Organic-inorganic hybrid matrix doped with alkenyldiazenido complexes of molybdenum

Article abstract of DOI:10.1016/j.jallcom.2006.12.085

New molecular systems based on conjugated groups, trans-[FMo(NNCHCHCHCHCHCH₂CH₃)(Ph₂PCH₂CH₂PPh₂)₂][BPh₄] have been synthesized and characterized. This complex was immo

Full text of DOI:10.1016/j.jallcom.2006.12.085

Journal of Alloys and Compounds 454 (2008) 72–77
Organic–inorganic hybrid matrix doped with
alkenyldiazenido complexes of molybdenum
∗
´
´
Antonio M. Fonseca, Carlos J.R. Silva, Natercia Nunes, Isabel C. Neves
Departamento de Qu´ımica, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
Received 13 October 2006; received in revised form 12 December 2006; accepted 15 December 2006
Available online 27 December 2006
Abstract
New molecular systems based on conjugated groups, trans-[FMo(NNCHCHCHCHCHCH2CH3)(Ph2PCH2CH2PPh2)2][BPh4] have been syn-
thesized and characterized. This complex was immobilized in hybrid organic–inorganic matrix obtained by sol–gel. The hybrid matrix of these
organically modified silicates, classified as ureasil formed by poly(oxyethylene) chains [POE, (OCH2CH2)n] grafted to siloxane domains by means
of urea cross-linkages. All of the new materials were characterized by thermal analysis (DSC), surface analysis (XPS) and spectroscopic methods
(FTIR and UV/vis), to gather information on the modifications introduced by the molybdenum complex in hybrid organic–inorganic matrix, as
well as on the immobilized complex integrity. The immobilization of the metal complex in the hybrid matrix has kept their integrity, although some
distortions can be observed caused by steric effects imposed by the structure of hybrid matrix or/and by interactions between the metal complex
and some groups of the hybrid matrix.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Molybdenum complexes; Hybrid matrix; Immobilization; Host–guest
1. Introduction
and mesoporous materials such as zeolites, clays and meso-
porous structures have been used as hosts for the encapsulation
or noncovalent attachment of metal complexes [8–13]. The com-
bination of the sol–gel method with the host–guest concept has
successfully led to production of a significant multifunctional
organic–inorganic framework with tunable design and appropri-
ate characteristics for heterogeneous catalysis [7,14,15]. These
systems combine many advantages of homogeneous (high activ-
ity, high selectivity and high reproducibility) and heterogeneous
(easy separation and recovery of the catalysts from the reaction
mixture) catalysis.
Metal transition complexes have been used as templates for
the structure of host organic–inorganic matrix, inducing the
formation of interaction with localized atoms of the macro-
molecular host structure and increasing the cohesion and order
of the resulting material. On the other hand, if the complexes
keep their structure inside the hybrid matrix and simultaneously
have catalytic properties, the novel materials can also be used in
heterogeneous catalysis [9].
The development of organic–inorganic hybrid materials has
been the subject of extensive research in the past decade because
these systems are found to have the advantage in many fields
of applications that they combine both inorganic and organic
characters [1–3]. The sol–gel process has been proven to be an
excellent candidate for development of the heterogeneous mate-
rials by incorporation of organometallic compounds to generate
specific catalysts and nanostructured materials for potentially
Green Chemistry catalytic systems, novel optical and electronic
applications [4–7].
Homogeneous catalysts have uniform and well-defined reac-
tive centers, which lead to high and reproducible selectivities.
The activity and efficiency of these catalysts are generally high.
However, one of the major problems is the separation of the
catalyst from the reaction mixture. To solve this problem, the
immobilization of metal complexes acting as homogeneous cat-
alysts, in porous supports provides an ideal solution. Micro-
Several complexes of molybdenum and tungsten with differ-
ent ligands were synthesized and their electronic and structural
properties studied [10–13]. Earlier work reports the heteroge-
nization of metal transition complexes in clays materials [15],
∗
Corresponding author. Tel.: +351 253604057; fax: +351 253678983.
E-mail address: ineves@quimica.uminho.pt (I.C. Neves).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.12.085

A.M. Fonseca et al. / Journal of Alloys and Compounds 454 (2008) 72–77
73
Scheme 1.
zeolites [16] and hybrid matrix [11,17], and have shown that
the methodologies used for encapsulation/immobilization of the
metal complexes are largely determined by the supports. The
analogy between microporous solid supports and hybrid matrix
obtained by sol–gel promoted us to explore the properties of the
former as supports for the immobilization of metal complexes.
