Functionalization in flexible porous solids: Effects on the pore opening and the host-guest interactions

Article abstract of DOI:10.1021/ja9092715

The synthesis on the gram scale and characterization of a series of flexible functionalized iron terephthalate MIL-53(Fe) type solids are reported. Chemical groups of various polarities, hydrophilicities, and acidities (-Cl, -Br, -CF₃, -CH₃, -NH₂, -OH, -CO₂H) were introduced through the aromatic linker, to systematically modify the pore surface. X-ray powder diffraction (XRPD), molecular simulations, thermogravimetric analyses, and in situ IR and ⁵⁷Fe Moessbauer spectrometries indicate some similarities with the pristine MIL-53(Fe) solid, with the adoption of the narrow pore form for all solids in both the hydrated and dry forms. Combined XRPD and computational structure determinations allow concluding that the geometry of the pore opening is predominantly correlated with the intraframework interactions rather than the steric hindrance of the substituent. Only (MIL-53(Fe)-(CF₃)₂) exhibits a nitrogen accessible porosity (S_BET ≈ 100 m² g^-1). The adsorption of some liquids leads to pore openings showing some very specific behaviors depending on the guest-MIL-53(Fe) framework interactions, which can be related to the energy difference between the narrow and large pore forms evaluated by molecular simulation.

Full text of DOI:10.1021/ja9092715

Published on Web 12/28/2009
Functionalization in Flexible Porous Solids: Effects on the
Pore Opening and the Host-Guest Interactions
,
†
†
†
‡
Thomas Devic,* Patricia Horcajada, Christian Serre, Fabrice Salles,
‡
§
†
§
Guillaume Maurin, B e´ atrice Moulin, Daniela Heurtaux, Guillaume Clet,
Alexandre Vimont, Jean-Marc Gren e` che, Benjamin Le Ouay, Florian Moreau,
Emmanuel Magnier, Yaroslav Filinchuk, Jer oˆ me Marrot, Jean-Claude Lavalley,
§
|
†
†
†
⊥
†
§
§
†
Marco Daturi, and G e´ rard F e´ rey
Institut LaVoisier, UMR CNRS 8180, UniVersit e´ de Versailles Saint-Quentin-en-YVelines, 45
aVenue des Etats-Unis, 78035 Versailles cedex, France, Institut Charles Gerhardt Montpellier,
UMR CNRS 5253, UM2, ENSCM, Place E. Bataillon, 34095 Montpellier cedex 05 France,
Laboratoire Catalyse et Spectrochimie, ENSICAEN, UniVersit e´ de Caen, CNRS, 6 Bd Mar e´ chal
Juin, 14050 Caen, France, Laboratoire de Physique de l’Etat Condens e´ , UMR CNRS 6087,
UniVersit e´ du Maine, 72085 Le Mans Cedex, France, and Swiss-Norwegian Beamlines at ESRF,
3
8043 Grenoble, France
Received November 3, 2009; E-mail: devic@chimie.uvsq.fr
Abstract: The synthesis on the gram scale and characterization of a series of flexible functionalized iron
terephthalate MIL-53(Fe) type solids are reported. Chemical groups of various polarities, hydrophilicities,
and acidities (-Cl, -Br, -CF
3
, -CH
3
, -NH
2
2
, -OH, -CO H) were introduced through the aromatic linker,
to systematically modify the pore surface. X-ray powder diffraction (XRPD), molecular simulations,
thermogravimetric analyses, and in situ IR and 57Fe M o¨ ssbauer spectrometries indicate some similarities
with the pristine MIL-53(Fe) solid, with the adoption of the narrow pore form for all solids in both the hydrated
and dry forms. Combined XRPD and computational structure determinations allow concluding that the
geometry of the pore opening is predominantly correlated with the intraframework interactions rather than
3 2
the steric hindrance of the substituent. Only (MIL-53(Fe)-(CF ) ) exhibits a nitrogen accessible porosity
2
-1
(S
BET ≈ 100 m g ). The adsorption of some liquids leads to pore openings showing some very specific
behaviors depending on the guest-MIL-53(Fe) framework interactions, which can be related to the energy
difference between the narrow and large pore forms evaluated by molecular simulation.
Introduction
magnitude, etc.) and the adsorption properties in a more complex
way. The present study concerns our series of flexible M
III
One method for modulating the storage/separation properties
of hybrid Porous Coordination Polymers (PCPs) or Metal
Organic Frameworks (MOFs)
component of their walls with groups of variable polarities,
acidities, etc., influencing the sorption and selectivity processes.
The functionalization of rigid MOFs has been investigated,
either starting simply from linkers containing halogen atoms,
hydroxo terephthalates denoted MIL-53(M) (M ) Cr, Al, Fe,
III
1
-3
Ga, In; MIL ) Materials of Institut Lavoisier) or (M (OH)-
is to functionalize the organic
(
O
2
C-C
built up from chains of corner-sharing MO
connected through terephthalate linkers to define diamond-
6
H
4
-CO
2
), which are three-dimensional porous solids
4
(OH) octahedra,
2
4-12
amine, amide, or alkyl groups
or by postsynthetic modifica-
(
4) Braun, M. E.; Steffek, C. D.; Kim, J.; Rasmussen, P. G.; Yaghi, O. M.
1
3-26
tions,
sorption properties
contrary, only a few studies concern flexible MOFs.
with, in some cases, an evaluation of their resulting
Chem. Commun. 2001, 2532–2533.
2
7,28
29,30
or catalytic activities.
On the
In this
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surfaces, and thus the strength of the host-guest interactions,
but also changes the flexible character (pore opening/closing,
2
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†
Institut Lavoisier.
Institut Charles Gerhardt Montpellier.
Universit e´ de Caen.
Universit e´ du Maine.
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15454–15455.
‡
§
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10.1021/ja9092715  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 1127–1136 9 1127

A R T I C L E S
Devic et al.
depending on both the nature of guests and the temperature,
evolve from a narrow pore (np) to a large pore (lp) form (see
Figure 1) with a variation of their cell volume (up to 40%)
without any bond breaking.
3
5,36,39,40
47
The explored guests up to now are water,
carbon
41,42
43-45
46
dioxide,
liquids.
linear alkanes,
Ibuprofen, xylenes, or other
Moreover, the nature of the form strongly depends
on the nature of the M metal, with a very different flexible
compared with its Cr,
While the Al, Cr dried solids possess
a large pore structure at room temperature that might shrink or
48-50
III
3
7,46
character for the MIL-53(Fe) solid
Al, or Ga analogues.
