Postsynthetic Modification of Metal-Organic Frameworks through Nitrile Oxide-Alkyne Cycloaddition

Article abstract of DOI:10.1021/acs.inorgchem.8b00126

Postsynthetic modification of metal-organic frameworks is an important method to tailor their properties. We report on the nitrile oxide-alkyne cycloaddition (NOAC) as a modification tool, a reaction requiring neither strained alkynes nor a catalyst. This is demonstrated with the reaction of nitrile oxides with PEPEP-PIZOF-15 and -19 at room temperature. PIZOF-15 and -19 are porous Zr-based MOFs (BET surface areas 1740 and 960 m² g^-1, respectively) consisting of two mutually interpenetrating UiO-type frameworks with linkers of the type ^-O₂C[PE-P(R¹,R²)-EP]CO₂^- (P, phenylene; E, ethynylene; R¹ and R², side chains at the central benzene ring with R¹ = R² = OCH₂C≡CH or R¹ = OCH₂C≡CH and R² = O(CH₂CH₂O)₃Me). Their syntheses, using benzoic acid as a modulator, and their characterization are reported herein. The propargyloxy (OCH₂C≡CH) side chains contain the ethyne moieties needed for NOAC. Formation of nitrile oxides through oxidation of oximes in aqueous ethanolic solution in the presence of PEPEP-PIZOF-15 and -19 resulted in the reaction of 96-100% of the ethyne moieties to give isoxazoles. Thereby the framework was preserved. The type of nitrile oxide RCNO was greatly varied with R being isopentyl, tolyl, 2-pyridyl, and pentafluorophenyl. A detailed NMR spectroscopic investigation showed the formation of the 3,5-disubstituted isoxazole to be clearly favored (≥96%) over that of the constitutional isomeric 3,4-disubstituted isoxazole, except for one example.

Full text of DOI:10.1021/acs.inorgchem.8b00126

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Postsynthetic Modification of Metal−Organic Frameworks through
Nitrile Oxide−Alkyne Cycloaddition
Tobias von Zons,† Luisa Brokmann,† Jann Lippke,‡ Thomas Preuße,† Miriam Hulsmann,†
̈
Andreas Schaate,‡ Peter Behrens,‡ and Adelheid Godt*,†
†Faculty of Chemistry and Center for Molecular Materials (CM ), Bielefeld University, Universitatsstraße 25, 33615 Bielefeld,
̈
2
Germany
‡Institute of Inorganic Chemistry and ZFM-Center for Solid State Chemistry and New Materials, Leibniz University Hannover,
Callinstraße 9, 30167 Hannover, Germany
S
* Supporting Information
ABSTRACT: Postsynthetic modification of metal−organic frameworks is an important method to tailor their properties. We
report on the nitrile oxide−alkyne cycloaddition (NOAC) as a modification tool, a reaction requiring neither strained alkynes nor
a catalyst. This is demonstrated with the reaction of nitrile oxides with PEPEP-PIZOF-15 and -19 at room temperature. PIZOF-
15 and -19 are porous Zr-based MOFs (BET surface areas 1740 and 960 m2 g−1, respectively) consisting of two mutually
−
−
interpenetrating UiO-type frameworks with linkers of the type O2C[PE-P(R1,R2)-EP]CO2 (P, phenylene; E, ethynylene; R1
and R2, side chains at the central benzene ring with R1 = R2 = OCH2CCH or R1 = OCH2CCH and R2
=
O(CH2CH2O)3Me). Their syntheses, using benzoic acid as a modulator, and their characterization are reported herein. The
propargyloxy (OCH2CCH) side chains contain the ethyne moieties needed for NOAC. Formation of nitrile oxides through
oxidation of oximes in aqueous ethanolic solution in the presence of PEPEP-PIZOF-15 and -19 resulted in the reaction of 96−
100% of the ethyne moieties to give isoxazoles. Thereby the framework was preserved. The type of nitrile oxide RCNO was
greatly varied with R being isopentyl, tolyl, 2-pyridyl, and pentafluorophenyl. A detailed NMR spectroscopic investigation
showed the formation of the 3,5-disubstituted isoxazole to be clearly favored (≥96%) over that of the constitutional isomeric 3,4-
disubstituted isoxazole, except for one example.
tetrazine,7 the reaction between a furan bearing MOF with
INTRODUCTION
■
maleimides,8 and, most recently, the reaction of a cyclo-
pentadiene bearing MOF with maleimides,9 whereas azide−
alkyne cycloaddition4,10−25 has been applied many times to
azide group bearing as well as to ethyne group bearing MOFs.
Azide−alkyne cycloaddition enjoys great popularity,26,27 not
only for PSM of MOFs but also, for example, in polymer
science28−30 and for bioconjugation,31−33 because alkynes and
organic azides are inert under the conditions of a wide range of
reactions and azide−alkyne cycloaddition is compatible with
many functional groups and substrate types (aliphatic,
aromatic), which allows an unlimited variability of the reaction
To make porous metal−organic frameworks (MOFs) useful
materials, important steps are their equipment with functional
moieties and the tuning of the chemical environment within the
pores. A powerful tool to tailor MOFs for a concrete
application is the postsynthetic modification (PSM) of linker
substituents.1−6 Among the PSM reactions, cycloaddition
reactions are especially attractive because they are of maximum
atom economy, are highly chemoselective and are thus
compatible with many functional groups, and most often
provide quantitative conversion. To date, the Diels−Alder
reaction7−9 and azide−alkyne cycloaddition4,10−25 (the most
prominent click reaction) have been applied for PSM at linker
substituents. Of the former only three examples have been
described, the reaction between an olefin bearing MOF with a
Received: January 14, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00126
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partners. However, the reaction comes with a severe drawback.
If not a strained alkyne is used,24,25 a copper salt is needed as a
catalyst for the reaction to proceed quickly under mild
conditions.34−36 To remove the copper salt completely can
be a major challenge and may never be achieved. Our
experiments with azide−alkyne cycloaddition8,37 applied to
Zr-based MOFs of the PIZOF type38,39 with propargyloxy
substituents at the linker showed us that even washing with
EDTA containing diethyl ether was insufficient to remove CuBr
and also the replacement of CuBr with Cu(MeCN)4PF6, a
complex soluble in organic solvents, did not provide copper-
free PIZOF despite Soxhlet extraction of the postsynthetically
modified product with THF and ethanol. As an alternative to
increasing the reactivity of the alkyne with the help of ring
strain, the reactivity of the dipolarophile can be increased
through the change from an azide to a nitrile oxide.40−44 Nitrile
oxide−alkyne cycloaddition (NOAC) neither requires a
strained alkyne, nor it requires a catalyst. A shortcoming of
NOAC can be that highly reactive nitrile oxides have to be
generated in situ in the presence of the alkyne in order to avoid
their dimerization to furoxanes.45,46 On the other hand, recent
work shows that even kinetically stable nitrile oxides undergo
smooth cycloaddition with terminal alkynes at temperatures as
low as those in refluxing CH2Cl2.47 NOAC has been applied
e.g. in polymer chemistry as a polymerization method48,49 and
for the modification of polymers,50,51 for the end-capping of
rotaxanes,47 and for biochemical purposes.43,52−56 Herein we
report on its application for the PSM of terminal alkyne bearing
MOFs using highly reactive, in situ generated nitrile oxides.
