Trinuclear Dioxidomolybdenum(VI) Complexes of Tritopic Phloroglucinol-Based Ligands and Their Catalytic Applications for the Selective Epoxidation of Olefins

Article abstract of DOI:10.1002/ejic.201800440

Four trinuclear dioxidomolybdenum(VI) complexes, [{Mo^VIO₂(H₂O)}₃ptk(bhz)₃] (1), [{Mo^VIO₂(H₂O)}₃ptk(fah)₃] (2), [{Mo^VIO₂(H₂O)}₃ptk(inh)₃] (3), and [{Mo^VIO₂(H₂O)}₃ptk(nah)₃] (4), based on the tritopic central 2,4,6-triacetylphloroglucinol (H₃ptk) ligands H₆ptk(bhz)₃ (I), H₆ptk(fah)₃ (II), H₆ptk(inh)₃ (III) and H₆ptk(nah)₃ (IV) (Hbhz = benzoylhydrazide, Hfah = 2-furanoylhydrazide, Hinh = isonicotinoylhydrazide and Hnah = nicotinoylhydrazide), respectively, are presented. All of the synthesized ligands, as well as their complexes, have been characterized by elemental, thermal, and electrochemical analyses, spectroscopic techniques (FTIR, UV/Vis, ¹H and ¹³C NMR), and single-crystal X-ray studies of [{Mo^VIO₂(H₂O)}{Mo^VIO₂(MeOH)}₂ptk(bhz)₃]·2H₂O·1.25MeOH (1a) and [{Mo^VIO₂(EtOH)}₃ptk(fah)₃]·3EtOH (2a). Each pocket of the ligands coordinates in a dibasic tridentate fashion through two oxygen atoms and one nitrogen atom to each metal center. Due to the presence of tridentate binding pockets in the ligands, each metal center conserves its octahedral structure by coordinating with water molecules in the synthesized complexes or by other solvent(s) in the crystal structures. These complexes were evaluated for the epoxidation of terminal and internal alkenes in the presence of H₂O₂ using NaHCO₃ as a promoter. Under the optimized reaction conditions, all alkenes were converted to the corresponding epoxides selectively in good yield and high turnover number.

Full text of DOI:10.1002/ejic.201800440

A Journal of
Accepted Article
Title: Trinuclear Dioxidomolybdenum(VI) Complexesof Tritopic
Phloroglucinol based ligands and their Catalytic Applications for
the Selective Epoxidation of Olefins
Authors: Mannar R. Maurya, Reshu Tomar, Lata Rana, and Fernando
Avecilla
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800440
Link to VoR: http://dx.doi.org/10.1002/ejic.201800440

10.1002/ejic.201800440
European Journal of Inorganic Chemistry
Table of contents
Trinuclear [MoVIO2]2+Complexes
Trinuclear
Dioxidomolybdenum(VI)
M.R. Maurya, R. Tomar, L. Rana and
F. Avecilla
Complexes
of
Tritopic
Phloroglucinol based ligands and
their Catalytic Applications for the
Selective Epoxidation of Olefins
Trinucleardioxidmolybdenum (VI)
complexes for the selective
epoxidation of terminal and internal
alkenes are reported
Keywords: Trinuclear molybdenum
complexes; Tritopic phloroglucinol
hydrazone ligands; Crystal structures;
Spectroscopy studies; Epoxidation of
various alkene.
1
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10.1002/ejic.201800440
European Journal of Inorganic Chemistry
Trinuclear Dioxidomolybdenum(VI) Complexes of Tritopic Phloroglucinol
based ligands and their Catalytic Applications for the Selective Epoxidation of
Olefins
Mannar R. Maurya,[*]a Reshu Tomar,[a] Lata Rana,[a] and Fernando Avecilla[b]
[a]Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee–247667, India. E–
mail: rkmanfcy@iitr.ernet.in; Fax: +91 1332 273560; Tel: +91 1332 285327;
https://www.iitr.ac.in/CY/rkmanfcy
[b]Departamento de Química Fundamental, Universidade da Coruña, Campus de A Zapateira,
15071 A Coruña, Spain
Keywords: Trinuclear dioxidomolybdenum complexes; Tritopic phloroglucinol hydrazone
ligands; Crystal structures; Spectroscopy studies; Epoxidation of various alkene.
Supporting information for this article is available on the WWW under https:// DOI:
10.1002/ejic.201800440
Abstract: Four trinuclear dioxidomolybdenum(VI) complexes, [{MoVIO2(H2O)}3ptk(bhz)3] (1),
[{MoVIO2(H2O)}3ptk(fah)3]
(2),
[{MoVIO2(H2O)}3ptk(inh)3]
(3),
and
[{MoVIO2(H2O)}3ptk(nah)3] (4), based on the tritopic central 2,4,6-triacetylphloroglucinol (H3ptk)
based ligands, H6ptk(bhz)3 (I), H6ptk(fah)3 (II), H6ptk(inh)3 (III) and H6ptk(nah)3 (IV) (Hbhz
= benzoylhydrazide, Hfah = 2-furanoylhydrazide, Hinh = isonicotinoylhydrazide and Hnah =
nicotinoylhydrazide), respectively, are presented. All of the synthesized ligands, as well as their
complexes, have been characterized by elemental, thermal and electrochemical analyses,
spectroscopic techniques (FT-IR, UV-Vis, 1H and 13C NMR), and single crystal X-ray studies of
[{MoVIO2(H2O)}{MoVIO2(MeOH)}2ptk(bhz)3(H2O)]·2H2O·1.25MeOH
(1a)
and
[{MoVIO2(EtOH)}3ptk(fah)3]·3EtOH (2a). Each pocket of the ligands acts as a dibasic tridentate,
coordinating through two oxygen atoms and one nitrogen atom to each metal center. Due to the
presence of tridentate binding pockets in ligands, each metal center conserves its octahedral
structure by coordinating with water molecule in synthesized complexes or by other solvent(s) in
the crystal structures. These complexes were evaluated for the epoxidation of terminal and internal
alkenes in the presence of H2O2 using NaHCO3 as promoter. Under the optimized reaction
2
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10.1002/ejic.201800440
European Journal of Inorganic Chemistry
conditions, all alkenes resulted in the formation of corresponding epoxide selectively in good yield
and high turnover number.
Introduction
Phloroglucinol (1,3,5-trihydroxybenzene) and its derivatives comprise a group of natural moieties
with bio-inspired approaches, including antimicrobial and antidepressant effects.[1]
Phloroglucinols can easily be transformed into 2,4,6-triketo or triformyl derivatives which are key
precursors for tricompartmental polytopic ligands. Homo- and heterometallic oligonuclear
complexes obtained from polytropic ligands have been engaged in various fields such as molecular
magnetism, biomimetic inorganic chemistry, electron transfer reactions and catalysts.[2] Several
polytopic ligands are Schiff bases which are generated from appropriate poly-keto/formyl and
polyamine moieties.[3] Tricompartmental ligands have attracted great attention because such a class
of ligands can bind three metal ions at a time. Such ligands have mainly been used by Glaser and
9,10]
co-workers.[4-8] and to some extent by Plass et al.[
in designing trinuclear metal complexes.
