Conformation change of the cyclohexanedicarboxylate ligand toward 2D and 3D La(III)-organic coordination networks

Article abstract of DOI:10.1039/b200658h

2D and 3D structures of La-1,4-cyclohexanedicarboxylates (chdc) coordination networks, have been constructed using flexible chdc ligands which can have cis and trans conformations of two dicarboxylate groups, controlled by the solution pH in hydrothermal

Full text of DOI:10.1039/b200658h

Conformation change of the cyclohexanedicarboxylate ligand toward 2D
and 3D La(III)-organic coordination networks
YooJin Kim and Duk-Young Jung*
Department of Chemistry-BK21 and the Institute of Basic Sciences, Sungkyunkwan University, Suwon
440-746, Korea. E-mail: dyjung@chem.skku.ac.kr
Received (in Columbia, MO, USA) 20th January 2002, Accepted 5th March 2002
First published as an Advance Article on the web 25th March 2002
2D and 3D structures of La-1,4-cyclohexanedicarboxylates
(chdc) coordination networks, have been constructed using
flexible chdc ligands which can have cis and trans conforma-
tions of two dicarboxylate groups, controlled by the solution
pH in hydrothermal reaction conditions.
(2 mmol) of 1 M KOH solution for [La2(trans-chdc22)(cis-
chdc22)2(H2O)2]·2H2O (3) were used, respectively. The hydro-
thermal reaction without KOH addition produces e,e-trans-
chdcH2 (1)8 and lanthanum ions remain in solution as the salt
forms. These compounds were formulated and characterized by
elemental microanalysis† and single crystal X-ray diffraction.‡
for crystallographic files in .cif or other format.
Rational design by incorporating appropriate structure-directing
ligands and coordination geometries of metal ions into the
extended structure offers possible production of novel metal–
organic materials of current interest with various crystal
structures and properties.1 Rigid spacer ligands such as benzene
di-,2 and tri-carboxylates3 successfully produce various ex-
tended structures with metal ions. A guiding principle of our
work is the attempt to control the conformations of flexible
spacer ligands in the architecture of the products. For instance,
we previously reported the novel 3D coordination networks of
transition metal alkyldicarboxylates with specific ligand con-
formations.4 We and other groups have used succinate ligands
as structure-templating ligands with anti/gauche conformations
in various open frameworks.5 We consider two basic aspects of
the structure-directing ligands with the expected conformations,
viz. selectivity and limited flexibility of ligand conformations.
In this context, we chose 1,4-cyclohexanedicarboxylic acid
(chdcH2) as a spacer ligand which possesses three possible
conformations of two carboxylate groups, a,a-trans-, e,e-trans-,
and e,a-cis-chdcH2. The e,e-trans-form is thermodynamically
more stable than the e,a-cis-form due to two equatorial
substituents and the a,a-trans-form is the least stable because of
1,3-diaxial hindrance.6 (Scheme 1)
Under hydrothermal condition using aqueous KOH solution
at 180 °C, we expect the a-protons located on the 1,4 carbons of
cyclohexane to be easily deprotonated to accelerate the
equilibrium state among different conformations of chdc
ligands. The two carboxylic protons of chdcH2 have different
acidity values (pKa1 and pKa2 are 4.18 and 5.42, respectively,
for e,e-trans-form), and the conformational equilibrium in-
volves the first deprotonated state of chdcH2,7 and is the key to
‘in-situ’ conformational modification of chdc ligands in this
study. The methodology allows us to create different structural
topologies, and here we report on the formation and properties
of three compounds representing the 1D, 2D and 3D net-
works.
The crystal structure of 1 reveals each trans-chdcH2 to be
hydrogen-bonded to two other molecules, one at each end, to
give 1D chains. In contrast, 2 involves 2D and 3 involves 3D
networks. The synthetic pathway for the prepared compounds
and the conformational isomers found in the prepared com-
pounds, is summarized in Scheme 2. We observed four
coordination modes of chdc produced from two different
conformations, viz., e,e-trans-chdcH2 (L0, protonated), e,e-
trans-chdc22 (L1, deprotonated), e,a-cis-chdcH2 (L2, partially
deprotonated), and e,a-cis-chdc22 (L3, deprotonated). The
structure of 2 consists of infinite edge-sharing LaO9 polyhedral
chains, interleaved by L1 and connected by L2 ligand (Fig. 1).
The coordination sphere of La(III) in 2 consists of L1 and L2,
alternately (L1+L2 = 1+1). Two carboxylate groups of L2 in 2,
coordinate to three La ions, one is bidentate and the other is
chelating. A partially deprotonated carboxylate group (L2) is
confirmed by FT-IR analysis and by distinguishable carbon–
oxygen bond lengths (1.293 and 1.214 Å). The crystal structure
of 3 is a 3D network built up by L1 and L3 with the ratio of L1+L3
= 1+2 (Fig. 2). The ligand environments of L1 and edge-sharing
LaO8(H2O) polyhedra chains in 3 present the same structural
moieties as those found in 2 except for hydrate coordination. 3
contains two types of water molecule, one is coordinated to La
Scheme 2
Three different chdc containing frameworks were prepared
using the hydrothermal method. 0.37 g (1.0 mmol) of
LaCl3·7H2O, 0.085 g (0.5 mmol) of 1,4-cyclohexanedicar-
boxylic acid (chdcH2, mixture of cis and trans), 5.0 mL of
distilled water and KOH solution were heated in a 23 mL
capacity Teflon-lined reaction vessel at 180 °C for 3 days and
then cooled to room temperature. 1.0 mL (0.5 mmol) of 0.5 M
KOH solution for La(trans-chdc22) (cis-chdcH2) (2) and 2 mL
Scheme 1
Fig. 1
This journal is © The Royal Society of Chemistry 2002
908
CHEM. COMMUN., 2002, 908–909

