Chemoselective transfer hydrogenation to nitroarenes mediated by cubane-type Mo3S4 cluster catalysts

Article abstract of DOI:10.1002/anie.201202584

Chemoselective cubes: Cubane-type [Mo₃S₄X ₃(dmpe)₃]⁺ clusters (dmpe=1,2-(bis) dimethylphosphinoethane), in combination with an azeotropic 5:2 mixture of HCOOH and NEt₃ as the reducing agent, act as selective cluster catalysts (X=H) or precatalysts (X=Cl) for the transfer hydrogenation of functionalized nitroarenes, without the formation of hazardous hydroxylamines. Copyright

Full text of DOI:10.1002/anie.201202584

Angewandte
Chemie
DOI: 10.1002/anie.201202584
Molybdenum Cluster Catalysis
Chemoselective Transfer Hydrogenation to Nitroarenes Mediated by
Cubane-Type Mo3S4 Cluster Catalysts**
Ivꢀn Sorribes, Gerrit Wienhçfer, Cristian Vicent, Kathrin Junge, Rosa Llusar,* and
Matthias Beller*
Functionalized anilines are valuable intermediates for the
manufacture of dyes, pigments, agrochemicals, and pharma-
ceuticals.[1] Furthermore, they represent key precursors for
the synthesis of important chemicals, such as imines[2] and
aromatic azo compounds.[3] Among the well-established
synthetic methods for the preparation of aromatic amines,
the chemoselective reduction of nitro groups is preferred on
both industrial and laboratory scales. In general, the most
frequently used catalysts for this benign nitro group hydro-
genation make use of noble metals such as Pt and Pd
supported on active carbon or alumina, although Ni-based
systems are applied as well.[4]
A crucial issue for the general application of catalysts for
nitro reduction is selectivity. In this respect, adding surface
modifiers, tailoring the metal particle size, or modifying the
support, can all improve the chemoselectivity of heteroge-
neous catalysts. Recently, Corma and co-workers demon-
strated that Au-based catalysts show high selectivity for the
reduction of the nitro arenes.[5] Nevertheless, the limited
availability of precious metals makes it desirable to search for
alternatives that are more economical. For example, signifi-
cant attention is being focused on iron-based catalysts as
a promising solution.[6] Compared to heterogeneous catalysis,
homogeneous catalysis often offers a more rational tuning of
the catalysts to obtain a broad functional group tolerance and
high activity under mild conditions.
reduction of nitroarenes.[7,8] Hence, a wide variety of organ-
ometallic catalysts based on Pd,[7] Ru,[7–9] Rh,[7] Co,[10] Cu,[10]
and even Fe[11] have been described. However, to the best of
our knowledge, molybdenum complexes have never been
used as catalysts following this protocol. Furthermore, few
examples of molybdenum-mediated reductions have been
reported and all employ hydrogen,[12] expensive silanes,[13] or
stoichiometric amounts of molybdenum compounds.[14]
Based on our experience with the transfer hydrogenation
mechanism for the reaction between trinuclear Mo3S4 cluster
hydrides and acids,[15,16] we became interested in pursuing
their potential applications as catalysts in the reduction of
organic substrates. Molecular complexes of the general
formula [M3S4X3(diphosphane)3]+, shown in Figure 1, are
air-stable and easily accessible, which makes them excellent
Figure 1. Cubane-type M3Q4 clusters.
Transfer hydrogenation using formic acid provides an
attractive alternative to catalytic hydrogenation, where auto-
claves and the handling of hydrogen are required for the
candidates for further mechanistic and catalytic investiga-
tions. The M3S4 cluster unit in these complexes is a robust
entity in solution and as such it does not coexist in equilibrium
with lower nuclearity species. In general, the reaction
of [M3S4H3(dmpe)3]+ (dmpe = 1,2-(bis)dimethylphosphino-
ethane) with acids occurs with the formation of dihydrogen-
bonded species, either as transition states or reaction inter-
mediates.
