Ruthenium Complexes for Catalytic Dehydrogenation of Hydrazine and Transfer Hydrogenation Reactions

Full text of DOI:10.1002/asia.201800315

CHEMISTRY
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Accepted Article
Title: Ruthenium complexes for catalytic dehydrogenation of hydrazine
and transfer hydrogenation reactions
Authors: Mahendra Kumar Awasthi, Deepika Tyagi, Soumyadip Patra,
Rohit Kumar Rai, Shaikh M. Mobin, and Sanjay Kumar Singh
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Chemistry - An Asian Journal
Ruthenium complexes for catalytic dehydrogenation of
hydrazine and transfer hydrogenation reactions
Mahendra Kumar Awasthi[a]†, Deepika Tyagi[a]†, Soumyadip Patra[a], Rohit
Kumar Rai[a], Shaikh M. Mobin[a] and Sanjay Kumar Singh*[a]
[a]Discipline of Chemistry, Indian Institute of Technology Indore, Simrol-453552,
Indore, India
Abstract
Catalytic dehydrogenation of hydrazine in THF and ethanol was achieved over ruthenium-
arene complexes containing Nimine-substituted iminopyridine ligands [(6-benzene)Ru(κ2-
L)Cl]+ (L = N-hydroxy-iminopyridine, N-methoxy-iminopyridine and N-iso-propyl-
iminopyridine). Our findings demonstrated that the catalytic activity for the release of H2 and
N2 gases, as confirmed by GC-TCD, from hydrazine depends on the temperature (30 ºC – 80
ºC), base (KOtBu and KOH), and solvents used (THF, ethanol and water). Moreover, the
studied complex displayed excellent recyclability for the dehydrogenation of hydrazine (in
THF) for six consecutive catalytic runs. 1H NMR and mass spectrometric studies revealed the
formation of hydrazine (and phenylhydrazine) coordinated Ru(II)-arene intermediate [(6-
C6H6)Ru(κ2-L)(RNHNH2)]+ (R = H or Ph), where the coordination of hydrazine to Ru center
and subsequent activation of hydrazine (Ea = 57.9 kJmol-1) was expected to be the key step
involved in achieving dehydrogenation of hydrazine. Exploring the unique ability to activate
hydrazine, these complexes were also employed for the catalytic transfer hydrogenation of
styrene and a wide range of nitro substrates in good yields in the presence of hydrazine,
further evidenced the involvement of Ru-hydride species during ruthenium catalyzed
activation of hydrazine for dehydrogenation and transfer hydrogenation reactions.
†Authors with equal contributions
Keywords: Hydrogen ∙ dehydrogenation ∙ hydrazine ∙ ruthenium ∙ homogeneous
catalysis ∙ iminopyridine ligands
1
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Introduction
Hydrazine coordinated transition metal complexes have been receiving huge scientific
attention because of the involvement of metal-hydrazine species as important intermediates in
several reactions of global importance including nitrogen to ammonia reduction.[1-2]
Therefore, such hydrazine coordinated complexes based on Fe, W, Mo, Ir, Ru and others
have been extensively explored for diverse applications.[1-27] For instance, Tyler et al. X-ray
crystallographically characterized a Fe-hydrazine complexes cis-[Fe(DMeOPrPE)2(η2-
N2H4)][BPh4]2 {DMeOPrPE = 1,2-bis[bis(methoxypropyl)phosphino]ethane} having η2-
coordinated hydrazine, which further undergoes disproportionation to ammonia in the
presence of acid.[9] On the other hand, upon base-promoted deprotonation, such Fe-ƞ2-N2H4
complexes afforded Fe-diazene (Fe-ƞ2-N2H2) complexes, which converted back to Fe-
hydrazine complex by treatment with a mild acid.[10-14] Ikariya et al. reported Fe complexes
containing proton responsive pyrazole pincer ligand, which also facilitated disproportionation
of N2H4 into nitrogen via a diazene intermediate.[15] Ir-based hydrazine complexes
[IrCl2(RNHNH2){P(OEt)3}(AsPh3)2]+ (R = H, Me, Ph), synthesized by treating
[IrCl2H{P(OEt)3}(AsPh3)2] with triflic acid followed by an excess of hydrazine, also
undergoes oxidation in the presence of Pd(OAc)2 to form Ir-diazene and Ir-hydride
complexes.[16] Very recently, Albertin et al. reported Ir-hydrazine complex [(η5-
C5Me5)Ir{P(OR)3}(N2H4)]+ which also exhibited Pb(OAc)4 assisted oxidation to Ir-diazene
