Effect of N-based additive on the optimization of liquid phase oxidation of bicyclic, cyclic and aromatic alcohols catalyzed by dioxidomolybdenum(VI) and oxidoperoxidomolybdenum(VI) complexes

Article abstract of DOI:10.1039/c5ra16490g

Two dioxidomolybdenum(vi) complexes, [Mo^VIO₂(L¹)(MeOH)] (1) and [Mo^VIO₂(L²)(MeOH)] (2) and their corresponding oxidoperoxidomolybdenum(vi) complexes, [Mo^VIO(O₂)(L¹)(MeOH)] (3) and [Mo^VIO(O₂)(L²)(MeOH)] (4) with ONO tridentate ligands, 4-[3,5-bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]benzoic acid (H₂L¹, I) and 3,5-bis(2-hydroxyphenyl)-1-phenyl-1,2,4-triazole (H₂L², II) have been synthesized and characterized by elemental analysis, spectroscopic techniques (infrared, UV-Vis, ¹H and ¹³C NMR) and thermogravimetric analysis. Structures of 1a (DMSO coordinated) and 2 (methanol coordinated) confirmed by single crystal X-ray study reveal that the tridentate ligands bind to the metal center through two oxygen atoms and a ring nitrogen atom. These complexes have been tested as catalysts for the homogeneous oxidation of bicyclic (isoborneol and fenchyl alcohol), aromatic (benzyl alcohol and cumic alcohol) and cyclic (cyclohexanol) alcohols, using 30% H₂O₂ as an oxidant. Various parameters such as amounts of catalyst, oxidant, solvent and temperature of the reaction mixture have been taken into consideration for the maximum conversion of substrates. Effect of N-based additive (NEt₃) on the conversion of substrates as well as selectivity of the corresponding product(s) under the optimized reaction conditions has also been checked and obtained results suggest that addition of an additive reduces time to achieve equilibrium and increases conversion of alcohols.

Full text of DOI:10.1039/c5ra16490g

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Effect of N-based additive on the optimization of
liquid phase oxidation of bicyclic, cyclic and
aromatic alcohols catalyzed by
dioxidomolybdenum(VI) and
Cite this: RSC Adv., 2015, 5, 101076
oxidoperoxidomolybdenum(VI) complexes†
a
Mannar R. Maurya, Neeraj Sainia and Fernando Avecillab
*
Two dioxidomolybdenum(VI) complexes, [MoVIO2(L1)(MeOH)] (1) and [MoVIO2(L2)(MeOH)] (2) and
their corresponding oxidoperoxidomolybdenum(VI) complexes, [MoVIO(O2)(L1)(MeOH)] (3) and
[MoVIO(O2)(L2)(MeOH)] (4) with ONO tridentate ligands, 4-[3,5-bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]
benzoic acid (H2L1, I) and 3,5-bis(2-hydroxyphenyl)-1-phenyl-1,2,4-triazole (H2L2, II) have been
synthesized and characterized by elemental analysis, spectroscopic techniques (infrared, UV-Vis, 1H and
13C NMR) and thermogravimetric analysis. Structures of 1a (DMSO coordinated) and 2 (methanol
coordinated) confirmed by single crystal X-ray study reveal that the tridentate ligands bind to the metal
center through two oxygen atoms and a ring nitrogen atom. These complexes have been tested as
catalysts for the homogeneous oxidation of bicyclic (isoborneol and fenchyl alcohol), aromatic (benzyl
alcohol and cumic alcohol) and cyclic (cyclohexanol) alcohols, using 30% H2O2 as an oxidant. Various
parameters such as amounts of catalyst, oxidant, solvent and temperature of the reaction mixture have
been taken into consideration for the maximum conversion of substrates. Effect of N-based additive
(NEt3) on the conversion of substrates as well as selectivity of the corresponding product(s) under the
optimized reaction conditions has also been checked and obtained results suggest that addition of an
additive reduces time to achieve equilibrium and increases conversion of alcohols.
Received 16th August 2015
Accepted 16th November 2015
DOI: 10.1039/c5ra16490g
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as catalysts for oxidation reactions,5–8 in general, and less toxic
dioxidomolybdenum(VI) complexes, in particular, are under
extensive study.9 Efficient and selective catalytic systems for
Introduction
Catalytic oxidation has been identied as a most efficient
method to generate products that are commercially valuable.1
Catalytic oxidation of alcohols to corresponding carbonyl
compounds is of great interest for both laboratory and synthetic
industrial diligences.2 However, developing green oxidation
process of alcohols is still a challenging task in catalysis.3
Homogeneous catalysts are oen much active and selective,
producing excellent yields, from economic and environmental
point of view.4 The development of transition metal complexes
oxidation of isoborneol to camphor and fenchyl alcohol to
fenchone are continuously sought aer.10 Further, fenchone is
a constituent of absinthe and the essential oil of fennel. Fen-
chone is also used as a avor in foods and in perfumery.
Oxidation of cyclohexanol is signicant as it renders interme-
diate that has usages as plasticizers, food additives and solvent
system in coating industry.11,12 Cyclohexanone was primarily
used in the production of precursors for Nylon 6 and Nylon
6,6.11 On the other hand oxidation of aromatic alcohols such as
benzyl alcohol and cumic alcohol give way to products that have
both commercial and biological implications. Benzaldehyde
and benzoic acid, distinctive products obtained by oxidation of
benzyl alcohol, have crucial role as intermediates in fragrances,
perfumes, ame retardants and pharmaceutical industry.13
Cuminaldehyde, an oxidation product of cumic alcohols, due to
its very pleasant smell, nd applications in perfume and other
cosmetic industry. In biological prospect, cuminaldehyde, as
a small molecule, inhibits the brillation of alpha-synuclein
and thus controls the progress of parkinson's disease,
dementia and multiple system atrophy.
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667,
India. E-mail: rkmanfcy@iitr.ernet.in; Fax: +91 1332 273560; Tel: +91 1332 285327
b
´
˜
Departamento de Quımica Fundamental, Universidade da Coruna, Campus de A
˜
Zapateira, 15071 A Coruna, Spain
† Electronic supplementary information (ESI) available: Table S1 (detail of
thermogravimetric analysis of complexes). Tables S2–S4 (data on the oxidation
of fenchyl alcohol, benzyl alcohol and cumic alcohol under various reaction
conditions, respectively). Fig. S1 (electronic spectra of ligands, H2L1 I and H2L2
II). Fig. S2–S5 (catalytic plots of fenchyl alcohol, benzyl alcohol, cumic alcohol
and cyclohexanol, respectively). CCDC 1433879 for 1a and 1414244 for 2. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra16490g
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and complexes were recorded on a Shimadzu 1601 single beam
spectrophotometer in methanol. 1H NMR spectra was obtained
in DMSO-d6 on a Bruker Avance 400 MHz spectrometer. Elec-
trochemical experiment has been carried out in a classic three-
electrode cell having platinum disc working electrode, platinum
wire as counter electrode, and Ag/AgCl as reference electrode on
CHI760 electrochemical workstation. Tetra-n-butylammonium
hexauorophosphate (0.1 M) was used as supporting electro-
lyte. Redox analysis of the compounds was carried out with scan
rate 0.1 V sꢀ1 in dry and degassed DMF with no trace of
decomposition as reected in smooth curve. The solutions were
purged with N2 for ca. 15 min before conducting experimental
analysis in order to remove the dissolved O2. The oxidation
products were analyzed with a Shimadzu 2010 plus gas-
chromatograph tted with an Rtx-1 capillary column (30 m ꢁ
0.25 mm ꢁ 0.25 mm) and a FID detector and the identity of the
products conrmed using the GC-MS Shimadzu QP-5000.
