Control of framework interpenetration for in situ modified hydroxyl functionalised IRMOFs

Article abstract of DOI:10.1039/c2cc35565e

By intimate control of reaction conditions, phase-pure crystalline porous metal-organic framework materials [Zn₄O(L)₃] with interpenetrated and non-interpenetrated structures can be synthesised. Under certain conditions, these reacti

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ARTICLE TYPE
Control of Framework Interpenetration for in situ Modified Hydroxyl
Functionalised IRMOFs
Damien Rankine,a Antonio Avellaneda,a Matthew R. Hill,b Christian J. Doonan*,a and Christopher J.
Sumby*,a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
30 anchoring catalytic moieties to the framework need to be
established. Two major challenges arise; 1) Increasing the pores
sizes of topologically identical MOFs commonly affords
interpenetration5 and, 2) utilizing organic links which bear metal-
coordinating groups can lead to unwanted side reactions or hinder
35 the formation of the desired networks. Thus, frameworks directly
synthesized with free metal-coordinating groups are rare.6-8
By intimate control of reaction conditions, phase-pure
crystalline porous metal-organic framework materials
[Zn4O(L)3] with interpenetrated and non-interpenetrated
10 structures can be synthesised. Under certain conditions, these
reactions occur with concomitant deprotection of masked
alcohols located on the organic links which yield accessible
‘metal-binding’ functional groups within the frameworks.
Recently, post-synthetic modification (PSM) has been
developed as a strategy to overcome these challenges and yield
open framework materials imbued with novel functionality.8 A
40 potential drawback of the PSM method is that performing
multiple reactions on the MOF crystals can lead to structural
decomposition and/or diminished surface area.9 Thus, to ensure
the MOFs retain optimal porosity and crystallinity it is preferable
that the most efficient synthetic procedure is employed. Here we
45 report the controlled one-pot synthesis and in situ ester hydrolysis
of non-interpenetrated and interpenetrated MOFs, [Zn4O(L1)3]
and -[Zn4O(L1)3], comprised of biphenyl-2,2’-diol links (L1)
(Scheme 1(a)). Notably, both materials can be synthesized in a
single-step procedure. It is also noteworthy that the metal co-
50 ordinating groups are freely accessible in both forms which we
demonstrate by the complexation of copper ions and enhanced
enthalpy of adsorption for -[Zn4O(L1)3] over IRMOF-1.
Metal-organic frameworks (MOFs) are constructed by a modular
15 synthetic approach whereby organic links and metal clusters are
assembled into porous extended networks via ‘one-pot’
reactions.1 This has led to the development of design principles
that yield precise control of pore dimensions and functionality by
utilizing links of identical connectivity but different lengths or
20 non-structural functional groups.2 Due to their high surface areas,
tuneable structure metrics and chemically mutable organic
building blocks, MOFs have been identified as materials with
great potential for gas separations3 and size-selective
heterogeneous catalysis.4 To fully realise their catalytic potential
25 MOFs with pores sufficiently large enough to simultaneously
allow anchoring of catalytic moieties and diffusion of chemical
reactants into the framework is requisite. Furthermore, given that
the majority of the metal clusters that comprise the ‘joints’ of
MOF structures are catalytically benign, reliable methods for
Scheme 1. (a) Synthesis of [Zn4O(L1)3] and -[Zn4O(L1)3]. Inset: Space-filling representations of the pores in (b) [Zn4O(L1)3] and (c)
55 -[Zn4O(L2)3]. (d) Images from the metallation studies to generate -[Zn4O(L1)3]Cu and show accessibility of the hydroxyl groups.
