Ligand-directed control over crystal structures of inorganic-organic frameworks and formation of solid solutions

Article abstract of DOI:10.1002/anie.201300440

Grounded in fact: Inorganic-organic frameworks with 3D Li-O-Li connectivity can form solid solutions through mechanochemical synthesis. High-resolution synchrotron powder X-ray diffraction and cross-polarization solid-state NMR spectroscopy demonstrate co

Full text of DOI:10.1002/anie.201300440

Angewandte
Chemie
DOI: 10.1002/anie.201300440
Hybrid Materials
Ligand-Directed Control over Crystal Structures of Inorganic–Organic
Frameworks and Formation of Solid Solutions**
Hamish H.-M. Yeung, Wei Li, Paul J. Saines, Thomas K. J. Kçster, Clare P. Grey, and
Anthony K. Cheetham*
The diversity of chemical and physical properties exhibited by
inorganic–organic frameworks has led to intense investigation
for many applications. Control of structure remains a central
element for the optimization of properties such as gas storage,
separations, catalysis, magnetism, luminescence, and conduc-
tivity.[1] Pore size, framework connectivity and dimensionality
can be tailored by variations in the metal nodes and organic
linkers.[2] Further control of pore shape and chemistry has
recently been achieved by placing ligands with different
functional groups on the same crystallographic site in a single
phase,[3] thereby enabling improved gas separation,[3c,d] cata-
lysis,[3a] and drug release.[3b] This development is analogous to
established solid solutions in important inorganic materials
such as ferroelectrics,[4] phosphors,[5] and catalysts.[6] In
inorganic–organic frameworks, however, control over stoi-
chiometry, phase segregation, and homogeneity remains
a challenge owing to crystallization kinetics, while differ-
entiation between compositions by X-ray diffraction has been
limited by the small variations in cell parameters that are
found in stable mixed-ligand phases.[3c]
compounds reveals that this compositional flexibility is
a result of structural flexibility in the metal coordination
sphere and ligand conformation.
The family of inorganic–organic frameworks with 3D Li–
O–Li connectivity, {Li2(flu)}n (1; flu = tetrafluorosuccinate),
{Li2(mal)}n (2; mal =l-malate), {Li2(met)}n (3; met = methyl-
succinate), and {Li2(suc)}n (4; suc = succinate), were prepared
by solution crystallization of lithium salts with the corre-
sponding ligand (see the Supporting Information for details).
Single-crystal X-ray diffraction studies showed that 1–3 are
ꢀ[9]
isostructural with 4, which adopts space group R3 (here
redetermined at 120 K; Figure 1).[10]
We report herein a new family of isostructural lithium-
based inorganic–organic frameworks, which form solid sol-
utions through mechanosynthesis, both in one-step reactions
and by combination of the end-member materials. By using
high-resolution synchrotron powder X-ray diffraction[7] and
cross-polarization solid-state NMR spectroscopy,[8] we dem-
onstrate apparently complete ligand mixing in the resulting
binary and ternary systems. Substitution of the ligands results
in changes of the unit cell volume of up to 8%, although
remarkably, the frameworks have 3D connectivity and con-
tain no solvent-accessible volume. Examination of the single-
crystal structures and mechanical properties of the parent
Figure 1. a) ORTEP extended asymmetric unit of 4; thermal ellipsoids
are set at 50% probability for C, Li, and O, and at 20% for H. b,c) The
bulk structure showing C, H, and O atoms and LiO4 tetrahedra in gray,
white, red, and green, respectively.
[*] H. H.-M. Yeung, Dr. W. Li, Prof. A. K. Cheetham
Department of Materials Science and Metallurgy
Cambridge University
The ligand substituents in 1–3 are accommodated by
distortions in the unit cell and atomic arrangement of the
parent compound 4 (Table 1). Substitution of all alkyl hydro-
Pembroke Street, Cambridge, CB2 3QZ (UK)
E-mail: akc30@cam.ac.uk
À
gen atoms by F atoms in 1 causes weakening of Li O bonds
and a reduction in the Li bond valence sum (BVS),[11] owing
to electron withdrawal from the carboxylate groups, thereby
resulting in expansion of LiO4 tetrahedra and an increase in
the cell volume (V) and the cell parameter ratio (a:c).
