Immobilization of Mo(IV) complex in hybrid matrix obtained via sol-gel technique

Article abstract of DOI:10.1016/S0925-8388(03)00348-7

A molybdenum(IV) complex, trans-bis-[1,2-bis(diphenylphosphino)ethane]-fluoro-(diazopropano)-molybdenum tetraphenylborate, [MoF(DIAZO)(dppe)₂][BPh₄], was prepared and immobilized in a hybrid matrix synthesized by the sol-gel process. The host matrix, designated as U(500), is an organic-inorganic network material, classed as ureasil, that combines a reticulated siliceous backbone linked by short polyether-based segments. Urea bridges make the link between these two components, and the polymerization of silicate substituted terminal groups generates the inorganic network. The free Mo(IV) complex and all new materials were characterized by spectroscopic techniques (FT-IR and UV-Vis) and thermal analysis (DSC). The ionic conductivity of the resulting material was also studied. The results indicate that immobilized Mo(IV) complex has kept its solid-state structure, although there is evidence of inter-molecular interactions between the Mo(IV) complex and some groups/atoms of the hybrid host matrix.

Full text of DOI:10.1016/S0925-8388(03)00348-7

Journal of Alloys and Compounds 360 (2003) 272–278
L
www.elsevier.com/locate/jallcom
Immobilization of Mo(IV) complex in hybrid matrix obtained
via sol–gel technique
a
a
C. Marquesa, A.M. Sousaa, C. Freireb, I.C. Nevesa , , A.M. Fonseca , C.J.R. Silva
*
a
´
ˆ
IBQF/Departamento de Quımica, Escola de Ciencias, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
b
´
ˆ
CEQUP/Departamento de Quımica, Faculdade de Ciencias, Universidade do Porto, 4169-007 Porto, Portugal
Received 25 March 2002; received in revised form 12 December 2002; accepted 27 February 2003
Abstract
molybdenum(IV) complex, trans-bis-[1,2-bis(diphenylphosphino)ethane]-fluoro-(diazopropano)-molybdenum tetraphenylborate,
A
[MoF(DIAZO)(dppe)2][BPh4], was prepared and immobilized in a hybrid matrix synthesized by the sol–gel process. The host matrix,
designated as U(500), is an organic–inorganic network material, classed as ureasil, that combines a reticulated siliceous backbone linked
by short polyether-based segments. Urea bridges make the link between these two components, and the polymerization of silicate
substituted terminal groups generates the inorganic network. The free Mo(IV) complex and all new materials were characterized by
spectroscopic techniques (FT-IR and UV–Vis) and thermal analysis (DSC). The ionic conductivity of the resulting material was also
studied. The results indicate that immobilized Mo(IV) complex has kept its solid-state structure, although there is evidence of
inter-molecular interactions between the Mo(IV) complex and some groups/atoms of the hybrid host matrix.
2003 Elsevier B.V. All rights reserved.
Keywords: Molybdenum(IV) complex; Complex immobilization; Sol–gel; Conductivity; FTIR
1. Introduction
after which polycondensation reactions took place. These
latter reactions can occur by two possible mechanisms,
either by a condensation reaction with the formation of an
oxygen bridge (oxolation),
The preparation of solid-state materials via sol–gel
processing is an important and successful method for
preparing new materials with enhanced properties, such as
non-linear optics properties, high ionic conductivities or
containing dispersed nano-particles, for application in
several fields such as lens coating and electronics [1–3].
The chemistry of the sol–gel method was initially based
in metal alkoxides, M(OR)4, where M stands for a metal
atom (Si, Ti, etc.) and OR for an alkoxy group
–OCnH2n11. The reactivity of the metal alkoxides, the
properties of the solvent used, the catalysts present and the
properties of the reactive media (such as temperature and
pH) have a strong influence on the kinetics of the process
and control the structure and morphology of the resulting
material.
(OR)3M–OH 1 XO–M(OR)3 → (OR)3M–O–M(OR)3 1
XOH (X 5 H or alkyl group)
(2)
by the formation of a hydroxo bridge (olation), or as an
addition reaction
(OR)3M–OH 1 HO–M(OR)3 → (OR)3M–(OH)2 –M(OR)3
(3)
The propagation of the polymerization process promotes
the size increase of the polymeric structure and the extent
of branch formation, giving this structure a fractal dimen-
sion. The final product is a gel where the polymeric
particles can trap solvents, molecules, colloidal solutions
or other particles.
