Reversible color changes of metal(ii)-N1,N3- di(pyridin-4-yl)isophthalamide complexes via desolvation and solvation

Article abstract of DOI:10.1039/c0dt00326c

Four metal(ii)-N¹,N³-di(pyridin-4-yl)isophthalamide (L) complexes formulated as [Cu(L)₂Cl₂]·5.5H ₂O (1), [Ni(L)₂Cl₂]·5.5H₂O (2), [Co(L)(m-BDC)]·DMF (3) (m-BDC = 1, 3-benzenedicarboxylic dianion) and [Cd₂(L)₂(m-BDC)₂]·DMF·0. 2H₂O (4) have been solvothermally synthesized and structurally characterized by single-crystal X-ray diffractions. Complexes 1 and 2 are isomorphous, both of them exhibit chain-like frameworks, in which ligand L shows a cis-conformation. Complex 3 displays a three dimensional architecture constructed by ligand L with intermediate-conformation linking different chains. Complex 4 shows a corrugated two dimensional layer in which ligand L with trans-conformation and m-BDC act as bridging groups linking different Cd(ii) centers. The four complexes all possess stable host frameworks, and complexes 1-3 show reversible color changes via desolvation and solvation.

Full text of DOI:10.1039/c0dt00326c

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www.rsc.org/dalton | Dalton Transactions
Reversible color changes of metal(II)-N1,N3-di(pyridin-4-yl)isophthalamide
complexes via desolvation and solvation†
Yun Gong,a,b Yuchao Zhou,b Jinghua Li,c Rong Cao,*a JianBo Qinb and Jian Lib
Received 19th April 2010, Accepted 12th August 2010
DOI: 10.1039/c0dt00326c
Four metal(II)-N1,N3-di(pyridin-4-yl)isophthalamide (L) complexes formulated as [Cu(L)2Cl2]·5.5H2O
(1), [Ni(L)2Cl2]·5.5H2O (2), [Co(L)(m-BDC)]·DMF (3) (m-BDC = 1, 3-benzenedicarboxylic dianion)
and [Cd2(L)2(m-BDC)2]·DMF·0.2H2O (4) have been solvothermally synthesized and structurally
characterized by single-crystal X-ray diffractions. Complexes 1 and 2 are isomorphous, both of them
exhibit chain-like frameworks, in which ligand L shows a cis-conformation. Complex 3 displays a three
dimensional architecture constructed by ligand L with intermediate-conformation linking different
chains. Complex 4 shows a corrugated two dimensional layer in which ligand L with trans-conformation
and m-BDC act as bridging groups linking different Cd(II) centers. The four complexes all possess
stable host frameworks, and complexes 1–3 show reversible color changes via desolvation and solvation.
conformations due to the rotation of the flexible C–N sin-
Introduction
gle bonds in the structure. Its three typical conformations
The reversible color or luminescence changes of chromophoric
are shown in Scheme 1. Based on the flexible ligand and
molecules in different solvents is called solvatochromism.1–2
Pt(II), Puddephatt’s group has obtained several metallamacro-
cycles and metallacages.7a–d Dastidar’s group has reported the
cis-conformation of the L ligand in Co(II)–L and Cd(II)–L
complexes.7e In the present paper, we obtained two isomorphous
1D porous MOFs: [Cu(L)2Cl2]·5.5H2O (1) and [Ni(L)2Cl2]·5.5H2O
(2). Utilizing the coligand, 1,3-benzenedicarboxylic acid (m-
H2BDC), we got two complexes: [Co(L)(m-BDC)]·DMF (3)
and [Cd2(L)2(m-BDC)2]·DMF·0.2H2O (4), which exhibit 3D and
2D architectures, respectively. In complexes 1 and 2, ligand L
shows a cis-conformation. In complexes 3 and 4, L displays an
intermediate- and a trans-conformation, respectively. The four
Solvatochromism is caused by the change in the energy gap
between a ground state and excited state of the compound, which
results from the specific interactions between chromophore and
solvent molecules. Organic conjugated polymers change their
conformations in certain solvents and give rise to a solvatochromic
response.1f Some transition metal complexes can undergo sol-
vatochromic changes and these changes often result from the
subtle structural changes of the coordination center due to the
solvent attack, leading to the energy alteration of the visible d–d
transition.2
Solvent indicator plays an important role in many fields.
Usually, the preparation of solvent indicator is based on the
principle of solvatochromism. However, solvatochromism usually
appears in solution and a large amount of solvent is involved,
which prevents the improvement of the sensitivity to solvent, and
it is not feasible for the recycling of the solvent indicator.3
As analogues of zeolites and clays, metal–organic frameworks
(MOFs) containing channels or voids have attracted current
interest because of their functional properties and practical
applications.4–5 Up until now, changes of properties6 such as color
changes of the stable porous MOFs during desolvation and re-
absorption has rarely been investigated.
