First lanthanide coordination polymers with N,N-dimethylformamide hydrolysis induced formate ligands

Article abstract of DOI:10.1016/j.ica.2011.12.030

A series of isostructural lanthanide organic frameworks with composition [Ln(tpa)(fa)] (Ln = Eu, Gd, Tb; H₂tpa = terephthalic acid, i.e. 1,4-benzenedicarboxylic acid; Hfa = formic acid, i.e. methanoic acid) has been synthesized through reaction of the corresponding lanthanide nitrates with H₂tpa in DMF/H₂O mixed solvent under solvothermal conditions (DMF = N,N-dimethylformamide). The formate ligand is in situ generated by hydrolysis of DMF. The dilanthanide-diformate subunits build two-dimensional layers which are shored up by terephthalates, leading to a three-dimensional pillar-layer network. These are the first examples of lanthanide complexes containing the formate ligand originated from hydrolysis of DMF. The dilanthanide-diformate subunit is also the first ever reported. The Eu and Tb complexes emit strong ligand sensitized f-f luminescence. Temperature variable magnetic susceptibility analysis reveals that the Gd(III) atoms are antiferromagnetically coupled (J = -0.0048 cm^-1, H = -2JS_Gd1·S_Gd2).

Full text of DOI:10.1016/j.ica.2011.12.030

Inorganica Chimica Acta 384 (2012) 333–339
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First lanthanide coordination polymers with N,N-dimethylformamide hydrolysis
induced formate ligands
a
Guang Huang a, Ping Yang a,b, Ning Wang c, Jian-Zhong Wu a, , Ying Yu
⇑
a School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China
b School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510631, China
c Research Institute of Tsinghua University in Shenzhen, Shenzhen 518055, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2011
Received in revised form 16 November 2011
Accepted 15 December 2011
Available online 26 December 2011
A series of isostructural lanthanide organic frameworks with composition [Ln(tpa)(fa)] (Ln = Eu, Gd, Tb;
H2tpa = terephthalic acid, i.e. 1,4-benzenedicarboxylic acid; Hfa = formic acid, i.e. methanoic acid) has
been synthesized through reaction of the corresponding lanthanide nitrates with H2tpa in DMF/H2O
mixed solvent under solvothermal conditions (DMF = N,N-dimethylformamide). The formate ligand is
in situ generated by hydrolysis of DMF. The dilanthanide-diformate subunits build two-dimensional
layers which are shored up by terephthalates, leading to a three-dimensional pillar-layer network. These
are the first examples of lanthanide complexes containing the formate ligand originated from hydrolysis
of DMF. The dilanthanide-diformate subunit is also the first ever reported. The Eu and Tb complexes emit
strong ligand sensitized f–f luminescence. Temperature variable magnetic susceptibility analysis reveals
Keywords:
Lanthanide
Carboxylate
Hydrolysis
Crystal structure
Photoluminescence
Magnetism
that the Gd(III) atoms are antiferromagnetically coupled (J = ꢀ0.0048 cmꢀ1, H = ꢀ2JSGd1ꢁSGd2).
ˆ
ˆ
ˆ
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
regards the lanthanide–terephthalate systems involving DMF
(N,N-dimethylformamide), it was firstly reported that standing
the DMF/methanol solution of terbium nitrate with terephthalic
acid in pyridine vapor at room temperature yielded microporous
Tb(tpa)(NO3)(DMF)2 where DMF and nitrate were also ligated [6].
In another paper, microporous frameworks Tb3(tpa)4.5(DMF)2
(H2O)3ꢁ(DMF)(H2O) and Ln3(tpa)4.5(DMF)2(H2O)3ꢁ(DMF)(H2O)0.5
(C2H5OH)0.5 (Ln = Dy, Ho, Er) were synthesized by the reaction of
lanthanide nitrates with H2tpa and triethylenetetramine in a
mixed solution of DMF, water and ethanol at 55 °C [12].
