Sulfur-tagged metal-organic frameworks and their post-synthetic oxidation

Article abstract of DOI:10.1039/b906170c

A series of sulfur-tagged zinc MOFs containing functionalised 4,4′-biphenyldicarboxylate ligands has been prepared: oxidative post-synthetic modification with dimethyldioxirane has converted the sulfide tags into sulfones, leaving the MOF networks intact.

Full text of DOI:10.1039/b906170c

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COMMUNICATION
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Sulfur-tagged metal–organic frameworks and their post-synthetic
oxidationw
Andrew D. Burrows,* Christopher G. Frost, Mary F. Mahon and Christopher Richardson
Received (in Cambridge, UK) 27th March 2009, Accepted 26th May 2009
First published as an Advance Article on the web 8th June 2009
DOI: 10.1039/b906170c
A series of sulfur-tagged zinc MOFs containing functionalised
4,40-biphenyldicarboxylate ligands has been prepared: oxidative
post-synthetic modification with dimethyldioxirane has converted
the sulfide tags into sulfones, leaving the MOF networks intact.
sulfones by oxidation while retaining the MOF framework.
This is, to the best our knowledge, the first example of an
oxidative PSM of a MOF.
We have prepared a series of sulfide-tagged ligands L1–3
(Fig. 1) based on the 4,40-biphenyldicarboxylate (bpdc)
backbone. In all cases the tag group is sufficiently remote
from the site of metal complexation for it to play a
non-structure defining role during framework assembly.
The solvothermal reaction of Zn(NO3)2ꢀ6H2O with H2L1 in
DMF (or DEF) at 100 1C over 24 h deposited colourless
crystals of a compound that was characterised crystallo-
graphically as [Zn4O(L1)3(DMF)2]ꢀ4DMF 1.z This forms a
doubly-interpenetrated cubic network, as shown in Fig. 2.
The gross structure is similar to those previously observed
for [Zn4O(bpdc)3] (IRMOF-9)9 and the aldehyde-tagged
analogue,8 demonstrating that, as anticipated, the sulfur tag
does not play a structure-defining role.
The first decade of the 21st century has witnessed a vast surge
of interest in metal–organic frameworks (MOFs).1 As MOF
chemistry matures, there has been an increasing shift towards
more complex structures and increased functionality. This has
given rise to problems in both synthesis and characterisation,
since pre-functionalised linker ligands may not give the desired
network topology, while crystal structures are often plagued
with disorder, preventing full structural characterisation.
Post-synthetic modification (PSM) has recently emerged as a
powerful tool for introducing functionality to MOFs by
covalent reactions involving functional groups that project
into the pores of pre-formed MOFs.2 A major advantage of
this approach is the ability to incorporate chemical functionality
that might not otherwise be possible. The straightforward
isolation procedures afforded by heterogeneous functionalisation
are also beneficial. Indeed, procedures and techniques developed
for, and commonly used with, resins and beads may prove
employable in the covalent PSM of MOFs.
In the structure of 1, Zn(3), one of the zinc centres of the
secondary building unit (SBU), has octahedral geometry due
to coordination of two DMF molecules in addition to three
carboxylates and the central oxygen atom. This leads to a
widening of the angle between two of the carboxylate groups,
which can be quantified by a C(16)ꢀ ꢀ ꢀO(1)ꢀ ꢀ ꢀC(16)0 angle of
1071, which compares with a range of 821–911 for the other cis
Cꢀ ꢀ ꢀO(1)ꢀ ꢀ ꢀC angles. The two interpenetrated networks are
closer together in 1 than those in [Zn4O(bpdc)3], with a closest
Znꢀ ꢀ ꢀZn distance of 6.93 A (cf. 8.09 A in [Zn4O(bpdc)3]).
