Post-synthetic modification of Zr-metal-organic frameworks through cycloaddition reactions

Article abstract of DOI:10.1002/chem.201103288

Cycloaddition reactions are highly attractive for post-synthetic modification of metal-organic frameworks (MOFs). We report herein on cycloaddition reactions with PIZOF(R¹,R²)s, which are porous interpenetrated Zr-based MOFs with Zr<

Full text of DOI:10.1002/chem.201103288

FULL PAPER
DOI: 10.1002/chem.201103288
Post-Synthetic Modification of Zr-Metal–Organic Frameworks through
Cycloaddition Reactions
Pascal Roy,[a] Andreas Schaate,[b] Peter Behrens,[b] and Adelheid Godt*[a]
Abstract: Cycloaddition reactions are
highly attractive for post-synthetic
modification of metal–organic frame-
works (MOFs). We report herein on
cycloaddition reactions with PIZOF-
of the ethyne groups. Reactions of
PIZOF(OMe,O(CH2)3furan) with mal-
Of all the reagents (NaOD/D2O,
D2SO4, Bu4NF, CsF, CsF/DCl, and
KHF2) tested for the disassembly of
the PIZOFs in [D6]DMSO, the combi-
nation of CsF and DCl was found to be
the best. The disassembly at room tem-
perature was fast (5–15 min), and after
the addition of K2CO3 the 1H NMR
data were identical to those of the di-
acids (=protonated linkers) dissolved
in pure DMSO. This allowed for simple
structure elucidation through data
comparison. CsF/DCl dissolves not
only PIZOFs but also the hydrolytical-
ly very stable UiO-66.
G
ACHTUNGTRENNUNG
eimide, N-methylmaleimide, and N-
phenylmaleimide converted 98, 99, and
89% of the furan moieties into the
Diels–Alder adducts. However, no re-
action occurred with maleic anhydride.
ACHTUNGTRENNUNG
1
Zr6O4(OH)
G
High-resolution H NMR spectra were
the dicarboxylates ÀO2C
(R1,R2)-
crucial in determining the conversion
and identifying the reaction products.
À
EP]CO2 (P: phenylene, E: ethyny-
lene; R1, R2: side chains at the central
phenylene unit) as the linkers. 1,3-Di-
polar cycloaddition between the pend-
ant ethyne moieties of PIZOF-
Keywords: cycloaddition
· Diels–
Alder reaction metal–organic
·
frameworks · post-synthetic modifi-
cation · zirconium
ꢀ
(OMe,OCH2C CH) and 4-methylben-
zyl azide resulted in 98% conversion
Introduction
tween alkynes and azides,[7,11c,16] presently the most popular
of the click reactions,[17] is just one example of the very
useful cycloadditions. Cycloadditions in general are highly
attractive for post-synthetic modifications for several rea-
sons. No inevitable second product is formed and therefore
none needs to be removed. Cycloadditions very often pro-
ceed quantitatively under mild conditions and are devoid of
side reactions. They are compatible with a large variety of
functional groups and the starting compounds can be broad-
ly varied. Quite recently, the Diels–Alder reaction was
added to the tools for post-synthetic modification with the
reactions of dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate with
IRMOF-3 and UMCM-1-NH2, both of which bear unsatu-
rated acyl substituents.[18,19] Herein, we present a second ex-
ample of the post-synthetic modification of MOFs through
a Diels–Alder reaction, namely the reaction between furan
covalently bound to the MOF framework and maleimides.
This reaction is demonstrated on a Zr-based MOF. Addi-
tionally, the modification of a Zr-based MOF through a 1,3-
dipolar cycloaddition is reported. Zr-based MOFs attract in-
terest because of their robustness against hydroly-
sis.[2k,m,12,20,21] This property, however, presents a challenge
when one wishes to analyze the products of post-synthetic
modifications. In this publication, we present and compare
methods for the disassembly of Zr-MOFs for analytical pur-
poses and disclose a method that allows product identifica-
tion through simple comparison of NMR data.
