Iridium catalyzed reversible dehydrogenation - Hydrogenation of quinoline derivatives under mild conditions

Article abstract of DOI:10.1016/j.jorganchem.2015.04.015

Abstract The potential of a hydrogen-based energy economy is limited by the fact that hydrogen gas is difficult to store and transport. Storing hydrogen in the form of liquid organic hydrogen-carriers (LOHCs) is a highly attractive alternative to current options but it requires the development of catalytic means of reversibly hydrogenating and dehydrogenating these carriers under mild conditions and ideally using a single catalyst for both processes. We report the optimization of two families of previously reported hydrogenation catalysts for the reverse reaction, dehydrogenation of N-heterocyclic substrates. These complexes are capable of catalyzing both dehydrogenation and hydrogenation reactions in alternation, giving high yields in both directions. Importantly, our complexes do not require high temperatures, high pressures of H₂ or strong base for the hydrogenation step.

Full text of DOI:10.1016/j.jorganchem.2015.04.015

Accepted Manuscript
Iridium Catalyzed Reversible Dehydrogenation – Hydrogenation of Quinoline
Derivatives Under Mild Conditions
Michael G. Manas, Liam S. Sharninghausen, Elisa Lin, Robert H. Crabtree
PII:
S0022-328X(15)00216-8
10.1016/j.jorganchem.2015.04.015
JOM 19001
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 26 January 2015
Revised Date: 9 April 2015
Accepted Date: 10 April 2015
Please cite this article as: M.G. Manas, L.S. Sharninghausen, E. Lin, R.H. Crabtree, Iridium Catalyzed
Reversible Dehydrogenation – Hydrogenation of Quinoline Derivatives Under Mild Conditions, Journal of
Organometallic Chemistry (2015), doi: 10.1016/j.jorganchem.2015.04.015.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Revision 2. 4-9-15
Iridium Catalyzed Reversible Dehydrogenation –
Hydrogenation of Quinoline Derivatives Under Mild
Conditions
+
+
Michael G. Manas , Liam S. Sharninghausen , Elisa Lin, Robert H. Crabtree*
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut
0
6520, United States
*robert.crabtree@yale.edu
+
These authors contributed equally
Keywords: Acceptorless Dehydrogenation; Hydrogen Storage; LOHCs;
Heterocycles
Abstract
The potential of a hydrogen-based energy economy is limited by the fact that
hydrogen gas is difficult to store and transport. Storing hydrogen in the form of liquid
organic hydrogen-carriers (LOHCs) is a highly attractive alternative to current options
but it requires the development of catalytic means of reversibly hydrogenating and
dehydrogenating these carriers under mild conditions and ideally using a single catalyst
for both processes. We report the optimization of two families of previously reported
hydrogenation catalysts for the reverse reaction, dehydrogenation of N-heterocyclic
substrates. These complexes are capable of catalyzing both dehydrogenation and
hydrogenation reactions in alternation, giving high yields in both directions. Importantly,
1

ACCEPTED MANUSCRIPT
2
our complexes do not require high temperatures, high pressures of H or strong base for
the hydrogenation step.
Introduction
The development of highly efficient catalysts for the reversible hydrogenation and
dehydrogenation of suitable organic molecules is of great interest to the field of
1
alternative energy research. Hydrogenated organic liquids can in principle be used to
transport hydrogen in the covalently bound form, with subsequent catalyzed release of
2
,3
free H
2
.
Ongoing research at GE Global Research and elsewhere is directed toward
4
using such compounds in fuel cells or flow batteries. The use of such liquid organic
hydrogen carrier molecules (LOHCs) holds promise as an attractive alternative to the
transportation and storage of gaseous hydrogen, which may otherwise require high
2
,5
pressure cylinders that give rise to a large weight penalty. The dehydrogenated carrier
molecule can then be recycled and regenerated by catalytic hydrogenation. The current
global energy transport and storage infrastructure utilizes liquid hydrocarbon fossil fuels
and could arguably be adapted for LOHCs with less disruption than for some other
2
-6
strategies.
Prior work indicates that the presence of a heteroatom in the LOHC
minimizes the unfavorable thermodynamics of the dehydrogenation reaction, allowing for
6
both hydrogenation and dehydrogenation to occur under mild conditions. In order to
make use of heterocycles as hydrogen storage molecules, both the catalytic
6
hydrogenation and dehydrogenation processes must be feasible under mild conditions.
7
,8,9,10
11,12,13,14
A number of catalysts, both heterogeneous
and homogeneous,
can bring
about either hydrogenation or dehydrogenation of N-heterocyclic substrates, but there are
2

