A molecular iron catalyst for the acceptorless dehydrogenation and hydrogenation of N-heterocycles

Article abstract of DOI:10.1021/ja504523b

A well-defined iron complex (3) supported by a bis(phosphino)amine pincer ligand efficiently catalyzes both acceptorless dehydrogenation and hydrogenation of N-heterocycles. The products from these reactions are isolated in good yields. Complex 3, the active catalytic species in the dehydrogenation reaction, is independently synthesized and characterized, and its structure is confirmed by X-ray crystallography. A trans-dihydride intermediate (4) is proposed to be involved in the hydrogenation reaction, and its existence is verified by NMR and trapping experiments.

Full text of DOI:10.1021/ja504523b

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Communication
A Molecular Iron Catalyst for the Acceptorless
Dehydrogenation and Hydrogenation of N-Heterocycles
Sumit Chakraborty, William W. Brennessel, and William D. Jones
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja504523b • Publication Date (Web): 30 May 2014
Downloaded from http://pubs.acs.org on June 4, 2014
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A Molecular Iron Catalyst for the Acceptorless Dehydrogenation and
Hydrogenation of N-Heterocycles
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Sumit Chakraborty, William W. Brennessel, and William D. Jones*
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
Supporting Information Placeholder
donors) for perdehydrogenation and perhydrogenation of fused
bicyclic N-heterocycles as well.6
ABSTRACT: A well-defined iron complex (3) supported by a
bis(phosphino)amine pincer ligand efficiently catalyzes both ac-
ceptorless dehydrogenation and hydrogenation of N-heterocycles.
The products from these reactions are isolated in good yields.
Complex 3, the active catalytic species in the dehydrogenation
reaction, is independently synthesized, characterized, and its
structure is confirmed by X-ray crystallography. A trans-
dihydride intermediate (4) is proposed to be involved in the hy-
drogenation reaction and its existence is verified by NMR and
trapping experiments.
Despite the recent progress with these iridium-based catalysts,
development of inexpensive, earth-abundant metal catalysts for
the dehydrogenation and hydrogenation of N-heterocycles is high-
ly desirable. In this context, iron fits well because of its low cost,
low toxicity, high abundance, and isoelectronic (d6 metal) nature
with the aforementioned Ir(III) catalysts. Additionally, iron-based
catalysts have been applied in dehydrogenation and hydrogenation
reactions in recent years.7 Therefore, we sought to carry out dehy-
drogenation of N-heterocycles with molecular iron catalysts. Re-
cently, the Beller8, Schneider/Hazari,9 and Guan10 groups have
independently reported bifunctional iron pincer complexes (1, 2,
and
related
chloride
derivatives)
supported
by
cceptorless catalytic dehydrogenation and hydrogenation
bis(phosphino)amine (PNP) ligands (Figure 1). Beller has demon-
strated base-assisted catalytic dehydrogenation of methanol with
these iron compounds.8 Remarkably, ppm levels of catalyst load-
ing could be employed and catalysis could be performed at a rela-
tively high temperature indicating the thermal stability of these
iron complexes and related intermediates. On the other hand,
Guan and coworkers carried out hydrogenation of unactivated
esters with the same iron catalysts.10 Encouraged by the dehydro-
genation and hydrogenation activities shown by these iron com-
plexes, we planned to explore their potential in catalytic dehydro-
genation and hydrogenation of N-heterocycles. Herein, we report
the acceptorless dehydrogenation and hydrogenation of N-
heterocycles with a single molecular iron catalyst.
of N-heterocycles are fundamentally important organic
A
transformations. In addition, these reactions find potential
applications in the field of organic hydrogen storage materials.1
Although the dehydrogenation of N-heterocycles is a thermody-
namically uphill process, experiments and computational studies
have shown that the presence of the N-atom decreases the endo-
thermicity of the reaction when compared to cycloalkanes.2
A
large positive entropic contribution comes from releasing H2 dur-
ing the reaction.2c So far, heterogeneous catalysts have mostly
been used for the dehydrogenation of N-heterocycles.3 However,
most of these heterogeneous systems require harsh conditions and
show poor functional-group tolerance. In addition, rational tuning
of catalysts is challenging because of the lack of mechanistic un-
derstanding.