For the study we used PEPEP-PIZOF-15 and PEPEP-
PIZOF-19, which provide propargyloxy substituents as the
reaction partner for the nitrile oxide. PEPEP-PIZOFs (porous
interpenetrated zirconium−organic frameworks)38,39 consist of
two independent, mutually interpenetrating UiO-type57 frame-
works with Zr6O4(OH)4(CO2)12 as the inorganic building units
(also called secondary building units) and the rodlike
In the following the syntheses and characterization of
PEPEP-PIZOF-19 and PEPEP-PIZOF-15 will be presented
first, and then their modification through NOAC will be
described.
RESULTS AND DISCUSSION
■
Syntheses of PEPEP-PIZOF-15 and PEPEP-PIZOF-19.
The organic building blocks for PEPEP-PIZOF-19 and PEPEP-
PIZOF-15, the diacids 4a,b, were synthesized starting from the
2,5-disubstituted-1,4-diiodobenzenes 1a,b,58 which contain
propargyloxy groups in a protected form (Scheme 1).
Sonogashira−Hagihara coupling of these building blocks with
methyl 4-ethynylbenzoate gave the diesters 2. Deprotection of
the ethyne moiety and finally ester hydrolysis yielded the
diacids 4. Whereas diester 2a was purified by applying our
routine protocol of column chromatography,38 diester 2b was
isolated simply through Soxhlet extraction, as we have recently
described for similar diacids.39 The latter procedure is clearly of
advantage with respect to the amount of work and waste and
also in the situation when large amounts of the diesters are
required. Whether this simple workup procedure can be applied
to diester 2a was not tested. After deprotection of the ethyne
moiety through reaction with Bu4NF in THF, precipitation or
recrystallization was sufficient to isolate the diesters 3. It is
important to keep the time in which the diesters are in contact
with Bu4NF short. Otherwise, isomerization of the propargyl
into a propa-1,2-dienyl group takes place.58 For ester hydrolysis
the diesters 3 were dissolved in THF and a solution of KOH in
methanol was added. The resulting precipitate was isolated. It
was suspended in THF, and trifluoroacetic acid (TFA) was
added until the solid dissolved (pH 2). Addition of water to the
solution resulted in precipitation of the diacid. Results, when
the same procedure of diester hydrolysis and product isolation
to structurally closely related dicarboxylic acids was applied,
strongly indicate that the mixture of THF, trifluoroacetic acid,
and water is a good solvent for the diacids, causing occasionally
a substantial amount of the diacid to stay in solution. Therefore,
if we were to redo the syntheses of diacids 4, we would remove
all volatiles after the treatment with trifluoroacetic acid, suspend
the residue in water, and isolate the undissolved diacids 4 by
filtration. This procedure was very successful for the isolation of
related diacids. Instead of TFA, one could use e.g. HCl for
acidification after the basic ester hydrolysis. We prefer TFA
because this allows us to detect residual acid simply by 19F
NMR spectroscopy. The presence of an acid other than the
dicarboxylic acid affects the degree of defects in the UiO
framework59 and is expected to leave similar traces in the
PIZOF framework.
−
dicarboxylates −O2C[PE-P(R1,R2)-EP]CO2 with P standing
for p-phenylene and E for ethynylene, the sequence of the
letters resembling the sequence of the corresponding structural
units in the backbone and R1 and R2 denoting the substituents
at the central benzene ring. Although the substituents R1 and R2
can be broadly varied38,39 through using the readily substituted
dicarboxylic acids in the PIZOF synthesis, even for PIZOFs,
PSM is highly valuable because it enlarges the pool of possible
substituents and promises a faster fine-tuning of the pore
interior, considering the imponderables of the MOF syntheses.
The reaction with nitrile oxides allowed us to modify the
PEPEP-PIZOFs with a broad range of substituents: aromatic,
heteroaromatic, aliphatic, and highly fluorinated substituents.
The very high conversions obtained with NOAC applied to
PEPEP-PIZOF-15, which has two propargyloxy substituents
(R1 = R2 = OCH2CCH) per linker, highlights the efficiency
of NOAC for PSM. The successful application of NOAC to
PEPEP-PIZOF-19, which carries one propargyloxy substituent
and one short linear polyethylene glycol (OlPEG3) substituent
per linker, gives an example of the combination of preinstalled
substituents of a specific property, here the highly hydrophilic
OlPEG3 chain, with substituents introduced in a postsynthetic
modification step, e.g. highly fluorinated side chains. Such a
combination of incompatible side chains on one linker calls for
failure when attempting to assemble the MOF with
dicarboxylates already bearing both substituents.
The reaction of the diacids 4a,b with ZrOCl2·8H2O in DMF
in the presence of benzoic acid as modulator under static
conditions at 120 °C (Scheme 1) gave PEPEP-PIZOF-19 and
PEPEP-PIZOF-15, respectively, as proven by the PXRD
diagrams (Figure 1). The synthesis conditions are very similar
to those described for other PIZOFs,38 however using ZrOCl2·
8H2O instead of ZrCl4 as the Zr source. When ZrCl4 was used
in the synthesis of PEPEP-PIZOF-15, the PIZOF formation
was slow, the solutions turned dark brown, and the precipitated
product was brown. The discoloration of the solution and the
product were minor when ZrOCl2·8H2O was employed. The
precipitate was washed by suspending it in DMF and then in
ethanol and subsequently extracting it with ethanol in a Soxhlet
extractor for 1 day. Despite this intensive extraction, some of
the benzoic acid remained in the PEPEP-PIZOFs: 9−26 mol %
B
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Scheme 1. Syntheses of Diacids 4a,b and of PEPEP-PIZOF-
a
19 and PEPEP-PIZOF-15
Figure 1. PXRD diagrams of (top) PEPEP-PIZOF-19 and NOAC-
modified PEPEP-PIZOF-19-mn(conversion of propargyl groups) and
(bottom) PEPEP-PIZOF-15 and NOAC-modified PEPEP-PIZOF-15-
mn(conversion of propargyl groups).
reduced to 0.5−9 mol % by a second Soxhlet extraction with
ethanol or methanol for 5−8 days. The PIZOFs used for
NOAC contained a maximum of 10 mol % benzoic acid.
PEPEP-PIZOF-19 and PEPEP-PIZOF-15 were characterized
with respect to porosity and composition through argon
sorption measurements (Figure 2 and Table 1) and
thermogravimetric analysis (TGA; Figures S1−S7 and Table
2), respectively. The sorption isotherm of PEPEP-PIZOF-19
shows a distinct hysteresis, whereas that of PIZOF-15 shows
only a very minor hysteresis. The latter displays clearly a two-
step adsorption and desorption, as has been found for PEPEP-
PIZOF-2 and PPPP−PIZOF-1.39 The BET specific surface area
(Table 1) of PEPEP-PIZOF-19 (960 m2 g−1) is significantly
smaller than that of PEPEP-PIZOF-15 (1740 m2 g−1), and both
are significantly smaller than that of PEPEP-PIZOF-2 (2350 m2
g−1).39 Obviously, the size increase of the substituents at the
linkers from methoxy, which is the substituent of PEPEP-
PIZOF-2, to propargyloxy and finally to OlPEG3 leads to a
decrease in pore volume and accessible surface. TGA in air
(Figures S1 and S6 and Table 2) revealed degradation of the
PIZOFs accompanied by mass loss to start at around 230 °C.
