Stereoselective Halogenation of Integral Unsaturated C-C Bonds in Chemically and Mechanically Robust Zr and Hf MOFs

Article abstract of DOI:10.1002/chem.201505185

Metal-organic frameworks (MOFs) containing Zr^IV-based secondary building units (SBUs), as in the UiO-66 series, are receiving widespread research interest due to their enhanced chemical and mechanical stabilities. We report the synthesis and extensive characterisation, as both bulk microcrystalline and single crystal forms, of extended UiO-66 (Zr and Hf) series MOFs containing integral unsaturated alkene, alkyne and butadiyne units, which serve as reactive sites for postsynthetic modification (PSM) by halogenation. The water stability of a Zr-stilbene MOF allows the dual insertion of both -OH and -Br groups in a single, aqueous bromohydrination step. Quantitative bromination of alkyne- A nd butadiyne-containing MOFs is demonstrated to be stereoselective, as a consequence of the linker geometry when bound in the MOFs, while the inherent change in hybridisation and geometry of integral linker atoms is facilitated by the high mechanical stabilities of the MOFs, allowing bromination to be characterised in a single-crystal to single-crystal (SCSC) manner. The facile addition of bromine across the unsaturated C-C bonds in the MOFs in solution is extended to irreversible iodine sequestration in the vapour phase. A large-pore interpenetrated Zr MOF demonstrates an I₂ storage capacity of 279 % w/w, through a combination of chemisorption and physisorption, which is comparable to the highest reported capacities of benchmark iodine storage materials for radioactive I₂ sequestration. We expect this facile PSM process to not only allow trapping of toxic vapours, but also modulate the mechanical properties of the MOFs.

Full text of DOI:10.1002/chem.201505185

DOI: 10.1002/chem.201505185
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Metal–Organic Frameworks |Hot Paper|
Stereoselective Halogenation of Integral Unsaturated C-C Bonds
in Chemically and Mechanically Robust Zr and Hf MOFs
Ross J. Marshall, Sarah L. Griffin, Claire Wilson, and Ross S. Forgan*[a]
Abstract: Metal–organic frameworks (MOFs) containing ZrIV-
based secondary building units (SBUs), as in the UiO-66
series, are receiving widespread research interest due to
their enhanced chemical and mechanical stabilities. We
report the synthesis and extensive characterisation, as both
bulk microcrystalline and single crystal forms, of extended
UiO-66 (Zr and Hf) series MOFs containing integral unsatu-
rated alkene, alkyne and butadiyne units, which serve as re-
active sites for postsynthetic modification (PSM) by halogen-
ation. The water stability of a Zr–stilbene MOF allows the
dual insertion of both ÀOH and ÀBr groups in a single, aque-
ous bromohydrination step. Quantitative bromination of
alkyne- and butadiyne-containing MOFs is demonstrated to
be stereoselective, as a consequence of the linker geometry
when bound in the MOFs, while the inherent change in hy-
bridisation and geometry of integral linker atoms is facilitat-
ed by the high mechanical stabilities of the MOFs, allowing
bromination to be characterised in a single-crystal to single-
crystal (SCSC) manner. The facile addition of bromine across
the unsaturated CÀC bonds in the MOFs in solution is ex-
tended to irreversible iodine sequestration in the vapour
phase. A large-pore interpenetrated Zr MOF demonstrates
an I2 storage capacity of 279% w/w, through a combination
of chemisorption and physisorption, which is comparable to
the highest reported capacities of benchmark iodine storage
materials for radioactive I2 sequestration. We expect this
facile PSM process to not only allow trapping of toxic va-
pours, but also modulate the mechanical properties of the
MOFs.
