Phosphine-free cobalt catalyst precursors for the selective hydrogenation of olefins

Article abstract of DOI:10.1039/c8cy02218f

Cobalt(ii) complexes bearing phosphine-free tridentate NNS ligands were prepared. Depending on the ligand, dimeric or monomeric complexes were isolated. Monomeric Co(NN^MeS)Cl₂ selectively catalysed the hydrogenation of olefins in the presence of reducible moieties such as ketones. Further investigation showed that this complex functions as a nanoparticle precursor under the reaction conditions.

Full text of DOI:10.1039/c8cy02218f

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Phosphine-free cobalt catalyst precursors for the
selective hydrogenation of olefins†
Cite this: Catal. Sci. Technol., 2019,
9, 61
a
a
Pim Puylaert,
Andrea Dell'Acqua, a Fatima El Ouahabi,
Anke Spannenberg,a Thierry Roisnel, b Laurent Lefort,
Sandra Hinze,
c
a
Received 26th October 2018,
Accepted 22nd November 2018
a
Sergey Tin a and Johannes Gerardus de Vries
*
DOI: 10.1039/c8cy02218f
rsc.li/catalysis
CobaltIJII) complexes bearing phosphine-free tridentate NNS ligands
were prepared. Depending on the ligand, dimeric or monomeric
complexes were isolated. Monomeric CoIJNNMeS)Cl2 selectively
catalysed the hydrogenation of olefins in the presence of reducible
moieties such as ketones. Further investigation showed that this
complex functions as a nanoparticle precursor under the reaction
conditions.
ligands used in hydrogenations were published earlier by
Gusev, Dub and Gordon (Scheme 1).5,6 There is a clear incen-
tive for combining such cheap, readily accessible ligands with
base metals in order to reap the benefits provided by both.
Midya, Balaraman and co-workers recently reported the
acceptorless dehydrogenative coupling of aminoalcohols and
alcohols to obtain heterocycles, which was catalysed by a
dimeric cobaltIJII) SNS complex (Scheme 2).7 At the time, we
were evaluating the use of our NNS ligands with cobalt for
catalytic hydrogenations, the results of which we report here.
The addition of 1 equivalent of ligand to a solution of
CoCl2 in ethanol afforded a clean reaction to the CoIJII)NNS
complexes 1 and 2 depicted in Scheme 3. Interestingly, the
secondary amine ligand L1 yielded the dimeric, chloride-
bridged complex 1, with the tridentate ligand coordinated in a
fac manner, while monomeric complex 2 was obtained with
the N-methylated ligand L2. Crystals suitable for X-ray diffrac-
tion analysis were obtained for both complexes, and their
solid-state molecular structures are shown in Fig. 1. Interest-
ingly, it appears that N-methylation of the ligand provided suf-
ficient steric hindrance to prevent complex 2 from dimerising.
We investigated the obtained complexes in the catalytic
hydrogenation of olefins. Several activation pathways were
The field of homogeneous hydrogenation has, over the last
decade, undergone a major shift away from noble metals
such as Rh, Ir, and Ru towards base metals like Fe, Co, and
Mn.1 Strongly electron-donating, stabilising pincer ligands
have played a key role in the development of such catalysts,
to the point where pincer complexes of most base metals
have now been applied for hydrogenation reactions.2,3
The impact on the environment and the costs of
phosphine- or carbene-based ligands, however, should not be
underestimated, and can even outweigh the advantages of
using base metals. Phosphine-based ligands also suffer from
shorter shelf-lives and can be hard to prepare on a large
scale, due to their sensitivity. In this light, we recently
reported the application of flexible alkylthio-N-(pyridine-2-
yl)methyl-1-ethanamine (NNS) tridentate ligands and their
ruthenium complexes in the selective hydrogenations of α,β-
unsaturated aldehydes, ketones, and even esters to the corre-
sponding allylic alcohols.4 Notable examples of similar
a Leibniz Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-
Straße 29a, 18055 Rostock, Germany. E-mail: johannes.devries@catalysis.de
b Université de Rennes 1, UMR “Sciences Chimiques de Rennes”, UR1-CNRS 6226,
Campus de Beaulieu, CS 74205, 35042 Rennes Cedex, France
c InnoSyn B.V., Urmonderbaan 22, 6167 RD, Geleen, The Netherlands
† Electronic supplementary information (ESI) available: Typical procedure for
olefin hydrogenation: oven-dried 4 mL glass vials were filled in the glovebox
with 2 (2.2 mg, 0.006 mmol) and NaBH4 (0.6 mg, 0.016 mmol). Solvent (1 mL),
dodecane (150 μL), and 1-octen-3-ol were added (50 μL, 0.33 mmol), and the
PTFE septum pierced with a needle. The vial was transferred to an autoclave,
which was flushed with Ar and then pressurised with H2 to 50 bar, and heated
to 100 °C overnight. Full experimental details and characterisation for isolated
products can be found in the ESI. CCDC 1864013 and 1864014. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c8cy02218f
Scheme
hydrogenations.
