Bis(3,5-dimethylpyrazol-1-yl)acetate bound to titania and complexed to molybdenum dioxido as a bidentate N,N0-ligand. Direct comparison with a bipyridyl analog in a photocatalytic arylalkane oxidation by O2

Article abstract of DOI:10.1007/s11243-012-9631-2

Bis(3,5-dimethylpyrazol-1-yl)acetate, well known as a tridentate N,N,O ligand, was bound to MoO2Cl2 in a bidentate N,N manner by anchoring covalently, at first, the ligand onto a titanium oxide surface, followed by its complexation with the molybdenum dioxo entity. It was fully characterized by ¹³CPMAS NMR and FT-IR spectroscopy. The tethered complex was directly compared with an analogous bipyridyl species in the catalytic ethylbenzene oxidation byO2 at room temperature, atmospheric pressure, and under visible light. The bis(pyrazol-1-yl) catalyst exhibited a significant initial higher oxygen atom transfer (OAT) capability, well in accord with its electron donating property. It was, unfortunately, hampered by a premature leaching phenomenon, totally absent in the comparable bipyridyl system. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11243-012-9631-2

Transition Met Chem (2012) 37:629–637
DOI 10.1007/s11243-012-9631-2
Bis(3,5-dimethylpyrazol-1-yl)acetate bound to titania
0
and complexed to molybdenum dioxido as a bidentate N,N -ligand.
Direct comparison with a bipyridyl analog in a photocatalytic
arylalkane oxidation by O2
Nelson J. Castellanos
Edgar A. P a´ ez-Mozo
Henri Arzoumanian
•
Fernando Mart ´ı nez
Fabio Ziarelli
•
•
•
Received: 14 June 2012 / Accepted: 29 June 2012 / Published online: 20 July 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Bis(3,5-dimethylpyrazol-1-yl)acetate, well known
as a tridentate N,N,O ligand, was bound to MoO Cl in a
Introduction
2
2
bidentate N,N manner by anchoring covalently, at first, the
ligand onto a titanium oxide surface, followed by its com-
plexation with the molybdenum dioxo entity. It was fully
Among the innumerable bidentate chelating ligands form-
ing complexes with transition metals, the planar and fairly
0
rigid 2,2 -bipyridine is certainly one of the most widely
1
3
characterized by CPMAS NMR and FT-IR spectroscopy.
The tethered complex was directly compared with an anal-
ogous bipyridyl species in the catalytic ethylbenzene oxi-
used. It has been studied with either low or high oxidation
state metals [1–3]. We have, as many others, extensively
used this ligand with complexes of molybdenum(VI) in
catalytic oxidation reactions [4–6]. Closely related, and
also frequently used, are (poly)pyrazolyl ligands. Since first
reported by Trofimenko, over 40 years ago [7], either the
(poly)pyrazolyl borates or the isoelectronic and isosteric
methane analog have been studied, mainly with low oxi-
dation state metals, for their distinct coordination behavior
due to their flexibility and their different catalytic activity
dation by O at room temperature, atmospheric pressure, and
2
under visible light. The bis(pyrazol-1-yl) catalyst exhibited a
significant initial higher oxygen atom transfer (OAT) capa-
bility, well in accord with its electron donating property. It
was, unfortunately, hampered by a premature leaching
phenomenon, totally absent in the comparable bipyridyl
system.
[
8]. More recently, high oxidation state transition metals
such as molybdenum(VI) dioxo complexes bearing these
ligands have also been synthesized [8] and their catalytic
activity measured in olefin epoxidation with tert-butyl
hydroperoxide (TBHP) as oxidant [9]. When compared
with other families of ligands such as diimine or bipyri-
dine, its O-transfer ability was found to be in the middle
range.
