Siloxane-based linkers in the construction of hydrogen bonded assemblies and porous 3D MOFs

Article abstract of DOI:10.1039/c7cc06022j

A siloxane-based hexacarboxylic acid (L1-H₆) has been prepared and applied in MOF construction. L1-H₆ itself crystallizes as an unusual interpenetrated 3D hydrogen-bonded framework. Reaction of L1-H₆ with Zn(ii) gave IMP-18-a 3D MOF incorporating Si-O-Si functionality. Cleavage of L1-H₆ gives a silanol-based triacid which is shown to give a coordination polymer (IMP-19) with Zn(ii).

Full text of DOI:10.1039/c7cc06022j

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Siloxane-based linkers in the construction of
hydrogen bonded assemblies and porous 3D
Cite this:DOI: 10.1039/c7cc06022j
MOFs†
Received 1st August 2017,
a
a
b
Luke C. Delmas,
Peter N. Horton,b Andrew J. P. White,
Simon J. Coles,
Accepted 22nd September 2017
Paul D. Lickiss *a and Robert P. Davies
*
a
DOI: 10.1039/c7cc06022j
rsc.li/chemcomm
A siloxane-based hexacarboxylic acid (L1-H6) has been prepared
and applied in MOF construction. L1-H6 itself crystallizes as an
unusual interpenetrated 3D hydrogen-bonded framework. Reaction
of L1-H6 with Zn(II) gave IMP-18 – a 3D MOF incorporating Si–O–Si
functionality. Cleavage of L1-H6 gives a silanol-based triacid which
is shown to give a coordination polymer (IMP-19) with Zn(II).
The rapid growth in research on metal–organic frameworks
(MOFs) has largely relied upon the use of commercially available
organic linkers.1,2 A further step towards the rational design of
functional MOFs (through fine-tuning of these materials’ topo-
logical and chemical properties) requires the synthesis of novel
ligands with unique structural and chemical features.3,4 We have
been interested for some time in the synthesis of highly branched
organosilicon linkers and their use in the assembly of MOFs,
largely due to their synthetic accessibility compared to their
carbon-centred analogues.5–10 However, despite the siloxane
linkage (Si–O–Si) being ubiquitous in many classes of materials
including zeolites,11,12 periodic mesoporous organosilicas,13,14
POSS hybrids and other porous materials,15,16 we were surprised
to find that there has been little interest in siloxane-based
Scheme 1 Synthesis of L1-H6 and L2-H3.
The synthesis of L1-H6 is outlined in Scheme 1. Firstly, tris(4-
linkers17–21 for MOFs, and that their incorporation into a porous bromophenyl)silane22 was converted to the corresponding hexa-
3D MOF has not been reported to date. We have thus recently substituted disiloxane via modification of an In(III)-catalysed
focussed upon the development of siloxane-based polycarboxy- protocol for hydrosilane oxidation.23 The sterically protected
lates, and report herein the synthesis of the novel pseudo- siloxane linkage withstood subsequent treatment with n-BuLi,
octahedral hexa(4-carboxyphenyl)disiloxane molecule (L1-H6), allowing reaction of the polylithiated intermediate with CO2 to
its unusual hydrogen bonded network assembly and its applica- afford L1-H6.8 Treatment of L1-H6 with base, followed by acid
tion in the preparation two new MOF materials.
