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Structural diversity and luminescent properties of
lanthanide 2,2- and 2,3-dimethylsuccinate frameworks3
Published on 19 September 2012. Downloaded on 28/08/2013 22:18:54.
Cite this: CrystEngComm, 2013, 15,
100
Paul J. Saines,a Mark Steinmann,a Jin-Chong Tan,ab Hamish H.-M. Yeunga
and Anthony K. Cheetham*a
The structures of fourteen new lanthanide frameworks (La, Ce, Eu, Tb, Y and Lu) containing the 2,2- or 2,3dimethylsuccinate ligands are reported. While the majority of the known 2,2-dimethylsuccinate
frameworks feature two dimensionally bonded layers, capped by hydrophobic methyl groups, several of
these new frameworks adopt quite different architectures. These include one dimensional inorganically
connected chains (La and Ce) with only non-covalent interactions in the other two dimensions, and three
dimensional covalently bonded frameworks (Eu and Lu) with spaces in their structure to accommodate the
Received 10th August 2012,
Accepted 18th September 2012
DOI: 10.1039/c2ce26279g
www.rsc.org/crystengcomm
bulky methyl groups. The new 2,3-dimethylsuccinate frameworks (La and Y) adopt three-dimensional
covalently bonded frameworks. The factors affecting the formation of structures with different
dimensionalities are examined and compared to previously reported transition metal frameworks. In
addition, the sequence of phases formed with changing lanthanide size, concentrations and temperatures
are rationalised. The luminescent properties of several 1- and 2-D frameworks doped with Eu and Tb are
reported, with the Y host exhibiting the most intense emission.
1. Introduction
Inorganic-organic (hybrid) framework materials have attracted
significant and sustained interest for over a decade due to
their fascinating range of structures and wide variety of
interesting properties.1,2 The structures of these hybrid
materials are influenced significantly by both their cation
and ligand building blocks, and they can adopt either porous
or dense structures with covalent connectivity of different
dimensions. Porous frameworks, often referred to as metal–
organic frameworks (MOFs), are of significant interest due to
their catalytic, gas storage and separation properties,2,3 while
dense frameworks attract interest for their ability to exhibit
cooperative behaviour more commonly associated with purely
inorganic materials, such as magnetic order, semiconductivity
and multiferroicity.4 Such frameworks can also exhibit a wide
range of structures with different dimensionalities, having
covalent connectivity in one, two or three dimensions;
a
Department of Materials Science and Metallurgy, The University of Cambridge,
Cambridge CB2 3QZ, United Kingdom. E-mail: [email protected];
Fax: +44(0) 1223 334567; Tel: +44(0) 1223 767061
b
Department of Engineering Science, University of Oxford, Parks Rd. Oxford OX1 3PJ,
United Kingdom
3 Electronic supplementary information (ESI) available: Synthetic details, single
crystal X-ray diffraction experimental and crystallographic information,
including CIF files, results from microanalysis, infrared and detailed
thermogravimetric analysis results and figures of powder X-ray diffraction
patterns, assymetric units, sorption results and thermogravimetric analysis. CIFs
for structures 1–8 and 10–14 have been deposited with the CCDC with deposit
numbers 895574–895586. See DOI: 10.1039/c2ce26279g
100 | CrystEngComm, 2013, 15, 100–110
furthermore, this covalent connectivity can either be organic
in nature (metal–ligand–metal) or inorganic (typically metal–
O–metal).5
Most studies of hybrid frameworks have focused on the
synthesis of single crystals and powders with micron-sized
particles, but significant attention has recently been drawn to
the possibility of making nano-sized particles of these
materials, principally as a means of incorporating them into
thin films for technological applications.6 There have been a
number of topical reports of exfoliation, via ultrasonication, of
layered frameworks with weak intra-layer interactions. This
‘‘top–down’’ approach produces nanosheets that are only a few
nanometres thick but have lateral dimensions on the order of
microns.7,8–10 Dense frameworks with simple linear dicarboxylate ligands that feature bulky substituent groups appear to
encourage the formation of such layered phases and are
therefore a particularly versatile platform from which
nanosheets can be exfoliated. A number of such compounds
feature the 2,2-dimethylsuccinate (2,2-DMS) and 2,3-dimethylsuccinate (2,3-DMS) ligands in combination with either
monovalent alkali or divalent transition metal cations.8–10 All
2,2-DMS frameworks have been found to adopt layered
structures suitable for exfoliation. The 2,3-DMS compounds,
however, appear to be a mixture of layered and three
dimensionally covalently bonded structures, depending on
whether chiral or meso-ligands are incorporated; this is due to
the preference for these isomers to adopt gauche or transarrangements, respectively.
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There have been a large number of studies of simple, linear
dicarboxylate frameworks in which the various factors influencing the formation of different structures have been
probed.8,9,11–13 The subtle role that the different DMS isomers
play in directing the formation of framework structures with
very different dimensionalities, however, highlights the need
to further explore this interesting family of compounds. This
will enable a better understanding of the effects of reaction
conditions, ionic radius and charge on the formation of low
dimensional structures. Additionally, the development of
novel, layered frameworks that are suitable for exfoliation is
an important area to on which to focus. The lanthanide 2,2DMS and 2,3-DMS frameworks are of particular interest since
they provide a ready means of examining the effect of ionic
radii on the dimensionalities of the structure formed as well as
a route towards frameworks with interesting luminescence
behaviour. In this work, therefore, we explore the synthesis
and structures of a number of lanthanide (Ln) 2,2-DMS and
2,3-DMS frameworks and report the luminescence properties
of a number of doped frameworks with one- and twodimensional covalent connectivity.
