Weili Wang

Tomas Guerrero

Saul R Merecias

Hector Garcia-Ortega

Rosa Santillan

Jean-Claude Daran

N Farfan

Dominique Agustin
email: dominique.agustin@iut-tlse3.fr.

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr.

Héctor García-Ortega

Norberto Farfán

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

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Introduction

Epoxides are among the most interesting and versatile intermediates in organic chemistry [START_REF] Jacobsen | Comprehensive Asymmetric Catalysis[END_REF]. 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 [START_REF] Sheldon | Green Chemistry and Catalysis[END_REF]. 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 [START_REF] Landau | [END_REF] 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 solventfree 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.

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)salicylaldehyde, cis-cyclooctene (98% Aldrich), cyclooctene oxide (Aldrich) and TBHP (70% in water, ACROS) were commercially available and used as received. Complex [MoO2(acac)2] was freshly synthesized as previously described [12]. The Schiff base ligands H2L 1-4 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(L 1 )]2 was synthesized as previously described [7c].

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 using DMSO-d6 as solvent, 1 H, 13 C 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 ciscyclooctene and the formation of cyclooctene oxide were calculated from calibration curves (r 2 = 0.999) relatively to an internal standard. 2-4 ]2. In a round bottom flask equipped with a magnetic stirrer bar, 9.1 mmol of the selected Schiff base H2L 1-4 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 1 H and 13 C NMR, IR, HRMS and TGA. The carbon atoms numbering used for the assignment of the 13 C NMR resonances of each complex is depicted in Scheme 1.

General procedure for the synthesis of molybdenum complexes [MoO2L

Scheme 1-Atom numbering for the assignment of the 13 C NMR resonances.

Characterization data for [MoO2(L

2 )]2. Yellow solid, 38% yield, mp >300 °C.

NMR 1 H (500 MHz, DMSO-d6)  9.43 (s, 1H, H-7), 8.02 (d,1H,J = 9.5 Hz,2H,2H,, 7.09 (ddd, 1H, J = 7.5, 6.5, 1.0 Hz, H-5), 6.95 (d, 1H, J = 6.5 Hz, H-3). 13 Characterization data for [MoO2(L 3 )]2. Brown solid, 44% yield, mp >300 °C. NMR 1 H (500 MHz, DMSO-d6)  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,7.04 (ddd,1H,J = 7.8,7.5,1.5 Hz,6.82 (ddd,1H,J = 7.8. 7.5,1.5 Hz,6.72 (dd,1H,J = 8,1.5 Hz,6.41 (dd,1H,J = 9.0,2.5 Hz,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). 13 4.4; N, 6.8. Found: C, 49.6; H, 3.8; N, 6.7%. 2.3.2. Characterization data for [MoO2(L 4 )]2. Brown solid, 42% yield, mp >300 °C.

NMR 1 H (500 MHz, DMSO-d6)  9.01 (s, 1H, H-7), 7.75 (d,1H,J = 9 Hz,7.74 (dd,1H,J = 8.9,2.3 Hz,, 7.49 (d,1H,J = 5.5 Hz,7.48 (d,1H,J = 2.3 Hz,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), 1.11 (t, 6H, J = 7 Hz, CH3). 13

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.

DFT Calculations.

The geometries of all species were optimized without any symmetry constraint with the Gaussian 09 program suite [START_REF] Frisch | Gaussian 09, Revision D.01[END_REF]. The input geometries were adapted from the X-ray structures of [MoO2(SAP)(EtOH)] [START_REF] Agustin | [END_REF], replacing one hydrogen by NEt2 and/or NO2 fragment depending on the molecule studied. The standard B3LYP threeparameter 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.