Covalent post-synthetic modification of metal–organic
frameworks (MOFs) has been recognized as a powerful tool
to chemically tailor the interiors of such materials.[1] Reports
on N-acylation,[2] O-acylation,[3] N-alkylation,[2g,4] conversion
of amino groups into isoACHTNUTGRNEUNG
(thio)cyanate[5] and azide groups,[6]
imine[7,8] and hydrazone formation,[9] reduction of the formyl
group,[7] thioether oxidation,[10] deprotection reactions,[11] ex-
change of bromo for cyano,[12] electrophilic aromatic substi-
tution such as nitration[13a] and sulfation,[13b] reduction of
a nitro group,[13a] photoinduced formation of nitrenes,[14] ad-
dition of bromine,[2c,15] and 1,3-dipolar cycloaddition be-
tween alkynes and azides,[7,11c,16] for example, testify the ver-
satility of this approach. The 1,3-dipolar cycloaddition be-
[a] Dr. P. Roy, Prof. Dr. A. Godt
Department of Chemistry
Bielefeld University
Universitꢀtsstrasse 25
33615 Bielefeld (Germany)
E-mail: godt@uni-bielefeld.de
[b] A. Schaate, Prof. Dr. P. Behrens
Institute of Inorganic Chemistry and ZFM –
Center for Solid State Chemistry and New Materials
Leibniz University Hannover
Callinstrasse 9
30167 Hannover (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103288.
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Results and Discussion
We recently obtained a series of isostructural porous inter-
penetrated Zr–organic frameworks [PIZOF
tion of ZrCl4 with the long rod-like diacids HO2C
(R1,R2)-EP]CO2H (P: phenylene, E: ethynylene; R1, R2:
(R1,R2)] by reac-
ACHTUNGTRENNUNG
AHCTUNGTRENGN[UN PE-P-
ACHTUNGTRENNUNG
side chains at the central phenylene unit) in DMF in the
presence of benzoic acid as a modulating agent.[21] The
PIZOF structure can be viewed as two interpenetrated net-
works of cubic closely packed Zr6O4(OH)4ACTHUNGTRNENUG(CO2)12 clusters,
with half of the tetrahedral voids of each of the two net-
works being occupied by the secondary building units
(SBUs) of the other, interpenetrating network. Alternative-
ly, the PIZOF structure can be described as two independ-
ent interpenetrating networks of the UiO-66 topology.[20a]
Despite the interpenetration, free pores with an internal di-
ameter of 19 ꢂ are present[21] and appear to be readily ac-
cessible by reaction partners. The side chains R1 and R2 can
be broadly varied, and PIZOFs containing the 1,3-dipolaro-
ꢀ
philic ethyne moiety [PIZOF(OMe,OCH2C CH)] (PIZOF-
3; Figure 1) and the enophilic furan moiety [PIZOF-
A
ACHTUNGTRENNUNG(CH2)3furan)] (PIZOF-8; Figure 2) were obtained
ꢀ
AHCTUNGTRENNUNG
(1a) and HO2C
(2a).[21]
[PE-P
N
ACHTUNGTRENNUNG
ꢀ
The ethyne moieties of PIZOF(OMe,OCH2C CH) react-
ed with 4-methylbenzyl azide in a CuI-catalyzed 1,3-dipolar
cycloaddition[22] at room temperature with a conversion of
98%. The powder X-ray diffraction (PXRD) pattern (Fig-
ure S1 in the Supporting Information) confirms the reten-
tion of the framework structure and crystallinity. The struc-
tural identity of the organic moiety and the degree of con-
1
version of the ethyne groups were determined by H NMR
spectroscopic analysis of the disassembled PIZOF (Fig-
ure 1c; for a discussion of the disassembly, see below). That
the reaction proceeds smoothly and nearly quantitatively is
very remarkable considering that our attempts to acylate
PIZOFACHTUNGTRENNUNG
(H,NH2) (PIZOF-1[21]) through treatment with acetic
anhydride or maleic anhydride in THF at 708C for 23 h
failed to give any reaction.[23] These findings indicate that in
PIZOFs the functional groups have to be at some distance
from the backbone of the linker to undergo reactions. The
reason may be simply a steric shielding of the amino groups
of one network in PIZOFACTHNUTRGNE(UNG H,NH2) through the SBUs and
the nearby terminal phenylene moieties of the linkers of the
interpenetrating network.[21] Alternatively, or additionally,
the reactivity of the amino group may be reduced due to an
electronic interaction of the amino-substituted benzene ring
with the SBU of the interpenetrating network. The bending
of the linkers towards these SBUs[21] indicates such an inter-
action. That interpenetration with the aforementioned steric
and electronic consequences is responsible for the inertness
ꢀ
Figure 1. a) 1,3-Dipolar cycloaddition between PIZOF(OMe,OCH2C CH)
and 4-methylbenzyl azide. b) 1,3-Dipolar cycloaddition between diacid
1a and 4-methylbenzyl azide. c) 1H NMR spectra ([D6]DMSO, 500 MHz,
room temperature): diacid 1a (bottom); cycloaddition product 1b ob-
tained through reaction of diacid 1a with 4-methylbenzyl azide (middle);
ꢀ
product from the reaction of PIZOF(OMe,OCH2C CH) with 4-methyl-
benzyl azide (top). The PIZOF was disassembled in [D6]DMSO through
the addition of CsF and a drop of aqueous DCl. K2CO3 was added to the
suspension prior to recording the NMR spectrum. The latter spectrum
proves nearly quantitative formation of PIZOF(OMe,OCH2triazole).