2. Experimental
The compounds examined in this work were made hydrothermally, from commercially available starting materials, in
23 mL Teflon lined Paar autoclaves heated at or below 200 uC
for three days. In total it was possible to grow single crystals
suitable for structure determination for 13 compounds.
Synchrotron X-ray diffraction data were collected in order to
determine the structure of framework 9, Lu2(C6H8O4)3(H2O)4,
which is isostructural with one of the Y containing phases (see
Table 1 for chemical formula and connectivity). The synthetic
conditions used to produce the crystals used for structure
determination and, where possible, pure phase samples, are
described in the supplementary information alongside experimental details of the single crystal X-ray structure determinations. The crystallographic parameters of phases studied using
single crystal X-ray diffraction are presented in Tables S1–S3,
ESI3 and the cation-anion bond distances of phases 1–13 are
listed in Tables S4–6, ESI.3
Powder X-ray diffraction (PXRD) patterns were collected of
all samples prepared in this study using Cu Ka radiation on a
Bruker D8 Advance diffractometer equipped with a position
sensitive linear detector. To indicate the high level of purity to
which samples of 1–6, 13 and 14 can be made, Le Bail fits to
PXRD patterns of these frameworks are presented in Fig. S1–8,
ESI.3 Microanalysis results, obtained from the Department of
Chemistry at the University of Cambridge, are presented in
Table S7, ESI.3 A synchrotron X-ray diffraction pattern of
compound 9 was collected using beamline I11 at the Diamond
Light Source UK.14 A wavelength of 0.827153(1) Å and the
Mythen position sensitive detector were used, and the
structure refined by use of the program GSAS with appropriate
C–O and C–C bond distance and angle restraints to retain
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Table 1 The formula and connectivity, using the notation of Cheetham et al.,5 of
all structures presented in this work. 1–12 contain 2,2-DMS and 13 and 14
feature meso-2,3-DMS. The O/Ln ratio includes both coordinated atoms and
those from pore water molecules, where present
Ln
coordination
Structure
Formula
Connectivity
1
La(C6H8O4)
(C6H9O4)(H2O)2
Ce(C6H8.5O4)2(H2O)2
Ce2(C6H8O4)3(H2O)2
Eu2(C6H8O4)3(H2O)3
Tb2(C6H8O4)3(H2O)3
Y2(C6H8O4)3
(H2O)4?H2O
Y2(C6H8O4)3(H2O)4
Y4(C6H8O4)6(H2O)
Lu2(C6H8O4)3(H2O)4
Lu2(C6H8O4)3(H2O)
Lu3(C6H8O4)4
(OH)(H2O)2
Lu3(C6H8O4)4(OH)
La2(C6H8O4)3(H2O)2
Y2(C6H8O4)3
I1O0
10
10
11O0
I1O1
I0O3
I0O3
I0O2
10
9
8
8
8
10
7
7.5
7.5
8.5
I0O2
I0O2
I0O2
I1O1
I0O2
8
6, 7, 8
8
8
7, 8
I0O3
I1O2
I1O2
7
9
8
2
3
4
5
6
7
8
9
10
11
12
13
14
O/Ln
8
6.25
8
6.5
6.3
5.7
7
6
typical dicarboxylate geometry15 (see supplementary information for further details of the refinement, Fig. S9, ESI3 for fit
and Table S8, ESI3 for crystallographic details).
Thermogravimetric analysis (TGA) of the bulk frameworks
was performed in air on a TA Instruments Q500 using a
heating rate of 10 uC min21; experimental details and results
from infra-red spectroscopy are described in the ESI.3 Nitrogen
sorption isotherms were measured at 77 K on a Micromeritics
ASAP 2020 instrument. Samples were degassed in vacuum for
at least 5 h at 100 uC before starting the gas uptake
measurements. The surface area was estimated using the
Brunauer–Emmett–Teller (BET) equation for the relative
pressure range (P/P0) of 0.002 to 0.3; the saturation pressure,
P0, corresponds to 103.88 kPa. Fluorescence spectra were
measured using a custom made Horiba Fluorolog-3 fitted with
a 450 W xenon lamp, a double monochromator in the
emission channel, and a red-sensitive R928 photomultiplier
tube detector.