N. J. Castellanos (&)
Facultad de Qu ´ı mica Ambiental, Grupo de Investigaciones
Ambientales para el Desarrollo Sostenible-GIADS,
Universidad Santo Tomas, Campus Universitario Floridablanca,
Santander, Colombia
e-mail: njc.castellanos@gmail.com;
nelson.castellanos@mail.ustabuca.edu.co
This O-transfer process is also possible with O as the
2
N. J. Castellanos ꢀ F. Mart ´ı nez ꢀ E. A. P a´ ez-Mozo
Escuela de Qu ´ı mica, Centro de Investigaci o´ n en Cat a´ lisis-
CICAT, Universidad Industrial de Santander, Km 2 v ´ı a El
Refugio, Piedecuesta, Santander, Colombia
direct oxygen atom donor [10]; thus, we have reported,
over the years, several molybdenum(VI) dioxo complexes,
bearing mostly bipyridyl ligands and studied their catalytic
activity in oxidation reactions with O [11–19]. Recently,
2
N. J. Castellanos ꢀ H. Arzoumanian (&)
we significantly optimized their activity by anchoring them
Chirosciences UMR CNRS 7313 Institut des Sciences
Mol e´ culaires de Marseille (iSm2), Aix-Marseille Universit e´ ,
Campus St J e´ r oˆ me, 13397 Marseille, France
e-mail: henri.arzoumanian@univ-cezanne.fr
covalently on the semiconductor solid matrix TiO and
2
established that the mechanism of the catalytic O-transfer
process is certainly different with a hydroperoxide or with
dioxygen as the oxidant [20–22]. Likewise, the influence of
a ligand in the coordination sphere can be expected to vary
in their different O-transfer processes. When comparing
F. Ziarelli
FR1739 CNRS, Aix-Marseille Universit e´ ,
Campus St J e´ r oˆ me, 13397 Marseille, France
123

6
30
Transition Met Chem (2012) 37:629–637
bipyridyl and bis-pyrazolyl ligands, many features must be
taken into account: the former has a rigid structure,
whereas the latter has a significant flexional capacity [23],
but what could be even more important in a catalytic
process are the electron donor and acceptor properties of
each one; the bipyridyl entity is an acceptor, whereas the
bis-pyrazolyl ligand is a good donor [24]. We therefore,
AAS method is 0.230 mg/L. Gas chromatography (CG)
analysis of the photo-oxidation products was performed
with a HP-6890 equipped with a flame ionization detector
and HP-INNOWAX column (30 m 9 0.32 mm 9 0.25
lm). Benzene was used as internal standard.
Synthesis and characterization
found opportune to compare directly TiO -covalently
2
bound molybdenum dioxo complexes bearing bipyridyl
Synthesis of bis(3,5-dimethylpyrazol-1-yl)acetic acid (1)
and bis-pyrazolyl ligands in the O catalytic oxidation of
2
arylalkanes. We report here the covalent grafting of
This compound was synthesized according to the literature
procedure [28, 29] starting from dichloroacetic acid instead
of dibromoacetic acid and with a reaction time of 72 h. FT-
bis(3,5-dimethylpyrazol-1-yl)acetate on anatase TiO , its
2
interaction with molybdenum(VI)dihalodioxido, as a
bidentate ligand, its full characterization and its compar-
ison with the bipyridyl analog in the O2 oxidation of
ethylbenzene, at room temperature and atmospheric
pressure.
-
1
1
IR (KBr, cm ) 1,737 (COOH), 1,562 (C=N). H NMR
(CDCl , 200 MHz): d 2.16 (s, 6H, CH ), 2.23 (s, 6 H,
3
3
1
CH ), 5.89 (s, 2H, H ), 6.93 (s, 1H, CH). C NMR
3
3
pz
(CDCl , 62.5 MHz): d 11.1 (CH ), 13.3 (CH ), 70.4 (CH),
3
3
3
1
3
1
07.8 (CH ), 141.9 (C ), 148.9 (C ), 165.1 (COOH).
pz
C
pz
pz
RMN/CPMAS (400 MHz, solid, 23 °C) d 165.3 (s,
Experimental
COOH), 150.4 (s, C ), 146.1(s, C ), 143.5 (s, C ), 142.3
pz
pz
pz
(s, C ), 108.9 (s, CH ), 107.4 (s, CH ), 71.7 (s, CHmet),
pz pz pz
All materials were commercial and were used without
further purification unless otherwise noted. All solvents
were thoroughly degassed prior to use. Acetonitrile was
distilled and kept under argon. Titanium oxide (Degussa-
13.6 (s, CH ), 13.2 (s, CH ), 12.1 (s, CH ), 10.5 (s, CH ).
3 3 3 3
Elemental analysis: C H N O Calc: C 58.1, H 6.5, N
12 16 4 2
22.6, Found C 59.4, H 7.0, N 22.2.
