Eco-friendly microwave synthesis of Mg(II) phenoxy carboxylic acid coordination compounds with specific motifs driven by multiple hydrogen bonding

Article abstract of DOI:10.1039/c6ra09589e

Four novel Mg(ii) coordination compounds [Mg(CPA)₂(H₂O)₄] 1, [Mg(H₂O)₆]·(MCPA)₂2, [Mg(DCPA)(H₂O)₅]·(DCPA) 3 and [Mg(TCPA)₂(H₂O)₄]·2H₂O 4 were designed by the strategy of hydrogen bonding driven self-assembly (HCPA = 2-chlorophenoxyacetic acid, HMCPA = 2-methyl-4-chlorophenoxyacetic acid, HDCPA = 2,3-dichlorophenoxyacetic acid and HTCPA = 2,4,6-trichlorophenoxyacetic acid). Single-crystal X-ray diffraction demonstrated that each coordination compound had its own specific motif driven by multiple hydrogen bonding: rectangle motif (compound 1), butterfly motif (compound 2), head and tail connected double heart-rectangle motif (compound 3) and alternative one side shared hexagon-rectangle motif (compound 4). The hydrogen bonding effects (O-H?Cl and O-H?O) on thermostability of the title compounds were well proved by TG analysis. The antibacterial activity of 1-4 has been explored, and all of them showed antibacterial efficiency against colibacillus at certain concentrations. The number of chlorides in the ligands seemed to affect the efficiency of the complexes while their position had no obvious effect. In addition, all the title compounds were evaluated for plant growth regulation by using the lettuce seedling growth bioassay. The results showed that these kinds of compounds had potential to serve as natural plant growth regulators because of their higher allelopathic activity. This research firstly applied the effective microwave method to synthesize Mg(ii) phenoxy carboxylic acid coordination compounds. The design strategy of hydrogen-bonding-driven self-assembly presented here gave a novel insight into the further structural prediction and biofunction achievement of Mg(ii) supramolecular assemblies.

Full text of DOI:10.1039/c6ra09589e

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Eco-friendly microwave synthesis of Mg(II) phenoxy
carboxylic acid coordination compounds with
specific motifs driven by multiple hydrogen
bonding†
Cite this: RSC Adv., 2016, 6, 67610
a
a
a
a
b
a
*
Xiuling Xu, Fan Hu, Saisai Yan, Jianguang Lin, Qing Li and Qi Shuai
Four novel Mg(II) coordination compounds [Mg(CPA)2(H2O)4] 1, [Mg(H2O)6]$(MCPA)2 2, [Mg(DCPA)(H2O)5]$(DCPA)
3 and [Mg(TCPA)2(H2O)4]$2H2O 4 were designed by the strategy of hydrogen bonding driven self-
assembly (HCPA ¼ 2-chlorophenoxyacetic acid, HMCPA ¼ 2-methyl-4-chlorophenoxyacetic acid,
HDCPA ¼ 2,3-dichlorophenoxyacetic acid and HTCPA ¼ 2,4,6-trichlorophenoxyacetic acid). Single-
crystal X-ray diffraction demonstrated that each coordination compound had its own specific motif
driven by multiple hydrogen bonding: rectangle motif (compound 1), butterfly motif (compound 2),
head and tail connected double heart-rectangle motif (compound 3) and alternative one side shared
hexagon-rectangle motif (compound 4). The hydrogen bonding effects (O–H/Cl and O–H/O) on
thermostability of the title compounds were well proved by TG analysis. The antibacterial activity of 1–4
has been explored, and all of them showed antibacterial efficiency against colibacillus at certain
concentrations. The number of chlorides in the ligands seemed to affect the efficiency of the
complexes while their position had no obvious effect. In addition, all the title compounds were
evaluated for plant growth regulation by using the lettuce seedling growth bioassay. The results
showed that these kinds of compounds had potential to serve as natural plant growth regulators
because of their higher allelopathic activity. This research firstly applied the effective microwave
method to synthesize Mg(II) phenoxy carboxylic acid coordination compounds. The design strategy of
hydrogen-bonding-driven self-assembly presented here gave a novel insight into the further structural
prediction and biofunction achievement of Mg(II) supramolecular assemblies.
Received 13th April 2016
Accepted 11th July 2016
DOI: 10.1039/c6ra09589e
www.rsc.org/advances
Moore and Winmill in 1912,9 has been proved to be one of the
most widely applied noncovalent interactions to assemble
supramolecular compounds.10–12 Although a great variety of H-
bonding-driven MOSAs with fascinating structures have been
reported so far,13–15 the prediction of desired H-bonding
frameworks still remains a far-reaching challenge because of
the broad ranges of H-bonding lengths and angles.16 To better
understand the packing patterns of MOSAs governed by
multiple H-bonding, more novel supramolecular compounds
with H-bonding interactions should be designed in crystal
engineering.