!=Signal assigned to the propargylic CH2 group of diacid 1a. fl=Signals
^
of the amino group of PIZOF
by the findings that tBuO2C[PE-P
diester corresponding to the linker of PIZOF
be acylated with maleic anhydride[24] and that the structural-
ly related, but non-interpenetrated, MOFs UiO-66
(NH2)[2k,m]
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
of THF. =Signal of H2O. =Signal of DMSO. + =Signal of CH Cl .
*
2
2
#=Signals of an unidentified contaminant of the batch of starting materi-
al 1a used to prepare 1b.
AHCTUNGTRENNUNG
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Post-Synthetic Modification and Disassembly of Zr-MOFs
FULL PAPER
and UiO-68
anhydride. Conversions of 75,[24] 88,[2k] and 100%[2m] of the
amino groups of UiO-66(NH2) have been reported with
acetic anhydride; conversions of 12[24] and 25%[2k] of the
amino groups of UiO-66
(NH2), and a conversion of 50%[24]
(NH2)[25] react with acetic anhydride and maleic
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
of the amino groups of UiO-68
with maleic anhydride.
ACHTUNGTREN(NUGN NH2) have been reported
What makes the cycloaddition reaction between azide and
alkyne so attractive is the inertness of alkynes and organic
azides under a wide range of reaction conditions and the
very high yields, but this is offset by the need to add copper
halide to accomplish the reaction under mild conditions.
Not unexpectedly, copper salt is still present even after ex-
tensive washing of the post-synthetically modified PIZOF
with DMF, THF, and MeCN, as is indicated by a slightly
greenish color of the material. Washing with amines or the
use of soluble CuI complexes may possibly yield PIZOFs
free from copper.[6] In contrast to the cycloaddition reactions
between azides and alkynes, Diels–Alder reactions often
take place under mild conditions without the aid of a cata-
lyst. To explore the option of using a Diels–Alder reaction
for post-synthetic modifications, we previously synthesized
PIZOF
Heating a suspension of PIZOFACHTUNGRTEN(NUNG OMe,OAHCTUNGTRENN(UNG CH2)3furan) and
(CH2)3furan).[21]
ACHUTGTNRNEUG(N OMe,OACHTUTGNRENNUGN
maleimide, N-methylmaleimide, or N-phenylmaleimide in
toluene at 608C for 7 days resulted in 89–99% conversion of
the furan moieties into the Diels–Alder (DA) adducts, with
the exo-isomers as the predominant products (Figure 2a).[26]
Product structures, degrees of conversion, and exo to endo
1
ratios were determined from H NMR spectra of the disas-
sembled PIZOF
sion of the disassembly, see below. Comparison of the
1H NMR spectrum of the disassembled PIZOF
(OMe,
(CH2)3-DA-adduct)s with those of the appropriate model
ACHTUGTNRENNUG(OMe,OACHTUNGTERN(NUGN CH2)3-DA-adduct)s. For a discus-
ACHTUNGTRENNUNG
O
ACHTUNGTRENNUNG
compounds 3b–d (Figure 2c, Figures S10 and S11 in the Sup-
porting Information) supported the product identification.
