Dehydrogenation of N-ethyl perhydrocarbazole catalyzed by PCP pincer iridium complexes: Evaluation of a homogenous hydrogen storage system

Article abstract of DOI:10.1016/j.jorganchem.2009.03.052

The iridium complexes IrH₂{C₆H₃-2,6-(CH₂PBu^t₂)₂ (1), IrH₂{C₆H₃-2,6-(CH₂PPrⁱ₂)₂ (2), and IrHCl{C₆

Full text of DOI:10.1016/j.jorganchem.2009.03.052

Journal of Organometallic Chemistry 694 (2009) 2854–2857
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Dehydrogenation of N-ethyl perhydrocarbazole catalyzed by PCP pincer iridium
complexes: Evaluation of a homogenous hydrogen storage system
Zhaohui Wang a, Ian Tonks b, Jack Belli c, Craig M. Jensen a,b,
*
a Hawaii Hydrogen Carriers, LLC, 531 Cooke Street, Honolulu, HI 96813, United States
b Department of Chemistry, University of Hawaii, 2545 McCarthy, Honolulu, HI 96822, United States
c Punahou School, 1601 Punahou Street, Honolulu, HI 96822, United States
a r t i c l e i n f o
a b s t r a c t
The iridium complexes IrH2{C6H3-2,6-(CH2PBu2t
)
(1), IrH2{C6H3-2,6-(CH2PPri2)2 (2), and IrHCl{C6H3-2,6-
Article history:
2
(OPBut2)2 (3) have been found to be highly active catalysts for the dehydrogenation of N-ethyl perhydrocar-
bazole at 200 °C. However, dehydrogenation to the fully unsaturated ethyl carbazole does not occur in most
instances. Complex 3 is the most active catalyst and shows reasonable activity even at 150 °C. No signs of
dehydrogenation were found in experiments conducted at 100 °C. This apparently reflects the thermody-
namic constraints imposed by the high enthalpy of dehydrogenation of the substrate.
Ó 2009 Elsevier B.V. All rights reserved.
Received 17 February 2009
Received in revised form 31 March 2009
Accepted 31 March 2009
Available online 12 April 2009
Keywords:
PCP pincer complex
Iridium complexes
Catalytic dehydrogenation
Hydrogen storage
Perhydrocarbazole
1. Introduction
cused on cycloalkanes [2–6]. The dehydrogenation of these
cheap and abundant materials to the corresponding aromatic
A major challenge to achieving the commercial viability of
hydrogen fuel cell powered automobiles is the development of a
safe and practical method for the onboard storage of hydrogen.
Cryogenic hydrogen storage systems are impractical due to volu-
metric constraints, safety concerns, and/or high costs. There has
been an extensive effort to develop hydrogen storage systems
based on hydrogen absorbing solids for vehicular application.
Unfortunately, no material has been found which has the requisite
combination of a high gravimetric hydrogen density; adequate
hydrogen-dissociation kinetics and thermodynamic properties;
reliability; and low cost [1].
The utilization of liquid aromatic organic compounds as
hydrogen carriers has remained a tantalizing but impractical
possibility for over 60 years [2–14]. A liquid hydrogen storage
media could be economically manufactured in the massive
quantities to meet the anticipated demand and eliminate many
of the problems commonly associated with systems based on
hydrogen absorbing solids including: thermal management on
charging and other issues relating to the compatibility with
existing infrastructure. In many ways such an onboard energy
carrier can be thought of as ‘‘recyclable gasoline”. Initial efforts
to develop liquid organic hydrogen carriers were primarily fo-
compounds releases approximately 7 wt% hydrogen. However,
the large enthalpy of dehydrogenation of cycloalkanes (ꢀ60 kJ/
mol H2) is a major drawback to their utilization in practical
systems. This thermodynamic constraint mandates that high
temperatures (>300 °C) are required to produce equilibrium
hydrogen pressure >1 atm. Additionally, the dehydrogenation
reaction suffers from extremely slow kinetics and high loadings
of heterogeneous precious metal catalysts are required to pro-
mote the evolution of hydrogen to rates that meet the demands
of an onboard fuel cell. In an attempt to circumvent these dif-
ficulties, we developed the ‘‘pincer catalyst”, IrH2{2,6-C6H3-
CH2PBut2
}
2
(1), the first homogenous catalyst for the dehydroge-
nation of
PBut2
H
Ir
H
PBut2
* Corresponding author. Address: Department of Chemistry, University of Hawaii,
2545 McCarthy, Honolulu, HI 96822, United States.
t
2 2
IrH {2,6-C H -CH PBu } (1)
2
6
3
2
E-mail address: jensen@hawaii.edu (C.M. Jensen).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.03.052