1671.8(4) Å3, Z = 4, m = 2.598 mm21, Dc = 1.908 Mg m23, Mo-Ka
radiation (l = 0.71073 Å), 3093 reflections collected, 2925 independent
(Rint = 0.0231) Final R1(I > 2s(I)) = 0.0412, wR2 (all data) = 0.1154.
¯
3: M = 860.37, triclinic, space group P1, a = 8.085(2), b = 9.0746(8),
c = 10.606(3) Å, a = 71.981(13), b = 83.635(14), g = 83.845(12)°, V =
733.2(2) Å 3, Z = 1, m = 2.948 mm21, Dc = 1.939 Mg m23, Mo-Ka
radiation (l = 0.71073 Å), 2729 reflections collected, 2530 independent
(Rint = 0.0202) Final R1(I > 2s(I)) = 0.0368, wR2(all data) = 0.0967. The
data collection was perfomed on a Siemens P4 automated four-circle
diffractometer. Data were corrected for absorption. The structures of 2 and
3 were solved by direct methods (SHELX-86) and standard difference
Fourier techniques (SHELX-97).9 All hydrogen atoms attached to carbon
positions were program generated and included in the refinements as a
riding model except those on water molecules which were refined
separately according to electron density difference. All non-hydrogen atoms
were treated anisotropically.
Fig. 2
CCDC reference numbers 175051 and 175052. See http://www.rsc.org/
suppdata/cc/b2/b200658h/
and the other physically adsorbed in the pore. All the water
molecules are interconnected by an extended hydrogen bonding
network, lending structural stability to compound 3. The water
molecules are reversibly desorbed and re-adsorbed in 3. The
TGA data of 2 show the first weight loss at ca. 400–420 °C
(19.7%) corresponding to removal of one chdc ligand per unit
(20.9%) and the second one at above 450 °C. The data for 3
present 7.3% (Calc: 8.3%) loss from 100–180 °C (dehydration
of both coordinate and non-coordinate H2O) and 55.9% (Calc:
53.9%) loss over 450 °C (decomposition of chdc ligand).
We believe that the construction of various dimensionalities
was possible thanks to the variable concentration of chdc
conformational isomers, controlled by the pH and temperature
applied. All three compounds basically contain the thermody-
namically most stable trans-forms (L1 or L0), which probably
have higher concentrations and a greater preference to bind La
ions due to the higher acidity. As pH increases, the cis-form
becomes deprotonated in the equatorial rather than the axial
position due to the higher acidity of equatorial one, which
results in the 2D network of 2. At higher pH ( > 7), all the
carboxylate groups of chdc (L3) are deprotonated and interlock
the 2D La L1 sheets into the 3D structure of 3.
The present work demonstrates that 1D, 2D and 3D
coordination networks were achieved by using chdc ligand with
flexible cis/trans conformations, which involve protonated,
partially and/or fully deprotonated carboxylates, depending on
the pH of the applied solution media. It is noteworthy that the
prepared compounds are the first examples of coordination
polymers constructed using both cis-chdc and trans-chdc in
different molar ratios. The sequential change of ligand con-
formations depending on the reaction conditions implies a
fundamental aspect of the crystallization mechanism of coor-
dination networks.
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We thank the KOSEF, Korea, for financial support through
the Electron Spin Science Center at POSTECH.
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with e,e-trans-form.
Notes and references
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† 2: 31% yield based on lanthanum used; elemental analysis calcd (%) for
LaC16H21O8: C, 39.98; H, 4.42. found: C, 39.62; H, 4.37. 3: 48% yield,
Calcd (%) for La2C24H38O16: C, 33.47; H, 4.46. Found: C, 32.56; H,
4.34.
Welvart, Soc. Chim. 5 serie, 1964, 2203.
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9 P. McArdle, SHELX-86 and SHELX-97 Users Guide; Crystallography
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‡
Crystallographic data: 2: M = 480.24, monoclinic, space group P21/n,
a = 10.699(2), b = 8.335(2), c = 18.8774(13) Å, b = 96.774(9)°, V =
CHEM. COMMUN., 2002, 908–909
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