In earlier work, the sol–gel process involved the hy-
drolysis of the alkoxide group [4]
Sol–gel syntheses are attractive because they offer a
lower processing temperature with high homogeneity and
purity of the resulting materials and the possibility of
various forming processes. The materials can exhibit
important physical properties, in particular: (i) high surface
(OR)3M–OR 1 H2O → (OR)3M–OH 1 ROH
(1)
*
Corresponding author. Tel.: 1351-253-604-386; fax: 1351-253-678-
983.
E-mail address: ineves@quimica.uminho.pt (I.C. Neves).
0925-8388/03/$ – see front matter
doi:10.1016/S0925-8388(03)00348-7
2003 Elsevier B.V. All rights reserved.

C. Marques et al. / Journal of Alloys and Compounds 360 (2003) 272–278
273
area; (ii) accessibility of molecular reactants to the nano-
scale domains throughout the aerogel via rapid mass
transport through the continuous volume of mesopores;
(iii) stabilization of nanoscale matter to coalescence or
agglomeration; and (iv) low densities, together with a high
thermal and chemical stability [5].
More recently, sol–gel methods have been used as a
new route to prepare organic–inorganic hybrid materials.
One of the steps in the preparation of these materials
includes the formation of a urea bridge between the
diamine and the substituted group of the organic-modified
alkoxide; an example of the preparation of one type of
organic–inorganic hybrid material is presented in Scheme
1. In the reaction, the organic-modified alkoxide (repre-
sented in the scheme as ICPTES) and the amine-substi-
tuted oligopolyoxyethylene (Jeffamine, ED-600 ) are
made to react in a mole proportion of 2 to 1.
The hybrid materials have also been used in combina-
tion with transition metal complexes. Metal complexes
have been used as templates for the structure of host
inorganic–organic matrix, inducing the formation of inter-
action with localized atoms of the macromolecular host
structure and increasing the cohesion and order of the
resulting material. On the other hand, if the complexes
keep their structure inside the host matrix and simul-
taneously have catalytic properties, the novel materials can
also be used in heterogeneous catalysis. Hence, sol–gel
techniques can be taken as a new route for the heterogeni-
zation of molecular catalysts.
Scheme
2. Molecular
structure
of
the
complex
trans-
[MoF(NN=CHCH2CH3)(dppe)2][BPh4].
work through urea bridges (–NHC(=O)NH–). These ma-
terials have been classed as ureasilicates [6].
The new material obtained, an amorphous and transpar-
ent monolith, was characterized by infrared (DRIFT) and
UV–visible (UV–Vis) spectroscopy, thermal analysis
(DSC) and total ionic conductivity. The data indicate that
the Mo(IV) complex trans-bis-[1,2-bis(diphenylphos-
phino)ethane]-fluoro-(diazopropano)-molybdenum
tetra-
phenylborate, [MoF(NN=CHCH2CH3)(dppe)2][BPh4], has
been immobilized inside the matrix, although there is
evidence for inter-molecular interactions between the
Mo(IV) complex and some groups/atoms of the hybrid
host matrix.
This paper describes the immobilization of a diazoal-
kane complex of molybdenum(IV), trans-bis-[1,2-bis-
(diphenylphosphino)ethane]-fluoro-(diazopropano)-molyb-
denum tetraphenylborate (Scheme 2), in a hybrid inor-
ganic–organic matrix material prepared by sol–gel tech-
nique. The macromolecular structure is composed of
oligopolyoxyethylene chains grafted onto a siliceous net-
2. Experimental
2.1. Reagents and solvents
A polyethylene oligopolymer with substituted amine
terminal groups, available commercially under the trade
name of Jeffamine ED-600 (Fluka) and a silicon alkoxide
Scheme 1. Synthesis of the ureasil precursor (ureapropyltriethoxysilane, UPTES).

