Molecular Defined Molybdenum-Pincer Complexes and Their Application in Catalytic Hydrogenations

Article abstract of DOI:10.1021/acs.organomet.8b00410

A family of low-valent molybdenum complexes, supported by the pincer ligand (ⁱPr₂PCH₂CH₂)₂NH, was prepared and characterized. After activation by NaBHEt₃ coordination compounds 2 and 3-Cl were found to be suitable catalysts for the hydrogenation of ketones and olefins.

Full text of DOI:10.1021/acs.organomet.8b00410

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Molecular Defined Molybdenum−Pincer Complexes and Their
Application in Catalytic Hydrogenations
Thomas Leischner, Anke Spannenberg, Kathrin Junge, and Matthias Beller*
Leibniz Institut fur Katalyse e.V. an der Universitat Rostock, Albert-Einstein-Straße 29a, 18059 Rostock, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A family of low-valent molybdenum complexes,
supported by the pincer ligand (iPr2PCH2CH2)2NH, was
prepared and characterized. After activation by NaBHEt3
coordination compounds 2 and 3-Cl were found to be suitable
catalysts for the hydrogenation of ketones and olefins.
INTRODUCTION
■
Pincer ligands represent a well-known class of tridentate
ligands, and their coordination chemistry has been intensively
studied since the 1990s.1 Especially in the past decade,
numerous transition-metal complexes were reported, in part
with interesting catalytic properties.2 Notably, even the first
industrial applications have been realized employing these
compounds.3
Among the huge variety of different pincer ligands,
complexes featuring PNP backbones have been shown to be
particularly well suited for homogeneous hydrogenation
reactions.4 Here, the “bifunctional” character of the metal−
ligand system allows for a facile H atom transfer onto
unsaturated substrates.5,6
In comparison to other metals, the organometallic chemistry
of group 6 PNP-pincer complexes, in particular of Mo, was
poorly developed for a long time.5 Although this type of
complex was already described for the first time in the late
1980s by Haupt and Ellermann, only three different
compounds of this type were known as late as 2006.7 Later
on, especially Kirchner and co-workers, but also the groups of
Nishibayashi, Schneider, Jones, Bernskoetter, and Berke
reported on the synthesis of molybdenum PNP complexes.
The vast majority of the known compounds feature pincer
ligands with a central pyridine moiety (Figure 1).5,8
Figure 1. Selected examples of Mo(PNP) complexes.
addition to the works of Nishibayshi on the reduction of
dinitrogen, Jones and co-workers established the acid-assisted
isomerization of terminal olefins in the presence of PONOP-
based heptacoordinated hydrido tricarbonyl Mo PNP cata-
lysts.8h,10
As part of our ongoing interest in base-metal catalysis, we
herein report the synthesis of a series of new Mo(PNP) pincer
complexes containing the commercially available ligand
(iPr2PCH2CH2)2NH (Figure 2).
In addition, we report the hydrogenation of various ketones
and styrenes using molecular dihydrogen with 3-Cl as catalyst.
In contrast to the well-studied metal organic chemistry of
these Mo(PNP) complexes, reports on their catalytic activity
remain scarce. As an example, Berke and co-workers showed
that their Mo amido pincer complex (Figure 1), generated
from the parent Mo chloride upon treatment with NaHMDS,
was active in the catalytic hydrogenation of secondary imines,
nitriles, and CO2.5,9 In addition, Bernskoetter described the
reduction of carbon dioxide to formate by a molybdenum PNP
complex in the presence of DBU, LiOTf, and dihydrogen.8i To
the best of our knowledge, these are the only examples of
Mo(PNP)-catalyzed reductions using molecular H2.
Figure 2. Selected examples of Mo(PNP) pincer complexes.
Special Issue: Organometallic Complexes of Electropositive Ele-
ments for Selective Synthesis
Apart from that, only two other reports demonstrated the
potential of this class of complexes in catalytic applications. In
Received: June 13, 2018
© XXXX American Chemical Society
A
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exchange using KBr and NBu4Br, respectively, were also
unsuccessful.