The model compounds 3b–d are the products of Diels–
Alder reactions between diester 3a and the corresponding
maleimides (Figure 2b). The diester 3a was used instead of
the diacid 2a because the latter is insufficiently soluble in
Figure 2. a) Diels–Alder additions between PIZOF
and maleimides and maleic anhydride. The listed yields were determined
through 1H NMR spectroscopy after disassembly of PIZOF
(OMe,
(CH2)3furan-DA-adduct). b) Diels–Alder additions between diester 3a
ACHUTGTNRNE(NUG OMe,OAHCTUNGTREN(NUGN CH2)3furan)
ACHTUNGTRENNUNG
OACHTUNGTRENNUNG
and maleimides or maleic anhydride. The listed yields were determined
through 1H NMR spectroscopy of the solution in toluene without any
work-up. c) 1H NMR spectra ([D6]DMSO, 500 MHz, room temperature):
c1) diacid 2a; c2) exo-isomer of cycloaddition product 3b; c3) product
from the reaction of PIZOF
c4) PIZOF(OMe,O(CH2)3-furan; retro) derived from PIZOF
(CH2)3-DA-adduct; X=NH). The PIZOF was disassembled in
ACHUTGTNRNEUG(N OMe,OACHTUGNTRENNNUG
A
R
ACHTUNGTRENNUNG
OACHTUNGTRENNUNG
[D6]DMSO through the addition of CsF and a drop of aqueous DCl, and
then K2CO3 was added. Spectrum c3 evidences a conversion of 98% and
the dominant formation of the exo-isomer. Spectrum c4 reveals a quanti-
tative retro-Diels–Alder reaction and complete removal of maleimide
from the pores. D=Signals assigned to the furan-2-yl group of diacid 2a.
^
fl=Signals of THF. =Signal of H O. =Signal of DMSO. £=Signals of
*
2
DMF. $=Signal of maleimide.
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A. Godt et al.
1
toluene for a reaction to occur. The H NMR spectral data
analysis of model compound 3c had tought us that retro-
Diels–Alder reaction commences at around 1258C. There-
fore, we refrained from attempts to obtain thermally activat-
of diacid 2a and diester 3a in DMSO are identical, apart
from the signal of the methyl ester group (Figure S9 in the
Supporting Information). Therefore, the NMR data of the
Diels–Alder products 3b–d can be used for comparison pur-
poses, despite the fact that 3b–d are diesters but the disas-
sembly of the PIZOFs gives diacids. Mass spectrometric
ed PIZOF
that might have undergone cycloreversion to an unknown
extent. Instead, we heated the PIZOF(OMe,O(CH2)3-DA-
ACHUTNGTRENNUG(OMe,OACHTUNGTRNE(NUGN CH2)3-DA-adduct)s and thus materials
A
ACHTUNGTRENNUNG
adduct)s to 2008C in air and kept them at this temperature
for 15 min before conducting the argon sorption experi-
ments. This thermal treatment had no significant influence
on the framework structure or crystallinity, as can be con-
cluded from the close similarity of the PXRD patterns of
the materials before and after thermal treatment (Figur-
es S3–S5 in the Supporting Information). The retro-Diels–
Alder reaction was quantitative and materials free from di-
enophile were obtained, as is evident from the 1H NMR
spectra of disassembled materials (Figure 2c, Figures S10c
and S11c in the Supporting Information). However, PIZOF-
data of disassembled PIZOFACTHUNGRTENNUG(OMe,OCAHTUNGTREN(NGUN CH2)3-DA-adduct)s
are also consistent with Diels–Alder product formation in
the PIZOF. The data given in Figure 2a and b reveal that
the Diels–Alder reactions between furan and maleimides
proceed in PIZOFACHTUNGTRENNUNG(OMe,OAHCTUNGTREN(NUGN CH2)3furan) as well as in solu-
tion. However, the ratios of exo to endo adducts differ dis-
tinctly with the exo isomer being more favored in the
PIZOF than in a homogeneous solution. The reason for this
difference is not known. It may be simply a steric effect.
