Solvent-assisted construction of diverse Mg-TDC coordination polymers

Article abstract of DOI:10.1039/c4ce02288b

Upon alteration of selected solvents, the reactions of 2,5-thiophenedicarboxylic acid (H₂TDC) and Mg(NO₃)₂·6H₂O afforded four diverse coordination polymers, namely Mg₂(TDC)₂(EG)_2.5·0.5EG (1), Mg(TDC)(DMSO) (2), (Me₂NH₂)[Mg₂(TDC)₂(Ac)]·1.5DMA·0.5H₂O (3), and [Mg₂(TDC)₂(DMF)₂(EtOH)(H₂O)₂]·DMF (4) (EG = ethylene glycol, DMSO = dimethyl sulfoxide, Ac = acetate anion, DMA = N,N′-dimethylacetamide, DMF = N,N′-dimethylformamide, EtOH = ethanol). Single-crystal X-ray diffraction analyses indicated that compounds 1-3 possessed a three-dimensional (3D) network while 4 adopted a two-dimensional (2D) layered structure. Noticeably, the coordinated solvent molecules adopt distinct coordination modes which play a vital role in constructing the Mg-TDC structures. In 1, the solvent EG molecules as bi-dentate bridging ligands help in interconnecting the Mg-TDC layers to a 3D framework, whereas the solvent DMSO molecules in 2 and the Ac^- anions generated from decomposition of DMA in 3 are coordinated in a μ₂-fashion and in a (k²-k²)-μ₃ mode, respectively, leading to infinite chains as secondary building units. In the layered structure of 4, the collaboration of coordinated DMF, EtOH and H₂O molecules assists in inducing the noncentrosymmetric structure. The compounds were fully characterized by PXRD, TGA, EA, and IR. The luminescence properties of 1-3 and the second-harmonic generation (SHG) properties of 4 were studied. 3 emitted bright green light upon the excitation of 365 nm UV light, while 4 displayed sound SHG response.

Full text of DOI:10.1039/c4ce02288b

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ARTICLE
Solvent-assisted Construction of Diverse Mg-
TDC Coordination Polymers
Cite this: DOI: 10.1039/x0xx00000x
Ying Song,ab Mei-Ling Feng,a Zhao-Feng Wuab and Xiao-Ying Huang*a
Received 00th January 2012,
Accepted 00th January 2012
Upon alteration of selected solvents, the reactions of 2,5-thiophenedicarboxylic acid (H2TDC)
and
Mg2(TDC)2(EG)2.5∙0.5EG (
), and [Mg2(TDC)2(DMF)2(EtOH)(H2O)2]∙DMF (4
Mg(NO3)2∙6H2O
afforded
four
diverse
), (Me2NH2)[(Mg2(TDC)2(Ac)]∙1.5DMA∙0.5H2O
) (EG = ethylene glycol, DMSO = dimethyl
coordination
polymers,
namely
DOI: 10.1039/x0xx00000x
1
), Mg(TDC)(DMSO) (
2
(
3
www.rsc.org/
sulfoxide, Ac = acetate anion, DMA = N, N’-dimethyl-acetamide, DMF = N, N’-dimethyl-
formamide, EtOH = ethanol). Single-crystal X-ray diffraction analyses indicated that compounds
1-3 possessed a three-dimensional (3D) network while
structure. Noticeably, the coordinated solvent molecules adopt distinct coordination modes which
play a vital role in constructing the Mg-TDC structures. In , the solvent EG molecules as bi-
dentate bridging ligand help interconnecting the Mg-TDC layers to a 3D framework. Whereas the
solvent DMSO molecules in are
and the Ac- anions generated from decomposition of DMA in
coordinated in a μ2-fashion and (k2-k2)-μ3 mode, respectively, leading to infinite chains as
secondary building units. In the layered structure of , the collaboration of coordinated DMF,
4 adopted a two-dimensional (2D) layered
1
2
3
4
EtOH and H2O molecules assists in inducing the noncentrosymmetric structure. The compounds
were fully characterized by PXRD, TGA, EA, IR and the luminescent properties of 1-3 and the
second-harmonic generation (SHG) property of
4 were studied. 3 emitted bright green light upon
the excitation of 365 nm UV light. While displayed sound SHG response.