This journal is © The Royal Society of Chemistry [year]
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Page 2 of 7
[Zn4O(L1)3] was synthesized by dissolving 30 mg of H2L1Ac
and 85 mg of Zn(NO3)2·6H2O (4 eq) in N,N’-diethylformamide
(DEF) (4.5 mL) in a tightly-capped 20 mL glass vial.† An aliquot
of 2.5 M aqueous NaOH solution (30 μL) was then added and the
solution heated at 90°C for 20 mins. The solution was then
transferred to a new 20 mL vial and heated at 100°C for 36 hours
yielding transparent pale yellow, cubic crystals of [Zn4O(L1)3]
with an empirical formula of (C42H24O19Zn4). Other more bulky
ester protected links (H2L1iBu) also generated the same non-
10 interpenetrated material. The structure of [Zn4O(L1)3] was
determined by single crystal X-ray diffraction. As anticipated, the
framework of [Zn4O(L1)3] is of cubic topology with Zn4O
clusters positioned at each of the vertices and biphenyl moieties
forming the organic backbone (Scheme 1). The formation of a
15 non-interpenetrated 3-D network is noteworthy given the size of
the pore channels (15.4 Å), and the lack of significant steric
congestion within the pores arising from the linker.
60 [Zn4O(L1)3] and -[Zn4O(L1)3] were treated with 0.05 M
solutions of CuCl2·2H2O (Scheme 1(d)). Within minutes,
crystalline samples of both materials took on a green colour
associated with complexation of copper ions by the links. The
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rate of complexation is consistent with binding to the free diol
65 links rather than incorporation into the metal-oxide nodes of the
framework.12 This colour was retained upon washing. Ester
protected [Zn4O(L1Ac)3] and -[Zn4O(L1Ac)3], synthesised by the
same methods for the hydrolysed MOFs but without the addition
of base, absorbed copper(II) ions although these could be readily
70 removed by washing. Similar behaviour was also observed for -
[Zn4O(L2)3] containing methoxy groups on the links (H2L2).
5
To ensure that the single crystal structure was representative of
the bulk material, samples of [Zn4O(L1)3] were examined by
20 powder X-ray diffraction (PXRD, Fig. S3).† The resultant peak
patterns were consistent with that expected for
a non-
interpenetrated framework. A further noteworthy feature of
[Zn4O(L1)3] is that the acetyl protecting groups are hydrolysed
during the one-pot synthesis to afford pores decorated with
25 accessible hydroxyl functional groups. 1H NMR spectra of
digested [Zn4O(L1)3] reveals all links have been fully deprotected
during the course of the reaction. Examples of permanently
porous MOFs synthesized from alcohol functionalized links are
uncommon,7, 8 as the free oxygen atoms can participate in metal
30 coordination.10 For example, we could not form a characterisable
MOF from only deprotected ligand (H2L1) and Zn(NO3)2·6H2O,
although MOFs with different topologies could be formed with
other metals or by incorporating other links. The choice of
reaction conditions is also critically important to the nature of the
35 product. Using an analogous procedure, but with DMF as the
solvent, we obtained phase-pure interpenetrated -[Zn4O(L1)3]
from both H2L1Ac and more sterically-demanding H2L1iBu. Unit
cell determinations for -[Zn4O(L1)3] indicated that in this case
the framework is two-fold interpenetrated. In addition, the PXRD
40 pattern obtained from bulk samples of -[Zn4O(L1)3] closely
matched that of the calculated peak positions and intensities for a
lower-symmetry two-fold interpenetrated framework.
Figure 1. N2 adsorption isotherms at 77 K for [Zn4O(L1)3]
(blue), and -[Zn4O(L1)3] (red). Filled and open circles represent
75 adsorption and desorption points, respectively.
The copper(II)-metallated forms, [Zn4O(L1)3]Cu and -
[Zn4O(L1)3]Cu, were analysed by Energy Dispersive
Spectroscopy (EDS). EDS confirmed the loading of Cu in
[Zn4O(L1)3]Cu and -[Zn4O(L1)3]Cu to have Cu:Zn ratios of
80 1:5 and 1:4 (27% and 33% of diol sites), respectively. EDS
analysis also revealed that both materials contained no chloride
and TGA-FTIR indicated that DMF, potentially within the
coordination sphere of the Cu ions, was lost at high temperatures.
[Zn4O(L1)3]Cu and -[Zn4O(L1)3]Cu are still permanently
85 porous with BET surface areas of 1675 and 851 m2/g-1,
respectively (Fig. S13). These reductions in surface area are also
consistent with in-channel complexation of the copper species.
