A Highly Active and Easily Accessible Cobalt Catalyst for Selective Hydrogenation of C=O Bonds

Article abstract of DOI:10.1021/jacs.5b04349

The substitution of high-price noble metals such as Ir, Ru, Rh, Pd, and Pt by earth-abundant, inexpensive metals like Co is an attractive goal in (homogeneous) catalysis. Only two examples of Co catalysts, showing efficient C=O bond hydrogenation rates, are described. Here, we report on a novel, easy-to-synthesize Co catalyst family. Catalyst activation takes place via addition of 2 equiv of a metal base to the cobalt dichlorido precatalysts. Aldehydes and ketones of different types (dialkyl, aryl-alkyl, diaryl) are hydrogenated quantitatively under mild conditions partially with catalyst loadings as low as 0.25 mol%. A comparison of the most active Co catalyst with an Ir catalyst stabilized by the same ligand indicates the superiority of Co. Unique selectivity toward C=O bonds in the presence of C=C bonds has been observed. This selectivity is opposite to that of existing Co catalysts and surprising because of the directing influence of a hydroxyl group in C=C bond hydrogenation.

Full text of DOI:10.1021/jacs.5b04349

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Communication
A highly active and easy accessible cobalt catalyst
for the selective hydrogenation of C=O bonds
Sina Rösler, Johannes Obenauf, and Rhett Kempe
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b04349 • Publication Date (Web): 16 Jun 2015
Downloaded from http://pubs.acs.org on June 17, 2015
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A highly active and easy accessible cobalt catalyst for the se-
lective hydrogenation of C=O bonds
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§
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Sina Rösler, Johannes Obenauf, Rhett Kempe *
§
Anorganische Chemie II - Katalysatordesign, Universität Bayreuth, 95440 Bayreuth, Germany
Supporting Information Placeholder
Hanson and coworkers reported a bis[2-(dicyclohex-
ylphosphino)ethyl]amine stabilized cobalt(II)-alkyl cata-
lyst (Scheme 1, left) for C=C, C=N and C=O reduction. The
ABSTRACT: The substitution of highly priced noble metals
such as Ir, Ru, Rh, Pd, and Pt by earth abundant, inexpensive
metals like Co is an attractive goal in (homogenous) catalysis.
Only two examples of cobalt catalysts, showing efficient C=O
bond hydrogenation rates are described. Here, we report on a
novel, easy to synthesize cobalt catalyst family. Catalyst acti-
vation takes place via addition of two equivalents of a metal
base to the Co dichlorido pre-catalysts. Aldehydes and ke-
tones of different types (dialkyl, aryl-alkyl, diaryl) are hydro-
genated quantitatively under mild conditions partially with
catalyst loadings as low as 0.25 mol%. A comparison of the
most active Co catalyst with the Ir catalyst stabilized by the
same ligand indicates the superiority of cobalt. A unique se-
lectivity towards C=O bonds in the presence of C=C bonds has
been observed. This selectivity is invers to that of existing Co
catalysts and surprising because of the directing influence of a
hydroxyl group in C=C bond hydrogenation.
7
pre-catalyst is activated with one equivalent of
F
.
8
F
-
H[BAr ] (Et O) (Brookhart’s acid, [BAr ] = tetrakis[(3,5-
4
2
2
4
trifluormethyl)-phenyl]borate) and reduces C=O bonds in-
volving 2.0 mol% catalyst loading within 1-4 bar H pres-
2
sure at room temperature. Notably, the dehydrogenation
9
of alcohols has been observed with this catalyst too.
A
heteroatom free arene-cobalt-ate catalyst for C=C, C=O
and C=N hydrogenation was developed by the groups of
1
0
Wolf and von Wangelin (Scheme 1, middle). Carbonyl
compounds are reduced in good to excellent yields with 5.0
mol% of the catalyst, 10 bar H pressure at 60°C without
2
previous activation of the catalyst.
The above-mentioned Co catalysts, able to mediate C=O
bond hydrogenation (Scheme 1), represent an impressive
progress in hydrogenation chemistry. Unfortunately, they
also suffer from disadvantages like labile ligand coordina-
tion, expensive activation agents and restricted capabilities
of ligand modifications.
Homogenous hydrogenation with molecular hydrogen is
a key step in the industrial synthesis of fine chemicals. Typ-
ically, highly priced noble metals such as Ir, Ru, Rh, Pd and
Pt play the leading role as catalytically active sites in hy-
Scheme 1. Known homogenous cobalt catalysts for
C=O bond hydrogenation (left and middle).
