Synthesis of New Coordination Polymers or MOFs

Synthesis of New Coordination Polymers or MOFs1.1 Design PrinciplesCoordination polymers as well as known as coat thorough frame ladders (MOFs) or metal-coordination ne 2rks be enhances constructed from organic ligands and metal ions connected by coordination bonds and other weak chemical bonds which fucking extend infinitely into one two or three dimensions 1-3. The ar guidement of the components in coordination polymers mostly exists only in self-coloured state which results from coordination interactions and weaker forces forming a smaller molecular units and growth occurs through self-assembly processes to give the final overall structure (Fig. 1) 4.There atomic number 18 four different kinds of building blocks of which coordination polymers are built ligands, metal ions or clusters, counter anions and response molecules. metallic element ions which are called nodes or connectors and ligands which act as linkers are the main components 7. They are the starting re compon ents which form the mind framework of the coordination polymer. The important characteristics of connectors and linkers are the number and orientation of their binding sites. Transition-metal ions are the most widely use connectors in the construction of coordination polymers. The choice of the metal and its oxidation state will determine the coordination geometries, which can be linear, T- or Y-shaped, tetrahedral, square-planar, square-pyramidal, trigonal-bipyramidal, octahedral, trigonal-prismatic, pentagonal-bipyramidal, and the corresponding distorted forms. Lanthanide ions are less employ because of the noble coordination number and variability coordination environment. However, the large coordination numbers from 7 to 10 and the polyhedral coordination geometry of the lanthanide ions will create new and queer net profit topologies. In addition, coordinatively unsaturated lanthanide ion centers can be generated by the removal of coordinated solvent molecules. The vacant sites could be utilized in chemical adsorption, heterogeneous catalysis, and sensors 8,9. Multidentate carboxylate functionalities tender rigid frameworks due to their ability to chelate metal ions and lock in to M-O-C clusters, which are referred as secondary building units (SBUs). alternatively of employing one transition metal ion at a network vertex, SBUs can assume extended frameworks of amply structural stability. Metal-complex connectors defecate the advantage of controlling the bond angles and restricting the number of coordination sites sites for no use can be blocked by chelating or macrocyclic ligands directly bound to a metal connector, and therefore, leave circumstantial sites free for linkers 25,10,11.Linkers are categorized in to three inorganic, organic, and organic-inorganic hybrid faces. Halides (F, Cl, Br, and I) are the smallest and simplest of all linkers. CN and SCN have identical bridging ability to halides 12,13.Typical organic ligands are shown in Fi g. 2 consisting of deaf(p), anionic and cationic organic ligands. The organic ligands act as bridging organic groups surrounded by the metal ions. Most famous neutral ligands are pyrazine (pyz) and 4,4-bpy 15-18. An congressman of a coordination polymer with the 4,4-bpy ligand is illustrated in Fig. 3. Recent efforts have been devoted to utilization of long bridging ligands with appropriate spacers 20-25.Among the anionic organic ligands di-,26-28 tri-,26, 29-32 tetra-,33,34 and hexacarboxylate 35,36 molecules are representative anionic linkers. Coordination polymers having nonsymmetric anionic ligands described as pyridine-X-COO (X=spacer) have been exhaustively studied 37. 1,4-Dihydroxy-2,5-benzoquinone and its derivatives stomach a variety of frameworks, in which they act as linear linkers 38.Coordination polymers with cationic organic ligands are precise rare, which is naturally a result of their very low coordination power for cationic metal ions 3943. Developed were swee t cationic ligands based on N-aryl pyridinium and viologen derivatives and were successfully employed 3941.Counter ions are present in the coordination frameworks when neutral bridging ligands are used as linkers to keep the neutrality in the overall put. Furthermore, other roles such(prenominal) as coordination and hydrogen bonding linker, guest for vacant spaces in the solid state are expected, eventually resulting in overall structure regulation.Solvent molecules are used not only for reaction media, but also the regulation of framework topology. It may co- crystallize, increasing the number of possible weak interactions in the final solid state packing, and can also act as guest molecules in the vacant space between polymer construct 7.SynthesisMany new coordination polymers or MOFs have been synthesized in the last few long time however, their rules of preparation and deduction were quite similar. Most of them are synthesized by employing a so called modular deductive reas oning, in which a mixture of metal precursors and appropriate ligands are combined under mild conditions to provide a crystalline porous network 26. In most of the resulting materials the solvent used during synthetic thinking is removed(p) by applying vacuum, heat, or exchange with volatile molecules, resulting in large pore volume and large fold up area accessible to guest molecules. Synthetic regularitys such as solvothermal synthesis (conventional start out), microwave synthesis 45, sonication synthesis 46, mechanochemical synthesis 47, and solid start synthesis 48 have been developed. Despite the simplicity of the synthesis, there are several(prenominal) challenges