NHC-Based Iridium Catalysts for Hydrogenation and Dehydrogenation of N-Heteroarenes in Water under Mild Conditions

Article abstract of DOI:10.1021/acscatal.7b03547

We present a set of iridium complexes containing triazolylidene ligands that are highly active for the reduction of quinoline under 5 atm of H₂ pressure and using water as a solvent. This reduction is effective also with a wide variety of quinolines having functionalities at the 2-, 3-, 6-, and 8-positions. One complex is active as well in catalyzing the reverse, viz., the dehydrogenation of tetrahydroquinoline, in high yields and in the same medium without the need of an external hydrogen scavenger. The use of a single catalyst for both hydrogenation and dehydrogenation processes is highly attractive for reversible hydrogen storage in liquid organic hydrogen carriers.

Full text of DOI:10.1021/acscatal.7b03547

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Letter
NHC-based Iridium Catalysts for Hydrogenation and
Dehydrogenation of N-Heteroarenes in Water under Mild Conditions
Ángela Vivancos, matthias Beller, and Martin Albrecht
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03547 • Publication Date (Web): 20 Nov 2017
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ACS Catalysis
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NHC-based Iridium Catalysts for Hydrogenation and Dehydrogena-
tion of N-Heteroarenes in Water under Mild Conditions
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a,b
b
,a
Ángela Vivancos, Matthias Beller and Martin Albrecht*
a
Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
Leibniz-Institut für Katalyse e.V. Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
b
ABSTRACT:
We present a set of iridium complexes containing triazolylidene ligands that are highly active for
the reduction of quinoline under 5 atm of H pressure and using water as a solvent. This reduction
2
is effective also with a wide variety of quinolines having functionalities at the 2-, 3-, 6-, and 8-
positions. One complex is active as well in catalyzing the reverse, viz the dehydrogenation of tet-
rahydroquinoline in high yields and in the same medium without the need of an external hydro-
gen scavenger. The use of a single catalyst for both hydrogenation and dehydrogenation pro-
cesses is highly attractive for reversible hydrogen storage in liquid organic hydrogen carriers.
KEYWORDS: NHC carbenes, iridium, hydrogenation, quinoline, dehydrogenation
for the reverse reaction, viz. the dehydrogenation of tetrahydroquin-
olines in the same solvent, achieving appreciable initial TOFs up to
Catalytic dehydrogenation and hydrogenation reactions of organic
molecules are fundamental and important processes in organic
chemistry. More recently, these reactions attracted also considerable
attention for alternative energy technologies from the viewpoint of
–1
1
8 h .
1
2
hydrogen storage. Some catalysts, both heterogeneous and homo-
3
,4
geneous, can bring about either hydrogenation or dehydrogena-
tion of N-heterocyclic substrates, but there are only very few homo-
geneous catalysts capable of carrying out both reactions under rela-
4
tively mild conditions. A single catalyst for both hydrogenation and
dehydrogenation requires activity in reduction as well as oxidation
catalysis. Accordingly, iridium complexes containing 1,2,3-triazole-
derived carbene ligands are attractive candidates as the triazolyli-
dene iridium platform has been shown to be highly active for several
5
stoichiometric and catalytic bond activation processes, such as
6
7
8
transfer hydrogenation, hydrosilylation, oxidative transformations
9
and water oxidation. However, to the best of our knowledge, tria-
zolylidene iridium complexes have not been investigated as catalysts
for (de)hydrogenation of quinolines as potential liquid organic hy-
drogen carriers (LOHC) with remarkably low dehydrogenation en-
thalpy. Of note, iridium(III) centers ligated by a benzimidazole-de-
rived N-heterocyclic carbene have shown activity in the catalytic hy-
drogenation of quinolines. Subsequent work indicated that these
complexes also catalyze the dehydrogenation of tetrahydroquino-
line, albeit at rather high temperatures (145 °C) and with moderate
Figure 1. NHC-based iridium complexes used in this study.