1
drogenation catalysts. The substitution of these metals by
inexpensive, earth abundant metals like cobalt would give
advantages in terms of costs and sustainability. Further-
more, the unique electronic structure properties of such
base metals may allow observing unusual activity/selectiv-
ity profiles. Despite these and other perspectives, the de-
velopment of well-defined homogenous cobalt hydrogena-
tion catalysts has been progressed slowly especially with
regard to the reduction of C=O bonds. However, with the
implementation of rational ligand design, a few new cobalt
catalysts for homogenous hydrogenation have been dis-
closed in recent years. Hydrogenation of olefins with co-
balt complexes have been described by Budzelaar and
We recently introduced (triazine-based) PN3-5P-Ir com-
plexes (an example of a cobalt complex is shown in Scheme
1
, right) as highly efficient homogenous catalysts for accep-
11
torless dehydrogenative condensation reactions. Haupt
and coworker introduced such PN
12
3
P ligands and Kirchner
2
3
coworkers as well as by Chirik and coworkers. A bis(phos-
phino)boryl cobalt catalyst (for which a boryl-metal coop-
erativity was observed) has been applied by Peters and
and coworker have demonstrated the broad applicability
13
of the ligand class. Reports on cobalt complexes stabilized
13e,14,15
by such ligands are rare.
4
coworkers in C=C bond hydrogenation. Very recently, Mil-
Herein we report on a novel, easy to synthesize, and
simply to activate homogenous cobalt C=O bond hydro-
genation catalyst family (Scheme 1, right). The pre-cata-
lysts can be synthesized quantitatively up to multi-gram
stein and coworkers reported on the Co complex catalyzed
5
hydrogenation of esters. To the best of our knowledge, ho-
mogenous Co catalysts for efficient C=O reduction with
6
molecular hydrogen are only described for two examples.
1
6
scale. They are air stable for a period of a few months as a
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crystalline material. The activation of the pre-catalysts pro- tertiary-butyl). Complex 3 was identified as the most active
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ceeds via salt elimination by adding two equivalents of a
base and is based on the ability of the used PN3-5P ligand
class to act as neutral, mono-anionic or di-anionic ligand.
The modularly assembled structure of the chosen ligands
allows the employment of catalyst libraries for activity
screenings. Selective hydrogenation of C=O bonds in the
presence of C=C bonds was observed despite (I) the direct-
ing influence of a hydroxyl group in olefin hydrogenation
pre-catalyst. A comparison of 3 and 6 reveals the beneficial
effect of the triazine ring. Beside the altered basicity of the
coordinating N-atom, an explanation can result from a sta-
bilization of the proton shuttle chain via hydrogen bond-
ing with the N-atoms of the triazine moiety of the PN P
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18
ligand backbone.
Table 1. Hydrogenation of acetophenone with several
[a]
cobalt(II) pre-catalysts (see Figure 1).
3a
for Co catalysts and (II) invers selectivity patterns as re-
0
1
2
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7c
ported for the Hanson catalyst.
The pre-catalyst synthesis is performed by addition of an
equimolar solution of the ligand to a suspension of anhy-
drous CoCl in THF. The desired pre-catalysts precipitate
as red crystalline solids in quantitative yields. Complexes
were characterized by X-ray crystal structure analysis
2
Entry
Pre-catalyst
Yield[b] [%]
1
1
7
(
XRD), elemental analysis, IR-spectroscopy and magnetic
measurements. All complexes show paramagnetic behav-
ior and an effective magnetic moment between 2.2 µ and
.3 µ (studied over a temperature range of 300-2 K using a
2
3
4
5
6
6
2
3
4
5
6
30
>99
28
0
B
2
B
17
SQUID magnetometer). The molecular structure of 3 is
shown in Figure 1. XRD indicates an pentacoordinated co-
balt(II) complex with a slightly distorted square pyramidal
23
0
coordination. The neutral PN P ligand is coordinated to
5
CoCl
2
the cobalt center in the typical tridentate mode with a P1–
Co1–P2 angle of 167.04(6)°. Both N–H hydrogen atoms
could be located in the difference electron density map. Se-
lected bond distances and angles are given in the caption
of Figure 1.