Scheme
1 Ligands, 4-[3,5-bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]
benzoic acid (R ¼ COOH: H2L1, I) and 3,5-bis(2-hydroxyphenyl)-1-
phenyl-1,2,4-triazole (R ¼ H: H2L2, II) used in this study.
Recently, we prepared homogeneous as well as heteroge-
neous catalysts based on dioxidomolybdenum(VI) complexes
and tested them for the oxidative bromination of salicylalde-
hyde, styrene, trans-stilbene and thymol, and oxidation of
methyl phenyl sulde, styrene, cyclohexene, benzoin and
different secondary alcohols.14 Considering the catalytic
potential of such complexes and in search of catalytically active
new molybdenum complexes, we have synthesized two dioxido-
and two oxidoperoxidomolybdenum(VI) complexes employing
dibasic tridentate ONO donor ligand systems; Scheme 1. These
complexes have been tested as catalysts for the homogenous
oxidation of bicyclic (isoborneol and fenchyl alcohol), aromatic
(benzyl alcohol and cumic alcohol) and cyclic (cyclohexanol)
alcohols under atmospheric conditions. Various parameters
such as amount of catalyst, oxidant, solvent and temperature of
the reaction mixture have been taken into consideration for the
maximum conversion of substrates. Addition of N-based addi-
tive to the reaction mixture reduces time considerably and
increases conversion of secondary alcohols.
X-ray crystal structure determination
Three-dimensional X-ray data were collected on a Bruker Kappa
Apex CCD diffractometer at room temperature for 1a and low
temperature for 2 by the f–u scan method. Reections were
measured from a hemisphere of data collected from frames,
each of them covering 0.3ꢂ in u. A total of 20 754 and 56 836
reections measured, all were corrected for Lorentz and polar-
ization effects and for absorption by multi-scan methods based
on symmetry-equivalent and repeated reections. Of them,
5145 for 1a and 7391 for 2 independent reections exceeded the
signicance level (rFr/srFr) > 4.0. Aer data collection, a multi-
scan absorption correction (SADABS)17 was applied, and the
structure was solved by direct methods and rened by full
matrix least-squares on F2 data using SHELX suite of
programs.18 Hydrogen atoms were located in a difference
Fourier map and le to rene freely, except for C(24) in 1a and
for C(1M) and C(2M) in 2, which were included in calculated
positions and rened in the riding mode. Hydrogen atoms of
C(1S) and C(2S) in compound 1a were located in a difference
Fourier map and xed to the carbon atoms. Renements were
done with allowance for thermal anisotropy of all non-hydrogen
atoms. A nal difference Fourier map showed no residual
Experimental
Material
Salicylic acid, salicylamide, 4-hydrazino-benzoic acid, phenyl-
hydrazine hydrochloride (Aldrich, USA), ammonium molybdate
tetrahydrate (Loba Chemie, India), thionyl chloride (SRL, India)
and 30% H2O2 (Rankem, India) were used as obtained. Catalytic
substrates i.e. isoborneol (1,7,7-trimethylbicyclo[2.2.1]heptan-2-
ol), fenchyl alcohol (1,3,3-trimethylbicyclo[2.2.1]heptan-2-ol)
and cumic alcohol (4-isopropylphenyl)methanol were
purchased from Aldrich, USA. Benzyl alcohol (phenylmethanol)
and cyclohexanol were incurred from Qualigens, India and used
as such without further purication. Ethanol (reagent grade) was
puried by distillation prior to use. Other solvents used were of
analytical reagent (AR) grade. [MoVIO2(acac)2],15 4-[3,5-bis(2-
hydroxyphenyl)-1,2,4-triazol-1-yl]benzoic acid (H2L1, I) and 3,5-
bis(2-hydroxyphenyl)-1-phenyl-1,2,4-triazole (H2L2, II)16 were
synthesized according to the methods reported in literature.
ꢀ3
˚
density: 0.787 and ꢀ0.362 e A for 1a and 0.702 and ꢀ0.828
e Aꢀ3 for 2. A weighting scheme w ¼ 1/[s2(Fo2) + (0.039600P)2 +
˚
0.498600P] for 1a and w ¼ 1/[s2(Fo2) + (0.033600P)2 + 0.322500P]
for 2, where P ¼ (|Fo|2 + 2|Fc|2)/3, were used in the latter stages
of renement. The crystal of 1a presents important disorders on
DMSO molecules. These disorders were resolved and the atomic
sites were observed and rened with anisotropic atomic
displacement parameters. More specically these disorders
were rened using 75 restraints (SADI, SIMU and DELU
restraints were used). The site occupancy factors were 0.91517
for S(1A) and 0.22651 for S(2A). Further details of the crystal
structure determination are given in Table 1.
Instrumentation and characterization procedures
Elemental analysis (C, H and N) of the ligands and complexes
were carried out on Elementar Analyser Vario-El-III. Thermog-
ravimetric analysis (TGA) of the complexes was carried out
using TG Stanton Redcro STA 780. Infrared spectra were
recorded as KBr pellets on a Nicolet 1100 FT-IR spectrometer
Preparations
[MoVIO2(L1)(MeOH)] 1. Ligand H2L1 (1.865 g, 0.005 mol) was
aer grinding the sample with KBr. UV-Vis spectra of ligands dissolved in 50 mL of methanol. A methanolic solution (30 mL)
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Table
(L1)(DMSO)]$DMSO 1a and for [MoVIO2(L2)(MeOH)]$MeOH 2
1
Crystal data and structure refinement for [MoVIO2-
solution of H2L1 (I, 1.865 g, 0.005 mol) dissolved in methanol
(50 mL) with slow stirring. The stirring was continued for addi-
tional 6 h. Reaction mixture was then aerated to reduce the
volume to ca. 15 mL and simultaneously light yellow solid sepa-
rated out. This was ltered off, washed with cold methanol and
dried in vacuum desiccator over silica gel. Yield: 2.10 g (77%).
(Found: C, 47.01; H, 2.89; N, 7.55%. Calc'd. for C22H17MoN3O8
(547.33): C, 48.28; H, 3.13; N, 7.68%). Selected IR bands
(KBr, nmax/cmꢀ1): 3255 (OH), 1716 (C]O), 1608 (C–C), 1588
(C]N), 1515 (C]C), 1325 (C–O), 1290 (C–N), 1049 (N]N), 949
(Mo]O), 853 (O–O), 754 (C–H), 624, 568 (Mo(O2)antisym and sym).
[MoVIO(O2)(L2)(MeOH)] 4. Complex 4 was prepared similarly
as outlined for 3 using 1.645 g (0.005 mol) of II. Yield: 1.99 g
(79%). (Found: C, 49.51; H, 3.23; N, 7.95%. Calc'd for
1a
2
Formula
Formula weight
T, K
C
26H27MoN3O8S2
C22H21MoN3O6
519.36
100(2)
0.71073
Triclinic
ꢀ
P1
7.7600(3)
9.9273(4)
14.1328(6)
90.030(2)
100.548(2)
99.031(2)
1056.59(7)
2
669.57
293(2)
0.71073
Triclinic
˚
Wavelength, A
Crystal system
Space group
ꢀ
P1
˚
a/A
11.771(2)
11.984(2)
12.499(2)
89.199(5)
69.782(4)
61.016(4)
1421.8(4)
2
˚
b/A
˚
c/A
a/ꢂ
b/ꢂ
g/ꢂ
3
˚
C
21H17MoN3O6 (503.32): C, 50.11; H, 3.40; N, 8.35%). Selected
V/A
IR bands (KBr, nmax/cmꢀ1): 3261 (OH), 1622 (C–C), 1590 (C]N),
1497 (C]C), 1359 (C–O), 1294 (C–N), 1029 (N]N), 936 (Mo]O),
855 (O–O), 749 (C–H), 616, 587 (Mo(O2)antisym and sym).