complex.[17] Seino and Mizobe et al. also reported a (η5-C5Me5)Ir coordinated bis-hydrazine
cluster [{(η5-C5Me5)Ir}4(μ3-S)2(μ2-H)2(N2H4)2]2+ having two hydrazine molecules
coordinated to a single (η5-C5Me5)Ir moiety connected through the bridging sulphur of the Ir3
cluster.[18] Analogously, several Ru-hydrazine complexes were also reported, which upon
oxidation may form corresponding diazene species,[19a-e] such as, Ru-hydrazine
[RuH(N2H4)L4]+ and Ru-bis-hydrazine complexes [Ru(N2H4)2L4]2+ (L= P(OEt)3, PPh(OEt)2,
2
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P(OMe)3 resulted in the formation of Ru-diazene complexes upon treatment of Pb(OAc)4 in
o
DCM at -80 C.[20] Similarly, hydrazine coordinated Ru-arene and analogous Os-arene
complexes [(6-p-cymene)M{PPh(OEt)2}Cl (1-N2H4)]+ (M = Ru and Os) were also reported
to be transformed into corresponding diazene complexes upon treatment with Pb(OAc)4 at -
30 C.[21] In similar fashion, Os-hydrazine complex [Os(PPh3)2{P(OMe)3}(CO)Cl(N2H4)]+
was also reported to be synthesized by direct treatment of N2H4
with
[OsHCl(CO)(PPh3)2{P(OMe)3}] in the presence of triflic acid, which can be further oxidized
to form corresponding Os-diazene complex.[22] Interesting to note, that these important
intermediates, including those containing N2H4 and N2H2 coordinated Fe complexes provided
substantial mechanistic insights on the role of these species in several reactions, such as
dehydrogenation reactions.
Scheme 1. Various hydrazine coordinated metal complexes – a key step to the
activation of hydrazine
In accordance with the above and several other precedent reports on the formation of
metal-hydride complex upon treatment of metal-diazene complex with a base, clearly
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evidenced a possible pathway for the dehydrogenation of hydrazine.[23-26] For instance, Chirik
et al. demonstrated the dehydrogenation of hydrazine upon coordination of hydrazine with
molybdenum-terpyridine-phosphine in [(PhTpy)(PPh2Me)2Mo(2-N2H4)] complex.[26] Field et
al. also investigated the base-induced dehydrogenation of hydrazine via a ‘coordination
induced activation pathway’ over Ru-tetradentate phosphine complexes, where ligand exerted
significant impact on the coordination of hydrazine to Ru (side-on [RuCl(2-N2H4)(3-
PP3ipr)]+ or end-on bound hydrazine [RuCl(1-N2H4)(4-PP3Ph)]+) and the reactivity pathway
of the resulting complexes.[27] Apart from homogeneous catalysts there are several reports are
available in literature for the dehydrogenation of hydrazine using heterogeneous catalyst but
exploration of homogeneous complexes for hydrazine activation are particular importance
because of several applications.[28,29] However, anhydrous hydrazine is explosive in nature
when exposed to a metal catalyst which makes it difficult to handle safely, whereas hydrous
hydrazine (H2NNH2·H2O), which also contains a large amount of hydrogen (7.9 wt %), is
much safer and easy to handle.[29]
Therefore, intrigued by aforementioned findings, herein, we report the reactivity of
iminopyridine based ruthenium-arene complexes with hydrazine and explored the base
1
assisted catalytic dehydrogenation of hydrazine. H NMR and mass spectrometric tools were
probed to investigate and evidence the possible interactions between hydrazine and Ru and
generation of any possible organometallic intermediates. Further, the application of the
ruthenium catalyzed activation of hydrazine was also explored for transfer hydrogenation of
nitro substrates and styrene.
Results and discussion
Synthesis and X-ray structure of ruthenium-arene complexes. To perform
ruthenium catalyzed dehydrogenation of hydrazine, Ru(II)-arene complexes, [(η6–
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benzene)Ru(N-hydroxy-iminopyridine)Cl]+
([Ru]-1),
[(η6–benzene)Ru(N-methoxy-
iminopyridine)Cl]+ ([Ru]-2), and [(η6–benzene)Ru(N-isopropyl-iminopyridine)Cl]+ ([Ru]-3),
were synthesized following earlier reports (Figure 1a).[30-32] Recrystallization of [Ru]-2 gave
orange colour crystals suitable for single-crystal structure determination (Figure 1b).