[
a] Reaction conditions: Acetophenone (3.0 mmol), 2.0 mol%
t
Co, NaO Bu (13 mg, 4.4 mol%), 2 mL THF, 20 bar H , room temper-
ature, 24 h. [b] determined via GC with dodecane as internal stand-
ard.
2
To understand the role of the base in the activation pro-
cess, a base loading screening was carried out (Supporting
Information [SI]). Two equivalents of the metal base
t
NaO Bu are necessary for generating a catalytically active
complex. The exact structure of this complex is not fully
clear yet. To gain inside, the activated species was trapped
with one equivalent of bipyridine. The resulting red-or-
ange complex (8, Figure 2) was analyzed by XRD. Both
chlorido ligands as well as both N–H protons were salt-
eliminated by the base. The PN P ligand acts as a di-anionic
5
ligand coordinating the cobalt again in the pincer like tri-
dentate manner with an P1–Co1–P2 angle of 158.18(5)°. The
oxidation state of the cobalt(II) center is not affected by
the activation procedure. Complex 8 has an effective mag-
netic moment of 1.9 µ , expected for an Co(II) low spin
B
Figure 1. Synthesized PN3-5P stabilized cobalt(II) chlorido
complexes and molecular structure of 3 with 50% probability
of thermal ellipsoids. Hydrogen atoms (except for the two N-
H groups) are omitted for clarity. Selected bond lengths [Å]
and angles [°]: Co1–P1 2.220(1); Co1–P2 2.220(1); Co1–Cl1
complex in a pentacoordinated environment.
Next, we became interested in comparing the activity of
the catalyst based on 3 with an Ir complex stabilized by the
same PN P ligand (Figure S2, SI). Under base free condi-
5
tions, the iridium catalyst shows no remarkable conver-
sion. With a Na to Ir ratio of 1 and 2 (Figure S2, SI, green
and blue graphs, respectively) a faster conversion is ob-
served. It increases at higher base concentration and is still
slower than Co (red graphs). Such activating effect of a
metal base is known for other iridium C=O bond hydro-
2
1
9
.437(1); Co1–Cl2 2.238(1); Co1–N1 1.925(4); P1–Co1–P2
67.04(6); N1–Co1–Cl1 93.0(1); N1–Co1–Cl2 160.3(1); N2–P1–Co1
8.8(2); N3–P2–Co1 99.1(2); C1–N2–P2 119.4(4); C3–N3–P2
118.6(4)
The catalytic activity of the complexes 1-6 (Figure 1, Ta-
19
ble 1) and the metal precursor CoCl
2
was investigated in the
genation catalysts. At very high base loadings (Na/Ir = 10,
t
hydrogenation of acetophenone (3.0 mmmol) using 2.0
mol% pre-catalyst in THF under 20 bar hydrogen pressure
and room temperature. The pre-catalysts were initially ac-
Figure S2 of the SI), the Ir catalyst is faster. NaO Bu addi-
tion higher than the two equivalent needed for its activa-
tion is not beneficial for 3. The comparison indicates that
different mechanistic pathways seem relevant and the Co
t
t
tivated with a slightly excess (4.4 mol %) of NaO Bu ( Bu =
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are focused towards a better understanding of the catalyt-
catalyst is superior under base free conditions or at low
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base concentrations including identical conditions. Base
free conditions are advantageous since metal bases like
NaO Bu can mediate side reactions of the educts like aldol
ically active species, the development of enantioselective
PN3-5P-Co hydrogenation catalysts, and other catalytic ap-
plications of the cobalt catalyst family described here.
t
type condensations.
Table 2. Hydrogenation of aryl-alkyl, diaryl and ali-
[
a]
phatic carbonyl compounds
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Entry
1
Product
Cat. loading [mol%]
0.25
Yield[b] [%]
>99
R= CH3
CH
CH3
2
3
4
5
R= CH
2
3
0.5
0.5
0.5
>99
>99
>99
R= (CH
2 4
)
R=H
0.5
>99
Figure 2. Synthesis and activation of 3 as well as trapping of
the activated catalyst with bipyridine (8). The molecular
structure of 8 is displayed with 50% probability of thermal el-
lipsoids. Hydrogens are omitted for clarity. Relevant bond
lengths [Å] and angles [°]: Co1 – P1 2.214(1); Co1 – P2 2.223(1);
Co1–N1 1.915(3); Co1–N6 1.930(3); Co1–N7 2.016(3); P1–Co1–P2
6
7
0
.25
>99
158.18(5); N1–Co1–N6 178.0(1); N1–Co1–N7 97.7(1); N2–P2–Co1
102.3(1); N3–P1–Co1 105.1(1); P1–N3–C3 112.0(3); P2–N2–C1
113.3(3);
[c]
1.0
98 (94 )
R=F
8
9
R=Cl
1.0
>99
64
Finally, we optimized the hydrogenation reaction condi-
tions and explored the substrate scope of our Co catalyst.