Due to the presence of meta-phenylene bridging unit, trinuclear Cu(II),[5], Ni(II)[6] and Co(II)[7]
complexes of triple salen based ligands exhibit ferromagnetic coupling of three metal ions through
the spin polarization mechanism.[11]
Our interest in molybdenum chemistry has led us to consider tricompartmental
phloroglucinol based ligands and study their trinuclear dioxiodomolybdenum (VI) complexes.
Dioxiodomolybdenum (VI) complexes exhibit potential catalytic activity in the olefins,[12] and bio-
inspired epoxidations,[13] alcohol oxidation, oxidative bromination of organic substrates and
sulphide oxidation.[14,15] Selective oxidation of olefins to the corresponding epoxides has been an
interesting area of research because alkene epoxides are useful intermediate in paint and polymer
industries.[15] Styrene oxide is employed in the synthesis of commercial products such as epoxy
resins, cosmetics, surface coatings, sweeteners, perfume, drugs etc. Similarly, cyclohexene oxide
is involved in synthesizing numerous products such as enantioselective drugs, epoxy paints, rubber
promoters and dyestuff. Catalytically, epoxides can be achieved by the reaction of alkenes with
H2O2 in the presence of transition metal based catalysts.[16-17] However, it has been observed that
aqueous H2O2 as terminal oxidant alone gives several products along with lower conversion and
poor turn over frequency, particularly using vanadium molybdenum, tungsten and manganese
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European Journal of Inorganic Chemistry
based catalysts.[18,19] Using NaHCO3 as a promoter along with H2O2, catalysts not only improved
their catalytic efficiency but also became selective for epoxidation of olefins.[20-21]
Herein, we introduce bio-mimetic [MoO2] group into trinucleating hydrazones (I –IV of Scheme
1) of 2,4,6-triacetylphloroglucinol (H3ptk) having tris(phenolic-O, enamine-N and amide-O donor)
functionalities, their characterization and reactivity. We also present the selective epoxidation of
olefins catalyzed by these complexes in the presence of H2O2 and NaHCO3 as a promoter.
O
O
OH
O
OH O
HN
NH
R
R
O
N
N
MeOH, reflux
3d
3
H2N-HN
R
HO
OH
HO
OH
N
O
R
HN
H3ptk
O
R
Ligands
R
Ligands
H6ptk(inh)3 (III)
H6ptk(nah)3 (IV)
H6ptk(bhz)3 (I)
H6ptk(fah)3 (II)
N
O
N
Scheme 1.Scheme for the preparation of trinucleating hydrazones designated as I, II, III and IV
used in this work.
Results and Discussion
Synthesis and Solid State Characterization of Complexes
Condensation of 2,4,6-triacetylphloroglucinol with hydrazides (cf. Scheme 1) in 1:3 molar ratio in
refluxing MeOH led to the formation of the analytically pure trinucleating ligands I - IV. These
ligands on reaction with [MoVIO2(acac)2] in a molar ratio of 1:3 in refluxing MeOH in open air
gave
corresponding
cis–tris[MoVIO2]2+
complexes,
[{MoVIO2(H2O)}3ptk(bhz)3](1),
[{MoVIO2(H2O)}3ptk(fah)3] (2), [{MoVIO2(H2O)}3ptk(inh)3] (3), and [{MoVIO2(H2O)}3ptk(nah)3]
(4) in good yields. The possible structures of these complexes are given in Scheme 2 which are
based on elemental, thermal and electrochemical analyses, and spectroscopic (FT-IR, UV-Vis, 1H
and 13C NMR) studies. Structures of 1 and 2 (considering association of different solvents in actual
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10.1002/ejic.201800440
European Journal of Inorganic Chemistry
X-ray structure, now abbreviated as 1a and 2a) have been confirmed by single crystal X-ray study.
These complexes are air-stable yellow solids and are soluble in MeOH, dimethylsulfoxide
(DMSO) and N,N-dimethylformamide (DMF) only.
Scheme 2. Proposed structure of complexes.
Thermal studies
All four cis–tris[MoO2]2+–complexes have very similar thermogravimetric patterns; Figure 1
presents their profiles and Table 1 presents thermos-analytical data. Thermal analysis profiles of
o
complexes under an oxygen atmosphere show their stability up to ca. 70 C and then lose mass
exothermically between 100 – 300 oC (for 1), 80 – 200 oC (for 2), 80 – 170 oC (for 3), and 70 –
o
200 C (for 4) equivalents to three coordinated water molecules, suggesting that these water
molecules are moderately/strongly bound to the metal ion. Existence of weakly solvent bound
octahedral or even solvent free trigonal bipyramidal [MoO2]2+–complexes are well known in the
literature.[22,23] Upon further increasing the temperature, solvent free complexes decompose in
two/three major fragments with exothermic mass loss and complete between 410 – 445 oC (Table
1) excepts 1 which decomposes in a single step between 300 – 370 oC. Part of the mass loss also
o
occurs between 410 – 730 C. The remaining mass obtained at this temperature equals their
corresponding MoO3 residue. Beyond 700 oC MoO3 residue starts volatilizing as expected.
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European Journal of Inorganic Chemistry
Figure 1. TGA profiles of complexes.
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European Journal of Inorganic Chemistry
Table 1. Detail of thermogravimetric analytical data of complexes.
Compound
Temp.
range
% Weight loss Group (Expected) group/ residue % Residue
Obs [a] (calcd) lost
Obs (Calcd)
[{MoO2(H2O)}3ptk(bhz)3]
100 – 300 4.6 (5.2)
300 – 370 54.8 (53.0)
370 – 700
3 H2O
L-3O
[{MoO2}3ptk(bhz)3]
3 MoO3
42.3 (41.6)
[{MoO2(H2O)}3ptk(fah)3]
[{MoO2(H2O)}3ptk(inh)3]
[{MoO2(H2O)}3ptk(nah)3]
80-200
5.0 (5.3)
3 H2O
[{MoO2}3ptk(fah)3]
200 – 420
420 – 700 55.6 (51.7)
L-3O
3 MoO3
41.8 (42.9)
42.1 (41.5)
41.6 (41.5)
80 – 170
5.0 (5.1)
3 H2O
[{MoO2}3ptk(inh)3]
200 – 410
410 – 730 53.8 (53.1)
L-3O
3 MoO3
70 – 200
5.0 (5.1)
3 H2O
[{MoO2}3ptk(nah)3]
200 – 445
445 – 700 54.35 (53.3)
L-3O
3 MoO3
[a] Total mass loss considering both temp range given in previous column.