Complex [Ru]-2 adopted a piano-stool geometry, with bond lengths (Ru-Cl, Ru-N1, and Ru-
N2, Ru-Ct) and bond angles (Npy-Ru-Nimine and Npy/imine-Ru-Cl ) are in the expected range of
the analogous ruthenium complexes (see supporting information).[31,32] Moreover, torsion
angle N1-C5-C6-N2 (-1.2°) is suggesting the planar arrangement of the bidentate
iminopyridine ligand.
(a)
(b)
Figure 1. (a) Ruthenium-arene catalyzed dehydrogenation of hydrazine and transfer
hydrogenation of nitro substrate and styrene, (b) X-ray crystal structure of complex
[Ru]-2 with 30% ellipsoid probability. Counter ion (Cl-) are omitted for sake of clarity.
Selected bond lengths [Å] and angles [deg]: Ru1-Cl1 2.402(11), Ru1-N1 2.098(4),
Ru1-N2 2.067(3), N2-C6 1.282(6), Ru1-Cavg 2.177, Ru1-Ct 1.444, N1-Ru1-N2
75.80(14), N1-Ru1-Cl1 84.61(10), N2-Ru1-Cl1 87.97(10), N1-C5-C6-N2 -1.2(6).
Ruthenium catalyzed dehydrogenation of hydrazine. At an outset, dehydrogenation
of hydrazine monohydrate (N2H4∙H2O) was attempted over [Ru]-1 complex in water by using
o
base (KOH) at 80 C, where the evolution of gas from the reaction mixture was observed
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volumetrically (Table 1, entry 1). Further, an enhancement in TON and a two-fold increment
in TOF were observed by performing the experiment in the presence of KOtBu instead of
KOH (Table 1, entry 2). In contrary to water, a significant enhancement in TON was
observed for the dehydrogenation reaction performed in ethanol (Table 1, entry 3). Notably,
appreciable amount of gas evolution from the hydrazine solution in ethanol was also
observed even in the absence of base, but the reaction was observed to be sluggish (Table 1,
entry 4). Further in the search of a suitable solvent medium, [Ru]-1 catalyzed
o
dehydrogenation of hydrazine was performed in THF-methanol (10:1 v/v) at 80 C in the
presence of different bases KOH, NaOH and KOtBu, where highest TON and TOF was
achieved with KOtBu (Table 1, entries 5-7). Based on the amount of gas evolved, selectivity
for H2 gas (X) was found to be 0.46.[28-29,33] Moreover, the presence of H2 in the released gas
was also analysed by GC-TCD (Figure S1). Therefore, the overall hydrazine decomposition
equation can be expressed which is as follows:
N2H4
→
0.72NH3 + 0.65N2 + 0.92H2
eq. (1)
It should be noted that gas evolution was not observed in the absence of the catalyst
(Table 1, entry 8, Figure S2). Furthermore, we performed control experiment under the
optimised reaction condition in the absence of hydrazine. There was no traces of gas release
was observed during the experiment, inferred that under the optimized reaction condition
methanol dehydrogenation does not occurred (Table 1, entry 9, and Figure S2). In addition,
analysis of reaction mixture obtained after the reaction does not show any other products,
such as CO, CO2, formate, acetal of formaldehyde, N-methylation of hydrazine. Moreover,
using methanol is advantageous, as it increases the solubility of the studied complex in the
solution. The above observations inferred that the observed gas release in the presence of
hydrazine was due to catalytic dehydrogenation of hydrazine only, and that the presence of
both catalysts and hydrazine is important for gas generation. Notably, in THF-methanol, an
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enhancement of three- and two-fold, respectively in the TOF was achieved compared to the
catalytic dehydrogenation reaction performed in water and ethanol. Further performing the
catalytic reaction at lower temperature (< 80 C), TON and TOF of the catalytic
dehydrogenation of hydrazine was significantly decreased (Table 1, entries 10-14). The
apparent activation energy (Ea = 57.9 kJmol−1) for the catalytic dehydrogenation of hydrazine