2
P25) with 50 m /g specific area and crystalline structure of
Trimethylsilylation of bis(3,5-dimethylpyrazol-1-yl)acetic
acid
8
5
0 % anatase and 20 % rutile was dehydrated at 60 °C and
9 10 mbar for 48 h prior to use. FT-IR (KBr) spectra
-4
1
13
were recorded on a Perkin-Elmer 1720XFT. H and
C
Bis(trimethylsilyl) urea (0.102 g, 0.5 mmol) was added to a
benzene suspension (40 mL) containing bis(3,5-dim-
ethylpyrazol-1-yl)acetic acid (1) (0.248 g, 1 mmol). The
mixture was refluxed for 4 h, cooled to room temperature,
and filtered. The filtrate was evaporated to yield the
product trimethylsilyl ester of bis(3,5-dimethylpyrazol-1-
NMR were obtained on Bruker Avance 200 and 400
spectrometers. All solid-state CPMAS NMR spectra were
obtained on a Bruker Avance 400 MHz NMR spectrometer
1
3
operating at a C resonance frequency of 101.6 MHz. The
1
3
C experiments were performed with a commercial Bruker
Double-bearing probe with zirconium dioxide rotors of
-mm outer diameter. The CP technique [25] was applied
during Magic Angle Spinning (MAS) of the rotor at
-
1
yl)acetic acid (3) as a white solid. FT-IR (KBr, cm
)
-
1,750 cm (C=O), 1,561 (C=N), 836 (–Si–CH ). H NMR
1
1
4
3
(CDCl , 200 MHz): d 6.82 (s, 1H, CH), 5.90 (s, 2H, H ),
3
pz
1
1
0 kHz. A ramped H-pulse starting at 100 % power and
2.24 (s, 6H, CH ), 2.20 (s, 6H, CH ), 0.155 (s, 9 H,
3
3
1
3
decreasing until 50 % was used during contact time in
order to circumvent Hartmann-Hahn mismatch [26, 27].
The contact time was 2 ms. To obtain a good signal-to-
(Si–(CH3)3).
C
NMR (CDCl3, 200 MHz):d 175.5
(C=O), 149.0 (C ), 141.6 (C ), 107.8 (C ), 42.4 (CH),
pz
pz
pz
13.3 (CH ), 11.1 (CH ).
3
3
1
3
noise ratio in C CPMAS experiments, 2048 scans were
1
3
accumulated using a delay of 2 s. The C chemical shifts
were referenced to tetramethylsilane and calibrated with
glycine carbonyl signal, set at 176.5 ppm. Thermogravi-
metric analysis was performed using a Micromeritics
instrument which has a thermogravimetric analyzer TGA
Chlorodioxido(bis(3,5-dimethylpyrazol-1-
yl)acetate)molybdenum(VI) (4)
A benzene solution (40 mL) containing the trimethylsilyl
ester of bis(3,5-dimethylpyrazol-1-yl)acetic acid (160.2 mg,
0.5 mmol) was added to a benzene suspension (20 mL)
containing MoO Cl (99.4 mg, 0.5 mmol). The mixture was
2
950 Hi-Re operating with a heating ramp of 10.00 °C/min
in a range between 30.0° and 900 °C. Molybdenum ele-
mental analysis was carried out with an atomic absorption
spectrophotometer (AAS) Thermo S4. The samples were
analyzed after acid digestion with previous calcination in a
muffle furnace at 500 °C for 5 h. The detection limit of the
2
2
stirred for 4 h at room temperature resulting in a clear
solution. Evaporation of the solvent yielded a light green
-
solid. FT-IR (KBr, cm ) 1,691, 1,300 (COO–), 1,558
1
1
(C=N), 942, 910 (MoO ). H NMR (CDCl , 200 MHz): d
2
3
1
23

Transition Met Chem (2012) 37:629–637
631
6
.49 (s, 1H, CH), 6.12 (s, 1H, CH ), 6.06 (s, 1H, CH ), 2.69
pz
Synthesis of dichlorodioxido(4,4-dimethoxycarbonyl-2,2-
bipyridyl)molybdenum (VI) (7)
pz
1
d, 6H, CH ), 2.46 (d, 6H, CH ). H NMR (DMSO,
(
3
3
3
00 MHz): d 7.12 (s, 1H, CH), 5.87 (s, 2H, CH ), 2.18 (s,
pz
1
3
H, CH ), 2.08 (s, 6H, CH ). C RMN/CPMAS (400 MHz,
6
This compound was synthesized according to the literature
3
3
-
procedure [16]. FT-IR (KBr, cm ) 1,727 (C=O), 1,566
1
solid, 23 °C): d 163.7 (s, COO), 155.7 (s, C ), 152.1 (s, C ),
pz
pz
1
1
6
9
3
44.5 (s, C ), 142.3 (s, C ), 111.8 (s, CH ), 107.7 (s, CH ),
(C=N), 944, 911 (Mo=O). H-RMN (CDCl , 200 MHz), d
3
pz
pz
pz
pz
6.5 (s, CHmet), 14.8 (s, CH ), 14.0 (s, CH ), 13.0 (s, CH ),
3
(ppm) 9.73 (d, 2H), 8.92 (s, 2H), 8.30 (d, 2H), 4.16 (t, 6H).