Z. Wang et al. / Journal of Organometallic Chemistry 694 (2009) 2854–2857
2855
cycloalkanes to arenes [15]. The unique reactivity of this espe-
cially robust and active catalyst can be ascribed to the P–C–P
pincer ligand that renders the metal center reactive with satu-
rated hydrocarbons but restricts its access to the ligand P–C
bonds. We have found that hydrogen can be evolved from solu-
tions of cycloalkanes containing catalytic amounts of 1 at tem-
peratures as low as 100 °C in a system in which the solution is
circulated through a ‘‘hot-tube” [16]. This ‘‘off-equilibrium” ap-
proach to cycloalkane dehydrogenation requires the utilization
of a Pd/Ag filter-tube that is selectively permeable to hydrogen.
Unfortunately, the system cannot supply hydrogen at rates ade-
quate for practical applications.
N
C2H5
N-ethyl perhydroethylcarbazole
combination of heterogeneous hydrogenation (Ru on C) and dehy-
drogenation (Pd on lithium aluminate) catalysts [8]. While this sys-
tem holds some promise for onboard hydrogen storage applications,
very high (5 wt%) precious metal catalyst loadings and heating to
200 °C are required in order to obtain only 4 wt% hydrogen at rates
that would be adequate for operation of an onboard PEM fuel cell.
Thus the 100 °C exhaust heat of a PEM fuel cell is inadequate to
drive the dehydrogenation half-cycle of storage apparatus based
on this system.
Pez et al. have calculated the enthalpy of formation of hydroge-
nation for a variety of aromatic heterocycles and their saturated
analogs [8]. Their calculations show that
DH of dehydrogenation
is significantly lowered upon introduction of a hetero-atom into
the ring system as it significantly reduces the aromaticity of the
dehydrogenated molecule. This has led several groups to explore
the dehydrogenation of heterocyclic aromatic compounds [7–15].
Pez et al. were able to achieve the reversible dehydrogenation of
N-ethyl perhydrocarbazole (EPHC) using a
Homogenous catalytic systems typically operate at significantly
lower temperatures than their heterogeneous counterparts. It has
Table 1
Dehydrogenation of N-ethyl perhydrocarbazole using pincer catalysts.
Entry
Catalyst
T (°C)
Time (h)
Yielda,b (%)
EOHC
TONc
ETHC
EC
1
2
3
4
5
6
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
3
3
3
3
3
3
3
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
150
150
150
150
150
150
150
150
3
6
9
12
24
48
72
96
3
8
14
17
21
51
65
73
69
16
34
36
47
58
65
62
23
0
3
4
5
13
19
23
31
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
18
16
40
50
62
154
206
238
262
32
7
8d
9
10
11
12
13
14d
15
16
17
18d
19
20
21e
22
23f
24g
25
26
27
28
29
30
31
32
6
9
0
5
68
92
12
24
48
72
3
12
24
35
38
26
30
30
34
40
28
82
142
212
270
276
150
236
260
268
280
512
436
0
0
9
27
38
68
87
104
110
115
6
9
58
70
66
60
72
0
12
24
18
216
24
24
3
no reaction
no reaction
9
23
28
48
65
78
90
85
3
3
3
3
3
3
3
3
3
0
2
5
10
11
13
10
15
0
0
0
0
0
0
0
0
9
12
18
24
36
48
72
a
Average of three runs as determined by GC–MS.
b
c
d
e
f
EOHC is N-ethyl octahydrocarbazole; ETHC is N-ethyl tetrahydrocarbazole; EC is N-ethyl carbazole.
TON is the molar ratio between the generated hydrogen gas and the catalyst.
All of the substrate, EPHC, has been consumed.
Initially, the reaction was set up according procedure B. After 9 h, another batch of EPHC (100 equivalent) was introduced into the reaction and heated for another 9 h.
The reaction was set up according procedure B without 3.
g
The reaction was set up according procedure B without NaOBut.

