Alkene Metalates as Hydrogenation Catalysts

Article abstract of DOI:10.1002/chem.201605222

First-row transition-metal complexes hold great potential as catalysts for hydrogenations and related reductive reactions. Homo- and heteroleptic arene/alkene metalates(1?) (M=Co, Fe) are a structurally distinct catalyst class with good activities in hydrogenations of alkenes and alkynes. The first syntheses of the heteroleptic cobaltates [K([18]crown-6)][Co(η⁴-cod)(η²-styrene)₂] (5) and [K([18]crown-6)][Co(η⁴-dct)(η⁴-cod)] (6), and the homoleptic complex [K(thf)₂][Co(η⁴-dct)₂] (7; dct=dibenzo[a,e]cyclooctatetraene, cod=1,5-cyclooctadiene), are reported. For comparison, two cyclopentadienylferrates(1?) were synthesized according to literature procedures. The isolated and fully characterized monoanionic complexes were competent precatalysts in alkene hydrogenations under mild conditions (2 bar H₂, r.t., THF). Mechanistic studies by NMR spectroscopy, ESI mass spectrometry, and poisoning experiments documented the operation of a homogeneous mechanism, which was initiated by facile redox-neutral π-ligand exchange with the substrates followed by H₂ activation. The substrate scope of the investigated precatalysts was also extended to polar substrates (ketones and imines).

Full text of DOI:10.1002/chem.201605222

A Journal of
Accepted Article
Title: Alkene Metalates as Hydrogenation Catalysts
Authors: Philipp Büschelberger, Dominik Gärtner, Efrain Reyes-
Rodriguez, Friedrich Kreyenschmidt, Konrad Koszinowski,
Axel Jacobi von Wangelin, and Robert Wolf
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To be cited as: Chem. Eur. J. 10.1002/chem.201605222
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10.1002/chem.201605222
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Alkene Metalates as Hydrogenation Catalysts
Philipp Büschelberger,[a] Dominik Gärtner,[b] Efrain Reyes-Rodriguez,[b] Friedrich Kreyenschmidt,[c]
Konrad Koszinowski,[c] Axel Jacobi von Wangelin,*[b] and Robert Wolf*[a]
Abstract: First-row transition metal complexes hold great potential as
catalysts for hydrogenations and related reductive reactions.
Homoleptic and heteroleptic arene/alkene metalates(1−) (M = Co, Fe)
are a structurally distinct catalyst class with good activities in
hydrogenations of alkenes and alkynes. The first syntheses of the
heteroleptic cobaltates [K([18]-crown-6)][Co(η4-cod)(η2-styrene)2] (5),
[K([18]crown-6)][Co(η4-dct)(η4-cod)] (6), and the homoleptic complex
[K(thf)2][Co(η4-dct)2] (7, dct = dibenzo[a,e]cyclooctatetraene, cod =
1,5-cyclooctadiene) are reported. For comparison, two cyclopenta-
dienylferrates(1−) were synthesized according to literature
procedures. The isolated and fully characterized monoanionic
complexes were found to be competent pre-catalysts in alkene
hydrogenations under mild conditions (2 bar H2, r.t., THF).
Mechanistic studies by NMR, electrospray-ionization (ESI) mass
spectrometry, and poisoning experiments documented the operation
of a homogeneous mechanism, which is initiated by facile redox-
neutral π-ligand exchange with the substrates followed by H2
activation. The substrate scope of the investigated pre-catalysts was
also extended to polar substrates (ketones and imines).
states. Budzelaar and coworkers reported the first application of
(pyridyldiimine)cobalt catalysts to hydrogenations of mono- and
disubstituted olefins (Figure 1, top left).[6] Significantly, Chirik and
coworkers introduced new catalyst derivatives and expanded the
scope to include bulky alkenes; they were also able to
hydrogenate geminal-disubstituted olefins enantioselectively
(Figure 1, top left).[7] Hanson and coworkers reported PNP-pincer
cobalt complexes to be active in the hydrogenation of alkenes,
aldehydes, ketones, and imines and to undergo transfer
hydrogenations (Figure 1, top center).[8] Iron and cobalt
complexes bearing bis(phosphine) ligands were also used for
(asymmetric) alkene hydrogenation (Figure 1, top right), while a
catalyst with a tridentate tris(phosphane) (= triphos) ligand was
shown to reduce esters and carboxylic acids.[9],[10] To date there
are many more examples especially for PNP-pincer complexes
which show impressive catalytic activities.[11] Recently, the groups
of Kempe and Kirchner used PNP-pincer cobalt and iron
complexes for selective hydrogenations of polar bonds with high
tolerance of other unsaturated bonds.[12] Moreover, effective
cobalt catalysts based on NNP, PBP-, and CCC-pincers have
been reported.[13],[14],[15]
Arenes are one of the most abundant and versatile classes of
unsaturated organic compounds and also entertain a rich
coordination chemistry with low-valent transition metals.[16],[17] Our
groups recently initiated a research program aiming at the
development of metalate catalysts that bear simple and cheap
arenes as stabilizing ligand motifs (Figure 1, bottom).[18] Initial
experiments focused on the homoleptic bis(η4-anthracene)
metalates 1 (M = Co) and 2 (M = Fe) originally reported by Ellis
and coworkers.[19],[20] The closed-shell 18-electron complex 1 and
the open-shell 17-electron complex 2 constitute two isolable
representatives of homogeneous Fe− and Co− sources.[17]
Introduction
Metal-catalyzed hydrogenations are among the largest technical
processes and constitute key operations in numerous chemical
syntheses.[1] Over the past decades, the use of highly active
platinum group metal catalysts has grown to maturity which
enabled efficient hydrogenations of unsaturated C=C and C=X
bonds.[2] Apart from nickel,[3] 3d transition metal catalysts have
received much less attention despite their higher abundance and
often lower toxicity.[4] The emphasis on stringent economic and
environmental criteria has placed the development of sustainable
hydrogenation methods with base-metal catalysts into the
limelight of current research activities.[5] Great progress was only
recently made with the development of low-valent iron group
metal catalysts (Fe, Co, Ni) for olefin hydrogenations under very
mild reaction conditions. Special ligand architectures allowed the
stabilization of the catalytically active species in low oxidation
H
N
BArF
4
*
N
R2P
PR2
N
N
Cy2P
PCy2
SiMe3
M
Co
M
R
R
R'
Me3Si
SiMe3
Budzelaar 2005 (M = Co)
Chirik 2012 (M = Fe)
Hanson 2012
Chirik 2013 (M = Co)
Chirik 2014 (M = Fe)
[a]
P. Büschelberger, Prof. R. Wolf
this work:
M = Fe, Co
University of Regensburg, Institute of Inorganic Chemistry
Universitätsstr. 31, 93040 Regensburg, Germany
E-mail: robert.wolf@ur.de
Dr. D. Gärtner, E. Reyes-Rodriguez, Prof. A. Jacobi von Wangelin
University of Regensburg, Institute of Organic Chemistry
E-mail: axel.jacobi@ur.de
F. Kreyenschmidt, Prof. K. Koszinowski, Georg-August-Universität
Göttingen, Institut für Organische und Biomolekulare Chemie,
Tammannstr. 2, 37077 Göttingen, Germany
labile
synthetic equivalent of Fe- and Co-
facile exchange with -substrates
olefin hydrogenation catalysis
π
-hydrocarbon ligands
M
π
[b]
[c]
?
Figure 1. Cobalt- and iron-based hydrogenation catalysts (top) and design
concept of alkene metalate catalysts (bottom).
Supporting information for this article is given via a link at the end of
the document.
- 1 -
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The bis(η4-anthracene)metalates 1 and 2 exhibited good
activity in hydrogenations of various alkenes under mild
conditions; cobaltate 1 was also active in catalytic hydrogenations
of alkynes, ketones, and imines.[18] Based on our preliminary
mechanistic investigations with the pre-catalysts 1 and 2, we
postulated a new catalytic approach to hydrogenation reactions
which involves (i) the facile synthetic access to a variety of
modular catalyst compositions from simple starting materials
(alkene/arene, metal salt, reductant), (ii) the presence of highly
reduced, anionic iron or cobalt species, providing sufficient
reducing power for the key H2 activation, (iii) the presence of a
cheap hydrocarbon ligand that can be easily replaced with the
structurally very similar substrates of olefin hydrogenations. The
exchange of the labile π-ligands with the substrates is redox-
neutral, requires only little structural reorganization, and can in
principle be traceless, if the ligands undergo complete
hydrogenation themselves under the reaction conditions
(Scheme 1). In an effort to explore the scope of this new
mechanistic paradigm further, we prepared a series of mono-
anionic alkene/arene metalates (M = Co, Fe) and studied their
catalytic activity in alkene hydrogenations. Here, we give a full
account of these catalytic studies and describe the results of
reaction monitoring and poisoning experiments designed to
reveal the catalyst activation step and the homogeneous or
heterogeneous nature of the catalytically active species.
potential intermediate of styrene hydrogenations with cobaltate
pre-catalysts (vide infra).
