Coordination polymers of flexible tetracarboxylic acids with metal ions. I. Synthesis of CH2- and (CH2)2-spaced bis(oxy)isophthalic acid ligands, and structural characterization of their polymeric adducts with lanthanoid i

Article abstract of DOI:10.1039/c0ce00474j

We report herein on the synthesis and structure of new tetracarboxylic ligands, 5,5′-methylene-bis(oxy)diisophthalic acid and 5,5′-(ethane-1,2-diyl)-bis(oxy)diisophthalic acid, bearing flexible spacers between the two isophthalic acid fragments, as well a

Full text of DOI:10.1039/c0ce00474j

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PAPER
Coordination polymers of flexible tetracarboxylic acids with metal ions. I.
Synthesis of CH2- and (CH2)2-spaced bis(oxy)isophthalic acid ligands, and
structural characterization of their polymeric adducts with lanthanoid ions†
*
Anirban Karmakar and Israel Goldberg
Received 8th April 2010, Accepted 3rd August 2010
DOI: 10.1039/c0ce00474j
We report herein on the synthesis and structure of new tetracarboxylic ligands, 5,50-methylene-
bis(oxy)diisophthalic acid and 5,50-(ethane-1,2-diyl)-bis(oxy)diisophthalic acid, bearing flexible spacers
between the two isophthalic acid fragments, as well as of the coordination polymers of these two ligands
with various lanthanoid metal ions. The formed compounds were characterized by X-ray
crystallography, IR spectroscopy and thermal analysis. The nine hybrid organic–inorganic
supramolecular assemblies reveal polymeric architectures of either two-dimensional or three-
dimensional connectivity.
unusual networking features and unique structural properties.6,7
Introduction
Indeed, the reported MOFs based on lanthanoid ion connectors
reveal interesting topological structures, such as 1D chains,8 2D
grids,9 3D porous frameworks, and interpenetrating networks.10
Some of these lanthanoid materials exhibit also unique photo-
physical properties, which can be attributed to f–f transitions
with an extremely narrow bandwidth.11 The interplay between
rigidity and flexibility of the organic component may have
various implications on the formed lattice. Rigidity is beneficial
for the generation of porosity, because it cannot permit struc-
tural distortion of the framework.12 On the other hand the use of
flexible multicarboxylate ligands as building blocks in the
assembly of coordination frameworks is attractive because some
conformational freedom of the ligand may offer various possi-
bilities for release of the steric strain imposed by the metal–ligand
association and relaxation of the network architecture.13
The design and synthesis of Metal–Organic Frameworks
(MOFs) have attracted great interest due their potential use as
gas storage, magnetism, catalysis activity, ion exchange, molec-
ular recognition, and optical properties.1 In this respect, various
theoretical predictions and network-based approaches to control
the network topology and geometries have been made to produce
useful functional materials.2 A remarkable progress has been
made in recent years in the development of porous metal–organic
frameworks by using multidentate aromatic carboxylic acid
ligands as building blocks, due to their robustness and thermal
stability.3 Moreover, these ligands can be readily deprotonated to
balance the charge of the metal ions they interact with, without
the need to include in the resulting framework lattice additional
uncoordinated counter ions which could occupy and block the
channel voids. These features of the metal–carboxylate lattices
could be quite useful in various applications such as size- and
shape-selective separations and catalysis.4 Until now, MOFs
utilizing common transition metal ions as inter-ligand connectors
have been studied in more depth, most probably due to their well
defined crystal field effects and preferred coordination geome-
tries. Analogous chemistry of the lanthanoid ions is still lacking
in scope.5 On the other hand, the high and variable coordination
numbers, flexible coordination geometries, oxophilic and hard
nature, and the presence of multi-single electrons of the lantha-
noids provide exceptional opportunities for the discovery of
In the latter context we describe the coordination polymeri-
zation features of two newly designed flexible tetracarboxylic
acid ligands. To this end we used methane or ethane spacer units
for interconnection between two oxyisophthalic acid moieties
through their ether linkage and generation of the tetracarboxylic
acid precursors: 5,50-methylene-bis(oxy)diisophthalic acid (H4L0)
and 5,50-(ethane-1,2-diyl)-bis(oxy)diisophthalic acid (H4L00)
(Schemes 1 and 2). This molecular design of the organic
component was based on the following considerations: (a) the
flexible and multifunctional coordination sites may generate
multidimensional structures; (b) the ligand can be singly, doubly,
triply or quadruply deprotonated to the corresponding carbox-
ylate species (H3Lꢀ, H2L2ꢀ, H1L3ꢀ, and L4ꢀ) to allow its diverse
coordination modes to the inorganic connectors. We report
herein on the synthesis and structure of the two ligands, H4L0 and
H4L00, and on the versatile architectures of the coordination
polymers obtained by reacting these ligands with a series of
School of Chemistry, Sackler Faculty of Exact Sciences, Tel-Aviv
University, Ramat-Aviv, 69978 Tel-Aviv, Israel. E-mail: anirban@iitg.
ernet.in; goldberg@post.tau.ac.il
† CCDC reference numbers 772531–772541 (for structures 1–11,
respectively) For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0ce00474j
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Scheme 1 Synthesis of 5,50-methylene-bis(oxy)diisophthalic acid (H4L0).
lanthanoid(III) nitrates under hydrothermal conditions. Prior to
the polymerization reactions with the lanthanoid ions, H4L0
could be crystallized as a co-crystal of this ligand with 1,2-
bi(4-pyridyl)ethane (bpe) and water (1 : 3 : 3), while H4L00 could
be crystallized as a monohydrate. The empirical formulae of the
resulting analyzed compounds include:
subsequent removal of the solvent gave the ethyl ester of H4L0.