4
as a guest being encapsulated in structural lattice to affect the
structural construction indirectly,6 or as a ligand that has a
significant impact on secondary building unit (SBU) assembly
and even on structural dimensionality7 and framework
topology.8 In addition, some acyl-amide solvents such as DMF,
DMA and DEF (N, N’-diethyl-formamide) may undergo in-situ
decomposition during the reaction process to produce small
ions which may also serve as a coordinated ligand and/or
charge-compensating agent in the resulting structure.9 Though
the Mg(II) ion has similar coordination nature with first-row
transition metal ions such as Co(II) and Zn(II), its coordination
bonds are of relatively more ionic nature.3b Indeed, the Mg(II)
ion is easily coordinated by polar solvent molecules with
oxygen donors. As a result, the variation of solvent molecules
may exert a significant effect on the formation of different Mg-
CPs.3b, 10
Introduction
The modular nature and crystallinity of coordination polymers
(CPs) facilitate the research of their structures at a molecular
level.1 Due to the wide choice of metal nodes and organic
linkers, various CPs with diverse structures and properties have
been synthesized and investigated.2 The majority of CPs is
focused on those based on the d-block, p-block and rare earth
metal ions, whereas the CPs based on s-block metal ions such
as Mg2+ were less explored.3 In fact, the Mg2+ has many
outstanding merits for constructing CPs; for instance, it has
light molecular mass and diverse coordination modes, and it is
eco-friendly and abundant in the earth’s crust. Recently, the
Mg-CPs have shown some excellent properties that are superior
to those of the CPs based on other metal ions;4 for example, a
Mg2(DOT) (DOT
= 2,5-dioxidoterephthalate) CP shows
excellent CO2 adsorption capacity outperforming its isomers
based on Zn(II), Co(II) and Ni(II) ions,4a and it can sufficiently
capture CO2 from CH4 which is essential for natural gas
purification.4b
The solvent used in the synthesis of CPs is a crucial
parameter which governs reaction kinetics and thermodynamics
during the coordination process.5 The solvent molecule can act
In this work, we attempted to exploit the different roles of
solvent molecules in the construction of Mg-CPs. By reacting
Mg(NO3)2∙6H2O with 2,5-thiophenedicarboxylic acid (H2TDC)
in the selected solvents, we isolated four Mg-TDC CPs with
varied structures, namely Mg2(TDC)2(EG)2.5∙0.5EG
Mg(TDC)(DMSO)
(
(
1
2
),
),
(Me2NH2)[(Mg2(TDC)2(Ac)]∙1.5DMA∙0.5H2O
(
3
),
and
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[Mg2(TDC)2(DMF)2(EtOH)(H2O)2]∙DMF (4) (EG = ethylene
Mg2(TDC)2(EG)2.5∙0.5EG (1). The reaction of a mixture of
glycol, DMSO = dimethyl sulfoxide, Ac = acetate anion, DMA 0.251 g Mg(NO3)2∙6H2O, 0.257 g 2,5-thiophenedicarboxylic
N, N’-dimethyl-acetamide, DMF N, N’-dimethyl- acid, 3 mL ethylene glycol and 3 mL methyl cyanide resulted in
formamide, EtOH = ethanol). In , the bi-dentate EG molecule brown bipyramidal crystals; yield 0.135 g, 46%. Elemental
=
=
1
as a bridging ligand participates in interlinking the Mg-TDC analysis (%): calc. for C18H22O14S2Mg2: C, 37.59%; H, 3.86%;
layers to three-dimensional (3D) framework, whereas the μ2- Found: C, 36.99%; H, 3.90%.
DMSO molecule in
2
and the Ac- anion with a (k2-k2)-μ3
Mg(TDC)(DMSO) (2). The reaction of a mixture of 0.261 g
fashion in contribute to constructing the one-dimensional (1D) Mg(NO3)2∙6H2O, 0.174 g 2,5-thiophenedicarboxylic acid, 4 mL
3
chain-like SBUs of the structures; finally the coexistence of dimethyl sulfoxide and 2 mL methanol resulted in colorless
multiple coordinated solvent molecules including DMF, EtOH block-like crystals; yield 0.150 g, 55%. Elemental analysis (%):
and H2O brings about symmetry-breaking, leading to the calc. for C8H8O5S2Mg: C, 35.25%; H, 2.96%; Found: C,
noncentrosymmetric (NCS) structure of
response.
4
with sound SHG 35.09%; H, 3.15%.
(Me2NH2)[(Mg2(TDC)2(Ac)]∙1.5DMA∙0.5H2O
mixture of 0.101 Mg(NO3)2∙6H2O, 0.067
(3)
g
.
A
2,5-
g
thiophenedicarboxylic acid and 4 mL N, N’-dimethylacetamide
was heated at 150 ˚C for 3 days follow by programmed cooling
in a rate of 0.67 ˚C/min to 30 ˚C. Light-yellow quadrangular
crystals were resulted in; yield 0.210 g, 42%. Elemental
analysis (%): calc. For C22H29.5N2.5O12S2Mg2: C, 41.69%; H,
4.69%; N, 5.52%; Found: C, 41.63%; H, 5.25%; N, 6.03%.