Introduction
Metal–organic supramolecular assemblies (MOSAs) controlled
through crystal engineering have emerged as a hot topic of
functional materials in application areas like drug delivery,1
molecular sensing,2 light-emitting devices,3 gas storage,4 and
semiconductors.5 Weak interactions such as hydrogen bonding
(H-bonding), p–p stacking, and van der Waals force have been
used to construct a large number of highly ordered supramo-
lecular architectures.6–8 The H-bonding, rstly proposed by
In 2009, Bharadwaj's group synthesized two Mg(II) coordi-
nation polymers based on pyridine-2,4,6-tricarboxylic acid
(ptcH3).17 It is reported that aer hydrothermal reaction at
aShaanxi Key Laboratory of Natural Products & Chemical Biology, College of Science,
Northwest A&F University, Yangling, Shaanxi 712100, People's Republic of China.
E-mail: shuaiqi@nwsuaf.edu.cn; Fax: +86 2987092769; Tel: +86 2987091196
bCollege of Foreign Languages, Qingdao Agricultural University, Qingdao 266109,
ꢀ
140 C for 72 h, Mg(NO3)2$6H2O (1 mmol) reacted with ptcH3
People's Republic of China
(1 mmol) in 5 mL of water, providing {Mg3(ptc)2$8H2O}n directly
from ltration, and {[Mg(ptc)(H2O)2]$1/2[Mg(H2O)6]$H2O}n by
further evaporation of ltrate. We are interested in phenom-
enon that from the same starting materials, just a little differ-
ence in reaction condition, two totally different products were
obtained in this paper. It seems that the intricate H-bonding
† Electronic supplementary information (ESI) available: X-ray crystallographic
les of 1–4 in CIF format.
A
summary of crystallographic data for 1–4.
˚
Additional selected bond lengths and angles for 1–4. Bond lengths (A) and
angles (ꢀ) of hydrogen-bond for 1–4 IR and NMR spectra of 1–4. CCDC number
1411705 for 1, 1411706 for 2, 1411707 for 3 and 1411665 for 4. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c6ra09589e
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arrayed in the two compounds plays an important role in antimicrobial agents and herbicides. It is expected that more
extending supramolecular assemblies with different structures. functional H-bonding frameworks could be predicted by
However, if we explore these compounds in deeper, we would making a deeper research on the relationship between structure
nd that the nature of Mg(II) contributed largely to the and bioactivity.
H-bonding driven of these kinds of MOSAs. As the smallest
group 2 ion with the second highest electronegativity among [Mg(H2O)6]$(MCPA)2 2, [Mg(DCPA)(H2O)5]$(DCPA)
In this work, four novel Mg(II) MOSAs [Mg(CPA)2(H2O)4] 1,
3
and
group elements, Mg(II) has a considerably strong interaction [Mg(TCPA)2(H2O)4]$2H2O 4 with specic motifs driven by
with neutral water molecules, so it forms more stable Mg–OH2 multiple H-bonding were reported. The synthesis method used
bonds than other metals.18,19 The strong affinity between water here was microwave synthesis, which was rstly applied in
and Mg(II) causes the coexistence of water molecules (including Mg(II) phenoxy carboxylic acids coordination compounds, with
coordination water or lattice water) and ligands, therefore the eco-friendly advantages of shorter reaction time, lower-energy
competition reaction to win the center metal Mg(II) between the consumption, and higher product yield.32–37 Using this
ligands and coordination water molecules occurs in water- method instead of the common used hydrothermal synthesis
containing solvent synthesis. More diverse H-bonding driven would inuence the equilibrium by different external stimuli,
MOSAs would be obtained by applying different synthesis which provides more opportunities to achieve structural diverse
methods with various external stimuli such as temperature, pH, Mg(II) coordination polymers. The H-bonding effects (O–H/Cl
or solvent effect to inuence the equilibrium.20–22 In fact, beside and O–H/O) inuence the thermostability of the title colorless
the strong Mg–OH2 interactions facilitated to multiple compounds, which was well proved by the TG analysis. The
H-bonding supramolecular assembles, divalent Mg(II) cation biological activities referring to antimicrobial efficiency and
also possesses other advantages such as low-cost, non-toxic, low plant growth regulation were also evaluated. The design strategy
molecular weight, biocompatibility, large abundances in the presented here gave novel insight into the further structural
earth's crust,23–26 and possible ability to improve the solubility of prediction and biofunction achievement of novel Mg(II) MOSAs
drugs.19,27 These merits would greatly improve the functionality driven by H-bonding.
of Mg(II) MOSAs and enhance their application in various areas.
