Orleans, LA, United States, April 7-11, 2013, INOR-1328.
10

(41) Dinh, L. V.; Li, B.; Kumar, A.; Schinski, W.; Field, K. D.; Kuperman, A.; Celik, F. E.; Goldman, A. S.
ACS Catalysis 2016, 2836.
(42) Goldman, A. S.; Dinh, L. V.; Schinski, W. L.; Rutgers, The State University of New Jersey, USA;
Chevron U.S.A. Inc. . 2013, p 13pp.; Chemical Indexing Equivalent to 158:679089 (US).
(43) Leitch, D. C.; Lam, Y. C.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2013, 135, 10302.
(44) Leitch, D. C.; Labinger, J. A.; Bercaw, J. E. Organometallics 2014, 33, 3353.
(45) Goldman, A. S.; Stibrany, R. T.; Schinski, W. L.; Chevron U.S.A. Inc., USA; Rutgers, The State
University of New Jersey . 2013, p 11pp.; Chemical Indexing Equivalent to 158:449690 (WO).
(46) Lyons, T. W.; Guironnet, D.; Findlater, M.; Brookhart, M. J. Am. Chem. Soc. 2012, 134, 15708.
(47) Kundu, S.; Lyons, T. W.; Brookhart, M. ACS Catalysis 2013, 3, 1768.
(48) Jia, X.; Huang, Z. Nature Chemistry 2016, 8, 157.
(49) Shi, Y.; Suguri, T.; Kojima, S.; Yamamoto, Y. J. Organomet. Chem. 2015, 799-800, 7.
(50) Lyons, T. W.; Bezier, D.; Brookhart, M. Organometallics 2015, 34, 4058.
(51) Zhang, X.; Fried, A.; Knapp, S.; Goldman, A. S. Chem. Commun. 2003, 2060.
(52) Yao, W.; Zhang, Y.; Jia, X.; Huang, Z. Angew. Chem., Intl. Ed. 2014, 53, 1390.
(53) Gupta, M.; C. Kaska, W.; M. Jensen, C. Chem. Commun. 1997, 461.
(54) Tanoue, K.; Yamashita, M. Organometallics 2015, 34, 4011.
(55) Chianese, A. R.; Mo, A.; Lampland, N. L.; Swartz, R. L.; Bremer, P. T. Organometallics 2010, 29,
3019.
(56) Chianese, A. R.; Shaner, S. E.; Tendler, J. A.; Pudalov, D. M.; Shopov, D. Y.; Kim, D.; Rogers, S. L.;
Mo, A. Organometallics 2012, 31, 7359.
(57) Chianese, A. R.; Drance, M. J.; Jensen, K. H.; McCollom, S. P.; Yusufova, N.; Shaner, S. E.; Shopov,
D. Y.; Tendler, J. A. Organometallics 2014, 33, 457.
(58) Knapp, S. M. M.; Shaner, S. E.; Kim, D.; Shopov, D. Y.; Tendler, J. A.; Pudalov, D. M.; Chianese, A. R.
Organometallics 2014, 33, 473.
(59) Gruver, B. C.; Adams, J. J.; Warner, S. J.; Arulsamy, N.; Roddick, D. M. Organometallics 2011, 30,
5133.
(60) Adams, J. J.; Gruver, B. C.; Donohoue, R.; Arulsamy, N.; Roddick, D. M. Dalton Trans. 2012, 41,
12601.
(61) Roddick, D. M. Top. Organomet. Chem. 2013, 40, 49.
(62) Zhang, Y.; Fang, H.; Yao, W.; Leng, X.; Huang, Z. Organometallics 2016, 35, 181.
(63) Gruver, B. C.; Adams, J. J.; Arulsamy, N.; Roddick, D. M. Organometallics 2013, 32, 6468.
(64) Allen, K. E.; Heinekey, D. M.; Goldman, A. S.; Goldberg, K. I. Organometallics 2013, 32, 1579.
(65) Allen, K. E.; Heinekey, D. M.; Goldman, A. S.; Goldberg, K. I. Organometallics 2014, 33, 1337.
(66) Kumar, A. G., Alan. S. In Abstracts of Papers, 245th ACS National Meeting & Exposition, New
Orleans, LA, United States, April 7-11, 2013, INOR-1202.
(67) Kumar, A. M., Oleg.; Saxton, Robert. J.; Goldman, Alan. S. In Abstracts of Papers, 248th ACS
National Meeting & Exposition, San Francisco, CA, United States, August 10-14, 2014, INOR-1041.
(68) Kumar, A.; Zhou, T.; Emge, T. J.; Mironov, O.; Saxton, R. J.; Krogh-Jespersen, K.; Goldman, A. S. J.
Am. Chem. Soc. 2015, 137, 9894.
(69) Kumar, A.; Moronov, O.; Saxton, R. J.; Goldman, A. S.; Rutgers, The State University of New Jersey,
USA; Chevron U.S.A. Inc. . 2015; Vol. US 20150251171, p 25pp.
(70) Kumar, A.; Goldman, A. S. Abstracts of Papers of the American Chemical Society 2013, 245.
(71) Kogan, S. B.; Herskowitz, M. Industrial & Engineering Chemistry Research 2002, 41, 5949.
(72) Renkema, K. B.; Kissin, Y. V.; Goldman, A. S. J. Am. Chem. Soc. 2003, 125, 7770.
11

