Tunable anisotropic thermal expansion of a porous zinc(II) metal-organic framework

Article abstract of DOI:10.1021/ja401671p

A novel three-dimensional metal-organic framework (MOF) that displays anisotropic thermal expansion has been prepared and characterized by single-crystal X-ray diffraction (SCD) and thermal analysis. The as-prepared MOF has one-dimensional channels containing guest molecules that can be removed and/or exchanged for other guest molecules in a single-crystal to single-crystal fashion. When the original guest molecules are replaced there is a noticeable effect on the host mechanics, altering the thermal expansion properties of the material. This study of the thermal expansion coefficients of different inclusion complexes of the host MOF involved systematic alteration of guest size, i.e., methanol, ethanol, n-propanol, and isopropanol, showing that fine control over the thermal expansion coefficients can be achieved and that the coefficients can be correlated with the size of the guest. As a proof of concept, this study demonstrates the realizable principle that a single-crystal material with an exchangeable guest component (as opposed to a composite) may be used to achieve a tunable thermal expansion coefficient. In addition, this study demonstrates that greater variance in the absolute dimensions of a crystal can be achieved when one has two variables that affect it, i.e., the host-guest interactions and temperature.

Full text of DOI:10.1021/ja401671p

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Communication
Tunable Anisotropic Thermal Expansion of
a Porous Zinc(II) Metal-Organic Framework
Ilne Grobler, Vincent J. Smith, Prashant M. Bhatt, Simon A. Herbert, and Leonard J. Barbour
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja401671p • Publication Date (Web): 12 Apr 2013
Downloaded from http://pubs.acs.org on April 18, 2013
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Tunable Anisotropic Thermal Expansion of a Porous Zinc(II)
Metal-Organic Framework
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Ilne Grobler, Vincent J. Smith, Prashant M. Bhatt, Simon A. Herbert and Leonard J. Barbour*
Department of Chemistry and Polymer Science, University of Stellenbosch, Stellenbosch 7600, South Africa
Supporting Information Placeholder
ics of different hostꢀguest inclusion complexes as a factor of temꢀ
perature can be studied systematically. Hostꢀguest materials can
have controllable physical properties if the guest component can
be removed and/or exchanged. An example of a potential benefit
of this occurring in a single crystal is that it can be used as a therꢀ
moꢀmechanical actuator or sensor in a singleꢀcrystal device.
ABSTRACT: A novel threeꢀdimensional metalꢀorganic frameꢀ
work (MOF) that displays anisotropic thermal expansion has been
prepared and characterized by singleꢀcrystal Xꢀray diffraction
(SCD) and thermal analysis. The asꢀprepared MOF has oneꢀ
dimensional channels containing guest molecules that can be reꢀ
moved and/or exchanged for other guest molecules in singleꢀ
crystal to singleꢀcrystal fashion. When the original guest moleꢀ
cules are replaced there is a noticeable effect on the host mechanꢀ
ics, resulting in an alteration of the thermal expansion properties
of the material. This study of the thermal expansion coefficients
of different inclusion complexes of the host MOF involved sysꢀ
tematic alteration of guest size, i.e. methanol, ethanol, nꢀpropanol
and isoꢀpropanol, showing that fine control over the thermal exꢀ
pansion coefficients can be achieved, and that the coefficients can
be correlated with the size of the guest. As a proof of concept, this
study demonstrates the realizable principle that a single crystal
material with an exchangeable guest component (as opposed to a
composite) may be used to achieve a tunable thermal expansion
coefficient. In addition, this study demonstrates that greater variꢀ
ance in the absolute dimensions of a crystal can be achieved when
one has two variables that affect it, i.e. the hostꢀguest interactions
and temperature.
Scheme 1. The solvothermal synthesis of 1. The framework is
shown in spacefilled representation with the methanol guest molꢀ
ecules omitted for clarity.
The porous metalꢀorganic framework [Zn(L)2(OH)2]nꢁnCH3OH
(1MeOH, Scheme 1), formed by the reaction of 4ꢀ(1Hꢀnaphtho[2,3ꢀ
d]imidazoleꢀ1ꢀyl)benzoic acid (L), with Zn(NO3)2, displays unuꢀ
sually large anisotropic thermal expansion with coefficients that
are amongst the highest reported values for MOFs.3a,4,11 The 3D
framework (space group: I) consists of eclipsed cubic 4,4ꢀnets
that are interconnected by polymeric ZnꢀhydroxyꢀZn coordination
along the c axis (Figure S1 and S2 (Supplementary information)).