With the aim to design functional MOSAs directly governed
by H-bonding, we picked up four phenoxyacetic acids of HCPA,
HMCPA, HDCPA and HTCPA to construct MOSAs with Mg(II)
Experimental section
Materials and physical measurements
2-Chlorophenol, 4-chloro-2-methylphenol, 2,3-dichlorophenol,
2,4,6-trichlorophenol and ethyl chloroacetate were purchased
from Aladdin Reagent Co. Ltd. (Shanghai, China). Microwave
synthesis was implemented using a CEM Discover SP single-
mode microwave synthesis reactor. Elemental analyses (C and
H) were performed on a Vario MACRO cube elemental analyzer.
IR spectra was recorded on a BRUKER TENSOR 27 spectro-
photometer within 400–4000 cmꢁ1 using the samples prepared
as pellets with KBr. TG analysis was performed on a SHIMADZU
TA-60ws from 25 to 1000 ꢀC at a heating rate of 10 ꢀC minꢁ1 in
N2 atmosphere. Viability was assessed by absorbance at 600 nm
using an Perlong DNM-9062 microplate reader.
centers in aqueous solution. The presence of exible –OCH2–
spacers together with –Cl substituent groups can cooperate with
water molecules Mg(II) to facilitate supramolecular assemblies
through multiple H-bonding (O–H/O or O–H/Cl). The various
coordination modes of –COOH, even more the changes of the
number or position of –Cl in ligands are benecial to structural
diversity in crystal engineering. What's more, the good antimi-
crobial efficiency28 and herbicidal activity29 of phenoxyacetic
acids can provide synergistic effects with metal centre to attain
the biofunctionality of the target complexes. In 2001, Kessis-
soglou's group gave us the rst example about the study of
antibacterial activity of coordination compounds constructed
by the copper(II) centres and a series of phenoxyalkanoic acids.
It's interesting that ve principle factors such as the chelate Synthesis of the ligand HCPA
effect inuenced on the antimicrobial activity of these copper
The ligands were prepared by the microwave synthesis in the
complexes.30 Subsequently, they continued to report the
manganese metallacrowns complexes based on auxin herbi-
following route (Scheme 1). In a microwave tube, 2-chlor-
ophenol (5 mmol, 642.80 mg) was dissolved in dry N,N-dime-
thylformamide (2 mL) under stirring followed by addition of 2
cides
2,4,5-trichlorophenoxyacetic
acid,
2-methyl-4-
chlorophenoxyacetic acid, and 2,3-dichlorophenoxyacetic acid.
Manganese metallacrown was picked up as the highest anti-
bacterial compound in this literature. The important informa-
tion obtained from Kessissoglou's group is that phenoxyacetic
acids served as a perfect building block to improve the anti-
microbial efficiency of the metal coordination compounds. The
number and position changes of –Cl substituent groups in
ligands seemed to have certain effect on antimicrobial effi-
ciency.31 Promoted by mentions above, we are interested in
designing novel H-bonding driven magnesium MOSAs func-
tionalized by bioactive phenoxyacetic acids with different –Cl
Scheme 1 Microwave synthesis of the ligands. HCPA: X1 ¼ Cl, HMCPA:
substituent groups and further exploring their application as X1 ¼ CH3, X3 ¼ Cl, HDCPA: X1 ¼ X2 ¼ Cl, HTCPA: X1 ¼ X3 ¼ X4 ¼ Cl.
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mL aqueous solution of K2CO3 (691.05 mg, 5 mmol) and KI (166 colorless, hexagon crystals of coordination compound 2 were
mg, 1 mmol). The reaction mixture was allowed to irradiate for 4 obtained directly on the lter paper. Anal. calc. for C18H28-
min at 100 ꢀC under 200 W. Then 5 mL of NaOH (2 M) was MgCl2O12: C, 40.63; H, 5.26%. Found: C, 40.64; H, 5.27%. IR
added to the mixture and irradiated for 5 min at 500 W. The data (cmꢁ1): 3311(b), 1617(s), 1457(s), 1430(s), 1136(s), 1056(s),
resulting solution was acidied (pH ¼ 6) by adding of a 1 M HCl 652(m).
solution under vigorous stirring aer mixture was cooled to
room temperature. Column chromatography provided the title
compound as white solids. Yield: 88%. Found: 1H NMR (DMSO/
TMS, 500 MHz, ppm): d 13.21 (s, 1H), 7.55–7.49 (m, 1H), 7.37
(dd, J ¼ 11.4, 4.4 Hz, 1H), 7.12 (d, J ¼ 8.3 Hz, 1H), 7.06 (t, J ¼ 7.6
Hz, 1H), 4.90 (s, 2H).
