Luminescent and Magnetic Properties of Coordination Polymers Induced by Coordinating Modes of a Bis(oxamate) Ligand

Article abstract of DOI:10.1002/cplu.201800470

The reactions of an unexploited N,N′-5-carboxyl-1,3-phenylenebis(ethyl oxamate) (HEt₂L) ligand with different Mn²⁺ and Cd²⁺ salts under solvothermal conditions afforded four three-dimensional (3D) coordination polymers (CP

Full text of DOI:10.1002/cplu.201800470

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Accepted Article
Title: Structural Diversity of CPs Induced by Coordinating Modes of a
New Bis(oxamate) Ligand: Luminescent and Magnetic Properties
Authors: Lei Hou, Li-Na Ma, Xiu-Yuan Li, Yong-Zhi Li, Wen-Juan Shi,
Ge Liu, and Yao-Yu Wang
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Structural Diversity of Coordination Polymers Induced by
Coordinating Modes of a New Bis(oxamate) Ligand: Luminescent
and Magnetic Properties
Li-Na Ma,[a] Xiu-Yuan Li,[a] Wen-Juan Shi,*[a] Yong-Zhi Li,[a] Ge Liu,[a] Lei Hou,*[a] and Yao-Yu Wang[a]
transmit metal ions. An oxamate group with two carboxylate O
atoms and one amide O atom can connect metal ions through
adopting monodentate, bidentate and tridentate fashions.[8]
Meanwhile, by introducing different substituents, such as pyridyl
and phenyl groups, on -NH2 group in oxamic acid, a variety of N-
substituent oxamic acid ligands were obtained, including N,N’-
1,3-phenylenebis(oxamic acid),[9] N,N’-1,4-phenylenebis(oxamic
acid)[10] and N,N’-2,6-pyridylbis(oxamic acid),[11] which were well
studied as organic linkers for the fabrication of functional CPs
with various architectures.
Abstract: The reactions of an unexploited N,N’-5-carboxyl-1,3-
phenylenebis(ethyl oxamate) (HEt2L) ligand with different Mn2+ and
Cd2+ salts under solvothermal conditions afforded four three-
dimensional (3D) coordination polymers (CPs): [Mn1.5L(H2O)2]·H2O
(1) [Mn2L(C2O4)0.5(NMP)(H2O)]·NMP (2), [Cd1.5L(H2O)2.5]·H2O (3) and
[Cd2L(C2O4)0.5(H2O)3]·H2O (4) (NMP = N-methyl pyrrolidone). L,
generated by the in-situ hydrolysis of HEt2L, displays four kinds of
coordinating modes through oxamate and carboxylate groups as
well as different syn-syn and syn-trans configurations. The resulting
novel (3,4,5)-, (3,4,6)-, (4,5)- and (4,4,4,5)-connected topologies for
1-4 were formed. Among them, 1 has interesting 1D metal-oxamate
zigzag chains, 2 contains 1D channels with a free void of 52.5%, 3
consists of an unprecedented Cd-oxamate layer, 4 exists a novel
Cd-oxamate-oxalate chain. The antiferromagnetic interactions of 1
In this work, we have designed and synthesized a new
oxamate
phenylenebis(oxamic acid) (H3L) (Scheme 1), for CP
constructions. Compared to the known N,N’-1,3-
phenylenebis(oxamic acid), H3L contains an additional 5-
luminescence with the maximum emission peaks at 483 and 479 nm, carboxyl group, therefore providing more coordinating modes.
group-based
ligand,
N,N’-5-carboxyl-1,3-
and
2 were observed, while 3 and 4 reveal the solid-state
respectively, under an excitation of 332 nm.
H3L possesses eight potential coordinating O atoms, which can
adopt chelating and bridging fashions to combine more metal
ions, giving rise to stable frameworks. In addition, according to
the spatial directions of two oxamic acid units about central
phenyl ring, H3L reveals changeable kinds of geometric
configurations of syn-syn, syn-trans and trans-trans. Thus
through tuning itself variability, H3L can adopt very rich fashions
to meet the coordination demand of metal ions in different
environments. Taking on this kind of research will not only give
us further insight into the coordination ability of oxamate ligand,
but also bring structural diversity. As expected, under different
reaction conditions, four distinct 3D CPs: [Mn1.5L(H2O)2]·H2O (1),
[Mn2L(C2O4)0.5(NMP)(H2O)]·NMP (2), [Cd1.5L(H2O)2.5]·H2O (3)
and [Cd2L(C2O4)0.5(H2O)3]·H2O (4), were obtained, in which L
arose from the in-situ hydrolysis of ethyl oxamate groups in
N,N’-5-carboxyl-1,3-phenylenebis(ethyl oxamate) (HEt2L). 1-4
possess varied novel topologies, and wherein L shows various
coordinating modes through oxamate and carboxylate groups as
well as distinct configurations. In addition, 1 and 2 show
antiferromagnetic interactions, while 3 and 4 exhibit intense
luminescence.
