Molybdenum complexes with a μ-O{MoO2}2 core: their synthesis, crystal structure and application as catalysts for the oxidation of bicyclic alcohols using N-based additives

Article abstract of DOI:10.1039/c6nj03162e

Three binuclear μ-oxidobis{dioxidomolybdenum(vi)} complexes, [(μ-O){Mo^VIO₂(MeOH)}₂(L¹)] 1, [(μ-O){Mo^VIO₂(MeOH)}₂(L²)] 2, and [(μ-O){Mo^VIO₂(MeOH)}₂(L³)] 3 with potential dibasic tetradentate azine ligands [H₂L¹ = azine of salicylaldehyde, H₂L² = azine of 3-methoxysalicyaldehyde and H₂L³ = azine of 3,5-ditertiarybutylsalicylaldehyde] are reported. The complexes are characterized via elemental analysis, various spectroscopic techniques (FT-IR, UV-Vis, ¹H and¹³C NMR), thermogravimetric analysis and single-crystal X-ray diffraction (2 and 3) studies. These complexes are binuclear having a μ-O{MoO₂}₂ core where ligands coordinate through a set of phenolic oxygen and azine nitrogen to each molybdenum, thus behaving as bis(bidentate). These complexes are investigated as catalysts for the oxidation of two bicyclic alcohols (fenchyl alcohol and isoborneol) using 30% H₂O₂ as the oxidant in the presence of NEt₃ as the additive. The conversion of bicyclic alcohols within the additives follows the order: NEt₃ > py > NH₃ > K₂CO₃ > KOH. In the presence of an N-based additive, fenchyl alcohol and isoborneol selectively give high yields of fenchone and camphor, respectively, while in the absence of the additive the conversion is relatively low.

Full text of DOI:10.1039/c6nj03162e

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Page 1 of 26
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Molybdednum complexes having µ-O{MoO } core: Synthesis, crystal
DOI: 10.1039/C6NJ03162E
2 2
structure and application as catalyst for the oxidation of bicyclic alcohols
using N-based additives
M.R. Maurya, L. Rana and F. Avecilla
Binuclear molybdenum(VI) complexes having µ-O{MoO2}2 core as catalyst for the oxidation
of bicyclic alcohols (fenchyl alcohol and isoborneol) using 30% H2O2 as oxidant in the
presence of NEt3 as additive are reported.
1

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Molybdednum complexes having µ-O{MoO } core: Synthesis, crystal
DOI: 10.1039/C6NJ03162E
2 2
structure and application as catalyst for the oxidation of bicyclic alcohols
using N-based additives†
Mannar R. Maurya,a* Lata Ranaa and Fernando Avecillab
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India.
E-mail: rkmanfcy@iitr.ac.in; Fax: +91 1332 273560; Tel: +91 1332 285327
bDepartamento de Química Fundamental, Universidade da Coruña, Campus de A Zapateira,
15071 A Coruña, Spain
† Electronic supplementary information (ESI) available: CCDC 1508583 for complex 2a and
1508582 for 3a. Spectral data and representative 1H NMR spectra of ligand and complex. For
ESI and crystallographic data in CIF or other electronic format see DOI: xxxxxxxx
((Abstract))
Three
binuclear
µ-oxidobis{dioxidomolybdenum(VI)}
1,
[(µ-O){MoVIO2(MeOH)}2(L2)]
complexes,
2, and
[(µ-
[(µ-
O){MoVIO2(MeOH)}2(L1)]
O){MoVIO2(MeOH)}2(L3)] 3 with potential dibasic tetradentate azine ligands [H2L1 = azine
of salicyaldehyde, H2L2 = azine of 3-methoxysalicyaldehyde and H2L3 = azine of 3,5-
ditertiarybutylsalicylaldehyde] are reported. The complexes are characterized by elemental
analysis, various spectroscopic techniques (FT-IR, UV-Vis, 1H and 13C NMR),
thermogravimetric and by single-crystal X-ray diffraction (2 and 3) studies. These complexes
are binuclear having µ-O{MoO2}2 core where ligands coordinate through a set of phenolic
oxygen and azine nitrogen to each molybdenum, thus behave as bis (bidentate). These
complexes are investigated as catalyst for the oxidation of two bicyclic alcohols (fenchyl
alcohol and isoborneol) using 30% H2O2 as oxidant in the presence of NEt3 as additive. The
conversion of bicyclic alcohols within the additives follows the order: NEt3 > py > NH3 >
K2CO3 > KOH. In the presence of N-based additive, fenchyl alcohol and isoborneol
selectively gives high yield of fenchone and camphor, respectively while in the absence of
additive the conversion is relatively low.
2

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DOI: 10.1039/C6NJ03162E
Introduction
Dioxomolybdenum(VI) complexes with cis-[MoVIO2] core exhibit high catalytic performance
towards the epoxidation / oxidation of various substrates viz. olefins,1 sulphides,2,3 phenol,4
phosphine,5 alcohols,6,7 hydrocarbons,8 and oxidative bromination of salicylaldehyde, styrene
and thymol,9 Such complexes also exhibit oxygen atom transfer properties and thus act as
functional mimics of molybdenum transferase enzymes which have active cis-[MoVIO2]
core.10 During past few years high nuclearity oxidomolybdenum complexes containing (a) µ-
O{MoO}2,11 (b) (µ-O)2{MoO}2,12 (c) µ-O{MoO2}2,13-15 and (d) (µ-O)2{MoO2}2,16,17 cores
(Scheme 1), and of other high nuclearity with no systematic patterns18,19 have been reported
in the literature. Modec et. al. have reported oxidomolybdenum(V) complexes with a basic
building unit of (µ-O)2{OMo–MoO} having one terminal and one bridged oxygen on each
molybdenum along with molybdenum-molybdenum bond.12(c),20 All these complexes are
effective catalysts for the epoxidation of olefins.