Synthesis of [Mg(DCPA)(H2O)5]$(DCPA) 3
The same synthetic procedure as that for 3 was used except that
the HCPA was replaced by HDCPA and the power for microwave
reaction is 110 W. Colorless, block crystals of coordination
compound 3 were obtained aer slowly evaporating the ltrate
at room temperature. Anal. calc. for C16H20MgCl4O11: C, 34.63;
H, 3.60%. Found: C, 34.64; H, 3.61%. IR data (cmꢁ1): 3368(b),
Synthesis of the ligand HMCPA
The procedure was similar to that of HCPA except 4-chloro-2- 1578(s), 1458(s), 1430(s), 1189(s), 1071(s), 698(m).
methylphenol was used instead of 2-chlorophenol. Column
1
chromatography provided white solids. Yield: 86%. Found: H
NMR (DMSO/TMS, 500 MHz, ppm): d 13.14 (s, 1H), 7.32 (d, J ¼
2.2 Hz, 1H), 7.29–7.22 (m, 1H), 6.93 (d, J ¼ 8.8 Hz, 1H), 4.80 (d, J
¼ 12.1 Hz, 2H), 2.28 (s, 3H).
Synthesis of [Mg(TCPA)2(H2O)4]$2H2O 4
The procedure was similar to that of 1 except that HTCPA was
used instead of HCPA. The power of microwave reaction is
150 W. Colorless, block crystals of coordination compound 4
were obtained aer slowly evaporating the ltrate at room
temperature. Anal. calc. for C16H20MgCl6O12: C, 29.94; H, 3.12%.
Synthesis of the ligand HDCPA
The procedure was similar to that of HCPA except 2,3-dichlor- Found: C, 29.93; H, 3.13%. IR data (cmꢁ1): 3279(b), 1734(s),
ophenol was used instead of 2-chlorophenol. Column chroma- 1709(s), 1461(s), 1425(s), 1141(s), 1084(s), 672(m) (Fig. 1).
tography provided white solids. Yield: 91%. Found: 1H NMR
(DMSO/TMS, 500 MHz, ppm): d 13.29 (s, 1H), 7.39 (t, J ¼ 8.2 Hz,
Single-crystal X-ray structure determinations
1H), 7.31 (dd, J ¼ 8.1, 1.3 Hz, 1H), 7.13 (dd, J ¼ 8.4, 1.2 Hz, 1H),
All crystallographic data were collected on a Bruker SMART CCD
area-detector diffractometer at 298(2) K with graphite mono-
4.95 (s, 2H).
˚
chromatized Mo-Ka radiation (l ¼ 0.71073 A) in u scan mode,
Synthesis of the ligand HTCPA
and the data reduction was performed using Bruker SAINT. The
The procedure was similar to that of HCPA except 2,4,6-tri- crystal structures were resolved by direct methods and rened
chlorophenol was used instead of 2-chlorophenol. Column with full-matrix least-squares renement on F2 with anisotropic
1
chromatography provided white solids. Yield: 83%. Found: H displacement parameters for non-H atoms using SHELXTL.38
NMR (DMSO/TMS, 500 MHz, ppm): d 13.25 (s, 1H), 7.81 (s, 2H), All non-hydrogen atoms and hydrogen atoms were rened
4.68 (s, 2H).
geometrically. Structural plots were generated with diamond. A
summary of crystallographic data for 1–4 is summarized in
ꢀ
˚
Table S2.† Bond lengths (A) and angles ( ) of hydrogen-bond for
1–4 are given in Table S3.† Selected bond lengths and angles for
1–4 are given in Table S4.†
Synthesis of [Mg(CPA)2(H2O)4] 1
An appropriate amount of 0.2 mmol NaOH solution was added
dropwise to HCPA ethanol solution under stirring to adjust the
pH value to approximately 7. Then the MgCl2$6H2O (0.1 mmol,
20.33 mg) aqueous solution was added into this mixture under
Determination of biological activity
stirring with the molar ratio of Mg2+ : HCPA ¼ 1 : 2. Finally, the The biological activity was determined against cultures of col-
mixture was transferred into a 25 mL microwave tube. The ibacillus strains. Briey, the colibacillus suspension (90 mL per
mixture was heated by microwave at 100 ꢀC for 1 h at 120 W and well) was prepared in Luria–Bertani culture medium, which was
then cooled to room temperature. The colorless, block crystals dispensed into 96-well plates. Then the bacteria were dispensed
suitable for the X-ray diffraction were obtained directly on the at a volume of 10 mL in triplicate with serial concentrations (2, 4,
lter paper. Anal. calc. for C16H20MgCl2O10: C, 41.06; H, 4.27%. 6, 8, 10 mg mLꢁ1) of four ligands (HCPA, HMCPA, HDCPA,
Found: C, 41.07; H, 4.28%. IR data (cmꢁ1): 3391(b), 1638(s), HTCPA) and coordination compounds 1–4. Aer incubationed
1488(s), 1431(s), 1135(s), 1074(s), 685(s).
in a carbon dioxide incubator at 37 ꢀC in a 5% CO2 atmosphere,
the optical density (OD) value of each well at 600 nm was
measured by a microplate reader aer 15 h.