3
9,51
41,43,44
not upon adsorption,
the iron solid exhibits a close narrow
pore dried form that turns into larger pore forms through two
39,45,48
structural transitions upon adsorption of guest molecules.
MIL-53(Ga) exhibits an intermediate behavior with a narrow
pore dried form at room temperature that further reopens at
5
1
higher temperature. It was shown that the narrow pore and
the large pore forms are rather close in energy and that the
host-guest interactions should overcome an energy barrier (∼20
-
1
40,43
kJ mol for MIL-53(Cr)
other.
) to switch from one form to the
Figure 1. View of the structure of MIL-53(M). Left: chains of corner
42
4 2
sharing MO (OH) octahedra. Right: the 3-dimensional framework shown
along the pores axis, in both its large pore (top) and narrow pore (bottom)
form.
First attempts of functionalization have concerned a basic
group (introduction of one -NH per linker for MIL-53(Al,
acidic groups (introduction of one -OH or two
2
3
2,52,53
Fe),
2
CO H per linker in MIL-53(Al) and (Fe) respectively),
35-38
shaped one-dimensional channels (Figure 1).
This structure
54
55
-
currently represents the archetype of flexible solids which,
and apolar groups (introduction of one phenyl or four -CH
3
56
57
groups per linker in MIL-53(Al) and (Cr) respectively). Polar
(
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1
128 J. AM. CHEM. SOC. 9 VOL. 132, NO. 3, 2010

Functionalization in Flexible Porous Solids
A R T I C L E S
a
n
Table 1. Synthesis Conditions of the MIL-53(Fe)-X Solids (X-Functionalized on the Phenyl Ring)
Linker
(mmol)
Iron(III) salt
(mmol)
Solvent + HF
5 M (mL)
Guest
exchange
Solid
Synthesis
Activation
b
c
MIL-53(Fe)-Cl
C
6
C
6
C
6
C
6
C
6
C
6
C
6
C
6
H O
3
H O
3
H O
3
H O
3
H O
3
H O
2
H O
2
H O
2
4
4
4
4
4
4
4
4
Cl (1)
Fe chloride (1)
DMF (5) +
HF (1)
Water (300)
Autoclave
(150 °C, 48 h)
Reflux, 48 h
No
-
c
MIL-53(Fe)-Cl
Cl (31)
Fe chloride (31)
Fe chloride (31)
Fe perchlorate (10)
Fe chloride (5)
DMF
100 °C, 24 h)
DMF
150 °C, 80 h
150 °C, 80 h
200 °C, 72 h
150 °C, 72 h
150 °C, 15 h
-
(
MIL-53(Fe)-Br
MIL-53(Fe)-CH
Br (31)
Water (300)
Reflux, 48 h
(
No
100 °C, 24 h)
c
3
CH
NH
(OH)
(COOH)
(CF
3
(10)
DMF (50) +
HF (1)
Water (50)
Autoclave
(150 °C, 24 h)
Autoclave
MIL-53(Fe)-NH
MIL-53(Fe)-(OH)
MIL-53(Fe)-(COOH)
2
2
(5)
Ethanol
(150 °C, 48 h)
No
(
150 °C, 72 h)
b
c
2
2
(6.8)
Fe perchlorate (4.6)
Fe perchlorate (10)
Fe chloride (2.5)
DMF (25) +
HF (1)
Water (50)
Autoclave
(100 °C, 16 h)
Autoclave
2
2
(10)
No
No
(
150 °C, 16 h)
Microwave
100 °C, 20 min)
c
MIL-53(Fe)-(CF
3
)
2
3
)
2
(2.5)
Water (25)
250 °C, 48 h
(
a
b
The general formula of these solids is [Fe(OH)(C
H O X
6 4-n 4 n
)]·G (G: guest). These syntheses provided single crystals of the as-synthesized solids.
c
These ligands were prepared according to the procedure reported in the Supporting Information.
Scheme 1. Modified Terephthalate Linkers BDC-X Used in the Present Study
n
5
5
groups first led to enhanced properties in terms of gas separation
53(Fe)-(CO
2
H)
2
(previously labeled MIL-82), were already de-
5
3
54
32
(
CO
2
/CH
4
), adsorption capacity (H
2
), or basic catalysis,
scribed, but their preparation required the use of hydrofluoric acid.
The present procedure is an HF-free alternative preparation. For
the adsorption of liquids by functionalized MIL-53(Fe) solids, their
activated forms were left in air for a few days, leading to their
hydrated form. 10 to 40 mg of these solids were poured into 5 mL
of a certain solvent (water, absolute ethanol, pyridine, and tetra-
chloroethane) and stirred for 24 h at room temperature. Small
fractions of the resulting slurries were thus transferred into 1 mm
glass capillaries, which were further sealed and left to stand for 5
days before an X-ray data collection.
Characterizations. The Supporting Information contains the
details of the different techniques (single crystal and powder X-ray
diffraction, thermal analyses (TGA, thermodiffractometry), and
infrared and Fe M o¨ ssbauer spectrometries) used for the charac-
terization of the solids described in this article, as well as details
of the molecular simulations.
whereas apolar groups emphasized the influence of the steric
hindrance of the groups grafted onto the phenyl ring on the
5
6,57
porosity of the material.
3 4
In MIL-53(Cr)-(CH ) , it induces
a rotation of the phenyl rings by 90°, which further prevents
5
7
any adsorption of guests.
Therefore, this paper aims at producing a large series of MIL-
3(Fe)-X functionalized solids (X ) -Cl, -Br, -CH
(CF , -NH , -(OH) , or -(CO H) ) presenting a wide range
5
n
n
3
,
-
3
)
2
2
2
2
2
of pore surface polarity, hydrophilicity, and acidity, to deeply
evaluate by X-ray diffraction, in situ IR spectroscopy, TG
5
7
analyses, and Fe M o¨ ssbauer spectrometry the effect of such
organic modifications both on the surface properties and on the
breathing phenomenon. Moreover, plausible structural models
for the different modified MIL-53(Fe) solids in the dry state
and in the presence of solvent molecules have also been
proposed by means of a computational structure determination
starting with the lattice parameters obtained from the refinement
of the experimental X-ray powder diffraction patterns (XRPD).
The first part of the paper is devoted to the characterization
and analysis of various parameters of the hydrated MIL-53(Fe)-
5
7
Results and Discussion
In addition to their different steric hindrances, the selected
terephthalic acid derivatives cover a broad spectrum of polarity,
hydrophilicity, and acidity (Scheme 1).