[Mg2(TDC)2(DMF)2(EtOH)(H2O)2]∙DMF (4). A mixture of
0.252 g Mg(NO3)2∙6H2O, 0.174 g 2,5-thiophenedicarboxylic
acid, 5 mL N, N’-dimethylformamide, 1 mL ethanol, 0.5 mL
distilled water was heated at 100 ˚C for 3.5 days followed by a
programmed cooling at a rate of 0.39 ˚C/min to 30 ˚C.
Colorless block-like crystals were obtained; yield 0.180 g, 52%.
Elemental analysis (%): calc. for C23H34N3O14S2Mg2: C,
40.08%; H, 4.97%; N, 6.10%; Found: C, 39.33%; H, 4.89%; N,
6.11%.
Experimental
Materials and physical measurements
All analytical grade chemicals employed in this study were
commercially available and used without further purification.
Powder X-ray diffraction (PXRD) patterns were recorded in the
angular range of 2θ = 3-65° on a Miniflex II diffractometer
using CuKα radiation. Thermogravimetric analyses were
carried out with a NETZSCH STA 449F3 unit at a heating rate
of 10 ˚C /min under a nitrogen atmosphere. The TG-MS was
carried out on a STA449C-QMS403C. Elemental analyses (EA)
for C, H, N were performed on a German Elementary Vario EL
III instrument. Fourier transform infrared (FT-IR) spectra were
taken on a Nicolet Magna 750 FT-IR spectrometer in the 4000–
400 cm-1 region by using KBr pellets. The fluorescence spectra
were recorded using an Edinburgh FLS920 fluorescence
spectrophotometer at room temperature. The powder Second-
Harmonic Generation (SHG) response was measured on
polycrystalline samples by using the experimental method
adapted from that reported by Kurtz and Perry.11 1064 nm
radiation generated by a Q-switched Nd:YAG solid-state laser
was used as the fundamental frequency light. The sample was
ground and then pressed into a disk with diameter of 8 mm
which was then put between glass microscope slides and
secured with tape in a 1 mm thick aluminum holder. The
second harmonic signals were compared with that of standard
nonlinear optical material potassium dihydrogen phosphate
(KDP = KH2PO4) to determine the relative SHG efficiency of 4.
Syntheses of compounds 1-4
All the four compounds were synthesized by solvothermal
methods. The reaction conditions for obtaining compounds
and are similar to each other with alteration of the solvents
used. Typically, mixture of Mg(NO3)2∙6H2O, 2,5-
1, 2
3
a
thiophenedicarboxylic acid and solvent was sealed in a stainless
steel reactor with a 20 (or 28) mL Teflon liner and heated in the
oven at 150 ˚C for 3 days, followed by cooling to room
temperature. The resulting crystals of 1-4 were selected by hand
and washed with anhydrous ethanol followed by drying in the
air. The yields were calculated based on Mg(NO3)2∙6H2O.
Fig. 1 Photographs of crystals of 1-4 under an optical microscope.
Single–crystal structure determination
Suitable single crystals of compounds 1-4 were carefully
selected under an optical microscope and glued to thin glass
fibres. The photographs of selected crystals for compounds 1-4
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are illustrated in Fig. 1. Data collections were performed on an compound
3
, the guest solvent molecules and cations were
Oxford Xcalibur Eos diffractometer with graphite- highly disordered, thus the PLATON/SQUEEZE13 was
monochromated MoKα radiation (λ = 0.71073 Å) for
room temperature and on a SuperNova CCD diffractometer the solvent region and thereby produced a set of solvent-free
with graphite-monochromated CuKα radiation (λ = 1.5418 Å) diffraction intensities. The final formula of was calculated
for and at 100 K, respectively. The structures were solved from the SQUEEZE results combined with elemental analysis
by direct methods and refined by full-matrix least-squares on F2 data and TGA-MS data (Fig. S1). In
the terminal DMF
1 and 4 at employed to calculate the contributions to the diffraction from
3
2
3
4
by using the program package SHELX-97.12 All non-hydrogen molecule was disordered over two positions with refined S.O.F.
atoms were refined anisotropically. The positions of hydrogen ratio of 0.659(9)/0.341(9), and the half of free DMF molecule
atoms were generated geometrically with assigned isotropic was disordered with S.O.F. of 0.5 (the O9 atom rides on a
thermal parameters, and allowed to ride on their respective mirror plane). The soft restraints of DFIX, SIMU, SADI, ISOR,
parent atoms before the final cycles of least–squares DELU were applied on the disordered atoms to keep their
refinements. In
1
, the C and H atoms on the half bridging EG geometries and atomic displacement parameters reasonable.
