A Cobalt(II) Complex Bearing the Amine(imine)diphosphine PN(H)NP Ligand for Asymmetric Transfer Hydrogenation of Ketones

Article abstract of DOI:10.1002/ejic.202000826

Novel chiral cobalt complex a containing amine(imine)diphosphine PN(H)NP ligand and complex b containing bis(amine)diphosphine PN(H)N(H)P ligand were synthesized. The structures of two complexes were characterized by X-ray crystallography and high resolution mass spectrometry. The catalytic performances of cobalt complexes a and b for asymmetric transfer hydrogenation (ATH) of ketones under mild conditions were evaluated using 2-propanolisopropanol as solvent and hydrogen source after being activated by 8 equivalents of base. Complex a showed a good reactivity for reduction of ketones, with a turnover number (TON) of up to 555, and a maximum enantiomeric excess (ee) value of up to 91 %. Complex b exhibited inertness for hydrogenation of ketones. Electronic structure studies on a and b were conducted to account for the function of ligands on the catalytic performances.

Full text of DOI:10.1002/ejic.202000826

Full Paper
doi.org/10.1002/ejic.202000826
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Asymmetric Hydrogenation | Very Important Paper |
A Cobalt(II) Complex Bearing the Amine(imine)diphosphine
PN(H)NP Ligand for Asymmetric Transfer Hydrogenation of
Ketones
Shangfei Huo,[a] Hong Chen,[a] and Weiwei Zuo*[a]
Abstract: Novel chiral cobalt complex a containing amine- and hydrogen source after being activated by 8 equivalents of
(imine)diphosphine PN(H)NP ligand and complex b containing base. Complex a showed a good reactivity for reduction of ket-
bis(amine)diphosphine PN(H)N(H)P ligand were synthesized. ones, with a turnover number (TON) of up to 555, and a maxi-
The structures of two complexes were characterized by X-ray mum enantiomeric excess (ee) value of up to 91 %. Complex
crystallography and high resolution mass spectrometry. The b exhibited inertness for hydrogenation of ketones. Electronic
catalytic performances of cobalt complexes a and b for asym- structure studies on a and b were conducted to account for the
metric transfer hydrogenation (ATH) of ketones under mild con- function of ligands on the catalytic performances.
ditions were evaluated using 2-propanolisopropanol as solvent
Introduction
was obtained.[6] Cobalt complex bearing P2C–PPh22NPh ligand
2
was applied for hydrogenation of ketones, and 93–99 % yields
were obtained under 30 bar H2 pressure at 50–100 °C.[7] Yang
et al. synthesized a cobalt–NHC pincer complex for transfer
hydrogenation of a broad range of ketones, using 2-propanol-
isopropanol as solvent and hydrogen source in the presence of
sodium tert-butoxide, and moderate to excellent yields were
obtained.[8]
Chiral alcohols are important to pharmaceutical, fragrance, and
agrochemical industries,[1] and they are conventionally pro-
duced by asymmetric hydrogenation of prochiral ketones via
catalysts based on precious metals.[2] But applications are lim-
ited for these precious metal complexes due to their high cost
and toxicity. Hence it is important to seek some earth abundant
and low or non-toxic metals as substitutions to these precious
metals. As an earth abundant metal, cobalt has its potential to
achieve this goal, and some examples of hydrogenation of
ketones with cobalt complexes have been reported.
Robter et al. developed cobalt complexes with chiral nitro-
gen containing ligands for asymmetric hydrogenation of aceto-
phenone using 2-propanol isopropanolas hydrogen source, and
ee value of up to 58 % was achieved, but with low conversion
of 8 %.[9] Zhang[10] et al. obtained chiral alcohols by combining
tetradentate aminophosphine ligand with CoCl2·6H2O as cata-
lyst for hydrogenation of ketones. Alcohols were obtained with
moderate to high yields and ee values at 2 mol-% catalyst load-
ing and 60 bar H2 pressure. However, the catalytic system re-
quired 400 mol-% of KOH to initiate the hydrogenation reac-
tion.