1
0
The activity of triazolylidene iridium as catalyst precursors for the
hydrogenation of quinoline 5a to form 1,2,3,4-tetrahydroquinoline
1
1
6
a was investigated initially using
–
1
12
activity (turnover frequency (TOF) of 0.8 h ).
Herein we describe the catalytic hydrogenation of quinolines as liq-
uid organic hydrogen carrier in the presence of the known triazole-
9
a,13
based iridium complexes 1–4 (Fig. 1) under 5 atm of H
2
without
the use of any additives. Some of these complexes are also efficient
1
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a
Table 1. Benchmark hydrogenation of quinoline: Variation of the reaction conditions.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Entry
H
2
(bar)
T (°C)
Additive (mol%)
Solvent
Conv. (%)b
Yield (%)b
i
1
2
50
50
50
50
50
50
50
50
50
50
50
50
50
50
35
20
20
10
5
120
90
90
90
90
90
90
90
90
90
90
90
90
70
70
70
90
90
90
90
90
90
-
-
-
PrOH
PrOH
PrOH
PrOH
PrOH
PrOH
PrOH
PrOH
100
48
53
51
51
50
51
31
38
3
10
32
99
98
99
64
100
100
100
15
100
18
100
46
48
49
49
49
49
30
38
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
i
c
i
3
i
4
5
6
7
8
CF
CF
3
3
COOH (7)
COOH (3)
i
i
CH
p-TsOH.H
3
COOH (15)
O (8)
i
2
i
Pyridinium p-toluensulfonate (2)
Pyridinium p-toluensulfonate (7)
i
9
PrOH
10
11
12
-
-
-
-
-
-
-
-
-
-
-
-
-
p-xylene
THF
DCE
8
30
95
97
98
54
98
96
85
15
96
18
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
0
water
water
water
water
water
water
water
water
water
water
1
5
5
d
2
1
2
e
2
a) General reaction conditions: quinoline (0.5 mmol), Ir complex 1a (1 mol%), solvent (2 mL), H
quinoline and yield of 1,2,3,4-tetrahydroquinoline calculated from GC samples using hexadecane as internal standard; c) 2 mol% Ir complex; d)
.5 mol% Ir complex; e) 0.1 mol% Ir complex.
2
(50–1 bar), 120-70 °C, 16 h; b) Conversion of
0
can be as low as 5 bar without compromising the conversion. At am-
the known iridium complex 1a (Table 1). This complex was selected
for the optimization of reaction conditions, as it has previously been
established as an effective catalyst for water oxidation. Initial reac-
tions were performed at 120 °C under high hydrogen gas pressure
(50 bar) using isopropanol as solvent (entry 1).
bient pressure, however the reaction is slow and reaches only 15%
conversion (entries 17–20). When keeping the pressure at 5 bar H2,
lowering of the catalyst loading to 0.5 mol% is feasible and increases
the turnover number to appreciable 200 (entry 21). Further lower-
ing of the catalyst loading provides similar turnover numbers and in-
complete conversion (entry 22), possibly indicating the current
limit of this catalytic system.
Having established optimized reaction conditions, catalytic quino-
line reduction was performed with different triazolylidene iridium
complexes, including neutral, mono-, and dicationic complexes con-
taining C,N-bidentate chelate motifs (1a, 1b) and C,C-bidentate
bonding triazolylidenes resulting from cyclometalation (complexes
or from bis(carbene) bonding (complex 4, see Fig. 1).
