Synthesis, Crystal Structures, Characterization and Catalytic Property of Manganese(II) Complexes Derived from Hydrazone Ligands

A new bromido-coordinated mononuclear manganese(II) complex [MnLBr2(OH2)] (1), and a new nitrato-coordinated mononuclear manganese(II) complex [Mn(L)2(ONO2)(OH2)]NO3 (2), with the hydrazone ligands 4-hydroxy-N’-(pyridin-2-ylmethylene)benzohydrazide (HL1) and N’-(pyridin-2-ylmethylene)isonicotinohydrazide (HL2), have been synthesized and structurally characterized by physico-chemical methods and single crystal X-ray determination. Single crystal structural analysis shows that the Mn atom in complex 1 is in octahedral coordination, and that in complex 2 is in pentagonal bipyramidal coordination. The catalytic property for epoxidation of styrene by the complexes was evaluated.


Physical Measurements
Infrared spectra (4000-400 cm -1 ) were recorded as KBr discs with a FTS-40 BioRad FT-IR spectrophotometer. Microanalyses (C, H, N) of the complex were carried out on a Carlo-Erba 1106 elemental analyzer. Solution electrical conductivity was measured at 298 K using a DDS-11 conductivity meter. GC analyses were performed on a Shimadzu GC-2010 gas chromatograph.

3. X-Ray Crystallography
Crystallographic data of the complexes were collected on a Bruker SMART 1000 CCD area diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 298(2) K. Absorption corrections were applied by using the multi-scan program. 5 The structures of the complexes were solved by direct methods and successive Fourier dif-HL 1 HL 2 Scheme 1. The preparation of the hydrazone ligands HL 1 and HL 2 .

6. Styrene Epoxidation
The epoxidation reaction catalyzed by the complexes was carried out at room temperature in MeCN under nitrogen atmosphere. The reaction mixture contains styrene (2.00 mmol), chlorobenzene (internal standard; 2.00 mmol), the complex (catalyst; 0.10 mmol) and iodosylbenzene or sodium hypochlorite (oxidant; 2.00 mmol), and MeCN (5.00 mL). When sodium hypochlorite was used as the oxidant, the solution was buffered to pH = 11.2. GC was used to determine the composition of reaction medium with styrene and styrene epoxide quantified by the internal standard method (chlorobenzene). For each catalyst, the reaction time for the maximum epoxide yield was determined by withdrawing periodically 0.1 mL aliquots from the mixture and this time was used to monitor the efficiency of the catalyst on performing at least two independent experiments. Blank experiments with each oxidant and using the same experimental conditions without catalyst were carried out.

1. Synthesis
The hydrazones were facile prepared by reaction of 2-pyridinecarboxaldehyde with 4-hydroxybenzohydrazide and 4-pyridylcarbonylhydrazine, respectively, in MeOH. The complexes 1 and 2 were synthesized from the hydrazones with manganese bromide tetrahydrate (for 1) and manganese nitrate tetrahydrate (for 2) in MeOH (Scheme 2). Notably, even though the synthetic procedures are different, the structure of the bromido-coordinated complex 1 is similar to the chlorido-coordinated manganese(II) complex. 6 In the synthesis of the chlorido-coordinated manganese(II) complex, triethylamine was added to remove the hydrogen of the amino group. To the best of our knowledge, it is no need to introduce triethylamine in the preparation of Schiff base complexes. The molar conductivities (Λ M = 35 Ω -1 cm 2 mol -1 for 1 and 138 Ω -1 cm 2 mol -1 for 2) are consistent with the values expected for non-electrolyte and 1:1 electrolyte. 7  (7) 9.1540 (13) 16.6952 (14) 10.3954 (15) 12.0488 (10) 14.4801 (17)

4. Synthesis of [MnL 1 Br 2 (OH 2 )] (1)
2-Pyridinecarboxaldehyde (1.0 mmol, 0.11 g) was reacted with 4-hydroxybenzohydrazide (1.0 mmol, 0.15 g) in methanol (20 mL) for 30 min at room temperature with stirring. Then, manganese bromide tetrahydrate (1.0 mmol, 0.29 g) was added, and the mixture was stirred at room temperature for another 30 min. The deep brown solution was evaporated to remove three quarters of the solvents under reduced pressure, yielding brown solid product of the complex. Yield: 63%. Well-shaped single crystals suitable for X-ray diffraction were obtained by recrystallization of the solid from methanol.

