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One pot synthesis of metal-Organic frameworks using N-Donor Ligand

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One pot synthesis of metal-Organic frameworks using N-Donor Ligand
   1 One pot synthesis of metal-Organic frameworks using N- Donor Ligand Tella, A.C 1 , Ameen O.A 2 1.   University of Ilorin, Ilorin kwara State. Nigeria, 2.   Kwara state Polytechnic Ilorin, Kwara State. ABSTRACT Synthesis of metal-Organic frameworks using 4,4-bipyridine ligand via mechanochemical and solvent based method is reported. The mechanochemical  product was obtained by grinding stoichiometric amount of zinc acetate with 4,4-bipyridine. Solvent based synthesis of the MOFs was carried out by reaction of zinc acetate with 4,4-bipyridine at 70 0 c in methanol using reflux method. The mechanochemical product was characterized by comparison of the melting, elemental analysis, spectroscopic data (IR, UV-visible and 1 H-NMR). Metal ion coordinates through the nitrogen of bipyridine and two oxygen of the carbonyl and hydroxyl group of carboxylate ligand. The MOFs exhibit  polymeric chain Key words : Mechanochemical, 4,4bipyridine, solvent-based metal-organic frameworks Introduction The evolution of a new class of coordination polymers known collectively as metal organic framework materials (MOFs) has been emerged in recent years. These materials are formed by coordinate bonds from multidentate ligands to mono- or polynuclear metal centers and may have extensive open-framework structures resulting in the inclusion of guest species (usually solvent) during synthesis. These species can be removed via desolvation and the resulting empty framework may maintain structural integrity giving a porous MOF material, which has a large apparent surface area. The srcinal solvent or other guest molecules can then be adsorbed into this porous structure. The MOF connectivities and topologies are controlled by the coordination preferences of metal and ligand, with the templating role of pore species being less  pronounced in structure direction than it is in zeolitic aluminosilicate and   2 metallophosphate systems governed by template directed assembly of purely tetrahedral building units. Porous media have potential applications in many fields including separation, heterogeneous catalysis, and gas storage. Porous networks are also of interest because of adsorption properties, where confined species may alter the physical properties of the host material. MOF materials have nanometer-sized channels with tailorable chemical functionality and are thus in principle attractive for the manipulation and transformation of a wide range of molecules. Applications of MOFs that have been demonstrated, in  proof of principle, include chemical sensing, chemical catalysis, light to electrical energy conversion and size selective membrane transport. 1.0 Experimental All materials were used as received from commercial sources (sigma-aldrich or fisher scientific): solvents were purified and dried according to standard methods. 1.1 Mechanochemical Synthesis of MOFs [Zn(oac) 2 (bipy) 2 ] 4,4-bipyridine (1mmol, 0.156 g) and Zincacetate (0.5mmol, 0.109g ) were weighed into a mortar and pestle that has been previously washed and dried. The reactants were ground 30minutes to a fine paste at room temperature. The progress of the reaction was monitored by the TLC to ascertain that no traces of reactant found in the product. The white powder obtained was washed with methanol to remove unreacting starting material and dried at room temperature. The equation of reaction is shown in scheme: 1 Zn(OAC) 2 .H 2 O + (4,4bipy) 2  [Zn(OAC) 2 (bipy) 2 ] Scheme 1: Mechanochemical Synthesis of [Zn(oac) 2 (bipy) 2 ] + Zinc acetate salt 4,4-bipyridine White powdered product of the reacting mixture Grinding in mortar and pestle  No solvent, RT 30 minutes Grinding in mortar and pestle  No solvent, RT 30 minutes   3 Yield 86.0%, M.WT=557g/mol, M.Pt 268 0 C; Analysis calculated for C 24 H 20  N 4 O 8 Zn: C, 51.70; H, 3.59; N, 11.05, O,22.98. found: C,50.16; H, 3.84;  N,11.68, O,23.32. IR (KBr, cm -1 ): 3481, 3105, 3055, 2528, 2362, 2185, 1716, 1653, 1491, 1458 1 H-NMR. δ (C 1 -H) 7.85ppm, d; δ (C 2 -H) 8.74, d Solvent based Synthesis of [Zn(oac) 2 (bipy) 2 ] (3b) A solution of 4,4-bipyridine (1mmol, 0.156 g) in methanol were added to a clear colourless solution of zincacetate ( 0.5 mmol, 0.109 g ) in methanol. The resulting solution was refluxed for 2hours at 70 0 C. A white precipitate was formed and allowed to evaporate for 3 days at room temperature. A white crystal was obtained as product, The equation of reaction is shown in scheme: 2.6 Zn(OAC) 2 .H 2 O + (4,4bipy) 2  [Zn(OAC) 2 (bipy) 2 ] Scheme 2.6: Solvent-based synthesis of [Zn(oac) 2 (bipy) 2 ] Yield 80.5%, M.WT=557g/mol, M.Pt 268 0 C; analysis calculated for C 24 H 20  N 4 O 8 Zn: C, 51.70; H, 3.59; N,11.05, O,22.98. found: C,50.16; H,3.84;  N,11.68, O,23.32. IR (KBr, cm -1 ): 3481, 3105, 3055, 2528, 2362, 2185, 1716, 1653, 1491, 1458. 