When the reaction is carried out with the di-cationic iridium com-
9
a
i
With 1 mol% of complex 1a (120 °C, PrOH), complete transfor-
mation into the hydrogenated product 1,2,3,4-tetrahydroquinoline
was observed after 16 h (entry 1). When decreasing the temperature
to 90 °C the conversion decreased to 48% (entry 2), and yields did
not improve even when increasing the catalyst loading to 2 mol%
9
a
(
entry 3). Likewise, the introduction of acetic acid, trifluoroacetic
acid, p-toluensulfonic acid or pyridinium p-toluensulfonate as addi-
tives did not increase the desired yields (entries 4–9). However, the
catalyst activity is strongly solvent-dependent, and conversions were
excellent in water (entry 13), whilst almost nil conversion was
achieved in p-xylene or THF (entries 10, 11) and just 32% was ob-
1
3a,b
13c
2, 3)
14
plex 1b as catalyst precursor, the yield is comparable to that obtained
1
5
with complex 1a (Table 2, entry 2 vs. 5). The essentially identical
performance of the two catalytic systems is in agreement with a rapid
solvolysis of the iridium chloride complex 1a when dissolved in
aqueous media, hence interconverting this complex to 1b (albeit
with a different counter ion). The neutral complex 2 containing an
anionic phenyl group as chelating ligand instead of a pyridyl frag-
ment in 1 induces only very low conversions when reactions are per-
formed in water (entry 6; cf entry 15 in Table 1), though switching
tained in 1,2-dichloroethane (entry 12). Further modification of
conditions involved variation of temperature and H
water as a solvent. The reaction temperature is critical and while full
conversion was reached even at 70 °C, a H pressure of 35 bar is re-
quired. At 20 bar H , the reaction was incomplete (entries 14–16).
When the reaction is performed at 90 °C, however, the H pressure
2
pressure using
1
6
2
2
2
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i
the solvent to PrOH enhanced yields considerably (Table 2, entries
7,8).
Table 2. Influence of ligand variation on the iridium-cata-
lyzed hydrogenation of quinoline.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
a
Full conversions were reached also when decreasing the catalyst
loading to 0.5% (entry 9). An appreciable 56% conversion of quino-
line was achieved even when the reaction was performed under just
1 atm of H
2
(entry 10), which is considerably higher than the 15%
conversion obtained with the monocationic complex 1a under the
1
7
same conditions. Cyclometalated complex 3 is markedly less active
than complexes 1 and 2 which is demonstrated in catalytic runs per-
formed at 70 °C (38% vs. > 99%; entries 1,7,11). With this complex,
the solvent had no critical influence (entries 11,12). As observed for
complex 1a, the cationic biscarbene complex 4 shows higher activity
H
2
T
Conv.
(%)b
99
100
15
100
96
6
100
100
100
56
38
32
Yield
(%)b
98
85
15
96
90
6
100
96
100
56
34
31
90
47
67
88
Entry Catalyst
Solvent
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(bar) (°C)
35
5
1
5
1
2
3
1a
1a
1a
1a
1b
2
2
2
2
70
90
90
90
90
70
70
90
90
90
70
70
90
70
70
90
water
water
water
water
i
in water than in PrOH (74% vs. 50% at 70 °C; entries 14,15), though
c
4
in both solvents, either complex 1 or 2 are more efficient.
i
5
6
7
8
5
PrOH
35
35
5
5
1
35
35
5
35
35
10
water
PrOH
PrOH
PrOH
PrOH
PrOH
water
PrOH
PrOH
water
water
The facile dissociation of the ancillary chloride ligand in complex 1a
offers some hint on the mechanism of the reaction. Ligand dissocia-
tion provides access to a free site in the iridium coordination sphere
for coordination of the N-heterocyclic substrate under catalytic con-
i
i
c
i
9
i
i
1
0
2
3
3
3
ditions. Indeed, when complex 1a was dissolved in D O in the pres-
2
11
12
13
ence of an excess of quinoline, formation of the N-adduct was easily
identified by the appearance of a new set of signals with a downfield
shifted doublet at 9.3 ppm as most prominent feature (Fig. S1).