Ln6(tpa)9(DMF)6(H2O)3ꢁ3DMF (Ln = La, Ce, Nd) or Ln2(tpa)3(DMF)2
(H2O)2 (Ln = Y, Dy, Eu) were obtained by slow diffusion of the
solution of Ln(NO3)3ꢁ6H2O in DMF/H2O into the solution containing
Na2tpa in DMF/H2O [14]. No study of the reactions involving the
relevant reactants under hydro(solvo)thermal conditions were
reported.
The hydro(solvo)thermal process provides a supercritical envi-
ronment with simultaneous high temperature and high pressure
and has been widely utilized recently in the synthesis of highly
stable and solvent insoluble coordination polymeric materials.
However, it is still a great challenge to control the products.
Further studies are needed to understand the mechanism and
discipline in hydro(solvo)thermal reactions. It is interesting that
in hydro(solvo)thermal conditions, some unique in situ reactions
involving the organic ligands may happen which seem to
unlikely occur in normal conditions [17–19]. This makes the
Research on synthesis and characterization of metal organic
coordination polymeric materials has attracted continuous inter-
ests for their potential applications and esthetic beauty associated
with the network structures [1–3]. The use of lanthanide salts and
organic linkers in the synthesis of polymeric materials has gained
much attention due to their unique photophysical and magnetic
properties, although lack of preferred coordination number and
geometry for lanthanide ions makes it difficult to predict the struc-
ture of the lanthanide coordination polymers. Choice of organic
linkers is essential for preparing novel lanthanide organic poly-
mers. The hard Lewis base nature of multicarboxylate-containing
ligands makes them suitable for connecting the lanthanide ions [4].
Terephthalic acid (H2tpa, 1,4-benzenedicarboxylic acid) has
drawn much attention in preparing lanthanide coordination poly-
mers, because its two carboxylate groups after double deprotona-
tion have a good affinity for lanthanide ions, and its rod-like
topology induces low steric hindrance, allowing formation of
porous frameworks [5–16]. Based on terephthalate and other
ligands, numerous mixed-ligand lanthanide complexes have been
prepared. It has been found that the reaction conditions have
significant influence on the structures of the final products. As
⇑
Corresponding author. Tel.: +86 20 31759373; fax: +86 20 85211642.
E-mail address: wujzh@scnu.edu.cn (J.-Z. Wu).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.030

334
G. Huang et al. / Inorganica Chimica Acta 384 (2012) 333–339
hydro(solvo)thermal synthesis a fascinating method to obtain novel
coordination polymers.
our knowledge, there is no report involving any lanthanide com-
plex with formate originated from hydrolysis of DMF, although a
few lanthanide complexes containing DMF generated dimethylam-
monium have been reported recently [36–38]. The three lantha-
nide complexes reported here represent the first examples of
lanthanide complexes with DMF generated formate ligand. It is
some interesting that when we used formic acid in place of DMF,
the product yields decreased vastly (less than 5%), which implies
that the slow hydrolysis of DMF may be important for the product
formation.
Based on the above considerations, we investigated the reaction
of lanthanide nitrates with H2tpa in DMF/H2O under hydro(solvo)
thermal conditions. Interestingly, an unexpected series of coordi-
nation polymers with composition [Ln(tpa)(fa)] (Ln = Eu, Gd, Tb;
fa = formate, i.e. methanoate) were obtained, where the ligand
formate is the hydrolysate of DMF. To the best of our knowledge,
these compounds represent the first lanthanide complexes
containing formate originated from hydrolysis of DMF.