Description of the Molecular Structure of the Compounds
ORTEP
diagrams
for
the
compounds
[{MoVIO2(H2O)}
{MoVIO2(MeOH)}2ptk(bhz)3(H2O)]·2H2O·1.25MeOH (1a) and [{MoVIO2(EtOH)}3ptk(fah)3]
·3EtOH (2a) are shown in Figures 2 and 3, respectively. Selected bond distances and angles are
given in Table 2. These compounds are trinuclear species with three Mo centres. The structure of
the complexes 1a and 2a adopt a distorted six-coordinated octahedral geometry around Mo centres
with two Ooxido terminal oxygen atoms and the ligands act as tridentate, coordinated through the
one Ophenolate, one Nazomethine and one Ohydrazone atoms. The axial sites are occupied by the oxido
atoms, O(2), O(7) and O(12), and by the oxygen atoms of the solvent molecules. In 1a, two
methanol molecules and one water molecule occupy these positions while in 2a, three ethanol
molecules are in these places. The equatorial plane is formed for the three atoms of the ligand and
one of the terminal oxygen atoms, and they are distorted respect to the planarity, [mean deviation
from the plane for N(1), O(1), O(3), O(4), 0.1489(20) Å, N(3), O(6), O(8), O(9),0.0356(24) Å and
N(5), O(11), O(13), O(14), 0.0241(22) Å in 1a, and 0.2983(10) Å,0.0391(11) Å and 0.0257(11) Å
in 2a, respectively]. The molybdenum atoms are displaced toward the apical oxido ligand, O(2),
from the equatorial plane defined by N(1), O(1), O(3) and O(4), 0.3568(20) Å, toward O(7), from
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10.1002/ejic.201800440
European Journal of Inorganic Chemistry
the equatorial plane defined by N(3), O(6), O(8), O(9), 0.3406(21) Å and toward O(12), from the
equatorial plane defined by N(5), O(11), O(13), O(14), 0.3227(22) Å in 1a, and 0.3415(20) Å,
0.3270 Å and 0.3387(19), respectively, in 2a. The Mo-Ooxido bond lengths [Mo(1)-O(1) and Mo(1)-
O(2) and for other metal centres (see Table 2) and bond angles [O=Mo=O angle are: 105.5(2)º,
106.0(3)º and 105.6(2)º in 1a, and 105.33(12)º, 105.07(11)º and 105.77(11)º in 2a] are within the
range for the bond distances typically observed in these type of compounds.[24]
- Interaction were not observed in the crystal packing of the two compounds.
Intermolecular hydrogen bonds were observed (see Table 3). The crystal packing is different in
the two compounds. In compound [{MoVIO2(H2O)}{MoVIO2(MeOH)}2ptk(bhz)3(H2O)]
·2H2O·1.25MeOH (1a), the trinuclear species are grouped in non-flat layers and are generated
hydrophilic holes, which include several water and solvent molecules. In
[{MoVIO2(EtOH)}3ptk(fah)3]·3EtOH (2a), ethanol molecules and van der Waals forces close these
holes and the structure is more compact (see Figures 4 and 5).
Figure 2. ORTEP for the complex [{MoVIO2(H2O)}{MoVIO2(MeOH)}2ptk(bhz)3]
·2H2O·1.25MeOH (1a). All the non-hydrogen atoms are presented by their 50% probability
ellipsoids. Hydrogen atoms are omitted for clarity.
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European Journal of Inorganic Chemistry
Figure 3. ORTEP for the complex [{MoVIO2(EtOH)}3ptk(fah)3]·3EtOH (2a). All the non-
hydrogen atoms are presented by their 50% probability ellipsoids. Hydrogen atoms are omitted for
clarity.
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Figure 4. ORTEP for the crystal packing of compound [{MoVIO2(H2O)}
{MoVIO2(MeOH)}2ptk(bhz)3(H2O)]·2H2O·1.25MeOH (1a). All the non-hydrogen atoms are
presented by their 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Hydrophylic
holes are occupied for water and methanol molecules.
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European Journal of Inorganic Chemistry
Figure 5. ORTEP for the crystal packing of compound [{MoVIO2(EtOH)}3ptk(fah)3]·3EtOH (2a).
All the non-hydrogen atoms are presented by their 50% probability ellipsoids. Hydrogen atoms
are omitted for clarity.
Table 2. Bond lengths [Å] and angles [°] for compounds[{MoVIO2(H2O)}
{MoVIO2(MeOH)}2ptk(bhz)3(H2O)]·2H2O·1.25MeOH
(1a)
and
[{MoVIO2(EtOH)}3ptk(fah)3]·3EtOH (2a).
Bond lengths
1a
2a
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-O(4)
1.707(4)
1.690(4)
1.947(4)
1.984(4)
1.683(2)
1.717(2)
1.943(2)
1.984(2)
Mo(1)-O(5)
Mo(1)-N(1)
Mo(2)-O(6)
2.376(4)
2.230(5)
1.710(5)
2.328(2)
2.237(2)
1.714(2)
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Mo(2)-O(7)
Mo(2)-O(8)
Mo(2)-O(9)
Mo(2)-O(10)
Mo(2)-N(3)
Mo(3)-O(11)
Mo(3)-O(12)
Mo(3)-O(13)
Mo(3)-O(14)
Mo(3)-O(15)
Mo(3)-N(5)
1.688(5)
1.948(4)
1.985(4)
2.300(5)
2.233(5)
1.714(4)
1.700(4)
1.940(4)
1.977(4)
2.272(4)
2.235(4)
1.686(2)
1.958(2)
1.991(2)
2.276(2)
2.237(2)
1.701(2)
1.708(2)
1.939(2)
1.983(2)
2.320(2)
2.233(2)
Angles
1a
2a
O(1)-Mo(1)-N(1)
O(2)-Mo(1)-N(1)
O(2)-Mo(1)-O(1)
O(3)-Mo(1)-N(1)
O(3)-Mo(1)-O(1)
O(3)-Mo(1)-O(2)
O(4)-Mo(1)-N(1)
O(4)-Mo(1)-O(1)
O(4)-Mo(1)-O(2)
O(4)-Mo(1)-O(3)
O(5)-Mo(1)-N(1)
O(5)-Mo(1)-O(1)
O(5)-Mo(1)-O(2)
O(5-Mo(1)-O(3)
O(5)-Mo(1)-O(4)
158.08(19)
95.3(2)
161.89(11)
92.26(11)
105.33(12)
77.93(9)
105.5(2)
79.09(16)
104.07(18)
98.41(18)
72.38(16)
96.36(19)
101.45(19)
146.42(16)
76.70(16)
82.54(19)
171.93(19)
79.33(15)
77.21(16)
102.87(10)
99.42(11)
72.75(9)
99.19(10)
100.82(11)
144.76(9)
79.11(8)
83.41(11)
171.21(10)
77.29(9)
78.35(9)
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Table
3.