ACCEPTED MANUSCRIPT
very few homogeneous catalysts capable of carrying out both reactions on model
1
2-14
quinoline derivatives under mild conditions.
Such a catalyst would be potentially
applicable to reversible hydrogen storage, by allowing for easy hydrogenation and later
dehydrogenation of an LOHC all in one reactor and without the need to add or remove
any additional compounds aside from H gas.
2
The iridium complexes reported by Fujita et al. are unusual in being very efficient
1
2,14
catalysts for both the hydrogenation and dehydrogenation of quinolines.
conditions reported for the hydrogenation of quinaldine were not ideal, requiring either
elevated temperature or H pressures of 3-10 atm, they were able to dehydrogenate
Although the
2
tetrahydroquinaldine in p-xylene at reflux (145°C) in less than 24 hours. Xiao and co-
workers recently found an iridium pincer complex to be highly active for
dehydrogenation of a range of tetrahydroquinolines under even milder conditions (74 ˚C)
1
1d
in 2,2,2-trifluoroethanol; however, the corresponding hydrogenation was achieved only
1
1e
after modification of the pincer ligand. Recently, the Jones group discovered a “two-
way” system for the sequential hydrogenation-dehydrogenation of quinoline derivatives
1
2
with an iron(II)-(PNP) pincer complex. While this system is not as active as the Fujita
system, it utilizes a much cheaper catalyst which makes it very attractive. Unfortunately,
this system also requires 10 mol % of strong base, in addition to 5-10 atm H , in order to
2
carry out the hydrogenation reaction. Interestingly, in contrast to the Fujita system, these
iron(II) complexes are not hindered by sterically unencumbered substrates which might
have been thought likely to bind to the active site via the N lone pair and inhibit the
reaction.
3

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Prior work in our lab has focused on two families of iridium catalysts capable of
efficiently catalyzing the conversion of quinaldine to 1,2,3,4-tetrahydroquinaldine under
1
5,16
1
atm of H at room temperature (Figure 1).
To our knowledge, these catalysts still
2
rank amongst the most efficient homogeneous hydrogenation catalysts for such reactions,
10e
only being surpassed recently by a family of Cp*Ir(III)(N-C)Cl complexes, and are
superior to the Fujita catalysts discussed above with regards to hydrogenation activity
under mild conditions. The first generation precatalyst, 1, and its activated form, 2
(
Figure 2), were found to be the more active of our two classes of complexes.
Compounds 1 and 2 are capable of catalyzing the reduction of a wider range of N-
heterocyclic substrates than complex, 3, but at the cost of solvent scope. Both classes of
catalysts are believed to operate via an outer-sphere mechanism entailing the sequential
delivery of a proton followed by a hydride to the unbound substrate from a coordinated
H molecule. As a natural extension of this work, we sought to explore the potential for
2
dehydrogenation of saturated N-heterocyclic substrates under mild conditions using these
same families of complexes. Based on the principles of microscopic reversibility, it is
plausible to think that these catalysts would be competent for the reverse reaction and that
the process could be optimized by variation of the conditions.
4

