Group 6 Metal Carbonyl Complexes Supported by a Bidentate PN Ligand: Syntheses, Characterization, and Catalytic Hydrogenation Activity

Article abstract of DOI:10.1021/acs.organomet.0c00612

We report on the preparation of a series of phosphorus-nitrogen donor ligand complexes [M(CO)4(PN)], where M = Cr, Mo, W and PN is 2-(diphenylphosphino)ethylamine. The organometallic compounds were readily obtained upon reacting the respective metal hexacarbonyls with equimolar amounts of the pertinent ligand in the presence of tetraethylammonium bromide. The PN-ligated metal carbonyls were fully characterized by standard spectroscopic techniques and X-ray crystallography. The ability of the title compounds to function as homogeneous hydrogenation catalysts was probed in the reduction of acetophenone and benzaldehyde derivatives to yield the corresponding alcohols. The reaction setup was easily assembled by simply combining the components in the autoclave on the bench outside an inert-gas-operated glovebox system.

Full text of DOI:10.1021/acs.organomet.0c00612

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Article
Group 6 Metal Carbonyl Complexes Supported by a Bidentate PN
Ligand: Syntheses, Characterization, and Catalytic Hydrogenation
Activity
Thomas Vielhaber, Kirill Faust, and Christoph Topf*
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ABSTRACT: We report on the preparation of a series of
phosphorus−nitrogen donor ligand complexes [M(CO)4(PN)],
where M = Cr, Mo, W and PN is 2-(diphenylphosphino)-
ethylamine. The organometallic compounds were readily obtained
upon reacting the respective metal hexacarbonyls with equimolar
amounts of the pertinent ligand in the presence of tetraethylam-
monium bromide. The PN-ligated metal carbonyls were fully
characterized by standard spectroscopic techniques and X-ray
crystallography. The ability of the title compounds to function as
homogeneous hydrogenation catalysts was probed in the reduction
of acetophenone and benzaldehyde derivatives to yield the corresponding alcohols. The reaction setup was easily assembled by
simply combining the components in the autoclave on the bench outside an inert-gas-operated glovebox system.
study toward the syntheses of group 6 metal complexes
incorporating the anionic [M(CO)3(PN)Br]− motif (M = Cr,
Mo, W). We anticipated this coordination unit to faithfully
reproduce the catalytic properties of the privileged manganese-
based congener mentioned above, while the preparation of the
pertinent compounds follows the rational guidelines as
outlined in the literature. The respective precursor salts
[M(CO)5Br][NEt4] are easily obtained upon decarbonylation
of the respective readily available metal hexacarbonyls with
tetrabutylammonium bromide in refluxing glyme or diglyme9
(vide infra), whereupon the notional catalysts [M(CO)3(PN)-
Br][NEt4] are obtained through ligand exchange reaction of 2
equiv of CO effected by the given bidentate PN ligand
(Scheme 1). Since the ability of the vast majority of PNP- and
PN-tagged metal carbonyls to cleave gaseous H2 depends on
the presence of a deprotonatable metal-bound NH motif,
pairing complexes [M(CO)3(PN)Br][NEt4] with a strong and
soluble base (e.g., t-BuOK or t-BuONa) is thought to give rise
to a hydrogenation catalyst assembly.
INTRODUCTION
■
The past decade has witnessed tremendous progress in the
syntheses of first row transition metal pincer complexes and
their catalytic applications in a variety of (de)hydrogenation
reactions.1 In this context, PNP-ligated metal atoms and ions
have played a dominant role by virtue of their intrinsically well
balanced stability−reactivity relationship and the synthetic
malleability of the given tridentate ligand. Driven by economic
considerations, the related scientific discourse was, to a great
degree, centered around the use of base metals as catalytically
competent centers.2 Among the latter, manganese represents
an attractive candidate owing to its good abundance, low price,
excellent biocompatibility, and rich coordination chemistry.3
Indeed, the first report on a PNP-tagged manganese carbonyl
hydrogenation catalyst4 stimulated a rapid and impressive
development in the field5 which will unequivocally continue to
excel in the future.
However, in 2017 Pidko and co-workers demonstrated that
the capability of tridentate PNP ligand−manganese assemblies
carries over to a congeneric but more truncated PN-based Mn
complex that effects the hydrogenation of esters in the
presence of substoichiometric amounts of t-BuOK.6 The
respective catalyst is prepared by reacting the commercial but
notoriously expensive [Mn(CO)5Br]7 with an equimolar
amount of 2-(diphenylphosphino)ethylamine. Furthermore,
Sortais, Bastin, and their co-workers devised a method for the
asymmetric transfer hydrogenation of ketones using chiral
aminophosphines and the aforementioned manganese pre-
cursor.8 This recent reports and the ongoing pursuit of cost-
effective catalytic protocols encouraged us to embark on a
It is worth noting that, while the coordination chemistry and
catalytic applications of chromium,10 molybdenum,11 and
tungsten11 complexes featuring a PNP backbone have already
been extensively described and are partially very well
Received: September 16, 2020
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indicated back-formation of solid (NEt4)Br. The latter was
filtered off, and thereafter a small amount of silver(I) nitrate
was added to the clear filtrate. Indeed, the addition of AgNO3
did not produce a precipitate of insoluble AgBr, and thus we
had to infer that the prepared PN complex did not contain any
bromide ligand. Finally, IR spectroscopy and X-ray crystallog-
raphy delivered certainty about the actual constitution of the
PN-ligated metal complexes (Figure 1). It turned out that
refluxing a mixture of [M(CO)5Br][NEt4] and the given
bidentate ligand solely resulted in the formation of the PN-
tagged tetracarbonyl complexes [M(CO)4(PN)] (M = Cr (1),
Mo (2), W (3)) (Scheme 2).