over [Ru]-1 catalyst, as obtained by an Arrhenius plot (Figure 2c (inset) and Figure S3), is in
good agreement with other literature findings.[33]
Table 1. Optimization of the reaction condition for the catalytic dehydrogenation of
hydrazine over [Ru]-1 catalysta
Entry Catalyst Solvent
Base
T (oC)/ Volume of TON
t (min.) gas (mL)
TOF
(h-1)
1
Water
KOH
KOtBu
KOtBu
-
80/53
80/36
80/70
8.0
-
-
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
-
2
Water
11.0
35.0
3.5
33.0
21.0
37.0
35.0
29.0
-
5.8
28.0
8.0
58.5
37.0
23.0
-
3
Ethanol
Ethanol
4
80/154 25.5
5
THF-methanol KOtBu
80/40
80/57
80/75
80/120
80/120
30/61
40/66
38.5
36.5
32.0
-
6
THF-methanol
THF-methanol
THF-methanol
THF-methanol
THF-methanol
THF-methanol
THF-methanol
THF-methanol
KOH
7
NaOH
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
8
9*
10
11
12
13
-
-
-
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
8
-
-
9.5
1.64
9.5
37.0
1.49
5.18
46.0
50/111 16.0
60/48 38.5
7
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14
THF-methanol
THF-methanol
THF-methanol
KOtBu
KOtBu
KOtBu
70/55
80/65
80/66
38.5
36.5
33.5
37.0
35.0
31.0
40.0
32.0
28.0
[Ru]-1
15
[Ru]-2
16
[Ru]-3
aReaction condition: N2H4·H2O (1.0 mmol), base (20 mol %), catalyst (2.5 mol %), solvent
(5.5 mL). THF-methanol (10:1 v/v). *Without hydrazine. TON = n(H2)/n(cat). TOF =
n(H2)/{n(cat)∙time}.
Further to evaluate the effect of substitutes at Nimine of iminopyrdine ligands in the
studied ruthenium complexes [Ru]-1 – [Ru]-3, catalytic dehydrogenation of hydrazine was
performed under the optimized reaction condition (hydrazine (1.0 mmol), catalyst (2.5 mol%)
and KOtBu (2.5 mol%) in THF-methanol (5.5 mL, 10:1 v/v) at 80 C) (Table 1, entries 15-16,
Figure 2a). Results inferred that among all the studied complexes having different steric and
electronic properties, the (6-benzene)Ru complex [Ru]-1 having Nimine-OH group
outperformed and showed higher TOF (h-1) over those having Nimine-OMe and Nimine-iPr
substitution (Figure 2b), which can be attributed to the plausible involvement of the hydroxyl
group in interaction with hydrazine coordinated with Ru and hence facilitating improved Ru
to hydrazine interactions.[34] Notably, the outperforming [Ru]-1 catalyst exhibited remarkably
high recyclability up to six consecutive catalytic runs for the dehydrogenation of hydrazine
under the optimized reaction condition (Figure 2d). Moreover, no remarkable change in the
catalytic efficacy was observed during the reaction performed in the presence of Hg,
suggesting the homogenous nature of the active form of the catalyst (Figure S4).
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(a)
1.6
1.2
0.8
0.4
0.0
(b)
60
40
20
0
[Ru]-1
[Ru]-2
[Ru]-3
[Ru]-1
[Ru]-2
[Ru]-3
0
20
40
60
80
Catalysts
Time (min)
(d)
(c)
1st
1.6
1.2
0.8
0.4
0.0
1.6
2nd
without base
NaOH
KOH
3rd
5th
4th
6th
1.2
0.8
0.4
0.0
1.8
1.6
1.4
1.2
KOtBu
2.85 2.90 2.95 3.00
1/T× [103 (K-1)]
0
2
4
6
8
10
0
25 50 75 100 125 150
Time (min)
Time (h)
Figure 2. (a-b) Comparative catalytic efficacy (a) amount of gas released vs time
and (b) TOF (h-1) for the dehydrogenation of hydrazine monohydrate over ruthenium-
arene catalysts. (c) Catalytic dehydrogenation of hydrazine monohydrate (1.0 mmol)
over [Ru]-1 catalyst (2.5 mol%) in the presence and absence of base (20 mol%)
(inset), Arrhenius plot for the dehydrogenation of hydrazine monohydrate over [Ru]-1
catalyst at 30, 40, 60 and 80 C to estimate the activation energy (R2 = 0.991). (d)
Recyclability experiments for the catalytic dehydrogenation of hydrazine
monohydrate over [Ru]-1 catalyst. Reaction condition: hydrazine monohydrate (1.0
mmol) over various ruthenium catalysts (2.5 mol%) in the presence of KOtBu (20
mol%) in THF-methanol (5.5 mL, 10:1 v/v) at 80 C.