Further evidence for in-channel copper incorporation was
provided by a decrease in the pore size distributions for -
90 [Zn4O(L1)3]Cu compared to -[Zn4O(L1)3].†
To determine the permanent porosity of [Zn4O(L1)3] and -
[Zn4O(L1)3] 77 K N2 isotherms were carried out on the respective
45 evacuated frameworks (Fig. 1). Based on their isotherms, the
BET surface areas of each material were calculated to be 2631
and 1790 m2/g-1 for [Zn4O(L1)3] and -[Zn4O(L1)3],
respectively. The isotherm of -[Zn4O(L1)3] is best described as
Type I, which is indicative of microporosity. A step in the
50 isotherm of [Zn4O(L1)3] is observed in the low pressure region
between P/P0 of 0.020 and 0.045 and can be attributed to the
larger pore volume expected for the non-interpenetrated
framework (Scheme 1(b) and (c)).11 Notably, the synthetic
procedure and activation conditions utilized in the present work
55 yielded a surface area for [Zn4O(L1)3] that is the highest reported
To further determine diol accessibility, the effect of hydroxyl
groups on CO2 adsorption was determined by collecting CO2
isotherms at 273 and 298 K on -[Zn4O(L1)3]. It is noteworthy
that the polar hydroxyl groups significantly increase the enthalpy
95 of adsorption for CO2 across the entire coverage range with
respect to non-functionalised IRMOF-1 (Fig. 2).13 This dramatic
increase demonstrates that the polar hydroxyl groups are
accessible to adsorbates and is due to the surface chemistry given
the similarity in pore sizes of the respective materials.
100
A one-pot synthesis of a non-interpenetrated 3D pillared MOF
with organic links bearing non-coordinating hydroxyl groups has
been previously reported.14 In that case, the authors suggest that
framework interpenetration was suppressed by the steric bulk of
the protecting groups. Similar observations on the control of
105 interpenetration in the IRMOF series have been made.15,16 Herein
however we observe that protecting groups of the same size
for
a
material based on an IRMOF-9 framework.
Thermogravimmetric analyses indicate [Zn4O(L1)3] and -
[Zn4O(L1)3] are thermally stable to 600 and 720 K respectively.†
To demonstrate accessibility of the diol chelating sites,
2
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45 b CSIRO Materials Science and Engineering, Clayton, VIC 3168,
Australia
† Electronic Supplementary Information (ESI) available: synthesis of
H2L1, H2L1Ac, H2L1iBu, and H2L2; synthetic details and activation
conditions for the MOFs; 1H NMR digestion studies; TGA and EDS data;
View Online
50 and crystal data and refinement details for [Zn4O(L1)3] and -
[Zn4O(L2)3]. See DOI: 10.1039/b000000x/.
1. S. Kitagawa, R. Kitaura, S-I. Noro, Angew. Chem. Int. Ed., 2004, 43,
2334; G. Ferey, Chem. Soc. Rev., 2008, 37, 191.
55 2. (a) M. Eddaoudi, J. Kim, N. L. Rosi, D. T. Vodak, J. Wachter, M.
O'Keeffe, O. M. Yaghi, Science, 2002, 295, 469; (b) H. Deng, C. J.
Doonan, H. Furukawa, R. B. Ferreira, J. Towne, C. B. Knobler, B.
Wang, O. M. Yaghi, Science, 2010, 327, 846.
3. K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D.
60
65
70
75
80
Bloch, Z. R. Herm, T.-H. Bae, J. R. Long, Chem. Rev., 2012, 112,
724; J.-R. Li, J. Sculley, H.-C. Zhou, Chem. Rev., 2012, 112, 869.
4. A. Corma, H. Garcia, F. X. Llabres, I. Xamena, Chem. Rev., 2010,
110, 4607; L. Ma, C. Abney, W. Lin, Chem. Soc. Rev., 2009, 38,
1248; A. U. Czaja, N. Trukhan, U. Müller, Chem. Soc. Rev., 2009,
38, 1284.
Figure 2. CO2 enthalpy of adsorption for
IRMOF-1.13
-[Zn4O(L1)3], and
controllably yield non-interpenetrated and interpenetrated
materials but in different solvents. This led us to propose that the
hydrolysis rate in the different solvents had a critical role in
control of interpenetration; specifically, reactions in DMF led to
faster hydrolysis and an interpenetrated product. Monitoring the
rate of hydrolysis of the ester protecting group for framework
5. O. M. Yaghi, Nature Materials, 2007, 6, 92.; S. R. Batten, R.
Robson, Angew. Chem. Int. Ed., 1998, 37, 1460.