Catalysis: ethylbenzene photo-oxidation
3
3
.8 (s, CH ). Elemental Analysis C H ClMoN O Calc: C
4 4
3
12 15
5.1, H 3.7, N 13.6, Found C 35.0, H 3.8, N 13.5.
Trimethylsilylation of TiO (Degussa P-25)
2
The photocatalytic oxidation of the arylalkanes was carried
out using a 10 mL batch micro-reactor (Ace-Glass model
T-121 m) with immersion lamp (Phenix, 220 V). A 0.083 M
with hexamethyldisilazane
Hexamethyldisilazane (0.5 mL, 2.4 mmol) was added to a
benzene suspension (40 mL) containing thoroughly dried
solution of ethylbenzene in CH
genated by bubbling N
gas for several hours at 5 °C before
the addition of 0.045 g of either MoO /TiO (6), a
‘‘mechanical’’ mixture of free molybdenum complex (4) and
TiO in the same proportion as (6), or pure TiO . In all cases,
3
CN was thoroughly deoxy-
2
-
5
(
60 °C, 10 mbar, 24 h) TiO (1 g, Degussa P-25, esti-
2
2
2
mated 0.5 mmol of OH/g). The suspension was slowly
stirred at room temperature for 24 h, and the resulting solid
was filtered, washed thoroughly with benzene (2 9 25 mL),
2
2
the mixture was kept in the dark and under nitrogenfor 1 h, to
assure adsorption/desorption equilibrium, before visible
light was turned on and kept for 4 h. Nitrogen was then
1
3
and dried under vacuum. C NMR/CPMAS: d –1.4 ppm.
Reaction of bis(3,5-dimethylpyrazol-1-yl)acetic acid (1)
with trimethylsilylated TiO2
replaced by O
2
in the dark. After a period of 2 h, the mixture
-free by bubbling N before the light
was rendered, again, O
2
2
was turned back on. The sequence was repeated as required.
In a separate experiment, complex (6) was tested in the
reaction of ethylbenzene following the same protocol. When
the catalytic activity stopped after 660 min, the suspension
Bis(3,5-dimethylpyrazol-1-yl)acetic acid (1) (124 mg,
0
.5 mmol) was added to a benzene suspension (50 mL)
containing freshly prepared trimethylsilylated TiO2,
obtained from TiO (1 g, Degussa P-25). The mixture was
2
was filtered and the solid (TiO ) washed with THF
2
stirred for 24 h at 60 °C, and the resulting mixture was
filtered. The solid (5) was thoroughly washed with benzene
(3 9 50 mL). The combined solutions were evaporated to
give a solid. Its FT-IR spectrum was different from (4) and
-
3 9 25 min). FT-IR (KBr, cm ) 1,650 (C=O), 1,566
1
-1
exhibited a strong absorption band at 751 cm , character-
(
1
3
(
C=N). C NMR/CPMAS (400 MHz, solid, 23 °C): d
65.7 (s, COO), 149.5 (s, C ), 147.4 (s, C ), 142.0 (s,
istic of a Mo–O–Mo bonding, similar to the one obtained
1
with the bis(l-oxo) dimer previously reported [19].
pz
pz
C ), 140.9 (s, C ), 108.7 (s, CH ), 107.7 (s, CH ), 72.7
pz
pz
pz
pz
(
s, CHmet), 12.0 (s, 2CH ), 10.2 (s, 2CH ).
3 3
Results and discussion
Reaction of anchored bis(3,5-dimethylpyrazol-1-yl)acetic
acid on TiO with MoO Cl
Synthesis and characterization
2
2
2
A THF solution freshly prepared containing MoO Cl (THF)
2
The ligand bis(3,5-dimethylpyrazol-1-yl)acetic acid (1) was
chosen as starting material, since, as reported [20–22],
through the carboxylate function it is possible to link cova-
2
2
(
0.10 g, 0.5 mmol) was added under inert atmosphere to a
THF suspension (40 mL) containing freshly prepared bis(3,5-
dimethylpyrazol-1-yl)acetic acid anchored on TiO , obtained
lently the ligand onto TiO
, thus permitting a direct com-
2
2
0
0
from silylated TiO (1 g). The mixture was stirred at room
2
parison with the 2,2 -bipyridyl-4,4 -dicarboxylic acid. This
ligand can easily be prepared from 3,5-dimethylpyrazol and
chloroacetic acid according to the literature procedure [28,
29]. However, and contrary to the bipyridyl analog, due to
the presence of the carboxylate function, it has the potential
of becoming a true tridentate N,N,O–ligand in the presence
of various transition metal species giving rise to hetero-
scorpionate complexes (2) [30, 31] (Scheme 1).