K(anthracene)
or
MBr2
Co
Fe
dme or thf
M = Co, Fe
[K(dme)2]+
[K([18]crown-6)(thf) ]+
2
1
2
Scheme 2. Synthesis of bis(anthracene) metalates 1 and 2.[19],[20],[21]
Reaction of [K([18]crown-6)][Co(η4-naphthalene)(η4-cod)] (3)
with 2.2 equiv. of styrene in THF at room temperature gave the
bis(η2-styrene) complex 5 in 61% yield (Scheme 3, top). The
analogous reaction of [K(thf)x][Co(η4-cod)2] (4) with styrene in
THF at room temperature required a large excess of styrene
(30 equiv.) and addition of [18]crown-6 to allow the isolation of the
bis(η2-styrene) complex 5 in 70% yield. Isolation of a solid product
was not possible in the absence of the crown ether. The formation
of a putative homoleptic complex [Co(η2-styrene)4]− was not
observed. Complex 5 crystallized as bright orange blocks from a
THF solution layered with n-hexane and was characterized by
single crystal X-ray diffraction (Scheme 3, bottom), NMR
spectroscopy, and elemental analysis. The compound is very air-
sensitive. Exposure of solid 5 to the air is followed by immediate
decomposition to a dark brown solid. A dark precipitate is formed
in solution upon contact with air or moisture.
L =
π
-acceptor
H2
precatalyst formation
(arene, alkene)
2
R
LnCo
Co
Co
R
R
R
1
H
H
R
Scheme 1. Catalytic concept: Activation of arene metalate pre-catalysts for
hydrogenation reactions by π-ligand exchange with olefinic substrates.
Results and Discussion
Pre-catalyst syntheses. The potassium bis(η4-anthracene)-
metalates 1 (M = Co) and 2 (M = Fe) were prepared in good yields
according to the method by Ellis and coworkers by reduction of
the metal dibromides with potassium in the presence of
anthracene (Scheme 2).[19],[20],[21] In a similar manner, treatment of
the in situ prepared [Co(η4-naphthalene)2]− with one equivalent of
cod (1,5-cyclooctadiene) gave the heteroleptic complex
[K([18]crown-6)][Co(η4-naphthalene)-(η4-cod)] (3).[22] Following a
protocol of Jonas and coworkers, we synthesized the homoleptic
[K(thf)x][Co(η4-cod)2] (4) by reduction of cobaltocene with a slight
excess of potassium in the presence of 3 equiv. of cod in THF.[23]
Upon ligand exchange of 3 and 4 with styrene, we succeeded in
Scheme 3. Synthesis and molecular structure of [K([18]crown-6)]
[Co(η4-cod)(η2-styrene)2] (5). Ellipsoids are at the 50% probability level; H atoms
are omitted for clarity.
the
first
preparation
of
the
heteroleptic
complex
[K([18]crown-6)][Co(η4-cod)(η2-styrene)2] (5), which constitutes a
- 2 -
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In the molecular structure of 5, the coordination environment
of the cobalt atom is distorted tetrahedral with a twist angle of
56.3°, which is somewhat smaller than the one for
[K([2,2,2]cryptand)][Co(η4-cod)2] (67.3°) reported by Ellis.[19] The
bite angle of the cod ligand is 90.0(3)°, and the angle between the
two styrene ligands and Co is 104.3(3)°. The average C=C bonds
length of the styrene ligands is 1.423(1) Å, which is 0.08 Å longer
than the value of free styrene.[24]
The 1H NMR spectrum of 5 (THF-d8) shows two sets of signals
with different intensities, which indicates the presence of a major
and a minor isomer in solution (SI, Figure S1). These isomers
likely arise from species with differing relative orientations of the
phenyl rings, but the same overall composition.[25] According to
1H NMR integration, the ratio between the main and the minor
isomer is 4:1.
coordinated dct molecules is very similar to the value found for
cod in [K([2,2,2]cryptand)][Co(η4-cod)2].[19]
The 1H NMR spectrum of the isolated product mixture
recorded in THF-d8 corroborates the composition of 6. The
spectrum clearly shows one set of signals assigned to 6 with the
expected broad multiplets for dct and cod ligands in a 1:1 ratio,
including the typical AA'BB' spin system arising from the arene
protons of dct (multiplets at 6.45 and 6.32 ppm). In addition, a
second set of minor signals can be assigned to
[K([18]crown-6)][Co(η4-dct)2].
Treatment of [K(thf)x][Co(η4-cod)2] (4) with dct (1.2 equiv.)
resulted in a mixture of unreacted 4, the mono-substitution
product
[K(solv)][Co(η4-cod)(η4-dct)],
and
homoleptic
[K(thf)2][Co(η4-dct)2] (7). The formation of such a mixture is
probably due to ligand exchange equilibria, which need to be
considered when using dct as a catalyst poison (vide infra).[27]
The desired homoleptic complex 7 was cleanly produced by
reacting [K(dme)2][Co(η4-anthracene)2] (1) with two equivalents of
dct in THF solution at room temperature (Scheme 5, top) and was
isolated in 19% yield by recrystallization from THF/n-hexane. The
relatively low yield is explained by the need for several
recrystallizations in order to remove free anthracene and dct. It
Similar to the preparation of 5, [K([18]crown-6)][Co-
(η4-cod)(η4-dct)] (6), containing the rigid, non-planar, tub-like
diene ligand dct (dibenzo[a,e]cyclooctatetraene),[26],[27] was
synthesized by adding 1 equiv. of dct to 3 in THF at room
temperature (Scheme 4, top). Ligand exchange is incomplete,
thus, 6 could not be obtained as a pure compound. Various
samples were contaminated with
a minimum of 18% of
[K([18]crown-6)][Co(η4-dct)2], even after several recrystallizations. seems noteworthy that the yield of 7 considerably increased when
styrene (2 equiv.) was added to the reaction mixture. In this case,
pure 7 was isolated in 62% yield after only one crystallization step
from the clear orange reaction solution. The higher yield in this
case might be due to the formation of an intermediary styrene
complex such as 5, which is subsequently converted to 7 by
reaction with dct.
Orange blocks of 7 suitable for X-ray crystallography were
obtained from THF/Et2O. Single-crystal X-ray analysis revealed
an ion-contact structure (Scheme 5, bottom) where the
coordination environment of cobalt is overall similar to that in
[K([2,2,2]cryptand)][Co(η4-cod)2].[19] The twist angle of 55.0(1)° is
significantly smaller than for the former compound (67.3°). One
1
set of dct signals is observed in the H NMR spectrum of 7 in
THF-d8 , consistent with the homoleptic structure of the complex.
Scheme 4. Synthesis and molecular structure of [K([18]crown-6)]
[Co(η4-cod)(η4-dct)] (6). Ellipsoids are at the 50% probability level; H atoms are
omitted for clarity.
X-ray quality crystals of yellow-orange 6 were obtained from
a THF solution layered with diethyl ether. The crystallographically
determined molecular structure (Scheme 4, bottom) is similar to
that of 5 and of [K([2,2,2]cryptand)][Co(η4-cod)2].[19] Cobalt has a
distorted tetrahedral coordination environment with a twist angle
Scheme 5. Synthesis and molecular structure of [K(thf)2][Co(η4-dct)2] (7).
Ellipsoids are at the 50% probability level; H atoms are omitted for clarity.
of 59.0°. The average C=C bond length (1.419 Å) of the
[a] yield of isolated compound obtained in the presence of styrene (2 equiv.).
- 3 -
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The aforementioned series of arene and alkene metalates
was complemented with two cyclopentadienyl iron complexes
(Scheme 6). [Li(thf)2][CpFe(η4-naphthalene)] (8, Cp = C5H5) was
prepared according to Jonas and coworkers from ferrocene by
reduction with Li in the presence of naphthalene.[28] The
compound was isolated in 60% yield. Its purity was confirmed by
1H NMR and elemental analysis. The synthesis of the related
(vide supra).[27] The strong coordination of dct, and to a lesser
extent of cod, to the formal Co− catalytic center slows down ligand
exchange with the substrate styrene. At the same time, dct is not
hydrogenated, and the hydrogenation of cod is slow. The iron
complexes 2, 8, and 9 showed slightly lower activity.
Table 1. Hydrogenation of alkenes with bis(anthracene) complexes 1 and 2.[a]
complex [K([18]crown-6)][Cp*Fe(η4-naphthalene)] (9, Cp*
=
C5Me5) was reported earlier by our group.[29] The reduction of
Cp*FeCl, in situ prepared from FeCl2(thf)1.5 and Cp*Li in DME,
with 2 equiv. of potassium naphthalenide in the presence of
[18]crown-6 at –60 °C in DME gave 9 in 40% yield.
