Lanthanide-polyphosphonate coordination polymers combining catalytic and photoluminescence properties

Article abstract of DOI:10.1039/c3cc42632g

A rapid, mild and high-yield microwave synthesis of 1D isotypical [Ln(H₄bmt)(H₅bmt)(H₂O)₂] ·3H₂O coordination polymers is presented. The La ³⁺-based material is highly active as a heterogen

Full text of DOI:10.1039/c3cc42632g

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Lanthanide-polyphosphonate coordination polymers
combining catalytic and photoluminescence
properties†
Cite this: DOI: 10.1039/c3cc42632g
Received 10th April 2013,
Accepted 24th May 2013
ab
a
a
Sergio M. F. Vilela, Ana D. G. Firmino,ab Ricardo F. Mendes, Jose A. Fernandes,
´
´
Duarte Ananias,a Anabela A. Valente,a Holger Ott,c Lu´ıs D. Carlos,d Joa˜o Rocha,a
DOI: 10.1039/c3cc42632g
b
a
´
Joa˜o P. C. Tome and Filipe A. Almeida Paz*
www.rsc.org/chemcomm
A rapid, mild and high-yield microwave synthesis of 1D isotypical
[La(H4bmt)(H5bmt)(H2O)2]Á3H2O (1) and [La2(H3bmt)2(H2O)2]ÁH2O
[Ln(H4bmt)(H5bmt)(H2O)2]Á3H2O coordination polymers is presented. The (2) could be produced from the reaction between H6bmt and LaCl3
La3+-based material is highly active as a heterogeneous catalyst in the hydrate using microwave-assisted synthesis (MWAS). In our previous
methanolysis of styrene oxide at nearly room temperature. Eu3+- and report on the 3D compound 2 we mentioned that 1 could also be
Tb3+-doped materials are very effective UV-to-visible light converters.
obtained.9 The reaction time and temperature in MWAS were, in this
way, systematically probed in order to investigate which typical
Coordination polymers, commonly known as Metal–Organic Fra- conditions could lead to either materials (Fig. 1). The 1D compound
meworks (MOFs), are known worldwide for their fascinating archi- 1 is readily isolated at low temperatures, mainly at 40 and 60 1C. Upon
tectures and properties,1 spanning from medical to industrial increasing both the reaction time and the temperature, compound 2
applications such as in gas storage,2 separation,3 catalysis,4 among starts to prevail as the main product. The latter material is, however,
others. Despite much of current research being still driven by the obtained as a pure phase only at 150 1C. Mixtures of both compounds
discovery of new frameworks, there is worldwide interest in finding are isolated at 90, 100 and 120 1C, with 2 dominating at higher
materials that can: (i) be prepared fast under very mild conditions reaction times (15 and 30 minutes). The crystal morphology can be
and in high yields (i.e., attractive to industry); (ii) combine several fine-tuned according to the synthetic conditions employed (see addi-
functions taking advantage of both the metallic centers and the tional details in the ESI† – Fig. S2).
organic ligands (and the framework itself); (iii) outperform known
The preparation of optically active materials with stoichiometric
compounds in industrial relevant applications while being ther- amounts of distinct lanthanides was performed under the optimal
mally and mechanically robust. It is, thus, not surprising to find conditions discovered for 1 while including 3% of Eu3+ or Tb3+ in the
MOFs used in exotic applications such as molecular sponges5 or reactive mixtures: [(La0.97Eu0.03)(H4bmt)(H5bmt)-(H2O)2]Á3H2O (3) and
new facile ways to produce nano-sized crystals.6 We have been [(La0.97Tb0.03)(H4bmt)(H5bmt)(H2O)2]Á3H2O (4) were obtained at 60 1C
investigating the use of polyphosphonate ligands,7 some designed using 5 minutes of irradiation. Phase purity of all materials and rare-
by us,8 with rare-earth cations to produce thermally and mechani- earth distribution were unequivocally confirmed using several tech-
cally robust MOFs using novel synthetic approaches. Herein we niques (see ESI†), including powder X-ray diffraction (Fig. S1, ESI†),
present a novel 1D isotypical series of materials based on tripodal electron microscopy (Fig. S6 and S7, ESI†), FT-IR (Fig. S8, ESI†) and
(benzene-1,3,5-triyltris(methylene))triphosphonic acid (H6bmt), solid-state NMR (Fig. S12 and S13, ESI†) spectroscopy.
which were prepared in a easy and fast way, and combine high
photoluminescence and catalytic activity.
a Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro,
Portugal. E-mail: filipe.paz@ua.pt; Fax: +351 234 401470; Tel: +351 234 401418
b Department of Chemistry, QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal
c Bruker AXS GmbH, Oestliche Rheinbrueckenstr. 49, 76187 Karlsruhe, Germany
d Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
† Electronic supplementary information (ESI) available: Detailed crystallographic
description, including figures, tables; CIF file; additional structural characterisa-
tion (FT-IR, TGA, thermodiffractometry, electron microscopy and powder X-ray
Fig. 1 Microwave-assisted hydrothermal synthesis optimisation of the reaction
temperature and time to obtain [La(H4bmt)(H5bmt)(H2O)2]Á3H2O (1) ( ) and the
diffraction); additional photoluminescence and catalytic studies. CCDC 932041.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3cc42632g
known [La2(H3bmt)2(H2O)2]ÁH2O (2)9
( ) materials.
c
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Fig. 2 (a) Mixed ball-and-stick and polyhedral representation of the neutral 1[La(H4bmt)(H5bmt)(H2O)2] one-dimensional coordination polymer consisting of the
N
crystal structure of 1. (b) The distorted {LaO8} square antiprismatic coordination environment of the crystallographically independent La3+ cation. (c) Schematic
representation of the brickwall-like fashion of the close packing of N1[La(H4bmt)(H5bmt)(H2O)2] polymers in the ac plane of the unit cell. Symmetry codes: (i) x, 1.5 À y,
1
2
z; (ii) Àx, 1 À y, Àz; (iii) Àx, + y, Àz. For selected bond lengths and angles see detailed crystal description in the ESI.†
The crystal structure of 1 was unequivocally determined from
single-crystal X-ray diffraction studies at 100 K in the monoclinic
centrosymmetric space group P21/m.† The structure is made up of a
single La3+ ion located on the crystallographic mirror plane, which
was coordinated to four symmetry-related organic ligands and two
water molecules (which are also located on the mirror plane). The
overall coordination environment, {LaO8}, resembles a slightly
distorted square antiprism (Fig. 2b). Connectivity through the
organic ligands leads to the formation of a zigzag one-dimensional
polymer [an angle of 143.0(1)1 between the La - La - La vectors –
Fig. 2a], imposing a LaÁÁÁLa intermetallic distance of 11.1804(10) Å
along the polymer. Individual polymers pack closely in the ac plane
of the unit cell in a typical brickwall-like fashion (Fig. 2c). Packing is
mediated by an extensive network of O–HÁÁÁO hydrogen bonds of
Fig. 3 Excitation (lEm. = 544 nm; black) and emission (lExc. = 276 nm; green)
various strengths involving coordinated and uncoordinated water
molecules and various PO–H groups from the organic linkers. A
detailed description of the crystal structure and the various supra-
molecular contacts present is provided in the ESI.†
spectra of [(La0.97Tb0.03)(H4bmt)(H5bmt)(H2O)2]Á3H2O (4) at ambient tempera-
ture. The inset shows the bright green emission of a small pellet of 4 under a
laboratory UV lamp (lExc. = 254 nm) and its corresponding CIE (Commission
International d’Eclairage) colour coordinates.
The crystal structure of the series remains unaltered up to
ca. 60 1C as revealed by TGA and thermodiffractometry (Fig. S10, Decay curves were fitted by single exponential functions, with
ESI†). Above this temperature a series of structural transformations lifetimes of 0.37 Æ 0.01 and 1.52 Æ 0.01 ms for 3 and 4, respectively.
of the micro-crystals occur, mostly motivated by the release of water
molecules (isolated materials remain crystalline).