Hydrogen
bonds
in
compounds
(1a)
[{MoVIO2(H2O)}
and
{MoVIO2(MeOH)}2ptk(bhz)3(H2O)]·2H2O·1.25MeOH
[{MoVIO2(EtOH)}3ptk(fah)3]·3EtOH (2a).
D-H...A
d(D-H)
d(H...A)
d(D...A)
<(DHA)
Complex 1
O(1W)-H(1WA)...O(11)#1
O(2W)-H(2W)...O(3W)
O(3W)-H(13W)...O(12)#2
O(3W)-H(23W)...O(5W)
O(10)-H(10)...O(2W)#3
O(15)-H(15A)...O(4W)#4
O(15)-H(15B)...O(3W)#4
Complex 2
0.87
2.31
2.06
2.02
1.77
1.82
1.91
1.87
3.012(8)
2.863(8)
2.860(7)
2.470(13)
2.712(8)
2.687(14)
2.772(7)
138.3
159.8
168.1
138.9
171.7
143.0
175.4
0.84
0.85
0.85
0.90
0.90
0.90
O(1S)-H(1S)...O(6)#1
O(2S)-H(2S)...O(1)#2
O(3S)-H(3S)...O(12)#3
O(5)-H(5S)...O(1S)
O(10)-H(10S)...O(3S)
O(15)-H(15S)...O(2S)
0.82
0.82
0.82
0.84
0.86
0.87
2.29
1.99
2.27
1.84
1.85
1.79
3.052(3)
2.810(3)
3.035(4)
2.662(4)
2.701(3)
2.661(3)
153.7
173.9
156.5
165.7
170.8
177.3
Symmetry transformations used to generate equivalent atoms for compound 1: #1 -x+1,-y,-z+1
#2 x,y+1,z #3 x,y-1,z #4 x-1,y-1,z.
Symmetry transformations used to generate equivalent atoms for compound 2: #1 -x+2,-y+1,-
z+1 #2 -x+2,-y,-z+2 #3 x-1,y,z.
FT-IR Spectroscopic Studies
A partial list of IR spectral data of ligands and complexes are presented in the experimental section
along with their synthesis while their details are given Table S1. All these complexes are well
characterized by two prominent peaks at 924–951 and 903–911 cm–1, which are assigned to
νasym(O=Mo=O) and νym(O=Mo=O) modes, respectively due to the cis-[MoO2] structure.[25] We
did not observed more than two bands for these modes, indicating symmetric nature of cis-[MoO2]
groups in trinuclear complexes. The IR spectra of four ligands show three characteristic bands at
3198, 1646, and 1570 cm–1 for H6ptk(bhz)3 (I), 3163, 1650, and 1573 cm–1 for H6ptk(fah)3 (II),
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3015, 1670, and 1571 cm–1 for H6ptk(inh)3 (III) and 3017, 1673, and 1596 cm–1 for H6ptk(nah)3 (
IV) due to ν(N–H), ν(C=O) (of the hydrazide moiety) and ν(C=N) stretches, respectively. The first
two bands endorse their ketonic nature in the solid state. Absence of bands due to ν(N–H) and
ν(C=O) in the IR spectra of all complexes indicates their enolisation and replacement of H by the
metal ion. A new band appearing in the region of 1256–1262 cm–1 is assigned due to the ν(C–O)
mode. The ν(C=N) of all ligands display at lower wave numbers in the corresponding complexes,
indicating the coordination of the azomethine nitrogen to the molybdenum. The broad band
appearing at ca. 3400 cm–1 due to the ν(O–H) in ligands is due to phenolic –OH while similar band
in complexes is possibly due to coordinated water.
UV/Vis Spectral Studies
The UV-Vis spectral data (absorption maxima with their extinction coefficients) of ligands and
their complexes are presented in Table 4. Ligands’ spectra were recorded in DMSO and metal
complexes in MeOH. Figure S1 (a and b) presents spectra of ligands and complexes. All ligands
shows two characteristic bands at ca. 260 and ca. 320 nm due to π-π* and n-π* transitions,
respectively. In complexes, these prominent bands appear at nearly same positions along with two
additional bands at ca. 220 and at ca. 420 nm. The former band is possibly arising due to φ→φ*
transition while the later one could be assigned due to the ligand to metal charge transfer transitions
(LMCT), originating from the filled p-orbital of the ligand to the vacant d-orbital of the
molybdenum ion of proper symmetry.
Table 4. UV/Vis spectral data for the prepared ligands and complexes.
Compound
Solvent λmax /nm (ɛ/M–1 cm–1)
DMSO
262 (2.13×104), 323(1.87×104)
H6ptk(bhz)3 (I)
[{MoO2(H2O)}3ptk(bhz)3] (1) MeOH
H6ptk(fah)3 (II) DMSO
[{MoO2(H2O)}3ptk(fah)3] (2) MeOH
H6ptk(inh)3 (III) DMSO
[{MoO2(H2O)}3ptk(inh)3] (3) MeOH
H6 ptk(nah)3(IV) DMSO
[{MoO2(H2O)}3ptk(nah)3] (4) MeOH
223(4.8×104), 315(3.1×104), 424(1.3×104)
263(2.67×104), 324(2.63×104)
219(3.8×104), 265(3.4×104), 318(2.7×104), 416(1.17×104)
262(2.14×104), 321(1.52×104)
220(4.2×104), 264(3.6×104), 315(2.9×104), 421(1.1×104)
260(2.19×104), 314(1.17×104)
216(3.9×104), 265(3.3×104), 314(2.3×104), 415(0.8×104)
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European Journal of Inorganic Chemistry
1H and 13C NMR Spectral Studies
The coordination functionalities in cis–tris[MoO2]2+– complexes were confirmed by
comparing 1H NMR spectral data of complexes with the corresponding ligands. The relevant data
collected in d6-[DMSO] are presented in Table 5 along with the J values for aromatic protons.
Figure 6 presents representative spectra (ligand IV and complex 4) of the series. A broad signal
obtained at δ 11.5 ppm for the three phenolic –OHs in the ligands spectra are not detected in the
spectra of complexes, indicating the coordination of these three phenolic oxygen atoms to the
molybdenum after deprotonation. A sharp singlet due to nine protons of the three methyl groups
of ligands appear at δ = 2.45 – 2.59 ppm and this shifts to δ = 2.73 – 2.95 ppm due to adjustment
of current arising out after coordination of nitrogen atom to molybdenum ion. Signals due to
aromatic protons in ligands as well as in complexes appear in the expected region with slight
variations.
1
Figure 6. H NMR spectra of ligand IV and complex 4 recorded in d6–[DMSO].* indicates the
proton impurity signal present in d6–[DMSO] at δ = 2.4 ppm and that of moisture at δ = 3.25 ppm.