i
i
98
50
74
97
1
1
1
4
5
6
4
4
4
When this sample was charged with 1 atm of H
changes were noticed by NMR spectroscopy. However, when the
-saturated mixture was heated to 90 °C, a new iridium species ap-
peared. After continued heating at this temperature for 18 h, the ma-
jor component was a new iridium species 1a–d with the ligand se-
2
, no significant
H
2
a) General reaction conditions: quinoline (0.5 mmol), Ir complex (1
mol%), solvent (2 mL), H (1–50 bar), 70–120 °C, 16 h; b) Conver-
1
2
lectively deuterated at the pyridyl C(6) position (Scheme 1). Deu-
sion of quinoline and yield of 1,2,3,4-tetrahydroquinoline calculated
from GC samples using hexadecane as internal standard; c) 0.5 mol%
Ir complex.
1
teration was indicated in the H NMR spectrum by the disappear-
ance of the doublet at 8.14 ppm, and the concomitant simplification
of the coupling pattern for the C(5)-H and C(4)-H protons, both
from ddd to a doublet of doublets (Fig. S2). No such deuteration
process was observed when complex 1a was exposed to the same
Scheme 1. Deuteration of iridium complexes 1a and 2, and proposed
intermediate in the rollover process leading to 1a–d .
1
conditions in the absence of H
base (1.5 mol equiv. DBU). These results therefore suggest the for-
mation of hydride species in which H/D exchange with the D O sol-
vent takes place. Deuteration of the pyridyl C(6) position can then
2
, even when heated in presence of a
In line with this model, exposure of a solution of complex 2 in iso-
propanol–d :D O (5:1 v/v) in presence of quinoline and H (1 bar)
afforded after 20 h complex 2–d , in which all ortho positons are deu-
2
8
2
2
1
8
3
1
9
occur through a reversible roll-over cyclometalation, which transi-
ently forms a 5-membered iridacycle with a C,C-bidentate coordi-
nating triazolylidene-pyridyl ligand. The reversibility of this roll-
over process accounts for the isotope scrambling and formation of
terated (Scheme 1; Fig. S3). As noted for complex 1a, deuteration is
evidenced by the disappearance of the doublet of doublet resonance
at 7.65 ppm, attributed to C(6)-H, as well as by the change of multi-
plicity of the signals for the coupled protons. Remarkably, isotope
exchange also occurred on the pendant phenyl ring, which suggests
transient metalation of either of the two aromatic rings during the
reaction. In the ground state, however, cyclometalation exclusively
involves the Ntrz-bound phenyl ring. Such reversible ligand cy-
clometalation may be key for the observed activity of these com-
plexes in quinoline hydrogenation.
1a-d
1
, and it rationalizes the selective deuteration of the ortho posi-
tion with respect to the triazolylidene linkage (Scheme S1).
+
+
+
H
2
presumably
via:
Ir
Ir
Ir
D
Cl
D O
Cl
N
2
N
N
N
N
N
N
N
N
N
N
N
D
With complexes 1a and 2 as the most active hydrogenation catalysts,
the scope of quinoline hydrogenation was investigated under opti-
mized conditions. The results are summarized in Table 3 and
demonstrate the efficient conversion of a variety of quinolines, in-
cluding substitutions at 2-, 3-, 4-, 6- and 8- positions as in 5b–f to
afford the corresponding 1,2,3,4-tetrahydroquinoline products (Ta-
ble 3, entries 1–6). Excellent yields were obtained, with the excep-
1a
1a–d1
+
+
H
2
Ir
Ir
Cl
iPrOD–d
Cl
8
D
2
D O
N
N
N
N
N
N
D
D
2
2–d3
4
a,20
tion of the 4-Me derivative. With that substrate, almost no reac-
tion was observed with complex 1a, while complex 2 achieved some
3
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Table 3. Hydrogenation of quinolines catalyzed by complexes the best results were achieved when the reaction was performed in
a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1a and 2.
water at reflux temperature, leading to 90% conversion after 20 h
(entry 6). This result implies that complex 1a provides a catalytic
system that is able to do both, the hydrogenation and the opposite
reaction in one and the same solvent, and that the course of the re-
action is only dependent on the partial pressure of the reaction me-
dium.