Scheme 1. (A) General Synthesis of Ionic Bromo
Pentacarbonyl Precursors [M(CO)5Br][NEt4] from
Commercial Carbonyls [M(CO)6] and Cheap
Tetrabutylammonium Bromide and (B) Formation of
Notional Catalysts [M(CO)3(PN)Br][NEt4] Thereof upon
CO Extrusion Effected by 2-
a
(Diphenylphosphino)ethylamine (M = Cr, Mo, W)
In order to deliberately synthesize the corresponding ionic
bromido pentacarbonyl metalate, the reaction was performed
under reduced pressure with the intention to provide
thermodynamic bias for CO extrusion from the metal core
while at the same time the dissociation of the bromido ligand
was intended to be suppressed. However, we again exclusively
obtained the neutral carbonyl complexes [M(CO)4(PN)] (M
= Cr, Mo, W) and never observed formation of the pursued
ionic species.
a
Dry glyme is a proper solvent for both reactions.
developed, related reports on PN-liganded kindred12 have
been hitherto almost unknown (vide infra).
In the interest of saving time, we then devised a convenient
in situ method which dispenses with the need for an extra
isolation step of the salts [M(CO)5Br][NEt4] and which
therefore allows for a quicker access to the PN ligand
supported notional catalysts. For the molybdenum compound,
the synthesis of the pertinent PN complex also succeeds with
the corresponding BF4 salt of the given PN ligand, i.e. 2-
(diphenylphosphino)ethylammonium tetrafluoroborate, pro-
vided that stoichiometric amounts of a mild base (triethyl-
amine) are present in the reaction mixture. This is of particular
practical interest, since the aforementioned PN-BF4 salt is an
easy to weigh solid, whereas the parent PN compound is a very
viscous oil that is not as readily portionable.
Related reports on the preparation and characterization of
group 6 metal chelates that incorporate tertiary amine or
pyridine based donor motifs14 already exist, but to the best of
our knowledge, other than mechanistic studies of their ring-
opening reactions,15 further accounts dealing with the
reactivity of generic complexes [M(CO)4(PN)] have not yet
been composed.
RESULTS AND DISCUSSION
■
Initially, we prepared a series of group 6 metal based starting
materials of the general formula [M(CO)5Br][NEt4] (M = Cr,
Mo, W) featuring an anionic metal carbonyl fragment which is
isoelectronic with the frequently used but high-priced
manganese-based precursor compound [Mn(CO)5Br] (vide
supra). The original literature method for the syntheses of the
pertinent bromopentacarbonyl metal salts relied on the use of
diglyme as the solvent,9 but we soon realized that its high
viscosity compromises the agitation of the reaction mixture
and residues of this solvent persistently stuck on the
synthesized compounds owing to the rather high boiling
point (162 °C)13 of the former, which requires annoyingly
prolonged drying periods on the vacuum pump. Accordingly,
we replaced this solvent with the more volatile but still
sufficiently well coordinating glyme, the boiling point of which
is 84 °C,13 and worked out an optimized reaction parameter
set for each goup 6 metal carbonyl (Table 1).
With pristine samples of the precursors [M(CO)5Br][NEt4]
in hand, we went straight away to assemble the projected
catalysts via reaction of the former with 2-
(diphenylphosphino)ethylamine (Scheme 1B). Surprisingly,
soon after starting the complex-generating reaction we noticed
the appearance of a colorless microcrystalline precipitate which
However, on adopting the literature protocol16 that
describes the direct reaction of the respective metal
hexacarbonyls [M(CO)6] (M = Cr, Mo, and W) with an
equimolar amount of the bidentate PN ligand, we soon realized
that application of the congeneric PN-BF4 salt does not work
for this particular synthesis of the chromium and tungsten
derivatives, most probably owing to the fact that the given salt
is almost insoluble in nonpolar solvents. Accordingly, we
followed the same protocol as outlined in the literature16 and
applied the free PN ligand as well. Finally, we obtained the
desired complexes as yellow crystalline powders in very good
yields and the thus-prepared coordination compounds were
characterized by infrared spectroscopy, as already described in
the report from Walsh et al.,16 and additionally by us through
X-ray crystallography, high-resolution mass spectrometry, and
1H, 13C, and 31P NMR spectroscopy.
Table 1. General Reaction Scheme for the Preparation of
the Requisite Precursor Salts Incorporating the Anionic
Coordination Unit [M(CO)5Br]− (M = Cr, Mo, W) along
with Optimized Reaction Conditions and Optical
a
Appearance of the Products
central
reaction
reaction time
t (min)
metal M temp T (°C)
yield (%)
Cr
Mo
W
120
80
130
90
150
80
84 (orange crystalline powder)
77 (yellow crystalline powder)
82 (yellow crystalline powder)
It is worth noting that the temperature for the CO extrusion
is significantly lowered if the syntheses of complexes 1−3
proceed via the respective intermittently generated [M-
(CO)5Br][NEt4] species (see the Experimental Section for
details).
a
Each reaction was performed with 0.68 mmol of the corresponding
[M(CO)6] starting material.
B
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Figure 1. ORTEP drawings of the prepared carbonyl complexes [M(CO)4(PN)], M = Cr, Mo, W. Thermal ellipsoids are drawn at the 50%
probability level.
In stark contrast to this, the activity completely ceased in
nonpolar solvents (entries 1−3) and when isopropyl ether was
used as the reaction medium the steric bulk of the alkyl groups
in the solvent also seemed to fully inhibit the catalytic
transformation (entry 4). Strikingly, upon a change in the
solvent from glyme to diglyme, the catalytic performance was
drastically increased and the substrate was fully converted to
the desired product with nearly quantitative yield after 18 h
(entries 9 and 10). This trend is clearly a result of the ability of
diglyme to effectively solubilize the potassium alcoholate by
providing three contiguous and regularly spaced oxygen donor
atoms to the K+ ions. We also observed complete conversion
for dimethylformamide and acetonitrile as solvents, but the
yields were rather poor owing to the formation of side
products, which escaped detection through the GC-MS device.
