Chiral Ni-Schiff Base Complexes inside Zeolite-Y and Their

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Chiral-Ni-Schiff Base Complexes Inside Zeolite-Y and Their Application in Asymmetric Henry Reaction: Effect of Initial Activation with Microwave Irradiation Mukesh Sharma, Biraj Das, Galla V. Karunakar, Lanka Satyanarayana, and Kusum K. Bania J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03457 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 13, 2016

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The Journal of Physical Chemistry

Chiral-Ni-Schiff Base Complexes Inside Zeolite-Y and Their Application in Asymmetric Henry Reaction: Effect of Initial Activation with Microwave Irradiation Mukesh Sharma,a Biraj Das,b Galla V. Karunakar,c L. Satyanarayana,d Kusum K. Bania* a,b,* c

Department of Chemical Sciences Tezpur University, Assam, India, 784028

Division of Crop Protection Chemicals, Indian Institute of Chemical Technology, Uppal

Road, Tarnaka, Hyderabad, Telangana 500007 d

Center for NMR and Structural Chemistry, Indian Institute of Chemical Technology, Uppal

Road, Tarnaka, Hyderabad, Telangana 500007

Abstract: Chiral Ni(II)-Schiff base complexes synthesized inside the cavity of zeolite-Y were used as heterogeneous chiral catalyst for asymmetric Henry reaction. Synthesized catalysts were characterized using various spectrochemical and physicochemical techniques. Solid state NMR analysis has been used to confirm the internal location of the metal complexes. To the best of our knowledge MAS NMR has not been used to characterize chiral diamagnetic Ni2+-Schiff base complex inside zeolite-Y. Catalytic activities of the materials were dependent on temperature, solvent and amount of catalyst. High catalytic transformation of aldehydes with nitromethane to nitro-aldol product (92% yield and 83% ee, S-isomer) was achieved at -10 °C.

Initial activation of the reaction with microwave

irradiation for 15 min leads to substantial decrease in the reaction time in comparison to normal stirring.

Heterogeneous catalysts were found to be advantageous over the

homogeneous counterparts in terms of recyclability of the catalyst. Most importantly product selectivity, percentage yield and enantioselectivity were found to be high with the heterogeneous catalyst. Catalytic activities of the metal complexes were influenced by the structural modification of the Schiff base ligands. Calculation of the energy barrier using density functional theory (DFT) suggests that activation barrier is less in case of the encapsulated complexes.

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Introduction Asymmetric Henry reaction or nitro-aldol condensation reaction has been considered as an important class of organic transformations due to the application of the aldol product in various natural product syntheses.1-5

Different homogeneous and heterogeneous chiral

catalysts are implemented to achieve such asymmetric transformation.6-10 Although various chiral homogeneous catalyst so far reported has been found to show good catalytic activity but they fail in terms of recyclability and in many cases homogeneous catalyst suffer from loss of catalytic activity.11-14 Now a days, many researchers use different techniques like use of ionic liquids, supercritical liquid etc, to recover the homogeneous chiral catalyst.15 But besides these methodologies, heterogenaization of homogeneous catalyst via encapsulation or immobilization into solid support has been found to be advantageous over various other techniques.16-18 Zeolites, MCM-41 etc. and such inorganic mantles are now a days found to be excellent support for heterogenaization of homogeneous catalysts.19-22 Out of the various Si-based materials, zeolite-Y a crystalline aluminosilicate with pore dimension of 7.4 Å and supercage of 13 Å has been found to be the most effective host for encapsulation of transition metal complexes. Starting from the pioneering work of Herron,23 quite a good number of works has been reported on encapsulation of transition metal complex inside the cavity of zeolite-Y. From our group we have reported for design of various metal complexes with 1, 10 phenanthroline, picolinates and with chiral and achiral Schiff base complexes inside zeolite-Y.24-28 Although a significant number of metal catalyst has been synthesized inside zeolite-Y cavity but very less number of reports are available on designing of chiral catalyst inside zeolite-Y. Recently, we have reported for synthesis of chiral vanadium Schiff base complex inside zeolite-Y29 and also for Cu-Cinchonidine complex supported on zeolite-Y.30 Vanadium Schiff base complexes were found to be efficient catalyst for oxidation of 2-