In this work, we used a flexible bis(pyridyl) ligand, N1,N3-
di(pyridin-4-yl) isophthal-amide (L), which possesses different
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian
350002, P. R. China. E-mail: rcao@fjirsm.ac.cn; Fax: +86-591-83796710
bDepartment of Chemistry, College of Chemistry and Chemical Engineering,
Chongqing University, Chongqing 400044, P. R. China
cDepartment of Pharmaceutical Chemistry, College of Chemistry and
Chemical Engineering, Chongqing University, Chongqing 400044, P. R.
China
† Electronic supplementary information (ESI) available: TGA diagrams,
PXRD patterns and colour changes of the compounds. Additional
data. CCDC reference numbers 707618 and 771991–771993. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00326c
Scheme 1 The cis- (a), intermediate- (b) and trans- (c) conformations of
the ligand L.
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complexes all possess stable host frameworks and complexes 1–3
show reversible color changes via desolvation and solvation.
X-Ray crystallography†
XRD data of complexes 1–4 were collected on a Bruker-AXS CCD
area detector-equipped diffractometer with graphite- monochro-
matized Mo Ka (l = 0.71073 A) radiation at room temperature.
Experimental
˚
An empirical absorption correction from y scan was applied.
All the structures were solved by direct methods and expanded
using Fourier techniques. The non-hydrogen atoms were refined
anisotropically. All of the hydrogen atoms were placed in the
calculated positions. All calculations were performed using the
SHELXTL-97 program. The solvent molecules in complexes 1
and 2 were highly disordered and were impossible to refine
using conventional discrete-atom models, thus the contribution of
partial solvent electron densities were removed by the SQUEEZE
routine in PLATON.8 The final chemical formula was estimated
from the SQUEEZE results combined with the TGA results.
Crystal data and structure refinements for complexes 1–4 are listed
in Table 1. Further details are provided in the ESI.†
Materials and methods
All chemicals purchased were reagent grade and used without
further purification. C, H, N elemental analyses were performed on
an Elementar Vario EL III analyzer. IR spectra were recorded as
KBr pellets on a PerkinElmer spectrometer. TGA was performed
on a NETZSCH STA 449C ther◦mogravimetric analyzer in flowing
N2 with a heating rate of 10 C min-1. The PXRD data were
collected on a RIGAKU DMAX2500PC diffractometer using
Cu-Ka radiation. UV-Vis spectra were measured on a Lambda-
900 UV spectro-photometer. All measurements were performed at
room temperature.
Synthesis
Results and discussion
Synthesis of C18H14N4O2. The ligand was prepared according
to the literature.7 Elemental Anal. Calcd. for C18H14N4O2: C, 67.92;
H, 4.40; N, 17.61%. Found: C, 68.02; H, 4.50; N, 17.88%. IR
(cm-1): 3234(s), 1950(w), 1684(s), 1600(s), 1509(s), 1415(s), 1329(s),
1290(s), 1207(s), 1073(s), 995(s), 828(s), 715(s), 580(m), 539(m).
Synthesis
Complexes 1 and 2 not only can be synthesized solvothermally, but
can also be prepared via diffusion technique at room temperature.
Dissolving MCl2 (M = Cu2+ or Ni2+) and L in DMF, the resulting
solution was added to a layer of CH2Cl2, after several days,
crystals suitable for study of X-ray diffraction were obtained. The
preparation of the two complexes was not susceptible to molar
ratio of reactants. The molar ratio of MCl2/L can range from
1 : 1 to 3 : 1. Complex 1 can be obtained even when CuCl was
used instead of CuCl2·2H2O. Complex 2 can selectively crystallize
from a mixture of L, NiCl2·6H2O and NiSO4·6H2O. It is indicated
that complexes 1 and 2 are thermally stable compounds under the
reaction condition.
Synthesis of [Cu(L)2Cl2]·5.5H2O (1).
A
mixture of
CuCl2·2H2O (0.5 mmol, 0.085 g), L (0.5 mmol, 0.159 g) and
◦
DMF (10 mL) was heated at 120 C in a Teflon-lined autoclave
for 2 days, followed by slow cooling to room temperature. The
resulting blue prismatic crystals were filtered off (yield: ca. 95%
based on L). Elemental Anal. Found: C, 49.35; H, 4.55; N,
13.03%. Calcd. for C36H39N8O9.5CuCl2: C, 49.66; H, 4.48; N,
12.87%. IR (cm-1): 3425(s), 1689(m), 1666(s), 1597(s), 1516(s),
1425(m), 1333(s), 1300(s), 1211(s), 1018(m), 841(m), 719(m),
542(m).