Most importantly, complex 1 did not not show any
hydrogenation activity in the absence of a strong base. Table
S1 in the Supporting Information contains additional
information on the hydrogenation of the benchmark substrate
in common basic N-donor solvents (pyridine, triethylamine,
and tetramethylethylenediamine), where in none of the
reaction media was proper catalytic activity observed.
To improve upon the results obtained in diglyme, we first
aimed to reduce the rather high amount of the auxiliary base
(20 mol % with respect to the substrate). Indeed, the reaction
also proceeds with full conversion of the substrate when an
equimolar amount of base versus complex 1 is present in the
reaction mixture (Table 3, entries 1 and 2). However, owing to
reproducibility problems in reactions that employed an
equimolar amount of t-BuOK versus 1, all subsequent base-
optimized reactions were performed with 6 mol % of the given
alcoholate and 5 mol % catalyst loading. Next, we lowered the
amount of compound 1, but to our dismay we observed a steep
decrease in the catalytic performance (entries 3−6). We then
continued our study by variation of reaction time, temperature,
and H2 pressure. Very interestingly, the catalyst enables full
conversion after a reaction period of only 4 h (entry 7) and,
encouraged by this finding, we also reduced the reaction
temperature and hydrogen pressure. However, when these
parameters were reduced, the activity of the given catalyst
dropped significantly (entries 8 and 9).
Scheme 2. Actual Reaction Outcome on Combining 2-
(Diphenylphosphino)ethylamine and Ionic Group 6 Metal
Carbonyl Compounds [M(CO)5Br][NEt4] in Refluxing
a
Glyme
a
The reaction exclusively produced the bromide-free complexes
[M(CO)4(PN)] (M = Cr (1), Mo (2), W (3)).
Catalytic Hydrogenation Reactions of Carbonyl
Derivatives with [Cr(CO)4(PN)] (1). In initial hydrogenation
experiments we tested PN-tagged complex 1 for its ability to
catalyze the reduction of ketones. We chose acetophenone as
the benchmark substrate and probed the catalytic performance
in different solvents (Table 2) while the catalytic reactions
were performed in the presence of excess t-BuOK versus the
catalyst. To our delight, complex 1 showed considerable
catalytic activity at 120 °C and 40−50 bar of H2 pressure,
provided the reaction was performed in coordinating solvents.
Table 2. Catalytic Hydrogenation of Acetophenone to
Afford 1-Phenylethanol Facilitated by the Neutral Carbonyl
Complex [Cr(CO)4(PN)] (1) in Different Solvents and in
a
the Presence of t-BuOK
reaction
time (h)
conversion
(%)
yield
(%)
entry
solvent
p (bar)
1
2
3
4
5
6
7
8
cyclohexane
n-heptane
toluene
isopropyl ether
tetrahydrofuran
1,4-dioxane
glyme
diglyme
glyme
diglyme
24
24
24
24
24
24
22
22
18
18
24
18
18
50
50
50
50
50
50
50
50
40
40
50
40
40
0
0
0
0
48
0
0
0
0
48
44
90
99
34
99
34
65
10
44
>99
>99
45
>99
>99
>99
35
In order to verify the homogeneous nature of complex 1 in
combination with t-BuOK, we performed a mercury-poisoning
experiment. For this purpose, the pertinent catalytic trans-
formation was conducted in the presence of a few drops of
elemental mercury. When the optimized reaction conditions
for this experiment were applied, we did not observe any loss
of catalyst activity. Hence, we have to infer that the given
catalyst system operates through a homogeneous pathway.
With the optimized reaction conditions in hand, coordina-
tion compound 1 was then compared with the congeneric
9
10
11
12
13
acetonitrile
dimethylformamide
pyridine
a
A 0.5 mmol amount of acetophenone was used. Conversions and
yields were determined by GC-MS analysis using n-dodecane as the
internal standard.
C
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Table 3. Optimization of the Reaction Conditions for the
Reduction of Acetophenone Catalyzed by 1
Table 5. Scope and Limitations for the Hydrogenation
a
a
Reaction of Ketones Catalyzed by 1
catalyst loading
(mol %)
t-BuOK
(mol %)
conversion
(%)
yield
(%)
entry
t (h)
1
2
3
4
5
6
7
5
5
2.5
2.5
1
1
5
5
5
10
5
10
5
10
5
6
6
6
6
6
18
18
18
18
18
18
4
6
4
4
2
>99
>99
30
15
25
8
>99
82
24
0
99
99
27
14
23
7
99
82
23
0
b
8
9
c
10
11
0
5
32
31
a
A 0.5 mmol amount of acetophenone was used. Conversions and
yields were determined by GC-MS analysis using n-dodecane as the
b
c
internal standard. The applied hydrogen pressure was 20 bar. The
reaction temperature was 100 °C.
group 6 metal complexes 2 and 3. Regrettably, these complexes
showed either very low or even no catalytic activity in the
reduction of acetophenone, which underpins the special role of
chromium as metal center for this particular catalytic reaction
(Table 4). Tables S2 and S3 in the Supporting Information
Table 4. Catalytic Hydrogenation of Acetophenone Using
the Group 6 Metal Carbonyl Complexes with the Optimized
a
Reaction Conditions
group 6 metal M in [M(CO)4(PN)]
conversion (%)
yield (%)
Cr
Mo
W
>99
4
0
99
4
0
a
A 0.5 mmol amount of acetophenone was used. Conversions and
yields were determined by GC-MS analysis using n-dodecane as the
internal standard.
summarize additional results for the acetophenone hydro-
genation that were obtained with complexes [Mo(CO)4PN]
and [W(CO)4PN] in combination with different solvents and
bases.