X-Ray Crystallographic Study. Single crystals of [MoO2(L 1 )(DMSO)] and

[MoO2(L 3 )(DMSO)] suitable for X-ray structural analyses were mounted on an Enraf Nonius Kappa-CCD and the data recorded at ambient temperature whereas the compound [MoO2(L 4 )(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 F 2 using SHELXL-2013 [START_REF] Sheldrick | SHELXL-2013[END_REF]. 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(L 1 )(DMSO)], the L 1 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. [START_REF] Farrugia | [END_REF] 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).

Results and Discussion

Synthesis of molybdenum complexes

The molybdenum complexes [MoO2L 1-4 ]2 were obtained by refluxing the corresponding Schiff bases H2L 1-4 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.

X-ray characterization

Tridentate Schiff bases complexes of type [MoO2L(DMSO)] are known to be easily crystallized in DMSO [24].

Catalysis results

Molybdenum tridentate complexes with structures very close to L 1 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(L 1 )]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(L 1-4 )]2 complexes. [a] Conditions: substrate/[Mo] = 400:1; T = 80 °C; time = 4 h. [b]calculated from initial slope (30 min).

Figure 4-. Kinetic profile of converted cyclooctene vs. time with dioxidomolybdenum(VI)

catalysts: [MoO2(L 1 )]2 (), [MoO2(L 2 )]2 (), [MoO2(L 3 )]2 (), [MoO2(L 4 )]2 (). Conditions: substrate/[Mo] = 400:1; T = 80 °C.
The cyclooctene conversion after 4 h decreases in the following order MoO2L 3 ]. The selectivity, however, is always high (always greater than 90%). The presence of the electron withdrawing NO2 group (ligand L 2 ) has a small beneficial effect on the activity, while the NEt2 donor group (ligand L 3 ) shows a weak detrimental effect. When both groups are present within the structure (ligand L 4 ), the activity is very close to that of the parent SAP backbone (ligand L 1 ). The [MoO2(L 1 )]•••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].

[MoO2L 2 ] > [MoO2L 4 ] ≈ [MoO2L 1 ] > [
The [Mo(OtBu)(OH)(L 1 )] 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.

Mechanistic rationalization through DFT calculations

We have extended the recently published DFT investigation of the catalytic cycle for the SAP (L 1 ) system, using ethylene as the model olefin, to the corresponding systems containing L 2 , L 3 and L 4 . As mentioned above, on the basis of experimental mechanistic information gathered on the SAP (L 1 ) system [7c] it was concluded that the active form of the catalyst is the mononuclear 5-coordinate [MoO2(L 1 )] 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 L 2 ligand, containing the electron-withdrawing nitro group, but the electron-donating diethylamino group in L 3 also provides a weak stabilizing effect. The very small extent of these changes is The most important parameter is the energy span of the catalytic cycle [26]. On the enthalpy scale, it goes in the order L 3 (23.8 kcal/mol) > L 1 (22.5 kcal/mol) ~ L 4 (22.3 kcal/mol) > L 2 (21.0 kcal/mol). The same trend is calculated on the Gibbs energy scale (L 3 , 37.0; L 1 , 35.5; L 4 , 35.5; L 2 , 34.2 kcal/mol), since the entropic contribution is essentially the same for all systems. This trend is in agreement with the experimentally observed faster epoxidation in the presence of the L 2 complex, with the slower epoxidation in the presence of the L 3 complex, and with an intermediate and nearly equivalent activity in the presence of the L 1 and L 4 complexes.

Conclusions

We have explored in this study the electronic effect of substituents on tridentate ONO-type

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

  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(L 1 )(EtOH)] complex,[7c, 18, 23] the slow release of the coordinated ethanol molecule leads to the dimeric complex [MoO2(L 1 )]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(L 1-4 )]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(L 1 )]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 1 H NMR investigations of the molybdenum complexes were performed in DMSO-d 6 , yielding the expected signals for the Schiff base ligand, displaced from those of the H2L molecule In

  Figures 1-3, and relevant bond distances and angles are compared in Table3.For all compounds, the system shows the presence of a cis-{MoO2} 2+ fragment surrounded by a tridentate ONO ligand in doubly deprotonated form (L 2-). 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 L 1 and L 3 , the DMSO ligand shows disorder. For [MoO2(L 1 )(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(L 3 )(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(L 3 )(DMSO)] and [MoO2(L 4 )(DMSO)] angles between the phenyl C6 ring and the NC2 fragment of 11.2(4)° and 4.8(5)°, respectively), showing π conjugation (sp 2 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(L 4 )(DMSO)] is also quasi coplanar with the aromatic ring, with a dihedral angle of 5.0(6)° for O11-N11-C14-C13.