Mass loss up to 200 °C is considered to be due to the removal
of guest molecules and the further mass loss (exp w/o guests in
a
The abbreviation lPEG3 stands for (CH2CH2O)3Me. For the
syntheses of the starting compounds 1a,b58 see the literature and
the Supporting Information.
relative to the dicarboxylate in the case of PEPEP-PIZOF-19
and 6−10 mol % in the case of PEPEP-PIZOF-15, as was
determined by H NMR spectroscopy of the disassembled
1
PIZOFs. The content of benzoic acid in PEPEP-PIZOF-19 was
C
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Figure 2. Linear scale plot of the argon sorption isotherms at 87 K (filled circles, adsorption; open circles, desorption): (a) PEPEP-PIZOF-19; (b)
PEPEP-PIZOF-19-m3(97%); (c) PEPEP-PIZOF-15 (this batch was extracted with ethanol in a Soxhlet extractor for 5 days prior to the sorption
experiment and was used for the preparation of PEPEP-PIZOF-15-m3(97%)); (d) PEPEP-PIZOF-15-m3(97%). The sorption isotherms of PEPEP-
PIZOF-19-m1, PEPEP-PIZOF-19-m2, and PEPEP-PIZOF-19-m4 are very similar to that of PEPEP-PIZOF-19-m3(97%). They are presented in
Figures S9, S10, and S12 in the Supporting Information, as are the pore size distributions of all of the studied materials (Figures S8−S14).
Table 1. Mass-Specific Surface Areas and Pore Volumes of
Unmodified and NOAC-Modified PEPEP-PIZOFs
Table 2. Experimentally (exp) Determined Mass Losses (%)
upon Heating of PEPEP-PIZOFs in Flowing Air and Data
Calculated for Defect-Free PEPEP-PIZOFs (calcd)
a
PEPEP-PIZOF(conversion of propargyl
groups)
pore volume
SBET (m2 g−1
)
(cm3 g−1
)
PEPEP-PIZOF(conversion of
propargyl groups)
exp w/o
guests
exp
calcd
-19
960
610
580
680
590
1740
730
0.47
0.35
0.33
0.39
0.34
0.79
0.38
-19-m1(100%)
-19-m2(89%)
-19-m3(97%)
-19-m4(100%)
-15
-19
guests
linker
2.9
75.4
21.7
3.7
77.7
22.3
82.2
17.8
residue
guests
linker
-19-m1(100%)
-19-m2(89%)
-19-m3(97%)
-19-m4(100%)
-15
75.0
21.3
4.2
77.9
22.1
84.7
15.3
-15-m3(97%)
residue
guests
linker
77.7
18.1
3.1
81.1
18.9
84.8
15.2
Table 2) to the dehydration of the inorganic building unit57,60
and the combustion of the linker. The residue of the thermal
decomposition in air is assumed to be ZrO2. The surprising
mass increase in the case of PEPEP-PIZOF-15 at 230 °C, a
phenomenon which occurred with all of the analyzed batches of
PEPEP-PIZOF-15, is assigned to the oxidation of the ethyne
groups. Table 2 presents the experimentally determined data
and the data calculated for the defect-free PIZOFs with the sum
formula Zr6O4(OH)4(dicarboxylate)12/2. In the case of PEPEP-
PIZOF-19, the experimentally determined amount of residue is
considerably larger than the calculated amount. Obviously,
PEPEP-PIZOF-19 contains defects. This conclusion is
corroborated by the hysteresis found in the sorption isotherm
of PEPEP-PIZOF-19 (Figure 2) as well as by the large amount
of benzoic acid in the samples of PEPEP-PIZOF-19 and the
tenacity with which the benzoic acid stayed in the material
during Soxhlet extraction. Benzoic acid is thought to coordinate
to the inorganic building units at sites where dicarboxylates are
residue
guests
linker
77.7
19.2
2.8
80.2
19.8
84.8
15.2
residue
guests
linker
80.6
16.6
3.1
82.9
17.1
86.4
13.6
residue
guests
linker
77.1
19.8
3.5
79.6
20.4
79.0
21.0
residue
guests
linker
-15-m3(97%)
83.3
13.3
86.2
13.8
84.9
15.1
residue
a
Details on the analysis of the TGA curves (Figures S1−S7) are
provided in the text and the Supporting Information.
D
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missing. In comparison with PEPEP-PIZOF-19, PEPEP-
PIZOF-15 contains much fewer, if any defects, as the
experimentally determined and the calculated TGA data
match very well, as do the curves of adsorption and desorption.
Only the amount of 6−10 mol % of benzoic acid present after 1
day of Soxhlet extraction may hint at structural defects in
PIZOF-15.
Scheme 2. Postsynthetic Modification of PEPEP-PIZOF-19
and PEPEP-PIZOF-15 through Nitrile Oxide−Alkyne
Cycloaddition (NOAC) between the Propargyloxy
Substituents at the Linkers and in Situ Generated Nitrile
a
Oxides 6
Nitrile Oxide Click Reaction. To explore the chemical
tailoring of PIZOF pores with NOAC, the nitrile oxides 6a−d
were selected as examples for hydrophobic, hydrophilic, highly
fluorinated, aliphatic, aromatic, and heterocyclic nitrile oxides
(Scheme 2). These nitrile oxides were expected to be unstable,
as they lack sterically stabilizing substituents and, therefore, to
require an in situ preparation. We opted for their generation via
oxidation of the oximes 5a−d.45,61,62 Oxime 5c was purchased;
the oximes 5a,b,d were synthesized from the corresponding
aldehydes, using hydroxylammonium chloride and a base.63
Among the several reagents described for the oxidation of
oximes to nitrile oxides,61 our first choice was phenyliodine
bis(trifluoroacetate) (PIFA)63,64 for its low toxicity and
instantaneous oxidation of oximes63 and because we expected
iodobenzene and trifluoroacetic acid, which are the two
products deriving from PIFA, to be easily removable by
extraction of the PIZOFs with an organic solvent. Adding PIFA
portionwise to mixtures of PEPEP-PIZOF-19 with the oximes
5a−d and of PEPEP-PIZOF-15 with the oximes 5b−d (for the
amounts of PIFA and oximes used see Table 3) in aqueous
methanol at room temperature gave PEPEP-PIZOF-19-mn
with n = 1−4 and PEPEP-PIZOF-15-mn with n = 2−4 with
disubstituted isoxazoles as linker substituents. With the
portionwise addition of PIFA the concentration of the
generated nitrile oxide and therefore the probability of their
dimerization were kept low. Indeed, the conversion of the
propargyl groups was significantly lower when all of the PIFA
was added at once. To determine the conversions of the
propargyl groups, the NOAC-modified PIZOFs were dis-
8
1
assembled using CsF and DCl in DMSO-d6, the H NMR
spectra of the solutions were recorded, and the intensities of the
signals caused by the proton of the isoxazole moiety (isoxazole
proton) and of the methylene unit of the propargyl group were
determined. The results are given in Table 3. Please note that
the reactions have not been optimized with respect to the
minimum amount of oxime and/or oxidant. Very high
conversions of 89−100% of the propargyl groups were achieved
with the aromatic oximes 5b−d. However, the combination of
the aliphatic oxime 5a with PEPEP-PIZOF-19 resulted in a
comparatively low conversion of 68%. A result comparable to
the latter finding provided NOAC applied to 1-methoxy-4-
(prop-2-yn-1-yloxy)benzene (7), which is the most simple
cutout of the PIZOF linkers. Using the aliphatic oxime 5a and
PIFA in a 5/1 mixture of methanol and water caused only
about 33% of model compound 7 to react, despite 3 equiv of
oxime and oxidant being used. In contrast, a complete
conversion of model compound 7 into the NOAC product
was achieved with sodium N-chloro-p-toluenesulfonamide
(chloramine T) as the oxidant in a 2/1 mixture of ethanol
and water. As in the experiment with PIFA, 3 equiv of oxidant
and of oxime were used. Interestingly, all of the chloramine T
could be added at once, whereas, as mentioned above, PIFA
had to be added portionwise. This suggests less competition
through nitrile oxide dimerization when chloramine T is used
and therefore a distinctly slower rate of oxime oxidation with
chloramine T than with PIFA. As in the case of model
a
The reactions were performed in aqueous methanol or aqueous
ethanol. Please note that for PEPEP-PIZOF-15-m4 only one of the
three isomeric cycloaddition products is shown (see discussion in the
text and Figure 3).