RESULTS AND DISCUSSION
■
Synthesis of Molybdenum(0) and Molybdenum(I)
Complexes. We started our studies by preparing the complex
[Mo(PNP-iPr)(CO)3] (1). This compound can be easily
synthesized in 91% yield by refluxing Mo(CO)6 with a slight
excess of the PNP ligand (iPr2PCH2CH2)2NH in toluene for
16 h (Figure 3). It has to be noted that, in 1989, Ellermann
and co-workers already reported the synthesis of the related
complex [Mo(PNP-Ph)(CO)3] in 66% yield, using a related
methodology.7b,11
The herein reported coordination compounds have all been
characterized using IR spectroscopy and elemental analysis.
Additionally, the solid-state structures of all reported
complexes have been determined by X-ray crystallography.
Structural views are depicted in Figures 4−7, respectively, with
Figure 4. Molecular structure of 1 in the crystal. Only one molecule
of the asymmetric unit is depicted. Displacement ellipsoids
correspond to 30% probability. Hydrogen atoms except those
bound to N are omitted for clarity. Selected bond lengths (Å) and
bond angles (deg): N1−Mo1 = 2.3505(16), P1−Mo1 = 2.5387(5),
P2−Mo1 = 2.5304(5), Mo1−C17 = 1.974(2), Mo1−C18 =
1.9489(19), Mo1−C19 = 1.948(2); P1−Mo1−N1 = 76.92(4), P2−
Mo1−N1 = 77.59(4), P1−Mo1−P2 = 100.778(16), N1−Mo−C17 =
94.21(7), N1−Mo1−C18 = 171.83(7), N1−Mo1−C19 = 100.13(7).
Figure 3. Synthesis of the reported Mo-PNP pincer complexes.
Legend: (a) toluene, reflux, overnight, 1.05 equiv of
(iPr2PCH2CH2)2NH; (b) (1) MeCN/benzene, 1.10 equiv of
C3H5Br, reflux, overnight, (2) 3.00 equiv of PPh3, MeCN, reflux, 1
h, (3) 1.05 equiv of (iPr2PCH2CH2)2NH, toluene, room temperature,
overnight; (c) (1) MeCN/benzene, 1.10 equiv of C3H5Br, reflux,
overnight, (2) 3.00 equiv of PPh3, MeCN, reflux, 1 h, (3) 1.05 equiv
of (iPr2PCH2CH2)2NH, DCM, room temperature, overnight; (d) (1)
MeCN/benzene, 1.10 equiv of C3H5Br, reflux, overnight, (2) 1.05
equiv of (iPr2PCH2CH2)2NH, toluene, room temperature, 24 h.
Next, we prepared the known molybdenum precursors
M o ( η 3 - a l l y l ) ( C O ) 2 ( C H 3 C N ) 2 B r a n d M o -
(PPh3)2(CO)2(CH3CN)2 according to literature procedures.12
Subsequent treatment with the aforementioned PNP ligand in
toluene at room temperature resulted in the formation of the
new Mo complexes 2 and 3-Br, respectively, in 82% and 11%
yields. The paramagnetic 17-electron complex 3-Br, which
originates from the reaction between Mo(η3-allyl)-
(CO)2(CH3CN)2Br and (iPr2PCH2CH2)2NH, features a
molybdenum center in the oxidation state +I, instead of the
expected oxidation number +II.13 When we used Mo-
(PPh3)2(CO)2(CH3CN)2 as the molybdenum source, we
obtained complex 2 as a pale yellow and poorly soluble
precipitate as the reaction product. Thereby, we were surprised
to find that the central molybdenum atom in 2 still coordinates
an acetonitrile molecule in place of an anticipated PPh3 ligand.
When we attempted to obtain X-ray-quality single crystals of
complex 2 via crystallization from DCM/toluene, the desired
complex was not obtained, however. Instead, the 17-electron
Mo(I) complex 3-Cl was formed.13 Presumably the chlorine
atom originates from the solvent.
Figure 5. Molecular structure of 2 in the crystal. Displacement
ellipsoids correspond to 30% probability. Hydrogen atoms except for
those bound to N are omitted for clarity. Selected bond lengths (Å)
and bond angles (deg): N1−Mo1 = 2.3228(13), P1−Mo1 =
2.4389(4), P2−Mo1 = 2.4296(4), Mo1−C19 = 1.9201 (16), Mo1−
C20 = 1.9156(17), Mo1−N2 = 2.2274(14); N1−Mo1−P1 =
79.28(3), N1−Mo1−P2 = 79.14(3), N1−Mo1−C19 = 174.57(6),
N1−Mo1−C20 = 99.55(6), N1−Mo1−N2 = 80.67(5).
selected bond distances and angles given in the captions.