Figure 1 -

 1 Figure 1 -ORTEP view of [MoO2(L 1 )(DMSO)]. The ellipsoids are drawn at the 30% probability level. The second form is represented by dotted lines.

Figure 2 -

 2 Figure 2-ORTEP view of [MoO2(L 3 )(DMSO)]. The ellipsoids are drawn at the 30% probability level. The second position of DMSO is represented by dotted lines.

Figure 3 -

 3 Figure 3-ORTEP view of [MoO2(L 4 )(DMSO)]. The ellipsoids are drawn at the 30% probability level.

Scheme 4 -

 4 Scheme 4 -Most plausible mechanism for the epoxidation catalytic cycle with [MoO2(L)] [7c].

  certainly related to the weakness of the Mo•••TBHP interaction, as suggested by the long Mo•••O distance and to the competing electronic effects on the Mo•••Oβ and Mo=O•••H interactions in the [MoO2(L 1 )]•••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 L 3 > L 4 > L 1 > L 2 , 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 L 1 ligand. Conversely, the Mo=O•••H becomes progressively longer along the same series: L 3 < L 4 < L 1 < L 2 . 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.

Figure 5 -

 5 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.

Figure 6 .

 6 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.

Figure 7 .

 7 Figure 7. Views and selected distances of the optimized transition states for the ratedetermining 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.

  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 electrondonating 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.

  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.

Table 1 .

 1 Crystal data and structure refinement for the complexes [MoO2(L 1 )(DMSO)],

	[MoO2(L 3 )(DMSO)] and [MoO2(L 4 )(DMSO)].
	Empirical formula	[MoO2(L 1 )(DMSO)](DMSO)	[MoO2(L 3 )(DMSO)]	[MoO2(L 4 )(DMSO)](H2O)0.5
		C17H21MoNO6S2	C19H24MoN2O5S	C19H24MoN3O7.5S
	Formula weight	495.41	488.40	542.41
	Temperature	293(2)	293(2)	293(2)
	Wavelength	0.71073	0.71073	0.71073
	Crystal system	monoclinic	monoclinic	triclinic
	Space group	P21/c	P21	P-1
	Unit cell parameters
	a (Å)	12.5387(5)	9.210	7.7016(11)
	b (Å)	6.8015(2)	7.947	12.3088(16)
	c (Å)	24.0157(10)	14.503	12.8230(19)
	 (deg)	90	90	117.249(5)
	 (deg)	93.460(2)	102.46	91.073(6)
	 (deg)	90	90	92.406(6)
	Volume (Å 3 )	2044.37(13)	1036.5	1078.7(3)
	Z	4	2	2
	ρcalcd (g cm -3 )	1.610	1.565	1.670
	μ (mm -1 )	0.878	0.766	0.754
	F(000)	1008	500	554
	Crystal size (mm 3 )	0.25 X 0.1 X 0.05	0.25 X 0.125 X 0.05	0.45 X 0.025 X 0.013
	θ range for data	2.91-27.48	2.91-27.48	2.65-26.43
	collection (°)
	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

Table 2 -

 2 Selected IR and 1 H NMR data for ligands and complexes.

	Compound	IR a		1 H NMR b
	C=N	Mo=O	Mo-O-Mo CH=N
	[MoO2(L 1 )]2 1608	935	808	9.25 (8.99)
	[MoO2(L 2 )]2 1603	922	798	9.43 (8.99)
	[MoO2(L 3 )]2 1615	927	844	8.89 (8.61)
	[MoO2(L 4 )]2 1614	934	809	9.01 (8.77)

a Solid state. b In DMSO-d 6 solution (values for the corresponding H2L molecule in parentheses).