compound 7, chloramine T was the reagent of choice for
NOAC on PEPEP-PIZOF-15 and -19 with the aliphatic nitrile
oxide 6a to achieve conversions of 98−99% of the propargyl
groups (Table 3, experiments 1a, 5a, and 5b). That it is the type
E
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Table 3. Results of NOAC on PEPEP-PIZOF-19 and PEPEP-PIZOF-15 with PIFA in MeOH/H2O (5/1) or with Chloramine T
a
in EtOH/H2O (2/1)
PEPEP-PIZOF(conversion of propargyl
groups)
oxime (equiv per propargyl
group)
oxidant (equiv per propargyl
group)
fraction of 3,5-disubstituted isoxazole
(%)
exp
1a
1b
2
-19-m1(100%)
-19-m1(68%)
-19-m2(89%)
-19-m3(97%)
-19-m4(100%)
-15-m1(99%)
-15-m1(100%)
-15-m2(97%)
-15-m3(97%)
-15-m3(96%)
-15-m4(100%)
5a (10)
5a (10)
5b (2)
5c (2)
5d (2)
5a (10)
5a (6)
5b (4)
5c (3)
5c (6)
5d (3)
chloramine T (10)
PIFA (6)
99
99
PIFA (2)
99
3
PIFA (2)
100
96
4
PIFA (2)
5a
5b
6
chloramine T (6)
chloramine T (6)
PIFA (2)
98
98
100
99
7a
7b
8
PIFA (2)
chloramine T (6)
PIFA (2)
97
b
59
a
The fractions of the 3,5-disubstituted isoxazole in the mixtures of 3,5- and 3,4-disubstituted isoxazoles and the conversions were determined
b
1
through H NMR spectroscopy. 18.5% 3,5/3,5-isomer, 80.5% 3,5/3,4-isomer, <1% 3,4/3,4-isomer.
of oxidant and not the amount that makes the difference can be
concluded from a comparison of the results of the experiments
1b and 5a in Table 3. Against our apprehension, the polar
toluene sulfonamide, which is the reduction product derived
from chloramine T, was nearly quantitatively removed through
Soxhlet extraction of the NOAC-modified PIZOFs. Further-
more, chloramine T can also be applied for the aromatic oxime
5c (the other aromatic oximes 5b,d were not tested) and gave
the same result as PIFA (Table 3, experiment 7b).
shift is very distinct for the two isomeric isoxazoles (see also
Table S1).
Quite a different situation was found for NOAC of PEPEP-
PIZOF-15-m4 with nitrile oxide 6d. The formation of 3,5-
disubstituted isoxazole is only slightly favored (59%) over that
of the 3,4-disubstituted isoxazole. The result is a PIZOF with
three constitutionally isomeric linkers: 3,5/3,5-isomer (18.5%),
3,5/3,4-isomer (80.5%), and 3,4/3,4-isomer (<1%) (Figure 3).
Because only very little of the 3,4/3,4-isomer formed and
NOAC of PEPEP-PIZOF-19 with nitrile oxide 6d gave the 3,5-
disubstituted isoxazole (96%) nearly exclusively, we assume
that the monoaddition product is a 3,5-disubstituted isoxazole
and that the isomeric isoxazole moiety is formed predominantly
upon the second cycloaddition at the linker. We tried to mimic
the 2-fold NOAC with the reaction of nitrile oxide 6d with
diester 3b in homogeneous solution under otherwise identical
conditions. This attempt failed because of an insufficient
conversion of the propargyl groups, most probably caused by
the low solubility of diester 3b and of the monoaddition
product. Hardly any diaddition product formed. Therefore, the
question remains whether the preference for the 3,4-
disubstituted isoxazole in the second cycloaddition step of
nitrile oxide 6d is a peculiarity of NOAC on PEPEP-PIZOF-15
or would occur as well in the homogeneous phase. In addition,
the cause for the difference in behavior of the perfluorinated
nitrile oxime 6d and the nonfluorinated, aliphatic, and aromatic
nitrile oxides 6a−c remains unknown.
1H NMR spectroscopy of the disassembled NOAC-modified
PIZOFs (Figure 4), revealed that, except for PEPEP-PIZOF-
Figure 3. Cutouts of the linkers of PEPEP-PIZOF-15-m4 displaying
the 3,5/3,5-, 3,5/3,4-, and 3,4/3,4-isomers, which formed in a ratio of
roughly 19/80/1.
The very close similarity between the powder X-ray
diffraction (PXRD) patterns (Figure 1) of all of the NOAC-
modified PEPEP-PIZOFs with those of the unmodified PEPEP-
PIZOFs strongly suggests that all these materials show the
same topology. The degree of crystallinity is, if at all, only very
slightly affected by the postsynthetic modification. In contrast
to the parent PIZOFs, the NOAC-modified PIZOFs were free
of benzoic acid. Argon sorption isotherms proved all NOAC-
modified PEPEP-PIZOFs to be porous (Figure 2 and Figures
S9−S12 and S14). The significant reduction of the BET surface
area and the pore volume upon NOAC (Table 1) is assigned to
the filling of the pores with the modified substituents. The
similarity of the sorption data for PEPEP-PIZOF-19-mn and
PEPEP-PIZOF-15-m1 is in line with the nearly quantitative
modification of both propargyl groups of the PEPEP-PIZOF-15
linker and supports the aforementioned view that the smaller
surface area of PEPEP-PIZOF-19 in comparison to PEPEP-
15-m4, one of the two possible cycloaddition products, 3,5- or
3,4-disubstituted isoxazole, had formed exclusively or nearly
exclusively (Table 3). Of these two possible isomers, the former
is reported to be the sole or at least main product of the
thermal reaction.53,63,65 However, an influence of the particular
environment of a MOF pore on the regioselectivity could not
be excluded. As outlined in the Supporting Information, the 1H
NMR data of the disassembled NOAC-modified PIZOFs in
1
combination with the H NMR data, including a 1H−1H-
NOESY spectrum of the NOAC product (model compound
8b) of diester 3a with nitrile oxide 6b (Scheme S1), provided
conclusive evidence that the main or sole product was the 3,5-
disubstituted isoxazole accompanied, if at all, by a maximum of
3% of 3,4-disubstituted isoxazole. The indicative 1H NMR
signals arise from the proton at the isoxazole ring and the
protons of the methylene group bound to the isoxazole ring at
the 5- or 4-position. As can be seen from Figure 4, the signal
F
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1
Figure 4. (top) H NMR spectra (DMSO-d6, 500 MHz, room temperature) of disassembled PEPEP-PIZOF-19, PEPEP-PIZOF-19-m1(100%);
1
PEPEP-PIZOF-19-m2(89%), PEPEP-PIZOF-19-m3(97%), and PEPEP-PIZOF-19-m4(100%). (bottom) H NMR spectra (DMSO-d6, 500 MHz,
room temperature) of disassembled PEPEP-PIZOF-15, PEPEP-PIZOF-15-m1(100%), PEPEP-PIZOF-15-m2(97%), PEPEP-PIZOF-15-m3(97%),
and PEPEP-PIZOF-15-m4(100%). Signals of 3,5-disubstituted isoxazole are indicated with + and those of 3,4-disubstituted isoxazole with #. In the
case of PEPEP-PIZOF-15-mn the signals assigned to the combinations of 3,5/3,5-, 3,5/3,4-, and 3,4/3,4-isoxazoles are marked with + /+, + /#, and
#/#, respectively. The singlets in the range of 5.0−5.5 ppm are the signals of the isoxazole−CH2O group. The signals in the range of 6.5−7.2 and
8.9−9.4 ppm are the signals of the isoxazole−H.