1 H-NMR. δ (C 1 -H) 7.85ppm, d; δ (C 2 -H) 8.74, d. Result and Discussion Metal-organic frame works of [Zn(bipy) 2 (oac) 2 ] The [Zn(bipy) 2 (oac) 2 ] was synthesised by mechanochemical grinding of Zn(CH 3 COO) 2 .2H 2 O and 4,4-bypridine in a mortar with pestle for 30 minutes at room temperature. While [Zn(bipy) 2 (oac) 2 ] (3b) which is solvent based  product was obtained by reaction of Zn(CH 3 COO) 2 .2H 2 O and 4,4-bipyridine  by refluxing the two reactants using magnetic stirrer for 2 hours at 70 0 C. The solid reaction (mechanochemical) was completed within a shorter time of 30minutes as compare to solvent based synthesis which took 2 hours to + Methanol Reflux Reflux Zinc acetate salt 4,4-bipyridine Crystal product of the reacting mixture Methanol   4 complete. No solvent and heat are used during the mechanochemical synthesis, it was carried out at room temperature. The products were generally obtained with no waste and no further purification process were required. The mechanochemical product was characterized by comparison of analytical data, spectral data, XRPD with those of free ligands and solution based product. The compounds were non hygroscopic, are soluble in organic solvent with melting point higher than the ligand. The elemental analyses (CHN) were found to be consistent with the formula unit of the product. It also appears in our result that both methods (mechanochemical and solvent-based) gave similar spectral data in comparison with the free ligand bipyridine. The comparison of solubility and melting point of sample synthesised via the two methods shows that there is no difference in there value. In addition the %yield of 3a is higher than the solvent based product 3b. 4,4-bipyridine possess two identical donor site i.e bipyridine nitrogen atom, it act has bidentate ligand. In all the compound synthesized using mechanochemical and solvent based method, bipyridine binds with zinc through the two nitrogen atom thereby forming octahedral geometry using two acetate ion, this is in agreement with the result obtained by Jungsook et al 107 The analytical data of 4,4-bipyridine and its MOFs complexes 3a and 3b with Zn(II) are collected in table below.   Table 3.7. Analytical data of 4,4-bipyridine, and there [Zn(bipy) 2 (oac) 2 ] (3a) and (3b) Analytical data 4,4-bipyridine [Zn(bipy) 2 (oac) 2 ] (Mechanochemical) [Zn(bipy) 2 (oac) 2 ] (Solvent base) Colour %Yield M.Pt ( 0 C) M.W (g/mol) % Metal Off white - 114 156.19 - White 86 346 0 c 518.92 14.0 White crystal 80.5 348 0 c 518.92 14.0 Elemental analysis - Anal.calc. (found)  % C H N 51.00 (50.16) 3.57 (3.84) 11.05 (11.68) Anal.calc. (found) % C H N 51.70 (50.16) 3.59 (3.84) 11.05 (11.68)   5 It can be seen from Table 10 that white colours were observed for both  products (mechanochemical and solvent base synthesis) the product of mechanochemical is a white powdered while the product of solvent-based give an off white crystals. The products are non-hydroscopic. The percentage yield of the mechanochemical product is higher than the solvent based product and melting point of the complexes are higher than the parent ligand. FT-IR spectroscopy result of [Zn(bipy) 2 (oac) 2 ] (3a) and (3b) Figure: shows a comparison of the FT- IR spectrum of the free 4, 4  bipyridine ligand with the MOFs of (3a) and (3b) obtained via mechanochemical and solvent based methods. Comparison of FT  –  IR spectrum of 4,4- bipyridine and it’s  MOFs [Zn(bipy) 2 (oac) 2 ] [3a&3b]. It can be seen that the comparison of FT  –   IR spectral of [Zn(bipy) 2 (oac) 2 ] [3a&3b] obtained via mechanochemical and solvent based method are identical. 40003500300025002000150010005000100200300    %    T  r  a  n  s  m   i   t   t  a  n  c  e Wavelent (nm)  [Zn(bipy) 2 (oac) 2 ] mechanocchemical [Zn(bipy) 2 (oac) 2 ] solvent-based 4,4-bipyridine   Wave number (cm -1 )
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