Yield: 73%. In the second step the isolated ester (2.44 g, 5 mmol)
and NaOH (0.6 g, 15 mmol) were dissolved in 20 mL of
EtOH : water (4 : 1). The reaction mixture was refluxed for 4 h at
80 ꢁC, after which the solvent was removed under reduced
pressure, 10 mL of water was added into it and the solution was
acidified (pH z 2) with dilute HCl solution. The obtained white
solid product H4L0 was filtered and washed with water until free
from acid. Yield: 62%. FT-IR (KBr, cmꢀ1): 3375 (bs, nO–H), 2987
(mb, nC–H), 1700 (s, nC]O asymmetric), 1598 (s, nC]C), 1456 (m,
nC]C), 1402 (m, nC]O symmetric), 1269 (bs, nC–O), 1128 (w),
1103 (w), 1024 (s), 907 (w), 759 (s), 727 (w), 687 (m), 666 (m),
(1) H4L0$3(bpe)$3H2O
(2) H4L00$H2O
(3) [La3+(H2O)$(HL0)3ꢀ
]
(4) [Nd3+(H2O)$(HL0)3ꢀ
]
(5) [Sm3+(H2O)2$(HL0)3ꢀ]$5H2O
(6) [Dy3+(H2O)2$(HL0)3ꢀ]$3H2O
(7) [Eu3+(H2O)2$(HL0)3ꢀ]$3H2O
(8) [La3+(H2O)3$(HL00)3ꢀ]$H2O
(9) [Nd3+(H2O)2$(HL00)3ꢀ]$3½H2O
(10) [Sm3+(H2O)2$(HL00)3ꢀ]$3½H2O
(11) [Ce3+(H2O)2$(HL00)3ꢀ]$xH2O
1
609 (w), 545 (w), 449 (w); H-NMR (DMSO-d6): 8.58 (2H, s),
8.26 (4H, s), 6.51 (2H, s); 13C-NMR (DMSO-d6): 166.58, 156.59,
133.47, 124.39, 121.06, 90.69.
The ligand H4L00 was synthesized by a similar pathway
(Scheme 2). In the first stage diethyl ester of 5-hydroxyisoph-
thalic acid (2.38 g, 10 mmol), dibromoethane (0.93 g, 5 mmol)
and anhydrous K2CO3 (1.65 g, 12 mmol) were placed in a round
bottom flask and then added dry acetone (20 mL). The reaction
mixture was stirred for 24 h at 60 ꢁC, the reaction progress being
monitored at regular intervals by TLC. Then, after removing
unreacted K2CO3 by filtration, and removing the solvent under
reduced pressure, a white solid was isolated. It was washed with
NaOH(aq.) (5%), extracted with dichloromethane, yielding the
ethyl ester of H4L00 in 64% yield. At the second stage 2.44 g
(5 mmol) of the latter and 0.6 g (15 mmol) of NaOH were dis-
solved in 20 mL of EtOH : water (4 : 1). As in the previous
Experimental
All reagents and solvents were purchased from commercial
sources and used without further purification. FT-IR spectra were
recorded in the range of 400–4000 cmꢀ1 from KBr pellets (Aldrich
99+%, FT-IR grade) using a Bruker PS15 spectrophotometer.
Synthesis of H4L0 and H4L00 ligands
The H4L0 compound was synthesized by a two step procedure
(Scheme 1).
ꢁ
procedure, after reflux of the reaction mixture for 4 h at 80 C,
followed by removal of the solvent at reduced pressure, acidifi-
cation with dilute HCl, washing with water and filtering of the
product, white solid of H4L00 was obtained in 81% yield. FT-IR
(KBr, cmꢀ1): 3355 (bs, nO–H), 2970 (mb, nC–H), 1701 (s, nC]O
asymmetric), 1596 (s, nC]C), 1421 (s, nC]O symmetric), 1280 (s,
nC–O), 1184 (m), 1134 (m), 1106 (w), 1057 (s), 887 (m), 760 (s),
In the first step diethyl ester of 5-hydroxyisophthalic acid
(2.38 g, 10 mmol), dibromomethane (0.86 g, 5 mmol) and
anhydrous K2CO3 (1.65 g, 12 mmol) were placed in a round
bottom flask and then added dry acetone (20 mL). The reaction
mixture was stirred for 8 h at 60 ꢁC (the reaction progress being
monitored at regular intervals by TLC). After completion of the
reaction, it was filtered off (to remove unreacted K2CO3). The
solvent was removed under reduced pressure and a white solid
was obtained. The isolated product was washed with NaOH (5%)
solution in water and then extracted with dichloromethane. The
organic extracts were collected over anhydrous sodium sulfate;
1
738 (m), 689 (m), 666 (m), 578 (w), 444 (w); H-NMR (DMSO-
d6): 8.09 (2H, s), 7.70 (4H, s), 4.48 (2H, s); 13C-NMR (DMSO-
d6): 166.34, 158.46, 132.66, 122.51, 119.22, 67.02.
Crystallization experiments of the two ligands proceeded as
follows.
Scheme 2 Synthesis of 5,50-(ethane-1,2-diyl)-bis(oxy)diisophthalic acid (H4L00).
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Compound 1. A mixture of H4L0 (20 mg, 0.05 mmol) and 1,2-
bis(4-pyridyl)ethane (bpe, 18 mg, 0.1 mmol) was placed in
a glass vessel and dissolved in 4 mL of MeOH/DMF/H2O (1/0.5/
0.5 v/v/v). The resulting yellow solution was kept for crystalli-
zation. After 1 week block shaped colorless crystals of the
hydrated H4L0-bpe adduct were obtained. Yield: 64%. FT-IR
(KBr, cmꢀ1): 3418 (mb, nO–H), 3058 (w, nC–H), 2927 (w, nC–H),
1931 (w, nC]C), 1709 (s, nC]O asymmetric), 1602 (s, nC]C), 1450
(m, nC]C), 1420 (m, nC]O symmetric), 1273 (bs, nC–O), 1228 (s),
1028 (s), 893 (w), 832 (m), 759 (m), 690 (w), 547 (s), 490 (w).
on Dy). FT-IR (KBr, cmꢀ1): 3464 (bs, nO–H coordinated water),
2925 (w, nC–H), 1693 (s, nC]O), 1626 (s, nC]O asymmetric), 1532
(s), 1457 (m, nC]C), 1396 (s, nC]O symmetric), 1283 (w), 1245 (m,
nC–O), 1150 (w), 1027 (s), 925 (w), 774 (s), 692 (m), 570 (w).