That the surrounding influences the stereochemical outcome
is not surprising, considering that in the case of Diels–Alder
reactions between furans and maleimides the kinetic differ-
entiation between the endo and exo adducts is marginal and
the retro-Diels–Alder reaction occurs readily.[27]
Whereas the reactions of the furan moieties of diester 3a
and of PIZOFACHTUNGTRENNUNG(OMe,OCAHTUNGTREN(NUGN CH2)3furan) with maleimides
reached very high degrees of conversion, the Diels–Alder
reaction of maleic anhydride with diester 3a under identical
conditions resulted in only 23% conversion of the furan
A
ACHTUGTNRENG(NU CH2)3-furan; retro) derived from PIZOFACHUTNGTERUNNNG(OMe,O-
AHCTUNGTRENNUNG
1
of additional, unidentified products with H NMR signals at
around 7.45 ppm, which may be thermal decomposition
products of released N-phenylmaleimide. We cannot un-
equivocally rule out that the signals were caused by a prod-
uct formed through the elimination of water from the Diels–
Alder product giving an N-phenylphthalimide moiety. How-
ever, if this were the case, we would expect additional dis-
1
moiety and no reaction occurred between PIZOF
(OMe,O-
tinct H NMR signals due to the phenylene moieties of the
A
linker backbone, and appropriate signals are missing. Irre-
spective of the nature of the N-substituent of the PIZOF-
ACHUTNGREN(NUG OMe,OACHTUNGTRENN(UNG CH2)3-DA-adduct), the cycloreversions were ac-
companied by an unidentified side reaction, as disclosed by
1H NMR signals of low intensity at 8.04, 7.11, and 6.24 ppm.
The same side products were formed to a minor extent
porting Information). When toluene is removed from the re-
action mixture containing diester 3a, maleic anhydride, and
Diels–Alder adduct 3e at 408C under reduced pressure, that
is, when the concentrations of the components are increased,
the conversion of diester 3a increases to 92%. This is indi-
cative of an especially pronounced thermodynamic instabili-
ty of the furan–maleic anhydride Diels–Alder adducts.
Diels–Alder adducts of furan with maleic anhydride and
maleimides are reported to be of low thermodynamic stabi-
lity.[27a] It may be that the Diels–Alder adduct is even more
unfavorable within the pores, and therefore PIZOF
O
planation that maleic anhydride was consumed through in
coordination to Zr ions is ruled out because the PXRD pat-
tern (Figure S2e in the Supporting Information)
shows that no changes in the framework occurred.
during the synthesis of (PIZOF
ure S9b in the Supporting Information).
The mass-specific surface areas of PIZOF
furan) and PIZOF(OMe,O(CH2)3-furan; retro) are within
ACHUTGTNRENN(UG OMe,OAHCTUNGTREN(NUGN CH2)3-furan) (Fig-
ACHUTNGREN(NUG OMe,OACHTUNGTRENNUNG(CH2)3-
A
ACHTUNGTRENNUNG
the same range and the pore volumes and the pore size dis-
tributions of these materials are very similar (Table 1, Figur-
es S6–S8 in the Supporting Information). We take this as an
indication that the framework structure remains essentially
unaffected by the Diels–Alder and retro-Diels–Alder reac-
ACHTUNGTNER(NUNG OMe,
ACHTUNGTRENNUNG
Table 1. Mass-specific surface areas and pore volumes of PIZOF
and of the retro-Diels–Alder product obtained through heating the PIZOF
(CH2)3-DA-adduct)s to 2008C at 38CminÀ1 and keeping them at this temperature
for 15 min in flowing air.
ACHUTGTNRNEUG(N OMe,OACHTUGNTRENNNUG
The
(CH2)3furan) and its Diels–Alder products,
PIZOF(OMe,O(CH2)3-DA-adduct)s, are very simi-
PXRD
patterns
of
PIZOFACTHNGUETRNNU(G OMe,
ACHTUNGTRENNUNG
O
ACHTUNGTRENNUNG
OACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
lar (Figure S2 in the Supporting Information).
