Catalytic Deoxygenation of Nitroarenes Mediated by High-Valent Molybdenum(VI)-NHC Complexes

Article abstract of DOI:10.1021/acs.organomet.0c00352

The high-valent molybdenum(VI) N-heterocyclic carbene complexes, (NHC)MoO2 (1) and (NHC)MoO(NtBu) (2) (NHC = 1,3-bis(3,5-di-tert-butyl-2-phenolato)-benzimidazol-2-ylidene), are investigated toward their catalytic potential in the deoxygenation of nitroarenes. Using pinacol as the sacrificial and green reductant, both complexes are shown to be very active (pre)catalysts for this transformation allowing a reduction of the catalyst loading down to 0.25 mol %. Mechanistic investigations show μ-oxo bridged molybdenum(V) complexes [(NHC)MoO]2O (4) and [(NHC)Mo(NtBu)]2O (5) as well as zwitterionic pinacolate benzimidazolium complex 6, with a doubly protonated NHC ligand, to be potentially active species in the catalytic cycle. Both 4 and 5 can be prepared independently by the deoxygenation of 1 and 2 using triethyl phosphine (PEt3) or triphenyl phosphine (PPh3) and were shown to exhibit an unusual multireferenced ground state with a very small singlet-triplet gap at room temperature. Computational studies show that the spin state plays an unneglectable role in the catalytic process, efficiently lowering the reaction barrier of the deoxygenation step. Mechanistic details, putting special emphasis on the fate of the catalyst will be presented and potential routes how nitroarene reduction is facilitated are evaluated.

Full text of DOI:10.1021/acs.organomet.0c00352

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Article
Catalytic Deoxygenation of Nitroarenes Mediated by High-Valent
Molybdenum(VI)−NHC Complexes
Shenyu Liu, Jorge Ivan Amaro-Estrada, Marc Baltrun, Iskander Douair, Roland Schoch, Laurent Maron,*
and Stephan Hohloch*
Cite This: Organometallics 2021, 40, 107−118
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sı Supporting Information
ABSTRACT: The high-valent molybdenum(VI) N-heterocyclic carbene
t
complexes, (NHC)MoO (1) and (NHC)MoO(N Bu) (2) (NHC = 1,3-
2
bis(3,5-di-tert-butyl-2-phenolato)-benzimidazol-2-ylidene), are investigated
toward their catalytic potential in the deoxygenation of nitroarenes. Using
pinacol as the sacrificial and green reductant, both complexes are shown to be
very active (pre)catalysts for this transformation allowing a reduction of the
catalyst loading down to 0.25 mol %. Mechanistic investigations show μ-oxo
bridged molybdenum(V) complexes [(NHC)MoO] O (4) and [(NHC)Mo-
2
t
(
N Bu)] O (5) as well as zwitterionic pinacolate benzimidazolium complex 6,
2
with a doubly protonated NHC ligand, to be potentially active species in the
catalytic cycle. Both 4 and 5 can be prepared independently by the
deoxygenation of 1 and 2 using triethyl phosphine (PEt ) or triphenyl
3
phosphine (PPh ) and were shown to exhibit an unusual multireferenced ground state with a very small singlet−triplet gap at room
3
temperature. Computational studies show that the spin state plays an unneglectable role in the catalytic process, efficiently lowering
the reaction barrier of the deoxygenation step. Mechanistic details, putting special emphasis on the fate of the catalyst will be
presented and potential routes how nitroarene reduction is facilitated are evaluated.
INTRODUCTION
Scheme 1. Generalized Scheme of the Deoxygenation of
Sulfoxides, Nitroarenes, and Vicinal Diols by Oxygen Atom
Transfer Catalysis
■
The development of new catalytic systems that can promote
the reduction of oxidatively over functionalized molecules
under environmentally benign and economic conditions is one
1
,2
of the key targets in industrial and academic research.
Despite many achievements have already been realized in this
3
−5
area,
the search for milder reaction conditions, the
reduction of the catalyst loading, or the avoidance of precious
metal catalysts, as well as pressurized reactors, still remain
important goals.
6
−9
While traditional methods such as direct hydrogenation
10
or transfer hydrogenation have been intensively explored in
this area, these methods still have some major draw backs, as
complicated
or the need for pressurized
reactors, which are not commonly available in all laboratories,
4
,11−15
Furthermore, OAT reactions focus only on oxygen containing
functional groups, which makes them ideal candidates for
chemoselective reductions compared to “classical” hydro-
genation reactions.
Prominent candidates facilitating oxygen atom transfer
catalysis (OATC) are high-valent molybdenum complexes as
the need for precious metal catalysts,
chemo-)selectivity strategies,
16−20
(
21
as well as the need for potentially hazardous hydrogen gas.
Although the latter can be overcome by using transfer
14−16,22−24
hydrogenation processes
with different hydrogen
delivering reagents, e.g., isopropanol or silanes, these protocols
often require a large excess of hydrogen suppliers and may
create large amounts of (hazardous) waste byproducts.
Received: May 20, 2020
Published: January 4, 2021
13,25−28
In this context, oxygen atom transfer reactions (OATR,
Scheme 1) hold great promise, not only because of their high
2
9−33
atom efficiency
but also due to simple executions and the
34−36
need for cheap, earth-abundant early transition metals.