Complexes 3 and 4 were obtained under similar conditions
except different metal salts were utilized. Their different structures
indicate the species of metal salts plays an important role in the
syntheses of coordination polymers.
Synthesis of [Ni(L)2Cl2]·5.5H2O (2). The preparation of com-
plex 2 was similar to that of complex 1 except that NiCl2·6H2O
was used instead of CuCl2·2H2O. The yield of the green prismatic
crystals is ca. 95% based on L. Elemental Anal. Found: C, 50.15;
H, 4.25; N, 13.13%. Calcd. for C36H39N8O9.5NiCl2: C, 49.94; H,
4.51; N, 12.95%. IR (cm-1): 3420(s), 1690(m), 1668(s), 1597(s),
1516(s), 1425(m), 1334(s), 1300(s), 1211(s), 1020(m), 843(m),
721(m), 542(m).
Crystal structures and thermal stability
Crystal structures and thermal stability of [Cu(L)2Cl2]·5.5H2O
(1) and [Ni(L)2Cl2]·5.5H2O (2). Single-crystal X-ray diffraction
analyses reveal that complexes 1 and 2 are isomorphous (Table 1).
Therefore, we will restrict our description to the copper complex 1
and only mention pertinent points for the nickel complex 2 where
appropriate.
Synthesis of [Co(L)(m-BDC)]·DMF (3). The synthesis of com-
plex 3 was carried out as described above for complex 1, but
starting with the mixture of Co(NO3)2·6H2O (0.05 mmol, 0.015 g),
L (0.05 mmol, 0.016 g), m-H2BDC (0.05 mmol, 0.008 g) and DMF
(8 mL). Yield: ca. 90% based on L. Elemental Anal. Found: C,
56.59; H, 4.02; N, 11.58%. Calcd. for CoC29N5H25O7: C, 56.68;
H, 4.07; N, 11.40%. IR(cm-1): 3435(s), 1666(s), 1597(s), 1516(s),
1426(m), 1334(s), 1299(s), 1211(s), 1020(m), 843(m), 512(w).
Complex 1 crystallizes in the trigonal space group P3121 (no.
152), whereas complex 2 crystallizes in the trigonal space group
P3221 (no. 154). The Flack parameters of the two complexes are
zero, indicating they possess opposite absolute structures.
In the copper complex 1, the asymmetric unit contains half
Cu(II), one L, one Cl-, one fourth dissociated water molecules.
The Cu(II) center is coordinated by four nitrogen atoms from four
Synthesis of [Cd2(L)2(m-BDC)2]·DMF·0.2H2O (4). The prepa-
ration of complex 4 was similar to that of complex 3 except using
CdSO4·8/3H2O (0.05 mmol, 0.013 g) instead of Co(NO3)2·6H2O.
Yield: ca. 90% based on L. Elemental Anal. Found: C, 52.29; H,
4.23; N, 9.85%. Calcd. for C55N9H43.4Cd2O13.2: C, 52.13; H, 4.11;
N, 9.95%. IR(cm-1): 3454(s), 1661(s), 1597(s), 1514(s), 1425(m),
1333(m), 1211(s), 1045(w), 831(w), 715(w), 544(m).
◦
˚
L [Cu–N 2.029(4)–2.030(4) A, N–Cu–N 89.0(2)–177.1(2) ] and
-
˚
two Cl to furnish an octahedral geometry [Cu–Cl 2.848 A] (Fig.
1). In the nickel complex 2, the Ni–N bond lengths range from
˚
2.103(4) to 2.106(4) A, the N–Ni–N bond angles are in the range
◦
˚
of 88.9(2)–177.4(2) , and the Ni–Cl bond length is 2.490(1) A.
9924 | Dalton Trans., 2010, 39, 9923–9928
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Table 1 Crystal data and structure refinements for complexes 1–4
Complex
1
2
3
4
Empirical formula
M
C36H39N8O9.5CuCl2
870.33
Trigonal
P3121
14.3159(4)
14.3159(4)
19.4452(11)
90
90
120
3451.3(2)
3
1.125
C36H39N8O9.5NiCl2
865.33
Trigonal
P3121
14.4083(3)
14.4083(3)
18.9964(8)
90
90
120
3415.29(18)
3
1.129
CoC29N5H25O7
614.47
Mono-clinic
C2/c
16.7156(17)
11.1345(12)
32.121(3)
90
91.554(2)
90
C55N9H43.4Cd2O13.2
1266.42
Mono-clinic
C2/c
30.151(3)
9.9652(5)
19.3515(16)
90
109.463(4)
90
5482.1(7)
4
1.525
0.848
2.21 to 25.01
-35 to 35
-11 to 11
-23 to 19
16 888
4813
380
2522
0.0535
0.1396
0.0699
0.1676
1.227
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
5976.2(10)
8
1.366
Z
Dc/Mg m-3
m/mm-1
q range/◦
Range hkl
0.632
0.585
0.627
2.66 to 27.48
-18 to 18
-18 to 18
-25 to 25
26 740
5280
237
2.70 to 27.46
-18 to 18
-18 to 18
-24 to 24
26 820
5208
241
2.28 to 25.01
-12 to 19
-11 to 13
-36 to 38
15 256
5210
421
2536
0.0640
0.1555
0.1111
Reflns collected
Unique reflns
No. of parameters
F(000)
1197
1194
R1
0.0793
0.2202
0.0895
0.2304
1.068
0.0776
0.2266
0.0851
0.2370
1.112
wR2 [I > 2s(I)]
R1
wR2 [all data]
GOF
0.1767
0.963
respectively. Two deviated L ligands form a cavity and different
Cu(II) centers are connected by two strands of L ligands to form
˚
a 1D porous framework with a Cu ◊ ◊ ◊ Cu separation of 14.316 A.