Z
F000
684
1.564
0.661
2.19 to 27.89
0.0327
528
1.632
0.666
1.47 to 34.79
0.0678
Dcalc/g cmꢀ3
m/mmꢀ1
q/(ꢂ)
Rint
Catalytic activity study-oxidation of alcohols
Crystal size/mm3
Goodness-of-t on F2
R1[I > 2s(I)]a
wR2 (all data)b
0.31 ꢁ 0.21 ꢁ 0.09
0.34 ꢁ 0.22 ꢁ 0.12
Conventional liquid phase method was employed for the oxida-
tion of bicyclic, aromatic and cyclic alcohols. Catalytic reactions
were carried out in a 50 mL round-bottom ask equipped with
a reux condenser. Under typical conditions, a relevant alcohol
(10 mmol), aqueous 30% H2O2 (1.69 g, 15 mmol) and catalyst
(0.002 g) were dissolved in acetonitrile (5 mL). Oxidation reac-
tions were carried out both in the absence or presence of N-based
1.035
1.061
0.0377
0.0913
0.0362
0.0829
Largest differencesꢀ3
0.787 and ꢀ0.362
0.702 and ꢀ0.828
˚
peak and hole (e A
)
P
P
P
a
b
R1
¼
||Fo|
.
ꢀ
|Fc||/ |Fo|. wR2
¼
{
[w(||Fo|2
ꢀ
|Fc|2|)2]|/
P
[w(Fo2)2]}1/2
ꢂ
additive i.e. NEt3 (0.05 mmol) at 80 C under magnetic stirring
for a particular time depending upon the substrates. The reac-
tion was monitored by withdrawing small aliquots of the reac-
tion mixture at denite time interval, extracting with hexane and
analyzing quantitatively by gas chromatograph. The identities of
the products were conrmed by GC-MS.
of [MoVIO2(acac)2] (1.63 g, 0.005 mol) was added drop wise to
the above solution with stirring and the nal reaction mixture
was reuxed for 2 h on a water bath. The obtained yellow
colored solution was concentrated on a water bath to 20 mL and
kept at room temperature for overnight. The separated crystal-
line yellow 1 was ltered, washed with cold methanol and dried
in vacuum desiccator over silica gel. Yield: 2.15 g (81%). (Found:
C, 46.79; H, 3.31; N, 7.54%. Calc'd. for C22H17N3O7Mo (533.01):
C, 48.73; H, 3.22; N, 7.91%). Selected IR bands (KBr, nmax/cmꢀ1):
3427 (OH), 1711 (C]O), 1604 (C–C), 1573 (C]N), 1527 (C]C),
1310 (C–O), 1266 (C–N), 1038 (N]N), 937 (O]Mo]Oantisym),
906 (O]Mo]Osym), 754 (C–H). X-ray diffraction quality crystals
for [MoVIO2(L1)(DMSO)]$DMSO 1a were obtained from its
DMSO solution at room temperature.
Results and discussion
Synthesis and characterization of complexes
The reaction between equimolar amounts of [MoVIO2(acac)2]
and H2L1 I or H2L2 II in reuxing methanol leads to the
formation of [MoVIO2]2+ complexes [MoVIO2(L1)(MeOH)] 1 and
[MoVIO2(L2)(MeOH)] 2, respectively (eqn (1) considering I as
a representative example). The in situ generated oxidoperox-
idomolybdenum(VI) species, by reacting MoO3 with H2O2, also
reacts with ligands I and II in methanol giving corresponding
[MoVIO2(L2)(MeOH)] 2. Complex 2 was prepared analogously
to 1 considering ligand II (1.645 g, 0.005 mol). Yield: 2.03 g
(83%). (Found: C, 50.99; H, 3.62; N, 8.39%. Calc'd. for
[MoVIO(O2)]2+ complexes, [MoVIO(O2)(L1)(MeOH)]
3
and
[MoVIO(O2)(L2)(MeOH)] 4, respectively (eqn (2) considering I as
representative example).
C
21H17MoN3O5 (489.02): C, 51.76; H, 3.52; N, 8.62%). Selected
IR bands (KBr, nmax/cmꢀ1): 3435 (OH), 1601 (C–C), 1574 (C]N),
1524 (C]C), 1306 (C–O), 1265 (C–N), 1000 (N]N), 924
(O]Mo]Oantisym), 904 (O]Mo]Osym), 753 (C–H). X-ray
diffraction quality crystals for 2 were obtained by slow evapo-
ration of its methanolic solution in air.
[MoVIO2(acac)2] + H2L1 + MeOH /
[MoO2(L1)(MeOH)] + 2Hacac (1)
[MoVIO(O2)(H2O)n]2+ + H2L1 + MeOH /
[MoVIO(O2)(L1)(MeOH)] + nH2O
(2)
[MoVIO(O2)(L1)(MeOH)]
3.
The
oxidoperoxidomoly-
bdenum(VI) precursor was prepared in situ by stirring MoO3
(0.720 g, 0.005 mol) in 10 mL of 30% H2O2 for 4 h at room
temperature and ltered. This was added drop wise to a ltered
Scheme 2 provides their possible structures which are based
1
on the spectroscopic (IR, UV/Vis, H and 13C NMR) data, ther-
mogravimetric and elemental analysis, and X-ray diffraction
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Scheme
2
Proposed structures of [MoO2]2+ and [MoO(O2)]2+
complexes.
Fig. 2 ORTEP plot of complex in [MoVIO2(L2)(MeOH)]$MeOH 2. All the
non-hydrogen atoms are presented by their 50% probability ellipsoids.
Hydrogen atoms are omitted for clarity.
study of 1a and 2. All these complexes exist as monomer and are
soluble in methanol, ethanol, DMF and DMSO.
Thermal study
bonds. Selected bond distances and angles are given in Table 2.
Both of complexes adopt a six-coordinated structure around Mo
atom in a distorted octahedral geometry. The coordination
sphere is formed by the phenolic oxygen atoms and one of the
triazole nitrogen atoms of the ligand. One DMSO molecule in 1a
and one methanol molecule in 2 and two terminal oxido groups
complete the coordination sphere. The O]Mo]O angle are
105.24(10)ꢂ in 1a and 105.48(6)ꢂ in 2 and Mo]O distances are:
The thermal stability of [MoO2]2+ and [MoO(O2)]2+complexes
has been studied under an oxygen atmosphere. Both types of
complexes are thermally stable at least up to ca. 135 ꢂC and
there aer lose weight roughly equal to one methanol molecule
in the temperature range 135–230 ꢂC. In [MoO(O2)]2+
complexes, the observed weight loss is slightly more than
calculated for methanol in this temperature range suggesting
the decomposition of peroxido moiety partly as well along with
the loss of methanol. In second step all complexes decompose
in two or three overlapping steps and convert into MoO3. Detail
of this study is presented in Table S1.†
˚
˚
˚
1.6992(18) A and 1.695(2) A in 1a, and 1.6998(12) A and 1.7070(12)
˚
˚
A in 2. The Mo–ODMSO distance is 2.2869(19) A in 1a. The Mo–O–C
angle and Mo–O distance for Mo–OHCH3 in 2 are 124.85(11)ꢂ and
˚
2.2831(12) A, respectively, similar to other examples in the liter-
ature.14a,19 Other DMSO molecule in 1a and other methanol
molecule in 2 are presents in the asymmetric unit.