3. Results and discussion
3.1. Synthesis and structures of the Ln 2,2-DMS frameworks
3.1.1. SYNTHESIS AND STRUCTURE OF LA 2,2-DMS. 1 is the
dominant phase produced when La is used as a cation,
forming between 90 uC and 200 uC. A secondary triclinic phase,
whose structure could not be determined due to severe
disorder, also formed at 200 uC along with 1. 1 consists of
one-dimensional covalently bonded chains, which are connected through edge-sharing LaO10 polyhedra and are surrounded by the methyl groups of the ligand. The only
interchain interactions are through van der Waals forces and
a single distinct relatively weak hydrogen bond (Odonor–
Oacceptor distance of 2.815(12) Å) between one of the oxygen
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atoms of one of the two distinct water molecules and a
carboxylate oxygen in a neighbouring chain (see Fig. 1). This is
the first 2,2-DMS or 2,3-DMS structure with only one
dimensional covalent connectivity, I1O0 according to the
notation of Cheetham et al.5 The asymmetric unit of 1
contains a La cation, two 2,2-DMS ligands and two water
molecules, both of which are coordinated to a single La atom
(see Fig. S10, ESI3). One of the two 2,2-DMS ligands appears to
have one of its carboxylate groups protonated, as required for
charge balance. This proton has a very short hydrogen bond
distance (Odonor–Oacceptor is 2.465(11) Å) with an oxygen atom
from the other distinct carboxylate ligand (although the
presence of the heavy La makes determining the precise
position of this hydrogen difficult and it is therefore possible
this may be somewhat disordered since neither of these
oxygen atoms bond to the La and their C–O bond lengths are
very similar). The La cations have bond valencies of 3.12 and
the 2,2-DMS ligands have backbones with near gauche
arrangements.16 Both 2,2-DMS ligands have (0112) connectivity, with one oxygen atom in both carboxylate groups bonding
to one La cation and the other two atoms bonding to none and
two lanthanide cations respectively.13
3.1.2. SYNTHESIS AND STRUCTURES OF THE CE 2,2-DMS PHASES. The
Ce containing frameworks are best synthesized below 125 uC
since temperatures above this appear to give rise to the
oxidation of Ce3+ to Ce4+ and the formation of a CeO2 impurity.
The formation of 2, Ce(C6H8.5O4)2(H2O)2, appears to be
favoured over 3, Ce2(C6H8O4)3(H2O)2, when lower reaction
temperatures and higher concentrations are used. 2 has a very
similar structure to 1, featuring edge-sharing chains of CeO10
Fig. 1 Crystal structure of 1 showing a) the arrangement of neighbouring chains
and b) the chain of edge-sharing polyhedra. The La atoms and polyhedra are
blue and the carbon, oxygen and hydrogen atoms are black, red and gray,
respectively.
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Fig. 2 Crystal structure of 2 showing a) the arrangement of neighbouring chains
and b) the chain of edge-sharing polyhedra. The Ce atoms and polyhedra are
yellow and all other colours are as in Fig. 1.
polyhedra that are decorated by the ligand (see Fig. 2).
Neighbouring chains only interact via weak van der Waals
forces and one distinct hydrogen bond (Odonor–Oacceptor
distance of 2.889(2) Å), leading to I1O0 connectivity overall.5
Unlike 1, the asymmetric unit of 2 only contains one unique
dicarboxylate, one aquo-ligand and a Ce cation that is on a 4c
Wyckoff special position (see Fig. S11, ESI3). The Ce cations
bond to two water molecules and eight oxygen atoms from
carboxylate groups with an overall bond valence sum of 3.16.16
Similarly to 1 the only distinct carboxylate ligand has a gauche
arrangement and (0112) connectivity.13 Charge balance
appears to be maintained by half of the carboxylate oxygen
atoms that do not bond to a Ce cation remaining protonated,
although since there is only one distinct ligand the proton site
has been set to 50% occupancy.
The structure of 3 features 1 nm thick covalently bonded
layers that are hydrophobically capped by the methyl groups of
2,2-DMS ligands, typical for all the 2,2-DMS structures
reported previously (see Fig. 3).9,10 The architecture of the
layers of 3, however, is novel and features sinusoidal chains of
CeO9 polyhedra, which are connected by alternating edge- and
face-sharing connectivity. These chains are bridged by the
backbone of the carboxylate groups at the points where
neighbouring chains are closest, leading to I1O1 connectivity
overall.5 The asymmetric unit of 3 has two Ce cations, three
2,2-DMS ligands and two water molecules, one of which is
involved in the edge-sharing connectivity (see Fig. S12, ESI3).
The two Ce cations bond to one and two water molecules and
have bond valencies of 3.28 and 2.92, respectively.16 The
backbones of all carboxylate ligands adopt the gauche
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Fig. 3 Crystal structure of 3 showing a) the arrangement of neighbouring layers
and b) the layer architecture. The colours are as in Fig. 2.
arrangement and two of the 2,2-DMS ligands have 1212
connectivity while the third has (0111) connectivity.13 All of the
aquo hydrogen atoms have intralayer hydrogen bonds with
carboxylate oxygen atoms.
3.1.3. SYNTHESIS AND STRUCTURES OF THE EU AND TB 2,2-DMS PHASES.
Frameworks 4, Eu2(C6H8O4)3(H2O)3, and 5, Tb2(C6H8O4)3(H2O)3,
are isostructural and are the first 2,2-DMS frameworks reported
to feature three dimensional covalent connectivity (see Fig. 4 and
S13, ESI3). They are the main phases formed when Eu and Tb are
used as cations, although in the case of Eu a small amount of a
triclinic phase, Eu2(C6H8O4)3(H2O)2, isostructural to compound
3, has also been found to form intermittently. 4 and 5 feature
alternating rows of dimers of LnO8 polyhedra and isolated pairs
of LnO8 polyhedra along the a-axis, and these are connected
through the ligand in a structure with I0O3 connectivity.5 Looking
down the c-axis there are oval shaped spaces that are occupied by
the methyl groups of the ligands, thereby allowing them to be
contained within a three dimensional structure. The asymmetric
units consist of two lanthanide cations, three 2,2-DMS ligands
and three water molecules, all of which are bonded to one cation
Fig. 4 The structure of compound 5 showing the a) ab and b) ac planes. The Tb
cations and TbO8 polyhedra are teal and all other colours are as in Fig. 1.