3.2. Structural description
2. Experimental
The single crystallographic analysis reveals that complexes 1, 2
and 3 are isostructural. As can be seen in Table 1, the crystal cell
size decreases on going from Eu to Gd and to Tb, which is consis-
tent with the lanthanide contraction effect. The variation of the
corresponding bond lengths as listed in Table 2 also provide direct
criteria for lanthanide contraction. In order to assist the explana-
tion of the magnetic property of the Gd complex 2 as will be dis-
cussed later, the structure of 2 is described here in detail. As
displayed in Fig. 1, a Gd(III) atom is eight-coordinated by four O
atoms each from a tpa2ꢀ ligand and four O atoms from three faꢀ li-
gand. The Gd–O bond lengths (Table 2) for O5 and O6 atoms of for-
mate [2.383(3)–2.578(3) Å] are significantly longer than those for
O1, O2, O3 and O4 atoms of terephthalate [2.279(3)–2.346(3) Å].
This can be explained by that the formate ligand forms an unstable
four-membered chelate ring with the gadolinium atom. As illus-
trated in Scheme 1, both terephthalate and formate function as
bridging ligands in the present complexes, connecting four and
three lanthanide atoms, respectively. But only the formate plays
also as a chelator here. The non-chelation tetradentate binding
mode of terephthalate in Scheme 1 is commonly seen in other lan-
thanide–terephthalate systems [5–7,12–14,16]. Other binding
modes of terephthalate involving four-membered chelation ring
formation [8–12,14–16] are not existed in the present complexes.
The continuous linkage between the formate ligands and the
Gd(III) atoms forms a two-dimensional layer. One can also regard
the layer as being built by alternately connected centrosymmetric
Gd2fa2 dimeric subunits, as emphasized by the circle shown in
Fig. 2. Such dilanthanide-diformate units are the first reported
examples to date. The Gdꢁ ꢁ ꢁGd distance, 3.9566(3) Å, is shorter
than those in other digadolinium-dicarboxylate units [39–60],
ascribable to smaller steric hindrance of formate than other car-
boxylates. The Gd2O2 diamonds are evenly separated by
7.2866(4) Å, forming a rhombic (4,4) grid network. The parallel
layers are overlapped if viewing down the a axis.
2.1. Materials and measurements
Ln(NO3)3ꢁ6H2O (Ln = Eu, Tb, Gd) were prepared by dissolving
the corresponding Ln2O3 in 1:1 nitric acid and then evaporating
the solvent to dryness. DMF was distilled and stored over molecu-
lar sieves. All the other reagents commercially available were used
without further purification. Elemental analysis results were
obtained on a Thermo Electron Flash EA 1112 Elemental Analyzer.
The luminescence spectra were recorded with a Hitachi F2500
Fluorescence Spectrophotometer. The magnetic susceptibility
measurement was carried out for polycrystalline samples on a
Quantum Design MPMS-XL5 SQUID magnetometer. Powder X-ray
diffraction (PXRD) patterns were recorded on a Rigaku D/M-
2200T automated diffractometer for CuK
a radiation (k =
0
1.54056 ÅA), with a scan speed of 4°/min and a step size of 0.02°
in the 2h range of 5–50°.
2.2. Synthesis
The complexes [Ln(tpa)(fa)] (Ln = Eu, 1; Gd, 2; Tb, 3) were syn-
thesized similarly as follows. The corresponding lanthanide nitrate
(0.2 mmol) and H2tpa (0.2 mmol) dissolved in a mixture of DMF
(4 mL) and water (6 mL) in a 23 mL Teflon-lined stainless steel
reactor was heated at 160 °C for 72 h. The reaction system was
cooled at a speed of 5 °C/h to room temperature. Block crystals
were collected and washed with distilled water, yields around
65% based on the lanthanide metals. Microanalysis: Calc. for
C9H5O6Eu: C, 29.93; H, 1.40. Found: C, 29.97; H, 1.38%. Calc. for
C9H5O6Gd: C, 29.50; H, 1.38. Found: C, 29.48; H, 1.42%. Calc. for
C9H5O6Tb: C, 29.37; H, 1.36. Found: C, 29.64; H, 1.51%.
