Organic-inorganic hybrids assembled from lanthanide and 1,4-phenylenebis(phosphonate)

Article abstract of DOI:10.1021/cg2008254

A series of novel organic-inorganic hybrids based on trivalent lanthanide (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho) and 1,4-phenylbis(phosphonate) formulated as Ln[O₃P(C₆H₄)PO₃H] has been obtained as single phases under hydrothermal conditions. In the praseodymium compound (Za1), single crystals have been obtained and the crystal structure has been determined. Za1 crystallizes in the monoclinic space group, C2/c, with a = 5.6060(4) A, b = 20.251(7) A, c = 8.2740(6) A, β = 108.52(1)°. All other compounds are isostructural to Za1 as confirmed by Rietveld refinement using X-ray powder diffraction data. Compounds are characterized by thermal analyses (TG-MS and SDTA), elemental analysis, IR spectra, and X-ray thermodiffraction analysis. Their visible photoluminescence properties are also discussed.

Full text of DOI:10.1021/cg2008254

ARTICLE
pubs.acs.org/crystal
OrganicꢀInorganic Hybrids Assembled from Lanthanide and
1,4-Phenylenebis(phosphonate)
Zakariae Amghouz,*,† Santiago García-Granda,† Josꢀe R. García,† Abraham Clearfield,*,‡ and Rafael Valiente§
†Departamentos de Química Física y Analítica y Química Orgꢀanica e Inorgꢀanica, Universidad de Oviedo-CINN, 33006 Oviedo, Spain
‡Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United States
§MALTA Consolider Team, Departamento de Física Aplicada, Universidad de Cantabria, 39005 Santander, Spain
S Supporting Information
b
ABSTRACT: A series of novel organicꢀinorganic hybrids based on trivalent lanthanide
(Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho) and 1,4-phenylbis(phosphonate)
formulated as Ln[O3P(C6H4)PO3H] has been obtained as single phases under
hydrothermal conditions. In the praseodymium compound (Za1), single crystals have
been obtained and the crystal structure has been determined. Za1 crystallizes in the
monoclinic space group, C2/c, with a = 5.6060(4) Å, b = 20.251(7) Å, c = 8.2740(6) Å,
β = 108.52(1)°. All other compounds are isostructural to Za1 as confirmed by
Rietveld refinement using X-ray powder diffraction data. Compounds are character-
ized by thermal analyses (TG-MS and SDTA), elemental analysis, IR spectra, and
X-ray thermodiffraction analysis. Their visible photoluminescence properties are
also discussed.
’ INTRODUCTION
magnetic properties. However, reports on lanthanide phospho-
nates are rather limited because lanthanide phosphonates nor-
mally have low solubility in water and organic solvents, and hence
it is difficult to obtain single crystals suitable for X-ray structural
determination.28 Nevertheless, the elucidation of the structures
of lanthanide phosphonates is very important since these com-
pounds may exhibit useful luminescent properties in both the
visible and near IR regions. In particular, reports on the structure
and photoluminescence properties of lanthanide (Ln) arylpho-
sphonates or aryldiphosphonates are still scarce,28,32,33 although
this kind of ligand may enhance the fluorescence of the lantha-
nide ions, via the so-called antenna effect.
Hydrothermal reaction of lanthanide(III) salts with 1,
4-phenylenebis(phosphonic acid), afforded a series of new
isostructural compounds, with three-dimensional networks for-
mulated as Ln[O3P(C6H4)PO3H] (where Ln = La, Ce, Pr, Nd,
Sm, Eu, Gd, Tb, Dy, Ho). Herein we report their synthesis,
crystal structure, thermal behavior, and luminescent properties
through luminescence and excitation spectroscopy and the excited-
state lifetimes.
Organicꢀinorganic hybrids in general and metalꢀorganic
frameworks (MOFs) in particular constitute an important class
of compounds in the exploratory research area of advanced
materials design. In particular, metal phosphonates represent a
particularly versatile field for investigation because of the great
latitude open to the investigator. During the past two decades, a
considerable amount of research attention has been devoted to
the chemistry of metal phosphonates for their potential applica-
tions in ion exchange, catalysis, and sensor devices.1ꢀ5 The wide
choice of metals (mono-, di-, tri-, tetra-, penta-, and hexavalent)
and the variety of organic ligands that can be utilized to prepare
the metal phosphonate mean that a wide latitude of choices is
possible. Because almost any desired organic compound may be
converted into a phosphonic acid by known reactions, it is thus
possible to introduce phosphonate ligands with different func-
tional groups into the hybrid structure, which suggests the
possibility of tailored structures for highly specific functions,6
and even with unusual properties.4 Often, metal phosphonates
exhibit polymeric structures consisting of chains, layers, or three-
dimensional networks. By utilizing diphosphonates, it is possible
to cross-link the layers and to space the cross-links by using
spacer molecules, such as phosphorous or phosphoric acid, to
create a porous structure. Recently, investigations of di-, tri-, and
tetravalent metal phosphonates have yielded exciting structural
diversity with interesting chemistries and applications,7ꢀ31
including open framework structures20ꢀ26 and porous pillared
layered structures.7,8 The synthesis of lanthanide phosphonates
has drawn the scientist’s attention for their possible optical and
’ EXPERIMENTAL SECTION
Synthesis of 1,4-Phenylenebis(phosphonic acid) (H4BDP).