1
2
Entry
1
2
3
4
5
6
7
8
Alkene
R
H
4-F
yield [%]
95
100
89
89
100
2
Li(naphthalene)
Fe
Fe
4-CO2Me
2-OMe
3-Me
4-NH2
OMe
OAc
Me
thf
95
96
27
97[b]
50
27
0
58
2
[Li(thf)2]+
8
69
K(naphthalene)
dme
100[c]
100[d]
100[c]
100[c]
76[d]
88[d, e]
92[d, e]
92[d]
-
FeCl2(thf)1.5 + Cp*Li
Cp*FeCl
Fe
9
dme
10
11
12
13
14
15
16
Ph
-
+
Me
-
[K([18]crown-6)]
9
Ph
-
CO2Et
n = 8
n = 12
-
Scheme 6. Synthesis of cyclopentadienylferrates 8 and 9.
73
72
-
Catalytic hydrogenations. Our preliminary study of catalytic
hydrogenations with 1 mol% of the potassium bis(η4-anthracene)
metalates(1-) 1 and 2 revealed superior activity of the cobaltate 1
(Table 1).[18] Various α-, β-, and ring-substituted styrenes were
hydrogenated in excellent yields in toluene solution at 2 bar H2
and room temperature. The conversion of terminal, internal, di-
and tri-substituted aliphatic alkenes and alkynes required a higher
catalyst loading as well as elevated pressure and temperature
(5 mol%, 10 bar H2, 60 °C). The 17 valence electron pre-catalyst
2 exhibited good activity only with unbiased styrenes and
1-alkenes, but fared much poorer with deactivated olefins (EDG-
substituted styrenes, internal alkenes). Rapid deactivation and
unwanted side reactions were observed when the substrate
contained ester and free amino groups. No significant effect of the
crown ether coordinated to the potassium counterion on the
catalytic activity was observed.
17
18
19
100[c]
63[d, f]
79[d]
-
-
< 5[d]
< 5[d, h]
20
99[d, g]
[a]
Standard conditions: 0.5 mmol substrate in 2 mL toluene; yields of
hydrogenation products determined by quantitative GC vs. internal
[b]
[d]
reference
n-pentadecane.
2 bar. [c] 60 °C, 2 bar, 24 h.
5 mol% cat.,
)-stilbene.
60 °C, 10 bar, 24 h. [e] < 8% 2-alkene. [f] 80 °C. [g] bibenzyl. [h]
(E
We then set out to evaluate the series of monoanionic alkene
and arene metalates 1 − 9 as pre-catalysts in parallelized olefin
hydrogenations under identical conditions. Styrene and
1-dodecene were chosen as model substrates (Table 2). The
standard conditions involved reaction with 5 mol% pre-catalyst
under an atmosphere of 2 bar H2 in THF (due to the better
solubiltity of the complexes compared to toluene) at room
temperature for 24 h in a stainless steel ParrTM reactor (Figure 2).
In general, styrene was converted in excellent yields with most
pre-catalysts except 6 and 7 containing dct as ligand. This
observation is in accord with the postulate that dct is a competent
catalyst poison for homogeneous low-valent monometal species
Figure 2. Parallelized hydrogenation setup in ParrTM pressure reactors.
- 4 -
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Table 2. Hydrogenation of alkenes with pre-catalysts 1−
9.[a]
Pre-catalyst
94
58 (27)
72
99
15 (7)
93 (0)
93
72
62 (29)
85 (8)
Figure 3. 1H NMR monitoring (THF-
catalyst 1 (dme: †); a) 3 h after the addition of 20 equiv. of styrene; b) 3 h after
d8: #) of styrene hydrogenation with pre-
addition of hydrogen.
With 1-dodecene, similarly good catalytic hydrogenation
activities were observed for the pre-catalysts 1, 3, 4, 5, and 8 with
up to 93% alkene hydrogenation and <29% alkene isomerization.
The best activity and selectivity was determined in the reaction
with 3, which resulted in no observable isomerization to internal
alkenes. Again, the bis(η4-dct)cobaltate 7 was catalytically
inactive due to the strong dct-coordination to the Co center, which
renders this complex inert with respect to ligand substitution and
ligand hydrogenation.[27] From both model reaction series, it
became obvious that, despite only small stereoelectronic
differences between the pre-catalysts 1 − 9, the nature of the π-
hydrocarbon ligands and the central metal ion has a strong
influence on the overall catalytic activity. Pre-catalysts containing
naphthalene or anthracene exhibited generally higher activity,
presumably due to the re-establishment of aromaticity upon
exchange of the polyarene ligand with the better π*-accepting
alkenes.[30] Cobaltate complexes were more active and selective
than their Fe counterparts.
Mechanistic studies. The investigated pre-catalysts 1 and 2
did not react with dihydrogen at ambient temperature (J. Young
NMR tube experiment, up to 4 bar H2, THF-d8). We therefore
believe that the proposed mechanism of alkene hydrogenation is
initiated by the substitution of the labile arene ligand by the
π-substrate followed by reaction of the resulting metal catalyst
with dihydrogen (Scheme 1). In preliminary studies with
bis(η4-anthracene)cobaltate 1, we monitored this catalyst
36
71 (24)
0
0 (0)
90
6
84 (15)
72 (24)
[a]
Standard conditions: 0.5 mmol substrate in 2 mL THF; yields of
hydrogenation products determined by quantitative GC vs. internal reference
-pentadecane. In parentheses: yield of alkene isomerization products.
n
- 5 -
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activation step by redox-neutral π-ligand exchange in
homogeneous phase through NMR experiments. Figure 3 shows
the 1H NMR spectra of pre-catalyst 1 in THF-d8, the reaction
mixture after 3 h resulting from the addition of 20 equiv. styrene
to the complex solution at room temperature (spectrum a), and
the reaction mixture after 3 h under 4 bar H2 pressure
(spectrum b). The observation of resonances of non-coordinated
anthracene in the spectra a) and b) clearly supports the notion of
ligand exchange prior to styrene hydrogenation. The signals of
ethylbenzene are apparent in spectrum b). There were no further
resonances observed in the high-field section which would
indicate the formation of hydride complexes under dihydrogen
atmosphere. The observed line broadening is tentatively
attributed to the slow formation of cobalt nanoparticles.
We extended the 1H NMR monitoring studies to the
[Co(η4-cod)(η4-L)]− complexes 3 (L = naphthalene) and 4 (L = cod).
When assuming a pre-catalyst activation by π-ligand exchange of
the weakest ligand with the substrate, both 3 and 4 should funnel
through the same catalytic intermediate. We tested this
mechanistic hypothesis by adding 20 equiv. of styrene to THF-d8
solutions of 3 and 4, respectively (Figures 4 and 5). Indeed, the
recorded 1H NMR spectra showed the clean formation of the
anticipated bis(η2-styrene)cobaltate 5 in both cases alongside
resonances of free naphthalene (from 3) and cod (from 4). This
observation strongly supports our mechanistic proposal. Upon
application of an atmosphere of H2 to the NMR scale reactions,
clean conversion of the substrate styrene was observed
(Figures 4 and 5). Furthermore, the rate of substrate conversion
can be qualitatively assessed from these experiments.
Figure 5. 1H NMR monitoring (THF-
catalyst 4; a) 1.5 h after the addition of 20 equiv. of styrene; b) 1.5 h, and c) 96 h
d8: #) of styrene hydrogenation with pre-
after addition of hydrogen.
The loss of the π-ligand styrene upon complete hydrogenation
with pre-catalyst 3 after 24 h at 2 bar H2 and a significantly slower
conversion of cod (and naphthalene) resulted in the reconstitution
of the original pre-catalyst 3 by naphthalene coordination as
indicated by the red color of the complex. This complex is difficult
to detect in the reaction mixture by 1H NMR, but its formation was
clearly proven by a separate experiment (Scheme 7, vide infra).
The NMR monitoring of the related 4-catalyzed hydrogenation of
styrene showed full conversion of the substrate and the ligand cod
after 96 h (Figure 5). Likewise, as in the case of 1, the NMR
monitoring of complexes 3 and 4 did not show any high-field
signals of hydride species.
We prepared and fully characterized the catalytically active
bis(η4-styrene) complex 5 (Scheme 3, vide supra), whose role as
a key intermediate in styrene hydrogenations with alkene
cobaltate pre-catalysts was obvious from the NMR-spectroscopic
experiments discussed above (Figures 4 and 5). Application of an
H2 atmosphere (1 bar) to a bright orange THF-d8 solution of 5
effected an immediate color change to black due to the
hydrogenative consumption of the π-ligands which stabilize this
1
cobaltate species (SI, Figure S4). H NMR spectra of the crude
mixture and GC analyses confirmed the instantaneous formation
of major amounts of ethylbenzene and cyclooctane and only
minor amounts of cyclooctene. With pre-catalyst 3, bearing a
much less reactive naphthalene ligand, the sufficiently differing
rates of hydrogenation, styrene > cod >> naphthalene allowed the
reconstitution of the original pre-catalyst by a release-catch
mechanism after the complete hydrogenation of the reactive
alkenes (Figure 4, top). A similar outcome was observed in
reactions of bis(η2-styrene) complex 5 with excess naphthalene
under 1 bar H2 pressure (Scheme 7). The chemoselective
conversion of styrene and inertness of naphthalene under the mild
hydrogenation conditions also led to the formation of the
(η4-cod)(η4-naphthalene)cobaltate 3, which was isolated as a dark
red solid by evaporation of the volatiles (SI, Figure S5).