The lattice has oneꢀdimensional squareꢀshaped channels with a
crossꢀsectional width of approximately 6 Å and an accessible
porosity of 16.0%. The wellꢀdefined channels of 1MeOH include
guest molecules, of which only the methanol molecules that hyꢀ
drogenꢀbond to the uncoordinated oxygen atoms of the carboxꢀ
ylate moieties of the ligand could be modeled.
When a material is heated it normally experiences positive
thermal expansion (PTE) along all three dimensions owing to an
increase in the anharmonic vibrations of the constituent atoms,
molecules or ions.1 In rare cases, specific structural features or
packing arrangements in the solid state may give rise to anomaꢀ
lous thermal expansion behavior such as exceptionally large
PTE,1b,2 zero thermal expansion (ZTE) or negative thermal expanꢀ
sion (NTE). Metalꢀorganic frameworks (MOFs) have recently
emerged as a new class of thermoꢀresponsive materials with
anomalous thermal expansion behavior. Anisotropic and excepꢀ
tionally large thermal expansion, in the order of that of
Ag3[Co(CN)6] and other Prussian blue analogues3 (>10–4 K–1), has
been reported for the flexible HMOFꢀ1, which has a latticeꢀfence
topology and hinged motion around the metal centre.4 Other types
of framework materials known to display isotropic NTE include
the cyanide bridged coordination polymers,5 the isoreticular
IRMOFs,6 and Cu3(1,3,5ꢀbenzenetricarboxylate)2 or Cu3(btc)2.7
The elucidation of the mechanisms of anisotropic thermal expanꢀ
sion provides information for the design of thermoꢀresponsive
materials. Provided that the mechanical response does not result
in the loss of singleꢀcrystal quality of the material, structure deꢀ
terminations from SCD at regular temperature intervals can afford
the necessary information to determine the mechanism involved.
Moreover, if the singleꢀcrystal quality of a material is maintained
during a guest exchange experiment, the changes in host mechanꢀ
The crystals of 1MeOH are extremely robust, remaining transparꢀ
ent and of diffractionꢀquality following removal of the guest molꢀ
ecules at 150 °C under reduced pressure. A crystal of the apohost
framework, 1apohost, placed under static vacuum exhibits excepꢀ
tionally large uniaxial PTE with αc=127 × 10–6 K–1 along the c
axis, measured over a temperature range of 100–370 K, Figure 1.
The PTE along the c axis is accompanied by biaxial NTE along
the a and b axes with αa,b = –21 × 10–6 K–1 for the same temperaꢀ
ture range. The exceptional PTE along the c axis, which is an
order of magnitude larger than conventional PTE, contributes to
an overall volumetric expansion of αV = 89 × 10–6 K–1.
Remarkably, the crystal mosaicity is not affected by the temꢀ
perature conditions or the anisotropic expansion itself. The mechꢀ
anism of the anisotropic expansion involves a substantial increase
in the intermolecular hostꢀhost interaction distances and the deꢀ
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formation of the labile coordination angles around the metal
nodes. Concerted atomic motion allows and facilitates the preserꢀ
vation of the integrity of a single crystal despite the significant
and reversible structural transformations. There are notable
changes in the metal geometry around the tetrahedral ZnII node.
Specifically, the four coordination angles shown in Figure 2
change considerably (see Table 1).
