A reagentless thermal post-synthetic rearrangement of an allyloxy-tagged metal-organic framework

Article abstract of DOI:10.1039/c2cc38176a

Direct heating of a metal-organic framework provides a simple, controllable way of effecting a covalent post-synthetic modification. Herein we report that an allyloxy-tagged zinc metal-organic framework undergoes a thermally-promoted aromatic Claisen rear

Full text of DOI:10.1039/c2cc38176a

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A reagentless thermal post-synthetic rearrangement of an allyloxy-
tagged metal-organic framework
Andrew D. Burrows,a Sally O. Hunter,b Mary F. Mahona and Christopher Richardson*b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
deprotection (PSD) chemistry through the instability of tert-
50 butoxycarbamates to unveil amine groups in MOFs, with the
additional benefit of concomitant pore enlargement.7 Very
recently, Vittal8 found that direct heating could dismember
cyclobutane rings via a reverse [2+2] cycloaddition process. We
sought to explore thermal post-synthetic rearrangement (PSR)
55 chemistry in MOFs as a simple and efficient way of bringing out
new functionality in these materials. We rationalised that all
atoms required for rearrangements could be installed at the point
of MOF formation through a direct synthesis and this would
completely eliminate the need for chemical reagents in the post-
60 synthetic step. All that is required to bring about the MOF
modification is just to heat it.
Direct heating of a metal-organic framework provides a
simple, controllable way of effecting a covalent post-synthetic
modification. Herein we report that an allyloxy-tagged zinc
metal-organic framework undergoes a thermally-promoted
10 aromatic Claisen rearrangement through which the
framework connectivity and porosity are maintained.
Metal-organic frameworks (MOFs) are porous solids constructed
from component bridging ligands and metal ions that assemble
into crystalline lattices.1 With tuneable pore metrics and chemical
15 functionality, MOFs provide unique environments and
opportunities for synthetic chemistry. The main thrust in this area
is using MOFs as heterogeneous catalysts wherein they may
convey advantages of substrate size selectivity and catalyst
recovery for chemical processes.2 The use of MOFs in
20 stoichiometric or sub-stoichiometric chemical transformations is
also possible. In this light, MOFs might be designed so that
unusual or novel reactivity is accessed to give products of high
value, while retaining the ability to recover and reuse the MOF.
Examples demonstrating this kind of reactivity have come from
25 the Fujita group.3
A particularly intriguing use of porous MOFs is their potential
to act as solid state reaction matrices. This is where the
framework backbone acts to hold chemical groups rigidly in
place and spatially separate them. Champness, and George and
30 co-workers, for example, used the spatial separation afforded by a
framework and immobilised rhenium complexes to enable photo-
physical studies.4 The advantage of immobilisation for avoiding
bimolecular catalyst deactivation are recognised in MOF
chemistry.5 Complexed MOF ligands can be viewed as being in a
35 protected form and, by virtue of their incorporation into a
framework, chemistry that would otherwise not be possible might
be enabled. Reactions on immobilised ligands in a preformed
network is, of course, covalent post-synthetic modification
(PSM).6
It is now 100 years since Claisen9 reported the thermal
rearrangement of allylphenyl ethers to ortho-allyl phenols – the
aromatic Claisen rearrangement. This celebrated reaction is now
65 the textbook example of sigmatropic rearrangements. The
aromatic Claisen rearrangement has many successful reaction
variants and yet, the classic conditions of heating in a high
boiling solvent remains in high use.10 In this communication we
report on the first example of thermally-promoted PSR chemistry
70 in a porous MOF through direct heating of the solid.
We prepared 2-(allyloxy)-[1,1'-biphenyl]-4,4'-dicarboxylic
acid H2L1† as a ligand with a linear biphenyl dicarboxylic acid
backbone and a pendant allyloxy tag group (Fig. 1) that would
allow us to target a cubic structure based on the well-known zinc
75 IRMOF series.11 The tag group was positioned on the ligand
backbone to project into the pore space of the resultant MOF
where it could attain the conformation needed to undergo the
rearrangement.