Transition metal clusters may lead to multiple metal–
metal reactivity, providing new families of catalysts that
exhibit novel or unique properties.[17] Well-defined cluster
complexes have also been postulated as reasonable models
for heterogeneous catalysis. Therefore, a better understand-
ing, in which mechanistic arguments prevail over heuristic
experiments, of the reaction steps governing these homoge-
neous processes is crucial for the improved design of more
efficient catalysts.
[*] I. Sorribes, Prof. Dr. R. Llusar
Departament de Quꢀmica Fꢀsica i Analꢀtica
Universitat Jaume I
Av. Sos Baynat s/n, 12071 Castellꢁ de la Plana (Spain)
E-mail: rosa.llusar@uji.es
Dr. C. Vicent
Serveis Centrals d’Instrumentaciꢁ Cientꢀfica
Universitat Jaume I
Av. Sos Baynat s/n, 12071 Castellꢁ de la Plana (Spain)
G. Wienhçfer, Dr. K. Junge, Prof. Dr. M. Beller
Leibniz-Institut fꢂr Katalyse e.V. an der Universitꢃt Rostock
Albert Einstein Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
[**] This work was supported by the state of Mecklenburg-Vorpommern,
the BMBF, the Deutsche Forschungs-gemeinschaft (Leibniz prize to
M.B.), the Spanish MICINN (CTQ2008-02670 and CTQ2011-23157,
and a predoctoral FPU fellowship to I.S.) and the Generalitat
Valenciana (ACOMP/2011/037 and PROMETEO/2009/053).
Herein, we present the first study on the molybdenum-
catalyzed transfer hydrogenation of nitroarenes. Our work
shows that trinuclear Mo3S4 hydrides functionalized with
outer diphosphane ligands are excellent catalysts for the
highly selective reduction of nitroarenes to the corresponding
anilines.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202584.
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At the start of our work, the reduction of nitrobenzene
was investigated as a benchmark system. Optimization of the
reaction conditions (temperatures, hydrogen sources, catalyst
loading, and solvents; see also the Supporting Information,
Table SI1) showed that the best results were obtained in
tetrahydrofuran at 708C, where full conversion and up to
99% yield of aniline were observed. Among the different
hydrogen sources (Table 1, entries 1–5) an azeotropic 5:2
1) direct reduction and 2) an indirect pathway involving
condensation of the initially formed nitrosoarene and the
hydroxylamine to produce azo derivatives.[18] The feasibility
of this second route was investigated by using azoxybenzene
or azobenzene as reactants. These experiments were carried
out using a higher cluster loading (20 mol%), because the
transient nature of these intermediates during the reaction
from nitrobenzene makes the relative amount of catalyst
much higher than the normal catalytic amount. The reduction
of azobenzene needed up to 18 h to reach a conversion of
90%, with only a 25% yield of aniline. When azoxybenzene
was used as a reactant, azobenzene appeared as the primary
product (20% yield) and aniline was only formed in 16%
yield. Therefore, we conclude that the formation of aniline in
the presence of the active molybdenum hydride cluster
preferentially takes place through the direct reduction route.
Obviously, cluster catalysis requires that the polynuclear
metal fragment remains intact during the catalytic cycle or at
least that mononuclear species, which are produced from
cluster decomposition, are not responsible for the catalytic
activity. To prove cluster catalysis, several experiments were
carried out using various lower nuclearity molybdenum
complexes (Table SI3) in the presence and absence of the
dmpe ligand under the optimized catalytic conditions. In all
cases, little or no reactivity was observed, which shows the
unique behavior of the Mo3S4 hydride. Further evidence of
cluster integrity during the catalytic reaction comes from
reaction monitoring by electrospray ionization mass spec-
trometry (ESI-MS) techniques, which in turn can be used to
extract valuable mechanistic information.[19] For ESI-MS
reaction monitoring, the recently reported pressurized
sample infusion (PSI) method was used.[20] Under catalytic
conditions, the base peak corresponds to Et3NH+, which
inhibits ionization of the rest of the cluster species under ESI
conditions. Consequently, higher cluster loading (50 mol%) is
needed when ESI-MS is used to unravel the nature of the
species present during the reaction. For this purpose, the
reaction mixture was heated to 708C and re-examined over
time for 90 min, whereupon a 25% yield of aniline was
formed (Figure 2).