To further investigate the role of Ru-catalyst in the catalytic dehydrogenation of
hydrazine, several controlled and catalytic experiments were performed to identify the crucial
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organometallic intermediates which might be generated during the reaction due to the
possible interactions between Ru-catalyst and hydrazine. In this context, control experiment
performed using [Ru]-1 catalyst in D2O with an excess of hydrazine added, showed a peak at
m/z 333.0 in mass spectra corresponding to the hydrazine coordinated Ru-species, [Ru]-1A.
Coordination of hydrazine to ruthenium was further evidenced by 1H NMR, where an upfield
shifting in the peaks corresponding to iminopyridine ligand in [Ru]-1 complex was observed
1
(Figure S5). Further, an analogous behaviour in H NMR was also observed when an excess
of hydrazine was added in a solution of [Ru]-1 complex in CD3OD, and the corresponding
mass data also showed m/z value of 333.0 corresponding to the [Ru]-1A species (Figure S6).
Moreover, mass spectral analysis of the reaction aliquot during the catalytic reaction
condition also evidenced the formation of a hydrazine coordinated ruthenium species (Figure
S7). The catalysts [Ru-1] having N–OH substitution outperformed over [Ru]-2 and [Ru]-3
having N-OMe and N-iPr substitutions respectively, possibly because of the presence of
protic group N–OH. In the presence of a base, the protic group N-OH deprotonated and
thereby stabilised a hydrazine coordinated monocationic ruthenium species [(η6–
benzene)Ru(N-hydroxy-iminopyridine)N2H4]+ ([Ru]-1A), which was well characterised by
mass (m/z 333.0) and NMR (Figure S5-S7). Analogously, [Ru]-2 and [Ru]-3 complexes also
showed the formation of corresponding hydrazine coordinated ruthenium species and due to
lack of protic group these ruthenium hydrazine species are dicataionc species [Ru]-2A (m/z
174.0) and [Ru]-3A (m/z 180.0) (Figure 3 and Figure S8-S9).
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333.0298
180.0391
Observed
174.0209
Observed
Observed
181.0389
175.0221
335.0285
2
173.0145
171.0111
177.0302
327.0261
336.9
333.0287
174.0258
180.0440
Simulated
Simulated
Simulated
335.0295
175.0262
181.0444
327.0316
171.0273
177.0455
170.0
172.5
175.0
325
330
335
178
180
182
m/z
m/z
m/z
Figure 3. Mass spectral analysis for the hydrazine coordinated ruthenium species,
[Ru]-1A, [Ru]-2A and [Ru]-3A, obtained by the addition of hydrazine to an aqueous
solution of the corresponding complexes [Ru]-1, [Ru]-2 and [Ru]-3, respectively.
Moreover, the addition of hydrazine to a pale yellow solution of [Ru]-1 in water
resulted in an intense wine-red solution, suggested the formation of ruthenium-hydrazine
1
species. The significant shift in the H NMR spectra and the presence of a mass peak at m/z
333.0 corresponds to the [Ru]-1A species clearly evidenced the formation of ruthenium-
hydrazine species. Further addition of an excess of dil. HCl in the above wine-red solution
immediately resulted in the regeneration of pale yellow solution, thus straight forward
evidenced the recovery of [Ru]-1, which was also supported by the reappearance of m/z peak
at 337.0 (in the mass spectral analysis of the pale yellow solution) corresponding to the [Ru]-
1 catalyst (Figure S10).