Scheme 1 Possible metal-oxido cores present in molybdenum complexes.
Recently our attention was drawn on dinuclear vanadium complex with azine ligand
where two vanadium ions are connected by rigid azine nitrogen atoms21 while ONO dibasic
tridentate behaviour of azines has earlier been inferred in dioxidomolybdenum(VI) complex22
and in oxidoperoxido complexes of molybdenum(VI), tungsten(VI), uranium(VI),
zirconium(IV) and thorium(IV) on the basis of spectroscopic studies.23 This prompted us to
re-consider azine ligands, I–III (Scheme 2) and prepare their molybdenum complexes.
Gonꢀalves et al. have observed that complexes of the type [MoO2Cl2L] (where L = mono or
bidentate ligands) having chloride ions coordinated to the molybdenum are potential
precursors to design a molybdenum complex of higher nuclearity.18,19 However, in the
present study, we have used [MoVIO2(acac)2] precursor to prepare molybdenum complexes
and report their complete characterization along with single crystal X-ray study of the two
complexes. In these complexes potential dibasic tetradentate ligands behave as bridging
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bis(monobasic ON bidenate) leading to hitherto rare binucleating molybdenum(VI)
DOI: 10.1039/C6NJ03162E
complexes having µ-O{MoO2}2 core.
Catalytic potential of these complexes towards the oxidation of bicyclic alcohols
(fenchyl alcohol and isoborneol) with aqueous 30% H2O2 has also been explored. Camphor
(1,7,7-trimethylbicyclo[2.2.1]heptan-2-one), an important bio product is obtained upon
oxidation of isoborneol (1,7,7-trimethylbicyclo[2.2.1]heptan-2-ol), while fenchone (1,3,3-
trimethylbicyclo[2.2.1]heptan-2-one), a main component of absinthe and the essential oil of
fennel is obtained upon oxidation of fenchyl alcohol (1,3,3-trimethylbicyclo[2.2.1]heptan-2-
ol). Catalytic oxidation of isoborneol and fenchyl alcohol have been studied7 earlier using
dioxidomolybdenum(VI) and other complexes as catalysts. The catalytic efficiency of these
catalysts could be increased by using N-based additive i.e. NEt3.7
Scheme 2 Azine ligands used in the present study.
Experimental Section
Materials
Analytical reagent grade hydrazine hydrate, triethylamine (Rankem, India), ammonium
molybdate, salicylaldehyde (Loba Chemie India), 2-hydroxy-3,5-ditertbutylsalicylaldehyde,
o-vanillin, fenchyl alcohol, isoborneol (Aldrich, USA) and 30% H2O2 (Qualizens, India) were
used as obtained. All other chemicals and solvents were of reagent grade and used without
purification. Metal precursor [MoVIO2(acac)2] was prepared as reported in the literature.24
Ligands H2L1 (I), H2L2 (II) and H2L3 (III) (Scheme 2) were synthesized following the
procedure described by Tarafder et al.23
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DOI: 10.1039/C6NJ03162E
Instrumentation and characterization procedures
Element analysis (C, H and N) of ligands and their complexes was performed on an
Elementar model Vario-EL-III. IR spectra were recorded on a Nicolet NEXUS Aligent 1100
FT-IR spectrometer. Electronic spectra were obtained in DMSO using a Shimadzu 1601 UV-
1
Vis spectrometer. H and 13C NMR spectra were obtained in DMSO-d6 using TMS as an
internal standard on JEOL 400 MHz spectrometer. Thermogravimatric analysis of complexes
was performed using oxygen environment on TG Stanton Redcroft STA 780. All catalytic
reactions were analysed using a Shimadzu 2010 plus gas-chromatograph fitted with a Rtx-1
capillary column (30 m × 0.25 mm × 0.25 μm) and a FID detector. The product identity was
confirmed by a GC-MS Perkin-Elmer, model Clarus 500 and comparing the obtained data
with electronic library.
Synthetic of complexes
[(µ-O){MoVIO2(MeOH)}2(L1)] 1. A suspension of H2L1 (0.30 g, 1 mmol) in methanol (10
mL) was added to a filtered solution of [MoVIO2(acac)2] (0.652 g, 2 mmol) in same solvent
(10 mL) and stirred initially at room temperature for 1 h and then refluxed on a water bath for
6 h where yellow solid slowly separated out. This was filtered washed with methanol and
dried in desiccator over silica gel. Yield 0.378 g (66%). (Found: C, 33.79; H, 3.14; N, 4.78%.
Calc’d. for C16H18N2O9Mo2 (574): C, 33.47; H, 3.16; N, 4.88).
[(µ-O){MoVIO2(MeOH)}2(L2)] 2. A light brown complex 2 was prepared following the
procedure described for 1 using H2L2 (0.240 g, 1 mmol) and [MoVIO2(acac)2] (0.652 g, 2
mmol). Yield 0.408 g (65%) (Found: C, 33.99; H, 3.45; N, 4.38 %. Calc’d. for
C18H22N2O11Mo2 (634): C, 34.08; H, 3.50; N, 4.42). Single crystals of 2 suitable for X-ray
analysis were grown from DMSO. Crystal of complex 2 is now formulated as [(µ-
O){MoO2(DMSO)}2(L2)] 2a.
[(µ-O){MoVIO2(MeOH)}2(L3)] 3. Complex 3 as light brown solid was prepared similarly as
described for 1 using H2L3 (0.46 g, 1 mmol) and [MoVIO2(acac)2] (0.652 g, 2 mmol). Yield
0.602 g (75%) (Found: C, 47.82; H, 6.30; N, 3.49 %. Calc’d. for C32H50N2O9Mo2 (798): C,
48.12; H, 6.31; N, 3.51). Single crystals of 3 suitable for X-ray analysis were grown from
DMF. Crystal of complex 3 is now formulated as [(µ-O){MoVIO2(DMF)}2(L3)]·3DMF 3a.