Structure description
ORTEP diagram of [MoVIO2(L1)(DMSO)]$DMSO 1a and
[MoVIO2(L2)(MeOH)]$MeOH 2 are shown in Fig. 1 and 2. Fig. 3
presents the interactions between the methanol molecules and
the complexes in the crystal packing through the hydrogen
Fig. 3 Crystal packing in the complex [MoVIO2(L2)(MeOH)]$MeOH 2.
Fig. 1 ORTEP plot of complex in [MoVIO2(L1)(DMSO)]$DMSO 1a. All the Intermolecular hydrogen bonds between O(1M) and O(2M) of meth-
non-hydrogen atoms are presented by their 50% probability ellipsoids. anol molecules and with O(4) of coordinated oxido groups are shown
Hydrogen atoms are omitted for clarity.
in dashed lines.
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Table 2 Bond lengths [A˚] and angles [ꢂ] for [MoVIO2(L1)(DMSO)]$DMSO
1a and for [MoVIO2(L2)(MeOH)]$MeOH 2
and 4), suggesting the coordination of azomethine nitrogen to
the molybdenum. This is further supported by the shi of the
n(N–N) stretch appearing at 1015 cmꢀ1 (in I) and 991 cmꢀ1 (in II)
to higher wave number due to reduced repulsion between the
lone pairs of adjacent nitrogen atoms. The coordination of
phenolic oxygen could not be assigned unequivocally as all
complexes also exhibit a broad band in the ca. 3400 cmꢀ1 region
due to coordinated methanol.
UV-vis spectral data of the triazole ring based ligands and
their corresponding complexes are presented in Table 4 and
spectra of complexes are presented in Fig. 4 (see Fig. S1† for
spectra of ligands). Both ligand systems exhibit very similar
three characteristic absorption bands at 207 (3 ¼ 1.88 ꢁ 104),
249 (3 ¼ 8.76 ꢁ 103) and 301 (3 ¼ 4.87 ꢁ 103) (in I) and at 208
(3 ¼ 1.97 ꢁ 104), 246 (3 ¼ 8.40 ꢁ 103) and 296 (3 ¼ 4.63 ꢁ 103)
nm (in II). Based on the extinction co-efficient values, these
characteristic peaks were designated as transition between
electronic energy levels, 4 / 4*, p / p* and n / p*,
respectively. In complexes, these prominent bands are also
observed with slight variations in their positions. In addition,
a new band of medium intensity appears around 400 nm, which
is assigned to a ligand to metal charge transfer (LMCT) band
originating from electrons movement from lled p-orbital of
ligand to the vacant d-orbital of the metal ion of proper
symmetry.21
Bond lengths
1a
2
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–N(1)
Mo(1)–O(5)
1.9616(18)
1.9473(18)
1.6992(18)
1.695(2)
2.259(2)
2.2869(19)
1.9489(12)
1.9242(12)
1.6998(12)
1.7070(12)
2.2483(13)
Mo(1)–O(1M)
2.2831(12)
Bond angles
1a
2
O(3)–Mo(1)–O(4)
O(3)–Mo(1)–O(2)
O(4)–Mo(1)–O(2)
O(3)–Mo(1)–O(1)
O(4)–Mo(1)–O(1)
O(2)–Mo(1)–O(1)
O(3)–Mo(1)–N(1)
O(4)–Mo(1)–N(1)
105.24(10)
105.48(6)
98.06(6)
99.48(6)
97.83(6)
95.11(6)
154.69(5)
158.74(6)
95.76(6)
79.04(5)
79.00(5)
85.48(5)
168.59(6)
81.77(5)
79.97(5)
73.26(5)
124.85(11)
99.29(9)
97.78(9)
98.37(8)
95.01(9)
154.61(8)
161.67(9)
93.07(9)
78.63(7)
78.86(7)
86.69(9)
168.02(8)
81.07(8)
81.89(7)
74.99(7)
O(2)–Mo(1)–N(1)
O(1)–Mo(1)–N(1)
O(3)–Mo(1)–O(5)/O(1M)
O(4)–Mo(1)–O(5)/O(1M)
O(2)–Mo(1)–O(5)/O(1M)
O(1)–Mo(1)–O(5)/O(1M)
N(1)–Mo(1)–O(5)/O(1M)
C(1M)–O(1M)–Mo(1)
The 1H NMR spectra of the ligands (Table 5) exhibit two
singlet at d ¼ 11.44 and 10.51 ppm (in I), and at d ¼ 10.90 and
10.07 ppm (in II) due to phenolic (–OH) protons. The absence of
these signals in [MoVIO2]2+ complexes (Fig. 5 for representative
spectra) are in agreement with the subsequent replacement of H
by the metal ion. Thus, 1H NMR data supports the coordination
of phenolic oxygen while IR spectral data conrms the coordi-
nation of ring nitrogen. The methyl and alcoholic protons of the
coordinated methanol in complexes appear at d ¼ 3.21–
3.23 ppm and at d ¼ 3.92–3.93 ppm, respectively. The charac-
teristic broad peak for carboxylic proton was observed at d ¼
12.00, 13.05 and 13.20 ppm in I, 1 and 3, respectively.
Steric requirements, derived from coordination sphere
around Mo and the ligand structure, prevent p–p interactions
between the phenol and phenyl rings in the two compounds.
Hydrogen bonds between methanol molecules and terminal
oxygen atom, O(4), of the complex determine the ordering of the
structure in the crystal packing of 2 (see Fig. 3 and Table 3).
Spectroscopic study
Comparison of the 13C NMR spectral data of ligand with the
corresponding complexes also provides useful information for
the elucidation of the structure of the complexes. Table 6
The [MoVIO2]2+ complexes are dominated by two prominent
peaks at 924–937 and 904–906 cmꢀ1 which are assigned to
nasym(O]Mo]O) and nsym(O]Mo]O) modes, respectively due
to cis-[MoVIO2] structure.20 The n(Mo]O) in [MoVIO(O2)]2+
complexes appears at 936–949 cmꢀ1 along with three new peaks
at 853–855 cmꢀ1 due to n(O–O), 616–624 cmꢀ1 due to nasym
[Mo(O2)] and at 568–587 cmꢀ1 due to nsym[Mo(O2)] mode.
