The effect of carboxylate position on the structure of a metal organic framework derived from cyclotriveratrylene

Article abstract of DOI:10.1039/c6ce01965j

Two cyclotriveratrylene-based ligands H₃L¹ and H₃L² have been synthesised using microwave heating and used in the formation of 1 [Zn₂(L¹)(DMA)₂(CH₃COO)] and 2 [Zn

Full text of DOI:10.1039/c6ce01965j

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The effect of carboxylate position on the structure
of a metal organic framework derived from
cyclotriveratrylene†
Cite this: DOI: 10.1039/c6ce01965j
Received 9th September 2016,
Accepted 1st October 2016
ac
Adam D. Martin,ab Timothy L. Easun,
Stephen P. Argent,a William Lewis,a
*
a
ad
*
Alexander J. Blake and Martin Schröder
DOI: 10.1039/c6ce01965j
www.rsc.org/crystengcomm
Two cyclotriveratrylene-based ligands H3L1 and H3L2 have been
synthesised using microwave heating and used in the formation of
1 [Zn2(L1)(DMA)2(CH3COO)] and 2 [Zn6(L2)4(DMA)6(H2O)5] (DMA = N,
N-dimethylacetamide). 1 displays an unusual trigonal paddlewheel
node geometry, while Zn(II) paddlewheels are observed in 2. How-
ever, the stacking of CTV molecules in 1 is replaced by an uncom-
mon molecular capsule structure in 2.
ciated sub-discipline of metal organic frameworks (MOFs).
MOFs are crystalline microporous materials that are attrac-
tive targets for applications such as carbon capture and mo-
lecular separation due to the wide variety of linkers and
metal node geometries that can be exploited.8–13 Here, a
change in the position of a functional group such as a car-
boxylate can result in a completely different structure as a re-
sult of the changed connectivity of the organic linker. This
has been observed for numerous linkers, ranging from sim-
ple substrates such as benzoic or pyridinecarboxylic
acids14–17 to more complex molecules such as porphyrinic
macrocycles.18,19
Introduction
Controlling self-assembly in the solid state is one of the major
goals of crystal engineering. A variety of methods have been
employed to achieve this aim, including ligand substitution,
metal coordination, hydrogen bonding and solvent
selection.1–5 Of these, ligand substitution or alteration can re-
sult in dramatically different structures under the same con-
ditions of self-assembly. Specifically, the position of a substit-
uent on an aromatic substrate can result in significantly
different properties of the resultant compounds, with perhaps
the best example of this being the xylenes, where the position
of the methyl groups on the benzene ring gives differences in
boiling point, viscosity and crystal structure.6
One area where systematic substitution of functional
groups is hugely important is that of supramolecular self-as-
sembly, in which complex structure and function arise during
materials synthesis from often delicately balanced structural
and electronic factors.7 This approach is particularly com-
mon in the synthesis of coordination polymers and the asso-
Macrocycles such as porphyrins,20 calixarenes,21 crown
ethers22,23 and cyclodextrins24 have been used as ligands for
the formation of MOFs. However, due to the increased com-
plexity associated with such substrates, this is a relatively
underexplored area. The use of macrocycles as linkers offers
advantages such as a higher density of functional sites per
molecule and additional coordination sites such as hydro-
phobic pockets or ion binding regions, when compared to
simpler substrates. Cyclotriveratrylenes (CTVs) are one such
class of bowl-shaped macrocycles containing a shallow, hy-
drophobic pocket and an upper rim which can be
functionalised in a number of ways.25
Whilst initial research into CTVs focused on their ability
to bind fullerenes,26,27 more recently the supramolecular
chemistry and metal coordination abilities of CTVs have been
explored, with a diverse range of metal–organic polyhedra
based upon the CTV scaffold reported.28–33 A few examples of
coordination polymers based upon CTVs have also been
reported, including low dimensional coordination polymers