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1
Therefore, mass and H NMR spectral results clearly evidenced the coordination of
hydrazine to Ru center is a crucial step for the activation of hydrazine, which further
transformed to Ru-H species via a well-established diazene intermediate. The temperature
1
dependent H NMR and corresponding mass analysis under the catalytic reaction condition
also inferred the appearance of a ruthenium-hydrazine species with the increase in the
reaction temperature (30-60 C) (Figures S11 and S12). Analogous to hydrazine coordinated
ruthenium species, phenylhydrazine also coordinated with the ruthenium metal, and such
phenylhydrazine
coordinated
ruthenium
species
[(η6–benzene)Ru(N-hydroxy-
1
iminopyridine){N2H3(C6H5)}]+ (m/z 409.0) were analysed by H NMR and mass. (Figure
S13-S14). (Figure S13 and Figure S14). Whilst we were unable to observe the Ru-H and Ru-
diazene intermediates during our investigations, presumably because of the lower stability of
these species in the reaction medium, such species are expected to be involved in the
dehydrogenation of hydrazine. Notably, X-ray structure of analogous ruthenium-diazene
species [(5-Cp*)Ru(2-HN=NH){P(OEt)3}2]+ was previously reported by Albertin et al..[4a]
Also, Hillhouse et al. demonstrated the formation of metal-diazene and metal-hydride species
from rhenium-hydrazine complexes.[5] In this regard, treating 4-methoxystyrene with
hydrazine over [Ru]-1 catalyst resulted in the formation of 1-ethyl-4-metoxy benzene in
water-ethanol at 80 ºC, suggesting the possible involvement of ruthenium-hydride species
during the transfer hydrogenation via an ruthenium catalyzed activation of hydrazine (Figure
S15). In a recent report, Milstein et al. also demonstrated the crucial role of a hydrazine-
coordinated Ru intermediate species [(tBuPNP)Ru(CO)(H)(N2H4)]+ during a Ru-pincer
complex catalyzed dehydrogenative coupling of alcohols with hydrazine with the release of
H2 gas.[25] Similar activation of hydrazine, by weakening of N-H bond of hydrazine upon
coordination of hydrazine with molybdenum-terpyridine-phosphine has also been
demonstrated.[26] It has been observed that such metal hydrazine complexation lower the
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nitrogen-hydrogen bond dissociation free energy (34.6 kcal mol-1) and hence enable H2
evolution and hydrogenation of styrene.[26] Analogous Ru-hydride and Ru-diazene
intermediates were also observed during dehydrogenation of hydrazine over Ru-tetradentate
phosphine complexes via a ‘coordination induced activation pathway’.[27] Based on the above
1
findings and spectral data (mass and H NMR) we proposed a plausible general reaction
pathway for the hydrazine activation with ruthenium complexes shown in the Scheme 2.
Moreover, the formation of hydrazine coordinated ruthenium species for all the complexes
[Ru]-1, [Ru]-2 and [Ru]-3, suggesting that these complexes probably follow an analogous
hydrazine dehydrogenation pathway, as illustrated in Scheme 2.
Scheme 2. Plausible reaction pathway for the dehydrogenation of hydrazine over the
studied ruthenium catalysts.
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Further, the transfer hydrogenation of 4-chloronitrobenzene (1a) was also investigated
over [Ru]-1 catalyst in the presence of hydrazine at 80 ºC, which led to the formation of 4-
chloroaniline (1b) in high yield (Table 2 and Table S4-S5). Notably, reaction could not
proceed in the absence of either the catalyst or hydrazine, suggesting the active role of the
catalyst in the activation of hydrazine and possibly generation of the most crucial Ru-hydride
intermediate to facilitate the catalytic transfer hydrogenation reaction (Scheme S1).[31a]
Further, we monitored the reaction progress and identified different possible intermediates by
using UV-vis and 1H NMR (Figure S16-S17). The UV-vis spectrum showed the formation of
a 4-chloronitrosobenzene intermediate (276 nm) after 2 h reaction progress, but we could not
1
find any absorbance for other possible intermediate (Figure S16). However, in H NMR
spectra we were able to identify different intermediates, such as phenylhydroxylamine,
nitrosobenzene, and azoxybenzene, formed during nitrobenzene reduction (Figure S17).[35]
The above results illustrated a step wise hydrogen transfer to nitro group of nitrobenzene,
which led to the formation of different intermediate phenylhydroxylamine, nitrosobenzene,
and azoxybenzene, which were further transformed to aniline (Scheme S1). Moreover, in
accordance to the dehydrogenation process, complexes other than [Ru]-1 were also found to
be poorly active for transfer hydrogenation of 1a. Worthy to mention, that the catalytic
transfer hydrogenation of nitro substrates was found to be a base free transformation. Along
with the efficient transformation of nitrobenzene (2a) to aniline (2b), a wide range of other
nitro substrates were also employed for the transfer hydrogenation reaction over [Ru]-1
catalyst. Nitro substrates with electron withdrawing substituents such as 4-chloronitrobenzene
(1a), 3-chloronitrobenzene (3a) and 4-bromonitrobenzene (4a) showed excellent conversion
to afford corresponding amines (1b, 3b, 4b) (Table 2). Nitro substrates with electron donating
groups, such as 4-nitrobenzylalcohol (5a), 4-methylnitrobenzene (6a), 3-methylnitrobenzene
(7a) and 4-methoxynitrobenzene (8a) were also transformed efficiently to corresponding
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amines (5b-8b) in excellent yields (Table 2). More interestingly, chemoselective transfer
hydrogenation of nitro-groups in 4-nitrobenzonitrile (10a) and 4-nitrobenzaldehyde (11a) to
corresponding amines, 4-aminobenzonitrile (10b) and 4-aminobenzaldehyde (11b) with 97%
and 80% yield respectively was also achieved (Table 2). 2-nitrobiphenyl (12a) and 8-
nitroquinoline (13a) were also efficiently converted into the corresponding amines, 12b and
13b with 96% and 90% yield respectively (Table 2).