However, we were only able to characterize complex 1 using
standard NMR techniques. Due to the extremely low solubility
of complex 2 in most organic solvents, including benzene,
toluene, acetonitrile, THF, acetone, DMSO, and methanol, we
were unable to obtain any NMR data of complex 2. Moreover,
complex 2 proved to be unstable in CD2Cl2 and CDCl3,
presumably due to the acidic nature of these solvents or
insertion into the C−Cl bond. Complexes 3-Cl and 3-Br are
We subsequently tried to prepare 3-Cl directly from
Mo(PPh3)2(CO)2(CH3CN)2 and (iPr2PCH2CH2)2NH by
using DCM as the solvent. Indeed, we were able to isolate
3-Cl in 68% yield as a pale brown powder. Attempts to
synthesize compound 3-Br via the same route using CH2Br2 as
the solvent resulted in the formation of an inseparable mixture
of different species, even at low temperatures. Additional
attempts to prepare 3-Br starting from 3-Cl via halide
B
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the most pronounced for 3-Br (164.55(10)°) and steadily
decreases from 3-Cl (171.20(9)°) to 2 (174.57(6)°).
Catalytic Hydrogenation of Acetophenones and
Styrenes with 3-Cl. In initial attempts we tested
molybdenum PNP pincer complex 1 for the reduction of
carbonyl moieties using acetophenone as the benchmark
substrate. Unfortunately, with or without additives (vide
supra) no conversion was obtained. Similarly, when 3-Cl was
used in the absence of additives or in the presence of 10 mol %
of KOtBu or NaHMDS, we did not observe any conversion of
the starting material. However, addition of 10 mol % of
NaBHEt3 resulted in full conversion of acetophenone under
the given conditions (Table 1). Using the same methodology,
Figure 6. Molecular structure of 3-Br in the crystal. Only one
molecule of the asymmetric unit is depicted. Displacement ellipsoids
correspond to 30% probability. Hydrogen atoms except for those
bound to N are omitted for clarity. Selected bond lengths (Å) and
bond angles (deg): N1−Mo1 = 2.298(2), P1−Mo1 = 2.5208(7), P2−
Mo1 = 2.4834(7), Mo1−C17 = 1.923(3), Mo1−C18 = 1.970(3),
Mo1−Br1 = 2.7435(3); N1−Mo1−P1 = 76.57(5), N1−Mo1−P2 =
78.96(5), N1−Mo1−C17 = 97.05(9), N1−Mo1−C18 = 164.55(10),
N1−Mo1−Br1 = 82.10(5).
Table 1. Catalytic Hydrogenation of Acetophenone Using 3-
Cl and Different Additives
a
b
c
additive
amount (mol %)
conversion (%)
yield (%)
none
0
3
5
100
10
0
0
0
97
8
KOtBu
10
10
10
10
NaHMDS
NaBHEt3
NaBHEt3
d
a
b
0.5 mmol of substrate was used. Conversions were determined by
c
GC analysis using hexadecane (20 mg) as internal standard. Yields
were determined by GC analysis using hexadecane (20 mg) as
internal standard. No catalyst was used.
d
we demonstrated that complex 2 gave the same result.
Nevertheless, due to practical reasons, we focused on 3-Cl in
the further course of our studies.
In order to rule out any heterogeneous side reactions during
the hydrogenation catalysis, a mercury poisoning experiment
was carried out. For this purpose, a drop of mercury was placed
in the reaction vial and the catalysis was carried out as
described above. To our delight, GC analysis of the resulting
reaction mixture still showed full conversion of the benchmark
substrate after 3.5 h, thus indicating that the developed system
follows a homogeneous pathway. Deterred by the harsh
conditions, we decided to investigate whether an increased
catalyst loading would allow for a lower reaction temperature.
Indeed, increasing the catalyst loading to 5 mol % led to full
conversion of acetophenone in toluene at 80 °C, though an
extended reaction time of 16 h was necessary. Additionally, we
were able to reduce the amount of NaBHEt3 used to 5 mol %.