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DOI: 10.1039/C6NJ03162E
Catalytic oxidation of bicyclic alcohols (fenchyl alcohol and isoborneol) with aqueous
30% H2O2
Catalytic oxidation of bicyclic alcohols (fenchyl alcohol and isoborneol) with aqueous 30%
H2O2 were performed in a 50 mL two necked round bottom flask fitted with a reflux
condenser and constant stirring at oil bath.
Oxidation of fenchyl alcohol
In a typical reaction, fenchyl alcohol (0.308 g, 2 mmol), aqueous 30% H2O2(0.678 g, 6
mmol), and catalyst (0.002 g) were mixed in a reaction flask containing 7 mL of MeCN and
o
reaction mixture was stirred at 80 C for 6 h. The oxidation of fenchyl alcohol was studied
both in the absence and in the presence of additive i.e. NEt3 (0.005 g, 0.05 mmol). The
progress of the reaction was examined by withdrawing small aliquot of the reaction mixture
at every 1.5 h interval, extracting sample with hexane and analysing quantitatively by gas
chromatograph equipped with a FID detector. The identity of products was confirmed by GC-
MS.
Oxidation of isoborneol
Isoborneol (0.308 g, 2 mmol), aqueous 30% H2O2 (0.904 g, 8 mmol), and catalyst (0.002 g)
were mixed in a flask containing 7 mL of MeCN and the reaction mixture was stirred at 80 oC
for 6 h. Again, the reaction was performed both in the absence and in the presence of additive
i.e. NEt3 (0.005 g, 0.05 mmol). The reaction progress was monitored by withdrawing small
aliquot of the reaction mixture at every 1.5 h interval, extracting sample with hexane,
analysing quantitatively and identifying the products as mentioned above.
X-Ray crystal structure determination
Three-dimensional X-ray data were collected on a Bruker Kappa Apex CCD diffractometer at
low temperature for 2a and at room temperature for 3a by the - scan method. Reflections
were measured from a hemisphere of data collected from frames, each of them covering 0.3º
in . A total of 113501 for 2a and 37100 for 3a reflections measured were corrected for
Lorentz and polarization effects and for absorption by multi-scan methods based on
symmetry-equivalent and repeated reflections. Of the total, 9313 for 2a and 9719 for 3a,
independent reflections exceeded the significance level (F/F) > 4.0. After data
6

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25
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DOI: 10.1039/C6NJ03162E
collection, in each case an multi-scan absorption correction (SADABS) was applied, and
the structure was solved by direct methods and refined by full matrix least-squares on F2 data
using SHELX suite of programs.26 Hydrogen atoms were included in calculation position
and refined in the riding mode in the two structures. Refinements were done with allowance
for thermal anisotropy of all non-hydrogen atoms. The crystal of 2a presents a disorder on
sulphur atom of one coordinated DMSO molecule. The disorder has been refined and two
atomic sites have been observed and refined with the anisotropic atomic displacement
parameters. The site occupancy factor was 0.78964 for S (1A). A final difference Fourier
map showed a residual density outside: 1.237 and –1.152 e.Å–3 for 2a, due to the disorder
around sulphur atom of DMSO molecule and 1.669 and –1.753 e.Å–3 for 3a, next to Mo
2
atoms, which cannot be refined. A weighting scheme w = 1/[σ2(Fo ) + (0.035000P)2 +
2
19.977900 P] for 2a and w = 1/[σ2(Fo ) + (0.163300 P)2 + 5.708200 P] for 3a, where P =
(|Fo|2 + 2|Fc|2)/3, were used in the latter stages of refinement. Further details of the crystal
structures determination are given in Table 1.
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Table
1
Crystal Data and Structure Refinement for the complDeOxI:e10s.103V9i[e/w(CµA6rN-ticJl0e3O1n6li2nEe
O){MoVIO2(DMSO)}2(L2)] 2a and [(µ-O){MoVIO2(DMF)}2(L3)]·3DMF 3a
2a
3a
Formula
C20 H26 Mo2 N2 O11 S2
726.43
C45 H77 Mo2 N7 O12
1100.01
296.0(2)
0.71073
Triclinic
P1
Formula weight
T, K
100(2)
Wavelength, Å
0.71073
Crystal system
Monoclinic
P21/c
Space group
a/Å
11.6534(3)
39.0383(12)
12.9675(4)
90
10.510(5)
15.733(7)
18.670(8)
79.12(2)
74.44(2)
77.50(2)
2875(2)
b/Å
c/Å
/º
β/º
116.5241(13)
90
γ/º
V/Å3
5278.4(3)
8
Z
2
F000
2912
1152
Dcalc/g cm-3
/mm-1
1.828
1.271
1.168
0.493
1.04 to 26.44
0.0764
1.34 to 28.46
0.0322
/ (º)
Rint
Crystal size/ mm3
Goodness-of-fit on F2
R1[I>2(I)] a
wR2 (all data) b
Largest differences peak and
hole (eÅ–3)
0.30 × 0.28 × 0.08
1.097
0.22 × 0.18 × 0.10
1.067
0.0431
0.0584
0.0985
0.2634
1.237 and –1.152
1.669 and –1.753
2 2
aR1 = Fo - Fc/Fo. bwR2 = [w(Fo2 -Fc2)2]/[w(Fo ) ]1/2
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Result and discussion
The dinuclear complexes 1–3 of general formula [(µ-O){MoVIO2(MeOH)}2(Ln)] (Scheme 3,
H2Ln = ligands I, II and III) having µ-O{MoO2}2 core are practicable by the reaction of
ligands with [MoVIO2(acac)2] (in 1 : 2 molar ratio) in refluxing methanol. Among these,
complexes 1 and 2 are only soluble in DMF and DMSO while complex 3 is additionally
soluble in MeOH and MeCN. These complexes are characterized by elemental, spectroscopic
1
13
(viz. FT-IR, UV-Vis , H and C NMR) and thermogravimetric analyses, and single crystal
X-ray diffraction study of complexes 2 and 3.
