Ion-exchange resin as a new tool for characterisation of coordination compounds and MOFs by NMR spectroscopy

Article abstract of DOI:10.1039/c9cc03721g

¹H and ¹³C NMR spectroscopy is used to investigate the organic constituents of metal complexes, MOFs and coordination compounds synthesised under solvothermal and precipitation conditions. The elucidation of the ligands in paramagnetic compounds bearing Cu²⁺ (d⁹), Gd³⁺ (f⁷), Eu³⁺ (f⁶), Fe³⁺ (d⁵), ions after treatment with a cationic exchange resin is possible. We prove the efficiency of two post-synthesis linker modifications on diamagnetic IRMOF-3 Zn²⁺ (d¹⁰) with ethyl isocyanate and benzyl bromide.

Full text of DOI:10.1039/c9cc03721g

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Reactivity differences of Sc
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DOI: 10.1039/C9CC03721G
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Ion-exchange resin as a new tool for characterisation of
coordination compounds and MOFs by NMR spectroscopy
a
b
b
Ayla Roberta Borges da Silva Galaço, Kleber Thiago de Oliveira, Maria Carolina Donatoni,
Received 00th January 20xx,
Accepted 00th January 20xx
c
a,d*
Richard I. Walton and Osvaldo Antonio Serra
DOI: 10.1039/x0xx00000x
www.rsc.org/
1H and 13C NMR spectroscopy is used to investigate the organic is chemically modified, and often long-range order and
constituents of metal complexes, MOFs and coordination crystallinity is lost. Furthermore, reactive functionality is
compounds synthesised under solvothermal and precipitation introduced that may be destroyed on acid or base treatment.
conditions. The elucidation of the ligands in paramagnetic MOFs represent a large class of porous compounds and they
2
+
9
3+
7
3+
6
3+
5
compounds bearing Cu (d ), Gd (f ), Eu (f ), Fe (d ), ions after offer enormous structural diversity and several possibilities to
6
-12
treatment with a cationic exchange resin is possible. We prove the create materials with specific properties;
hence accurate
efficiency of two post-synthesis linker modifications on chemical analysis is needed for characterisation. Because the
2
+
10
diamagnetic IRMOF-3 Zn (d ) with ethyl isocyanate and benzyl syntheses are carried out at high temperatures, for various
bromide.
days, some ligands may decompose, to generate new
1
3,14,15
unexpected structures.
The compositional analysis of coordination compounds, which
now includes the large class of coordination polymers and
metal-organic frameworks, can be challenging in the absence of
single crystal data. Many compounds bearing transition metal
and lanthanoid cations have relevant applications in chemistry
and apart from being present in biological systems, these ions
can also combine with thousands of organic ligands, to result in
compounds that deserve intense scientific study.
paramagnetic effect limits investigation by nuclear magnetic
resonance (NMR) and limits the use of this very elucidative
In this communication, we demonstrate that H and 13C NMR
1
spectroscopy can be employed to characterise complexes and
metal organic frameworks (MOFs) of paramagnetic ions without
the need to use conventional strong acid or base during
chemical processing. We determine the ligands present in the
previously reported samples of MOFs and coordination
2+
9
3+ 6
complexes containing paramagnetic Cu (d ), Eu (f ) and
1
-3
The
3+ 7
Gd (f ), Figure 1. We also report a study using a diamagnetic
Zn (d ) MOF, to which two different post-synthesis
2+ 10
modifications are performed. Here, we propose a soft
procedure based on a cationic ion exchange resin to determine
the organic constituents of coordination compounds and MOFs,
4
technique to shed light on chemical structures. The usual
strategy to analyse complexes containing paramagnetic ions is
to add a strong acid or base to the complex, to cleave the M-L
Figure 2. The metal ions bind to the resin sulfonyl groups (SO
3
)
5
bonds and to release the ion and the ligands. However, this
through electrostatic interactions, to release hydrogen ions to
protocol can partially or completely destroy the organic ligand,
to culminate in very difficult and unreliable analysis. A second
situation where analysis of ligand chemistry is challenging is
when metal-organic frameworks (MOFs) have been subjected
to post-synthesis modification: here, a proportion of the ligand
-
3
the medium. Resins displaying negative groups, such as SO ,
16-17
are classified as strongly acid cationic.
They were first used
to purify water and to separate ions, and they played an
important role during the Manhattan Project, which dealt with
18
uranium purification. In recent years, synthetic polymeric ion
exchangers have been applied as efficient adsorbents in
industrial wastewater treatment because they are easy to
remove and to regenerate for repeated use. Our group has
employed the Dowex 50WX12 ion exchange resin to dissolve
a. University of São Paulo, Chemistry Department, FFCLRP, Ribeirão Preto, Av.
Bandeirantes, 3900, 14040-901, Ribeirão Preto – SP, Brazil *oserra@usp.br
Departamento de Química, Universidade Federal de São Carlos, São Carlos, SP,
b.
1
9
1
3565-905, Brazil.
and to analyse rare earth trimetaphosphates.
c.