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(see Fig. S14, ESI3). The Ln cations bond to one or two water
molecules, with the rest of their coordination sphere completed
by bonds to oxygen atoms from different carboxylate groups.
They have bond valencies of 3.36 and 3.34 for Eu and 3.29 and
3.24 for Tb, respectively.16 The carbon backbones of the 2,2-DMS
ligands all adopt an arrangement that is close to gauche and
coordinate to Eu and Tb cations in an (1111) fashion.13 Five of
the six distinct hydrogen atoms from the water molecules have a
hydrogen bond with a carboxylate oxygen atom.
3.1.4. SYNTHESIS AND STRUCTURES OF THE Y 2,2-DMS PHASES.
Three distinct Y phases were encountered under different
synthetic conditions. The most commonly formed phase is 6,
Y2(C6H8O4)3(H2O)4?H2O, which forms in all reactions conducted in this study. 6 features 1 nm thick covalently bonded
layers capped by methyl groups with only van der Waals forces
acting between layers (see Fig. 5). In contrast to the layered 2,2DMS and 2,3-DMS phases reported previously,8–10 the architecture of the layers includes pores that are occupied by one
water molecule for every cation; the waters are disordered over
two sites. The pores are accommodated in layers that feature
dimers of YO8 polyhedra bridged by two carboxylate groups;
these dimers are connected to each other through the back
bone of the dicarboxylate ligand, giving rise to I0O2 connectivity overall.5 The asymmetric unit of 6 features one Y
cation, one and a half 2,2-DMS ligands, the half being
disordered over two sites, half a disordered oxygen from a
pore water molecule, and two coordinated water molecules
that each bond to one Y cation (see Fig. S15, ESI3). The cations
have a bond valence of 3.24 and both the dicarboxylate ligands
exhibit (1111) connectivity and have carbon backbones that
adopt either gauche and trans arrangements, respectively, the
latter being required by symmetry.13,16 The residual factors
from the refinement of this structure are higher than the other
compounds examined in this work; this is attributed to the
high level of disorder present in this material.
Fig. 5 Crystal structure of 6 showing a) the arrangement of neighbouring layers
and b) the layer architecture. Both components of the disordered 2,2-DMS
ligand are displayed. The Y cations and YO8 polyhedra are blue and all other
colours are as in Fig. 1.
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When reactions are carried out using higher concentrations
of reagents and lower temperatures, framework 7,
Y2(C6H8O4)3(H2O)4, forms along with compound 6. 7 is
another example of a 2,2-DMS framework with covalently
bonded layers capped by methyl groups, although these layers
are more corrugated than found in frameworks 3 and 6 (see
Fig. 6). Similarly to 6, the architecture of the layers features
dimers of YO8 polyhedra bridged by two carboxylate groups;
dimers within a layer are coupled through the backbone of the
ligand, giving rise to I0O2 connectivity.5 The arrangement of
the polyhedra and 2,2-DMS ligand within a layer are, however,
subtly different, such that the pores found in 6 do not exist in
7. Consequently the asymmetric unit of 7 features one Y
cation, two coordinated water molecules, and one and a half
2,2-DMS ligands (the methyl groups on the half ligand are
disordered with 50% occupancy, see Fig. S16, ESI3). The Y
cations bond to two water molecules, with the remainder of its
coordination completed by carboxylate oxygen atoms; they
have a bond valency of 3.18.16 The 2,2-DMS ligands both have
(1111) connectivity to Y cations and their carbon backbones
adopt arrangements close to gauche and trans, the latter being
required by the symmetry of the structure.13 Each of the
hydrogen atoms from the water molecules has one hydrogen
bond with an oxygen atom from a nearby carboxylate group.
An isostructural Er phase has previously been reported by
Bernini et al.,17 although they suggest it adopts noncentrosymmetric P1 symmetry in which the methyl groups of
the ligand that adopts the trans-structure are not disordered.
In the case of the Y compound, however, attempts to solve the
structure in the non-centrosymmetric space group indicated
that there is equivalent electron density in all four possible
positions for the methyl carbon atoms, suggesting that the
disorder is genuine, at least in the case of the Y framework.
Fig. 6 Crystal structure of 7 showing a) the arrangement of neighbouring layers
and b) the layer architecture. Both components of the disordered methyl
substituents are displayed. The colours are the same as Fig. 5.
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Reactions carried out with higher concentrations and
elevated temperatures lead to the formation of a mixture of
frameworks 6 and 8, Y4(C6H8O4)6(H2O). 8 is another compound that features hydrophobically capped covalently
bonded layers that only interact with each other through van
der Waals forces (see Fig. 7). Similarly to 7, it does not feature
any pore solvent but the surfaces of its layers are somewhat
smoother. The architecture of the layers of 8 are quite
complex, consisting of tetramers of edge-sharing YO8 polyhedra with the outer polyhedra sharing corners with YO7
polyhedra, giving rise to inorganically connected discrete
chains of six YOx polyhedra. The YO7 polyhedra are also
connected to two YO6 octahedra, via one and three carboxylate
groups. It is through these linkages that neighbouring chains
are connected, giving rise to a structure with I0O2 connectivity
overall.5 To accommodate this structural complexity, the
asymmetric unit of 8 features four distinct Y cations, six
distinct carboxylate groups, and one water molecule that
coordinates to one of the YO8 polyhedra (see Fig. S17, ESI3).