Some Remarks on the As-Synthesized Solids. Their optimized
syntheses allow us to obtain them at the 1-2 g scale, or even
larger (10 g) for MIL-53(Fe)-X (X ) Cl, Br) (see Figures S1
and S2-A for XRPD and IR characteristics of the as-synthesized
solids). However, depending on the synthesis conditions (mainly
the nature of the solvent, but not exclusively), the guests are
n
X solids, and the second to the dry ones. The effect of external
stimuli (here adsorption of various liquids) on the flexible
character of the functionalized MIL-53(Fe) solids is explored
in the third part. Finally, the impact of the functionalization on
the energetics of the narrow pore-large pore transition is
discussed in light of the energy difference between the two
structural forms simulated for each grafted group.
2
different (H O, DMF, or even terephthalic acid). They can
therefore induce specific and additional interactions with the
framework, which partially conceals the inherent effect of
functionalization on the breathing. This is the reason why, after
their guest exchange and activation, all the solids were exposed
to air for rehydratation, providing a homogeneous series with
the same guest, suitable for establishing the effects of func-
Experimental Section
Syntheses. Table 1 summarizes the main characteristics of the
synthesis conditions which are described in detail in the Supporting
5
2
Information. Two solids, namely MIL-53(Fe)-(NH
2
)
and MIL-
J. AM. CHEM. SOC. 9 VOL. 132, NO. 3, 2010 1129

A R T I C L E S
Devic et al.
Figure 2. Single X-ray crystal structures of the as-synthesized MIL-53(Fe)-Cl (left) and MIL-53(Fe)-(OH)
2
(right) solids. Pore content was discarded
during the refinement using the SQUEEZE procedure.
tionalization on their flexibility. IR spectra confirm the elimina-
tion of most of the free terephthalic acid and DMF molecules
and the presence of adsorbed water in the rehydrated solid
solids first indicates that the adsorbed water molecules can be
easily eliminated. Upon outgassing at room temperature, the
-1
2
band associated with the adsorbed water (δ(H O) ≈ 1620 cm )
(
Figure S2-B).
disappears, while those related to the bridging hydroxyl group
The information on the as-synthesized solids comes from the
(Fe-O(H)-Fe) ((ν(OH) and δ(OH) modes at approximately
-
1
single crystal X-ray diffraction study of (MIL-53(Fe)-(OH)
2
and
3640 and 850 cm respectively) become more resolved. In the
hydrated form, these bands are broader and are located at
MIL-53(Fe)-Cl) (see Tables 1 and S1). Their structural char-
acteristics (bond distances, etc.) are quasi-invariant (Figure 2),
with the only difference being in the position of the phenyl rings.
-
1
approximately 3300 and 1050 cm (Figure 3) due to weak
hydrogen bonds between the water molecules and the bridging
hydroxyl groups of the skeleton, as already observed for the
2
Indeed, in the case of the BDC-(OH) linker, the phenyl ring
5
1
lies parallel to the axis of the tunnels with the hydroxo groups
statistically disordered over the 2 + 2 available positions on
the phenyl ring. For the BDC-Cl derivative, a similar positional
disorder is observed, but associated with an ∼20° tilt of the
phenyl ring relative to the pore axis, indicating an influence of
the steric hindrance between the adjacent linkers.
MIL-53(Ga, Al) solids.
All solids are stable up to 200 °C under vacuum, except MIL-
53(Fe)-(OH) , as indicated by IR spectroscopy (see Figure S4).
2
At a higher temperature (250 °C), destabilization of the
structures begins, as evidenced from the decrease in intensity
of the ν(OH) and δ(OH) bands of the hydroxyl groups in
conjunction with a broadening of some terephthalate bands
n
Hydrated Forms of MIL-53(Fe)-X . The unit cells of the (air)
-1
hydrated functionalized MIL-53(Fe) solids were determined by
XRPD. They are reported in Table 2 which gathers the
crystallographic data on the dry, hydrated, and different post-
(1600 and 1150 cm ), corresponding to both a dehydroxylation
and an amorphization of the structure. In the case of MIL-
2
53(Fe)-(OH) , the amorphization starts at lower temperatures
solvated forms of the MIL-53(Fe)-X
solids exhibit the narrow pore form
n
solids. All the hydrated
(150-200 °C) (Figure S4), a phenomenon that could be related
3
5,36,39
58
with a monoclinic
to the redox reactivity of the 1,4-dihydroxyterephthalate.
unit cell (space group C2/c (n°15)) and parameters very similar
to those determined for the pristine hydrated MIL-53(M) (M )
Cr, Al, Fe) solids. The slight variations of the parameters with
Further, the thermogravimetric analyses are consistent with
the IR observations. All solids except MIL-53(Fe)-(OH) exhibit
2
a similar behavior (Figure S5), with the first weight loss at low
temperature (T < 100 °C) associated with the water’s departure
and the second one at higher temperature associated with the
collapse of the framework. This alteration of the solid takes
-
n
(X) relate to the size and number of functional groups per
iron and to the magnitude of the rotation of the phenyl ring,
which will be discussed in the part devoted to the dry forms of
the functionalized MIL-53(Fe) solids. It is worth noting that,
2
place at 150-160 °C for MIL-53(Fe)-(OH) and in the range
3
whereas all the cell volumes are close to 1000 Å , X ) CF
3
is
250-300 °C (lower than the nonfunctionalized MIL-53(Fe)
(300-320 °C)) for the other solids. The residue corresponds to
3
an exception (V ≈ 1300 Å ), probably due to the huge steric
hindrance of the CF group. Indeed, in the case of MIL-53(Fe)-
CF , a mixture of two forms (two pore openings) was
3
2 3
a poorly crystallized Fe O . The resulting weight losses (see
(
3
)
2
Table S2) are consistent both with the chemical formulation
Fe(OH)(BDC-X ) and with the presence of roughly one water
molecule per formula unit in the tunnels (except for MIL-53(Fe)-
(CF , see below), as for the pristine MIL-53(M) materials.
observed. Once it was verified by IR spectroscopy as well as
chemical and TG analyses that the guest was only water, this
phenomenon was attributed to either a very slow adsorption of
water after activation (as a consequence of the high hydropho-
n
3 2
)
Finally, the removal of free water is surprisingly easier for
most of the functionalized frameworks (100 °C or less)
compared with the pristine solid (150 °C). This behavior shows
that the introduction of functional groups does not lead to a
drastic change of stability or large activation barriers which
would otherwise prevent their use for gas sorption/separation
bicity of the -CF
corresponding to different arrangements of the terephthalate
cores as a result of different -CF /-CF steric repulsion along
3
groups) or the coexistence of two phases,
3
3
the chain axis, giving rise to two pore openings (see Figure
S8). Nevertheless, our computations show that two different
arrangements of the -CF
3
groups considered initially in the
applications. In the case of MIL-53(Fe)-(OH) , the difference
between the first and second weight loss is less pronounced and
may indicate that the collapse of the structure takes place before
the complete dehydration (see below).