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Figure 1. Compounds 1 and 3 are highly active precatalysts for quinaldine hydrogenation
under mild conditions.
Results and Discussion
Precatalyst 1-PF
6
was first screened for the dehydrogenation of 1,2,3,4-
tetrahydroquinaldine in toluene in the presence of 1 equivalent of triphenylphosphine. It
was found that in reactions run below reflux temperature, no appreciable reactivity was
observed; however, small quantities of dehydrogenated product were observed when the
reaction was refluxed (Table 1, Entries 1-2). This is believed to arise in large part from
the reflux action itself sweeping the product H out of the solution, driving the
2
equilibrium in the dehydrogenative direction by mass action, as previously seen in
1
7
catalytic alkane dehydrogenation.
proposed active catalyst, was preformed according to a previously published procedure
by stirring 1-PF under 1 atm of H and in the presence of 1 equivalent of
In order to increase the catalytic activity, 2, the
6
2
triphenylphosphine (Figure 2). Indeed, 2 gave a marked increase in yield under identical
5

ACCEPTED MANUSCRIPT
conditions (Table 1, Entries 3-4). By changing the solvent to chlorobenzene (Table 1,
Entry 5) or p-xylene and thus increasing the reflux temperature, activity increased further,
with complete conversion observed after 72 hours (Table 1, Entry 7). In contrast, under
these optimized conditions, less conversion was observed for 1,2,3,4-tetrahydroquinoline
(Table 1, Entry 8) than for the more sterically hindered 1,2,3,4-tetrahydroquinaldine,
having an additional methyl group at the 2-position. This result follows the trend
observed in our hydrogenation work where substrates lacking a bulky substituent at the 2-
position give lower yields, presumably due to coordination of the nitrogen lone-pair to
the metal, leading to catalyst deactivation.
Figure 2. Formation of active species 2 by hydrogenation in the presence of PPh . The
3
conversion from can be tracked visually by a distinct color change.
6

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Table 1. Dehydrogenation of Tetrahydroquinoline Derivatives by 1
Entry
Solvent
Toluene
Toluene
Toluene
Toluene
R
Temp. (°C)
80
Time (h)
24
Yield
1
2
3
4
5
6
7
8
Me
Me
Me
Me
<2%
115
24
7%
a
a
80
24
<2%
115
24 / 72
24/ 72
24
10 / 20%
45/ 85%
12%
a
Chlorobenzene Me
135
p-Xylene
p-Xylene
p-Xylene
Me
Me
H
145
a
a
145
24 / 72
24 / 72
48 / ~95%
19 / 61%
145
Experiments run under 1 atm N
2
, 1,2,3,4-tetrahydroquinaldine or 1,2,3,4-tetrahydroquinoline (0.112 mmol),
a
1
and PPh
3
(both 5 mol% with respect to substrate), solvent (3 ml). Experiments run with pre-activation of
2
catalyst by heating to 100 ˚C and bubbling H gas for 20 minutes prior to catalysis. Yields determined by
1
H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard. Control experiments for
these reactions can be found in Table S1.
7

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1
6
Following our prior work, derivatives of 3 were also screened for the
dehydrogenation of tetrahydroquinoline compounds (Table 2). Variants of complex 3 are
generally less active as hydrogenation catalysts than 1 but do not require the addition of
phosphine additives or pre-activation with H . In analogy to our prior hydrogenation
2
studies, 1 was more active than 3b (Table 2); over 24 hours in p-xylene at 145°C, 1 gave
almost twice the yield that 3b did. Change of the counterion from PF to tetrakis[3,5-
6
F
bis(trifluoromethyl)phenyl]borate (B(Ar )
4
) had little effect (Table 2, Entries 1-2, 6-7).
Significantly, changing the solvent to chlorobenzene gives a marked increase in yield
Table 2, Entry 5), likely due to the increased solubility of the complex or possibly also to
(
its mild coordinating power. As was observed with 1, increasing the temperature and
reaction time increased the overall yield and reactivity proved to be greater with 1,2,3,4-
tetrahydroquinaldine than with 1,2,3,4-tetrahydroquinoline (Table 2, Entries 2-5).
Additionally, both variants of 3 were tested for dehydrogenation in the presence of a H2
accepting substrate (N-benzylideneaniline) in an attempt to further drive the
dehydrogenation (Table 2, Entries 6-8). A slight increase in activity was observed but
yields remained relatively low.
8