MOFs[13] has been known to be tricky; however, coordination
modulation[14] has greatly improved the ability to access both
bulk microcrystalline[15] and single crystal forms.[16]
Introduction
Metal–organic frameworks (MOFs)[1] are multidimensional coor-
dination networks comprised of metal nodes separated by or-
ganic linkers that have received widespread interest over the
past 10-15 years,[2] mainly attributable to their permanent po-
rosity, leading to potential application in areas such as gas cap-
ture and storage,[3] catalysis[4] and drug delivery.[5] The judicious
choice of both the organic and inorganic constituents of MOFs
enables vast opportunities for framework design,[6] leading to
materials with intrinsically variable structures[7] and proper-
ties.[8] Research focused on improving the stabilities of MOFs[9]
has resulted in the utilisation of high valent metal cations in
the secondary building units (SBUs). Most notably, ZrIV MOFs,
in particular the so-called UiO-66 series[10] wherein dicarboxy-
late ligands connect Zr6O4(OH)4 clusters, demonstrate increased
chemical[11] and mechanical[12] stabilities. The synthesis of Zr
The chemical stabilities of Zr MOFs make them attractive
candidates for postsynthetic modification (PSM), whereby
chemical transformations are performed on pre-synthesised
MOFs whilst maintaining crystallinity.[17] PSM offers a vast array
of opportunities for the functionalisation of Zr MOFs, although
this has typically been limited to ligand/metal-ion exchange,[18]
ligand metalation[13a,19] and covalent modifications of pendant
functional groups.[20] Across all MOFs, postsynthetic bromina-
tion has been attempted mainly on tethered moieties,[21] with
limited success obtained for integral units.[22] In contrast, we re-
cently communicated the stereoselective bromination of inte-
gral alkene and alkyne units contained within the Zr MOFs
1 and 2, respectively, (Figure 1) in a single-crystal to single-
crystal (SCSC) manner, resulting in a mechanical contraction of
the frameworks accompanied by an increase in ligand flexibili-
ty, which in turn alters mechanical compliance.[23]
[a] R. J. Marshall, S. L. Griffin, Dr. C. Wilson, Dr. R. S. Forgan
WestCHEM, School of Chemistry, The University of Glasgow
University Avenue, Glasgow G12 8QQ (UK)
E-mail: Ross.Forgan@glasgow.ac.uk
Herein we significantly broaden the scope of both the MOFs
and functional groups that can be postsynthetically modified,
as well as the chemical transformations that are utilised. Con-
tinuing with our interest in introducing flexibility into typically
rigid UiO-66 type MOFs,[24] we describe the synthesis (see Sup-
porting Information, Sections S2 and S3) of a new Zr MOF 3,
Homepage: www.forganlab.com
Supporting information and ORCID for one of the authors for this article is
available on the WWW under http://dx.doi.org/10.1002/chem.201505185.
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons At-
tribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
constructed from
a butadiyne-containing organic ligand,
alongside a pair of analogous interpenetrated isoreticular Zr
and Hf MOFs,[25] 4 and 5, respectively, containing an extended
Chem. Eur. J. 2016, 22, 4870 – 4877
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Figure 1. a) Abbreviation scheme of the ligands and MOFs used throughout
this study. Note that 4 and 5 display two-fold interpenetration. b) Represen-
tation of the crystal structure of 1, which can be brominated in a SCSC
manner to c) 1-Br2. Atom colouring: Zr (cyan); C (grey); O (red); Br (brown).
H atoms are omitted for clarity.
It is therefore surprising that few efforts have focused on the
introduction of hydroxyl groups, either through protection–de-
protection mechanisms[28] or postsynthetically.[29] To that end,
reaction conditions were tailored to promote conversion to 1-
Br-OH, with increased amounts of NBS resulting in slightly
better conversion (entry 4), but the yield of 1-Br-OH remained
low. Deliberate addition of water (entry 5) gave significantly
more bromohydrinated product, while the combined presence
of both water and DPT gave full conversion and up to 67%
bromohydrination (entries 6–8) versus bromination. Reduced
quantities of NBS (entries 9 and 10) gave similar product distri-
butions; around 60–65% of 1-Br-OH and 35–40% of 1-Br2.
Similar product distributions were obtained when the reaction
was performed on a larger scale (entry 11).
4,4’-[1,4-phenylenebis(ethyne-2,1-diyl)]-dibenzoate
(peb2À
)
linker (Figure 1a). The new MOFs are extensively characterised
using a number of techniques, including single-crystal X-ray
diffraction (SCXRD), and postsynthetic halogenation of their in-
tegral reactive sites has also been thoroughly investigated.
