Crystal structures of chiral mono- and dinuclear salan-Cu(II) complexes and the application to catalytic asymmetric Henry reaction

Article abstract of DOI:10.1016/j.inoche.2012.03.020

Two chiral salan-Cu(II) complexes, dinuclear {[(C₂₈H ₂₆CuN₂O₂)₂·2CH ₃OH·CH₃COOH] (1)} and mononuclear {[C ₅₆H₅₅CuN₄O₄·1/2CH ₃

Full text of DOI:10.1016/j.inoche.2012.03.020

Inorganic Chemistry Communications 20 (2012) 259–262
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Crystal structures of chiral mono- and dinuclear salan–Cu(II) complexes and the
application to catalytic asymmetric Henry reaction
a
a
Yan Shi a, Zhijie Mao a, Qicai Xue a, Chengjian Zhu a,b, , Hongwen Hu , Yixiang Cheng
⁎
a
School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, China
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, 200032, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Two chiral salan–Cu(II) complexes, dinuclear {[(C28H26CuN2O2)2 ·2CH3OH∙CH3COOH] (1)} and mononuclear
{[C56H55CuN4O4∙½CH3OH∙CH3COOH∙H2O] (2)} were synthesized and structurally characterized. The complex
1 was obtained with the ratio 1:1.2 of the metal source and the ligand, in which the two copper atoms were
bridged by μ–O atoms forming a four-membered binuclear Cu2O2 square–planar geometry. However, using
two equivalents of the ligand led to the formation of the complex 2, in which one copper atom coordinated
with two salan ligands bound in a five-coordinate square–pyramidal. The asymmetric Henry reaction was
studied by these in situ formed complexes. The complex 1 provided moderate to high enantioselectivities
(up to 83%) when the reaction was carried out in EtOH at room temperature for 48 h, whereas the complex
2 gave a modest enantioselectivity.
Received 11 February 2012
Accepted 8 March 2012
Available online 19 March 2012
Keywords:
Crystal structures
Copper
Salan ligands
Asymmetric catalysis
Henry reaction
© 2012 Elsevier B.V. All rights reserved.
In the past decades, enantioselective catalysis has experienced an
unprecedented advance in response to the demand for enantiopure
compounds such as fine-chemicals and material science [1–3]. Among
them, chiral metal–salen/salan complexes have been applied to a
wide variety of reactions [4–6]. However, the salen ligands have some
disadvantage because of the hydrolysis of the C=N bond in solution
[7,8]. This can be avoided by reducing the imine functional group. Our
group has been studying metal–salan complexes for several years and
some progress have been made [9–11].
Enantioselective copper-catalyzed reaction is one of the most
powerful carbon–carbon bondforming methods to construct biologi-
cally active and natural compounds [12,13]. In the past few years,
highly efficient copper catalytic systems have witnessed impressive
results. Reported here is the synthesis, structure of mono- and dinuc-
lear Cu(II) complexes based on salan ligand and the application to
asymmetric nitroaldol reaction.
One equivalent of salan ligand reacted with 1.2 equivalents of
Cu(OAc)2·H2O in methanol, in anticipation of generating complex 1,
Scheme 1. When preparing the complex 2, two equivalents of salan li-
gand was used. For both complexes, the mixture was stirring for one
hour at room temperature and during this period the color of it turned
to deep green. The filtrate was left in the air for a few days, dark-green
crystals were obtained. They are stable in air at room temperature and
soluble in a wide variety of solvents.
The complex 1 displays no prominent IR signal in the region
around 3400 cm−1 whereas the complex 2 shows a quite prominent
vibration at 3454 cm−1 indicating a phenolic O–H vibration. These
results indicate that the coordination environment of the Cu ion
with salan ligands is totally different in the formation process of cop-
per complexes 1 and 2. For complex 1, all phenolic alcohol groups in
salan ligands are deprotonated whereas only one phenolic alcohol
groups for complex 2 is deprotonated to participate in the formation
of Cu–O bonds. The structure refinement is also assured by single
crystal X–ray diffraction.
The CD spectra of the salan–Cu complexes are identical or similar
to the salan–parent ligand.(Fig. 1) Comparing the CD spectra of the
three complexes, the bands observed in the ligand are still observed
in the relevant metal complexes. However, the peak 1 was split and
negative cotton effects were observed. When it comes to the peak 2,
the band around 250–275 nm red shift to 275 to 300 nm. Two new
bands, weak but remarkable, can be examined in the CD spectroscopy
of the salan–Cu complexes [14,15].
Thermal curves of the two complexes are shown in Fig. 2. Thermal
properties of these complexes have been studied in the range of
25–700 °C using thermal techniques. Their thermal decomposition
reveals that the complexes contain the adsorbed CH3OH and CH3-
COOH molecules respectively, which are consistent with the crystal
structure. The two complexes are stable in air at room temperature.
The mass loss of these copper complexes starts at around 250 °C.
As shown in Fig. 3 and Table 1, the complex 1[16] is noncentro-
symmetric and dimeric in which two copper ions are bridged by
μ–O atoms forming a four-membered binuclear Cu2O2 square–planar
geometry. Each distorted square–pyramidal in the binuclear complex
is constructed by pentadentate linkage with two nitrogen atoms and
⁎
Corresponding author at: School of Chemistry and Chemical Engineering, State Key
Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, China.
Tel./fax: +86 25 83594886.
E-mail address: cjzhu@nju.edu.cn (C. Zhu).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.03.020