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Table 2. Dehydrogenation of Tetrahydroquinoline Derivatives by 3a and 3b
Entry
Complex Solvent
R
Temp. (°C) Time
hour)
Yield
(
1
2
3
4
5
6
7
8
3a
3b
3b
3b
3b
3b
3a
3b
Toluene
Toluene
Toluene
p-Xylene
H
115
115
115
145
135
145
115
115
24 / 48
24 / 48
24
8 / 17%
9 / 18%
16%
H
Me
Me
24
21%
Chlorobenzene Me
24 / 72
24
46 / 66%
25%
a
p-Xylene
Me
H
a
Toluene
24
13%
a
Toluene
H
24 / 48
18 / 29%
Experiments run under 1 atm N , 1,2,3,4-tetrahydroquinaldine or 1,2,3,4-tetrahydroquinoline (0.112 mmol),
2
a
3
a or 3b (both 5 mol% with respect to substrate), solvent (3 ml).
Experiments run with N-
1
benzylideneaniline as a H
2
acceptor. Yield determined by H NMR spectroscopy using 1,3,5
trimethoxybenzene as an internal standard. Control experiments for these reactions can be found in Table
S1.
9

ACCEPTED MANUSCRIPT
In order to investigate the robustness of complexes 1 (preactivated to form 2) and 3b
under the reaction conditions, which relates to their potential to perform 2-way
dehydrogenation-hydrogenation reactions, we monitored their activity over time using a
gas-burette at low catalyst loading (0.5 mol %) (Figure 3 and SI, page S2). Over 72
hours, 1 and 3b give TONs of 43 and 32, respectively, similar to TONs achieved by other
1
1d,12,13
heterocycle dehydrogenation systems.
The complexes show similar, approximately
-1
linear activity for the first ~30 h, with a TOF of approximately 0.8 h ; however at longer
times, complex 3b decreases in activity. This is in agreement with the 24 and 72 hour
experiments in chlorobenzene (Table 1 entry 5; Table 2, Entry 5), where yield is similar
for the two complexes after 24 h, but is much higher for 1 after 72 h. The latter complex
1
may therefore better resist deactivation. Indeed, H NMR spectra of a catalytic reaction
mixture containing 3b show a hydride peak corresponding to 3b after 24 h (~ 70 % of
added amount), while at 72 h, this peak is no longer present, consistent with
decomposition into inactive species. Because of the durability of 1, as well as its ability
to perform the dehydrogenation reaction in high yield, we attempted dehydrogenation
followed by hydrogenation using this complex. Notably, no activity for hydrogenation
was observed after exposure to H at room temperature following a 72 h dehydrogenation
2
reaction in p-xylene or chlorobenzene.
10

ACCEPTED MANUSCRIPT
Figure 3. Reaction Profiles for Dehydrogenation with Pre-catalysts 1 and 3b
Generated by Gas-burette.
Experiments run with iridium complex (0.02 mmol, 0.5 mol %), tetrahydroquinaldine (4 mmol) and
chlorobenzene (10 mL). Precatalyst 1 was pre-activated by adding 1 eq. PPh , heating to 100 ˚C and
3
bubbling H
2
gas for 20 minutes prior to catalysis. Reaction progress was monitored by gas burette, with 1
produced per molecule of catalyst (See SI for details).
TON defined as 2 molecules of H
2
In the hope of accomplishing dehydrogenation faster and potentially retaining
enough active catalyst for the subsequent hydrogenation, we ran reactions at higher
1
1