Table 2. Functional group tolerance for the catalytic transfer hydrogenation of nitro
substrate[a]
aReaction conditions: nitro substrate (1 mmol), [Ru]-1 (2.5 mol%), N2H4·H2O (2 - 10
1
equiv.), ethanol-water (5 mL, 1:1 v/v), conversion determined by H NMR, isolated
yields (%) are given in parenthesis, [c] 6h, [d] 2 equiv. N2H4·H2O, [e] 5 equiv.
N2H4·H2O, [f] 10 equiv. N2H4·H2O
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Conclusions
We demonstrated catalytic dehydrogenation of hydrazine over ruthenium-arene
catalysts containing N-substituted iminopyridine ligands at 80 ºC in ethanol and THF-
methanol in the presence of a base. Our findings suggested a ligand-accelerated
dehydrogenation of hydrazine, where the ruthenium complex [Ru]-1 containing N-
hydroxyiminopyridine ligand outperformed over others. The evolved gas was analysed by
1
GC-TCD. Our experimental findings along with H NMR and mass studies evidenced the
presence of a hydrazine coordinated ruthenium species. These observations suggesting a
possible coordination assisted activation of hydrazine N-H bond over ruthenium catalyst to
release hydrogen gas via a Ru-hydride species. Subsequently, the involvement of Ru-hydride
species was further confirmed by achieving efficient transfer hydrogenation of nitro
substrates and styrene in good yields over [Ru]-1 catalyst in the presence of hydrazine.
Further efforts to explore more into the mechanistic aspect of the ruthenium catalyzed
dehydrogenation of hydrazine are underway.
Experimental Section
Materials. All experiments were carried out using the chemicals of higher purity purchased
from Sigma Aldrich, TCI and Alfa Aesar unless otherwise specified, except the ligands and
complexes prepared by us. Ru(II)-arene complexes were synthesized according to previous
reports using, Ru(II)-arene precursors, [{(η6-benzene)RuCl2}2].[36]
Instrumentation. 1H NMR (400 MHz) spectra were recorded at 298 K using CDCl3, D2O
and DMSO-d6 as a solvent on a Bruker Avance 400 spectrometer using tetramethylsilane
(TMS) as an external standard. Chemical shifts (in ppm) are relative to the center of the
1
singlet at 7.26 ppm for CDCl3, 4.75 ppm for D2O and 2.49 for DMSO-d6 in H NMR
respectively. Coupling constant (J) values are reported in hertz (Hz), and the splitting patterns
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10.1002/asia.201800315
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are designated as follows: s (singlet); d (doublet); t (triplet); m (multiplet); br. (broad). ESI-
mass spectra (Positive mode) were recorded on a micrOTF-Q II mass spectrometer. UV-
visible absorption spectra were recorded on a Carry-60 UV-visible Spectrophotometer using
10 mm quartz cuvettes. GC−TCD analysis was performed on a Shimadzu GCMS-QP2010
Ultra and GC-2010 Plus system in EI (electron impact) mode using RT-Msieve 5A column.
