Transformation from non- to double-interpenetration in robust Cd(ii) doubly-pillared-layered metal-organic frameworks

Article abstract of DOI:10.1039/c4cc07112c

Two known Cd(ii) non-interpenetrated doubly-pillared metal organic frameworks are shown to undergo a change of degree of interpenetration upon loss of lattice solvent molecules to yield doubly-interpenetrated frameworks. The conversion is inferred from powder X-ray diffraction (PXRD) patterns and supported by additional evidence from single-crystal diffraction (SCD) data. This journal is

Full text of DOI:10.1039/c4cc07112c

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Transformation from non- to double-interpenetration
in robust Cd(II) doubly-pillared-layered metal–organic
frameworks†
Cite this: Chem. Commun., 2014,
50, 14543
Received 9th September 2014,
Accepted 28th September 2014
Himanshu Aggarwal, Prem Lama and Leonard J. Barbour*
DOI: 10.1039/c4cc07112c
www.rsc.org/chemcomm
Two known Cd(II) non-interpenetrated doubly-pillared metal organic been reported that exhibit less interpenetration than their network
frameworks are shown to undergo a change of degree of interpenetration topologies can allow.13 However, the phenomenon of dynamic
upon loss of lattice solvent molecules to yield doubly-interpenetrated conversion between different orders of interpenetration has received
frameworks. The conversion is inferred from powder X-ray diffraction little attention to date.
(PXRD) patterns and supported by additional evidence from single-crystal
diffraction (SCD) data.
We recently reported how a highly porous doubly-interpenetrated
MOF converts to a less porous triply-interpenetrated form upon loss
of solvent molecules from the channels.14 Since it seems that
We1 and others2 have long been interested in solid–solid phase switching of degree of interpenetration upon activation is generally
transformations resulting from guest insertion/removal into/from assumed to be improbable, we became interested in investigating
inclusion compounds. Indeed, such phenomena are particularly whether such occurrences are restricted to only a handful of specific
important for the utility of porous crystalline systems such as systems, or if the phenomenon might be more general. We thus
metal–organic frameworks (MOFs). Some of the most important expanded our study to include a supposedly more robust system,
attributes of a MOF include its rigidity, free volume and surface area. and its analogue.
Typically, a MOF would be prepared3 (usually under solvothermal
Here we report on our investigation of two closely-related
conditions) and then ‘activated’ by removal of the solvent molecules. non-interpenetrated MOFs which have previously been described by
In ideal cases the material would retain its framework connectivity other groups. Frameworks 1 and 2 have the formulae [Cd(tp)-
without collapse or distortion of the network – MOFs with metal (4,40-bpy)]11,15 and [Cd(atp)(4,40-bpy)],10 respectively (Scheme 1,
cluster nodes (i.e. secondary building units) are generally thought to tp = terephthalate; 4,40-bpy = 4,40-bipyridine and atp =
be more robust to the activation process.4
2-aminoterephthalate).
Interpenetration5 or catenation of MOFs6 is a well-known
These frameworks are comprised of Cd(II) ions coordinated
phenomenon; when the geometry of the system allows, two or to carboxylate linkers to form two dimensional networks, which
more independent frameworks can become interwoven to yield are further linked by means of 4,40-bipyridine ligands to yield
double-, triple- or even higher-order interpenetration. For a given infinite three-dimensional doubly-pillared structures. Both systems
framework the free volume and surface area depend on the total have been reported to form non-interpenetrated as well as doubly-
number of nets involved. Since gas sorption7 is considered one interpenetrated structures (Fig. 1). However, the possibility has not
of the major applications of MOFs, interpenetration is usually been suggested that the corresponding structures might interconvert
undesirable because it results in reduction of the guest-accessible by switching their degrees of interpenetration.
space8 (i.e. less porosity). To overcome this problem, several groups
have tried to control the degree of interpenetration using various
techniques such as liquid epitaxy,9 variation of temperature10,11
and concentration,11 and the use of bulkier solvents for crystal-
lisation.12 As a result of these strategies, a number of MOFs have
Department of Chemistry and Polymer Science, University of Stellenbosch,
Private Bag X1, Matieland, 7602, South Africa. E-mail: ljb@sun.ac.za
† Electronic supplementary information (ESI) available: PXRD patterns, thermal
analysis, crystallographic information, Rietveld refinement parameters and addi-
Scheme 1 Synthetic scheme for the preparation of frameworks 1 and 2.