The Y cations all have bond valencies between 3.11 and 3.30
and the backbone of the 2,2-DMS ligands adopt an arrangement close to gauche.16 The oxygen atoms in the carboxylate
ligand adopt different coordination modes to the Y cations,
with three having (1111) connectivity, two having (1112)
connectivity, and the remaining ligand adopting (1212)
connectivity.13 All of the hydrogen bonds from the hydrogen
atoms in the water molecule are with oxygen atoms from the
carboxylate groups.
3.1.5. SYNTHESIS AND STRUCTURES OF THE LU 2,2-DMS PHASES. Five
different phases were encountered amongst the Lu 2,2-DMS
frameworks although, unlike the Y compounds, it appears that
temperature is the main factor that controls which of these
Fig. 7 Crystal structure of 8 showing a) the arrangement of neighbouring layers
and b) the layer architecture. The colours are the same as Fig. 5.
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phases form. Between 90 uC and 125 uC two phase mixtures of
Lu analogues of compounds 6, Lu2(C6H8O4)3(H2O)4?H2O, and
7, Lu2(C6H8O4)3(H2O)4, form, although at 150 uC the analogue
of 7 can be made in pure form. As this was the only Lu phase
that could be made in pure form, its structure and properties
were investigated further and from here on it will be referred
to as compound 9. The refinement of the structure of
compound 9 from the synchrotron X-ray diffraction pattern
indicates the only significant change, as might be expected
from the smaller ionic radii of Lu, is that the Lu–O bond
distances are mostly smaller than those found in the Y–O
analogue. Despite this shortening, the bond valence is reduced
from 3.18 for Y in 7 to 2.91 for Lu in 9.16
Increasing the temperature to 180 uC leads to the formation
of a two phase mixture of frameworks 10, Lu2(C6H8O4)3(H2O),
and 11, Lu3(C6H8O4)4(OH)(H2O)2, and increasing the reaction
temperature further to 200 uC leads to the formation of 12,
Lu3(C6H8O4)4(OH), instead of 10. Attempts were made to alter
the conditions and temperatures to obtain each of these
phases in pure form. This did not fundamentally change the
reaction mixtures obtained but merely confirmed that 10 and
12 form sequentially, alongside 11, as temperature is increased
above 150 uC. Similarly to 9, frameworks 10 and 11 both
feature covalently bonded layers capped with methyl groups
with only van der Waals interactions acting between layers,
although their layers are smoother. (see Fig. 8 and 9). The layer
architecture of 10 features edge-sharing dimers of LuO8
polyhedra, which are connected to neighbouring dimers by
Fig. 8 Structure of compound 10 showing a) the arrangement of neighbouring
layers and b) the architecture of an individual layer, highlighting the sinusoidal
inorganic chains. The Lu cations and LuO8 polyhedra are pink and the other
colours are as in Fig. 1.
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Fig. 9 The structure of framework 11 showing a) the arrangement of
neighbouring layers and b) the architecture of an individual layer. The colours
are the same as in Fig. 8.
their corners to form a sinusoidal shaped chain. Neighbouring
chains are bridged by the backbones of the dicarboxylate
ligands giving rise to a structure with I1O1 connectivity.5 It is
isostructural to an Er phase that was previously reported by
Bernini et al.,17 and the asymmetric unit of 10 features two Lu
cations, three 2,2-DMS ligands, and a water molecule that
coordinates to one of the Lu cations (see Fig. S18, ESI3). The Lu
cations have bond valencies of 3.16 and 3.21, the latter being
the Lu cation that bonds to the water molecule.16 One of the
2,2-DMS ligands has a carbon backbone that adopts gauchegeometry. The other two have intermediate arrangements and
are the first two ligands in the structures reported in this study
to have torsion angles more than 15u away from an ideal
gauche or trans-arrangement (77.8(6)u and 83.4(6)u). The ligand
adopting a gauche-like arrangement adopts (1212) connectivity
while other two ligands adopt (1112) and (1111) typearrangements.13 The aquo-hydrogen form hydrogen bonds
with oxygen atoms from the carboxylate groups.
The asymmetric unit of 11 features three Lu cations, four
2,2-DMS ligands, two water molecules and a hydroxide group
(see Fig. S19, ESI3). The three Lu are connected inorganically
into trimers with each Lu1O7 polyhedra connected to a Lu2O7
polyhedra via a corner, and Lu2O7 and Lu3O8 polyhedra
sharing edges. The closest oxygen to the Lu2O7 polyhedra that
is bonded to Lu1 but not Lu2 is disordered over two sites that
are very close to each other and are each fixed at 50%
occupancy. This disorder along with the displacement parameter of the carbon atom in the carboxylate group suggests
that there may be some rotation of the Lu1O7 polyhedra
towards that of the Lu2O7 groups. These LuOx trimers are
connected into rows along the a-axis by two carboxylate groups
with connectivity within a layer being completed through the
backbone of the dicarboxylate ligand, giving rise to a structure
that has I0O2 connectivity overall.5
In 11, Lu1 coordinates to both water molecules, the
hydroxide anion, and four oxygen atoms from different
carboxylate groups; Lu2 bonds on one hydroxide anion and
six oxygens from five different carboxylate groups, and Lu3
bonds only to oxygen atoms from carboxylate groups. The Lu
cations have bond valencies of 3.19, 3.09 and 3.17 for Lu1-3,
respectively.16 The coordinated water and hydroxide groups
bond to one and two Lu cations, respectively, and the 2,2-DMS
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ligands, which all adopt arrangements that are close to gauche,
either adopt (1111) or (1112) connectivity, with two of each
type of coordination found in the structure.13 Three of the four
hydrogen atoms from the water molecules have hydrogen
bonds with carboxylate oxygen atoms and the fourth has one
with an oxygen atom from the crystallographically identical
water molecule.