Fig. 1. The C5 and C6 region of GC profiles obtained in the transfer dehydrogenation of n-
pentane (4.1 M)/TBE (4.1 M) at 200 °C catalyzed by (a) (tBu4PCP)Ir(H4) (1-H4) (1 mM) and
(b) (iPr4PCP)Ir(C2H4) (2- C2H4) (7 mM) after 180 minutes
12

31
Fig. 2. Comparison of P NMR spectra at t = 0 and t = 100 min in the (iPr4PCP)Ir(C2H4) (2-
C2H4) (7 mM) catalyzed transfer dehydrogenation of pentane (4.1 M) at 200 °C with TBE
(4.1 M)
13

Fig. 3. Comparison of 31P NMR spectra at t = 0 and t = 30 min in the transfer-
dehydrogenation of pentane (4.1 M) with TBE (4.1 M) catalyzed by (tBu4PCP)Ir(H4) (1-H4) (7
mM) at 200 °C
14

Scheme 1. Pincer-Ir catalysts investigated in this study
15

Table 1
Transfer Dehydrogenation of n-Pentane (4.1 M) with TBE (4.1 M) by Various Pincer-Iridium
Catalysts (200 °C)
Catalyst
(1 mM)
Time
Min
TBA
mM
Total Olefin
mM
1-pentene
mM
2-pentenes 1,3-pentadiene
Entry
1
mM
mM
0
0
0
0
0
0
0
5
10
40
55
70
55
70
25
30
25
30
360
75
75
20
50
5
1-H4
5-H4
6-H4
8
0
10
40
0
45
60
0
35
45
0
10
10
0
25
35
0
0
0
2
3
4
5
360
70
55
5
50
0
0
10
40
0
30
50
0
25
40
0
5
10
0
20
30
0
0
0
360
60
55
10
45
0
0
10
40
0
275
695
0
270
620
0
65
80
0
190
450
0
15
90
360
1380
1140
95
780
260
0
10
40
0
810
1310
0
665
1020
0
135
135
0
400
600
0
130
280
360
1950
1540a
135
920
485
2-C2H4
0
0
0
0
0
0
10
40
570
1230
465
905
115
125
280
545
70
235
6
360
1910
1320
125
765
425
7-C2H4
9-C2H4
3-H4
0
10
40
0
635
1310
0
545
1060
0
95
110
0
360
700
0
90
250
7
8
9
360
1680
1280
110
850
315
0
10
40
0
125
170
0
110
150
0
50
50
0
60
100
0
0
0
180
190
160
45
115
0
0
10
40
0
495
1310
0
395
915
0
55
75
0
270
540
0
75
300
360
2420b
1710a
100
930
675
4-H4
aTwo best TONs, bHighest dehydrogenation yield = 59% (with respect to TBE consumed)
16

Table 2
Transfer Dehydrogenation of n-Pentane (4.1 M) with TBE (4.1 M) Catalyzed by
Pr4PCP)Ir(C2H4) (2-C2H4) and (tBu4PCP)Ir(H4) (1-H4) at Various Concentrations (200 °C)
i
(
Catalyst
Conc.
Time TBA
Total Olefin 1-pentene 2-pentenes 1,3-pentadiene Diels Alder
Entry
Min
mM mM
mM
mM
mM
mM
0
0
0
0
0
0
0
10
40
180
810
1310
1800
665
1010
1310
140
135
135
400
600
770
130
280
400
0
0
0
1
2
2-C2H4
2-C2H4
2-C2H4
1-H4
360
1950
1540
135
920
490
0
1 mM
0
10
40
180
360
960
0
0
965
1460
1830
1860
1820
0
0
0
0
0
0
15
50
90
1320
2130
2760
2950
2930
140
130
130
125
120
560
815
1010
1060
1080
265
510
650
580
445
3.5 mM
7.0 mM
1 mM
0
10
40
180
360
960
0
0
0
0
830
1190
1380
1440
1220
0
0
0
10
75
130
350
2020
3110
3750
3870
3950a
1470
2170
2530
2600
2480
140
135
130
135
105
495
825
880
775
460
3
4
0
0
0
0
0
0
0
5
5
0
0
0
0
10
40
360
55
70
75
55
70
75
25
30
20
25
30
50
0
0
0
0
0
0
0
5
10
145
170
200
125
140
165
40
35
30
75
10
10
15
0
0
0
40
100
125
1-H4
180
7.0 mM
aHighest dehydrogenation yield = 96% (with respect to TBE consumed)
17

Table 3
Transfer Dehydrogenation of n-Pentane Catalyzed by (iPr4PCP)Ir (2) (7 mM) with 1:1 and 1:2
ratios of n-Pentane:TBE (200 °C)
Diels Alder
Total Olefin 1-pentene 2-pentenes 1,3-pentadiene
Time TBA
Entry pentane:TBE
min
mM
mM
mM
mM
mM
mM
0
0
0
0
0
0
0
10
40
180
360
960
2020
3110
3750
3870
3950
1470
2170
2530
2600
2480
140
135
130
135
105
830
495
825
880
775
460
0
1
2
1:1
1190
1380
1440
1220
10
75
130
350
4.1 M : 4.1 M
0
0
0
0
0
0
0
5
15
115
160
10
40
180
360
1570
2620
3500
3630
1170
1800
2290
2200
105
95
80
80
560
715
770
795
505
965
1210
1010
1:2
2.7 M : 5.5 M
18