Next, a variety of para-substituted acetophenones was
subjected to the reaction conditions developed. For the
given starting materials excellent conversions (90−100%)
and isolated yields (84−91%) were obtained in all cases
(Table 2).
Figure 7. Molecular structure of 3-Cl in the crystal. Displacement
ellipsoids correspond to 30% probability. Hydrogen atoms except for
those bound to N are omitted for clarity. Selected bond lengths (Å)
and bond angles (deg): N1−Mo1 = 2.3029(15), P1−Mo1 =
2.5002(5), P2−Mo1 = 2.4877(5), Mo1−C17 = 1.9118(19), Mo1−
C18 = 1.954(2), Mo1−Cl1 = 2.5817(4); N1−Mo1−P1 = 77.10(4),
N1−Mo1−P2 = 78.80(4), N1−Mo1−C17 = 100.53(7), N1−Mo1−
C18 = 171.20(9), N1−Mo1−Cl1 = 81.05(4).
NMR silent due to their paramagnetic nature and therefore
could not be analyzed by this method.
In accordance with the initial report of Ellermann and co-
workers on the related compound [Mo(PNP-Ph)(CO)3],
complex 1 also adopts a fac coordination geometry in the solid
state.7b The recorded IR spectrum likewise shows three
absorption bands at 1924, 1805, and 1758 cm−1, respectively.
The IR spectra for 2, 3-Cl, and 3-Br all show medium to
strong carbonyl absorption bands between 1889 and 1676
cm−1.
Complexes 2, 3-Br, and 3-Cl all exhibit a distorted-
octahedral arrangement of the donor atoms around the Mo
center with the CO ligands being in a cis orientation. The N1−
Mo, P1−Mo, and P2−Mo bond lengths as well as the N1−
Mo−P1 and N1−Mo−P2 bond angles are in the same range
for all of the mentioned compounds. Noteworthy are the
deviations of the bond angles N1−Mo−C18 for 3-Br and 3-Cl
and N1−Mo−C19 for 2 from ideal linearity. This deviation is
When we used chalcone (Table 2, entry 7) as a substrate, in
order to investigate whether our catalyst system displayed any
selectivity toward either the CC double bond or the
carbonyl moiety, we found complete reduction of both
functional groups and were able to isolate 1,3-diphenyl-1-
propanol in 91% yield.
On the basis of this result, we decided to investigate the
general reactivity of our catalyst system toward terminal and
internal CC double bonds in more detail. Here, 1-dodecene,
C
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Table 2. Hydrogenation of Various Acetophenones with 3-
Cl/NaBHEt3
Table 3. Hydrogenation of Various Sytrenes with 3-Cl/
a
a
NaBHEt3
a
b
0.5 mmol of substrate was used. Yield determined by GC using
c
hexadecane (20 mg) as internal standard. NMR yield determined by
d
19F NMR of the crude mixture using C6F6 as internal standard. NMR
yield determined by 1H NMR of the crude mixture using 1,3,5-
trimethoxybenzene as internal standard.
benzonitrile proceeded with a modest conversion of 42% and
gave N-benzylidenebenzylamine as the major reaction product
with a yield of 29% along with 13% of benzylamine. All yields
and conversions reported in this paragraph were determined by
GC using hexadecane (20 mg) as internal standard.
CONCLUSIONS
■
a
b
0.5 mmol of substrate was used. Conversions determined by GC
In conclusion, we have synthesized a series of four structurally
new molybdenum complexes featuring the commercially
available PNP-pincer ligand (iPr2PCH2CH2)2NH. These new
coordination compounds can all be prepared in a straightfor-
ward manner by starting from known molybdenum precursors
and were characterized by IR spectroscopy, elemental analysis,
and X-ray crystallography. We demonstrated that the Mo(I)
species 3-Cl is a suitable catalyst for the hydrogenation of
various acetophenones and styrenes in the presence of
NaBHEt3 as an additive. Moreover, we could show that 3-Cl
is active in the hydrogenation of diphenylacetylene and
benzonitrile, though low selectivities were observed.
c
using hexadecane (20 mg) as internal standard. Isolated yields given.
d
24 h reaction time.
styrene, and cis- and trans-stilbene were applied as model
substrates. While 1-dodecene, as well as cis- and trans-stilbene,
only showed low conversions of 4%, 25%, and 29%,
respectively, styrene gave 76% conversion into ethylbenzene.