2.3. X-ray crystallography
Single crystals of complexes were carefully selected under a
microscope and glued at the tip of a thin glass fiber with cyanoac-
rylate adhesive. X-ray diffraction was performed on a Bruker Apex-
All the terephthalate ligands incline between the layers. For
each terephthalate ligand, two O atoms in a carboxylate group con-
nect two Gd(III) atoms in the same Gd2O2 diamond in one layer,
while the two O atoms in the para-carboxylate group link two
Gd(III) atoms which come from two adjacent Gd2O2 diamonds.
The interlayer distance is 10.8849(8) Å. So the terephthalate li-
gands function like pillars to shore up the layers, resulting in
three-dimensional ‘‘pillar-layer’’ network, as shown in Fig. 3. Such
pillar-layer network is quite compact and no remarkable porosity
is existed.
II CCD X-ray diffractometer with MoK
a radiation operating at
50 kV and 30 mA. The crystal structures were solved by direct
method and refined on F2 by full-matrix least-squares method.
All non-hydrogen atoms were refined anisotropically. All the H
atoms for organic ligand were placed at the calculation positions.
Further details of the X-ray structural analysis are given in Table 1.
3. Results and discussion
3.3. Bulky purity and photoluminescence properties
3.1. Synthesis
The purity of the bulky products of the complexes was con-
firmed by elemental analysis (see Section 2) and powder X-ray dif-
fraction. As shown in Fig. 4, the room temperature diffraction
patterns of the europium complex 1 as synthesized are in good
agreement with the calculated diffractogram based on the single
crystal data, implying that the bulky sample is phase pure. The
DMF is a frequently used solvent in preparing coordination
compounds. But it could occasionally hydrolyze in acidic solution,
under either normal or solvothermal conditions, giving rise to the
dimethylammonium cation [20–31] or sometimes the formate an-
ion [32–35] as a component in the final complexes. To the best of

G. Huang et al. / Inorganica Chimica Acta 384 (2012) 333–339
335
Table 1
Crystal data and structure refinement.
Complex
1
2
3
Empirical formula
Formula weight
T (K)
C9H5EuO6
361.09
298(2)
C9H5GdO6
366.38
298(2)
C9H5O6Tb
368.05
298(2)
k (Å)
0.71073
monoclinic
P21/c
0.71073
monoclinic
P21/c
0.71073
monoclinic
P21/c
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
10.6699(13)
12.9403(16)
6.7635(8)
95.4450(10)
929.6(2)
4
2.580
6.756
680
10.6678(8)
12.9319(9
6.7191(5)
95.2900
922.98(12)
4
2.637
7.195
684
10.6621(9)
12.8903(11)
6.7157(6)
95.2710
918.9(14)
4
2.660
7.705
688
b (°)
Volume (Å3)
Z
Density (Mg mꢀ3
)
l
(mmꢀ1
F(000)
)
Reflections collected
Independent reflections
Rint
5004
1819
0.0187
1.128
4938
1805
0.0234
0.959
4779
1703
0.0184
1.018
Goodness-of-fit (GOF) on F2
R1, wR2 [I > 2
R1, wR2 (all data)
r
(I)]
0.0204, 0.0445
0.0229, 0.0453
0.0209, 0.0549
0.0230, 0.0563
0.0222, 0.0506
0.0243, 0.0516
Table 2
Bond lengths (Å) and angles (°).