The present work aims to assess the effect of metal complex
immobilization on their physical–chemical properties and their
interaction with the supports.
The hybrid matrix used in this work, designated as U(600),
is an organic–inorganic network material, classified as an ure-
asil that combines a reticulated siliceous backbone linked
by short polyether-based segments. Urea bridges form the
link between these two components, and the polymerization
of silicate substituted terminal groups generates the inor-
ganic network. In the reaction the organic-modified alkoxide,
3-isocyanatepropyltriethoxysilane (ICPTES), and the amine-
substituted oligopoly(oxyethylene) (Jeffamine) are made to
react in a mole proportion of 2 to 1 (Scheme 1).
and trans-[MoF(NNH2)(dppe)2][BF4] were prepared by from published method
[19].
2.2. Sample preparation
2.2.1. Preparation of Mo(IV) complex
The stirred solution of the complex trans-[MoF(NNH2)(dppe)2][BF4]
(0.50 g, 0.49 mmol) in 20 mL of THF solvent, was added 2,4-heptadiene-1-al
(63 ␮L, 1.5 mmol) at room temperature. After 6 h, the resulting brown
solution was filtered and hexane (4 mL) was added to the filtrate to give
brown–green crystals, which were filtered off, washed with ether and
hexane, and then dried, in vacuum. The product was dissolved in methanol
(20 mL) and NaBPh4 was added to the stirred solution. The complex,
trans-[MoF(NNCHCHCHCHCHCH2CH3)(dppe)2][BPh4] precipitates from
the reaction solution as a green solid. This was filtered off, washed with
methanol (3 × 5 mL), ether (3 × 5 mL) and re-crystallized with CH2Cl2/Et2O
and dried in vacuum. Yield (0.36 g, 54%) Anal. found for MoFC83H78N2P4B
(calcd.): C, 73.41 (73.74); H, 5.59 (5.77); N, 2.15 (2.07). NMR (CDCl3): 1H
(300.0 MHz), δ 1.1 (t, 3H, CH3), 1.7 (m, 2H, CH2CH3), 2.6–3.0 (2 m, 8H,
PCH2CH2P), 4.1 (br, 1H, NCHCH), 5.3 (m, 4H, olefinic protons) and 7.0–7.5
1
(m, 60H, PPh2 and BPh4). 31P-{ H} (121.7 MHz), δ 93.0 (s). FTIR (ν, cm−1):
1545 [␯(C N)], 1210 [␯(C C)]. UV/vis (λmax, nm): 255, 265, 272, 320 and
510. FABMS (m/z): 1033 [M+].
This work reports, the effect of the concentration in the immo-
bilization of a diazoalkene molybdenum complex, trans-bis-[1,
2-bis(diphenylphosphine)ethane]-fluoro-(2,4-diazoheptadiene)
molybdenum of tetraphenylborate (trans-[FMo(NNCHCHC-
HCHCHCH2CH3)(Ph2PCH2CH2PPh2)2][BPh4]), into a high
molecular weight di-ureasil structure, U(600), which contains
about 8.5 oxyethylene repeating units. The macromolecular
structure is composed by oligopoly(oxyethylene) chains grafted
ontoasiliceousnetworkthroughureabridges[ NHC( O)NH ]
[18].
2.2.2. Preparation of hybrid matrix containing the complex
In the first stage of the synthesis the sol–gel materials, the hybrid matrix was
prepared by reaction of a doubly functional amine of Jeffamine with ICPTES, to
give ureapropyltriethoxysilane (ureasil—UPTES) [18]. Finally, different con-
centrations of metal complex solution (initial concentration of metal complex,
1.29 × 10−3 mol/L), were incorporated in the UPTES solution. The immobi-
lization of the metal complex is achieved simultaneously by the hydrolysis and
condensation reactions. Details of the preparation of the synthesis ureasil pre-
cursor (UPTES) with metal complex can be found in the authors’ previous work
[17]. A sample that corresponds to the pure hybrid matrix was prepared using
the same methodology, but without the addition of the metal complex.
The nomenclature for the diazoalkene molybdenum complex was denoted
[MoL][BPh4] where Mo = FMo(Ph2PCH2CH2PPh2)2 and L is a diazoalkene
ligand (NNCHCHCHCHCHCH2CH3). The ureasil samples, have been iden-
tified by designation U(600)n[MoL][BPh4] where U originates from the word
“urea”, 600 indicates the average molecular weight of diamine used, and n
represents the molecular ratio between the Jeffamine (the limiting reagent
used in the ureasil synthesis) and the metal complex. Samples with n = 200
(2.16 × 10−3 mmol of metal complex), 250 (1.75 × 10−3 mmol of metal com-
plex) and 500 (8.74 × 10−4 mmol of metal complex) were prepared. U(600)
corresponds to the pure matrix. The obtained materials were a flexible, non-rigid
and brittle homogeneous transparent film with brown–green coloration.