Table 3 -

 3 Selected bond lengths and angles for all compounds.

		[MoO2(L 1 )(DMSO)] [MoO2(L 3 )(DMSO)] [MoO2(L 4 )(DMSO)]
	Bonds
	Mo=O	1.696(4)	1.687(4)	1.701(3)
		1.686(4)	1.698(3)	1.707(2)
	Mo-O	1.929(4)	1.926(3)	1.941(3)
		1.943(4)	1.973(3)	1.980(3)
	Mo-	2.267(6)	2.23(1)	2.318(3)
	O(DMSO)
	Mo-N	2.281(8)	2.266(3)	2.245(3)

Table 4 .

 4 Relevant results for the cyclooctene epoxidation catalysis. a

	Complex	Conversion	Epoxide selectivity	TON	TOF b (h -1 )
	[MoO2(L 1 )]2	71	94	286	195
	[MoO2(L 2 )]2	86	96	346	344
	[MoO2(L 3 )]2	62	93	252	167
	[MoO2(L 4 )]2	73	91	293	192


Weili Wang

Thibaut Vanderbeeken

Dominique Agustin
email: dominique.agustin@iut-tlse3.fr.

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr.

Tridentate ONS vs. ONO salicylideneamino(thio)phenolato MoO2L complexes for catalytic solvent-free epoxidation with aqueous TBHP

Introduction

Within the quest of green chemistry, the implementation of solvent-free processes is one of the best options, especially with respect to the E-factor [1]. In addition, the elimination of solvents is a safety issue when these are potentially toxic (such as for instance the chlorinated solvents often used in academic research for epoxidation). Epoxides are interesting chemical intermediates, used extensively in fine chemistry through ring-opening and as precursors to different polymers [2,3]. Epoxides are classically obtained using organic oxidants (m-CPBA, CH3CO3H, PhIO), needing the use of organic solvents (often chlorinated) or ionic liquids, but transition metal catalyzed epoxidation offers the possibility to use greener oxidants such as THBP, H2O2, or even O2. Although being by-products of the reaction, tBuOH and water can be recycled, tBuOH being in addition useful for the synthesis of the gasoline additive MTBE [4].

Among the metals used for epoxidation catalysis, molybdenum is the most used both in academic research [5] and in industry [6]. In this area, we have shown catalytic efficiency of different types of molybdenum [7][8][9] and vanadium [10] complexes as well as polyoxometalates [11] for the epoxidation of cyclooctene and bio-sourced substrates [12], using aqueous TBHP as oxidant and no added organic solvent. Molybdenum(VI) complexes with the salicylideneaminophenolato (SAP) Schiff base ligand (a tridentate ONO ligand) where tested in organic solvents [13] and without solvent [9,12]. The combination of the SAP ligand and molybdenum(VI) yielded reaction efficiency and catalyst stability. We were intrigued by the availability of an open coordination site in the 5-coordinate [MoO2(SAP)], which is easily generated by dissociation of a weakly coordinating ligand (L) such as an alcohol from the 6-coordinate [MoO2(SAP)(L)] pre-catalyst, whereas most other active catalyst do not have the same possibility. Concerning the tridentate ONS donor set, only few contributions have dealt with their use in molybdenum catalyzed epoxidation [7,14,15] but only one of them, to the best of our knowledge, has used solvent-free conditions. The exception consists of a pyridoxal thiosemicarbazonato ligand having a different scaffold from SAP and a direct comparison of performance between the two systems is not possible [7]. The present article compares the SAP system with its sulfur analogue, the salicylideneaminothiophenolato (SATP) ligand, revealing that the latter is much more active, and analyzes the possible reasons for this difference.