, and were confirmed by the
molecule and one of the terminal EG molecules were The empirical formulae of
1
,
2
4
disordered over two positions with refined S.O.F. ratios of thermo-gravimetric analyses (TGA) and element analyses (EA)
0.787(16)/0.213(16) and 0.537(9)/0.463(9), respectively. results. Details of crystallographic data and structural
Whereas in
2
, the S, C and H atoms of the four unique DMSO refinement parameters are summarized in Table 1. CCDC nos.
), 1032294 ( ), 1032295 ( ) and 1032296 (
molecules were all disordered over two positions with refined 1032293 (
1
2
3
4)
S.O.F. ratios of 0.866(4)/0.134(4), 0.737(5)/0.263(5), contain the supplementary crystallographic data for this paper.
0.867(4)/0.133(4) and 0.766(5)/0.234(5), respectively. For
Table 1 Crystallographic data for 1-4.
1
C18H22Mg2O14S2
575.10
triclinic
P-1
9.6484(6)
11.7939(7)
11.8603(5)
73.671(4)
67.426(5)
74.356(5)
1175.85(11)
2
2
C32H32Mg4O20S8
1090.30
triclinic
P-1
10.6233(3)
14.7624(5)
15.8152(4)
76.453(2)
74.011(2)
70.088(3)
2214.52(11)
2
3
C22H29.5Mg2N2.5O12S2
633.72
tetragonal
P42/nmc
22.2398(1)
22.2398(1)
12.3010(1)
90.00
4
C23H34Mg2N3O14S2
689.27
monoclinic
Cm
Empirical formula
Formula Mass
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
12.9363(6)
15.8580(8)
9.0474(7)
90
120.978(10)
90
90.00
90.00
6084.18(6)
8
γ/°
V/Å3
1591.28(16)
2
Z
T/K
293(2)
/
596
1.624
0.352
100(2)
/
1120
1.635
100(2)
/
2648
1.384
293(2)
-0.07(12)
722
1.439
0.276
Flack parameter
F(000)
ρcalcd/g cm-3
μ/mm-1
4.983
2.530
Measured refls.
Independent refls.
No. of parameters
Rint
9808
4806
357
0.0269
0.0499
0.1238
1.007
23275
9212
705
0.0224
0.0626
0.1585
1.011
17000
3179
134
0.0228
0.0308
0.0898
1.013
3600
2569
269
0.0242
0.0443
0.1087
1.038
R1 (I> 2σ(I))a
wR(F2) (I> 2σ(I))b
GOF
2
[a] R1 = ∑║Fo│–│Fc║/∑│Fo│. [b] wR2 = [∑w(Fo -Fc2)2/∑w(Fo2)2]1/2
.
viscosity of the reaction solution. While
140~150 ˚C in a mixed solvent of DMSO and methanol; at a
higher temperature of 170 ˚C, the DMSO decomposed and the
2 could be isolated at
Results and discussion
Syntheses
crystals of MgSO4 were identified. Noticeably, at 150 ˚C
2
The effects of temperature and solvents on the syntheses of four
compounds have been explored. The various reactions for the
preparation of compounds 1-4 are listed in Table S1. The
could always be obtained in DMSO no matter whether the
auxiliary solvent was presented or not. The auxiliary solvents
could be polar ones such as DMF, ethanol, acetonitrile, and
nonpolar ones such as benzene. However, it was evident that
the presence of auxiliary solvents such as methanol can help
experiments showed that compound
1 could be obtained at
150~160 ˚C in a mixed solvent of EG and acetonitrile. In
addition to EG, an auxiliary solvent with low boiling point
(such as acetonitrile, acetone and methanol) was indispensable
increasing the yield of
2 greatly. To our knowledge, the
acetamide-type solvents like DMF and DMA may decompose
to produce HCOO- (CH3COO-) and [Me2NH2]+ with or without
to obtain the crystals of
1 which might help in decreasing the
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the existence of H2O above a specific temperature (about 100 center), two TDC2- ligands, two terminal coordinated EG
˚C or higher) under solvothermal conditions and this was molecules, half of an EG molecule as bridging ligand and half
further confirmed by the syntheses of
3 and 4. 3 could be of a free EG molecule (each with an inversion center passing
obtained in DMA at 120 ~ 150 ˚C; at 150 ˚C the presence of a through the middle of C-C bond). All the Mg2+ ions adopt
micro amount of H2O and methanol did not affect the formation octahedral geometry, Fig. S2a. The Mg1 located in an inversion
of 3. As for 4
, it could be obtained at 100 ˚C in a mixed solvent center is coordinated by six oxygen atoms from six TDC2-
of DMF, ethanol and H2O, but when the temperature was ligands. Whereas the coordination sphere of Mg2 includes five
increased to 120 ˚C and above the DMF decomposed and a oxygen donors from four TDC2- ligands and one oxygen atom
well-known Mg-formate, namely (Me2NH2)[Mg(HCOO)3] was from an EG molecule that acts as a bridging ligand linking to
obtained.14 Since the H2O existed in
4, our experiments showed another Mg2 ion. The Mg3 also lies in an inversion center; its
that crystals of
solvent system.