©
2021 American Chemical Society
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1
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intermediate 6 (in case of 4). Both, 4 and 5 were found to
exhibit a thermally accessible open-shell triplet state which is
responsible for an unexpected paramagnetism in these
complexes. Mechanistic details of the catalytic deoxygenation
of nitroarenes will be discussed, shedding light on the fate of
the catalyst and potential routes how nitroarene reduction is
facilitated.
Figure 1. Overview of high-valent Mo-NHC complexes recently
reported by our group.
RESULTS AND DISCUSSION
■
As already mentioned, a crucial factor a (pre)catalyst must
meet to be active in OATC, is to bind oxygen reversibly.
Additionally, it has been shown in the past that deoxygenated
metal complexes are prone to undergo (undesired) ligand
this element combines three advantages: (i) It is environ-
mentally friendly and earth abundant, (ii) it is relatively
cheap compared to many other 4d metals
capable of binding oxygen reversibly. Thus, complexes of the
general formula MoO Cl (L) (L = DMF, DPPE, and DME)
have recently been shown to be prominent catalysts for the
deoxygenation of sulfoxides, phosphine oxides, and N-oxides,
including nitroarenes.
3
7
3
7−39
and (iii) it is
83
decompositions. To ensure that our NHC complexes 1 and 2
meet both requirements, we initially examined their OAT
reactivity using phosphines as oxygen atom acceptors. Using
both triethyl- or triphenylphosphine (PEt or PPh ) resulted in
2
2
2
3
3
5
,40−45
However, one major drawback of
the clean formation of two new complexes 4 and 5 at 100 °C
(Scheme 2). Complex 4 could be isolated as a reddish brown
(Figure S29), and complex 5 could be isolated as a dark green
(Figure S30) powder in moderate yields of 56−81%. In
these systems is that although molybdenum is an environ-
mentally friendly metal a large amount of catalyst (between 5
4
6,47
and 10 mol %)
is still necessary to efficiently perform
5
OATC, especially for the reduction of nitroarenes to anilines.
One strategy that has been successfully employed in the past
years to reduce catalyst loadings and enhance the catalytic
potential of metal complexes is the introduction of one or
more N-heterocyclic carbene (NHC) ligands.
high-valent molybdenum(VI)-oxo NHC complexes having
been found to be rare and lack stability,
and co-workers have shown that NHC ligands are indeed
valuable supporting ligands in high-valent molybdenum- and
contrast, using trimethylphosphine (PMe ) at various temper-
3
atures (room temperature to 100 °C) only resulted in the
isolation of complicated mixtures which could not be further
separated. Upon further characterization of 4 and 5, we found
both complexes to display unexpected (paramagnetic) NMR
characteristics (Figure 2).
48−54
Despite
55−58
Buchmeiser
Both NMRs show paramagnetically shifted peaks ranging
from 10.3 to 1.30 ppm for complex 4 and from 8.50 to −1.11
ppm for complex 5. The presence of twelve distinct resonances
(Figure 2) furthermore indicates the formation of an
asymmetric (dinuclear) complex in solution. This is
unexpected, since previous deoxygenations of molybdenum-
(VI) dioxo complexes resulted in the formation of either
mononuclear Mo(IV) or symmetric dinuclear μ-oxo Mo(V)
5
9−67
tungsten-catalyzed metathesis reactions.
Closing the gap
for dioxo, oxo-imido, and bis-imido molybdenum complexes
we have recently discovered the facile and simple synthesis of a
series of high-valent molybdenum(VI) NHC complexes 1−3
6
8,69
(
compare Figure 1)
taking advantage of anionic tethered
7
0−79
35
NHC-ligands,
using a pincer-type OCO-benzimidazolyli-
complexes. The latter are formed if the steric bulk of the
dene ligand originally introduced by Bellemin-Laponnaz and
Bercaw.
Herein, we will evaluate the catalytic potential of 1 and 2 in
the OATC converting nitroarenes into anilines using pinacol as
a green and environmentally friendly reductant. Deoxygenation
of 1 and 2 yields the oxo-bridged dimers 4 and 5, via a putative
ligands is too small, allowing putative molybdenum(IV)
complexes to comproportionate with the remaining
molybdenum(VI) complexes to form μ-oxo bridged dinuclear
8
0−82
30
molybdenum(V) complexes. While solid-state structures
(vide infra) suggested the presence of a C symmetric species,
2v
only C symmetry is observed in solution. To test if large
s
Scheme 2. Deoxygenation of Complex 1 or 2 to Corresponding Oxo-Bridged Dimers 4 and 5 Using Either Triethyl- Or
Triphenylphosphine
1
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Figure 2. Proton NMR signatures of the new dimeric oxo bridghed complexes 4 and 5 showing unexpected paramangetically shifted resonances.
rotational barriers, e.g., along the μ-oxo unit, could be
responsible for lowering the symmetry in solution, we
performed variable-temperature (VT) NMR in the range of
we have only been able to obtain low-quality crystals, and the
pronounced air sensitivity of 5 (<1 min in the crystalline form)
prevented us from obtaining high-quality data, prohibiting a
reliable discussion of any bond parameters in 5. Nevertheless,
the connectivity in 5 could be unambiguously determined, and
from the similarity of the NMR characteristics of complexes 4
and 5, it can be assumed that the structural parameters follow
the same tendencies. Both complexes crystallize in the
2
43−333 K (instrumentation limits). Although we have seen
an approach between the resonances in this temperature range,
coalescence of the signals could not be reached (Figures S2
and S9).