O
O
Me O
O
Me
H
O
O
O
R′ = R′′ = H: 1
Mo
Mo
H
O
R′ = H, R′′ = OMe: 2
R′ = R′′ = t-but: 3
"R
O
R"
N
N
R'
R'
Scheme 3 Structure of complexes 1–3.
Solid state characterizations
ORTEP diagrams of [(µ-O){MoVIO2(DMSO)}2(L2)] 2a and [(µ-O){MoVIO2(DMF)}2(L3)] 3a
are shown in Figs. 1 and 2, respectively. The asymmetric unit of 3a also contains three DMF
molecules. The asymmetric unit of 2a contains two dinuclear complexes (we only discuss the
structure around Mo(1) and Mo(2) atoms for simplicity). Selected bond distances and angles
are given in Table 2. These dinuclear complexes present {Mo2N2O9} cores. The two Mo(VI)
centers in each complex, exhibit highly distorted octahedral coordination environments. The
2-bridging oxido group is responsible for the conformation of dinuclear cores, which are
formed by two octahedra, with {MoNO5} cores, they share a corner.
The equatorial planes are occupied by the one terminal oxido group [O(2) in Mo(1) and O(6)
in Mo(2)], 2-bridging oxido group [O(4)], one phenolic oxygen atom [O(3) in Mo(1) and
O(7) in Mo(2)] and one oxygen atom from DMSO molecule in 2a and from DMF molecule
in 3a. The apical positions are occupied by the other terminal oxido group [O(1) in Mo(1)
and O(5) in Mo(2)] and by Nazine nitrogen atoms. These latter groups present a well-known
trans effect [Mo-Nazine distances, 2.337(4) Å for Mo(1) and 2.363(4) Å for Mo(2) in 2a and
2.367(4) Å for Mo(1) and 2.331(4) Å for Mo(2) in 3a,], with the Mo(1) and Mo(2) centers
displaced by ca. 0.29 Å from the plane described for the equatorial atoms in the two
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compounds. The two equatorial planes in each compound form a dihedral angle of 60.37(11)º
DOI: 10.1039/C6NJ03162E
and 59.29(11)º, respectively. The separation between Mo atoms in dinuclear cores are:
3.628(4) Å in 2a and 3.563(3) Å in 3a, and is shorter than in other similar complexes,27 due
to the conformation of the ligand and to closer 2-bridging oxido, which imposes a Mo-O-Mo
"kink" angle of 146.18(17)º in 2a and 138.5(2)º in 3a.
Intermolecular hydrogen bonds and - interactions do not appear in the crystal
packing of the compounds 2a and 3a.
Table 2 Bond lengths [Å] and angles [°] for [(µ-O){MoVIO2(DMSO)}2(L2)] 2a and [(µ-
O){MoVIO2(DMF)}2(L3)]·3DMF 3a
Bond lengths
2a
3a
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-O(4)
Mo(1)-N(1)
Mo(1)-O(1D)
Mo(1)-O(8)
N(1)-N(2)
Mo(2)-O(4)
Mo(2)-O(5)
Mo(2)-O(6)
Mo(2)-O(7)
Mo(2)-N(2)
Mo(2)-O(2D)
Mo(2)-O(9)
1.700(3)
1.702(3)
1.969(3)
1.909(3)
2.337(3)
1.733(4)
1.683(4)
1.967(4)
1.883(4)
2.367(4)
2.303(4)
2.265(3)
1.414(5)
1.883(3)
1.715(3)
1.699(3)
1.986(3)
2.363(3)
1.424(6)
1.927(4)
1.702(4)
1.684(4)
1.986(4)
2.331(4)
2.290(4)
2.251(3)
Angles
2a
3a
O(2)-Mo(1)-O(1)
O(2)-Mo(1)-O(4)
O(1)-Mo(1)-O(4)
O(2)-Mo(1)-O(3)
O(1)-Mo(1)-O(3)
O(4)-Mo(1)-O(3)
O(2)-Mo(1)-O(8)
O(1)-Mo(1)-O(8)
O(4)-Mo(1)-O(8)
O(3)-Mo(1)-O(8)
O(2)-Mo(1)-N(1)
O(1)-Mo(1)-N(1)
O(4)-Mo(1)-N(1)
O(3)-Mo(1)-N(1)
O(8)-Mo(1)-N(1)
Mo(1)-O(4)-Mo(2)
105.41(14)
97.99(14)
98.68(13)
94.53(14)
98.99(13)
154.76(12)
165.16(13)
88.69(13)
84.13(12)
78.41(12)
90.72(13)
163.84(13)
79.59(12)
78.41(12)
75.15(12)
146.18(17)
105.3(2)
99.0(2)
100.9(2)
97.29(19)
98.37(19)
150.37(16)
167.5(2)
86.98(19)
80.57(17)
78.22(16)
92.20(19)
162.35(18)
77.89(16)
76.91(15)
75.43(15)
138.5(2)
O(2)-Mo(1)-O(1D)
O(1)-Mo(1)-O(1D)
O(4)-Mo(1)-O(1D)
O(3)-Mo(1)-O(1D)
O(1D)-Mo(1)-N(1)
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Fig. 1 ORTEP plot of the complex [(µ-O){MoVIO2(DMSO)}2(L2)] 2a. All the non-hydrogen
atoms are presented by their 50% probability ellipsoids. Hydrogen atoms are omitted for
clarity.
Fig. 2 ORTEP plot of the complex [(µ-O){MoVIO2(DMF)}2(L3)] 3a. All the non-hydrogen
atoms are presented by their 50% probability ellipsoids. Hydrogen atoms are omitted for
clarity.