Compound 7. A mixture of Eu2(SO4)3$xH2O (17.7 mg,
0.03 mmol), H4L0 (4 mg, 0.01 mmol) and H2O (5 mL) was placed
in a capped glass vessel and heated at 100 ꢁC for 48 h. After the
reaction mixture was cooled to room temperature (at 0.2 ꢁC
minꢀ1 rate), the rod like colorless crystals were obtained, which
were washed with water and dried in air. Yield: 20% (based on
Eu). FT-IR (KBr, cmꢀ1): 3398 (bs, nO–H), 2921 (w, nC–H), 1672 (s,
nC]O), 1598 (s, nC]O asymmetric), 1540 (s), 1457 (m, nC]C), 1396
(s, nC]O symmetric), 1309 (w), 1240 (m, nC–O), 1142 (w), 1024 (s),
921 (w), 769 (s), 700 (m), 541 (w).
Compound 2. A mixture of H4L00 (20 mg, 0.05 mmol), La(N-
O3)3$6H2O (21 mg, 0.1 mmol), 0.1 mL of HCl (3 M), and H2O
(7 mL) was placed in a 8 mL capped glass vessel, at pH ¼ 2. It
was heated at 100 ꢁC for 2 days, anticipating a coordination
product between the organic and inorganic components. Instead,
upon subsequent cooling to room temperature of the product
sample colorless crystals of hydrated H4L00 were obtained inad-
vertently.
Compound 8. Compound 8 was prepared by a hydrothermal
reaction. A mixture of La(NO3)3$6H2O (4.33 mg, 0.01 mmol),
H4L00 (2 mg, 0.005 mmol) and H2O (7 mL) was placed in a cap-
ped glass vessel and heated at 120 ꢁC for 48 h. After the sample
was cooled gradually to room temperature (0.2 ꢁC minꢀ1),
washed with water and air-dried, block-shaped colorless crystals
were obtained. Yield: 32% (based on La). FT-IR (KBr, cmꢀ1):
3361 (w, nO–H coordinated water), 2935 (w, nC–H), 1660 (s, nC]O),
1537 (s, nC]O asymmetric), 1448 (s, nC]C), 1386 (s, nC]O
symmetric), 1280 (m, nC–O), 1229 (w), 1076 (w), 1036 (m),
985 (m), 784 (m), 722 (m), 682 (w), 521 (w).
Synthesis of the coordination polymers
All the metal–ligand coordination reactions were carried out in
aqueous environments.
Compound 3. This complex was prepared by a hydrothermal
reaction. A mixture of La(NO3)3$6H2O (4.33 mg, 0.01 mmol),
H4L0 (2 mg, 0.005 mmol) and H2O (7 mL) was placed in a capped
glass vessel and heated at 120 ꢁC for 48 h. After cooling the
sample to room temperature (0.2 ꢁC minꢀ1), block-shaped
colorless crystals appeared, were washed with water and dried in
air. Yield: 22% (based on La). FT-IR (KBr, cmꢀ1): 3376 (bs, nO–H
coordinated water), 1713 (s, nC]O), 1613 (m, nC]O asymmetric),
1536 (s), 1462 (m, nC]C), 1380 (s, nC]O symmetric), 1320 (m),
1243 (s, nC–O), 1198 (s), 1144 (m), 1062 (m), 1025 (s), 933 (w),
778 (s), 722 (m), 683 (s), 501 (w), 442 (w).
Compounds 9, 10 and 11. These three products were prepared
by the same synthetic strategy.
A
mixture of either
Nd(NO3)3$6H2O (8.6 mg, 0.02 mmol) or Sm(NO3)3$6H2O
(9 mg) or Ce(NO3)3$6H2O (8.7 mg), H4L00 (4 mg, 0.01 mmol),
2,20-bipyridine (1.5 mg, 0.01 mmol) and H2O (7 mL) was placed
in a capped glass vessel and heated at 100 ꢁC for 48 h. The yields
of the corresponding crystalline products obtained after slow
cooling (at 0.2 ꢁC minꢀ1 rate) were approximately 21% based on
Ln. For (9), FT-IR (KBr, cmꢀ1): 3386 (bs, nO–H coordinated
water), 2955 (w, nC–H), 1664 (s, nC]O), 1608 (m) 1545 (s, nC]O
asymmetric), 1454 (s, nC]C), 1390 (s, nC]O symmetric), 1332 (w),
1265 (m, nC–O), 1132 (m), 1063 (s), 930 (w), 772 (s), 733 (w), 684
(m). For (10), FT-IR (KBr, cmꢀ1): 3421 (bs, nO–H coordinated
water), 2945 (w, nC–H), 1667 (s, nC]O), 1607 (m) 1554 (s, nC]O
asymmetric), 1455 (s, nC]C), 1391 (s, nC]O symmetric), 1267
(m, nC–O), 1132 (m), 1065 (s), 1005 (w), 772 (s), 728 (w). For (11),
FT-IR (KBr, cmꢀ1): 3428 (bs, nO–H coordinated water), 3172 (bs,
nC–H), 1714 (s, nC]O), 1608 (m) 1551 (s, nC]O asymmetric), 1387
(s, nC]O symmetric), 1330 (w), 1273 (m, nC–O), 1203 (s), 1133 (s),
1065 (m), 1002 (m), 894 (w), 820 (w), 769 (m), 728 (s).