Experimental

Details on the starting materials and the characterization techniques can be found in the supporting information.

Synthesis of [MoO2(SATP)]2

In a 100 mL Erlenmeyer flask, salicylaldehyde (0.50 g, 4.09 mmol) was dissolved in 50 mL of methanol and 2-aminothiophenol (0.50 g, 4.03 mmol) was added. After magnetic stirring for 0.5 h at room temperature, [MoO2(acac)2] (1.34 g, 4.11 mmol) was added. The resulting mixture was further stirred for 4 h at room temperature. The resulting brown precipitate was separated by filtration, washed with CH2Cl2 and dried under reduced pressure for 24 h (Yield: 0.76 g, 53.3 %). TGA Δm = exp(theo) 58.5 (59.5). IR (KBr, ν(cm -1 )) 781(Mo-O-Mo), 920 (Mo=O), 1602 (C=N). 1 H NMR (DMSO-d6, δ(ppm)): 6.89-7.81 (m, 7 H, Ar-H), 9.06 (s, 1 H, CH=N). 13 (CH-Ar), 123.1 (Cq), 120.9 (CH-Ar), 119.6 (CH-Ar), 119.1 (CH-Ar).

Catalytic Procedures.

Routine experiments. CAUTION: a mixture of TBHP with organic compounds (especially without a solvent) is potentially explosive. In a typical experiment, cyclooctene (1 equiv) and the catalyst (x equiv, see tables) were mixed together and stirred in air in a round bottom flask. Acetophenone was added as internal standard. The reaction temperature (see tables) was regulated and then wet THBP (70% in water, y equiv, see tables) was added to the mixture, starting the reaction. Both [MoO2(SAP)]2 and [MoO2(SATP)]2 are 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.

Water effect. The effect of water was tested using the same experimental conditions as the routine experiments, plus additional water was added to the reaction mixture, changing the water/TBHP ratio.

Computational Details.

The geometries of all species were optimized without any symmetry constraint with the Gaussian 09 program suite [START_REF] Frisch | Gaussian 09, Revision D.01[END_REF]. The input geometries were adapted from the X-ray structures of [MoO2(SAP)(EtOH)] [START_REF] Agustin | [END_REF], replacing one oxygen atom by a sulfur atom. The standard B3LYP three-parameter functional was used in conjunction with the 6-31G** basis set for C, H, N, O and S 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.

Results and Discussion

The molybdenum complex of the ONO tridentate ligand SAP, [MoO2(SAP)]2, has been reported previously [9,12,18]. The corresponding ONS ligand [19] has also been previously used to synthesize various complexes of Ti, Mn, Tc, Re, Co, Ni, Cu, Zn, [19,20] and also a molybdenum complex [MoO2(SATP)(L)] (L = substituted imidazole) [21]. A ligand-free [MoO2(SATP)]2 derivative, however, has apparently not yet been described. Because of the air sensitivity of the H2SATP ligand [20], the complex was obtained in a one-pot process from salicylaldehyde, 2-aminothiophenol and the freshly prepared molybdenum precursor [MoO2(acac)2] in methanol at room temperature (Scheme 1).

Scheme 1-Synthetic routes for ligands and complexes

The spectroscopic and other characterization shows identical properties for complex [MoO2(SATP)]2 and the previously reported [MoO2(SAP)]2 analogue. Formation and coordination of the SATP ligand is indicated by the C=N stretching vibration at 1602 cm -1 (vs. 1608 cm -1 for the SAP analogue). The vibrational pattern of the {MoO2 2+ } fragment is also characteristic: whereas mononuclear [MoO2(SAP)(L)] complexes show two Mo=O stretching vibrations between 850 and 940 cm -1 , dinuclear [MoO2(SAP)]2 exhibits only one Mo=O vibration at 937 cm -1 and a lower frequency band at 790 cm -1 corresponding to a vibrational mode of the bridging Mo-O-Mo unit [START_REF] Agustin | [END_REF]. For the [MoO2(SATP)]2 complex, these vibrations are observed at lower frequencies, 920 and 781 cm -1 respectively, suggesting greater electron donating ability for SATP. These data also exclude a monomeric 5-coordinate geometry for the compound, at least in the solid state. The 1 H NMR spectrum of [MoO2(SATP)]2 in d 6 -DMSO (which probably generates the monomeric [MoO2(SATP)(d 6 -DMSO)] in situ) shows the CH=N resonance at δ 9.06 (vs. 9.23 for the SAP analogue). The absence of an aldehydic resonance at δ > 10 and broad OH resonances confirms the generation of the desired complex from the one-pot procedure.

