High reactivity of metal-organic frameworks under grinding conditions: Parallels with organic molecular materials

Article abstract of DOI:10.1002/anie.200906965

Interconversion made easy: Metalorganic frameworks (MOFs) are surprisingly reactive under grinding conditions and can perform various rearrangements (see picture). In this respect, the results reveal clear parallels between MOFs and organic molecular mate

Full text of DOI:10.1002/anie.200906965

Communications
DOI: 10.1002/anie.200906965
Mechanochemistry
High Reactivity of Metal–Organic Frameworks under Grinding
Conditions: Parallels with Organic Molecular Materials**
ˇˇ ´
Wenbing Yuan,* Tomislav Friscic,* David Apperley, and Stuart L. James*
Although it has recently been found that extended metal–
organic frameworks (MOFs) can be prepared by grinding
with minimal or no added solvent (mechanochemistry), the
reactivity of MOFs themselves under these conditions has not
yet been probed.[1,2] Herein we report strikingly high reac-
tivity of MOFs under mechanochemical conditions: We show
that they can undergo complete reconstruction into different
topologies within minutes by grinding with small amounts of
liquid (liquid assisted grinding, or LAG) or with additional
solid ligands in the complete absence of added solvent. As
well as the efficiency and practical utility of these trans-
formations, the findings point to previously unrecognized
similarities between MOFs and organic molecular materials
under grinding conditions. This finding naturally suggests that
methodologies established for each class of material may be
applied or adapted to the other.
Figure 1. Synthesis of 1–3 using liquid-assisted grinding (LAG). Coor-
The three primary materials used in this study, 1–3, are
shown in Figure 1. These known MOFs,[3] were prepared by
grinding 1,4-benzenedicarboxylic acid (H2bdc) with ZnO[2b]
dinated water molecules are shown in light gray, O in black, C in gray,
and Zn in dark gray. H atoms and the DMF molecules included in the
channels of 2 are omitted for clarity. DMF=N,N-dimethylformamide.
[4]
or basic zinc carbonate [ZnCO3]2·[Zn(OH)2]3 in a ball mill
in the presence of a small amount of added liquid (100 mL of
H2O, MeOH, or DMF) for 20 minutes.[5] As previously
observed in related reactions between fumaric acid and
ZnO,[2b] the nature of the added liquid determined the
product: [Zn(bdc)(H2O)2] (1; CSD code DIKQET[3a]) was
obtained using added water, [Zn(bdc)(H2O)]·DMF (2; CSD
code GECXUH[3b]) with DMF, and [Zn(bdc)(H2O)] (3; CSD
code IFABIA[3c]) with methanol. These structures have one-,
two-, and three-dimensional connectivity, respectively. For 2,
although 40 minutes of grinding with 200 mL of DMF still left
some ZnO starting material,[6] use of [ZnCO3]2·[Zn(OH)2]3
gave the product quantitatively after 20 minutes. 1–3 were
identified by comparison of the experimental X-ray powder
diffraction (XRPD) patterns with patterns simulated from
single-crystal structures in the Cambridge Structural Data-
base (see the Supporting Information, Figures S2–S4). Solid-
[*] Dr. W. Yuan
1
state H and 13C NMR also confirmed the syntheses to be
Department of Chemical Engineering, Hainan University
Haikou (P. R. China)
E-mail: hnyuanwb@126.com
quantitative (Figures S5–S12).
A key observation was that formation of 1 occurred in a
stepwise manner through 3 when reduced amounts of added
water were present (Figure S13). At the molecular level, the
change from 3 to 1 involves the addition of one water
molecule per Zn center and a change in carboxylate
coordination from bidentate bridging to monodentate. This
stepwise process with 3 as an intermediate phase indicated
that MOFs themselves may be highly labile under grinding
conditions. Indeed, other stepwise mechanisms during grind-
ing have recently been seen in a closely related MOF
synthesis.[2b] More generally however, this type of lability is
reminescent of organic co-crystals which can also form by
stepwise mechanisms[7] and which readily interconvert
between different structural forms upon grinding.[7b]
Dr. W. Yuan, Dr. S. L. James
Centre for the Theory and Application of Catalysis (CenTACat)
School of Chemistry and Chemical Engineering
Queen’s University Belfast
David Keir Building, Stranmillis Road, Belfast, BT9 5AG (UK)
E-mail: s.james@qub.ac.uk
Homepage: http://www.ch.qub.ac.uk/staff/james/index.html
ˇˇ ´
Dr. T. Friscic
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: tf253@cam.ac.uk
Dr. D. Apperley
Department of Chemistry, Durham University
South Road, Durham, DH1 3LE (UK)
[**] W.Y. would like to acknowledge the NSFC (grant no. 20761003) for a
visiting researcher scholarship to Queen’s University Belfast. Dr.
Mark Nieuwenhuyzen is acknowledged for assistance with XRPD.
Remarkably, several additional MOF interconversions
between 1, 2, and 3 could also be induced efficiently by
grinding as shown by the green arrows in Figure 2. For
example, the reverse reaction, 1!3, was easily induced by
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906965.
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Angewandte
Chemie
be a result of increasing the available surface area and
potentially amorphization. Amorphization raises the free
energy of the solid compared to the crystalline state and can
result in higher kinetic solubility as widely observed for
molecular organic materials[7,9] and in rare cases some
inorganic extended materials.[10] Consistent with this possi-
bility, grinding 3 without solvent for 20 minutes increased its
(kinetic) solubility to 0.58 mmolLꢀ1.
The lability of MOFs under grinding conditions could be
exploited synthetically and for regenerating materials which
have lost crystallinity. To probe the synthetic applications
further, MOFs 1–3 were ground with additional bridging