Due to the paramagnetic nature of complexes 4 and 5, clear
assignments of the NMR signals cannot be made. However, for
complex 5 we have been able to observe resonances at δ 392.3
monoclinic system in the space groups P2 /n (for 4) and
1
C2/c (for 5) with various strongly disordered solvent
molecules (benzene and hexane) in the lattice, which needed
to be removed by SQUEEZE techniques (see the Supporting
Information for further details). For complex 4, we have also
been able to detect 0.5 equivalents of non-coordinated
triethylphosphine in the asymmetric unit. Both complexes 4
and 5 form μ-oxo bridged dimers in solution resulting in a 5-
fold coordination of each molybdenum center creating a
1
3
1
and 391.5 ppm in the C{ H} spectrum most likely
corresponding to the benzimidazolylidene carbene atoms.
Additionally, for complex 5 we have been able to observe the
1
5
1
15
N chemical shift using H− N-HMBC NMR spectroscopy,
showing a signal at δ 502.7 ppm corresponding to the N-tert-
butyl imido ligand. The presence of two distinct carbene
resonances in 5 further supports the presumption of an
asymmetric dinuclear species in solution; the abnormal
chemical shifts supports the paramagnetic nature of the
complexes. To unambiguously prove the paramagnetic nature
of the complexes, we measured its magnetic moment using the
Evans method in solution. This revealed an effective magnetic
moment of 2.22 μ (1.11 μ per Mo) and 1.96 μ (0.98 μ per
slightly distorted square pyramidal (τ = 0.10 for Mo1 and τ =
5
5
0.03 for Mo1A for complex 4, Figure 3) environment around
the molybdenum ions. The molybdenum carbene distances
Mo1−C1 and Mo1−C1A in complex 4 were found to be
2.181(4) and 2.182(4) Å and are slightly shorter compare to 1
68
(2.193(4) Å). The oxo bridge in complex 4 was found to be
symmetric, displaying Mo−O distances of 1.903(3) and
1.892(3) Å for Mo1−O10 and Mo1A−O10, respectively,
with a Mo−O−Mo angle of 154.6(2)°. Further details on the
crystallographic structure and structural parameters can be
found in Tables S1 and S2.
B
B
B
B
Mo-ion) μB for 4 and 5 respectively. These values are
substantially lower than the spin only values of 1.73 μ
B
1
84
expected for an isolated d configured Mo(V) ion, which
would be anticipated if no antiferromagnetic coupling between
the molybdenum centers existed. 2D-NMR techniques
(
COSY, HSQC, and HMBC, Figures S4−S6 for 4 and Figures
In summary, the structural data indicate a molybdenum-
(V)−molybdenum(V) configuration of the complex. However,
in the past, μ-oxo bridged molybdenum(V)−molybdenum(V)
complexes have been found to be diamagnetic, exhibiting
strong antiferromagnetic coupling between the two molybde-
1
S11−S13 for 5) as well as H DOSY NMR (Figures S7 and
S15) furthermore unambiguously proved the presence of a
single (asymmetric) complex in solution. It is noteworthy at
this point that in the case of complex 4 H- and P-NMR
spectra show the presence of uncoordinated triethylphosphine
1
31
85−90
num centers.
This is in stark contrast to the paramagnetic
3
1
oxide (δ P = 46.2 ppm), which we have not been able to
remove despite copious hexane washing or recrystallization.
X-ray quality crystals of both complexes were grown by slow
diffusion of pentane (for 4) or hexane (for 5) into a
concentrated benzene solution of the complexes at room
temperature. To our delight, the structural investigation,
together with the results from NMR spectroscopy (vide
supra) unambiguously proved that our NHC ligand is inert
toward metal deoxygenations. Unfortunately, for complex 5,
NMR signatures and the obtained magnetic moments in
solution of our systems. To get further insight into the
electronic structure of the complex, computational inves-
tigations were carried out at both the DFT (B3PW91) and
CASSCF level (see Figure 4; for further details, see the
Supporting Information, section 8). The geometries of
complexes 4 and 5 were optimized at the DFT level for
different spin states (singlet, triplet, and quintet). For complex
4, the geometry agrees with the experimental one (see Table
1
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Mo1A angle is a little bit too open (167° vs 154°). Although it
was not possible to obtain high-quality X-ray data for complex
5
, the computational structure was compared with that of
complex 4. The bridging Mo1−O10 distance is slightly longer
in 5 than that in 4 (1.90 Å vs 1.88 Å) but within the same
range. Similarly, the Mo−C1 distance is essentially the same
(
2.17 Å in 5 vs 2.18 Å in 4) and the Mo1−O−Mo1A angle is
found to be around 153° (see Table S2 for further detail). The
electronic structures of the two complexes were then
investigated using CASSCF calculations. CAS(6,5) calculations
were then conducted for the two complexes in order to check
the nature of the ground state and of the excited states. In both
cases, the ground state is found to be an open-shell singlet
1
formed from two Mo(V) d centers. This open-shell singlet is
therefore responsible for a TIP behavior at low temperature,
that would account for the low magnetic moment found for the
91−95
two complexes, as demonstrated by Booth.