12 is the second 2,2-DMS framework architecture to feature
three dimensional covalent connectivity. Like compounds 4
and 5, 12 has I0O3 connectivity.5 It features trimers of LuO7,
which all bond to a central hydroxide group, and two of the
polyhedra in a trimer share edges with the third (see Fig. 10).
The trimers are connected to each other along the a- and c-axis
via carboxylate groups and the offset of trimers by half a unit
cell down the c-axis creates space between a grid of four
trimers that accommodates the bulky methyl groups (see
Fig. 10a). The asymmetric unit of 12 features three Lu cations,
four 2,2-DMS ligands, and a single hydroxide group (see Fig.
S20, ESI3). The bond valencies of the Lu cations are all between
3.15 and 3.21 and the 2,2-DMS ligands, whose carbon
backbones all adopt an arrangement close to gauche, either
bond to Lu cations in a (1111) or (1112) fashion, with half
adopting each configuration.13,16 The hydrogen from the
hydroxide anion has hydrogen bonds with a carboxylate
oxygen from a neighbouring trimer (Odonor–Oacceptor distance
of 2.974(7) Å).
CrystEngComm
Fig. 11 The structure of 13 showing the a) ab and b) ac plane. The colours are
the same as in Fig. 1 and both components of the disorder ligand are displayed.
3.2. Synthesis and structures of the Ln 2,3-DMS frameworks
Both of the frameworks synthesized with the 2,3-DMS ligands
feature three dimensional covalent connectivity, although this
may be related to the apparent necessity to use high
temperatures to obtain crystals suitable for structure determinations in these La and Y frameworks. Both frameworks can
also be made from either a mixture of 2,3-DMS isomers or
using
the
pure
meso-ligand.
Framework
13,
La2(C6H8O4)3(H2O)2, features corner sharing chains of LaO9
polyhedra aligned along the c-axis (see Fig. 11). Neighbouring
chains are connected along the b-axis through three atom
carboxylate groups bridges and along the a-axis by the
backbone of the 2,3-DMS ligands, leading to a structure with
I1O2 connectivity.5 Viewed down the c-axis there are hexagonal
Fig. 10 The structure of framework 12 viewed in the a) ab and b) bc plane. The
colours are the same as in Fig. 8.
106 | CrystEngComm, 2013, 15, 100–110
pores, which are occupied by the methyl groups, enabling
them to be incorporated into a phase with three-dimensional
covalent connectivity. These pores, however, unlike those in
other three dimensional 2,2-DMS and 2,3-DMS frameworks,
are not fully occupied, leading to a structure featuring 4.2%
solvent accessible voids according to the SOLV routine in the
program PLATON.18 Nitrogen sorption measurements indicate
a surface area of only 1.6 m2 g21, possibly due to the tortuous
nature of the cavity which renders the pores inaccessible at
atmospheric pressures (see Fig. S21, ESI3).
The asymmetric unit of 13 has one La cation, one and a half
2,3-DMS ligands, and one water molecule that coordinates to
one La (see Fig. S22, ESI3). The 2,3-DMS ligand, whose two
halves are crystallographically identical, provides the connectivity along the a-axis and is disordered over two sites. Both the
2,3-DMS ligands in the structure adopt the meso-isomer, but
the backbone of the disordered ligand adopts a transconfiguration and features (1212) connectivity to the La
cations; the other distinct ligand adopts a gauche-configuration and has 1112 connectivity.13 The La cations have bond
valencies of 3.19 and the hydrogen atoms from the water
molecule each have a hydrogen bond with a nearby carboxylate
group.16
Framework 14 is the only phase found in the present study
where the dicarboxylate ligand is the only ligand present.
Y2(C6H8O4)3 features chains of edge-sharing YO8 polyhedra,
which are bridged in the other two dimensions via the
backbone of the ligand, giving rise to a structure with I1O2
connectivity overall (see Fig. 12).5 The YO8 chains are arranged
into a square array with space inside the square to
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Fig. 12 The structure of 14 illustrating a) the ab and b) the ac planes. The
colours are the same as Fig. 5 and, for clarity, only half of the disordered ligands
are shown.
accommodate the methyl groups of the ligands. The asymmetric unit of 14 includes three Y sites, two of which are on
special positions, and three ligands, all of which are
disordered over two sites with approximately half occupancy
of each component present. The high apparent level of
disorder in the structure means that the details of the
structure must be examined carefully, but two of the three
ligands clearly coordinate to Y in an (1112) fashion, while the
third adopts a (1212) arrangement.13 All the ligands are the
meso-isomer with carbon backbones close to a trans-arrangement.
3.3. Structural trends in the Ln 2,2-DMS and 2,3-DMS
frameworks
There are a number of significant changes in the structures
adopted by the Ln frameworks with changing cation size,
synthesis temperature and reagent concentration. All previously reported 2,2-DMS phases have adopted layered
structures, which is attributed to the methyl groups always
being on the same side and closer together in this isomer,
making for formation of such structures energetically favourable.8–10 In this study, however, a wider range of dimensionalities is reported for the 2,2-DMS phases, although the
geometry of the carboxylate groups of the ligand is generally
close to a gauche-geometry where it is not required by
symmetry to be trans.