Conversion was nil after 20 h when complex 2 was used for the de-
hydrogenation reaction using the same conditions as applied for the
reverse quinoline hydrogenation, viz. PrOH at 90 °C (entry 7), pre-
Conversion
Isolated
Yield (%)
96 / 100b
85 d
i
b
Entry
Substrate
(%)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
sumably because this solvent adversely affects the dehydrogenation
equilibrium. With 2 mol% of catalyst loading and using 1,2-dichloro-
methane, 64% conversion was achieved (entry 8), which is lower
than the yield obtained with complex 1a (cf entry 4). Most efficient
dehydrogenation with complex 2 took place when the reaction was
performed in water (entries 9–11), with good yields even in the pres-
ence of as little as 0.5 mol% of catalyst. Time-dependent monitoring
1
a / 2
1
2
3
4
5
6
7
8
9
5a (R = H)
100 / 100
100 / 100
100 / 100
7 / 51
5b (R = 2-Me)
5c (R = 3-Me)
5d (R = 4-Me)
5e (R = 6-Me)
5f (R = 8-Me)
5g (R = 6-Cl)
96 d
-
100 / 100
100 / 100
100 / 100
2 / 72
86 d
85 d
98 e
–
1
of the conversion indicates turnover frequencies up to 20 h for the
quinoline dehydrogenation (Fig. S4), which is more than one order
of magnitude larger than related iridium complexes featuring a less
electron-donating and monodentate coordinating benzimidazolyli-
5h (R = 6-NO
2
)
-
c
87 e
–1 4a,12
5i (R = 6-COOH)
nd / 100
dene ligand or a pyridinol system (TOF ca. 1 h ), or a cobalt sys-
–
1
4c
1
0
5j (R = 2-Me, 6-OMe)
40 / 46
-
tem (TOF ca. 0.1 h ). These values emphasize the high capacity of
the triazolylidene iridium platform for mediating bond activation
a) Reaction conditions: substrate (0.5 mmol), catalyst (0.5 mol%), sol-
vent (2 mL), H (5 bar), 90 °C, 16 h; solvent = water for reactions with
2
i
Table 4. Dehydrogenation of tetrahydroquinoline catalyzed by
complexes 1a or 2.
complex 1, and PrOH for complex 2; b) conversion and yields calcu-
lated from GC samples using hexadecane as internal standard, isolated
yields only determined for near-quantitative conversion; c) nd = not de-
termined, due to substantial formation of side products; d) isolated
yields from runs with complex 1a; e) isolated yields from runs with com-
plex 2.
a
[
Ir] (1 mol%)
solvent (2 mL)
20 h
N
N
H
6a
5a
5
1% conversion. Tentatively, we suggest that the C,N-chelating lig-
T
(°C)
95
Conv.
(%)b
17
Yield
(%)b
15
and in complex 1 prevents coordination of the C3–C4 double bond,
while the Ir–CPh bond may easily be hydrogenated during catalyst
activation to yield a monodentate carbene iridium complex as active
Entry
[Ir]
Solvent
1
2
1a
1a
1a
1a
1a
1a
2
2,2,2-TFE
p-xylene
1,2-DCB
1,2-DCB
Water
150
160
160
100
100
90
7
6
species. Functional groups such as –Cl, –NO and –COOH substit-
2
uents (entries 7–9) are also tolerated, and the resulting hydrogen-
ated heterocycles were obtained in good to excellent yields when cat-
alyst 2 was used. Complex 1 was narrower in substrate scope and did
not induce significant conversion with nitro and carboxylate func-
tional groups. Only moderate performance was observed with both
complexes 1 and 2 in the hydrogenation of the di-substituted quin-
oline 6-methoxy quinaldine 5j (entry 10).