Hence, the framework structure is preserved and
the degree of crystallinity is hardly affected by the
post-synthetic modification. Additional support for
this conclusion was gained from argon adsorption
isotherms of materials that had undergone thermal-
ly induced retro-Diels–Alder reaction, that is,
mass-specific
pore volume
[cm3 gÀ1
surface area [m2 gÀ1
]
G
]
PIZOF
PIZOF
derived from:
PIZOF
PIZOF
PIZOF
G
G
1530
0.707
1210
1370
990
0.618
0.693
0.610
PIZOF
ACHTUNGTRENNUNG(OMe,OACHTUNGTRENNUNG
[a] The as-synthesized PIZOF was extracted with ethanol for 24 h in a Soxhlet extrac-
tion apparatus. The powder was then dried at 1208C for 1 h under reduced pressur-
verted into PIZOF
prior to sorption measurements. Thermogravimetric e.[21b]
G
ACHTUNGTRENNUNG
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Post-Synthetic Modification and Disassembly of Zr-MOFs
FULL PAPER
tions. Differences may be attributed to the content of non-
volatile by-products formed during cycloreversion, especially
in the case of PIZOFACHTNUGTRENNUG(OMe,OACHTUGNTRENN(UNG CH2)3-DA-adduct; X=NPh).
For structure elucidation and determination of conversion,
high-resolution NMR spectra were crucial. To obtain such
spectra, disassembly of the PIZOFs was necessary. When we
started our experiments, no method for the disassembly of
Zr-MOFs had been reported. Furthermore, the PIZOFs
were found to be very stable towards hydrolysis. Working
with N-acylated and N-alkylated UiO-66ACTHNUTRGNEUNG
(NH2),[2k,m,24] we
found that these Zr-MOFs are disassembled in a mixture of
NaOD, D2O, and [D6]DMSO, which is in agreement with
a recent report by Lillerud et al.[20b] These MOFs are also
disassembled in a mixture of concentrated D2SO4 and
[D6]DMSO.[25] Whereas the use of NaOD resulted in the for-
mation of a precipitate, probably zirconium (oxo)hydrox-
ides, the use of D2SO4 gave clear solutions. Both methods
are incompatible with many functional groups. Additionally,
the chemical shifts vary significantly with the ratio of com-
ponents, reagents, and solvents, which complicates structure
identification based on NMR data obtained from the corre-
sponding independently synthesized modified diacids or die-
sters. A much better method was found during an attempt
to desilylate PIZOF(OMe,OCH2C CTIPS) (PIZOF-4)[21]
Figure 3. 1H NMR spectra ([D6]DMSO, 500 MHz, room temperature):
a) Cycloaddition product 1b obtained through reaction of diacid 1a with
4-methylbenzyl azide. b) PIZOF(OMe,OCH2triazole) was disassembled
through addition of CsF. c) PIZOF(OMe,OCH2triazole) was disassem-
bled through addition of CsF and aqueous DCl. Then K2CO3 was added.
d) PIZOF(OMe,OCH2triazole) was disassembled through addition of
KHF2. e) Same sample as in d after addition of K2CO3. Comparison of
these spectra demonstrates that disassembly of Zr-MOFs with CsF and
aqueous DCl and addition of K2CO3 prior to recording of the NMR spec-
trum is best for easy comparison of NMR data. !=Signal assigned to the
^
ꢀ
with nBu4NF in THF, which resulted not only in desilylation
but additionally in disassembly of the PIZOF. This led to
the idea of using fluoride for disassembling the PIZOFs.
nBu4NF works very well. Unfortunately, the intense signals
of the butyl groups necessitate long acquisition times to
1
obtain H NMR spectra of sufficient quality. For this reason,
we tested CsF. CsF in [D6]DMSO works well, albeit with
slow disassembly of the PIZOFs at room temperature, most
probably due to the low solubility of CsF in DMSO. Heating
enhances the rate significantly. Another way to accelerate
the disassembly is the addition of a drop of aqueous DCl
(35 wt% in D2O). In this case, as soon as the PIZOF was
disassembled, which took about 5–15 min, solid K2CO3 was
added to neutralize the acid. The main advantage of this
latter method over the former ones is that the chemical
shifts are well reproducible and so the data can be com-
pared with those of the modified diacids dissolved in pure
[D6]DMSO, making structural identification straightforward
(Figure 3, Figure S3 in the Supporting Information). We also
tested KHF2 in [D6]DMSO. It was found to disassemble the
PIZOF within 1 h at room temperature. However, the pres-
propargylic CH2 group of diacid 1a. =Signal of H2O. =Signal of
*
DMSO. fl=Signal of THF. + =Signal of CH2Cl2. #=Signals of an un-
identified contaminant of the batch of starting material 1a used to pre-
pare 1b.