A cobalt(II) complex of [(PNHPCy)Co(CH2SiMe3)]BArF4 {BArF
=
4
B(3,5-(CF3)2C6H3)4} and its analogs were applied in hydrogenat-
ing some ketones to obtain alcohols at 25–60 °C with high
yields.[3] Potassium bis(anthracene) cobaltate was used as cata-
lyst to reduce aromatic and aliphatic ketones under H2 atmos-
phere, and good yields were obtained, but with no enantio-
selectivity.[4] Sina et al. designed their triazine-based PN3–5P co-
balt complexes to hydrogenate ketones under H2 atmosphere,
and good to high yields were obtained, yet with no enantio-
selectivity.[5]
The development of cobalt-catalyzed hydrogenation of ket-
ones was slow relative to that with precious metals and iron,
and the examples of cobalt complexes for hydrogenation of
ketones, especially for asymmetric hydrogenation of ketones
are limited. Herein, we report a new type of cobalt complex
that contains an amine(imine)diphosphine PN(H)NP ligand[11]
for asymmetric transfer hydrogenation of ketones. The structure
of novel cobalt pre-catalyst a bearing amine(imine)diphosphine
PN(H)NP ligand is showed in Figure 1. For comparison, complex
b containing bis(amine)diphosphine PN(H)N(H)P ligand is also
synthesized and showed in Figure 1. We postulated that amido-
ene(imido) structure in the real catalyst derived from deproto-
nation of complex a by base could activate the cobalt metal
center for catalysis, which has been demonstrated in the related
Liu et al. developed a cobalt catalytic system that was gener-
ated by combining pincer NHC ligand with CoCl2 in situ for
hydrogenation of ketones, and maximum TON of up to 2160
[a] S. Huo, H. Chen, Prof. Dr. W. Zuo
State Key Laboratory for Modification of Chemical Fibers and Polymer
Materials, College of Materials Science and Engineering, Donghua
University,
2999 North Renmin Road, Songjiang District, Shanghai, 201620,
P. R. China
E-mail: zuoweiwei@dhu.edu.cn
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000826.
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iron catalysts.[12] In this work, the catalytic performance of two there is a C=N double bond. For b, the bond lengths are as
catalysts were tested. Results indicated that cobalt complex a follow: N1–C2 (1.488 Å), N2–C5 (1.483 Å). The length of two
could hydrogenate a large broad of ketones to obtain corre-
sponding chiral alcohols with moderate TON and moderate to
good ee values.
Figure 1. Structures of cobalt pre-catalysts of a and b.
Results and Discussion
Complex a was synthesized via a template reaction between
PN(H)N(H2)[11] ligand and diphenylphosphino-acetaldehyde in
the presence of cobalt(II) bromide hydrate, where diphenyl-
phosphino-acetaldehyde was released by diphenylphosphino-
acetaldehyde hydrochloride dimer upon it treatment with
Figure 2. The molecular structure of complex a with the thermal ellipsoids
set at 50 % probability. Hydrogen atoms are omitted for clarity. Selected
bond lengths [Å] and angles [°] are as follows: Co(1)–N(1) 1.934(3), Co(1)–N(2)
1.981(3), Co(1)–P(1) 2.1910(11), Co(1)–P(2) 2.2116(11), Co(1)–Br(1) 2.5619(6),
P(1)–C(1) 1.846(4), P(2)–C(6) 1.837(4), N(1)–C(2) 1.267(5), N(2)–C(5) 1.476(5),
C(1)–C(2) 1.487(6), C(5)–C(6) 1.539(6), N(1)–Co(1)–N(2) 84.59(13), N(1)–Co(1)–
P(1) 83.74(10), N(2)–Co(1)–P(1) 161.54(11), N(1)–Co(1)–P(2) 171.47(10), N(2)–
Co(1)–P(2) 86.92(10), P(1)–Co(1)–P(2) 104.28(4), N(1)–Co(1)–Br(1) 88.23(10),
N(2)–Co(1)–Br(1) 96.80(11), P(1)–Co(1)–Br(1) 97.07(3), P(2)–Co(1)–Br(1)
93.58(3).
NaOMe. Complex b was synthesized via reaction between
PN(H)N(H)P[13] ligand and cobalt(II) bromide hydrate in MeOH
(Scheme 1). Complexes a and b were purified by crystallization
and characterized by FT-IR and high resolution mass spectrome-
try. The structures of the complexes a and b were further deter-
mined by X-ray single crystal diffraction and the molecular
structures are shown in Figure 2 and Figure 3, respectively. It
could be seen that one bromine atom connects to Co center,
while the other bromine atom is dissociative, in both a and b.
