Unexpected coordination chemistry of bisphenanthroline complexes within hybrid materials: A mild way to Eu2+ containing materials with bright yellow luminescence

Article abstract of DOI:10.1021/ja075277e

A hybrid organic-inorganic material containing bisphenanthroline units forming tetrahedral cavities was prepared. We show that these cavities not only allow the incorporation of Cu²⁺ and Eu³⁺ ions but also induce the reduction of these ions in Cu⁺ and Eu²⁺, respectively. The Eu²⁺ containing material shows an extremely bright yellow luminescence and might be therefore a candidate for future applications. Copyright

Full text of DOI:10.1021/ja075277e

Published on Web 10/03/2007
Unexpected Coordination Chemistry of Bisphenanthroline Complexes within
Hybrid Materials: A Mild Way to Eu2+ Containing Materials with Bright Yellow
Luminescence
Laurence Raehm,† Ahmad Mehdi,*,† Claudia Wickleder,‡ Catherine Reye´,† and Robert J. P. Corriu†
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS, Chimie Mole´culaire et Organisation du Solide, UniVersite´
Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 5, France, and Anorganische Chemie,
UniVersita¨t Siegen, 57068 Siegen, Germany
Received July 16, 2007; E-mail: ahmad.mehdi@univ-montp2.fr
Scheme 1
Nanostructured hybrid materials based on silica and obtained
by sol-gel process have attracted considerable attention during the
past decade, as they constitute a unique class of materials combining
the properties of organic moieties and inorganic matrix.1 They are
obtained by hydrolysis and polycondensation of bis- or poly-
(trialkoxysilyl)organic precursors, leading to materials in which the
organic fragments are integrated into the silica matrix by covalent
bonds.2 The interest of this class of materials resides in that their
properties can be modified by changing the nature of the bridging
organic groups, which renders them very attractive.3
We have been interested by the introduction of groups able to
chelate transition-metal or lanthanide ions4 as such materials could
present various physical properties (optic, electric, or magnetic, for
incorporated after the demetalation to adopt unusual geometries,
example) depending on the nature of the salts. They could also be
which could induce unexpected results.
promising in metal-ion separation, including actinides.
We show that the incorporation of Cu2+ and Eu3+ ions into hybrid
In the course of our investigations in this field, some results
materials containing free cavities is not only possible but also
highly suggested that the coordination chemistry in the solid is
induces the reduction of these ions in Cu+ and Eu2+, respectively.
different from that in solution. For instance, we previously observed
For this purpose, ligand 2 was prepared in a 62% yield by
that it is possible to incorporate Eu3+ ions directly inside the solid
silylation of 2,9-diphenol-1,10-phenanthroline8 by a Williamson-
resulting from the hydrolytic polycondensation of tetrasilylated
type reaction with 3-iodopropyltriisopropoxysilane.9 The coordina-
cyclam 1, with a cyclam/Eu3+ ratio of 2, while complexation of
tion of a copper(I) ion with two bidentate phen units was
Eu3+ by the precursor 1 was not possible in solution.4c That
accomplished by treating 2 with a stoichiometric amount of [Cu-
suggested that, in the solid where the mobility of chelating species
(CH3CN)4]BF4 in CH3CN/CH2Cl2 at room temperature, leading
is very limited, the formation of complexes originated essentially
quantitatively to the tetrasilylated bisphen copper(I) complex 2-
from the short-range arrangement of the chelating moieties. These
[Cu] as a deep-red colored solid (Scheme 1). The latter was fully
results prompted us to extend our investigations in the field of the
1
characterized by H, 13C, and 29Si NMR spectroscopies as well as
coordination chemistry in the solid. Phenanthroline (phen) has been
by elemental analysis. The geometry around the copper atom is
used for many years by Sauvage and co-workers to stabilize various
essentially tetrahedral as expected for bisphen copper(I) complexes.7a
oxidation states of transition metals5 as well as to create chelating
Sol-gel polycondensation of 2[Cu] was performed in ethanol
electroactive films,6 therefore, we decided to explore the chelating
in the presence of a stoichiometric amount of H2O and tetrabuty-
properties of this rigid ligand incorporated in hybrid organic-
lammonium fluoride (TBAF) as catalyst (Scheme 1). The gelation
inorganic materials.
occurred in a few minutes. After the usual workup, the material
S2[Cu] was obtained quantitatively as a deep-red colored powder.
Demetalation of S2[Cu], that is, removal of the metal from the
cavities of the material, was attempted by treating the material with
a large excess of KCN in a mixture of CH3CN and H2O.
Surprisingly, demetalation, which occurs within minutes at room
temperature for bisphen copper(I) complexes in solution,8 failed