Compound 4. Compound 4 was prepared by the same proce-
dure as 3, but using Nd(NO3)3$6H2O (4.38 g, 0.01 mmol). Yield:
29% (based on Nd). FT-IR (KBr, cmꢀ1): 3159 (bw), 1702
(s, nC]O), 1615 (m, nC]C), 1571 (s, nC]O asymmetric), 1545 (s),
1466 (m, nC]C), 1386 (s, nC]O symmetric), 1234 (w, nC–O), 1145
(w), 1026 (s), 932 (m), 773 (s), 681 (s).
Compound 5. Compound 5 was prepared by the same method
described for 3 and 4, using Sm(NO3)3$6H2O (4.44 mg, 0.01
mmol) instead of the La/Nd salts. Yield: 35% (based on Sm). FT-
IR (KBr, cmꢀ1): 3209 (bs, nO–H coordinated water), 1700 (s,
nC]O), 1620 (s, nC]C), 1573 (s, nC]O asymmetric), 1537 (s), 1469
(s, nC]C), 1387 (s, nC]O symmetric), 1225 (m, nC–O), 1146 (w),
1027 (s), 932 (m), 888 (w), 773 (s), 716 (m), 682 (s).
Crystal structure determinations
The X-ray measurements (Nonius KappaCCD diffractometer,
MoKa radiation) were carried out at approximately 110 K on
crystals coated with a thin layer of amorphous oil to minimize
crystal deterioration, possible structural disorder and related
thermal motion effects, and to optimise the precision of the
structural results. These structures were solved by direct methods
(SIR-97) and refined by full-matrix least-squares (SHELXL-
97).15 All non-hydrogen atoms were refined anisotropically. The
Compound 6. Dy(NO3)3$6H2O (4.56 mg, 0.01 mmol), H4L0
(2 mg, 0.005 mmol) and H2O (3 mL) were mixed in a 5 mL
beaker. After being stirred for 2 h, the mixture was sealed in the
bomb reactor in thermal conditiꢁons, heated at 120 ꢁC for 3 days,
and then cooled gradually (0.2 C minꢀ1) to room temperature.
After filtration, the product (colorless crystals) was washed with
distilled water and dried at room temperature. Yield: 54% (based
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hydrogen atoms attached to carbon were located in idealized/
calculated positions and were refined using a riding model, with
Uiso ¼ 1.2Ueq of the parent atom. Those attached to O and N
atoms, which are involved in hydrogen bonding (except for the
disordered species) were located in difference-Fourier maps; then
their O–H and N–H distances were restrained to common values.
Crystallographic refinements of all the structures converged to
acceptable R-values, representing precisely determined structural
models of compounds 1–11 and allowing reliable characteriza-
tions of the molecular structures and supramolecular binding
motifs. Crystals of the two ligands and most of the polymeric
assemblies contain non-coordinated molecules of water as crys-
tallization solvent, which engage in extensive supramolecular
hydrogen bonding schemes. In 1 (co-crystal of H4L0 and bpe), the
bipyridyl moieties (the central bond in one of them being
orientationally disordered) are also involved in extensive H-
bonding with the tetracarboxylic ligand. In 11, most of the water
solvent was found severely disordered, and could not be
modelled by discrete atoms. Correspondingly, the contribution
of the disordered solvent moieties was subtracted from the
diffraction pattern by the SQUEEZE procedure and PLATON
software.16 The crystallographic and experimental data for 1–11
are given in Table 1. The uniform identity of the formed crystal
lattices (1–11) in a given reaction was confirmed in each case by
repeated measurements of the unit-cell dimensions from different
single crystallites.
Results and discussion
General features of the FT-IR spectra for compounds 1–11 are
almost identical through the region from 400 to 4000 cmꢀ1 14
The
.
characteristic strong bands of coordinated carboxylate groups
are shown in the range of 1537–1626 cmꢀ1 for asymmetric
stretching and 1386–1421 cmꢀ1 for symmetric stretching. C–O
stretching of coordinated carboxylate groups comes into view in
between 1225 and 1280 cmꢀ1. A strong absorption band between
1660 and 1714 cmꢀ1 appeared due to the C]O stretching
frequency of uncoordinated carboxylate groups. The O–H
stretching frequency of carboxylic acid and water molecules are
visible in the region of 3209–3464 cmꢀ1. The nC–H vibration
modes of –CH2– groups of methane or ethane spacers unit
appear as a weak band in between 2925 and 3172 cmꢀ1. The
bands in the region 1607–1620 cmꢀ1 and 1448–1469 cmꢀ1 are
attributed to the C]C stretching frequency of aromatic rings. In
compound 1, weak absorption was observed at 3058 cmꢀ1 due to
nC–H of the free 1,2-bis(4-pyridyl)ethane.
The molecular structures of the two ligands in compounds 1
and 2 are shown in Fig. 1, revealing their conformations. H4L0 is
characterized by a V-shape geometry and has a C2 symmetry,
with the peripheral carboxylic functions at the top of the
molecular framework pointing sideways in opposite directions.
In 1, they are engaged in extensive COOH/N and COOH/O
hydrogen bonding interactions with the surrounding bpe and
Fig. 1 Ball-and-stick illustrations of the molecular structures of (a) H4L0 in 1 and (b) H4L00 in 2.
