Synthesis of amino acid derivative Schiff base copper(II) complexes by microwave and wet mechanochemical methods

Article abstract of DOI:10.1016/j.jics.2021.100004

As resource- and time-saving and environmentally friendly synthetic methods than conventional one in a solution, microwave, and wet mechanochemical synthesis are tested for L-amino acid derivative Schiff base copper(II) complexes. Herein, we systematicall

Full text of DOI:10.1016/j.jics.2021.100004

Journal of the Indian Chemical Society 98 (2021) 100004
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Synthesis of amino acid derivative Schiff base copper(II) complexes by
microwave and wet mechanochemical methods
Nao Otani, Tetsundo Furuya, Natsuki Katsuumi, Tomoyuki Haraguchi, Takashiro Akitsu *
Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
As resource- and time-saving and environmentally friendly synthetic methods than conventional one in a solution,
microwave, and wet mechanochemical synthesis are tested for L-amino acid derivative Schiff base copper(II)
complexes. Herein, we systematically compared efficiency (low-temperature, time, and yield (if possible to
detect)) for both conventional solution method and microwave or mechanochemical methods. The wet mecha-
nochemical synthesis promoted fast reaction (typically 20 min by mechanochemical vs 4 h by conventional) by a
little amount of solvent for preparations of amino acid derivative Schiff base copper(II) complexes. New crystal
structure of a five-coordinated square pyramidal copper(II) complex as one of the products of microwave method
was also reported.
Microwave synthesis
Mechanochemical synthesis
Amino acid
Schiff base
Copper complex
For effective preparation of materials, in the era of SDGs, environ-
mentally friendly (e.g. tailor-made synthesis of amino acids [1,2] by
mehanochemical method [3,4]) than microfluid including droplet
method (needing large equipment) [5–7] have been required for syn-
thesizing not only metal complexes besides conventional method
(so-called liquid phase method in a solution) but also hybrid artificial
metalloproteins which is easy to decompose by heating condition. As for
the preparation of the L-amino acid derivative Schiff base copper(II)
complexes, having potential application for photocatalysts [8,9], we
have generally employed two-step reactions, namely (1) imine conden-
sation of primary amine and aldehyde; (2) coordination of copper(II) ion
from acetate source (Scheme 1). Herein, we compared with microwave
[10] and wet mechanochemical syntheses [11–13] for the L-amino acid
derivative Schiff base copper(II) complexes.
The merits of microwave synthesis are faster reaction speed by
controlled heat transfer, safety, fewer reactants, improved reactivity,
high yield, selectivity of heating, and reproducibility. Known L-amino
acid chiral Schiff base complexes [14–20] were synthesized using the
microwave synthesis apparatus (Biotage Initiatorþ). The green-colored
products were characterized with IR and UV–vis spectra and so on (not
shown). In the conventional liquid phase method (298 K), it took about
(2 þ 2 ¼ ) 4 h to complete the two-step reaction of Scheme 1 continu-
ously, while about 10 min by microwave one-pot synthesis (358 K) as
listed in Table 1. The products obtained in shorter time and higher yield
were identified to be identical to that by conventional methods by
comparing IR spectra. Microwave synthesis method may be effective in
particular for 2, 7, 14, and 15 (due to soluble leucine and
electron-withdrawing 3,5-dichlorosalicylaldehyde). For all complexes,
full characterization for new compounds has already reported in the
original papers. Therefore, we confirmed that the same compounds could
be prepared by different methods only using limited (mainly IR spectral)
data. Unreported new crystal structure of 1 solved using programs of
SHELXL [21] and so on is depicted in Fig. 1 and found to afford a
five-coordinated square pyramidal geometry to form a chain structure.
Dpending on bulkiness of ligands and conditions of preparation, coor-
dination modes except for the tridentate Schiff base ligand can vary
significantly.
Crystallographic data for 1 (CCDC 2045315): C13H15CuNO3, Mono-
clinic, space group P21 (#4), Z ¼ 2, a ¼ 9.7026(3), b ¼ 5.0944(2), c ¼
13.6481(5) Å, β ¼ 109.5110(10)ꢀ, V ¼ 635.87(4) Å3, ρcalc ¼ 1.550
gcmꢁ3
,
μ
¼ 1.716 mmꢁ1, F(000) ¼ 306, S ¼ 1.086, R1[I > 2
σ(I)] ¼
0.0490, wR2 ¼ 0.1157, Flack parameter ¼ ꢁ0.002(16), T ¼ 293 K.
On the other hand, the mechanochemical can be carried out under
solvent-free or small amounts of solvent [11], mass transfer in mecha-
nochemical reaction can proceed through a gas, a liquid, a solid phase, or
any combination of [12] (microwave cannot be used for gas-emitting
reactions due to a closed container). However, there are also some dis-
advantages of this method, for example, reactants and products stick to
the mortar wall, low yield, difficult to adjust reaction temperature, and
unreacted reactants need to be separated. Indeed, James and co-workers
* Corresponding author.
E-mail address: akitsu2@rs.tus.ac.jp (T. Akitsu).
https://doi.org/10.1016/j.jics.2021.100004
Received 9 November 2020; Received in revised form 23 November 2020; Accepted 27 November 2020
0019-4522/© 2021 Indian Chemical Society. Published by Elsevier B.V. All rights reserved.