Four atoms of PNNP form a plane, and occupy four points in the
equatorial plane, composing a pentahedron forming a square-
pyramidal geometry with cobalt atom. There is a difference be-
tween two complexes where complex a contains unsaturated
C=N bond but b does not. In the case of a, the bond lengths
are as follow: N1–C2 (1.267 Å), N2–C5 (1.476 Å). The length of
two carbon–nitrogen bonds is significantly different, indicating
Figure 3. The molecular structure of complex b with the thermal ellipsoids
set at 50 % probability. Hydrogen atoms are omitted for clarity. Selected
bond lengths [Å] and angles [°] are as follows: Co(1)–N(1) 1.986(3), Co(1)–
N(2) 1.985(2), Co(1)–P(1) 2.2022(9), Co(1)–P(2) 2.2107(9), Co(1)–Br(1) 2.5853(5),
P(1)–C(1) 1.833(3), P(2)–C(6) 1.836(3), N(1)–C(2) 1.488(4), N(2)–C(5) 1.483(4),
C(1)–C(2) 1.529(4), C(5)–C(6) 1.531(5), N(1)–Co(1)–N(2) 84.43(10), N(1)–Co(1)–
P(1) 85.66(8), N(2)–Co(1)–P(1) 158.49(8), N(1)–Co(1)–P(2) 170.41(8), N(2)–
Co(1)–P(2) 87.10(8), P(1)–Co(1)–P(2) 103.80(4), N(1)–Co(1)–Br(1) 83.54(8), N(2)–
Co(1)–Br(1) 96.80(8), P(1)–Co(1)–Br(1) 101.00(3), P(2)–Co(1)–Br(1) 93.03(3).
Scheme 1. Synthesis of complexes a and b.
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carbon–nitrogen bonds is almost same, indicating there are two of potassium tert-butoxide. Asymmetric transfer hydrogenation
C–N single bonds.
Asymmetric transfer hydrogenation of ketones with com-
plexes a and b were performed using 2-propanolisopraponal as
of acetophenone with complex a afforded (R)-1-phenylethanol
with 74 % conversion of acetophenone (Table 1). Interestingly,
ee value was 70 % at 10 min. When the reaction was in equilib-
solvent and hydrogen source after activated with 8 equivalent rium at 12 h, ee value was 72 %, a little better higher than the
Table 1. Asymmetric transfer hydrogenation of ketones catalyzed by complexes a and b.[a]
[a] General conditions for a: [a] = 6.79 × 10–4
by gas chromatography or HPLC by comparison to known standards. The conversion was calculated based on GC analysis, which was calibrated by comparing
with a working curve. [b] Conditions for b: [b] 6.79 × 10–4 , [KOtBu] = 5.52 × 10–3
, [ketone] = 0.422 , [iPrOH] = 12.4 , 25 °C. [c] Conditions for a:
[a] 6.79 × 10–4 , [KOtBu] = 5.52 × 10–3 , 25 °C. [d] Conditions for a: [a] 6.79 × 10–4 , [KOtBu] = 5.52 × 10–3
, [ketone] = 0.042 , [iPrOH] = 12.4 , [ketone] =
0.141 , [iPrOH] = 12.4 , 25 °C.
M M, [ketone] = 0.422 M, [iPrOH] = 12.4 M, 25 °C. The absolute configurations were obtained
, [KOtBu] = 5.52 × 10–3
M
M
M
M
M
M
M
M
M
M
M
M
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value in the beginning. This value is slightly superior to these
catalysts of chiral salen–Co(II) complexes and cobalt complex
with tetradentate aminophosphine ligand[10,14] in asymmetric
reduction. There is totally no reactivity of b in the hydrogen-
ation. Different performance between a and b on hydrogen-
ation could be explained withby the different structure be-
tween a and b. We postulated that a was activated by excess
base to afford a1 containing an amido-ene(amido) ligand
(Scheme 2). The conversion of amine(imine) to amido-ene-
(amido) ligand by base has been demonstrated by Morris et al.
in iron-based complexes.[12] Similar to the case of iron,[15] to
elucidate the function of ligands on catalytic performance, DFT
calculations at the B3LYP/LANL2DZ level were employed to
study the electronic structures. It turned out that the SOMO-1
of a1 possesses 10.9 % cobalt character and 29.4 % amido N
atom character (Figure 4). This suggests the N atom underwen-
tundergoes a process of π donation with the metal d orbital to
some extent, i.e. ligand-to-metal π donation. In addition, the
same SOMO-1 orbital also contains 2.0 % CH=CH–N ene(amido)
π* character, indicating the existence of a π-backdonation inter-
action with the ene(amido) group. The similar mechanism has
been demonstrated in iron-based catalysts recently by our
group.[15] The multicomponent SOMO-1 orbital could be re-
garded as a hybrid orbital where the negative charge from the
amido N atom could be relayed to the vacant ene(amido) π*-
antibonding orbitals via a cobalt d orbital to avert high electron
density on the cobalt center. In the case of iron, this electron
density transfer was assumed to reduce the high electron den-
sity on the metal center, so as to stabilize the complex against
decomposition.[15] This π-backdonation interaction does not oc-
cur on b1 due to lack of ene(amido) π*-antibonding orbitals
(see supporting information section for details).