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The thermal study of all binuclear molybdenum complexes were studied under
DOI: 10.1039/C6NJ03162E
o
oxygen environment at a temperature increment rate of 10 C/ min. These symmetric
complexes lose mass exothermically between 195-300 oC equivalents to loss of one methanol
molecule from each metal center. Increasing temperature further, the methanol free
complexes decompose in two-three exothermic overlapping steps, and finally form MoO3
above 400 oC.
Spectroscopic studies
All complexes exhibits two characteristic IR bands at 889-915 and 933-950 cm-1 assignable
to the symmetric and asymmetric stretching modes, respectively, for cis-[MoO2] core.28 An
additional band centred at 767-773 cm-1 is attributable to a ν(Mo-O-Mo) stretching mode.
The IR spectra of ligands exhibit two intense bands at 1572-1591 and 963-974 cm-1due to
ν(C=N)azomethine and ν(N-N)azine stretches, respectively (Table S1).
Upon complexation ν(C=N)azomethine band shifted to lower frequency23 while a positive
shift occurred in ν(N-N)azine stretch, revealing that both nitrogen atoms participate in
coordination to the metal center.29 A broad band covering the area 3040−3150 cm−1 is
assigned to intramolecular hydrogen bonded ν(OH) mode. Absent of this band in complexes
indicates the breaking of the hydrogen bond. However, the unequivocal identification of the
coordination of the phenolic oxygen atom could not be established due to a distinct peak at
3422-3431 cm–1 in the IR spectra of all complexes which is assignable to ν(OH) stretch
possibly due to the presence of coordinated MeOH.
Fig. S1 replicates the UV-visible spectra of complexes and complete spectral data of
ligands and their complexes are presents in Table S2. Two spectral bands exhibited at 297-
308 nm and 358-376 nm in the UV region are assigned due to * and n* transitions,
respectively. Both bands shift to higher wave lengths and appear at 320-323 and 363-371 nm;
the latter one merges with a new band appearing at ca. 410-416 nm and makes the overall
appearance as broad. The characteristic band at ca. 410-416 nm is assigned due to the ligand
to metal charge transfer transition.
1
Table 3 presents the H NMR spectral data of ligands and the corresponding
complexes. The 1H NMR spectra of H2L3 III and its complex 3 are shown in Fig. S2. Due to
centre of symmetry, one singlet each due to phenolic protons at δ = 11.12-11.50 ppm and
1
azomethine protons at δ = 8.77-9.96 ppm appears in the H NMR spectra of ligands. The
12

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absence of phenolic protons in complexes suggests the replacement of protons by the metal
DOI: 10.1039/C6NJ03162E
atom during complexation. All aromatic protons in ligands as well as in complexes appear in
the expected region with slight variations. Tertiary butyl groups give two intense peaks at δ =
1.40 and 1.30 ppm in complex 3. In addition, the spectra of the complexes also display a peak
at δ = 3.18-3.22 ppm equivalent to six methyl protons which confirm that each molybdenum
is coordinated to one methanol. This is further supported by the appearance of a broad peak at
δ=4.09-4.12 ppm due to methanolic OH.
Further, endorsement of the coordination modes of the ligands to the metal complexes
are obtained by recording the 13C NMR spectra of ligands and complexes. The 13C NMR
spectra of a representative ligand (H2L1) and its complex 1 are depicted in Fig. 3 while Table
4 provides all spectral data. All binuclear complexes unveil a distinct peak at  = 47.0-48.88
ppm which corresponds to the carbon of coordinated methanol. A considerable change in the
chemical shifts [Δδ = δ (complex) – δ (free ligand)] of the carbon atoms present in the
vicinity [i.e. C1/C1′, C2/C2′, and C7/C7′ in Table 4] of the coordinating atoms (i.e. Ophenolic
and Nazine) has been observed due to coordination of these functionalities with the
molybdenum. The carbon atoms of the tertiary butyl groups in H2L3 and its complex 3, and
methoxy carbon in H2L2 and its complex 2 appear well within the expected region. Signals
for other aromatic carbon atoms of the ligands and complexes also appear in the expected
region.
1
Table 3 H NMR chemical shifts (δ in ppm) of the ligands and their complexes recorded in
DMSO-d6
Compound
-OH
Tert-butyl
OCH3
-OCH3
-CH=N-
Aromatic protons
(phenolic/
methanol
11.12
(Methanol)
-
-
-
-
-
-
-
8.96
(s, 2H)
7.42-7.69 (q, 2H), 7.38-7.40 (q, 2H),
6.96 (d, 2H), 6.95 (d, 2H)
I
(br, 2H)
4.10
(br, 2H)
3.15
(s, 6H)
8.99
(s, 2H)
7.28-7.30 (q, 2H), 7.26 (m, 2H), 7.20
(m, 2H), 7.12-7.14 (m, 2H)
1
II
2
11.10
(br, 2H)
3.83
(s, 6H)
-
8.99
(s, 2H)
7.27-7.30 (q, 2H), 7.12-7.14 (m, 2H),
6.89-6.93 (m, 2H)
4.12
(br, 2H)
3.83
(s, 6H)
3.12
(s, 6H)
8.99
(s, 2H)
7.28-7.30 (q, 2H), 7.20-7.26 (m, 2H),
6.88-7.14 (m, 2H)
11.50
(br, 2H)
4.10
1.37 (s, 36 H),
1.28 (s, 36 H)
1.40 (s, 36 H),
1.30 (s, 36 H)
-
9.96
(s, 2H)
8.79
7.63-7.62 (m, 2H), 7.56-7.55 (m, 2H)
7.54-7.55 (m, 2H), 7.52-7.53 (m, 2H)
III
3
-
3.16
(s, 6H)
(br,2H)
(s, 2H)
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DOI: 10.1039/C6NJ03162E
Fig. 3 13C NMR spectra of ligand H2L1 (I) and complex 1 recorded in DMSO-d6.