Conclusion
PIZOFs have been post-synthetically modified through 1,3-
dipolar cycloaddition between an alkyne and an azide and
through Diels–Alder cycloadditions between furan moieties
and maleimides. The results demonstrate that the functional
groups of PIZOFs are chemically addressable as long as
they are spatially remote from the linker backbone, and em-
phasize the value of the Diels–Alder reaction as a tool for
post-synthetic modification. Additionally, a protocol for the
disassembly of Zr-MOFs has been developed, that is, the
disassembly of the MOF in [D6]DMSO through the addition
of CsF and DCl followed by the addition of K2CO3, which
1
ence of KHF2 influences the shifts of the H NMR signals
(Figure 3), and this effect is not compensated by the addi-
tion of K2CO3. A CsF/aqueous DCl mixture as well as KHF2
also dissolve the hydrolytically very stable UiO-66.[20b,28]
These methods are preferable to the use of highly toxic HF,
which was recently reported as a reagent for this purpo-
se.[2k,12]
1
allows very simple structure elucidation by H NMR spec-
troscopy.
Chem. Eur. J. 2012, 00, 0 – 0
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A. Godt et al.
Acknowledgements
Experimental Section
Financial support through the DFG priority program 1362 and technical
support by Ms. M. Hꢄlsmann is gratefully acknowledged. We thank Mr.
G. Platz for the measurement of the Ar sorption isotherms.
Experimental details concerning the synthesis of the model compounds,
as well as figures showing the PXRD patterns, sorption data, and further
NMR data, are presented in the Supporting Information.
Disassembly of PIZOFs with CsF and aqueous DCl: The PIZOF (1–
6 mg) and CsF (6–11 mg) were suspended in [D6]DMSO (0.65 mL). A
drop of aqueous DCl (35 wt% in D2O, 20–25 mg) was then added. As
soon as the yellow solid had disappeared, that is, as soon as the PIZOF
had been disassembled (5–15 min; CsF is a white solid and can be easily
distinguished from the yellow PIZOF), K2CO3 (10–15 mg) was added.
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ꢀ
1,3-Dipolar cycloaddition with PIZOF(OMe,OCH2C CH)
ꢀ
Washing of PIZOF: PIZOF(OMe,OCH2C CH) (PIZOF-3) was washed
extensively prior to the cycloaddition reaction in order to remove all of
the benzoic acid that had been used as a modulator in its synthesis. To
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[21]
ꢀ
this end, the as-synthesized PIZOF(OMe,OCH2C CH) was suspended
in THF (1–2 mL). After standing for 6–14 h at room temperature, the
suspension was centrifuged and the solvent was removed by decanting.
This procedure was carried out five times. The powder was then dried
under high vacuum. The thus obtained PIZOF was free from benzoic
acid, as proven by 1H NMR spectroscopic analysis of a sample disassem-
bled in warm [D6]DMSO through the addition of CsF.
1,3-Dipolar cycloaddition: 4-Methylbenzyl azide (10.1 mg, 0.069 mmol)
and CuBr (10.0 mg, 0.070 mmol) were added to a suspension of PIZ-
ꢀ
ꢀ
OF(OMe,OCH2C CH) (11.7 mg, 0.021 mmol of C CH) in DMF
(0.8 mL). After 19 h at room temperature without stirring, the suspension
was centrifuged for 5 min at 6000 rpm and the solvent was removed by
decanting. The residue was washed with DMF (3ꢃ1 mL), THF (3ꢃ
1 mL), and finally MeCN (4ꢃ1 mL) as described above for PIZOF(O-
ꢀ
Me,OCH2C CH). The greenish powder (12.1 mg) was dried at room
temperature under reduced pressure. HRMS (MALDI): m/z: calcd for
C36H27N3O6Na+: 620.17921; found: 620.17895 [M+Na+]; calcd for
C36H28N3O6+: 598.19726; found: 598.19674 [M+H+].