In order to establish the scope and limitations of the
catalytic system described herein, we applied a broad array of
structurally diverse ketones (Table 5). We soon realized that
the optimized reaction conditions for the acetophenone
reduction were applicable to only a limited number of
substrates, i.e. benzophenone 4a, the benzoannelated con-
geners 4b,c, propiophenone 4d, and acetophenones equipped
with methyl groups at the aromatic ring 4f−h. However, it
seems that increased steric bulk in close vicinity of the carbonyl
group is well accommodated by the Cr-based catalyst, provided
the substituent has an electron-releasing nature (entries 1, 4,
and 6). When the cycloaliphatic derivative 4e was used as the
substrate for the hydrogenation, we had to significantly
a
b
A 0.5 mmol amount of the ketone was used. Conditions A: t-BuOK
(6 mol %), 40 bar of H2, 4 h. Conditions B: t-BuOK (6 mol %), 50
bar of H2, 12 h. Conditions C: t-BuOK (20 mol %), 50 bar of H2, 20
h; Conditions D: t-BuOK (7.5 mol %), 50 bar of H2, 14 h.
c
Conversions were determined by GC-MS analysis using n-dodecane
as the internal standard. For 5f,g n-hexadecane was used as the
D
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to hydrodehalogenation under the reaction conditions (entries
9, 12, and 15). Notably, decent catalytic activity and medium
yields were only observed if the halide group was placed in the
meta position, but this assertion does not apply to the bromo
derivative 4p (entries 10, 13, and 16). The notoriously reactive
compound 4r did not produce any desired alcohol; instead, we
only detected the untagged acetophenone in the GC-MS
spectrum as the result of hydrodebromination. To our delight,
substrate 4s bearing an electron-withdrawing CF3 group in the
meta position was neatly converted to 5s, albeit with harsher
reaction conditions in comparison to those for the parent
propiophenone (entry 19). Interestingly, the regioisomer 4t
with a shifted keto group produced the alcohol 5t in only 48%
yield under the same conditions (entry 20).
Table 5. continued
d
internal standard. Yields were determined by GC-MS using n-
dodecane as internal standard; isolated yields are givenin parentheses.
e
Hydrodehalogenation was observed.
increase the reaction time (20 h), base loading (20 mol %),
and H2 pressure (50 bar) in order to maintain both complete
conversion and high yield (entry 5). Electron-donating methyl
groups at either position of the phenyl ring did not affect the
catalyst performance (entries 6−8), and accordingly we
obtained full conversions with excellent yields under the
optimized reaction conditions. Consequently, the effect of
electron-withdrawing groups located on the aromatic ring
(fluoro-, chloro-, and bromoacetophenones) was then
established on using the respective halide derivatives. The
pertinent substituent in the ortho position gave rise to very low
conversions and yields, whereas substrates 4l,o were also prone
We then tested N-heterocyclic ketones (4v−x) as substrates;
in these experiments the desired alcohol was only observed
when the nitrogen atom was fixed in the meta position (Table
5, entries 22−24). When carbocyclic indanone (4y) was
a
Table 6. Hydrogenation of Aldehydes Catalyzed by Cr-Based Catalyst 1
a
A 0.5 mmol amount of the aldehyde was used. Conversions and yields were determined by GC analysis using n-dodecane as the internal standard.
E
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subjected to the hydrogenation procedure, we obtained a deep
blue solution on completion of the reaction but the secondary
alcohol 5y was only formed in a trace amount (3%, entry 25).
The fact that 4-acetylbenzonitrile (4z) does not give rise to any
product 5z proves that the given catalyst system is not
compatible with CN groups.
EXPERIMENTAL SECTION
■
General Information. All chemicals were purchased from Merck
(including Sigma-Aldrich), Acros Organics, Alfa Aesar, VWR, Roth,
TCI, Lancaster Synthesis, or Chem Lab and were used as received
without further purification. Hydrogenation reactions were carried out
in a 300 mL autoclave from Parr Instruments GmbH, and the
hydrogen employed with a purity of 5.0 was purchased from Linde
Gas GmbH. Dry solvents were received from a MB-SPS-7 solvent
system from M. Braun GmbH or purchased from Acros Organics.
GC-MS analysis was carried out on a Shimadzu GC-MS QP-2020
instrument with helium (5.0 purity from Linde Gas GmbH) as the
carrier gas. Infrared spectroscopy was performed in the solid state on
a Bruker Tensor 27 ATR, and high-resolution mass spectra were
recorded on a Thermo Fisher Scientific LTQ Orbitrap XL. NMR
measurements were performed on a Bruker Avance 300 and 500 MHz
spectrometer. Spectra for different cores were recorded as follows:
300 MHz for 1H NMR, 75.5 or 125.8 MHz for 13C NMR, and 121.54
MHz for 31P NMR. Chemical shifts are listed in parts per million
(ppm) on the delta scale (δ). Axis calibration of the 1H and 13C NMR
spectra is based on the nondeuterated solvent as reference. For the
irradiation procedure in the respective product isolation steps an
EvoluChem LED spotlight device (450 nm, 30 W) was used.
Safety Statement Concerning High-Pressure Hydrogena-
tion. The pressurized H2 container (200 bar, 50 L) was placed in a
safety storage cabinet with an integrated tapping point. The pressure
cylinder was connected to a control panel for the fine adjustment of
the H2 pressure used for the hydrogenation reactions. The loading of
the autoclaves was performed in a fume hood that was equipped with
a hydrogen sensor which was wired to a magnetic valve that
instantaneously stopped the gas supply in case of any H2 leakage that
might occur during the filling procedure. Furthermore, both optical
and acoustic alarm signals were triggered whenever free H2 was
detected inside the hood.