14

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New Journal of Chemistry
Table 4 13C NMR spectral data of ligands and complexes
View Article Online
DOI: 10.1039/C6NJ03162E
Compou C1/C1' C2/C2' C7/C7' C5/C5' C3/C3' C4/C4' C6/C6'
nda
C8/C8'(m
ethanol)
C
(t-but.)
C
(methoxy)
118.24 163.05 158.71 119.92 116.55 132.84 130.95
120.63 155.33 162.62 147.77 118.34 149.39 148.31
-
-
-
-
-
I
47.95
1
(Δδ)
II
(2.39)
(-7.72) (3.91)
121.47 162.32 162.88 118.32 147.81 115.78 122.47
125.48 155.40 165.84 125.25 157.66 120.60 149.48
-
-
-
55.46
55.83
48.48
2
(Δδ)
III
(4.01)
(-6.92) (2.96)
117.33 154.20 158.84 140.55 137.35 129.55 127.84
-
34.51,3
3.31
-
-
119.85 145.57 161.45 138.53 129.42 128.77 119.81
48.46
34.90,
33.95
3
(Δδ)
(2.52)
(-8.63) (2.61)
a[Δδ = [δ (complex) – δ (free ligand)]
Catalytic activity studies: Oxidation of fenchyl Alcohol and isoborneol
The oxidative conversion of bicyclic alcohols eg. fenchyl alcohol and isoborneol catalysed by
dinuclear molybdenum complexes in the presence of H2O2 and additive (NEt3) in MeCN led
to the formation of fenchone and camphor, selectively (Scheme 4).
Scheme 4 Oxidation products of fenchyl alcohol and isoborneol.
Considering [(µ-O){MoVIO2(MeOH)}2(L3)] 3 as a representative catalyst precursor
for the oxidation of fenchyl alcohol, parameters, like amounts of catalyst, 30 % aqueous H2O2
15

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solvent and temperature of the catalytic reaction have been studied for obtaining the
DOI: 10.1039/C6NJ03162E
maximum conversion of fenchyl alcohol.
Thus, for 2 mmol (0.308 g) of fenchyl alcohol, three different amounts of catalyst
precursor 3 (1.25×103, 1.87×103 and 2.5×103 mmol) and 30 % aqueous H2O2 (2, 4 and 6
mmol) were taken in three different volumes of solvent MeCN (5, 7 and 9 mL) and reaction
was carried out at three different temperatures (60, 70 and 80 oC)in the presence of NEt3 (0.05
g, 0.05 mmol). About 6 h was required to achieve the equilibrium. Fig. 4 presents conversion
profiles under different reaction conditions and Table 5 summarises conversion of fenchyl
alcohol under a particular set of conditions. It is clear from the data of the table that the
optimized reaction conditions for the conversion of 2 mmol (0.308 g) of fenchyl alcohol are
those using substrate to catalyst molar ratio of 1 : 2.5×103 and substrate : H2O2 : NEt3 molar
o
ratios of 1 : 3 : 0.025 in 7 mL of MeCN at 80 C (i.e. entry 9 of Table 5). Under these
conditions 82 % conversion was achieved in 6 h of reaction time. In the absence of additive
about 24 h was required to achieve 78 % conversion under above conditions. Thus, additive
not only enhances the conversion, it helped in reducing the reaction time. Only 16 %
conversion was obtained in the absence of catalyst but in presence of NEt3, while in the
absence of catalyst as well as NEt3 both only 4 % conversion was obtained.
Table
5
Conversion of fenchyl alcohol (0.308 g,
2
mmol) using [(µ-
O){MoVIO2(MeOH)}2(L3)] 3 as catalyst precursor in the presence of NEt3 (0.05 g, 0.05
mmol) in 6 h of reaction time under reaction different parameters
Entry
No.
1
Catalyst
[g (mmol)]
H2O2
[g (mmol)]
0.226 (2)
MeCN
[mL]
5
Temp.
[oC]
60
Conv.
[%]
42
0.0010 (1.25 × 103)
0.0015 (1.87 × 103)
0.0020 (2.50 × 103)
0.0020(2.50 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50 × 103)
2
3
4
5
6
7
8
9
0.226 (2)
0.226 (2)
0.452 (4)
0.672 (6)
0.672 (6)
0.672 (6)
0.672 (6)
0.672 (6)
5
5
5
5
7
9
7
7
60
60
60
60
60
60
70
80
48
52
73
80
81
74
72
82
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DOI: 10.1039/C6NJ03162E
(b)
(a)
(c)
(d)
Fig. 4 (a) Effect of amount of catalyst variation (0.0010, 0.0015 and 0.0020 g) on the
oxidation of fenchyl alcohol. Reaction conditions : fenchyl alcohol (0.308 g, 2 mmol),
Catalyst 3, 30 % aqueous H2O2 (0.226 g, 2 mmol), MeCN (5 mL), NEt3 (0.05 g, 0.05 mmol)
and reaction temp (60 oC) for 6 h. (b) Effect of amount of oxidant variation (30 % H2O2)
(substrate to oxidant ratio: 1:1, 1:2 and 1:3) on the oxidation of fenchyl alcohol. Reaction
conditions : fenchyl alcohol (0.308 g, 2 mmol), Catalyst 3 (0.002 g), MeCN (5 mL), NEt3
o
(0.05 g, 0.05 mmol) and reaction temp (60 C) for 6 h. (c) Effect of amount of solvent
variation (5, 7 and 9 mL) on the oxidation of fenchyl alcohol. Reaction conditions : fenchyl
alcohol (0.308 g, 2 mmol), Catalyst 3 (0.0020 g), 30 % aqueous H2O2 (0.672 g, 6 mmol),
o
NEt3 (0.05 g, 0.05 mmol) and reaction temp (60 C) for 6 h. (d) Effect of temperature
o
variation (60, 70 and 80 C) on the oxidation of fenchyl alcohol. Reaction conditions :
fenchyl alcohol (0.308 g, 2 mmol), Catalyst 3 (0.0020 g), 30 % aqueous H2O2 (0.672 g, 6
mmol), MeCN (7 mL), NEt3 (0.05 g, 0.05 mmol) for 6 h.