G
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4.79 ppm (1H). For 19F NMR spectra C6F6 (δ(19F) −169 ppm) was
used as an internal standard, however, it was contained in a separate
capillary. Signal assignments are supported by DEPT-135 experiments,
calculations using increments,71 and comparison with data from closely
related compounds.38,39,58 All of the named components accompany-
ing the main component, such as a solvent or a side product, were
identified and quantified through 1H NMR spectroscopy. Signals
assigned to solvents are not listed with the NMR data. The signals
listed below are the signals assigned to the highly dominant
component.
ESI mass spectra were recorded using an Esquire 3000 ion trap
mass spectrometer equipped with a standard ESI source. Samples were
introduced by direct infusion with a syringe pump. Nitrogen served as
both the nebulizer gas and the dry gas.
Elemental analyses were performed by using HEKAtech EURO EA
2010 CHNS elemental analyzer.
PEPEP-PIZOF-15 and PEPEP-PIZOF-19. ZrOCl2·8H2O (0.111 g,
0.343 mmol) and benzoic acid (1.256 g, 10.287 mmol) were dissolved
in DMF (20 mL) by using ultrasound. Diacid 4b (0.163 g, 0.343
mmol) or diacid 4a (0.195 g, 0.343 mmol) was added and dissolved by
the application of ultrasound. The screw-capped bottle (100 mL,
VWR, Schott, Borosilicate 3.3, ISO 4796; PTFE-coated seal) was
tightly closed and put into an oven preheated to 120 °C. The reaction
mixture was kept at this temperature under static conditions for 24 h.
The suspension was removed from the hot oven and cooled to room
temperature. The precipitate was isolated by centrifugation. The
yellowish solid was suspended in DMF, the suspension was
centrifuged, and the solution was decanted off. The residual solid
was washed with ethanol in the same way as described for the washing
with DMF. The solid was extracted in a Soxhlet extractor with ethanol
for 1 day. The solid was dried at room temperature and reduced
pressure. PEPEP-PIZOF-19 was extracted with ethanol or methanol in
a Soxhlet device for another 5−8 days. PEPEP-PIZOF-15 contained
6−10 mol % and PEPEP-PIZOF-19 contained 0.5−9 mol % benzoic
acid (mol % refers to the moles of the MOF linker).
Disassembly of PEPEP-PIZOFs. PEPEP-PIZOF (1−6 mg) and
CsF (6−11 mg) were suspended in DMSO-d6 (ca. 0.65 mL). A drop
of aqueous DCl (35 wt % in D2O, 20−25 mg) was added. The
disappearance of the yellow solid indicated disassembly of the PEPEP-
PIZOF (5−15 min). CsF is a white solid and can be easily
distinguished from the yellow PEPEP-PIZOF. In some cases, the
suspension had to be heated to 80 °C to induce disassembly. Finally,
K2CO3 (10−15 mg) was added and the solid was removed through
decantation.
Nitrile Oxide−Alkyne Cycloaddition (NOAC) with PEPEP-
PIZOFs. General Procedures. Using PIFA. PIZOF and oxime 5 were
suspended in a 5/1 mixture of MeOH and water. [Bis-
(trifluoroacetoxy)iodo]benzene (PIFA) was added portionswise, and
the suspension was stirred at room temperature for the time stated.
The solid was isolated by filtration, if not otherwise stated, and
extracted in a Soxhlet extractor.
Using Chloramine T. PIZOF and oxime 5 were suspended in a 2/1
mixture of EtOH and water. Chloramine T·3H2O was added, and the
suspension was stirred at room temperature for the time stated. The
solid was isolated by filtration, if not otherwise stated, and extracted in
a Soxhlet extractor.
PIZOF-15 is caused by the space-demanding OlPEG3
substituents.
Like PEPEP-PIZOF-19, all of the four PEPEP-PIZOF-19-mn
show a sorption isotherm with a hysteresis pointing to the
presence of mesopores and therefore framework defects.
Another sign of framework defects is the mismatch between
experimentally determined and calculated masses for linker and
inorganic components (Table 2 and Figures S2−S5 and S7).
The courses of hystereses in the sorption isotherms of PEPEP-
PIZOF-19 and PEPEP-PIZOF-19-mn are very similar. Only the
extent of the hystereses varies from material to material. A
comparatively very slight hysteresis is found in the sorption
isotherm of PEPEP-PIZOF-15-m3 which is slightly larger than
that of the parent PEPEP-PIZOF-15. The experimentally
determined TGA data of PEPEP-PIZOF-15-m3 agree well with
the calculated data. Please note that all of the four PEPEP-
PIZOF-19-mn were prepared from independent batches of
PEPEP-PIZOF-19. Therefore, the differences in sorption
isotherms and TGA data may simply be rooted in the variation
of defect number among the different batches of PEPEP-
PIZOF-19. On the other hand, the sorption isotherm of
PEPEP-PIZOF-15 shown in Figure 2 comes from the material
that was used to prepare PEPEP-PIZOF-15-m3. Therefore, the
slight increase of the hysteresis in this case is understood as an
indication that NOAC is not completely harmless. Including
PXRD data and TGA data, we estimate the extent of damage to
be rather small.
CONCLUSION
■
The family of PIZOFs has been enlarged by the PEPEP-
PIZOFs-15 and -19, which have one and two propargyloxy
substituents per linker, respectively. With these MOFs the
postsynthetic modification of MOFs through nitrile oxide−
alkyne cycloaddition (NOAC) was demonstrated. NOAC
works with the unstrained propargyloxy substituent, does not
require a catalyst, and yielded conversions of 89−100%.