Chem. Commun., 2014, 50, 14543--14546 | 14543
tional figures. See DOI: 10.1039/c4cc07112c
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Fig. 1 Packing diagrams of (a) non-interpenetrated 1, (b) doubly-inter-
penetrated 1, (c) non-interpenetrated 2 and (d) doubly-interpenetrated 2.
As expected, the non-interpenetrated structures are more
porous than their corresponding doubly-interpenetrated forms.
As part of our investigation we prepared the non-interpenetrated
form of framework 1 using the reported procedure.11 The crystal
structure possesses free volume of 59.3% of the unit cell11,16 with
dimethyl formamide (dmf) and water molecules occupying the
channels. Thermogravimetric analysis (TGA) shows continuous
Fig. 2 (a) Simulated PXRD pattern of non-interpenetrated 1, (b) experi-
mental PXRD pattern of as-synthesised 1, (c) experimental PXRD pattern of
1 activated at 150 1C and (d) simulated PXRD pattern of the reported crystal
structure of doubly-interpenetrated 1.
weight loss up to 160 1C, which corresponds to the loss of all the 2-aminoterephthalate, but the overall coordination environment
solvent molecules. The PXRD pattern of the bulk material was around the central metal atoms remains the same. We prepared
compared with that simulated from the reported SCD structure11 to crystals of non-interpenetrated [Cd(atp)(4,40-bpy)] by following
establish phase purity of the crystals. In order to completely remove the reported procedure;10 the PXRD pattern of the as-synthesised
the solvent molecules from the channels, the as-synthesised crystals crystals was compared with the simulated PXRD pattern for the
were evacuated at 150 1C under dynamic vacuum for 12 hours. known structure in order to confirm phase purity of the bulk
The crystals remained transparent upon desolvation, yielding a material. The free volume in framework 2 is 61.2% of the unit
PXRD pattern different from that of the as-synthesised material cell10,16 and TGA shows continuous weight loss until 220 1C,
but matching that simulated from the reported twofold- corresponding to the complete loss of dmf and water molecules
interpenetrated crystal structure of 115 (Fig. 2). This finding from the channels. Similar to the treatment of 1, as-synthesised
indicates that the as-synthesised non-interpenetrated framework bulk crystals of 2 were evacuated at 150 1C under dynamic
transforms to its doubly-interpenetrated form upon activation.
vacuum for 12 hours. The PXRD pattern of the resulting material
This result is consistent with our previous observation that the differed considerably from that of the as-synthesised crystals
loss of solvent from MOF channels can result in switching of degree (ESI†) but it did not match that simulated for the reported
of interpenetration, showing that the phenomenon is not limited to doubly-interpenetrated SCD structure of 2.10 Further activation
only one system. Although we attempted to record SCD intensity was carried out at 270 1C, after which the PXRD pattern matched
data for the activated crystals, they had been subjected to a relatively that of 2 (Fig. 3). The harsher conditions required for the
high activation temperature and the quality of the data was therefore activation of 2 can be attributed to the presence of the amino
insufficient for acceptable structure solution and refinement. Never- groups directed towards the framework channels; i.e. more
theless, comparison of the unit cell parameters of the activated energy is required for the process of solvent removal and
crystals with those of the reported twofold-interpenetrated structure concomitant disassembly/reassembly of the framework. As
confirmed that the crystals had indeed undergone a change in noted for the transformation of 1, it was possible to determine
degree of interpenetration (ESI†). It seems reasonable to propose the unit cell parameters of 2 by means of SCD data, but the
that the change in interpenetration occurs as a result of the empty diffraction record was of insufficient quality for rigorous crystal
space created upon solvent removal – the architecture of the frame- structural analysis (ESI†).
work can accommodate interpenetration and the coordination
linkages are sufficiently labile to allow the process to proceed once interpenetration of [Zn2(ndc)2(4,40-bpy)],14 we can postulate a
the space becomes available. possible mechanism for the transformations experienced by 1
Based on our previous report on the change in mode of
As a continuation of this study, we also investigated the and 2. It is well known that molecular crystal solvates are
closely analogous system 2, where terephthalate is replaced by generally stabilised by the presence of the solvent molecules.