-
Table 4 UV-vis spectral data of ligands and complexes
Compounds
lmax/nm (3/Mꢀ1cmꢀ1
)
The band appearing at 1583 cmꢀ1 (in I) or 1591 cmꢀ1 (in II)
due to n(C]N) stretch moves to lower wave numbers and
appears at 1573–1588 cmꢀ1 (in 1 and 3) or 1574–1590 cmꢀ1 (in 2
H2L1 I
301 (4.87 ꢁ 103), 249 (8.76 ꢁ 103),
207 (1.88 ꢁ 104)
H2L2 II
296 (4.63 ꢁ 103), 246 (8.40 ꢁ 103),
208 (1.97 ꢁ 104)
[MoVIO2(L1)(MeOH)] 1
[MoVIO2(L2)(MeOH)] 2
[MoVIO(O2)(L1)(MeOH)] 3
[MoVIO(O2)(L2)(MeOH)] 4
412 (2.24 ꢁ 103), 328 (9.65 ꢁ 103),
245 (2.06 ꢁ 104), 210 (4.42 ꢁ 104)
421 (2.59 ꢁ 103), 325 (9.09 ꢁ 103),
246 (1.74 ꢁ 104), 209 (3.66 ꢁ 104)
407 (2.03 ꢁ 103), 299 (6.63 ꢁ 103),
245 (1.11 ꢁ 104), 206 (2.12 ꢁ 104)
415 (2.76 ꢁ 103), 294 (5.43 ꢁ 103),
246 (9.80 ꢁ 103), 207 (2.31 ꢁ 104)
Table 3 Hydrogen bonds for [MoVIO2(L2)(MeOH)]$MeOH 2a
D–H/A
d(D–H)
d(H/A)
d(D/A)
:(DHA)
O(1M)–H(1M)/O(2M)
O(2M)–H(2M)/O(4)#1
0.75(2)
0.70(3)
1.90(2)
2.05(3)
2.6489(19)
2.7423(18)
172(3)
172(3)
a
Symmetry transformations used to generate equivalent atoms: #1 x + 1,
y, z.
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Fig. 4 (a) Electronic spectra of [MoVIO2]2+- and [MoVIO(O2)]2+-complexes with ligand (H2L1) recorded in methanol {conc. of complex 1 : 2.6 ꢁ
10ꢀ4 M and conc. of complex 3: 2.9 ꢁ 10ꢀ4 M}. (b) Inset shows LMCT band of complex 3 (conc. 5.7 ꢁ 10ꢀ5 M) recorded in methanol. (c) Electronic
spectra of [MoVIO2]2+- and [MoVIO(O2)]2+-complexes with ligand (H2L2) recorded in methanol {conc. of complex 2: 3.4 ꢁ 10ꢀ4 M and conc. of
complex 4: 2.7 ꢁ 10ꢀ4 M}. (d) Inset shows LMCT band of complex 4 (conc. 6.4 ꢁ 10ꢀ5 M) recorded in methanol.
provides 13C NMR spectral data of ligands and their [MoVIO2]2+ Ag/AgCl at room temperature. Relevant data are presented in
and [MoVIO(O2)]2+ complexes; 13C NMR spectra of H2L1 and Table 7 and representative cyclic voltammograms of [MoVIO2-
[MoVIO2(L1)(MeOH)] 1 are given in Fig. 6. Assignments of the (L1)(MeOH)] 1 and [MoVIO2(L2)(MeOH)] 2 recorded in DMF at
peaks are based on the coordination-induced shis [Dd ¼ room temperature are presented in Fig. 7. The voltammograms
d(complex) – d(free ligand)] of the signals for carbon atoms in the of the complexes exhibit two reductive responses within the
vicinity of the coordinating atoms22 and Chemdarw® program. potential window ꢀ0.38 V to ꢀ1.68 V, which are ascribed to
Ligands I and II display 11 and 10 13C NMR signals corre- MoVI/MoV and MoV/MoIV processes, respectively.23–26 Reductive
sponding to 21 and 20 carbon atoms, respectively. All expected responses corresponding to MoVI/MoV and MoV/MoIV processes
signals are also present in complexes. A large coordination in the polar aprotic solvent are irreversible in nature.26 Both
induced shi of the signals for the carbon atoms associated with reduction processes may be exemplied as a metal-centered
phenolic oxygen (C2/C20) and triazole nitrogen (C7/C70) (see one-electron transfer involving the MoVI, MoV and MoIV oxida-
Table 6) conrms the coordination of these functionalities to the tion states.26–28 The lack of anodic response, at a higher scan
molybdenum. The signals for carbon atoms (C1/C10) closure to rate, is possibly due to rapid decomposition of the reduced
the above carbons are only slightly affected. The signal due to species.23–26,28 The more negative the cathodic reduction
methyl carbons (C14/C15) of coordinated methanol was potential, the more difficult for the [MoVIO2]2+ complexes to be
observed at d ¼ 49.12–53.18 ppm in all complexes.
reduced. The electron withdrawing group in the ligand leads to
the shiing of the cathodic reduction potential in [MoVIO2]2+
complex towards less negative value.29
Electrochemical study
Cyclic voltammetry measurements were carried out to investi-
gate the redox stability of complexes in solution state. Voltam-
Catalytic activity study
mograms of redox active [MoVIO2]2+- and [MoVIO(O2)]2+
-
Using molybdenum complexes as catalyst precursors, oxidation
complexes (1–4) were recorded in the range 1.8 V to ꢀ1.8 V vs. of bicyclic alcohols (isoborneol and fenchyl alcohol), cyclic
Table 5 1H NMR spectral data of ligands and complexesa
Compounds
–OH (phenolic)
Aromatic protons
I
11.44 (s, 1H), 10.51 (s, 1H)
10.90 (s,1H), 10.07 (s,1H)
12.00 (br, 1H), 8.06–8.03 (d, 1H) (J ¼ 12 Hz), 7.87–7.85 (d, 2H) 7.83–7.81 (t, 2H),
7.56–7.55 (d, 1H), 7.46–7.43 (t, 2H), 7.05–7.01 (m, 3H) (J ¼ 12 Hz), 6.88–6.87 (d, 1H)
8.04–8.02 (dd, 1H) (J ¼ 8 Hz), 7.48–7.42 (m, 6H) (J ¼ 24 Hz), 7.37–7.34 (t, 2H), 7.02–
6.98 (t, 2H), 6.95–6.92 (d, 1H), 6.87–6.85 (d, 1H)
13.05 (br, 1H), 8.23–8.19 (d, 1H) (J ¼ 16 Hz), 8.11–8.09 (d, 2H), 7.88–7.83 (t, 2H),
7.47–7.44 (t, 2H), 7.39–7.32 (m, 3H) (J ¼ 28 Hz), 6.98–6.95 (d, 1H), 6.80–6.77 (d, 1H)
8.23–8.18 (d, 2H) (J ¼ 20 Hz), 8.12–8.09 (dd, 1H) (J ¼ 12 Hz), 7.88–7.81 (m, 4H) (J ¼
28 Hz), 7.47–7.43 (t, 2H), 7.09–7.05 (t, 2H), 6.98–6.95 (d, 1H), 6.80–6.76 (d, 1H)
13.20 (br, 1H), 8.15–8.11 (d, 1H) (J ¼ 16 Hz), 8.02–7.98 (d, 2H), 7.89–7.84 (t, 2H),
7.51–7.54 (d, 1H), 7.41–7.36 (t, 2H), 7.03–6.97 (m, 3H) (J ¼ 24 Hz), 6.86–6.85 (d, 1H)
8.07–8.04 (dd, 1H) (J ¼ 12 Hz), 7.51–7.43 (m, 6H) (J ¼ 32 Hz), 7.40–7.37 (t, 2H), 7.05–
7.01 (t, 2H), 6.97–6.94 (d, 1H), 6.85–6.84 (d, 1H)
II
1
2
3
4
a
Letters given in parentheses indicate the signal structure: s ¼ singlet, d ¼ doublet, dd ¼ doublet of doublet, m ¼ multiplet and br ¼ broad.
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Fig. 5 1H NMR spectra of [MoVIO2(L2)(MeOH)] 2 and H2L2 II. Signal with star (*) is due to protons of water/methyl groups of DMSO.
