CHEMCATCHEM
COMMUNICATIONS
DOI: 10.1002/cctc.201402599
Mesoporous Metal Complex–Silica Aerogels for
Environmentally Friendly Amination of Allylic Alcohols
Aida Grau,[a] Alejandro Baeza,[b] Elena Serrano,[a] Javier Garcꢀa-Martꢀnez,*[a] and
Carmen Nꢁjera*[b]
Two series of mesoporous hybrid iron(III) complex–silica aero-
gels were prepared in one-pot synthesis by using the sol–gel
coordination chemistry approach. The use of the ligands 3-(2-
aminoethylamino)propyltrimethoxysilane and 2-(diphenylphos-
phino)ethyltriethoxysilane, both with terminal triethoxysilyl
groups, were used to incorporate metal complexes in situ into
the framework of silica, through their co-condensation with
a silicon alkoxide during the aerogel formation. This methodol-
ogy yielded optically translucent hybrid mesoporous gels with
homogeneous metal incorporation and excellent textural prop-
erties. The catalytic performance of these materials was tested
in the direct amination of allylic alcohols in water as a target
reaction, with activities comparable or even higher than those
corresponding to the homogeneous iron(III) complex. Further-
more, these catalysts were stable and maintained their catalyt-
ic activity after six reaction cycles.
merous application fields, including thermal insulators, drug re-
lease, drug delivery, catalysis and adsorption.[4]
Herein, we report the synthesis of hybrid iron(III) complex–
silica gels by using the sol–gel coordination approach, in
which a metal complex with ligands containing terminal trial-
koxysilane groups is hydrolysed either in the presence or ab-
sence of other silica sources to produce a coordination com-
plex–silica network.[5] The main advantages of this approach
are that it does not require anaerobic or anhydrous environ-
ments, and it is performed under mild conditions, preserving
the integrity of the metal complexes in the framework, ensur-
ing an homogeneous dispersion along the support and avoid-
ing the pore blocking.[5] This methodology has already been
used to incorporate metal nanoparticles and metal complexes
in mesoporous silica and organosilica supports.[5b–e] Monomer
complexes have been prepared under the same reaction con-
ditions and characterised fully before the formation of the
mesoporous materials by spectroscopic (FTIR, Raman, UV/Vis)
and spectrometric techniques. Once the gels containing 1 wt%
FeIII were synthesised and characterised, the activity of these
materials as supported heterogeneous catalysts was evaluated.
The catalytic performance of these materials has been tested
in the direct amination of allylic alcohols in water as solvent as
the target reaction.[6] This transformation, which yielded allylic
amines with water as the only by-product, has recently been
described with FeCl3·6H2O as catalyst, obtaining the corre-
sponding amination products in high yields.[7] In addition, we
have compared the results herein to those obtained with the
homogeneous catalyst and the metal-free hybrid gels to eluci-
date which iron(III) species are the active phase in the reaction
in aqueous media
Homogeneous catalysts exhibit high activity and selectivity but
they are difficult to recover and reuse.[1] Heterogenisation of
metal complexes on mesoporous supports overcomes these
limitations by easing catalyst separation and reuse, avoiding
leaching and retaining the activity and selectivity of the homo-
geneous catalysts.[2] Traditionally, mesoporous silicas have
been used as a catalyst support.[3] More recently, silica aerogels
are becoming ideal candidates to be used as catalyst supports
because their high specific surface area, high porosity, good
accessibility and simple surface functionalisation.[4,5] One of the
properties that characterises an aerogel is the major accessibili-
ty of reagents and products to the catalyst when supported
owing to the open-pore structures and interconnectivity in
three dimensional networks.[4f] So, they are widely used in nu-
Iron(III) complexes
[a] A. Grau, Dr. E. Serrano, Dr. J. Garcꢀa-Martꢀnez
Department of Inorganic Chemistry
Molecular Nanotechnology Lab.
University of Alicante
Monomeric FeIII complexes were obtained by stirring an
FeCl3·6H2O solution with two different ligands, 3-(2-aminoeth-
ylamino)propyltrimethoxysilane (AEAPTS, L1) and 2-(diphenyl-
phosphino)ethyltriethoxysilane (PPETS, L2), in 1:2 and 1:3
metal/ligand molar ratios under the same conditions as those
used for the synthesis of the hybrid mesoporous metal com-
plex–silica gels (i.e., by using a ethanolic aqueous solution at
room temperature). The colour of the solution with L2
changed from yellow (initial ethanolic FeCl3·6H2O solution) to
blue but that with L1 remained unchanged, according to the
substitution of chloride ligands by L1 or L2 after the addition
of the ligands. Both solutions with L1 and L2 in water were
a yellow–orange colour, owing to the incorporation of water
Ap. 99, E-03690 Alicante, Spain
Fax: (+34)965903454
[b] Dr. A. Baeza, Prof. C. Nꢁjera
Department of Organic Chemistry
Grupo de Procesos Catalꢀticos en Sꢀntesis Orgꢁnica
University of Alicante
Ap. 99, E-03690 Alicante, Spain
Supporting information for this article is available on the WWW under
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 7
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