To a three-necked round-bottom flask, 5 g of 4,40-dibromobenzene
Received: June 30, 2011
Revised:
September 29, 2011
Published: October 05, 2011
r
2011 American Chemical Society
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Table 1. Crystallographic Data for Za1
empirical formula
formula weight/g molꢀ1
temperature/K
wavelength/Å
crystal system
space group
C6H5P2O6Pr
375.95
115(2)
0.71073
monoclinic
C2/c
unit cell dimensions
a (Å)
5.6060(4)
20.251(7)
8.2740(6)
90
b (Å)
c (Å)
α (°)
β (°)
γ (°)
cell volume/Å3
108.522(11)
90
890.7(3)
4
Z
calc density/mg mꢀ3
absorption coefficient/mmꢀ1
F(000)
2.804
5.833
692
crystal size (mm3)
theta range for data collection/°
index ranges
0.22 ꢁ 0.09 ꢁ 0.06
2.01ꢀ27.1
ꢀ7 e h e 7, ꢀ25 e k e 25,
ꢀ10 e l e 10
reflections collected
4543
independent reflections
completeness to thetamax
absorption correction
max and min transmission
refinement method
971 [R(int) = 0.115]
98.30%
refined (XABS2)
1 and 0.3984
full-matrix least-squares on F2
971/5/85
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
1.053
R1 = 0.054, wR2 = 0.122
R1 = 0.112, wR2 = 0.151
1.449 and ꢀ1.914
largest diff peak and hole/e Åꢀ3
3
(Aldrich, 98%) and 50 mL of 1,3-diisopropylbenezene (TCI America)
were added. The system was flushed with N2 and 0.5 g of NiBr2
(Aldrich) was added. The reaction was slowly heated to 160 °C, while
10 mL of triethyl phosphate (Aldrich, 98%) was added over a 6-h period.
The system was heated to 180 °C and stirred for 24 h. NiBr2 and
triethylphosphite were sequentially added twice, with half the amount
each time. After such additions, the temperature was reduced to 120 °C
and the system was left open to the atmosphere for 48 h. The reaction
was distilled to remove solvents and byproducts; extractions were
completed using hexanes. After recrystallization from hexanes, a hydro-
lysis using a 1:1 ratio of H2O and HCl was carried out at 110 °C. The
product was then washed and dried.10 Yield: ca. 2 g (40%).
Figure 1. SEM images of [Ln(HO6P2C6H4)], (where Ln = La (a),
Ce (b), Pr (c), Nd (d), Sm (e), Eu (f), Gd (g), Tb (h), Dy (i), and
Ho (j)).
Synthesis of Lanthanide 1,4-Phenylenebis(phosphonate)
(Ln[O3P(C6H4)PO3H]). In a typical synthesis, 0.2 mmol (0.05 g) of
1,4-phenylylenebis(phosphonic acid), 0.2 mmol of Ln(NO3)3 xH2O
3
(x = 5, 6) (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho), and 6 mL of
water were mixed. The reaction mixture was stirred until a homogeneous
gel was formed (pH = 1ꢀ2), sealed in a PTFE-lined stainless steel
autoclave with an internal volume of 15 mL and heated at 180 °C for
5 days under autogenous pressure. After cooling, the resulting products
were filtered off, washed with water, and dried in an oven at 80 °C. In the
case of Pr[O3P(C6H4)PO3H] (Za1), the product contains a mixture of
pale green powder and tiny plate-shaped crystals. A suitable single crystal
was selected to perform the single-crystal X-ray diffraction studies.
Figure 2. Powder XRD patterns of [Ln(HO6P2C6H4)] compared with
that calculated for Pr[O3P(C6H4)PO3H].
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Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for
Elemental analyses were performed in a CꢀHꢀN Perkin-Elmer model
2400B elemental analyzer, the calculated and found percentages of
CꢀH are represented in Table 5.
Za1a
Pr1ꢀO3
2.371 (9)
2.435 (8)
2.516 (9)
1.519 (10)
1.542 (9)
1.574 (9)
4.2503(6)
107.5 (5)
107.9 (5)
110.6(5)
103.0(5)
110.7 (5)
116.9 (5)
Single-Crystal X-ray Diffraction Studies. Data collection was
performed at 115 K on a Bruker Smart APEX-II CCD diffractometer
with a Mo Kα source operated at 40 kV and 40 mA. The crystal structure
was solved by direct methods. The refinement was performed using full-
matrix least-squares on F2. All non-H atoms were anisotropically refined.
All H atoms were either geometrically placed riding on their parent
atoms or located from the difference Fourier map, with isotropic
displacement parameters set to 1.2 times the Ueq of the atoms to which
they are attached. Crystallographic calculations were carried out using
the following programs: APEX234 for data collection, SAINT34 for cell
refinement and data reduction, SADABS35 for empirical absorption
correction, SHELXS-9736 for structure solution, XABS237 for refined
absorption correction, SHELXL-9736 for structure refinement and to
prepare materials for publication, PLATON38 for the geometrical calcula-
tions, and Diamond39 for molecular graphics.