Fluorination has also been shown to cause the torsion angle
Dr. P. J. Saines
Inorganic Chemistry Laboratory, Oxford University
South Parks Road, Oxford, OX1 3QR (UK)
Dr. T. K. J. Kçster, Prof. C. P. Grey
Department of Chemistry, Cambridge University
Lensfield Road, Cambridge, CB2 1EW (UK)
between carboxylate groups and the carbon backbone, dCO
,
2
[**] This work was supported by the EPSRC (research studentship to
H.Y.) and ERC (Advanced Investigator Award to A.K.C.). We would
like to thank Prof. Chiu Tang for assistance on beamline I11 at
Diamond Light Source (UK).
to become perpendicular,[12] but the distortion of angles in the
LiO4 tetrahedron, dtet,[13] is constant within experimental
error. In contrast, inter-ligand hydrogen bonding between
trimers of OH groups in chiral 2 causes a contraction in the ab
plane, thus resulting in a decrease in a:c and V. In this case, the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300440.
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frameworks. In addition, Adams et al.[20] showed that it offers
fine control of the cell metrics of perhalocobalt bipy (bipy =
4,4’-bipyridine) coordination polymers through variation of
the Cl:Br ligand ratio. In this work, we believe that the
solvent-free conditions[21] and constant disruption of partic-
ulates in the grinding process enable kinetic stabilization of
homogeneous mixed-ligand phases. This may be contrasted
with the high temperatures used in the synthesis of inorganic
solid solutions; these temperatures prevent phase separation
by raising the entropic contribution to free energy, TDS, but
are not compatible with inorganic–organic framework mate-
rials owing to their lower stability.
The binary solid solution {Li2(suc)1Àx(flu)x}n (denoted as
4·1 (x), where x = 0–1) and the ternary system {Li2(suc)x-
(mal)y(met)z}n (denoted as 4·2·3 (x:y:z), where x + y + z = 1)
were synthesized by grinding stoichiometric amounts of the
component precursors for 30 min (see the Supporting Infor-
mation for details). High-resolution synchrotron powder X-
ray diffraction (HRS-PXRD) data were obtained for 4·1 in
stepwise increments of x, showing large shifts in cell
parameters (Figure 2). Rietveld refinement gave accurate
Table 1: Structural distortions and changes in unit cell parameters (1, 3,
and 4 in space group R3; 2 in R3) at 120 K in 1–4.
ꢀ
1
2
3
4
Subst.
4ꢀF
OH
1309.5(2)
1.10801(15)
1.09, 1.09
CH3
H
V [ꢁÀ3
a:c
]
1418.3(10)
1.17241(6)
0.99
1375.8(3)
1.1226(2)
1.02
1332.03(14)
1.14040(9)
1.09
Li BVS
dCO [8]
dtet [8]
89.9(2)
12.9(9)
93.5(3), 73.1(3)
10.9(15), 10(2)
63.8(10)
11.1(11)
91.75(14)
10.9(6)
2
carboxylate group furthest from the OH group of major
occupancy twists, while the Li BVS and dtet remain the same.
The bulky, hydrophobic methyl substituent in 3, which
underwent in situ racemization, causes large amounts of
strain, both in V and in the atomic structure. The observation
that crystals of the equivalent chiral structure do not form,
suggests that the steric interaction between neighboring
methyl groups is reduced when a mixture of enantiomeric
forms is present.
Compounds 1, 2, and 4 formed large, well-defined single
crystals, which allowed detailed investigation of the mechan-
ical properties by nanoindentation (Table 2; see Figures S1–
Table 2: Mechanical properties measured by nanoindentation and
calculated packing indices of 1, 2, and 4.
1
2
4
Substituent
4ꢀF
82.7
33.2(10)
2.35(7)
0.860(13)
OH
79.8
29.8(7)
2.28(6)
1.62(2)
H
73.5
20.3(6)
1.65(6)
1.116(15)
Packing index[a] [%]
Young’s modulus, E [GPa]
Hardness, H [GPa]
Yield stress, sy [GPa]
[a] Calculated by using PLATON;[17] only the moiety of the disordered
ligand with major occupancy in 2 was considered.