274
C. Marques et al. / Journal of Alloys and Compounds 360 (2003) 272–278
with a substituted isocyanate group, the 3-isocyanatep-
ropyltriethoxysilane (ICPTES, Aldrich), were used in the
preparation of the matrix. Both reagents were dried under
dynamic vacuum for 6 h at room temperature. Ethanol and
tetrahydrofuran (both from Merck) were used as received
and stored over molecular sieves. High purity water (k.18
MV) obtained by a purification system (Barnstead E-Pure)
was used in all experiments.
ml, 3.50 mmol) was dissolved in tetrahydrofuran (THF, 1
ml) previously dried under dynamic vacuum for 6 h at
room temperature. The solution was stirred in a sealed
glass flask for 30 min. The completion of the reaction was
monitored by infrared spectroscopy using the extinction of
the strong band at 2277 cm21, assigned to the stretching
vibration of the R3 –Si–(CH2)3 –NCO group [11].
Mo(IV)
complex,
trans-
All experiments in the preparation of Mo(IV) complex
were carried out under an inert atmosphere of argon using
Schlenk techniques. The solvents from Merck were freshly
distilled from appropriate drying agents under dinitrogen
before use.
[MoF(NN=CHCH2CH3)(dppe)2][BPh4] (0.60 mg), was
dissolved in a mixture of 2.4 ml of ethanol and 2.0 ml of
THF by using an ultrasound bath for 10 min. After
complete dissolution, 360 ml of water was added.
The latter solution was added to the UPTES solution and
the mixture was stirred for 10 min, and then poured in two
Teflon moulds, covered with Parafilm and left in a fume
cupboard for 24 h. When the gelification had begun, which
took $2 days, the mould was transferred to an oven
equipped with a gas re-circulation system, through a
column containing dried type 4A (Fluka) molecular sieves,
for a period of 7 days at 70 8C. The obtained material was
a yellow flexible and brittle homogeneous transparent film.
The ureasil samples, which corresponded to the pure
matrix, are denominated as U(500). Using the same
nomenclature the prepared material containing the im-
2.2. Preparation of Mo(IV) complex
The
[MoF(NN=CHCH2CH3)(dppe)2][BPh4] was prepared by
dissolving the precursor complex trans-
complex
trans-
[MoF(NNH2)(dppe)2][BF4], previously prepared by a li-
terature method (0.45 g, 0.44 mmol) [7] in dichlorome-
thane (10 ml) at room temperature. An excess of prop-
ionaldehyde (100 ml, 1.5 mmol) was added and the
reaction mixture stirred overnight. During this period, the
solution color changed from red to brown-green. The
solution was filtered and ethyl ether (10 ml) was added to
the filtrate and a green solid was obtained. The product
was dissolved in methanol (20 ml), and an excess of
NaBPh4 (Merck) was added to induce the precipitation of
the product trans-[MoF(NN=CHCH2CH3)(dppe)2][BPh4].
The complex was filtered, washed with methanol (235
ml), hexane (23|5 ml) and dried under vacuum for
several hours.
mobilized
[MoF(DIAZO)(dppe)2][BPh4],
Mo(IV)
complex,
denominated as
is
U(500)n[MoF(DIAZO)(dppe)2][BPh4] where n53434
represents the molecular ratio between the Jeffamine (the
limiting reagent used in the ureasil synthesis) and the
Mo(IV) complex.
2.4. Characterization
The total ionic conductivity of the solid samples was
determined by an impedance spectroscopy technique and
was carried out under a dry argon atmosphere, within
high-integrity gloveboxes containing argon. The measure-
ments were carried out using a Frequency Response
Analyser (Solartron 1250) and an Electrochemical Inter-
face (Solatron 1286) controlled by a PC computer. The
conductivity data were extracted from complex plane
impedance plots analysis using an suitable program. Impe-
dance measurements were made using a sine wave func-
tion, in a frequency range between 65 kHz and 0.1 Hz, at
open circuit potential.
trans-[MoF(NN=CHCH2CH3)(dppe)2][BPh4]:
yield
(73%) found (calc. for MoFC79H76N2P4B): C, 72.6 (72.8);
1
H, 5.6 (5.8); N, 2.1 (2.2). NMR (CDCl3): H NMR (300.0
MHz d, ppm); d50.5 (t, 3H, CH3), 1.2 (m, 2H, CH2CH3),
]
]
2.6–2.8 (2 m, 8H, PCH2CH P), 5.1 (t, 1H, NCHCH2) and
] ]2
]
6.8–7.6 (m, 60H, PPh2 and BPh4). 31P-h1Hj (121.7 MHz),
d 97.5 (s). FTIR (n/cm21): 1523 [n(C=N)]. This band is
characteristic of diazoalkane complexes confirming that the
metal have the oxidation state four [8–10].
