Reversible and selective amine interactions of [Cd(μ2-N,O-p- NH2C6H4SO3)2(H 2O)2]n

Article abstract of DOI:10.1039/b400790p

[Cd(μ₂-N,O-p-NH₂C₆H₄SO ₃)₂(H₂O)₂]_n (1) is a layered coordination compound. The solid-vapor reactions between crystalline 1 and a series of volatile amines were investigated and the corresponding amine adducts were characterized by EA, TGA, PXRD and IR. Among them, the C ₂H₅NH₂ and C₃H₇NH ₂ adducts, namely [Cd(C₂H₅NH₂) ₄(H₂O)₂](H₂NC₆H ₄SO₃)₂ (3) and [Cd(C₃H ₇NH₂)₄(O-p-H₂NQH₄SO ₃)₂]-C₃H₇NH₂ (4), grew into single crystals in situ from the solid-vapor reaction processes and their crystal structures were characterized. In both cases, 4 mol equiv. of amine molecules coordinate to Cd(II) via replacing the N,O-p-NH₂C ₆H₄SO₃ ligands or coordinated water molecules. The single-phase product suggests that the solid-vapor reaction between the metal sulfonate and volatile alkylamines could be used as a green process to synthesize monoamine-coordinated Cd(II) complexes without any solvent and routine separation. Finally, the substitution reaction is reversible at room conditions and selective for primary alkylamines.

Full text of DOI:10.1039/b400790p

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Reversible and selective amine interactions of
[Cd(ꢀ2-N,O-p-NH2C6H4SO3)2(H2O)2]n †
Jin-Sen Zhou, Jiwen Cai,* Li Wang and Seik-Weng Ng
School of Chemistry & Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275,
P. R. China
Received 16th January 2004, Accepted 5th March 2004
First published as an Advance Article on the web 31st March 2004
[Cd(µ2-N,O-p-NH2C6H4SO3)2(H2O)2]n (1) is a layered coordination compound. The solid–vapor reactions
between crystalline 1 and a series of volatile amines were investigated and the corresponding amine adducts
were characterized by EA, TGA, PXRD and IR. Among them, the C2H5NH2 and C3H7NH2 adducts, namely
[Cd(C2H5NH2)4(H2O)2](H2NC6H4SO3)2 (3) and [Cd(C3H7NH2)4(O-p-H2NC6H4SO3)2]ؒC3H7NH2 (4), grew into
single crystals in situ from the solid–vapor reaction processes and their crystal structures were characterized. In
both cases, 4 mol equiv. of amine molecules coordinate to Cd() via replacing the N,O-p-NH2C6H4SO3 ligands or
coordinated water molecules. The single-phase product suggests that the solid–vapor reaction between the metal
sulfonate and volatile alkylamines could be used as a green process to synthesize monoamine-coordinated Cd()
complexes without any solvent and routine separation. Finally, the substitution reaction is reversible at room
conditions and selective for primary alkylamines.
different from what we obtained from X-ray single crystal
Introduction
structural analysis, which is [Cd(C3H7NH2)4(H2NC6H4SO3)2]ؒ
C3H7NH2 (4). The EA results reveal that 1 can quantitatively
uptake primary alkylamines. In particular, 1 took up to 4 equiv.
of CH3NH2 and C2H5NH2 and 5 equiv. of C3H7NH2. It is noted
that there is a sharp decrease in the molar amount of amines
adducted to 1 from 5 mol equiv. for C3H7NH2 (4) to 0.5 mol
equiv. for iso-C3H7NH2 (5), and 0.5–1.5 mol equiv. for the
C4H9NH2 isomers (6–8).
All the amine adducts are crystalline materials, with moder-
ate to strong reflection intensities, as shown in Fig. 1. TGA
results, shown in Fig. 2, reveal that 2–4 release amine molecules
below 150 ЊC, well before the decomposition of the framework
around 350 ЊC. Complexes 5–8 release amine molecules in mul-
tiple steps, with the second batch of release followed by the
decomposition of the framework. The TGA behaviour of the
CH3NH2 adduct (2) is similar to those of both the C2H5NH2 (3)
and C3H7NH2 (4) adducts, suggesting that the CH3NH2 adduct
2 could have a similar coordination structure as that of 3 and 4,
with 4 mol equiv. of CH3NH2 coordinated to Cd(). Since the
EA and TGA results match better with the formula of [Cd-
(NH2C6H4SO3)(CH3NH2)4], it is obvious that the coordinated
water molecules have been replaced in the coordination sphere.