Figure 2. HRS-PXRD data for the binary solid solution {Li2(suc)1Àx
(flu)x}n (4·1 (x=0–1); l=0.826124 ꢁ).
-
S6 in the Supporting Information). Values for the Youngꢀs
modulus (E) and hardness (H) of {0,1,-1} facets correlate well
with packing index, thus indicating that short-range inter-
ligand interactions are important for reducing mechanical
compliance in these materials. In contrast, values for the
directional yield stress (sy) correlate with framework bond-
cell parameters for each sample and fractional occupancies
for H and F atoms consistent with elemental analysis and the
reagent stoichiometry (see Figures S7–S8, S13–S14). V varies
linearly with x, thus obeying Vegardꢀs law.[22] Cell parameters
a and c vary nonlinearly (Figure 3), thereby demonstrating
anisotropic structural strain owing to ligand mixing. The
HRS-PXRD data reveal some peak broadening in the solid
solutions compared with the end members, not observable in
laboratory data. This could simply be due to local strain
effects, or it might reflect a slightly inhomogeneous distribu-
tion of the ligand within individual particles.
À
ing: longer Li O bonds in 1 are broken more easily than those
in 2 and 4, thereby resulting in the lowest sy value. 2 and 4
À
have similar Li O bond strengths, but the additional hydro-
gen bonding between ligands in 2 causes an increase in its sy
value. While 1, 2, and 4 exhibit negligible creep owing to their
3D Li–O–Li network, they are more compliant than most
nonporous inorganic–organic frameworks previously stud-
ied.[14] The ionic lithium coordination sphere and torsional
degrees of freedom associated with the succinate-based
ligands impart more flexibility than, for example, covalent
copper-based units[15] and rigid oxalate ligands.[16]
13C–1H cross-polarization (CP) solid-state MAS-NMR
spectroscopy was used to confirm ligand mixing in the sample
4·1 (0.5), thereby giving qualitative insight into the spatial
The flexibility, both compositional and structural, of this
family of compounds suggested that it could be possible to
synthesize mixed-ligand phases with continuous variation in
properties between two or more pure end members. Mecha-
nochemistry has recently been employed by our group both in
the synthesis[18] and amorphization[19] of zeolitic imidazolate
proximity of suc and flu ligands. In the CP experiment, the 13
C
nuclei were polarized indirectly by a magnetization transfer
from coupled protons. The two different carbon atoms of suc
ligands in 4·1 (0.5) as well as the carboxylate carbon atom in
the flu ligand (168 ppm) were observed. Since the flu anion
does not contain any protons, the observed signal must be due
2
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Figure 5. a) FTIR of 4·1 (x=0–1) in the carboxylate region.
Figure 3. Unit cell parameters for 4·1 (x=0–1) from Rietveld refine-
ment of HRS-PXRD data. Error bars are smaller than markers.
increases, which forces the suc and flu ligands into progres-
sively larger and smaller coordination environments, respec-
tively. Thermogravimetric analysis (TGA) of 4·1 shows
a decrease in decomposition temperature as the ligands are
mixed, with 4·1 (x = 0.8) having the lowest stability (see
Figure S12 in the Supporting Information).
to a polarization transfer from suc protons to flu 13C nuclei.
The intensity of peaks in the 13C spectrum was measured as
a function of cross-polarization contact time; a signal at
168 ppm was observed, which corresponds to flu 13C nuclei
excited by protons on nearby suc ligands (Figure 4). The
The cell parameters of the ternary system 4·2·3 were
determined by Le Bail fitting of HRS-PXRD data. V varies
most with increasing z, owing to the steric bulk of methyl
substituents, whereas it remains almost unchanged when
x and y vary independently of z (Figure 6). The largest
stepwise increase in V is seen from 4·2·3 (0:1:0) to (0:0.8:0.2),
which is due to the simultaneous disruption of cooperative
hydrogen bonding and addition of methyl groups to the mal-
ligand-rich sample.
Figure 4. 13C–1H CP MAS-NMR of 4·1 (0.5) with variable contact time.