Furthermore, NOAC was found to be applicable to very
different types of nitrile oxidesaliphatic, aromatic, hetero-
aromatic, and highly fluorinatedand thus appears as highly
suited for customizing the pore environment for concrete
demands, including the introduction of functional groups. The
first two examples of the latter are PEPEP-PIZOF-15-m3 and
PEPEP-PIZOF-19-m3, which provide a bidentate ligand for
metal ions such as palladium and copper ions. Metal ion loaded
MOFs are of interest as catalysts.19,66−70 The modification of
PEPEP-PIZOF-15 opens access to a MOF with amphiphilic
pores and, in the case of a self-sorting process through rotation
of the central benzene ring, to a MOF with alternating pores of
e.g. highly polar and fluorophilic pores. We expect post-
synthetic modification through NOAC to be applicable to all
MOFs which are compatible with aqueous alcoholic solutions.
In addition, it may be applicable to MOFs only compatible with
water-free solvents, when preformed, kinetically stabilized
nitrile oxides are used47 or one of the other methods to form
nitrile oxides is applied.
PEPEP-PIZOF-19-m1(100%). See the general procedure for NOAC
using chloramine T: PEPEP-PIZOF-19 (153 mg, 0.22 mmol propargyl
moieties), oxime 5a (222 mg, 2.19 mmol), EtOH/H2O 2/1 (20 mL),
chloramine T·3H2O (629 mg, 2.23 mmol). Reaction time: 94 h.
Soxhlet extraction with EtOH (70 mL) gave a yellow solid (113 mg).
1H NMR (500 MHz, DMSO-d6) (Figure S26): δ 7.95 and 7.92 (two
halves of two AA′XX′ spin systems, 2 H each, ArH ortho to CO2),
7.60 and 7.56 (two halves of two AA′XX′ spin systems, 2 H each, ArH
meta to CO2) 7.36 and 7.25 (2 s, 1 H each, ArH ortho to OCH2), 6.47
(s, 1 H, isoxazole-H), 5.31 (s, 2 H, isoxazole-CH2O), 4.16, 3.76, 3.63,
EXPERIMENTAL SECTION
The syntheses of diacids 4 and of oximes 5 are described in the
Supporting Information.
General Considerations. NMR spectra were recorded at room
temperature if not otherwise mentioned. They were calibrated using
the solvent as an internal standard: CDCl3, 7.25 ppm (1H) and 77.0
ppm (13C); DMSO-d6, 2.49 ppm (1H) and 39.50 ppm (13C), D2O,
■
l
3.46, 3.42, and 3.31 (6 t-like, 2 H each, CH2 of PEG3), 3.13 (s, 3 H,
3
OCH3), 2.45 (d, J = 7.1 Hz, 2 H, CH2CH(CH3)2), 1.84 (m, 1 H,
3
CH2CH(CH3)2), 0.82 (d, J = 6.7 Hz, 6 H, CH2CH(CH3)2). 13C
NMR (500 MHz, DMSO-d6) (Figure S27): δ 167.4 (CN), 166.84
H
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and 166.80 (CO2), 163.1 (HCCO), 154.2 and 152.4 (CArOCH2),
131.70 and 131.69 (CArH meta to CO2), 130.93 and 130.87 (CArCO2),
129.93 and 129.87 (CArH ortho to CO2), 126.9 and 126.8 (CAr para to
CO2), 118.4 and 117.0 (CArH ortho to OCH2), 114.1 and 113.3
(CArCC ortho to OCH2), 104.5 (HCCO), 94.8 and 88.8 (C
mL), PIFA (274 mg, 0.64 mmol). PIFA was added in five portions
within a period of 4 h and a sixth portion after 21 h. Reaction time
after the addition of the last portion of PIFA: 72 h. Soxhlet extraction
first with EtOH (70 mL) and then with THF (70 mL) gave a yellow
solid (223 mg). 1H NMR (500 MHz, DMSO-d6) (Figure S32): δ 7.94
and 7.89 (two halves of two AA′XX′ spin systems, 2 H each, ArH
ortho to CO2), 7.60 and 7.55 (two halves of two AA′XX′ spin systems,
2 H each, ArH meta to CO2) 7.43 and 7.24 (2 s, 1 H each, ArH ortho
OCH2), 7.00 (s, 1 H, isoxazole-H), 5.47 (s, 2 H, isoxazole-CH2O),
4.16, 3.76, 3.63, 3.46, 3.42, and 3.31 (6 t-like, 2 H each, CH2), 3.12 (s,
3 H, OCH3). 13C NMR (500 MHz, DMSO-d6) (Figure S33): δ 169.4
(CN), 166.9 and 166.8 (CO2), 163.3 (HCCO), 154.4 and 152.3
(CArOCH2), 151.6, 145.5, and 143.5 (3 m, CF), 131.72 and 131.70
(CArH meta to CO2), 130.82 and 130.79 (CArCO2), 129.9 and 129.8
(CArH ortho to CO2), 126.9 and 126.8 (CAr para to CO2), 118.6 and
117.0 (CArH ortho to OCH2), 114.3 and 113.4 (CArCC ortho to
OCH2), 105.5 (HCCO), 99.9 (CArCN), 94.9, 94.8, 88.7, and 88.4
l
C), 71.5, 70.5, 70.1, 69.8, 69.4, and 69.3 (CH2 of PEG3), 62.5
(isoxazole-CH2O), 58.3 (OCH3), 34.3 (CH2CH(CH3)2), 27.6
(CH2CH(CH3)2), 22.3 (CH2CH(CH3)2. MS (ESI): m/z 704.3 [M
+ Na]+, 680.2 [M − H]−.
PEPEP-PIZOF-19-m1(68%). See the general procedure for NOAC
using PIFA: PIZOF-19 (148 mg, 0.22 mmol propargyl moieties),
oxime 5a (216 mg, 2.13 mmol), MeOH/H2O 5/1 (20 mL), PIFA
(563 mg, 1.31 mmol). PIFA was added in five portions within a period
of 4 h and a sixth portion after 18 h. Reaction time after the addition of
the last portion of PIFA: 71 h. Soxhlet extraction with EtOH gave a
yellow solid (104 mg).
PEPEP-PIZOF-19-m2(89%). See the general procedure for NOAC
using PIFA: PEPEP-PIZOF-19 (150 mg, 0.22 mmol propargyl
moieties), oxime 5b (60 mg, 0.44 mmol), MeOH/H2O 5/1 (15
mL), PIFA (189 mg, 0.44 mmol). PIFA was added in five portions
within a period of 4 h and a sixth portion after 17 h. Reaction time
after the addition of the last portion of PIFA: 73 h. Soxhlet extraction
with EtOH (70 mL) gave a yellow solid. 1H NMR (500 MHz, DMSO-
d6) (Figure S28): δ 7.95 and 7.90 (two halves of two AA′XX′ spin
systems, 2 H each, ArH ortho to CO2), 7.68 (half of AA′XX′ spin
system, 2 H, ArH of tolyl), 7.61 and 7.58 (two halves of two AA′XX′
spin systems, 2 H each, ArH meta to CO2), 7.43 and 7.26 (2 s, 1 H
each, ArH ortho to OCH2), 7.26 (half of AA′XX′ spin system, 2 H,
ArH of tolyl), 7.08 (s, 1 H, isoxazole-H), 5.40 (s, 2 H, isoxazole-
CH2O), 4.17, 3.77, 3.63, 3.46, 3.42, and 3.31 (6 t-like, 2 H each, CH2),
3.13 (s, 3 H, OCH3), 2.31 (s, 3 H, CH3 of tolyl). 13C NMR (500 MHz,
DMSO-d6) (Figure S29): δ 168.6 (CN),166.83 and 166.79 (CO2),
162.1 (HCCO), 154.3 and 152.4 (CArOCH2), 140.4 (CArCH3),
131.7 (CArH meta to CO2), 130.79 and 130.75 (CArCO2), 129.97,
129.91, and 129.87 (CArH ortho to CO2 and CArH of tolyl), 127.2 and
126.9 (CAr para to CO2), 126.8 (CArH of tolyl), 125.6 (CArCN),
118.5 and 117.1 (CArH ortho to OCH2), 114.2 and 113.6 (CArCC
ortho to OCH2),102.5 (HCCO), 94.8, 94.7, 88.8, and 88.5 (CC),
71.4, 70.5, 70.1, 69.8, 69.4, and 69.3 (CH2 of lPEG3), 62.3 (isoxazole-
CH2O), 58.3 (OCH3), 23.3 (CH3 of tolyl). MS (ESI): m/z 738.2 [M
+ Na]+, 714.0 [M − H]−.
l
(CC), 71.5, 70.5, 70.1, 69.8, 69.5, and 69.3 (CH2 of PEG3), 62.6
(isoxazole-CH2O), 58.3 (OCH3).