For details please see the SI. To our delight, a broad prod-
uct scope was observed. Aryl-alkyl ketones (Table 2, entry
R=Br
2.0
1
0
3.0
91
1
-11), diaryl ketones (Table 2, entry 12-14) as well as aliphatic
1
1
ketones (Table 2, entry 15-20) are reduced to the corre-
sponding alcohols in quantitative yields, tolerating diverse
functional groups. In most cases, the catalyst loading
amounts 0.25 or 0.5 mol%. In case of unsaturated carbonyl
compounds (Table 2, entry 21-24), a distinct selectivity to-
wards the C=O bond is observed. No directing effect of a
3.0
95
1
2
0.25
>99
R=H
>99 (97[c])
(
generated) hydroxyl group, as described by Chirik and co-
13
R= Me
R= OMe
0.5
0.5
3
a
workers, is noticed. The selectivity we observe is invers to
the Hanson catalyst for which the selective hydrogenation
of the C=C bond in 2-methyl-5-(prop-1-en-2-yl)cyclohexa-
none was reported and for which a bifunctional mecha-
nism has been proposed.
1
1
4
5
97
98
0.5
7c
1
6
7a,20
0.5
>99
>99
In conclusion, we reported on an easy accessible, inex-
pensive to activate and highly active cobalt catalyst for the
homogenous hydrogenation of C=O bonds. The chosen
PN3-5P ligand family allows an easy fine tuning or optimiz-
ing of the catalyst performance and its flexibility with re-
gard to the protonation or charge allows an efficient gen-
eration of the catalytically active species. The best catalyst
operates under mild conditions and addresses a broad sub-
strate scope covering dialkyl, diaryl and aryl-alkyl ketones.
The hydrogenation of C=O bonds in the presence of C=C
bonds can proceed highly selective. Further investigations
1
1
7
0.25
R= H
[c]
8
R= Ph
0.5
>99 (93 )
19
0.5
>99
20
>99 (92[c])
1.0
21
0.5
>99
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4685–4689; (f) Sonnenberg, J. F.; Lough, A. J.; Morris, R. H. Organo-
[
c]
1
.0
>99 (97 )
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2
0
.5
.5
>99
24
[c]
0
>99 (95 )
[a] Reaction conditions: 3.0 mmol carbonyl compound, 2.0 mL
-methyl-2-butanol, NaO Bu (2.0 equiv. with respect to the pre-
catalyst), 20°C, 24h, 20 bar H , pre-catalyst 3, [b] determined via
t
2
2
GC with dodecane as internal standard, [c] isolated yield.
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ASSOCIATED CONTENT
Supporting Information
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Additional experimental procedures, spectroscopic and crys-
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at http://pubs.acs.org.
(
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AUTHOR INFORMATION
Corresponding Author
*
E-mail: kempe@uni-bayreuth.de. Fax: +499212157.
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Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank the Deutsche Forschungsgemeinschaft, DFG, KE
56/23-2, for financial support, Charles Lochenie for the mag-
netic measurements, Toni Hille for synthesizing 4-methylpyr-
idine-2,6-diamine, Detlef Heller for helping with the kinetic
analysis and the reviewers for their valuable suggestions. We
7
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Ziller, J. W.; Goddard, W. A.; Nielsen, R. J.; Yang, J. Y. Inorg. Chem.
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2
dedicate our work to Manfred Scheer on the occasion of his
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Chirik, P. J. Chem. Sci. 2014, 5, 1956-1960; (d) Semproni, S. P.; Mils-
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th
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birthday.
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(
(
(
16) The ligand synthesis is similarly easy. For instance, 96% isolated
yield starting from commercially available educts for the PN P ligand
5
of 3.
(17) The spin only value for a cobalt(II) complex in a pentacoordinated
ligand environment in the low spin state is 1.7 μB.
(
(
18) Qu, S.; Dang, Y.; Song, Ch.; Wen, M.; Huang, K.W.; Wang, Z.-X. J.
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(
5
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6) For examples of C=O bond hydrogenation catalysis based on PNP-
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