BH3
On the other hand, well-defined homogeneous catalysts for
the acceptorless dehydrogenation of N-heterocycles are rare in the
literature and advances have been primarily made with iridium-
based precious metal catalysts.4,5 Fujita and Yamaguchi first re-
ported a [Cp*Ir(2-hydroxypyridine)] catalyst (Cp*: 1,2,3,4,5-
pentamethyl cyclopentadienyl) capable of dehydrogenating tetra-
hydroquinoline derivatives in refluxing p-xylene.4a DFT studies
have supported a ligand-promoted hydrogen abstraction from the
substrate without changing the oxidation state of the metal.4c No-
ticeably, the same iridium complex can also serve as a catalyst for
the hydrogenation of quinaldine, although a [Cp*Ir(Cl)(µ-H)]2
complex has been shown to be the active intermediate. Xiao and
coworkers have developed cyclometalated [Cp*IrCl(imino)] com-
plexes as efficient dehydrogenation-hydrogenation catalysts for a
broad range of N-heterocycles.4e Catalysis was performed with a
low catalyst loading (0.1 mol%) in refluxing 2,2,2-
H
Br
P(iPr)2
CO
P(iPr)2
P(iPr)2
H
H
N
Fe
N
Fe CO
H
N
Fe CO
H
H
P
P
P
(iPr)2
(iPr)2
(iPr)2
3
1
2
Figure 1. Fe-catalysts employed in the current work.
We initially tested the catalytic activity of complex 1 and 2
with a model substrate 1,2,3,4-tetrahydroquinaldine (C) (Table 1).
When the dehydrogenation of C was carried out with 5 mol% of 1
in refluxing toluene while purging the solution slowly with nitro-
gen, only 10% conversion to the desired product was observed in
24 hours (entry 1). Complex 2 in the presence of KOtBu (20
mol%) afforded only 1% conversion after a day in THF (entry 2).
o
o
However, gratifyingly, when xylene (avg. bp 138.5 C) was used
trifluoroethanol (TFE, bp 78 C). TFE appeared to play multiple
instead of toluene or THF, 82% conversion of the starting material
was observed with catalyst 1 (5 mol%) after 24 hours (entry 3).11
Moreover, under the same catalytic conditions, quantitative con-
roles in this reaction that include promoting halide dissociation
and facilitating the H2-release step. Recently, Fujita et al. have
reported Cp*(L)Ir(bipyridonate) catalysts (where L: neutral σ-
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Page 2 of 5
version was achieved in 30 hours with an even lower catalyst
firmed the formation of H2 from the dehydrogenation reaction. In
1
2
3
4
5
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7
8
loading (3 mol% of 1, entry 4). An attempt to further decrease the
amount of catalyst (1 mol%) led to an incomplete reaction even
after a longer reaction time (entry 5). Control experiments without
catalysts did not yield any product. Furthermore, the homogene-
ous nature of this system was indicated by performing a mercury-
poisoning experiment12 and an experiment with added PNP lig-
and.13
order to test the catalytic activity of the remaining iron species
after the first catalytic run, a successive addition experiment was
performed with a second batch of substrate C. Unfortunately,
much lower conversion (~42% by NMR) was observed this time
suggesting a significant amount of catalyst degradation after the
first run. To test the thermal stability of the iron catalyst, two
separate protio-xylene solutions of 1 were heated at 140 oC in the
absence and presence of C (33 equivalents with respect to 1) and
the reaction was monitored by 31P NMR spectroscopy. Interest-
ingly, for the reaction in the presence of C, a new pincer iron
complex (3, located at 110.2 ppm) was found to be the major
remaining species after 20 hours, whereas the one without any
substrate led to complete decomposition within four hours (see
Supporting Information). These results suggest that the thermal
decomposition of 1 is greatly suppressed when the substrate is
present; but the same iron complex degrades much more rapidly
in the absence of any substrate. This accounts for the much lower
catalytic activity observed when the substrate was introduced for
the second time.