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Catalyst precursor 3 was also used to optimise the reaction conditions for the
DOI: 10.1039/C6NJ03162E
maximum oxidation of isoborneol. Table 6 presents various conditions applied and the
conversion of isoborneol obtained under each set of conditions while Fig. 5 provides
conversion profiles with time under different reaction conditions. It is clear from the table
that the optimized conditions for the oxidation of 2 mmol (0.308 g) of isoborneol are those
using substrate to catalyst molar ratio of 1 : 2.5×103 and substrate : H2O2 : NEt3 molar ratios
of 1 : 2 : 0.025 in 7 mL of MeCN at 80 oC (i.e. entry 9 of Table 6) Under these conditions
reaction also required 6 h to attain the equilibrium and gave 84 % conversion. Only 4 mmol
of H2O2 was required in place of 6 mmol compared to the optimised reaction conditions for
fenchyl alcohol. Without additive, reaction required 24 h to achieve 69 % conversion. In the
absence of catalyst, but in the presence of NEt3 only 12% conversion and in absence of both
only 7 % conversion were obtained.
Catalytic potential of other complex precursors i.e. complexes 1 and 2 under specified
conditions for fenchyl alcohol as well as for isoborneol have also been tested, and results are
summarised in Table 7. Their catalytic potentials are equally good (84-89%) for both
substrates in the presence of additive. Under similar reaction conditions
[MoVIO2(acac)2](6.13×103 mmol) and MoO3(13.8×103 mmol) give 66-70% conversion.
Along with lower conversion (73-81 %), reaction again required 24 h in the absence of
additive (NEt3) to achieve equilibrium Thus, additive helps in reducing the reaction time from
24 h to 6 h and improving the potential of catalysts in the oxidation of fenchyl alcohol and
isoborneol.
Table 6 Conversion of isoborneol (0.308 g, 2 mmol) using [(µ-O){MoVIO2(MeOH)}2(L3)] 3
as catalyst precursor in the presence of NEt3 (0.05 g, 0.05 mmol) in 6 h reaction time under
different reaction parameters
Entry
No.
1
Catalyst
H2O2
[g (mmol)]
0.226 (2)
MeCN
[mL]
5
Temp.
[oC]
60
Conv.
[%]
51
[g (mmol)]
0.0010 (1.25 × 103)
0.0015 (1.87 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50 × 103)
0.0020 (2.50×103)
2
3
4
5
6
7
8
9
0.226 (2)
0.226 (2)
0.452 (4)
0.672 (6)
0.452 (4)
0.452 (4)
0.452 (4)
0.452 (4)
5
5
5
5
7
9
7
7
60
60
60
60
60
60
70
80
54
60
70
71
76
72
79
84
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DOI: 10.1039/C6NJ03162E
(b)
(a)
(c)
(d)
Fig. 5 (a) Effect of amount of catalyst variation (0.0010 g, 0.0015 and 0.0020 g) on the
oxidation of isoborneol. Reaction conditions : isoborneol (0.308 g, 2 mmol), Catalyst 3, 30 %
aqueous H2O2 (0.226 g, 2 mmol), MeCN (5 mL), NEt3 (0.05 g, 0.05 mmol) and reaction temp
(60 oC) for 6 h. (b) Effect of amount of oxidant variation (30 % H2O2) (substrate to oxidant
ratio: 1:1, 1:2 and 1:3) on the oxidation of isoborneol. Reaction conditions: isoborneol (0.308
g, 2 mmol), Catalyst 3 (0.0020 g), MeCN (5 mL), NEt3 (0.05 g, 0.05 mmol) and reaction
o
temp (60 C) for 6 h. (c) Effect of amount of solvent variation (5, 7 and 9 mL) on the
oxidation of isoborneol. Reaction conditions: isoborneol (0.308 g, 2 mmol), Catalyst 3
(0.0020 g), 30 % aqueous H2O2 (0.452 g, 4 mmol), NEt3 (0.05 g, 0.05 mmol) and reaction
o
temp (60 oC) for 6 h. (d) Effect of temperature variation (60, 70 and 80 C) on the oxidation
of isoborneol. Reaction conditions: isoborneol (0.308 g, 2 mmol), Catalyst 3 (0.0020 g), 30
% aqueous H2O2 (0.452 g, 4 mmol), MeCN (7 mL), NEt3 (0.05 g, 0.05 mmol) for 6 h.
19

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Table 7 Conversion of bicylic alcohol (0.308 g, 2 mmol) and TOF calculated withVieawnAdrticle Online
DOI: 10.1039/C6NJ03162E
without additive
Bicyclic alcohol Catalyst
(conc. in mmol)
With additivea
Conv.[%] TOF[h-1]b Conv. [%] TOF [h-1]c
Without additive
Fenchyl alcohol 1 (3.48×10-3)
2 (3.15×10-3)
89
84
85
85
89
81
75
19
20
26
36
15
-
3 (2.50×10-3)
[MoO2(acac)2](6.13×10-3) 70
MoO3(13.8×10-3)
113 78
38
16
-
67
63
4
66
16
82
89
85
Blank
Isoborneol
1 (2.50×10-3)
2 (3.48×10-3)
3 (3.15×10-3)
109 69
23
18
19
35
85
90
38
75
73
65
64
7
[MoO2(acac)2](6.13×10-3) 69
MoO3(13.8×10-3)
67
12
16
15
Blank
-
-
a NEt3 = 0.05 mmol
bTOF values calculated at 6 h of reaction time for both alcohols.
cTOF values calculated at 24 h of reaction time.