Synthesis of [Mg(H2O)6]$(MCPA)2 2
A mixture containing HMCPA (0.2 mmol, 0.04 g), MgCl2$6H2O
(0.1 mmol, 20.33 mg), NaOH (0.2 mmol, 8 mg) and deionized
Allelopathic bioassay
water (8 mL) was placed in a microwave reaction tube. The The seeds of lettuce (Lactuca sativa) which belongs of herba-
mixed solution was sealed and heated at 100 ꢀC under 140 W for ceous plants were used for the bioassay. The procedure was
1 h in a microwave reactor. Aer cooling to room temperature, conducted according to the reported protocol.39 The lettuce
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Fig. 2 The coordination environment and coordination polyhedron of
Mg(II) ion in 1.
As is shown in Fig. 3, all the carboxylate groups show the
same coordination mode of monodentate bridging in 1 and
further crosslink the neighboring Mg(II) with coordination
water molecules via O1W–H1WB/O2 and O3W–H3WB/O5
hydrogen bonds, forming the 1D chains with the rectangle-
shaped assembly repeat units. The carboxyl group oxygen
atoms of O2 and O5 serve as acceptors, and the oxygen atoms of
O1W and O3W from coordinated water molecules play a role of
donors in the O–H/O hydrogen bonds for detail. Notably, the
2D layers have been assembled by countless 1D chains, such as
the 2D layer on ac plane extended by the parallel blue, yellow,
and green 1D chains along a axis in the Fig. 4.
Fig. 1 Synthesis of the coordination compounds 1–4.
seeds was washed with running water for 2 h, soaked in 0.5%
KMnO4 for 15 min, and ushed until they were colorless. The
compounds, positive control and blank solvent acetone, were
added to 12-well plates with lter paper to nal concentrations
of 500, 250, 150 and 50 ppm. Aer the evaporation of acetone,
the lettuce seeds were sown in the microdishes of 12-well plates
and irrigated with deionized water. The plates were then incu-
The crystal structure of [Mg(H2O)6]$(MCPA)2 2
X-ray diffraction reveals that 2 is a co-crystal structure and
crystallizes in orthorhombic space group C2/c. The asymmetric
ꢀ
bated at 25 C for 90 h, and the allelopathic effects [response
index (RI)] were calculated according to the following equations:
If T > C, then RI ¼ 1 ꢁ C/T; if T < C, then RI ¼ T/C ꢁ 1
where T is the length of the treatment, C is the length of the
blank control, and RI is the response index. gp stands for
glyphosate and ck for the blank.
Fig. 3 A view of the 1D chain along a axis in 1.
Results and discussion
The crystal structure of [Mg(CPA)2(H2O)4] 1
Single-crystal X-ray diffraction analysis reveals that 1 crystallizes
in monoclinic crystal system, P2(1) space group. The coordi-
nation environment and coordination polyhedron of Mg(II) is
presented with atom numbering scheme (Fig. 2). The asym-
metric unit consists of one crystallographically independent
Mg(II) ion, two CPA ligands and four coordinated water mole-
cules. The Mg(II) ion is six-coordinated with a slightly distorted
octahedral arrangements, formed by two oxygen atoms of
carboxylate groups (O1, O4) from two CPA ligands with a mon-
odentate bridging mode, and four oxygen atoms from coordi-
nated water molecules (O1W, O2W, O3W, O4W). The axial
positions of the octahedron are occupied by O1W and O3W with
O1W–Mg–O3W bond angles of 179.44(9)ꢀ, and the Mg–O bond
˚
distance ranging from 2.0363(16) to 2.1048(18) A. The bond
angles and bond distances are similar to the Mg(II) coordination
compounds reported previously.40
Fig. 4 A view of the 2D layer on ac plane in 1.