3
4c
48
82
69
90
0
40
67
68
83
0
5
6c
Water
iPrOH
7
8c
9c
2
1,2-DCB
water
160
100
100
100
64
99
85
66
53
96
80
64
2
10
11d
2
water
Having established the effectiveness of complexes 1a and 2 for quin-
oline hydrogenation, we explored the potential of these complexes
as catalyst precursors for the reverse reaction, i.e. the dehydrogena-
tion of saturated N-heterocyclic substrates. In a first set of experi-
ments, complex 1a was used for the oxidation of 1,2,3,4-tetrahydro-
quinoline under different reaction conditions. The results indicate
again a strong solvent dependence as was observed for the hydro-
genation reaction. While conversions were low when p-xylene or
2,2,2-trifluoroethanol were used as solvents (Table 4, entries 1, 2), a
moderate 50% conversion of 1,2,3,4-tetrahydroquinoline was ob
tained when the reaction was performed in 1,2-dichlorobenzene at
160 °C (entry 3), yet conversions were almost complete when the
catalyst loading was raised from 1 to 2 mol% (entry 4). Interestingly,
2
water
a) Reaction conditions: substrate (0.5 mmol), catalyst (1 mol%), sol-
vent (2 mL), 20 h (TFE = trifluoroethanol, DCB = dichlorobenzene);
b) Conversion and yield calculated by GC using hexadecane as internal
standard; c) 2 mol% of catalyst; d) 0.5 mol% of catalyst.
processes.
In conclusion, we have developed a homogeneous system consist-
ing of a triazolylidene-based iridium complex for efficient hydro-
genation and dehydrogenation of quinoline derivatives without the
use of any additive. The reversible catalytic transformation takes
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place favorably in aqueous media and swapping between hydrogena-
T.; Xu, L.; He, Y.; Fan, Q.-H.; Pan, J.; Gu, L.; Chan, A. S. C. Angew. Chem.
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tion and dehydrogenation processes exclusively depends on the
presence or absence of dihydrogen gas. Such a simple and reversibly
operating catalytic system holds great promise for reversible hydro-
gen storage in N-heterocyclic compounds. Investigations on further
improving the catalyst performance for the reversible dehydrogena-
tion-hydrogenation reaction of quinolines as well as other liquid or-
ganic hydrogen carriers containing a larger hydrogen storage capac-
Int. Ed. 2008, 47, 8464–8467. (c) Manaeka, Y.; Suenobu, T.; Fukuzumi, S. J.
Am. Chem. Soc. 2011, 134, 367–374. (d) Wu, J.; Talwar, D.; Johnston, S.;
Yan, M.; Xiao, J. Angew. Chem. Int. Ed. 2013, 52, 6983–6987. (e) Wu, J.; Bar-
nard, J. H.; Zhang, Y.; Talwar, D.; Robertson, C. M.; Xiao, J. Chem. Commun.
2
013, 49, 7052–7054. (f) Cai, X.-F.; Huang, W.-X.; Chen, Z.-P.; Zhou, Y.-
G. Chem. Commun. 2014, 50, 9588–9590. (g) Wendlandt, A. E.; Stahl, S. S.
J. Am. Chem. Soc. 2014, 136, 11910–11913. (h) Fujita, K.; Tanaka, Y.;
Kobayashi, M.; Yamaguchi, R. J. Am. Chem. Soc. 2014, 136, 4829–4832. (i)
Adam, R.; Cabrero-Antonino, J. R.; Spannenberg, A.; Junge, K.; Jackstell, R.;
Beller, M. Angew. Chem. Int. Ed. 2017, 56, 3216–3220.
1
0
ity as one of the many requirements for efficient LOHCs are cur-
rently in progress.
(
4) (a) Yamaguchi, R.; Ikeda, C.; Takahashi, T.; Fujita, K. J. Am. Chem.
0
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9
0
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5
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7
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Soc. 2009, 131, 8410–8412. (b) Chakraborty, S.; Brennessel, W. W.; Jones,
W. D. J. Am. Chem. Soc. 2014, 136, 8564–8565. (c) Xu, R.; Chakraborty, S.;
Yuan, H.; Jones, W. D. ACS Cat. 2015, 5, 6350–6354.