Reversible solid–gas or solid–vapor reactions of metal–organic
compounds have attracted much attention recently due to their
potential application as functional materials, such as chemical
sensors for toxic gases or volatile organic compounds.1–5 The
guest up-taking capabilities of the metal–organic frameworks
of interest are caused by vacant or expanded coordination sites
of the metal centers,2 also, by the accessible lattice space of
the crystalline supramolecular frameworks.4,5 Reversible solid–
vapor substitution reaction occurring under mild conditions
represents another unique category with only a few known
examples.1
On the other hand, metal sulfonates are a class of com-
pounds showing interesting functional properties6 contributed
by the flexible and soft interaction between the metal atom and
the sulfonate groups. Côté and Shimizu have reviewed recent
progress in this field.7 Herein, as part of our systematic investi-
gation into the solid state chemistry of metal sulfonates,8,9 we
report the reversible solid–vapor substitution reactions of a
neutral polymeric compound [Cd(µ2-N,O-p-NH2C6H4SO3)2-
(H2O)2]n (1) with volatile alkylamines with C ≤ 4 at room
conditions.
Results and discussion
Characterization of the amine adducts
The matrix compound 1 does not react with NH3, (C2H5)2NH,
PhNH2, pyridine, as well as CH3OH, CH3COOC2H5 and HCl,
indicating that the reaction of 1 with primary amines is select-
ive. The stoichiometry of the reaction product was established
by elemental analyses and the amine adducts were further
characterized by TGA, IR and PXRD. The matrix complex 1,
and C2H5NH2 and C3H7NH2 adducts 3 and 4 were also char-
acterized by X-ray single crystal structural analyses. Since the
C3H7NH2 adduct releases amine molecules once it is removed
from the reaction vessel, the chemical formula derived from
EA and TGA, which is Cd(H2NC6H4SO3)2(C3H7NH2)2.5 (4Ј), is
† Electronic supplementary information (ESI) available: Fig. S1: IR
spectra of [Cd(µ2-N,O-p-NH2C6H4SO3)2(H2O)2]n (1) and the amine
adducts (2–8). Fig. S2: The reaction vessel containing the C3H7NH2
adduct 4, showing the wet crystals. Table S1: elemental analysis results.
See http://www.rsc.org/suppdata/dt/b4/b400790p/
Fig. 1 PXRD of [Cd(µ2-N,O-p-NH2C6H4SO3)2(H2O)2]n (1) and the
amine adducts (2–8).
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 1 4 9 3 – 1 4 9 7
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produce a two-dimensional layered structure, as shown in
Fig. 3(a). Inter-layered hydrogen bonds formed between the
coordinated water molecules and the –NH2 groups with the
Ϫ
free –SO3 oxygen atoms generate an extended 3-D structure,
as shown in Fig. 3(b), which is very similar to that of [Cd-
(1,5nds)(H2O)2]n (1,5nds = 1,5-naphthalenedisulfonate).9 TGA
analysis shows that 1 is stable until 100 ЊC when the water
molecules are released. As shown in Fig. 4, the dehydrated
compound 1Ј is also a crystalline material, in which the
inter-layered distance shrinks from 9.37 Å in 1 to 8.77 Å in
1Ј. Interestingly, 1Ј is a stable phase at room conditions, as
shown by the unchanged PXRD pattern after air exposure for
12 h. However, when 1Ј is exposed to water moisture, it can
absorb water and rearrange to 1. Therefore, the dehydration
process of 1 is reversible.
Fig. 2 TGA of [Cd(µ2-N,O-p-NH2C6H4SO3)2(H2O)2]n (1) and the
amine adducts (2–8).
The nature of the amine interactions with 1 is more com-
plicated in the cases of iso-C3H7NH2 and the C4H9NH2 isomers
(5–8). Nevertheless, the amines released at a much higher
temperature than that of their corresponding boiling points,
indicating that coordinative interactions are involved.