temperature for 24 h. The resulting yellow solid (6) was fil-
tered off, washed with THF (4 9 25 mL), and dried under
-1
vacuum. FT-IR (KBr, cm ) 1,649 (C=O), 1,559 (C=N), 947,
1
3
9
05 (Mo=O). C NMR/CPMAS (400 MHz, solid, 23 °C): d
(
ppm) 164.4 (s, COO), 154.5 (s, 2 C ), 144.2 (s, 2 C ), 108.2
pz
pz
(
s, CH ), 103.5 (s, CH ), 69.4 (s, CHmet), 12.7 (s, 2CH3),
pz pz
1
1.1 (s, 2CH3).
123

6
32
Transition Met Chem (2012) 37:629–637
trimethylsilyl ester to be transformed into the heteroscor-
pionate complex (4) could partially undergo such a trans-
formation under the reaction conditions during the
complexation with MoO Cl . Once the solvent and the
N N
N N
N N
N N
O
M
COOH
M
X
O
2
2
compound (4) are removed, the solid matrix (6) is stable.
The assessment of the supported complex (6) according
to that represented in scheme 3 was clearly established by
(
1)
(2)
1
3
C CPMAS NMR. The comparison between the spectrum
Scheme 1 Synthetic route to obtain heteroscorpionate complexes
of a physical mixture of bis(3,5-dimethylpyrazol-1-
yl)acetic acid (1) with TiO (Fig 1a) and the spectrum of
This otherwise interesting property becomes a hindrance
if one wants to preserve the tethering ability of the car-
boxylate group. It was thus attempted to esterify the acid
function with a trimethylsilyl group (3), followed by
complexation with MoO Cl and reaction with the Ti–OH
2
the anchored ligand (Fig 1b), evidences an important shift
of the signals attributed to the pyrazol rings, situated in the
spectral region between 140 and 150 ppm. This confirms
the covalent bond formed with the TiO surface. Moreover,
2
2
2
entities on the surface of titania as reported for the bipyr-
idyl analog [22] (Scheme 2).
this new bond can be confirmed by the slight broadness of
all the NMR signals (Fig. 1b), due to the reduced mobility
of the anchored molecule.
This anticipated pathway, however, failed since the reac-
tion of the trimethylsilyl ester with molybdenum dichloro
dioxidoresulted, nevertheless, in the quantitative formation of
the heteroscorpionate complex (4). Apparently, (4) is rapidly
formed when the ester is reacted with the molybdenum dioxo
entity, and consequently, the carboxylate function is no longer
available for reaction with the surface groups. One must point
out that, under the reaction conditions, MoO Cl does not
A more important broadness of these signals is observed
in (Fig 1c) where the complexation with Mo reduces
drastically the mobility of the pyrazol ring. In addition, an
important chemical shift variation is observed, in particu-
lar, for the signals of the pyrazol ring carbons close to Mo
center (from 147.4–149.5 ppm to 154.5 ppm). Both the
broadness of signals and the chemical shift variation shown
in the spectrum (Fig. 1c) allow us to exclude the presence
of a heteroscorpionate species (Fig. 1d) in (6).
2
2
remain attached to the matrix after washing.
The desired tethered bis(pyrazol-1-yl) complex on TiO2
6) was, however, obtained by modifying the procedure,
namely by anchoring the bis(3,5-dimethylpyrazol-1-
(
The FT-IR spectral analysis of each intermediate leading
to complexes (4) and (6) is depicted in Fig. 2 and corrobo-
1
rates the CMAS NMR results. The covalent anchoring of
3
yl)acetic acid on a trimethylsilylated titania surface to give
(
5), prior to its treatment with MoO Cl [22] (Scheme 3).
2
the free acid (1) (Fig. 2a) onto titania to give (5) (Fig. 2c)
2
-
1
The loading of Mo complex bis(3,5-dimethylpyrazol-1-
yl)acetate (6) was 0.0425 mmol/g of TiO which is mea-
results, as expected, in a shift from 1,737 to 1,650 cm for
the C=O stretching mode. The spectrum also indicates, as in
the NMR analysis, the absence in (5) of either free acid (1) or
any trimethylsilyl functionality. The heteroscorpionate
complex (4) (Fig. 2b) exhibits absorption bands around
2
surably lower than the value obtained with the bipyridyl
analog [22]. A plausible explanation could be that the
tethered complex (6), although much less prone than the
Scheme 2 Reaction of the
trimethylsilyl ester with
molybdenum dichloro dioxide.