In cases where we did not observe any alcohol formation but
significant substrate conversion, we invoke the generation of
high-molecular-weight products which were poorly volatile
such that they could not be detected by the GC-MS device.
Given the high base loading and a temperature of 120 °C,
competing and predominant polycondensation reactions might
occur that eventually produce such products which were
sometimes deeply colored (cf. the hydrogenation of 4y).
Finally, we extended the substrate scope to aldehydes (Table
6), where significantly harsher reaction conditions with respect
to the base loading had to be applied because the reaction
under the acetophenone-optimized conditions led to only a
low conversion of benzaldehyde (6a). As already mentioned
for acetophenone derivatives, the catalyst readily copes with
steric hindrance and the corresponding alcohol was obtained in
medium yields ranging from 63 to 79% (entries 1 and 4−7).
Furthermore, it is worth noting that Cr catalyst 1 exhibits only
mediocre catalytic activity against heterocyclic aldehydes such
as furfural (6h) and thiophene-2-carboxaldehyde (6i). Due to
the high t-BuOK loading, base-induced polycondensation and/
or ring-opening reactions are likely to take place that
significantly contribute to the substrate conversion. These
processes give rise to low-volatility products and clearly
exacerbate the selective formation of the primary alcohols
from the respective aldehydes.
General Procedure for the Synthesis of Precursor Salts
[M(CO)5Br][NEt4] (M = Cr, Mo, W). These complexes were prepared
according to a modified literature method.9 The respective group 6
metal carbonyl [M(CO)6] (0.68 mmol, 1.5 equiv), where M = Cr,
Mo, W, and tetraethylammonium bromide, NEt4Br (95 mg, 0.45
mmol, 1.0 equiv), were placed in a flame-dried Radleys reaction tube,
and the reactants were then suspended in dry glyme (8 mL).
Thereafter, the reaction mixture was heated to a certain temperature
(Cr, 120 °C; Mo, 80 °C; W, 130 °C), which was kept for a given
period of time (Cr, 90 min; Mo, 150 min; W, 80 min). The reaction
solution was then filtered off while still hot in order to remove residual
solids, and the clear filtrate was warmed to room temperature.
Precipitation of the title compounds was initiated upon slow addition
of n-pentane (20 mL), and the resulting suspension was then cooled
to −40 °C for 2 h. The crystals that formed were filtered off, washed
with three small portions of diethyl ether, and finally dried in vacuo.
Tetraethylammonium Bromopentacarbonylchromate(0), [Cr-
(CO)5Br][NEt4]. In the synthesis, 150 mg of [Cr(CO)6] was used
and the reaction mixture was refluxed at 120 °C for a period of 90
min. This afforded 152 mg (0.38 mmol, 84% yield) of the title
compound, which was isolated as an orange crystalline powder. IR
CONCLUSION
■
Our study toward the development of [Mn(CO)3(PN)Br]-
emulated catalysts based on group 6 metals resulted in the
preparation of a series of the three halide-free organometallic
complexes [M(CO)4(PN)], where M = Cr, Mo, W. The
coordination compounds were synthesized in a straightforward
manner from the respective metal carbonyl and commercially
available 2-(diphenylphosphino)ethylamine, whereas the com-
plexes were then characterized through X-ray crystallography,
HR-MS, and IR and NMR spectroscopy. A decent catalytic
activity in the homogeneous hydrogenation of acetophenone
to afford the corresponding secondary alcohol was demon-
strated with the compound [Cr(CO)4(PN)] in diglyme,
provided that an excess of t-BuOK versus the catalyst was
present in the reaction mixture. The molybdenum congener
displayed only minor activity in the pertinent catalytic
transformation, whereas the corresponding tungsten com-
pound was not catalytically active at all. The privileged
[Cr(CO)4(PN)] complex was thereafter applied to the related
hydrogenation of aromatic aldehydes to yield the correspond-
ing primary alcohols, whereas the catalytic performance of the
given catalyst was significantly weaker in comparison to the
acetophenone reduction. The optimized catalytic protocol
described herein is free from the requirement for adding high
loadings of a sensitive auxiliary hydride reagent that usually
plagues related catalytically active systems. Given the fact that
reports on homogeneous chromium-based hydrogenation
catalysts are still scarce, we think that our contribution might
open new vistas for the development of efficient catalytic
protocols facilitated by this abundant non-noble metal.
(ATR, cm−1): 2060 (νCO
), 1898 (νCO), 1865 (νCO). The measured
̃ ̃ ̃
values are well in accord with the literature data.9
Tetraethylammonium Bromopentacarbonylmolybdate(0), [Mo-
(CO)5Br][NEt4]. The synthesis relied on the use of 180 mg of
[Mo(CO)6], and the reaction mixture was heated to 80 °C for a
period of 150 min. This afforded 159 mg (0.36 mmol, 80% yield) of
the title compound, which was isolated as a yellow crystalline powder.
IR (ATR, cm−1): 2068 (νCO
̃
), 1899 (νCO
), 1861 (νCO). The measured
̃ ̃
values are well in accord with the literature data.9
Tetraethylammonium Bromopentacarbonyltungstate(0), [W-
(CO)5Br][NEt4]. For the preparation of the title compound 239 mg
of [W(CO)6] was applied and the reaction mixture was refluxed at
130 °C for a period of 80 min. This afforded 205 mg (0.38 mmol,
85% yield) of the ionic carbonyl complex, which was isolated as a
yellow crystalline powder. IR (ATR, cm−1): 2066 (νCO
), 1897 (νCO),
̃ ̃
F
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1857 (νCO). The measured values are well in accord with the
̃
(νNH
̃
), 3284 (νNH
̃
), 2015 (νCO
̃
), 1913 (νCO
̃
), 1872 (νCO
̃
), 1822
2
2
literature data.9
(νCO); HR-MS (ESI+) m/z: calcd for C17H17MoNO3P, 411.9995 [M
̃
+ H − CO]+; found, 411.9986.