˚
˚
The size of the cavity is ca. 9.0 A¥ 9.9 A.
Different 1D chains are not stacked parallel, but in a staggered
mode to form a 3D supramolecular network (Fig. 2). Strong
H bond interactions between chains are observed in complex 1.
˚
˚
For example: H4–N4: 0.86 A, H◦4 ◊ ◊ ◊ Cl1A: 2.38 A, N4 ◊ ◊ ◊ Cl1A:
˚
3.20 A, ∠N4–H4 ◊ ◊ ◊ Cl1A: 157.9 (atom with additional label A
refers to the symmetry operations: y - 1, x, -z + 2). Due to the
molecular stacking, the solvent-accessible volume of the unit cell
3
˚
˚
of complex 1 was estimated to be 1212.7 A , which is approximately
Fig. 1 1D porous framework of complex 1 (C, N, O atoms and {CuN4Cl2}
octahedra are black, blue, red and green, respectively, H atoms and water
molecules are omitted for clarity).
3
9
35.1% of the unit-cell volume (3451.3(7) A ) (PLATON program ).
Strong H bond interactions also exist between solvent water
˚
and 1D chains, for example: H3–N3: 0.86 A, H3 ◊ ◊ ◊ OWB:
◦
˚
˚
In complex 1, the crystallographically unique ligand L exhibits
a cis-conformation (Scheme 1(a) and Fig. 1). The dihedral angles
between its phenyl ring and two pyridine rings are 41.5 and 40.9◦,
2.06 A, N3 ◊ ◊ ◊ OWB: 2.88 A, ∠N3–H3 ◊ ◊ ◊ OWB: 159.9 (atom
with additional label B refers to the symmetry operations: y, x,
-z + 1).
Fig. 2 1D chains are stacked in a staggered mode to form a 3D supramolecular network in complex 1 (H atoms and water molecules are omitted for
clarity).
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In complex 2, the corresponding dihedral angles are 37.6 and
O–Co–O bond angles range from 58.9(1) to 160.3(1)◦. The two N
atoms from L ligands occupy the axial positions of {CoO4N2}
octahedron and the equatorial positions are occupied by the
four carboxylate O atoms. Two of the O atoms are from the
chelating carboxylate group and the other two are from two bis-
monodentate carboxylate groups (Fig. 3). Different Co(II) centers
are linked by two strands of m-BDC to form a chain with an
◦
˚
37.7 , respectively. The Ni ◊ ◊ ◊ Ni separation is 14.408 A and the
˚
size of the cavity is ca. 9.2 ¥ 9.7 A. The solvent-accessible volume
3
˚
of the unit cell of complex 2 was estimated to be 1174.4 A ,
3
˚
which is approximately 34.4% of the unit-cell volume (3415.3 A )
(PLATON program9). Strong H bonds are also observed not
only between chains, but also between water and 1D chains in
˚
˚
˚
complex 2. For example: H4–N4: 0.86 A, H4 ◊ ◊ ◊ Cl1C: 2.45 A,
alternating Co ◊ ◊ ◊ Co separation of 4.421 and 7.029 A (Fig. 3).
◦
˚
˚
◦
N4 ◊ ◊ ◊ Cl1C: 3.26 A, ∠N4–H4 ◊ ◊ ◊ Cl1C: 157.6 ; H2–N2: 0.86 A,
As shown in Fig. 4, the 1D chains run through several directions.
L ligand links different chains and complex 3 exhibits a 3D
architecture. In complex 3, L shows an intermediate-conformation
(Scheme 1(b) and Fig. 4) and the dihedral angles between the
phenyl ring and two pyridine rings are 45.0 and 34.7◦, respectively
(Fig. 4). According to PLATON program,9 the solvent-accessible
˚
˚
H2 ◊ ◊ ◊ O3: 2.11 A, N2 ◊ ◊ ◊ O3: 2.92 A, ∠N2–H2 ◊ ◊ ◊ O3: 157.3
(atom with additional label C refers to the symmetry operations:
y, x, -z).