Moreover, the
first excited state is in both cases, the associated triplet and is
very close in energy (0.09 eV for complex 4 and 0.07 eV for
complex 5). Thus, the triplet state can be populated at room
temperature which increases the magnetic moment of the two
complexes. The other excited states, accounting for lower
oxidation state of Mo, are at least 2.1 eV higher in energy.
Thus, the computational investigations of complexes 4 and 5
are clearly indicating the presence of two molybdenum(V) ions
that are antiferromagnetically coupled in an open-shell singlet
ground state but with the open-shell triplet state being
thermally accessible, leading to the magnetic moments
observed by Evans method (vide supra).
Figure 3. Molecular structures of complex 4 (top) and 5 (bottom).
Hydrogen atoms, tert-butyl groups and lattice solvents have been
omitted for clarity.
Catalytic Deoxygenations. Knowing that complexes 1
and 2 can be deoxygenated and that the NHC ligand is not
affected under these conditions, we next investigated the
catalytic potential of these complexes in the deoxygenation of
nitroarenes. As an initial starting point, we have tried to
deoxygenate p-nitrotoluene using 1 mol % catalyst (1 or 2) and
triphenylphosphine as a sacrificial oxygen atom acceptor at 130
°
C. Unfortunately, these conditions were found to be very
inefficient, resulting in no conversion at all. However, taking a
deeper look into the stoichiometry of this reaction, this is not
surprising as the efficient conversion of nitroarenes to anilines
requires the presence of at least two protons. Switching from
triphenylphosphine to pinacol as a green and environmentally
9
6
benign reducing agent, we found the clean and fast
conversion of p-nitrotoluene to toluidine within 8 h using
only 1 mol % loading of the corresponding catalysts at 130 °C
(
Figure S33). Lower temperatures, e.g., 110 °C, or omitting
the catalysts were both found to be detrimental to the catalytic
reaction (Table 1). Inspired by the relatively low catalyst
loading of only 1 mol %, we were further interested how much
the catalyst loading could be decreased.
Indeed, using only 0.25 mol % resulted in similar results, and
we found full conversion of p-nitrotoluene after 8 h of reaction
time (Table 1 and Figure S33). To set this into relation, we
also calculated the corresponding TON and TOF values. At
0
.25 mol % loading, the highest TOF we observed corresponds
Figure 4. Frontier orbitals of complex 4 (left) and 5 (right) and the
corresponding energy differences between the open-shell singlet
ground state and the open-shell triplet excited state of complex 4 and
to about 50 turnovers per hour. This is a roughly 5-fold
increase compared to previous reports by Sanz and co-
96
workers. Comparison of the reaction time profiles performed
on the reaction with 0.25 mol % of catalyst using either
complex 1 or 2 as a precatalyst shows a very similar
progression of the reaction (Figure S33), further indicating
that the active species forming might be identical for both of
these complexes.
5
calculated by CASSCF.
S2) and appears to be not much influenced by the spin state.
Among others, the Mo1−C1 distance is very well reproduced
(
2.18 Å) as is the Mo1−O10 distance (1.88 Å). The Mo1−O−
1
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Table 1. Condition Screening for the Deoxygenation of p-
Nitrotoluene Using Complexes 1 or 2 as (Pre-)Catalysts
Scheme 3. Scope of the Catalytic Deoxygenation Reactions
Using Complex 1 as a Precatalyst
a
cat.
mol % T (°C) conv.
TON
TOF (h−1)
1
2
1
2
1
2
1
2
1
1
1
1
0.5
0.5
0.25
0.25
0
130
130
110
110
130
130
130
130
130
130
99%
99%
0%
100
100
0
12.5
12.5
0
0%
0
0
99%
99%
99%
99%
0%
200
200
400
400
0
25
25
50
50
0
b
MoO Cl (DMF)
2
5
93%
18.6
9
2
2
a
Conversions are determined via GC using mesitylene as an internal
b
standard. Reaction has been carried out for 2 h in xylene; see ref 96.
Since water is liberated during the reaction (vide supra),
hydrolysis of the imido group in complex 2 to form complex 1
is very likely to happen. Due to these circumstances and
because the synthesis of complex 1 is more convenient
6
8
compared to 2, we decided to focus on complex 1 for further
studies, regarding the mechanism and the substrate scope of
the catalytic reaction.