Pr1ꢀO2
Pr1ꢀO1
P1ꢀO3i
P1ꢀOii
P1ꢀO1iii
Pr1ꢀPr1
O3iꢀP1ꢀC1
O1iiiꢀP1ꢀC1
O2ii ꢀP1ꢀC1
O2iiꢀP1ꢀO1iii
O3iꢀP1ꢀO1iii
O3iꢀP1ꢀO2ii
a Symmetry codes: (i) ꢀx ꢀ 1/2, ꢀy + 1/2, ꢀz + 1; (ii) ꢀx + 1/2, ꢀy +
1/2, ꢀz + 1; (iii) ꢀx, y, ꢀz + 1/2.
Table 3. Hydrogen-Bond Geometry in Za1 (Å, °)a
Thermal Characterization. A Mettler-Toledo TGA/SDTA851e
and a DSC822e were used for the thermal analyses in an oxygen dynamic
atmosphere (50 mL/min) at a heating rate of 10 °C/min. In all cases, ca.
10 mg of powder sample was thermally treated, and blank runs were
DꢀH
A
DꢀH [Å]
H
A [Å]
D
A [Å] DꢀH A [deg]
3 3 3
3 3 3
1.54(8)
3 3 3
2.47(1)
3 3 3
O1ꢀH10 O1 I
1.0(1)
151(14)
3 3 3
a Symmetry codes: (i) ꢀx + 1, +y, ꢀz + 1/2.
Figure 3. (a) Asymmetric unit of [Pr(HO6P2C6H4)]. (b) Coordination modes of 1,4-phenylbis(phosphonate) ligand [μ6 - 2(η1: η2: η1)]. (c, d)
Perspective view of the coordination environment of Pr3+. (e) Perspective view of the inorganic layer along the b-axis. (f, g) Projection of the structure
along the a-axis (g) and c-axis (f), respectively.
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performed. In TG tests, a Pfeiffer Vacuum ThermoStar GSD301T mass
spectrometer was used to determine the evacuated vapors. The masses
18 (H2O), 44 (CO2), and 78 (C6H6) were tested by using a detector
C-SEM, operating at 1200 V, with a time constant of 1 s.
Morphological Characterization. Micrographs and X-ray mi-
croanalysis (SEM/EDX) were recorded with a JEOL JSM-6100 electron
microscope operating at 20 kV coupled with an INCA Energy-200
energy dispersive X-ray microanalysis system (EDX) with a PentaFET
ultrathin window detector. SEM images show that the particles are
nanoplates with diameters between 200ꢀ400 nm (see Figure 1), and
EDX confirmed that the ratio Ln/P is ca. 1:2.
Infrared Absorption Spectra. The infrared data were collected
at room temperature (RT) using a FT-IR Bruker Tensor-27 spectro-
meter from KBr pellets. The spectra were collected over the range
4000ꢀ400 cmꢀ1 by averaging 15 scans at a maximum resolution
of 4 cmꢀ1
.
Powder X-ray Diffraction Studies. Powder X-ray diffraction
(PXRD) patterns were recorded on a Bruker D8 Advance powder X-ray
diffractometer with CuꢀKα radiation. The samples were gently ground
in an agate mortar in order to minimize the preferred orientation.
Routine XRD data were collected at room temperature over the angular
2θ range 4ꢀ40° with a step of 0.009° and a counting time of 0.15 s/step.
High-resolution X-ray diffraction (XRD) measurement collected at RT
over the angular 2θ range 4ꢀ90° with a step of 0.013° and a counting
time of 7 s. The PXRD patterns of the compounds were compared with
the calculated one (see Figure 2), indicating that the products have been
successfully obtained as isostructural pure crystalline phases. Rietveld
refinement was carried out by the FullProf program.40 Single crystal
structure of Za1 was used as the initial model in all cases. The PXRD
profiles have been modeled as a Cauchy (Lorentzian) function in the
range 2θ = 6ꢀ70°. The zero offset, the scale factor, six background
terms, fwhm parameters, the unit cell, asymmetry correction for peaks
below 2θ = 20°, and the positions of Ln, O, and P atoms (excepting C
and H) were refined.
Powder X-ray Thermodiffraction Studies. Powder X-ray
thermodiffraction studies were performed in air. The sample was placed
in an Anton Paar XRK 900 reactor chamber on a Bruker D8 Advance
diffractometer with DAVINCI design, using Cu Kα radiation, equipped
with a LynxEye detector. Each powder pattern was recorded in the
7ꢀ60° range (2θ) from RT to 900 °C with a step of 0.02° and a counting
time of 0.4 s. The temperature ramp between two consecutive tempera-
tures was 10 °C/min.
Gas Adsorption Study. Nitrogen adsorption data were performed
volumetrically at 77 K using a Micromeritics ASAP 2020 analyzer.