2
large pore form lead to energy minimized structures with pore
openings which differ by only 0.3 Å, a difference significantly
weaker than the one observed experimentally. The first hypoth-
esis (slow hydration) might therefore be the most realistic.
IR spectroscopy performed on the activated and rehydrated
(
58) Costentin, C.; Robert, M.; Saveant, J. M. J. Am. Chem. Soc. 2006,
128, 8726–8727.
under air MIL-53(Fe)-(X)
n
3 2 2 3
(X ) Cl, Br, CH , NH , CO H, CF )
1
130 J. AM. CHEM. SOC. 9 VOL. 132, NO. 3, 2010

Functionalization in Flexible Porous Solids
A R T I C L E S
Table 2. Unit-Cell Dimensions of the Functionalized MIL-53(Fe) Solids with Various Pore Fillings Determined from X-ray Powder Diffraction
(
* Corresponds to Mixtures of Narrow Pore and Large Pore Forms)
3
Solvent
S.G.
a (Å)
b (Å)
c (Å)
ꢀ (deg)
V (Å )
MIL-53(Fe)
3
9
dry
C2/c
C2/c
C2/c
Imcm
C2/c
21.269(1)
19.320(1)
21.119(2)
16.183(2)
19.196(2)
18.739(3)
6.759(1)
6.884(1)
6.835(1)
6.830(1)
6.895(1)
6.885(1)
7.050(1)
114.62(1)
96.305(1)
114.87(1)
-
108.92(1)
92.68(1)
899.6(1)
3
9
8
hydrated (air)
water (liquid)
ethanol
pyridine
tetrachloroethane
2 × 7.518(1)
2 × 986.8(1)
1003.0(2)
1584.9(3)
1395.4(2)
1217.0(2)
4
7.664(1)
4
8
4
14.204(2)
11.161(1)
9.320(2)
8
1
P2 /m
MIL-53(Fe)-Cl
dry
C2/c
C2/c
C2/c
Imcm
C2/c
Imcm
20.112(4)
20.116(2)
20.094(3)
16.819(1)
19.301(1)
16.673(3)
7.424(2)
7.668(1)
7.684(1)
13.371(1)
11.169(1)
13.509(2)
6.898(2)
6.859(1)
6.853(1)
6.913(1)
6.931(1)
6.946(1)
105.89(1)
106.00(1)
106.00(1)
-
108.99(1)
-
990.5(4)
hydrated (air)
water (liquid)
ethanol
pyridine
tetrachloroethane
1017.0(2)
1017.1(2)
1554.7(2)
1412.8(1)
1564.6(5)
MIL-53(Fe)-Br
dry
C2/c
C2/c
C2/c
Imcm
C2/c
Imcm
20.164(1)
20.227(1)
20.254(1)
16.711(1)
19.238(1)
16.288(1)
7.996(1)
7.832(1)
7.897(1)
13.575(1)
11.245(1)
13.721(1)
6.914(1)
6.906(1)
6.912(1)
6.923(1)
6.941(1)
6.886(1)
106.95(1)
106.62(1)
106.84(1)
-
108.75(1)
-
1066.3(1)
1048.2(1)
1058.1(1)
1570.4(1)
1421.9(1)
1539.0(1)
hydrated (air)
water (liquid)
ethanol
pyridine
tetrachloroethane
MIL-53(Fe)-CH
3
dry
C2/c
C2/c
C2/c
Imcm
C2/c
C2/c
20.059(3)
20.075(2)
20.078(1)
16.627(1)
19.168(1)
20.082(3)
7.947(2)
7.913(1)
7.957(1)
13.620(1)
11.372(1)
7.903(1)
6.897(1)
6.887(1)
6.890(1)
6.929(1)
6.911(1)
6.880(1)
106.43(1)
106.21(1)
106.29(1)
-
19.49(1)
106.28(1)
1054.6(3)
1050.7(2)
1056.6(3)
1569.1(2)
1440.2(1)
1048.2(2)
hydrated (air)
water (liquid)
ethanol
pyridine
tetrachloroethane
MIL-53(Fe)-NH
2
dry
C2/c
C2/c
C2/c
C2/c
Imcm
C2/c
C2/c
20.148(1)
19.993(1)
20.003(1)
20.029(2)
16.323(2)
19.219(1)
20.124(1)
7.901(1)
7.724(1)
7.737(1)
7.747(1)
13.998(1)
11.157(1)
7.913(5)
6.978(1)
6.848(1)
6.847(1)
6.860(1)
6.903(1)
6.909(1)
6.930(4)
106.52(1)
105.32(1)
105.38(1)
105.52(1)
-
1065.1(1)
1019.9(1)
1021.7(1)
1025.7(1)
1577.2(3)
1403.0(1)
1057.0(1)
hydrated (air)
water (liquid)
ethanol*
pyridine
tetrachloroethane
108.73(1)
106.64(3)
MIL-53(Fe)-(OH)
2
a
dry
C2/c
C2/c
Imcm
Imcm
C2/c
C2/c
Imcm
21.3
6.9
6.9
115
107.17(1)
-
-
107.11(1)
107.15(1)
-
917
hydrated (air)
water (liquid)
ethanol
pyridine
tetrachloroethane*
19.909(2)
17.368(1)
16.148(1)
19.924(3)
20.078(1)
16.275(1)
9.123(1)
12.707(5)
14.404(1)
9.138(1)
9.010(1)
13.947(2)
6.832(1)
6.891(1)
6.892(1)
6.827(1)
6.858(1)
6.889(1)
1185.5(2)
1520.7(1)
1603.1(1)
1187.9(3)
1185.5(2)
1563.8(4)
MIL-53(Fe)-(CO
H)
2 2
dry
C2/c
C2/c
C2/c
C2/c
C2/c
C2/c
20.013(1)
20.302(3)
20.318(1)
20.321(1)
20.306(1)
20.307(1)
8.069(1)
8.116(1)
8.123(1)
8.125(1)
8.120(1)
8.117(1)
6.967(1)
6.955(1)
6.960(1)
6.963(2)
6.956(3)
6.957(1)
106.40(1)
107.32(1)
107.34(2)
107.34(1)
107.33(2)
107.32(1)
1079.5(1)
1094.1(3)
1096.5(1)
1097.3(1)
1094.9(1)
1094.6(1)
hydrated (air)
water (liquid)
ethanol
pyridine
tetrachloroethane
3 2
MIL-53(Fe)-(CF )
dry*
C2/c
Imcm
C2/c
19.429(1)
16.490(1)
19.425(2)
19.462(2)
16.49 (1)
16.746(1)
15.917(2)
16.455(1)
10.355(1)
13.653(2)
10.408(1)
10.341(1)
13.623(6)
13.605(1)
14.704(2)
13.793(1)
6.911(1)
6.883(1)
6.905(1)
6.913(1)
6.882(2)
6.929(1)
6.933(1)
6.915(1)
108.57(1)
1317.9(1)
1549.5(3)
1319.5(2)
1319.2(2)
1546(4)
1578.6(2)
1622.5(3)
1569.5(2)
-
hydrated (air)*
water (liquid)*
109.1(1)
C2/c
108.53(1)
Imcm
Imcm
Imcm
Imcm
-
-
-
-
ethanol
pyridine
tetrachloroethane
a
Estimated from thermodiffraction.