These structural features help accommodate the tag groups
within the pores and were also observed in the hydrazone-
functionalised MOF reported previously.8 Although there is
The first examples of covalent PSM of MOFs were the
conversion of a pendant alcohol into an ester, reported in
1999,3 and the N-alkylation of a pyridine ring, reported
in 2000.4 There were no further reports until 2007, and
subsequently several groups have reported transformations
of amines,5 including conversions to amides, ureas and
salicylidenes. Coupling of azides and alkynes has also been
demonstrated6 and
a borohydride reduction has been
accomplished.7 Recently we brought forward our paradigm
of tagging MOFs for subsequent PSM and used the
classic colorimetric test for the presence of an aldehyde, the
formation of its 2,4-dinitrophenylhydrazone, to illustrate this.8
We define a tag as a group that is stable and non-structure
defining during MOF formation, but that can be transformed
by PSM. In this communication, we present our efforts to
prepare a series of MOFs with sulfur tags. We show that these
compounds form a topologically identical series of networks,
and demonstrate that the sulfide tags can be converted to
Department of Chemistry, University of Bath, Claverton Down, Bath,
UK BA2 7AY. E-mail: a.d.burrows@bath.ac.uk;
Fax: +44 (0)1225 386231; Tel: +44 (0)1225 386529
w Electronic supplementary information (ESI) available: Syntheses of
H2L1–6, 1–4 and the post-synthetic modifications of 1 and 3; X-ray
crystallographic data for 1, 2 and 4. CCDC 725565–725567. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b906170c
Fig. 1 The functionalised dicarboxylates L1–6. L1–3 were used to
generate the sulfur-tagged MOFs 1–3, whereas L4 and L6 were
generated by PSM.
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Fig. 2 The structure of 1. Part (a) shows the SBU and (b) shows the
doubly-interpenetrated network, with DMF molecules and hydrogen
atoms omitted for clarity.
Scheme 1 Post-synthetic oxidation of 1 and 3. (i) DMDO, acetone,
20 1C.
considerable rotational disorder of the phenyl rings in 1, the
sulfide tag groups are remarkably ordered, considering the
scope for disorder.
Treatment of Zn(NO3)2ꢀ6H2O with H2L2–3 under similar
conditions produced crystals of [Zn4O(L2)3]ꢀ5DMF 2 and
[Zn4O(L3)3]ꢀnDMF 3, respectively.z Compounds 2 and 3 also
have doubly-interpenetrated cubic structures, but in both cases
data quality and disorder of the linkers prevented the tags
from being reliably located.
demonstrating that the material derived from PSM is iso-
structural to that formed by direct reaction, and also confirming
that PSM occurs with maintenance of crystallinity and structural
integrity of the sample (Fig. 3).
Bulk digestion of the sample in DCl–D2O followed
by analysis by 1H NMR spectroscopy gave a quantitative
measure of the reaction. Comparison of the spectrum with
those of authentic samples of D2L1 and D2L4 gave a ratio of
L1 : L4 of 77 : 23, with a small amount of the intermediate
sulfoxide also present.
Compounds 1 and 3 were selected for oxidative PSM.
In order to help analyse the products of these reactions,
the sulfone-functionalised dicarboxylic acids H2L4–6 were
prepared independently. L4 was converted into a MOF
through reaction with Zn(NO3)2, and the product was struc-
turally characterised as [Zn4O(L4)3(DMF)2]ꢀ2DMF 4, which
again adopts a doubly-interpenetrated cubic network.z One of
the zinc centres in 4 is coordinated to two DMF molecules, so
it displays a distorted octahedral geometry, similar to that
observed for Zn(3) in 1. Significantly, the pendant sulfonyl
tags of the L4 ligands were located in the crystallographic
study and refined as being disordered over two positions in a
1 : 1 ratio. As with 1, the distance between the two networks
(closest Znꢀ ꢀ ꢀZn contact 7.29 A) and the opening up of angles
proximate to the octahedral zinc centre help to accommodate
the functionalities within the pores.
When 3 was treated with DMDO under the same conditions
for 3 h, the ESI mass spectra of individual crystals digested in
HCl–H2O showed [HL6]ꢂ with no evidence for [HL3]ꢂ. The
quantitative conversion of 3 to [Zn4O(L6)3] 30 was confirmed
by analysing the 1H NMR spectra of bulk samples digested in
DCl–D2O. These showed only D2L6, with no evidence for
D2L3 or the corresponding sulfoxide. Hence, in this case, the
post-synthetic oxidation is 100% efficient.
In conclusion, we have prepared a series of functionalised
MOFs that are derived from linear 4,40-biphenyldicarboxylate
ligands bearing substituent sulfur tags. Topologically identical
doubly-interpenetrated MOFs are formed from ligands
containing chemically different tags (sulfide or sulfone),
different numbers of pendant sulfide tags, and tags with
different steric and spatial requirements (cyclic or pendant).
