Amidines as effective ancillary ligands in copper-catalyzed hydrogenation of carbon dioxide

Article abstract of DOI:10.1246/cl.190873

Mononuclear Cu(II) complexes bearing a bidentate bisamidine ligand were newly synthesized and characterized. The catalytic activity was evaluated in the hydrogenation of carbon dioxide to formate salts. A substantial enhancement of the catalyst turnover number was achieved by the imidazoline-based complex, indicating that amidines serve as effective ancillary ligands for homogeneous copper catalysis.

Full text of DOI:10.1246/cl.190873

CL-190873
Received: November 28, 2019 | Accepted: December 24, 2019 | Web Released: December 28, 2019
Amidines as Effective Ancillary Ligands in Copper-catalyzed Hydrogenation of Carbon Dioxide
1
2
2
Ryo Watari,* Shigeki Kuwata, and Yoshihito Kayaki*
1
Environmental Chemistry Sector, Environmental Science Research Laboratory, Central Research Institute
of Electric Power Industry, 1646 Abiko, Abiko, Chiba 270-1194, Japan
2
Department of Chemical Science and Engineering, School of Materials and Chemical Technology,
Tokyo Institute of Technology, 2-12-1-E4-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
E-mail: r-watari@criepi.denken.or.jp (R. Watari), ykayaki@o.cc.titech.ac.jp (Y. Kayaki)
Mononuclear Cu(II) complexes bearing a bidentate bisami-
dine ligand were newly synthesized and characterized. The
catalytic activity was evaluated in the hydrogenation of carbon
dioxide to formate salts. A substantial enhancement of the
catalyst turnover number was achieved by the imidazoline-based
complex, indicating that amidines serve as effective ancillary
ligands for homogeneous copper catalysis.
The involvement of DBU-coordinated copper species in
the hydrogenation step was not postulated in either system;
however, the catalytic activity of copper hydrides is possibly
evoked by coordination of the amidine structure, as studied
previously in the heterogeneous C=O bond hydrogenation using
11
copper nanoparticles supported by polystyrene-bound DBU.
Considering that amidines have rarely been utilized as com-
1
2
petent ligands in transition metal catalysis, we envisioned that
molecules containing proximal amidine units will behave as
rigid and viable ligands suited for Cu-based catalysis. In this
context, Iwasawa and co-workers reported the oxidative coupling
Keywords: Carbon dioxide | Copper complex
| Amidines
Carbon dioxide (CO2) utilization driven by redundant
renewable energy sources has drawn increasing attention to
of CO with alkynes or allenes using a stoichiometric amount
2
1
13
attaining net zero carbon emissions in the near future. Hydro-
of bisamidine-Ni(0) species. Recently, Onishi, Himeda, and
genation of CO2 is one of the most promising and significant
ways directed toward large-scale applications. Recently, earth
co-workers demonstrated that the catalytic hydrogenation of
CO2 was accelerated by a Cp*Ir complex bearing a bisimidazo-
line, 4,4¤,5,5¤-tetrahydro-1H,1¤H-2,2¤-biimidazole. Herein, we
report the synthesis of novel copper complexes with a bidentate
bisamidine ligand and their catalytic activities in the CO2
hydrogenation.
2
1
4
abundant first-row transition metals, such as Mn, Fe, Co, Ni,
and Cu, have emerged as alternatives to the well-established
3
hydrogenation catalysts based on precious metals. For example,
the rational design of Fe catalysts contributed to achieve
excellent turnover number (TON) of 58990 in the hydrogenation
of CO2 to formate salts.4
We targeted bisamidines (Scheme 1), which are accessible
in a single step from commercially available materials. In a
1
5
In this context, we have disclosed the potential of homoge-
reported work by Weisman and Wong, the tricyclic bisamidine
(1) derived from dithiooxamide and N,N¤-bis(3-aminopropyl)-
1,2-ethylenediamine reacted with Cu(ClO4)2, to form dicationic
bis- and tris-chelate complexes with an appropriate bite angle. A
5
neous copper catalysts to promote the hydrogenation of CO2.
Among the base additives tested, 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU) specifically assisted to exert the catalytic function,
1
3
even though homogeneous copper catalysts were considered to
be less effective for the CO2 hydrogenation than other 3d
propylidene-linked bis(1-methylimidazoline) (2) was conven-
iently obtained from dimethylmalononitrile with 2 equiv of a
monotosylate salt of N-methylethylenediamine with modifica-
tion of the reported procedure for synthesis of polymethyl-
6
metals. The combined use of Cu(OAc)2 and DBU with a molar
ratio of 1:500 led to a satisfactory TON up to 167. Aside from a
primary role in trapping the formic acid product as its salt, we
conceived that DBU can behave as a supporting ligand to form a
DBU-Cu species. Actually, we identified the first DBU-ligated
copper complex, (dbu)2CuI, which showed a decent activity for
the CO2 hydrogenation. In the subsequent studies, analogous
marked enhancement by addition of DBU was observed in the
CO2 hydrogenation with phosphine- and cyclic (alkyl)(amino)-
carbene (CAAC)-Cu complexes. Appel and co-workers reported
1
6
ene-linked bisimidazolines (see the Supporting Information).
Treatment of an acetonitrile solution of Cu(OAc)2¢H2O with
1 equiv of the bisamidine followed by crystallization from
CH2Cl2/Et2O gave blue crystals of the expected mononuclear
N
N
N
N
Cu(OAc)
L:
·H O
+
2
L
2
1
equiv CH CN, rt
that a tridentate phosphine-Cu complex, [(triphos)Cu(CH CN)]-
3
3
Cu
[PF6]
(triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane),
AcO
3, 65% yield
OAc
effectively catalyzed the formate synthesis with TONs up to
N
N
7
5
00. The roles of DBU were proposed to promote heterolytic
cleavage of H and to suppress the formation of catalytically
inactive dinuclear copper species by its reversible coordination.
N
N
N
N
2
1
8
N
N
Bertrand’s group achieved the best catalytic activity in the Cu-
catalyzed hydrogenation by combination of a CAAC-Cu com-
N
N
Cu
AcO
4, 74% yield
OAc
9
N
N
plex and tris(pentafluorophenyl)borane (TON = 1881). A frus-
2
trated Lewis pair-like activation of H2 by DBU and the borane
was considered to be conducive to the formation of copper
hydrides and formates.10
Scheme 1. Synthesis of bis(acetato)copper(II) complexes bear-
ing a bidentate bisamidine ligand.
252 | Chem. Lett. 2020, 49, 252–254 | doi:10.1246/cl.190873
© 2020 The Chemical Society of Japan

