Synthesis, crystal structure and DFT studies of a new dioxomolybdenum(VI) Schiff base complex as an olefin epoxidation catalyst of a new as

________________________________________________________________ A cis-dioxomolybdenum(VI) complex was prepared from Mo(acac) 2 and a Schiff base ligand derived from 2-hydroxy-1-naphthaldehyde (Naph) and L-histidine (His) in ethanol and was designated as MoO 2 (Naph-His). Characterization of MoO 2 (Naph-His) was carried out by means of elemental analysis, 1 H NMR, 13 C NMR, , FT-IR, UV–Vis and TGA techniques. The crystal structure of MoO 2 (Naph-His), determined by single-crystal X-ray crystallography, revealed that the coordination of Mo in the complex is a distorted octahedron, formed by a tetradentate Naph-His Schiff base ligand and two binding oxygen atoms. The optimized geometrical parameters obtained by DFT calculations are in good agreement with the single XRD data. It was found that MoO 2 (Naph-His) successfully catalyzes the epoxidation of cyclooctene, cyclohexene and norbornene with 80-100% conversions and 54-100% selectivities. Based on the obtained results, the heterogeneity and reusability of the catalyst seems promising. The synthesis, characterization, crystal structural and DFT studies of a new Mo complex using Mo(acac) 2 and a Schiff base ligand derived from 2-hydroxy-1-naphthaldehyde and L-histidine in ethanol and its usage as an epoxidation catalyst are presented.


Introduction
The enzymatic role of molybdenum compounds in biological reactions has created a tremendous impetus in the synthesis of a number of model complexes mimicking oxotransferase molydoenzymes [1][2][3][4]. In this regard, many stable molybdenum complexes with oxygen-, nitrogen-and sulfur-containing ligands have been prepared. Molybdenum(VI) Schiff base complexes with a cis-MoO 2 core are excellent enzyme model systems for the active sites of molybdo-enzymes, such as xanthine oxidase, nitrogenase and sulfite oxidase. Possessing an Mo=O unit, they have widely been used as catalysts in industrial processes, such as epoxidation, sulfoxidation and hydroxylation of olefins, [5][6][7][8][9].
Coordination complexes of Mo with Schiff base ligands have wide applications in electrochemistry [10] and biological modelling [11], as antioxidant [12] and antibacterial agents [13], and as catalysts for hydrogen generation [14], alkene epoxidations [15,16] and sulfide oxidations [17]. In order to mimic biological systems, a number of dioxomolybdenum complexes have been synthesized and characterized [18][19][20][21][22]. The activity of these complexes varies with the ligand type and coordination sites. In spite of the synthesis of many Mo complex Schiff bases, there are few reports on the preparation of Schiff bases with amino acids [23][24]. Amino acids are efficient biologically active and cytotoxic agents and are considered as anticancer and antibacterial reagents [25,26]. Amino acid Schiff bases are sensitive to moisture and decompose when exposed to air. Therefore, they are usually generated immediately prior to use for complexion. It is particularly significant that isolated crystalline amino acid Schiff bases have rarely been reported [27].
The catalytic epoxidation of alkenes is a reaction of great industrial interest because epoxides are widely used as intermediate chemicals for making valuable products, such as chiral pharmaceuticals, epoxy resins, epoxy paints, surfactants, pesticides, agrochemicals, perfume materials and sweeteners. Epoxides have also been numerously applied as precursors in the production of fine chemicals [28][29][30][31][32][33][34]. The type of ligand structure present in the complex and the catalytic reaction conditions have a significant effect on the catalytic activity of complexes involving the Mo(VI) metal center [35]. As a result, several research groups have focused on the design of new Mo(VI) complexes and their potential applications.
In this study, an attempt has been made to prepare a new Schiff base complex of Mo with His and Naph and using it as a catalyst for the epoxidation of some alkenes.

Materials and characterization
All materials were of commercial reagent grade and used without further purification. t-Butyl hydroperoxide (TBHP) was purchased from Fluka, 2-hydroxy-1-naphthaldehyde (Naph), Lhistidine (His), sodium acetate, hydrogen peroxide (30%), cyclooctene, diethyl ether and ethanol were purchased from Merck Chemical Company. FT-IR spectra were recorded on a Bruker Tensor 27 FT-IR spectrometer using KBr pellets over the range 4000-400 cm -1 . The UV-Vis measurements were performed on a double beam UV-Vis Perkin Elmer Lambda 35 spectrophotometer. Single crystal measurement was performed on an Agilent Super Nova Dual single crystal diffractometer. Intensity data were collected using graphite monochromatised Mo K α radiation (k = 0.71073Å). Chemical analyses of samples were determined with a Perkin Elmer atomic absorption spectrometer (AAS). TGA thermal curve measurement was carried out with a Perkin Elmer Pyris 1. Oxidation products were analyzed by GC and GC-MS using an Agilent 6890 Series with an FID detector, HP-5, 5% phenylmethyl siloxane capillary and an Agilent 5973 Network, mass selective detector, HP-5 MS 6989 Network GC system.