A typical X-ray thermodiffractogram of a modified MIL-
3(Fe) solid is illustrated in Figure 4 (here MIL-53(Fe)-Br, see
(-X ) -OH), 260 °C (-X ) -NH
2
3
), or 300 °C (-X ) -CH ,
5
-Br, -Cl, -CF
3
2
and -CO H). The latter change indicates the
Figures S6 and S7 for the other solids). The Bragg peaks
associated with the hydrated solids first slightly shift upon
heating, as a consequence of both the departure of the water
destruction of the hybrid frameworks, in agreement with the
conclusions drawn from the IR and TG analyses.
Noteworthy, at variance to the pristine MIL-53(Fe) solid,
39,40
5
9
and the thermal expansion, and finally vanish above 220 °C
is the fact that no intermediate hemihydrated form is observed
J. AM. CHEM. SOC. 9 VOL. 132, NO. 3, 2010 1131

A R T I C L E S
Devic et al.
of each quadrupolar components and the nature of the substitu-
ent. Nevertheless, these results show that the perturbation on
the iron site not only depends on the hydration state (as already
39
observed in the nonmodified MIL-53(Fe) ) but also on the local
perturbation in the environment of the iron(III) ion induced by
the presence of substituents on the organic part.
Dry Forms of MIL-53(Fe)-X
forms of MIL-53(Fe)-X (X ) Cl, Br, CH
were extracted from synchrotron XRPD at 303 K (see Table
). The lower stability of MIL-53(Fe)-(OH) prevented its
n
. The cell parameters of the dry
n
3
, NH , CO H, CF
2
2
3
)
2
2
isolation in the dry state, and the cell parameters were thus
estimated from the thermodiffraction data. All solids exhibit
comparable monoclinic unit cells (space group C2/c, a )
1
1
9.42-21.3 Å, b ) 6.9-10.3 Å, c ) 6.83-6.98 Å, ꢀ )
05.8°-115°), similar to those observed for the nonmodified
3
9
solid. These cell parameters therefore correspond to the ones
of the narrow pore form, in agreement with the absence of any
significant permanent porosity (Table S4). MIL-53(Fe)-CF
3
,
despite its large cell volume and noticeable BET surface area
2
-1
(
96 m g ), which is largely determined by the steric hindrance
of the CF group, corresponds also to a narrow pore form (i.e.,
3
noticeably contracted compared to an orthorhombic large pore
form). This behavior is also supported by our modeling
approach, which shows that when the unit cell parameters of
3 2
the large pore form of MIL-53(Fe)-(CF ) are fully relaxed,
3
one obtains a final structure with a unit cell volume of 1283 Å
close to the experimental one, while, for the other modified
structures, the same procedure leads to optimized structures with
Figure 3. FTIR spectra of the activated functionalized MIL-53(Fe) solids.
Dotted line: hydrated forms under air. Full line: dry forms outgassed at
3
unit cell volumes ranging from 835 to 1213 Å , similar to the
4
23 K (MIL-53(Fe)-(OH)
2
) or 473 K (MIL-53(Fe), MIL-53(Fe)-CH
3
, MIL-
).
ones experimentally observed. Furthermore, starting from the
experimental parameters, a plausible structural model for each
dry functionalized-MIL-53(Fe) (Crystallographic Information
Files provided in the Supporting Information) was built up using
our computational approach described in the Supporting Infor-
mation.
53(Fe)-Br, MIL-53(Fe)-Cl, MIL-53(Fe)-(CF
3 2
) , and MIL-53(Fe)-(COOH)
2
As stated in the introduction for MIL-53(Cr)-(CH
05) with the same topology, the functionalization of the phenyl
rings (hereafter noted Φ) can induce rotations of the rings (noted
3 4
) (or MIL-
1
57
θ in Figure 6) with regard to the axis of the octahedral chains.
They can reach 90°, a priori depending on the number, the
nature, and the steric hindrance of the substituent. Figure 6,
related to the dry forms, illustrates that, in all the cells, the Φ
rings are rotated (range 15°-35° depending on the nature of
Figure 4. X-ray thermodiffractogram of the MIL-53(Fe)-Br solid performed
under air (Co KR, λ ≈ 1.7906 Å).
-
X) with respect to the axis of the tunnels. The same feature
occurs with the large pore forms, which exhibit different values
for the angle of rotation (see Supporting Information). This
implies that the host-guest interactions play a role in determin-
ing the dynamics of the structure. This calls for a reexamination
of the effects of the functionalization on the rotations which
occur around the Φ rings.
The first concerns the carboxylate function, which represents
the linking part between the Φ rings and the inorganic chains
Fe-OH-Fe. In terms of geometry, the COO group can rotate
with respect to Φ. This rotation depends on the specific steric
hindrance of the function grafted onto Φ and not directly on its
volume, to take into account the anisotropy of the shape of the
substituent. The larger the hindrance, the larger the γ angle
between the COO group and Φ (Figure 7b).