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Table 5. 1H NMR (400 MHz) data (δ in ppm) of ligands and complexes
Compounds ˗OH
-CH3
Aromatic protons
I
10.45 (br, 3H)
2.45 (s, 9H)
7.90 (d,J = 8.5 Hz 6H), 7.57 (d,J = 8.4 Hz, 6H), 7.45
(m, 3H)
1
II
2
---
2.73 (s, 9H)
2.52 (s, 9H)
2.82 (s, 9H)
8.3 (d,J = 7.1 Hz, 6H), 7.59 (d, J = 8.8 Hz, 6H), 7.50
(m,3H)
11.27 (br, 3H)
---
7.98 (d, J = 2.2 Hz, 3H), 7.36 (d,J = 3.5 Hz,3H), 6.73
(dd, J = 3.5, 1.7 Hz,3H)
7.99 (d, J = 5.3 Hz, 3H), 7.27 (d,J = 3.1 Hz, 3H), 6.72
(dd, J = 3.5, 1.7 Hz,3H)
III
3
11.54 (br, 3H)
---
2.56 (s, 9H)
2.95 (s, 9H)
2.59 (s, 9H)
8.80 (d, J = 4.8 Hz, 6H), 7.96 (d, J = 4.6 Hz 6H)
8.78 (d, J = 3.8 Hz, 6H), 7.95 (d,J = 5.9 Hz, 6H)
9.10 (s, 3H), 8.79 (d,J = 6.1, 3H), 8.31 (d,J = 8.0 Hz,
3H), 7.58 (dd, J = 7.7, 4.8 Hz, 3H)
IV
11.56 (br, 3H)
4
---
2.83 (s, 9H)
9.22 (s, 3H), 8.78 (d, J = 3.8 Hz, 3H), 8.37 (d, J = 9.7
Hz 3H), 7.56 (dd,, J = 7.8, 4.9 Hz, 3H)
The binding modes of the ligands were further ascertained by studying the coordination-induced
13C NMR chemical shifts. All spectra were recorded in DMSO-d6 and their assignments are given
in Table 6. Figure7 presents the spectra for IV and 4 as the representative spectra for the ligands
and cis–tris[MoO2]2+– complexes under study. A significant downfield shifts, Δδ = [δ(complex) −
δ(ligand)], was witnessed in the signals of the carbon atoms in the vicinity of the coordinating
atoms. A large coordination induced shift of the signals for the carbon atoms associated with the
phenolic oxygen atoms (C1/C1’/C1’’), the azomethine nitrogen atoms (C3/C3’/C3’’) and the
enolic oxygen atoms (C4/C4’/C4’’) (see Table 6) confirms the coordination of these functionalities
to the molybdenum ion. The signals for carbon atoms (C5/C5’/C5’’), (C8/C8’/C8’’),
(C7,7/C7’,7’/C7’’,7’’), (C6,6/C6’,6’/C6’’,6’’), (C2/C2’/C2’’) closure to the above carbons are
only slightly affected due to the electronic arrangement.
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European Journal of Inorganic Chemistry
Figure 7. 13C NMR spectra of ligand H6ptk(nah)3 (IV) and its molybdenum complex
[{MoO2(H2O)}3ptk(nah)3] (4).
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Table 6. 13C NMR spectral data of ligands and complexes.
Compounds
C3/C3’/C3’’ C1/C1’/C1’’ C4/C4’/C4’’ Other carbons
-CH3
H6ptk(bhz)3 (I)
166.49
154.74
152.88
132.29(C5/C5’/C5’’),131.70(C8/C8’/C8’’),
19.74(C9/C9’/C9’’)
22.12(C9/C9’/C9’’)
19.18C9/C9’/C9’’)
20.14(C9/C9’/C9’’)
129.08(C7,7/C7’,7’/C7’’,7’’),127.92(C6,6/
C6’,6’/C6’’,6’’),127.33(C2/C2’/C2’’)
132.52(C5/C5’/C5’’),130.07(C8/C8’/C8’’),
129.16(C7/C7’C7’’),128.49(C6/C6’/C6’’),1
11.56(C2/C2’/C2’’)
[{MoO2(H2O)}3ptk(bhz)3] (1)
168.33
(1.84)
165.43
(10.69)
167.01
(14.13)
(Δδ)
H6ptk(fah)3 (II)
164.41
153.74
151.37
147.54(C5/C5’/C5’’),145.84(C8/C8’/C8’’),
137.50(C6/C6’/C6’’),135.36(C7/C7’/C7’’),
109.90(C2/C2’C2’’)
[{MoO2(H2O)}3ptk(fah)3] (2)
166.54
(2.13)
161.37
(7.63)
165.03
(13.66)
147.60(C5/C5’/C5’’),144.82(C8/C8’/C8’’),
138.37(C6/C6’/C6’’),136.87(C7/C7’/C7’’),
111.42(C2/C2’C2’’)
(Δδ)
H6ptk(inh)3 (III)
164.33
156.42
154.61
150.58(C7,7/C7’,7’/C7’’,7’’),140.48(C5/C5 19.05(C8/C8’/C8’)
’/C5’’),121.41(C6,6/C6’,6’/C6’’,6’’),108.89
(C2/C2’/C2’’)
[{MoO2(H2O)}3ptk(inh)3] (3)
166.70
(2.37)
162.79
(6.37)
164.40
(9.79)
152.23(C7,7/C7’,7’/C7’’,7’’),143.05(C5/C5 21.31(C8/C8’/C8’)
’/C5’’),123.31(C6,6/C6’,6’/C6’’,6’’),110.65
(C2/C2’/C2’’)
(Δδ)
H6 ptk(nah)3( IV)
165.01
153.18
152.30
150.30(C9/C9’/C9’’),149.15(C8/C8’/C8’’),
135.83(C6/C6’/C6’’),128.28(C5/C5’/C5’’),
123.78(C7/C7’/C7’’),109.49(C2/C2’/C2’’)
150.36(C9/C9’/C9’’),149.17(C8/C8’/C8’’),
136.45(C6/C6’/C6’’),126.45(C5/C5’/C5’’),
124.11(C7/C7’/C7’’),111.76(C2/C2’/C2’’)
19.50(C10/C10’/C10’’)
[{MoO2(H2O)}3ptk(nah)3] (4)
166.64
(1.63)
166.23
(13.05)
167.13
(14.83)
22.23(C10/C10’/C10’’)
(Δδ)
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Electrochemical Properties
Electrochemical proerties of complexes 1–4 have been stuided by cyclic voltametry in DMF
solution to investigate the redox stabilty of complexes in solution state. Voltammograms of redox
active [MoVIO2] complexes (1–4) were recorded in the range of 1.8V to -1.8V vs. SCE at room
temperature. Relevant data are presented in Table 7 and a representative cyclic voltammogram of
H6ptk(nah)3 (IV) and [{MoO2(H2O)}3ptk(nah)3] (4) are presented in Figure 8 (for other CV plots
see Figure S2). The cyclic voltagrames of all trinulcear dioxidomolybdenum (VI) complexes show
almost similar pattern. The response at −0.95V can be attributed to the MoVI → MoV reduction.