(cyclohexanol) and aromatic alcohols (benzyl alcohol and cumic (0.05 g, 0.0005 mol) at three different temperatures (60, 70 and
alcohol) have been carried out under atmospheric conditions. 80 ꢂC). Fig. 8 and Table 8 summarize all the conditions and the
Effect of N-based additive (i.e. NEt3) on the rate of reaction and conversion obtained under a particular set of conditions. It is
conversion has also been checked.
clear from the Table 8 that the optimized reaction conditions
Oxidation of isoborneol. Catalytic oxidation of isoborneol, for 0.010 mol (1.54 g) of isoborneol are: catalyst 1 (0.002 g, 3.8
using [MoVIO2]2+ complexes prepared here in the presence of ꢁ 10ꢀ6 mol), 30% H2O2 (3.39 g, 0.030 mol), MeCN (5 mL), NEt3
H2O2, gives camphor (1,7,7-trimethylbicyclo[2.2.1]heptan-2- (0.05 g, 0.0005 mol) and reaction temperature (80 ꢂC) (i.e. entry
one) selectively; Scheme 3. Considering 1 as a representative no. 2 of Table 8). Under these conditions, reaction requires 6 h
catalyst precursor, parameters like, amounts of catalyst, oxidant to attain equilibrium and gave a maximum of 75% conversion.
and solvent and temperature of the reaction mixture have been In the absence of NEt3 under above reaction conditions,
optimized for the maximum oxidation of substrate.
almost similar conversion (66–80%) was obtained but ca. 24 h
Thus, for 0.010 mol (1.54 g) of isoborneol; three different was required to achieve the equilibrium. In the absence of
amounts of catalyst precursor (i.e. 0.001, 0.002 and 0.003 g) catalyst as well as NEt3, only 5% conversion were achieved in
and aqueous 30% H2O2 (0.020, 0.030 and 0.040 mol) were 24 h while in the absence of catalyst but in the presence of
taken in three different volumes of solvent (MeCN: 5, 7 and 9 NEt3, a maximum of 14% conversion was obtained in 6 h of
mL) and the reaction was carried out in the presence of NEt3 reaction.
Table 6 13C NMR spectral data of ligands and complexes
Compoundsa
C15
C1/C10
C2/C20
C7/C70
Ar-C
H2L1 I
119.74
156.93
164.55
167.09, 134.35, 130.77, 130.40, 123.43,
123.01, 119.05, 117.20
166.72, 133.66, 132.48, 130.78, 127.98,
123.61, 120.17, 116.93
166.98, 133.13, 132.03, 130.73, 127.33,
123.98, 120.02, 116.65
131.31, 129.19, 128.66, 126.62, 123.75,
119.23, 116.05
132.61, 131.65, 129.28, 127.20, 121.05,
120.36, 115.43
132.82, 131.52, 129.71, 127.15, 121.28,
119.77, 115.08
[MoVIO2(L1)(MeOH)] 1 (Dd)
[MoVIO(O2)(L1)(MeOH)] 3 (Dd)
H2L2 II
52.93
49.12
120.40 (0.66)
120.25 (0.51)
119.66
155.30 (ꢀ1.63)
155.23 (ꢀ1.70)
156.30
165.61 (1.06)
165.76 (1.21)
159.53
[MoVIO2(L2)(MeOH)] 2 (Dd)
[MoVIO(O2)(L2)(MeOH)] 4 (Dd)
53.18
51.74
120.57 (0.91)
120.39 (0.73)
154.43 (ꢀ1.87)
154.39 (ꢀ1.91)
160.62 (1.09)
160.71 (1.18)
a
For numbering of carbons see Fig. 6.
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Fig. 6 13C NMR spectra of [MoVIO2(L1)(MeOH)] 1 and H2L1 I. Signal with star (*) is due to methyl carbon of DMSO.
Table 7 Cyclic voltammetric results for dioxido- and oxidoperox-
idomolybdenum(VI) complexes at 298 K
Complexes
Epca [V]
[MoVIO2(L1)(MeOH)] 1
[MoVIO2(L2)(MeOH)] 2
[MoVIO(O2)(L1)(MeOH)] 3
[MoVIO(O2)(L2)(MeOH)] 4
ꢀ0.38, ꢀ1.01
ꢀ1.15, ꢀ1.44
ꢀ1.27, ꢀ1.68
ꢀ0.96, ꢀ1.41
Fig.
7
Cyclic voltammograms of [MoVIO2(L1)(MeOH)]
1
and
a
Solvent: DMF; working electrode: platinum disc; counter electrode:
platinum wire; reference electrode: Ag/AgCl; supporting electrolyte:
0.1 M TBAP; scan rate: 0.1 V sꢀ1, quiet time: 2 s, temperature: 298 K.
Potential range: 1.8 to ꢀ1.8 V. Epc is the cathodic peak potential.
[MoVIO2(L2)(MeOH)] 2 recorded in DMF at room temperature.
Under the optimized reaction conditions, catalytic potentials
of complexes 2, 3 and 4 have also been tested for the oxidation
of isoborneol and fenchyl alcohol, and results are summarized
in Table 9. With 71–82% conversion of isoborneol, the catalytic
potentials of 2, 3 and 4 are equally good and comparable to 1. A
slightly lower conversion (69–76%) than isoborneol has been
obtained for fenchyl alcohol with 2, 3 and 4 but their catalytic
potentials are again comparable to 1 for this particular reaction.
However, within these complexes, catalytic potentials of
[MoVIO(O2)]2+ complexes are little better than [MoVIO2]2+
complexes. This is expected as [MoVIO2]2+ complexes have to
pass through an active intermediate species i.e. [MoVIO(O2)]2+
complexes by their reaction with H2O2 while peroxide
complexes can start catalytic activity instantly.
Oxidation of fenchyl alcohol. Molybdenum complexes 1, 2, 3
and 4 also catalyze the oxidation of fenchyl alcohol to fenchone
(1,3,3-trimethylbicyclo[2.2.1]heptan-2-one) selectively (Scheme 4).
Again various parameters as mentioned above have been taken
into account to optimize the reaction conditions while taking 1 as
a catalyst precursor. Reactions were carried out in the presence as
well as in the absence of additive.
As presented in Table S2 and Fig. S2,† the optimized reaction
conditions for the oxidation of 0.010 mol (1.54 g) of fenchyl
alcohol are: catalyst 1 (0.003 g, 5.6 ꢁ 10ꢀ6 mol), oxidant 30%
H2O2 (3.39 g, 0.030 mol), N-based additive NEt3 (0.05 g,
0.0005 mol), MeCN (5 mL) and reaction temperature (80 ꢂC) (i.e.
entry no. 3 of Table S2†). These conditions are very similar to
the one already concluded for the oxidation of isoborneol.
Under these conditions reaction requires 6 h to attain equilib-
rium and give a maximum of 74% conversion. In the absence of
catalyst as well as NEt3, only 19% conversion was achieved in
24 h and in the absence of catalyst but in the presence of NEt3,
only 13% conversion was obtained in 6 h.
Scheme 3 Oxidation of isoborneol to camphor.