Results and Discussion
Postsynthetic bromination of integral alkenes within MOFs has
been highlighted as an attractive route for the stereocontrolled
synthesis of bromoalkanes. Bromination of a Zn–stilbene MOF
has previously been reported, however the forcing conditions
required for quantitative conversion (1008C) resulted in degra-
dation of the MOF.[22a] We recently communicated the quantita-
tive bromination of the Zr–stilbene MOF [Zr6O4(OH)4(sdc)6]n (1)
to [Zr6O4(OH)4(meso-sdc-Br2)6]n (1-Br2), as both bulk microcrys-
talline and single-crystal material (Figure 1b and 1c).[23b] Con-
sidering the harsh bromination conditions employed (neat Br2),
we decided to investigate the use of N-bromosuccinimide
(NBS) as a milder, alternative brominating agent (Table 1).[26]
Addition of 15 equivalents of NBS (entry 2) was necessary to
quantitatively brominate 1. Addition of N,N’-diphenylthiourea
(DPT), a known catalyst for activation of NBS, gave small
amounts of the bromohydrinated product, 1-Br-OH, alongside
the brominated material 1-Br2 (entry 3), presumably resulting
from excipient water in the reagent or solvent. Incorporation
of hydroxyl units within MOFs allows pore hydrophilicity to be
tuned, yet direct synthesis of such MOFs is not always possible
due to coordination of the hydroxyl group to the metal ions.[27]
Considering the similarity of bromohydrination to bromina-
2À [23b]
tion of 1, which results exclusively in meso-sdc-Br2
,
we
assume that only the enantiomeric (R,S)/(S,R)-isomers are ob-
tained (see Supporting Information, Section S4). The fact that
1 remains crystalline during the transformation highlights its
high mechanical stability, as it is able to withstand a change in
geometry of the central carbon atoms as their hybridisation
changes from sp2 to sp3. The chemical stability of 1 is also ap-
parent as it retains its framework structure in the presence of
significant amounts of water. The incorporation of hydroxyl
groups generates a platform for subsequent functionalisation
of MOFs, with alkoxide formation[30] and incorporation of cata-
lytic units[31] previously described. Whilst NBS has allowed us
to bromohydrinate 1, enabling the insertion of orthogonal
functionality into the MOF in one-step, we have chosen Br2 as
a favoured brominating agent for Zr-MOFs bearing integral un-
saturated functionalities.

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Inspired by the work of Anderson and co-workers, in which
they show that butadiyne units are highly flexible and can
adopt non-linear geometries,[32] we decided to synthesise 4,4’-
(buta-1,3-diyne-1,4-diyl)-dibenzoic acid (bdb-H2) for incorpora-
tion into a UiO-66 series MOF. Bulk microcrystalline and single-
crystal samples of [Zr6O4(OH)4(bdb)6]n (3) were synthesised in
the presence of 30 equivalents of benzoic acid (see Supporting
Information, Section S3).[33] SCXRD reveals that 3 adopts the
typical UiO-66 topology, with Zr6O4(OH)4 clusters connected
The transformation to 3-Br4 was evidenced by PXRD (Fig-
ure 2c); excellent agreement between the predicted (from the
single-crystal structure) and experimental PXRD patterns of 3
reveal that the bulk microcrystalline samples are of excellent
phase purity and, upon comparison with the brominated ma-
terial 3-Br4, it is clear that a structural transition has occurred
due to distinct changes in both peak positions and their rela-
1
tive intensities. H NMR spectra of acid-digested samples of 3
and 3-Br4 (Figure 2d) provide evidence of quantitative bromi-
nation to a single isomer, and we assume the spectrum to
comprise trans,trans-bdb-Br4-H2, due to the steric restrictions
imposed on the linker when bound within the MOF. The pres-
ence of only aromatic protons provides limited information
and, unfortunately, 13C NMR spectra of digests of 3 were not
possible due to the limited solubility of bdb-H2. In the 13C NMR
spectrum of 3-Br4, however, seven signals are observed, which
correspond to the carbon atoms of the brominated product,
alongside two small peaks around d=130 ppm, which repre-
sent aromatic carbon atoms of residual benzoic acid modulator
(see Supporting Information, Section S5).[33] The number of res-
onances indicate only one symmetric product is formed, while
the very similar values for the chemical shifts of the trans-di-
bromoalkene carbon atoms in 3-Br4 compared to the analo-
gous MOF 2-Br2 (d=117.6 ppm vs. d=118.2 ppm, respectively)
suggests that the single species is the trans,trans-bdb-Br4-H2
product; that is, the bromination of 3 occurs stereoselectively.