Diels–Alder cycloadditions with PIZOF
N
ACHTUGNTRNEN(UNG CH2)3furan)
Washing of PIZOF: PIZOF(OMe,OMe,OAHCUTNGTRENNUNG
washed extensively prior to the cycloaddition reaction in order to remove
all of the benzoic acid that had been used as a modulator in its synthesis.
(CH2)3furan)[21] was
To this end, the as-synthesized PIZOFACTHNUTRGENNUG(OMe,OACHTUNGTRENNUGN
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washed with DMF (5ꢃ1 mL) and THF (16ꢃ1 mL), as described above
ꢀ
for PIZOF(OMe,OCH2C CH). The thus obtained PIZOF was free from
benzoic acid, as proven by 1H NMR spectroscopic analysis of a sample
disassembled in [D6]DMSO by the method employing CsF, DCl, and
K2CO3.
Diels–Alder cycloaddition: A suspension of PIZOFACTHNUTRGENN(UG OMe,OACHTUNGTERN(NUNG CH2)3furan)
(32 mg, 0.048 mmol of furan moiety) and the dienophile (30 equiv per
furan moiety) in toluene (2 mL) under argon atmosphere was stirred at
608C for 7 d. Thereafter, the suspension was centrifuged for 5 min at
6000 rpm and the solvent was removed by decanting. The residue was
washed with DMF (3ꢃ1 mL) and THF (14ꢃ1 mL) and finally dried at
room temperature under reduced pressure.
For analytical purposes, a small amount of the obtained solid was disas-
sembled in [D6]DMSO by the method employing CsF, DCl, and K2CO3.
For NMR data, see Figure 2c and Figures S10–S12 in the Supporting In-
formation. Conversions and endo/exo ratios were calculated from the
1H NMR signal intensities of the olefinic and allylic protons of the prod-
uct and of the protons of the furan moiety of the starting material.
HRMS (ESI) of the product from the reaction with maleimide: m/z:
calcd for C36H26NO9À: 616.16131; found: 616.15997. HRMS (ESI) of the
product from the reaction with N-methylmaleimide: m/z: calcd for
C37H28NO9À: 630.17696; found: 630.17655. HRMS (ESI) of the product
2À
from the reaction with N-phenylmaleimide: m/z: calcd for C42H31NO9
:
345.59266; found: 345.59177.
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Post-Synthetic Modification and Disassembly of Zr-MOFs
FULL PAPER
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1
is the only or predominant isomer formed. Assignments of H NMR
signals to the exo- and endo-isomers are in agreement with data
from refs. [27b,c].
ˇ
ˇ
ˇ
ˇ
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[28] UiO-66 employed in the disassembly test was prepared according to
the modulation approach described in ref. [25] and therefore con-
tained benzoic acid. However, we do not expect benzoic acid to
have an impact on the disassembly.
cation describes
a photochemical [2+2] cycloaddition between
double bonds that are part of the linker backbone. Such a crystal-to-
crystal transformation differs distinctly from the post-synthetic
modifications that are achieved when the linkers react with an
added component, as is the focus of this publication. The reaction
Received: October 18, 2011
Revised: January 21, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
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www.chemeurj.org
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A. Godt et al.
MOF Modification
P. Roy, A. Schaate, P. Behrens,
A. Godt* ....................... &&&& — &&&&
Post-Synthetic Modification of Zr-
Metal–Organic Frameworks through
Cycloaddition Reactions
Modifying and disassembling MOFs:
Porous interpenetrated Zr-organic
frameworks (PIZOFs) with large
pores, have been post-synthetically
modified through 1,3-dipolar cycload-
dition and Diels–Alder reactions (see
scheme, left). Additionally, a simple
procedure is reported for the disassem-
bly of the hydrolytically very stable
UiO-66- and PIZOF-type Zr-based
MOFs for analytical purposes (see
scheme, right).
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These are not the final page numbers!