40
MOFs have considerable thermal stabilities, often to several
hundred degrees Celsius. Perhaps inevitably, the thermal
stabilities of MOFs are drawn in comparison to other porous solid
state materials, such as zeolites, which are stable to very high
temperatures. As a consequence, MOFs are considered to have
45 only ‘moderate’ thermal stabilities. However, temperatures of up
to 350 °C give access to many thermal organic transformations.
Thermally-promoted post-synthetic chemistry of MOFs has seen
few reports. Telfer has pioneered thermolytic post-synthetic
80
Fig 1 The structure of H2L1 and synthetic conditions to form 1-3. One
network and the major ligand component of each PSR is shown.
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H2L1 was reacted with Zn(NO3)2·6H2O in hot DMF‡ to give
40
The heating protocol was modified to hold the sample at 260
ºC† and the PSR product, 2, was recovered. The 1H NMR spectra
slightly opaque cubic crystals of 1 that were analysed by single
crystal X-ray diffraction. 1 crystallises in the space group C2/m
with similar unit cell parameters to those we have observed for
zinc MOFs with aldehyde-, sulfide- and sulfone-tagged biphenyl
of the recovered material† showed the presence of unreacted
H2L1, the desired PSR product H2L2, and a product containing a
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5
furan ring, H2L3 (Fig. 1). The optimum conditions for the
dicarboxylate ligands.12 The structure exists as a pair of 45 generation of H2L2 required a hold time of 20 minutes, which
interpenetrating cubic networks (Fig. 2), separated by ca. 4–5 Å
between phenyl rings of the linkers, and with a framework
formula of [Zn4O(L1)3(DMF)2]. One of the zinc centres
typically gave a ratio of H2L1 : H2L2 : H2L3 of 1 : 7 : 0.5,
respectively, resulting in
a
formulation for
2
of
[Zn4O(L1)0.33(L2)2.47(L3)0.17]. Thus, this modification is efficient
10 coordinates 2 DMF ligands and adopts a distorted octahedral
with only around 10% of starting allyloxy groups remaining. The
geometry. The Zn4O SBU falls across the mirror plane of the 50 rearrangement temperature of 260 ºC was definitively established
space group as does a bridging ligand and this contributes to
rotational disorder of the linker. Remarkably, despite this
disorder, the first three atoms of the pendant allyloxy tag
15 stemming from this ligands backbone could be located
as when the material was heated to 250 ºC, analysis of the the
1
material through digestion and solution H NMR spectroscopy,
showed only H2L1.
To examine the crystallinity of the frameworks, 1 and 2 were
crystallographically. Typically, the positions of the atoms of tag 55 analysed by powder X-ray diffraction (PXRD) (Fig. 4). The
groups are not observed due to disorder. The terminal carbon
atom of the allyloxy group could not be reliably located in the
final difference Fourier electron density map, but this is not
20 surprising given the disorder present. However, through digestion
excellent correspondence of peaks between the samples indicates
the connectivity of the framework has not changed as a result of
the PSR.
1
and solution H NMR analysis of 1 it was confirmed that L1 was
unchanged upon incorporation into the MOF.
60
Fig. 4 PXRD patterns for 1 and 2.
Fig. 2 The doubly interpenetrated structure of 1.
Direct evidence that porosity is retained during the
rearrangement from 1 to 2 comes from nitrogen gas adsorption
measurements at 77 K (Fig. 5). After activation at 145 ºC, 1
shows a Type I isotherm that gives a Brunauer-Emmett-Teller
25
Following solvent exchange and vacuum drying,‡ powdered 1
was analysed by simultaneous thermogravimetric–differential
thermal analysis (TG–DTA) under an atmosphere of nitrogen gas
(Fig. 3). The TG curve shows a mass loss of 3.5% between room
temperature and 100 ºC, corresponding to some residual solvent
30 being driven from the framework, before a broad plateau of
stability to around 380 ºC, whereupon the onset of decomposition
occurs. In the DTA curve the endothermic solvent loss is
followed by a prominent exotherm which is observed to occur
with no concurrent mass loss. This is consistent with the process
35 of a rearrangement taking place where no atoms are lost or
gained.