Hence, we tested a number of different styrenes, bearing
electron-donating and -withdrawing substituents on the
aromatic ring (Table 3). In general, we obtained medium to
good yields (57−85%), though harsher conditions in
comparison to those for the reduction of acetophenones
were needed. Notably, no cleavage of the benzyl ether moiety
was observed during the reduction of 3-benzoyloxy-3-
methoxystyrene (Table 3, entry 5).
In order to evaluate the potential of complex 3-Cl in a
broader sense, we finally tested diphenylacetylene and
benzonitrile as substrates under the conditions shown in
Table 3. When diphenyleneacetylene was applied, we obtained
a conversion of 75%. However, the reaction turned out to be
unselective, yielding a mixture of 41% 1,2-diphenylethane, 17%
cis-stilbene, and 17% trans-stilbene. The hydrogenation of
EXPERIMENTAL SECTION
■
General Experimental Information. Unless otherwise stated, all
reactions were performed under an argon atmosphere with exclusion
of moisture and air from reagents and glassware using standard
Schlenk and glovebox techniques for manipulating air-sensitive
compounds. Dry and oxygen-free solvents (acetonitrile, DCM,
toluene, THF, and heptane) were collected from an Innovative
Technology PS-MD-6 solvent purification system and stored over 3 Å
molecular sieves. CD2Cl2 was purchased from Eurisotop, degassed by
freeze−pump−thaw techniques, and subsequently dried over 3 Å
molecular sieves. All other chemicals were purchased and used
D
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without further purification. Mo(CO)6, NaBHEt3 (1 M in THF),
acetophenone, 4-F-acetophenone, 4-MeO-acetophenone, 4-CF3-ace-
tophenone, 4-MeS-acetophenone, chalcone, 3-benzoyloxy-3-methox-
ystyrene, and trans-stilbene were purchased from Sigma-Aldrich.
(iPr2PCH2CH2)2NH was obtained from Strem (10 wt % solution in
THF). Styrene, 4-F-styrene, 4-MeO-styrene, and 4-CF3-styrene were
purchased from TCI. 4-Ethylacetophenone and cis-stilbene were
purchased from Alfa Aesar. 1-Dodecene was obtained from Fluka.
M o ( η 3 - a l l y l ) ( C O ) 2 ( C H 3 C N ) 2 B r 1 2 a n d M o -
suspended in 5 mL of toluene and cooled to 0 °C in an ice bath.
(iPr2PCH2CH2)2NH (1.54 mL of a 10 wt % solution in THF, 444.8
μmol, 1.05 equiv) was added dropwise, and the reaction mixture was
warmed to room temperature overnight. The resulting opaque liquid
was filtered, and the solid residue was extracted two times with 2 mL
of toluene. The combined filtrates were dried under reduced pressure,
and the resulting orange-brown solid was washed two times with 5
mL of heptane. The remaining residue was dissolved in the minimum
amount of DCM and layered with heptane to afford 16.2 mg (11%
yield) of 3-Br as tiny brownish crystals. Crystals suitable for X-ray
analysis were obtained by slowly allowing a layer of heptane to diffuse
into a saturated solution of 3-Br in DCM at room temperature. Anal.
Calcd for C18H37BrMoNO2P2: C, 40.24; H, 6.94; N, 2.61. Found: C,
40.42; H, 7.01; N, 2.65. IR (ATR, cm−1): 1894 (νCO), 1780 (νCO),
1751 (νCO).
Synthesis of 3-Cl. In a 50 mL Schlenk flask, Mo-
(PPh3)2(CO)2(CH3CN)2 (540.2 mg, 712.0 μmol, 1.00 equiv) was
suspended in 15 mL of DCM and cooled to 0 °C in an ice bath.