Synthetic route failed to obtain
dioxomolybdeno(VI) complex
Cl
Mo
Cl
O
C
N
N
N
N
O
Cl
TiO2
N
N
N
N
O
O
(
6) covalently anchored on TiO
2
Mo
Cl
COOSiMe3
O
O
TiO2
(
6)
N
N
N
N
MoO Cl
COOSiMe3
2
2
HOSiMe3
Cl
N
N
N
O
O
O
(3)
Mo
Cl-SiMe3
O
N
(4)
1
23

Transition Met Chem (2012) 37:629–637
633
Scheme 3 Synthesis of
covalently grafted bis(pyrazol-
1-yl) Mo(VI) complex
on TiO (6)
Cl
Mo
Cl
O
C
N
N
N
N
2
N
N
N
N
N
N
N
N
O
C
O
TiO2
2
MoO Cl2
O
COOH
O
O
TiO2
TiO2
(6)
(
5)
(
1)
HOSiMe3
1
3
Fig. 1 CPMAS NMR spectra
of 1 (a), 5 (b), 6 (c), and 4 (d)
2'
1,1’- 3,3’
2, 2’
5
A
1
'
3'
O
N
N
N
C
5
OH
N
1
3
2
(1)
B
O
C
N
N
O
TiO2
N
N
(5)
C
Cl
Mo
Cl
O
C
N
N
N
N
O
O
TiO2
O
(6)
D
Cl
N
N
N
O
O
O
Mo
N
O
(4)
123

6
34
Transition Met Chem (2012) 37:629–637
Fig. 2 FT-IR (KBr) spectra of
1
A
B
(a), 4(b), 5(c), and 6(d)
1
126
1
558
1
237
1
051
1159
1
737 1562 1412
1390
1
691
940
1223
908
2000
1800
1600
1400
1200
1000
800
600
2000 1800 1600 1400 1200 1000
800
600
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
C
D
1
559
1
1258
104
1241
1
557
1649
1423
1155
1040
1014
1
650
1416
947
905
2000
1800
1600
1400
1200
1000
800
600
2000 1800 1600 1400 1200 1000
800
600
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
-
,700 and 1,300 cm assigned tentatively to the symmetric
1
1
When a carboxylic acid group is attached to these
ligands, the species obtained upon complexation with the
and asymmetric stretching mode of a carboxylate structure
0
[
32]. The cis-MoO stretching vibrations are observed at
2
molybdenum dioxo entity differ. For the 2,2 -bipyridyl-
0
-
40 and 908 cm [33]. Complex (6) (Fig. 2d) despite the
1
9
4,4 dicarboxylic acid, it is necessary to methylate the acid
low concentration bound on the matrix gave observable
signals corresponding to the main vibrations of the com-
plex anchored. The presence of a C=O stretching mode
function (7) in order to attain complexation [21]. Whereas
the bis(3,5-dimethylpyrazolyl)acetic acid, upon reaction
with MoO Cl , is immediately transformed into a N,N,O-
2
2
-
1
analogous to (5) is confirmed at 1,649 cm , as well as
-
tridentate ligand, yielding quantitatively the heteroscorpi-
onate (4) [8]. Although the molybdenum environment is
slightly different in these two complexes (4, 7), a com-
parison was, nevertheless, made for the oxidation of eth-
ylbenzene. The only observed product is acetophenone, as
previously reported with various dioxomolybdenum com-
plexes, arising from a four electron oxidation, via the
alcohol intermediate [15]. It was followed, stepwise, in a
sequence in which a stoichiometric O-transfer occurs under
nitrogen, at room temperature and under visible light, fol-
lowed by a period under dioxygen, in the dark, during
which no O-transfer occurs but where the reduced Mo(IV)
oxo species can be reoxidized to a Mo(VI) oxo-peroxo
entity to complete the cycle [21, 22] (Scheme 4).
1
two weak cis-MoO dioxo bands at 947 and 905 cm
2
.
More importantly, the FT-IR Spectrum of (6) excludes
unequivocally any presence of either (1), (4) or (5)
attached to the matrix.