65 (BET) surface area of 1348 m2 g-1 (Langmuir, 1464 m2 g-1). The
sample was retained in the instrument and heated to 260 ºC and
the BET surface area was found to be 1360 m2 g-1 (Langmuir,
1
1477 m2 g-1) with a near identical isotherm. H NMR analysis
following digestion showed that the composition of the MOF
70 material from this experiment was [Zn4O(L1)0.26(L2)1.96(L3)0.78].
Fig. 5 Nitrogen gas adsorption/desorption isotherms for the in situ
conversion of 1 (left) to 2 (right). Closed squares/circles adsorption, open
squares/circles desorption.
75
When H2L2 was used as a ligand in MOF synthesis under the
same reaction conditions used for H2L1 there was no formation of
any solid product. This result indicates that post-synthetic
modification provides the best, and, perhaps, the only way to
prepare 2. We also tried to prepare a zinc MOF from 2-hydroxy-
Fig. 3 TG—DTA curves for heating dried 1 to 550 °C. The blue curve
shows the TG response. The red curve shows the DTA response.
2
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‡ MOF synthesis H2L1 (100 mg, 0.34 mmol) and Zn(NO3)2·6H2O (300
[1,1'-biphenyl]-4,4'-dicarboxylic acid in DMF solution and found
60 mg, 1.0 mmol) were dissolved in DMF (18 cm3) and heated in an oven for
24 hours. The crystals were solvent exchanged with fresh portions of
anhydrous DMF then with CH2Cl2 and finally with benzene. Samples
that no solid was produced in this reaction. However, during the
course of our study it was reported that when using DEF as the
reaction solvent the interpenetrated cubic structure is formed.13
MOFs with non-coordinated hydroxyl groups are relatively rare13,
were left under benzene until required whereupon they were freeze dried.
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5
A typical yield from this process was 100 mg. Found: C, 49.77; H, 3.07;
65 N. 0.00. [Zn4O(L1)3]·3.5H2O requires C, 49.80; H, 3.51; N, 0.00.
14
but are desirable as groups for tuning the adsorptive
1
S. Kitagawa, R. Kitaura and S.-I. Noro, Angew. Chem. Int. Ed., 2004,
43, 2334; G. Ferey, Chem. Soc. Rev., 2008, 37, 191.
performance of MOF materials. Recently
a method with
seemingly general applicability for hydroxyl functionalised
MOFs has appeared.15
2
M. Yoon, R. Srirambalaji and K. Kim, Chem. Rev., 2012, 112, 1196;
A. Corma, H. García and F. X. Llabrés i Xamena, Chem. Rev., 2010,
110, 4606.
70
10
We became interested in the observation of L3 within the PSR
process. Coordinated L3 comes from L2 via a cyclisation pathway
and longer hold times leads to increased proportions of L3 at the
expense of L2. Much better results were obtained, however, by
employing higher temperatures and we found that by heating to
3
Y. Inokuma, G.-H. Ning and M. Fujita, Angew. Chem. Int. Ed., 2012,
51, 2379; Y. Inokuma, N. Kojima, T. Arai and M. Fujita, J. Am.
Chem. Soc., 2011, 133, 19691; Y. Inokuma, S. Yoshioka, and M.
Fujita, Angew. Chem. Int. Ed., 2010, 49, 8912.
75 4 A. J. Blake, N. R. Champness, T. L. Easun, D. R. Allan, H. Nowell,
15 300 ºC, or above, L3 becomes the dominant product. Remarkably,
crystallinity is still maintained in this process as evidenced by the
PXRD patterns.† Thus this sequence is controllable through
extending the heating time but shows better response to
temperature. The best conversions to L3 were obtained with a
20 heating rate of 15 °C min-1 to 320 ºC and a hold time of 20
M. W. George, J. Jia and X.-Z. Sun, Nature Chem., 2010, 2, 688.
5
S.-H. Cho, B.-Q. Ma, S. T. Nguyen, J. T. Hupp, and T. E. Albrecht-
Schmitt, Chem. Commun., 2006, 2563; A. Shultz, O. Farha, D.