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unit contains one crystallographically independent Mg(II) ion,
six coordinated water molecules, and two uncoordinated MCPA
ligands. One Mg(II) ion and six water molecules (O1W, O1WA,
O2W, O2WA, O3W, O3WA) form the Mg(II)–water clusters
[Mg(H2O)6]2+ with the Mg–O distance ranging from 2.031(4) to
˚
2.073(3) A (Fig. 5), which are consistent with the literature re-
ported.41 The co-crystal structure formed by MCPA and
[Mg(H2O)6]2+ is in ratio of 2 : 1. To the best of our knowledge,
such a co-crystal structure involving both Mg(II)–water clusters
and phenoxyacetic acid has not been reported.
Each of Mg(II)–water cluster in 2 further connected the
neighboring deprotonated MCPA ligands by multiple hydrogen-
bond to generate a neutral 1D chain along the c axis with a non-
Fig. 7 A view of the 2D layer on ac plane in 2.
˚
bonding Mg/Mg distance of 7.460 A. The most striking feature
of 1D chain is buttery-shaped metal–ligand extending cross-
linked by hydrogen-bonds, while O2W–H2WB/O3 and O3W–
H3WA/O3 form a pair of bigger wings, O1W–H1WB/O1 and
O3W–H3WB/O2 form the two smaller wings (Fig. 6). Further-
more, adjacent chains stack together to form 2D layers, as is
shown in Fig. 7, three parallel chains such as the blue, yellow,
and green chains self-assembled to the 2D layer on ac plane.
molecules and one uncoordinated ligand in the asymmetric
unit. The coordination environment and coordination poly-
hedron of Mg(II) ion in 3 is shown in Fig. 8. The Mg(II) ions are
also six-coordinated with a distorted octahedron geometry where
the axial positions were occupied by O3W and O1W, and Mg(II)
ions were coordinated by one oxygen atom from one mono-
dentate carboxylate group (O1) and ve oxygen atoms (O1W,
O2W, O3W, O4W, O5W) from the coordinated water molecules.
Bond distances around the central Mg(II) ions vary from 2.035(5)
The crystal structure of [Mg(DCPA)(H2O)5]$(DCPA) 3
˚
˚
to 2.098(6) A, and the average distance is 2.074 A which is in
agreement with other coordination compounds containing
Mg(II) centers.42
Coordination compound 3 crystallizes in the triclinic crystal system
ꢀ
with the space group of P1. There are one crystallographically
independent Mg(II) ion, one DCPA ligand, ve coordinated water
An interesting structural feature for 3 is the formation of 1D
chain by hydrogen-bond interactions involving the uncoordi-
nated DCPA molecules and coordinated water molecules. One
DCPA ligand act as monodentate bridging ligand (Fig. 9, the
yellow part) to coordinate with the Mg(II) center, while another
DCPA ligand (Fig. 9, the dark blue part) has no coordination
interactions with metals. The uncoordinated DCPA molecules
play an important role of balancing the charge, and they also
bridge the neighboring Mg(II) by the formation of hydrogen-
bonds (O2W–H2WB/O4, O3W–H3WA/O4, O3W–H3WB/
O4W) between coordinated water molecules and carboxyl
oxygen atoms. It's worth noting that, coordinated water mole-
cules (O2W) can donate free hydrogen atoms (H2WA) to chlo-
rine atoms of DCPA (the o-position chlorine atoms), then form
Fig. 5 The coordination environment and coordination polyhedron of
Mg(II) ion in 2.
˚
O2W–H2WA/Cl3 hydrogen-bond with a distance of 3.472 A,
which has not been found in 1 and 2. The metal–ligand
extending of 1D chain is head and tail connected double heart-
rectangle-shaped assembly consisted of two Mg(II) centers,
a part of uncoordinated DCPA, hydrogen-bonds of O–H/O and
Fig. 8 The coordination environment and coordination polyhedron of
Mg(II) ion in 3.
Fig. 6 A view of the 1D chain along c axis in 2.
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Fig. 11 The coordination environment and coordination polyhedron
of Mg(II) ion in 4.
Fig. 9 A view of the 1D chain in 3.
O2, O1W–H1WB/O3W, O4W–H4WB/O1, O4W–H4WB/
O1W) to generate a neutral 1D chain with the feature of one side
shared hexagon-rectangle-shaped extending along the a axis in
Fig. 12. Moreover, the 1D chains such as the green, yellow and
blue one further extend into the stable 2D layer represented in
Fig. 13.
O–H/Cl in the red ring of Fig. 9. Adjacent two parallel 1D
chains are further held together to form a 2D layer, as is shown
in Fig. 10, two parallel chains such as the green, yellow chains
inserted with the uncoordinated DCPA self-assembled to the 2D
layer on ab plane.
The crystal structure of [Mg(TCPA)2(H2O)4]$2H2O 4
Comparison of Mg(II) coordination compound 1–4 structures
Coordination compound 4 crystallizes in the monoclinic space
group P2(1)/c. The asymmetric unit contains one crystallo-
graphically independent Mg(II) ion, two TCPA ligands, four
coordinated water molecules and two lattice water molecules.