2856
Z. Wang et al. / Journal of Organometallic Chemistry 694 (2009) 2854–2857
been reported that 1 and related iridium ‘‘PCP pincer” complexes
catalyze dehydrogenation of secondary and tertiary amines to imi-
nes and enamines [17,18] and ammonia borane [19]. We were
therefore interested in exploring 1 as a catalyst for the dehydroge-
nation of perhydrocarbazoles in a homogeneous systems. We re-
port here the results of our investigation the activity of 1 and the
120
100
80
60
40
20
0
related complexes, IrH2{C6H3-2,6-(CH2PPr2i 2
(2), and IrH2{C6H3-
)
2,6-(OPBut2)2 (3), as catalysts for the dehydrogenation of EPHC to
N-ethyl octahydrocarbazole (EOHC) and N-ethyl tetrahydrocarbazole
(ETHC).
PBut2
PPri2
O
H
H
H
Ir
Ir
0
20
40
60
80
H
PPri2
PBut2
Time (h)
O
Fig. 2. Dehydrogenation of EPHC catalyzed by 3 at 150 °C.
IrH2{C6H3-2,6-(CH2PPri2)2 (2)
IrH2{C6H3-2,6-(OPBut2)2 (3)
occurred in only 9 h when the reaction was catalyzed by 3 com-
pared to the 78 and 48 h required when 1 and 2 were used, respec-
tively. This finding is consistent with the earlier observation that
among the three pincer complexes, 3 catalyzes the transfer dehy-
drogenation of cyclooctane at the fastest rate [22].
2. Results and discussion
A further study of the catalytic activity of the 3 was conducted
at 150 °C, a temperature that is much closer to the operating tem-
peratures that are required for a practical hydrogen storage sys-
tem. As seen in Table 1 and Fig. 2, 3 catalyzes the dehydro-
genation of EPHC to EOHC at 150 °C. However, much lower cata-
lytic activity was observed at the lower temperature. For example,
the dehydrogenation of a full charge of EPHC (50% of the charged
used in the 200 °C experiments) requires 48 h. Additionally, analy-
sis of the product mixture shows it to consist of only 10% ETHC and
90% EOHC as opposed 30% ETHC, 70% EOHC distribution observed
at the time of total consumption EPHC. Thus only 104 molar equiv-
alents H2 were released in 48 h as opposed to 260 molar equiva-
lents in 9 h at 200 °C.
Complexes 1 and 2 were synthesized and purified by the litera-
ture methods [20,21]. Complex 3 was generated from the in situ
reaction of IrHCl{C6H3-2,6-(OPBut2 2 with NaOBut [22]. It has been
)
established the pincer complexes all show significant activity as cat-
alysts for the dehydrogenation of cycloalkanes at 200 °C [20–22].
Therefore, our initial kinetic comparison studies were conducted at
that temperature. The results of the studies of the catalytic activity
of the pincer complexes are summarized in Table 1. All three cata-
lysts show the ability to dehydrogenate EPHC completely to EOHC
and to further dehydrogenate 30–40% of the EOHC to ETHC. In the
case of complex 3, as shown in entry 21, the catalyst was found to
be capable of dehydrogenating a second loading of EPHC after the
initial loading has been consumed without loss of activity or any sig-
nificant difference in the distribution of products. Furthermore,
product inhibition, a common problem in acceptorless aliphatic al-
kane dehydrogenation systems [20–22], was not observed in the
catalytic dehydrogenation of EPHC.
The activity of complex 3 was also studied 100 °C. After two
weeks time no dehydrogenated products were observed in the
reaction mixture. This finding is consistent with the calculations
of Pez et al. that predict the lowest thermodynamically allowed
dehydrogenation temperature is 120 °C [8].
Fig. 1 compares the kinetics observed for the dehydrogenation
reactions catalyzed by the pincer complexes. While similar turn-
over numbers (TON, molar hydrogen/catalyst ratio) were achieved
for all three complexes, the consumption of the full charge of EPHC
3. Conclusions
These three iridium complexes 1–3, all exhibit excellent activity
as catalysts for the dehydrogenation of N-ethyl perhydrocarbazole
at 200 °C. However, dehydrogenation to the fully unsaturated ethyl
carbazole does not occur in most instances. Complex 3 is the most
active catalyst and shows a reasonable activity even at 150 °C. No
signs of dehydrogenation were found in experiments conducted at
100 °C. This apparently reflects the thermodynamic constraints im-
posed by the high enthalpy of the substrate.
300
250
200
150
4. Experimental
100
complex 1
4.1. General details
50
0
complex 2
complex 3
All manipulations were carried out under an argon atmosphere,
using either glovebox or double-manifold Schlenk techniques. Pen-
tane (Fisher Scientific) was distilled from lithium aluminum hy-
dride under nitrogen. Temperatures were maintained to within
5 °C using a silicon oil bath. All dehydrogenation reactions were
performed in re-sealable glass vessels equipped with one or several
0
20
40
60
80
100
Time (h)
Fig. 1. Dehydrogenation of EPHC catalyzed by complexes 1–3 at 200 °C.