2.3. Preparation of hybrid matrix containing the Mo(IV)
complex
The cell and the procedure used are described in the
literature [12]. A thin film of the prepared material (|230
mm) was placed between two gold electrodes discs (f513
mm and 100 mm thickness) in a home-made cell support
and heated for 6 h at a constant temperature of 60 8C. The
cell was dried overnight and the conductivity measure-
ments were performed at between 25 and 100 8C, in
intervals of 15 8C.
The matrix was prepared by reaction of a doubly
functional amine, O,O9-bis(2-aminopropyl)-polyethylene
glycol-500, (Jeffamine ED-600 ), with 3-isocyanatep-
ropyltriethoxysilane (ICPTES), to give ureapropyltriethox-
ysilane (UPTES) (Scheme 1) [6]. The synthesis was
performed in a fume cupboard, and was started by drying
the two reagents under dynamic vacuum for 6 h at room
temperature. A stoichiometric proportion of 1 mol of
Jeffamine (1.00 ml, 1.75 mmol) to 2 mol of ICPTES (0.87
The
specific
conductivity
of
[MoF(DIAZO)(dppe)2][BPh4] complex (0.60 mg) in a
mixture of 2.4 ml of ethanol, 2.0 ml of THF and 360 ml of

C. Marques et al. / Journal of Alloys and Compounds 360 (2003) 272–278
275
water was measured using a Philips PW 9527 Digital
Conductivity Meter.
Disk sections were removed from ureasil films and
sealed within 20-ml aluminium differential scanning
calorimeter (DSC) pans inside a preparative glovebox. The
purge gas used in throughout the analysis was high purity
argon supplied at a 20 ml min21 flow rate. Thermal
analysis was carried out with a Mettler TC11 controller
and a DSC20 Mettler oven equipped with a cooling
accessory. All samples were subjected to a 5 8C min21
heating rate and were characterized between 25 and
350 8C.
Room temperature FTIR spectra of free complex and
sol–gel materials were obtained from powered samples
without dilution in a Bomem MB104 spectrometer, using a
reflectance (DRIFT) cell. The spectra were collected over
the range 4000–400 cm21, by averaging 20 scans at a
maximum resolution of 8 cm21. The reflectance spectra
were calculated as the ratio (R) of sample reflectance to
that of background and expressed as f(R) 5 (1 2 R)2 /2R,
the Kubelka-Munk function.
Reflectance UV–Vis spectra of free complex and sol–
gel materials were obtained from powered samples and
Fig. 1. Temperature dependence of ionic conductivity of U(500) and
U(500)n[MoF(DIAZO)(dppe)2][BPh4].
were recorded on
a Shimadzu UV/3101PC spec-
trophotometer at room temperature in the range 1500–200
nm using MgO as reference.
individual mobility and charge. However, there are several
mechanisms that can be responsible for the decrease of
conductivity. Ion–ion interactions will produce the forma-
tion of ionic pairs, which contribute to reduce the con-
centration of free mobile charges, although the electrostatic
nature of these interactions could result in the formation of
transient form species. Such interactions will promote the
reduction of the ionic mobility, as the free ions will be
subject to the electrical field of other ionic species.
Another mechanism that can contribute to the decrease of
conductivity is ion–molecule interactions, which can in-
volve, in particular, hydrogen cations and BPh4 anions.
These interactions will induce the formation of chemical
bonds or dipole–ion interactions, which produce the same
effects on concentration of the free ionic species, as that
referred to for ion–ion interactions.
3. Results and discussion
3.1. Ionic conductivity
The
ionic
conductivity
of
the
[MoF(DIAZO)(dppe)2][BPh4] complex in a CH3CH2OH/
THF/H2O solution is 0.38331026 S cm21 (k50.91
cm21), which indicates that the free complex in solution
behaves as a 1:1 electrolyte [13]. This result suggests that
Mo(IV) complex added to the ureasils solution is largely
dissociated in the Mo(IV) complex cation and BPh4 anion.