2. Experimental
2.1. Reagents and solvents
O,Oꢀ-Bis(2-aminopropyl)-polyethylene glycol-500 (Jeffamine ED-600®,
Fluka), the polyethylene oligopolymer with substituted amine terminal
groups and the silicon alkoxide with a substituted isocyanate group, the 3-
isocyanatepropyltriethoxysilane (Aldrich) were used in the preparation of the
matrix. Both of the reagents were dried under dynamic vacuum during 30 min
immediately before used. All the reaction synthesis of the complex was car-
ried out under nitrogen or argon atmosphere with the use of standard Schlenk
techniques unless otherwise stated. The others chemicals, tetrahydrofuran (THF,
Merck) and Et2O were dried by distilling over sodium/benzophenone, ethanol
(CH3CH2OH, Merck) and dichloromethane were dried by distilling over CaH2
2.3. Physical measurements
Thermal analyses (DSC, differential scanning calorimetry) were carried out
with a Mettler TC11 controller and a DSC20 Mettler oven equipped with a cool-
˚
and stored over 4 A molecular sieves. The complexes, trans-[Mo(N2)(dppe)2]

74
A.M. Fonseca et al. / Journal of Alloys and Compounds 454 (2008) 72–77
Scheme 2.
ing accessory under high purity argon supplied at a constant 50 mL min−1 flow
rate. Samples obtained from ureasil films were sealed within a 0.2 mL aluminum
pan inside a preparative glovebox under argon atmosphere. All samples were
subjected to a 10 ◦C min−1 heating rate and were characterized between 25 and
350 ◦C.
The NMR spectra were recorded using a Varian Unity Plus spectrometer
at 300 MHz, chemical shifts being given in ppm. Mass spectra (FAB) were
obtained using an Analytical Fison Instruments Auto Spec VG spectrome-
ter, using 3-nitrobenzyl alcohol as the matrix. Elemental chemical analysis
was performed using a Leco CHNS-932. The FTIR spectra of free complex
and ureasil samples were obtained from powdered samples on KBr pellets,
using a Bomem MB104 spectrometer in the range 4000–500 cm−1 by aver-
aging 20 scans at a maximum resolution of 4 cm−1. X-ray photoelectron
spectroscopy analyses were obtained at the C.A.C.T.I. from Vigo University
(Spain) in a VG Scientific ESCALAB 250iXL spectrometer using monochro-
matic Al K␣ radiation (1486.92 eV). Reflectance UV/vis spectra of sol–gel
materials were obtained from solid sample films and were recorded on a
Shimadzu UV/2501PC spectrophotometer at room temperature in the range
800–200 nm using a matrix as a reference. For the free complex, the UV/vis
absorption spectra were obtained from complex solution in ethanol using quartz
cells.
Fig. 1. DSC thermograms: (a) U(600), (b) n = 200, (c) n = 250 and (d) n = 500.
*
(␲–␲ transition) and diazoalkene-p␲ to the metal-d␲ charge-
transfer transitions, respectively.
3. Results and discussion
3.2. Characterization of materials doped with molybdenum
complex
3.1. Synthesis and characterization of molybdenum
complex
The preparation of metal complex based hybrid materials was
made in two steps: (i) reaction of the alkoxysilane precursor
(ICPTES) with the polyethylene oligopolymer with substituted
amine terminal groups (Jeffamine ED-600, with a + c = 2.5 and
b = 8.5) in a mole proportion of 2 to 1 in THF, to give the urea
cross-linked organic–inorganic hybrid precursor, ureapropyltri-
ethoxysilane, UPTES and (ii) addition to the latter precursor
solution, the metal complex in THF and the solvents (ethanol
and water) that would induce the hydrolysis and condensation
reactions to give the xerogels.
The complex of molybdenum, [MoL][BPh4] (Scheme 2),
was obtained by condensation of hydrazido(2-) complex,
trans-[MoF(NNH2)(dppe)2][BF4], with 2,4-heptadiene-1-al
compound.
The complex has been characterized by elemental analyses,
1H and 31P NMR, FTIR, UV/vis and FABMS spectroscopy. The
elemental analyses agree with their chemical formula, and the
1
FABMS spectra clearly show the molecular ion M+. 31P-{ H}
NMR spectroscopy shows a single resonance which demon-
strates that the complex retains the square-planar array of P
atoms about the molybdenum [20]. 1H NMR spectrum revealed
the presence of all the H of the diazoalkene ligand in similar
positions for other analogous complexes of literature [21].