(iPr2PCH2CH2)2NH (2.6 mL of a 10 wt % solution in THF, 747.6
μmol, 1.05 equiv) was added dropwise, and the reaction mixture was
warmed to room temperature overnight. The resulting clear brown
liquid was filtered, and the solvent was removed under reduced
pressure until product precipitation occurred. The supernatant was
filtered off, and the remaining ocher solid was washed three times
with 5 mL of heptane/DCM 3/1 (v/v) and three times with 5 mL of
THF. Removal of residual solvent under reduced pressure
subsequently gave 238.6 mg (68% yield) of 3-Cl as a pale brown
powder. Crystals suitable for X-ray analysis were obtained from a
saturated solution of 3-Cl in acetonitrile at 4 °C. Alternatively single
crystals can be obtained by slowly allowing a layer of heptane to
diffuse into a saturated solution of 3-Cl in DCM at room temperature.
Anal. Calcd for C18H37ClMoNO2P2: C, 43.87; H, 7.57; N, 2.84.
Found: C, 43.89; H, 7.30; N, 2.75. IR (ATR, cm−1): 1889 (νCO),
1782 (νCO), 1747 (νCO).
General Procedure for Catalytic Experiments. Under an
argon atmosphere, a 4 mL glass vial containing a stirring bar was
charged with complex 3-Cl (12.5 mg; 5 mol %). Afterward, the
reaction vial was capped with a septum and equipped with a syringe
needle and toluene (2 mL) was added. The resulting brown
suspension was treated with NaBHEt3 (0.5 M in THF, 0.05 mL, 5
mol %) and stirred for 10 min, and the corresponding substrate was
subsequently added to this mixture. The reaction vial was transferred
into an autoclave. Once sealed, the autoclave was purged 10 times
with 10 bar of dihydrogen, before the pressure was set to the desired
value (50 bar). The reaction mixture was stirred for 16 h in a
preheated aluminum block at 80 °C for ketones and 24 h at 130 °C
for styrenes, respectively. Afterward, the autoclave was cooled in an
ice bath and the remaining gas was released carefully. The solution
was subsequently diluted with ethyl acetate and filtered through a
small pad of Celite (1 cm in a Pasteur pipet). The Celite was washed
with methanol (2 mL), and the combined filtrates were evaporated to
dryness afterward. The remaining residue was purified by column
chromatography (SiO2, heptane/EtOAc, gradient 100/0 → 0/100).
For the characterization of the products of the catalysis, see the
Supporting Information.
12
(PPh3)2(CO)2(CH3CN)2 were prepared according to literature
1
procedures. H, 13C, and 19F NMR spectra were recorded on Bruker
AV 300 and Bruker AV 400 spectrometers. Chemical shift values in
1H and 13C NMR spectra were referenced internally to the residual
solvent resonances, whereas 19F and 31P NMR spectra were referenced
externally to CCl3F and H3PO4, respectively. Abbreviations used in
the reported NMR experiments are as follows: b, broad; s, singlet; d,
doublet; t, triplet; m, multiplet. All measurements were carried out at
room temperature. Infrared spectra were recorded in the solid state on
a Bruker Alpha P FT-IR spectrometer. Elemental analyses were
carried out using a Leco TruSpec Micro CHNS device. Gas
chromatography was performed on a HP 6890 instrument with an
HP5 column (Agilent) unless stated otherwise. All hydrogenation
reactions were set up under Ar in a 300 mL autoclave (PARR
Instrument Co.). In order to avoid unspecific reductions, all catalytic
experiments were carried out in 4 mL glass vials, which were set up in
an alloy plate and placed inside the autoclave.