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Fig. 4 Comparison of corresponding parts of an individual network
representing framework 1 in both its non-interpenetrating and doubly-
interpenetrating forms: arrangement of metal clusters and tp units in the (a) non-
interpenetrated and (b) doubly-interpenetrated forms and arrangement of the
4,40-bpy ligands in the (c) non-interpenetrated and (d) doubly-interpenetrated
forms.
Fig. 3 (a) Simulated PXRD pattern of non-interpenetrated 2, (b) experi-
mental PXRD pattern of as-synthesised 2, (c) experimental PXRD pattern of
2 activated at 270 1C and (d) simulated PXRD pattern of the reported
crystal structure of doubly-interpenetrated 2.
pyridyl rings. Upon activation of non-interpenetrated 1 at 150 1C,
rotation of two of the four terephthalate linkers (L2 and L4) takes
place. This is concomitant with coordination bond breaking and
That is, the solvent molecules play a structural role until they are formation within the metal cluster, thereby altering the orienta-
removed, whereupon the host molecules rearrange to form a closely- tions of the diagonally placed metal-cluster nodes C2 and C4. In
packed apohost phase. We believe that the same holds true for some the non-interpenetrated structure all four tp units have one
MOF structures; i.e. that the frameworks are partially supported by carboxylate coordinated to the metal centre in chelating mode
the solvent molecules. When an attempt is made to activate the whereas the second carboxylate group is coordinated in both
crystals by removing the solvent molecules, the vacant spaces thus bridging and chelating fashion. However, in the corresponding
created allow the coordination bonds to become distorted. In doubly-interpenetrated structure, both of the carboxylate groups
extreme cases, the coordination environment about the metal of L2 and L4 are coordinated to the metal centre in chelating
centre can undergo modification.17 In even more severe cases, the fashion only, whereas both of the carboxylate groups of L1 and
secondary building units may disassemble and reassemble, which L3 are coordinated in bridging as well as chelating mode.
could also involve changes in entanglement of the coordination Furthermore, the pyridyl rings of the 4,40-bpy ligands are twisted
networks.14,18 Such a process involves rapid bond dissociation and relative to one another with a dihedral angle of 38.7(8)1. Owing to
formation to yield a thermodynamically more stable structure with the changes in metal cluster nodes C2 and C4, all four dinuclear
a higher degree of interpenetration. It is interesting to note the Cd(II) clusters (C1 to C4) assume the same general orientation in
differences between the current systems and our previously the twofold-interpenetrated framework. During the transformation
reported examples where the change of interpenetration was from of 1 from being non-interpenetrated to doubly-interpenetrated
doubly- to triply- in singly-pillared-layered paddlewheel structures. the diagonal distances between the metal cluster centres (i.e. the
Systems 1 and 2 are more robust owing to their doubly-pillared- CdꢀꢀꢀCd centroid) changes from 21.29(7) ꢁ 12.95(7) Å to 20.17(4) ꢁ
layered structures and the change in interpenetration is from 16.11(3) Å (ESI†). We also note that the originally planar
non- to doubly-. Moreover, in the case of [Zn2(ndc)2(4,40-bpy)]14 geometry of the ring C1–C4 becomes distorted as a result of
the conversion took place under ambient conditions whereas the the transformation.
transformation requires elevated temperatures for 1 and 2.
A similar rearrangement of the metal clusters and ligand coordi-
Close inspection of the non-interpenetrated [Cd(tp)(4,40-bpy)] nation mode occurs during the transformation of 2 (Fig. 5).
structure shows that four dinuclear Cd(II) clusters along with The diagonal distances between the metal clusters change from
four tp units form a cyclic metallo-organic unit with cluster C1 in 21.10(3) ꢁ 13.70(2) Å to 20.48(6) ꢁ 15.95(2) Å on conversion from
the same orientation as C3 and cluster C2 in the same orienta- non-interpenetrated to twofold interpenetrated 2 (ESI†).
tion as C4 (Fig. 4). All of the aromatic units of the terephthalate
We have shown that robust doubly-pillared structures can
ligands (L1 to L4) are in the metal–carboxylate plane – i.e. also undergo change in degree of interpenetration upon loss of
parallel to (001) and the doubly-pillaring bipyridyl linkers are solvent molecules from the channels. The transformation occurs
parallel to [001], with a dihedral angle of 2.1(1)1 between the two at 150 1C in the case of framework 1, whereas the analogous
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14546 | Chem. Commun., 2014, 50, 14543--14546
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