Fig. 1 depicts the logarithm of the total ionic con-
ductivity as a function of the inverse of absolute tempera-
ture, for the matrix, U(500), and the prepared material
containing
the
Mo(IV)
complex,
In the pure matrix, U(500), the ionic conductivity is
assigned to the most efficient charge carrier present in the
material, the free protons. As in the metal complex based
material, a significant decrease in the total conductivity is
observed, it can be proposed that strong chemical inter-
action between the charge-carriers (free protons) with
atoms or groups present either in polymer chain or in the
Mo(IV) complex, may be responsible for the observed
behavior. Interaction between free protons and NN atoms
of the DIAZO group of Mo(IV) complex and formation of
ionic pairs between free protons and BPh24 ions are two
different factors which may contribute to the decrease of
the conductivity of the matrix by incorporation of Mo(IV)
BPh4 salt.
U(500)n[MoF(DIAZO)(dppe)2][BPh4]. In both materials
the variation of the logarithm of conductivity with 1/T
shows a similar curved shape described as Vogel-Tamman-
Fulcher (VTF) behavior, which is typical of amorphous
ion-conductive polymeric materials [14–16]. This behavior
was previously observed in sol–gel materials with similar
matrix [6], containing highly soluble salts.
As observed, the pure matrix shows a conductivity
which is two orders of magnitude greater than that
exhibited by the films of the material containing the
Mo(IV) complex.
The ionic conductivity is the sum of all mobile charged
species (charge-carriers) contributions: their concentration,

276
C. Marques et al. / Journal of Alloys and Compounds 360 (2003) 272–278
is observed, which can be assigned to the reorganization of
partial fused oxyethylene chains.
DSC curves indicate that both materials are clearly
largely amorphous and the incorporation of the complex in
the hybrid host does not modify the morphology of the
matrix. Moreover, the presence of the complex
[MoF(DIAZO)(dppe)2][BPh4] in the matrix causes a slight
reduction in the decomposition temperature of the material:
the pure matrix is thermally stable up to 350 8C, whereas
in the presence of the Mo(IV) the onset temperature is
|325 8C.
3.3. FTIR spectroscopy
Fig.
2. DSC
traces
of
[MoF(DIAZO)(dppe)2][BPh4],
The
infrared
spectra
of
free
complex,
U(500)n[MoF(DIAZO)(dppe)2][BPh4] and U(500).
U(500)n[MoF(DIAZO)(dppe)2][BPh4] and the hybrid ma-
trix U(500) in the spectral region 2000 to 500 cm21 are
shown in Fig. 3.
3.2. Thermal analysis
The spectrum of the free complex is dominated by the
bands characteristic of diphenylphosphine ligands (1480,
1435, 735 and 695 cm21) and the BPh4 anion (1412, 744
and 720 cm21) [17]. Other important bands are observed at
1523 cm21, which is assigned to n(C=N), and the weak
band at 1190 cm21 is assigned to n(N–N) of the DIAZO
group.
The
thermograms
of
free
complex,
U(500)n[MoF(DIAZO)(dppe)2][BPh4] material and hybrid
matrix U(500) are presented in Fig. 2. The most significant
feature observed in the DSC curve of the free complex is
the wide exothermic peak around 380 8C, attributed to the
decomposition of the complex.
DSC curves of the pure matrix, U(500) (Tonset 5347 8C),
and that of metal complex based material (Tonset 5310 8C)
are very similar. In both materials a broad endothermic
peak is observed at $90 8C, which is often assigned to the
evaporation of water and ethanol molecules [6]. At higher
temperatures, between 180 and 200 8C, an exothermic peak
The spectra of the sol–gel materials are dominated by
bands due to the hybrid matrix U(500): the strongest band
at 1160 cm21 is attributed to N–CO–N stretching and in
the spectral region between 1050 and 800 cm21 are
observed bands corresponding to the vibration modes
associated with the matrix. Others bands are summarized
Fig. 3. Infrared spectra of [MoF(DIAZO)(dppe)2][BPh4], U(500)n[MoF(DIAZO)(dppe)2][BPh4] and U(500).