5
6. J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon, K. Kim,
Nature, 2000, 404, 982; C.-D. Wu, A. Hu, L. Zhang, W. Lin, J. Am.
Chem. Soc., 2005, 127, 8940; B. Chen, L. Wang, Y. Xiao, F. R.
Fronczek, M. Xue, Y. Cui, G. Qian, Angew. Chem. Int. Ed., 2009, 48,
500; X.-P. Zhou, Z. Xu, M. Zeller, A. D. Hunter, Chem. Commun.,
2009, 5439; E. D. Bloch, D. Britt, C. Lee, C. J. Doonan, F. J. Uribe-
Romo, H. Furukawa, J. R. Long, O. M. Yaghi, J. Am. Chem. Soc.,
2010, 132, 14382.
7. K. L. Mulfort, O. K. Farha, C. L. Stern, A. A. Sarjent, J. T. Hupp, J.
Am. Chem. Soc., 2009, 131, 3866; T. Devic, P. Horcajada, C. Serre,
F. Salles, G. Maurin, B. Moulin, D. Heurtaux, G. Clet, A. Vimont, J-
M, Greneche, B. Le Ouay, F. Moreau, E. Magnier, Y. Filinchuk, J.
Marrot, J-C, Lavalley, M. Daturi, G. Ferey, J. Am. Chem. Soc., 2010,
132, 1127; K. S. Jeong, Y. B. Go, S. M. Shin, S. J. Lee, J. Kim, O.
M. Yaghi, N. Jeong, Chem. Sci., 2011, 2, 877.
10 synthesis reactions using H2L1Ac in DMF and DEF revealed this
was not the case (Fig. S7). In fact, crystal growth (6-8 hrs)
proceeds considerably faster than link deprotection (up to 36 hrs)
indicating the links are deprotected within the framework. While
Kim and co-workers have demonstrated that MOF topology is
15 sensitive to small changes in the reaction pH,17 we can rule out
this effect for the diol systems as both [Zn4O(L1)3] and -
[Zn4O(L1)3] can be synthesised in the presence of base and acid.
Also in contrast to previous work,2a, 18 the reactions give phase
pure products independent of concentration. Thus, these
20 observations, coupled with the fact that H2L1Ac and H2L1iBu form
both types of frameworks, point to steric effects deriving from
both the link and the solvent having a role in the control of
interpenetration. Recently, solvent-mediated control of
interpenetration has been observed for other MOF systems.19
This work describes the controlled ‘one-pot’ syntheses of
MOF materials with two different pore structures bearing
accessible alcohol functional groups. Our studies show that
suppression of framework interpenetration and control of pore
architectures is not only the result of simple steric arguments for
30 the ligand but points toward other mechanisms such as solvent
‘templating’. These findings elucidate novel methodology for
synthesizing alcohol functionalized MOFs with controlled pores
sizes whereby high molecular weight solvents may be used to
control interpenetration. We are currently exploring masking
35 group strategies and solvent combinations to facilitate
simultaneous control of pore chemistry and interpenetration in
MOFs constructed from longer organic links.
8. Z. Wang, S. M. Cohen, Chem. Soc. Rev., 2009, 38, 1315; S. M.
Cohen, Chem. Rev., 2012, 112, 970.
85 9. M. J. Ingleson, J. P. Barrio, J. B. Guilbaud, Y. Z. Khimyak, M. J.
Rosseinsky, Chem. Commun., 2008, 23, 2680.
10. S. R. Caskey, A. G. Wong-Foy, A. J. Matzger, J. Am. Chem. Soc.,
2008, 130, 10870; N. L. Rosi, J. Kim, M. Eddaoudi, B. L. Chen, M.
O’Keeffe, O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 1504.
90 11. K. S. Walton, R. Q. Snurr, J. Am. Chem. Soc., 2007, 129, 8552.
12. C. K. Brozek, M. Dincă, Chem. Sci., 2012, 3, 2110; X. Song, T. K.
Kim, H. Kim, D. Kim, S. Jeong, H. R. Moon, M. S. Lah Chem.