Preparation of [MoO 2 (Naph-His)]
Initially, Naph (0.172 g, 1 mmol) was dissolved in ethanol (2 mL). This solution was then added to a solution of His (0.155 g, 1 mmol in 2 mL water). Upon addition of an aqueous sodium acetate solution (0.164 g, 2 mmol in 1 mL water), the color changed to yellow. To this solution was added MoO 2 (acac) 2 (0.278 g, 0.85 mmol in 2 mL ethanol, prepared according to the literature method [36]), followed by heating at reflux for 3 h. The yellow resultant solid was then filtered, washed with water, ethanol and diethyl ether, and then dried in air at room temperature.

General procedure for the oxidation of alkenes
All oxidation reactions of alkenes (norbornene, cyclohexene and cyclooctene) were carried out in a round bottom flask equipped with a magnetic stirrer and a water-cooled condenser.
Typically, [MoO 2 (Naph-His)] as catalyst (0.025 g) in CCl 4 (5 mL) and alkene (10 mmol) with TBHP (14 mmol) were mixed and the mixture was heated at reflux for 8 h. After separation of the catalyst, the filtrate was subjected to GC and GC-Mass for analyses.

X-ray crystallography
Crystallographic data were collected on a MAR345 dtb diffractometer equipped with image plate detector using Mo-Kα radiation (0.71073 Å). The structure was solved by direct methods using SHELXS-97 and refined using the full-matrix least-squares method on F 2 , SHELXL-97 [40]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added at ideal positions and refined using a riding model. Crystallographic data, details of collected data and structure refinement are listed in Table 1. Selected bond lengths and angles are shown in Table 2.

Computational details
All calculations were carried out with the Gaussian program series 2003 [41]. The optimization of the geometry was performed employing a hybrid Hartree-Fock-density functional scheme, the adiabatic connection method-Becke three-parameter with Lee-Yang-Parr (B3LYP) [42] density functional theory (DFT) [43] with the two standard 3-21G and 6-31G* basis sets. Full optimizations were performed without any symmetry constrains. The harmonic vibrational frequencies were computed to confirm that an optimized geometry correctly corresponds to a local minimum that has only real frequencies. The partial charges were obtained using the NBO method. The solvent effect was investigated with the PCM method [44] at the B3LYP/6-31G* level. Solvation calculations were carried out for ethanol with the geometries optimization for this solvent. found to be entirely consistent with its composition as determined by X-ray crystallography.

Geometry optimization of the (Naph-His) Schiff base ligand
The structure of the Naph-His Schiff base ligand was fully optimized by the B3LYP method using two basis set, including 3-21G and 6-31*, with no initial symmetry restrictions and assuming the C 1 point group. The optimized geometry of the ligand and some structural details are given in Fig. S6 and Table S1, respectively (see supplementary data). Calculation of the vibrational frequencies has confirmed a stationary point with no negative eigenvalue observed in the force constant matrix. . In order to compare the reliability of our predicted results, a comparison between the theoretical and experimental data is presented in Table 2. As indicated in this Table, the standard deviation of bond distances, bond angles and dihedral angles for the B3LYP/3-21G and B3LYP/6-31G* levels are very similar. In fact, there is a good agreement between the solid structure and gas phase calculated values, the small differences are due to the X-ray crystal diffraction being applied in the solid state. Recall that calculations were performed in the gas state.

Geometry optimization of the [MoO 2 (acac) 2 ] and [MoO 2 (Naph-His)] complexes
To reduce the computational cost, the B3LYP method with the 3-21G basis set can be applied to optimize the molybdenum complexes. Calculation of vibrational frequencies has confirmed a stationary point with no negative eigenvalue observed in the force constant matrix.
In the next step, the fully optimized geometry of the [MoO 2 (Naph-His)] complex in the gas phase was re-optimized by considering the solvent effect (E = 431569.12 Kcal/mol) using the PCM method. The calculated results indicated that the complex is stabilized by 38.5 kcal/mol in ethanol, perhaps due to rather high complex dipole moment (10.41 D).

Details of atomic net charge
The calculated net charges on the [MoO 2 (Naph-His)] complex are given in Table S3 ( Table S3. Of particular significance is the partial charge of 1.19 observed on the molybdenum atom when Naph-His is coordinated. In fact, the calculated electron density computed on the donor oxygen and nitrogen atoms is less than expected, while the electron density computed on the central ion is more than expected. Thus, the net charge results confirm an electron transmission from the donor atoms of the Naph-His ligand to the central Mo ion.