Synthesis of Tetracarbonyl(2-(diphenylphosphino)-
ethylamine)chromium(0), [Cr(CO)4(PN)] (1). Method A. The
solids [Cr(CO)6] (100 mg, 0.45 mmol, 1.5 equiv) and NEt4Br (64
mg, 0.31 mmol, 1.0 equiv) were placed in a flame-dried Radleys glass
tube and then suspended in dry glyme (10 mL). Afterward, the
reaction mixture was heated to 120 °C and kept at reflux for a period
of 80 min, whereupon the solids were filtered off while the suspension
was still hot. The clear filtrate was transferred into an argon-flushed
Schlenk flask (25 mL), and thereafter a solution of 2-
(diphenylphosphino)ethylamine (86 mg, 0.38 mmol, 1.2 equiv) in
dry glyme (1 mL) was added. The mixture was agitated at 50 °C for
40 min and then refluxed at 120 °C for a further period of 2 h. After
that, the suspension was cooled to room temperature, whereupon the
recovered NEt4Br was removed by filtration. To the resulting yellow
solution was then added n-pentane (25 mL) in order to initiate
product precipitation. Storage in the refrigerator (−40 °C) overnight
and subsequent filtration afforded 44 mg (0.11 mmol, 36% yield) of
the title compound as a yellow microcrystalline powder.
Synthesis of Tetracarbonyl(2-(diphenylphosphino)-
ethylamine)tungsten(0), [W(CO)4(PN)] (3). Method A. In a
flame-dried 20 mL Radleys glass tube were placed [W(CO)6] (160
mg, 0.45 mmol, 1.5 equiv) and NEt4Br (63 mg, 0.30 mmol, 1.0
equiv), whereupon the solids were suspended in dry glyme (8 mL).
Under an argon atmosphere, the mixture was then stirred at 130 °C
for a period of 90 min, whereupon the suspension turned yellow upon
heating and agitation. Then the reaction mixture was filtered while
still hot and the clear filtrate was directly transferred into a flame-dried
Schlenk flask (25 mL). After that, 2-(diphenylphosphino)ethylamine
(75 mg, 0.33 mmol, 1.1 equiv) was added and the mixture was heated
to 120 °C and kept at reflux for 5 h. The reaction solution was then
cooled to room temperature, upon which n-pentane (20 mL) was
slowly added in order to initiate product precipitation. The resulting
suspension was stored in the refrigerator (4 °C) overnight, and the
crude product was thereafter recrystallized from a hot acetone/water
mixture (1/1 by volume). This afforded 61 mg (0.12 mmol, 39%
yield) of the title compound as yellow needles, which were collected
on filter paper and eventually dried with a vacuum pump.
Method B.16 A dry pressure tube was initially charged with 2-
(diphenylphosphino)ethylamine (229 mg, 1.00 mmol, 1.0 equiv).
Under an argon atmosphere, the ligand was dissolved in 10 mL of dry
n-octane and [Cr(CO)6] (220 mg, 1.00 mmol, 1.0 equiv) was added
to the ligand solution. The resulting mixture was stirred at 140 °C
overnight. The precipitate that formed was filtered off, washed with 10
mL of n-pentane, and finally dried with a vacuum pump. This afforded
338 mg (0.86 mmol, 86% yield) of the title compound as a yellow
crystalline powder. Crystals suitable for X-ray analysis were obtained
via slow diffusion of n-pentane into a solution of [Cr(CO)4(PN)] in
Method B.16 Under an argon atmosphere, 2-(diphenylphosphino)-
ethylamine (115 mg, 0.50 mmol, 1.0 equiv) was dissolved in 10 mL of
dry mesitylene in a pressure tube. To this solution was added
[W(CO)6] (176 mg, 0.50 mmol, 1.0 equiv), and the resulting mixture
was stirred at 190 °C overnight. Afterward, the precipitate was filtered
off, washed with 10 mL of n-pentane, and subsequently dried with a
vacuum pump. This afforded 200 mg (0.38 mmol, 76% yield) of the
title compound as a yellow crystalline powder. Crystals suitable for X-
ray crystallography were obtained through slow diffusion of n-pentane
1
THF. H NMR (300 MHz, CD2Cl2, 20 °C): δ 7.71−7.64 (m, 4H),
7.45−7.42 (m, 6H), 2.92−2.77 (m, 2H), 2.39−2.32 (m, 4H) ppm.
31P NMR (121.5 MHz, CD2Cl2, 20 °C): δ 63.3 ppm. 13C{1H} NMR
(125.8 MHz, (CD3)2CO, 20 °C): δ 229.1, 229.0, 228.0 (d, J = 2.6
Hz), 220.2 (d, J = 14.2 Hz), 138.2 (d, J = 33.4 Hz), 132.5 (d, J = 11.3
Hz), 130.6 (d, J = 2.0 Hz), 129.5 (d, J = 9.1 Hz), 44.6 (d, J = 12.8
1
into a solution of [W(CO)4(PN)] in toluene. H NMR (300 MHz,
CD2Cl2, 20 °C): δ 7.71−7.64 (m, 4H), 7.45−7.43 (m, 6H), 3.17−
3.07 (m, 2H), 3.03 (s, 2H), 2.38 (q, J = 6.4 Hz, 2H) ppm. 31P NMR
(121.5 MHz, CD2Cl2, 20 °C): δ 35.9 ppm. 13C{1H} NMR (125.8
MHz, (CD3)2SO, 20 °C): δ 211.4, 211.2, 211.2, 203.8 (d, J = 7.4 Hz),
135.9 (d, J = 38.8 Hz), 131.7 (d, J = 12.3 Hz), 130.1 (d, J = 1.6 Hz),
128.7 (d, J = 9.6 Hz), 45.6 (d, J = 11.4 Hz), 28.3 (d, J = 24.0 Hz)
Hz), 28.9 (d, J = 18.4 Hz) ppm; IR (ATR, cm−1): 3336 (νNH
̃
), 3288
2
(νNH ), 2005 (νCO), 1906 (νCO), 1866 (νCO), 1818 (νCO). HR-MS
(ESI+) m/z: calcd for C18H16CrNO4P, 393.0217 [M]+; found,
393.0218.