100
190
280
370
126.7(1)
128.5(1)
129.6(2)
130.7(2)
115.4(1)
108.7(1)
109.5(1)
109.6(2)
111.6(1)
112.0(1)
112.4(1)
113.5(3)
105.9(1)
105.0(1)
104.6(2)
104.2(2)
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The exceptional PTE results from the large increase in the Zn–
O(H)–Zn angle. The angle increases by 4.0° over the temperature
range 100–370 K, resulting in an increase in the distance between
two Zn atoms across the hydroxyl bridge. The Zn∙∙∙Zn separation
increases from 3.431 Å to 3.482 Å. The difference of 0.051 Å
corresponds to an expansion coefficient of 55 × 10–6 K–1. There
are two Zn–O(H)–Zn angles along the c axis of the unit cell and
the summation corresponds to an overall Zn∙∙∙Zn expansion coefꢀ
ficient of 110 × 10–6 K–1 which does , in part, represent the coeffiꢀ
cient determined for the c axis (137 × 10–6 K–1) . The Zn–O(H)–
Zn bridges form a polymeric chain along the c axis and the enꢀ
largement of this angle results in the “colossal” PTE along that
5.35
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5.20
5.15
5.10
26.60
26.55
26.50
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26.40
26.35
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a, b (Å)
c (Å)
direction. There is
a
concomitant reduction in the
N–Zn–OH and Ocarb–Zn–OH angles by 5.8° and 1.7°, respectiveꢀ
ly. The N–Zn–OH, N–Zn–Ocarb and Ocarb–Zn–OH angles are more
or less in the ab plane and it is believed that their distortion leads
to the observed NTE along these directions. A structural model
that explains the material’s thermal response is shown in Figure 3.
The enlargement of the Zn–O(H) –Zn angle at elevated temperaꢀ
tures leads to the elongation of the framework along the c axis,
analogous to the stretching of an accordion.
100
150
200
250
300
350
400
T (K)
Figure 1. The temperatureꢀdependent a = b (triangles) and c (cirꢀ
cles) unit cell parameters of 1apohost. Error bars show the estimated
standard deviations.
b
c
0
Figure 3. A schematic twoꢀdimensional representation of the
framework topology with the Zn metal centres depicted as blue
circles and the hydroxyl bridges as red circles.
Close examination of the intermolecular framework interacꢀ
tions reveals that the interaction distances are temperature–
dependent and are involved with the anisotropic thermal expanꢀ
sion of the framework itself. Intermolecular interactions have
shallower potential energy wells and are subject to larger thermal
expansion than covalent or coordination bonds.8 There are strong
hydrogenꢀbonding interactions along the [Zn–OH–]n chain, Figure
4.
Figure 2. The coordination environment around the tetrahedral
ZnII node in relation to the unit cell axes. (a) The Zn–O(H) –Zn
angles along the c axis expand with increasing temperature. (b)
The Ocarb–Zn–OH and N–Zn–OH angles that are in the plane of
the a and b axes contract upon heating. Hydrogen atoms are omitꢀ
ted for clarity.
Figure 4. The [ZnꢀOHꢀ]n coordination chain viewed perpendicuꢀ
lar to the c axis. Hydrogen bonds that propagate approximately
parallel to the c axis are shown as dashed red lines.
Table 1. Coordination angles (°) around the Zn(II) node of
The hydrogen bond distance between the hydroxyl group and
the coordinated carboxyl oxygen atom increases from 2.875(3) Å
at 100 K to 3.065(8) Å at 370 K. Thus the increase in the hydroꢀ
genꢀbond distances and the concomitant changes in the coordinaꢀ
tion angles around the ZnII node are responsible for the anisoꢀ
1apohost
.
T / K
ZnꢀO(H)ꢀZn NꢀZnꢀOH NꢀZnꢀOcarb
OcarbꢀZnꢀ
OH
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tropic thermal expansion observed. That is, the expansion of the
T range: hostꢀguest complexes
(a)
5.50
5.45
5.40
5.35
5.30
5.25
5.20
5.15
5.10
5.05
5.00
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intermolecular framework interactions and the lability of the coꢀ
ordination angles are both requirements for the large PTE and
NTE effects.
. As anticipated, the length of the needleꢀshaped crystal of
1MeOH, Figure 5, increases from 1.10 mm at 100 K to 1.16 mm at
370 K. The macroscopic increase in length corresponds with αc of
202.0 × 10–6 K–1 calculated from the unit cell parameters for the
100–370 K temperature range. At the same time; the width deꢀ
creases to a lesser extent due to the NTE along the a and b axes.
c (Å)
9
Since the material is microporous, it affords us the opportunity
to study the effects that different guest molecules situated within
the channels may have on the thermal expansion. Single crystals
of 1MeOH were suspended in vials containing ethanol, nꢀpropanol
or isoꢀpropanol for two weeks prior to SCD structure determinaꢀ
tion. After initial structure determination at 100 K in each case to
confirm guest exchange, the unit cell parameters were measured
at 15 K intervals upon heating to 370 K. Across this series of
alcohol solvates the guest occupancy is the same (four molecules
of guest per ~300 Å3 channel section or one guest molecule per
asymmetric unit), even with an increase in the size of the guest.