Luminescence and Excitation Measurements. RT lumines-
cence and excitation spectra were measured using a standard spectro-
fluorometer (Jobin-Yvon Fluorolog-2). The sample was excited with a
150 W Xe-lamp by an excitation monochromator and the dispersed
emitted light was detected with a photomultiplier (Hamamatsu R928) at
a 90° configuration to minimize direct light from the lamp. Excitation
spectra were corrected for the lamp response at different wavelengths.
Luminescence spectra were not corrected for the detection system
response (monochromator + detector). Emission lifetimes were mea-
sured in a FLSP920 fluorescence spectrometer (Edinburgh Inst.),
equipped with a microsecond xenon flashlamp and a cooled R928P
photomultiplier (Hamamatsu), based on the multichannel scaling tech-
nique (repetition rate 50 Hz and pulse width of 1 μs).
Figure 4. Topological representation of the binodal frameworks of
[Ln(HBDP)]: P corresponds to 4-connected nodes and Pr is 6-con-
nected nodes. Internodal distances: P f P 6.44 Å; P f Pr 3.24 Å, 3.78 Å,
and 3.84 Å.
simple containing half Pr3+ cation and half the HBDP3ꢀ ligand as
displayed in Figure 3a. This is a consequence of the fact that Pr3+
cation is situated on a crystallographic 2-fold axis along the
b-direction, while HBDP3ꢀ resides on an inversion center. The
coordination mode of the HBDP3ꢀ ligand is depicted in Figure 3b.
We should mention that this coordination mode has not yet been
reported. However, the same coordination mode has been
reported with another similar ligand.29 The Pr3+ cation is bonded
to eight oxygen atoms that come from six phosphonate crystal-
lographically equivalent groups (Figure 3c). Its coordination
geometry may be described as a distorted bicapped trigonal-prism,
{PrO8} (Figure 3d), out of six phosphonate groups surrounding
Pr3+: (i) two of them are coordinated in chelating modes via the
oxygen atoms O1 and O2, (ii) the other two act as a bridge
between two adjacent Pr3+ cations within the oxygen atom O2,
(iii) and the two remaining are bonded via oxygen atom O3.
The distances PrꢀO range from 2.371(9) to 2.516(9) Å, with
average values of 2.44(1) Å. The Pr3+ polyhedra alternate in a
chain edge-to-edge parallel to the [101] direction, and the distance
between adjacent Pr3+ cations in the chain is 4.2503(6) Å. These
chains are in turn connected via phosphonate groups along the
[101] direction forming the inorganic layer (see Figure 3e). Con-
nection between adjacent layers is ensured by HBDP3ꢀ organic
pillars leading to an alternation along c-axis of organic and
inorganic fragments as depicted in Figure 3f,g, forming channels
running along the a- and c-axis, with approximate dimensions of
6.5 ꢁ 4.2 Å2 and 6.5 ꢁ 5.6 Å2. Gas (N2/77 K) adsorption study
’ RESULTS AND DISCUSSION
The detailed crystallographic data of and the structure refine-
ment parameters of Pr-compound (Za1) are summarized in
Table 1. Selected bond distances, angles, and hydrogen bonds are
given in Tables 2 and 3. The asymmetric unit of Za1 is relatively
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Table 4. Refinement Indexes of Agreement, Cell Parameters (a, b, c, β, and Volume), and Distances LnꢀLn and LnꢀO of the
Ln-Compounds Compared with Single-Crystal Data of Za1
Ln
Pr (single-crystal)
1.126
La
Ce
1.143
Pr
1.126
Nd
1.109
Sm
1.079
RShannon/Å
RBragg
1.16
2.62
2.37
4.41
3.48
4.91
3.92
Rf
3.62
2.30
3.67
3.41
a (Å)
5.6060(4)
20.251(7)
8.2740(6)
108.52(1)
890.7(3)
4.2503(6)
2.516(9)
2.435(8)
2.650(9)
2.370(9)
5.6184(4)
20.149(1)
8.3824(5)
108.674(3)
898.9(1)
4.2901(5)
2.621(9)
2.501(8)
2.669(9)
2.33(1)
5.5897(9)
20.073(3)
8.325(1)
108.573(7)
885.4(2)
4.2682(9)
2.65(2)
2.52(1)
2.74(2)
2.32(2)
5.5746(6)
20.056(2)
8.2868(8)
108.588(4)
878.2(2)
4.2505(8)
2.49(1)
2.48(1)
2.66(2)
2.21(2)
5.555(1)
20.050(4)
8.244(2)
108.496(8)
870.7(3)
4.223(1)
2.54(3)
2.59(2)
2.66(2)
2.29(3)
5.5196(9)
19.973(3)
8.179(1)
108.469(7)
855.3(2)
4.1970(9)
2.53(2)
2.44(2)
2.61(2)
2.41(2)
b (Å)
c (Å)
β (°)
cell volume/Å3
LnꢀLn
LnꢀO(1)
LnꢀO(2)
LnꢀO(2)
LnꢀO(3)
Ln
Pr (single-crystal)
1.126
Eu
Gd
Tb
Dy
Ho
RShannon/Å
RBragg
1.066
1.053
1.04
1.027
1.015
3.13
6.03
4.56
6.70
8.14
Rf
2.13
4.15
3.92
4.67
5.09
a (Å)
5.6060(4)
20.251(7)
8.2740(6)
108.52(1)
890.7(3)
4.2503(6)
2.516(9)
2.435(8)
2.650(9)
2.370(9)
5.4997(7)
19.955(3)
8.158(1)
108.353(5)
849.8(2)
4.1904(9)
2.53(2)
2.52(2)
2.62(2)
2.27(2)
5.480(1)
19.943(4)
8.132(2)
108.367(8)
843.5(3)
4.173(1)
2.59(3)
2.62(2)
2.69(2)
2.42(3)
5.4735(7)
19.894(2)
8.116(1)
108.323(5)
838.9(2)
4.1764(9)
2.51(2)
2.47(1)
2.61(2)
2.13(2)
5.4462(9)
19.869(3)
8.086(1)
108.237(6)
831.1(2)
4.169(1)
2.48(2)
2.26(2)
2.64(2)
2.11(3)
5.4309(5)
19.844(2)
8.0629(6)
108.208(3)
825.4(1)
4.1512(9)
2.49(2)
2.28(1)
2.56(1)
2.06(2)
b (Å)
c (Å)
β (°)
cell volume/Å3
LnꢀLn
LnꢀO(1)
LnꢀO(2)
LnꢀO(2)
LnꢀO(3)
was performed in order to check the accessibility of channels, and
the results prove they are not accessible (SBET = 29 m2/g).