Mater. 2012, 10.1021/cm301605w.
25
13. J.-S. Choi, W.-J. Son, J. Kim, W.-S. Ahn, Micropor. Mesopor.
95
100
105
110
Mater. 2008, 116, 72.
14. T. Yamada, H. Kitagawa , J. Am. Chem. Soc., 2009, 131, 6312.
15. J. M. Roberts, O. K. Farha, A. A. Sarjeant, J. T. Hupp, K. A. Scheidt,
Cryst. Growth Des., 2011, 11, 4747.
16. R. K. Deshpande, J. L. Minnaar, S. G. Telfer, Angew. Chem., Int. Ed.,
2010, 49, 4598; R. K. Deshpande, G. I. N. Waterhouse, G. B.
Jameson, S. G. Telfer, Chem. Commun., 2012, 48, 1574; A. S. Gupta,
R. K. Deshpande, L. Liu, G. I. N. Waterhouse, S. G. Telfer,
CrystEngComm, 2012, 10.1039/c2ce25854d.
17. H. Kim, S. Das, M. G. Kim, D. N. Dybtsev, Y. Kim, K. Kim, Inorg.
Chem., 2011, 50, 3691.
18. J. Zhang, L. Wojtas, R. W. Larsen, M. Eddaoudi, M. J. Zaworotko, J.
Am. Chem. Soc., 2009, 131, 17040.
MRH, CJD and CJS gratefully acknowledge the Australian
Research Council for funding.
19. M. Guo, Z.-M. Sun, J. Mater. Chem., 2012, 22, 15939; J. Duan, J.
Bai, B. Zheng, Y. Li, W. Ren, Chem. Commun., 2011, 47, 2556; L.
Ma, W. Lin, J. Am. Chem. Soc., 2008, 130, 13834; J. M. Falkowski,
C. Wang, S. Liu, W. Lin, Angew. Chem. Int. Ed., 2011, 50, 8674.
40 Notes and references
a School of Chemistry & Physics, The University of Adelaide, Adelaide,
Australia. CJD: Phone +61 8 8303 5770. Fax. +61 8 8303 4358. Email:
christian.doonan@adelaide.edu.au; CJS: Phone +61 8 8303 7406. Fax.
+61 8 8303 4358. Email: christopher.sumby@adelaide.edu.au
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ARTICLE TYPE
Control of Framework Interpenetration for in situ Modified Hydroxyl
Functionalised IRMOFs
Damien Rankine,a Antonio Avellaneda,a Matthew R. Hill,b Christian J. Doonan*,a and Christopher J.
Sumby*,a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
30 anchoring catalytic moieties to the framework need to be
established. Two major challenges arise; 1) Increasing the pores
sizes of topologically identical MOFs commonly affords
interpenetration5 and, 2) utilizing organic links which bear metalꢀ
coordinating groups can lead to unwanted side reactions or hinder
35 the formation of the desired networks. Thus, frameworks directly
synthesized with free metalꢀcoordinating groups are rare.6ꢀ8
By intimate control of reaction conditions, phase-pure
crystalline porous metal-organic framework materials
[Zn4O(L)3] with interpenetrated and non-interpenetrated
10 structures can be synthesised. Under certain conditions, these
reactions occur with concomitant deprotection of masked
alcohols located on the organic links which yield accessible
‘metal-binding’ functional groups within the frameworks.
Recently, postꢀsynthetic modification (PSM) has been
developed as a strategy to overcome these challenges and yield
open framework materials imbued with novel functionality.8 A
40 potential drawback of the PSM method is that performing
multiple reactions on the MOF crystals can lead to structural
decomposition and/or diminished surface area.9 Thus, to ensure
the MOFs retain optimal porosity and crystallinity it is preferable
that the most efficient synthetic procedure is employed. Here we
45 report the controlled oneꢀpot synthesis and in situ ester hydrolysis
of nonꢀinterpenetrated and interpenetrated MOFs, [Zn4O(L1)3]
and αꢀ[Zn4O(L1)3], comprised of biphenylꢀ2,2’ꢀdiol links (L1)
(Scheme 1(a)). Notably, both materials can be synthesized in a
singleꢀstep procedure. It is also noteworthy that the metal coꢀ
50 ordinating groups are freely accessible in both forms which we
demonstrate by the complexation of copper ions and enhanced
enthalpy of adsorption for αꢀ[Zn4O(L1)3] over IRMOFꢀ1.