In the IR spectra of all the amine adducts, there are alkyl
C–H stretching bands observed around 2900–3000 cmϪ1 con-
tributed by the alkylamines, confirming the existence of the
adducted amine molecules.
Structure of the matrix compound [Cd(ꢀ2-N,O-p-NH2C6H4-
SO3)2(H2O)2]n (1)
The crystal structure of 1 is shown in Fig. 3. Its Co2ϩ and Zn2ϩ
analogues have been reported and all three of them have similar
coordination and crystal structures.10 Cd() is coordinated
by two –NH2 and two –SO3 groups from four different
Fig. 4 PXRD showing the dehydration and hydration process of
[Cd(µ2-N,O-p-NH2C6H4SO3)2(H2O)2]n (1). Its dehydrated form is
denoted as 1Ј.
Ϫ
NH2C6H4SO3 ligands, and two water molecules in the axial
positions, resulting in an octahedral coordination sphere. The
Ϫ
p-NH2C6H4SO3 anions behave as µ2 bridging ligands to
It is interesting to note that the title compound 1 has very
similar layered structure as that observed in [Cd(1,5nds)-
(H2O)2]n,9 in which the Cd() is also coordinated octahedrally
with two water molecules occupying the axial positions and
the layers were connected by hydrogen bonds formed between
the coordinated water molecules and the sulfonate oxygens.
However, these two cadmium sulfonates show different
reactivity toward NH3 and secondary amines. 1 does not react
with either NH3 or the secondary amine (C2H5)2NH. In
contrast, [Cd(1,5nds)(H2O)2]n can uptake up to 3 mol equiv. of
NH3 and also react with secondary amines.9
Crystal structures of 3 and 4
In the case of C2H5NH2 exposure, the powder matrix com-
pound 1 became solidified, and then the volume expanded.
Single crystals of [Cd(C2H5NH2)4(H2O)2](H2NC6H4SO3)2 (3)
suitable for X-ray analyses were found among the final powder
crystalline product. Structural analysis shows that 4 equiv. of
Ϫ
amine molecules substitute the p-NH2C6H4SO3 ligands from
the Cd() coordination sphere, while the water molecules
remained coordinated to the metal center, as shown in Fig. 5.
The Cd–N distances are 2.334(2) and 2.3505(19) Å, while the
bond length of Cd–OH2 is 2.4629(17) Å.
When crystalline material 1 was exposed to C3H7NH2 vapor,
it became solidified, damp, and then crystalline material
appeared which further grew into single crystals suitable for
X-ray structural analyses. The crystal structure of [Cd(C3H7-
NH2)4(O-p-H2NC6H4SO3)2]ؒC3H7NH2 (4) is shown in Fig. 6(a).
4 equiv. of amine molecules replaced the two coordinated
Fig. 3 The 2-D layered structure of [Cd(µ2-N,O-p-NH2C6H4SO3)2-
(H2O)2]n (1) (a), and its 3-D extended structure showing the inter-
layered hydrogen bonds formed between the –SO3 oxygen and both the
water and –NH2 groups.
Ϫ
water molecules and –NH2 groups of the two p-NH2C6H4SO3
ligands. The sulfonate group remained coordinated to Cd(),
D a l t o n T r a n s . , 2 0 0 4 , 1 4 9 3 – 1 4 9 7
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Ϫ
reaction, where the sulfonate groups of the p-NH2C6H4SO3
ligands still remain coordinated to Cd(). Further experiments
are required in order to decipher this behaviour, which could
be caused by steric and electronic factors. On the other hand,
based upon what we have documented about the readily
modified coordination mode of sulfonate due to its weak
coordination nature,8 what is observed here is not unexpected.
When single crystals of 1 were used directly for amine uptake,
the crystals fractured and became a white powder immediately.
In the case of C2H5NH2, as shown in Fig. 7, the PXRD simu-
lated by the single crystal structure is very similar to the PXRD
pattern of the sample obtained by grinding the polycrystalline
material and single crystals of 3 together. The EA results also
match the formula of the single crystal, indicating that the
reaction is complete with 3 as the only phase. However, in
the case of C3H7NH2, since 4 released amine molecules rapidly
when it was removed from the reaction vessel, characterization
of the batch product 4 could not be conducted. Nevertheless,
according to the consistency of the crystalline appearance,
it can be tentatively assumed that the resulting complex consists
of a uniform phase. Two single crystals selected from the same
batch were submitted for X-ray analyses and showed very
similar cell parameters.