N. Otani et al.
Journal of the Indian Chemical Society 98 (2021) 100004
Scheme 1. Typical reaction scheme in a solution for the L-amino acid derivative Schiff base copper(II) complexes. In the case of microwave synthesis, 40 ꢀC, 3 h are
85 ꢀC, 20 min.
Table 1
Summary of the results. Up: conventional IR bands, middle: microwave, down: mechanochemical. “AA” denotes amino acid. “Aldehyde” indicates substitution groups of
salicylaldehyde (H denotes salicylaldehyde). “Time” indicates sum of reaction time of the first and the second steps. IR and UV (spectra) denote predominant bands of
–
C
N and
π
-
π*, respectively.
–
Entry [Ref.]
AA
aldehyde
H
Time/min
Yield/%
IR/cmꢁ1
UV–vis/nm
Temperature/ꢀC
1 [15]
Leu
120 þ 120
5 þ 5
53.9
45.1
31.1
50.7
31.2
53.9
65.5
65.3
24.6
64.0
49.2
27.4
19.2
67.0
13.0
27.8
75.0
34.2
6.8
85.9
21.6
31.6
77.7
21.2
50.5
85.9
21.6
55.5
66.7
24.0
42.2
87.4
35.8
44.8
80.2
28.5
25.4
18.8
33.1
44.3
26.4
17.3
10.0
31.2
44.2
33.1
52.8
34.0
1617
1646
1611
1636
1636
1621
1634
1645
1609
1636
1646
1645
1616
1640
1622
1613
1644
1608
1631
1633
1611
1628
1636
1606
1615
1639
1632
1623
1636
1627
1630
1637
1620
1621
1636
1621
1644
1633
1633
1635
1636
1633
1634
1636
1637
1635
1626
1636
400
378
378
385
386
386
364
386
386
377
397
399
372
372
372
388
388
388
390
377
377
400
401
401
354
367
367
354
418
383
354
377
377
358
405
396
370
369
369
370
380
380
370
372
370
380
380
380
45
85
25
45
85
25
45
85
25
45
85
25
40
85
25
40
85
25
40
85
25
40
85
25
60
85
25
60
85
25
60
85
25
60
85
25
60
85
25
60
85
25
60
85
25
45
85
25
5 þ 25
2 [15]
3 [15]
4 [15]
5 [16]
6 [16]
7 [17]
8 [17]
9 [19]
10 [19]
11 [19]
12 [19]
13 [18]
14 [18]
15 [18]
16 [14]
Leu
Ser
3,5-Cl
H
120 þ 120
5 þ 5
10 þ 10
120 þ 120
5 þ 5
3 þ 20
Ser
3,5-Cl
H
120 þ 120
5 þ 5
10 þ 10
120 þ 120
5 þ 5
*Ala
Ala
5 þ 25
3,5-Cl
H
120 þ 120
5 þ 5
10 þ 10
180 þ 180
5 þ 5
*Thr
*Thr
*Val
*Val
*Val
*Val
Arg
Arg
Arg
Lys
2 þ 12
3,5-Cl
H
180 þ 180
5 þ 5
10 þ 10
180 þ 120
5 þ 5
5 þ 35
5-Cl
5-Br
5-MeO
H
180 þ 120
5 þ 5
10 þ 15
180 þ 120
5 þ 5
10 þ 15
180 þ 120
5 þ 5
5 þ 10
120 þ 160
5 þ 5
1 þ 3
5-Br
5-MeO
3,5-Cl
120 þ 160
5 þ 5
1 þ 6
120 þ 160
5 þ 5
2 þ 10
120
5 þ 5
5 þ 5
reported a one pod two-step mechanochemical synthesis of Schiff base
metal complexes at room temperature [13]. Synthesis by mechano-
chemical method and characterization of the same complexes are also
listed (Table 1). Mechanochemical synthesis generally took only about
20 min at 298 K. Thus the formation of Schiff base copper(II) complex
would depend on and may be supported by metal ion (Scheme 2). As for
low yield, it could not be compared between previous and present data
properly due to different conditions of concentration and temperature
(solubility) of reaction solutions. Attempt to obtain good crystals suitable
for X-ray analysis directly was unsuccessful from the product of
mechanochemical synthesis, contrary to microwave method [22]. Thus,
we could elucidate that mechanochemical synthesis may be effective for
12, 14, and 16. Promoting by a little amount of solvent and local heat
induced by mechanical pressure, which was also different from the mi-
crowave method, may attribute to these results.
For * marked complexes, single crystals of these complexes were
isolated as imidazole coordinated ones for X-ray analysis in original pa-
–
pers. Therefore, IR C N bands was mainly used for confirming the
–
products and comparing the original reports.
We intend on prepare artificial metalloenzymes including these
2