ACCEPTED MANUSCRIPT
substrate and precatalyst concentrations (Table 3). Chlorobenzene was selected as solvent
because it dissolves the iridium complexes at high concentration and has a relatively high
boiling point. Indeed, for both 1 and 3b, increasing the tetrahydroquinaldine
concentration from 0.04 to 1.1 M gives increased conversion for dehydrogenation over 24
h at 5 % catalyst loading (Table 3, Entries 3 and 6). Following this improved
dehydrogenation step, subsequent hydrogenation is possible under 1 atm H at ambient
2
temperature, giving >95 % yield for 1 and 75 % yield for 3b, thus demonstrating the
ability of our catalysts to carry out both reactions in alternation as appropriate for a
hydrogen storage application. Importantly, our complexes do not require high
temperatures, high pressures of H or strong base for the hydrogenation step as in
2
previous “2-way” systems, while giving slightly lower but comparable yields in both
1
2,13
directions.
Thus, we have developed a system for reversible quinaldine hydrogenation
that functions under unprecedentedly mild conditions for the hydrogenation step.
12

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Table 3: Reversible dehydrogenation of tetrahydroquinaldine in chlorobenzene with
5
mol % iridium complex under mild conditions
Entry
Complex
Substrate
Concentration
Dehydrogenation Hydrogenation
b
b
yield (%)
yield (%)
(M)
c
a
1
2
3
4
5
6
1
0.04
0.24
1.1
45
63
73
46
60
85
-
c
a
1
-
c
1
>95
a
3b
3b
3b
0.04
0.24
1.1
-
a
-
76
Experiments run with chlorobenzene, iridium complex (5 mol %), tetrahydroquinaldine, and 1,3,5-
trimethoxybenzene as internal standard. Dehydrogenation reaction performed at reflux (135 ˚C) for 24 h.
Subsequently, for entries 3 and 6, reaction mixture cooled to room temperature and hydrogenation carried
a
out by stirring for 24 h at ambient temperature under 1 atmosphere of H . Hydrogenation not attmepted for
2
b
1
these entries. Yields determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal
standard; Hydrogenation yield calculated as amount of quinaldine hydrogenated divided by initial amount
c
of quinaldine after dehydrogenation step. Complex pre-activated by adding 1 eq. PPh
3
, heating to 100 ˚C
and bubbling H gas for 20 minutes prior to catalysis.
2
13

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Conclusion
Two iridium catalysts previously developed by our group for the hydrogenation of N-
heterocycles have been applied to the reverse reaction, dehydrogenation of the saturated
N-heterocycles. In addition, these complexes are able to perform reversible
dehydrogenation-hydrogenation reactions in sequence with high yields for both
directions. Neither system surpassed the best prior “two-way” literature catalyst for the
1
2,14
hydrogenation/dehydrogenation of quinaldine in terms of dehydrogenation efficiency
but both 1 and 3 are much more efficient than the prior “two-way” systems for the
hydrogenation of quinaldine.
Experimental
General Methods. The syntheses of the metal complexes were conducted under nitrogen
using dry degassed solvents unless otherwise indicated. Solvents were dried and degassed
prior to use. Iridium species used in these experiments were synthesized according to
1
4,15
literature procedures (see SI).
All other reagents were commercially available from
Airgas, Sigma-Aldrich, Alfa-Aesar and Strem Chemicals and used as received unless
otherwise indicated. NMR spectra were recorded at room temperature on a 400 or 500
MHz Bruker or Varian spectrometer.
Procedure for dehydrogenation of 1,2,3,4-tetrahydroquinaldine and 1,2,3,4-
tetrahydroquinoline using 1. To a flame-dried 15 ml Schlenk tube under nitrogen was
added a stir bar, 1,2,3,4-tetrahydroquinaldine or 1,2,3,4-tetrahydroquinoline (16.2 µl and
1
1
4.1 µl respectively, 0.112 mmol), trimethoxybenzene (10.0 mg, internal H NMR
14