Introduction
Coordination polymers (CPs) have recently become a hot spot
for researchers in the field of crystal engineering, not only due to
the variety of interesting architectures but also excellent
properties with potential applications in gas storage and
separation,[1] catalysis,[2] molecular recognition,[3] luminescence,[4]
and magnetic properties.[5] Self-assembly of designed organic
linkers with transition metal ions allows the generation of CPs
with the defined topology and intriguing property.[6] However, this
process in building novel CPs with desired topology still presents
some challenges, mainly because many factors affect the
formation of CPs, such as the ligand shape, reaction
temperature, solvent, pH and so on.[7] A slight change in any one
condition may produce different products. The selection of
ligands and the control of solvent systems play a crucial role.
Therefore the investigation about the influence of reaction
conditions and coordinating configurations/modes on the
structures and properties of CPs is important and desirable.
The oxamic acid that combines carboxylic acid and amide
groups possesses strong coordinating ability with various
[a]
L.-N. Ma, Dr. X.-Y. Li, Dr. W.-J. Shi, Dr. Y.-Z. Li, Prof. L. Hou, Prof.
Y.-Y. Wang. Key Laboratory of Synthetic and Natural Functional
Molecule Chemistry of the Ministry of Education, National
Demonstration Center for Experimental Chemistry Education
(Northwest University), Shaanxi Key Laboratory of Physico-
Inorganic Chemistry, College of Chemistry & Materials Science,
Northwest University
Xi'an 710069, P. R. China
E-mail: swjuan2000@126.com, lhou2009@nwu.edu.cn
Scheme 1. Different configurations between two oxamic acid groups and
phenyl ring in H3L: a) syn-syn, b) syn-trans, c) trans-trans.
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Results and Discussion
Synthesis
HEt2L
was
synthesized
by
condensation
of
ethyl
chloroglyoxalate with 3,5-diaminobenzoic acid in THF. To
produce complexes, the hydrolysis of ethyl oxamate groups in
HEt2L is required. Our initial experiments were failed to obtain
complexes based on the ligands originated from the in-situ
hydrolysis of HEt2L, so the solvothermal synthesis conducted at
a high reaction temperature and pressure was adopted to gain
CPs 1-4. It found that the hydrolysis of HEt2L produces negative
trivalent L anionic ligands in 1-4. Although 1 and 2 (in NMP-H2O)
or 3 and 4 (in DMA-H2O) were prepared in the same solvents,
however, different types of metal salts of MnSO4 and MnCl2 or
CdCl2 and CdCO3 lead to various structures of 1 and 2 or 3 and
4, which can be related to different solubilities of these salts in
reaction solvents to some extent. Interestingly, compared to 1
and 3 obtained at 105 °C, for 2 and 4, the higher temperature of
120 °C causes the in-situ hydrolysis of ethyl oxamate groups in
HEt2L to form not only oxamate groups in L but also oxalate
anions.[14] This observation can be attributed to that the high
temperature effectively promotes the dicleavage of C-N and C-O
bonds of ethyl oxamate groups in HEt2L. In contrast, the similar
experiments performed at a lower temperature of 105 °C almost
cannot generate 2 and 4. As the strong binding ability of oxalate
leads to more Mn2+ and Cd2+ ions were coordinated in 2 and 4,
there is a higher molar ratio of Mn/Cd: L = 2 : 1 in the molecular
formula of 2 and 4 relative to that (Mn/Cd: L = 1.5 : 1) in 1 and 3.
Above facts illustrated that the temperature and metal ion type
play a crucial role to influence the hydrolysis of ethyl oxamate
groups and consequently form various frameworks.
Scheme 2. Synthesis routes of 1-4.