4
could not be isolated without water in the coordination mode is quite special in that it is surrounded by
two oxygen atoms from two TDC2- ligands in the apical
positions and four oxygen atoms from four terminal EG
Crystal structure descriptions
molecules in the equatorial plane. Thus, the Mg3 actually can
belongs to the P-1 be regarded as a two-connected node, Fig. S3a. The four
a 3D framework distinct EG molecules existing in the structure can be denoted
Mg2(TDC)2(EG)2.5∙0.5EG (1). Compound
space group. The structure features
1
constructed by bridging solvent EG molecules and two- as EG1, EG2, EG3 and EG4, corresponding to the EG
connected Mg2+ nodes interconnecting Mg-TDC layers, Fig. 2. molecules that act as bridging ligands (EG1), terminal ligands
The asymmetric unit contains one formula unit, including one (EG2 and EG3) and free guest molecules (EG4), Fig. S4.
and two halves of Mg2+ ions (each located in an inversion
Fig. 2 (a) View of a Mg-TDC layer in the ac plane fabricated by TDC2- ligands interconnecting Mg2-Mg1-Mg2 clusters in
the 3D structure along the a axis showing the linking roles of EG1 molecules and Mg3 nodes. The four terminal coordinated EG molecules of Mg3 are omitted for
clarity. (d) View of the 3D structure of along the c axis with free EG molecules located in the voids in a space-filling mode. All the H atoms are omitted for clarity.
1. (b) Simplified net of the layer in 1. (c) View of
1
There are two independent TDC2- ligands denoted as L-I and unit is evident as SBU in a Mg2-Mg1-Mg2 sequence. Then
L-II, respectively. The L-I connects to four Mg2+ ions (Mg1, each [Mg3] unit is interlinked to other four same [Mg3] units by
Mg2, Mg2, Mg3) in a (k1-k1-μ2)-(k1-k1-μ2)-μ4 mode, whereas the TDC2- ligands in a “double-bridge” mode to give rise to a 2D
L-II links to four Mg2+ ions (Mg1, Mg2, Mg1, Mg2) in a (k1-k1- layer along the ac plane, Fig. 2a; the linking modes among the
μ2)-(k1-k2-μ2)-μ4 mode, Fig. S3b. As a result, a tri-nuclear [Mg3] SBUs are further demonstrated as a simplified net in Fig. 2b.
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Then, the adjacent layers are interlinked to a 3D framework by (k1-k1-µ2)-µ4 mode, Fig. S5b. Similarly, the four independent
EG1 molecules that coordinate to two Mg2 ions from two DMSO molecules all bind to two Mg2+ ions in a μ2-fashion, Fig.
adjacent layers and Mg3 ions that bind to two carboxylic O3 3a.
atoms of L-I from two adjacent layers, Fig. 2c. The EG2 and
EG3 molecules are located in the inter-layered spaces and the two (COO)- groups from TDC2- ligands in a “bi-bridge” mode
Along the b axis, adjacent Mg2+ ions are interconnected by
EG4 molecules are filled in the voids along the c axis, Fig. 2d.
and one DMSO molecule in a μ2-fashion, respectively, to form
Interestingly, the EG molecules play different roles
simultaneously in constructing the structure of
1, that is, as
guest molecules (EG4), terminal ligands (EG2 and EG3), and
bridging ligands (EG1). In the reported CPs, there are several
examples in which the EG molecules act as bridging ligands in
the structure where the roles of them can be sorted to two types:
1) to take part in forming SBUs, exemplified by the formation
of 1D chains in [Co3(BDC)3(EG)4]15 and {Na2NiPr(μ4-
ClO4)(μ2-EG))-(μ4-ptc)2(H2O)8}∙4.5H2O}n;16
2)
to
help
increasing structural dimensionality, exemplified by
17
{[Mn(C14H6O6S)(C2H6O2)(H2O)2]∙C4H9NO}n in which the
EG molecules interlink 1D Mn-L chains to 2D layers, and
[Cd(HBTC)(EG)∙8(H2O)]18 in which they act as pillars to link
2D layers of Cd-L to 3D framework. The latter is further
illustrated in
Mg(TDC)(DMSO) (2). The structure of
1.
Fig. 3 (a) The linking mode of DMSO molecules with Mg2+ ions in
2. (b) The
2
belongs to the P-1
space group and features a 3D network constructed by TDC2-
ligands and DMSO molecules interlinking Mg2+ ions, Fig. 3.