Figure 1. ORTEP drawings of the Cu complex 3 (A: top view,
B: front view). The asymmetric unit contains only one-half
of the molecule. The hydrogen atoms are omitted for clarity.
Thermal ellipsoids are shown at the 30% probability level.
Selected bond lengths (¡) and angles (°): Cu­N1, 1.974(5); Cu­
O1, 1.961(4); N1­Cu­N1¤, 81.3(2); O1­Cu­O1¤, 93.96(15); N1­
Cu­O1, 93.71(18); N1­Cu­O1¤, 166.15(10); Cu­N1­C1,
Figure 2. ORTEP drawings of the Cu complex 4 (A: top view,
B: side view). The hydrogen atoms are omitted for clarity.
Thermal ellipsoids are shown at the 30% probability level.
Selected bond lengths (¡) and angles (°): Cu­N1, 1.944(4); Cu­
N3, 1.963(5); Cu­O1, 1.967(4); Cu­O3, 1.977(4); N1­Cu­N3,
8
7.74(19); O1­Cu­O3, 90.35(19), N1­Cu­O3, 92.50(19); N3­
Cu­O1, 94.42(19); N1­Cu­O1, 165.5(2); N3­Cu­O3, 159.9(2);
Cu­N1­C1, 123.2(4); Cu­N1­C3, 127.5(3); C1­N1­C3,
126.4(3); Cu­N1­C4, 115.0(5); C1­N1­C4, 118.3(4); C3­N2­
C4, 119.8(4); C3­N2­C5, 116.8(4); C4­N2­C5, 119.8(5).
1
08.1(4); Cu­N3­C5, 120.3(4); Cu­N3­C7, 127.2(4); C5­N3­
C7, 108.3(5); C2­N2­C3, 109.3(5); C2­N2­C4, 117.9(4); C3­
N2­C4, 132.2(5); C6­N4­C7, 109.3(5); C6­N4­C8, 118.3(5);
C7­N4­C8, 132.5(6).
Cu(II) complexes (3 and 4) in reasonable yields of 65% and
7
4%, respectively.
a
The isolated complexes were characterized by elemental
Table 1. Cu-catalyzed hydrogenation of CO to formate.
2
analysis or high-resolution mass spectroscopy as well as X-ray
Cu cat
_
+
–
1
7
CO
2
+
H
2
+
base
[base H] [HCO ]
2
crystallography. As shown in Figures 1 and 2, the crystal
structures revealed that these Cu(II) complexes adopt a slightly
twisted square-planar geometry around the metal bonded to two
neutral imine nitrogen atoms of the cyclic amidines and two
1,4-dioxane, 21 h
P
T
Yield
Entry
Cat
Base Base/Cat
TON
[
atm]b [°C] [%]c
30/30 100 32 163
30/30 100 16 81
30/30 100 41 210
30/30 80 12 65
20/40 100 46 239
10/50 100 39 212
20/40 100 <1 <1
20/40 100 <1 <1
1
oxygen atoms of the acetato group in a ¬ -coordination mode.
1
2
3
4
5
6
7
8
9
Cu(OAc)
2
¢H
2
O
DBU
DBU
DBU
DBU
DBU
DBU
NEt3
500
500
500
500
500
500
500
500
500
250
Sums of the angles around the Cu atom are 362.7° for 3 and
3
4
4
4
4
4
4
4
4
4
365.0° for 4, displaying a moderate distortion from planarity.
Comparison of the four-coordinate geometry index τ4, which can
range from an ideal square planar arrangement (τ = 0) to a
4
tetrahedral structure (τ4 = 1),18 indicates that 4 (τ4 = 0.25) has
a more distorted geometry, compared to 3 (τ4 = 0.20). The
complex 3 crystallized in the space group C2 and contains half
of the molecule in the asymmetric unit. The structure of 3
exhibits a C2 symmetry represented by an enantiomerically pure
conformer, as seen in the front view of Figure 1(B). The near
coplanarity of bonds at the coordinating nitrogen atoms for 3
was indicated by the sum of the bond angles being 359.7°,
whereas 4 showed a bent structure, as seen in the side view of
BTMG
TBD
20/40 100 19
20/40 100
95
27
1660
0d
t
1
11
KO Bu
5
4
DBU
40000 20/40 100
a
Reaction conditions: base (10 mmol), 1,4-dioxane (5.0 mL),
b
c
2
1 h. P(CO2)/P(H2). Molar ratios of the product/initial DBU
1
d
t
2
determined by H NMR. KO Bu (5 mmol).
Figure 2(B). The average sum of the bond angles around the sp
nitrogen atoms is 357.3°, that is in accord with the pyramidal-
ized configuration. The non-conjugated bisamidine ligand in 4
possibly offers a stronger σ-donation to the Cu atom.
non-conjugated bisamidine contributed to the superior formate
productivity. Although the hydrogenation at a lower reaction
temperature of 80 °C decreased the yield to 12% (Entry 4),
1
9
The isolated complexes were applied to the hydrogenation
of CO2 to formate salts in the presence of a base, as summarized
in Table 1. We initially examined the effect of bisamidine
ligands on the catalytic activities by comparison with the
hydrogenation using Cu(OAc)2¢H2O (Entry 1). Both complexes
catalyzed the formate production using a 2.0 M solution of DBU
in 1,4-dioxane with a DBU/catalyst ratio of 500 at 100 °C under
a mixture of CO2 (30 atm) and H2 (30 atm) for 21 h (Entries 2
and 3). The TON for 3 and the yield of formate based on the
added DBU were 81 and 16%, respectively, displaying a lower
reduction of the CO partial pressure had a positive influence on
2
the catalytic performance of 4. Among the reactions under a total
pressure of 60 atm, a P(CO2)/P(H2) ratio of 20/40 provided an
optimal TON of 239 (Entry 5).
Changing the additive base from DBU to triethylamine and
a sterically hindered guanidine, 2-tert-butyl-1,1,3,3-tetramethyl-
guanidine (BTMG), hampered the formate synthesis and resulted
in formation of copper precipitates after the hydrogenation
(Entries 7 and 8). The use of 1,5,7-triazabicyclo[4.4.0]dec-5-ene
(TBD) was effective for the formate synthesis, affording a
catalytic activity than that of Cu(OAc)2¢H2O (TON = 163 and
5
3
2% yield). In contrast, the hydrogenation was surely enhanced
t
in the presence of 4 to attain the TON of 210 (Entry 3).
Presumably, the relatively strong electron-donating ability of the
moderate TON of 95 (Entry 9). A strong solid base, KO Bu, did
not exhibit promotional effects (Entry 10), possibly due to a
Chem. Lett. 2020, 49, 252–254 | doi:10.1246/cl.190873
© 2020 The Chemical Society of Japan | 253