Based on the emission spectra, 5D0 lifetimes and empirical
radiative and non-radiative transition rates, assuming that only
The excitation spectra of the stoichiometric mixed-lanthanide non-radiative and radiative processes are involved in the depopula-
5
compounds 3 and 4, recorded at ambient temperature while tion of the D0 state, the 5D0 quantum efficiency, q,10 has been
monitoring the Eu3+ 5D0 - F2 and Tb3+ 5D4 - F5 transitions determined for 3. The theoretical rationale employed in these
(Fig. S14, ESI† and Fig. 3), are dominated by a broad UV band (240– calculations is provided in subsection 2.8.1 in the ESI.† The number
288 nm) attributed to p–p* transitions. In the emission spectra of 3 of water molecules (nw) coordinated to Eu3+ and Tb3+ may be
and 4 recorded at ambient temperature (excited at 276 nm; Fig. determined using the empirical formulae of Horrocks and Kimura,
S15† and Fig. 3) the sharp lines are assigned to transitions between respectively.11 All data for 3 and 4, including the absolute emission
the first excited non-degenerate 5D0 state and the 7F0–4 levels of the quantum yield (f, measured experimentally), are collected in
fundamental Eu3+ septet and to the first excited state 5D4 and the Table S4 (ESI†). Compound 3 has a relatively low quantum efficiency
7F6–0 levels of Tb3+, respectively, for 3 and 4. The local-field splitting (8%). The measured maximum absolute emission quantum yield
of the Eu3+ 7F1,2 levels into three and clearly less than five Stark reaches a maximum of 5% at 393 nm. In contrast, compound 4
components, respectively, and the predominance of the 5D0 - 7F2 reaches a relatively high absolute emission quantum yield of 44% at
7
7
transition relatively to the 5D0
-
7F1 (Fig. S15, ESI†), indicate 276 nm excitation. This relatively high value is presumably due to a
the presence of a single low-symmetry Eu3+ environment in the good superposition between the interconfigurational 4f75d1 excited
structure, as supported by the crystal structure. The ambient states of Tb3+, namely the high spin states commonly positioned at
5
5
temperature D0 and D4 lifetimes of Eu3+ and Tb3+ of 3 and 4 around 280 nm, with the emitting S1 - S0 transition of the ligand
were determined by monitoring the emission decay curves within (zero-phonon level at ca. 277 nm).9
the maximum of the 5D0
-
7F2 and 5D4
-
7F5 transitions,
The catalytic performance of 1 was explored in the methano-
using excitation at 393 and 376 nm, respectively (Fig. S16, ESI†). lysis of styrene oxide (PhEtO) (Fig. 4 – top). It was found that the
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It also outperforms nano-sized HKUST-1 (40 1C, 90% conversion
at 2.5 h).13 The superior catalytic performance of 1 may, thus,
be due to a favorable compromise between the acid properties
and active site accessibility.
In summary, we have presented a fast and simple synthetic route
to novel 1D lanthanide-polyphosphonate materials. The inclusion
of stoichiometric amounts of optically active lanthanide cations
produces highly efficient light converting materials: the Tb3+-based
material has an absolute emission quantum yield of 44%. We show
that these materials are outstanding heterogeneous catalysts in
the methanolysis of styrene oxide, outperforming known MOF
structures: high activity at 35 1C with nearly complete conversions
at 30 min of reaction without the need for catalyst regeneration. We
are exploring synthetic ways to improve the photoluminescence in
the red and blue regions of the spectrum.
We thank FCT, the European Union, QREN, FEDER, COMPETE
and CICECO (PEst-C/CTM/LA0011/2011). We further wish to
thank FCT for the R&D project PTDC/QUI-QUI/098098/2008
(FCOMP-01-0124-FEDER-010785), the PhD grants no. SFRH/
BD/66371/2009 (to SMFV), SFRH/BD/84495/2012 (to ADGF),
Fig. 4 Reaction of styrene oxide (PhEtO) with methanol to afford 2-methoxy-2-
phenylethanol in the presence of the heterogeneous catalyst [La(H4bmt)(H5bmt)-
(H2O)2]Á3H2O (1) in four consecutive batch runs. Reaction conditions: 0.4 M PhEtO,
20 g(1) LÀ1, 1 h, 55 1C, 800 rpm.
¨
active sites are likely of Bronsted-type and associated with its organic SFRH/BD/84231/2012 (to RFM), and the post-doctoral grant
component (see ESI† for additional details). The catalytic reaction at SFRH/BPD/63736/2009 (to JAF).
55 1C gave 2-methoxy-2-phenylethanol (MeOPhEtOH) in quantitative
yield within 30 min of reaction (Table S5 in the ESI†); without a
catalyst no reaction occurred. The catalytic stability of 1 was
Notes and references
investigated by reusing the washed–dried catalyst: a slight decrease
in the PhEtO conversion and MeOPhEtOH yield at 30 min of
reaction was observed for four consecutive batch runs. These
differences are less pronounced compared to the results for 1 h of
reaction (Fig. 4). The structural and morphological features of 1 after
each catalytic batch run were investigated using powder X-ray
diffraction and electron microscopy (Fig. S17, ESI†). Despite an
apparent decrease in the average crystal size of the recovered catalyst
1 after the first catalytic run, the morphology is maintained and
the crystalline structure is preserved in subsequent batch runs.
Morphological changes may lead to the observed slight catalyst
deactivation. The catalytic performance of 1 in recycling runs is fairly
good without using dedicated catalyst regeneration steps.
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c
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Chem. Commun.