̃
̃
̃
̃
̃
2
ppm. IR (ATR, cm−1): 3326 (νNH
̃
), 3278 (νNH ), 2010 (νCO), 1905
̃
̃
2
2
(νCO), 1862 (νCO), 1815 (νCO). HR-MS(ESI+) m/z: calcd for
̃
̃
̃
C18H17WNO4P, 526.0399 [M + H]+; found, 526.0399.
Synthesis of Tetracarbonyl(2-(diphenylphosphino)-
ethylamine)molybdenum(0), [Mo(CO)4(PN)] (2). Method A. In
a flame-dried Schlenk flask (25 mL) were placed [Mo(CO)6] (75 mg,
0.28 mmol, 1.0 equiv) and 2-(diphenylphosphino)ethylammonium
tetrafluoroborate (90 mg, 0.28 mmol, 1.0 equiv), which were then
suspended in n-heptane (8 mL). Triethylamine was then added (50
μL, 0.36 mmol, 1.3 equiv), and thereafter the reaction mixture was
heated to 130 °C and kept at this temperature for a period of 19 h
with stirring. The crude product was filtered off and recrystallized
from a hot acetone/water mixture (1/1 by volume). This afforded 78
mg (0.18 mmol, 64% yield) of the title compound which was obtained
as a yellow crystalline powder.
Hydrogenation Reactions Conducted with Complexes
Incorporating Group 6 Metals (1−3). A 4 mL glass vial was
initially charged with a magnetic stirring bar, 1 (0.025 mmol), the
substrate (0.5 mmol), and n-dodecane (12 mg) as internal standard.
After that, 2 mL of diglyme was added and the resulting yellow
solution was treated with t-BuOK (0.03−0.125 mmol). The reaction
vessel was then sealed with a septum cap that was equipped with a
syringe needle. The glass vial was placed in a drilled Al plate and
transferred into the 300 mL autoclave. After that, the autoclave was
tightly sealed, purged three times with 30 bar of H2, and finally
pressurized to the required value. The autoclave was placed on a
preheated stirring plate and heated to 120 °C. On completion of the
catalytic transformation, the autoclave was cooled in a water bath.
Afterward, the H2 pressure was slowly released and the solutions were
degassed upon stirring in air for 5 min. Finally, a 30 μL aliquot was
taken from each vial, mixed with 1 mL of acetone, and eventually
analyzed by GC-MS. For hydrogenation experiments that involved
product isolation in the final step, the reaction vials were charged
without n-dodecane.
General Procedure for the Isolation of the Hydrogenation
Products. For the product isolation we fruitfully exploited the
photoinstability of complex 1 at 450−490 nm (blue region). First, the
yellow reaction solution was irradiated (450 nm, 30 W) until
complete decoloration and then the resulting suspension was diluted
with 1 mL of dichloromethane (DCM). The mixture was filtered
through a plug of silica (2 cm in a Pasteur pipet) that was then
washed with 2 mL of DCM. Finally, the clear filtrate was evaporated
to dryness, leaving behind the corresponding alcohol.
Method B.16 A dry pressure tube was charged with a magnetic
stirring bar and 2-(diphenylphosphino)ethylamine (115 mg, 0.50
mmol, 1.0 equiv). Under an argon atmosphere, the ligand was
dissolved in 10 mL of dry n-heptane and [Mo(CO)6] (132 mg, 0.50
mmol, 1.0 equiv) was added to the solution. The resulting mixture
was heated to 130 °C and stirred overnight. The precipitate was
filtered off, washed with 10 mL of n-pentane, and finally dried in
vacuo. This afforded 180 mg (0.41 mmol, 82% yield) of the title
compound as a yellow crystalline powder. Crystals of X-ray quality
were obtained via slow diffusion of n-pentane into a solution of
[Mo(CO)4(PN)] in toluene. 1H NMR (300 MHz, CD2Cl2, 20 °C): δ
7.71−7.64 (m, 4H), 7.45−7.42 (m, 6H), 3.05−2.90 (m, 2H), 2.65 (s,
2H), 2.36 (q, J = 6.4 Hz, 2H) ppm. 31P NMR (121.5 MHz, CD2Cl2,
20 °C): δ 42.7 ppm. 13C{1H} NMR (125.8 MHz, (CD3)2SO, 20 °C):
δ 220.7, (d, J = 7.6 Hz), 218.4, 218.2, 209.1 (d, J = 9.4 Hz), 136.2 (d,
J = 32.7 Hz), 131.6 (d, J = 13.0 Hz), 129.9, 128.7 (d, J = 9.3 Hz), 43.6
(d, J = 12.0 Hz), 27.6 (d, J = 20.4 Hz) ppm. IR (ATR, cm−1): 3335
G
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Article
Diphenylmethanol (5a). The title compound was synthesized
according to the standard procedure described above using 90.9 mg
(0.499 mmol) of benzophenone 4a: colorless crystals, 81.3 mg (0.441
mmol, 88% yield). 1H NMR (300 MHz, CD2Cl2, 20 °C): δ 7.40−7.24
(m, 10H), 5.83 (s, 1H), 2.41 (s, 1H) ppm. 13C{1H} NMR (75.5
MHz, CD2Cl2, 20 °C): δ 144.6, 128.8, 127.8, 126.8, 76.4 ppm.