Adhikari, A. Sarjeant, J. T. Hupp and S. Nguyen, Inorg. Chem.,
2011, 50, 3174
Z. Wang and S. M. Cohen, Chem. Soc. Rev., 2009, 38, 1315; S. M.
Cohen, Chem. Rev., 2012, 112, 970.
R. K. Deshpande, J. L. Minnaar and S. G. Telfer, Angew. Chem. Int.
Ed., 2010, 49, 4598; D. J. Lun, G. I. N. Waterhouse and S. G. Telfer,
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L. Liu, G. L. N. Waterhouse and S. G. Telfer, CrystEngComm, 2012,
14, 5701.
80
85
6
7
minutes. This gave
a
MOF with the formulation of
1
[Zn4O(L2)0.45(L3)2.55] 3 as judged through H NMR spectroscopy
of the digested product.† At these temperatures some terminal-to-
internal alkene isomerisation is observable, but only in very small
25 amounts (<5%). Significantly, this transformation cannot be
efficiently carried out by direct heating of H2L1 itself, as it
undergoes decomposition before these temperatures. Mass
spectrometry of the digested samples shows a signal at twice the
expected mass, suggesting that not all of the rearrangement
30 processes have complete reticular fidelity and that there are inter-
reticular reactions between pairs of ligands close by in the
interpenetrated framework. This is an interesting type of
reactivity as it results in covalently linking the networks together.
In summary, we have shown that high temperature organic
35 chemistry is accessible in porous MOF systems through direct
heating in a controllable way. This method thus provides a
potentially valuable addition to the existing methods for covalent
PSM in MOFs. Demonstrated here is an example whereby a
chemical transformation can be done on the linker because of
40 increased stability via framework incarceration. Considering the
high temperatures and possible competing reaction pathways, the
PSRs described herein occur remarkably cleanly and in high
yields. The success of this example has focussed our attention to
establishing the generality of thermally-promoted PSRs in MOFs
45 and we are now investigating other examples.
8
A. Chanthapally, G. K. Kole, K. Qian, G. K. Tan, S. Gao and J. J.
Vittal, Chem. Eur. J., 2012, 18, 7869.
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10 A. M. Martín Castro, Chem. Rev., 2004, 104, 2939; H. Ichikawa and
K. Maruoka, in The Claisen Rearrangement: Methods and
Applications, ed. M. Hiersemann, U. Nubbemeyer, Wiley, 2007, ch.
3, pp. 45-116.
95 11 M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe
and O. M. Yaghi, Science, 2002, 295, 469.
12 A. D. Burrows, C. G. Frost, M. F. Mahon and C. Richardson, Angew.
Chem. Int. Ed., 2008, 47, 8482; A. D. Burrows, C. G. Frost, M. F.
Mahon and C. Richardson, Chem. Commun., 2009, 4218.
100 13 R. K. Deshpande, G. I. N. Waterhouse, G. B. Jameson and S. G.
Telfer, Chem. Commun., 2012, 48, 1574.
14 K. L. Mulfort, O. K. Farha, C. L. Stern, A. A. Sargeant and J. T.
Hupp, J. Am. Chem. Soc., 2009, 131, 3866; K. S. Jeong, Y. B. Go, S.
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Sumby, Chem. Commun., 2012, 48, 10328.
We are grateful to Dr Deanna M. D'Alessandro of the University
of Sydney for recording the gas adsorption data.
Notes and references
a Department of Chemistry, University of Bath, Claverton Down, Bath
50 BA2 7AY, UK.
b School of Chemistry, University of Wollongong, Wollongong NSW 2522,
Australia; Fax: +61 2 4221 4287; Tel:+ 61 2 4221 3254; E-mail:
chris_richardson@uow.edu.au
55 † Electronic Supplementary Information (ESI) available: Synthesis of
H2L1, TG–DTA data, PXRD patterns for 1 and 3; selected single crystal
data for 1; 1H NMR digestion spectra. CCDC 910203. See
DOI: 10.1039/b000000x/
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