Thermogravimetric analysis of the asꢀsynthesized material
(1MeOH) showed that guestꢀloss occurs at room temperature and
therefore the unit cell parameters of 1MeOH start to converge with
those of 1apohost from 295 K onwards. However, we determined
that all of the hostꢀguest inclusion complexes are stable over the
temperature range of 100–295 K.
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T (K)
(b)
T range: hostꢀguest complexes
26.90
26.80
26.70
26.60
26.50
26.40
26.30
26.20
26.10
a, b (Å)
100
150
200
250
300
350
400
T (K)
Figure 6. The temperatureꢀdependent (a) c and (b) a=b unit cell
parameters of 1MeOH (red squares), 1EtOH (green triangles), 1n
(purple crosses), 1apohost (black diamonds) and 1iso
cles). Error bars show the estimated standard deviations.
-PrOH
(blue cirꢀ
-PrOH
Table 2. The PTE coefficients and selected structural inforꢀ
mation of the hostꢀguest complexes.
Solvate Guest volume
αc
Zn–O(H)–Zn Hꢀbond
(Å3)*
×106
(K–
(°)
(Å)***
1)**
38.8
54.0
70.7
–
166(4)
144(5)
138(4)
137(5)
76(6)
123.7(1)
123.8(2)
124.6(1)
126.7(1)
127.4(1)
2.808(4)
2.812(5)
2.832(8)
2.875(3)
2.915(3)
1MeOH
1EtOH
Figure 5. Photomicrographs of a crystal of 1MeOH mounted on a
glass fibre and recorded at 100 K (left) and 370 K (right).
1n
-PrOH
Relative to 1apohost there is an increase in the already unusually
large PTE and NTE in the presence of methanol, ethanol and nꢀ
propanol guest molecules (1MeOH, 1EtOH, 1n-PrOH) while the bulky
isoꢀpropanol molecules (1iso-PrOH) suppress the extent of thermal
expansion (Figure 6). More significantly, the PTE coefficients for
the different guests correlate to the size/van der Waals volume of
the guest involved. Table 2 summarizes the PTE coefficients for
the host material when it includes guests of increasing van der
Waals volume; the Zn–O(H)–Zn angles at 100 K are also listed.
1apohost
69.1
1iso
-
PrOH
*The guest volume was calculated using the van der Waal’s surꢀ
face.
**αc was calculated for the temperature range100–295 K.
***The intraꢀframework hydrogenꢀbond distance between the
hydroxyl group and the coordinated carboxylꢀoxygen atom at 100
K (as shown in Figure 4).
The PTE coefficient decreases in the order: 1MeOH >1EtOH >1n
-PrOH
> 1apohost >1iso
.
-PrOH
The guest size, shape and the hydrogenꢀbonding interactions
with the host framework concurrently have an effect on the host
mechanics, as is evidenced by the differing coordination environꢀ
ment around the Zn(II) nodes and the different strengths of the
intraꢀframework hydrogen bonds in the different solvates. There is
an induced fit of the guest within the framework channels. The
framework has the ability to adjust its metal geometry and chanꢀ
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Page 4 of 5
nel size according to the size of the guest molecules. The differꢀ
lishes a novel concept of guestꢀtunable thermal expansion, with
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ences in the metal geometry and the intraꢀframework hydrogen
bonding distances at 100 K relative to the apohost framework
(listed in Table 2) ultimately result in the different responses to
temperature. At 100 K the Zn–O(H)–Zn angle of 1MeOH, 1EtOH
implications for enhanced utility of materials based on ‘single
crystals’.
ASSOCIATED CONTENT
Supporting Information
and 1n
is smaller than that of 1apohost resulting in a shorter c
-PrOH
axis of the unit cell. These solvates are in a “contracted” state
relative to the apohost framework. Effectively, the initial metal
geometry at 100 K allows for a greater extent of PTE to occur in
Details of synthesis, thermal analysis, crystallographic inforꢀ
mation. This material is available free of charge via the Internet at
http://pubs.acs.org.
these complexes. However, in 1iso
the Zn–O(H)–Zn angle is
-PrOH
9
already enlarged due to the presence of the bulky guest, and the
framework is already in a slightly “expanded” state which renders
the framework less flexible towards further thermal expansion.