1 Previously reported N,S-tridentate ligands used for
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genative coupling (reflux in m-xylene) made us suspect that
the dimeric complex may well be very stable in solution, and
therefore would require elevated temperatures to dissociate
into an active, monomeric form.7 In order to test if the dimer
could be dissociated at lower temperatures, we included the
addition of monodentate ligand into the screening of activat-
ing additives. In an initial high-throughput screening (HTS),
the hydrogenation of 1-octene, acetophenone and 1-octen-3-ol
were investigated (see ESI†). After these initial experiments,
1-octen-3-ol (3a) was selected as a model substrate since its
isomerisation to the corresponding ketone (4b) was observed
during the HTS.10 Consequently, we used this substrate to ex-
plore the activity of the Co catalysts in both hydrogenation
and isomerisation. The results of the study of the activation
of the Co precatalyst by different additives are summarised
in Table 1. Interestingly, the best results were obtained using
2 together with a reductant (NaBH4, entry 18). Subsequently,
several reaction solvents were investigated, showing that iso-
propanol gave the highest yield and selectivity (Table S3†).
Scheme 2 Co-SNS dimers reported by Balaraman.7
Scheme 3 Synthesis of Co(NNS) complexes 1 and 2.
For all reactions where full conversion with
2 was
obtained, a black residue was observed after the experiment.
This suggested the formation of cobalt nanoparticles, either
after consumption of the substrate or during the reaction. In
the latter case, the hydrogenation was possibly catalysed by
cobalt nanoparticles formed in situ.11 To investigate this
possibility, we observed the reaction progress by monitoring
Table 1 Activation studies for catalysts 1 and 2 in the hydrogenation of
1-octen-3-ol
a
#
Cat.
Base
Additive
Yield 4a : 4bb (%)
Fig. 1 ORTEP drawings (30% probability ellipsoids, hydrogen atoms,
except the N–H hydrogen for 1, were omitted for clarity). Selected
bond lengths (Å) and angles (°) for a) complex 1: Co1–S1 2.5162(4),
Co1–N1 2.1159IJ11), Co1–N2 2.1704IJ11), Co1–Cl1 2.4186(3), Co1–Cl1A
2.4991(3), Co1–Cl2 2.3633(3); N1–Co–N2 78.41(4), N1–Co1–S1 83.76(3),
N2–Co1–S1 83.72(3), Cl1–Co1–Cl1A 91.547IJ12), Cl2–Co1–Cl1 95.210IJ13),
C6–N2–C7 115.33(11) and b) complex 2: Co1–S1 2.5612(8), Co1–N1
2.102(2), Co1–N8 2.122(2), Co1–Cl1 2.2785(8), Co1–Cl2 2.2885(8); N1–
Co1–N8 78.14(8), N1–Co1–S1 161.69(6), N8–Co1–S1 84.06(6), Cl1–Co1–
Cl2 118.99(3), C7–N8–C10 111.2(2).†
1
2
3
4
5
6
7
8
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
—
—
—
PPh3
NaBH4
Zn
0 : 0
0 : 1
0 : 0
12 : 0
3 : 0
KOtBu
NaOH
NaOH
NaOH
—
—
—
0 : 0
0 : 1
KOtBu
KOtBu
KOtBu
NaOEt
NaOEt
NaOH
NaOH
NaOEt
NaOH
NaOH
NaOH
—
PPh3
NaBH4
PPh3
NaBH4
PPh3
NaBH4
Zn
PPh3
NaBH4
—
41 : 0
20 : 0
94 : 3
94 : 1
90 : 5
95 : 1
12 : 0
41 : 4
72 : 2
0 : 0
9
10
11
12
13
14
15c
16c
17
18
envisioned. Firstly, a strong base is often added to activate
hydrogen via a bifunctional mechanism.6b,8 This pathway
should enable the hydrogenation of polarised, unsaturated
moieties such as carbonyl groups.8 Another strategy to
activate CoIJII) complexes is their reduction, be it in situ or ex
situ, to the corresponding CoIJI) complexes, which has recently
been reported to lead to increased activities.9 The reaction
conditions reported by Balaraman et al. for their dehydro-
NaBH4
95 : 0
a
Reaction conditions: 0.33 mmol 1-octen-3-ol, 1.0 mL THF, 1 mol%
1 or 2 mol% 2, 5 mol% of additive and base, 50 bar H2, 100 °C, 16 h
b
reaction time. Determined by GC using dodecane as internal
standard. c 1 mol% of 2.