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Fig. 8 (a) Effect of variation of amount (0.001, 0.002 and 0.003 g) of catalyst 1 on the oxidation of isoborneol. Reaction conditions: isoborneol
(1.54 g, 0.010 mol), 30% H2O2 (3.39 g, 0.030 mol), MeCN (5 mL), NEt3 (0.05 g, 0.0005 mol) and reaction temp (80 ꢂC) for 6 h. (b) Effect of amount
of oxidant (30% H2O2) (substrate to oxidant ratio: 1 : 2, 1 : 3 and 1 : 4) on the oxidation of isoborneol. Reaction conditions: isoborneol (1.54 g,
0.010 mol), 1 (0.002 g), MeCN (5 mL), NEt3 (0.05 g, 0.0005 mol) and reaction temp (80 ꢂC) for 6 h. (c) Effect of variation of amount of solvent (5, 7
and 9 mL) on the rate of the oxidation of isoborneol. Reaction conditions: isoborneol (1.54 g, 0.010 mol), 30% H2O2 (3.39 g, 0.030 mol), 1
(0.002 g), NEt3 (0.05 g, 0.0005 mol) and reaction temp (80 ꢂC) for 6 h. (d) Effect of different temp (60, 70 and 80 ꢂC) on the oxidation of
isoborneol. Reaction conditions: isoborneol (1.54 g, 0.010 mol), 1 (0.002 g), 30% H2O2 (3.39 g, 0.030 mol), MeCN (5 mL) and NEt3 (0.05 g,
0.0005 mol) for 6 h.
Catalytic potentials of these complexes have also been tested alcohol and cumic alcohol. While benzyl alcohol gives three
under optimized conditions in the absence of N-based additive products (benzaldehyde, benzoic acid and benzylbenzoate),
(NEt3) (Table 9). In the absence of N-based additive (NEt3), 68, cumic alcohol gives cuminaldehyde selectively (Scheme 5). All
66, 80 and 73% conversion of isoborneol with catalyst 1, 2, 3 and
4, respectively were obtained in 24 h of reaction time. Similarly,
63–70% conversion of fenchyl alcohol was obtained in 24 h with
these catalysts in the absence of N-based additive. Thus,
N-based additive reduces the reaction time for oxidation of
isoborneol as well as fenchyl alcohol from 24 h to 6 h.
Oxidation of benzyl alcohol and cumic alcohol. We have also
optimized reaction conditions for the oxidation of benzyl
Scheme 4 Oxidation of fenchyl Alcohol to fenchone.
Table 8 Conversion of isoborneol (1.54 g, 0.010 mol) using [MoVIO2(L1)(MeOH)] 1 as catalyst precursor in presence of NEt3 (0.05 g, 0.0005 mol)
in 6 h of reaction time under different reaction conditions
Entry no.
Catalyst [g (mmol)]
H2O2 [g (mol)]
MeCN [mL]
Temp. [ꢂC]
Conv. [%]
1
2
3
4
5
6
7
8
9
0.001 (1.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.003 (5.6 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
)
)
)
)
)
)
)
)
)
3.39 (0.030)
3.39 (0.030)
3.39 (0.030)
2.26 (0.020)
4.52 (0.040)
3.39 (0.030)
3.39 (0.030)
3.39 (0.030)
3.39 (0.030)
5
5
5
5
5
7
9
5
5
80
80
80
80
80
80
80
70
60
66
75
78
68
80
72
63
69
59
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Table 9 Oxidation of different alcohols and TOF's using different molybdenum(VI) complexes as catalyst precursor
With additivea
Conv. [%]
Without additive
Substrates
Catalyst (conc. in mmol)
TOFb [hꢀ1
]
Conv. [%]
TOFc [hꢀ1
]
Isoborneol (0.77 g, 5 mmol)
1 (3.8 ꢁ 10ꢀ3
2 (3.8 ꢁ 10ꢀ3
3 (3.8 ꢁ 10ꢀ3
4 (3.8 ꢁ 10ꢀ3
1 (5.6 ꢁ 10ꢀ3
2 (5.6 ꢁ 10ꢀ3
3 (5.6 ꢁ 10ꢀ3
4 (5.6 ꢁ 10ꢀ3
)
)
)
)
)
)
)
)
75
71
82
79
74
69
76
71
328
311
359
346
217
205
226
211
68
66
80
73
67
63
70
68
74
71
87
79
49
47
52
50
Fenchyl alcohol (0.77 g, 5 mmol)
a
NEt3 ¼ 0.05 mmol. b TOF values calculated at 6 h of reaction time for both substrates. c TOF values calculated at 24 h of reaction time.
experimental details are presented in Tables S3, S4, Fig. S3 and cumic alcohol and results are presented in Table 10. In the
S4.† The optimized reaction conditions for the maximum presence of additive (NEt3), conversions of these substrates are
oxidation of both alcohols are same [i.e. for 0.005 mol (0.54 g) of 61–74% and 53–68%, respectively while in the absence of
benzyl alcohol or 0.005 mol (0.75 g) of cumic alcohol: catalyst 1 additive, the conversions are 54–68% and 48–61%, respectively.
(0.002 g, 3.8 ꢁ 10ꢀ6 mol), 30% aqueous H2O2 (1.13 g, Though catalytic potential of these complexes towards the
0.010 mol), MeCN (5 mL) and NEt3 (0.05 g, 0.0005 mol) at 80 ꢂC; oxidation of both substrates are not very high, the time required
entry no. 2 of Table S3† for benzyl alcohol and entry no. 4 of to achieve the equilibrium in the absence and in the presence of
Table S4† for cumic alcohol] along with the conversion of additive (i.e. 24 h vs. 5–8 h), suggests that additive plays an
alcohols.
important role in improving the catalytic efficiency of
Under the optimized reaction conditions other catalysts have complexes.
also been tested for the oxidation of benzyl alcohol (for the
Oxidation of cyclohexanol. The oxidation of cyclohexanol
selectivity of different reaction products see Table S5†) and catalyzed by complexes 1, 2, 3 and 4 using H2O2 as oxidant in
the presence of NEt3 gave cyclohexanone selectively (Scheme 6).
Using complex 1, the best suited reaction conditions, aer
various trials (Table 11, Fig. S5†), concluded for the maximum
oxidation of 0.005 mol (0.5 g) of cyclohexanol (entry no. 2 of
Table 11) are: [MoO2(L1)(MeOH)] 1 (0.002 g, 3.8 ꢁ 10ꢀ6 mol),
30% aqueous H2O2 (1.69 g, 0.015 mol), MeCN (5 mL), NEt3
ꢂ
(0.05 g, 0.0005 mol) at 80 C. About 4 h was required to attain
Scheme 5 Oxidation of benzyl alcohol to (a) benzaldehyde, (b) ben-
zoic acid, (c) benzylbenzoate, and oxidation of cumic alcohol to
cuminaldehyde.
Scheme 6 Oxidation of cyclohexanol to cyclohexanone.
Table 10 Oxidation of different aromatic alcohols and TOF's using different molybdenum(VI) complexes as catalyst precursor
With additivea
Conv. [%]
Without additive
Conv. [%]
Substrates
Catalysts (conc. in mmol)
TOFb [hꢀ1
]
TOFc [hꢀ1
]
Benzyl alcohol (0.54 g, 5 mmol)
1 (3.8 ꢁ 10ꢀ3
2 (3.8 ꢁ 10ꢀ3
3 (3.8 ꢁ 10ꢀ3
4 (3.8 ꢁ 10ꢀ3
1 (3.8 ꢁ 10ꢀ3
2 (3.8 ꢁ 10ꢀ3
3 (3.8 ꢁ 10ꢀ3
4 (3.8 ꢁ 10ꢀ3
)
)
)
)
)
)
)
)
66
61
74
69
59
53
68
60
173
160
194
181
97
87
111
99
59
54
68
63
55
48
61
57
32
29
37
34
30
26
34
29
Cumic alcohol (0.75 g, 5 mmol)
a
b
c
NEt3 ¼ 0.05 mmol. TOF values calculated at 5 h and 8 h of reaction time for benzyl alcohol and cumic alcohol, respectively. TOF values
calculated at 24 h of reaction time.