1
2
3
Eu(1)–O(1)#1
Eu(1)–O(2)
2.335(3)
Gd(1)–O(1)#1
Gd(1)–O(2)
2.333(3)
2.279(3)
2.311(3)
2.346(3)
2.554(3)
2.470(3)
2.578(3)
2.383(3)
Tb(1)–O(1)#1
2.330(3)
2.272(3)
2.313(3)
2.352(4)
2.558(4)
2.462(3)
2.575(4)
2.383(4)
2.285(3)
2.321(3)
2.354(3)
2.574(3)
2.471(3)
2.588(3)
2.392(3)
Tb(1)–O(2)
Eu(1)–O(3)#2
Eu(1)–O(4)#3
Eu(1)–O(5)
Gd(1)–O(3)#2
Gd(1)–O(4)#3
Gd(1)–O(5)
Tb(1)–O(3)#2
Tb(1)–O(4)#3
Tb(1)–O(5)
Eu(1)–O(5)#4
Eu(1)–O(6)
Gd(1)–O(5)#4
Gd(1)–O(6)
Tb(1)–O(5)#4
Tb(1)–O(6)
Eu(1)–O(6)#5
Gd(1)–O(6)#5
Tb(1)–O(6)#5
O(1)#1–Eu(1)–O(4)#3
O(1)#1–Eu(1)–O(5)
O(1)#1–Eu(1)–O(5)#4
O(1)#1–Eu(1)–O(6)
O(1)#1–Eu(1)–O(6)#5
O(2)–Eu(1)–O(1)#1
O(2)–Eu(1)–O(3)#2
O(2)–Eu(1)–O(4)#3
O(2)–Eu(1)–O(5)
145.44(10)
80.59(9)
75.26(9)
O(1)#1–Gd(1)–O(4)#3
O(1)#1–Gd(1)–O(5)
O(1)#1–Gd(1)–O(5)#4
O(1)#1–Gd(1)–O(6)
O(1)#1–Gd(1)–O(6)#5
O(2)–Gd(1)–O(1)#1
O(2)–Gd(1)–O(3)#2
O(2)–Gd(1)–O(4)#3
O(2)–Gd(1)–O(5)
144.99(10)
80.44(9)
O(1)#1–Tb(1)–O(4)#3
O(1)#1–Tb(1)–O(5)
O(1)#1–Tb(1)–O(5)#4
O(1)#1–Tb(1)–O(6)
O(1)#1–Tb(1)–O(6)#5
O(2)–Tb(1)–O(1)#1
O(2)–Tb(1)–O(3)#2
O(2)–Tb(1)–O(4)#3
O(2)–Tb(1)–O(5)
145.49(12)
80.69(11)
75.33(12)
122.69(12)
135.83(12)
78.06(12)
100.05(13)
101.59(13)
156.93(11)
77.41(12)
152.78(11)
78.53(12)
76.13(12)
136.39(12)
83.36(13)
151.24(12)
71.71(13)
71.78(13)
90.88(13)
71.05(12)
71.75(13)
76.07(13)
88.64(7)
75.10(10)
122.75(9)
136.25(10)
77.92(10)
100.08(11)
101.72(12)
156.42(9)
77.07(9)
152.91(9)
78.62(10)
76.35(10)
136.61(10)
83.68(10)
151.26(10)
71.65(10)
72.10(10)
90.69(11)
70.84(10)
71.88(11)
76.08(10)
88.45(5)
122.55(10)
135.83(10)
78.24(10)
100.30(10)
101.34(11)
157.01(9)
77.24(9)
152.80(9)
78.44(10)
76.23(10)
136.35(10)
83.14(10)
151.28(10)
71.51(10)
71.76(10)
91.06(11)
71.06(9)
71.94(10)
76.10(10)
88.83(5)
122.48(9)
49.80(9)
123.71(9)
133.82(10)
74.36(11)
O(2)–Eu(1)–O(5)#4
O(2)–Eu(1)–O(6)
O(2)–Gd(1)–O(5)#4
O(2)–Gd(1)–O(6)
O(2)–Tb(1)–O(5)#4
O(2)–Tb(1)–O(6)
O(2)–Eu(1)–O(6)#5
O(3)#2–Eu(1)–O(1)#1
O(3)#2–Eu(1)–O(4)#3
O(3)#2–Eu(1)–O(5)
O(3)#2–Eu(1)–O(5)#4
O(3)#2–Eu(1)–O(6)
O(3)#2–Eu(1)–O(6)#5
O(4)#3–Eu(1)–O(5)
O(4)#3–Eu(1)–O(5)#4
O(4)#3–Eu(1)–O(6)
O(4)#3–Eu(1)–O(6)#5
O(5)#4–Eu(1)–O(5)
O(5)#4–Eu(1)–O(6)
O(5)–Eu(1)–O(6)
O(2)–Gd(1)–O(6)#5
O(3)#2–Gd(1)–O(1)#1
O(3)#2–Gd(1)–O(4)#3
O(3)#2–Gd(1)–O(5)
O(3)#2–Gd(1)–O(5)#4
O(3)#2–Gd(1)–O(6)
O(3)#2–Gd(1)–O(6)#5