Z. Wang et al. / Journal of Organometallic Chemistry 694 (2009) 2854–2857
2857
3-mm Teflon vacuum stopcocks (Rotaflo or Kontes). These reaction
vessels were immersed into a hot silicon oil bath during the reac-
tions and the reaction mixtures were stirred with small Teflon-
coated stirring bars.
Identity and purity of the products were examined by gas chro-
matography with a gas chromatograph GC HP 5890 Series II with a
mass selective detector HP 5971 (GC–MS), equipped with a HP-
1MS capillary column (30 m) from Hewlett Packard.
the pentane was boiled off at 55 °C over 30 min. After the pentane
was removed, the flask was placed in an oil bath at 200. For the
reactions carried out at 150 °C the half the amount of N-ethyl per-
hydrocarbazole (88 mg, 0.42 mmol) was used in the reaction mix-
ture. After the prescribed reaction time, the reaction mixture was
removed from the flask and analyzed GC–MS.
Acknowledgment
4.2. Preparation of N-ethyl perhydrocarbazole
This work was supported by General Motors Corporation.
EPHC was prepared by a slight modification of the literature
method [8]. About 5 g (30 mmol) of N-ethylcarbazole was added
to a 150 mL autoclave along with 1 g of a 5% Ru/C catalyst. The
reactor was pressurized with H2 and vented 3 times to purge the
system of oxygen. The reactor was then pressurized again with
50 bar H2 pressure and the autoclave was heated to 160 °C stirred
for two days. The resulting yellow oil was extracted twice with
50 mL acetone and dried in vacuo overnight. GC–MS analysis
showed this procedure produces >99% N-ethylperhydrocarbazole
(EPHC) in nearly quantitative yield.
References
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Procedure A: A solution of 1 or 2 (0.0085 mmol) in 2 mL of pen-
tane was combined with a solution of N-ethylperhydrocarbazole
(176 mg, 0.85 mmol) in 2 mL of pentane and placed into a 10 mL
Schlenk flask with an integrated water condenser. The flask was
connected to an oil bubbler, and the pentane was boiled off at
55 °C over 30 min. After the pentane was removed, the flask was
placed in an oil bath at 200 °C. After a prescribed period of reaction
time, the reaction mixture was removed from the flask and ana-
lyzed by GC–MS.
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1804.
Procedure B: A solution of 3 (5.3 mg, 0.0085 mmol) and NaOBut
(1.0 mg, 0.01 mmol) in 2 mL of pentane was combined with a solu-
tion of N-ethyl perhydrocarbazole (176 mg, 0.85 mmol) in 2 mL of
pentane and placed into a 10 mL Schlenk flask with an integrated
water condenser. The flask was connected to an oil bubbler, and