Figure 4. 1H NMR monitoring (THF-
d8: #) of styrene hydrogenation with pre-
catalyst 3; a) 1.5 h after the addition of 20 equiv. of styrene; b) 1.5 h, and c) 24 h
after addition of hydrogen; the spectrum of a clean sample of 5 is shown on top.
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Filtration of the freshly prepared pre-catalyst solution through a
PTFE syringe filter (pore size <0.1 µm) prior to the addition of the
substrate gave unaltered hydrogenation activity of pre-catalyst 1.
The addition of 300 mol% mercury did only slightly affect the
catalyst activity. However, the formation of amalgams between
mercury and 3d transition metals is very slow.[31] A pronounced
reaction inhibition was only observed by addition of dct to the
catalytic hydrogenation with the bis(η4-anthracene)cobaltate
pre-catalyst 1. This suggests the formation of a catalytically
inactive homoleptic cobaltate bearing dct ligands, which is in
perfect agreement with the observation of 0% conversion in
alkene hydrogenations with the pre-catalyst [K(thf)2][Co(η4-dct)2]
(7, see Table 2). The rapid formation of 7 from 1 and dct was
already demonstrated (Scheme 6). Further support comes from
1H NMR experiments of a THF solution of 7 and 20 equiv. of
styrene, which showed no substitution of the dct ligands over a
course of 1.5 h (Figure 9).
1 bar H2, THF
Co
2
+
Co
2.2 equiv.
Ph
Ph
[K([18]crown-6)]+
+
[K([18]crown-6)]
5
3
Scheme 7. Demonstration of the ligand release-catch concept by conversion of
5 to 3 upon chemoselective hydrogenation of styrene.
Figure 6. Reaction progress analysis: 1-catalyzed hydrogenation of styrene
under standard conditions without any detectable induction period. Dashed lines
are only visual guides.
As the abovementioned results do not rule out the operation
of
a heterogeneous catalytic pathway as a background
reaction,[27] we turned to reaction progress analyses by
quantitative GC analysis of all reaction components (Figure 6).
The early reaction phase of the 1-catalyzed hydrogenation of
styrene (<20 min) showed no induction period and no sigmoidal
curvature, which would indicate a nucleation step en route to
nanoclusters and nanoparticles. An identical behavior was
observed from the hydrogenation of styrene with 5 mol% of pre-
catalyst 3. Without any detectable induction period, styrene was
completely hydrogenated within 45 min at 2 bar H2. The
conversion of the ligands cod and naphthalene largely
commenced after the substrate styrene had been entirely
converted to ethylbenzene (Figure 7).
To gain further information with respect to the homogeneous
vs. heterogeneous nature of the operating catalyst, we performed
kinetic poisoning studies with a scavenger reagent that is
selective for mononuclear late transition metal species in low
oxidation states: dibenzo[a,e]cyclooctatetraene (dct).[26],[27] Upon
addition of only 2 mol% dct to a catalytic hydrogenation of styrene
with 1 mol% 1 after 35 min (~17% conversion), a complete
inhibition of catalyst turnover was observed, which is indicative of
a homogeneous mechanism (Figure 8, vide supra). Inhibition of a
potential heterogeneous pathway by amalgamation was not
observed.[27]
Figure 7. Reaction progress analysis: 3-catalyzed hydrogenation. Conversions
of styrene (a), cod (b), and naphthalene (c). Dashed lines are only visual guides.
In an extended study, we performed the two model reactions
(styrene, 1-dodecene) with the two most active pre-catalysts 1
and 3 in the presence of scavengers (Hg, PMe3 and dct; Table 3).
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transition metal complexes.[33] Indeed, negative-ion mode ESI of
a solution of 1 in THF afforded the free [Co(anthracene)2]− anion
in high signal intensity (SI, Figure S6). In addition, the potassium-
bound dimer [K{Co(anthracene)2}2]− was also observed.
Presumably, this species was not present in the diluted sample
solution, but formed due to the concentration increase during the
ESI process; similar behavior has been found in other cases as
well.[32],[34] ESI of a solution of the heteroleptic complex 3
produced not only [Co(η4-cod)(η4-naphthalene)]− as well as small
quantities of [K{Co(η4-cod)(η4-naphthalene)}2]−, but also its
homoleptic counterpart [Co(η4-cod)2]¯ (SI, Figure S7). This
observation clearly demonstrates the operation of intermolecular
exchange process in solution. ESI-mass spectrometric analysis of
solutions of 4 and 5, respectively, also resulted in the detection of
the expected anionic complexes as main peaks (SI, Figures S8
and S9).
Figure 8. Poisoning studies with pre-catalyst 1 by addition of 300 mol% Hg and
After treating solutions of 1 and 3 with an excess of styrene,
we observed the formation of the cobaltates 10 and 5,
respectively (Figure 10a and 10b). In both complexes, two
styrene molecules had replaced one of the originally bound
ligands (also compare Figure 4). For the heteroleptic complex 3,
only naphthalene, but not the cod ligand was released. This
behavior is fully in line with the higher binding energy of the latter,
which we had already derived from the NMR spectroscopic
experiments. The reaction of 1 with styrene also furnished the
homoleptic complex [Co(styrene)3]− in very small abundance. The
lack of any detectable [Co(styrene)4]− suggests that this species
did not form in solution or that its stability was too low to survive
the ESI process. When 1 was treated with an excess of 1-
dodecene, the replacement of naphthalene by 1-dodecene
proceeded only to a small extent (Figure 10c). This finding is
consistent with the lower reactivity of 1-dodecene observed in the
synthetic studies (vide supra).
2 mol% dct, respectively. Dashed lines are only visual guides.
Table 3. Poisoning experiments of hydrogenations with arene cobaltate pre-
catalysts 1 and 3.[a]
Catalyst
manipulation
1
3
1
3
none
94
99
58[b]
93
<0.1 μm filter
300 mol% Hg
91
81
-
46[c]
29[d]
-
75
40
1.25 mol%
PMe3
69
91
81
47[e]
94
66
11 mol% dct
14
3[f]
[a]
Standard conditions: 0.5 mmol substrate in 2 mL THF. Isomerization:
[b] 27%, [c] 34%, [d] 34%, [e] 6%, [f] 31%.
The observation of good catalytic activity of a mixture of pre-
catalyst 3 and dct in Table 3 is a direct consequence of the
presence of the strongly coordinating ligand cod in 3, which
undergoes little or no substitution with equimolar dct. This results
in the exclusive substitution of the naphthalene ligand of 3 by dct
and formation of the heteroleptic cobaltate 6 as the dominant
catalyst species. Our catalytic experiments showed that
[K([18]crown-6)][Co(η4-cod)(η4-dct)] (6) has a good activity in
hydrogenations of styrene and 1-dodecene (Table 2, vide supra).
Given the anionic nature of the putative catalyst species, we
also used negative-ion mode ESI mass spectrometry for their
selective detection and analysis. Under carefully optimized
conditions, this method is well capable of detecting even highly
reactive organometallics in intact form,[32] including low-valent
Figure 9. 1H NMR spectrum (THF-
d8: #) of complex 7, and a) 1.5 h after the
addition of 20 equiv. of styrene.
Interestingly, the cobaltate complexes incorporating two
molecules of styrene were accompanied by ions whose m/z ratios
were shifted by 2 u to lower values, which obviously resulted from
dehydrogenation reactions. According to the principle of
microscopic reversibility, the catalytic activity of the cobaltate
complexes with respect to hydrogenation reactions implies that
they can also catalyze dehydrogenations.[35] The absence of any
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ions with m/z ratios shifted by 4 u moreover indicates that the
dehydrogenation reactions involved a coupling of two styryl units,
which most likely gave 1,4-diphenylbuta-1,3-diene. Possibly, this
diene originated from the dehydrogenation of one cobalt-bound
styrene molecule and the addition of a second cobalt-bound
styrene to the resulting C≡C triple bond. Low-valent cobalt
complexes are known to catalyze related C−H activation
reactions.[36]
Finally, we probed the unimolecular gas-phase reactivity of
the mass-selected cobaltate complexes. These experiments have
the advantage of excluding any interference from dynamic
equilibria, counter-ion or solvent effects, which may operate in
solution. Gas-phase fragmentation of the complex 3 led to the loss
of cod and naphthalene, whereas 5 and 10 only released styrene
(SI, Figures S10−S13).