The Mg(II) ions are six-coordinated by monodentate carboxylate
groups (O4, O1) and four oxygen atoms (O1W, O2W, O3W, O4W)
from the coordinated water molecules. The average Mg–O bond
First of all, all of the Mg(II) ions in 1–4 are six-coordinated and
show distorted octahedron geometry. Secondly, as shown in
molecule formulas [Mg(CPA)2(H2O)4] 1, [Mg(H2O)6]$(MCPA)2
2, [Mg(DCPA)(H2O)5]$(DCPA) 3 and [Mg(TCPA)2(H2O)4]$2H2O
4, both of HCPA and HTCPA ligands have coordination effect
with Mg(II) centers in 1 and 4, while HMCPA ligand in 2 has no
coordination effect with Mg(II) centers, only electrostatic force
could be found among [Mg(H2O)6]2+ units and MCPA units.
Moreover, HDCPA ligand in 3 produces not only the coordi-
nation style but also the lattice style. It could be noticed that
whether the ligands coordinate to the metal centers or not, all
of them have the H-bonding effects with water molecules,
which are benecial to the H-bonding-driven MOSAs. The
distance is 2.076 A as literature reported,43 and the central Mg(II)
˚
ion shows a distorted octahedron geometry where the axial
positions occupied by O3W and O2W (Fig. 11). The carboxylic
groups of TCPA are totally deprotonated and displayed the
monodentate bridging coordination modes.
The neighboring [Mg(TCPA)2(H2O)4]n repeat units further
connected with each other by hydrogen-bond (O1W–H1WA/
Fig. 10 A view of the 2D layer on ab plane in 3.
Fig. 12 A view showing the 1D chain along the a axis in 4.
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Thermogravimetric analysis of coordination compounds 1–4
The TG analysis was performed in N2 atmosphere at a heating
rate of 10 ꢀC minꢁ1 on powder samples of 1–4, and the TG
curves are shown in Fig. 15. The TG results reveal that all of the
four compounds have three weight-loss stages. The rst step
can be attributed to the removal of coordinated water and lattice
water molecules. Upon further heating, the following two
weight losses can be ascribed to the collapse of the frameworks.
The remaining residue corresponds to formation of MgO. The
data of thermal analysis for 1–4 are given in Table S1.† The nal
ꢀ
decomposition temperature is in the order of 730.86 C for 3 >
706.18 ꢀC for 4 > 581.33 ꢀC for 1 > 569.61 ꢀC for 2, which
corresponds to the thermostability of 3 > 4 > 1 > 2. These results
agree quite well with the structural feature. From the structural
analysis above, we can clearly see that even the O–H/O
hydrogen-bonds exist in all the four title compounds, the
O–H/Cl hydrogen-bonds only coexist with O–H/O hydrogen-
Fig. 13 A view of the 2D layer in 4.
O–H/O hydrogen-bonds exist in all the title compounds,
however, only compound 3 has O–H/Cl hydrogen-bond, the
existence of uncoordinated ligands HDCPA in 3 provided o-
position chlorine atoms as acceptor to form the O–H/Cl
hydrogen-bond with coordinated water molecules, which
achieves the H-bonding controlling by perfect steric effects.
Finally, four different supramolecular 1D chains with specic
motifs (Fig. 14) were found in 1–4. They are the rectangle-
shaped motif for 1, buttery-shaped motif for 2, head and
tail connected double heart-rectangle-shaped motif for 3 and
one side shared hexagon-rectangle-shaped motif for 4. All the
motifs are governed by multiple H-bonding, which displays
successful controlling of H-bonding driven MOSAs in
this work.
Fig. 15 TG curves of the coordination compounds 1–4.
Fig. 14 The different metal–ligand extending of 1D chains in 1–4.
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bonds in compound 3, resulting in the more stable structure of
3 than 1, 2 and 4. The O–H/O hydrogen-bonds in 4 are more
complex than 1, so the thermostability of 4 is higher than 1.
Mg(II) ions in 2 have not coordinated with MCPA ligand, and the
weak interaction forces between them resulting in the lowest
thermostability of 2.
Evaluation of allelopathic activity
All isolated compounds were evaluated for allelopathic activity
against lettuce (Lactuca sativa) seeds by determining the seed-
ling growth (root elongation) with respect to the control of
glyphosate, which is a broad-spectrum systemic herbicide. The
RI was selected as an evaluation indicator, which ranges from
ꢁ1 to 1, with positive values indicating stimulation by the
treatments and negative values indicating inhibition by them.