with S2[Cu] even when treated with a very large excess (40 equiv)
of KCN for 3 days at 60 °C. This astonishing behavior underlines
In this contribution, we describe the preparation of bisphen
the difference of reactivity between solution and the solid state.
transition-metal complexes (Cu+, Ag+) with a tetrahedral geometry7
This unexpected result10 prompted us to prepare the silver(I)
bearing four hydrolyzable Si(OiPr)3 groups as well as their
complex in place of the Cu(I) one in order to allow the demetalation.
hydrolytic polycondensation which gives rise to hybrid organic-
2[Ag] was prepared by reacting AgBF4 with 2 equiv of 2 in a
inorganic materials containing transition-metal ions. The demeta-
mixture of CH2Cl2 and MeOH, leading to an off-white complex.
lation of these materials was examined in order to study the
The corresponding beige material S2[Ag] was prepared quantita-
coordination properties of the resulting materials. Indeed, we
tively in a similar manner to S2[Cu] (Scheme 1).
guessed that the rigid framework of 1,10-phen might force the ions
The Si/N as well as Si/Ag ratios in the solid S2[Ag] determined
by elemental analysis were found to be 1.13 and 4.17, which is
very close to the expected values (1 and 4, respectively) showing
† Universite´ Montpellier II.
‡ Universita¨t Siegen.
9
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J. AM. CHEM. SOC. 2007, 129, 12636-12637
10.1021/ja075277e CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
probable different Eu2+ coordination. Additionally, Eu3+ transitions
are observed in all cases leading to a rather bad color quality. It is
worth noting that Zhang and co-workers13 prepared a hybrid
material containing Eu3+ phen complexes covalently bonded to a
silica matrix. This material was obtained by hydrolytic polycon-
densation of tetraethoxysilane and phen bearing two Si(OEt)3
groups, in the presence of Eu3+ ions. Interestingly, the emission
spectrum of this material was characteristic of Eu3+ ions. This result
points out the role of the tetrahedral cavities in our case.
The observed reduction of Eu(III) to Eu(II) results from the
unusual geometry, which was imposed by the procedure. Reduction
of Cu2+ to Cu+ was previously observed within hybrid materials.
Guilard and co-workers demonstrated that the reduction occurred
through electronic transfer between ions and ethanol.14 As ethanol
was also the reaction solvent in the present study, we propose a
similar process to explain the electron transfer.
Figure 1. Emission (left) and excitation spectra (right) of [Eu]S2.
Scheme 2
In conclusion, we described the preparation of hybrid materials
containing bisphen transition-metal complexes (Cu+, Ag+) with a
tetrahedral geometry. The demetalation-remetalation of the Ag+
containing material was possible. We showed that the incorporation
of Cu2+ and Eu3+ ions within hybrid material containing free
tetrahedral cavities induced their reduction into Cu+ and Eu2+. The
luminescence of Eu2+ containing material revealed an extremely
bright emission in the yellow range. That renders this air-stable
material of particular interest. Indeed, it could be a promising
candidate for luminescence applications such as energy saving
LEDs.
that demetalation did not occur during the sol-gel process.
Treatment of S2[Ag] with a large excess of KCN led quantitatively
to metal-free potentially chelating material S2 (Scheme 1) as
revealed by elemental analysis.
Remetalation of the material by ions, which do not usually adopt
a tetrahedral geometry, was attempted in order to investigate the
consequences of such a coordination. Treatment of S2 with an
ethanolic solution containing 1 equiv of Cu(BF4)2 gave rise to the
brown-red material [Cu]S2 after a 12 h reflux (Scheme 2). Titration
revealed that 60% of the metal-free sites were remetalated. As the
brown-red color of the material suggested the reduction of Cu(II)
into Cu(I), ESR measurements were carried out. The amount of
Cu(II) within [Cu]S2 determined by this method was found to be
very low (<1%) as indicated by the weak absorption at 3009 G (g
) 2.248) (spectrum available as Supporting Information). In
addition, the UV/vis absorption spectrum of [Cu]S2 was obtained.
The absence of the band at 680 nm corresponding to the Cu(II)
complex5 as well as the similarity between this spectrum and that
of S2[Cu] led us to suggest that reduction of Cu(II) to Cu(I)
occurred during the remetalation process (both spectra and the one
of S2 are available as Supporting Information).
Acknowledgment. Dedicated to the memory of Jean-Marc
Kern. We are very grateful to Dr. J.-P. Sauvage for fruitful
discussions.
Supporting Information Available: EPR spectra of [Cu]S2, UV-
vis spectra of S2, S2[Cu], and [Cu]S2, and details of the luminescence
measurements. This material is available free of charge via the Internet
at http://pubs.acs.org.
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