t
rapid formation of KOCO2 Bu from CO2. Further optimization
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of 1660 (Entry 11), which is comparable to the Bertrand system
comprised of CAAC-Cu species and tris(pentafluorophenyl)-
borane. Although the beneficial effect of DBU underlies the
CO hydrogenation in analogy with Appel’s and our previous
systems, the marked catalytic performance of 4 surpassing the
original DBU-Cu catalysts clearly indicates that the amidines
play a pivotal role as the supporting ligand during the H and
4
5
6
7
8
9
9
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copper complexes bearing a bidentate amidine ligand as catalyst
precursors for CO2 hydrogenation using DBU. The complex 4
with a non-conjugated bis(1-methylimidazoline) chelate showed
a satisfactory activity and achieved the optimal TON of 1660,
due to the relatively strong σ-donating ability of the amidine unit
to enhance the hydridicity of catalytic species. This work opens
up the potential applications of bidentate amidine ligands to
extensive Cu-promoted reactions. Further studies on the rational
design of multidentate amidines compatible with transition
metal catalysts are in progress in our laboratory.
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Number JP19K15614.
Supporting Information is available on https://doi.org/
1
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254 | Chem. Lett. 2020, 49, 252–254 | doi:10.1246/cl.190873
© 2020 The Chemical Society of Japan