Table 1. Catalytic activities of iron complexes (1 and 2) for
the dehydrogenation of 1,2,3,4-tetrahydroquinaldine
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entry catalyst (loading) solvent time (h) conversion (%)
1
2a
3
1 (5 mol%)
2 (5 mol%)
1 (5 mol%)
1 (3 mol%)
1 (1 mol%)
toluene
THF
24
24
24
30
48
10
1
xylene
xylene
xylene
82
100
45
4
5
a KOtBu (20 mol%) was added as an additive.
Interestingly, when the dehydrogenation of C was performed
in the presence of external H2 (5 atm), a complete loss of catalytic
activity was observed. This result suggested to us that perhaps the
hydrogenation of unsaturated N-heterocycles15 could also be per-
formed utilizing the same iron catalysts. Gratifyingly, in the pres-
ence of 2 (3 mol%) and KOtBu (10 mol%), quinaldine (K) was
quantitatively converted to the reduced product, 1,2,3,4-
tetrahydroquinaldine (C), with only 5 atm of H2 pressure in 24
hours. THF was used as the solvent in this case and the reaction
was performed at 80 oC. The same reaction was also catalyzed by
Having established the optimized reaction conditions for the
catalytic dehydrogenation of C, we explored the substrate scope
for this system using 3 mol% of 1 in refluxing xylene. As shown
in Figure 2, several N-heterocyclic compounds (A-H) were dehy-
drogenated successfully under this oxidant-free condition and the
corresponding products were isolated in decent yields (58-91%).
For all of the substrates, fully dehydrogenated products were iso-
lated after the reaction. Partially dehydrogenated products were
not observed. Positions of the –CH3 groups in the N-heterocycles
(C, E, and F) did not seem to be critical as all of these substrates
gave quantitative conversions after 30 hours. Apart from the tet-
rahydroquinoline derivatives, 2-methylindoline (G) and 2,6-
dimethylpiperidine (H) were also successfully dehydrogenated.
For substrate H, complete dehydrogenation led to the formation of
the corresponding pyridine derivative. Remarkably, formation of
quinoline or pyridine derivatives, potential ligands for iron, did
not hamper the catalysis.
o
1 (3 mol%) in toluene at 110 C; however a lower conversion
(89%) was achieved after the same reaction time. Under the best
catalytic conditions, substrates I-O were successfully hydrogenat-
ed (Figure 3) under 5-10 atm of H2 pressure and the desired re-
duced products were obtained with good isolated yields (60-92%).
Although most of the substrates (I-M) could be reduced with 5
atm of H2, substrates N and O required higher hydrogen pressure
to fully convert. No products resulting from the over reduction of
the aromatic ring was observed.
1
2 (3 mol%)
(3 mol%)
KOtBu (10 mol%)
R1
R1
R2
R2
+
R1
R2
R1
R2
2 H2
5-10 atm
2 H2
+
reflux, 30 h
xylene
80 oC, 24 h
THF
N
N
N
H
N
H
isolated yields
isolated yields
NH
N
H
Ph
N
H
N
N
N
N
N
H
B(72%)
J(66%)
K(92%)
L(71%)
I(77%)
A(69%)
D(83%)
C(91%)
H
N
H
N
N
N
N
N
H
N
H
H
O(60%)
M(85%)
N(69%)
F(89%)
G(86%)
E(78%)
H(58%)
Figure 3. Hydrogenation of Unsaturated N-Heterocycles. Condi-
tions: 2 (25 μmol), KOtBu (83 μmol), substrate (0.83 mmol), THF
(2 mL). H2 pressure: 5 atm (for I-M), 10 atm (for N and O).
Numbers in the parentheses represent isolated yields of the hydro-
genated N-heterocycles.