The inset shows the region at 160–180 ppm. Cd is not observed owing
to a lack of 19F decoupling; asterisks mark spinning sidebands from Ca
and Cc.
build-up of this peak was slower than those of suc nuclei
owing to their different distances from suc protons, as
expected from the crystallographic structure (see Figur-
es S9,S10 in the Supporting Information). Notably, owing to
poor 1H–13C coupling, the 13C–1H CP MAS-NMR spectrum of
tetrafluorosuccinic acid contains no peaks (see Figure S11 in
the Supporting Information), thus confirming that the flu
signal in 4·1 (0.5) arises because of the proximity of the
different ligands on the molecular level in the same crystalline
phase.
Figure 6. Trends in cell volume (V) of the ternary system 4·2·3 (x:y:z).
In summary, our work represents significant progress in
the area of inorganic–organic frameworks for four important
reasons. Firstly, through the combination of HRS-PXRD and
CP MAS-NMR spectroscopy we demonstrate complete solid
solutions in the binary and ternary systems, {Li2(suc)1Àx(flu)x}n
and {Li2(suc)x(mal)y(met)z}n, respectively. Secondly, we pres-
ent a simple, cheap, and efficient method, namely mechano-
chemical synthesis, used for the first time to produce mixed-
ligand materials. Thirdly, through examination of the single-
crystal structures and mechanical properties of the isostruc-
The ratio of peaks from suc and flu ligands in the FTIR
spectra of 4·1 varies smoothly with x (Figure 5). As a further
À
indication of ligand mixing, the CO2 antisymmetric stretch
assigned to one ligand shifts to higher frequency owing to
perturbation of the structure as the fraction of the other
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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[4] G. Catalan, J. F. Scott, Adv. Mater. 2009, 21, 2463 – 2485.
[5] H. A. Hçppe, Angew. Chem. 2009, 121, 3626 – 3636; Angew.
Chem. Int. Ed. 2009, 48, 3572 – 3582.
[6] A. K. Cheetham, G. Fꢁrey, T. Loiseau, Angew. Chem. 1999, 111,
3466 – 3492; Angew. Chem. Int. Ed. 1999, 38, 3268 – 3292.
[7] S. P. Thompson, J. E. Parer, J. Marchal, J. Potter, A. Birt, F. Yuan,
R. D. Fearn, A. R. Lennie, S. R. Street, C. C. Tang, J. Synchro-
tron Radiat. 2011, 18, 637 – 648.
[8] H. C. Hoffmann, M. Debowski, P. Mꢂller, S. Paasch, I. Senkov-
ska, S. Kaskel, E. Brunner, Materials 2012, 5, 2537 – 2572.
[9] H. Klapper, H. Kuppers, Acta Crystallogr. Sect. B 1973, 29, 21 –
26.
[10] Crystal data for 1 – 4 can be found in the Supporting Informa-
tion. CCDC 910413, 910414, 910415, 910416 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[11] N. E. Brese, M. OꢀKeeffe, Acta Crystallogr. Sect. B 1991, 47,
192 – 197.
tural parent compounds, we show that this compositional
flexibility is a result of structural flexibility in the metal
coordination environment and ligand torsional angles. Finally,
this work is the first of its kind involving a 3D, dense
inorganic–organic framework system, opening up the possi-
bility of similar investigations into materials that were to date
thought to be unsuitable for ligand substitution. These
materials have potential applications owing to their conduct-
ing, ferroelectric, luminescent, and magnetic behavior.[1a,b]
Received: January 17, 2013
Published online: && &&, &&&&
Keywords: inorganic–organic frameworks · ligand effects ·
.
lithium · nanoindentation · solid solution
ˇˇ ´
[12] T. Friscic, D. G. Reid, G. M. Day, M. J. Duer, W. Jones, Cryst.
[1] a) C. Janiak, Dalton Trans. 2003, 2781 – 2804; b) C. N. R. Rao,
A. K. Cheetham, A. Thirumurugan, J. Phys. Condens. Matter
2008, 20, 083202; c) S. Horike, S. Shimomura, S. Kitagawa, Nat.
Chem. 2009, 1, 695 – 704.
