OH-substituted tridentate ONO Schiff base ligands and related molybdenum(VI) complexes for solvent-free (ep)oxidation catalysis with TBHP as oxidant

Article abstract of DOI:10.1016/j.molcata.2016.02.021

Molybdenum complexes with ONO tridentate Schiff base ligands based on salicylideneaminophenolato (SAP) backbone with OH substitution on the salicyl arene ring have been synthesized and characterized. The molecular structures of the full series of OH substituted molybdenum complexes were determined by X-ray crystallography as monomers stabilized by one solvent molecule, [MoO₂L(D)]. All dimeric complexes [MoO₂L]₂ were tested for the epoxidation of cyclooctene and cyclohexene under organic solvent-free conditions using aqueous TBHP as oxidant. The position of the OH on the SAP-modified ligand influenced the catalytic activity of the respective complexes. DFT calculations for the catalytic cycle yield energy spans in agreement with the experimentally observed activity trend.

Full text of DOI:10.1016/j.molcata.2016.02.021

Accepted Manuscript
Title: OH-Substituted Tridentate ONO Schiff base ligands and
related molybdenum(VI) complexes for solvent-free
(ep)oxidation catalysis with TBHP as oxidant.
Author: Weili Wang Jean-Claude Daran Rinaldo Poli
Dominique Agustin
PII:
S1381-1169(16)30055-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.02.021
MOLCAA 9787
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
18-1-2016
10-2-2016
19-2-2016
Please cite this article as: Weili Wang, Jean-Claude Daran, Rinaldo Poli,
Dominique Agustin, OH-Substituted Tridentate ONO Schiff base ligands
and related molybdenum(VI) complexes for solvent-free (ep)oxidation
catalysis with TBHP as oxidant., Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.02.021
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OH-Substituted Tridentate ONO Schiff base ligands and related molybdenum(VI) complexes for
solvent-free (ep)oxidation catalysis with TBHP as oxidant.
Weili Wang,a,b, Jean-Claude Daran,a Rinaldo Poli,a,c Dominique Agustin*a,b
a
CNRS; LCC (Laboratoire de Chimie de Coordination); Université de Toulouse; UPS, INPT, 205,
route de Narbonne, F-31077 Toulouse, France Fax: +33-561553003
b
Université de Toulouse; Institut Universitaire de Technologie Paul Sabatier - Département de
Chimie, Av. Georges Pompidou, BP 20258, F-81104 Castres Cedex, France E-mail:
dominique.agustin@iut-tlse3.fr.
c Institut Universitaire de France 103, bd Saint-Michel, 75005 Paris, France

Graphical abstract

Highlights
 New molybdenum complexes have been synthesized and characterized
 Their catalytic activity was studied under organic solvent-free conditions.
 DFT calculations confirmed the observed trends.
Abstract
Molybdenum complexes with ONO tridentate Schiff base ligands
based
on
salicylideneaminophenolato (SAP) backbone with OH substitution on the salicyl arene ring have been
synthesized and characterized. The molecular structures of the full series of OH substituted
molybdenum complexes were determined by X-ray crystallography as monomers stabilized by one
solvent molecule, [MoO2L(D)]. All dimeric complexes [MoO2L]2 were tested for the epoxidation of
cyclooctene and cyclohexene under organic solvent-free conditions using aqueous TBHP as oxidant.
The position of the OH on the SAP-modified ligand influenced the catalytic activity of the respective
complexes. DFT calculations for the catalytic cycle yield energy spans in agreement with the
experimentally observed activity trend.
Keywords
Molybdenum, solvent-free, epoxidation, DFT, X-ray structures

1. Introduction
Epoxides are useful reagents for different applications in fine chemistry and polymer chemistry
[1]. They are generally obtained through classical organic reactions using organic oxidants (meta-
chloroperbenzoic acid or m-CPBA, CH3CO3H or PhIO), with the use of organic solvents (often
chlorinated) or ionic liquids [2]. The organic by-products produced by these oxidants and the solvents
can be considered as waste, increasing the E factor [3,4]. In addition, certain chlorinated solvents are
potentially toxic and their use in several industrial sectors is not recommended [5]. Therefore, using a
lower amount of solvent or a safer one or, even better, no organic solvent at all represents a
challenging benefit for green chemistry. Catalysis brings several advantages to green chemistry in
general, and in the particular case of epoxidation processes also through the possibility to replace the
above mentioned oxidants with others having lower environmental impact (e.g. tert-
butylhydroperoxide or TBHP, H2O2, O2). There are, however, economic and safety issues about the
amount (cost) of the needed catalyst and its toxicity. Molybdenum is the most commonly used
transition metal in academic and industrial applications of epoxidation catalysis [6], in combination
with many different supporting ligands. We and others have been interested in the use of tridentate
ONO and ONS Schiff base ligands in combination with molybdenum in organic solvents [7] and
without solvent [8, 9]. We have recently shown high catalytic efficiencies at relatively low metal
loadings for different types of molybdenum and vanadium complexes [Error! Bookmark not defined.,
10] as well as for polyoxometalates [11] for the epoxidation of cyclooctene as a model substrate and of
bio-sourced compounds [12] using aqueous TBHP as oxidant and no added organic solvent. Several
advantages can be pointed out for these processes. The by-product and waste product (tBuOH and
water) are non-toxic and can be recycled, tBuOH being also used as additive in the gasoline industry
as well as for the synthesis of methyl tert-butyl ether (MTBE) [13]. The salicylideneaminophenolato
(SAP) and salicylideneaminothiophenolato (SATP) ligands have yielded good results in terms of
reaction efficiency and catalyst stability [Error! Bookmark not defined.] under these organic
solvent-free conditions. Using DFT calculations, we have also established that the intimate mechanism
of the catalytic cycle involves activation of TBHP by coordination to the catalytic metal without
proton transfer followed by oxygen transfer to the olefin in a way that closely resembles the
mechanism proposed by Bartlett for the stoichiometric epoxidation with peracids [14]. Slight
improvements of these results could be achieved by tuning the SAP ligand through the addition of
electron donor (NEt2) and/or acceptors (NO2) substituents on the ligand backbone [15]. Therefore, we
have wondered whether the introduction of simple OH groups on the SAP ligand backbone could also
positively affect the catalyst performance. The synthesis and characterization of the modified systems
as well as the catalytic test on different substrates will be discussed herein.

2. Results and discussion
2.1. Synthesis and characterization
2.1.1.Synthesis
Reaction of the previously reported [16, 17, 18, 19] H2L1-3 molecules with [MoO2(acac)2] in
refluxing alcohol gave, by analogy with the parent H2SAP ligand [Error! Bookmark not defined.a,
Error! Bookmark not defined.] and with other related systems [20], either mononuclear alcohol
adducts, [MoO2L(ROH)], or oligonuclear [MoO2L]n depending on the nature of the ligands and on the
reaction conditions (see Scheme 1). The reaction of H2L1 in EtOH and H2L2 in MeOH yielded the new
mononuclear complexes [MoO2L1(EtOH)] and [MoO2L2(MeOH)], respectively, which could be
transformed to the alcohol free systems upon drying under vacuum at high temperatures. The
recovered product of the reaction of H2L3 and [MoO2(acac)2] was dried directly to yield [MoO2L3]n.
2.1.2.IR Characterizations
The most relevant IR spectroscopic properties of the new complexes are collected in Table 1
and compared with those of the previously reported ligands and [MoO2(SAP)]2 complex. The key
feature in the IR spectrum of the H2SAP and H2L1-3 molecules is the C=N stretching vibration around
1600-1630 cm-1. Bonding of L2- to the {MoO22+} fragment is accompanied by a small red shift of this
band to the 1600-1610 cm-1 region. The vibration pattern of the {MoO22+} fragment observed between
750 and 1100 cm-1 gives additional information about the form of the isolated complex, particularly
those associated to the terminal Mo=O bonds that appear in the 850-950 cm-1 region. For the solvent
stabilized monomeric species of general formula [MoO2L(D)], two sharp bands are observed around
910 and 940 cm-1, whereas the dimeric or oligomeric [MoO2L]n feature only one Mo=O vibration in
this region, plus another band corresponding to a Mo-O-Mo vibration at lower frequency (760-820
cm- 1) [21].
2.1.3.NMR Characterizations
1
The H NMR spectrum of the H2L1-3 molecules in DMSO-d6 shows distinct resonances for the
three OH groups of the substituted salicylidene ring, indicating their slow mutual exchange.
Complexation to molybdenum leads to the disappearance of two of the three OH resonances in each
case and to a weak shift of the imine resonance (see SI). Note that, since all NMR measurements of the
dioxomolybdenum complexes were carried out in DMSO-d6, the observed spectrum of each product,
whether mononuclear alcohol adduct or alcohol-free oligomer, corresponds to the [MoO2L(DMSO)]
adduct.

2.1.4.TG analyses
The thermogravimetric analyses of the isolated [MoO2L1-2(ROH)] complexes show loss of the
alcohol molecule below 200°C, followed by loss of the Schiff base ligand at higher temperatures (up
to 600°C). The second process is also observed for the alcohol free oligomers, leaving MoO3 as a
residue as previously observed with similar ligands [22]. The relevant data are collected in Table 2, in
comparison with the parent [MoO2(SAP)]2 compound.[Error! Bookmark not defined.a]
2.2. X-ray structures
All Ln systems (n = 1, 2, 3) have given crystals suitable for a structural determination by X-ray
diffraction in the form of solvent-stabilized mononuclear complexes. For n = 1 and 3, crystals were
obtained from DMSO as [MoO2Ln(DMSO)], whereas for ligand L2 the crystal was obtained from an
acetonitrile/water mixture as the water adduct, [MoO2L2(H2O)]. The complex [MoO2L2(DMSO)] was
also crystallized but its structure could not be satisfactorily refined because of a severe disorder
problem. The common feature of all these complexes is a pseudo-octahedral geometry around the
molybdenum center and a cis arrangement for the {MoO2}2+ fragment, which is surrounded by an
ONO tridentate ligand in a mer arrangement and by a sixth donor molecule D (D being DMSO or
H2O), in accordance with the structures of similar compounds [Error! Bookmark not defined.,
Error! Bookmark not defined.h, Error! Bookmark not defined., 23]. Views of the molecules are
shown in Figure 1.
The [MoO2L1(DMSO)] complex crystallizes as an H-bonded dimer, the interaction being
established between the free OH group of the ligand (O61 atom) of one molecule as a proton donor
with one of the oxido ligands (O2 atom) of the second one as a proton acceptor, see Figure 2 and
relevant bonding parameters in the supporting information (Table S1).
On the other hand, the L3 complex does not show any intermolecular H-bonding but crystallizes
with two additional DMSO molecules per complex in the crystal lattice, one of which is linked to the
free OH group of the L3 ligand through an O-H···O interaction (Figure 3 and Table S2 in the SI).
Finally, complex [MoO2L2(H2O)] crystallizes with one molecule of acetonitrile, which is H-
bonded to the aqua ligand by a O1-H···N2 interaction. In addition, as for compound
[MoO2L1(DMSO)], intermolecular H-bonding involving the free salicyl OH group (atom O51) of one
molecule as a proton donor with one of the oxo groups (atom O2) of a second molecule as a proton
acceptor is also established, yielding dimers in which the two ONO ligands are essentially coplanar.

These dimers are further linked together in a 3-dimensional network via additional H-bonds involving
the water molecule (O1) of one complex as a proton donor and the OH of a complex in another dimer
(O51) as a proton acceptor (Figure 4 and Table S3 in SI).
All relevant bond lengths and angles for the pseudo-octahedral [MoO2Ln(D)] molecules are
collected in Table 3. Interatomic distances around the molybdenum atom are very similar for all
complexes. Some slight differences occur for the angles. The cis conformation of the MoO2 moiety is
observed with O=Mo=O angles of 103.99° and 103.88° for L1 and L3 respectively, whereas a slightly
wider angle of 105.2(1)° is observed for the L2 complex (see) Table 3 and in accordance with similar
structures [Error! Bookmark not defined.e,g,h, 24]. The L ligands deviate slightly from coplanarity,
with very similar dihedral angles for the L1 (10.6°) and L3 (10.4°) complexes, and even smaller for the
L2 complex (5.5°).
2.3. Catalysis studies
2.3.1.Epoxidation of cyclooctene
All synthesized complexes have been tested for the epoxidation of the model substrate
cyclooctene by aqueous TBHP (Scheme 2) at 80°C without added organic solvent. Given the high
concentration (70% w/w) of the aqueous TBHP and the higher affinity of TBHP for cyclooctene, the
reaction mixture consists of a majority of organic phase containing cyclooctene and TBHP and a small
proportion of essentially neat water. The complexes are only slightly soluble in cyclooctene at room
temperature but appear to dissolve completely in the organic phase after addition of TBHP at the
reaction temperature. The organic phase is colored during the entire reaction procedure, whereas the
water phase is colorless or very faintly colored. Standard catalytic tests were carried out using two
equivalents of TBHP per cyclooctene and a 0.5% Mo/substrate ratio.
Introduction of a free OH group in the SAP system produced the effects illustrated in Figure 5.
The selectivity and activity data are detailed in Table 4. While the OH addition at the ortho (L1) and
para (L3) positions of the salicyl ring (particularly the former) increased the catalytic activity relative
to the reference SAP catalyst, substitution at the meta (L2) position had essentially no effect. The
selectivity towards cycloctene oxide is high (>85%) with all catalysts. In the case of the L2 ligand,
there is no significant difference between the ethanol-stabilized mononuclear and the oligomeric
systems, suggesting that the two pre-catalysts may generate the same active species. This result is in
agreement with our previous mechanistic work on the non-substituted SAP system, where it was
concluded that the active catalyst is the 5-coordinate mononuclear [MoO2(SAP)] species, with
growing concentrations of added MeOH having only minor negative effect on the activity [Error!

Bookmark not defined.a]. The observed differences could be intuitively attributed to either different
catalyst solubility or at an electronic effect of the OH substituent. However, all complexes appeared
totally soluble at the reaction temperature. The major acceleration when the free OH group lies in
ortho position, followed by the OH substitution at the para position, seems to indicate a favorable
electronic effect and suggests a transmission of this effect by a combination of the π delocalization and
inductive mechanisms.
The catalytic runs were also carried out with different catalyst loadings down to 0.1% for
[MoO2(SAP)]2 and 0.05% for [MoO2L1]2 (Figure 6). The selectivity remained approximately constant
in the 92-95% range (see details in Table S5 in the SI) upon catalyst dilution for the experiments run
with [MoO2L1]2, whereas it slightly deteriorated from 93% at 1% catalyst to 73% at 0.1% catalyst for
the experiments run with [MoO2(SAP)]2. The observed conversion after 4 h was approximately the
same in the high catalyst concentration range and slightly decreased only when using very low
loadings (< 0.2%), while the TOF correspondingly increased. This phenomenon is not expected for a
stable and fully soluble catalyst, for which the rate is expected to be first order in catalyst and the TOF
should not dependent on the catalyst concentration. The constant conversion in the higher catalyst
concentration range suggests a saturation effect, which seems however contradicted by the visual
evidence of full catalyst dissolution in the reaction medium.
Decreasing the amount of catalyst while keeping the substrate and TBHP amounts constant
entails an increase of the water/[Mo] ratio. In order to probe the possible effect of water, additional
experiments were carried out for catalysts [MoO2(SAP)]2 and [MoO2L1]2 with constant amounts of
catalyst, substrate and TBHP and (0.5/100/200) in the presence of increasing amounts of additional
water. These catalytic runs yield only slightly lower TOF values at higher water/[Mo] ratios, showing
a negative effect of water on the catalysis (for full details, see Table S6 in SI). Therefore, the dramatic
TOF increase observed in Figure 6 at lower catalyst loading (greater water/[Mo] ratios) cannot be
attributed to a chemical activation of the catalyst by water.
2.3.2.Epoxidation of cyclohexene
Cyclooctene oxide is quite robust compared to cyclohexene oxide relative to ring opening in the
presence of water. Thus, cyclohexene oxidation by TBHP was also investigated with same catalysts.
The catalytic procedure was carried out under the same conditions as for cyclooctene (at 80°C with 2
equivalent of aqueous TBHP) except for the use of a catalyst loading of only 0.1% mol vs. substrate,
the catalysts being less soluble in this reaction mixture. The kinetic profiles for the substrate

conversion are shown in Figure 7 and the main activity and selectivity results are collected in Table 5.
Three main products detected within the organic phase for the oxidation of cyclohexene are
cyclohexene oxide (CHO) from epoxidation, trans-1,2-cyclohexanediol (CHD) (due to CHO
hydrolysis) and small traces of 2-cyclohexen-1-ol (CHol) from allylic oxidation, Error! Reference
source not found. [25].
In the case of the reactions catalyzed by [MoO2(SAP)]2 and [MoO2L1]2, the catalyst is totally
dissolved, giving similar bright orange solutions. The [MoO2L3]2 catalyst dissolves slowly but is
eventually fully dissolved in the reaction mixture. Qualitatively, the trend of the dissolution rate is
[MoO2L1]2 ~ [MoO2(SAP)]2 > [MoO2L2]2 > [MoO2L3]2. Like for the epoxidation of cyclooctene, the
highest activity is for the epoxidation of cyclohexene is exhibited by [MoO2L1]2 (see Figure 7). The
slower conversion observed with the other catalysts, especially at the beginning of the reaction, might
be due to the slow catalyst dissolution in the organic phase. The activity trend among the OH
substituted compounds is the same as for the cyclooctene. The catalysts affording the highest
cyclohexene conversion lead to a greater extent of ring opening. Indeed, [MoO2L1]2 and [MoO2L3]2
give less CHO than CHD, while [MoO2L2]2 and [MoO2(SAP)]2 give more CHO than CHD.
2.4. DFT calculations
A cycle for the epoxidation process catalyzed by the [MoO2L] system with L = tridentate Schiff
base has already been proposed on the basis of DFT calculations for the SAP system, using ethylene as
model olefin [Error! Bookmark not defined.a]. On the basis of this cycle, which is recalled in
Scheme 4, the resting state is the TBHP adduct [MoO2L(TBHP)] (I) and the rate-determining
transition state (TS) corresponds to transfer of the TBHP Oα-atom to the outer-sphere olefin, for an
energy span of 22.5 kcal/mol on the ΔH scale. We have recently communicated the greater activity of
the SATP system (L5) and have shown by DFT calculations that the same proposed cycle gives a
lower energy span of 22.3 kcal/mol [Error! Bookmark not defined.b]. We have also recently
reported a beneficial effect of an electron-withdrawing substituent on the aminophenol ring (NO2) and
a negative effect of an electron donating substituent on the salicyl ring (NEt2) on the catalytic activity
of modified SAP catalysts; the DFT calculations, based again on the same mechanism, provided
variation of the energy spans in qualitative agreement with the experimental results (21.0 kcal/mol for
the former, 23.8 kcal/mol for the latter) [Error! Bookmark not defined.]. We have therefore wished
to verify whether this mechanism is also able to reconcile the observed activity trend of the [MoO2L1-
3] systems.
All the key structures involved in the catalytic cycle have been recalculated for the L1, L2 and L3
systems; optimized geometries and energies are reported in the SI (Tables S7 and S8). Relative to the

5-coordinate [MoO2L], TBHP coordination gives very similar stabilization ranging from 8.3 to 8.5
kcal/mol on the ΔH scale for all systems (SAP, -8.4; L1, -8.5; L2, -8.5; L3, -8.3). The relative TS
enthalpy (G) is also very similar for all systems (SAP, 14.1; L1, 13.9; L2, 14.4; L3, 14.3). The result for
the enthalpy span, ESH = ΔH(TS)-ΔH(I), is roughly consistent with the experimentally observed
activity differences when cyclooctene is the substrate: ESH(L1) (22.4) < ESH(SAP) (22.5) < ESH(L3)
(22.6) < ES(L2)H (22.9).
3. Conclusions
The effect of OH substituents at different positions of the salicyl ring of the SAP ligand has
been evaluated for the epoxidation of cyclooctene and cyclohexene by aqueous TBHP catalyzed by
dimeric molybdenum complexes [MoO2L]2 under organic solvent-free conditions. The position of the
OH substituent and the nature of the substrate have a slight influence on the catalytic activity. DFT
calculations on the catalytic cycle give a trend of energy spans in accordance with the experimental
results. Further ligand tuning remains of interest for several purposes. The optimization of catalytic
activity and selectivity are to be improved, as well suitable groups for grafting on solid supports and
heterogeneization of the most stable and active complexes in order to allow efficient catalyst recovery.
4. Experimental part
4.1. Materials and methods
All preparations were carried out in air. Water was distilled before use. Organic solvents (ethanol,
methanol, diethylether and acetrontrile) and organic compounds (TBHP (70% in water, ACROS),
cyclooctene (98%, Aldrich), cyclooctene oxide (98%, Aldrich), cyclohexene (purity, supplier),
cyclohexene oxide (purity, supplier), trans-cyclohexanediol (purity, supplier), 2-cyclohexen-1-ol
(purity, supplier), 2-aminophenol (99%, Aldrich), 2,5-dihydroxybenzaldehyde (98%, Aldrich), 2,3-
dihydroxybenzaldehyde (97%, Aldrich), 2,4-dihydroxybenzaldehyde (98%, Aldrich), salicylaldehyde
(98%, Aldrich) were used as received without any purification. The H2SAP and H2L1-3 compounds
were synthesized and characterized according to literature (see SI) [Error! Bookmark not defined.a,Error!
Bookmark not defined.-Error! Bookmark not defined.]]. [MoO2(acac)2] was synthesized as previously
described [26] and used freshly prepared. Complex [MoO2(SAP)]2 was prepared as described earlier
[Error! Bookmark not defined.a]. The thermogravimetric analyses were performed on a SETARAM TGA
92-16.18 thermal analyzer. The sample was placed into a nickel/platinum alloy crucible and heated at
0.83 °C 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 in KBr matrices at room temperature with a Mattson Genesis II FTIR
1
spectrometer. H spectra were recorded at 200.1 MHz on a Bruker Avance DPX-200 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, cyclohexene and the
formation of cyclooctene oxide, cyclohexene oxide and cyclohexanediol were calculated from
calibration curves (r2= 0.999) relatively to an internal standard.
4.2. Synthesis of the Mo complexes
[MoO2L1(MeOH)] and [MoO2L1]2. In a 100 mL Erlenmeyer flask, H2L1 (0.35 g, 1.53 mmol) was
dissolved in 20 mL of methanol and then [MoO2(acac)2] (0.50 g, 1.53 mmol) was added. The mixture
was refluxed under magnetic stirring for 4 hours. The resulting red solution was left to stand
overnight, leading to the separation of orange needles characterized as [MoO2L1(MeOH)] (80%).
These needles were treated under reduced pressure at 80°C for 12 hours, leading to [MoO2L1]2 as
brown solid. Addition of MeOH to this solid led a color change indicating the regeneration of
[MoO2L1(MeOH)]. [MoO2L1(MeOH)]: TGA: exp(theo) Δm1 over 20-120 °C = 8.3 (9.8), Δm2 over
250-600 °C = 62.8 (63.4) %. IR (KBr): 908, 933 (Mo=O) 1612 (C=N) cm-1. [MoO2L1]2: TGA:
exp(theo) Δm over 200-600 °C = 59.4 (59.5) %. IR (KBr):919 (Mo=O) 780 (Mo-O-Mo), 1612 (C=N)
cm-1. Anal. Calcd. for C13H9MoNO5 (Mr = 355.15): C, 43.96; H, 2.55; N, 3.9 %. Found: C, 43.01; H,
1
1.97; N, 3.90 %. H NMR (300 MHz, DMSO-d6, δ(ppm)): 6.82-6.96 (m, 3H, ArH), 7.05-7.08 (m, 1H,
Ar-H), 7.19-7.24 (m, 2H, Ar-H), 7.80-7.82 (m, 1 H, Ar-H), 9.21 (s, 1 H, CH=N), 9.32 (s, 1 H, Ar-OH).
1
This H NMR spectrum in DMSO-d6 is the same as that for [MoO2L1(MeOH)], suggesting that for
both compounds the stable species in solution is [MoO2L2(DMSO)]. 13C NMR (300 MHz, DMSO-d6,
δ(ppm)): 160.3 (Cq-O), 157.3 (CH-N), 150.3 (Cq-O), 147.0 (Cq-O), 135.9 (Cq-N), 130.5 (CH-Ar), 125.7
(CH-Ar), 122.9 (Cq), 121.5 (CH-Ar), 121.4 (CH-Ar), 120.7 (CH-Ar), 117.7 (CH-Ar), 116.8 (CH-Ar).
[MoO2L2(EtOH)] and [MoO2L2]2. In a 100 mL Erlenmeyer flask, 0.70 g of H2L2 (3.05 mmol) was
dissolved in 25 mL of ethanol and 1.00 g of [MoO2(acac)2] (3.06 mmol) was added. The mixture was
refluxed under magnetic stirring over night. The resulting precipitate was separated by filtration giving
orange needles of [MoO2L2(EtOH)]. When dried under reduced pressure at 120°C for 12 hours,
[MoO2L2]2 is obtained as a red-brown solid. Addition of EtOH to this solid led a color change
indicating the regeneration of [MoO2L2(EtOH)]. [MoO2L2(EtOH)]: TGA: exp(theo) Δm1 over 20-200
°C = 11.6 (11.5) %, Δm2 over 200-600 °C = 62.8 (63.4) %. IR (KBr): 908, 945 (Mo=O), 1600 (C=N)
cm-1. [MoO2L2]2: TGA: exp(theo) Δm over 200-600 °C = 61.6 (59.5) %. IR (KBr): 821(Mo-O-Mo),
904 (Mo=O), 1598 (C=N) cm-1. Anal. Calcd. for C13H9MoNO5 (Mr = 355.15): C, 43.96; H, 2.55; N,
3.9 %. Found: C, 43.93; H, 2.10; N, 3.92 %. 1H NMR (300 MHz, DMSO-d6, δ(ppm)): 6.28 (s, 1H, Ar-
H), 6.49-7.18 (m, 4H, Ar-H), 7.55-7.74 (m, 2 H, Ar-H), 9.03 (s, 1 H, CH=N), 10.65 (s, 1 H, Ar-OH).

1
This H NMR spectrum in DMSO-d6 is the same for monomer and dimer, suggesting that for both
compounds the stable species in solution is [MoO2L2(DMSO)]. 13C NMR (300 MHz, DMSO-d6,
δ(ppm)): 165.4 (CH-N), 163.6 (Cq-O), 159.9 (Cq-O), 156.3 (CH-Ar) , 137.4 (CH-Ar), 136.7 (Cq-N),
129.4 (CH-Ar), 120.8 (CH-Ar), 117.4 (CH-Ar), 116.1 (CH-Ar), 115.2 (Cq), 110.6 (CH-Ar), 105.1 (CH-Ar).
[MoO2L3]2. In a 100 mL Erlenmeyer flask, H2L4 (0.45 g, 1.96 mmol) was dissolved in 30 mL of
methanol and then [MoO2(acac)2] (0.64 g, 1.96 mmol) was added. The mixture was refluxed under
magnetic stirring for 4 hours. The resulting brown precipitate was separated by filtration. The
precipitate was dried (80 °C) under reduced pressure for 24 hours. Yield 87%. TGA: exp(theo) Δm
over 300-530°C = 58.7 (59.5) %. IR (KBr): 804 (Mo-O-Mo), 912 (Mo=O), 1613 (C=N) cm-1. Anal.
Calcd. for C13H9MoNO5 (Mr = 355.15): C, 43.96; H, 2.55; N, 3.9 %. Found: C, 43.84; H, 1.83; N, 3.84
1
%. H NMR (DMSO-d6, δ(ppm)): 6.75-7.24 (m, 6H, Ar-H) 7.78-8.82 (m, 1 H, Ar-H), 9.15 (s, 1 H,
CH=N), 9.43 (s, 1 H, Ar-OH). 13C NMR (300 MHz, DMSO-d6, δ(ppm)): 160.6 (Cq-O), 156.9 (CH-N),
155.0 (Cq-O),151.4 (Cq-O), 136.0 (Cq-N), 130.6 (CH-Ar), 123.8 (CH-Ar), 122.5 (Cq), 120.6 (CH-Ar),
119.7 (CH-Ar), 119.4 (CH-Ar), 117.8 (CH-Ar), 116.9 (CH-Ar).
4.3. X-ray structural analyses.
Single crystals of each compound [MoO2L1(DMSO)], [MoO2L2(H2O)](MeCN) and
[MoO2L3(DMSO)]·2DMSO were mounted under inert perfluoropolyether at the tip of glass fiber and
cooled in the cryostream of a Bruker APEX2 CCD diffractometer. The structures were solved by
direct methods (SIR97 [27]) and refined by least-squares procedures on F2 using SHELXL-2013 [28].
In both compounds, the DMSO molecule linked to the molybdenum is disordered over two positions
related by a pseudo mirror plane. This disorder has been modelised using the tools available in
SHELXL-2013. The two disordered fragments were included in two different parts. Then the
occupancy factor for the two sites was refined using the free variable restraining the sum of the
occupancy factors to be equal to 1. The ratio of the occupancy is 8/2. All H atoms attached to carbon
were introduced in calculation in idealised positions and treated as riding models. The drawing of the
molecules was realised with the help of ORTEP32 [29]. Crystal data and refinement parameters are
shown inTable 6.
4.4. Catalytic Procedures.
Routine experiments. In a typical experiment, cyclooctene or cyclohexene (1 equiv) and catalyst (x
equiv, see tables) were mixed together and then stirred in air in a round bottom flask. Acetophenone
was added as internal standard. The temperature was regulated at 80°C (see Tables 4, 5, S4 and S5)
and then THBP (70% in water, y equiv, see tables) was added, starting the reaction. Samples of the
organic phase (0.1 mL) were periodically withdrawn. The reaction was quenched by addition of MnO2

(2 mg), followed by addition of diethylether (2.5 mL) and removal of the manganese oxide and
residual water by filtration through silica before GC analysis.
Water effect. The effect of water was tested for the catalytic runs with [MoO2(SAP)]2 and [MoO2L1]2
using the same experimental conditions as for the routine experiments except for adding additional
water (seeTable S6).
4.5. Computational Details.
All input geometries were adapted from the X-ray structures of [MoO2(SAP)(EtOH)] [Error! Bookmark
not defined.] and optimized without any symmetry constraint using the Gaussian 03 program suite [30].
The calculations used the standard B3LYP three-parameter functional [31] in conjunction with the
6-31G** basis set [32] for the C, H, N, and O atoms and the CEP-31G* basis set [33] 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 stationary points and local minima (for stable molecules and 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.
5. Acknowledgements
We acknowledge the Centre National de la Recherche Scientifique (CNRS) and the Institut
Universitaire de France (IUF) for financial support, the Université Paul Sabatier and its Institut
Universitaire Technologique for the facilities, and the China Scholarship Council for the fellowship of
W.W. (No.201208370239). This work was granted access to the HPC resources of CINES under the
allocation 2010-086343 made by GENCI (Grand Equipement National de Calcul Intensif) and to the
resources of the CICT (Centre Interuniversitaire de Calcul de Toulouse, project CALMIP).

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Figure 1. Molecular view of compound [MoO2L1(DMSO)] (left), [MoO2L2(H2O)] (center) and
[MoO2L3(DMSO)] (right) with the atom labelling scheme. Displacement ellipsoids are drawn at the
50% probability level. For the sake of clarity, only one component of the disordered DMSO is
represented.
Figure 2. View of the dinuclear unit formed by intermolecular hydrogen bonds between two
[MoO2L1(DMSO)] molecules. Symmetry code: (i) -x+2,-y,-z.

Figure 3. View of compound [MoO2L3(DMSO)].2 DMSO showing the hydrogen bond (dashed line)
between the complex and one of the DMSO solvent molecule. Displacement ellipsoids are drawn at
the 50% probability level. H atoms are represented as small spheres of arbitrary radii. The disordered
part is omitted for clarity.
Figure 4. View of the intermolecular H-bonding network for compound [MoO2L2(H2O)]·CH3CN.

100
80
60
40
20
0
0
60
120
180
240
Time (min)
Figure 5. Cyclooctene conversion vs. time with different ONO based catalysts: [MoO2(SAP)]2 (),
[MoO2L1]2 (),[MoO2L2]2 (),[MoO2L2(EtOH)] () and [MoO2L3]2 ( ). For the reaction conditions,
see
Table 3. Relevant bond lengths around the molybdenum.
[MoO2L1(DMSO)] [MoO2L2(H2O)] [MoO2L3(DMSO)]
Relevant Bonds (Å)
Mo=O(2)
Mo=O(5)
Mo-N
Mo-O(3)
Mo-O(4)
Mo-O(1)
1.714(2)
1.698(2)
2.277(2)
1.957(2)
1.943(2)
2.285(2)
1.731(3)
1.696(3)
2.252(3)
1.948(3)
1.944(3)
2.325(3)
1.7082(15)
1.7038(15)
2.2752(15)
1.9673(13)
1.9302(14)
2.289(3)
Relevant angles (°)
O(2)-Mo-O(5)
O(2)-Mo-N(1)
O(3)-Mo-O(4)
O(5)-Mo-O(1)
103.98(11)
164,32(10)
151,30(9)
169.11(10)
105.2(1)
163.0(1)
152.6(1)
169.5(1)
103.90(8)
164.78(7)
152.29(6)
172.14(10)
Table 4.
100
80
60
40
20
0
250
200
150
100
50
100
1400
1200
1000
800
600
400
200
0
(b)
(a)
80
60
40
20
0
0
0
0.1
0.2
0.3
0.4
0.5
0
0.2
0.4
0.6
0.8
1
[Mo]/cycloctene (%)
[Mo]/cycloctene (%)
Figure 6. Conversion after 4 h ( ) and initial TOF () vs. [Mo]/cyclooctene ratio for the processes
catalyzed by [MoO2(SAP)]2 (a) and [MoO2L1]2 (b).

100
80
60
40
20
0
0
60
120
180
240
300
360
Time (min)
Figure 7. Kinetic profile of converted cyclohexene vs. time in the presence of
dioxidomolybdenum(VI) catalysts: [MoO2(SAP)]2 (□), [MoO2L1]2 (), [MoO2L2]2 (×), [MoO2L3]2().
Conditions: substrate/TBHP/[Mo] = 100:200:0.1; T = 80°C.
Scheme 1. Access routes to the H2L pro-ligands and molybdenum complexes.

Scheme 2. Epoxidation of cyclooctene.
Scheme 3. Epoxidation of cyclohexene.

Scheme 4. Proposed cycle for the olefin epoxidation catalyzed by [MoO2L] [Error! Bookmark not
defined.].

Table 1. Selected IR vibrations (ν, cm-1) of the H2SAP, H2L1-3, [MoO2Ln(ROH)] (n = 1,2),
[MoO2(SAP)]2 and [MoO2L1-3]2 compounds.
C=N
1633
1618
Mo=O
-
-
Mo-O-Mo
-
-
Pro-ligands H2SAP
H2L1 [Error!
Bookmark not
defined.]
H2L2 [Error!
Bookmark not
defined.]
1603
1612
1608
-
-
H2L3 [Error!
Bookmark not
defined.]
-
-
Complexes
[MoO2(SAP)]2
937
790
[Error!
Bookmark not
defined.a]
[MoO2L1(MeOH)] 1612
908, 933
919
908, 945
904
[MoO2L1]2
1612
1600
1598
1613
780
[MoO2L2(EtOH)]
[MoO2L2]2
821
804
[MoO2L3]2
912
Table 2. Mass losses of the molybdenum complexes by TG analyses.
Complex
m1 (< 250°C) m2
Exp
Theo
Exp
Theo
-
-
58.5
57.6
[MoO2(SAP)]2 [Error!
Bookmark
not
defined.a]
[MoO2L1(MeOH)]
[MoO2L1]2
8.3
-
11.6 11.5
-
-
9.8
-
62.8
59.1
65.0
59.2
58.7
63.4
59.5
64.1
59.5
59.5
[MoO2L2(EtOH)]
[MoO2L2]2
-
-
[MoO2L3]2
Table 3. Relevant bond lengths around the molybdenum.
[MoO2L1(DMSO)] [MoO2L2(H2O)] [MoO2L3(DMSO)]
Relevant Bonds (Å)
Mo=O(2)
Mo=O(5)
Mo-N
Mo-O(3)
1.714(2)
1.698(2)
2.277(2)
1.957(2)
1.731(3)
1.696(3)
2.252(3)
1.948(3)
1.7082(15)
1.7038(15)
2.2752(15)
1.9673(13)

Mo-O(4)
Mo-O(1)
1.943(2)
2.285(2)
1.944(3)
2.325(3)
1.9302(14)
2.289(3)
Relevant angles (°)
O(2)-Mo-O(5)
O(2)-Mo-N(1)
O(3)-Mo-O(4)
O(5)-Mo-O(1)
103.98(11)
164,32(10)
151,30(9)
169.11(10)
105.2(1)
163.0(1)
152.6(1)
169.5(1)
103.90(8)
164.78(7)
152.29(6)
172.14(10)
Table 4. Results of cyclooctene epoxidation by aqueous TBHP catalyzed by different molybdenum
complexes.a
Conversion Selectivity TOFb TON
[%]
76
92
73
74
81
[%]
93
93
85
90
91
[h-1]
70
251
71
68
[MoO2(SAP)]2
[MoO2L1]2
153
184
146
148
163
[MoO2L2]2
[MoO2L2(EtOH)]
[MoO2L3]2
127
a Conditions : Mo/TBHP/cyclooctene = 0.5/100/200; T = 80°C; t
= 4 h. b TOF is calculated on the time interval with maximum
slope for the conversion plot.

Table 5. Results of cyclohexene oxidation by [MoO2(SAP)]2 and [MoO2L1-3]2 .
Yield (%)
Conversion Selectivity
TOF
(h-1)
catalyst
TON
(%)
(%)/ oxide
CHO CHD CHol
[MoO2(SAP)]2
[MoO2L1]2
[MoO2L2]2
[MoO2L3]2
48.8
68.8
53.4
61.0
41.6
22.8
35.8
22.7
20.3
15.7 21.8
19.1 8.1
13.9 21.2
8.7
1.1
2.1
1.0
1.8
447 150a
687 377b
535 187b
612 165a
a1 h, b0.5 h

Table 6. Crystal data and structure refinement parameters for all compounds.
Formula
[MoO2L1(DMSO)]
C15H15MoNO6S
436.30
[MoO2L2(H2O)]·CH3CN [MoO2L3(DMSO)]· 2DMSO
Empirical formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
a, Å
C15H14 MoN2O6
414.22
180(2)
C19H27MoNO8S3
589.54
173(2)
180(2)
0.71073
Monoclinic
P 21/c
0.71073
Monoclinic
I 2/a
0.71073
Monoclinic
P 21/n
7.4084(4)
20.8805(12)
10.8957(6)
90.0
21.202(5)
6.599(5)
24.110(5)
90.0
15.1752(4)
7.5571(2)
22.2331(6)
90.0
b, Å
c, Å
.
, °
Β, °
99.961(2)
90.0
106.582(5)
90.0
105.963(2)
90.0
, °
Volume, Å3
Z
1660.06(16)
4
3233(3)
8
2451.39(11)
4
Density (calc), Mg/m3
Abs. coefficient, mm-1
F(000)
1.746
1.702
1.597
0.947
0.845
0.834
884
1664
1208
Crystal size, mm3
θ range, °
0.500 x 0.200 x 0.040 0.625 x 0.075 x 0.05
0.37 x 0.35 x 0.20
1.457 to 30.031
96079
2.134 to 28.281
27592
3.210 to 29.520
15189
Reflections collected
Indpt reflections (Rint)
Completeness, %
Absorption correction
Max. /min. transmission
Refinement method
4114 (0.0360)
99.6
10994 (0.040)
99.9
7181 ( 0.0291)
100.0
Multi-scan
0.7461 and 0.6777
F2
Multi-scan
1.0 and 0.948
F2
Multi-scan
0.7474 / 0.6860
F2
Data /restraints/parameters 4114 / 6 / 234
10994 / 3 / 220
1.042
7181 / 8 / 322
1.224
Goodness-of-fit on F2
R1, wR2 [I>2(I)]
1.164
0.0360, 0.0923
0.0464, 0.1004
1.068 / -1.019
0.0315, 0.0819
0.0424, 0.0840
0.570 / -0.466
0.0305, 0.0792
0.0343, 0.0815
0.989 / -0.915
R1, wR2 (all data)
Residual density, e.Å-3