1-Naphthylethanol (5b). The compound was prepared according
to the standard procedure described above using 85.1 mg (0.500
mmol) of 1-acetonaphthone 4b: colorless oil, 77.8 mg (0.452 mmol,
Accession Codes
CCDC 2011626−2011628 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.
1
90% yield). H NMR (300 MHz, CD2Cl2, 20 °C): δ 8.13−8.10 (m,
AUTHOR INFORMATION
■
1H), 7.92−7.88 (m, 1H), 7.81−7.78 (m, 1H), 7.69−7.66 (m, 1H)
7.55−7.46 (m, 3H), 5.63 (q, J = 6.5 Hz, 1H), 2,40 (s, 1H), 1.63 (d, J
= 6.5 Hz, 3H) ppm. 13C{1H} NMR (75.5 MHz, CD2Cl2, 20 °C): δ
142.1, 134.2, 130.7, 129.2, 128.1, 126.3, 125.9, 125.9, 123.7, 122.4,
67.2, 24.7 ppm.
Corresponding Author
Christoph Topf − Institute of Catalysis (INCA), Johannes
Kepler University (JKU), 4040 Linz, Austria; orcid.org/
0000-0001-7595-3074; Email: christoph.topf@jku.at
2-Naphthylethanol (5c). The compound was synthesized accord-
ing to the standard procedure using 85.0 mg (0.499 mmol) of 2-
acetonaphthone 4c: white crystalline powder, 78.1 mg (0.453 mmol,
Authors
Thomas Vielhaber − Institute of Catalysis (INCA), Johannes
Kepler University (JKU), 4040 Linz, Austria
Kirill Faust − Institute of Catalysis (INCA), Johannes Kepler
University (JKU), 4040 Linz, Austria
1
91% yield). H NMR (300 MHz, CD2Cl2, 20 °C): δ 7.90−7.86 (m,
4H), 7.56−7.48 (m, 3H), 5.09 (q, J = 6.4 Hz, 1H) 2.10 (s, 1H), (d, J
= 6.5 Hz, 3H) ppm. 13C{1H} NMR (75.5 MHz, CD2Cl2, 20 °C): δ
144.0, 133.8, 133.3, 128.5, 128.2, 128.0, 126.5, 126.1, 124.3, 124.1,
70.7, 25.5 ppm.
Complete contact information is available at:
1-Phenylpropan-1-ol (5d). The title compound was synthesized
according to the standard procedure using 67.1 mg (0.500 mmol) of
propiophenone 4d: colorless oil, 27.4 mg (0.201 mmol, 40% yield).
1H NMR (300 MHz, CD2Cl2, 20 °C): δ 7.35−7.26 (m, 5H), 4.58 (t, J
= 6.5 Hz, 1H), 1.93 (s, 1H), 1.83−1.67 (m, 2H), 0.90 (t, J = 7.4 Hz,
3H) ppm. 13C{1H} NMR (75.5 MHz, CD2Cl2, 20 °C): δ 145.4,
128.7, 127.7, 126.3, 76.1, 32.5, 10.3 ppm.
https://pubs.acs.org/10.1021/acs.organomet.0c00612
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1-(3-Methylphenyl)ethanol (5h). The alcohol was prepared
following the standard procedure using 67.1 mg (0.500 mmol) of
3-methylacetophenone 4h: colorless oil, 43.3 mg (0.318 mmol, 64%
We gratefully thank Univ.-Prof. Dr. Marko Hapke from the
INCA at JKU for pleasant and fruitful discussions and the
generous support. Moreover, we are much obliged to assoc.
Univ.-Prof. DI Dr. Markus Himmelsbach from the department
of Analytical Chemistry at JKU for performing the HR-MS
measurements of the pertinent PN-liganded group 6 metal
carbonyl complexes.
1
yield). H NMR (300 MHz, CD2Cl2, 20 °C): δ 7.25−7.07 (m, 5H),
4.84 (q, J = 6.4 Hz, 1H), 2.35 (s, 3H) 1.92 (s, 1H), 1.45 (d, J = 6.4
Hz, 3H) ppm. 13C{1H} NMR (75.5 MHz, CD2Cl2, 20 °C): δ 146.5,
138.5, 128.6, 128.4, 126.4, 122.7, 70.6, 25.5, 21.6.
1-(Ferrocenyl)ethanol (5u). The title compound was synthesized
according to the standard procedure, but the product isolation was
performed in a slightly different manner. The orange reaction mixture
was irradiated until it turned into a brownish suspension, which was
then diluted with 1 mL of diethyl ether and filtered through a small
pad of Celite (2 cm in a Pasteur pipet). The loaded Celite was washed
with 2 mL of diethyl ether, and the filtrate was thereafter evaporated
to dryness, leaving behind the desired alcohol. For the synthesis, 114.1
mg (0.500 mmol) of 4u was applied: orange needles, 111.3 mg (0.484
mmol, 97% yield). 1H NMR (300 MHz, CDCl3, 20 °C): δ 4.53 (q, J =
6.4 Hz, 1H), 4.22−4.16 (m, 9H), 1.92 (s, 1H) 1.44 (d, J = 6.4 Hz
3H) ppm. 13C{1H} NMR (75.5 MHz, CDCl3, 20 °C): δ 94.9, 68.4,
68.1, 68.0, 66.3, 66.2, 65.7, 23.8. HRMS (ESI+) m/z: calcd for
C12H14FeO [M]+, 230.0394; found, 230.0392.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00612.
Additional hydrogenation experiments in coordinating
solvents conducted with complexes 1−3 in the presence
1
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Organometallics XXXX, XXX, XXX−XXX