This reductive response corresponding to MoVI/MoV processes in the polar aprotic solvent is
irreversible in nature.[26,27] Metal centerd oxidation peak is not observerd during the anodic scan,
which is possibly due to rapid decomposition of the reduced species.[28] The CV trace of the
complexes also display two redox responses within the potential window −0.688 to −0.958V. The
reduction potential near −0.68V in the complexes is possibly due to the redox behavior of the
ligand. To confirm it we have also recorded CV of ligands and they exhibit one irreversible
oxidative peak around 1.10 V and one reductive peak at -0.688V in the cyclic voltammograms
recorded in the potential range +1.8 to −1.8 V, confirming above assumption. Differential pulse
voltammetry (dpv) was also performed to support the above data. The CV traces in Figure 8(a)
when compared with dpv traces (also see Figure S3), it was observed that the reduction potential
values are not exactly similar to CV values which conclude that the system is irreversible in nature
having one electron reduction and one electron oxidation. In Figure 8(b), at −0.95V there is one
electron reduction from MoVI → MoV. Metal centerd oxidation peak is not observerd during the
anodic scan neither in CV nor in dpv studies (Figure 8(b)). Ligands also have irreversible oxidative
peak at 1.00 V which is not overlapping on dpv data confirming its irreversible nature having
unsymmetrical peaks.
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(b)
(a)
Figure 8. Comparative study of cyclic voltammograms and differential pulse voltametric for
H6ptk(nah)3 (IV) and [{MoO2(H2O)}3ptk(nah)3] (4) – scan rate 100 mV/s recorded in DMF in the
range 1.8 V to -1.8 V vs. Ag/AgCl at room temperature with 0.1 M TBAPF6 as a supporting
electrolyte.
Table 7. Cyclic voltammetric and differential pulse voltametric results for dioxidomolybdenum
(VI) complexes at 298 K.
Complexes
Epc[V]
Epa[V]
dpv
H6ptk(bhz)3 (I)
-0.688, -0.850
-0.958
1.10
-0.872, 0.750
-0.876, -0.468
-0.850, 0.836
-0.896, -0.472,
-1.70, 0.836
-0.848, -0.489
-0.840, 1.05
-0.868, -0.484
[{MoO2(H2O)}3ptk(bhz)3] (1)
H6ptk(fah)3 (II)
-
1.08
-
-0.655, -0.893
-0.949
[{MoO2(H2O)}3ptk(fah)3] (2)
H6ptk(inh)3 (III)
-0.667, -0.797
-0.948
1.04
-
[{MoO2(H2O)}3ptk(inh)3] (3)
H6ptk(nah)3 (IV)
-0.688, -0.775
-0.891
1.00
-
[{MoO2(H2O)}3ptk(nah)3] (4)
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Catalytic Study- Oxidation of Alkenes
Considering the importance of epoxides and quite a high production requirement every year,
catalytic competence of the synthesized cis–tris[MoO2]2+– complexes was established for the
epoxidation of various alkenes. Complex [{MoVIO2(H2O)}3ptk(inh)3] (3) was used as a
representative catalyst to study the epoxidation of cyclohexene as a representative oxidation
substrate and different reaction parameters were optimized viz. the amounts of catalyst, oxidant
(30% aqueous H2O2), promoter (NaHCO3), solvent and the temperature at which the reaction was
carried out while maintaining stirring speed of around 100 rpm in all reactions. Thus, four different
amounts of catalyst 3 (0.0005 g, 0.0010, 0.0015 and 0.0020 g), for 5 mmol of cyclohexene and
aqueous 30% H2O2 (10, 15 and 20 ml) were taken in the presence of NaHCO3 (1, 2 and 3 mmol)
in three different volumes of MeCN solvent (5, 7 and 9 mL) at three different temperatures (50,
60 and 70 °C).
First, the amount of catalyst were optimized for ideal catalytic activity. For this, four
sequential reactions were examined using different amounts of representative catalyst (3), viz.
0.0005, 0.0010, 0.0015, and 0.0020 g whereas all parameters were kept constant, i.e. 5 mmol
(0.410 g) cyclohexene, 20 mmol (2.270 g) of 30 % H2O2, 2 mmol (0.168 g) NaHCO3 and 5 mL of
MeCN at 60 °C for 2 h. Results show that no significant increment in conversions were obtained
from 0.0005 g to 0.0020 g through 0.0010 to 0.0015 g (96-98 %) of catalyst which suggests that a
large amount of catalyst is not an essential condition to improve the oxidation. Thus the catalyst
amount 0.0005 g was deliberated as optimized catalyst amount for obtaining the best results (entry
no.4 of Table 8).
In order to achieve best suited reaction conditions for the maximum oxidation of olefins the
effect of promoter concentration on the oxidation is demonstrated by performing the reactions
using different amount of promoter. For this, three different amounts of NaHCO3 viz. 1 mmol
(0.084 g), 2 mmol (0.126 g) and 3 mmol (0.168 g) were taken, keeping all factors constant i.e. 5
mmol (0.410 g) cyclohexene, 20 mmol (2.270 g) of 30 % H2O2, 2 mmol (0.168 g) NaHCO3 and 5
mL of MeCN at 60 °C for 2 h. After 2h of reaction time only a slight increase in conversion from
96-99 % was obtained on increasing the amount of promoter from 1 mmol to 3 mmol. No
significant enhancement was observed due to higher amount of promoter. Thus, 1 mmol of
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NaHCO3 was considered as optimized amount to achieve maximum conversion (entry no. 6 of
Table 8).
To see the effective changes in the oxidation of olefins as well as on their selectivity, the
amount of oxidant have been varied by performing the reactions with different amount of oxidant
i.e.10 mmol (1.130 g), 15 mmol (1.69 g), 20 mmol (2.270 g) of 30% H2O2, having 1:2, 1:3, 1:4
molar ratios with substrate cyclohexene (0.410 g, 5 mmol) keeping all the factors constant. It shows
significantly enhanced conversion to 99% and 90% for 10 mmol and 15 mmol of oxidant,
respectively, though one should expect the higher conversion with 15 mmol. This is possibly due
to the catalyst nature. Thus, 10 mmol of oxidant is sufficient enough for best results (entries 6 and
7 of Table 8).