Thermogravimetric analyses (TGA) were carried out to examine
the thermal stability of the two complexes (Fig. S1, ESI†). The
samples were heated up to 1000 ◦C in N2.
3
˚
volume of the unit cell of complex 3 was estimated to be 1824.9 A ,
3
˚
It is found that the cavities of complexes 1 and 2 are occupied by
5.5 solvent water molecules per unit, as estimated by SQUEEZE
and TGA. The TGA curves of both complexes 1 and 2 show a one
which is approximately 30.5% of the unit-cell volume (5976.2 A ).
Strong H bonds are observed not only within the 3D architecture,
but also between solvent DMF and the 3D architecture. For
◦
˚
˚
˚
˚
˚
step weight loss of 11.4% between 30 and 210 C corresponding
example, H1–N1: 0.86 A,◦H1 ◊ ◊ ◊ O1D: 2.10 A, N1 ◊ ◊ ◊ O1D: 2.92 A,
to the loss of the water molecules (calc. 11.4 wt%) in the cavities
(Fig. S1(a) and (b), ESI†). The number of the water molecules and
the TG results are in agreement with the data for the isomorphic
Co(II)–L complex in previous work.7e Strong H bonds between
the solvent molecules and the host frameworks as well as the small
voids resulted from the staggered 1D chains in the structures of
complexes 1 and 2 can prevent solvent molecules to be easily lost.
As for complexes 1 and 2, the dehydrate MOFs remain stable
up to ~310 ◦C without any weight loss (Fig. S1(a) and (b), ESI†),
showing that 1D frameworks could retain structural integrity to
ca. 310 ◦C. It is evidenced by the powder XRD patterns of the
products, which are in good accordance with the simulated powder
XRD patterns of complexes 1 and 2 (Fig. S2(a) and (b), ESI†).
It can be concluded that complexes 1 and 2 has excellent thermal
stability in the class of MOFs.4–5 The high stability of the porous
1D MOFs may be attributed to the L-bridged double chains and
strong H bonds between chains.
∠N1–H1 ◊ ◊ ◊ O1D: 160.0 ; H3–N3: 0.86◦A, H3 ◊ ◊ ◊ O8: 2.09 A,
˚
N3 ◊ ◊ ◊ O8: 2.82 A, ∠N3–H3 ◊ ◊ ◊ O8: 143.4 (atom with additional
label D refer to the symmetry operations: D -x + 1, -y + 2,
-z + 1).
Fig. 4 1D chains in several directions (denoted in blue) linked by L ligands
to construct a 3D architecture in complex 3 (H atoms and solvent DMF
omitted for clarity).
Crystal structure and thermal stability of [Co(L)(m-BDC)]·DMF
(3). Complex 3 crystallizes in C2/c space group, its asymmetric
unit contains one crystallographically unique Co(II) ion, one
unique L ligand, one unique m-BDC and two unique half disso-
ciated DMF molecules. There are two crystallographically unique
carboxylate groups, one of them exhibits a chelating coordination
mode, and the other shows a bis-monodentate coordination mode
to link two Co(II) centers (Fig. 3).
As shown in Fig. S1(c), ESI,† complex 3 loses its solvent DMF
molecules in the range from 150 to 250 ◦C with a loss of 12.0 wt%
(calc. 11.9 wt%). The desolvated MOF doesn’t lose weight up to
~350 ◦C. The PXRD pattern of the desolvated sample is in good
accordance with the simulated PXRD pattern of complex 3 (Fig.
S2(c), ESI†), indicating the porous framework is retained in the
absence of guest DMF molecules.
Crystal structure and thermal stability of [Cd2(L)2(m-
BDC)2]·DMF·0.2H2O (4). Complex 4 crystallizes in C2/c space
group, its asymmetric unit contains one Cd(II), one L, one m-BDC,
half dissociated disordered DMF and one tenth dissociated water
molecule.
Cd(II) center in complex 4 exhibits a square pyramidal geometry
and is five coordinated by three oxygen atoms from two car-
boxylate groups of two m-BDC and two nitrogen atoms from
two L ligands. One of the two carboxylate group exhibits a
chelating coordination mode and another shows a monodentate
coordination mode with one O atom uncoordinated. The O–Cd–
O bond angles range from 54.1(1) to 125.1 (2)◦. The N–Cd–N
bond angle is 96.8(2)◦, indicating the neighboring two L ligands
are almost perpendicular to each other. The crystallographically
Fig. 3 1D chain linked by m-BDC in complex 3 with the Co ◊ ◊ ◊ Co
separation indicated by dashed lines (H atoms, solvent DMF and ligand
L omitted for clarity).