Elucidating the substrate scope of the catalytic deoxygena-
tion reaction, we decided to raise the catalyst loading to
1
mol %, to make sure challenging nitroarenes can also be
efficiently converted to the corresponding anilines. The
catalytic reactions have been carried out using four equivalents
of pinacol per nitro group at 130 °C for 24 h. The results have
been summarized in Scheme 3. We found that complex 1
efficiently catalyzes the deoxygenation of unfunctionalized,
alkyl substituted nitroarenes, including o-nitrotoluene as well as
unsubstituted nitrobenzene. This gave access to the corre-
sponding anilines in good yields, after a short workup (see the
Supporting Information). Turning our focus on the functional
group tolerance, we initially examined o- and p-nitro-
acetophenone, which could both be efficiently converted into
the corresponding anilines in isolated yields of 94% (ortho)
and 89% (para). In contrast, using p-nitrobenzaldehyde and
despite the full consumption of p-nitrobenzaldehyde, we have
not been able to isolate any p-aminobenzaldehyde from the
reaction. The crude reaction mixture indicated the formation
of a complex mixture of products, displaying various imine
a
Yields of the corresponding nitroarenes determined by GC
b
measurements using mesitylene as an internal standard. Conversion
of 4-nitrobenzaldehyde. No formation of the corresponding aniline
could be detected by GC. A total of 8 equiv of pinacol have been
applied. Values in brackets are isolated yields.
c
we also investigated the reduction of 4-(tert-butyl)-2,6-
dinitrophenol with 8 equiv of pinacol (4 equiv per nitro
group) which resulted in the full conversion of nitroarene. The
latter two examples show that protic phenols (and potentially
other substrates up to pK = 7) can be efficiently deoxygenated
a
under the conditions present. In summary, our substrate
screening shows that even at catalyst loadings of only 1 mol %
complex 1 is an efficient (pre)-catalyst for the deoxygenation
of various nitroarenes with excellent group tolerance.
Mechanistic Investigations. Having established our
molybdenum complexes 1 and 2 to be indeed very active
(pre)catalyst for the deoxygenation of nitroarenes, we next
turned our interest toward how the reactions proceeds
mechanistically. Since the time-reaction profiles recorded
(Figure S33) indicate that the active species formed could be
the same for both (pre)catalysts 1 and 2, the following
investigations will be limited to complexes 1 and 4. Special
emphasis was put on the question of whether isolated μ-oxo
complex 4 could be a potential intermediate in the catalytic
cycle. Therefore, two major question needed to be addressed:
(a) Is the oxo-bridged complex 4 also formed if pinacol is used
as the sacrificial reductant? (b) Is the μ-oxo-bridged complex 4
capable of deoxygenating nitroarenes and other possible
intermediates, e.g., nitrosoarenes, formed during the deoxyge-
nation processes?
1
signals in the low-field region of its H NMR. We assume that
due to the high temperature necessary for the reaction to
proceed, the aldehyde condensates directly with the formed
amine to (polymeric) imines. Polymerization of amino-
97
benzaldehydes has also been seen previously in the literature.
Turning to more electron-withdrawing substituents such as
fluorides and esters, we have also observed moderate to good
conversions of the corresponding nitroarenes. However, this
indicates that for strongly electron-withdrawing substituents
the higher catalyst loadings might be necessary to reach full
conversion of the substrates. Switching to electron-donating
substituents, we found 70% conversion of 2,6-dimethyl-4-
nitrophenol, which could be isolated in a yield of 45% after
workup from the reaction mixture. Probing the efficiency of
our catalyst in the reduction of multiple nitro groups at once,
1
11
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Scheme 4. Test Reaction under Various Conditions to Prove the Formation of Complex 4 Using Pinacol as the Sacrificial
Reductant
In analogy to Sanz and co-workers, who were able to isolate
96
4
μ-oxo-bridged complexes MoO Cl (DMF) from the reac-
3
4
tion between MoO Cl (DMF) and pinacol, we also heated
2
2
2
complex 1 in the presence of excess pinacol at 130 °C
overnight. However, instead of forming complex 4, this only
resulted in the formation of intractable mixtures. Switching to
less harsh conditions, the reaction between equimolar amounts
of pinacol and complex 1 at room temperature affords a new
complex, 6, which can be isolated in good yields of 96%
1
(
Method A, see SI) as a white solid. The H NMR spectrum of
complex 6 showed C symmetry in solution and the presence
1
of four singlets between 1.11 and 0.5 ppm integrating to
3
:3:3:3 protons clearly indicate the presence of coordinated
13
pinacol. Surprisingly, the C NMR showed no resonance
above 155.4 ppm, which suggests the absence of an NHC
ligand. Aside from the structural proof (vide infra), the
assignment of the proposed structure of complex 6 in Scheme
4
can be confidently made based on several features in the 2D-
NMR data of the isolated material. Protonation of the
benzimidazolium unit can be unambiguously assigned by the
presence of a cross peak at 8.87/143.5 ppm in the H- C
HSQC NMR (see Figure S19). Protonation of a phenolate
unit is furthermore indicated by the presence of three features:
First, no coupling of the proton resonating at 8.25 pm to any
1
13
81
1
other arene proton is observed neither in the 1D- H NMR nor
in H− H-COSY NMR. Second, the proton was found to
1
1
1
13
couple to a quarterny arene carbon atom in the H− C
HMBC at 8.25/141.8 ppm, which was also found to couple to
one tert-butyl group at 1.51/141.8 ppm (Figure S20). Third,
while the carbon atom at 155.4 ppm corresponding the
deprotonated two C−O group was found to couple to only
two arene protons (7.57/155.4 and 7.32/155.4 ppm), the
carbon atom of the protonated C−O group resonates at 149.7
ppm showing couplings to two arene protons (7.61/149.7 and
Figure 5. Molecular structures of complexes 6 (top) and 7 (bottom).