Fig. 2 Hydrogen bonding interactions (dashed lines) of the H4L0 and H4L00 ligands with the surrounding moieties. Wireframe representation, except for
the water species, (spheres) and the central H4L00 molecule in (b) (ball-and-stick)0.0 (a) In 1, H4L0 forms four O–H/N and two O/H–O (water) hydrogen
ꢁ
ꢁ
bonds at 2.579(3)–2.586(3) A and 2.808(4) A, respectively. (b) In 2, each H4L molecule is directly involved in six O–H/O ligand–ligand hydrogen
ꢁ
ꢁ
ꢁ
bonds at 2.588(2)–2.678(2) A, and additional O–H/O (water) ¼ 2.593(2) A, O–H (water)/O (carboxy) ¼ 2.732(2) A, and O–H (water)/O (ether) ¼
ꢁ
3.019(2) A interactions.
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The extensive hydrogen bonding interactions in all the
analyzed compounds 1–11 are detailed in the Crystallographic
Information Files (CIFs)† of the corresponding structures.
All the lanthanoid metal ions that reacted with H4L0 and H4L00
appear in their most common oxidation state of 3+. A 1 : 1
reaction between the organic and the inorganic components is
associated, due to charge balance requirements, with triple
3ꢀ
deprotonation of the organic species to a HL03ꢀ or HL00 state.
The hydrogen bonding capacity of the remaining carboxylic acid
function is then normally utilized in hydrogen bonding to
neighboring ligand moieties or to the water solvent. Compounds
3 and 4 are isomorphous (Table 1), representing 1 : 1 two-
dimensional coordination polymerization of H4L0 with La and
Nd ions, respectively (Fig. 3, hydrothermal reactions between
Ce(NO3)3$6H2O or PrCl3 with H4L0 also led to isomorphous
materials, but their crystals were of lower quality). The metal
ions are nine-coordinate and arrange in pairs across centers of
inversion. Eight carboxylate groups are coordinated to the two
inversion-related metal centers. The latter constitute a di-nuclear
core acting as a secondary building block unit in the construction
of the two-dimensional polymeric assembly. Every metal ion
coordinates through one monodentate, one chelating, one m2–h1–
h1-bridging and finally one m2–h2–h1-bridging carboxylate groups
(see below). In addition its ninth coordination site is occupied by
a water molecule.
Fig. 3 The two-dimensional bilayered coordination polymerization in 3
(4 is isomorphous). The metal ions and their water ligands are denoted by
small spheres.
water moieties (Fig. 2a), forming a 3D hydrogen-bonded motif
throughout the crystal. The overall conformation of H4L0 can be
best characterized by two parameters: the dihedral angle between
the two phenyl rings of d0 ¼ 61.4(1)ꢁ, and the Cphenyl–O/O–
Cphenyl torsion angle in the central part of the molecule of s0 ¼
124.7(3)ꢁ. It should be pointed out, however, that the actual
conformation of the free ligand when not ion-paired with the bpe
Lewis base could be somewhat different. The observed structure
of H4L00 in 2 is characterized by gauche conformation around the
central CH2–CH2 bond with the O–C–C–O torsion angle of s00 ¼
69.2(2)ꢁ, and dihedral angle d00 between the two C6-phenyl rings
of 59.97(4)ꢁ. As a result, the four carboxylic acid molecular
recognition groups point roughly at tetrahedral directions with
respect to the central C–C bond. The intermolecular organiza-
tion in 2 is directed by numerous ligand–ligand as well as ligand–
water O–H/O hydrogen bonds (Fig. 2b), which represents
a continuous supramolecular assembly of the component species.
The isophthalic acid paddle-like residues in 1 and 2 are
approximately planar, with only a small twist of the carboxy-
lic groups with respect to the plane of their proxime phenyl-
rings.
ꢁ
The Ln–O bond lengths are within 2.431(3)–2.671(3) A in 3
ꢁ
and 2.374(3)–2.620(3) A in 4. The La/La and Nd/Nd
distances between the inversion-related ions are 4.1850(5) and
ꢁ
4.0958(4) A, respectively. The polymeric array adopts a bilayered
nature with respect to the metal-connected ligand components.
The latter are characterized by a flattened geometry to conform
to the 2D coordination scheme, as it is reflected in the corre-
sponding conformation indicators d0 ¼ 30.2(2)ꢁ and 29.5(5)ꢁ, and
s0 ¼ 142.3(3)ꢁ and 142.5(3)ꢁ in 3 and 4, respectively. The upper
and lower parts of the bilayered ensemble are related to one
another by crystallographic inversion, as are the individual metal
linkers within the dinuclear lanthanoid cluster. The water ligands
are oriented perpendicular to the polymeric bilayers as flag poles
on both sides of the bilayer. They provide additional molecular
recognition sites for further intermolecular association, and
Fig. 4 Compound 5. (a) Face-on view of the two-dimensional polymeric assembly. (b) Edge-on view (down the b-axis) of three adjacent polymers,
illustrating the tight packing of the corrugated layered ensembles along the c-axis (horizontal). The Sm ions and the water ligands bound to them are
denoted by small spheres. The non-coordinated water species residing in the interligand void grids are omitted.
344 | CrystEngComm, 2011, 13, 339–349
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impart roughness to layer surface. In fact, neighboring bilayers
interdigitate one into the other with their water ‘‘poles’’. Water
molecules of one layer penetrate between those of an adjacent
layer and hydrogen bond to the carboxylate groups that line the
concave zones of the latter.
The isomorphous structures 6 and 7 with heavier lanthanoid
ions represent
a different polymeric aggregation than in
compounds 3–5. They exhibit continuous coordination poly-
merization in three (rather than two) dimensions, forming
genuine framework solids (Fig. 5).