We have also studied the effect of different additives on the conversion of fenchyl
alcohol and isoborneol under above optimised reaction conditions and results are summarised
in Table 8. Overall conversions for both substrates are poor using pyridine, NH3, K2CO3 and
KOH than those obtained using NEt3. However, conversion within the additives follows the
order: NEt3> py> NH3> K2CO3> KOH. This observation possibly suggests that only mild
base is required to accelerate the conversion rate.
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Table 8 Oxidation of bicylic alcohol and % conversion with different additives inDpOrIe: 1s0e.1n03c9e/Co6fNJ03162E
catalyst 1
Additive
Fenchyl alcohol
Conv. [%]
TOF [h-1]
Isoborneol
Conv. [%]
TOF [h-1]
Pyridine
NH3
49 65
38 50
21 28
1925
46
37
25
21
61
49
33
28
K2CO3
KOH
Reactivity of molybdenum complexes with H2O2 and possible reaction intermediate for
the oxidation of bicyclic alcohols
The cis-[MoVIO2]2+ complexes are known to undergo transformation into
[MoVIO(O2)]2+ upon reaction with H2O2 in suitable solvent and in some cases solid peroxido
complexes have also been isolated.30 The in situ formation of such peroxido species has also
been established spectrophotometrically by reacting complex 3 with H2O2 in DMSO. In a
typical reaction, 25 mL of 1.24×10–6 M solution of 3 in DMSO was treated with one drop
portions of 30 % aqueous H2O2 (0.096 g, 5 mmol) dissolve in 5 mL of DMSO and the
obtained spectral changes are shown in Fig 6 (a). Thus, the band at 320 nm shifted to 305 nm
along with increase in intensity while 270 nm band showed only increase in intensity. The
intensity of the band at 371 remained almost unchanged. Simultaneously, the intensity of the
weak and broad charge transfer band appearing at 416 nm slowly decreased along with
further broadening and shifting to 420 nm [inset of Fig 6 (a)]. These changes along with the
generation of an isosbestic point at 396 nm indicate the interaction of 3 with H2O2 and the
plausible formation of the corresponding peroxido complex in DMSO. Very similar spectral
changes were obtained after adding only few drops of a mixture of NaHCO3 and H2O2 which
indicates the quick formation of oxidoperoxido species. As reported, NaHCO3 reacts with
–
H2O2 instantly and produces HCO4 (peroxymonocarbonate)31, 2(b) which actually reacts with
dioxidomolybdenum(VI) complex to give oxidoperoxidomolybdenum(VI) intermediate more
rapidly as compared to H2O2 alone. Similar changes have also been noted for complexes 1
and 2; Figs. 6 (b) and 6 (c). Thus, the catalytic reaction is likely to proceed through the
[MoVIO(O2)]2+ intermediate formed upon reaction of H2O2 with complexes which may
transfer oxygen to the substrates during the catalytic oxidation.
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DOI: 10.1039/C6NJ03162E
(b)
(a)
(c)
Fig. 6 (a) Plots representing the spectral changes during the titration of [(µ-
O){MoVIO2(MeOH)}2(L3)] 3 with H2O2. Spectra recorded after successive addition of one
drop portions of 30% aqueous H2O2 (0.096 g, 0.50 mmol) dissolved in 5 mL DMSO to 25 mL
of 1.24×10– 6 M solution of 3 in DMSO. Inset shows expanded region (375 – 500 nm). (b)
Spectral changes during the titration of [(µ-O){MoVIO2(MeOH)}2(L1)] 1 with H2O2. Spectra
observed after successive addition of one drop portion of 30 % aqueous H2O2 (0.076 g, 3.3
– 6
mmol) dissolved in 5 mL DMSO to 25 mL of 1.25×10
M solution of 1in DMSO. Inset
shows the expanded (300-500 nm) region. (c) Spectral changes during the titration of [(µ-
O){MoVIO2(MeOH)}2(L2)] 2 with H2O2. Spectra recorded after successive addition of one
drop portion of 30 % aqueous H2O2 ( 0.102 g, 3.6 mmol) , dissolved in 5 mL DMSO to 25
mL of 1.25×10 – 6 M solution of 2 in DMSO. Inset shows the expanded (320-500 nm) region.
22

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DOI: 10.1039/C6NJ03162E
Conclusions
Three binuclear molybdenum (VI) complexes, [(µ-O){MoVIO2(MeOH)}2(L1)] 1, [(µ-
O){MoVIO2(MeOH)}2(L2)] 2 and [(µ-O){MoVIO2(MeOH)}2(L3)] 3 with µ-O{MoO2}2 core
have been prepared from dibasic ONNO tetradentate ligands. The X-ray crystal structures of
2 (DMSO solvated) and 3 (DMF solvated) confirm the distorted octahedral environment on
each molybdenum center and the bis (monobasic ON bidentate) behaviour of ligands. Each
cis-[MoO2] unit is linked through bridged-oxygen. These complexes are good catalyst
precursors for the oxidation of bicyclic alcohols (fenchyl alcohol and isoborneol)in the
presence of 30 % aqueous H2O2 and a range of N-base additives to give bio-active product
fenchone and camphor, respectively. The N-base additive plays an important role in
abstracting hydrogen from H2O2 and generate the oxidoperoxidomolybdenum(VI) species
which easily facilitate the transfer of oxygen to the substrate resulting in good yield of
oxidised product. However, conversion of bicyclic alcohols within the additives follows the
order: NEt3> py > NH3> K2CO3> KOH. Much poor conversion of substrates in the absence
of additive was noted which demonstrate the importance of additive for such a catalysts.
Acknowledgements
L.R. thank the Council of Scientific and Industrial Research, New Delhi for financial support.
Gas chromatograph used in this study was procured from the grant of Department of Science
and Technology (SR/S1/IC-32/2010), Government of India, New Delhi.
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