Each Co(II) ion is six-coordinated by four O atoms from three
carboxylate groups of three m-BDC and two N atoms from
two L ligands to furnish a distorted octahedral geometry. The
˚
Co–O bond lengths range from 2.006(4) to 2.349(4) A and the
9926 | Dalton Trans., 2010, 39, 9923–9928
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unique L ligand exhibits a trans-conformation (Scheme 1(c) and
Fig. 5) and the corresponding dihedral angles are 70.4 and 48.2◦,
respectively. Therefore complex 4 shows a corrugated 2D layer in
which bent L ligand and m-BDC act as bridging groups linking
different Cd(II) centers (Fig. 5).
via desolvation and salvation is probably due to the flexible L
ligand in the structure. The flexible C–N single bond can rotate
freely to some extent and it is possible not to damage the original
framework when solvent molecules in the channel are removed.
Color change and UV-Vis spectra
At room temperature, the color of complexes 1–4 is blue, green, red
and white, respectively. After being evacuated at 100 ◦C in vacuum
for 2 h, the color of complexes 1–3 becomes green, off-white and
purple, respectively except that the color of complex 4 is unchanged
and still white. The PXRD patterns of the evacuated samples are
similar from those simulated using the single-crystal data, which
indicates that the host frameworks of the four complexes are kept
via desolvation (Fig. S2, ESI†). The experiment shows the color
of the hydrated and dehydrated complex 1 is blue and green,
respectively (Fig. S3, ESI†). Immerse the desolvated complex 1
into water and the green solid becomes blue immediately. Whereas
immerse in other solvents such as DMF, acetone, tetrahydrofuran,
the color of the green solid is unchanged (Fig. S3, ESI†). The
PXRD pattern of the re-hydrated sample is similar to the simulated
PXRD pattern of complex 1 (Fig. S2(a), ESI†). It is indicated that
the solvent water molecules can reversibly enter into the cavities of
complex 1, accompanying with the color change of the sample. No
color change is observed in other solvents such as DMF, acetone,
tetrahydrofuran, etc., it is probably due to their bigger size than
that of water and they can’t enter into the cavities of complex
1 freely. The phenomenon is in agreement with our single-crystal
data and TG result, it is indicated that only solvent water molecules
can locate in the cavities of the complex.
Fig. 5 The corrugated 2D layer of complex 4 (H atoms, dissociated DMF
and water omitted for clarity).
Different layers in complex 4 stacked in an AA sequence
via strong interlayer H-bonds and complex 4 shows a 3D
supramolecular architecture (Fig. 6). For example: H2–N2:
˚
◦
˚
˚
0.86 A, H2 ◊ ◊ ◊ O1E: 2.28 A, N2 ◊ ◊ ◊ O1E: 3.05 A, ∠N2–H2 ◊ ◊ ◊ O1E:
˚
˚
˚
149.7 ; H4–N4: 0.86 A, H4 ◊ ◊ ◊ O2F: 2.12 A, N4 ◊ ◊ ◊ O2F: 2.96 A,
∠N4–H4 ◊ ◊ ◊ O2F: 163.4◦ (atom with additional labels E, F refer to
the symmetry operations: E -x + 1/2, y - 1/2, -z + 3/2; F -x + 1/2,
-y + 3/2, -z + 1). According to PLATON program,9 the solvent-
accessible volume of the unit cell of complex 4 was estimated to be
3
˚
1103.5 A , which is approximately 20.1% of the unit-cell volume
3
˚
(5482.1 A ). However, no strong H bond is observed between
As for complex 2, the color of the hydrated and dehydrated
sample is green and off-white, respectively (Fig. S4, ESI†). More
interestingly, when the dehydrated complex 2 is left to stand in an
open vessel, the dehydrated off-white sample can absorb the water
in air and becomes green quickly. When dried in oven, the color
is recovered to be off-white again (Fig. S4, ESI†). The PXRD
patterns of the dehydrated and re-hydrated samples are similar
to the simulated PXRD pattern of complex 2 (Fig. S2(b), ESI†),
indicating water molecules can reversibly enter into the cavities of
complex 2. The obvious color change and the sensitivity to water
molecules shows the two complexes, complexes 1 and 2 are good
candidates for water indicator. Their stable host frameworks make
it feasible to be recycled as water indicator.
solvents and the 2D layers.
As for complex 3, the color of the solvated and desolvated
sample is red and purple, respectively (Fig. S5, ESI†). The PXRD
patterns of the solvated and desolvated samples are similar to
the simulated PXRD pattern of complex 3 (Fig. S2(c), ESI†),
indicating the host framework of complex 3 is stable. However,
when immerse the desolvated sample in solvents such as DMF
and water, only subtle color changes happen, and no obvious color
change can be observed by naked eyes, indicating the color change
via re-absorption is not pronounced and sensitive, and complex 3
is not a feasible candidate for solvent indicator.