Taking as network nodes the phosphorus atoms and Ln3+ cations,
Ln-compounds can be envisaged as binodal frameworks built-up
from one type of 4-(P-atom) and 6-connected (Ln3+ cation)
nodes (Figure 4).
All the other members of the family have been obtained as
microcrystalline powders. Rietveld refinement shows the iso-
structurality of all Ln-compounds, tuned by the influence of the
Ln3+ cations (see Table 4). The final values obtained of figures of
merit are within acceptable ranges,41 as the model used adjusts
very well with the experimental data. The final Rietveld
refinements plots for all compounds are shown in Figure 5.
The variation of unit cell parameters show dependency to the
Shannon’s effective ionic radii,42 and the expected tendencies are
observed: the ionic radii decrease linearly as the unit cell
parameters (a, b, c, β, and volume) and distances LnꢀLn do.
These tendencies are illustrated in Figure 6.
Selected IR spectra of Ln-compounds are depicted in Figure
S1 (see Supporting Information). The presence of the HBDP3ꢀ
ligand in the Ln-compounds is confirmed by two bands at
1140 cmꢀ1 (strong) and 1046 cmꢀ1(medium) characteristic
of the stretching vibrations of the phophonate PꢀO3 groups,
as well as the broad band at around 1634 cmꢀ1 characteristic of
PdO stretching vibrations. The band at 1260 cmꢀ1 may
attributed to PdO deformation vibration. Bands between
600 cmꢀ1 and 400 cmꢀ1 main correspond to the bending modes
of the PO3 group. However, the band at 538 cmꢀ1 may be
assigned to Ln—O stretching vibration. The band at 975 cmꢀ1 is
characteristic of PꢀC stretching vibrations. Bands at 1385 cmꢀ1
and 663 cmꢀ1 are, respectively, characteristic of CꢀC stretching
vibrations of the phenylene group and in-plane deformation
vibrations of para-disubstituted aromatic ring with identical
groups. Bands at 836 cmꢀ1 and 819 cmꢀ1 are attributed to the
CꢀH out-of-plane vibrations for para-disubstituted phenylene
rings. The broad bands around 3600 and 3000 cmꢀ1, arising from
the OꢀH stretching vibrations of the protonated phosphonate
oxygens O(1), suggest that extensive hydrogen bonds are present
in the Ln-compounds because of the absence of OꢀH deformation
band around 1568 cmꢀ1, as they do not contain either coordinated
or lattice water molecules.10,11,43ꢀ46
The thermal stability in air of Ln-compounds was investigated.
The TG curves, and some typical mass spectrometry analysis
curves, are depicted in Figure S2a,b (see Supporting In-
formation). According to the difference in their thermal stability,
TG curves may be divided into two groups named hs (high
stability) and ms (medium stability). TG curves of Ln-com-
pounds reveal the observed total mass losses, presented in the
Table 5, from room temperature up to 1200 °C, and a 1:1
mixture of LnPO4 and Ln(PO3)3 as the final product. The TG
curves of Ln-compounds very closely resemble each other and
show that this series of compounds is characterized by high
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Figure 5. Observed (red) and calculated (black) profiles for the final Rietveld refinement of [Ln(HO6P2C6H4)] compounds (Ln = La, Ce, Pr, Nd, Eu,
Dy, Ho). The difference plot (blue) is on the same intensity scale.
thermal stability. A noticeable increase in thermal stability has
been observed in the case of Dy- and Ho-compounds. From
room temperature to ca. 600 °C in the case of hs, and to ca.
