Efficient synthesis of new asymmetric tripodal ligands using microwave irradiation, and their crystal structures

Article abstract of DOI:10.1039/c4ra07346k

An efficient method for the synthesis of asymmetric tripodal ligands from aryl aldehydes and (2-hydro-xybenzyl-2-pyri-dylmethyl)amine was developed. It has the advantages of short reaction times, high yields and a simple methodology. The title ligands were characterized using a combination of ¹H NMR, ¹³C NMR, FT-IR and H RMS spectroscopies. The structures of some of the ligand were confirmed by single-crystal X-ray diffraction. This journal is

Full text of DOI:10.1039/c4ra07346k

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Efficient synthesis of new asymmetric tripodal
ligands using microwave irradiation, and their
crystal structures†
Cite this: RSC Adv., 2014, 4, 42211
a
a
a
c
Zeli Yuan,ab Xiaoqing Yang, Lei Wang, Jiandong Huang and Gang Wei
Received 20th July 2014
Accepted 15th August 2014
*
*
DOI: 10.1039/c4ra07346k
www.rsc.org/advances
An efficient method for the synthesis of asymmetric tripodal ligands complexes.5 Asymmetric substitution of the pyridyl units by a
from aryl aldehydes and (2-hydro-xybenzyl-2-pyri-dylmethyl)amine phenoxyl unit changes the reactive site,5a and the selective
was developed. It has the advantages of short reaction times, high introduction of substituents on different units enables ne
yields and a simple methodology. The title ligands were characterized control of the steric and electronic properties of the ligand.
using a combination of 1H NMR, 13C NMR, FT-IR and H RMS spec- However, previously reported asymmetric methods suffer from
troscopies. The structures of some of the ligand were confirmed by drawbacks such as long reaction times, low yields, high
single-crystal X-ray diffraction.
temperature, and large numbers of side products. The devel-
opment of a new and effective methods for introducing asym-
metry into tripodal ligands using inexpensive and
environmentally friendly reagents is therefore needed. To the
best of our knowledge, there have been few reports on the
synthesis of asymmetric nitrogen- and oxygen-rich tripodal
ligands.5a,6 Here, we focus on asymmetric tripodal ligands in
which one ligand arm differs from the others, introducing the
possibility of geometric isomers.
Organic reactions assisted by microwave irradiation have
attracted considerable attention in recent years.7 Modern
scientic microwave equipment can be used to control many
reaction parameters such as temperature, pressure and reaction
times accurately. Numerous microwave-assisted organic reac-
tions have been performed,8 including Michael additions,
acylation and alkylation reactions, condensations, enzymatic
catalysis, rearrangements, oxidations and reductions, and
regioselective cycloadditions.
Nitrogen-rich tripodal ligands and their derivatives form stable
complexes with a wide range of metal ions and their study
continues to be of interest to researchers.1 Their versatile
coordination modes lead to diverse roles in biological, catalytic
and photoactive applications. These ligands have been investi-
gated for use as catalysts in the sequential oxidation and
asymmetric alkylation of alcohols by Ru3+ complexes.2 They
have also been used as photochemotherapeutic agents in
nuclear medicine, using Fe3+, Cu2+ or V4+ chelates;3 some of the
ligands have been linked to suitable photosensitizer molecules,
such as anthracenyl or ferrocene for activation by near FT-IR
light, to enable greater tissue penetration.
Numerous complexes containing the basic N,N-bis-(2-pyr-
idylmethyl)amine (DPA, Fig. 1) tripodal ligand has been
included, and metal complexes containing this ligand have
been synthesized, and metal complexes containing this ligand
exhibit a wide variety of unusual physical and chemical prop-
erties.4 The denticity and donor types of these ligands deter-
mine the structures and reactivities of the corresponding metal
The commonest of the method for the tripodal ligand
synthesis is alkylation of DPA with 9-(chloromethyl)anthracene,
using triethylamine as base.9 Usually, a large excess of one
reagent and a long reaction time are needed to ensure high
yields and to avoid undesired by-products. However, the
aCollege of Chemistry, Fuzhou University, Fuzhou 350108, P. R. China. E-mail:
jdhuang@fzu.edu.cn; Fax: +86 591 22866227
bSchool of Pharmacy, Zunyi Medical University, Zunyi, Guizhou, 563003, P. R. China.