work-up, gave the tris-carboxylic acid silanol derivate L2-H3. The
presence of strongly polar Si–OH groups in the pores of MOFs
could potentially lead to enhanced non-covalent interactions with
gas molecules giving rise to attractive adsorption properties.24,25
In some situations, polycarboxylic acids have been shown to
form porous frameworks on crystallisation through hydrogen
bonding.26 Moreover, some of these materials have demon-
strated potential for gas storage and separation applications.27,28
Given L1-H6 is a rare example of a hexatopic ligand exhibiting an
octahedral arrangement of carboxylic acid groups,8,29 we sought to
explore the solid-state structure of this compound. Crystallization of
a Department of Chemistry, Imperial College London, South Kensington,
London SW7 2AZ, UK. E-mail: r.davies@imperial.ac.uk, p.lickiss@imperial.ac.uk
b EPSRC Crystallographic Service, Department of Chemistry,
University of Southampton, Highfield, Southampton SO17 1BJ, UK
† Electronic supplementary information (ESI) available: Full experimental details
of the synthesis, characterisation data and the crystallographic protocols employed
in this study. PXRD and TGA plots for IMP-18 and IMP-19 and N2 sorption isotherm
for IMP-18. Details of additional topological analyses for structures are also
presented. CCDC 1566035–1566037. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c7cc06022j
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L1-H6 from a mixture of acetic acid and methanol afforded packing is achieved by mutual interpenetration of two additional
crystals suitable for X-Ray analysis (see ESI†). L1-H6 crystallizes nets to afford a triply interlocked system (Fig. 1c). In addition,
%
in the triclinic space group P1 (No. 2) and the unit cell contains one dimensional channels still remain along the [1 0 1] crystal
three crystallographically distinct molecules of L1-H6. Two direction and these are filled by infinite 1D-chains of L1-H6
of these are linear (with the central oxygen atoms sitting on molecules that hydrogen bond through only two carboxyl groups
centres of symmetry) while the third molecule is slightly bent as shown in Fig. 1b. The remaining four carboxylic acid groups
with a Si–O–Si bond angle of 170.76(18)1. In all three cases, the on these molecules do not engage in any inter-molecular inter-
tris(4-carboxyphenyl)silyl groups are staggered with respect to actions. The overall porosity for this structure is consequently
one another (looking down the respective Si–O–Si axes) disposing low, with a calculated solvent-accessible void volume of 4709 Å3
the carboxylic acid groups in a pseudo-octahedral geometry. per unit cell or just 22% of the unit cell volume.33 Efforts to
The acid groups in L1-H6 aggregate through dimeric hydrogen- exploit L1-H6 in the preparation of a non-interpenetrated hydrogen-
bonding interactions into a cubic crystalline network with bonded array are ongoing.
H-bonding OÁ Á ÁO distances in the range 2.57–2.69 Å, typical
We then turned our attention to the complexation of L1-H6
with transition metals for the generation of MOF materials.
of strong hydrogen bonds.30–32
The molecules of L1-H6 assemble via dimeric hydrogen Reaction of L1-H6 with Zn(NO3)2Á6H2O in DMF at 85 1C in a
bonding interactions to give a unique structure comprising of sealed vial for 2 days afforded colourless needle-like crystals of
triply interpenetrating 3D nets with 1D polymeric chains sited IMP-18, where IMP is short for Imperial College London. These
within the remaining pores (Fig. 1). The 3D assemblies can be crystals were characterised by single crystal X-ray analysis to be
considered as 6-c pcu nets with the 6-c vertices centred on the [Zn3(L1)(H2O)3]Á7DMF and the bulk purity of the sample was
siloxane oxygen of the L1-H6 molecules (Fig. 1a). All six octa- confirmed by powder X-ray diffraction (Fig. S3, ESI†).34 The
hedrally disposed carboxylic acids on the L1-H6 molecules are crystals were found to comprise of a porous 3D-connected MOF
involved in hydrogen bonding to neighbouring molecules within built from discrete bimetallic zinc paddle-wheel nodes linked
these nets. An alternative topological interpretation in which together by the fully deprotonated ligand L1 (Fig. 2). The siloxane
each 6-c vertex is replaced by two 4-c vertices, each centred on linkages in the L1 ligands are close to linear [+Si–O–Si 176.0(12)
a tetrahedral silicon atom, gives the derived 4-c dia net. The and 167.9(17)1] with staggered aryl groups, resulting in the six
calculated accessible void space33 for one of these 3D-structures carboxylate branches being arranged in a pseudo-octahedral
is approximately 85% of its overall volume, and satisfactory disposition. In order to better understand the framework topol-
ogy the Zn2-dimer can be reduced to a 4-c node and the L1 ligand
reduced to a 6-c node to give a (4,6)-c fsy basic net, rarely
encountered in MOFs.35,36 After theoretical removal of both the
coordinated and non-coordinated solvent, PLATON33 estimates
the solvent-accessible void volume for IMP-18 to be 9006 Å3 or
59% of the unit cell volume. MOFs with hexatopic linkers remain
relatively few in number and the majority of those reported
feature ligands of hexagonal shape.29,37 An alternative topologi-
cal description in which the 6-c ligand based vertex is replaced by
two 4-c silicon centred vertices gives a new (4,4,4)-connected
topology whose point symbol is {4Á63Á82}4{42Á62Á82}2{62Á84} (see
also Fig. S2, ESI†). PXRD measurements on a desolvated IMP-18
sample showed retention of the structure, albeit with slight
broadening of the diffraction peaks. Initial sorption analysis
studies on IMP-18 show a reversible type-I N2 adsorption
isotherm at 77 K (Fig. S5, ESI†) with a calculated BET surface
area of 358 m2 gÀ1
.