Growth Des. 2011, 11, 972 – 981.
[13] M. Harding, Acta Crystallogr. Sect. D 2000, 56, 857 – 867.
[14] J.-C. Tan, A. K. Cheetham, Chem. Soc. Rev. 2011, 40, 1059 –
1080.
[15] J.-C. Tan, C. A. Merrill, J. B. Orton, A. K. Cheetham, Acta
Mater. 2009, 57, 3481 – 3496.
[16] J.-C. Tan, J. D. Furman, A. K. Cheetham, J. Am. Chem. Soc.
2009, 131, 14252 – 14254.
[17] A. L. Spek, Acta Crystallogr. Sect. D 2009, 65, 148 – 155.
[18] P. J. Beldon, L. Fabian, R. S. Stein, A. Thirumurugan, A. K.
[2] a) M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M.
OꢀKeeffe, O. M. Yaghi, Science 2002, 295, 469 – 472; b) L. N.
Appelhans, M. Kosa, A. V. Radha, P. Simoncic, A. Navrotsky, M.
Parrinello, A. K. Cheetham, J. Am. Chem. Soc. 2009, 131,
15375 – 15386; c) P. J. Saines, P. Jain, A. K. Cheetham, Chem. Sci.
2011, 2, 1929 – 1939; d) P. J. Saines, M. Steinmann, J.-C. Tan,
H. H.-M. Yeung, W. Li, P. T. Barton, A. K. Cheetham, Inorg.
Chem. 2012, 51, 11198 – 11209.
ˇˇ ´
Cheetham, T. Friscic, Angew. Chem. 2010, 122, 9834 – 9837;
Angew. Chem. Int. Ed. 2010, 49, 9640 – 9643.
[19] T. D. Bennett, S. Cao, J.-C. Tan, D. A. Keen, E. G. Bithell, P. J.
[3] a) W. Kleist, F. Jutz, M. Maciejewski, A. Baiker, Eur. J. Inorg.
Chem. 2009, 3552 – 3561; b) K. M. L. Taylor-Pashow, J. Della
Rocca, Z. G. Xie, S. Tran, W. B. Lin, J. Am. Chem. Soc. 2009, 131,
14261; c) T. Fukushima, S. Horike, Y. Inubushi, K. Nakagawa, Y
Kubota, M. Takata, S. Kitagawa, Angew. Chem. 2010, 122, 4930 –
4934; Angew. Chem. Int. Ed. 2010, 107, 19033 – 16038; d) H. X.
Deng, C. J. Doonan, H. Furukawa, R. B. Ferreira, J. Towne, C. B.
Knobler, B. Wang, O. M. Yaghi, Science 2010, 327, 846 – 850;
e) S. Marx, W. Kleist, J. Huang, M. Maciejewski, A. Baiker,
Dalton Trans. 2010, 39, 3795 – 3798.
ˇˇ ´
Beldon, T. Friscic, A. K. Cheetham, J. Am. Chem. Soc. 2011, 133,
14546 – 14549.
[20] C. J. Adams, M. F. Haddow, M. Lusi, A. G. Orpen, Proc. Natl.
Acad. Sci. USA 2010, 107, 16033 – 16038.
[21] Liquid drops of ca. 50 mL and water produced during the
reaction are not thought to dissolve reagents enough to be
considered solvents.
[22] L. Vegard, Z. Phys. 1921, 5, 17 – 26.
4
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Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Hybrid Materials
Grounded in fact: Inorganic–organic
frameworks with 3D Li–O–Li connectivity
can form solid solutions through mecha-
nochemical synthesis. High-resolution
synchrotron powder X-ray diffraction and
cross-polarization solid-state NMR spec-
troscopy demonstrate complete ligand
mixing in the resulting binary and ternary
systems (see picture for trends in unit cell
volume (V) of the ternary system {Li2-
(suc)x(mal)y(met)z}n).
H. H.-M. Yeung, W. Li, P. J. Saines,
T. K. J. Kçster, C. P. Grey,
A. K. Cheetham*
&&&& — &&&&
Ligand-Directed Control over Crystal
Structures of Inorganic–Organic
Frameworks and Formation of Solid
Solutions
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!