The infrared spectra of molybdenum complex display mul-
tiple peaks in the range 1500–650 cm−1. These peaks are most
likely associated with the diphenylphosphine ligands, ν(PPh2)
(1484, 1435, 721 and 690 cm−1) and the BPh4 anion (1052, 744
and 720 cm−1) in these complexes [19]. On the other hand, the
bands recorded at 1545 and 1210 cm−1 are assigned to ␯(C N)
and ␯(C C) groups of diazoalkene ligand.
The final ureasil samples obtained after the immobilization of
the molybdenum complex were characterized by thermal anal-
ysis, surface analysis (XPS) and spectroscopic methods (FTIR
and UV/vis).
3.2.1. Thermal analysis
For understanding the effect of molybdenum complex con-
centration on thermal properties of hybrid matrix, studies using
(DSC) are performed. The DSC thermograms of all samples
(Fig. 1) clearly show that the processes are totally amorphous in
the studied temperature range.
The DSC profiles obtained for the pure matrix and ure-
asil samples with different concentrations of metal complex
shows that these materials have also a high thermal stability,
In the electronic spectroscopic data of molybdenum complex,
the major feature displayed by these complexes [22] is multiple
bands in the range 250–300 nm and a shoulder peak in 510 nm.
These absorptions are assigned to the diphenylphosphine ligands

A.M. Fonseca et al. / Journal of Alloys and Compounds 454 (2008) 72–77
75
Table 1
Areas under the XPS bands in the O 1s, C 1s, Si 2p, N 1s and Mo 3d for the
UREASIL samples
Sample
XPS (atomic %)
O 1s C 1s Si 2p N 1s F 1s B 1s Mo 3d
U(600)
16.9 72.9
6.9
3.3
5.9
5.7
5.5
–
–
–
U(600)200[MoL][BPh4] 16.2 65.9 10.5
U(600)250[MoL][BPh4] 16.6 65.6 10.7
U(600)500[MoL][BPh4] 17.1 65.4 11.0
0.4
0.4
0.3
0.3
0.3
0.2
0.8
0.7
0.5
confirming the expected properties assigned on similar mate-
rials based on this hybrid organic–inorganic matrix [17]. For
U(600), an endothermic band between 40 and 120 ◦C is observed
which could be assigned to the evaporation of solvent molecules
trapped inside the hybrid matrix. This kind of thermal process
is less intense on the samples with immobilized metal complex.
This difference of behaviors was observed on previous studies
[17,23], and suggests that the metal complex contribute to the
modification of the matrix properties resulting, in particular, on
a more dense structure were less free space is available for host-
ing small molecules. This apparent increase of matrix cohesion
may result from localized chemical interactions between atoms
host matrix and metal complex.
The broad exothermic peak observed on the hybrid matrix,
which is localized in the 150–200 ◦C region could be a result of
an oxidative process involving the polyether group of the ure-
asilicate network. The presence of the metal complex apparently
inhibits this oxidative process, giving to the enhancement of the
material chemical stability.
Fig. 2. FTIR spectra in the range 2000–600 cm−1 of: (a) U(600), (b) n = 200,
(c) n = 250, (d) n = 500 and (e) [MoL][BPh4].
bonded to silicon and to carbon atoms, as well as from the dou-
ble bonded oxygen of the [ NHC( O)NH ] group (Scheme 1).
The N 1s high resolution spectrum shows a symmetrical band
at 400.1 eV which corresponds to the nitrogen atoms from the
[ NHC( O)NH ] group and also a symmetrical band centred
at 102.5 eV is observed in the Si 2p high resolution spectrum
due to the [ (CH2)3 Si(O )3] groups (Scheme 1).
The XPS analysis detected the presence of the molybdenum
in all ureasil samples. After the immobilization molybdenum
complexes no significant changes were observed in the C 1s, O
1s and N 1s profile spectra. Low Mo contents at surface levels
observed indicate that the outermost surface is not enriched in
Mo. The metal complex is immobilized into the ureasil structure.
The binding energy value of Mo 3d5/2 was found to be equal to
230.6 eV, which confirms that the metal is in oxidation state four,
in agreement with the molybdenum coordination sphere of the
complex [15].
At temperature higher than 250 ◦C a drift on the thermo-
gram lines towards an exothermic direction may indicate the
approach of degradation of the material, and this process may
be assigned to the immobilized metal complex as the exten-
sion of this process increases in intensity with increased metal
complex concentration in the sample.