In conclusion, our investigations on catalytic alkene
hydrogenations documented the formation of 18 valence electron
(18e) bis(alkene) complexes in the reaction mixtures. These
species presumably are resting states, which serve as the
reservoir for the catalytically active cobalt species. One may
speculate that H2 activation is initiated by loss of an alkene ligand,
forming an unsaturated and reactive 16e monoalkene complex.
Table 4. Hydrogenation of ketone and imine with pre-catalysts 1−
9.[a]
Pre-catalyst
1
2
3
4
5
7
8
9
11 (91)
14 (14)
4 (65)
5 (60)
5
0 (17)
6 (4)
2 (4)
0 (99)
0 (2)
0 (15)
0 (3)
3 (3)
0 (26)
0 (15)
4 (6)
[a]
Standard conditions: 0.5 mmol substrate in 2 mL THF; yields of
hydrogenation products determined by quantitative GC vs. internal
reference -pentadecane. Yields from reactions at 10 bar H2, 60 °C in
n
parentheses.
Methodology extensions. We also applied the pre-catalysts
1 − 9 to hydrogenations of ketones and imines. Generally,
hydrogenations of such polar unsaturated compounds are
accelerated by the presence of a Lewis acidic catalyst in higher
oxidation states. However, the pre-catalyst complexes contain a
weakly Lewis acidic K+ counterion. We observed very poor
catalytic activities under the standard conditions at 2 bar H2 and
room temperature. Elevated pressure and temperature (10 bar H2,
60 °C, see Table 4) led to good activity of potassium
bis(anthracene)cobaltate
1
in
the
hydrogenation
of
dibenzylketone and N-benzylideneaniline (>91% yield). The cod-
containing complexes 3 and 4 exhibited moderate activity in the
ketone hydrogenation (60-65%). Surprisingly, both complexes
were rather inactive in the hydrogenation of the imine.
Figure 10. Negative-ion mode ESI mass spectra of the products formed upon
reaction of: a) 3 (7.5 mM) with 10 equiv. of styrene; b) 1 (7.5 mM) with 20 equiv.
of styrene; c) 1 (7.5 mM) with 20 equiv. of 1-dodecene (dod) in THF.
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Table 5. Hydrogenation of ketones and imines with cobaltate pre-catalyst 1.
(in DMSO).[37] After the first turnover, this is very likely to alter the
catalytic mechanism by catalyst oxidation and H2 evolution
(Scheme 8).[38] Considering a catalyst oxidation after the direct
electron transfer to the ketone or after the first hydrogenation
catalysis turnover, we postulate the formation of a cobalt(+I)
catalyst which displays lower catalytic activity and therefore
requires harsher conditions. The formation of dihydrogen was
Entry
Substrate
R
yield [%]
1
observed by H NMR in an equimolar reaction between 1 and
1
2
Me
Bn
99, 90[b]
96
1,3-diphenyl-2-propanol. Furthermore, a transfer hydrogenation
experiment between 4-methylstyrene and 4 equiv. of
1,3-diphenyl-2-propanol afforded 18% yield of ethylbenzene in
the presence of 5 mol% 1.[18]
Direct SET reduction of acetophenone was observed in the
presence of 1 and 2, respectively, to give the pinacol product in
good yields (Scheme 9a).[39] With an olefinic radical probe, such
behavior was much less pronounced under standard reaction
conditions (Scheme 9b). Catalyst 1 showed no ring-opening of
α-cyclopropylstyrene but clean hydrogenation of the double bond.
Significant radical character was observed in reactions with the
cod-bearing catalysts 3 and 4.
3
4
5
100
88
71
6
7
8
9
10
11
H
96,100[b]
98
100
100
79[c]
0
2-Me
3-Me
4-OMe
CO2Et
Br
[a]
a) Pinacol coupling:
Standard conditions: 0.5 mmol substrate in 2 mL toluene; yields of
hydrogenation products determined by quantitative GC vs. internal
n
-pentadecane. [b] Solvent: THF [c] 7.5 mol% 1, 70 °C, 10 bar H2.
33 mol%
(
)
reference
[LnM-]
[LnM+]
OH
O
X
Ph
Ph
Ph
THF, r.t., 3 h
R
1st turnover
OH
(0.1 mmol)
K+[LnCo]-
H2
cat. 1: 0.021 mmol (64%)*
cat. 2: 0.025 mmol (74%)*
X
SET-initiated
radical reactions
pKa !
XH
* ref. to 2e transfer from [LnM-]
R
XH
K+[LnCo]-
+
R
X-K+
b) Ring opening:
R
R
R
R
5 mol% [cat.]
XH
R
XK
[LnCo]
+
Ph
Ph
Ph
1/2 H2
2 bar H2
+
X
THF, r.t., 3 h
R
XH
cat. 1: 95%
cat. 3: 50%
cat. 4: 75%
-
H2
15%
10%
higher oxidation state catalysis
Scheme 9. Observation of radical side reactions.
Scheme 8. Change of mechanism, H2 evolution and catalyst oxidation in the
hydrogenation of polar substrates.
Conclusions
The most active ketone hydrogenation catalyst 1 was
subjected to a series of other carbonyl compounds (Table 5).[18]
Good catalytic activity was only observed at elevated temperature
and pressure. Importantly, the employment of carbonyl
compounds as hydrogenation substrates could in principle trigger
three unwanted side reactions: deprotonation at the α-carbonyl
position, direct reduction of the carbonyl moiety by metalate
addition or single-electron transfer (SET), and deprotonation of
any formed alcohol. Indeed, we have observed the operation of
the latter two pathways under the present reaction conditions. The
catalytic hydrogenation reaction generates an acidic proton in the
resulting alcohol and amine products, both with pKa values of ~29
We showed that the bis(η4-anthracene)metalates 1 and 2 exhibit
good activity in catalytic hydrogenation reactions. The
bis(η4-anthracene)cobaltate 1 is a highly active pre-catalyst for the
hydrogenation of a variety of alkenes, ketones and imines at
ambient H2 pressure and temperatures. The iron analogue
[K([18]crown-6)][Fe(η4-anthracene)2] (2) showed significantly
lower catalytic activity.[18]
In a greatly extended study, we have now compared the
catalytic activity of 1 and 2 with that of structurally related alkene
and arene metalates [K([18]crown-6)][Co(η4-naphthalene)-
(η4-cod)] (3), [K(thf)x][Co(η4-cod)2] (4), [K([18]crown-6)][Co-
(η4-cod)(η2-styrene)2] (5), [K([18]crown-6)]Co(η4-cod)(η4-dct)] (6),
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[K(thf)x][Co(η4-cod)2] (4) varies according to 1H NMR and elemental
analysis (x = 0.15 − 0.3).
[K(thf)2][Co(η4-dct)2] (7), [Li(thf)2][CpFe(η4-naphthalene)] (8), and
[K([18]crown-6)][Cp*Fe-(η4-naphthalene)]
(9). 5 − 7 were
[K([18]crown-6)][Co(η4-cod)(η2-styrene)2] (5). Method 1 (from 3): A
solution of styrene (57.4 mg, 0.551 mmol, 2.20 equiv.) in THF (2 mL) was
added dropwise to a solution of [K([18]crown-6)][Co(η4-naphthalene)-
(η4-cod)] (3) (150 mg, 0.251 mmol, 1.00 equiv.) in THF (5 mL) at room
temperature. The resulting clear orange solution was stirred overnight.
Afterwards the solvent was removed in vacuo. The solid orange residue
was washed several times with diethyl ether (10 mL overall). The crude
product was dissolved in THF (5 mL), the resulting solution was filtered
and concentrated. Orange, Xray-quality crystals of 5 formed after layering
of the THF solution with n-hexane (1:2). Yield: 105 mg (62%). M.p. 125 °C
(decomp.). Elemental analysis calcd for C36H52O6CoK (M = 678.84):
synthesized for the first time. 1 − 6 as well as 8 and 9 are
competent pre-catalysts for the hydrogenation of alkenes under
mild conditions. Unlike 1 and 2, the bis(η4-styrene) complex 5
rapidly reacts with H2 (1 bar) with release of ethylbenzene. Kinetic
studies and 1H NMR monitoring experiments presented now lead
to the conclusion that the olefin hydrogenation reaction is initiated
by the substitution of one labile arene ligand by a π-acceptor
substrate. Further, we proved the concept of the release-catch
mechanism of catalyst activation by 1H NMR monitoring of π-
ligand exchange reactions and by negative-ion mode ESI mass
1
C 63.70, H 7.72, found C 63.04, H 7.47. H NMR (300.13 MHz, THF-d8),
spectrometry investigations. The selective formation of
a
major isomer: δ = –0.15 (d, J = 11.1 Hz, 2H, styrene CH2), –0.02 (d,
J = 7.3 Hz, 2H, styrene CH2), 0.92-1.16 (m, 2H, cod CH), 1.22-1.44 (m, 2H,
cod CH2), 1.78-2.04 (m, 2H, cod CH2), 2.62 (dd, J = 13.4, 7.9 Hz, 2H, cod
CH2), 2.27-2.93 (m, 2H, cod CH), 3.27-3.39 (m, 2H, cod CH2), 4.77 (dd,
J = 11.1, 7,3 Hz, 2H, styrene CH), 6.38-6.64 (m, 2H, styrene p-Ar-H), 6.89
(t, J = 7.4 Hz, 4H, styrene m-Ar-H) , 7.15-6.97 (m, 4H, styrene o-Ar-H),
minor isomer: –0.63 (d, J = 6.8 Hz), –0.26 (d, J = 11.2 Hz), 0.55 (d,
J = 11.2 Hz), 0.65 (d, J = 6.8 Hz), 0.87-0.89 (m), 2.29-2.39 (m), 3.02-3.10
(m), 4.17-4.27 (m), 4.50-4.62 (m). 13C{1H} NMR (100.61 MHz, 300 K,
THF-d8): δ = 29.4 (cod CH2), 37.8 (cod CH2), 47.0 (styrene CH2), 60.4
(styrene CH), 71.0 ([18]crown-6 CH2), 81.6 (cod CH), 89.5 (cod CH), 117.7
(styrene p-Ar-CH), 124.2 (styrene m-Ar-CH), 127.0 (styrene o-Ar-CH),
154.6 (styrene Cquart.-Ar); minor isomer: δ = 29.0, 29.4, 38.5, 117.6, 118.0,
124.4, 126.9, 127.5.