C. Marques et al. / Journal of Alloys and Compounds 360 (2003) 272–278
277
Table 1
Selected IR band locations (cm21) and assignments for the matrix [U(500)]
Wavenumber (cm21
)
Designation
3000–3500 (m, br)
2900–2940 (S, br)
2850–2870 (S, br)
1640–1670 (m, br)
1560 (m)
R–CO–NH– stretching vibrations
CH asymmetric stretching vibrations
CH symmetric stretching vibrations
C=O asymmetric stretching vibration
N–H deformation and symmetric stretching of C=O
Si–CH2 stretching vibration
1500 (s)
1450 (m)
CH deformation
1350 (m)
CH2 groups of the (CH2CH2O) units of the diamine
Si–OCH2CH3 stretching vibration
CH2 rocking to C–C and C–O stretching vibrations and asymmetric stretching of Si–O–Si
Asymmetric stretching of Si–OH
1260 (m, br)
1050 (s, br)
960 (s)
750–800 (s)
Symmetric stretching of Si–O–Si
br, broad; m, medium; s, small; S, strong.
in Table 1 [18–21]. As no change in the matrix bands was
observed after immobilization of the complex, we suppose
that the complex has promoted no significant variation on
the structure of the matrix.
ligand charge transfer transitions, dp(MoIV) →p*(LD)
(LD5DIAZO ligand), and the broad band at lower energy
to d–d transitions.
Electronic
data
for
hybrid
matrix
U(500),
New low intensity bands due to the Mo(IV) complex are
observed superimposed upon those of the matrix after
immobilization of the complex. When compared to those
of the free complex, there is a shift of the phosphine bands
U(500)n[MoF(DIAZO)(dppe)2][BPh4] and the free com-
plex are summarized in Table 2. No electronic bands due
to
the
complex
could
be
detected
in
U(500)n[MoF(DIAZO)(dppe)2][BPh4], probably due to
the low concentration of the complex in the matrix.
at 1435 and 1480 cm21 upwards to 1448 and 1498 cm21
,
respectively, and the band at 1523 cm21 characteristic of
C=N stretching vibration, shifts upwards to 1531 cm21
.
The presence of these bands indicates that metal complex
has been immobilized in the hybrid host and primarily has
kept its integrity.
The frequency shift observed in the bands of the
complex upon immobilization can be attributed to: (i) the
presence of a small distortion on the complex; or (ii)
interactions between the hybrid matrix and the ether
oxygens of the polyethylene chain segments of the hybrid
matrix and the Mo(IV) metal ion; and/or (iii) the protona-
tion of NN atoms of the DIAZO group.
4. Conclusion
This paper explores a new approach to the heterogeniza-
tion of a Mo(IV) complex in hybrid organic–inorganic
materials, classed as ureasils. The complex has been
immobilized successfully by the sol–gel technique and the
resulting material was obtained as amorphous transparent
thermally stable monoliths. Combination of all techniques
suggests that the immobilization process does not affect
morphology and structure of the matrix. However, there is
some evidence for host–guest interaction between the
Mo(IV) complex and the matrix, which only affects the
geometry of the complex to a small extent.
3.4. UV–Vis spectroscopy
Room
temperature
electronic
spectrum
of
[MoF(DIAZO)(dppe)2][BPh4] exhibits two intense, high
energy bands at lmax 5380 and 470 nm and a less intense
broad band at lmax 5650–700 nm. The spectrum is analo-
gous to those observed for Mo(IV) complexes with similar
coordination sphere as described in the literature [22,23].
The two bands of higher energy are assigned to metal–
Acknowledgements
The authors thank M. Silva and S. Cerqueira from the
Departamento de Quımica da Universidade do Minho for
their help in conductivity studies.
´
Table 2
Electronic spectral data for the samples
Samples
lmax (nm)
U(500)
250a
250a
300a
300a
U(500)n[MoF(DIAZO)(dppe)2][BPh4]
[MoF(DIAZO)(dppe)2][BPh4]
380a
470a
670b
a dp –p* electronic transitions.
b d–d electronic transitions.

278
C. Marques et al. / Journal of Alloys and Compounds 360 (2003) 272–278
[12] M. Smith, C. Silva, Port. Electrochim. Acta 9 (1991) 225.
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