Fig. 5 Coordination structure of [Cd(C2H5NH2)4(H2O)2](H2NC6-
H4SO3)2 (3).
Fig. 7 PXRD tracking the reversible transformations between [Cd-
(µ2-N,O-p-NH2C6H4SO3)2(H2O)2]n (1) and the C2H5NH2 adduct
[Cd(C2H5NH2)4(H2O)2](H2NC6H4SO3)2 (3).
A = C2H5NH2 vapor.
Reflections contributed by the third phase are marked by *.
It is remarkable that both 3 and 4 grew into single crystals
in situ in the solid–vapor reactions. However, with the replaced
water molecules and the amine deposited from the vapor as the
potential solvent, it is not clear whether the transformation
between the matrix compound 1 and its final amine adduct is a
solid-to-solid transformation or localized dissolution of one
phase with subsequent growth of another is involved.
Fig.
6
Coordination structure of [Cd(C3H7NH2)4(O-p-H2NC6H4-
SO3)2]ؒC3H7NH2 (4) (a) and its packing structure showing the included
amine molecules in channels running along the a axis (b).
with the Cd–O distance expanded significantly from 2.294(2) Å
in 1 to 2.474(4) and 2.453(4) Å in 4. Interestingly, molecules
packed along the crystallographic a axis form void spaces of
200 Å3 per unit cell, which accommodates one free amine
molecule, as shown in Fig. 6(b). In other words, 1 can take
up to 5 equiv. of C3H7NH2 and form crystalline 4. Both the
substitution and inclusion reactions occur during the reaction
process. The calculated crystal density of compound 4 is
1.314 g cmϪ3, which is significantly lower than that of the
matrix compound 1 with a density of 2.033 g cmϪ3. When an
unprotected crystal of 4 was exposed to the air, it lost its
diffraction intensity quickly.
As a closely related family, water-coordinated metal phos-
phonates also display interesting amine up-taking properties
after dehydration and have attracted extensive investi-
gations.11,12 The amine intercalation process could be topotactic
or imparted by a guest-directed phase transformation. How-
ever, in most cases, the solid-state structures of the resulting
adducts formed via solid–vapor or solid–liquid reaction cannot
be explored directly. The up-taking of guest molecules causes
considerable stress and intermolecular reorganization in the
metal–organic framework. With the loss of crystallinity, the
characterization of the solid-state structures becomes quite
challenging. One of the common approaches to overcome this
problem is to grow single crystals of the targeted compound
and compare the simulated PXRD pattern with that of the
reaction product.4 This method is only achievable when the
targeted compound can be crystallized and the phases obtained
It is noted that complex 1 shows a distinct reactivity towards
C2H5NH2 and C3H7NH2. Four equiv. of C2H5NH2 exclude the
Ϫ
p-NH2C6H4SO3 ligands from the Cd() coordination sphere.
However, this does not occur in the case of the C3H7NH2
D a l t o n T r a n s . , 2 0 0 4 , 1 4 9 3 – 1 4 9 7
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from the solid state reaction and from solution are the same.
Therefore, our results reported herein, with the growth of single
crystals in situ of a solid–vapor reaction process is rather
unique, which made direct structural characterization possible.
a series of volatile primary amine molecules with C ≤ 4. The
corresponding amine adducts were characterized. The results
show that single phase monoamine coordinated Cd() com-
plexes could be obtained via solid–vapor reaction between the
metal sulfonate and amine vapor, without using any solvent and
routine separation. Moreover, the substitution reaction process
is reversible under mild conditions and selective for primary
alkylamines, which could be regarded as an advantage to
further explore the potential application of the stable crystal-
line material 1 as a chemical sensor or switch for volatile
primary alkylamines. As far as we are aware, the fact that 3
and 4 were obtained as single crystals in situ from a solid–
vapor substitution reaction has never been reported for a
coordination compound.
Investigation into the intercalation process and its reversibility
PXRD recorded every 24 h showed that after C2H5NH2
exposure for one day, reflections contributed by 3 appeared,
together with reflections contributed by 1 and a third phase.