Method 2 (from 4): Styrene (2.09 mL, 18.2 mmol, 30.0 equiv.) was added
to a solution of [K(thf)x][Co(η4-cod)2] (4) (200 mg, 0.608 mmol, 1.00 equiv.)
and [18]crown-6 (162.5 mg, 0.608 mmol, 1.00 equiv.) in THF (5 mL) at
room temperature. The resulting clear, orange solution was stirred for 5 h.
All volatile components were removed in vacuo afterwards. The resulting
orange solid was washed with diethyl ether (5 mL), taken up in THF and
layered with n-hexane. 5 was obtained as orange blocks by storage at
room temperature. Yield: 290 mg (70%). The 1H NMR spectrum of the
sample prepared by method 2 was identical with those prepared by
method 1.
[K([18]crown-6)][Co(η4-cod)(η4-dct)] (6). A solution of dct (73.6 mg,
0.360 mmol, 1.50 equiv.) in THF (7 mL) was added dropwise to
[K([18]crown-6)][Co(η4-naphthalene)(η4-cod)] (3) (143.7 mg, 0.240 mmol,
1.00 equiv.) in THF (10 mL) at room temperature. The resulting clear
yellow solution was stirred overnight. Afterwards the solvent was removed
in vacuo. The yellow-orange solid residue was washed three times with
diethyl ether (15 + 10 + 5 mL). The crude product was dissolved in THF
(7 mL) and filtered. Yellow-orange, X-ray-quality crystals of 6 formed after
layering the filtrate with diethyl ether (1:1). Compound 6 is contaminated
with a varying amount of 7, which could not be removed by crystallization.
A minimum of 18% impurity was observed. Yield: 76.3 mg (46%), referring
to a mixture of 6 (82%) and 7 (18%). 1H NMR (400.13 MHz, THF-d8):
δ = 1.98-2.06 (m, 4H, CH2 of cod or dct ), 2.24-2.34 (m, 4H, CH2 of cod or
dct), 2.71 (br s, 4H, alkene-CH of cod or dct), 2.93 (s, 4H, alkene-CH of
cod or dct), 6.27-6.36 (m, 4H, Ar-H), 6.42-6.49 (m, 4H, dct Ar-H); in
addition one set of signals assigned to the [Co(dct)2]− anion of
[K([18]crown-6)][Co(η4-dct)2] can be observed.
[K(thf)2][Co(η4-dct)2] (7). Method 1 (from 1): A solution of dct (733 mg,
3.59 mmol, 2.00 equiv.) in THF (60 mL) was added to a solution of 1
(1.14 g, 1.79 mmol, 1.00 equiv.) in THF (100 mL) at –80 °C, and the
mixture was slowly warmed to room temperature. The resulting black
suspension was concentrated, filtered and layered with n-hexane. A dark
precipitate was isolated after 3 days. Repeated recrystallization (3x from
THF/n-hexane 1:3) was necessary to remove remaining dct and
anthracene. 7 was obtained as bright orange crystals. Yield: 220 mg (19%).
M.p. 112 °C (decomp.). Elemental analysis calcd for C40H40O2CoK
(M = 650.79): C 73.82, H 6.20, found C 73.45, H 6.04. 1H NMR (400.13
bis(η2-monoalkene)cobaltate is believed to be key to a rapid
dihydrogen activation because, unlike coordinated cod and
naphthalene or anthracene, the monoalkene ligands of such a
species are readily hydrogenated.
Poisoning experiments with dct and mercury support the
hypothesis that the active species have a homogeneous nature.
The validity of Crabtree’s dct test for cobaltate complexes was
confirmed by the formation of [K(thf)2][Co(η4-dct)2] (7) and
[K([18]crown-6)][Co(η4-cod)-(η4-dct)] (6). Bis(η4-dct) complex 7 is
not a competent pre-catalyst, presumably because the dct ligands
are not substituted or hydrogenated under the reaction conditions.
By contrast, 6 still showed some catalytic activity because of its
more labile cod ligand.
Extensions to polar substrates (ketones and imines) were also
investigated, but these reactions most likely proceed via a
different mechanism than alkene hydrogenations because of the
operation of unwanted radical and acid-base reactions. Both
pathways most likely involve oxidation of the metalate complexes
to a higher oxidation manifold which ultimately exhibits lower
catalytic activity. However, the rapid onset of SET reactions with
polar substrates appears to be a promising entry to future studies
of radical reactions catalyzed by such alkene metalates. The
general concept of redox-neutral alkene ligand substitution with
metalate complexes has only recently been tapped for catalytic
reaction developments. Further variations of this motif in the
context
of
small
molecule
hydrogenation
and
hydrofunctionalization will be reported in due course.
Experimental Section
General Procedures: All experiments were performed under an
atmosphere of dry argon by using standard Schlenk and glovebox
techniques. Solvents were purified, dried, and degassed by standard
techniques. NMR spectra were recorded (300 K) with Bruker Avance 300
and Avance 400 spectrometers internally referenced to residual solvent
resonances. NMR assignments are based on COSY, HSQC and NOESY
2D NMR experiments.Melting points were measured on samples in sealed
capillaries and are uncorrected. Elemental analyses were determined by
the analytical department of the University of Regensburg.
Pre-catalysts [K(dme)2][Co(η4-anthracene)2] (1),[19],[21] [K([18]crown-6)]-
[Fe(η4-anthracene)2] (2),[20] [K([18]crown-6)][Co(η4-naphthalene)(η4-cod)]
(3),[22] [K(thf)x][Co(η4-cod)2] (4),[23] [Li(thf)2][CpFe(η4-naphthalene)] (8),[28]
and [K([18]crown-6)][Cp*Fe(η4-naphthalene)] (9)[29] were prepared
according to literature procedures. The THF content of
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MHz, THF-d8): δ = 1.77 (m, THF), 3.45 (s, 8H, dct CH), 3.61 (m, THF),
6.45-6.48 (m, 8H, dct Ar-H), 6.56-6.58 (m, 8H, dct Ar-H). 13C{1H} NMR
(100.61 MHz, 300 K, THF-d8): δ = 26.3 (THF), 68.1 (THF), 87.6 (CH),
122.8 (C-Ar), 124.9 (C-Ar), 152.9 (Cquart-Ar).
applied.[32] Mass spectra were recorded over a typical m/z range of
50−1000. Gas-phase fragmentation was accomplished by subjecting the
mass-selected ions to excitation voltages of amplitudes Vexc and allowing
them to collide with the helium gas.
Method 2 (from 1): A solution of dct (600 mg, 2.94 mmol, 2.00 equiv.) and
styrene (612 mg, 5.88 mmol, 4.00 equiv.) in THF (50 mL) was added to a
THF (120 mL) solution of 1 (932 mg, 1.47 mmol, 1.00 equiv.) at room
temperature. The mixture was stirred overnight and filtered. Concentration
of the clear orange solution to 60 mL and layering with diethyl ether (1:1)
gave 7 as orange crystals. The isolated compound had the composition
[K(thf)0.75][Co(η4-dct)2] after drying in vacuo for one hour according to
1H NMR and elemental analysis. Yield: 512 mg (62%). The 1H NMR
spectrum of samples prepared by this method was identical with those of
samples prepared by method 1.
General procedure for hydrogenation reactions. A dry 5 mL vial with a
screw cap and PTFE septum was charged with a magnetic stir bar and a
solution of the pre-catalyst (0.025 mmol) in THF (1 mL). After adding a
solution of the substrate (0.5 mmol) in THF (1 mL) with a pipette, the vial
was closed and the septum was punctured with a short needle (Braun).