Reactivity
The comparative reactivity study between the bis(3,5-
0
0
dimethylpyrazolyl)acetate and 2,2 -bipyridyl-4,4 dicarbox-
ylate ligands bound to a molybdenum dioxo center, in the
catalytic oxidation of ethylbenzene with O , was investi-
2
gated both under homogeneous and TiO bound hetero-
2
geneous conditions. An analogous comparison between
0
bis-pyrazolyl and 2,2 -bipyridine ligands, lacking the car-
When a physical mixture of TiO and complex (4) was
2
boxylate function, has been reported in an olefin epoxida-
tion reaction catalyzed by a molybdenum dioxo complex
and t-butylhydroperoxide as the oxidant [9]. The TOF
obtained with the bis-pyrazolyl complex was shown to be
slightly higher than for the bipyridyl analog.
reacted with ethylbenzene as a suspension, following the
above protocol and compared with the corresponding
dimethylester-bipyridyl molybdenum dioxo complex (7)
(Fig. 3), the estimated rate of O-transfer to give aceto-
phenone was shown to be higher for the complex bearing
1
23

Transition Met Chem (2012) 37:629–637
635
Scheme 4
O
H O
2
Inert Dimer
Cl
Mo
Cl
Cl
VI
O
O
IV
O
2
L
2
L
Mo
N ; uv-vislight
2
Cl
O
dark
N2
uv-vis light
L=Bis-pyrazolyl,
Bipyridyl
O2
Cl
O
VI
Mo
L
O
O
Cl
the tridentate ligand by a factor of nearly two to three. This
is in accord with a reported similar comparison made in the
case of a TBHP phosphine oxidation with an analogous
molybdenum heteroscorpionate complex [34]. The pres-
ence of greater electron density in the carbonyl function
directly bonded to the metal center, apparently, accelerates
the OAT significantly.
oxidant. This inconvenience limited, therefore, the com-
parison between the two ligands to a stoichiometric OAT.
This handicap was eliminated with the titanium oxide
surface bound molybdenum dioxo bis-pyrazolyl complex
(6). Its reaction with ethylbenzene using the same reaction
protocol was compared with the analogous TiO bound
2
bipyridyl system (8) previously reported [21, 22]. Here, the
comparison is more direct than with the two homogeneous
systems since the bidentate bonding mode is identical for
both ligands.
However, one other issue was revealed from this study,
indeed, as shown in Fig. 3, the O-transfer process nearly
stops for both systems after about 660 min. This propensity
to give unreactive species, characterized as Mo(V) bis(l-
oxo) dimers, under homogeneous conditions, has been
The results depicted in Fig. 4 show a significantly higher
initial rate of acetophenone formation which can be
reported by us and others [10, 16] when O is the direct
2
light
N2
dark
O2
light
N2
dark
O2
light
N2
light
N2
dark
O2
light
N2
dark
O2
light
N2
4
00,0
1
1
1
1
60,0
40,0
20,0
00,0
350,0
300,0
250,0
200,0
150,0
100,0
8
6
4
2
0,0
0,0
0,0
0,0
5
0,0
0,0
0
120
240
360
480
600
720
840
960
0
,0
0
120
240
360
480
600
720
840
960
Time (min)
Tíme (min)
MoO2Cl-bis-pyrazolyl-COO + TiO2 (4)
MoO2Cl2-Bis-pyrazolyl-COO/TiO2 (6)
MoO2Cl2-bipyridyl-COO/TiO2 (8)
MoO2Cl2-Bipyridyl-COOCH3 + TiO2 (7)
Fig. 4 Evolution of [mol of ketone]/[mol of MoO
time for TiO modified with dioxomolybdenum(VI) complexes (6)
and (8)
2
] as a function of
Fig. 3 Evolution of [mol of ketone]/[mol of MoO
time for dioxomolybdenum(VI) complexes (4) and (7)
2
] as a function of
2
123

6
36
Transition Met Chem (2012) 37:629–637
light
N2
dark
O2
light
N2
dark
O2
light
N2
2
1
1
1
1
1
00,0
80,0
60,0
40,0
20,0
00,0
1558
1
734
1254
1
440
1
669
1
169
9
58
7
97
8
6
4
2
0,0
0,0
0,0
0,0
1
115
0
,0
0
120
240
360
480
600
720
840
960
Time (min)
2
000
1800
1600
1400
1200
1000
800
600
MoO2Cl-bis-pyrazolyl-COO + TiO2 (4)
-
1
Wavenumber (cm )
MoO2Cl2-Bis-pyrazolyl-COO/TiO2 (6)
Fig. 6 FT-IR (KBr) spectra of solid obtained after ethylbenzene
catalytic oxidation reaction using (6)
Fig. 5 Evolution of [mol of ketone]/[mol of MoO
time for physical mixture (4) ? TiO and anchored complex (6)
2
] as a function of
2
1
00
considered as the emanation of the real role of the ligand
which is in accord with the electron donating property of
the pyrazolyl ring. This result might also suggest that in the
homogeneous system, the tridentate character, given as
responsible for its higher activity, is not the only factor to
be taken into account.