This behavior suggests that the presence of metal complex
contribute to matrix stabilization and this is probably due to the
increase of localized chemical interactions between atoms from
the [MoL][BPh4] and those from the matrix structure.
3.2.3. Infra-red spectroscopy
The FTIR spectra of ureasil samples with different concen-
trations of metal complex and free complex in the spectral region
of 2000–600 cm−1 are presented in Fig. 2.
3.2.2. Surface analysis
The XPS technique can provide information about the pres-
ence and distribution of molybdenum complexes in matrix
as well as on the oxidation state of the metal. All samples
revealed the presence of oxygen, carbon, silicon and nitrogen
in their XPS resolution spectra. In low resolution XPS spec-
tra of the U(600)n[MoL][BPh4] samples, the presence of the
molybdenum, boron and fluorine, from the metal complexes
were detected. The surface atomic contents obtained by the area
of the relevant bands in XPS spectra are presented in Table 1.
The high resolution XPS spectra of U(600) sample shows an
intense band in the C 1s region at 285.3 eV, which corresponds
to aliphatic carbons, carbon atoms covalently bonded by a single
bond to electronegative atoms and carbon atom from the group
[ NHC( O)NH ] of the UPTES matrix. A symmetrical band
at 532.7 eV in the O 1s region was due to the oxygen single
FTIRspectraofallsamplesaredominatedbythetypicalsetof
intense bands assigned to the vibration of hybrid matrix U(600).
TheIRspectraofhostedmatrixhavebeenpreviouscharacterized
[11]. Shift or broadening of these host structure-sensitive vibra-
tions is observed upon inclusion of the complex, which provides
evidence, at the molecular level, that the hybrid matrix changed
uponimmobilizationofthemolybdenumcomplex. Forhighcon-
centrations of complex in hybrid matrix an increase of the broad
band attributed to [ NHC( O)NH ] group at 1160 cm−1 was
observed. The bands due to the molybdenum complex can only
be observed in the regions where the hybrid matrix does not
absorb, i.e. from 1050 to 800–600 cm−1. The IR bands of immo-
bilized [MoL][BPh4] in the range 1050–600 cm−1, which are
assigned to the diphenylphosphine ligands and [BPh4]− anion

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A.M. Fonseca et al. / Journal of Alloys and Compounds 454 (2008) 72–77
denum complex occurs in hybrid organic–inorganic matrix
obtained by sol–gel method. The highly transparent, flexible
and amorphous films sol–gel doped with different concentra-
tion of molybdenum complex in U(600) di-ureasil host structure
instead of poly(oxyethylene) allowed us to produce materials
with improved mechanical proprieties and thermal characteris-
tics. The analysis data suggest that the matrix ureasil structure
exhibits host–guest interaction between the molybdenum com-
plex and the matrix. These results confirm the interaction with
macromolecular host structure and contributing to increase the
cohesion and order of the resulting material.
The strategy used for the immobilization of molybdenum
complexes can be extended to other metals with known cat-
alytic properties. These novel materials may thus be used as
heterogeneous catalysts, indicating that it is possible to extend
the potential applications of sol–gel materials in the field of
catalysis.
Fig. 3. UV/vis spectra in the range 230–400 nm of: (a) U(600), (b) n = 200, (c)
n = 250, (d) n = 500 and (e) [MoL][BPh4].
occuratfrequencies, whichareshiftedrelativetothoseofthefree
complex. These observations confirm the presence of the molyb-
denumcomplexandsuggestthatthestructureoftheimmobilized
complex is distorted when compared with that of the structure
of free complex. This is a consequence of host–guest interac-
tions with the hybrid matrix, eventually to the matrix carbonyl
oxygen atoms of the urea bridges [24]. In our previous work,
with tungsten complex immobilized in the same hybrid matrix,
the same behavior was observed; the complexes are distorted
as a consequence of physical constraints imposed by the matrix
[17]. Despite these limitations, infrared spectroscopy provide
compelling evidence that the molybdenum complexes have been
immobilized within hybrid organic–inorganic matrix, but failed
to yield molecular information on the immobilized molybdenum
species.
Acknowledgments
´
This work was supported by the Centro de Quımica (Uni-
˜
ˆ
versity of Minho, Portugal) and by Fundac¸ao para a Ciencia e
Tecnologia (FCT-Portugal), under programme POCTI-SFA-3-
686. Dr. Carmen Serra Rodrigues (C.A.C.T.I., Vigo University,
Spain) is gratefully acknowledged for performing and analyzing
the XPS measurements.
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