The vial was placed into a high-pressure reactor (Parr Instr.), which was
sealed, removed from the glove box, placed on a magnetic stirrer plate,
and purged with hydrogen. After 24 h at room temperature under an
atmosphere of hydrogen (2 bar) the pressure was released, the vial
removed, and the reaction quenched with saturated aqueous NaHCO3
(1 mL). For quantitative GC-FID analysis, n-pentadecane was added as
an internal standard. The mixture was extracted with diethyl ether and the
combined organic layers were dried over Na2SO4.
General procedure for poisoning and filtration experiments. The
poisoning experiments were carried out according to the general
procedure for hydrogenation reactions. In the case of poisoning with PMe3
the pre-catalyst was dissolved in 0.5 mL THF before a THF stock solution
of the phosphane (0.5 mL, c = 1.25 ñ 10-2 mol/L) was added. For the
experiments with dct the catalyst poison was added to the solid pre-
catalyst before dissolving both together in THF. When using elementary
mercury as the catalyst poison, the liquid metal was added directly to the
dissolved pre-catalyst with a syringe before the addition of the substrate
solution. For the filtration experiments, the pre-catalyst solution was
filtered through a PTFE syringe filter (Puradisc 13, Whatman, pore size
< 0.1 µm) before the substrate solution was added.
General procedure for 1H NMR monitoring. Reaction monitoring by
1H NMR was carried out in a sealed J. Young NMR tube. A solution of the
pre-catalyst (5 ñ 10-3 mmol, 5 mol%) in THF-d8 (0.5 mL) was transferred
into a NMR tube, and the first 1H NMR spectrum was measured. In the
glovebox, styrene (10 mg, 0.1 mmol, 1.0 equiv.) was added to the
pre-catalyst solution. After storing the sample for 90 min, the second
1H NMR spectrum was recorded. Subsequently, the atmosphere was
exchanged with dihydrogen by the freeze-pump-thaw technique.
Subsequent spectra were recorded after further 90 min and then at
irregular intervals until the substrate was fully consumed or until no further
consumption was detected.
X-ray crystallography. The single crystal X-ray diffraction data were
recorded on an Agilent Technologies SuperNova diffractometer in case of
compound 5 and on an Agilent Technologies Gemini Ultra diffractometer
in case of 6 and 7, using CuKα radiation for 5 and 6 and MoKα radiation
for 7. Empirical multi-scan and analytical absorption corrections were
applied to the data.[40],[41] Using Olex2,[42] the structures were solved with
SHELXS or SHELXT.[43],[44] Least-square refinements were carried out
with SHELXL.[43] CCDC 1513657, 1513658, and 1513659 contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was generously supported by the Deutsche
Forschungsgemeinschaft (DFG: WO 1496/6-1, JA 1107/6-1, KO
2875/8-1) and the European Research Council (ERC: 683150).
E.R.R. is a doctoral fellow of the Deutsche Bundesstiftung Umwelt
(DBU). We thank Christoph Ziegler and Julia Märsch for the
preparation and activity studies of complexes 5 and 6 as a part of
their B.Sc. projects, Matteo Villa for the preparation of
α-cyclopropylstyrene, and Stefan Pelties for crystallographic
support.
Keywords: cobalt • iron • hydrogenations • transition metal
catalysis • reaction mechanism
[1]
[2]
a) Catalytic Hydrogenation (Ed.: L. Cerveny), Elsevier, Amsterdam,
1986; b) The Handbook of Homogeneous Hydrogenation (Eds.: J. G. de
Vries, C. J. Elsevier), Wiley-VCH, Weinheim, 2007.
a) J. A. Osborn, F. H. Jardine, J. F. Young, G. Wilkinson, J. Chem. Soc.
1966, 1711-1732; b) W. S. Knowles, M. J. Sabacky, B. D. Vineyard, D.
J. Weinkauff, J. Am. Chem. Soc. 1975, 97, 2567–2568; c) H. Doucet, T.
Ohkuma, K. Murata, T. Yokozawa, M. Kozawa, E. Katayama, A. F.
England, T. Ikariya, R. Noyori, Angew. Chem. Int. Ed. 1998, 37, 1703–
1707; d) Á. Molnár, A. Sárkány, M. Varga, J. Mol. Catal. 2001, 173, 185–
221.
[3]
[4]
a) M. Raney, US patent 1628190 1927; b) I. M. Angulo, A. M. Kluwer, E.
Bouwman, Chem. Commun. 1998, 2689–2690; c) S. Kuhl, R. Schneider,
Y. Fort, Organometallics 2003, 22, 4184–4186; d) T. Hibino, K. Makino,
T. Sugiyama, Y. Hamada, ChemCatChem 2009,
a) W. Hess, J. Treutwein, G. Hilt, Synthesis 2008, 40, 3537-3562; b) M.
S. Holzwarth, B. Plietker, ChemCatChem 2013, , 1650–1679; c) I.
1, 237–240.
5
Bauer, H. J. Knölker, Chem. Rev. 2015, 115, 3170-3387; d) P. Röse, G.
Hilt, Synthesis 2016, 48, 463-492.
[5]
[6]
S. Enthaler, K. Junge, M. Beller, Angew. Chem. Int. Ed. 2008, 47, 3317–
3321.
Q. Knijnenburg, A. D. Horton, H. van der Heijden, T. M. Kooistra, D. G.
H. Hetterscheid, J. M. M. Smits, B. de Bruin, P. H. M. Budzelaar, A. W.
Gal, J. Mol. Catal. A: Chem. 2005, 232, 151–159
General procedure for reaction progress analysis. Reaction progress
was monitored in a 50 mL Schlenk flask. A solution of styrene (260 mg,
2.50 mmol, 1.00 equiv.) in THF (5 mL) was added to a solution of the pre-
catalyst (0.125 mmol, 5 mol%) in THF (5 mL). For quantitative GC-FID
analysis, n-pentadecane was added as an internal standard. The reaction
was started by replacing the atmosphere in the Schlenk flask by
dihydrogen (2 bar). Samples of 0.1 mL were taken at regular intervals
through a septum. Each sample was worked up according to the general
procedure for hydrogenation reactions. Quantification of starting material
and hydrogenation products was performed by GC-FID analysis.
ESI mass spectrometry. Sample solutions were transferred into a gas-
tight syringe and infused into the ESI source of a HCT quadrupole-ion trap
mass spectrometer (Bruker Daltonik) at a flow rate of 8 μLmin−1. For the
ESI process and the transfer of the ions into the helium-filled quadrupole
ion trap, mild conditions similar to those reported previously were
[7]
[8]
[9]
a) S. Monfette, Z. R. Turner, S. P. Semproni, P. J. Chirik, J. Am. Chem.
Soc. 2012, 134, 4561–4564; b) R. P. Yu, J. M. Darmon, J. M. Hoyt, G.
W. Margulieux, Z. R. Turner, P. J. Chirik, ACS Catal. 2012, 2, 1760–
1764.
a) G. Zhang, B. L. Scott, S. K. Hanson, Angew. Chem. Int. Ed. 2012, 51
,
12102–12106; b) G. Zhang, K. V. Vasudevan, B. L. Scott, S. K. Hanson,
J. Am. Chem. Soc. 2013, 135, 8668–8681; c) G. Zhang, S. K. Hanson,
Chem. Commun. 2013, 49, 10151-10153.
a) M. R. Friedfeld, M. Shevlin, J. M. Hoyt, S. W. Krska, M. T. Tudge, P.
J. Chirik, Science 2013, 342, 1076–1080; b) M. R. Friedfeld, G. W.
Margulieux, B. A. Schaefer, P. J. Chirik, J. Am. Chem. Soc. 2014, 136
,
13178–13181; c) J. M. Hoyt, M. Shevlin, G. W. Margulieux, S. W. Krska,
M. T. Tudge, P. J. Chirik, Organometallics 2014, 33, 5781–5790.
- 12 -
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10.1002/chem.201605222
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FULL PAPER
[10] T. J. Korstanje, J. I. van der Vlugt, C. J. Elsevier, B. de Bruin, Science
2015, 350, 298–302.
Possibly, these species formed in the ESI source, where no hydrogen
atmosphere could be maintained.
[11] Selected examples: a) X. Yang, ACS Catal. 2011,
1
, 849–854; b) G.
[36] B. J. Fallon, E. Derat, M. Amatore, C. Aubert, F. Chemla, F. Ferreira,
A. Perez-Luna, M. Petit, J. Am. Chem. Soc. 2015, 137, 2448-2451.
[37] a) http://www.chem.wisc.edu/areas/reich/pkatable/ (October 15th,
2016); b) W. N. Olmstead, Z. Margolin, F. G. Bordwell, J. Org. Chem.
1980, 45, 3295-3299; c) F. G. Bordwell, D. J. Algrim, J. Am. Chem. Soc.
1988, 110, 2964-2968.