Figure 4. Qualitative molecular SOMO-1 orbital diagram of complex a1 ob-
tained from restricted DFT calculations at the B3LYP/LANL2DZ level.
was furnished on hydrogenation of 3′-bromoacetophenone,
slightly more than the result reported by Zhang et al.[10]
Hydrogenation of 3′-(trifluoromethyl)acetophenone exhibited
an excellent conversion once ratio of pre-catalyst to substrate
was improved. Relative low conversion of hydrogenation of 3,5-
bis(trifluoromethyl)acetophenone was obtained due to stronger
electron absorption capacity of substituents.
Moderate ee value(72 %) and high conversion (91 %) were
obtained on hydrogenation of propiophenone, and these val-
ues have advantage over asymmetric borohydride reduction.[14]
A moderate ee value of 66 % was obtained on reduction
of 1-acetonaphthone, morehigher than Tomohiko's result,[16]
and the reduced product can be used for the synthesis of (+)-
compactin and 1233A.[17] A good ee value of 78 % was achieved
on hydrogenation of 1-tetralone, and this value is better than
the reported result of Co-catalyzed borohydride reduction.[18]
Fortunately, when 2-chlorobenzophenone was used as feed-
stock, the corresponding alcohol was obtained with 91 % ee
value, much more than the result of Co-catalyzed borohydride
reduction,[19] and the enantiopurity erosion did not happened
in this reaction. But when chlorine substituent was replaced by
methyl, ee value was cut down to 80 % on hydrogenation
of 2-methylbenzophenone, slightly lower than the result of
hydroboration.[20] Finally, hydrogenation of alkyl-alkyl ketone
was also performed. When 3-methyl-2-butanone was selected
as substrate, product was obtained with an ee value of 41 %.
Conclusion
Scheme 2. The proposed reactions of cobalt pre-catalysts a and b with
2 equiv. of KOtBu to produce the putative amido complexes of a1 and b1.
A new cobalt(II) complex bearing amine(imine)diphosphine
PN(H)NP ligand for asymmetric transfer hydrogenation of ket-
Complex a could hydrogenate a wide range of ketones. ones was developed. This catalyst could prompt asymmetric
Unfortunately, when substituents were introduced on the transfer hydrogenation of a wide range of ketones under very
benzenephenyl ring of acetophenone, ee values decreased mild conditions after being activated by base and exhibited a
more or less for most substrates. Good conversions were ob- maximum ee value of up to 91 %. The electronic structure stud-
tained on hydrogenation of acetophenones which contain elec- ies based on DFT calculations revealed that a π-backdonation
tron donor groups. When substituent groups were replaced by interaction between the cobalt d orbital and the π* antibond-
electron withdrawing groups, the catalytic efficiency decreased ing orbitals of amido-ene(imido) group is a key factor to enable
under the identical general conditions. The ee value of 68 % its catalytic properties.
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Computational Details
Experimental Section
Geometry optimizations and frequency calculations were per-
formed with the Gaussian09 program package using a series of
functionals and basis sets in the vacuum state. Optimized ground
states were found to have no imaginary frequencies while transition
states had only one imaginary frequency. A tightly converged crite-
rion (1 × 10–8 Eh in energy, 1 × 10–7 Eh in density change, and
1 × 10–7 in the maximum element of the DIIS error vector) was used
for the self-consistent-field calculations. A pruned (99,590) integra-
tion grid was used for density functional theory calculations. Three
dimensional visualizations of calculated structures were generated
by ChemCraft. The analysis of MO compositions was performed us-
ing the natural atomic orbitals method. Orbitals from the Gaussian
calculations were plotted with the ChemCraft program.
General Comments
NMR spectra were recorded at ambient temperature and pressure
using BRUKER AVANCE III, 31P (242 MHz). The 31P NMR spectra were
referenced to 85 % H3PO4 (0 ppm). The electrospray ionization mass
spectrometry (ESI-MS) data were collected on an Xevo G2-XS
QTOFMS of mass spectrometer with an ESI source at the Instrumen-
tal Analysis Center of Shanghai Jiao Tong University. Single-crystal
X-ray diffraction data were collected using a Nonius Kappa-CCD
diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences.