3
9,55
in the series. Incidentally, we previously showed
that, for
Fe(III)-containing hybrid porous solids, the presence of water
in the pores influences the characteristics of the M o¨ ssbauer
spectra. A similar study was therefore undertaken for the
investigated series. Figure 5 illustrates the M o¨ ssbauer spectra
recorded at 300 K and ambient pressure with a small velocity
range to improve the resolution of the hyperfine structure. All
the resulting spectra recorded at 77 K are rather similar. While,
III
in all the structures described here, the Fe ions are on a single
crystallographic site, the quadrupolar structure always consists
of two main quadrupolar doublets, except that of MIL-53(Fe)-
(
2
COOH) which seems to exhibit a single quadrupolar compo-
nent. The isomer shift values are consistent with both the
3+
presence of only octahedral high spin Fe sites and the absence
of dense inorganic impurities such as iron oxide/hydroxides.
Under vacuum at 300 K, all spectra exhibit a significant decrease
of the outer quadrupolar component. Unfortunately, there is no
clear quantitative correlation between the ratio of the intensities
This rotation immediately influences the tilting of the Fe
octahedra around the Fe-Fe axis since each oxygen of the
(
59) Barthelet, K.; Marrot, J.; Riou, D.; F e´ rey, G. Angew. Chem., Int. Ed.
2002, 41, 281–284.
1
132 J. AM. CHEM. SOC. 9 VOL. 132, NO. 3, 2010

Functionalization in Flexible Porous Solids
A R T I C L E S
Figure 5. M o¨ ssbauer spectra of the modified MIL-53(Fe) solids (hydrated form) recorded at 300 K under air. They are compared to the one of the nonmodified
MIL-53(Fe) solid (bottom right).
Figure 6. Projection along the Fe chain axis of the simulated crystal structures of the dry MIL-53(Fe)-X
n
solids. Values of the b crystallographic parameter
in both the dry and hydrated (parentheses) forms is given, as well as the θ angle in the dry form (see the definition in Figure 7).
O-C-O group is bound to one Fe of the chain. This means
also that the θ angle between Φ and the Fe chains, which
characterizes the projection of the rotation in Figure 6, differs
from the γ angle between Φ and COO.
Within the functionalized frameworks, different interactions
also affect the rotation of Φ. The first of them (noted 1 in Figure
7b) relates to the coupling of -X groups with the OH of the
Fe-OH-Fe chains. Depending on the nature of -X, it can give
J. AM. CHEM. SOC. 9 VOL. 132, NO. 3, 2010 1133

A R T I C L E S
Devic et al.
Figure 7. (a) Definition of the angles of rotation between the COO group
and Φ (γ) and between the Fe-OH-Fe chains and Φ (θ); the incidence of
the rotation of the COO group on the tilting of the octahedral chains is
noted by black arrows. (b) Interactions occurring in the hydrated and dry
forms of MIL-53(Fe)-X (the OH groups of the skeleton are in blue and
gray, and the substituents X in green). Interactions 3 and 4 between water
and the skeleton disappear in the dry solids in which only subsist the
interactions 1 (called “inorganic-organic intraframework interaction” in
the text) between X and the OH groups of the framework and 2 (called
“
organic-organic intraframework interaction” in the text) corresponding
to the interactions between two Φ-X rings.
additional inorganic-organic intraframework hydrogen bonds
(
see legend of Figure 7) which will modify the position of the
Φ rings. Moreover, the most stable optimized structures always
come with substituting groups distributed on the opposite sides
of the phenyl rings which face each other. This steric repulsion
Figure 8. Infrared spectra of functionalized MIL-53(Fe)-X
73 K (except for MIL-53(Fe)-(OH)
Left part: ν(OH) range, right part δ(OH) range (not presented for MIL-
3(Fe)-NH , MIL-53(Fe)-(OH) , MIL-53(Fe)-(COOH) ).
n
evacuated at
2
, 423 K) under secondary vacuum.
(along the b axis, noted 2 in Figure 7b) is the second interaction
4
which plays a role on the orientation of the Φ rings. The final
conformation is thus a complex compromise between the
geometrical factors, inorganic-organic intraframework OH · · ·X
interactions, and organic-organic intraframework ΦX· · ·ΦX
steric repulsions.
5
2
2
2
bond acceptors (the aromatic ring and the halogen atom), the
first one can be discarded, since no interactions are observed in
MIL-53(Fe) and MIL-53(Fe)-CH . The halogen atoms are thus
3
To examine the potential of hydrogen bonds, the local
structure of the dry forms was probed by IR, focusing on the
bridging hydroxyl group (Figure 8). Different situations,
depending on the nature of the functional group, are manifested.
In the absence of any interaction ν(OH) and δ(OH) bands
are located at approximately 3645 and 850 cm- respectively,
involved in weak OH · · ·X (X ) Cl, Br) hydrogen bonds. This
is in accordance with the short OH · · ·X contacts deduced from
the simulated structures (H · · ·Cl ) 2.52 Å, O · · ·Cl ) 3.47 Å
62,63
and H· · ·Br ) 2.61 Å, O· · ·Br ) 3.57 Å).
The Fe-OH
2 2
band of the MIL-53(Fe)-(CO H) compound, rather broad, is
1
-1
situated at 3614 cm . By analogy with results relative to Br
and Cl, we assign it to the Fe-OH group in H-bonding
interaction with the carboxylic group. No significant ν(OH) band
as in the case of MIL-53(Fe) and MIL-53(Fe)-CH
3
. Their
sharpness indicates a homogeneous environment. Note that a
-
1
broad and weak ν(OH) band is also observed at 3300 cm on
MIL-53(Fe)-CH which might be due to some impurities (this
appears in the spectra of MIL-53(Fe)-(OH)
2
and MIL-53(Fe)-
-1
3
NH
above 3550 cm (the two sharp bands at 3491 and 3382
2
band indeed is still present when the compound is decomposed
-1
cm are assigned to NH
the Fe-OH entities of both samples are H-bonded with an
H-acceptor site (OH or NH group respectively). In the case of
MIL-53(Fe)-(OH) and MIL-53(Fe)-(COOH) , other OH groups
2
stretching modes). This shows that
after the thermal treatment above 300 °C.) In the case of MIL-
5
3(Fe)-(CF ) , the ν(OH) and δ(OH) bands are situated at 3628
3 2
2
-
1
and 879 cm . Such shifts are explained by the higher acidity
2
2
of the Fe-OH group as shown by the adsorption of deuterated
are present (phenolic or carboxylic). They also give rise to very
6
0
acetonitrile. Spectra of the MIL-53(Fe)-Cl and MIL-53(Fe)-
Br both reveal a splitting of the ν(OH) and δ(OH) bands.