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naphthol and styrene. Looking into the advantage of zeolite-Y encapsulated chiral metal Schiff base complexes, in this study we report for the synthesis and characterization of the Ni2+-Schiff base complexes inside zeolite-Y and their application in asymmetric Henry reaction. Experimental Section Materials. Nickel (II) chloride dihydrate, NiCl2.2H2O procured from SRL was used as a source of nickel. Salicylaldehyde and Chiral 1, 2-diamines used for synthesis of chiral Schiff base ligands were purchased from Alfa Aesar and Sigma Aldrich, respectively. Tetrabutyl ammonium bromide (TBAB) of analytical grade (AR) used as electrolyte in cyclic voltammetry analysis was obtained from E-Merck. NaY zeolite was purchased from Sigma Aldrich.

Aldehydes used for asymmetric Henry reaction were purchased from Sigma

Aldrich. All the solvents used during the synthesis of materials and in asymmetric Henry reactions were purified by standard procedure prior to their use. Preparation of Schiff Base Ligands. Details of the synthesis of four Schiff base ligands namely L1, L4, L3 and L4 are provided in supporting information along with their 1H and 13

C-NMR data (Figure S1-Figure S4). The structures of the four ligands are shown in

Scheme 1. Synthesis of the Ni2+-Schiff Base Complexes. To a solution of Schiff base ligands (L1, L2, L3 or L4) dissolved in 5 ml of acetonitrile (CH3CN), equimolar solutions of NiCl2.H2O (dissolved in DMF) was added in drop-wise. The resultant clear reddish-brown solution was stirred for 6 hour and was left for recrystallization. Dark brown red crystals were obtained after 6 days which was then washed with methanol. The neat Ni-Schiff base complexes herein are designated as Ni2+-L1, Ni2+-L2, Ni2+-L3, Ni2+-L4, Scheme1. Synthesis of Ni2+ - exchanged zeolite, Ni2+-Y. Ni2+ exchanged zeolite was prepared by heating Na-Y zeolite in air at a rate of 1 K min-1 and was further heated for 24 hours at 773 K



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to remove the impurities. 1 g of Zeolite-Y was dissolved in a round bottom flask with 50 ml of 0.01 M aqueous solution of NiCl2.2H20 and the resultant mixture was stirred for 24 hours at room temperature. To prevent metal hydroxide preparation, the pH of the solution was maintained in between 3-5. After 24 hours, the solutions were filter by using whatman no. 1 filter paper and were washed thoroughly with warm deionized water to remove all dissolved chloride ions or until it gives negative test with AgNO3. The Ni2+ exchanged zeolite were then dried for 15 hours in an oven at 373 K for further use. Synthesis of Zeolite-Y Encapsulated Ni(II)-Schiff Base Complexes. Zeolite-Y supported vanadium Schiff base complexes were synthesized following the “Ship in a bottle” synthesis method.29

In a general procedure, to the Ni2+@NaY chiral 1,2 diammine and 2-

hydroxybenzaldehyde or its dervative in either case dissolved in DMF were added in stoichiometric amount and reflux at 40 °C for 32 h under nitrogen atmosphere. The light brownish powders so obtained were subjected to Soxhlet extraction for several hours using acetonitrile, ethanol and diethyl ether as solvents. The color of the samples did not change after Soxhlet extraction and prolonged exposure to air indicating the formation of metal complexes inside zeolite-Y. The powders were then dried under vacuum and and kept for 32h in a desicator for further characterization.