In contrast, the liquid-phase bromination of the dimethyl ester
of the ligand (bdb-Me2), using conditions similar to those em-
ployed for the bromination of 3 (neat Br2, CHCl3 solution), re-
12-fold by bdb2À ligands (Figure 2a) in the Fm3m space group
¯
with unit cell edge a=33.3694(3) Š. N2 adsorption isotherms
performed on bulk samples at 77 K prove the permanent po-
rosity of 3, which displays a type I isotherm with stepwise ad-
sorption observed at low partial pressures, characteristic of fill-
ing of the smaller tetrahedral and larger octahedral pores of
UiO-66 type MOFs (Figure 2b). The high porosity of 3 is evi-
dent from its BET surface area of 3850 m2 gÀ1, in line with what
was expected when compared with 2, which contains shorter
edb2À connecting ligands and has a BET surface area of
3300 m2 gÀ1 [23a]
.
With 3 in hand, we envisaged that bromination of its buta-
diyne units should be possible to form 3-Br4. There exists little
literature concerning the bromination of butadiynes to tetra-
bromodialkene units,[34] but nonetheless the bromination of 3
was initially attempted by exposing bulk microcrystalline sam-
ples to solutions of neat Br2 (15 equivalents per alkyne unit) in
CHCl3, with the transformation followed using a number of ex-
perimental techniques (see Supporting Information, Sec-
tion S5).
Figure 2. a) Representation of the single-crystal structure of 3. b) N2 adsorption isotherm at 77 K of 3, with the insert highlighting the stepwise adsorption ob-
served at low partial pressures. c) Comparison between predicted and experimental PXRD patterns of 3, alongside the experimental pattern of 3-Br4. Compar-
isons of the d) 1H NMR spectra (the asterisk denotes residual benzoic acid) and e) Raman spectra of 3 and 3-Br4, with both techniques suggesting quantitative
conversion. f) Crystal structure of a rare tetrabromodialkene-containing compound (CCDC deposition RUWROR)[34] highlighting both the vertical and horizon-
tal displacements as a result of the steric bulk of the Br atoms. g) A molecular dynamics minimised (UFF) representation of the similarly expected arrangement
of bdb-Br4-H2. Atom colouring: Zr (cyan); C (grey); O (red); N (blue); H (yellow); Br (brown). H atoms in parts a) and f) are omitted for clarity.
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1
sulted in a H NMR spectrum containing resonances for three
tion of the crystal structures of 1-Br2 and 2-Br2, it was only
possible to collect unit cell parameters for 3-Br4. We would
expect the lowest energy arrangement of the bdb-Br4 linker
species, which we have assigned to be the trans,trans, cis,trans
and cis,cis geometric isomers of bdb-Br4-Me2 (see Supporting
Information, Section S6).
2À
to result in the tetrabromodialkene unit being geometrically
frustrated both horizontally and vertically as a consequence of
the steric bulk of the bromo substituents; this conformation
has been observed in the crystal structure of a tetrabromodial-
kene analogue[34] (Figure 2 f) and in our energy minimisation of
bdb-Br4-H2 (Figure 2g). As the linker sits along a linear vector
in the framework, the resultant MOF 3-Br4 will exhibit signifi-
cant disorder. Despite this, comparison of the unit cell edges
of 3 (a=33.3694(3) Š) and 3-Br4 (a=32.7864(7) Š) clearly
shows a mechanical contraction associated with the bdb2À
linker shortening upon bromination, consistent with the bro-
minations of 1 and 2.[23] Unfortunately, we have found that 3-
Br4 is not porous and so cannot examine the mechanical modi-
fication by gas uptake. We hypothesise that the large mechani-
cal contraction combined with the significant ligand disorder
brought on by bromination may result in pore collapse.
Raman spectroscopy (Figure 2e) also suggests quantitative
conversion, with the complete disappearance of the peak at
2230 cmÀ1, representative of the butadiyne functionality, com-
bined with the emergence of a distinct shoulder on the alkene
peak at about 1600 cmÀ1, attributable to the formation of the
tetrabromodialkene moiety as 3 is brominated to 3-Br4. Exclud-
ing initial solvent loss (<2008C), thermogravimetric analysis
(TGA) in air reveals that 3 is thermally stable to about 4608C,
typical of UiO-66 type MOFs (see Supporting Information, Sec-
tion S5). The TGA profile of 3-Br4 displays an additional mass
loss event that we expect to correspond to debromination,
but which could not be fully deconvoluted from the overall
decomposition. The total mass loss between 200–5508C
(82.8%) corresponds very closely to the theoretical mass loss
(82.9%) expected upon decomposition of 3-Br4 to ZrO2, in con-
trast to the 70.7% mass loss for 3 over the same temperature
range (expected 69.3%), which, along with bromine analysis
(Br content 44.3% calculated; 43.1% found) and the spectro-
scopic data, suggests quantitative bromination.