At room temperature (t = 0 min), a prominent peak
centered at m/z = 870.9 is initially observed, which is assigned
to [Mo3S4H3(dmpe)3]+ (1+) hydride on the basis of the m/z
value as well as its characteristic isotopic pattern. Additional
signals of the monosubstituted [Mo3S4H2(OOCH)(dmpe)3]+
(2+) and disubstituted [Mo3S4H(OOCH)2(dmpe)3]+ (3+) for-
mate complexes at m/z = 914.8 and m/z = 959.8, respectively,
are also observed. After 15 min, the trisubstituted [Mo3S4-
(OOCH)3(dmpe)3]+ (4+) formate complex is also identified at
m/z = 1000.8. After 30 min (up to 90 min), only 1+ and 4+ are
observed and their relative intensities remain largely
unchanged, indicating that a stationary state has been
achieved. Apparently, 1+ is the active species, which is
subsequently regenerated from the 4+ formate cluster. Addi-
tional support for this hypothesis is provided by investigation
of the characteristic gas-phase dissociation of 4+ by collision
induced dissociation (CID) experiments, which are commonly
used to unravel elementary steps in catalytic cycles.[21] In
general, these experiments demonstrate that Mo–formate
Table 1: Reductant testing for the reduction of nitrobenzene catalyzed by
molybdenum clusters.[a]
Entry
X
Reducing agent
Conversion [%][b]
Yield [%][b]
1
2
3
H
H
H
H
H
H
H
Cl
Cl
Cl
HCOOH/Et3N (5:2)
HCOO(NH4)
HCOOH
PhSiH3
>99
32
23
>99
9
73
41
–
65
>99
>99
29
22
75
5
73
35
–
4[c]
5[d]
6[e]
7[f]
8[f]
9
H2
HCOOH/Et3N (5:2)
–
–
HCOOH/Et3N (5:2)
HCOOH/Et3N (5:2)
63
>99
10[g]
[a] Reaction conditions: nitrobenzene (0.1 mmol), reducing agent
(3.5 equiv), catalyst (3 mol%), THF (2 mL), 10 h, 708C. [b] Determined
by GC analysis using n-hexadecane as an internal standard. [c] Catalyst
(8 mol%), 18 h. [d] 5 or 30 bar H2, 15 h, 808C. [e] Catalyst (2 mol%).
[f] Catalyst (55 mol%), 18 h. [g] 19 h.
mixture of HCOOH and NEt3 gave the best result as
a reducing agent. Longer reaction times are required when
formic acid or formates were used for the reduction. The
reaction selectivity decreased (75% yield) when expensive
silanes such as phenylsilane are employed. Relevant mecha-
nistic information is obtained from the lack of reactivity
observed when the reaction is carried out under H2, as only
traces of aniline (5%) are detected. This result clearly
indicates that Mo3S4-mediated nitro reduction catalysis
occurs by transfer hydrogenation, discarding the possibility
of direct hydrogenation by hydrogen generated from formic
acid decomposition.
Furthermore, optimal quantities of reducing agent were
found by gradually increasing the concentration of formic
acid using the azeotropic HCOOH/NEt3 5:2 mixture. In the
presence of stoichiometric amounts (3 equiv) of HCOOH,
a 97% yield of aniline is obtained; thus indicating that there is
basically no unproductive decomposition of formic acid to
hydrogen (Table SI2).
Because of safety concerns, the formation of hydroxyl-
amine as a reaction intermediate constitutes a drawback in
some catalytic reductions of nitroarenes. Fortunately, in our
case the conversion of nitrobenzene occurred almost quanti-
tatively with > 99% yield of aniline. No traces of hydroxyl-
amine were observed during catalysis (< 1%; Figure SI1).