ACCEPTED MANUSCRIPT
standard), triphenylphosphine (5.5 mol% with respect to 1,2,3,4-tetrahydroquinaldine, 1.6
mg) and 1 (5 mol% with respect to 1,2,3,4-tetrahydroquinaldine, 4.7 mg). Dry solvent (3
ml of toluene or p-xylene, sparged with N ) was then added to the flask. The flask was
2
purged with H by bubbling through the solution for ~20 minutes, activating the catalyst;
2
a distinct color change from red-orange to yellow was observed. The H atmosphere was
2
removed with three cycles of evacuation and N replacement, leaving the system under 1
2
atm of nitrogen with an oil bubbler outlet. The reaction mixture was then stirred and
heated to just above the reflux temperature of the solution (115°C for toluene, 145°C for
p-xylene) and aliquots were collected at 24, 48 and 72 hours. The solvent was removed in
vacuo and the resulting mixture was redissolved in CD Cl . Conversion was assessed by
2
2
comparative integration of the product and starting material peaks to the internal standard
1
in the H NMR spectrum. Spectra of pure quinaldine and 1,2,3,4-tetrahydroquinaldine
from commercially obtained samples were used as peak references in identifying the
products.
Procedure for dehydrogenation of 1,2,3,4-tetrahydroquinaldine and 1,2,3,4-
tetrahydroquinoline using 3a and 3b. To a flame dried 15 ml Schlenk tube under
nitrogen was added a stir bar, 1,2,3,4-tetrahydroquinaldine or 1,2,3,4-tetrahydroquinoline
1
(
16.2 µl and 14.1 µl respectively, 0.112 mmol), trimethoxybenzene (10.0 mg, internal H
NMR standard) and 3a or 3b (5 mol% with respect to 1,2,3,4-tetrahydroquinaldine, 0.1
ml of 9.95 mg 3a or 5.9 mg 3b in 1 ml stock solution of appropriate solvent). For
reactions run with a hydrogen acceptor, N-benzylideneaniline (20.3 mg, 0.112 mmol)
2
was added. Dry solvent (3 ml of toluene or p-xylene, sparged with N ) was then added to
the flask. The mixture was then stirred and heated to just above its reflux temperature the
15

ACCEPTED MANUSCRIPT
reac (115°C for toluene, 145°C for p-xylene) and aliquots were collected at 24 and 48
hours. The solvent was removed in vacuo and the resulting mixture was treated as above.
Procedure
for
2-way
dehydrogenation-hydrogenation
of
1,2,3,4-
tetrahydroquinaldine For the optimized reaction run at 1.1 M substrate loading, the
dehydrogenation was carried out as described above, using 0.022 mmol of either 1 or 3b,
6
6 ꢀL (0.44 mmol) tetrahydroquinoline, 0.5 mL chlorobenzene and 5 mg 1,3,5
trimethoxybenzene as internal standard. After heating at reflux under N
2
for 24 h, the
1
reaction mixture was cooled to room temperature and a 50 ꢀL aliquot was taken for H
NMR analysis. 0.5 mL additional chlorobenzene, was added to the reaction mixture, and
the reaction was stirred under an atmosphere of H for 24 h.
2
Acknowledgements:
This material is based in on work supported by the Chemical Sciences, Geosciences, and
Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of
Energy (DE- FG02-07ER15909) (L.S.S., R.H.C.) as well as U.S. Department of Energy (DoE),
Office of Science, Office of Basic Energy Sciences catalysis award (DE-FG02-84ER13297,
catalysis) (M.G.M., R.H.C.).
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829-4832.
1
5
Dobereiner, G. E.; Nova, A.; Schley, N. D.; Hazari, N.; Miller, S. J.; Eisenstein, O.;
Crabtree, R. H., J. Am. Chem. Soc. 2011, 133, 7547-7562.
1
6
Manas, M. G.; Graeupner, J.; Allen, L. J.; Dobereiner, G. E.; Rippy, K. C.; Hazari, N.;
Crabtree, R. H.,. Organometallics 2013, 32, 4501-4506.
1
7
Fujii, T.; Saito, Y. Chem. Comm. 1990 757-758.
19