Figure 2. Acceptorless Catalytic Dehydrogenation of N-
Heterocycles. Conditions: 1 (25 μmol), N-heterocyclic substrate
o
(0.83 mmol), xylene (1 mL), refluxed at 140 C. Numbers in the
parentheses represent isolated yields of the dehydrogenated N-
heterocycles.
A proposed catalytic pathway for the dehydrogenation of N-
heterocycles is outlined in Scheme 1. A penta-coordinated iron
hydride species 3 is postulated to be the active catalytic interme-
diate in this reaction.16 To verify its identity and involvement in
the catalysis, complex 3 was independently synthesized by a reac-
tion of 2 with KOtBu (eq 1). This complex has been completely
characterized by NMR, IR spectroscopy, elemental analysis, and
X-ray crystallography (see Figure 3). Complex 3 adopts a pseudo-
In order to confirm the identity of the evolved gas from the
dehydrogenation reaction, it was introduced into a separate reac-
tion vessel containing cyclooctene, 5 mol% of RhCl(PPh3)3, and 3
o
mL of THF.14 The later reaction was performed at 60 C. Analy-
sis of the product from the later reaction indicated a clean for-
mation of cyclooctane (89% conversion by GC), demonstrating in
turn H2 being produced from the dehydrogenation reaction. In
addition, the head-space gas analysis by GC unambiguously con-
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Journal of the American Chemical Society
trigonalbipyramidal geometry in the solid state. Most noticeably,
to occur in this system. No dehydrogenation activity with 1,2-
1
2
3
the Fe-N bond distance in 3 was found to be significantly shorter
(1.86 Å) than other related octahedral iron complexes8-10 (for ex-
ample, 2.07 Å for 18).
dihydronaphthalene indicates that having a relatively weak allylic
C-H bond (~85 kcal/mol)18 is not sufficient for the dehydrogena-
tion with our iron catalysts.
However, when 3,4-dihydroisoquinoline was subjected to ca-
talysis, the fully dehydrogenated product, isoquinoline (J), was
formed as the sole product (eq 5). This result demonstrates that
the presence of the N-atom in cycloalkanes is critical for catalysis
and also points to the second scenario involving a sequential
isomerization-amine dehydrogenation process.
4
5
6
7
8
9
Br
P(iPr)2
CO
P(iPr)2
H
rt, 30 min
THF
KOtBu
1.1 equiv
N
Fe
(1)
N
Fe CO
+
H
P
P
H
(iPr)2
(iPr)2
3 (70%, isolated)
X-ray structure
2
H
1 or 3
(3 mol%)
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N
N
(2)
Scheme 1. Proposed Catalytic Pathway for the Dehydro-
genation of N-Heterocycles.
reflux, 30 h
N
H
xylene,
N
2 H2
100% conversion
P(iPr)2
CO
1 or 3
(3 mol%)
N
Fe
(3)
+
H
reflux, 30 h
xylene,
H2
P
(iPr)2
3
N
H
H
not formed
(pathway a)
H2
dehydrogenation
[Fe]
1 or 3
H2
N
(3 mol%)
H
(4)
(5)
P(iPr)2
N
reflux, 30 h
xylene,
H2
H
not observed
during catalysis
N
N
Fe CO
not formed
N
H
H
P
1 or 3
(3 mol%)
H
[Fe]
(iPr)2
H2
4
N
reflux, 30 h
xylene,
H2
(pathway b)
N
N
100% conversion
For the hydrogenation reaction, a trans-Fe(H)2 species (4) is
proposed to be the active intermediate. Guan and coworkers have
proposed the same species as the active form of the catalyst in
ester hydrogenation.10 Although we could not isolate this species
independently, its existence was supported by NMR spectroscopy
and trapping experiments. Treatment of the independently synthe-
sized complex 3 with H2 (1 atm) generated the trans-dihydride
intermediate at room temperature (Scheme 2). Compound 4 ex-
hibited a multiplet resonance centered at -9.32 ppm for the two
Fe-Hs in the 1H NMR spectrum (see Supporting Information). In a
separate reaction, it was also shown that the trans-dihydride in-
termediate readily loses H2 under vacuum and is converted to 3.