Deposition Numbers 2013827 (for a) and 2014174 (for b) contain
the supplementary crystallographic data for this paper. These data
are provided free of charge by the joint Cambridge Crystallographic
Data Centre and Fachinformationszentrum Karlsruhe Access Struc-
tures service www.ccdc.cam.ac.uk/structures.
Acknowledgments
Synthesis of Complex a: In an argon glovebox, a solution of di-
phenylphosphino-acetaldehyde hydrochloride dimer (353.6 mg,
0.67 mmol) in 10 mL of MeOH was added into a suspension of
NaOMe (71.9 mg, 1.3mmol) in 5 mL of MeOH with stirring in a
100 mL of Schlenk flask. Then the solution of PN(H)N(H2)[11] ligand
(about 82 % purity, 750.0 mg, 1.44 mmol) in 30 mL of MeOH was
added into the Schlenk flask. Finally, the solution of cobalt(II) brom-
ide hydrate (291.6 mg, 1.3 mmol) in 10 mL of MeOH was added
into the mixture solution, and dark brown solution was obtained.
The solution was condensed to 10 mL after stirring for 3 h and
filtered through a syringe filter with PTFE membrane (pore size
0.225 μm). Then the rest of solvent was removed under vacuum,
and brown solid was obtained and transferred into a new flask.
Subsequently, 50 mL of acetone was poured into the flask with
stirring to dissolve the solid. The solution was stirred for further
12 h, and the precipitate appeared in the bottom of flask. Pure
complex a (432.0 mg, 35 % yield) was collected by filtering with
filter paper in glovebox, washing with acetone twice. The complex
a was dissolved with 4 mL of dichloromethane, and acetone was
let to slowly diffuse into the solution to afford brown crystals of a,
which is suitable for X-ray diffraction. HRMS (ESI-TOF, CH2Cl2) m/z
calculated for [(C42H40Br2CoN2P2)]: 853.0350, found 853.0345. FTIR
(ATR, cm–1): 3047w, 3008w, 2851w, 1624w, 1486w, 1453m, 1432s,
1187w, 1094s, 1073m, 1020w, 994w, 966m, 849w, 742s, 685s, 642s,
575s, 525s. Anal. Calcd for C42H40Br2CoN2P2: C, 59.11 %, H, 4.72 %,
N, 3.28 %; found C, 59.23 %, H, 4.77 %, N, 3.32 %.
This work was sponsored by National Natural Science Founda-
tion of China (21772021), “Shanghai Rising-Star Program”
(16QA1400100), The Program for Professor of Special Appoint-
ment (Eastern Scholar) at Shanghai Institutions of Higher Learn-
ing (TP2014035), the National Key Research and Development
Program of China (Nos. 2016YFA0201702 of 2016YFA0201700),
International Joint Laboratory for Advanced Fiber and Low-
dimension Materials (18520750400).
Keywords: Asymmetric transfer hydrogenation · Cobalt ·
Homogeneous catalysis · π-Backdonation · Non-precious
metals · Metal activation
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Synthesis of Complex b: In an argon glovebox, 1.60 g of
PN(H)N(H)P[13] ligand (about 60 % purity, 1.50 mmol) was dissolved
in MeOH (30 mL) in a 100 mL Schlenk flask. Then the solution of
cobalt(II) bromide hydrate (344 mg, 1.6mmol) in 20 mL of MeOH
was added into the flask. Dark brown solution was obtained and
stirred for further 3 h. Then solvent was removed under vacuum,
and brown solid was obtained. After that, 50 mL of acetone was
poured into the flask to dissolve the solid. The solution was stirred
for further 12 h, and the precipitate appeared in the bottom of
flask. Pure complex b (361.0 mg, 28 % yield) was collected by filter-
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complex b was dissolved with 4 mL of dichloromethane, and acet-
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CH2Cl2) m/z calculated for [(C42H42Br2CoN2P2)]: 855.0506, found
855.0490. FTIR (ATR, cm–1): 3054w, 2963w, 2873w, 2368w, 1699m,
1581w, 1482w, 1449m, 1432s, 1354w, 1183m, 1094s, 1069m, 1020w,
959m, 916m, 845m, 803w, 735s, 689s, 628s, 557s. Anal. Calcd for
C42H42Br2CoN2P2: C, 58.97 %, H, 4.95 %, N, 3.27 %; found C, 59.08 %,
4.97 %, 3.30 %.
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Received: September 2, 2020
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