500 °C in the case of ms, no noticeable mass loss (or a slight mass
loss of ca. 3%) was detected. Between ca. 600 and 1100 °C in the
case of hs, and between ca. 500 and 1000 °C in the case of ms,
the mass loss proceeds in continuous and overlapping stages and
corresponds to the loss of the phenyl group. The associated mass
spectrometry m/z 18 (H2O), 44 (CO2), and 78 (C6H6) curves
are in a good agreement with TG curves. The very slight mass loss
below 600 and 500 °C for hs and ms, respectively, is explained
by the m/z 78 curve which shows the desorption of the phenyl
groupintheranges500ꢀ750 °C(hs) and400ꢀ700 °C(ms), with
first maxima at around 560 °C (hs) and 528 °C (ms). After 600 °C
(hs) or 500 °C (ms), the mass loss increases rapidly due to the
combustion of phenyl groups and that is manifested by a series of
bands on the m/z 18 and 44 curves in the ranges of 600ꢀ1100 °C
(hs) and 500ꢀ1000 °C (ms).
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The X-ray thermodiffraction analysis of the Ho-compound
carried out under air heating up to 900 °C (Figure 7) has
confirmed that no significant structural change occurs up
600 °C. At 650 °C, a noticeable diffraction intensities drop is
observed and peak broadening is observed, and continues
decreasing and broadening until 850 °C. At 900 °C, practically
no diffraction peaks have been observed. Nevertheless, by main-
taining the temperature at 900 °C for a longer time as well as after
cooling down to 30 °C, two new phases have been formed
characterized by a series of broad diffraction peaks corresponding
to the formation of HoPO4 and Ho(PO3)3.47,48 The X-ray thermo-
diffraction study is consistent with TG analysis and confirms
the very high thermal stability of the Ln-compounds reported
in this paper.
The RT luminescence and excitation spectra of the Eu- and
Tb-compounds as powders are shown in Figure 8a,b. The
emission peaks in the luminescence spectra are assigned to the
5
5
electronic transitions from the D0 and D4 multiplet to lower
energy states 7FJ of Eu3+ and Tb3+, respectively. The excitation
spectra correspond to transitions from the ground states — 7F0
and 7F6 Eu3+ and Tb3+, respectively — to excited states according
to the Dieke’s diagram (see insets of Figure 8a,b).49 In addition to
these fꢀf transitions, the excitation spectra show strong bands at
wavelengths smaller than 340 nm, which are tentatively assigned
to an energy transfer process from the ligand. The presence of
organic ligands usually leads to an absence of luminescence due
to the strong competition with nonradiative relaxation processes
Figure 6. Variation of cell parameters (a, b, c, and β), volume, and
distance LnꢀLn versus Shannon’s ionic radii (Å) of Ln3+ cations in the
isostructural Ln-compounds.
Table 5. Calculated and Found Percentages of CꢀH and Total Mass Losses of [Ln(HO6P2C6H4)] Compounds
calculated
experimental
compound
Mr (g/mol)
C (%)
H (%)
weight loss (%)
C (%)
H (%)
weight loss (%)
La[O3P(C6H4)PO3H]
Ce[O3P(C6H4)PO3H]
Pr[O3P(C6H4)PO3H]
Nd[O3P(C6H4)PO3H]
Sm[O3P(C6H4)PO3H]
Eu[O3P(C6H4)PO3H]
Gd[O3P(C6H4)PO3H]
Tb[O3P(C6H4)PO3H]
Dy[O3P(C6H4)PO3H]
Ho[O3P(C6H4)PO3H]
373.95
375.16
375.95
379.29
385.41
387.01
392.30
393.97
397.55
399.98
19.25
19.19
19.15
18.98
18.86
18.60
18.35
18.27
18.11
18.00
1.34
1.33
1.33
1.32
1.30
1.29
1.27
1.27
1.26
1.25
20.62
20.55
20.51
20.33
20.01
19.92
19.65
19.57
19.40
19.28
19.1
19.4
18.7
19.2
18.7
18.9
18.1
18.6
18.2
18.1
1.2
1.1
1.4
1.3
1.2
1.1
1.3
1.2
1.3
1.2
21.8
21.2
21.3
20.8
21.0
20.4
20.2
20.4
19.6
18.7
Figure 7. X-ray thermodiffractogram of Ho-compound recorded in air heating up from 50 to 900 °C. Color code corresponding to the structural changes.
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(Figure S3, see Supporting Information) with lifetimes of
1.89 ( 0.02 ms and 1.397 ( 0.015 ms, respectively.
’ CONCLUSIONS
The hydrothermal synthesis and the structural character-
ization of a series of novel isostructural metalꢀorganic hybrids
based on lanthanide and 1,4-phenylenebis(phosphonate)
Ln[O3P(C6H4)PO3H] (where Ln = La, Ce, Pr, Nd, Sm, Eu,
Gd, Tb, Dy and Ho) have been reported. These compounds have
been obtained as single phases and exhibit very high thermal
stability, and their photoluminescence properties are also
discussed. Currently, our work is focused on using the extended
analogue l,4-biphenylenebis(phosphonate) and spacer mol-
ecules as a second ligand, such as phosphorous or phosphoric
acid, to create porous structures, as these may be promising
materials in the fields of gas storage and photoluminescence.