Considering that the hydrogen bond interaction (i) increases the
δ(OH) frequency and (ii) decreases the frequency of the ν(OH)
band, while the band increases in intensity and broadens, the
splitting suggests the formation of a hydrogen bond with the
-1
broad ν(OH) bands below 3400 cm , demonstrating that they
are not free but involved in a hydrogen bonding interaction with
equivalent substituent groups (or remaining water for MIL-
5
3(Fe)-(OH)
These intraframework interactions above can for instance
explain why the unit cell volume of MIL-53(Fe)-(CO H) is
only slightly larger than those observed for MIL-53(Fe)-Br and
MIL-53(Fe)-NH , whereas the functional groups are bulkier;
2
).
2
2
-
1
hydroxyl group of the chains. Thus, the 3620 and 865 cm
bands on MIL-53(Fe)-Cl and the 3593 and 875 cm- bands on
MIL-53(Fe)-Br are assigned to ν(OH) and δ(OH) vibrations of
hydrogen bonded hydroxyl groups. The linear relation between
the shift and the full width at half-maximum (fwhm) of the
perturbed ν(OH) band, the main characteristics of the hydrogen
bond interaction, supports this assumption (see Supporting
1
2
this could be directly related to the O-H· · · ·O hydrogen bonds
between the free carboxylic groups of facing linkers, which force
55
the pores to close.
(61) Huggins, C. M.; Pimentel, G. C. J. Phys. Chem. 1956, 60, 1615–
6
1
1
619.
Information for details). Among the two potential hydrogen
(
62) Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001,
, 277–290.
(63) Kov a´ cs, A.; Varga, Z. Coord. Chem. ReV. 2006, 250, 710–727.
1
(
60) Vimont, A.; Clet, G., unpublished results.
134 J. AM. CHEM. SOC. 9 VOL. 132, NO. 3, 2010
1

Functionalization in Flexible Porous Solids
A R T I C L E S
to the strong intraframework hydrogen bond network mentioned
5
5
above, which forces the pores to remain closed. Otherwise,
for all the others, as already observed on the nonmodified MIL-
4
8
5
3(Fe) solid, the position of the Bragg peaks (and thus the
pore opening) for a given phase strongly depends on the nature
of the adsorbed liquid (Figure S10, top). Moreover, for a given
liquid, the XRPD pattern depends on the nature of the functional
groups (Figure S10, bottom). Unless noticed, no mixture of
forms was observed in the liquid-immersed samples. As already
noticed for the nonmodified MIL-53(Fe), depending on the
nature of the liquid, not only the hydrated narrow pore and
large pore forms but also intermediate pore openings were
observed. This latter could correspond to narrow pore forms,
whose volumes have been shown to be dependent on the steric
4
3,45
hindrance of the guest (size and shape).
Figure 9. Unit cell volume of the MIL-53(Fe) and its modified analogues
in their (air) hydrated form and immersed in various solvents. TCE, 1,1,2,2-
tetrachloroethane; EtOH, ethanol; H O, water; Pyr, pyridine.
2
The adsorption of ethanol leads to the large pore form for
all the modified forms with a very similar unit cell volume
3
36,43,48
within the 1550-1560 Å range,
the maximum opening of the structure. In the case of MIL-
3(Fe)-NH , this form is in equilibrium with the narrow pore
which corresponds to
Among the series, both hydrated and dry MIL-53(Fe)-X solids
exist in a narrow pore configuration. Dehydration is associated
with only negligible increases or decreases of the unit cell
volumes (Table S5). This suggests that the volume of the guest
5
2
hydrated form, which could be the consequence of strong
water-framework interactions (hydrogen bonds) in the starting
material, giving rise to only partial water-ethanol exchange.
On the other hand, in water, most of the solids capture only a
very limited amount of liquid and remain in the narrow pore
form with unit cell volumes almost identical to those of the
(
water) does not play a significant role in the unit cell volume
variations, at variance to the intraframework short contacts. This
is in sharp contrast with our previous findings on other MIL-
5
3(M) derivatives (M ) Cr, Al, Ga), whose structures evolve
3
hydrated (air) forms, i.e. lower than 1100 Å in most cases and
from a narrow pore to a large pore form upon dehydration.
3
64
equal to 1319 Å for MIL-53(Fe)-(CF ) (a volume correspond-
Despite some previous qualitative hypotheses, the reasons for
this discrepancy between Fe and the other metals currently
remains unfortunately unclear. Such a difference may find a
possible explanation in the electronic configuration of the 3d
orbitals of the various trivalent cations involved in the MIL-
3 2
ing to a closed form, as seen above). A noticeable exception
concerns MIL-53(Fe)-(OH) which captures large amounts of
2
water and exhibits the large pore form with a unit cell volume
3
of 1550 Å , which is consistent with additional hydrogen bonds
3
+
between hydroquinolic OH groups and free water molecules.
In pyridine, once again, MIL-53(Fe)-(OH) , is a special case:
2
no adsorption occurs, and the hydrated narrow pore structure
5
3-type structure. Fe , in particular, presents a stable, sym-
5
metric half filled 3d orbital, difficult to be perturbed, whereas
Cr , for example, possesses a 3d configuration, open to host
electrons from the guest molecules.
3
+
3
is maintained. For the others, a significant uptake is observed
3
3
with an intermediate (V≈1400 Å ) or complete (V ≈ 1600 Å )
pore opening. In the halogenated solvent (TCE), the adsorption
is significant for the halogenated MIL-53(Fe) solids (-Cl, -Br,
Adsorption by Immersion in Liquids. For pristine MIL-
5
3(Fe), this discrepancy no longer exists when liquids are used
4
5,48
instead of gases and water.
The question of whether or not
-
CF
3
) leading to the large pore form, whereas a few other
, with similar
the functionalization of MIL-53(Fe) can drastically change the
swelling properties during the adsorption of liquids is addressed
here. As already done for the nonmodified MIL-53(Fe) solid,
the adsorption of liquids was performed on the hydrated (air)
forms of the modified MIL-53(Fe)s rather than on the dehydrated
ones. This allows starting from a homogeneous series of
compounds, whereas dehydration, if not performed completely,
may lead to heterogeneous results. As a consequence, this study
concerns not only the adsorption of a given guest but also the
competition between the adsorption (or coadsorption) of this
given guest and water. The (air) hydrated forms of the
functionalized MIL-53(Fe) solids were therefore suspended in
various solvents (water, ethanol, pyridine, and 1,1,2,2-tetra-
chloroethane (TCE)) with variable polarity or acidity, registering
drastically different XRPD patterns.
structures, such as the apolar MIL-53(Fe)-CH
3
steric hindrance, do not take any TCE and remain in the hydrated
narrow pore form.