The thermogravimetric analysis (TGA) is the simplest method to determine whether the compound contains a donor solvent molecule. For instance, complex [MoO2(SAP)(EtOH)] shows release of the weakly coordinated EtOH molecule at ca. 100°C [START_REF] Agustin | [END_REF]. The TGA of the isolated SATP complex shows the absence of a mass loss at low temperatures and the presence of a single mass loss at T ca. 249-543 °C (cf. 245-543°C for the corresponding SAP system), related to the loss of the SATP ligand with formation of MoO3 as residue.

Interestingly, while labile alcohol adducts are generally obtained under these conditions with SAP and other similar ONO Schiff base ligands [START_REF] Agustin | [END_REF]18,22], a methanol-free product is obtained with SATP. This suggests that the O/S substitution makes the formation of a monomeric MeOH adduct thermodynamically less favorable.

Most of the previously published catalytic epoxidation of Mo VI complexes of ONO and ONS ligands were conducted in organic solvents [13][14][15][22][23][24][25][26][27][28][29][30][31]. Complex [MoO2(SATP)]2 was tested for the epoxydation of cyclooctene by aqueous TBHP under solvent-free conditions at 80°C (Scheme 2) and the results compared with those of the previously investigated SAP analogue, as well as with [MoO2(acac)2], under the same operating conditions. The results are summarized in Figure 1. The most active catalyst is [MoO2(acac)2] (96% conversion after 4 h, initial TOF = 270 h -1 ). Compound [MoO2(SATP)]2, however, shows only slightly lower activity (90% conversion after 4 h, TOFinit. = 253 h -1 ), but much greater than [MoO2(SAP)]2 (76% conversion, TOFinit. = 70). The epoxide selectivity is high and similar for all three complexes (91, 93 and 94% for [MoO2(acac)2], [MoO2(SAP)] and [MoO2(SATP)], respectively). In spite of its higher activity, however, complex [MoO2(acac)2] is of lesser interest because of its decomposition under the catalytic conditions as pointed out before [9]. A decrease of catalyst loading down to 0.025% (250 ppm) while maintaining the same substrate amount and TBHP/substrate ratio still allowed to attain high conversions after 4 h for the [MoO2(SATP)]2 catalyst, higher than with a greater loading of the [MoO2(SAP)]2 catalyst, see Figure 2. The selectivity remains high for the [MoO2(SATP)]2 catalyst while it is slightly eroded at the lowest loadings of [MoO2(SAP)]2, see Table S1. The increase of initial TOF as the catalyst loading is decreased (up to 2697 h -1 with 0.025% of the SATP complex) suggests that at high concentrations the catalyst is not entirely in its active form, although optical evidence is in favor of complete dissolution and confinement in the organic phase at the high temperature of the catalytic runs. A possible interpretation is equilibrium dissociation of the stable dimer into catalytically active monomer. No catalyst activation phase is apparent in any of these runs. Since a decrease of catalyst loading entails an increase of the water/catalyst ratio, the effect of water was independently investigated while keeping the catalyst loading and TBHP/substrate ratio constant. Increasing amounts of water have only a slightly negative effect on the conversion (see Table S2 in SI), which could simply be related to a decreasing TBHP concentration in the olefin phase. The selectivity does not appear to be significantly affected by the water amount. The results of the calculations on the SATP system, summarized in Figure 3, completely parallel those of the analogous SAP system, pathway A being again characterized by the lowest energy rate-determining transition state. The association of the pentacoordinated monomer with either TBHP, leading to the path A intermediate IA, or with itself, yielding the di(µ-oxido)bridged dimer, has lower exothermicity for the SATP system (stabilizations by 5.0 and 1.9 kcal/mol, respectively) than for the SAP analogue (8.4 and 4.9 kcal/mol). This is consistent with the experimental trends discussed above, indicating reduced electron deficiency for the 5-coordinate SATP complex. Like for the SAP system, the IB and IC intermediates of the other two pathways, produced by proton transfer activation of TBHP, are located at higher energy relative to the 5-coordinate dioxido complex. The transition states of the three pathways (energies in kcal/mol from the [MoO2(SATP)] monomer), TSA (17.3), TSB (33.6) and TSC (30.0), have structures (details in Table S3 in the SI) and relative energies comparable to those of the SAP catalyst. A detailed comparison of the TSA geometries for the SAP and SATP systems (details in SI, Figure S6) shows very similar features, with a slightly more pronounced Mo-OtBu bond formation and a less pronounced O-ethylene bond formation for the TS with the ONS ligand. Like for the SAP system, an organometallic intermediate as originally proposed by Mimoun could not be located and the reaction coordinate involved instead a direct O atom transfer to the external olefin to produce the epoxide product, which remains coordinated in a 6-coordinate adduct, plus tBuOH in a single step.

Scheme 2 -Solvent-free epoxidation of cyclooctene

The energy span [35] for the lowest energy pathway, [E(TSA) -E(IA)], 22.3 kcal/mol, is quite close to that of the SAP analogue, 22.5 kcal/mol [9]. It is difficult to make firm conclusions out of a calculated energy difference of only 0.2 kcal/mol, but we nevertheless observe that this difference is in the right direction (smaller barrier for the SAPT complex).

Assuming that the TS contribution to the activation Gibbs energy is identical for the two catalysts, application of the Eyring expression on this Δ(ΔE) yields a reactivity ratio (kSATP/kSAP) of 17 at 80°C, which is close to the ratio of ca. 10 from the TOF of the two catalysts determined under the conditions where the dinuclear precatalyst is most extensively dissociated, i.e. at the lowest catalyst loading (Table S1).

Figure 3 -Energy profile of the reaction along the three catalytic cycles of Scheme 3.

Conclusions

In conclusion, we have reported the first direct comparison of two ligands that are related by an O/S atom substitution in Mo VI -catalyzed olefin epoxidation and have shown that the S ligand yields a much more efficient catalytic system than the O ligand. This difference is related to a subtle electronic effect of the O/S substitution that slightly lowers the energy span of the catalytic cycle.

1 (

 1 C NMR (300 MHz, DMSO-d6): 163.0 (CH-N), 161.3 (Cq-O), 148.Cq-S), 138.6 (CH-N), 137.0 (CH-Ar), 136.8 (CH-Ar), 129.8 (CH-Ar), 129.1 (CH-Ar), 125.8

Figure 1 -

 1 Figure 1 -Cyclooctene conversion vs. time catalyzed by [MoO2(acac)2] () , [MoO2(SAP)]2 () and [MoO2(SATP)]2 (). Experimental conditions: Mo/TBHP/cyclooctene = 0.5/100/200; T = 80°C.

Figure 2 -Scheme 3 -

 23 Figure 2 -Cyclooctene conversion after 4 h as a function of the catalyst loading for [MoO2(SAP)]2 () and [MoO2(SATP)]2 (). Conditions: T = 80°C, [Mo]/cyclooctene/TBHP = x/100/200.

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, the Syndicat Mixte de la Communauté d'Agglomération Castres-Mazamet for the housing facilities of T.V. and the Chinese 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).