’ ASSOCIATED CONTENT
S
Supporting Information. IR spectra of Ln(HBDP) com-
b
pounds, TG curves of Ln(HBDP) with representative MS curves
for Ho-compound and La-compound, room temperature of
Eu3+ emission decay curve, and X-ray crystallographic infor-
mation (CIF files) for the Ln(HBDP) compounds can be
obtained free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: amghouz.uo@uniovi.es (Z.A.); clearfield@chem.tamu.
edu (A.C.).
Figure 8. RT emission (up) and excitation (down) spectra of (a) Eu-
and (b) Tb-compounds. Emission spectrum was obtained upon excita-
tion at 464 and 376 nm, while excitation spectrum was performed
detecting luminescence at 615 and 545 nm, for Eu- and Tb-compounds,
respectively. Energy level scheme of Eu3+ and Tb3+ ion with the
transitions responsible for the luminescence and excitation spectra are
depicted in the inset.
’ ACKNOWLEDGMENT
We acknowledge financial support from Spanish Ministerio de
Ciencia e Innovaciꢀon and FEDER (Projects MAT2010-15094,
MAT2006-01997, UCAN08-4E-008, Factoría de Cristalizaciꢀon ꢀ
Consolider Ingenio 2010, and FPI grant BES-2007-14340 to Z.A.).
A.C. wishes to thank NSF under grant DMR-0652166 and the R. A.
Welch Foundation Grant No. A-0673 for which grateful acknowl-
edgement is made.
induced by high energy ligand vibrations. Moreover, it is well-
known that a strong quenching of luminescence occurs for high
concentration of lanthanide ions due to energy migration of
the excitation to quenching impurities (killer traps).50 This is
especially relevant for two- or three-dimensional lattices. There-
fore, the observation of visible luminescence in a solid form in
concentrated lanthanide compounds at room temperature is an
outstanding result itself. Additionally, the sharp lines observed in
luminescence and excitation spectra are a clear demonstration
of the good crystalline quality of the samples. The absence of
’ REFERENCES
(1) Cao, G.; Hong, H.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420.
(2) Clearfield, A. Curr. Opin. Solid State Mater. Sci. 1996, 1, 268.
(3) Clearfield, A. Progress in Inorganic Chemistry; Karlin, K. D., Ed.;
John Wiley & Sons, Inc.: New York, 1998; Vol. 47, pp 371ꢀ510.
(4) Clearfield, A. Curr. Opin. Solid State Mater. Sci. 2002, 6, 495.
(5) Maeda, K. Microporous Mesoporous Mater. 2004, 73, 47.
(6) Clearfield, A. In Design of New Materials; Cocke, D. L., Clearfield,
A., Eds.; Plenum: New York, 1986.
(7) Gꢀomez-Alcꢀantara, M. M.; Cabeza, A.; Moreno-Real, L.; Aranda,
M. A. G.; Clearfield, A. Microporous Mesoporous Mater. 2006, 88, 293.
(8) Wang, Z.; Heising, J. M.; Clearfield, A. J. Am. Chem. Soc. 2003,
125, 10375.
(9) Clearfield, A.; Wang, Z.; Bellinghausen, P. J. Solid State Chem.
2002, 167, 376.
(10) Poojary, D. M.; Zhang, B.; Bellinghausen, P.; Clearfield, A.
Inorg. Chem. 1996, 35, 4942.
(11) Poojary, D. M.; Zhang, B.; Bellinghausen, P.; Clearfield, A.
Inorg. Chem. 1996, 35, 5254.
(12) Poojary, D. M.; Zhang, B.; Clearfield, A. An. Quim.-Int. Ed.
1998, 94, 401.
5
5
emission from the excited state D1 and D3 to the ground
state in Eu3+ and Tb3+, respectively, can be explained by the
energy gap’s law taking into account the maximum phonon
energy of the lattice or by resonant energy transfer processes
between adjacent Eu3+ or Tb3+ ions. In this process the excited
5
5
electron in D1 can relax to D07multiplet and excites the
7
ground state F0 electron to the F3 state in a neighboring
Eu3+ ion since the energy difference between these states
is the same. In the same manner, a Tb3+ ion can relax from
the 5D3 level to the 5D4 and excites a near neighbor Tb3+ from
the 7F6 to the 7F1,0 51 The same process can be responsible for
.
the lack of visible emission in all the other members of the
series. Temporal dependence of Tb3+ and Eu3+ lumines-
5
5
cence, corresponding to transitions from the D4 and D0
to lower energy levels, shows a single exponential behavior
5296
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Crystal Growth & Design
ARTICLE
(13) Clearfield, A.; Wang, J. D.; Tian, Y.; Campbell, F. L., III, Peng,
G. Z. In Perry, D., Ed.; Materials Synthesis and Characterization; Plenum:
New York, 1997.
(14) Clearfield, A.; Wang, Z. J. Chem. Soc., Dalton Trans. 2002, 15, 2937.
(15) Distler, A.; Sevov, S. C. Chem. Commun. 1998, 959.
(16) Fredoueil, F.; Massiot, D.; Poojary, D. M.; Bujoli-Doeuff, M.;
Clearfield, A.; Bujoli, B. Chem. Commun. 1998, 175.