This part of the study shows that, in functionalized MIL-
n
53(Fe)-X solids, the flexibility (and therefore the adsorption)
is governed to a different extent by the host (including X)-guest
interactions, and not by steric considerations. Their strength can
sometimes counterbalance the intraframework interactions
(which always lead to a narrow pore form in the original solids),
allowing in this case the opening of the structures. The whole
set of observations clearly proves that the selectivity for the
liquid adsorption in modified MIL-53(Fe) solids depends on the
solvent-host framework interaction. This conclusion suggests
that, by using an adequate functionalization, an “ a` la carte” liquid
purification/separation seems possible.
The corresponding unit-cell dimensions are reported in Table
2
. The unit cell volumes for each solid/liquid pair are sum-
Energetics of the np-lp Transition in Modified MIL-53(Fe)
Solids. The pore opening in the functionalized MIL-53(Fe) solids
triggered by the adsorption of guest molecules is governed by a
critical balance between the intrinsic stability of the narrow and
large pore forms and the guest-framework interactions. As a
preliminary step, a computational structure determination of the
large pore forms of each functionalized MIL-53(Fe) solid was first
conducted starting from the experimental unit cell parameters
marized in Figure 9 and Table S6.
One can note that MIL-53(Fe)-(CO
2
2
H) is an exception. It
remains in its narrow pore form regardless of the liquid used,
without any significant liquid uptake. This is probably related
(64) Liu, Y.; Her, J.-H.; Dailly, A.; Ramirez-Cuesta, A. J.; Neumann, D. A.;
Brown, C. M. J. Am. Chem. Soc. 2008, 130, 11813–11818.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 3, 2010 1135

A R T I C L E S
Devic et al.
Table 3. Evolution of the Energy Difference between the Large
Pore (lp) and the Narrow Pore (np) Forms for the Different
MIL-53(Fe) Structures
etc.). This is clearly illustrated by the fact that MIL-53(Fe)-
CH and MIL-53(Fe)-(CF ) do not easily open in the selected
3 3 2
solvents (see Table 2).
Structures
[E(lp) - E(np)] (kcal mol-
1
)
MIL-53(Fe)-Cl
MIL-53(Fe)-Br
MIL-53(Fe)-NH
MIL-53(Fe)-(OH)
MIL-53(Fe)
285
270
230
180
60
Conclusion
The synthesis, on a large scale and HF-free when possible,
activation, and structural analysis of a series of functionalized
flexible MIL-53(Fe) hybrid solids have been reported. As
expected from previous studies on the nonmodified parent MIL-
53(Fe), all functional solids keep their flexible character upon
adsorption/desorption, nevertheless being modulated by the
presence of additional functional groups, with a complex
combination of steric hindrance considerations and intraframe-
work interactions that lead to slightly different breathing
characters. Finally, the study of the adsorption of various liquids
showed that the pore opening is selective and strongly depends
on the guest-framework affinity, making these inexpensive and
environmentally friendly nontoxic solids good candidates not
only for liquid phase separation (or drug delivery) but also for
gas sorption applications envisaged from the larger accessibility
of the pores in their dried forms. Similar work on another series
2
2
MIL-53(Fe)-CH
MIL-53(Fe)-(CH
hypothetical)
MIL-53(Fe)-(CF
3
–50
–150
3 4
)
(
3
)
2
–170
obtained for an ethanol-containing framework (Table 2; see
Supporting Information for the crystallographic data of each MIL-
3(Fe)-X solid). These additional calculations allowed us to
n
estimate for each functionalized form the energy difference between
the lowest energetic structures obtained for the narrow pore and
large pore forms. The resulting values are reported in Table 3.
First, this energy difference is positive for all solids except for those
5
substituted with -CH
3
3
and -CF groups, which emphasizes that
the most stable structure corresponds to the narrow pore form
observed in the dry state. The lower the energy difference, the easier
the MIL-53(Fe) structure will switch toward the large pore form
in the presence of solvent molecules.
6
5
of functionalized flexible solids (MIL-88) is currently under
way in our laboratories.
Acknowledgment. Dr. P. L. Llewellyn and Dr. S. Bourrelly
are thanked for their help in powder X-ray measurements of the
activated solids, Dr. F. Dumur for his fruitful advice on linker
synthesis, and Dr. S. Bauer and Prof. N. Stock for their help in the
The energy cost for opening the structure thus increases in
the following sequence: MIL-53(Fe)-(CF
MIL-53(Fe) < MIL-53(Fe)-(OH) <MIL-53(Fe)-NH
3(Fe)-Br < MIL-53(Fe)-Cl. One should keep in mind that this
energetic sequence is only qualitative since it has been obtained
using the generic UFF force field. From the IR results, it is
worth noting that the materials with a structure which is easy
)
3 2
< MIL-53(Fe)-CH
3
<
5
2
2
< MIL-
synthesis of MIL-53(Fe)-NH
2
. The authors acknowledge the
financial support of the French ANR ‘SAFHS’ (ANR-07-BLAN-
0
0
284-02), ‘NOMAC’ (ANR-06-CO2-008), ‘CONDMOFs’ (ANR-
7-BLAN-1-203677) and the UE through the FP6-STREP ‘De-
to open (-CH
contrast, the most energetically stable materials in the narrow
pore form (-Br, -Cl, -(OH) ) present OH groups involved in
3
3 2
, -(CF ) ) present only free OH groups. In
SANNS’ (SES6-020133). The ESRF is acknowledged for providing
access to the Swiss-Norwegian beamline.
2
inorganic-organic intraframework hydrogen bonds. This sug-
gests that the presence of hydrogen bonded OH groups
influences the narrow pore form stability. In these compounds,
the corresponding energy barriers should be overcome by the
guest-MIL-53(Fe) framework interactions when one introduces
the solvent molecule. The functional groups favoring the large
Supporting Information Available: Full experimental details
synthesis, characterization, XRD data, structural description,
simulation methodology, cif files).
This material is available free of charge via the Internet
at http://pubs.acs.org.
(
pore form (-CH
3
3
and -CF ) only weakly interact with guests
JA9092715
through van der Waals interactions, thus giving rise to a low
energy gain through the host-guest interaction, contrarily to
(
65) Surbl e´ , S.; Serre, C.; Mellot-Draznieks, C.; Millange, F.; F e´ rey, G.
the groups stabilizing the narrow pore form (-Br, -NH
2
, -OH,
Chem. Commun. 2006, 284–286.
1
136 J. AM. CHEM. SOC. 9 VOL. 132, NO. 3, 2010