From a mechanistic point of view, the (transfer) hydro-
genation of nitroarenes can occur through two routes:
2
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As shown in Scheme 1, we tentatively
propose the following catalytic cycle for the
molybdenum cluster species involved in the
reduction of nitrobenzene: Initially,
HCOOH reacts with the cluster hydride
site to generate an unstable dihydrogen
bonded species. This hypothesis is sup-
ported by our previous studies on the
mechanism of proton transfer to molybde-
num and tungsten diphosphino M3S4 cluster
hydrides.[15,16] Next, hydrogen is transferred
to nitrobenzene to afford the formate-sub-
stituted cluster complex. This last step is
favored in the presence of formate in
solution. Subsequent b-hydride elimination
accompanied by CO2 release regenerates
the cluster hydride. In the specific case
where the Mo3S4 cluster chloride is used as
a catalyst precursor, the chlorine atom is
replaced by formate to afford the corre-
sponding hydride active cluster.
Figure 2. ESI mass spectra from monitoring of the nitrobenzene reduction at a) t=0 min ,
b) t=15 min, and c) t=90 min.
Next, we were interested in the func-
tional group tolerance of this novel catalyst
system. Hence, more than 30 different nitro-
functional groups in the trinuclear 4+ cluster are prone to be
transformed into Mo–hydride groups through the liberation
of CO2, very likely by a b-hydride elimination process (Figure
SI2).[11,22] Once it was shown that degradation of the hydride
cluster does not take place during the catalytic process, some
additional experiments were done to confirm 1+ as the active
species in the catalytic cycle (Table 1, entries 7–8). Following
our typical protocol, the reaction of 1+ (55 mol%) in the
absence of any additional reducing agent affords aniline in
35% yield, but no reaction is observed when the hydride
cluster is replaced by its chloride analogue. On the other
hand, when the chloride cluster is reacted in combination with
HCOOH and NEt3, an excellent yield of aniline (> 99%) is
obtained, although longer reaction times are needed (19 h). In
consequence, all of this experimental evidence allows us to
conclude that the Mo3S4 hydride cluster is in fact the active
species and that this complex can also be generated from its
halide analogue upon reaction with formic acid (Table 1,
entries 9–10).
substituted arenes and heteroarenes were tested under the
optimized conditions. As shown in Table 2, various alkyl-
substituted nitroarenes are reduced to afford the correspond-
ing anilines in up to 99% yield (Table 2, entries 1–4). When
the nitro group is sterically hindered, an increased catalyst
loading is needed (Table 2, entries 3 and 4). Interestingly, no
dehalogenation processes are observed during the reduction
of halogenated nitroarenes, which give the halogen-substi-
tuted aniline products in 97–99% yield (Table 2, entries 5–
12), independently of the number of halogens and their ring
position. Notably, 4-nitrobenzotrifluoride and 5-bromo-2-
nitrobenzotrifluoride are also completely reduced to the
corresponding anilines in excellent yield (Table 2, entries 13–
14). This cluster catalyst is also able to reduce nitroanilines to
diaminoanilines in 90–99% yields (Table 2, entries 15–18),
which can also be obtained from the reduction of 2,4-
dinitrotoluene in quantitative yields (Table 2, entry 19).
From a synthetic point of view, the selective transfer
hydrogenation of nitroarenes functionalized with other
reducible groups is more challenging. In this respect, we
tested nitroarenes with different functional groups, such as
nitrile, olefin, ketone, ester, amides, and even aldehydes. To
our delight, the corresponding anilines are obtained in good
to excellent yields (Table 2, entries 20–31). Finally, we applied
different heterocyclic and biphenyl nitroarenes, which also
gave the respective anilines in 95–99% yield (Table 2,
entries 32–35).
In conclusion, cubane-type [Mo3S4X3(dmpe)3]+ clusters
have been established for the first time as excellent catalysts
(X = H) or precatalysts (X = Cl) for the reduction of func-
tionalized nitroarenes using formates as a reducing agent.