The asymmetric unit contains four formula units though the
four independent Mg2+, TDC2- and DMSO all adopt similar
coordination modes. This may probably be due to the difference
in the disordering degree of DMSO molecules. The four unique
Mg2+ ions are all six-coordinated (Fig. S2b) by four TDC2-
ligands in the equatorial plane and two DMSO molecules in the
apical positions, which differ slightly in Mg-Ocarboxylate bond
lengths and O-Mg-O angles from each other, Fig. S5a. While
the four unique TDC2- ligands all adopt the common (k1-k1-μ2)-
[Mg(COO)2DMSO]n infinite chain in
atoms are omitted for clarity.
2. (c) View of the 3D structure of 2. All the H
a 1D infinite chain, Fig. 3b. In the chain, the four unique Mg2+
ions are periodically ordered to form a Mg4-Mg3-Mg2-Mg1-
Mg4-Mg3-Mg2-Mg1 array. Then, each chain is interlinked to
other four chains by TDC2- ligands to give rise to a 3D
framework, in which the rhombic tunnels along the b axis are
formed filled by the S, C, H atoms of the bridging DMSO
molecules, Fig. 3c. Similarly, the DMSO molecules also act as
bridging ligands in [Mn(DTDC)(μ2-DMSO)]19 and
Fig. 4 (a) The linking mode of Ac- anions with Mg2+ ions in showing the sequence of Mg2+ centers. (c) View of the 3D framework of
3. (b) The Mg-COO-Ac chain in 3 3
along the c axis. [Me2NH2]+ cations, DMA and H2O molecules in the channels are not shown for clarity. (d) The topology of
3. All the H atoms are omitted for clarity.
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[Pb(p-BDC)(μ2-DMSO)].20 For other solvent molecules playing characteristic of Mg1-Mg2-Mg1-Mg2 array, Fig. 4b. Each
the similar role, there is also a Mg-based CP, namely chain is interlinked to other four chains by TDC2- ligands along
Mg(ndc)(μ2-nmp)∙0.5H2O (nmp = 1-methyl-2-pyrrolidinone).10
the a and b axes. As a result, a 3D anionic framework is formed
(Me2NH2)[(Mg2(TDC)2(Ac)]∙1.5DMA∙0.5H2O (3). Compound with large tunnels along the c axis in which the [(Me)2NH2]+
belongs to the P42/nmc space group. It features a 3D anionic cations, the free DMA molecules and H2O molecules are
open framework fabricated by TDC2- ligands and Ac- anions located, Fig. 4c. Note that the structure of
is iso-structural
linking Mg2+ ions, with [Me2NH2]+ as counter ions located in with a Mn-based CP, namely (Me2NH2)[(Mg2(TDC)2(Ac)].22
the tunnels along the c axis, Fig. 4. Its asymmetric unit contains The coordination mode of Ac- anion in
can also be found in
half of the formula unit including two halves of Mg2+ ions, two other compounds.23 Taking the [Mg3Ac] unit as a 4-connected
3
3
3
halves of TDC2- ligands and half of Ac- anion, each of which is node and TDC2- ligand as a bridging linker, the framework of
3
located in an inversion center. Both the Mg2+ ions adopt an could be simplified to a 4-connected uni-nodal net with a crb
octahedral configuration, Fig. S2c. The Mg1 is coordinated by topology, Fig. 4d, and the Schläfli symbol is {4; 6^5}.
four TDC2- ligands in the equatorial plane and two Ac- anions [Mg2(TDC)2(DMF)2(EtOH)(H2O)2]∙DMF (4). Compound
4
in the apical positions, while the Mg2 is connected by four crystallizes in the noncentrosymmetric Cm space group
TDC2- and one Ac- anion in a chelating mode, Fig. S6a. The featuring a 2D layered structure, Fig. 5. The asymmetric unit
TDC2- ligands all adopt a (k1-k1-μ2)-(k1-k1-μ2)-μ4 coordination contains half of the formula unit including two halves of Mg2+
mode, Fig. S6b. The Ac- anion was produced by the ions (each located in an inversion center), one TDC2- ligand,
decomposition of DMA solvent during the reaction.21 It adopts one terminal coordinated DMF molecule, half of free DMF
a (k2-k2)-μ3 coordination mode connecting three Mg2+ ions by molecule, half of terminal EtOH molecule, half of terminal
chelating to Mg2 and linking to two Mg1 in both sides to form coordinated H2O molecule and half of μ2-coordinated H2O
a [Mg3(Ac)] unit, Fig. 4a.
molecule; the free DMF molecule and the terminal EtOH
Along the c axis, the Ac- anions together with TDC2- ligands molecule are positional disordered by a mirror, and the O atom
connect the Mg1 and Mg2 ions to form a chain-like SBU of the two halves unique H2O molecules are located in a mirror.