Synthesis of 1. Under an argon atmosphere, a 50 mL Schlenk
flask, equipped with a magnetic stirring bar and a reflux condenser,
was charged with Mo(CO)6 (105.6 mg, 400.0 μmol, 1.00 equiv). A 10
mL portion of toluene was added, and the suspension was
subsequently treated dropwise with (iPr2PCH2CH2)2NH (1.46 mL
of a 10 wt % solution in THF, 420.0 μmol, 1.05 equiv) at room
temperature. The reaction mixture was heated to reflux overnight, and
the resulting orange-red solution was cooled to room temperature
again. The solvent was removed under reduced pressure until product
precipitation occurred. The orange precipitate was filtered off, washed
two times with 5 mL of toluene, and afterward redissolved in the
minimum amount of DCM. Heptane was added until product
precipitation occurred again. The resulting yellow solid was filtered
off, washed two times with 5 mL of heptane/DCM 3/1 (v/v), and
dried in vacuo to yield 176.7 mg (91%) of complex 1 as an intense
yellow solid. Crystals suitable for X-ray analysis were obtained from a
saturated solution of 1 in acetonitrile at 4 °C. Alternatively single
crystals can be obtained by slowly allowing a layer of heptane to
diffuse into a saturated solution of 1 in DCM or toluene at room
1
temperature. H NMR (400 MHz, CD2Cl2): δ 3.23−3.10 (m, 2H),
2.47−1.41 (m, 1H), 2.18−1.95 (m, 8H), 1.40−1.31 (m, 2H), 1.25−
1.11 (m, 24H). 13C{1H} NMR (101 MHz, CD2Cl2): δ 232.1 (t, J =
6.1 Hz), 218.8 (t, J = 10.5 Hz), 216.2 (t, J = 9.0 Hz), 53,57 (t, J = 8.1
Hz), 30.71 (t, J = 13.1 Hz), 29.06 (t, J = 9.1 Hz), 26.47 (t, J = 10.1
Hz), 19.93 (t, J = 3.0 Hz), 19.43 (t, J = 3.0 Hz), 18.99 (t, J = 3.0 Hz),
18.73 (s). 31P{1H} NMR (162 MHz, CD2Cl2): δ 75.6. Anal. Calcd for
C19H37MoNO3P2: C, 47.01; H, 7.68; N, 2.89. Found: C, 47.35; H,
7.86; N, 2.84. IR (ATR, cm−1): 1924 (νCO), 1805 (νCO), 1758 (νCO).
Synthesis of 2. In
a 25 mL Schlenk flask, Mo-
(PPh3)2(CO)2(CH3CN)2 (304.8 mg, 401.7 μmol, 1.00 equiv) was
suspended in 10 mL of THF and treated dropwise with
(iPr2PCH2CH2)2NH (1.45 mL of a 10 wt % solution in THF,
421.8 μmol, 1.05 equiv) at room temperature. The reaction mixture
was stirred overnight, and the precipitated solid was filtered from the
supernatant. Subsequent washing of the obtained pale yellow solid
with three 5 mL portions of THF followed by removal of residual
solvent under reduced pressure gave 126.3 mg (81% yield) of 2 as a
pale yellow solid. Crystals suitable for X-ray analysis were obtained by
slowly allowing a layer of toluene to diffuse into a saturated solution
of 2 in THF at −30 °C. Anal. Calcd for C20H40MoN2O2P2: C, 48.19;
H, 8.09; N, 5.62. Found: C, 48.26; H, 8.30; N, 5.54. IR (ATR, cm−1):
2246 (νCN), 1762 (νCO), 1676 (νCO).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00410.
General procedure for hydrogenation reactions, charac-
terization of the hydrogenated products, NMR spectra
for complex 1, IR spectra for complexes 1, 2, 3-Br, and
3-Cl, X-ray crystal structure analysis, and NMR spectra
of the isolated products of the catalysis (PDF)
Synthesis of 3-Br. In a 10 mL Schlenk flask, Mo(η3-allyl)-
(CO)2(CH3CN)2Br (150.4 mg, 423.6 μmol, 1.00 equiv) was
E
DOI: 10.1021/acs.organomet.8b00410
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Organometallics
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Accession Codes
CCDC 1847003−1847006 contain the supplementary crys-
tallographic 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 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.B.: Matthias.Beller@catalysis.de.
■
ORCID
Matthias Beller: 0000-0001-5709-0965
Notes
The authors declare no competing financial interest.
(6) (a) Ben-Ari, E.; Leitus, G.; Shimon, L. J. W.; Milstein, D. Metal-
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PNP Ir(I) System: Facile Ligand Dearomatization-Aromatization as
Key Steps. J. Am. Chem. Soc. 2006, 128, 15390−15391.
(b) Khusnutdinova, J. R.; Milstein, D. Metal-Ligand Cooperation.
Angew. Chem., Int. Ed. 2015, 54, 12236−12273.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the support from the Federal
Ministry of Education and Research of Germany, the State of
Mecklenburg-West Pomerania, and the European Union
(ERC-Advanced Grant 670986-NoNaCat). We thank the
analytical department (S. Schareina, S. Buchholz, A. Lehmann)
of the Leibniz-Institute for Catalysis, Rostock.
̈
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