Synthesized zeolite supported metal

complexes are represented herein as Ni2+-L1@NaY, Ni2+-L2@NaY, Ni2+-L3@NaY and Ni2+L4@NaY, Scheme 1. General Procedure for Asymmetric Henry Reaction. In a mixed solvent of acetonitrile and ethanol 15 mg of the (Ni-L1 or Ni2+-L1@NaY) catalyst was dissolved followed by addition of 4-nitrobenzaldehyde (5 mmol) in a typical round bottom flask used for microwave treatment. The resulting mixture was stirred for few minutes and nitromethane (5 mmol) was added dropwise into it.

The whole reaction mixture was then subjeted to mirowave

irradiation with 50% power (425W) at 31 °C for 15 min in a scientific multimode microwave


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reactor equipped with digital power and temperature control unit. After irradiation with microwave the reaction mixture was magnetically stirred for 5 hrs at -10ᵒ C.

Product

formation was confirmed by checking the progress of the reaction using TLC. Resultant nitoaldol produt was seperated by washing the reaction mixture for several times with 9:2 mixture of hexane ethyl acetate solution. Heterogeneous catalyst was recovered by simple filtration and completely dried under vaccum and stored in desiccator for further use. Physical Measurements and Computational Details. Details of the physical measurements and different characterization techniques used in this study are given in supporting informations along with the methods we implemented for the density functional theory (DFT) calculations using DMOL3 programme.31 Results and Discussion The synthesized chiral Ni2+ Schiff base complexes were characterized via various spectrochemical and physicochemical analysis prior to its application in asymmetric catalysis. Zeolite-Y encapsulated complexes were initially characterized by performing the elemental analyses. The results of the elemental analyses are presented in Table 1. The Si/Al ratio in zeolite-Y was found to be ~ 2.5 consistent with the previously reported values.29 The amounts of metal content in the encapsulated system were found to be less than those of the nickel exchanged zeolite-Y, Ni2+@NaY due to participation of Ni2+ ion in the formation of metal Schiff base complex inside zeolite-Y. Carbon (C), hydrogen (H) and nitrogen (N) contents in the encapsulated complexes were found to be consistent with the expected molecular formula of the Ni2+ Schiff base complexes. In particular the C/N ratios in all the encapsulated Ni2+-Schiff base complexes were found to be approximately same as that in the neat complexes. BET surface area analysis was conducted to determine the surface area of the encapsulated complexes. The corresponding adsorption/desorption isotherm for the four



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encapsulated complexes are comparatively shown in Figure 1 and the surface area, pore dimension values are given in Table 1 along with elemental analysis.

Pattern of the

adsorption/desorption isotherms for all the complexes were found to be more or less identical. However, there exist significant difference in the surface area, pore size and pore volume among the encapsulated systems. Apart from BJH, the pore size distribution (PSD) was also obtained from the adsorption branch using the Non-Local Density Functional Theory (NLDFT), assuming a cylindrical pore shape. The PSD was determined via a NLDFT model using nitrogen adsorption data.32-34 BJH and NLDFT micropore PSD were almost found to be comparable. In all the encapsulated complexes pore size, pore volume and BET-surface area were found to be significantly less in comparison to neat NaY, Table 1. Decrease in surface area and pore volumes of parent NaY clearly indicates the presence of metal complex inside the cavity of zeolite-Y.28 It is pertinent to mention that the surface area reported herein for NaY zeolite is taken from our previously reported results.29 PXRD-pattern of the Na-Y, Ni2+-NaY and the encapsulated complexes are shown in the Figure 2. The PXRD pattern of the Na-Y and Ni2+-NaY were almost similar, Figure 2a and 2b, respectively. However in the case of encapsulated complexes, the intensity of I220 and I311 plane observed at 10° and 12°, 2θ values got reversed, Figure 2c. In case of the Na-Y and Ni2+-NaY, intensity of 220 plane was greater than 311 plane i.e I220>I311. Reversal in the intensity of 220 and 311 plane i.e. I220

Chiral Ni-Schiff Base Complexes inside Zeolite-Y and Their

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