Results and Discussion
Crystal Structure of 1
1 with monoclinic space group C2/c contains one and a half
Mn2+ ions, one L, two coordinating H2O molecules and one
lattice H2O molecule in an asymmetric unit. Both Mn1 and Mn2
are coordinated by six O atoms to form distorted octahedral
geometries (Figure 1a). Mn1 is ligated by two carboxylate and
two oxamate groups of four L. Mn2 is connected with one
carboxylate and two oxamate groups as well as two H2O
molecules. Two oxamate units of L in 1 form a syn-syn
configuration about central phenyl ring, and disclose the same
chelating bidentate and monodentate coordinating modes
through three O atoms (two carboxylate O and one amide O
atoms). The carboxylate group in L shows a monodentate
coordinating mode. Neighboring Mn2 along the b axis are
bridged by oxamates to form a 1D zigzag chain, which was
previously unrecorded in CPs based on metal-oxamate building
Figure 1. a) Coordination environment of Mn2+ ions in 1 (Symmetry codes: #1: 0.5-x,1.5-y,1-z; #2: 1-x,2-y,1-z; #3: 0.5+x,0.5+y,z; #6: x,1-y,-0.5+z), b) 2D layer, c)
3D framework viewed along the b axis, and d) (3,4,5)-connected topology (blue: L, purple: Mn1, green: Mn2).[22]
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Figure 2. a) Coordination environment of Mn2+ ions in 2 (Symmetry codes: #1 = 1+x, y, z; #2 = 0.5+x, 0.5-y, -0.5+z; #3 = 3-x, -y, -z; #4 = -1+x, y, z; #5 = -0.5+x,
0.5-y, 0.5+z, b) 2D layer, c) 3D framework viewed along the a axis, and d) (3,4,6)-connected topology (blue: L, green: Mn1, purple: Mn2). [22]
topological net with an unprecedented point symbol of
blocks. Adjacent chains are interconnected through 1,3-
(43)(43·6 2·8)(46·6 6·8 3) (Figure 2d).
phenylenebis(oxamate) units of L to produce a grid layer parallel
2 contains 1D channels along the a axis with a void of 52.5%
to the bc plane (Figure 1b). The interlayer coordination between
occupied by H2O and NMP molecules (Figure 2c). To obtain the
5-carboxylate groups of L and Mn1 atoms gives rise to a 3D
desolvated framework, we attempted to exchange the bulky
framework (Figure 1c). Each L connecting five Mn2+ ions can be
NMP molecules with other solvents, however, it was failed
viewed as a 5-connected node, while Mn1 and Mn2 can be
because the lattice NMP are locked in channels by H2O and
considered as 4- and 3-connected nodes, respectively. Thus 1
NMP ligands. Alternatively, when a high heating temperature
forms a trinodal (3,4,5)-connected topological net with a new
was adopted to expel the solvents, the obtained sample is not
type of point symbol of (4·82)2(42·8 3·10)(46·8 3·10)2 (Figure 1d).
adsorptive for N2 and CO2, possibly resulting from the framework
distortion or collapse under a harsh activated procedure.
Crystal Structure of 2
2 belongs to monoclinic space group P21/n, and its asymmetric
Crystal Structure of 3
unit is composed of two Mn2+ ions, one L, one coordinating NMP
3 crystallizes in orthorhombic space group Aba2, and consists of
and one lattice NMP, one H2O ligand as well as one and a half
one and a half Cd2+ ions, one L, two and a half coordinating H2O
oxalate anionic ligand that is from the hydrolysis of ethyl
molecules and one lattice H2O molecule in an asymmetric unit.
oxamate groups in HEt2L under a high reaction temperature.[12]
Cd1 is coordinated by seven O atoms from two oxamate groups
Both Mn1 and Mn2 show MnO6 octahedral geometries (Figure
of two L and three H2O molecules (Figure 3a). Cd2 is connected
2a). Mn1 is ligated by one carboxylate and two oxamate groups
by eight O atoms from three oxamate and one carboxylate
of three L in equatorial plane, and H2O and NMP molecules
groups of four L and one H2O molecule. Two oxamate units of L
located in axial apexes. Mn2 is connected by two oxamate and
reveal a syn-trans configuration, and display two kinds of
one carboxylate groups of three L as well as one chelating
coordinating modes: double chelating bidentate, and chelating
oxalate anion. Resembling to L in 1, the oxamate unit of L in 2
bidentate and monodentate, respectively, distinguishing from the
also displays a syn-syn configuration and the similar chelating
situation of L in 1 and 2. The carboxylate group in L shows a
bidentate and monodentate coordinating modes through three O
chelating bidentate mode. Further analysis on 3 observes the
atoms, while the carboxylate group adopts a bridging bidentate
formation of a novel trinuclear Cd3(HNCOCOO)2 cluster based
mode. Each L connects six Mn2+ ions to produce a 2D layer
on two oxamates combining one Cd1 and two Cd2 atoms. The
parallel to the ac plane (Figure 2b). Adjacent layers are jointed
similar clusters were not recorded in previous complexes
together by the interlayer coordination between oxalates and
according to the latest CCDC research. Each cluster is
Mn2 atoms to afford a 3D layer-pillared framework (Figure 2c).
connected by the other type of four oxamates to create an
Mn1, Mn2, and L can be simplified as 3-, 4- and 6-connected
unprecedented Cd-oxamate layer parallel to the ac plane (Figure
nodes, respectively, so 2 forms a trinodal (3,4,6)-connected
3b). The interlayer clusters are further connected by L, resulting
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