Hydrogen atoms, tert-butyl groups, and additional lattice solvents
have been removed for clarity.
the former carbene center. Additionally, the benzimidazolium-
2H proton can be found in the Fourier map, unambigously
confirming the protonation of the benzimidazolylidene unit.
Notably, the protonated phenol group of the ligand displays
strong hydrogen bonding to one of the MoO oxo ligands
(compare Figure 5, top structure). Interestingly, the proto-
nation of the NHC unit is in strong contrast to our and
Buchmeiser’s previous findings demonstrating the Mo−NHC
7
.09/149.7 ppm) and the additional OH-proton at 8.25/149.7
ppm. This coupling pattern can only be observed if one of the
phenolate residues is protonated. Protonation of the
benzimidazolylidene unit as well as the phenolate unit would
67,68
last explain the obtained C symmetry in solution. Ultimate
bond stable toward protic reagents and water.
However, it
1
proof for the protonation of the benzimidazolylidene moiety in
might be worth mentioning that in Buchmeiser’s case, the
inherent stability of the cationic Mo−NHC complexes toward
protic substrates can be partly explained by the delocalization
of the positive charge across the NHC ligand, rendering it to
6
was obtained by X-ray diffraction analysis. X-ray quality
crystals of 6 were obtained after two days from a solution of 1
in benzene layered by a solution of pinacol in benzene at room
temperature. The complex crystallizes in the monoclinic space
group C2/c with two molecules of benzene in the asymmetric
unit. The molybdenum center in 6 is five-coordinate by only
oxygen donors in a strongly distorted square pyramidal fashion
be less basic. Additionally, the MoO fragment is more Lewis
2
acidic compared to the imido-alkylidene fragment, which
facilitates protonation of the NHC moiety in such complexes.
This is in line with seminal reports by Herrmann and co-
workers, stating that the NHC ligand in dioxo-molybdenum
(
τ = 0.45). The Mo1−C1 distance was found to be 3.684(3)
5
55
Å clearly showing no interaction between the metal center and
NHC complexes can be easily hydrolyzed. Thus, these
1
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Scheme 5. Stochiometric Reactions between Complex 4 and o-Nitro/Nitrosotoluene and Azobenzene to Investigate the
a
Mechanism of the Catalytic Deoxygenation Reaction of Nitrotoluene
a
VI
V
Color coding: Mo : yellow, Mo : turquoise. Note that the triethylphosphine oxide ligand in 7 results from an unremovable impurity of
triethylphosphine oxide in complex 4.
findings suggest that the stability of the NHC-Mo bond cannot
be generally determined and is depending on various effects
like the substitution pattern on the Mo(VI) fragment and the
coordination mode, e.g., potential chelators such as pinacol, of
the protic substrate used.
nitrotoluene using complex 5 is completed within a few
seconds, and mechanistic investigations using an internal
standard proves that 2 equiv of anticipated complex 2 are
reformed under these conditions (Figure S37 and Figure S41)
The fact that complex 4 is indeed capable of efficiently
deoxygenating o-nitrotoluene suggests it to be a potential
intermediate in the catalytic deoxygenation of nitroarenes.
Since Arnold and co-workers have recently shown that metal
complexes with multiconfigurational ground states can exhibit
To seek further evidence if complex 4 can be formed from
complex 6, we heated 6 to 130 °C which resulted in
catastrophic decompositions (Figure S35). Acknowledging
the fact that 4 most likely forms via the synproportionation of a
Mo(IV) and a Mo(VI) complex, we repeated the experiment
in the presence of 1 equiv of complex 1. Notably, these
conditions led to the formation of desired compound 4 (see
Figure S36). However, we want to emphasize that the reaction
under these conditions is far from clean, and given the fact that
the carbene unit can be protonated under catalytic conditions,
the formation of other molybdenum complexes without NHC
coordination, which could be potential catalysts for the
99−101
unique and unexpected reactivity patterns,
we were
curious if the spin state (singlet vs triplet) of our complexes
would also have an influence on the activation barrier of the
deoxygenation reaction. Therefore, we calculated the energy
profile of the deoxygenation reaction of nitrobenzene mediated
by complex 4 in dependency of the spin state (Figure S45).
This revealed the coordination of nitrobenzene to be
exothermic, leading to a splitting of 4 to make complex 1
and the molybdenum(IV) complex 8 (see also Scheme 6).
Processing along the reaction coordinate revealed that effective
barrier for the deoxygenation process is lower by 5 kcal/mol if
a triplet state of the catalyst is assumed compared to the singlet
state. This strongly indicates that the spin state of the catalyst
has a major impact on the catalytic potential of these metal
complexes, which might be another explanation for the high
catalytic potential of the complexes (vide supra).
96
deoxygenation reaction, cannot be fully excluded (vide infra).