Interpenetrated Zr MOFs containing substituted derivatives
of the extended, alkyne-containing ligand 4,4’-[1,4-phenylene-
bis(ethyne-2,1-diyl)]-dibenzoate (peb2À), were reported in
2011.[25] Here, we detail the synthesis of the analogous unsub-
stituted Zr MOF [Zr6O4(OH)4(peb)6]n (4) and the Hf derivative
[Hf6O4(OH)4(peb)6]n (5) (See Supporting Information, Sec-
tion S3).[35] Single crystals obtained using l-proline modulation
Single crystals of 3 were suspended in CHCl3 and exposed to
Br2 for four days, thereafter the CHCl3 was replenished several
times before finally resolvating in DMF. In contrast to our solu-
Figure 3. a) Packing diagram of 4 derived from its single crystal structure, with two independent frameworks coloured red and blue to highlight the inter-
penetrated structure. b) Comparison between predicted and experimental PXRD patterns of 4, alongside the experimental pattern of 4-Br4. Comparisons of
the c) 1H NMR and d) Raman spectra of 4 and 4-Br4, with both techniques suggesting quantitative conversion. e) TGA profiles highlighting the presence of
a debromination step that has been introduced upon bromination of 4 to 4-Br4. f) Adsorption isotherms at 77 K highlighting the decrease in N2 uptake as
a result of the mechanical contraction induced upon bromination of 4 to 4-Br4. Analogous data for 5 and 5-Br4 can be found in the Supporting Information,
Section S7.
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were smaller (50 mm) than those from conventional benzoic
acid modulated syntheses (100 mm), and so benzoic acid
modulated crystals were analysed by SCXRD. Both 4 (Fig-
(see Supporting Information, Section S7), suggesting that the
transformation from alkyne to dibromoalkene units results in
a mechanical contraction of the ligand and thus the MOF.
Single crystals of both 4 and 5 were suspended in CHCl3 and
exposed to Br2 for four days, thereafter the CHCl3 was replen-
ished several times before finally resolvating in DMF (see Sup-
porting Information, Section S9). Both MOFs were successfully
brominated in a single-crystal to single-crystal manner with
data collections possible, resulting in an accurate description
of the structural alterations introduced during bromination.
¯
ure 3a) and 5 crystallise in the cubic Fd3m space group with
unit cell edges of a=39.8116(7) Š and a=39.806(5) Š, respec-
tively, and are structurally similar to their functionalised ana-
logues.[25] The high flexibility of the peb2À ligands is evident, as
they bow in and out of the linear plane separating adjacent
M6O4(OH)4 (M=Zr or Hf) clusters. Experimental PXRD patterns
of bulk samples of 4 and 5 prepared by l-proline modulation
show excellent agreement (Figure 3b) with the patterns pre-
dicted from their single-crystal structures.
¯
The brominated MOFs were found to retain the same Fd3m
space group as their parent structures, with bromination re-
sulting in a mechanical contraction of the MOFs, as observed
through reductions in the unit cell edges from 39.8116(7) Š to
39.067(7) Š as 4 is brominated to 4-Br4 and from 39.806(5) Š
to 39.0451(3) Š as 5 is brominated to 5-Br4. The solid-state
structures of 4-Br4 and 5-Br4 exhibit significant disorder in
their linker components, as would be expected due to the
number of frustrated positions the dissymmetric linker could
adopt along the linear vector.