with lanthanide ions,33 a covalent organic framework (COF)
based upon a triformyl CTV derivative34 and a MOF featuring
one of the ligands used in this study, (L1)3−, coordinated to a
copper node, giving rectangular one-dimensional pores.35
Here, we report the synthesis of the linkers H3L1 and
H3L2, in which a cyclotriveratrylene scaffold is functionalised
with para and meta-benzoic acid group, respectively. Notably,
a School of Chemistry, The University of Nottingham, University Park, Nottingham,
NG7 2RD, UK
b School of Chemistry, The University of New South Wales, Sydney, NSW, 2052,
Australia
c School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10
3AT, UK. E-mail: EasunTL@cardiff.ac.uk
d School of Chemistry, University of Manchester, Oxford Road, Manchester, M13
9PL, UK. E-mail: M.Schroder@manchester.ac.uk
† Electronic supplementary information (ESI) available. CCDC 1470195–1470197.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c6ce01965j
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using a microwave Suzuki coupling procedure, we are able to Results and discussion
achieve greater yields and shorter reaction times than those
1-Zn crystallises in the orthorhombic space group Pbca, with
the asymmetric unit containing one molecule of [L1]3−, two
ZnIJII) ions, two coordinated DMA solvent molecules and an
acetate molecule (Fig. 1a), giving an empirical formula of
[Zn2(L1)(DMA)2CH3COO]. The acetate molecule is coordinated
to Zn(II) in a bidentate fashion, and in the absence of acetic
acid no crystalline material is formed. This suggests that the
acetate molecule plays an important role in the stabilisation
of this particular crystal phase.
In 1-Zn, the two ZnIJII) metal centres display different coor-
dination environments. One metal centre has a distorted tet-
rahedral geometry, with coordination to a solvent DMA mole-
cule and three carboxylates from [L1]3− molecules. The other
ZnIJII) cation possesses a distorted octahedral geometry, with
coordination to a solvent molecule and three carboxylates in
previously reported for Suzuki couplings involving CTVs as a
substrate.36 We have used these linkers in solvothermal reac-
tions and confirm that the structures of the materials
obtained are strongly dependent on the position of the car-
boxylate. For the para benzoic acid-substituted H3L1, a colum-
nar stacking of CTV molecules is observed. However, the
change in directionality provided by the meta benzoic acid
component in H3L2 results in the formation of a molecular
capsule arrangement which has not been observed previously
for coordination polymers.
Experimental section
Tribromocyclotriveratrylene (CTV-Br3) was prepared from
3-hydroxybenzoic acid in 15% yield according to a literature
procedure,37 and this compound was used as the substrate
for Suzuki couplings. Couplings were performed under an in-
ert atmosphere using a microwave reactor at 100 °C for 30
min, with 5 mol% PdIJPPh3)4 as catalyst with cesium fluoride
and six equivalents of x-ethoxycarbonylphenylboronic acid
(x = 4 for H3L1, 3 for H3L2). Following purification and hydro-
lysis of the ester groups (full experimental details can be
found in the ESI†), H3L1 and H3L2 were obtained as racemic
mixtures in 75% and 58% yield, respectively (Scheme 1).
The complex 1-Zn was obtained by solvothermal reaction
of H3L1 with Zn(NO3)2·6H2O in a solvent mixture of N,N-
dimethylacetamide (DMA) and ethanol (2.5 : 1) in the pres-
ence of acetic acid at 90 °C. Upon cooling and filtration,
colourless needles suitable for X-ray diffraction analysis were
obtained. 1 was obtained as isostructural cobalt(II) and zinc(II)
complexes, 1-Zn and 1-Co, respectively. However only 1-Zn
will be discussed here in detail. 2-Zn was synthesised in a
similar manner but in the absence of acetic acid, where after
cooling and filtration, colourless hexagonal plates of suitable
quality were obtained for X-ray diffraction studies. The addi-
tion of acetic acid to this second solvothermal reaction mix-
ture resulted in no crystalline material being produced.