This demonstrates that the series is robust in its design
principle and formation. We were able to post-synthetically
oxidise the sulfides to sulfones, and in the conversion of L3 to
L6, this oxidation goes to completion. To the best of our
knowledge these are the first examples of oxidative PSM in
MOF chemistry, and this type of reaction increases the scope
Dimethyldioxirane (DMDO) is a frequently employed
reagent for sulfide to sulfone conversion.10 Given its properties
and size we rationalised that it was an appropriate choice of
reagent for the oxidative PSM of sulfur-tagged MOFs. The
reaction of 1 with DMDO was carried out by soaking
‘as-synthesised’ crystals of 1 in portions of acetone to exchange
for DMF prior to introducing excess DMDO as a dilute
solution in acetone (B0.05 M, B7–8 equiv.) and standing at
4 1C (Scheme 1).z We were able to follow the progress of the
modification by digesting individual crystals in HCl–H2O and
analysing their composition using electrospray ionisation
(ESI) mass spectrometry. During the early stages of the
reaction we observed the presence of [HL1]ꢂ, [HL4]ꢂ and
considerable amounts of the intermediate oxidation product,
the sulfoxide. Near complete conversion of L1 to L4 was
achieved in 2–3 h, and the reaction was allowed to run to
completion overnight as the reagent expired.
The modified crystals 10 were analysed by powder X-ray
diffraction (after standing under DMF) and compared with an
authentic sample of 4. The traces are virtually identical,
Fig. 3 X-Ray powder diffraction patterns for 10 and 4 together with
the pattern for 4 simulated from the X-ray single crystal data.
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reflections collected of which 3663 are independent [Rint = 0.0999]. R1
= 0.0666, wR2 = 0.1686 for 1722 data with I 4 2s(I). GOF = 0.954
based on F2. Crystal data for 4: C60H64N4O23S3Zn4, M = 1566.81,
of the PSM strategy. Future work on this series of MOF
compounds for their inclusion properties and ability to bind
metal complexes is currently being pursued.
monoclinic, C2/m,
a = 25.1670(4) A, b = 23.2000(3) A,
c = 17.1020(3) A, b = 93.561(1)1, U = 9966.1(3) A3, T = 150 K,
Z = 4. 88 639 reflections collected of which 11 661 are independent
[Rint = 0.0592]. R1 = 0.0884, wR2 = 0.2743 for 8102 data with
I 4 2s(I). GOF = 1.104 based on F2. 1, 2 and 4 all contain severely
disordered DMF molecules in the pores, and following the use of the
SQUEEZE routine these have been taken as 4, 5 and 2 molecules,
respectively.
The EPSRC and the Leverhulme Trust are thanked for
financial support.
Notes and references
z MOF synthesis: H2L1 (0.105 g, 0.35 mmol) and Zn(NO3)2ꢀ6H2O
(0.315 g, 1.06 mmol) were dissolved in either DMF or DEF (15 cm3)
and heated in an oven at 100 1C for 24 h. The reaction mixture was
allowed to cool slowly to room temperature and crystals of
[Zn4O(L1)3(DMF)2]ꢀ4DMF 1 were collected by filtration, washed with
fresh solvent and air dried. Compounds 2–4 were prepared in an
analogous manner.
Post-synthetic oxidation of 1 to 10: A sample of 1 (0.044 g,
0.027 mmol) was soaked in portions of acetone (3 ꢃ 3 cm3) over
1.5 h to exchange DMF for acetone. The crystals were then covered
with a solution of DMDO in acetone (5 cm3 of 0.05 M solution), and
the solution allowed to stand at 4 1C. Individual crystals were taken
out, digested in HCl–H2O and the resulting solutions analysed
by ESI-MS. The solvent was removed from the bulk sample by
decantation, the crystals washed with acetone and dried under reduced
pressure. A sample was digested in DCl–D2O–d6-DMSO, and analysis
by 1H NMR spectroscopy revealed a sulfide : sulfoxide : sulfone ratio
of 19 : 15 : 66.
Post-synthetic oxidation of 3 to 30: This was undertaken in a similar
manner to the conversion of 1 to 10. A crystal was taken out after 3 h,
digested in HCl–H2O and the resulting solution analysed by ESI-MS.
The solvent was removed from the bulk sample by decantation, the
crystals washed with acetone and dried under reduced pressure.
A sample was digested in DCl–D2O–d6-DMSO, and analysis by
1H NMR spectroscopy revealed complete conversion of sulfide to
sulfone.
Crystal data for 1: C66H78N6O19S3Zn4, M = 1617.00, monoclinic,
C2/m, a = 25.0111(9) A, b = 22.9843(9) A, c = 17.1959(3) A,
b = 96.404(4)1, U = 9823.6(5) A3, T = 170 K, Z = 4. 83 061
reflections collected of which 10 421 are independent [Rint = 0.0698].
R1 = 0.0765, wR2 = 0.2190 for 6615 data with I 4 2s(I).
GOF = 0.997 based on F2. Crystal data for 2: C69H83N5O18S6Zn4,
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4220 | Chem. Commun., 2009, 4218–4220