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the hydrogen uptake, and performed two types of poisoning
experiments.
The hydrogen uptake curve at 1 mol% catalyst loading
(Fig. 2a) shows an induction period of around 30 minutes,
during which no hydrogen was consumed. After this time,
the reaction proceeded to full conversion and >99% yield
through a roughly sigmoidal curve, which is often indicative
for nanoparticle-catalysed reactions.12 Mercury poisoning ex-
periments provided no conclusive evidence (details can be
found in the ESI†). The hydrogenation reaction still
proceeded to a certain extent. However, it has been reported
that Co nanoparticles do not readily form an amalgam with
mercury.12,13 Consequently, we performed a second poison-
ing experiment with PMe3 instead, the results of which are
shown in Fig. 2b.14 After the induction period, a sub-
stoichiometric amount of PMe3 (0.15 eq. w.r.t. 2) was injected
into the autoclave using excess of H2 pressure, leading to an
immediate stop of the hydrogen consumption. This is a
strong indication that the reaction is catalysed by cobalt
nanoparticles, which are poisoned when their surface gets
saturated with PMe3.
A variety of olefins were reduced with full conversions,
and the products isolated in good to excellent yields, as illus-
trated in Fig. 3.
Fig. 3 Substrate scope and limitations of the hydrogenation of olefins
catalysed by 2. Reaction conditions: substrate (5 mmol), iPrOH (15 mL),
2 (33.9 mg, 2 mol%), NaBH4 (9.5 mg, 5 mol%), 50 bar H2, 100 °C, 16 h
reaction time. a) Full conversions, but isolated yields are low due to
volatility of the products being comparable to that of iPrOH. b) The
isolated product is the isopropyl ester. c) The reactions never reached
full conversions, and as such the products were not isolated.
During our initial HTS, we observed that our catalysts
were inactive in the hydrogenation of acetophenone. There-
fore, we decided to explore the selectivity of the Co nano-
particles for olefins in the presence of carbonyl functionali-
ties (Fig. 3). Ketones (3j) and esters (3h) were not reduced
even under these harsh conditions. Excellent chemo-
selectivities were observed for α,β-unsaturated ketones
(3f–3g), with no reduction of the carbonyl even after 16
hours. Aldehydes appear to inhibit the reaction, only 25 and
40% conversion was observed in the hydrogenation of
cinnamaldehyde (3k) and hexen-2-al (3l). Even here, the sole
product observed was the saturated aldehyde. No conversion
was observed in the attempted hydrogenation of an imine
(3m), quinoline (3n) or enamine (3o). These reactivities con-
trast with the activity of cobalt pincer complexes containing
strongly donating phosphine ligands such as those recently
described by Junge et al., which hydrogenated esters
readily.9a The cobalt nanoparticles recently reported by the
Jacobi von Wangelin group reduced carbonyl groups
Fig. 2 a) Hydrogen uptake curve at 1 mol% catalyst loading under
normal reaction conditions (corrected for heating to 100 °C). b)
Hydrogen uptake under the same conditions, where a PMe3 solution was
injected after the induction period, which initially increased the total
pressure, but then remained stable. At this point a conversion of 65%
had been achieved (determined by GC, dodecane as internal standard).
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efficiently, and under far milder reaction conditions.11a,b In
our case, it is likely that L2 stabilises the nanoparticles and
partially deactivates them, leading to chemoselectivity to-
wards olefinic double bonds.
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Conclusions
Two new cobaltIJII) complexes bearing non-phosphorus NNS li-
gands were synthesised, characterised and their reactivities
in catalytic hydrogenation were investigated. The dimeric
complex 1 was nearly inactive for hydrogenation, but complex
2 proved useful as catalyst precursor for the selective hydro-
genation of CC bonds in the presence of other reducible
moieties.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful to Dr. Ralf Jackstell and Mr. Rui Sang
for assistance with the PMe3 poisoning experiment, and to
Mrs. Susann Buchholz, Mrs. Susanne Schareina, Mr. Andreas
Koch, and Dr. Christine Fischer (all at the Likat) for their in-
dispensable analytical support. ADA acknowledges the Euro-
pean Union Erasmus+ Traineeship Programme for the ex-
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64 | Catal. Sci. Technol., 2019, 9, 61–64
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