Amount of solvent is also considered as an important parameter in conversion. On
increasing the solvent the conversion lowers down from 70 to 61% in 7 mL and 9 mL MeCN,
respectively. This probably happens because upon dilution the catalyst gets least space to react at
target site and reactivity becomes slow which may further influences the steric and electronic
properties of the intermediate species due to favorable modest adsorption effects on the active
catalyst surface. Thus, 5 mL of MeCN was chosen as a best solvent for best results. (entries 8 and
9 of Table 8)
Temperature of the reaction mixture has also influenced on the performance of reaction
mixture of catalyst. Amongst three different temperatures of 50, 60 and 70 °C under the optimized
reaction conditions, 60 °C significantly improved the conversion to 99 % in 2h (entries 10 and 11
of Table 8).
Figure 9 presents the detailed profiles of conversion with time under different reaction
conditions. From the data of Table 8, the optimized reaction conditions for the epoxidation for 5
mmol of cyclohexene can be concluded as (entry 7): catalyst (0.0005 g, 4.8×10–4 mmol), 30%
H2O2 (1.130 g, 10 mmol), NaHCO3 (0.084 g, 1 mmol) in 5 mL of MeCN at 60 °C reaction
temperature. Under these conditions, reaction reaches to the equilibrium in 2 h with almost 99%
of mass balance and gave a maximum of 99% conversion with 100% selectivity of cyclohexene
oxide. We have also extended the reaction time of the above reaction up to 24 h and chromatograms
recorded during the oxidation of cyclohexene for 2 h, 6 h and 24 h (Figures S4-7) revealed that
cyclohexene oxide is the main reaction product as no other products were spotted by GC
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10.1002/ejic.201800440
European Journal of Inorganic Chemistry
technique.[29] Other olefins were also tested after 24 h reaction time and they also show no other
product(s) formation. (Figures S8, 9).
Three types of controlled experiments were also performed under the optimized reaction
conditions to investigated the effect of catalyst as well as promoter (NaHCO3) on the oxidation of
cyclohexene, (i) the controlled experiment in the absence of catalyst but in the presence of
NaHCO3 (0.084 g, 1 mmol) and H2O2 (1.130 g, 10 mmol) which gave 19% of conversion with
100% selectivity of epoxide (entry 12 of Table 8), (ii) the controlled experiment in the presence of
catalyst (0.0005 g, 1.1×10–3 mmol) and H2O2 (1.130 g, 10 mmol) but in the absence of NaHCO3
and this gave 7% conversion of epoxide (entry 13 of Table 8) and, (iii) the controlled experiment
in the absence of both catalyst as well as promoter but in the presence of H2O2 (1.130 g, 10 mmol)
and in this case only 6% conversion was obtained within 2 h of reaction time (entry 14 of Table
8). These experiments demonstrate the importance of promoter and the catalytic efficiency of
complex in its presence.
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European Journal of Inorganic Chemistry
Figure 9. (a) Effect of amount of catalyst on the oxidation of cyclohexene. Reaction conditions:
cyclohexene (0.410 g, 5 mmol), 30% H2O2 (2.270 g, 20 mmol), NaHCO3 (0.168 g, 2 mmol),
MeCN (5 mL) and temp. (60˚C). (b) Effect of amount of oxidant on the oxidation of cyclohexene.
Reaction conditions: cyclohexene (0.410 g, 5 mmol), catalyst (0.0005 g), NaHCO3 (0.084 g,
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10.1002/ejic.201800440
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1mmol), MeCN (5 mL) and temp. (60˚C). (c) Effect of NaHCO3 amount on the oxidation of
cyclohexene. Reaction conditions: cyclohexene (0.410 g, 5 mmol), catalyst (0.0005 g), 30% H2O2
(1.130 g, 10 mmol), MeCN (5 mL) and temp. (60˚C). (d) Effect of temperatures on the oxidation
of cyclohexene. Reaction conditions: cyclohexene (0.410 g, 5 mmol), catalyst (0.0005 g), 30%
H2O2 (1.130 g, 10 mmol), NaHCO3 (0.084 g, 1 mmol) and MeCN (5 mL). (e) Effect of amount of
solvent on the oxidation of cyclohexene. Reaction conditions: cyclohexene (0.410 g, 5 mmol),
catalyst (0.0005 g), 30% H2O2 (1.130 g, 10 mmol), NaHCO3 (0.084 g, 1 mmol) and temp. (60 ˚C).
(f) Plots representing conversion of styrene in the presence of different [MoVIO2]2+-complexes as
catalyst precursors under optimized reactions conditions (entry 7 of Table 8).
Table 8. Conversion of cyclohexene (0.41 g, 5 mmol) using [{MoO2(H2O)}3ptk(inh)3](3) as a
catalyst precursor in the presence of NaHCO3 in 2 h of reaction time under different reaction
conditions
Entry
Catalyst
H2O2
NaHCO3
[g (mmol)]
0.168 (2)
0.168 (2)
0.168 (2)
0.168 (2)
0.252 (3)
0.084 (1)
0.084 (1)
0.084 (1)
0.084 (1)
0.084 (1)
0.084 (1)
0.084 (1)
-
Temp.
[°C]
60
MeCN
[mL]
5.0
Conv.
[%]
98
96
96
96
99
98
99
70
61
67
57
19
7
[g (mmol)]
[g (mmol)]
1
0.0020 (1.9×10-3) 2.27 (20)
0.0015 (1.4×10-3)
2.270 (20)
0.0010 (9.6×10-4) 2.270 (20)
2
60
5.0
3
60
5.0
4
0.0005 (4.8×10-4)
0.0005 (4.8×10-4)
0.0005 (4.8×10-4)
0.0005 (4.8×10-4)
0.0005 (4.8×10-4)
0.0005 (4.8×10-4)
0.0005 (4.8×10-4)
0.0005 (4.8×10-4)
-
2.270 (20)
2.270 (20)
2.270 (20)
1.130 (10)
1.130 (10)
1.130 (10)
1.130 (10)
1.130 (10)
1.130 (10)
1.130 (10)
1.130 (10)
60
5.0
5
60
5.0
6
7[a]
60
5.0
60
5.0
8
60
7.0
9
60
10
10
11
12
13
14
70
5.0
50
5.0
60
5.0
0.0005 (4.8×10-4)
60
5.0
-
-
60
5.0
6
[a] Optimized reaction conditions.
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After concluding the optimized reaction conditions for complex 3 as catalyst precursor,
other catalyst precursors i.e. complexes 1, 2 and 4, were also tested for the epoxidation of
cyclohexene under the same reaction conditions. The resulting conversion, selectivity and turn
over frequency of product (calcd. after 2 h) are presented in Table 9. Thus, the performance
[TOF/h–1] of catalyst precursors follows the order: 1(5156) = 3(5156)>2 (5104) >4 (4739). Further,
selectivity of epoxide is almost 100% in all cases.
Table 9. Conversion of cyclohexene and their selectivity towards epoxide using various complexes
as catalyst precursors.