These results are consistent with Guan’s observation on the reac-
tivity of the in-situ formed complex 3.10 Nevertheless, complex 4
was trapped (Scheme 2) in the presence of stoichiometric amount
Figure 4. X-ray structure of 3.
When 3 was used directly as the dehydrogenation catalyst, a
very comparable catalytic activity was observed (TON of 33 in 30
hours) under the same conditions. This result provides direct evi-
dence that complex 3 is indeed an active intermediate in the dehy-
drogenation reaction of N-heterocycles.
.
of BH3 THF to form the iron hydrido-borohydride complex (1).
As complex 4 could be directly generated from 3 under H2 pres-
sure, we successfully carried out hydrogenation of quinaldine (K)
using a catalytic amount of 3 (3 mol%). Therefore, complex 3
serves as the single molecular iron complex that can catalyze both
dehydrogenation and hydrogenation of N-heterocycles. To the
best of our knowledge, there is no report of a single first-row tran-
sition metal-based catalyst capable of performing both of the reac-
tions.
Next, we examined the catalytic activity of 1 and 3 towards a
cyclic secondary amine to probe the amine dehydrogenation step
involved in the cycle. Consistent with our mechanistic hypothe-
sis, when 1,2,3,4-tetrahydroquinoxaline was subjected to the cata-
lytic conditions, the oxidized product quinoxaline was formed in
quantitative amount (eq 2). In order to release the second equiva-
lent of H2 from substrate A, a C-H bond activation step must oc-
cur as no N-H bond remains in 3,4-dihydroquinoline. Two differ-
ent pathways were considered (Scheme 1) that could lead to the
final product: (a) a direct alkane dehydrogenation17 by iron from
the partially oxidized substrate (pathway a) and (b) isomerization
of the initially formed C=N to a C=C, followed by the second
dehydrogenation from the cyclic secondary amine fragment
(pathway b).4e To test this hypothesis, we carried out dehydro-
genation reactions of the following substrates: (i) 1,2,3,4-
tetrahydronaphthalene, (ii) 1,2-dihydronaphthalene, and (iii) 3,4-
dihydroisoquinoline. For the first two substrates, no dehydro-
genation activity was observed with either 1 or 3 (eqs 3 and 4),
suggesting the direct alkane dehydrogenation pathway is unlikely
Scheme 2. NMR and Trapping Experiments to Support
the Formation of the trans-Dihydride Intermediate
BH3
H
H
P(iPr)2
P(iPr)2
P(iPr)2
CO
BH3 THF
(1.2 equiv)
H
H
H
+
2
N
Fe CO
N
Fe CO
N
Fe
C6D6, rt
H2
H
P
P
P
H
H
(iPr)2
(iPr)2
(iPr)2
1
3
4
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(4) For homogeneous dehydrogenation and hydrogenation of tetrahy-
In summary, we have demonstrated reversible dehydrogena-
tion-hydrogenation of N-heterocycles with inexpensive and earth-
abundant iron-based molecular catalysts. Products from both the
dehydrogenation and hydrogenation reactions were isolated in
good yields. The penta-coordinated iron hydride species 3, the
proposed dehydrogenation intermediate, was isolated by an inde-
pendent route and its direct involvement in the catalysis was
demonstrated. Substrate-driven mechanistic studies support the
initial amine-dehydrogenation step and provide evidence against
direct alkane dehydrogenation from the partially oxidized N-
heterocycles. The presence of the N-atom seems to be critical for
a successful catalytic dehydrogenation. On the other hand, a
trans-dihydride species (4) was invoked as the active catalyst for
the hydrogenation of N-heterocycles. NMR and trapping experi-
ments support the formation of such a species. Currently, our
efforts are focused on further understanding the reaction mecha-
nism and a detailed study will be reported in the future. We are
also applying these iron complexes as efficient catalysts for the
dehydrogenation of other organic substrates and these studies are
forthcoming.
droquinolines and quinolines see: (a) Yamaguchi, R.; Ikeda, C.;
Takahashi, T.; Fujita, K.-I. J. Am. Chem. Soc. 2009, 131, 8410. (b) Li, H.;
Jiang, J.; Lu, G.; Huang, F.; Wang, Z.-X. Organometallics 2011, 30, 3131.