PEPEP-PIZOF-15-m1(100%). See the general procedure for NOAC
using chloramine T: PIZOF-15 (30 mg, 0.10 mmol propargyl
moieties), oxime 5a (62 mg, 0.61 mmol), EtOH/H2O 2/1 (4 mL),
chloramine T·3H2O (176 mg, 0.62 mmol). Reaction time: 72 h.
Soxhlet extraction first with EtOH (70 mL) and then with THF (70
mL) gave a yellow solid (17 mg). 1H NMR (500 MHz, DMSO-d6): δ
7.84 and 7.36 (AA′XX′ spin system, 4 H each, ArH ortho and meta to
CO2, respectively), 7.35 (s, 2 H, ArH ortho to OCH2), 6.51 (s, 2 H,
isoxazole-H), 5.34 (s, 4 H, isoxazole-CH2O), 2.49 (signal strongly
overlapping with the signal of DMSO, CH2CH(CH3)2), 1.90 (hept, 3J
= 6.5 Hz, 2 H, CH2CH(CH3)2), 0.87 (d, 3J = 6.6 Hz, 6 H,
CH2CH(CH3)2).
PEPEP-PIZOF-15-m1(99%). See the general procedure for NOAC
using chloramine T: PIZOF-15 (30 mg, 0.10 mmol propargyl
moieties), oxime 5a (110 mg, 1.09 mmol), EtOH/H2O 2/1 (4 mL),
chloramine T·3H2O (176 mg, 0.62 mmol). Reaction time: 68 h.
1
Soxhlet extraction with EtOH (70 mL) gave a yellow solid. H NMR
(500 MHz, 80 °C, DMSO-d6) (Figure S38): δ 7.94 and 7.57 (AA′XX′
spin system, 4 H each, ArH ortho and meta to CO2, respectively), 7.37
(s, 2 H, ArH ortho to OCH2), 6.43 (s, 2 H, isoxazole-H), 5.33 (s, 4 H,
isoxazole-CH2O), 2.48 (d partially overlapping with the signal of
DMSO, CH2CH(CH3)2), 1.89 (hept-like, 3J = 6.7 Hz, 2 H,
PEPEP-PIZOF-19-m3(97%). See the general procedure for NOAC
using PIFA: PEPEP-PIZOF-19 (120 mg, 0.17 mmol propargyl
moieties), oxime 5c (44 mg, 36 mmol), MeOH/H2O 5/1 (8 mL),
PIFA (153 mg, 0.36 mmol). PIFA was added in six portions within a
period of 5 h. Reaction time after the addition of the last portion of
PIFA: 115 h. Soxhlet extraction with THF (70 mL) gave a yellow solid
(100 mg). 1H NMR (500 MHz, DMSO-d6) (Figure S30): δ 8.70 (m, 1
H, ArH of pyridine), 8.03 (d, 3J = 7.8 Hz, 1 H, ArH of pyridine), 7.98
(m partially overlapping with the signal of an AA′XX′ spin system, 1
H, ArH of pyridine), 7.97 and 7.93 (two halves of two AA′XX′ spin
systems, 2 H each, ArH ortho to CO2), 7.63 and 7.61 (two halves of
two AA′XX′ spin systems, 2 H each, ArH meta to CO2), 7.54 (m, 1 H,
ArH of pyridine), 7.47 and 7.28 (2 s, 1 H each, ArH ortho to OCH2),
7.16 (s, 1 H, isoxazole-H), 5.49 (s, 2 H, isoxazole-CH2O), 4.19, 3.78,
3.65, 3.48, 3.44, and 3.33 (6 t-like, 2 H each, CH2), 3.14 (s, 3 H,
OCH3). 13C NMR (500 MHz, DMSO-d6) (Figure S31): δ 169.1 (C
N of isoxazole), 166.79 and 166.75 (CO2), 162.7 (HCCO), 154.2
and 152.2 (CArOCH2), 149.8 and 147.1 (CArH of pyridine and q-C of
pyridine), 138.3 (CArH of pyridine), 131.7 and 131.6 (CArH meta to
CO2), 130.74 and 130.69 (CArCO2), 129.84 and 129.80 (CArH ortho
to CO2), 126.9 and 126.7 (CAr para to CO2), 125.6 and 121.8 (CArH
of pyridine), 118.3 and 117.0 (CArH ortho to OCH2), 114.1 and 113.3
(CArCC ortho to OCH2), 102.9 (HCCO), 94.74, 94.66, 88.7, and
3
CH2CH(CH3)2), 0.87 (d, J = 6.7 Hz, 12 H, CH2CH(CH3)2). The
differences in the shifts in comparison to the data reported above for
disassembled PEPEP-PIZOF-15-m1(100%) are ascribed to the
difference in temperature and pH of the solution.
PEPEP-PIZOF-15-m2(97%). See the general procedure for NOAC
using PIFA: PEPEP-PIZOF-15 (30 mg, 0.10 mmol propargyl
moieties), oxime 5b (28 mg, 0.21 mmol), MeOH/H2O 5/1 (1.5
mL). PIFA (807 mg, 1.88 mmol) was added in five portions within a
period of 7 h and a sixth portion after 22 h. Reaction time after the
addition of the last portion of PIFA: 3 days. The yellow solid was
isolated by centrifugation. Soxhlet extraction with MeOH (70 mL)
gave a yellow solid (21 mg). 1H NMR (500 MHz, DMSO-d6) (Figure
S39): δ 7.84 (half of AA′XX′ spin system, 2 H, ArH ortho to CO2),
7.73 (half of AA′XX′ spin system, 2 H, ArH of tolyl), 7.43 (s, 2 H, ArH
ortho to OCH2), 7.40 (half of AA′XX′ spin system, 2 H, ArH meta to
CO2), 7.31 (half of AA′XX′ spin system, 2 H, ArH of tolyl), 7.11 (s, 2
H, isoxazole-H), 5.45 (s, 4 H, isoxazole-CH2O), 2.34 (s, 6 H, CH3).