A bilayered two-dimensional polymerization motif, which
extends parallel to the (ab) plane of the crystal, characterizes also
compound 5, wherein dinuclear clusters of inversion-related Sm
ions interconnect by coordination between six H4L0 ligand units
(Fig. 4a). The metal ions are nine-coordinate, with four ligand
species bridging between them. The coordination sphere of every
metal ion is complemented by an additional carboxylate group
and two water ligands. The Sm–O coordination bonds are within
Molecules of the non-coordinated water solvent occupy intra-
lattice channels that propagate through the polymeric assembly
parallel to the c-axis. These two structures are characterized by
mononuclear inorganic connectors (Dy or Eu, the distance
between closest ions in the polymeric lattice are longer than
0
ꢁ
5.0 A), and a V-shape conformation of the H4L ligand is very
similar to that observed in 1. It is characterized by the C–O/O–
C torsion angle in the central part of the ligand, and the dihedral
angle between the two phenyl rings, of s0 ¼ 125.5(5)ꢁ (in 6) and
123.3(4)ꢁ (in 7), and d0 ¼ 58.2(2)ꢁ (in 6) and 59.3(1)ꢁ (in 7),
correspondingly. The metal ions have a coordination number of
8, doubly coordinating to two carboxylate groups of different
ligands, and singly coordinating to the carboxylates of two other
ligand species and to two water molecules at Dy/Eu–O distances
ꢁ
2.356(2)–2.631(2) A, and the Sm/Sm distance across the
ꢁ
inversion is 4.0205(3) A.
However, in 5 the observed metal–ligand connectivity scheme
is somewhat different than in 3 and 4. The ligand adopts
a considerably more bent V-geometry with d0 ¼ 85.9(1)ꢁ and s0 ¼
103.3(3)ꢁ, and utilizes only three of its carboxylate groups for
coordination to the lanthanoid linkers. The fourth carboxylic
group as well as the methylene center of every ligand are turned
away from the metal center, lining the surface of the polymeric
layers on both sides. The top and bottom layers of the polymeric
assembly are related to each other by inversion, the organic
ligands converging on the dinuclear metal–ion clusters in the
center of the bilayer. Edge-on view of the formed layers (Fig. 4b)
shows a grid-like structure of the polymeric array associated with
the sharp bending of the ligands (with a nearly perpendicular
alignment of the two phenyl residues). It consists of a square-type
organization with the organic ligands and inorganic connectors
located in opposite corners of a given square. The metal-coor-
dinated water ligands point inward into the interligand voids,
which are also accommodated by molecules of non-coordinated
water moieties. All the water entities are involved in an extensive
array of hydrogen bonds. The corrugated polymeric layers pack
tightly along the c-axis, with the convex zones of one layer fitting
tightly into the concave surfaces of adjacent layers from above
and below.
ꢁ
ꢁ
within 2.234(14)–2.2445(4) A in 6 and 2.282(3)–2.479(3) A in 7.
The three carboxylate groups of H4L0 are coordinated to four
metal ions. One of the carboxylates on each phenyl ring is doubly
coordinated to Dy/Eu, while the third one is bound to, and
bridges between, two ions. The carboxylic acid function of H4L0
is oriented into the periodically spaced water-filled channels and
hydrogen bonds to them.
Ligand H4L00 is considerably more flexible than H4L0 due to
the extended aliphatic bridge that connects between the two
phenyl fragments. Correspondingly, the relative disposition of
the carboxylic molecular recognition functions can vary from
coplanar to tetrahedral. The latter is favorable for the induction
of three-dimensional coordination patterns, particularly when
combined with metal ion connectors of complimentary tetrahe-
dral binding geometry. With the lanthanoid metals that lack
crystal field effects, the situation is less predictable. A single
example of a 3D-framework polymerization of H4L00 with La
Fig. 6 Crystal structure of the framework coordination polymer 8. The
metal ions and the coordinated water molecules are denoted by small
spheres. Molecules of the non-coordinated water solvent are omitted for
clarity. Note that flat shape of the organic ligand which extends with its
four carboxylic/carboxylate groups along the horizontal direction, while
the metal–ligand binding the other two directions is facilitated by the
spatially extended coordination valency of the La ions.
Fig. 5 Crystal packing in framework-3D coordination polymer 7 (6 is
isomorphous), viewed approximately down the c-axis. The metal ions
and the water molecules (metal-ligated as well as uncoordinated solvent)
are denoted by small spheres.
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ions is provided by compound 8 (Fig. 6). In this compound the
La ion is nine-coordinate and binds to eight ligating molecules
(three water molecules, one doubly coordinated H4L00, and four
singly coordinated H4L00), with La–O distances ranging from
and a nearly coplanar orientation of the two phenyl rings of the
ligand: s00(O–C–C–O) ¼ 178.7(4)ꢁ in 9, 179.4(4)ꢁ in 10, and
177.8(4)ꢁ in 11; d00 (phenyl–phenyl) ¼ 13.4(3)ꢁ in 9, 13.7(3)ꢁ in 10,
and 16.6(2)ꢁ in 11. The coordination pattern is also similar in the
three compounds. The metal ion connectors are nine-coordinate.
They coordinate doubly to two carboxylate groups of two
different ligands, and singly to the carboxylate groups of three
other H4L00 molecules as well as to two water ligands. The cor-
ꢁ
ꢁ
2.457(3) A to 2.622(3) A. The relatively large size of the metal ion
allows it to coordinate simultaneously to five different H4L00
ligands, giving rise to the expansion of the coordination poly-
merization in three dimensions. Moreover, every ligand species
coordinates to five different La ions, one of the carboxylate
ꢁ
responding Ln–O bond distances are within 2.359(4)–2.770(4) A
ꢁ
groups bridging between two of the ions 5.813(1) A apart from
ꢁ
ꢁ
in 9, 2.325(3)–2.780(3) A in 10, and 2.414(3)–2.651(3) A in 11.