Fig. 5 (a) The bi-nuclear [Mg2(COO)4(µ2-H2O)] SBU in
4. (b) A single layer of 4. (c) View of the 3D supramolecular structure of 4 with the three layers in different colors
highlighting the “head to tail” packing mode of adjacent layers. (d) The intra-SBU and inter-layer H-bonding interactions between two SBUs from adjacent layers in
4.
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Both of Mg2+ centers adopt an octahedral geometry, Fig. S2d. supramolecular structure, Fig. 5d. The solvent molecules are
Mg1 is occupied by four TDC2- ligands in the equatorial plane, located in the spaces between and within layers, Fig. S10.
one EtOH molecule and one μ2-H2O molecule in the apical
positions. Whereas Mg2 is coordinated by two TDC2- ligands,
two DMF molecules in the equatorial plane, one terminal and
one μ2-H2O molecules in the apical positions, Fig. S7a. The
TDC2- ligand presents a common (k1-k1-μ2)-(k1-μ1)-μ3 mode
with one carboxylic oxygen atom uncoordinated, Fig. S7b.
The Mg1 and Mg2 are connected by four TDC2- ligands and
one μ2-H2O to form a 4-connected bi-nuclear unit (Fig. 5a)
which is quite different from the familiar paddle-wheel type
Zn(COO)4 unit in MOF-5.24 In the unit, two carboxylate groups
(O1, O2) of two TDC2- ligands link Mg1 and Mg2 in a bi-
bridge mode, while the other two carboxylate groups in another
two ligands only bind to Mg1 (each with O4) in a mono-dentate
mode with the uncoordinated O3 atoms forming strong H-
Fig. 6 The SBU of
4 in this work and the typical “paddle-wheel” type SBU in
reported structures.25
Generally, to ensure a layered structure without an inversion
center, two conditions must be satisfied including the NCS
nature of a single layer and packing of these layers in a way
without an inversion center.2e Lin et al. proved that for rhombic
grids, the noncentrosymmetric metal coordinating centers could
guarantee the NCS nature of the grids.2e Thrillingly, the μ2-
bonds (O5-H5O3 = 2.604(3) Å,
∠O5-H5-O3 = 153.2°) with
the μ2-H2O molecule. While the left coordination sites of two
Mg2+ ions of the unit are occupied by solvent molecules (with
EtOH molecule binding to Mg1, and H2O and DMF molecules
binding to Mg2, respectively). This dimmer can be viewed as
the secondary building unit (SBU) of 4. In the bc plane, each
SBU is interlinked to another four ones by TDC2- ligand to
form a layer with rhombic grids, Fig. 5b and Fig. S8. Then,
upon the H-bonding interactions among the terminal H2O (O6)
molecules in one layer and the carboxylate oxygen (O4) atoms
H2O-containing SBU in
rejects an inversion center. Accordingly, each layer of
unsymmetrical. In comparison, the typical “paddle-wheel”
shape [M2(COO)4] SBU usually has a higher symmetry with an
inversion center,25 Fig. 6.
4
has a C2v symmetry which absolutely
is
4
in the other layer (O6-H6O4 = 2.797(4) Å,
∠O6-H6-O4 =
With a translation operation along the c axis, these layers
145.3°, Fig. 5c), these layers accumulate in the same direction,
accumulate to give the NCS structure of 4.
that is, in a “head to tail” fashion, to give rise to a 3D
Scheme 1 Multi-roles of the solvent molecules in the construction of structures 1-4. The data in the lower right corner: Oscilloscope traces of SHG signals of
compound and KDP (marker) in the particle size of 210−300 μm.
4
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We presume the reasons why these layers prefer this packing
mode as follows: i). the steric hindrance stemming from
coordinated solvent molecules (DMF and H2O in one side and
ethanol in the other) is quite different in both sides of the layer
which makes their packing prefer a “head to tail” fashion to
maximally fill the void space within each layer, Fig. S10. There
are reported layered structures with noncentrosymmetric layers
featuring similar rhombic grids as that in 4, but these layers
have similar steric hindrance in both sides thus a “head to head”
packing mode upon an inversion center results in
centrosymmetric structures.26 ii). this packing mode also
facilitates the formation of inter-layer H-bonding interactions to
further stabilize the whole structure. In a word, the multiple
coordinated solvent molecules play vital roles in the symmetry-
breaking process of forming 4.