Single crystal X-ray diffraction Studies. Single crystal X-ray structure studies of
complex [Ru]-2, from the suitable crystals grown by slow diffusion of diethyl ether into the
methanol solution of complex [Ru]-2 was accomplished on a CCD Agilent Technologies
(Oxford Diffraction) SUPERNOVA diffractometer. Crystal data for complexes [Ru]-2 was
collected at 293(2) by the standard ‘phi-omega’ scan techniques and were scaled and reduced
using CrysAlisPro RED software, using graphite-monochromated Cu Kα radiation (λ =
1.5418 Å) based diffraction. The extracted data was evaluated using the CrysAlisPro CCD
software. The structures were solved by direct methods using SHELXS-97, and refined by
full matrix least-squares with SHELXL-97, refining on F2.[37] Direct methods determined the
positions of all the atoms. All non-hydrogen atoms were refined anisotropically. The
remaining hydrogen atoms were placed in geometrically constrained positions and refined
with isotropic temperature factors, generally 1.2Ueq of their parent atoms. The CCDC
number 1535290 contains the supplementary crystallographic data for [Ru]-2. The data is
freely available at www.ccdc.cam.ac.uk (or can be procured from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB21 EZ, UK; Fax: (+44) 1223-
336-033; or deposit@ccdc.cam.ac.uk).
Catalytic dehydrogenation of hydrazine over ruthenium catalysts. A two neck
round bottom flask attached to a condenser and equipped with a magnetic bar was charged
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with ruthenium catalyst (2.5 mol %), potassium tertiary butaoxide (20 mol%), THF-methanol
(5.5 mL, 10:1 v/v). The resulting solution was heated at 80 °C for 15 minute until the colour
changes from yellow to dark brown. Now addition of hydrazine in the solution resulted in a
colour change from dark brown to the red brown. The gas released during the reaction was
passed through a hydrochloric acid solution (2.0 M) before measured volumetrically by a
water displacement method. Later, the presence of hydrogen in the generated gas was
confirmed by GC-TCD.
Mass spectral studies to identify the hydrazine-coordinated ruthenium species
[Ru]-1A. In a separate experiment, complex [Ru]-1 (0.020 mg, 0.05 mmol) and an excess of
hydrazine monohydrate (25 µL, 0.5 mmol) were mixed in 1 mL solution of H2O. The
reaction mixture was stirred at room temperature for 5 minute, where a colour change from
yellow to wine-red was observed. An aliquot of the reaction mixture was analysed by mass
spectrometry to identify a hydrazine coordinated Ru(II)-arene species ([Ru]-1A) at m/z
333.0. Similarly, under the catalytic reaction condition, aliquot from the crude reaction
mixture was taken out to analyse by mass spectrometry and similar intermediate species
[Ru]-1A was observed.
NMR spectral studies to identify the hydrazine-coordinated ruthenium species
[Ru]-1A. To confirm the identity of a plausible hydrazine coordinated species [Ru]-1A by
1H NMR, complex [Ru]-1 (0.010 g) in D2O (500 µL) and hydrazine monohydrate (12.5 µL)
were taken into an NMR tube and analysed by 1H NMR.
Recyclability experiments. For the recyclability experiment after every cycle the 20
mol% potassium tertiary butoxide added along with 1 mmol of hydrazine and gas was
produced measured by volumetrically.
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Catalytic transfer hydrogenation of nitro substrates. Ru(II)-arene complex [Ru]-1
(0.0093 g, 2.5 mol%) was dissolved in water-ethanol mixture (5.0 mL, 1:1 v/v), followed by
the addition of nitro substrate (1 mmol) and hydrazine monohydrate (2 – 10 equiv.) in
graduated test tube. The reaction mixture was led to stir at 80 °C for desired reaction time in
open air condition. Progress of reaction was monitored by thin layer chromatography (TLC)
to observe the complete conversion of substrate. After complete reaction, the reaction mixture
was extracted with ethyl acetate (3 × 10 mL). The combined organic layer was dried over
sodium sulfate, and solvents were removed in vacuo to afford the desired product. All the
products were identified by 1H NMR spectroscopy.
Acknowledgements
Authors gratefully acknowledge the financial support from IIT Indore, SERB-DST and CSIR.
Discipline of Chemistry, IIT Indore and SIC, IIT Indore are acknowledged for laboratory and
instrumentation facilities. M.K.A. thanks MHRD and INSPIRE-DST for his fellowship. D.T.
thanks UGC-CSIR for her SRF grant. S.P. and R.K.R. thanks MHRD and IIT Indore,
respectively for their fellowships.
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Table of Content
Catalytic dehydrogenation of hydrazine was achieved over iminopyridine ligated ruthenium-
arene complexes, where the release of H2 gas, as confirmed by GC-TCD, from hydrazine
depends on reaction temperature, base, and solvents. NMR and mass studies evidenced the in
situ generation of a hydrazine coordinated ruthenium species, a key intermediate of hydrazine
dehydrogenation via a coordination assisted activation pathway.
23
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