Then, in each of these three compounds the organic ligands
connect to five different metal ions, two of the carboxylates bind
doubly to two of the ions, the third to another metal ion, and the
fourth carboxylate function links to, and bridges between, two
additional metal ions. These metals do not seem to interact
strongly with one another, the distances between the carboxy-
each other. The 3D architecture formed in 8 can be attributed
mainly to the high coordination capacity of the La ion, as the
H4L00 organic ligand in it is characterized by a relatively flat
shape. The conformation of H4L00 in 8 is characterized by a s00 ¼
58.4(4)ꢁ torsion angle about the central bond and a dihedral
angle between the phenyl rings of only d00 ¼ 23.2(2)ꢁ. As in the
previous example, molecules of non-coordinated water are
trapped in the interstitial voids of, and hydrogen bond to, the
polymeric assembly.
ꢁ
late-bridged ions being Nd/Nd ¼ 5.293(1) A in 9, Sm/Sm ¼
ꢁ
ꢁ
5.303(1) A in 10, and Ce/Ce ¼ 6.043(1) A in 11. As observed in
compounds 3–5, the coordination polymers have a double-layer
nature (in terms of the organic ligand arrangement in them), with
the metal-bound water ligands directed outward on both sides
(Fig. 7). Compounds 9 and 10 are isomorphous and isometric.
Compound 11 crystallizes with a unit-cell of somewhat different
dimensions and reveals slightly different ligand conformation
and metal–ligand coordination distances. However, these varia-
tions have little effect on the overall coordination motif. All three
compounds contain non-coordinated and partly disordered
water solvent species (hydration layer) in the interface between
the layered polymeric assemblies.
Only 2D coordination polymerization takes place, however, in
the reactions of H4L00 with Ce, Nd and Sm ions (9–11). It is
associated with an almost entirely flat conformation of the
organic ligand in the three compounds. The latter is character-
ized primarily by anti torsion angles about the central fragment
In order to understand the structures of metal–organic
frameworks, it would be valuable to explore the binding mode
variety between the metal centers and the carboxylate ligands.17
In this study we observe that the two ligands H4L0 and H4L00 are
linked to the lanthanoid centers in four different ways (Fig. 8): (i)
in a monodentate coordination mode the carboxylic acid coor-
dinates to one metal center via the carbonyl O-atom without
deprotonation; (ii) in a chelating bidentate mode, two oxygen
atoms of a carboxylate group are chelating the same metal
center; (iii) in the m2–h1–h1-bridging coordination mode, two
oxygen atoms of the carboxylate group connect with, and bridge
between, two different lanthanoid ions; and (iv) in the m2–h2–h1-
bridging coordination mode one O-atom of the carboxylate
group forms a bifurcated bond with two different lanthanoid
ions, the other O-atom connects to only a single metal ion. In this
Fig. 7 Compound 9. (a) Face-on view of the two-dimensional polymeric
assembly. (b) Edge-on view (approximately down the a–c axis) of three
adjacent polymers, illustrating the outward orientation of the metal-
bound water ligands into the interface between the bilayered ensembles.
The Sm ions and the water ligands bound to them are denoted by small
spheres. The non-coordinated water species residing in the interligand
void grids as well as at the interface between adjacent polymeric arrays
are omitted. Similar two-dimensional coordination polymerization
characterizes compounds 10 and 11.
Fig. 8 Coordination modes of H4L0 and H4L00 with lanthanoid ions in
compounds 3–11.
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case, then, the carboxylate group binds simultaneously to two
metals.
carboxylates ligands have also been reported in literature.26
Among them difunctional organic linkers as 4,40-biphenyldi-
carboxylate27 and naphthalene dicarboxylate28 have been widely
investigated. Other interesting coordination networks of
lanthanoid ions with 6,60-dichloro-2,20-diethoxy-1,10-binaph-
thalene-4,40-dicarboxylic acid,29 S,S-dioxodibenzothio-phen-
3,7-dicarboxylic acid,30 1,3,5-benzenetrisbenzoate,31 and 3,30,4,40-
biphenyltetracarboxylate17b have also been reported. These
compounds exhibit a wide diversity of lanthanoid–carboxylate
coordination patterns, the correlation of which is beyond the
scope of the present discussion. The focus in this article is on the
design and application of new functional building blocks for
crystal engineering of related hybrid organic–inorganic coordi-
nation networks, while utilizing similar coordination synthons,
and to demonstrate if and how they materialize.
Similar type of torsionally flexible tetracarboxylic acids, with
central butyl and butenyl spacer units are reported in literature.18
The ligand with butyl spacer exhibited a planar conformation
upon coordination with metal ions, while the acid with butenyl
spacer adopted either a planar or a gauche arrangement
depending on the particular coordination mode. Tetracarboxylic
acid with a flexible aryloxy group as spacer has been also
reported in the literature.19 Two selected literature examples are
of a particular relevance to our present observations. One relates
to a 3D lanthanide–organic framework involving Eu3+ ions and
the 2,5-pyridinedicarboxylic and 1,4-phenylenediacetic acids as
organic ligands,20 as their polymeric assembly is tessellated by di-
nuclear cores of similar type to that found in compound 3. The
Eu ions in this compound are nine-coordinate and are paired
across centers of crystallographic inversion. The di-nuclear Eu/
Eu node coordinates through one monodentate, one chelating
(involving also the N-atom), and one m2–h1–h1-bridging pyr-
idinedicarboxylic acids. The remaining coordination sites are
occupied by one m2–h2–h1-bridging phenylenediacetic acid and
one water molecule. Another example involves a 3D coordina-
tion polymer of Yb3+ with 1,2,4,5-benzenetetracarboxylic acid,
which has a structural resemblance with compounds 6 and 7,
revealing a rather similar binding pattern around the metal ion.5a
Two types of coordination modes of benzenetetracarboxylic acid
ligands are present in that structure: (i) a bidentate chelating
mode and (ii) a m2–h2–h1-bridging coordination mode. Each Yb-
atom is coordinated by eight oxygens, four from two chelating
carboxylate groups, two from bridging carboxylate groups of
different benzenetetracarboxylate units, and two coordinated
water molecules.5a
The topology of the coordination networks described above
has been analyzed with the TOPOS (v. 4.0) software.21 The
relatively high complexity of these arrays evolves from the
multiple binding features exhibited by the lanthanoid ions.