Molecular orbital analysis point of view
To find the nature of binding between the Naph-His ligand and Mo 2+ , molecular orbital (MO) analysis has been employed. This analysis is useful in presenting the factors influencing the stability of this complex. Taking into account the above discussion, we focused mostly on the [MoO 2 (Naph-His)] complex. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the [MoO 2 (Naph-His)] complex are shown in Fig. 5.
While the LUMO is mainly based on molybdenum 3d orbitals, the HOMO includes contributions from both Mo 2+ and interacting ligand heteroatoms. The presented HOMO orbital reveals an antibonding interaction between the metal ion and ligand orbitals. These antibonding molecular orbitals arise from the interaction between the dσ metal orbital and the lone pairs of ligand oxygen atoms.

Catalytic studies
In order to evaluate the catalytic activity of [MoO 2 (Naph-His)] in epoxidation reactions, cyclooctene was selected as a model alkene. For this reaction, the effects of the amount of catalyst, time and solvent were studied (Figs. 7-9). As indicated in Fig. 7, increasing the amount of catalyst from 10 to 25 mg increases the conversion from 37 to 100% with 100% selectivity.
Therefore, all epoxidation reactions were carried out using 25 mg of catalyst. Similarly, increasing the reaction time from 1 to 8 h using 25 mg of catalyst was found to increase the cyclooctene conversion from 23 to 100% with 100% selectivity (Fig. 8). To test the effect of solvent, the epoxidation reaction was carried out in different solvents (Fig. 9). The trend of activity was found to be CCl 4 ˃ CHCl 3 ˃ CH 2 Cl 2 ˃ n-hexane ˃ ethanol ˃ CH 3 CN. It is evident that while coordinating solvents inhibited the reaction completely (ethanol or CH 3 CN), epoxidation proceeded in non-coordinating solvents (CH 2 Cl 2 , CHCl 3 or CCl 4 ). Based on the conversion percentage, CCl 4 was found to be the best solvent. Since CCl 4 has the highest boiling point within these three solvents, a higher rate based on the Arrhenius equation is expected.
Beside boiling point, decreasing polarity from CH 2 Cl 2 to CHCl 3 and CCl 4 may not be overlooked. As such, the epoxidation transition state seems to be less polar than the starting material since such reactions should be decelerated in a more polar solvent (CH 2 Cl 2 ) in comparison to a less polar solvent (CCl 4 ) [52].
Epoxidation of norbornene and cyclohexene with TBHP was then carried out with [MoO 2 (Naph-His)] as the catalyst in CCl 4 under the optimum reaction conditions (Fig. 10). We have included the cyclooctene epoxidation results in Fig. 10 in order to make a comparison more convenient. As seen in this Figure, the alkene epoxidation reactions proceeded with 80-100 % conversions and 54-100% selectivities.
To give an insight into the reaction mechanism, epoxidation of cyclooctene was carried out in the presence of diphenylamine as a radical scavenger. Observation of suppression of the epoxidation yield clearly revealed that this reaction has proceeded via a radical mechanism [53,54]. Based on the operation of a radical mechanism, one may rationalize the results obtained in Fig. 10. Whereas cyclooctene exclusively gives the corresponding epoxide, cyclohexene undergoes allylic oxidation and epoxidation, concomitantly (Fig. 10.). It has been reported that the high epoxidation selectivity typically observed for cyclooctene is related to a poor σ C-αH -π C=C orbital overlap in the predominant conformation, disfavoring αH-abstraction by radical species (Scheme 2a) [55]. Therefore, allylic-site oxidation of cyclooctene is not in competition with epoxidation. On the other hand, cyclohexene exist in the half-chair conformation in which allylic carbons lie in the plane of the double bond (Scheme 2b) [56]. Therefore, cyclohexene undergoes two competing oxidations, epoxidation and allylic-site. Norbornene is particularly significant, selectively affording the corresponding epoxide (Fig. 10). If norbornene undergoes allylic-site oxidation through αH-abstraction by radical species, it affords a bridgehead radical which has been shown to be unstable due to Bredt's rule (Scheme 2c) [57]. Therefore, norbornene conclusively undergoes an epoxidation reaction, affording the corresponding epoxide.
The recyclability of the [MoO 2 (Naph-His)] complex as a catalyst was investigated under the optimum reaction conditions. After each reaction cycle, the catalyst was recovered by centrifugation, washed several times with ethanol and dried under vacuum. Catalyst activity was found to be unchanged (100% conversion and 100% selectivity) after three runs, beyond which a slight decrease in conversion (98%) was observed, but with 100% selectivity after five runs.
Since no catalytic activity was observed when the filtrate of each run was subjected to the epoxidation conditions in the absence of catalyst, it can be concluded that the catalyst is truly functioning heterogeneously in nature.