Fig. 1 View of the asymmetric unit of 1 (a), metal node geometry of 1
displaying a trigonal paddlewheel (b), three different types of channels
in 1 as viewed down the b axis (c), and hexagonal superstructure of
channels outlined in red (d). Hydrogen atoms in b–d have been omitted
for clarity.
Scheme 1 Synthesis of H3L1 and H3L2 starting from 3-hydroxybenzoic
acid. For L1, R = COOEt, R1 = COOH, R′ = R1′ = H. For L2, R′ = COOEt,
R1′ = COOH, R = R1 = H.
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addition to a bidentate acetate molecule (Fig. 1b). Together,
this arrangement of metal centres can be thought of as an
asymmetric trigonal paddlewheel, where on one side, in addi-
tion to an axial solvent (in this case DMA) molecule, there is
bidentate coordination to an acetate. Trigonal paddlewheel
structures are uncommon,38–40 and this is the first example
of a metal organic framework containing a metal node bear-
ing this trigonal paddlewheel geometry.
fords no crystalline material in our hands. The arrangement
of the CTV ligands in 2-Zn is distinctly different to that ob-
served in 1-Zn; a molecular capsule arrangement of [L2]3− li-
gands is observed in 2-Zn, with no stacking interactions ob-
served between CTV ligands in this structure. The molecular
capsules are connected to each other through four-blade
Zn(II) paddlewheels, as opposed to the trigonal paddlewheel
structure observed for 1-Zn. The zinc paddlewheels are axially
coordinated by one water molecule and one DMA molecule,
with two unbound DMA molecules found inside each molec-
ular capsule, and one addition DMA molecule exo to the
capsules.
A capsular arrangement of CTVs in the solid state is un-
common, and although some examples have been shown for
cyclotricatechylene (CTC),42,43 the only examples reported for
CTV-based structures incorporate complementary binding
groups at their upper rim.44,45 In this structure, each molecu-
lar capsule is composed of one left handed and one right
handed [L2]3− ligand, and when viewed down the c axis it can
be seen that these capsules pack into a hexagonal arrange-
ment (Fig. 2b), with one molecular capsule positioned at each
corner of the hexagon. These hexagons stack in an alternat-
ing abab manner, which reduces the available porosity for
this structure. However, the solvent accessible volume was
calculated to be 21%, which is markedly higher than that for
1-Zn (which is 5.5%) This is due to the presence of two types
of rectangular pores in 2-Zn of dimensions 7.57 × 6.28 Å and
12.55 × 10.57 Å (Fig. 2c), as well as the interior of the
capsules.
The molecules of [L1]3− stack in a columnar arrangement,
with a distance of 4.9 Å between hydrophobic CTV pockets.
This motif is commonly observed in crystal structures of
molecular CTV-based species,28–30 presumably because it max-
imises the π–π interactions between CTV cores. In the colum-
nar arrangement in this structure, left and right handed mol-
ecules of [L1]3− stack alternately. This reduces any steric
hindrance associated with extending the CTV cores through
the addition of the benzoic acid moiety.
When viewed down the b axis, three types of channels are
visible: a rhombic channel (8.65 × 9.97 Å), a pentagonal chan-
nel (5.68 (pentagon base width) × 12.63 (width of channel at
its widest point) × 13.43 (channel height) Å) and a hexagonal
channel (12.30 × 13.89 Å) (Fig. 1c). The rhombic channel is
occupied by two solvent molecules and the pentagonal chan-
nel by two solvent molecules and two acetate molecules, but
the hexagonal channel remains free of any coordinated sol-
vents. Using a 1.2 Å probe, the solvent accessible volume for
this structure is calculated to be 5.5% using the Mercury pro-
gram from the CCDC software package,41 which is solely due
to the unoccupied hexagonal pores. During this calculation,
metal coordinated solvent molecules were not removed.