260
Y. Shi et al. / Inorganic Chemistry Communications 20 (2012) 259–262
Scheme 1. Syntheses of the Cu(II) complexes.
Fig. 3. The ORTEP diagram of the complex 1 indicating atom-numbering scheme with
thermal ellipsoids at 30% probability.
two oxygen atoms from the same ligand, and one oxygen atom from
the other one. The Cu–O and Cu–N distances in this complex range
from 1.913(3) to 2.180(3) Å, and from 1.973(3) to 2.004(4) Å. The
six-member chelate ring which having been puckering amplitudes
is different from similar structures that have been reported in the lit-
erature[17].
All bond lengths and angles are comparable with the literature
values of the reported structures containing five–coordinated Cu
complexes. Atoms O(1), O(2), N(1) and N(2) in the same ligand
form a coplane on the base of the pyramid, whereas another atom
O(1) from the other one occupies apical position. In this way, a dimer-
ic structure with Cu2O2 square–planar geometry is built up from five-
coordinate distorted square–pyramid through three-bridge oxygen
atom vertexes. The acetic acid and methanol molecules exist in the
isolated form in the molecular structure of the complex 1.
Fig. 1. CD spectroscopy of the complexes.
As shown in Fig. 4 and Table 2, the complex 2 [18] is noncentro-
symmetric and mononuclear. One copper atom is coordinated with
two salan ligands bound in
a pentadentate fashion (Cu–O(2),
1.944(4) Å; Cu–N(1), 2.087(4) Å, Cu–N(2), 2.004(4) Å; Cu–N(3),
2.054(4) Å); Cu–N(4), 2.291(4) Å). Unlike the complex 1, in which
Table 1
Selected bond lengths, bond angles of complex 1.
1-O(1)
1-N(2)
1-O(1)
1.913(3)
2.004(4)
2.180(3)
1.133(8)
1.340(10)
1.498(6)
1.478(5)
83.18(14)
92.96(15)
153.04(14)
123.78(15)
98.16(14)
1-N(1)
1-O(2)
1.973(3)
2.077(3)
1.339(6)
1.347(6)
1.470(6)
1.476(6)
90.60(15)
174.52(17)
93.27(14)
95.07(14)
85.34(15)
133.5(3)
O(1)-C(1)
O(2)-C(9)
N(1)-C(7)
N(2)-C(28)
O(1)-1-O(2)
O(1)m-1-N(1)
N(1)-1-O(2)
N(1)-1-O(1)
N(1)-1-N(2)
C(1)-O(1)-1
C(31)-O(4)n
C(29)-O(3)
N(1)-C(14)
N(2)-C(21)
O(2)-1-O(1)
O(1)m-1-N(2)
N(2)-1-O(2)
N(2)-1-O(1)
1m-O(1)-1
Fig. 2. The Thermal curves of the Cu(II) complexes.
[a] m : −x+1, −y, z ; n : −x+1, −y+1, z.