ligands, 4,4’-bipyridine (bipy) and 1,4-diazabicyclo-
[2.2.2]octane (dabco), which could give rise to other materials
in a targeted manner (anticipating the formation of mixed-
ligand pillared MOFs [Zn(bdc)(bipy)] (5)[3d,e] and [Zn(bdc)-
(dabco)(H2O)] (6);[3f] Figure 3). In the absence of any added
liquid, 1 and 3 reacted cleanly with 1 equivalent of solid bipy
to give the mixed-ligand framework 5 within 20 minutes.
Interestingly, under these conditions, 2 gave a new, unidenti-
fied structure. All three MOFs reacted cleanly with solid
dabco without added liquid to give the mixed-ligand frame-
work 6 within 20 minutes. It is significant that none of these
reactions could be performed by immersion in solvent
containing bipy or dabco, which gave instead different
unidentified products. It is also notable that attempts at
single-step reactions to obtain 5 or 6 directly from ZnO (or
[Zn(CO3)]2·[Zn(OH)2]3), bipy, and H2bdc by grinding were
generally not successful: Although ZnO reacted with bipy
and H2bdc to give 5 cleanly, the other reactions gave impure
products containing large amounts of H2bdc–dabco co-
crystals[11] or unidentified phases (Figure S15). Therefore,
stepwise syntheses which exploit the lability of MOFs under
Figure 2. Interconversions between 1, 2, and 3 induced by grinding
(inner, green arrows) compared with simple immersion in excess of
the liquid (outer, blue arrows).
grinding 1 with a small amount of methanol (200 mL) for
40 minutes. A difference to the forward reaction is that
conversion of 1 into 3 cannot be driven by coordination or
inclusion of the added liquid (methanol is not present in 3),
and therefore seems to be purely a rapid recrystallization
through small amounts of a methanolic phase. Both 1 and 3
could be converted into 2 by grinding with DMF (75 mL) for
20 minutes. Although DMF is not directly coordinated to Zn
in 2, it is included through H bonding in the channels and
therefore plays the role of reactant (as well as presumably
solvent) in these cases. Finally, completing the cycle of
interconversions, 2 could be converted back into 1 by grinding
with H2O (75 mL) for 20 minutes. At the molecular level, the
change 2!1 is similar to 3!1, that is, coordination of a
second water molecule to each Zn center, and a change of the
carboxylate coordination mode from bridging bidentate to
terminal monodentate. The only interconversion that could
not be induced by grinding was 2!3.
Figure 2 summarizes the results of liquid-assisted grinding
(green arrows) and compares them with the results of simple
immersion in an excess of the same liquid (blue arrows). Only
two immersion experiments replicated interconversions
induced by grinding, specifically 2!1 and 3!1, and they
are far slower. In some cases immersion has no effect even
after several days (i.e. 1
6! 3 and 26! 3) or it gives alternative
products which could not be identified by XRPD methods
(i.e. 1!4 and 3!4, where 4 is an unidentified crystalline
product, see Figure S14).
The driving force for the LAG-induced interconversions
may be dominated by formation of the least soluble product.
Consistent with this, the solubility of
3
in methanol
(0.51 mmollꢀ1
)
is indeed lower than that of
1
(1.21 mmollꢀ1).[8] Furthermore, 2 has similar solubility in
methanol (0.56 mmollꢀ1) relative to 3, consistent with its lack
of conversion into 3 by LAG. The marked acceleration of
interconversion by LAG compared to simple immersion may
Figure 3. Synthesis of 5 and 6 by grinding MOFs 1, 2, or 3 with bipy or
dabco, and comparison with simple immersion in solutions containing
an excess of the ligands.
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Communications
Received: December 10, 2009
Published online: April 26, 2010
grinding conditions can extend the scope of grinding-based
syntheses.
Overall, the observed transformations point to striking
Keywords: grinding · mechanochemistry ·
similarities between MOFs and molecular organic materials
under grinding conditions. Specifically, their formation,
structural interconversions[12,13] (including stepwise formation
mechanisms)[7] and solubilities are all similarly achieved or
increased by brief neat or liquid-assisted grinding for both
classes of materials.[12] Specific analogies are indicated in
Figure 4. These two classes of materials therefore exhibit
similar degrees of reactivity under grinding conditions.
.
metal–organic frameworks · zinc
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Figure 4. Analogous behaviors of a) molecular crystals and b) MOFs
under grinding conditions.
In summary, we report that MOFs can be surprisingly
labile under grinding conditions. This lability is shown by
1) several rapid, interconversions between MOF structures
induced by liquid-assisted grinding, and 2) synthesis of mixed-
ligand materials by grinding MOFs with additional ligands
and no solvent. The results reveal clear parallels between
MOFs and organic molecular materials under these reaction
conditions. Overall, the findings improve our insight into the
possibilities of grinding-induced transformations and extend
the application of grinding as a convenient solvent-free or
minimal-solvent method.
[5] Reactions between Zn(OAc)2·2H2O and H2bdc gave unidenti-
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[6] The reaction between ZnO and H2bdc was sensitive to the water
content of the DMF; increasing the water content to 10% (v/v)
gave 2 quantitatively after 30 min.
Experimental Section
Standard reaction conditions involved 1 mmol of reactant, ground
with a Retsch MM400 shaker mill in a 20 mL steel vessel with a
10 mm steel ball at 25 Hz for the specified time.
ˇ ´
ˇˇ ´
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Chemie
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1
ˇˇ ´
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