Metalꢀorganic frameworks (MOFs) are constructed by a modular
15 synthetic approach whereby organic links and metal clusters are
assembled into porous extended networks via ‘oneꢀpot’
reactions.1 This has led to the development of design principles
that yield precise control of pore dimensions and functionality by
utilizing links of identical connectivity but different lengths or
20 nonꢀstructural functional groups.2 Due to their high surface areas,
tuneable structure metrics and chemically mutable organic
building blocks, MOFs have been identified as materials with
great potential for gas separations3 and sizeꢀselective
heterogeneous catalysis.4 To fully realise their catalytic potential
25 MOFs with pores sufficiently large enough to simultaneously
allow anchoring of catalytic moieties and diffusion of chemical
reactants into the framework is requisite. Furthermore, given that
the majority of the metal clusters that comprise the ‘joints’ of
MOF structures are catalytically benign, reliable methods for
Scheme 1. (a) Synthesis of [Zn4O(L1)3] and αꢀ[Zn4O(L1)3]. Inset: Spaceꢀfilling representations of the pores in (b) [Zn4O(L1)3] and (c)
55 αꢀ[Zn4O(L2)3]. (d) Images from the metallation studies to generate αꢀ[Zn4O(L1)3]⊃Cu and show accessibility of the hydroxyl groups.
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[Zn4O(L1)3] was synthesized by dissolving 30 mg of H2L1Ac
and 85 mg of Zn(NO3)2ꢁ6H2O (4 eq) in N,N’ꢀdiethylformamide
(DEF) (4.5 mL) in a tightlyꢀcapped 20 mL glass vial.† An aliquot
of 2.5 M aqueous NaOH solution (30 ꢂL) was then added and the
solution heated at 90°C for 20 mins. The solution was then
transferred to a new 20 mL vial and heated at 100°C for 36 hours
yielding transparent pale yellow, cubic crystals of [Zn4O(L1)3]
with an empirical formula of (C42H24O19Zn4). Other more bulky
ester protected links (H2L1iBu) also generated the same nonꢀ
10 interpenetrated material. The structure of [Zn4O(L1)3] was
determined by single crystal Xꢀray diffraction. As anticipated, the
framework of [Zn4O(L1)3] is of cubic topology with Zn4O
clusters positioned at each of the vertices and biphenyl moieties
forming the organic backbone (Scheme 1). The formation of a
15 nonꢀinterpenetrated 3ꢀD network is noteworthy given the size of
the pore channels (15.4 Å), and the lack of significant steric
congestion within the pores arising from the linker.
60 [Zn4O(L1)3] and αꢀ[Zn4O(L1)3] were treated with 0.05 M
solutions of CuCl2ꢁ2H2O (Scheme 1(d)). Within minutes,
crystalline samples of both materials took on a green colour
associated with complexation of copper ions by the links. The
rate of complexation is consistent with binding to the free diol
65 links rather than incorporation into the metalꢀoxide nodes of the
framework.12 This colour was retained upon washing. Ester
protected [Zn4O(L1Ac)3] and αꢀ[Zn4O(L1Ac)3], synthesised by the
same methods for the hydrolysed MOFs but without the addition
of base, absorbed copper(II) ions although these could be readily
70 removed by washing. Similar behaviour was also observed for αꢀ
[Zn4O(L2)3] containing methoxy groups on the links (H2L2).