99
9
9
9
9
9
9
9
8
7
6
5
4
3
2
It is also interesting to compare the reaction pattern
obtained with the physical mixture (4)/TiO2 with the
anchored complex (6). As depicted in Fig. 5, a higher rate
is clearly observed with the tethered species which can be
attributed to the direct interaction between the semicon-
ductor and the organometallic entity in the OAT process.
Continuing the catalytic cycle (Fig 4), one observes that
0
100
200
300
400
500
600
700
800
Temperature (C)
MoO2Cl2-bispyrazolyl (6)
MoO2Cl2-bispyrazolyl (6) used
after a period under O in the dark, during which the
2
Mo(IV) is reoxidized, the formation of acetophenone pro-
ceeds as expected at a rate comparable with the bipyridyl
Fig. 7 TG analysis for as-made and used TiO
dioxomolybdenum(VI) complex (6)
2
matrix modified with
system. However, after a second period under O , contrary
2
to the bipyridyl system, the catalysis abruptly stops. A
plausible explanation of this reaction pattern is a progres-
sive formation of an inactive dimer which could arise from
the transformation of (5) into the heteroscorpionate (4),
which is soluble in the medium and, as shown earlier,
prone to be dimerized rapidly. This hypothesis was verified
in a separate experiment, in which the suspension, when
the catalytic activity ends after 660 min, was filtered and
evaporated to dryness yielding a solid. Its FT-IR spectral
analysis (Fig. 6) is different from (4) and exhibits a strong
oxidation. An additional plausible factor for the higher
stability of (8) compared to (6) could be the slow reoxi-
dation of the bis-pyrazolyl Mo(IV) entity by O in the dark
2
leaving more time for the heteroscorpionate (4) to be
formed. This lower rate could be rationalized, also, by the
poor p-acceptor property of the pyrazolyl ring.
Conclusion
-
1
absorption band at 797 cm characteristic of a Mo–O–Mo
bonding mode and similar to the bis(l-oxo) dimers previ-
ously characterized and reported as being inert to reoxi-
dation [17, 35–37]. Further evidence was given by a TGA
analysis of the matrix before and after reaction (Fig. 7),
showing clearly that no significant amount of complex
remains attached to the surface thus justifying the lack of
Bis(pyrazolyl)acetate compounds are known to give rise to
N,N,O-tridentate ligands in the presence of several transi-
tion metals [8, 9]. Compound (1) was, nevertheless,
maintained as a bidentate N,N-ligand with MoO Cl via
2
2
the prior esterification onto a titanium oxide surface to give
the tethered complex (6). This was fully characterized by
1
CMAS NMR spectroscopy, and its reactivity directly
3
1
23

Transition Met Chem (2012) 37:629–637
637
compared with the analogous TiO -bound bipyridyl [21,
2
11. Arzoumanian H (2011) Current Inorg Chem 1:140–145
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1
2
2] species in the O oxidation of ethylbenzene at room
2
3
temperature and atmospheric pressure. The initial OAT rate
of acetophenone formation seems to be significantly higher
with complex (6), which is well in accord with the electron
donating property of the pyrazolyl ring [24]. This behavior,
however, rapidly deteriorates, most probably due to the
leaching of the bis(pyrazolyl) moiety to give the soluble
heteroscorpionate complex (4) which in turn is quickly
transformed into an inactive dimer, stopping any catalytic
activity. This leaching phenomenon is certainly due to the
intrinsic instability of (5) in the presence of solvent, but it
could plausibly be, also, assisted by a slow reoxidation step
1
3. Arzoumanian H, Agrifoglio G, Kreintzien H Capparelli M (1995)
J Chem Soc Chem Commun 655–656
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0:699–705
2
1
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accepting character of the pyrazolyl species, leaving more
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H
(2008) Catal Today
From a practical point of view, this study showed a
significantly higher OAT capability for the bis(pyrazolyl)
bound complex compared to its bipyridyl analog, as long as
it remains catalytically active. This was also observed in a
comparable homogeneous system. It also emphasized the
importance of choosing the correct balance of electron
donation and abstraction of the ligand system in a catalytic
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Acknowledgments Part of this work was performed under the
auspices of COLCIENCIAS (Fondo Apoyo a Doctorados Nacionales)
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Industrial de Santander.
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