In the titled structures, Mg2+ ions all adopt the characteristic
octahedral geometry, Fig. S2. The Mg-OCarboxylate bond
distances of four structures varies from 1.995(2) to 2.182(2) Å
for
1
, 2.020(3) to 2.063(3) Å for
2
, 2.0227(10) to 2.081(1) Å for
, all of which fall in the
3
and 2.044(3) to 2.139(3) Å for
4
normal range of literature vales.3b The outstanding feature of
these structures is that the coordinated solvent molecules as
important structural components promote formation of the
diverse structures, as summarized in Scheme 1.
PXRD patterns and thermal stability analyses
The phase purity of compounds 1-4 was confirmed by PXRD
characterizations (Fig. 7a) and elemental analyses.
Thermogravimetric analyses (TGA) were performed on the
pure powder samples of 1-4 from 30 to 650 °C, Fig. 7b. The
multi-step weight losses of
corresponded to the losses of free and terminal coordinated EG
molecules and those as bridging ligands (about 32.6%, calc.
1 from room temperature to 385 °C
Fig. 7 The PXRD patterns (a) and TGA curves (b) of 1-4
.
455 nm (λex = 407 nm and 375 nm), respectively, Fig. 8a.
Monitored at 300 nm excitation wavelength, the free H2TDC
ligand features a blue light emission with a narrow peak centred
at 350 nm, and the emission can be attributed to the π* → n
transition within the ligand.27 Compared with the emission
spectra of H2TDC ligand, a bathochromic shift of the
luminescence is observed in the three compounds. Considering
the electronic configuration of Mg2+, the red-shift of emission
may be derived from the increase of the ligand conjugation
degree after coordinated with Mg2+ ions.2b, 28
Nonlinear optics (NLO) possess practical importance due to
its applications in telecommunications, optical storage,
information processing and so on.29 One of the most common
NLO behaviours is second-harmonic generation (SHG), in
which a NLO material can amplify light signal by doubling the
frequency.2e NLO properties require that the crystal structure
belongs to the noncentrosymmetric space group.30 Considering
32.4%).
2 was stable up to 300 °C ; the sharp weight loss from
300 to 400 °C corresponded to the release of μ2-DMSO
molecules (about 28.7%, calc. 28.6%) which necessarily
brought about the collapse of the framework. The open
framework of
3 began to release free DMA molecules at room
temperature followed by the escape of CH3COOH molecules
from the major framework, which inevitably caused the
decomposition of the whole structure. In the temperature range
of 90 °C to 390 °C,
4 orderly released terminal EtOH molecules,
terminal H2O molecules, and free and terminal DMF molecules
(weight loss: 43.1%, calc. 43.6%). All the four complexes are
unstable at ambient conditions probably because of the strong
interaction between Mg2+ ions in the frameworks with water
molecules in the air which may cause the collapse of the
framework.
Optical properties
the acentric nature of the structure of 4, the SHG properties of 4
Photoluminescence (PL) properties of compounds 1-3 in the
solid state were investigated at room temperature. Crystals of
emits bright green light centred at 475 nm (monitored at λex
415 nm), Fig. 8, which is near the white light section convinced
by the CIE chromaticity diagram, Fig. S11.4f While
and
was investigated on the powder samples by using the Kurtz–
Perry method.11 An approximate estimation was carried out on
a pulsed Q-switched Nd:YAG laser at a wavelength of 1064 nm.
The intensity of the green light (λ = 532 nm) produced by the
3
=
1
2
show relatively weak emission with peaks centred at 480 and
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crystal powder of
marker, which is sound among other SHG materials, Fig. S12.
4 is about 0.5 times that of a KDP (KH2PO4)
Notes and references
a
State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian, 350002, P.R. China
b
University of Chinese Academy of Sciences, Beijing, 100049, P.R.
China
* E-mail: xyhuang@fjirsm.ac.cn; Fax: (+86)591–83793727
†
Electronic Supplementary Information (ESI) available: A list of
additional reactions, TG-MS figure, more structural figures, CIE
chromaticity diagram, SHG figure and IR spectra for compounds 1-4
CCDC reference numbers 1032293 ( ), 1032294 ( ), 1032295 ( ) and
1032296 ( ). See DOI: 10.1039/b000000x/
.
1
2
3
4
1
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Conclusions
Four Mg-TDC CPs were solvothermally synthesized in
different mixed solvents. Compounds 1-3 feature a 3D infinite
framework while
4 has a 2D layered structure. The solvent
2014, 2, 6426-6431.
molecules play vital roles in the formation of these structures
by contributing to the extension of the structural dimensionality
5
6
7
M. Mazaj, T. B. Celic, G. Mali, M. Rangus, V. Kaucic and N. Z.
Logar, Cryst. Growth Des., 2013, 13, 3825-3834.
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structure. Photoluminescence emissions and SHG response
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8
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Table of Contents
Four Mg-TDC CPs have been isolated solvothermally in which the solvent molecules
play vital roles in constructing the diverse structures.