University of Warwick, Department of Chemistry, Coventry CV4 7AL, U.K
Universidade Federal do ABC, CCNH, Av. dos Estados, 5001, 09210-580, Santo
André – SP, Brasil
In this study, we used a strongly acid cationic ion exchange resin
to analyse the organic constituents of paramagnetic
compounds (Figure 2). To this end, the solid samples of
complexes or MOFs were mixed with the ion-exchange resin, to
d.
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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result in protonation of the anionic ligand (L) and release in its
acid form (HL) with trapping of the cations by the resin. This was
View Article Online
DOI: 10.1039/C9CC03721G
2
carried out in either D O or deuterated DMF or DMSO,
depending on the solubility of HL, and after separation from the
1
13
resin, the solutions were analysed by H NMR and C NMR
spectroscopy. The process is depicted in Figure 2. Compound
3 2
HKUST-1 (Cu (btc) ) (btc = benzene-1,3,5-tricarboxylate) was
2
0
the first to be subjected to this protocol. The resin captured
the Cu2+ ions, to release benzene-1,3,5-tricarboxylic acid, which
1
13
was then analysed by H NMR and C NMR (Figure 3). The
solvents, acids and water, were deuterated; the ion exchange
resin Dowex 50WX12 with 200-400 mesh and exchange
capacity of 1.7 meq/mL was employed.
Figure 2: Illustration of paramagnetic ions bonded to an ion exchange resin – (a)
non-strongly bonded ligands; (b) strongly bonded ligands
.
(
b)
(
a)
The PXRD (Figure S11) indicates the presence of at least one
impurity. However, this is not unreacted 2,4,6-ptc. The TGA
(Figure S2) is in agreement with the bulk composition; the
impurity phase(s) must be present in small amounts, or are
polymorphs of the major phase.
aldrich btc.001.001.1r.esp
DEUTERIUM OXIDE
(
a)
(
d)
1.00
(
c)
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
Chemical Shift (ppm)
aldrich btc.001.001.1r.esp
(
b)
.5H
.
.
Figure 1: Structure of (a) [Eu O (H
2 3 n
(Cu (btc) )] (d) IRMOF-3 [Zn O(bdc-NH ) ] . Oxygen atoms are shown in red, carbon
2
(ptc)
2
2
2 n 2
O)] (b) [Gd(ofd) 8H O] (c) HKUST-1
[
2
3
n
4
atons grey, and the metals as the third colour in each panel. See text for definition of
liagnds ptc, ofd, btc and bdc.
1.00
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
Chemical Shift (ppm)
1
We treated the Dowex 50WX12 cation exchanger with DCl 1.0 Figure 3: H NMR of benzene-1,3,5-tricarboxylate in D
mol.L solution three times, washed it with D O four times, and
2
2
O from (a) MOF HKUST-1
after treatment with the resin and (b) commercial standard obtained from Aldrich.
-1
dried it at 25 °C after filtration. Subsequently, we added 0.5 mL
of the treated resin (≈ an excess of 20 times), 0.5 mL of
deuterated N,N-dimethylformamide, and 0.5 mL of D O to 10
2
1
The H NMR signal at 8.78 ppm indicated the presence of C-H in
the benzene ring (Figure 4a). The signals at 128.1, 142.8, 150.1,
66.5 and 166.7 ppm present in the C NMR analysis confirmed
the presence of pyridine tricarboxylic acid (Figure S17). We
1
3
1
mg of MOF in a screw-caped recipient. After two days, the
solution was filtered off into an NMR-tube and analysed at 400
1
1
13
recorded the H NMR spectra of the ligands released from the
and 100 MHz for H and C NMR, respectively.
2
+
3+
1
Cu and Eu MOFs and compared them with the standard
ligands. We measured the T from the  of the signals at 8.68
ppm (H btc) and 8.78 (H ptc) (see SI, Fig S25). The same values
were obtained for both released and standard ligands with 1.25
Hz for H btc (T = 254 ms) and 1.37 Hz for H ptc (T = 232 ms).
The H NMR signal at 8.7 0.02 ppm (Figure 3a) perfectly fitted
1
with the signal expected for benzene-1,3,5-tricarboxylic acid
and was corroborated with a commercial standard (Figure 3b).
2
3
.
.
Similarly, treatment of 10 mg of the MOF[Eu
2 2 2 2 n
(ptc) 5H O) H O]
2
1
3
1
(ptc = 2,4,6-pyridinetricarboxylate) by the same protocol
All these results indicate the absence of paramagnetic ions in
solution after the ion exchange treatment.
allowed us to determine the presence of the ligand (for details
see the SI).21
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be achieved by direct synthesis alone.23,24 After the reaction the
MOFs often become polycrystalline in nature and, therefore,
rpm_2.001.001.1r.esp
DEUTERIUM OXIDE
View Article Online
DOI: 10.1039/C9CC03721G
determination of the modification becomes a challenge in the
absence of crystallographic methods. To analyse the post
modification method, we first synthesised the well-known
4 2 3 2
IRMOF-3 ((Zn O(bdc-NH ) ) (bdc-NH = 2-amino-benzene-1,4-
2.00
dicarboxylate) and then functionalised it with ethyl isocyanate
8
.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
Chemical Shift (ppm)
2
5-26
(IRMOF-3-EISCN) (details in the SI).