ASSOCIATED CONTENT
Supporting Information. NMR spectra from deuteration and reaction
intermediates, time-conversion profile for dehydrogenation, NMR
spectra of products, and experimental procedures. This material is avail-
able free of charge on the ACS Publication website at DOI:
(
5) (a) Lalrempuia, R.; Müller-Bunz, H.; Albrecht, M. Angew. Chem. Int.
Ed. 2011, 50, 9969–9972. (b) Donnelly, K. F.; Lalrempuia, R.; Müller-Bunz,
H.; Clot, E.; Albrecht, M. Organometallics 2015, 34, 858−869.
(6) Bolje, A.; Hohloch, S.; Van der Meer, M.; Kosmrlj, J.; Sarkar, B. Chem.
Eur. J. 2015, 21, 6756–6764.
(7) Petronilho, A.; Woods, J. A.; Müller-Bunz, H.; Bernhard, S.; Albrecht,
M. Chem. Eur. J. 2014, 20, 15775–15784.
1
0.1021/xxxxx.
AUTHOR INFORMATION
Corresponding Author
(
8) Valencia, M.; Pereira, A.; Müller-Bunz, H.; Belderraín, T.; Pérez, P. J.;
Albrecht, M. Chem. Eur. J. 2017, 23, 8901–8911.
(9) (a) Woods, J. A.; Lalrempuia, R.; Petronilho, A.; McDaniel, N. D.;
Müller-Bunz, H.; Albrecht, M.; Bernhard, S. Energy Environ. Sci. 2014, 7,
*
E-mail: martin.albrecht@dcb.unibe.ch
ORCID
2
316–2328. (b) Lalrempuia, R.; McDaniel, N. D.; Müller-Bunz, H.; Bern-
hard, S.; Albrecht, M. Angew. Chem. Int. Ed. 2010, 49, 9765–9768.
10) (a) Preuster, P.; Papp, C.; Wasserscheid, P. Acc. Chem. Res. 2017,
0, 74–85. (b) Graetz, J.; Wolstenholme, D.; Pez, G.; Klebanoff, L.;
Angela Vivancos: 0000-0001-9375-8002
Matthias Beller: 0000-0001-5709-0965
Martin Albrecht: 0000-0001-7403-2329
(
5
McGrady, S.; Cooper, A. in Hydrogen Storage Technology (Klebanoff, L. ed.),
CRC press, 2013.
(11) 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.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
(
12) Manas, M. G.; Sharninghausen, L. S.; Lin, E.; Crabtree, R. H. J. Or-
We acknowledge generous financial support from the European Re-
search Council (CoG 615653), from the Swiss National Science Foun-
dation (200021_162868), and from COST Action CM1205 for funding
a STSM research visit to A.V. (ECOST-STSM-CM1205-260915-
ganomet. Chem. 2015, 792, 184–189.
(13) (a) Donnelly, K. F.; Lalrempuia, R.; Müller-Bunz, H.; Albrecht, M.
Organometallics 2012, 31, 8414−8419. (b) See Supporting Information for
the synthesis of complex 3 and ref. 9b for related complexes. (c) Vivancos,
A.; Albrecht, M. Organometallics 2017, 36, 1580−1590.
(14) Levin, E.; Ivry, E.; Diesendruck, C. E.; Lemcoff, N. G. Chem. Rev.
2015, 115, 4607−4692.
0
62826). We thank K. F. Donnelly for the synthesis of complex 3.
(
15) The reaction with catalyst 1b has not been tried in H O, as solvolysis
2
REFERENCES
of complex 1a in water is fast and produces in fact 1b. For reactions in aque-
ous phase, it therefore is irrelevant whether 1a or 1b is the starting material
as both compounds produce the di-cationic aqua compound 1b.