E-mail: zlyuan@zmc.edu.cn; Fax: +86 8528609343
cCSIRO Materials Science and Engineering, PO Box 218, Lindeld, NSW 2070,
Australia. E-mail: gang.wei@csiro.au
† Electronic supplementary information (ESI) available: Full experimental
procedures and data. CCDC 966462, 966463 and 966464. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra07346k
Fig. 1 Structures of DPA and HBPA.
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Table 2 The reaction of HBPA with some aromatic aldehydea
preparation of 9-(chloromethyl)anthracene is inconvenient and
expensive, and silica gel column chroma-tography using CH2Cl2
as eluant is used for product purication.
Secondary amines react readily with aromatic aldehydes to
form enamines, which can then be reduced with appropriate
reductant; this is an atom economic reaction,10 and therefore is
in line with the philosophy of green chemistry. We therefore
used this reaction, with microwave irradiation as the heat
source, to prepare new asymmetric tripodal ligands.
Entry
1
RCHO
Color
White
Time (min)
30
Yieldb (%)
83
The desired product was isolated in high yield from the
reaction using methanol (5 mL) as the solvent and NaBH3CN as
ꢀ
a reductant, under microwave irradiation (60 C, 300 W initial
power) for 15 time (Table 1, entry 1). Further investigation
showed that increasing the temperature to 70 or 80 ꢀC (Table 1,
entries 2 and 3) did not signicantly changes the yield. When
the reaction time and power were reduced (5 min and 100 W),
lower yields were obtained (Table 1, entries 4 to 7). The
optimum reaction time and power for synthesizing 5 were
therefore 15 min and 300 W, respectively. When NaBH4 and
NaBH(OAC)3 was used as reductant under the same conditions,
the product was not obtained (Table 1, entries 8 and 9). Addi-
tionally, increasing the material to 10 mmol or 20 mmol led to
minor decrease in the yield (Table 1, entries 10 and 11).
However, the yields were still satisfactory.
Based on the above results and our intention to us a green
approach in this study, we performed the reaction using a
shorter reaction time and less solvents, an almost quantitative
yield was obtained (Table 1, entry 1). Having established the
optimum conditions for our reaction, a series of aromatic
aldehydes were tested in tripodal ligand synthesis of aromatic
2
White
30
89
3
4
5
White
20
15
15
94
93
97
Yellow
Pale yellow
6
Pale white
Pale white
15
50
96
90
aldehydes
and
(2-hydro-xybenzyl-2-pyri-dylmethyl)amine
(HBPA) at 300 W in methanol (Table 2). Polycyclic aromatic
7
Table 1 The reaction of HBPA with 9-anthracene formaldehydea
a
Conditions: HBPA (5 mmol), aromatic aldehyde (5 mmol), and
methanol (5 mL). b Yield of isolated yields.
aldehydes gave the corresponding products in high yields
(Table 2, entries 3 to 7).
The asymmetric tripodal ligands were analyzed by 1H and
13C NMR, FT-IR and HRM spectra (Fig. S4–S31 in the ESI†).
Single crystals of the 3–5 suitable for X-ray diffraction, was
obtained in 1 week by diethyl ether vapor diffusion into
CHCl3 solution. The molecular structures of 3–5 are shown in
Fig. 2. Crystal data, structure renement details, and
hydrogen bond interaction are listed in Tables S1–S3 and
Fig. S1–S3 in ESI.†
Entry
Time (min)
Temp (ꢀC)
Power (W)
Yieldf (%)
1b
15
15
15
5
10
15
15
15
15
15
15
60
70
80
60
60
60
60
60
60
60
60
300
300
300
300
300
100
200
300
300
300
300
97
97
97
NR
78
NR
56
NR
NR
95
2b
3b
4b
5b
6b
7b
The ligand structures belong to monoclinic space group,
P21/c. All bond distances and angles are all within the normal
ranges. There are two intramolecular hydrogen bonds,
between the phenol and pyridine or the tertiary amine
present in the crystal lattices: (a) phenol ring and corre-
8c
9d
10b,e
11b,f
94
a
b
c
Yield of isolated yields. NaBHCN3 as a reductant. NaBH4 as a
ꢀ
˚
d
e
sponding tertiary amine (O1–H/N2 ¼ 152.04 , 2.811 A), and
reductant. NaBH(OAC)3 as a reductant. 10 mmol of materials for
each other. f 20 mmol of material for each other.
ꢀ
˚
phenol ring and pyridyl ring (O1–H/N1 ¼ 133.64 , 3.121 A),
42212 | RSC Adv., 2014, 4, 42211–42214
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Acknowledgements
We thank the Natural Science Foundation of China (Grant no.
21172037), SRFDPHEC (Grant no. 201135141001), and the
Natural Science Foundation of Fujian, China (Grant no.
2011J0ss1040) for nancial support.
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42214 | RSC Adv., 2014, 4, 42211–42214
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