Compounds 3 and 4 are characterized by the same type of 14-
connected uninodal net with the Schlafli point symbol (345.444.52).
Their corresponding vertex symbol is [3.3.3.3.3.3.3.3.3.3.3.
3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.4.
4.4.4.4.4.4.4.4(2).4(2).4(2).4(2).4(2).4(2).5.5.5.5.N.N.N.N.N.N.
N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N].
Compounds 5, 9, 10 and 11 have similar type of 11-connected
uninodal net, point (Schlafli) symbol (327.426.52), and vertex symbol
[3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.4.4.4.4.5.5.5.N.
N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N].
Compounds 6 and 7 represent 10-connected uninodal nets
with point symbol (315.422.58) and FeB topology. The
extended point symbol is [3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.
4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4(2).4(2).4(2).4(2).4(2).4(2).5(2).
5(3).5(3).5(3).5(3).5(3).5(3).5(3)], and the vertex symbol is
[3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.4.4.4.4.4.4.4.4.5.5.5.5.5.5.N.N.N.
In addition to lanthanoid polymeric compounds with
benzene–polycarboxylates,5c,5d,22–25 a few examples of polymeric
arrays constructed with lanthanide centers and multiphenyl
Fig. 9 Thermogravimetric analysis in compounds 3, 5, 6, 8 and 9, showing weight loss of the tested samples during the dehydration processes.
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N.N.N.N.N.N.N.N.N.N.N.N]. Finally compound 8 is
characterized by Ca0.3 Sn1.7-type inter-metallic topology with
one kind of 14-connected uninodal net, point symbol
(333.454.54), and vertex symbol [3.3.3.3.3.3. 3.3.3.3.3.3.3.3.
3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.3.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.
4.4.4.4.4.4.4.4.4.5.5.5.5.5.5. N.N.N.N.N.N.N.N.N.N.N.
N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N.N].
polymeric assemblies, by adjusting the shape of the ligand to the
spatial coordination requirements of the metal bridging unit.
However, due to this flexibility factor, as well as the high affinity
of the lanthanoid ions to supplement their first coordination
sphere with an unpredictable number of water ligands from the
aqueous reaction mixture, it is practically impossible to control
the preferential formation of a given coordination scheme.
Compounds 3–5 and 9–11 contain coordination networks of 2D
connectivity, and loose crystallinity on removal of the crystalli-
zation solvent. Compounds 6–8 are characterized by open
architectures of 3D connectivity, yet they also represent soft
materials. Their structures collapse upon heating and dehydra-
tion into either polycrystalline or amorphous phases. Current
findings are in some contrast with our earlier observations on the
coordination polymerization features of the more rigid tetra(4-
carboxyphenyl)porphyrin and tetra(3-carboxy)phenylporphyrin
moieties with lanthanoid ions.6f,6g In that case most of the formed
porphyrin–lanthanoid polymers represented open 3D-frame-
work materials, where the metal–carboxylate interaction
schemes provided robust construction pillars to sustain the
porphyrin-layered (dictated by the large aromatic frameworks of
these building blocks) multi-level architectures. It is evident that
the delicate interplay between molecular rigidity vs. flexibility is
one of the major factors to consider in targeted formulations of
metal–organic framework coordination polymers with the tet-
raacid ligands.
Thermogravimetric analyses were carried out under argon in
the range from room temperature to about 400 ꢁC at heating rate
of 10 K minꢀ1. Features of the thermal stability of compounds 3,
5, 6, 8 and 9 are illustrated in Fig. 9. Compound 3 shows weight
loss of 4.0% between 151 to 184 ꢁC, corresponding to the loss of
one molecule of water (calcd: 3.4%). Upon further heating, the
ꢁ
anhydrous compound is stable up to 410 C, and then decom-
poses above this temperature. Compound 6 exhibits weight loss
of 14.8% in the 91–264 ꢁC temperature range, which accounts for
the total removal of the two metal-coordinated and three non-
coordinated water molecules (calcd: 14.4%). The remaining
material shows no further weight loss up to 400 ꢁC. Compound 8
looses 12.7% of its weight within 128–177 ꢁC, most likely due to
the de-sorption of the four water molecules (calcd: 12.0%), the
dehydrated residue remains stable up to about 395 ꢁC, and starts
to decompose above this temperature. Compound 9 shows
ꢁ
weight loss of 14.3% within 45–299 C, and then remains stable
ꢁ
up to 400 C. The observed weight loss corresponds to the five
water molecules (calculated weight loss: 14.3%) in this
compound. The only apparent discrepancy between the crystal-
lographic (at 110 K) and TGA data relates to compound 5. In
spite of the relatively high water content in compound 5 (see
above), it shows weight loss of only 4.7% within the 100–255 ꢁC
temperature range, due the apparent loss of two water molecules
(calcd: 5.3%). It is possible that in this case the five non-coordi-
nated water species accommodated in the wide channel voids of
the polymeric lattice diffused out of the crystalline material
during the drying process of the analyzed sample. In such case
the observed weight loss can be attributed to the two metal
coordinated molecules. As expected, relatively high temperatures
are required to release the metal-coordinated water ligands. After
completion of the dehydration processes, no other phase tran-
sitions have been observed in DTA and DSC diagrams prior to
decomposition of the metal–ligand coordination frameworks
near 400 ꢁC.
Acknowledgements
This research was supported by The Israel Science Foundation
(Grant No. 502/08).
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