Stoichiometric Deoxygenation Reactions. Having
shown that the formation of μ-oxo complex 4 is indeed
possible under catalytic conditions, our next focus was to
explore if 4 would be a potentially active species in the
deoxygenation of nitroarenes. Notably, Sanz and co-workers
stated earlier that MoO Cl (DMF) has similar catalytic
3
4
4
properties as the parent MoO Cl (DMF) when pinacol is
2
2
2
96
used as a sacrificial reductant. To our delight, mixing
stoichiometric amounts of 4 with o-nitrotoluene resulted in the
complete consumption of complex 4 after 96 h at room
temperature (see Figure S38). However, NMR spectroscopy
revealed that not the expected complex 1 is reformed under
these conditions, but a new complex 7 is formed. (Scheme 5).
Remembering that complex 4 could only be obtained with
uncoordinated triethylphosphine oxide as an impurity, we have
been able to identify this new complex to be triethylphoshine
Despite showing that complexes 4 and 5 could be both
catalytically active species in the deoxygenation of nitroarenes,
we were further interested in the role of complex 6 during the
catalytic reaction. Although we have shown that 4 can be made
under catalytic conditions by heating an equimolar mixture of
complexes 1 and 6 (vide supra), the large excess of pinacol
(compared to precatalyst 1) used in the reaction suggests that
complex 6 might also play an important role in the catalytic
reaction. Despite we have not been able to isolate any species
from the thermal decomposition of complex 6 at 130 °C
(Figure S35), adding an equimolar amount of 4-nitrotoluene
prior to heating facilitated the clean reformation of complex 1
at 130 °C after 18 h, along with the consumption of the
nitroarene and the liberation of 2 equiv of acetone (resulting
from the reductive splitting of the pinacol bound to the Mo
center in 6, compare Figure S43). This clearly states that not
only complex 4 could be a potential active species but also 6
might also play a vibrant role during the catalytic cycle.
Pathways of Nitroarene Deoxygenation Catalysis.
Looking deeper into how the reduction of nitroarenes could be
facilitated mechanistically, there are two different potential
routes that can occur (Scheme 6). Pathway A would cover the
reduction of nitroarenes via nitrosoarenes and its direct
1
31
oxide adduct 7 by H and P NMR spectroscopy. It is
noteworthy that complex 7 can also be obtained independently
9
8
by the reaction of 1 with 1 equiv of triethylphosphine oxide.
1
3
1
The presence of a resonance at δ = 194.5 ppm in the C{ H}
NMR spectrum unambiguously confirms the assignment of a
NHC complex. The phosphorus signal shifts from 46.2 to 55.7
ppm, supporting the coordination of triethylphosphine oxide.
X-ray quality crystals of 7 could be obtained by storing a very
concentrate solution of 7 in toluene at −40 °C for 3 days
(
Figure 5). Since the structural metrics resemble the ones
68
found in complex 1, we would like to resign on a detailed
discussion here. Additional information regarding the bond
angles and crystal metrics can be found in Table S1 and S2. We
would like to note at this point that the deoxygenation of o-
1
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Scheme 6. Possible Catalytic Pathways and Intermediates of the Deoxygenation of Nitroarenes to Anilines Mediated by
a
Complex 1 via either 4 or 6 as a potentially active species.
a
VI
V
IV
Color coding: Mo : yellow, Mo : turquoise, Mo : brown.
conversion to the corresponding aniline. However, since the
catalytic reaction is carried out at 130 °C, it is also possible that
once small amounts of anilines have formed these could
condensate with nitrosoarenes present to form azoarenes. The
latter could then be activated by complex 4 or 6, and
hydrolysis of the corresponding imido complex 9 would lead to
the desired aniline, reforming complex 1 (Pathway B, see
Scheme 6).
fairly well that the azobenzene route (pathway B) plays only a
minor role for the catalytic reduction of nitroarenes to anilines
mediated by NHC molybdenum complex 1. This can be
further rationalized by the fact that o-nitrosotoluene is
efficiently deoxygenated by complex 4 already at room
temperature (vide supra), preventing the build up of higher
nitrosoarene concentration and thus lowering the chances for
the successful condensations of nitrosoarenes and anilines to
form the corresponding azobenzenes under catalytic con-
ditions (130 °C).
Since complexes 4 and 5 were already shown to effectively
deoxygenate o-nitrotoluene, we next examined its reactivity
toward o-nitrosotoluene. Indeed, full conversion from 4 to 7
was achieved within 1 h at room temperature (Figure S39). To
exclude that Pathway B, via the condensation to azobenzene,
now plays an important role in the catalytic cycle, if 4 would be
the active species in the catalytic cycle, we have investigated
the stoichiometric reaction between 4 and azobenzene (Figure
S40). Indeed, no reaction occurred even after 2 days at 130 °C,
confirming complex 4 being incapable of splitting azobenzene.