The presence of alkyne functionalities within the frameworks
prompted us to attempt postsynthetic bromination of both 4
and 5, and we have analysed the transformation using
a number of techniques, which are discussed below for 4-Br4
but are also described for 5-Br4 in the Supporting Information,
Section S7. The PXRD pattern of the brominated material, 4-
Br4, reveals that crystallinity is retained during the transforma-
tion (Figure 3b) and it is clear that peak positions have moved
to slightly higher values of 2q upon bromination, indicative of
the conversion of alkyne to dibromoalkene units and the me-
chanical contraction that results.[23a]
In the crystal structure (Figure 4a) of 5-Br4, it is possible, de-
spite the linker disorder resulting in multiple bromine posi-
1
The presence of a single species in the H NMR spectrum of
acid-digested 4-Br4 (Figure 3c), with altered chemical shifts rel-
ative to the parent framework, suggests that postsynthetic
bromination has again occurred quantitatively and stereoselec-
tively. Considering the geometrical constraints imposed on the
ligand when bound within the MOF, we assume that the trans,-
2À
trans-peb-Br4 isomer is formed exclusively. Conversely, the so-
lution-phase bromination of the dimethyl ester of the ligand,
peb-Me2 to peb-Br4-Me2, occurs non-stereoselectively, with all
three possible geometrical isomers (trans,trans-peb-Br4-Me2,
cis,trans-peb-Br4-Me2 and cis,cis-peb-Br4-Me2) obtained (see
Supporting Information, Section S8). Raman spectra of the
MOFs (Figure 3d) suggest quantitative conversion of the
alkyne to dibromoalkene units, with the complete disappear-
ance of the alkyne peak at 2220 cmÀ1 combined with a broad-
ening of the alkene peak at about 1600 cmÀ1 indicative of the
alkene and the newly formed dibromoalkene peaks being su-
perimposed.
Excluding solvent loss (<2008C), TGA reveals that 4 is ther-
mally stable to about 4708C, typical of Zr MOFs (see Support-
ing Information, Section S7). The TGA profiles of 4 and 4-Br4
are clearly different (Figure 3e), with two distinct mass loss
events observed for 4-Br4. Assuming that the first mass loss of
4-Br4 (200–4658C) represents debromination, there is excellent
agreement between the observed mass loss of approximately
39.1% and the theoretical Br content of 40.1%, which, in con-
cert with a bromine content of 41.9% by elemental analysis,
suggests quantitative bromination has been achieved. N2 ad-
sorption isotherms prove the permanent porosity of 4 which
Figure 4. a) Representation of the crystal structure of 5-Br4, with the two in-
terpenetrated nets coloured red and blue. Disorder in the peb-Br4 linker
2À
has not been removed. b) Despite the disorder, it is clear that peb-Br42À is
found in the trans,trans configuration, although rotation of the central ben-
zene ring results in multiple bromine positions. c) The disorder model pro-
posed for 5-Br4 is validated by the correlation between the PXRD pattern
predicted from the single crystal structure and the experimental pattern.
Atom colouring: Hf (pale blue); C (grey); O (red); Br (brown). H atoms are
omitted for clarity.
has
a
BET surface area of 2650 m2 gÀ1, decreasing to
1440 m2 gÀ1 upon bromination to 4-Br4 (Figure 3 f). The higher
mass of 4-Br4 cannot fully account for this decrease in gravi-
metric surface area, and calculated pore size distributions
show a reduction in the major pore diameter from 14.2 to 11 Š
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2À
tions, to visualise the trans,trans-configuration of the peb-Br4
linker (Figure 4b). The contraction on bromination brings the
two interpenetrated nets closer together, narrowing the pores
as can be seen in the N2 adsorption isotherms (Figure 3 f). The
disorder model of 5-Br4, as well as that of the more disordered
4-Br4 (see Supporting Information, Section S9), indicates that
the central benzene ring of the linker is free to rotate. Compar-
ison of the experimental PXRD pattern with the pattern calcu-
lated from our structure model of 5-Br4 shows good agree-
ment (Figure 4c), suggesting that the disorder model is valid
and that bromination occurs quantitatively to produce phase-
pure material.
The facile addition of bromine across the integral functional
units of 1–4 prompted us to consider the irreversible trapping
of harmful radioactive iodine released from the nuclear fission
of uranium in nuclear processing applications.[36] Several
porous materials have been investigated for physisorption of
radioactive iodine, including MOFs (e.g. ZIFs,[37] HKUST-1[38]), al-
though charged porous aromatic frameworks (PAFs) have re-
cently been shown to be promising candidates with PAF-24
able to capture 276% iodine by weight.[39] To investigate the
iodine storage capacities of the Zr MOFs 1–4, microcrystalline
samples were exposed to iodine vapours (see Supporting In-
formation, Section S10), with the total iodine uptakes mea-
sured through gravimetric investigations, alongside 1H NMR
spectroscopic analysis to establish the percentage chemisorp-
tion (Figure 5a).