O(4)#3–Gd(1)–O(5)
O(4)#3–Gd(1)–O(5)#4
O(4)#3–Gd(1)–O(6)
O(4)#3–Gd(1)–O(6)#5
O(5)#4–Gd(1)–O(5)
O(5)#4–Gd(1)–O(6)
O(5)–Gd(1)–O(6)
O(2)–Tb(1)–O(6)#5
O(3)#2–Tb(1)–O(1)#1
O(3)#2–Tb(1)–O(4)#3
O(3)#2–Tb(1)–O(5)
O(3)#2–Tb(1)–O(5)#4
O(3)#2–Tb(1)–O(6)
O(3)#2–Tb(1)–O(6)#5
O(4)#3–Tb(1)–O(5)
O(4)#3–Tb(1)–O(5)#4
O(4)#3–Tb(1)–O(6)
O(4)#3–Tb(1)–O(6)#5
O(5)#4–Tb(1)–O(5)
O(5)#4–Tb(1)–O(6)
O(5)–Tb(1)–O(6)
122.63(9)
50.30(9)
124.22(9)
133.41(9)
74.29(10)
122.35(12)
49.90(11)
123.70(11)
133.85(12)
74.25(13)
O(6)#5–Eu(1)–O(5)
O(6)#5–Eu(1)–O(5)#4
O(6)#5–Eu(1)–O(6)
O(6)#5–Gd(1)–O(5)
O(6)#5–Gd(1)–O(5)#4
O(6)#5–Gd(1)–O(6)
O(6)#5–Tb(1)–O(5)
O(6)#5–Tb(1)–O(5)#4
O(6)#5–Tb(1)–O(6)
Symmetry transformations used to generate equivalent atoms: #1, x, ꢀy + 3/2, z ꢀ 1/2; #2, ꢀx, ꢀy + 2, ꢀz; #3, x + 1, y, z; #4, x, ꢀy + 3/2, z + 1/2; #5, ꢀx + 1, ꢀy + 2, ꢀz.
pure crystalline samples are thus suitable for studies of photolumi-
nescent and magnetic properties.
Photoluminescence is one of important properties of the lantha-
nide complexes with aromatic ligands since a highly absorptive li-
gand may sensitize the f–f transition of the lanthanide atom. The
emission spectra, especially the environment relevant transitions
of the Eu(III) atom provide important structural information [61].
As depicted in Fig. 5, the ligand-centered luminescence is com-
pletely suppressed, whereas the typical narrow emission bands
of the Eu(III) or Tb(III) ions can be detected upon excitation of

336
G. Huang et al. / Inorganica Chimica Acta 384 (2012) 333–339
Fig. 1. ORTEP plot (50% thermal ellipsoids) of the coordination environment around a Gd(III) atom in 2 with atom labeling. All hydrogen atoms are omitted for clarity.
Symmetry codes: #1, x, ꢀy + 3/2, z ꢀ 1/2; #2, ꢀx, ꢀy + 2, ꢀz; #3, x + 1, y, z; #4, x, ꢀy + 3/2, z + 1/2; #5, ꢀx + 1, ꢀy + 2, ꢀz.
and relatively insensitive to environment) also reflects a low sym-
metry environment. The hypersensitive 5D0 ? 7F2 electric dipole
transition was found to be the strongest emission and split to
two or three peaks (the 616 nm peak not resolved well), strongly
supporting that the central europium ion locates at site in low
symmetry without inversion centre. These features agree well with
the X-ray structural analysis as discussed above.