As shown in Table 1, all of the RI values for the ligands and
compounds 1–4 (RI values range of ꢁ0.42 to ꢁ0.99) were
negative, which showed higher inhibit ability referencing to the
positive control glyphosate (RI values of ꢁ0.40 at 50 ppm and
ꢁ0.90 at 500 ppm). For compound 1, at high concentrations, it
can cause signicant roots elongation which is almost equal to
HCPA ligands. Compound 2 showed the prominent inhibition
of shoot root elongation (ꢁ0.81 to ꢁ0.91) which is equivalent to
HMCPA ligand. Compounds 3 and 4 have obvious inhibiting
ability for lettuce, which are higher than their ligands. These
results revealed that compound 1–4 could be identied as
potential natural plant growth regulator.
Antibacterial activity
Antibacterial assays against colibacillus showed that the four
coordination compounds 1–4 exhibited a moderate activity. As
is shown in Fig. 16, the x-axis represents the concentration of
drugs range from 2–10 mg mLꢁ1. The y-axis represents OD value
while the shorter bars correspond to higher activity. The z-axis
represents ligands and their coordination compounds use eight
different colours. The results are listed as following: (1) both
ligands and coordination compounds have certain antibacterial
activity against colibacillus, with the obvious rising trend along
with the increase of drug's concentration. (2) The antibacterial
efficiency of coordination compound 4 is superior to HTCPA
ligand, while this rule is not clear in other three groups. (3) The
total antibacterial efficiency order for 1–4 and the correspond-
ing ligands is: 4 > HTCPA > HDCPA > 3 > HMCPA > 2 > HCPA > 1,
which reveals that the number of the chlorides in the ligands
seemed to affect the efficiency of the complexes while their
position had no obvious effect on it.
Conclusions
Four novel Mg(II) MOSAs based on phenoxyacetic acids were
designed by the strategy of H-bonding driven self-assembly.
Specic motifs driven by multiple H-bonding dominate
greatly to the formation of 1D chains in 1–4, that is: the
rectangle-shaped motif in 1, the buttery-shaped motif in 2, the
head and tail connected double heart-rectangle-shaped motif in
3 and the one side shared hexagon-rectangle-shaped motif
appearing alternatively in 4. The co-crystal structure involving
both Mg(II)–water clusters [Mg(H2O)6]2+ and MCPA in 2 was
rstly reported in this work. From the structural analysis, it can
be clearly seen that even the O–H/O hydrogen-bonds exist in
all the four title compounds, the O–H/Cl hydrogen-bonds only
coexist with O–H/O hydrogen-bonds in compound 3, these
H-bonding (O–H/Cl and O–H/O) inuence the thermosta-
bility of the title compounds, which is well proved by the TG
analysis. Moreover, the total antibacterial efficiency order
against colibacillus for 1–4 and the corresponding ligands is: 4 >
HTCPA > HDCPA > 3 > HMCPA > 2 > HCPA > 1, which would be
affected by the number of the chlorides in the ligands. Allelo-
pathic bioassay test proved that compound 1–4 can be identi-
ed as potential natural plant growth regulators because they
can inhibit the growth of lettuce (Lactuca sativa). Notably, the
synthesis method used here was eco-friendly microwave
synthesis, which was for the rst applied in Mg(II) phenoxy
Fig. 16 Antibacterial activity of the coordination compounds 1–4 and
corresponding ligands.
Table 1 Allelopathic effects on lettuce (Lactuca sativa)
Ligands
50 ppm
150 ppm
250 ppm
500 ppm
Compounds
50 ppm
150 ppm
250 ppm
500 ppm
CPA
ꢁ0.52
ꢁ0.81
ꢁ0.62
ꢁ0.62
ꢁ0.4
ꢁ0.68
ꢁ0.91
ꢁ0.82
ꢁ0.73
ꢁ0.5
ꢁ0.73
ꢁ0.91
ꢁ0.9
ꢁ0.89
ꢁ0.92
ꢁ0.92
ꢁ0.89
ꢁ0.9
1
2
3
4
ꢁ0.42
ꢁ0.81
ꢁ0.86
ꢁ0.91
0
ꢁ0.55
ꢁ0.87
ꢁ0.9
ꢁ0.75
ꢁ0.89
ꢁ0.95
ꢁ0.94
ꢁ0.05
ꢁ0.90
ꢁ0.91
ꢁ0.99
ꢁ0.96
ꢁ0.06
MCPA
DCPA
TCPA
gp
ꢁ0.78
ꢁ0.64
ꢁ0.92
ꢁ0.02
ck
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Acknowledgements
The authors gratefully acknowledge the nancial supports from
the National Natural Science Foundation of China (Grant No.
21473136, Grant No. 21103140), and the PhD. Programs Foun-
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