Notably, the procedure is highly chemoselective in the
presence of other reducible functional groups. Hence, the
corresponding functionalized anilines are obtained in 89–
99% yield. Furthermore, through the use of a pressurized
Scheme 1. Proposed catalytic cycle for the molybdenum cluster species
involved in the reduction of nitrobenzene.
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Table 2: Reduction of nitroarenes catalyzed by molybdenum clusters in
Experimental Section
the presence of several functional groups.[a]
General procedure for the reduction of nitrobenzene: Under an
argon atmosphere, dry nitrobenzene (10 mL, 0,097 mmol), dry
n-hexadecane (20 mL; added as an internal standard) and HCOOH
(3.5 equiv), as a 5:2 mixture of HCOOH/Et3N (29.4 mg), were added
to a red solution of [Mo3S4(dmpe)3H3]BPh4 (3.47 mg, 0.0029 mmol) in
dry THF (2 mL). The reaction mixture was heated to 708C for 10 h.
Then the mixture was cooled down and a sample was taken from the
resulting green solution. All catalytic reactions were performed at
least twice to ensure reproducibility. To determine the yield of
isolated products for different functionalized anilines, the general
procedure was scaled up by the factor of five and no internal standard
was added. After full conversion was achieved, diethyl ether (70 mL)
was added to the reaction mixture to precipitate the cluster catalyst.
The resultant suspension was filtered through celite and the solvent of
the filtrate was removed under vacuum. In a general protocol,
triethylamine was separated from the anilines by column chromatog-
raphy (silica gel; n-hexane/ethyl acetate, 9:1!6:4). Alternatively, for
solid anilines, triethylamine can be removed by washing the solid
residue that remains after solvent evaporation with n-hexane.
Trinuclear [Mo3S4X3(dmpe)3](BPh4) (X = H, Cl) clusters were
prepared according to literature methods.[16,23]
Entry Substrate
Catalyst
(mol%)
Conversion[b] [%] Yield[b] [%]
1
2
3
R=4-Me
R=4-tBu
R=2-iPr
3
3
6
10
3
3
3
3
3
3
3
3
3
3
3
6
3
3
6
3
3
3
3
3
3
3
3
>99
>99
>99
98
>99 (96)[c]
>99
>99
97
4
5
R=2,5-Me
R=2-Cl
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
98 (93)[c]
>99
>99
97 (95)[c]
97
6
R=4-Cl
7
R=3-Cl
8
R=2-I
9
R=4-Br
10
11
12
13
14
15
R=4-F
R=3,4,5-Cl
R=3-Cl;4-F
R=4-CF3
R=2-CF3;4-Br
R=3-NH2;4-Me
98 (92)[c]
99
93
>99
>99
>99
90
94
>99
93
Received: April 3, 2012
Revised: May 10, 2012
Published online: && &&, &&&&
16[d] R=2-Me;5-NH2
17
18
R=4-NH2
R=2-NHMe
19[e] R=2-Me;5-NO2
Keywords: cluster catalysis · molybdenum · nitrobenzene ·
20
21
22
23
24
25
26
27
R=4-OMe
R=4-SMe
R=4-CN
R=3-CHCH2
R=4-CHCHPh
R=4-COMe
R=4-CO2Me
R=4-
.
96
90
reduction · transfer hydrogenation
>99 (95)[c]
>99
>99
>99 (96)[c]
99
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Molybdenum Cluster Catalysis
I. Sorribes, G. Wienhçfer, C. Vicent,
K. Junge, R. Llusar,*
M. Beller*
&&&& — &&&&
Chemoselective Transfer Hydrogenation
to Nitroarenes Mediated by Cubane-Type
Mo3S4 Cluster Catalysts
Chemoselective cubes: Cubane-type
act as selective cluster catalysts (X=H)
or precatalysts (X=Cl) for the transfer
hydrogenation of functionalized nitro-
arenes, without the formation of hazard-
ous hydroxylamines.
[Mo3S4X3(dmpe)3]+ clusters (dmpe=1,2-
(bis)dimethylphosphinoethane), in com-
bination with an azeotropic 5:2 mixture of
HCOOH and NEt3 as the reducing agent,
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!