In contrast to the bright red emission of 1, the terbium complex
3 displays bright green luminescence, with emission peaks cen-
tered at 489 nm (5D4 ? 7F6), 546 nm (5D4 ? 7F5), 584 nm
Scheme 1. Binding modes of terephthalate (left) and formate (right) in 1, 2 and 3.
(5D4 ? 7F4), 622 nm (5D4 ? 7F3) (Fig. 5b). The slight splitting of
these peaks also verifies the low symmetry around the terbium
atoms.
the ligand-centered absorption at 317 nm, showing strong ligand
sensitized f–f emission. For the europium complex 1 (Fig. 5a), the
presence of the weak unsplit 5D0 ? 7F0 electric-dipole transition
3.4. Magnetic property
indicates only one type of europium site and the site symmetry
is not especially high (i.e. the europium atom is not on an inversion
As Gd(III) has the largest spin among the lanthanide ions, it is
worthy to study the magnetic behavior of the Gd(III) complex.
centre). The doublet 5D0 ? 7F1 transition (magnetic-dipole allowed
Fig. 2. Two-dimensional layer formed by formates and Gd(III) atoms in 2. (a) Projection down the a axis showing the (4,4) net. (b) View as a layer down the c axis.

G. Huang et al. / Inorganica Chimica Acta 384 (2012) 333–339
337
Fig. 3. Projection down the c axis, showing the three dimensional pillar-layer
network in 2 with terephthalates shoring up the Gd-formate layers.
Fig. 5. Solid-state emission spectra of
kex = 317 nm.
1
(a) and
3
(b) at room temperature.
Fig. 4. The experimental and simulated PXRD patterns of 1 at room temperature.
The temperature variable magnetic susceptibility measurement for
the gadolinium complex 2 was performed in the range of 2–300 K
under a field of 1000 Oe, and the magnetic behavior is shown in
Fig. 6 as plots of vMꢀ1 vs. T and
v
MT vs. T, where
susceptibility per mononuclear ꢀG1d unit.
The thermal evolution of vM obeys Curie–Weiss law,
vM is the magnetic
vM = C/
(T ꢀ h), over the whole temperature range with Curie constant
C = 7.995 cm3 molꢀ1 K and Weiss constant h = ꢀ0.411 K. The small
negative value of the Weiss constant implies the weak antiferro-
magnetic coupling between the Gd(III) ions.
The
v
MT value at room temperature is 7.98 cm3 molꢀ1 K, close
8
to the expected value for an uncoupled Gd(III) ion with S7/2
ground state (7.88 cm3 molꢀ1 K, g = 2). With the decreasing of
ꢀ1
Fig. 6. vM vs. T and
vMT vs. T plots for 2.
temperature,
vMT decreases slowly till about 10 K and then drops
sharply to 7.08 cm3 molꢀ1 K at 2 K. This is a typical behavior for
weak antiferromagnetic interaction, consistent with the indication
of the Weiss constant.
connected intra-layer Gd2O2 units with even distance 7.2866(4) Å
(Fig. 2) is also possible, whereas the magnetic interaction between
the terephthalate connected inter-layer Gd2O2 units with long dis-
tance 10.8849(8) Å is negligible.
As can be seen from the structural details discussed above, the
magnetic coupling of the Gd(III) ions is mainly realized through the
dimeric Gd2O2 unit which has
3.9566(3) Å. The magnetic interaction between the formate
a
short Gdꢁ ꢁ ꢁGd distance,
For the Gd2O2 diamond system, the simple dimer model based
on spin Hamitonian H = ꢀ2JSGd1ꢁSGd2 with quantum numbers
ˆ
ˆ
ˆ

338
G. Huang et al. / Inorganica Chimica Acta 384 (2012) 333–339
SGd1 = SGd2 = 7/2, as reported previously [62–66], can be used here.
terephthalate ligands and two l
O:j2O,O0 formate ligands as the
The molar magnetic susceptibility can be expressed as
bridges. The Gd–O–Gd angle is 105.7(1)°, less than 110°. The
Gdꢁ ꢁ ꢁGd distance, 3.9566(3) Å, is among the smallest ones in the
Gd2O2 cores. Such ‘‘compact’’ digadolinium structure favors signif-
icant increasing of the overlap integral of the magnetic orbitals and
thus the weak antiferromagnetic contribution to the exchange
coupling.