Treatment of 10 mg of
Figure 4: 1H NMR of pyridine tricarboxylic acid (H
emerged at 8.78 ppm.
3
ptc). Signal relative to ptc
the IRMOF-3-EISCN was done using the ion-exchange protocol
1
outlined before. The H NMR signals at 1.48 ppm indicated the
The Gd3+ ion has seven unpaired electrons, which makes its
coordination compounds strongly paramagnetic. We
synthesised the coordination compound Gd
o-phenylenedioxydiacetate) in order to test the efficiency of the
ion-exchange towards such a compound (SI). In this case, the
resin does not exchange the cation due to the high binding
presence of CH
attested to the presence of CH
Figure 6a). Finally, the 13C NMR chemical shifts at 16.2, 35.02
3
group, whereas the signals at 3.50 ppm
2
group and NH at 8.65 ppm
.
2
(ofd)
3
8H
2
O (ofd =
(
and 151.6 ppm confirmed the success of the post-synthetic
modification (Figure S14).
2
2
3
+
affinity between Gd and the organic ligand. In order to release
3
+
the Gd from the coordination complex, we exchanged it with
3
+
14
3+
Lu (f ). Among rare earth cations, diamagnetic Lu provides
the highest stability constants with most organic ligands.
Therefore, accomplishing the resin treatment described above,
the Gd3+ goes to the resin and the Lu diamagnetic
coordination compound can equally be analysed by NMR,
Figure 2b.
3+
O
O
OH
OH
O
gd2opd3_lucl3_x12.015.001.1r.esp
O
DMSO-d6
1
Figure 6: H NMR of the MOF IRMOF-3 post-synthesis functionalised with ethyl
isocyanate.
It is expected that the ligands in the centre of the crystal will be
difficult to reach during functionalisation, so only partial
modification occurs. By NMR it was possible to quantitatively
determine the amount of effective post modification.
Integrating the signals of the H at 7.59 ppm (1H) and 8.03 ppm
1
70
160
150
140
130
120
110
100
90
80
70
60
50
40
Chemical Shift (ppm)
Figure 5: 13C NMR of o-phenylenedioxydiacetic acid from the Gd
complex.
3 2
(ofd) O
.8H
2
(0.13 H), respectively, unmodified and modified compound, we
found the value of 11.5% of modified ligand, Figure 7, in
accordance with the TGA analysis (11.3%), (details in the SI).
Analysis of the 13C NMR spectra (Figure 5) allowed us to assign the Another post-synthetic modification was performed, with
chemical shifts at 115.1, 123.5, and 147.7 ppm to the aromatic ring benzyl bromide, one not previously reported, and was
carbons. The signal at 66.6 ppm corresponds to aliphatic carbon, and confirmed by the synthesis of the monomer standard as
1
illustrated in Scheme 1 (synthesis described in the SI). The H
the signal at 173.1 ppm referred to the C=O carbon. The strong
paramagnetism of compounds containing Gd3+ leads to broad NMR
peaks. If Gd3+ exchange for Lu3+ is not satisfactory, NMR peak
NMR signals at 5.07 ppm indicated the presence of CH group,
which allowed us to confirm the post modification. The signals
between 7 to 8 ppm are attributed to the H at the benzene
rings, Figure 8. For quantitative determination of the post-
modification, the integration of the signals at 7.65 (1H 0.22) and
2
1
broadening occurs even if only a small amount of gadolinium is
present in the medium. We also analysed the stable MOF MIL-53
3
+
[
Fe(OH)0.2F0.8(bdc)] containing paramagnetic Fe , by using the same
8
.34 (1H 0.71) lead to the value of 23.3%, Figure 5S(b), in
procedure, and the ligand bdc (benzene-1,4-dicarboxylate) was
confirmed as its acid by H NMR and 13C NMR spectroscopy. The data
are presented in Figure S18 in the SI.
agreement with the value found by TGA, 24%, SI.
1
In summary, the use of cationic ion exchange resin to remove
the paramagnetic ions proved to be a useful tool to elucidate
“
Post-synthetic” modification of MOFs can provide a method the ligands present in MOFs and coordination compounds.
for introducing functional groups into the materials, generating Unmodified organic constituents were determined in all cases.
a greater range of physical and chemical properties than could
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irmof_3_ethylisocianate.001.001.1r.esp
Notes and references
View Article Online
DOI: 10.1039/C9CC03721G
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8.05
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7.55
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Chemical Shift (ppm)
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Figure 7: H NMR Quantitative determination of post-functionalisation (IRMOF-3-
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bromide.
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1
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1.00 4.13 1.44 1.09 0.22
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2.07
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8.5
7.0
6.5
Chemical Shift (ppm)
6.0
5.5
4.5
18
1
2
2
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2
2
2
9
0
1
2
3
4
5
6
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Figure 8: H NMR of the MOF IRMOF-3 post functionalised with benzyl bromide.
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We acknowledge the Brazilian agencies CAPES, FAPESP, and
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to thank Vinicius Palaretti for NMR analysis.
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Conflicts of interest
There are no conflicts to declare.
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