N. Otani et al.
Journal of the Indian Chemical Society 98 (2021) 100004
Fig. 1. Crystal structure of 1 as a product of microwave method. Hydrogen atoms are omitted for clarity. Selected bond distances (Å): Cu1–O3 ¼ 1.880(4), Cu1–N1 ¼
1.923(5), dashed line Cu1–O2 (symmetry codes: 1-x, y-1/2, 2-z) ¼ 1.982(4), Cu1–O2 ¼ 2.012(4) (, and dashed line Cu1–O1 (symmetry codes: x, y-1, z) ¼ 2.472(4)).
Scheme 2. Proposed mechanism of the L-amino acid derivative Schiff base copper(II) complexes for mehanochemical reaction.
3

N. Otani et al.
Journal of the Indian Chemical Society 98 (2021) 100004
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complex at present.
18942–7.
€
~
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Declaration of competing interest
There is no conflict of interest.
Acknowledgment
[6] Tanaka D, Sawai S, Yoon DH, Sekiguchi T, Akitsu T, Shoji S. RSC Adv. 2017;7:
39576 (and references therein).
[7] Tanaka D, Sawai S, Hattori S, Nozaki Y, Hyun Yoon D, Fujita H, Sekiguchi T, Shoji T
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[9] Miyagawa Y, Tsatsuryan A, Haraguchi T, Shcherbakov I, Akitsu T. New J. Chem.
2020;44:16665.
The authors thank Ms. Yuika Onami at Tokyo University of science for
her help of X-ray crystallography. This work was partly supported by
Grant-in-Aid for Scientific Research (A) KAKENHI (20H00336). This
work was also performed under the Cooperative Research Program of
“Network Joint Research Center for Materials and Devices”.
[10] Tsaturyan A, Machida Y, Akitsu T, Gozhikova I, Shcherbakov I. J. Mol. Struct. 2018;
1162:54.
[11] Howard JL, Cao Q, Browne DL. Chem. Sci. 2018;9:3080.
[12] Kaupp G. CrystEngComm 2003;5:117 (and references therein).
[13] Ferguson M, Giri N, Huang X, Apperley D, James SL. Green Chem. 2014;16:1374.
[14] Yamamoto S, Akitsu T. Asian Chem. Lett. 2011;15:203.
[15] Nakayama T, Minemoto M, Nishizuru H, Akitsu T. Asian Chem. Lett. 2011;15:215.
[16] Watanabe Y, Akitsu T. Asian Chem. Lett. 2012;16:9.
[17] Kurata M, Yoshida N, Fukunaga S, Akitsu T. Contemporary Eng. Sci. 2013;6:255.
[18] Takeshita Y, Nogami A, Akitsu T. World Sci. Echo 2014;1:20.
[19] Takeshita Y, Takakura K, Akitsu T. Int. J. Mol. Sci. 2015;16:3955.
[20] Takeshita Y, Akitsu T. Pure and Appl. Chem. Sci. 2015;3:11.
[21] Sheldrick GM. Acta Crystallogr. A. 2008;64:112.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.jics.2021.100004.
[22] Katsuumi N, Onami Y, Pradhan S, Haraguchi T, Akitsu T. Acta Crystallogr. 2020;
E76:1539.
[23] Kashiwagi K, Tassinari F, Haraguchi T, Banerjee-Gosh K, Akitsu T, Naaman R.
Symmetry 2020;12:808.
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