Ng2b2
vM0
¼
3kBT
2J
6J
42J
k
56J
k
T
k
T
T
k T
B
B
B
B
6e þ 30e þ 84e þ 180e þ 330e þ 546e þ 840e
ꢁ
2J
6J
12J
42J
T
56J
k
T
k
T
k
T
T
k
k T
B
B
B
B
B
B
1 þ 3e þ 5e þ 7e þ 9e þ 11e þ 13e þ 15e
4. Conclusions
where N, b, kB, g, J and T have their usual meanings. Considering the
two-dimensional square lattice coupling [67,68], the magnetic sus-
ceptibility can be further rewritten as
The reactions of europium, gadolinium or terbium nitrates with
terephthalic acid in DMF/H2O at solvothermal conditions yielded
three new isostructural coordination polymers with mixed tere-
phthalate and formate ligands. These complexes represent the first
lanthanide complexes containing the formate ligand originated
from hydrolysis of DMF. The dilanthanide-diformate dimeric unit
is also the first ever found in literature. The connection of these di-
meric units constitute (4,4) layers which are shored up by tereph-
thalates. The europium and terbium complexes possess ligand
sensitized red and green luminescence, respectively. The gadolin-
ium complex shows a weak antiferromagnetic behavior between
the Gd(III) ions due to the bridging motif with small Gd–O–Gd an-
gle and Gdꢁ ꢁ ꢁGd distance involving the dimeric unit.
22 þ TIP
Ng2b2
3kBT
ð1 þ uÞ
ð1 ꢀ uÞ
vM
¼
ꢁ SðS þ 1Þ ꢁ
Ng2b2
SðS þ 1Þ ¼ v0M
ꢁ
3kBT
J1SðS þ 1Þ
kBT
J1SðS þ 1Þ
u ¼ coth
ꢀ
kBT
where J1 and TIP represent the magnetic exchange parameter be-
tween the adjacent Gd2O2 diamonds and temperature-independent
paramagnetism, respectively. The best fitting result is g = 2.00,
J = ꢀ0.0048 cmꢀ1
,
J1 = ꢀ0.0025 cmꢀ1
,
TIP = 8.04 ꢂ 10ꢀ4 cm3 molꢀ1
Acknowledgments
and R (residue sum of squares) = 4.99 ꢂ 10ꢀ4. The very small nega-
tive values of the coupling constants indicate very weak antiferro-
magnetic interaction between the Gd(III) ions.
Financial supports from the National Natural Science Foundation
(20975041), Guangdong Natural Science Foundation(5005935) and
South China Normal University are gratefully acknowledged.
Recently the magneto-structural studies of oxo-carboxylate
bridged Gd(III) complexes have made remarkable progress
[66,69–72]. It has been found that there is a strong relationship be-
tween a certain bridging motif and the magnetic exchange interac-
tion [66,69,72]. The structural data for the Gd2O2 diamond,
especially the Gd–O–Gd angle or the Gdꢁ ꢁ ꢁGd distance, seem to
have crucial effect on the computed magnetic coupling parameters.
Based on structural and magnetic data for a number of oxo-carbox-
ylate bridged Gd(III) complexes, Ruiz-Pérez and her coworkers
have compiled a detailed table for comparison and concluded that
the Gd–O–Gd angle smaller than 110° may lead to an antiferro-
magnetic behavior [69]. The bridging motif in a dimeric unit of 2
Appendix A. Supplementary material
CCDC 820774, 820775 and 820776 contain the supplementary
crystallographic data for the complexes 1, 2 and 3. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.12.030.
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