Fig. 6 The AA stacking mode of the wave-like layers in complex 4 (H
atoms, solvent DMF and water omitted for clarity).
Complex 4 shows a one step weight loss of 6.1% between 30 and
230 ◦C corresponding to the loss of the water and DMF molecules
(calc. 6.0 wt%) in the structure. The desolvated MOF doesn’t lose
◦
weight up to ~390 C. (Fig. S1(d), ESI†). The PXRD pattern of
the desolvated MOF in excellent agreement with the simulated
PXRD pattern of complex 4 (Fig. S2(d), ESI†), shows it retains
the structural integrity up to ca. 300 ◦C.
As described above, complexes 1–4 all possess stable host
frameworks. In terms of Kitagawa’s classification, they belong to
the second generation of porous MOFs.5 The structural retention
As for complex 4, the color of the solvated and desolvated
sample is the same (white). Though the PXRD patterns of the
solvated and desolvated samples are similar to the simulated
PXRD pattern of complex 4 (Fig. S2(d), ESI†), indicating the
host framework of complex 4 is stable, it is not possible to use the
complex to act as solvent indicator.
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9923–9928 | 9927
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The UV-Vis spectrum of complex 1 in solid state exhibits
a strong reflection peak at 475 nm in the range of 200–800
nm, whereas the reflection peak of the UV-Vis spectrum of the
dehydrated sample shifts to 496 nm. Complex 2 and its dehydrated
sample display visible radiation with lmax at 481 and 450 nm,
respectively. Complex 3 shows an intense reflection peak at 408
nm, a weak shoulder peak at 604 nm and a weak reflection peak
at 790 nm. In the UV-Vis spectrum of the desolvated sample, the
reflection peaks shift to 416, 630 and 830 nm, respectively. As for
complex 4, no obvious peaks are found in the UV-Vis spectra of
the solvated and desolvated samples. The UV-Vis spectra results
are agreement with the color changes observed by naked eyes in
our experiment.
As described above, complexes 1–3 all show reversible color
changes via desolvation and salvation, whereas complex 4 doesn’t
possess the property. What is the origin of the color changes? Re-
cently, Tao’s group has reported a metal-complex, Fe(tpa)(NCS)2
(tpa = tris(2-pyridylmethyl)amine), which exhibits a dramatic color
change upon exposure to methanol vapour. The color change is
due to the strong H bond interactions between the metal-complex
and the solvent.10 Similarly, as described above, in our work, strong
H bond interactions are found between the host frameworks and
solvents in the structure of complexes 1–3. Whereas in complex 4,
no strong solvent-induced H bond is observed. The mechanism
of color change is probably based on the solvent-induced H-
bonds which change the crystal packing of MOFs and lead to the
molecular distortion to some extent,10 then the energy alteration
of the visible d–d transition is expected.2
Financial support from the 973 Program (2006CB932903,
2007CB815303), NSFC (20731005, 20821061, and 20873151),
NSF of Fujian Province (E0520003), Key Project from CAS,
the Fundamental Research Funds for the Central Universities
(No. CDJZR10 22 00 09), Chongqing University Postgraduates’
Science and Innovation Fund (No. CDJXS10 22 11 43) and
Scientific Research Start-up Foundation of Chongqing University
are gratefully acknowledged.
References
1 (a) C. Reichardt, Chem. Rev., 1994, 94, 2319; (b) V. W. Yam, K. M. Wong
and N. Zhu, J. Am. Chem. Soc., 2002, 124, 6506; (c) K. Balakrishnan,
A. Datar, T. Naddo, J. Huang, R. Oitker, M. Yen, J. Zhao and L. Zang,
J. Am. Chem. Soc., 2006, 128, 7390; (d) J. C. Nelson, J. G. Saven, J. S.
Moore and P. G. Wolynes, Science, 1997, 277, 1793; (e) D. G. Yablon
and A. M. Schilowitz, Appl. Spectrosc., 2004, 58, 843; (f) J. Aimi, Y.
Nagamine, A. Tsuda, A. Muranaka, M. Uchiyama and T. Aida, Angew.
Chem., Int. Ed., 2008, 47, 5153.
2 (a) R. W. Soukup and K. Sone, Bull. Chem. Soc. Jpn., 1987, 60, 2286;
(b) Y. Fukuda, M. Yasuhira and K. Sone, Bull. Chem. Soc. Jpn., 1985,
58, 3518; (c) W. Linert, R. F. Jameson and A. Taha, J. Chem. Soc.,
Dalton Trans., 1993, 3181.