Figure 5. a) Summary of the physisorption and chemisorption I2 storage ca-
pacities of 1–4. Representations of the solid-state structures of b) trans-edb-
I2-H2, c) trans-trans-bdb-I4-H2, and d) trans-trans-peb-I4-H2, isolated from di-
gested samples of 2–4, respectively, after iodination. Atom colouring: C
(grey); O (red); H (yellow); I (purple).
Surprisingly we find that 1 is unable to chemisorb I2 across
its integral alkene units, despite both bromination and bromo-
hydrination occurring in a facile manner in solution; however,
a maximum I2 physisorption storage capacity of 107% w/w
was recorded. Both 2 and 3 demonstrate much larger total I2
storage capacities as they can both chemi- and physisorb I2,
and the presence of twice as many alkyne units in 3 is reflect-
ed in the chemisorption capacities, with 2 demonstrating
a maximum of 57% w/w irreversible trapping of I2, compared
to 88.5% w/w for 3. The interpenetrated material 4 shows the
highest storage value, with a maximum uptake of 279% w/w
recorded, comparable to the recent benchmark capacity of
276% w/w set by PAF-24.[39] The I2 chemisorption capacity of 4
is lower than that of 3, even though both materials contain
a similar alkyne content; hence, the superior uptake is the
result of a high tendency of I2 to physisorb within the pores,
possibly due to the high density of Zr6 clusters as a result of in-
terpenetration. Chemisorption by 3 may also lead to partial
pore collapse, as the analogous 3-Br4 is not porous, thus hin-
dering later I2 uptake.
cert with reactive chemisorptive sites on the linkers, generates
potential candidates for I2 storage/capture applications. Addi-
tionally, proof-of-concept experiments show that chemisorbed
halogens can be abstracted by pyrrolidine to regenerate the
parent MOF; 1-Br2 can be completely converted back to 1 by
soaking in an acetone solution with 15 equivalents of pyrroli-
dine for 42 h (see Supporting Information, Section S12).
Conclusion
In conclusion, we have demonstrated that the combined
chemical and mechanical stabilities of Zr and Hf MOFs enable
their postsynthetic halogenation across integral unsaturated
linker functionalities. Both solution-phase bromination and
vapour-phase iodination proceed stereoselectively and in ex-
cellent yields across alkenes, alkynes, butadiynes and in inter-
penetrated frameworks, while the water stability of the Zr
MOFs allows aqueous bromohydrination of alkene units. Bro-
mination is shown to be reversible by abstraction with pyrroli-
dine to restore the parent material, making it an unusual ex-
ample of reversible covalent postsynthetic modification, and
we are currently investigating the potential for multiple cycles
of sequestration and regeneration of the MOFs. The chemi-
sorption of I2 vapours by the MOFs, combined with their high
porosity, makes them excellent candidates for sequestration of
The stereoselectivities of these vapour phase iodinations are
analogous to the solution phase brominations, as can be seen
1
by the H NMR spectra of digested samples used to monitor
the extent of chemisorption. Additionally, crystals of iodinated
linkers separated from a number of the [D6]DMSO/D2SO4
digest solutions (see Supporting Information, Section S11), al-
lowing us to crystallographically characterise trans-edb-I2-H2,[23a]
trans,trans-bdb-I4-H2 and trans,trans-peb-I4-H2, from the iodina-
tion of 2, 3 and 4, in turn (Figure 5b-5d). These results unam-
biguously show that the chemical stability of Zr MOFs, in con-
Chem. Eur. J. 2016, 22, 4870 – 4877
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4875 ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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Experimental details for this work can be found in the Supporting
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Acknowledgements
R.S.F. thanks the Royal Society for receipt of a University Re-
search Fellowship and the University of Glasgow for funding.
The research was supported by EPSRC (EP/L004461/1). We
thank the EPSRC UK National Crystallographic Service for
single-crystal data collection.[40] The data which underpin this
work can be obtained free of charge from The University of
Glasgow.
Keywords: gas adsorption
frameworks · postsynthetic modification · zirconium
·
iodine
·
metal–organic
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Published online on February 24, 2016
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4877 ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim