Substituent effects on solvent-free epoxidation catalyzed by dioxomolybdenum(VI) complexes supported by ONO Schiff base ligands

Article abstract of DOI:10.1016/j.ica.2014.12.022

Molybdenum complexes of type [MoO₂L]_n (L = tridentate ONO Schiff base ligands based on the salicylidene-2-aminophenolato structure) have been synthesized and used as catalysts for the solvent-free epoxidation of olefins. The effect o

Full text of DOI:10.1016/j.ica.2014.12.022

Accepted Manuscript
Substituent effects on solvent-free epoxidation catalyzed by dioxomolybde-
num(VI) complexes supported by ONO Schiff base ligands
Weili Wang, Tomas Guerrero, Saul R. Merecias, Héctor García-Ortega, Rosa
Santillan, Jean-Claude Daran, Norberto Farfán, Dominique Agustin, Rinaldo
Poli
PII:
S0020-1693(15)00022-5
http://dx.doi.org/10.1016/j.ica.2014.12.022
ICA 16369
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 September 2014
8 December 2014
10 December 2014
Please cite this article as: W. Wang, T. Guerrero, S.R. Merecias, H. García-Ortega, R. Santillan, J-C. Daran, N.
Farfán, D. Agustin, R. Poli, Substituent effects on solvent-free epoxidation catalyzed by dioxomolybdenum(VI)
complexes supported by ONO Schiff base ligands, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/
j.ica.2014.12.022
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Substituent effects on solvent-free epoxidation catalyzed by dioxomolybdenum(VI)
complexes supported by ONO Schiff base ligands
Weili Wanga,b, Tomas Guerreroc, Saul R. Mereciasc, Héctor García-Ortegac, Rosa Santilland,
Jean-Claude Darana, Norberto Farfánc*, Dominique Agustina,b*, Rinaldo Polia,e*
a - CNRS; LCC (Laboratoire de Chimie de Coordination); Université de Toulouse; UPS,
INPT, 205, route de Narbonne, F-31077 Toulouse, France. Fax: +33-561553003. Email:
rinaldo.poli@lcc-toulouse.fr.
b - Université de Toulouse; Institut Universitaire de Technologie Paul Sabatier - Département
de Chimie, Av. Georges Pompidou, BP 20258, F-81104 Castres Cedex, France. Fax : +33-
563351910. Email: dominique.agustin@iut-tlse3.fr.
c - Facultad de Química, Depto. de Química Orgánica, Universidad Nacional Autónoma de
México, 04510, México, D. F., México
d - Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN,
07000, Apdo. Postal. 14-740, México D. F. México.
e - Institut Universitaire de France 103, bd Saint-Michel, 75005 Paris, France
Abstract
Molybdenum complexes of type [MoO2L]n (L= tridentate ONO Schiff base ligands based on
the salicylidene-2-aminophenolato structure) have been synthesized and used as catalysts for
the solvent-free epoxidation of olefins. The effect of donor (diethylamino) and/or acceptor
(nitro) substituents has been studied.
1

Graphical abstract
Highlights
•
Synthesis and characterization of substituted tridentate Schiff-base
dioxomolybdenum complexes
•
•
•
These [MoO2L]2 complexes are active epoxidation catalysts
Analysis of the molecular structures
Electronic effect of donor and acceptor substituents on the catalytic activity
Keywords
molybdenum / tridentate Schiff-base ligands / epoxidation / organic solvent-free / catalysis
2

1. Introduction
Epoxides are among the most interesting and versatile intermediates in organic
chemistry [1]. Traditionally, they are obtained from olefins using strong oxidants such as m-
CPBA, PhIO, and NaOCl in organic solvents (often chlorinated ones) [2]. These procedures
are time consuming, produce waste and do not comply with the principles of green chemistry
[3]. Metal catalysis with use of milder oxidants (H2O2, TBHP or O2) is a way to promote
environmentally friendlier processes [4]. Molybdenum has been shown to be one of the most
active metals for olefin epoxidation catalysis and is currently used in industrial processes. The
use of milder oxidant generates less toxic or useful by-products and less waste [5] but the
majority of the industrial processes that have been implemented so far need the use of an
organic solvent [6]. Diminishing or avoiding the use of organic solvents is one of the
challenges of green chemistry to be taken into account. We are currently studying the solvent-
free epoxidation of olefins using coordination complexes of molybdenum [7], vanadium [8]
and polyoxometalates [9]. Within these investigations, we have shown that the molybdenum
complexes with the salicylidenaminophenolato (SAP) Schiff base ligand are stable and active
under solvent-free conditions. Theoretical calculations on the catalytic cycle have shown that
these complexes promote O atom transfer from TBHP to the olefin in a way reminiscent of
Bartlett’s mechanism [10] for the stoichiometric epoxidation by peracids. Since it is known
that modification of the ligand backbone through simple functionalization affects the
electrochemical properties of the molybdenum complexes [11], we wondered whether the
addition of electron donor and/or withdrawing substituents in the SAP ligand would affect the
catalytic performance of this molybdenum system. Thus, dimethylamino (donor) and/or nitro
(withdrawing) groups have been introduced in the SAP backbone and the resulting complexes
have been tested and compared in epoxidation catalysis.
2. Experimental part
2.1. General. All preparations were carried out in air. Water was deionised twice before
use. Organic solvents (ethanol, methanol, diethylether from ACROS) were used as received:
2-amino-phenol, salicylaldehyde, 2-amino-5-nitrophenol, 4-(N,N-diethylamino)salicyl-
aldehyde, cis-cyclooctene (98% Aldrich), cyclooctene oxide (Aldrich) and TBHP (70% in
3

water, ACROS) were commercially available and used as received. Complex [MoO2(acac)2]
was freshly synthesized as previously described [12]. The Schiff base ligands H2L1-4 were
prepared according to the literature procedures [13,14,15,16]. The physical and spectroscopic
properties of these compounds are given in the SI. Complex [MoO2(L1)]2 was synthesized as
previously described [7c].
2.2. Characterization methods. The thermogravimetric analyses were performed on a
SETARAM TGA 92-16.18 thermal analyzer. The sample was placed into an alumina crucible
and heated at 0.83 K s-1 in a reconstituted air flow from 15°C to 700°C. An empty crucible
was used as a reference. Infrared spectra were recorded using the ATR technique with a
Perkin Elmer FTIR/FIR 400 spectrometer. NMR experiments were carried out in solution
1
using DMSO-d6 as solvent, H, 13C and 2D NMR experiments were recorded on a Varian
Unity Inova 7.2 (300 and 400 MHz) and a JEOL ECA 500 (500 MHz) spectrometer. High
resolution mass spectra were recorded in an Agilent G1969A mass spectrometer. Catalytic
reactions were followed by gas chromatography on an Agilent 6890A chromatograph
equipped with FID detector, a HP5-MS capillary column (0.30 m x 0.25 mm x 0.25 m) and
automatic sampling, or on a Fisons GC 8000 chromatograph equipped with FID detector and
with a SPB-5 capillary column (30 m x 0.32 mm x 0.25 m). The GC parameters were
quantified with authentic samples of the reactants and products. The conversion of cis-
cyclooctene and the formation of cyclooctene oxide were calculated from calibration curves
(r2= 0.999) relatively to an internal standard.
2.3. General procedure for the synthesis of molybdenum complexes [MoO2L2-4]2. In a
round bottom flask equipped with a magnetic stirrer bar, 9.1 mmol of the selected Schiff base
H2L1-4 and 10 mmol of [MoO2(acac)2] were suspended in 50 mL of ethanol. The resulting
suspension was stirred at reflux temperature for two hours. After that time, two thirds of the
solvent were evaporated and the solution was cooled in an ice bath. The solid obtained was
filtered and washed with cold ethanol and diethyl ether. The compounds were then dried
under vacuum at 50°C for several days. The molybdenum complexes were characterized by
1H and 13C NMR, IR, HRMS and TGA. The carbon atoms numbering used for the assignment
of the 13C NMR resonances of each complex is depicted in Scheme 1.
4

Scheme 1- Atom numbering for the assignment of the 13C NMR resonances.
2.3.1. Characterization data for [MoO2(L2)]2. Yellow solid, 38% yield, mp >300 °C.
NMR 1H (500 MHz, DMSO-d6) δ 9.43 (s, 1H, H-7), 8.02 (d, 1H, J = 9.5 Hz, H-13), 7.82-7.79
(m, 2H, H-6, H-10), 7.62-7.58 (m, 2H, H-4, H-12), 7.09 (ddd, 1H, J = 7.5, 6.5, 1.0 Hz, H-5),
6.95 (d, 1H, J = 6.5 Hz, H-3). 13C (125 MHz, DMSO-d6) δ 162.0 (C2), 161.5 (C7), 160.4
(C9), 148.3 (C11), 141.8 (C8), 137.5 (C4), 136.6 (C6), 122.3 (C1), 121.9 (C5), 119.6 (C3),
117.7 (C13), 115.8 (C12), 112.6 (C10). IR (ATR) ν (cm-1) 1603 (C=N), 1511 (NO2), 1342
(NO2), 922 (MoO2), 798 (Mo-O-Mo). HRMS (ESI) calcd. for C13H9N2O6Mo, [M++1]
386.9509, found 386.9511 [M++1]. TGA: 37.1 % MoO3 residue (requires 37.4 %). Anal.
Calc. for C13H8MoN2O6 (Mr = 384.15): C, 40.6; H, 2.1; N, 7.3. Found: C, 40.5; H, 1.5; N,
7.1%.
1
Characterization data for [MoO2(L3)]2. Brown solid, 44% yield, mp >300 °C. NMR H (500
MHz, DMSO-d6) δ 8.89 (s, 1H, H-7), 7.59 (dd, 1H, J = 8.0, 1.0 Hz, H-13), 7.44 (d, 1H, J =
9.0 Hz, H-6), 7.04 (ddd, 1H, J = 7.8, 7.5, 1.5 Hz, H-11), 6.82 (ddd, 1H, J = 7.8. 7.5, 1.5 Hz,
H-12), 6.72 (dd, 1H, J = 8, 1.5 Hz, H-10), 6.41 (dd, 1H, J = 9.0, 2.5 Hz, H-5), 6.07 (d, 1H, J =
2.5 Hz, H-3), 3.39 (q, 4H, J = 7 Hz, NCH2), 1.09 (t, 6H, J = 7 Hz, CH3). 13C (125 MHz,
DMSO-d6) δ 163.5 (C2), 159.7 (C4), 154.8 (C7), 154.2 (C9), 137.6 (C8), 137.1 (C6), 128.0
(C11), 120.5 (C12), 117.0 (C10), 115.6 (C13), 111.6 (C1), 106.4 (C5), 99.5 (C3). 44.7
(NCH2), 13.1 (CH3). IR (ATR) ν (cm-1) 1615(C=N), 927 (MoO2), 844 (Mo-O-Mo). HRMS
(ESI), calcd. for C17H19N2O4Mo [M++1] 413.0393, found 413.0400 [M++1]. TGA: 35.6 %
MoO3 residue (requires 35.1 %). Anal. Calc. for C17H18MoN2O4 (Mr = 410.28): C, 49.8; H,
4.4; N, 6.8. Found: C, 49.6; H, 3.8; N, 6.7%.
2.3.2. Characterization data for [MoO2(L4)]2. Brown solid, 42% yield, mp >300 °C.
1
NMR H (500 MHz, DMSO-d6) δ 9.01 (s, 1H, H-7), 7.75 (d, 1H, J = 9 Hz, H-13), 7.74 (dd,
1H, J = 8.9, 2.3 Hz, H-12), 7.49 (d, 1H, J = 5.5 Hz, H-6), 7.48 (d, 1H, J = 2.3 Hz, H-10), 6.50
(dd, 1H, J = 9.0, 2.5 Hz, H-5), 6.12 (d, 1H, J = 2.5 Hz, H-3), 3.44 (q, 4H, J = 7 Hz, NCH2),
5

13
1.11 (t, 6H, J = 7 Hz, CH3). C (125 MHz, DMSO-d6) δ 164.1 (C2), 159.3 (C9), 157.4 (C7),
155.5 (C4), 146.0 (C11), 144.3 (C8), 138.3 (C6), 116.1 (C12), 115.5 (C13), 111.9 (C1), 111.5
(C10), 107.5 (C5), 99.4 (C3), 44.9 (NCH2), 13.2 (CH3). IR (ATR) ν (cm-1) 1614 (C=N), 1503
(NO2), 1325 (NO2), 934 (MoO2), 809 (Mo-O-Mo). HRMS (ESI) calcd. for C17H18N3O6Mo,
[M++1] 458.0244, found 458.0250 [M++1]. TGA: 30.8 % MoO3 residue (requires 31.6 %).
Anal. Calc. for C17H17MoN3O6 (Mr = 455.27): C, 44.8; H, 3.8; N, 9.2. Found: C, 44.7; H, 3.2;
N, 9.1%.
2.4. Catalysis experiments. In a typical experiment, cyclooctene (1 equiv) and the
catalyst (0.0025 equiv) were mixed and stirred in air in a round bottom flask. Acetophenone
was added as internal standard (ca. 2% vs. substrate). After the reaction temperature was
regulated at 80°C, wet THBP (70% in water, 2 equiv) was added to the mixture, starting the
reaction. All complexes are only slightly soluble at room temperature but appear to
completely dissolve after addition of TBHP at the reaction temperature. The color indicates
that both complexes are confined in the organic phase, whereas the water phase is colorless or
only slightly colored. Samples of the organic phase were periodically withdrawn. The reaction
was quenched by addition of MnO2, followed by the addition of diethylether and removal of
the manganese oxide and residual water by filtration through silica before GC analysis.
2.5. DFT Calculations. The geometries of all species were optimized without any
symmetry constraint with the Gaussian 09 program suite [17]. The input geometries were
adapted from the X-ray structures of [MoO2(SAP)(EtOH)] [18], replacing one hydrogen by
NEt2 and/or NO2 fragment depending on the molecule studied. The standard B3LYP three-
parameter functional was used in conjunction with the 6-31G** basis set for C, H, N and O
atoms and with the CEP-31G* basis set for molybdenum. The transition states were optimized
using a preliminary scan of a relevant internal coordinate, followed by full optimization of the
TS guided by knowledge of such coordinate. All optimized geometries were confirmed to be
local minima (for stable molecules or reaction intermediates) or first order saddle points (for
the TS’s) by frequency analyses. For the TS’s, analysis of the imaginary frequency confirmed
the expected motion along the reaction coordinate. The calculated frequencies were also used
to derive the thermochemical parameters at 298 K according to the standard ideal gas
approximation.
2.6. X-Ray Crystallographic Study. Single crystals of [MoO2(L1)(DMSO)] and
[MoO2(L3)(DMSO)] suitable for X-ray structural analyses were mounted on an Enraf Nonius
6

Kappa-CCD and the data recorded at ambient temperature whereas the compound
[MoO2(L4)(DMSO)] was mounted under inert perfluoropolyether on the tip of a loop and
cooled in the cryostream of an Enraf Nonius fitted with a Bruker ApexII CCD detector. The
three structures were solved by direct methods using SHELXS97 [19] or SIR97 [20] and
refined by least-squares procedures on F2 using SHELXL-2013 [21]. All H atoms attached to
the C, atoms were introduced at idealised positions and treated as riding on their parent atoms
in the calculations. In the first two compounds, the DMSO ligand is disordered over two
positions, moreover in [MoO2(L1)(DMSO)], the L1 ligand is disordered by pseudo symmetry
around a pseudo mirror which is also involved in the disorder of the DMSO. All disordered
parts were treated with the tools available in SHELXL-2013. The drawing of the molecules
was realised with the help of ORTEP3.[22] Crystal data and refinement parameters are shown
in Table 1. Crystallographic data (excluding structure factors) have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 1025022-
1025024. Copies of the data can be obtained free of charge on application to the Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
Table 1. Crystal data and structure refinement for the complexes [MoO2(L1)(DMSO)],
[MoO2(L3)(DMSO)] and [MoO2(L4)(DMSO)].
Empirical formula
[MoO2(L1)(DMSO)](DMSO) [MoO2(L3)(DMSO)]
[MoO2(L4)(DMSO)](H2O)0.5
C19H24MoN3O7.5S
C17H21MoNO6S2
C19H24MoN2O5S
Formula weight
Temperature
Wavelength
Crystal system
Space group
495.41
293(2)
0.71073
monoclinic
P21/c
488.40
293(2)
0.71073
monoclinic
P21
542.41
293(2)
0.71073
triclinic
P-1
Unit cell parameters
a (Å) 12.5387(5)
b (Å) 6.8015(2)
c (Å) 24.0157(10)
9.210
7.947
14.503
90
102.46
90
1036.5
2
1.565
0.766
7.7016(11)
12.3088(16)
12.8230(19)
117.249(5)
91.073(6)
92.406(6)
1078.7(3)
2
1.670
0.754
90
93.460(2)
90
2044.37(13)
4
1.610
0.878
1008
0.25 X 0.1 X 0.05
2.91-27.48
α (deg)
β (deg)
γ (deg)
Volume (Å3)
Z
ρcalcd (g cm–3)
µ (mm–1)
F(000)
500
554
Crystal size (mm3)
θ range for data
collection (°)
0.25 X 0.125 X 0.05
2.91-27.48
0.45 X 0.025 X 0.013
2.65-26.43
Reflections collected 12136
10413
29410
Reflections
4053 [R(int) = 0.0498]
3770 [R(int) = 0.0275] 4574 [R(int) = 0.0736]
independent
Completeness to θmax 96.7
99.3 97.0
7

(%)
Absorption
correction
multiscan
multiscan
multiscan
Max./min.
transmission
Refinement method
0.9591/0.8953
0.9496 / 0.8971
0.7454 /0.5136
Full-matrix least-squares on
Full-matrix least-
Full-matrix least-squares on
2
2
2
F
squares on F
F
Data / restraints /
parameters
4053 / 9/ 304
3770 / 8 / 295
4574 / 2 / 299
Goodness–of–fit on
1.049
1.045
1.047
2
F
Final R indices
[I>2σ(I)]
R indices (all data)
R1 = 0.0520
wR2 = 0.1144
R1 = 0.0741
wR2 = 0.1279
R1 = 0.0267
wR2 = 0.0622
R1 = 0.0301
wR2 = 0.0636
R1 = 0.0638
wR2 = 0.1616
R1 = 0.0727
wR2 = 0.1710
Largest diff. peak /
hole
-3
-3
-3
1.022 / -0.768 e.Å
0.304 /-0.415 e.Å
2.860 / -2.130 e.Å
3. Results and Discussion
3.1. Synthesis of molybdenum complexes
The molybdenum complexes [MoO2L1-4]2 were obtained by refluxing the corresponding
Schiff bases H2L1-4 in ethanol with [MoO2(acac)2] (Scheme 2). After two hours of reflux,
evaporation of the solvent yielded red-orange coloured solids, presumably corresponding to
the monomeric complexes [MoO2(L)(EtOH)]. As previously shown for the
[MoO2(L1)(EtOH)] complex,[7c, 18, 23] the slow release of the coordinated ethanol molecule
leads to the dimeric complex [MoO2(L1)]2. For accurate stoichiometry control in the catalytic
experiments and easier handling, the ethanol molecule was completely removed under
reduced pressure from each product to yield dark solids corresponding to the binuclear
complexes [MoO2(L1-4)]2. The IR and TGA analyses confirmed the absence of residual
ethanol in the isolated products. Thermal analyses gave one mass loss with MoO3 as residue,
as seen earlier with [MoO2(L1)]2 [7c,18]. The IR spectra of the dinuclear complexes (see
Table 2) show a band for the C=N vibration in the 1603-1615 cm-1 range and two major and
narrow Mo-O bands, one corresponding to the Mo=O stretching vibration (around 922-935
cm-1) and another one to a Mo-O-Mo vibration (around 798-844 cm-1) indicating the dimeric
nature of these species. Indeed, for polymeric forms the latter vibration is broader and
displaced to lower frequencies. Mass spectrometry showed in all cases the presence of the
[MoO2L] fragment with a m/z value corresponding to the mass of the molecular ion + 1. The
1H NMR investigations of the molybdenum complexes were performed in DMSO-d6, yielding
the expected signals for the Schiff base ligand, displaced from those of the H2L molecule In
8

particular, the imine signal was significantly downfield shifted (see Table 1). It should be
noted that under these conditions the observed compounds are probably the mononuclear
DMSO adducts, since these can be obtained as single crystals from the DMSO solutions (see
the X-ray characterization in the next section). The imine proton shows a characteristic shift
depending on the substituent present in the ligand backbone.
Scheme 2- Synthesis pathway of dimeric Mo complexes.
Table 2 - Selected IR and 1H NMR data for ligands and complexes.
Compound
IRa
Mo=O
1H NMRb
Mo-O-Mo CH=N
C=N
[MoO2(L1)]2 1608
[MoO2(L2)]2 1603
[MoO2(L3)]2 1615
[MoO2(L4)]2 1614
935
922
927
934
808
798
844
809
9.25 (8.99)
9.43 (8.99)
8.89 (8.61)
9.01 (8.77)
a Solid state. b In DMSO-d6 solution (values for the corresponding H2L molecule in
parentheses).
3.2. X-ray characterization
Tridentate Schiff bases complexes of type [MoO2L(DMSO)] are known to be easily
crystallized in DMSO [24]. Single crystals of compounds [MoO2(L1)(DMSO)],
[MoO2(L3)(DMSO)] and [MoO2(L4)(DMSO)] have been obtained by slow evaporation of
ethanol from ethanol/DMSO solutions and analyzed X-ray diffraction. The resulting
9

molecular structures are described herein. While compound [MoO2(L1)(DMSO)] crystallized
with one additional DMSO molecule and compound [MoO2(L4)(DMSO)] with one water
molecule per two molecules of complex, the structure of compound [MoO2(L3)(DMSO)] did
not contain any solvent of crystallization. Views of the three molecular structures are in
Figures 1-3, and relevant bond distances and angles are compared in Table 3.
For all compounds, the system shows the presence of a cis-{MoO2}2+ fragment
surrounded by a tridentate ONO ligand in doubly deprotonated form (L2-). An octahedral
geometry around the molybdenum is observed, two cis-positions of the octahedron being
occupied by two oxido oxygen atoms (angles in the range 104.8(2)°-105.4(1)°), and three
mer-positions being occupied by the ONO tridentate ligand. The sixth position is occupied by
a molecule of DMSO with binding through its O atom.
In the case of L1 and L3, the DMSO ligand shows disorder. For [MoO2(L1)(DMSO)],
the DMSO disorder may be related to a disorder of the imine function linking the two
aromatic rings. The Mo(1)-O(1) and Mo(1)-O(2) bonds are identical and the aromatic rings of
the two forms are superimposed. The two different ligand positions around the molybdenum
atom, particularly the different orientations of the Mo-N bond in the two possible
arrangements, possibly induce the DMSO positional disorder. The same explanation,
however, cannot be valid for the [MoO2(L3)(DMSO)] structure, since there is no disorder in
the Schiff base orientation. The diethylamino group is quasi-coplanar with the aromatic group
in the structures of [MoO2(L3)(DMSO)] and [MoO2(L4)(DMSO)] (dihedral angles between
the phenyl C6 ring and the NC2 fragment of 11.2(4)° and 4.8(5)°, respectively), showing π
conjugation (sp2 hybridization for the N atom), which is confirmed by the N atom trigonal
planarity with a sum of the three bond angles of 360.0(4)° and 359.8(4)°, respectively).
Finally, the NO2 group in the structure of [MoO2(L4)(DMSO)] is also quasi coplanar with the
aromatic ring, with a dihedral angle of 5.0(6)° for O11-N11-C14-C13.
10

Figure 1 - ORTEP view of [MoO2(L1)(DMSO)]. The ellipsoids are drawn at the 30%
probability level. The second form is represented by dotted lines.
Figure 2- ORTEP view of [MoO2(L3)(DMSO)]. The ellipsoids are drawn at the 30%
probability level. The second position of DMSO is represented by dotted lines.
Figure 3- ORTEP view of [MoO2(L4)(DMSO)]. The ellipsoids are drawn at the 30%
probability level.
Table 3 - Selected bond lengths and angles for all compounds.
[MoO2(L1)(DMSO)] [MoO2(L3)(DMSO)] [MoO2(L4)(DMSO)]
Bonds
Mo=O
1.696(4)
1.686(4)
1.929(4)
1.943(4)
2.267(6)
1.687(4)
1.698(3)
1.926(3)
1.973(3)
2.23(1)
1.701(3)
1.707(2)
1.941(3)
1.980(3)
2.318(3)
Mo-O
Mo-
O(DMSO)
Mo-N
2.281(8)
2.266(3)
2.245(3)
11

CH=N
Angles
1.27(1)
1.297(5)
1.308(5)
O=Mo=O
O-Mo-O
104.8(2)
151.9(2)
105.2(2)
152.7(1)
105.4(1)
152.4(1)
3.3. Catalysis results
Molybdenum tridentate complexes with structures very close to L1 have been tested in
epoxidation by several authors in organic solvents [25]. With the aim of designing a cleaner
process, we are exploring solvent-free epoxidation conditions [7-9], i.e. using aqueous TBHP
as oxidant (water only being the oxidant carrier and not a solvent for the catalytic reaction)
and no added organic solvent.
All complexes have been tested under the same experimental solvent-free conditions
and using the same substrate (cyclooctene) already used for the previously reported
epoxidation with [MoO2(L1)]2 [7c] in order to evaluate the potential role of the substituents in
the ligand backbone on the catalytic activity. In order to ensure solubility of all complexes in
the reaction medium, a very low catalyst/substrate ratio (0.25/100) was used. While the
complexes do not dissolve in the substrate phase at room temperature, they become fully
soluble in the organic phase upon warming in the presence of the TBHP oxidant, which is also
transferred to the organic substrate phase. The small amount of water originating from the
oxidant solution remains as a separate colourless phase. The results have been compiled in
Table 4 and the kinetic results are shown in Figure 4.
Scheme 3 – Catalyzed epoxidation reaction with [MoO2(L1-4)]2 complexes.
Table 4. Relevant results for the cyclooctene epoxidation catalysis.a
Complex
[MoO2(L1)]2
[MoO2(L2)]2
[MoO2(L3)]2
[MoO2(L4)]2
Conversion
Epoxide selectivity
TON
286
346
252
293
TOFb (h-1)
71
86
62
73
94
96
93
91
195
344
167
192
12

[a] Conditions: substrate/[Mo] = 400:1; T = 80 °C; time = 4 h. [b]calculated from initial slope
(30 min).
100
75
50
25
0
0
60
120
180
240
Time (min)
Figure 4-. Kinetic profile of converted cyclooctene vs. time with dioxidomolybdenum(VI)
catalysts: [MoO2(L1)]2 (ꢀ), [MoO2(L2)]2 (ꢁ), [MoO2(L3)]2 (∆), [MoO2(L4)]2 (×). Conditions:
substrate/[Mo] = 400:1; T = 80 °C.
The cyclooctene conversion after 4 h decreases in the following order [MoO2L2] >
[MoO2L4] ≈ [MoO2L1] > [MoO2L3]. The selectivity, however, is always high (always greater
than 90%). The presence of the electron withdrawing NO2 group (ligand L2) has a small
beneficial effect on the activity, while the NEt2 donor group (ligand L3) shows a weak
detrimental effect. When both groups are present within the structure (ligand L4), the activity
is very close to that of the parent SAP backbone (ligand L1).
13

Scheme 4 – Most plausible mechanism for the epoxidation catalytic cycle with [MoO2(L)]
[7c].
The mechanism of the catalytic cycle for the reaction catalyzed by complex
[MoO2(L1)]2, as suggested by experimental evidence and a DFT study, is recalled in Scheme
4 [7c]. The catalytically active species is the 5-coordinate [MoO2(L1)] monomer, which is
able to activate the oxidant by coordination with assistance by H-bonding to one oxido ligand.
The [MoO2(L1)]···TBHP adduct is able to transfer the Oα atom to the external olefin in a way
reminiscent of the Bartlett mechanism of stoichiometric olefin epoxidation by peracids [10].
The [Mo(OtBu)(OH)(L1)] intermediate generated by the Oα atom transfer step then closes the
cycle by eliminating tBuOH. Under the hypothesis that the mechanism of is the same for all
complexes , we can intuitively estimate that the electron-withdrawing nitro substituent on the
SAP ring favours the approach of TBHP to the molybdenum atom and logically also the atom
transfer. However, since the cycle energy span is given by the gap between the TBHP adduct
(resting state) and the O atom transfer transition state, it is necessary that the NO2 stabilizes
the latter more than the former in order to provide an accelerating effect. The opposite
situation would occur upon introduction of the electron-donating diethylamino group. These
assumptions have been supported by theoretical calculations.
14

3.4. Mechanistic rationalization through DFT calculations
We have extended the recently published DFT investigation of the catalytic cycle for
the SAP (L1) system, using ethylene as the model olefin, to the corresponding systems
containing L2, L3 and L4. As mentioned above, on the basis of experimental mechanistic
information gathered on the SAP (L1) system [7c] it was concluded that the active form of the
catalyst is the mononuclear 5-coordinate [MoO2(L1)] species and the most plausible (lowest
energy span) pathway is recalled in Scheme 4, with an energy span of 22.5 kcal/mol on the
enthalpy scale.
The results of the calculations with the four systems have been summarized in Figure 5.
The coordination of TBHP to [MoO2(L)] stabilizes the system by 8.4-8.6 kcal/mol on the
enthalpy scale, the electronic effect of the ligand substituents being very small. Entropic
effects are rather equivalent for the various systems, yielding Gibbs energy changes that are
also restrained within a small range. The greater stabilization is provided by the L2 ligand,
containing the electron-withdrawing nitro group, but the electron-donating diethylamino
group in L3 also provides a weak stabilizing effect. The very small extent of these changes is
certainly related to the weakness of the Mo···TBHP interaction, as suggested by the long
Mo···Oβ distance and to the competing electronic effects on the Mo···Oβ and Mo=O···H
interactions in the [MoO2(L1)]···TBHP adduct (I in Scheme 4). Figure 6 shows views of these
resting states for the four different catalysts, with relevant distances. Indeed, the Mo···Oβ
distance follows the trend L3 > L4 > L1 > L2, the longer one being assosciated to the ligand
with the electron-donating Et2N group and the shorter one to the electron-withdrawing NO2
group. The presence of both groups yields an intermediate distance, close to that observed
with the parent L1 ligand. Conversely, the Mo=O···H becomes progressively longer along the
same series: L3 < L4 < L1 < L2. The O-O and O-H distances in the TBHP molecule and the
M=O distance in the pentacoordinated [MoO2(L)] species do not appear significantly altered
by the electronic effects of the ligand substituent.
15

Figure 5 – Energy profile of the reaction along the three catalytic cycles of Scheme 4. The
indicated values are relative enthalpies (with Gibbs energies in parentheses) at 298 K in
kcal/mol.
16

Figure 6. Views and selected distances of the optimized [MoO2(L)] ···TBHP adducts for the
four different catalysts. All H atoms except the H atom of the TBHP reagent have been
removed for the sake of clarity.
On the other hand, the effect of the ligand substituents on the energy of the rate
determining transition state is more substantial. Taking the pentacoordinated [MoO2(L)]
complex as the reference for the energy scale, the more electron withdrawing L2 ligand
stabilizes the TS by 1.7 kcal/mol relative to the parent L1 ligand, whereas the more electron
donating L3 ligand destabilizes it by 0.8 kcal/mol. The combination of both substituents in L4
provides a very weak stabilizing effect of 0.2 kcal/mol. The structural analysis of the TS
geometries (see Figure 7) rationalizes the stronger energetic effect as being essentially related
to the stronger Mo···Oβ interaction. Indeed, the trend observed for the Mo···Oβ distance (L3 >
L4 > L1 > L2) is the same as for intermediate I. Although the span of the Mo···Oβ distance in
17

the transition state on going from L3 to L2 (0.055 Å) is in fact smaller than in intermediate I
(0.180 Å), the effect on the energy span is of course greater because of the greater strength of
the incipient covalent bond. The Mo=O···H distances in the TS are significantly shorter than
in the corresponding intermediates I because of the incipient proton transfer from TBHP to
the oxido ligand, but the interaction remains weak and the electronic effect of the substituents
in the two systems is similar (span from L2 to L3 of 0.032 Å in the TS, vs. 0.029 Å in
intermediate I). Thus, the competing effect on the O···H interaction is less relevant than for
intermediate I. The contribution of the O···O breaking process does not seem to be affected
by the L substituents, as judged by the relatively uniform O···O distance.
Figure 7. Views and selected distances of the optimized transition states for the rate-
determining O atom transfer step with the four different catalysts. All H atoms except the H
atom involved in the proton transfer process have been removed for the sake of clarity.
The most important parameter is the energy span of the catalytic cycle [26]. On the
enthalpy scale, it goes in the order L3 (23.8 kcal/mol) > L1 (22.5 kcal/mol) ~ L4 (22.3
kcal/mol) > L2 (21.0 kcal/mol). The same trend is calculated on the Gibbs energy scale (L3,
37.0; L1, 35.5; L4, 35.5; L2, 34.2 kcal/mol), since the entropic contribution is essentially the
18

same for all systems. This trend is in agreement with the experimentally observed faster
epoxidation in the presence of the L2 complex, with the slower epoxidation in the presence of
the L3 complex, and with an intermediate and nearly equivalent activity in the presence of the
L1 and L4 complexes.
4. Conclusions
We have explored in this study the electronic effect of substituents on tridentate ONO-type
Schiff base ligands (L), which are based on the salicylidene-2-aminophenolato structure, on
the solvent-free molybdenum catalyzed epoxidation of cyclooctene. The presence of an
electron-withdrawing nitro group increases the catalytic activity, whereas the electron-
donating diethylamino group decreases it, although the measured effects are rather small. A
DFT computational analysis, building on the recently established catalytic cycle that involves
the [MoO2(ONO)]···TBHP adduct as resting state and a Bartlett-style oxygen atom transfer to
external olefin as the rate-determining transition state of the cycle, suggests that the
substituent electronic effects affect the cycle energy span mostly through their effect on the
transition state energy, whereas their effect on the resting state energy is relatively
unimportant, given the weakness of the Mo-THBP interaction and the compensation of the
opposite effects on the Mo···Oβ and Mo=O···H interactions.
5. Acknowledgements
The present work has been carried out within the framework of the French-Mexican
International Laboratory (LIA) LCMMC-CONACYT. We acknowledge the Centre National
de la Recherche Scientifique (CNRS) and the Institut Universitaire de France (IUF) for
financial support. We thank the Universidad Nacional Autónoma de México, the Université
Paul Sabatier and its Institut Universitaire Technologique for the facilities, the China
Scholarshop Council for the fellowship of W.W. (No.201208370239), and CONACyT for the
grant (No. 318435) to T. G.
6. References
19

[1] E. N. Jacobsen, A. Pfaltz, H. Yamamoto, Comprehensive Asymmetric Catalysis,
Springer, Berlin, (1999).
[2] (a) R. N. McDonald, R. N. Steppel, J. E. Dorsey, Org. Synth. 50 (1970) 15. (b) E. Rose,
B. Andrioletti, S. Zrig, M. Quelquejeu-Etheve, Chem. Soc. Rev., 34 (2005) 573. (c) M.
Bagherzadeh, L. Tahsini, R. Latifi, L. K. Woo, Inorg. Chim. Acta, 362 (2009) 3698.
[3] (a) P. Tundo and al, P. Appl. Chem. 72 (2000) 1207. (b) P.T. Anastas, J.C.
Warner,Green Chemistry: Theory and Practice, Oxford University Press: New York,
(1998) 30.
[4] R.A. Sheldon, I. Arends, U. Hanefeld, Green Chemistry and Catalysis, Wiley VCH,
2007
[5] R. Landau, G. A. Sullivan, D. Brown, Chemtech, 9 (1979) 602.
[6 ] J. Kollar (Halcon International), US 3350422, 1967;J. Kollar (Halcon International), US
3351635, 1967;J. Kollar (Halcon International), US 3507809, 1970;J. Kollar (Halcon
International), US 3625981, 1971.
[7] (a) J. Pisk, D. Agustin, V. Vrdoljak, R. Poli, Adv Synth Catal, 353 (2011) 2910. (b) J.
Pisk, B. Prugovečki, D. Matković-Čalogović, R. Poli, D. Agustin, V. Vrdoljak,
Polyhedron, 33 (2012) 441. (c) J. Morlot, N. Uyttebroeck, D. Agustin, R. Poli,
ChemCatChem, 5 (2013) 601. (d) M. Loudidi, D. Agustin, A. Benharref, R. Poli, C.R.
Chimie, 17 (2014) 549. (e) J. Pisk, B. Prugovečki, D. Matković-Čalogović, T. Jednačak,
P. Novak, D. Agustin, V. Vrdoljak, RSC Adv., 4 (2014) 39000. (f) W. Wang, T.
Vanderbeeken, D. Agustin, R. Poli, Cat. Commun. (2014), in press, doi:
10.1016/j.catcom.2014.08.018. (g) V. Vrdoljak, J. Pisk, D. Agustin, P. Novak, J. Parlov
Vuković, D. Matković-Čalogović. New J. Chem., 38 (2014) 6176.
[8] C. Cordelle, D. Agustin, J.-C. Daran, R. Poli, Inorg Chim Acta, 364 (2010) 144.
[9] B. Guérin, D. Mesquita Fernandes, J.-C. Daran, D. Agustin, R. Poli: New. J. Chem. 37
(2013) 3466.
[10] P. D. Bartlett, Rec. Chem. Prog. 11 (1950) 47.
[11] (a) J. Topich, Inorg. Chem. 20 (1981) 3704. (b) J. Topich, Inorg. Chim. Acta, 46 (1980)
L37.
[12] G. J. J. Chen, J. W. McDonald, W. E. Newton, Inorg. Chem. 15 (1976) 2612.
[13] B. O. Öztürk, E. Bucak, S. Karabulut, J. Mol. Cat. A: Chem. 376 (2013) 53.
[14] M. Rodríguez, M. E. Ochoa, R. Santillán, N. Farfán, V. Barba, J. Organomet. Chem.
690 (2008) 2975.
20

[15] K. A. Neeraj; V. Kumar, R. Prajapati, S.K. Asthana, K. K. Upadhyay, J. Zhao, Dalton
Trans. (2014) 5831.
[16] B. Muñoz, R. Santillán, M. Rodríguez, J. M. Méndez, M. Romero, N. Farfán, P.
Lacroix, K. Nakatani, G. Ramos-Ortíz, J. L. Maldonado, J. Organomet. Chem. 693
(2008) 1321.
[17] M. J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M.
Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J.
J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K.
Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N.
J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J.
Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J.
Cioslowski, D. J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT,
2009.
[18] D. Agustin, C. Bibal, B. Neveux, J.-C. Daran, R. Poli, Z. Anorg. Allg. Chem. 635
(2009) 2120.
[19] G. M. Sheldrick, Acta Cryst. A 64 (2008)112.
[20] A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C. Giacovazzo, A. Guagliardi,
A. G. G. Moliterni, G. Polidori, R. Spagna, J. Appl. Cryst. 32 (1999)115.
[21] G. M. Sheldrick, SHELXL-2013 (2013) University of Göttingen.
[22] (a) L. J. Farrugia, J. Appl. Cryst. 30 (1997) 565. (b) M. N. Burnett, C. K. Johnson,
ORTEPIII. Report ORNL-6895, Oak Ridge National Laboratory, Tennessee, USA,
(1996)
[23] O. A. Rajan, A. Chakravorty, Inorg. Chem. 20 (1981) 660.
[24] M. Cindric, N. Strukan, V. Vrdoljak, B. Kamenar, Boris, Z. Anorg. Allg. Chem. 630
(2004) 585.
[25] (a) M. Bagherzadeh, S. G. Esfahani, Scientia Iranica C, 17 (2010) 131. (b) C. Bibal, J.-
C. Daran, S. Deroover, R. Poli, Polyhedron, 29 (2010) 639. (c) J. M. Sobczak, J. J.
Ziolkowski, Appl. Catal., A: Gen. 248 (2003) 261. (d) D. D. Agarwal, S. Shrivastava,
21

Polyhedron 7 (1988) 2569. (e) D. D. Agarwal, J. Mol. Catal. 44 (1988) 65. (f) Z.
Dawoodi, R. L. Kelly, Polyhedron 5 (1986) 271. (g) A. Rezaeifard, M. Jafarpour, H.
Raissi, M. Alipour, H. Stoeckli-Evans, Z. Anorg. Allg. Chem. 638 (2012) 1023. (h) A.
Rezaeifard, I. Sheikhshoaie, N. Monadi, M. Alipour, Polyhedron 29 (2010) 2703. (i) A.
Rezaeifard, I. Sheikhshoaie, N. Monadi, H. Stoeckli-Evans, Eur. J. Inorg. Chem. (2010)
799. (j) Y. Sui, X. Zeng, X. Fang, X. Fu, Y. Xiao, L. Chen, M. Li, S. Cheng, J. Mol.
Catal. A: Chem., 270 (2007) 61. (k) X. Zhou, J. Zhao, A. M. Santos, F. E. Kuehn, Z.
Naturforsch., B, 59 (2004) 1223. (l) J. Zhao, X. Zhou, A. M. Santos, E. Herdtweck, C.
C. Romao, F. E. Kuehn, Dalton Trans. (2003) 3736. (m) L. Casella, M. Gullotti, A.
Pintar, S. Colonna, A. Manfredi, Inorg. Chim. Acta 144 (1988) 89.
[26] S. Kozuch, S. Shaik, Acc. Chem. Res., 44 (2011) 101.
22

Highlights
•
Synthesis and characterization of substituted tridentate Schiff-base
dioxomolybdenum complexes
•
•
•
These [MoO2L]2 complexes are active epoxidation catalysts
Analysis of the molecular structures
Electronic effect of donor and acceptor substituents on the catalytic activity