(17) Burwell, D. A.; Thompson, M. E. Chem. Mater. 1991, 3, 14.
(18) Silvestre, J. P.; El Messbahi, N.; Rochdaoui, R.; Dao, N. Q.; Lee,
M. R.; Neuman, A. Acta Crystallogr. 1990, C46, 986.
(19) Poojary, D. M.; Zhang, B.; Cabeza, A.; Aranda, M. A. G.;
Bruque, S.; Clearfield, A. J. Mater. Chem. 1996, 6, 639.
(20) Soghomonian, V.; Chen, R.; Haushalter, R. C.; Zubieta, J.
Angew. Chem., Int. Ed. Engl. 1995, 34, 223.
(21) Maeda, K.; Akimoto, J.; Kiyouzumi, Y.; Mizukami, F. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1199.
(22) Finn, R. C.; Zubieta, J. J. Chem. Soc., Dalton Trans. 2000, 1821.
(23) Serre, C.; Ferey, G. Inorg. Chem. 1999, 38, 5370.
(24) Loshe, D. L.; Sevov, S. C. Angew. Chem., Int. Ed. Engl. 1997,
36, 1619.
(25) Wharmby, M. T.; Mowat, J. P. S.; Thompson, S. P.; Wright,
P. A. J. Am. Chem. Soc. 2011, 133, 1266.
(26) Rocha, J.; Almeida Paz, F. A.; Shi, F. N.; Ananias, D.; Silva,
N. J. O.; Carlos, L. D.; Trindade, T. Eur. J. Inorg. Chem. 2011, 13, 2035.
(27) Vivani, R.; Alberti, G.; Costantino, F.; Nocchetti, M. Micropor-
ous Mesoporous Mater. 2008, 107, 58.
(28) Mao, J. G. Coord. Chem. Rev. 2007, 251, 1493.
(29) Shi, F. N.; Trindade, T.; Rocha, J.; Almeida Paz, F. A. Cryst.
Growth Des. 2008, 8, 3917.
(30) Adelani, P. O.;Albrecht-Schmitt, T. E.Inorg. Chem. 2009, 48, 2732.
(31) Diwu, J.; Nelson, A. G. D.; Wang, S.; Campana, C. F.; Albrecht-
Schmitt, T. E. Inorg. Chem. 2010, 49, 3337.
(32) Wang, R. C.; Zhang, Y.; Hu, H.; Frausto, R. R.; Clearfield., A.
Chem. Mater. 1992, 4, 864.
(33) Song, S. Y.; Ma, J. F.; Yang, J.; Cao, M. H.; Zhang, H. J.; Wang,
H. S.; Yang, K. Y. Inorg. Chem. 2006, 45, 1201.
(34) APEX2 and SAINT; Bruker AXS Inc.: Madison, Wisconsin,
2007.
(35) Sheldrick, G. M. SADABS, Program for Absorption Correction of
Area Detector Frames, Bruker AXS, Inc., Madison, WI.
(36) Sheldrick, G. M. SHELXL-97, Program for Refinement of Crystal
Structures; University of G€ottingen: Germany, 1997.
(37) Parkin, S.; Moezzi, B.; Hope, H. J. Appl. Crystallogr. 1995,
28, 53.
(38) PLATON/PLUTON: (a) Spek, A. L. Acta Crystallogr., Sect. A
1990, 46, C34. (b) Spek, A. L. PLATON: A Multipurpose Crystallographic
Tool; Utrecht University: Utrecht, The Netherlands, 1998.
(39) Brandenburg, K. DIAMOND, Version 3.1; Crystal Impact
GbR: Bonn, Germany, 2007.
(40) RodríguezꢀCarvajal, J. Physica B 1993, 192, 55.
(41) Toby, B. H. Powder Diffr. 2006, 21, 67.
(42) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(43) Bellamy, L. J. The Infra-Red Spectra of Complex Molecules, 3rd
ed; Chapman and Hall: London, 1975.
(44) Socrates, G. Infrared Characteristic Group Frequencies; Wiley:
New York, 1994.
(45) Abdel Kerim, F. M.; Aly, H. F.; El-Agramy, A. Proc. Indian Acad.
Sci. 1977, 85, 559.
(46) Amicangelo, J. C.; Leenstra, W. R. Inorg. Chem. 2005, 44, 2067.
(47) Milligan, W. O.; Mullica, D. F.; Beall, G. W.; Boatner, L. A.
Inorg. Chim. Acta 1983, 70, 133.
(48) Hoeppe, H. A. J. Solid State Chem. 2009, 182, 1786.
(49) Dieke, G. H. In Spectra and Energy Levels of Rare Earth Ions in
Crystals; Wiley Interscience: New York, 1968.
(50) Berdowski, P. A. M.; Blasse, G. J. Lumin. 1984, 29, 243.
(51) Hayakawa, T.; Kamata, N.; Yamada, K. J. Lumin. 1996, 68, 179.
5297
dx.doi.org/10.1021/cg2008254 |Cryst. Growth Des. 2011, 11, 5289–5297