The ionic radii of the lanthanide cation clearly plays a
significant role in the dimensionality of the structure adopted,
although the trend is not as simple as with some alkaline earth
frameworks, such as those formed by the thiazolothiazole
dicarboxylates reported by Falcão et al.19 The 10-fold coordination spheres found with the largest Ln cations, La and Ce, form
frameworks with two 2,2-DMS ligands per cation, with one of
these ligands retaining its acidic proton. This leads to the
formation of the first 2,2-DMS frameworks with one-dimensional covalent connectivity, which is necessary to accommodate the additional methyl groups. The second Ce phase, 3,
features 9 coordinate cations, and as the lanthanide ionic
radius decreases 8-fold coordination becomes dominant.
Several of the Y and Lu phases have cations with 6- and 7-fold
coordination, consistent with trends observed amongst the
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LnCl3?xH2O and some lanthanide dicarboxylate frameworks.20
Structures 3–12 have a maximum of 1.5 ligands per cation,
reducing the number of methyl groups that need to be
accommodated in the structure; in most cases this results in
the anticipated two-dimensional layered structures.8–10 The
intermediate sized Ln, Eu and Tb, however, adopt 2,2-DMS
structures with three dimensional covalent connectivity. As
with 13 (see below), the formation of architectures where their
larger polyhedra are arranged with three-dimensional connectivity creates spaces that are suitable for accommodating
the methyl groups.
Almost all of the 2,2-DMS structures featuring the smaller Y
and Lu cations have two-dimensional covalent connectivity,
although the architectures of their layers are quite distinct. In
the Y case, low reagent concentrations lead to the formation of
6, which features pore water within a two dimensional
framework. As might be expected, however, increasing the
reagent concentrations results in phases from which the pore
water is eliminated. Structure 8 has higher condensation, as
measured by its lower O/Ln ratio, and is more dehydrated than
7. Its formation at higher temperatures is therefore consistent
with previous studies of succinate-related frameworks.9,11
Five Lu 2,2-DMS phases were observed during this study and
the trends among them are quite complex. The lowest
temperature phase is a Lu analogue of 6, which forms in
combination with 9, a Lu analogue of 7. The fraction of 9 in
the mixtures increases with increasing temperature, as
expected for the more dehydrated phase 9. Above 150 uC,
however, 10, 11 and 12 form with increasing temperature; all
of these phases are significantly more dehydrated and
condensed than 9. Bernini et al.17 have previously suggested
that the Er analogue of 10 is more thermodynamically stable
than the corresponding analogue of 9, with the latter being
favoured kinetically. In the present work, this is consistent
with 10 forming at higher temperatures than 9. The similar
degree of condensation in 10, 11 and 12 may be related to
issues encountered in obtaining these phases in pure form.
Condensation increases in the order that these three phases
appear, consistent with expected behaviour with increasing
temperature. The overall connectivity of the anhydrous, high
temperature phase, 12, is three dimensional, while all the
other Lu phases are two dimensional and hydrated, consistent
with previously reported studies.9,11 It should, however, be
noted that 10 is the only Lu phase that has any inorganic
connectivity. In addition, it has fewer water molecules in its
structure than we find in 11 (1/2 per cation compared to 2/3),
despite preferring to form at lower temperatures. This is the
opposite of the behaviour expected from previous studies, but
seems to be compensated by the inclusion of a hydroxide
ligand in 11, which is not found in 10; this ensures that 11 has
slightly higher condensation overall, along with higher
density.9,11 It is likely that the overall O/Ln ratio (Table 1) is
a more accurate insight into which phases will form with
increasing temperature in cases where the extent of hydration
is similar.
While the three dimensional structures formed by compounds 13 and 14 are consistent with those formed by the
other meso-2,3-DMS frameworks synthesized to date, 13 does
this despite two thirds of its ligands adopting gauche-
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arrangements.8 Amongst the transition metal 2,2-DMS frameworks, gauche-geometry was only adopted by the D- and Lligands, which prefer to form two dimensional, covalently
bonded structures. The meso-ligand, however, always adopts
the trans-geometry in the transition metal systems. 13 defies
this trend by forming a 3-D structure which contains both
conformations, suggesting that the larger lanthanide cations
can form structures with larger voids that are suitable for
accommodating methyl groups on the same side of the ligand.
This enables three dimensional connectivity and gauchearrangements to be simultaneously achieved.