The siloxane linkage in L1-H6 is susceptible to base cleavage.
Indeed treatment of L1-H6 with an excess of 1 M NaOH affords,
after protonation, tris(4-carboxyphenyl)silanol (L2-H3). L2-H3 is
a novel and interesting linker since, in addition to the coordi-
nating abilities of the carboxylate groups, the Si–OH group is a
good hydrogen bond donor38 and a potential site for further
functionalization of the ligand (or post-synthetic modification
of derived MOFs). Silanol-based linkers are not common in the
MOF literature but Sun and co-workers have reported two 3D
frameworks based on biphenyl-substituted silanols.39,40
Fig. 1 (a) H-Bonding of L1-H6 molecules to afford a pcu net and the
equivalent topology diagram (b) arrangement of linear L1-H6 molecules to
form one-dimensional supramolecular polymeric chains (c) a topological
view of the triply interpenetrated network with 1D chains residing in the
pores along [1 0 1]. Colour scheme for chemical structures: O, red; C, grey;
Si, green; H-bonds represented by dashed lines.
The reaction of tricarboxylic acid L2-H3 with Zn(NO3)2Á6H2O
in a 2 : 1 mixture of DMF and H2O at 80 1C for 2 days afforded
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Fig. 3 (a) Structure of tetranuclear zinc SBUs in IMP-19 – disorder,
solvent molecules and non-silanol hydrogen atoms omitted for clarity.
Colour scheme: Zn, gold; O, red; C, grey; Si, green; H, pink. (b) Portion of
a single layer of IMP-19 showing pores along the crystallographic b axis.
(c) Schematic representation of the flu network showing the 6-connected
Zn SBU nodes (red) and the 4-connected silicon-based nodes (blue) with
hydrogen bonding interactions in yellow.
and the non-coordinated oxygen atom of the L2 monodentate
carboxylate group with a typical D–HÁ Á ÁA distance of 1.88 Å.41
Considering these hydrogens bonds as topological connections,
the overall supramolecular structure of IMP-19 is determined by
TOPOS42 to be a 4,8-connected net of flu topology. The IMP-19
framework has solvent-filled channels with the largest window
Fig. 2 (a) Coordination environment of L1 in IMP-18 and corresponding
representation in topology diagram (b) arrangement of binuclear zinc
paddlewheel SBUs in IMP-18 (c) section of IMP-18 viewed from the 010
crystallographic direction – disorder, solvent molecules and hydrogen atoms
omitted for clarity. Colour scheme: Zn, gold; O, red; C, grey; Si, green
(d) schematic representation of the fsy network showing the 4-connected size being ca. 12 Â 12 Å2. After theoretical removal of both
the coordinated and non-coordinated solvent, the PLATON33
Zn SBU nodes (red) and the 6-connected silicon-based nodes (blue).
estimated solvent-accessible void volume for IMP-19 is 2070 Å3
or 58% of the unit cell volume. However, in practice evacuation
colourless blade-like crystals. These crystals were characterised of the material led to its decomposition and a complete loss of
by single crystal X-ray analysis to be [Zn2(L2)(OH)(H2O)(DMF)]Á porosity.
2H2OÁ3DMF (IMP-19) and the bulk purity of the sample was
In summary, the novel siloxane-based hexacarboxylic acid
confirmed by powder X-ray diffraction (Fig. S7, ESI†). The L1-H6 has been prepared and shown to self-assemble into a 3D
crystals were found to comprise 2D polymeric sheets built from triply interpenetrated hydrogen-bonded organic framework with
tetrametallic nodes (comprising four Zn atoms held together by guest 1D H-bonded chain polymers in its pores. Treatment of
two m3-bridging hydroxides) linked together by the triply depro- L1-H6 with Zn(II) gives IMP-18, which in our knowledge is the first
tonated L2 (Fig. 3) to give a (3,6)-connected net of kgd topology. 3D-connected MOF structure which incorporates a disiloxane-
These 2D sheets can be considered to further assemble into a based linker. We believe these initial results will lead to the use
3D networked structure through hydrogen bonding between the of other siloxane scaffolds in the assembly of porous MOFs,
peripheral silanol group of an adjacent layer (H-bond donor) including the use of cyclic and cage-like polysiloxane linkers.
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The Imperial College President’s PhD Scholarship Scheme (LD)
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