(c) Zhang, X.-B.; Xi, Z. Phys. Chem., Chem. Phys. 2011, 13, 3997. (d)
Wu, J.; Talwar, D.; Johnston, S.; Yan, M.; Xiao, J. Angew. Chem. Int. Ed.
2013, 52, 6983. (e) Luca, O.; Huang, D. L.; Takase, M. K.; Crabtree, R. H.
New J. Chem. 2013, 37, 3402.
(5) For catalytic dehydrogenation of other N-heterocycles see: (a) (b)
Tsuji, Y.; Kotachi, S.; Huh, K. T.; Watanabe, Y. J. Org. Chem. 1990, 55,
580. (b) (b) Hara, T.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K.
Tetrahedron Lett. 2003, 44, 6207. (c) Dean, D.; Davis, B.; Jessop, P. G.
New J. Chem. 2011, 35, 417.
1
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(6) Fujita, K.-I.; Tanaka, Y.; Kobayashi, M.; Yamaguchi, R. J. Am.
Chem. Soc. 2014, 136, 4829.
(7) For iron-based catalysts in hydrogenation and dehydrogenation see:
(a) Iron Catalysis in Organic Chemistry: Reactions and Applications, ed.
Bernd Plietker, Wiley-VCH, Weinheim, 2008. (b) Nakazawa, H.; Itazaki,
M. Top. Organomet. Chem. 2011, 33, 27 and references therein. (c) Dar-
wish, M.; Wills, M. Catal. Sci. Technol. 2012, 2, 243 and references there-
in. (d) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282 and references
therein.
(8) Alberico, E.; Sponholz, P.; Cordes, C.; Nielsen, M.; Drexler, H.-J.;
Baumann, W.; Junge, H.; Beller, M. Angew. Chem. Int. Ed. 2013, 52,
14162.
(9) Koehne, I.; Schmeier, T. J.; Bielinski, E. A.; Pan, J. C., Lagaditis, P.
O.; Bernskoetter, W. H.; Takase, M. K.; Wurtle, C.; Hazari, N.; Schneider,
S. Inorg. Chem. 2014, 53, 2133.
(10) Chakraborty, S.; Dai. H.; Bhattacharya, P.; Fairweather, N. T.;
Gibson, M. S.; Krause, J. A.; Guan, H. J. Am. Chem. Soc. 2014, 136,
ASAP, DOI: 10.1021/ja504034q. Provisional US patent application with
a serial number of 61/972927, 2014.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures, product characterization data, and X-
ray crystallographic data for 3 (CCDC#1001232) are included.
This material is free of charge via the internet at
http://pubs.acs.org.
(11) Yamaguchi and coworkers have reported similar sharp increase in
yields when they switched the solvent from toluene to p-xylene. See refer-
ence 4a.
AUTHOR INFORMATION
(12) (a) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003,
198, 317. (b) Crabtree, R. H. Chem. Rev. 2012, 112, 1536. It should be
noted that Hg(l) does not always inhibit iron nanoparticle catalysts. See:
(c) Sonnenberg, J. F.; Coombs, N.; Dube, P. A.; Morris, R. H. J. Am.
Chem. Soc. 2012, 134, 5893. 5899 (d) Bedford, R. B.; Betham, M.; Bruce,
D. W.; Davis, S. A.; Frost, R. M.; Hird, M. Chem. Commun. 2006, 1398.
(e) Rangheard, C.; Fernández, C. J.; Phua, P.-H.; Hoorn, J.; Lefort. L.; de
Vries, J. G. Dalton Trans., 2010, 39, 8464.