Catalysts
Conversion
[%]
TOF [h-1][a]
Selectivity [%] of
cyclohexene epoxide
[{MoO2(H2O)}3ptk(bhz)3] (1)
[{MoO2(H2O)}3ptk(fah)3] (2)
[{MoO2(H2O)}3ptk(inh)3] (3)
[{MoO2(H2O)}3ptk(nah)3] (4)
[a] Calcd at 2 h as [h−1].
99
98
99
91
5156
100
100
100
100
5104
5156
4739
Under the above optimized reaction conditions, other alkenes (cyclooctene, styrene, trans-
stilbene 1-octeneand cyclohexenone) have also been tested at 2 h and 24 h of reaction time using
all catalyst precursors and results are summarized in Table 10. In general, conversion followed
the order: cyclohexene > cyclooctene > 1-octene >1-hexene >styrene >trans-stilbene>
cyclohexenone, and within catalyst, the best conversion was obtained with 3. Complex
[MoVIO2(acac)2] employed as a catalyst precursor under optimized conditions provides a much
lower conversion (56-65%) of these substrates. A blank reaction (i.e. in the presence of H2O2 and
NaHCO3 but without catalyst) with all alkenes provides 15-35% conversion under optimized
conditions. All these studies indicate that complexes have very good catalytic potentials and that
promoter NaHCO3 plays a very important role in enhancing catalytic potential of complexes 1, 2,
3, and 4.
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Table 10. Oxidation of olefins (5 mmol each) by different Mo-catalyst (0.0005 g) and 30% aqueous
H2O2 (10 mmol) as a terminal oxidant and NaHCO3 (1 mmol) as promoter under the optimized
reaction conditions
Entry Substrate
1.
catalyst
Product
Time Conv.
(h)
Selectivity[a]
[2h/24h] [2h/24h]
%
%
100/100
1
2
2
2
2
2
2
99/100
2
98/100
99/100
91/100
61
100/100
100/100
100/100
100
3
4
[MoVIO2(acac)2]
Blank
19
100
2.
3.
4.
1
2
2
2
2
2
2
99/100
99/100
99/100
97/100
65
100/100
100/100
100/100
100/100
100
2
3
4
[MoVIO2(acac)2]
Blank
24
100
1
2
2
2
2
2
2
98/100
99/100
97/100
99/100
56
100/100
100/100
100/100
100/100
100
2
3
4
[MoVIO2(acac)2]
Blank
34
100
1
2
2
2
2
2
2
99/100
98/100
99/100
97/100
62
100/100
100/100
100/100
100/100
100
2
3
4
[MoVIO2(acac)2]
Blank
15
100
27
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10.1002/ejic.201800440
European Journal of Inorganic Chemistry
5.
1
2
2
2
2
2
2
92/100
90/100
88/100
82/100
69
100/100
100/100
100/100
100/100
100
2
3
4
[MoVIO2(acac)2]
Blank
16
100
6.
1
2
2
2
2
2
2
94/100
92/100
96/100
95/100
62
100/100
100/100
100/100
100/100
100
2
3
4
[MoVIO2(acac)2]
Blank
12
100
7.
1
2
2
2
2
2
2
98/100
96/100
98/100
97/100
64
100/100
100/100
100/100
100/100
100
2
3
4
[MoVIO2(acac)2]
Blank
14
100
[a] Calculated as ‘‘[Yield of epoxide’’/’’Conversion’’]×100 %[30]
The catalytic performance of complexes examined in the present paper for the epoxidation
particularly of cis-cyclooctene using H2O2 and NaHCO3 compares favorably with the reported
literature data. For example, mononuclear complex [MoO2Cl2(pbim)] used as a catalyst gives
82/98% conversion at 6h/24h[31] while dinuclear complex [Mo2O6(4,4′-di-tert-butyl-2,2′-
bipyridine)2] offers ca. 93/100% conversion at 6h/24h.[32]
For the polynuclear
dioxidomolybdenum (VI) complexes [Mo2O6(HpypzA)]n 57/82% at 6/24h,[33] and [Mo3O9(2-
[3(5)-Pyrazolyl]pyridine)]n, the epoxidation rate of olefins tends to be lower (29/51% at 6/24h) [34]
possibly due to poor catalyst solubility and difficulty in accessibility of the substrate and oxidant
to reach to the active site. It is to be noted that trinuclear molybdenum complexes reported here
have an open system where all three metal centres have one solvent molecule which are easily
accessible to the reactants and hence present good performance.
28
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800440
European Journal of Inorganic Chemistry
Reactivity of Dioxidomolybdenum(VI) Complexes with H2O2 and Possible Reaction
Mechanism for the Catalytic Oxidation of Substrates
Generation of [MoVIO(O2)]2+ species has been established when mononuclear- or dinuclear
dioxidomolybdenum(VI) complexes reacted with aqueous H2O2 and in some cases such peroxido
complexes have been isolated. [35] We wanted to establish the formation of such peroxido species
with trinuclear complexes 1–4 as well. To accomplish this objective, a methanolic solution of
[{MoVIO2(H2O)3ptk(bhz)3] (1) was titrated with H2O2 and the resulting changes were monitored
by electronic absorption spectroscopy. Thus, the progressive addition of 30% H2O2 (0.085 g, 2.5
mmol) dissolved in 10 ml of MeOH to a 10 ml of ca. 1×10–5 M solution of
[{MoO2(H2O)}3ptk(bhz)3] (1) resulted in decreases in the intensity of the LMCT band at λ = 416
nm along with the formation an isosbestic point at λ =345 nm [Figure 10(a)]. Simultaneously, the
321 nm band increases its intensity moderately along with the formation of another isosbestic point
at 283 nm [inset of Figure 10(b] while 209 nm band shows a considerable increase in the
intensity[Figure.10(b)]. A very similar trends have also been observed in other complexes on their
reaction with H2O2 (Figures S10- S12). These observations are very similar to the ones observed
earlier for mononuclear- as well as for dinuclear dioxidomolybdenum(VI) complexes and thus are
interpreted as the formation of oxidoperoxidomolybdenum(VI) intermediate species. This
intermediate moiety is responsible for transfer of oxygen to the substrate during catalytic
epoxidation. The promoter NaHCO3 enhances the formation oxidoperoxidomolybdenum(VI)
intermediate as compared to H2O2 alone through other possible intermediate (Scheme 3) which
ultimately affect the catalytic conversion of substrate(s) and the selectivity of reaction product.[18-
21]
-
-
H2O2
HCO4
HCO3
H2O
N
O
N
O
N
O
OH2
O
O
O
OH
Mo
Mo
O
Mo
O
C
O
-H2O
O
O
O
O
O
O
-
O
HCO3
Scheme 3. Proposed mechanism for the formation of oxidoperoxidomolybdenum(VI) complex
by the reaction of complex with H2O2 in the presence of NaHCO3. Only one unit of trinuclear
complex is shown here.
29
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