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Table 11 Conversion of cyclohexanol (0.5 g, 0.005 mol) using [MoVIO2(L1)(MeOH)] 1 as catalyst precursor in the presence of NEt3 (0.05 g,
0.0005 mol) in 4 h of reaction time under different reaction conditions
Entry no.
Catalyst [g (mmol)]
H2O2 [g (mmol)]
MeCN [mL]
Temp. [ꢂC]
Conv. [%]
1
2
3
4
5
6
7
8
9
0.001 (1.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.003 (5.6 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
0.002 (3.8 ꢁ 10ꢀ3
)
)
)
)
)
)
)
)
)
1.69 (15)
1.69 (15)
1.69 (15)
1.13 (10)
2.26 (20)
1.69 (15)
1.69 (15)
1.69 (15)
1.69 (15)
5
5
5
5
5
7
9
5
5
80
80
80
80
80
80
80
70
60
45
53
51
44
56
50
47
49
51
Fig. 9 (a) Spectral changes recorded after successive addition of one drop portion of 30% H2O2 (0.053 g, 0.47 mmol) dissolved in 10 mL of MeOH
(final concentration of H2O2; 4.6 ꢁ 10ꢀ2 M) to 20 mL of (4.3 ꢁ 10ꢀ5 M) solution of [MoVIO2(L1)(MeOH)] 1. (b) Inset: spectral changes with 30% H2O2
(0.064 g, 0.56 mmol) dissolved in 10 mL of methanol (final concentration of H2O2; 6.1 ꢁ 10ꢀ2 M) to 20 mL of methanolic solution (7.8 ꢁ 10ꢀ5 M) of
complex 1. (c) Spectral changes with 30% H2O2 (0.062 g, 0.55 mmol) dissolved in 10 mL of methanol (final concentration of H2O2; 5.4 ꢁ 10ꢀ2 M) to
20 mL of methanolic solution (6.0 ꢁ 10ꢀ5 M) of [MoVIO2(L2)(MeOH)] 2. (d) Inset: spectral changes with 30% H2O2 (0.069 g, 0.61 mmol) dissolved in
10 mL of methanol (final concentration of H2O2; 5.6 ꢁ 10ꢀ2 M) to 20 mL of methanolic solution (8.1 ꢁ 10ꢀ5 M) of complex 2.
Table 12 Oxidation of cyclic alcohol and TOF's using different molybdenum(VI) complexes as catalyst precursor
With additivea
Conv. [%]
Without additive
Conv. [%]
Substrates
Catalysts (conc. in mmol)
TOFb [hꢀ1
]
TOFc [hꢀ1
]
Cyclohexanol (0.50 g, 5 mmol)
1 (3.8 ꢁ 10ꢀ3
2 (3.8 ꢁ 10ꢀ3
3 (3.8 ꢁ 10ꢀ3
4 (3.8 ꢁ 10ꢀ3
)
)
)
)
53
50
61
57
174
164
201
187
47
42
55
51
26
23
30
28
NEt3 ¼ 0.05 mmol. b TOF values calculated at 4 h of reaction time. c TOF values calculated at 24 h of reaction time.
a
the equilibrium with 53% conversion and TOF value of 174 hꢀ1
.
ligands I and II (see experimental). The generation of such
Complex 2, 3 and 4 gave 50% (TOF ¼ 164 hꢀ1), 61% (TOF ¼ species has also been established in methanol by reacting
201 hꢀ1) and 57% (TOF ¼ 187 hꢀ1) conversion, respectively in 4 [MoVIO2(L1)(MeOH)] 1 and [MoVIO2(L2)(MeOH)] 2 with H2O2
h of reaction time. Without additive, as observed above also, it and monitoring progress of the reaction by electronic absorp-
requires 24 h and the obtained conversions are 47% (TOF ¼ 26 tion spectroscopy. In a typical reaction, 20 mL of 4.3 ꢁ 10ꢀ5
M
hꢀ1), 42% (TOF ¼ 23 hꢀ1), 55% (TOF ¼ 30 hꢀ1) and 51% (TOF ¼ solution of 1 was treated with one drop portion of 30% aqueous
28 hꢀ1) for complexes 1, 2, 3 and 4, respectively (Table 12). In the H2O2 (0.053 g, 0.47 mmol) dissolved in 10 mL of methanol and
absence of catalyst, conversion obtained was only 10%.
the resultant spectroscopic changes are presented in Fig. 9.
Thus, the band at 210 nm shows considerable increase in
intensity and slightly shis to 220 nm. Simultaneously, the
intensity of bands at 245 and 298 nm increases along with
a generation of isosbestic point at 313 nm; Fig. 9(a). In the
presence of excess of H2O2 the 245 band slowly disappears;
Reactivity of dioxidomolybdenum(VI) complexes with H2O2
and possible reaction intermediate
The [MoVIO(O2)]2+ complexes have been isolated by reacting in
situ generated oxidoperoxidomolybdenum(VI) species with
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Fig. 9(a). Considering higher concentration of 1 (7.8 ꢁ 10ꢀ5 M)
where 412 nm band is visible and adding drop wise portions of
6.1 ꢁ 10ꢀ2 M H2O2 causes slow decrease of 412 nm band along
with the generation of an isosbestic point at 313 nm; Fig. 9(b).
This band nally disappears up on the addition of excess of
H2O2 solution. Complex 2 presents very similar spectral
changes upon treating with a solution of H2O2 in methanol. The
nal spectral patterns {see Fig. 9(c) and (d)} are similar to the
one recorded for the peroxido complexes 3 and 4 indicating the
formation of similar peroxido complexes in solution upon the
addition of H2O2 to complexes 1 and 2, respectively.
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Conclusions
The [MoVIO2]2+ complexes, [MoVIO2(L1)(MeOH)] 1 and [MoVIO2-
(L2)(MeOH)] 2, and their corresponding [MoVIO(O2)]2+ complexes,
[MoVIO(O2)(L1)(MeOH)] 3 and [MoVIO(O2)(L2)(MeOH)] 4 have
been prepared from potential tridentate ligands, 4-[3,5-bis(2-
hydroxyphenyl)-1,2,4-triazol-1-yl]benzoic acid (H2L1, I) and 3,5-
bis(2-hydroxyphenyl)-1-phenyl-1,2,4-triazole (H2L2, II) and char-
acterized. The single crystal X-ray study of 1a (i.e. DMSO coordi-
nated in place of methanol) and 2 conrms the dibasic tridentate
behavior of ligands. These complexes emerge out as efficient and
selective homogenous catalysts for the oxidation of bicyclic, cyclic
and aromatic alcohols in MeCN in the presence of N-based
additive (NEt3) where about 4–8 h was required to achieve the
equilibrium. However, in the absence of additive reactions take
about 24 h to reaching equilibrium. The N-based additive, NEt3,
abstracts hydrogen from H2O2 due to its strong basic nature
which in turns accelerates the formation of peroxido interme-
diate complex. This way it reduces the time period of the catalytic
reaction with equally good conversion.30 Thus, the additive plays
an important role in improving the catalytic efficiency of
complexes and reduces the time of oxidation considerably (24 h
to 4–8 h). Amongst the two type of complexes studied, the cata-
lytic potentials of [MoVIO(O2)]2+ complexes are slightly better than
[MoVIO2]2+ complexes. Reactivity of the [MoVIO2]2+ complexes
with H2O2 provides evidence for the possible peroxido interme-
diate species formation during the catalytic action.
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