Bauer, K. A. Kirchner, Angew. Chem. Int. Ed. 2011, 50, 5798–5800; c)
R. Langer, G. Leitus, Y. Ben-David, D. Milstein, Angew. Chem. Int. Ed.
2011, 50, 2120–2124; d) R. Xu, S. Chakraborty, H. Yuan, W. D. Jones,
ACS Catal. 2015, 5, 6350–6354; e) A. Z. Spentzos, C. L. Barnes, W. H.
Bernskoetter, Inorg. Chem. 2016, 55, 8225–8233..
[12] a) S. Rösler, J. Obenauf, R. Kempe, J. Am. Chem. Soc. 2015, 137
,
[38] a) G. E. Dobereiner, R. H. Crabtree, Chem. Rev. 2010, 110, 681-703;
b) G. Zhang, S. K. Hanson, Org. Lett. 2013, 15, 650-653.
[39] a) B. S. Terra, F. Macedo, Jr., ARKIVOC 2012, 134-151; b) B. E. Kahn,
R. D. Rieke, Chem. Rev. 1988, 88, 733-745.
[40] a) SCALE3ABS, CrysAlisPro, Aglient Technologies Inc., Oxford, UK,
2015; b) G. M. Sheldrick, SADABS, Bruker AXS, Madison, USA, 2007.
[41] a) R. C. Clark, J. S. Reid, Acta Crystallogr. A 1995, 51, 887; b)
CrysAlisPro, version 171.37.35, Agilent Technologies Inc., Oxford, UK,
2015.
7998–8001; b) N. Gorgas, B. Stöger, L. F. Veiros, K. Kirchner, ACS
Catal. 2016, , 2664–2672. [1]
6
[13] a) D. Srimani, A. Mukherjee, A. F. G. Goldberg, G. Leitus, Y. Diskin-
Posner, L. J. W. Shimon, Y. Ben David, D. Milstein, Angew. Chem. Int.
Ed. 2015, 54, 12357–12360; b) Z. Shao, S. Fu, M. Wei, S. Zhou, Q. Liu,
Angew. Chem. Int. Ed. DOI: 10.1002/ange.201608345; c) S. Fu, N.-Y.
Chen, X. Liu, Z. Shao, S.-P. Luo, Q. Liu, J. Am. Chem. Soc. 2016, 138
,
8588–8594.
[14] a) T.-P. Lin, J. C. Peters, J. Am. Chem. Soc. 2013, 135, 15310–15313;
b) T.-P. Lin, J. C. Peters, J. Am. Chem. Soc. 2014, 136, 13672–13683.
[15] K. Tokmic, C. R. Markus, L. Zhu, A. R. Fout, J. Am. Chem. Soc. 2016,
138, 11907–11913.
[42] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H.
Puschmann, J. Appl. Cryst. 2009, 42, 339-341.
[43] G. M. Sheldrick, Acta Cryst. A 2008, 64, 112-122.
[44] G. M. Sheldrick, Acta Cryst. A 2015, 71, 3-8.
[16] a) S.-B. Choe, K. J. Klabunde, J. Organomet. Chem. 1989, 359, 409–
418; b) S. Sun, C. A. Dullaghan, G. B. Carpenter, A. L. Rieger, P. H.
Rieger, D. A. Sweigart, Angew. Chem. Int. Ed. Engl. 1995, 34, 2540–
2542; c) S. Sun, L. K. Yeung, D. A. Sweigart, T.-Y. Lee, S. S. Lee, Y. K.
Chung, S. R. Switzer, R. D. Pike, Organometallics 1995, 14, 2613–2615;
d) J. K. Seaburg, P. J. Fischer, V. G. Young Jr., J. E. Ellis, Angew. Chem.
Int. Ed. 1998, 37, 155-158.
[17] review: J. E. Ellis, Inorg. Chem. 2006, 45, 3167–3186.
[18] D. Gärtner, A. Welther, B. R. Rad, R. Wolf, A. Jacobi von Wangelin,
Angew. Chem. Int. Ed. 2014, 53, 3722–3726.
[19] Preparation of [K(L)][Co(anthracene)2] and related compounds: a) W. W.
Brennessel, V. G. Young Jr., J. E. Ellis, Angew. Chem. Int. Ed. 2002, 41
1211–1215; b) W. W. Brennessel, J. E. Ellis, Inorg. Chem. 2012, 51
9076–9094.
,
,
[20] [K([18]crown-6)(thf)2)][Fe(anthracene)2]: W. W. Brennessel, R. E. Jilek, J.
E. Ellis, Angew. Chem. Int. Ed. 2007, 46, 6132–6136.
[21] Compound 1 was prepared according to a slightly modified procedure
detailed in ref. [18].
[22] W. W. Brennessel, V. G. Young, J. E. Ellis, Angew. Chem. Int. Ed. 2006,
45, 7268–7271.
[23] a) K. Jonas, R. Mynott, C. Krüger, J. C. Sekutowski, Y.-H. Tsay, Angew.
Chem. 1976, 88, 808-809 b) K. Jonas, US patent 4169845 1979.
[24] W. Caminati, B. Vogelsanger, A. Bauder, J. Mol. Spectrosc. 1988, 128
,
384–398.
[25] In total, four isomers with differing relative orientations of the phenyl
groups are conceivable for 5, two of which have 2 symmetry while two
have s symmetry.
[26] a) S. Chaffins, M. Brettreich, F. Wudl, Synthesis 2002,
C
C
9, 1191–1194;
b) G. Franck, M. Brill, G. Helmchen, Org. Synth. 2012, 89, 55-65.
[27] a) D. R. Anton, R. H. Crabtree, Organometallics 1983,
2, 855–859; b)
J. A. Widegren, R. G. Finke, J. Mol. Catal. 2003, 198, 317-341; c) R. H.
Crabtree, Chem. Rev. 2011, 112, 1536–1554.
[28] a) A. J. Frings, PhD dissertation, University of Bochum, Germany, 1988;
b) K. Jonas, Pure Appl. Chem. 1990, 62, 1169-1174.
[29] E.-M. Schnöckelborg, M. M. Khusniyarov, B. de Bruin, F. Hartl, T. Langer,
M. Eul, S. Schulz, R. Pöttgen, R. Wolf, Inorg. Chem. 2012, 51, 6719–
6730.
[30] a) S. Zhang, J. K. Shen, F. Basolo, T. D. Ju, R. F. Lang, G. Kiss, C. D.
Hoff, Organometallics 1994, 13, 3692–3702; b) J. K. Seaburg, P. J.
Fischer, J. Young Victor G., J. E. Ellis, Angew. Chem. Int. Ed. 1998, 37
,
155–158; c) G. Zhu, K. E. Janak, J. S. Figueroa, G. Parkin, J. Am. Chem.
Soc. 2006, 128, 5452–5461.
[31] P. Paklepa, J. Woroniecki, P. K. Wrona, J. Electroanal. Chem. 2001, 498
,
181–191.
[32] a) B. H. Lipshutz, J. Keith, D. Buzard, J. Organometallics 1999, 18, 1571-
157; b) K. Koszinowski, P. Böhrer, Organometallics 2009, 28, 100-110;
c) A. Putau, K. Koszinowski, Organometallics 2011, 30, 4771-4778; d) T.
K. Trefz, M. A. Henderson, M. Linnolahti, S. Collins, J. S. McIndoe, Chem.
Eur. J. 2015, 21, 2980-2991; e) C. Schnegelsberg, S. Bachmann, M.
Kolter, T. Auth, M. John, D. Stalke, K. Koszinowski, Chem. Eur. J. 2016,
22, 7752-7762.
[33] T. Parchomyk, K. Koszinowski, Chem. Eur. J. 2016, 22, 15609-15613.
[34] a) N. G. Tserkezos, J. Roithová, D. Schröder, M. On
čák, P. Slavíček,
Inorg. Chem. 2009, 48 6827-6296; b) A. Putau, H. Brand, K.
,
Koszinowski, J. Am. Chem. Soc. 2012, 134, 613-622.
[35] We also analyzed sample solutions prepared under
a hydrogen
atmosphere (1 bar), which should disfavor dehydrogenation reactions.
However, we still observed the peaks of the dehydrogenated complexes.
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Entry for the Table of Contents
FULL PAPER
A series of nine alkene and arene
metalates (M = Fe, Co) were prepared
and studied as pre-catalysts in
hydrogenation reactions. Excellent
catalytic activities in olefin
hydrogenations were observed. The
catalyst activation mechanism by
redox-neutral π-ligand exchange was
monitored by NMR and ESI-MS
experiments. With carbonyl
P. Büschelberger, D. Gärtner, E. Reyes-
Rodriguez, F. Kreyenschmidt, K.
Koszinowski, A. Jacobi von Wangelin,*
and R. Wolf*
M = Fe, Co
labile π-ligands
formal source of M-
hydrogenation
catalysts
M
Page No. – Page No.
Alkene Metalates as Hydrogenation
Catalysts
substrates, catalyst species of higher
oxidation states are likely formed by
SET and deprotonation reactions.
H
R
H
H2
R
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