5
To ensure that the single crystal structure was representative of
the bulk material, samples of [Zn4O(L1)3] were examined by
20 powder Xꢀray diffraction (PXRD, Fig. S3).† The resultant peak
patterns were consistent with that expected for
a nonꢀ
interpenetrated framework. A further noteworthy feature of
[Zn4O(L1)3] is that the acetyl protecting groups are hydrolysed
during the oneꢀpot synthesis to afford pores decorated with
25 accessible hydroxyl functional groups. 1H NMR spectra of
digested [Zn4O(L1)3] reveals all links have been fully deprotected
during the course of the reaction. Examples of permanently
porous MOFs synthesized from alcohol functionalized links are
uncommon,7, 8 as the free oxygen atoms can participate in metal
30 coordination.10 For example, we could not form a characterisable
MOF from only deprotected ligand (H2L1) and Zn(NO3)2ꢁ6H2O,
although MOFs with different topologies could be formed with
other metals or by incorporating other links. The choice of
reaction conditions is also critically important to the nature of the
35 product. Using an analogous procedure, but with DMF as the
solvent, we obtained phaseꢀpure interpenetrated αꢀ[Zn4O(L1)3]
from both H2L1Ac and more stericallyꢀdemanding H2L1iBu. Unit
cell determinations for αꢀ[Zn4O(L1)3] indicated that in this case
the framework is twoꢀfold interpenetrated. In addition, the PXRD
40 pattern obtained from bulk samples of αꢀ[Zn4O(L1)3] closely
matched that of the calculated peak positions and intensities for a
lowerꢀsymmetry twoꢀfold interpenetrated framework.
Figure 1. N2 adsorption isotherms at 77 K for [Zn4O(L1)3]
(blue), and αꢀ[Zn4O(L1)3] (red). Filled and open circles represent
75 adsorption and desorption points, respectively.
The copper(II)ꢀmetallated forms, [Zn4O(L1)3]⊃Cu and αꢀ
[Zn4O(L1)3]⊃Cu, were analysed by Energy Dispersive
Spectroscopy (EDS). EDS confirmed the loading of Cu in
[Zn4O(L1)3]⊃Cu and αꢀ[Zn4O(L1)3]⊃Cu to have Cu:Zn ratios of
80 ∼1:5 and ∼1:4 (∼27% and ∼33% of diol sites), respectively. EDS
analysis also revealed that both materials contained no chloride
and TGAꢀFTIR indicated that DMF, potentially within the
coordination sphere of the Cu ions, was lost at high temperatures.
[Zn4O(L1)3]⊃Cu and αꢀ[Zn4O(L1)3]⊃Cu are still permanently
85 porous with BET surface areas of 1675 and 851 m2/gꢀ1,
respectively (Fig. S13). These reductions in surface area are also
consistent with inꢀchannel complexation of the copper species.
Further evidence for inꢀchannel copper incorporation was
provided by a decrease in the pore size distributions for αꢀ
90 [Zn4O(L1)3]⊃Cu compared to αꢀ[Zn4O(L1)3].†
To determine the permanent porosity of [Zn4O(L1)3] and αꢀ
[Zn4O(L1)3] 77 K N2 isotherms were carried out on the respective
45 evacuated frameworks (Fig. 1). Based on their isotherms, the
BET surface areas of each material were calculated to be 2631
and 1790 m2/gꢀ1 for [Zn4O(L1)3] and αꢀ[Zn4O(L1)3],
respectively. The isotherm of αꢀ[Zn4O(L1)3] is best described as
Type I, which is indicative of microporosity. A step in the
50 isotherm of [Zn4O(L1)3] is observed in the low pressure region
between P/P0 of 0.020 and 0.045 and can be attributed to the
larger pore volume expected for the nonꢀinterpenetrated
framework (Scheme 1(b) and (c)).11 Notably, the synthetic
procedure and activation conditions utilized in the present work
55 yielded a surface area for [Zn4O(L1)3] that is the highest reported
To further determine diol accessibility, the effect of hydroxyl
groups on CO2 adsorption was determined by collecting CO2
isotherms at 273 and 298 K on αꢀ[Zn4O(L1)3]. It is noteworthy
that the polar hydroxyl groups significantly increase the enthalpy
95 of adsorption for CO2 across the entire coverage range with
respect to nonꢀfunctionalised IRMOFꢀ1 (Fig. 2).13 This dramatic
increase demonstrates that the polar hydroxyl groups are
accessible to adsorbates and is due to the surface chemistry given
the similarity in pore sizes of the respective materials.