(13) When 5 mol% of free PNP ligand was added with 1 mol% of cata-
lyst 1 (conditions of entry 5 in Table 1), 92% conversion of tetrahydroqui-
naldine was observed after 30 hours. Without the added free ligand, only
34% conversion was achieved after 30 hours under these conditions. If
iron nanoparticles are being generated, one would not expect to see such a
significant difference between these two experiments. Furthermore, a
PMe3 poisoning experiment was not conducted as complex 1 itself de-
composed in the presence of excess of PMe3 and therefore, the result from
this experiment would be inconclusive.
Corresponding Author
jones@chem.rochester.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was funded by the Center for Electrocatalysis,
Transport Phenomena, and Materials (CETM) for Innovative
Energy Storage, an Energy Frontier Research Center funded by
the U.S. Department of Energy, and the ESD NYSTAR program.
S.C. would like to thank James Kovach (University of Rochester)
for helpful discussion on the dehydrogenation mechanism.
(14) Yamaguchi and coworkers reported similar dual experiments in
their studies. See references 4a and 6.
REFERENCES
(15) For hydrogenation of N-heterocycles see: (a) 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 and references cited therein. (b)
Wu, J.; Barnard, J. H.; Zhang, Y.; Talwar, D.; Robertson, C. M.; Xiao, J.
Chem. Commun. 2013, 49, 7052 and references cited therein. For hydro-
genation of imines see: Lagaditis, P. O.; Sues, P. E.; Sonnenberg, J. F.;
Wan, K. Y.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2014, 136,
1367.
(16) Complex 3 was also proposed to be the active catalytic intermedi-
ate in Beller’s methanol dehydrogenation chemistry (see reference 7); but
no experimental evidence for the formation of such a species was provided
in that report.
(1) (a) Eberle, U.; Felderhoff, M.; Schüth, F. Angew. Chem. Int. Ed.
2009, 48, 6608. (b) Sartbaeva, A.; Kuznetsov, V. L.; Wells, S. A.;
Edwards, P. P. Energy Environ. Sci. 2008, 1, 79. (c) Makowski, P.;
Thomas, A.; Kuhn, P.; Goettmann, P. Energy Environ. Sci. 2009, 2, 480.
(d) Teichmann, D.; Arlt, W.; Wasserscheid, P.; Freymann, R. Energy
Environ. Sci. 2011, 4, 2767. (e) Armaroli, N.; Balzani, V. ChemSusChem
2011, 4, 21. (f) Fukuzumi, S.; Suenobu, T. Dalton Trans. 2013, 42, 18.
(2) (a) Crabtree, R. H. Energy Environ. Sci. 2008, 1, 134 and references
cited therein. (b) Jessop, P. Nat. Chem. 2009, 1, 350 and references cited
therein. (c) Watson, L. A.; Eisenstein, O. J. J. Chem. Educ. 2002, 79,
1269.
(3) (a) Adkins, H.; Lundsted, L. G. J. Am. Chem. Soc. 1949, 71, 2964.
(b) Lu, S. M.; Wang, Y. Q.; Han, X. W.; Zhou, Y. G. Chin. J. Catal. 2005,
26, 287. (c) Wang, Z.; Tonks, I.; Belli, J.; Jensen, C. M. J. Organomet.
Chem. 2009, 694, 2854. (d) Mikami, K.; Ebata, K.; Mistudome, T.; Mi-
zugaki, T.; Jitsukawa, K.; Kaneda, K. Heterocycles 2011, 82, 1371. (e)
Sotoodeha, F.; Huberb, B. J. M.; Smith, K. J. Appl. Catal., A, 2012, 420,
67.
(17) For a comprehensive review on alkane dehydrogenation see: Choi,
J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011,
111, 1761.
(18) Kerr, J. A. Chem. Rev. 1966, 66, 465.
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TOC Graphic for
A Molecular Iron Catalyst for the Acceptorless Dehydrogenation and
Hydrogenation of N-Heterocycles
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Sumit Chakraborty, William W. Brennessel, and William D. Jones*
2 H2
+ 2 H2
N
N
H
P(iPr)2
CO
N
Fe
H
P
(iPr)2
well-defined catalyst
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