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3.4. Thermal stabilities of the 2,2-DMS and 2,3-DMS phases
The thermal stabilities of the frameworks that could be
obtained with high purity were examined using thermogravimetric analysis (TGA). The results from this are summarized in
Table 2, with the details of their thermal stabilities and
decomposition being presented in Fig. S23–31, ESI.3 The first
noticeable result is that the 1-D frameworks, compounds 1 and
2, lose their coordinated water at lower temperatures and then
begin to decompose at much lower temperatures than the
other compounds examined. Interestingly, particularly in the
case of the La compound, it seems that one of the 2,2-DMS
ligands begins to decompose before the other. Compounds 6
and 13 also lose their water molecules at lower temperatures
than the other compounds, which is likely due to the presence
of pore water in 6 and the elongated bond distance between
the water molecule and the La cation in 13. Framework
decomposition, excluding any rearrangement caused by the
loss of coordinated water, occurs for most two and three
dimensional compounds at around 350 uC. However, 9
appears to be slightly more stable, only beginning to
decompose at 380 uC, while 3 and 13 decompose at
significantly lower temperatures than this. The particularly
low decomposition temperature of 3 is likely related to the loss
of the coordinated water at lower temperatures, which plays an
important role in the inorganic connectivity in this structure.
significant fluorescent behaviour, and nominal doping with
Eu and Tb above 3% led to the formation of phases
isostructural with the three dimensional architectures, 4 and
5. Only samples of 1, 6 and 9 doped with 3% nominal Eu or Tb
were characterized further as they retained pure phases with
one or two dimensional architectures that can potentially be
made into fluorescent nanosheets or chains.
Florescence measurements of the samples doped with a
nominal percentage of 3% Eu3+ revealed that the most
efficient excitation wavelength above 350 nm for all samples
is at 394 nm, which excites a transition from the 7F0 ground
state to the 5D3 (see Fig. 13).21 Non-radiative, energy transfer to
the 5D0 state via multi-phonon relaxation then occurs, with
subsequent sharp emission from the 5D0 level to the 7FJ, J = 0,
1, 2, 3 and 4, states. The strongest emission is to the 7F2 state,
which results in the red emission at around 615–620 nm. The
emission from the doped sample of 6 is strongest and is clearly
perceptible to the naked eye. The sample of 9 has about half
the emission intensity of 6, and the doped sample of 1 emits
only very weakly with about a quarter of the intensity of 6. The
excitation of these samples with lower wavelength radiation
does not qualitatively change the emission spectra, showing
that emission occurs from the 5D0 level regardless of the
energy level to which the Eu is initially excited.
378 nm excitation of samples of 1, 6 and 9 doped with 3%
nominal Tb3+ leads to an excitation from the 7F6 ground state
of Tb to the 5D3 state (see Fig. 14).21 Subsequent emission
occurs from the 5D4 state to the 7FJ, J = 6, 5, 4 and 3 states, with
distinct green luminescence to the 7F5 level being observed at
544 nm. As with the case of Eu doping, emission is strongest
from the doped sample of 6, with the emission of 9 being
slightly over 50% of that seen from 6 and 1 emitting
approximately half that of 9. Excitation with higher energy
ultraviolet radiation does not qualitatively change the spectra,
indicating that all emission occurs from the 5D4 state with any
additional energy again lost via multi-phonon relaxation.
3.5. Luminescence properties
Compounds 1, 6 and 9 were doped with Ce3+, Eu3+ and Tb3+ to
investigate their potential suitability as low dimensional,
florescence materials. Ce doping did not result in any
Table 2 Connectivity and temperature at which the liberation of water and
framework decomposition begins for each sample examined using TGA
Compound
Connectivity
Water
liberated (uC)
Framework
decomposition (uC)
1
2
3
4
5
6
9
13
14
I1O0
11O0
I1O1
I0O3
I0O3
I0O2
I0O2
I1O2
I1O2
100
80
150
150
140
110
130
110
N/A
150
150
280
350
350
350
380
320
360
108 | CrystEngComm, 2013, 15, 100–110
Fig. 13 Emission (main) and excitation (insert) spectra of samples of compounds
1, 6 and 9 doped with nominal 3% Eu.
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Paper
Diamond Light Source for access to Beamline I11, which,
contributed to the results presented here and to Alistair Lennie
and Stephen Thompson for their assistance in the use of the
beamline. P.J.S. would like to thank Jack Clegg for his advice
with regards to disorder modelling.
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References
Fig. 14 Emission (main) and excitation (insert) spectra of samples of compounds
1, 6 and 9 doped with nominal 3% Tb.
4. Conclusions
This work reports the synthesis and structures of fourteen new
Ln 2,2-DMS and 2,3-DMS frameworks, and shows that the Ln
compounds exhibit more structural diversity than is encountered amongst the analogous transition metal hybrids. While
both the 2,3-DMS frameworks are three dimensional, as
expected for frameworks incorporating the meso-isomer, the
2,2-DMS frameworks exhibit covalent bonding in one, two or
three dimensions.
In contrast to the clear correlation between cation radius
and inorganic connectivity that has been seen in alkaline earth
frameworks,19 the trends in the Ln-DMS systems are more
complex. The 2,2-DMS frameworks containing the largest
lanthanides form structures with one-dimensional inorganically connected chains and inclusion of two DMS ligands per
metal. As the ionic radius of the lanthanides decreases the
coordination number of the lanthanide also decreases
significantly and the ligand to metal ratio reduces to 1.5 : 1.
These frameworks form structures with two- or three-dimensional covalent connectivity, with the lanthanides with
intermediate ionic radii favouring the latter. The compounds
featuring the smallest cations, Y and Lu, appear to exhibit
more structural diversity, despite predominantly adopting two
dimensional layered structures; increasing reactant concentrations and temperature favour more condensed and, generally,
more dehydrated structures. Samples of 1, 6 and 9 doped with
Eu3+ and Tb3+ at the 3% level were also made. These exhibit
characteristic red and green emissions, respectively, which was
strongest in all cases for compound 6.
Acknowledgements
The authors would like to thank the European Research
Council for financial support. We would like to thank
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