ACCEPTED MANUSCRIPT
•
•
•
Reversibility of catalytic heterocycle hydrogenation demonstrated
Potential application to liquid organic hydrogen carriers
Unusual outer sphere mechanism proposed

ACCEPTED MANUSCRIPT
Supplementary Information: Iridium Catalyzed Reversible
Dehydrogenation – Hydrogenation of Quinoline Derivatives Under
Mild Conditions
Michael G. Manas, Liam S. Sharninghausen, Elisa Lin, Robert H. Crabtree*
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520,
United States
*
robert.crabtree@yale.edu

ACCEPTED MANUSCRIPT
Procedure for gas burette experiments.
i
Reactions were monitored using the same gas burette setup described previously. To a 50 mL
Schlenk tube equipped with a stir bar was added iridium compound (0.021 mml, 0.5 mol%). The flask
was placed under a N atmosphere and chlorobenzene (10 mL) and tetrahydroquinaldine (610 µL, 4.2
2
mmol) were added. For reactions using 1, the iridium compound was pre-activated prior to
tetrahydroquinaldine addition as described previously. The Schlenk was then coupled to a condenser
connected to a 50 mL gas burette. After evacuating and filling the system with argon 3-5 times, the
reaction flask was heated to 115 ˚C. Subsequently, the Schlenk flask was closed to the argon line and the
condenser was opened to the burette using a 3-way stopcock. The water level in the burette was recorded
as a function of time. Gas volume was converted to moles of H using the Van der Waals equation (eq.
2
1
S1). After stopping the reaction, the amount of quinaldine in the reaction mixture was calculated by H
NMR by integration against an internal standard. This was compared with the moles of gas produced
with good agreement (assuming 2 moles of gas produced per mole of quinaldine) (> 85 %). Turnovers
were calculated assuming 2 equivalents of H per turnover.
2
ꢂꢃ
ꢄ
ꢆ
ꢇ
ꢀ
ꢁ
=
+ ꢅ − =24.21
ꢂꢃ
ꢁꢈꢉ
ꢋ
ꢌꢍ
R = 8.3145ꢊm Pa ∙ mol
T = 295 K
p = 101,325 Pa
a = 2.49 ∗ 10ꢌꢍꢎPa ∙ m ∙ mol
ꢋ
ꢌꢏ
ꢌꢐ
ꢋ
ꢌꢍ
b = 26.7 ∗ 10 m ∙ mol
Equation S1. Van der Waals equation used for calculation of H molar volume.
2

ACCEPTED MANUSCRIPT
Table S1. Dehydrogenation of Tetrahydroquinoline Control Reactions
Entry
Solvent
Toluene
Toluene
p-Xylene
p-Xylene
R
Temp. (°C)
115
Time (h)
24
Yield (%)
<1
1
2
3
4
5
Me
H
115
24
<1
Me
H
145
24 /168
24 /168
24
<1 / 2
<1 / 2
<1
145
a
Chlorobenzene Me
135
Conditions: Experiments run under 1 atm N , 1,2,3,4-tetrahydroquinaldine or 1,2,3,4-
2
tetrahydroquinoline (0.112 mmol), solvent (3 ml). Control reactions run in the absence of
all iridium, all other conditions were identical to those reported in Tables 1 and 2. Yield
1
determined by H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal
1
standard. Reaction run at 1.1 M substrate loading.
(
1) Sharninghasen, L.S.; Campos, J.; Manas, M. G.; Crabtree, R. H. Nat. Commun. 2014, 5, 5084.