When viewed down the b axis, a hexagonal superstructure
composed of four of each type of channel is visible, with each
side being approximately 22 Å (Fig. 1d).
It can be seen that in the structures of 1-Zn and 2-Zn there
is a balance between the preferred ligand geometry for the
CTV ligand. In 1-Zn the CTV molecules stack on top of one
another to maximise π–π interactions, with a preferred metal
node geometry incorporating a trigonal ZnIJII) paddlewheel. It
has been observed previously that minor differences in ligand
substitution, either from the addition of a phenyl or methyl
group to an aromatic core, or through post-synthetic modifi-
cation, can result in significant differences in separation be-
haviour, proton conductivity or dye adsorption in the
resulting frameworks.46–48
Compound 2-Zn crystallises in the triclinic space group
¯
P1, with the asymmetric unit containing four molecules of
[L2]3−, six ZnIJII) cations, six molecules of DMA and five water
molecules (Fig. 2a), thus yielding a product of formula
[Zn6(L2)4(DMA)6(H2O)5]. Unlike the reaction with [L1]3−, in the
presence of acetic acid the reaction of [L2]3− with Zn(II) af-
For the case of the para benzoic acid appended CTV,
[L1]3−, the stacking interactions between CTV molecules are
more favoured than the formation of four-blade ZnIJII)
paddlewheels. This results in the formation of an unusual tri-
gonal paddlewheel metal node, where each ZnIJII) cation in
the paddlewheel has a different coordination number. This is
in contrast to the previously reported MOF using this li-
gand,35 where [L1]3− is connected through CuIJII) paddlewheels
(where the copper displays octahedral coordination) and a
five-coordinate, square bipyramidal CuIJII) to form rectangu-
lar, one dimensional nanotubes. In that structure, no stack-
ing of the CTV molecules is observed.
Fig. 2 View of the asymmetric unit of 2 (a), hexagonal arrangement of
molecular capsules as viewed down the c axis (b) and two types of
rectangular pores in the structure as viewed down the a axis (c).
Hydrogen atoms and uncoordinated solvent molecules in b and c have
been removed for clarity.
For 2-Zn, the formation of the four-blade ZnIJII)
paddlewheel structure is more favoured, resulting in an un-
common molecular capsule arrangement for the meta
benzoic acid appended CTV, which results in a higher level
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of porosity. As the reaction conditions in both cases are al-
most identical, it must be concluded that it is the position of
the carboxylic acid which plays the major role determining
the structural outcome of the synthesis.
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In this work we have synthesised the para and meta benzoic
acid appended cyclotriveratrylenes H3L1 and H3L2, respec-
tively, through a microwave assisted Suzuki coupling reac-
tion. The use of microwave irradiation affords markedly
faster reaction times and greater yields when compared with
previously reported syntheses. We then used these ligands in
solvothermal reactions with Zn(NO3)2 to give the correspond-
ing coordination polymers 1-Zn and 2-Zn. For both of these
frameworks, it can be seen that there is a finely balanced
compromise between optimal ligand packing and metal node
geometry. For 1, the common CTV packing motif is observed
where CTV molecules stack on top of one another, which re-
sults in an unusual trigonal paddlewheel metal node geome-
try. For 2-Zn, the common four-blade paddlewheel geometry
is seen, although this yields an uncommon molecular cap-
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firmed that the position of the carboxylic acid, here meta or
para, plays a critical role in the assembly of CTV-based MOFs.
This is important for the future design of porous materials
based upon macrocyclic scaffolds, where maintaining poros-
ity and functionality is crucial.
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Acknowledgements
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We thank the EPSRC for support. MS gratefully acknowledges
support from the ERC for an Advanced Grant. TLE gratefully
acknowledges the Royal Society for the award of a University
Research Fellowship. ADM thanks the Australian Government
for the award of an NHMRC-ARC Fellowship (APP1106751).
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