AUTHOR INFORMATION
Corresponding Author
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The magnified thermal response when there are certain guests
within the channel is an unusual result. The guestꢀfree frameꢀ
works of Cu3(btc)2 and cadmium cyanide (Cd(CN)2) are known to
display isotropic NTE, which is dampened by the presence of
guest molecules such that only unremarkable PTE is observed.7b,10
In both of these materials, a purely vibrational mechanism involvꢀ
ing transverse vibrations of the organic linkers is associated with
the NTE and the authors report that open channels are necessary
ljb@sun.ac.za
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank the National Research Foundation of South Africa for
financial support. PB thanks the Claude Leon foundation for a
postdoctoral fellowship.
for the framework flexibility.7b In contrast, in the case of 1MeOH
1EtOH and 1n the guest molecules enhance the flexibility of
,
-PrOH
the framework by means of altering the coordination environment
and the strength of framework interactions. The fact that the NTE
effect is enhanced by guest inclusion in some cases (Table S7) is
evidence that the guest molecules do not simply act to increase the
overall volume thermal expansion as was the case in the studies of
Cu3(btc)2 and cadmium cyanide (Cd(CN)2). Instead the NTE
coefficient of 1apohost, αa,b = –26(5) × 10–6 K–1, is increased to αa,b
= –41(3) × 10–6 K–1 in 1MeOH, which suggests that hostꢀguest
interactions play a role in the thermal response of the various
solvates.
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Eckert, J. A. K. Howard, O. M. Yaghi, Science 2005, 309, 1350ꢀ1354.
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Kemner, C. J. Kepert, Angew. Chem. Int. Ed. 2010, 49, 585ꢀ588; (b) Y.
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Lock, P. D. Southon, C. J. Kepert, Angew. Chem.Int. Ed. 2008, 120, 9061ꢀ
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(8) (a) I. D. Brown, A. Dabkowski, A. McClearly, Acta Cryst. Sect. B
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36, 7ꢀ13.
A study of the effect of guest molecules on the apparent therꢀ
mal expansion of a MOF has been reported by Omary et al. They
observed an increase in the apparent NTE and PTE with the seꢀ
quential filling of the pores of a fluorous MOF (FMOFꢀ1) with N2
molecules.11 In contrast to 1, FMOFꢀ1 displays unusual twoꢀstep
breathing, involving a volumetric NTE process in one temperature
range (90–119 K) and an overall PTE process in another temperaꢀ
ture range (119–295 K). The anomalous thermal expansion was
correlated to the location of the N2 within the channels and the
reversal in the thermal response was, in part, attributed to the
localization of more N2 guest molecules such that guestꢀguest
repulsion becomes significant. In the case of 1MeOH, 1EtOH, 1n
-PrOH
and 1iso-
a constant trend is observed and the initial guest
PrOH
occupancy is presumed to remain constant from 100 to ~295 K.
To our knowledge, the current study is the first example of a conꢀ
trolled experiment in which a series of related guest molecules of
increasing size is included with a constant hostꢀguest ratio over a
broad temperature range. The effect of temperature on the host
mechanics was monitored by structural analyses.
In conclusion, we have shown that 1MeOH has the combined
properties of being porous and having unusual anisotropic thermal
expansion. We have demonstrated that the choice of guest can be
used to alter the thermal expansion behavior of a host material
such that fine control over the thermal expansion coefficients can
be achieved. The study also demonstrates that guest inclusion
does not only alter the linear thermal expansion coefficient but it
also creates an array of possible absolute physical dimensions of a
single crystal by the control of two variables; the type of guest
and the temperature. If one considered the PTE of the c axis in
terms of a “guest expansion coefficient” as well as a “thermal
expansion coefficient” then a two dimensional plot will create a
surface of potential c axis values or crystal lengths that are accesꢀ
sible (Figure S13 (Supplementary Information)). This study estabꢀ
(10) A. E. Phillips, A. L. Goodwin, G. J. Halder, P. D. Southon, C. J.
Kepert, Angew. Chem Int. Ed. 2008, 47, 1396ꢀ1399.
(11) C. Yang, X. Wang, M. A. Omary, Angew. Chem. Int. Ed. 2009, 48,
2500ꢀ2505.
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