In contrast, exploring the catalytic cycle and potentially
intermediates with the pinacol complex 6, we found that 6 is
besides deoxygenating nitroarenes (vide supra and Figure S43)
also capable of splitting azobenzene stoichiometrically (Figure
S44), Interestingly, we have not been able to isolate the
anticipated oxo-imido complex 9 from the reaction mixture,
but dioxo complex 1 was reisolated. Although this was
unexpected in the first place, it can be rationalized by the
fact that somewhere during the catalytic process water must be
eliminated to reform the metal−carbene and metal−phenolate
bonds. At 130 °C reaction temperature, water is very likely to
hydrolyze any imido complex to reform the dioxo complex
CONCLUSION
■
In conclusion, we have shown molybdenum(VI) complexes 1
and 2, reported earlier by our group, are very active
68
(
pre)catalysts for the deoxygenation of nitroarenes to form
anilines using pinacol as a green sacrificial reductant. Under the
present conditions, we have been able to reduce the catalyst
46
loading by up to a factor of 20, compared to previous reports
with an excellent tolerance of functional groups, except for p-
nitro-benzaldehyde. For the latter, the high temperatures
needed for the deoxygenation to proceed most likely induces
the polymerization of the anticipated p-amino-benzaldehyde.
Mechanistic investigations have shown that under catalytic
conditions both, new dinuclear μ-oxo molybdenum(V)
complexes 4 and 5 as well as the unexpected pinacolate
complex 6 with a protonated benzimidazolium unit can be
formed. Notably, 4 and 5 can also be formed by deoxygenation
of parent complexes 1 and 2 with triethylphosphine as an
oxygen atoms acceptor. Both complexes were found to adopt
an unusual multiconfigurational configuration, with the open-
shell-triplet state to be only marginally above the (expected)
open-shell-singlet state in energy which allowed thermal
population of the former state. This results in an unexpected
paramagnetism for both complexes 4 and 5. Both complexes, 4
and 5, have been found to be excellent oxygen atoms acceptors
at room temperature, efficiently deoxygenating nitroarenes and
nitrosoarenes reforming parent complexes 1 and 2. Addition-
ally, complex 6 deoxygenates nitroarenes at 130 °C (under
catalytic conditions), also reforming complex 1. These results
indicate that both the μ-oxo complexes as well as
1
02
1.
The potential of 6 to split azobenzene is furthermore
undermined by the fact that complex 1 reduces azobenzene to
aniline catalytically using 4 equiv of pinacol as a reducing agent
and a catalyst loading of 1 mol % within 4 h at 130 °C (Figure
S34). Nevertheless, on re-examining the time conversion data
(Figure S33), no azobenzene could be detected in the reaction
mixtures. Although this is not clear proof that pathway A is the
dominant path (it is possible that the reduction of azobenzene
is extremely fast in solution, prohibiting the accumulation of a
detectable concentration of it in solution), these results suggest
1
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benzimidazolium complex 6 could play an important role in
the catalytic cycle. The activation barrier of the deoxygenation
step was further shown to be dependent on the spin state of
the complexes, and we found the barrier for complex 4 to be
about 5 kcal/mol lower for the triplet state compared to that
for the singlet state, which might further explain the excellent
catalytic potential of complexes 1 and 2. In addition, we
shedded light on the mechanism of the reduction process,
which most likely favors a stepwise, direct reduction from
nitroarenes via nitrosoarenes to anilines. However, the results
do not fully exclude the possibility of azoarenes as
intermediates in the catalytic cycle as it has been observed
Roland Schoch − Paderborn University, Faculty of Science,
Department of Chemistry, 33098 Paderborn, Germany;
orcid.org/0000-0003-2061-7289
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00352
Author Contributions
The project was designed by S.H. Experimental work was
conducted by S.L., M.B., and S.H. Theoretical investigations
were performed by J.I.A.-E., I.D., and L.M. X-ray crystal
analysis was performed by S.H. and R.S. The manuscript was
written by S.L., S.H., and L.M. and proofread by all authors.
15
for example in (transfer-)hydrogenations.
Future work is necessary to determine the effect of the
Notes
69
central carbene unit on the activity of the molybdenum(VI)
complexes in OAT reactions/catalysis, to lower the temper-
atures needed for effective OATC to proceed, to further
elucidate and distinguish between the two possible active
species. Additonally, we are aiming to indentify further modes
of reactivity prompted by the unusual electronic structure of
dinuclear complexes 4 and 5.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Daimler and Benz Foundation, to the
Fonds der Chemischen Industrie, to the Young Academy of
the North Rhine-Westphalian Academy of Sciences, Human-
ities and the Arts, to the Paderborn University and to the
University of Innsbruck for funding of this work. Prof. Dr. Jan
Paradies is kindly acknowledged for helpful discussion. The
authors thank Philipp Boos for synthesis of ligand precursor
ASSOCIATED CONTENT
sı Supporting Information
■
*
[
H L]Cl. Our special thanks go to Dr. Hans Egold from
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00352.
3
Paderborn University and to Prof. Dr. Christoph Kreutz from
the University of Innsbruck for assistance with NMR
measurements. Finally, S.H. expresses his special thanks to
Prof. Dr. Wolfgang Kaim whose lectures, passion for
unexpected electronic structures of metal complexes, and
continuous support have been one of the foundation stones for
pursuing a career in academia. This work is dedicated to Prof.
Dr. Wolfgang Kaim on the occasion of his 70th birthday.
Compound structures (XYZ)
Synthetic details, NMR spectra, IR spectra, UV−vis
spectra, GC traces, and crystallographic details for all
compounds (PDF)
Accession Codes
CCDC 1993053, 1993054, and 2044177 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
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