(
1) (a) Crabtree, R. H. Energy Environ. Sci. 2008, 1, 134–138. (b) Eberle,
U.; Felderhoff, M.; Schüth, F. Angew. Chem. Int. Ed. 2009, 48, 6608–6630;
Angew. Chem. 2009, 121, 6732–6757. (c) Teichmann, D.; Arlt, W.; Wasser-
scheid, P.; Freymann, R. Energy Environ. Sci. 2011, 4, 2767–2773.
(
16) Solvolysis of this complex has been established, see: Petronilho, A.;
Llobet, A.; Albrecht, M. Inorg. Chem. 2014, 53, 12896–12901.
(
2) (a) Mikami, K.; Ebata, K.; Mistudome, T.; Mizugaki, T.; Jitsukawa,
(17) When the reaction is performed under the same conditions without
K.; Kaneda, K. Heterocycles 2011, 82, 1371–1377. (b) Ren, D.; He, L.; Yu,
L.; Ding, R.-S.; Liu, Y.-M.; Cao, Y.; He, H.-Y.; Fan, K.-N. J. Am. Chem. Soc.
H
2
gas no conversion is observed, discharging the possibility of being trans-
ferring hydrogen from the solvent.
18) (a) Cooper, A. C.; Eisenstein, O.; Gaulton, K. G. New J. Chem. 1998,
07−309. (b) Maseras, F.; Lledos, A.; Clot, E.; Eisenstein, O. Chem. Rev.
000, 100, 601–636.
19) (a) Butschke, B.; Schwarz, H. Chem. Sci. 2012, 3, 308−326. (b)
2
012, 134, 17592–17598. (c) Dell’Anna, M. M.; Capodiferro, V. F.; Mali,
M.; Manno, D.; Cotugno, P.; Monopoli, A.; Mastrorilli, P. Appl. Catal. A,
014, 481, 89–95. (d) Zhu, D.; Jiang, H.; Zhang, L.; Zheng, X.; Fu, H.; Yuan,
(
3
2
2
M.; Chen, H.; Li, R. ChemCatChem 2014, 6, 2954–2960. (e) Zhang, Y.; Zhu,
J.; Xia, Y.-T.; Sun, X.-T.; Wu, L. Adv. Synth. Catal. 2016, 358, 3039–3045.
(
Zucca, A.; Maidich, L.; Canu, L.; Petretto, G. L.; Stoccoro, S.; Cinellu, M. A.;
Clarkson, G. J.; Rourke, J. P. Chem. Eur. J. 2014, 20, 5501−5510.
(20) 4-quinolines are known as a challenging substrate: Chen, F.; Surkus,
A.-E.; He, L.; Pohl, M.-M.; Radnik, J.; Topf, C.; Junge, K.; Beller, M. J. Am.
Chem. Soc. 2015, 137, 11718−11724.
(
2
f) Tang, M.; Deng, J.; Li, M.; Li, X.; Li, H.; Chen, Z.; Wang, Y. Green Chem.
016, 18, 6082–6090. (g) Cui, X.; Li, Y.; Bachmann, S.; Scalone, M.; Surkus,
A.-E.; Junge, K.; Topf, C.; Beller, M. J. Am. Chem. Soc. 2015, 137, 10652–
1
2
0658. (h) Sun, X.-T.; Zhu, Y.; Xia, Y.-T.; Wu, L. ChemCatChem 2017, 9,
463–2466.
(
3) (a) Wang, W. B.; Lu, S. M.; Yang, P. Y.; Han, X. W.; Zhou, Y. G. J. Am.
Chem. Soc. 2003, 125, 10536–10537. (b) Zhou, H.; Li, Z.; Wang, Z.; Wang,
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De)Hydrogenation: Triazolylidene iridium complexes are efficient catalysts for both the hydrogenation of quinolines to tetrahydro-
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quinolines and for the oxidation of the hydrogenated products. These reactions take place in aqueous media under mild conditions.
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