3 (a) M. A. Rahman, M. S. Won, N. H. Kwon, J. H. Yoon, D. S. Park and
Y. B. Shim, Anal. Chem., 2008, 80, 5307; (b) P. J. Skrdla, S. S. Saavedra,
N. R. Armstrong, S. B. Mendes and N. Peyghambarian, Anal. Chem.,
1999, 71, 1332; (c) B. Garc´ıa, S. Aparicio, R. Alcalde, R. Ruiz, M.
J. Da´vila and J. M. Leal, J. Phys. Chem. B, 2004, 108, 3024; (d) D.
G. Cho, J. H. Kim and J. L. Sessler, J. Am. Chem. Soc., 2008, 130,
12163.
4 (a) O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi
and J. Kim, Nature, 2003, 423, 705; (b) S. L. James, Chem. Soc. Rev.,
2003, 32, 276; (c) G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange,
J. Dutour, S. Surble and I. Margiolaki, Science, 2005, 309, 2040.
5 (a) S. K. Ghosh, B. Sareeya and S. Kitagawa, Angew. Chem., Int. Ed.,
2008, 47, 3403; (b) M. Ohba, W. Kaneko, S. Kitagawa, T. Maeda and
M. Mito, J. Am. Chem. Soc., 2008, 130, 4475; (c) S. K. Ghosh, W.
Kaneko, D. Kiriya, M. Ohba and S. Kitagawa, Angew. Chem., Int. Ed.,
2008, 47, 8843; (d) R. Matsuda, R. Kitaura, S. Kitagawa, Y. Kubota,
R. V. Belosludov, R. C. Kobayashi, H. Sakamoto, T. Chiba, M. Takata,
Y. K aw a z o e a n d Y. M i t a , Nature, 2005, 436, 238; (e) S. Kitagawa and
R. Matsuda, Coord. Chem. Rev., 2007, 251, 2490.
6 (a) B. L. Chen, Y. Yang, F. Zapata, G. Lin, G. D. Qian and E. B.
Lobkovsky, Adv. Mater., 2007, 19, 1693; (b) Y. Q. Huang, B. Ding, H.
B. Song, B. Zhao, P. Ren, P. Cheng, H. G. Wang, D. Z. Liao and S. P.
Yan, Chem. Commun., 2006, 4906; (c) M. H. Zeng, X. L. Feng, W. X.
Zhang and X. M. Chen, Dalton Trans., 2006, 5294.
7 (a) Z. Q. Qin, M. C. Jennings and R. J. Puddephatt, Chem. Commun.,
2002, 354; (b) Z. Q. Qin, M. C. Jennings and R. J. Puddephatt, Inorg.
Chem., 2003, 42, 1956; (c) N. L. S. Yue, D. J. Eisler, M. C. Jennings
and R. J. Puddephatt, Inorg. Chem., 2004, 43, 7671; (d) N. Yue, Z.
Qin, M. C. Jennings, D. J. Eisler and R. J. Puddephatt, Inorg. Chem.
Commun., 2003, 6, 1269; (e) N. N. Adarsh, D. K. Kumar and P.
Dastidar, CrystEngComm, 2009, 11, 796.
8 (a) A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7; (b) X. F. Wang, Y. B.
Zhang, H. Huang, J. P. Zhang and X. M. Chen, Cryst. Growth Des.,
2008, 8, 4559.
9 A. L. Spek, Acta Crystallogr., Sect. A: Found. Crystallogr., 1990, 46,
C34The free water molecules were removed before the cited solvent
accessible volume was calculated by PLATON.
Conclusion
In conclusion, using L ligand, metal(II) salts and coligand,
m-H2BDC, four metal(II)-L complexes have been solvother-
mally synthesized and structurally characterized by single-crystal
X-ray diffraction. Complexes 1 and 2 are isomorphous and exhibit
chain-like frameworks. Complex 3 displays a 3D architecture
and complex 4 shows a corrugated 2D layer. Complexes 3 and
4 were prepared under similar condition except using different
metal salts. The work shows the species of metal salts plays an
important role in the structures of coordination polymers. Owing
to the flexibility of L ligand, L shows a cis-conformation in
complexes 1 and 2, an intermediate-conformation in complex 3
and a trans-conformation in complex 4. All the four complexes
possess stable host frameworks, and complexes 1–3 show reversible
color changes via desolvation and solvation. The obvious color
change of complexes 1 and 2 via dehydration and re-hydration,
and the sensitivity and selectivity to water as well as the convenient
recycling of the two complexes indicate complexes 1 and 2 are
potential good candidates for water indicator. This work shows
choosing suitable metal ions and ligands, it is possible to construct
stable porous MOFs for solvent indicators.
10 B. Li, R. J. Wei, J. Tao, R. B. Huang, L. S. Zheng and Z. P. Zheng, J.
Am. Chem. Soc., 2010, 132, 1558.
9928 | Dalton Trans., 2010, 39, 9923–9928
This journal is
The Royal Society of Chemistry 2010
©