Y. Shi et al. / Inorganic Chemistry Communications 20 (2012) 259–262
261
Scheme 2. Enantioselective Henry reaction catalyzed by copper–salan complexes.
asymmetric Henry reaction between different benzaldehydes and ni-
tromethane was tested using in situ method (Scheme 2). The results
of which are shown in Table 3. All reactions were performed
on a 0.17 mmol scale with 12 mol % of salan ligand and 10 mol % of
Cu(OAc)2 ·H2O using 10 equiv. of nitromethane in ethanol at room
temperature over a period of 48 h.
As a modest catalytic result was achieved by complex 2 (ee 57%),
we then focused our attention on complex 1. The electron properties
and location of the substituent on the aromatic rings have an influ-
ence on the enantioselectivity in this reaction. Generally, substrates
bearing electron-donating substituents tend to provide better results
in terms of ee values (Table 3, Entries 3–10). Moreover, among the
electron-withdrawing aldehydes, the 3-substituted ones give the
best ee values (Table 3, Entries 3, 5, 7, 8). The highest ee (83%) was
obtained when 3j was used (Table 3, Entry 11).
Fig. 4. The ORTEP diagram of the complex 2 indicating atom-numbering scheme with
Two salan–copper complexes have been prepared and structurally
characterised. The complex 1 is dinuclear in which two copper ions
are bridged by μ–O atoms forming a four-membered binuclear
Cu2O2 square–planar geometry. The complex 2 is monoclear, where
one copper atom is coordinated with two salan ligands bound to a
pentadentate fashion. Each central metal is five-coordinate. Both the
complexes were tested for Henry reaction, the highest ee is 83%.
thermal ellipsoids at 30% probability.
all four phenolic alcohol groups in two salan ligands are deprotonated
forming a covalent Cu–O bond, only one of the four phenolic alcohol
groups is deprotonated in complex 2. The other type of O–H group
is confirmed by the IR spectrum. The Cu–O bond length is consistent
with complexes bearing similarly structure of salan–copper that
reported before [19]. The apical positions of the square–pyramidal
of the copper atom are occupied by N(4). The Cu–N(4) bond distance
of 2.291(4) Å is longer than Cu–N bond length reported so far involv-
ing a deprotonated amine group [20]. The O(2), N(1), N(2) and N(3)
atoms are almost located within a coplane. The pentadentate coordi-
nation of Cu(II) atom forces the salan ligand out of the equatorial
plane and causes a quite asymmetric structure of the molecule. The
acetic acid, methanol and water molecules exist in the isolated form
in the molecular structure of the complex 2.
Acknowledgements
We gratefully acknowledge the National Natural Science Foundation
of China (20832001, 20972065, 21074054, 21172106) and the National
Basic Research Program of China (2010CB923303) for their financial
support.
Appendix A. Supplementary data
Crystallographic data (excluding structure factors) for the structural
analysis have been deposited with the Cambridge Crystallographic Data
Center as supplementary publication Nos. CCDC 816439 and 812238.
Copies of the data can be obtained free of charge via www.ccdc.ac.uk/
conts/retrieving.html (or from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK, Fax: +44-1223-336-033. E-mail: deposit@
ccdc.cam.ac.uk). Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.inoche.2012.03.020.
Catalytic studies
As a traditional method for carbon–carbon bond formation, Henry
reaction plays an important role in the synthetic chemistry. Since Shi-
basaki group first reported asymmetric nitroaldol reaction catalyzed
by rare earth metal alkoxides, a lot of efforts had been engaged in
this reaction [21–26]. Cu(II) complexes have been extensively utilised
in the asymmetric Henry reaction [27–31]. The salan–Cu(II) catalyzed
Table 2
Table 3
Selected bond lengths, bond angles of complex 2.
Catalytic results for the Henry reaction with nitromethane.
1-O(2)
1-N(3)
C(1)-O(1)
1.944(4)
2.054(4)
1.388(8)
1.500(6)
1.387(7)
1.513(7)
1.358(8)
93.15(16)
178.05(16)
82.03(16)
94.06(17)
81.03(15)
110.1(3)
113.8(4)
105.6(3)
112.2(3)
111.3(4)
127.1(3)
1-N(2)
1-N(1)
C(7)-N(1)
2.004(4)
2.087(4)
1.534(6)
1.345(6)
1.507(6)
1.520(6)
1.248(19)
88.58(16)
153.56(17)
96.02(16)
99.73(15)
112.37(15)
109.5(3)
115.1(3)
111.4(4)
108.7(3)
106.3(3)
120.1(3)
Entry
Ar
Cat
Product
Yield(%)a
ee (%)b
1
2
3
4
5
6
7
8
Ph-
Ph-
1
2
1
1
1
1
1
1
1
1
1
5a
5a
5b
5c
5 d
5e
5f
5 g
5 h
5i
22
29
84
10
36
30
45
43
75
70
30
63
57
54
72
70
81
75
76
67
60
83
C(22)-N(2)
C(29)-O(3)
C(36)-C(37)
C(56)-O(4)
O(2)-1-N(2)
N(2)-1-N(3)
N(2)-1-N(1)
O(2)-1-N(4)
N(3)-1-N(4)
C(8)-N(1)-1
C(22)-N(2)-C(15)
C(15)-N(2)-1
C(35)-N(3)-1
C(43)-N(4)-C(50)
C(50)-N(4)-1
C(28)-O(2)
C(35)-N(3)
C(36)-N(3)
C(57)-O(7)
O(2)-1-N(3)
O(2)-1-N(1)
N(3)-1-N(1)
N(2)-1-N(4)
N(1)-1-N(4)
C(7)-N(1)-1
C(22)-N(2)-1
C(35)-N(3)-C(36)
C(36)-N(3)-1
C(43)-N(4)-1
C(28)-O(2)-1
4-ClPh-
4-OCH3Ph-
4-BrPh-
4-CH3Ph-
3-ClPh-
3-BrPh-
2-ClPh-
2-BrPh-
9
10
11
5j
12
1
5 k
45
80
[a] The yields were determined by the isolated product. [b] The ee value was
determined by HPLC.

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