After 2 weeks, both reflections contributed by 1 and the third
phase disappeared, and 3 was the only phase obtained. As
shown in Fig. 7, when 3 was exposed in the air for one day,
reflections contributed by 3 almost disappeared, while those
corresponding to 1 appeared, together with reflections contri-
buted by the third phase observed during the 1 to 3 transform-
ation process, as represented by reflections around 10.0, 21.0
and 23.4Њ. After 8 days, reflections contributed by the third
phase also disappeared and the diffraction pattern became
identical to that of 1 simulated by the single crystal structure.
This observation indicates that the substitution reaction,
including the breakdown and formation of four coordinative
bonds, is completely reversible at room conditions. Moreover,
the phase transformations between 1 and 3 involve a third
phase associated with either 1 or 3.
Similar amine-releasing processes at room temperature were
observed in all the other complexes. In the case of both 2 and 4,
the phases obtained after losing all the amine molecules have to
be exposed to moisture in order to form the phase identical
to that of 1. This observation is consistent with the fact that the
two water molecules were substituted by the amine molecules
and are not in the coordination sphere anymore, which is differ-
ent from that observed in 3. In order to accelerate the amine-
releasing process, amine adducts 2–4 were heated to 150–200 ЊC
to remove the amine molecules. As shown in Fig. 8, the phases
generated after amine releasing of 3 and 4 are identical to that
of 1Ј. By contrast, the phase 1Љ generated by 2 is different.
However, after moisture exposure, 1Љ rearranged back to 1.
The readily achieved transformations between 1 and its
amine adducts could be contributed by the compatible co-
Ϫ
ordination strength of the –NH2 and –SO3 groups of the
Ϫ
NH2C6H4SO3 ligand, the –NH2 group of the alkylamine and
the water molecule. By nature, the reaction process of metal
sulfonates with amine vapor could be understood as a solid-
state substitution reaction driven by Lewis acid–base inter-
actions. Primary experiments show that cadmium salts other
than the sulfonates, such as CdCl2 and CdCO3, do not react
with amine vapor under the same reaction condition
investigated for 1 and [Cd(1,5nds)(H2O)2]n, 9 indicating that the
sulfonate group is critical to drive the amine intercalating
process.
Experimental
Materials and methods
All materials were commercially available and used without
further purification. Elemental analyses (C, H, N) were con-
ducted by the Vario EL elemental analyzer. PXRD (Powder
X-ray diffraction) was performed on a Rigaku D/Max-IIIA
diffractometer operated using a 0.02Њ step scan from 3 to 70Њ in
2θ and 0.2 s preset time. Infrared spectra were obtained as KBr
pellets on a Bruker EQUINOX 55 FTIR spectrometer. TG-IR
(thermogravimetry-infrared) was carried out with a Netzsch
TG-209 analyzer under flowing nitrogen.
Synthesis
[Cd(µ2-p-N,O-NH2C6H4SO3)2(H2O)2]n (1): sulfanilic acid
(17.319 g, 0.1 mol) was dissolved in distilled water (400 mL) at
343 K and cadmium acetate dihydrate (13.317 g, 0.05 mol) was
added to the above solution. After stirring for 1 h and concen-
tration, the white precipitate was collected as [Cd(µ2-N,O-p-
NH2C6H4SO3)2(H2O)2], yield 59%. Single crystals suitable for
diffraction data collection were obtained via slow evaporation
of the saturated aqueous solution.
Solid–vapor reaction
Crystalline 1 and the amine liquid were placed in two different
but connected vessels. The reaction process was monitored via
the net weight gain. It took a few days to 2 weeks for the
reaction process to be complete. The adducts were placed in an
N2 flow to remove the deposited amine and moisture on the
crystalline surface before submission for characterization. All
solid–vapor reactions were performed at room temperature.
Single crystals of the C2H5NH2 adduct 3 were found among the
polycrystalline product. In the case of C3H7NH2, all the adduct
4 grow into large and shiny crystals. For the reversibility study,
the amine adduct was either exposed in the air and its PXRD
was recorded every day, or heated at 150–200 ЊC for 2 h and its
PXRD was recorded.