2. Description of the Structure of Complex 1
Single-crystal X-ray analysis reveals that compound 1 is a bromido-coordinated mononuclear manganese(II) complex. The ORTEP plot of the complex is shown in Figure 1. The manganese atom is in a distorted octahedral geometry, which is coordinated by the N 2 O donor atoms of the hydrazone ligand and one Br atom in the equatorial plane, and one Br atom and one water O atom in the axial positions. The distortion of the octahedral coordination of the structure can be observed from the bond angles (Table  2) (Table 3), to generate chains along the c axis ( Figure 2).

Description of the Structure of Complex 2
Single-crystal X-ray analysis reveals that compound 2 is a nitrato-coordinated mononuclear manganese(II) complex. The compound contains a [Mn(L 2 ) 2 (ONO 2 ) (OH 2 )] cation and a nitrate anion. The ORTEP plot of the complex is shown in Figure 3. The manganese atom is in a Scheme 2. The preparation of the complexes.   pentaganol-bipyramidal geometry, which is coordinated by the N 2 O donor atoms of one hydrazone ligand and the NO donor atoms of the other hydrazone ligand in the equatorial plane, and one nitrate O atom and one water O atom in the axial positions. The distortion of the pentagonal bipyramidal coordination of the structure can be observed from the bond angles (Table 2) (Table 3), to generate a network (Figure 4).

5. Catalytic Epoxidation Results
Epoxidation of styrene was carried out at room temperature with complexes 1 and 2 as the catalysts and PhIO and NaOCl as oxidants. The brown color of the solutions containing the catalysts and the substrate was intensified after the addition of oxidant indicating the formation of oxo-metallic intermediates of the catalysts. After completion of the oxidation reaction, the solution regains its initial color. The percentage of conversion of styrene, selectivity for styrene oxide, yield of styrene oxide and reaction time to obtain maximum yield using both the oxidants are given in Table 4. The data reveals that the complexes as catalysts convert styrene most efficiently in the presence of both oxidants. Nevertheless, the catalysts are selective towards the formation of styrene epoxides despite of the formation of by-products which have been identified by GC-MS as benzaldehyde, phenylacetaldehyde, styrene epoxides derivative, alcohols etc. From the data it is also clear that the complexes exhibit excellent efficiency for styrene epoxide yield. When the reactions are carried out with PhIO and NaOCl, most of the oxidation was occurred in the first one hour. When the reaction time was prolonged to two hours for complex 1 and three hours for complex 2, the styrene conversions were about 89 and 77% for complex 1, and 78 and 70% for complex 2, respectively. It is evident that between PhIO and NaOCl, the former acts as a better oxidant with respect to both styrene conversion and styrene epoxide selectivity. The epoxide yields for the complexes 1 and 2 using PhIO and NaOCl as oxidants are 77 and 65%, and 73 and 57%, respectively. It is also obvious that complex 1 has better catalytic property than complex 2. Nitrogeneous ligands are reported to lengthen and weaken the M-O bond in the oxidized form of the catalyst by donating electron density into the M-O antibonding orbital, which can account for the improved reactivity. 13 Kochi et al. reported epoxide yields of 50-75% for the epoxidation of various types of olefins, including substituted styrenes, stilbenes, and cyclic and acyclic alkenes, within 15 min at room temperature in acetonitrile using PhIO as the oxidant and several Mn(III)-salen complexes as catalysts. 14 Hosseini-Monfared et al. reported the cyclohexene epoxide yield ranging from 43-68% in presence of PhIO as oxidant. 6 Lei and Yang reported the styrene oxide yields of 75 and 60%, respectively, with the oxidant PhIO and NaOCl. 15 Thus, manganese complexes with Schiff base and hydrazone ligands are a kind of excellent catalysts for the oxidation reactions.

Conclusion
A new bromido-coordinated mononuclear manganese(II) complex and a new nitrato-coordinated mononuclear manganese(II) complex derived from hydrazone ligands were prepared and characterized. Single crystal X-ray analysis indicates that the Mn atom in complex 1 is in octahedral coordination, and that in complex 2 is in pentagonal bipyramidal coordination. The complexes have effective catalytic property for the epoxidation of styrene.