PEPEP-PIZOF-15-m3(97%). See the general procedure for NOAC
using PIFA: PEPEP-PIZOF-15 (90 mg, 0.31 mmol propargyl
moieties), oxime 5c (76 mg, 0.62 mmol), MeOH/H2O 5/1 (16
mL). PIFA (400 mg, 0.93 mmol) was added in five portions within a
period of 5 h and a sixth portion after 18 h. Reaction time after the
addition of the last portion of PIFA: 49 h. Soxhlet extraction with
EtOH (70 mL) gave a pale yellow solid (84 mg). 1H NMR (500 MHz,
DMSO-d6) (Figure S40): δ 8.69 (d, 3J = 4.5 Hz, 2 H, ArH of
pyridine), 8.01 (d, 3J = 7.8 Hz, 2 H, ArH of pyridine), 7.94 (t-like, 3J =
7.8 Hz, 2 H, ArH of pyridine), 7.91 and 7.57 (AA′XX′ spin system, 4
H each, ArH ortho and meta to CO2, respectively), 7.52−7.51 (m, 3
l
88.4 (CC), 71.4, 70.4, 70.3, 69.7, 69.3, and 69.2 (CH2 of PEG3),
62.4 (isoxazole-CH2O), 58.2 (OCH3). MS (ESI): m/z 725.3 [M +
Na]+, 701.1 [M − H]−.
PEPEP-PIZOF-19-m4(100%). See the general procedure for NOAC
using PIFA: PEPEP-PIZOF-19 (223 mg, 0.32 mmol propargyl
moieties), oxime 5d (135 mg, 0.64 mmol), MeOH/H2O 5/1 (15
I
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H, ArH of pyridine and ArH ortho to OCH2), 7.15 (s, 2 H, isoxazole-
H), 5.51 (isoxazole-CH2O).
(3) Marshall, R. J.; Forgan, R. S. Postsynthetic Modification of
Zirconium Metal-Organic Frameworks. Eur. J. Inorg. Chem. 2016,
2016, 4310−4331.
PEPEP-PIZOF-15-m3(96%). See the general procedure for NOAC
using chloramine T: PIZOF-15 (30 mg, 0.10 mmol propargyl
moieties), oxime 5c (78 mg, 0.64 mmol), EtOH/H2O 2/1 (4 mL),
chloramine T·3H2O (176 mg, 0.62 mmol). The yellow solid was
isolated by centrifugation. Reaction time after the addition of the last
portion of PIFA: 50 h. Soxhlet extraction with EtOH gave a yellow
solid (24 mg). For NMR data see PEPEP-PIZOF-15-m3 (97%).
PEPEP-PIZOF-15-m4(100%). See the general procedure for NOAC
using PIFA: PEPEP-PIZOF-15 (27 mg, 0.09 mmol propargyl
moieties), oxime 5d (44 mg, 0.21 mmol), MeOH/H2O 5/1 (4 mL).
PIFA (152 mg, 0.32 mmol) was added in five portions within a period
of 5 h and a sixth portion after 18 h. Reaction time after the addition of
the last portion of PIFA: 48 h. The yellow solid was isolated by
centrifugation. Soxhlet extraction with EtOH (70 mL) gave a yellow
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1
solid (25 mg). H NMR (500 MHz, DMSO-d6) (Figure S41): signals
assigned to the 3,5/3,4-isomer δ 9.42 (s, 1 H, isoxazole-H of the 3,4-
isoxazole), 7.86 and 7.41 (AA′XX′ spin system, 4 H each, ArH ortho
and meta to CO2, respectively), 7.34 and 7.26 (2 s, 1 H each, ArH
ortho to OCH2), 7.03 (s, 1 H, isoxazole-H of the 3,5-isoxazole), 5.48
(s, 2 H, isoxazole-CH2O of the 3,5-isoxazole), 5.25 (s, 2 H, isoxazole-
CH2O of the 3,4-isoxazole); signals assigned to the 3,5/3,5-isomer δ
7.84 and 7.31 (AA′XX′ spin system, 4 H, ArH ortho and meta to CO2,
respectively), 7.40 (s, 2 H, ArH ortho to OCH2), 7.06 (s, 1 H,
isoxazole-H), 5.53 (s, 4 H, isoxazole-CH2O).
ASSOCIATED CONTENT
■
S
* Supporting Information
(12) Goto, Y.; Sato, H.; Shinkai, S.; Sada, K. ″Clickable″ Metal-
Organic Framework. J. Am. Chem. Soc. 2008, 130, 14354−14355.
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Jeanneau, E.; Lecocq, V.; Pinel, C.; Farrusseng, D. Generic
Postfunctionalization Route from Amino-Derived Metal-Organic
Frameworks. J. Am. Chem. Soc. 2010, 132, 4518−4519.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00126.
Syntheses of diacids 4 and of oximes 5, PXRD diagrams,
and NMR spectra (PDF)
(14) Savonnet, M.; Kockrick, E.; Camarata, A.; Bazer-Bachi, D.; Bats,
N.; Lecocq, V.; Pinel, C.; Farrusseng, D. Combinatorial synthesis of
metal−organic frameworks libraries by click-chemistry. New J. Chem.
2011, 35, 1892.
(15) Tuci, G.; Rossin, A.; Xu, X.; Ranocchiari, M.; van Bokhoven, J.
A.; Luconi, L.; Manet, I.; Melucci, M.; Giambastiani, G. Click” on
MOFs: A Versatile Tool for the Multimodal Derivatization of N3-
Decorated Metal Organic Frameworks. Chem. Mater. 2013, 25, 2297−
2308.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for A.G.: godt@uni-bielefeld.de.
ORCID
Adelheid Godt: 0000-0001-8453-1439
Author Contributions
(16) Li, B.; Gui, B.; Hu, G.; Yuan, D.; Wang, C. Postsynthetic
Modification of an Alkyne-Tagged Zirconium Metal-Organic Frame-
work via a ″Click″ Reaction. Inorg. Chem. 2015, 54, 5139−5141.
(17) Chen, C.-X.; Qiu, Q.-F.; Cao, C.-C.; Pan, M.; Wang, H.-P.;
Jiang, J.-J.; Wei, Z.-W.; Zhu, K.; Li, G.; Su, C.-Y. Stepwise engineering
of pore environments and enhancement of CO2/R22 adsorption
capacity through dynamic spacer installation and functionality
modification. Chem. Commun. 2017, 53, 11403−11406.
(18) Kokado, K.; Ishiwata, T.; Anan, S.; Sada, K. Unidirectional
compression and expansion of a crosslinked MOF crystal prepared via
axis-dependent crosslinking and ligand exchange. Polym. J. 2017, 49,
685−689.
T.v.Z., L.B., T.P., and M.H. worked on the linker synthesis and
developed the PSM. J.L. developed the synthesis of the PIZOFs
and characterized the modified PIZOFs. All authors con-
tributed to the interpretation of the data and to the preparation
of the manuscript and have given approval to the final version
of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(19) Wu, S.; Chen, L.; Yin, B.; Li, Y. ″Click″ post-functionalization of
a metal-organic framework for engineering active single-site heteroge-
neous Ru(III) catalysts. Chem. Commun. 2015, 51, 9884−9887.
(20) Zhang, X.; Xia, T.; Jiang, K.; Cui, Y.; Yang, Y.; Qian, G. Highly
sensitive and selective detection of mercury (II) based on a zirconium
metal-organic framework in aqueous media. J. Solid State Chem. 2017,
253, 277−281.
We thank the Deutsche Forschungsgemeinschaft DFG (SPP
1362, GO-555/5-2, BE-1664/17) for funding this project. We
are grateful for support by Ms. B. Brosent with respect to the
linker syntheses.
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