100
A oneꢀpot synthesis of a nonꢀinterpenetrated 3D pillared MOF
with organic links bearing nonꢀcoordinating hydroxyl groups has
been previously reported.14 In that case, the authors suggest that
framework interpenetration was suppressed by the steric bulk of
the protecting groups. Similar observations on the control of
105 interpenetration in the IRMOF series have been made.15,16 Herein
however we observe that protecting groups of the same size
for
a
material based on an IRMOFꢀ9 framework.
Thermogravimmetric analyses indicate [Zn4O(L1)3] and αꢀ
[Zn4O(L1)3] are thermally stable to 600 and 720 K respectively.†
To demonstrate accessibility of the diol chelating sites,
2
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45 b CSIRO Materials Science and Engineering, Clayton, VIC 3168,
Australia
*This article is part of the ChemComm 'Metalꢀorganic frameworks' web
themed issue.
† Electronic Supplementary Information (ESI) available: synthesis of
50 H2L1, H2L1Ac, H2L1iBu, and H2L2; synthetic details and activation
conditions for the MOFs; 1H NMR digestion studies; TGA and EDS data;
and crystal data and refinement details for [Zn4O(L1)3] and αꢀ
[Zn4O(L2)3]. See DOI: 10.1039/b000000x/.
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2334; G. Ferey, Chem. Soc. Rev., 2008, 37, 191.
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O'Keeffe, O. M. Yaghi, Science, 2002, 295, 469; (b) H. Deng, C. J.
Doonan, H. Furukawa, R. B. Ferreira, J. Towne, C. B. Knobler, B.
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Wang, O. M. Yaghi, Science, 2010, 327, 846.
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Bloch, Z. R. Herm, T.ꢀH. Bae, J. R. Long, Chem. Rev., 2012, 112,
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Figure 2. CO2 enthalpy of adsorption for
αꢀ[Zn4O(L1)3], and
IRMOFꢀ1.13
controllably yield nonꢀinterpenetrated and interpenetrated
materials but in different solvents. This led us to propose that the
hydrolysis rate in the different solvents had a critical role in
control of interpenetration; specifically, reactions in DMF led to
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rate of hydrolysis of the ester protecting group for framework
5
5. O. M. Yaghi, Nature Materials, 2007, 6, 92.; S. R. Batten, R.
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was not the case (Fig. S7). In fact, crystal growth (6ꢀ8 hrs)
proceeds considerably faster than link deprotection (up to 36 hrs)
indicating the links are deprotected within the framework. While
Kim and coꢀworkers have demonstrated that MOF topology is
15 sensitive to small changes in the reaction pH,17 we can rule out
this effect for the diol systems as both [Zn4O(L1)3] and αꢀ
[Zn4O(L1)3] can be synthesised in the presence of base and acid.
Also in contrast to previous work,2a, 18 the reactions give phase
pure products independent of concentration. Thus, these
20 observations, coupled with the fact that H2L1Ac and H2L1iBu form
both types of frameworks, point to steric effects deriving from
both the link and the solvent having a role in the control of
interpenetration. Recently, solventꢀmediated control of
interpenetration has been observed for other MOF systems.19
This work describes the controlled ‘oneꢀpot’ syntheses of
MOF materials with two different pore structures bearing
accessible alcohol functional groups. Our studies show that
suppression of framework interpenetration and control of pore
architectures is not only the result of simple steric arguments for
30 the ligand but points toward other mechanisms such as solvent
‘templating’. These findings elucidate novel methodology for
synthesizing alcohol functionalized MOFs with controlled pores
sizes whereby high molecular weight solvents may be used to
control interpenetration. We are currently exploring masking
35 group strategies and solvent combinations to facilitate
simultaneous control of pore chemistry and interpenetration in
MOFs constructed from longer organic links.
75
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MRH, CJD and CJS gratefully acknowledge the Australian
Research Council for funding.
110 19. M. Guo, Z.ꢀM. Sun, J. Mater. Chem., 2012, 22, 15939; J. Duan, J.
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40 Notes and references
a School of Chemistry & Physics, The University of Adelaide, Adelaide,
Australia. CJD: Phone +61 8 8303 5770. Fax. +61 8 8303 4358. Email:
christian.doonan@adelaide.edu.au; CJS: Phone +61 8 8303 7406. Fax.
+61 8 8303 4358. Email: christopher.sumby@adelaide.edu.au
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