Fig. 8 PXRD showing that the phases obtained via heating the amine
adducts at 150–200 ЊC are identical to that of dehydrated [Cd(µ2-N,O-p-
NH2C6H4SO3)2(H2O)2]n (1Ј) in the cases of [Cd(C2H5NH2)4-
(H2O)2](H2NC6H4SO3)2 (3) and [Cd(C3H7NH2)4(O-p-H2NC6H4SO3)2]ؒ
C3H7NH2 (4), or different from that of 1Ј but can be converted back to
1 after moisture exposure.
Summary
We have presented the systematic investigation into the solid–
vapor substitution reactions between the title compound 1 and
D a l t o n T r a n s . , 2 0 0 4 , 1 4 9 3 – 1 4 9 7
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Table 1 Crystallographic data
Compound
1
3
4
Empirical formula
Mr
C12H16CdN2O8S2
492.79
C20H44CdN6O8S2
673.13
C27H57CdN7O6S2
752.32
T /K
λ/Å
293(2)
0.71073
293(2)
0.71073
293(2)
0.71073
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/c
Triclinic
P1
¯
a/Å
b/Å
c/Å
α/Њ
7.210(4)
5.962(3)
18.975(10)
90
99.336(9)
90
804.9(8)
2, 2.033
1.663
10.680(3)
7.863(2)
18.906(6)
90
100.152(6)
90
1562.8(8)
2, 1.430
7.9686(9)
15.4694(19)
15.7489(19)
88.932(2)
88.548(2)
78.565(2)
1902.0(4)
2, 1.314
β/Њ
γ/Њ
V/Å3
Z, Dc/g cmϪ3
µ/mmϪ1
0.880
0.728
F(000)
492
700
792
Crystal size/mm
θ Range/Њ
Limiting indices, hkl
Reflections collected
0.50 × 0.37 × 0.28
3.57–27.03
Ϫ8/9, Ϫ7/6, Ϫ22/24
4696
0.47 × 0.26 × 0.20
2.81–29.87
Ϫ6/14, Ϫ10/9, Ϫ26/25
10887
0.43 × 0.29 × 0.12
2.59–25.00
Ϫ9/9, Ϫ18/17, Ϫ18/18
14060
Unique (Rint
Completeness (%)
)
1732 (0.0391)
98.2
4171 (0.0170)
92.7
6686 (0.0246)
99.5
Data/restraints/parameters
Goodness-of-fit
Final R indices [I > 2σ(I )]
1732/0/118
1.195
R1 = 0.0278
wR2 = 0.0735
4171/0/177
1.032
R1 = 0.0301
wR2 = 0.0791
6686/12/371
1.020
R1 = 0.0458
wR2 = 0.1178
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2003, 19, 81; J. W. Cai, C. H. Chen, C. Z. Liao, X. L. Feng and
C. M. Chen, Acta Crystallogr., Sect B, 2001, 57, 520.
9 J. Cai, J.-S. Zhou and M.-L. Lin, J. Mater. Chem., 2003, 13, 1806.
10 V. Shakeri and S. Haussuhl, Z. Kristallogr., 1992, 198, 165.
11 D. M. Poojary and A. Clearfield, J. Am. Chem. Soc., 1995, 117,
11278.
X-Ray crystallography
Single crystal X-ray data were collected on a Bruker SMART
1000 CCD diffractometer at room temperature. Absorption
corrections were applied. The space groups were determined
from systematic absences and confirmed by the results of
refinement. The structures were solved using direct methods
and all non-H atoms were refined with anisotropic displace-
ment parameters.13 All H atoms of the organic ligands were
placed at idealized positions and refined as riding atoms. The
crystallographic data were listed in Table 1. The single crystal
of 4 was covered by wax and therefore protected from
decomposition before it was submitted for data collection.
CCDC reference numbers 222142-222144.
See http://www.rsc.org/suppdata/dt/b4/b400790p/ for crystal-
lographic data in CIF or other electronic format.
Acknowledgements
We are thankful for the financial support from the National
Natural Science Foundation of China (Grant Nos. 20271053
and 20131020).
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D a l t o n T r a n s . , 2 0 0 4 , 1 4 9 3 – 1 4 9 7
1497