in the preparation of new materials related to the optimization of the reaction conditions that lead to the desired MOF, in high yield and crystallinity. The following parameters can influence MOFs optimization and synthesis temperature, solvent compositions, reaction times, reagent ratios, reagent concentration s, and pH of the co-solvent issue 44. Accordingly, any change in any of these parameters can result in large number of network connectivities, many of which are nonporous and have adverse effect on the gas storage and time interval applications. Therefore, large numbers of reactions trails are required to discover the new desired MOFs in which the reaction parameters are systematically varied. As a result high throughput technologies have been employed for the synthesis of new MOFs in the recent years 49,50.1.2.1 Solvothermal SynthesisSolvothermal methods have been confirmed to be among the most effective and convenient routes under relatively mild conditions, in particular for the crystal growth of coordination polymers 50-52. Solvothermal reactions are carried out in unkindly vessels under autogenous pressure above the boiling point of the solvent. In most cases, high-boiling organic solvents have been used for solvothermal reactions. The most commonly used being dimethyl forma mide, diethyl formamide, acetonitrile, acetone, ethanol, and methanol etc. Mixtures of solvents have also been used to tune the solution polarity and the kinetics of solvent-ligand exchange, effecting enhanced crystal growth. Solvothermal reactions can be carried out in different temperature ranges, depending on the requirement of the reaction 53. When water is used as the solvent, the reactions are referred to as hydrothermal. The hydrothermal method has been used successfully for the synthesis of an enormous number of inorganic compounds and inorganic organic hybrid materials 54. collectable to their unique advantageous properties such as high thermal stability, air and moisture non-sensitivity, non-volatility, low reactivity, and templating and charge balancing ability of ionic unstables, they can be chosen as solvothermal reaction media. Solvothermal synthesis in ionic liquids is specifically referred to as ionothermal synthesis 55. The coordination polymer Cu(I)(bpp)BF4 bpp = 1,3-bis(4-pyridyl)propane 56 was prepared by solvothermal reaction using the ionic liquid bmimBF4 (bmim = 1-butyl-3-methylimidazolium).The synthesis methods employed for different structures of coordination polymers (MOFs) and their key findings are listed in Table 1.1.2.2 Microwave-assisted synthesisMicrowave-assisted synthesis has attracted much(prenominal) attention as it provides a very rapid method for the synthesis of MOFs and has been used extensively to produce nanosize metal oxides 59. Such processes involve heating a solution with microwaves for a period of about an moment to produce nanosized crystals. The microwave-assisted synthesis has been termed microwave-assisted solvothermal synthesis for the preparation of MOFs. Microwave-assisted processes generally produce the same qualities of crystals as those begined by the unshakable solvothermal processes, but much quicker 60-64.The root coordination polymer reported to be synthesized by microwave synthesis was Cr-MIL- 100 65. The compound was synthesized in 4 h at 220 oC with 44% yield, which is comparable with that of conventional hydrothermal synthesis (220 oC and 4 days). The write expanded this method to synthesis of Cr-MIL-101 at 210 oC in less than 60 min, and reported similar physicochemical and textural properties compared with the standard material synthesized using the conventional electrical heating method 66. Another coordination polymer, MOF-5, was also synthesized by applying microwave irradiation increase in microwave irradiation time, power level, and concentration of the substratums beyond an best condition led to a reduction in synthesis time at the expense of crystal quality 67. Microwave-assisted heating was found to be the method of choice to rapidly synthesize HKUST-1 crystals in the range of 10-20 m in high yields (90%) within 1 h 68. Fe-MIL-53 69, Fe-MIL-101-NH2 70, IRMOF-3 (H2BDC-NH2) 71, and ZIF-8 (HMeIm) 72 were also synthesized using microwave-assisted synthesis meth od.1.2.3 Sonochemical SynthesisSonochemical methods can also achieve a reduction in crystallization time and importantly smaller particles size than those by the conventional solvothermal synthesis by homogeneous and accelerated nucleation 73,74. A substrate solution mixture for a given MOF structure is introduced to a horn-type Pyrex reactor fitted to a sonicator bar with an adaptable power output without external cooling. After sonication, formation and collapse of bubbles will be formed in the solution which produces very high local temperatures (5,000 K) and pressures (1,000 bar) 74,75, and results in extremely fast heating and cooling place (1010 K/s) producing fine crystallites 76.High-quality MOF-5 crystals in the 5-25m range were obtained within 30min by sonochemical synthesis using NMP (1-methyl-2-pyrrolidone) as the solvent 77. Detailed characterization and comparison with a conventionally synthesized sample showed almost identical somatogenetic properties. HKUST-1 was also prepared using DMF/EtOH/H2O mixed-solution in an ultrasonic bath 78. High-quality Mg-MOF-74 crystals (1,640m2/g BET go up area) with particle size of ca. 0.6 m were successfully synthesized in 1 h by a sonochemical method after triethylamine (TEA) was added as a deprotonating agent. Interestingly, mesopores were formed, probably due to the competitive binding of TEA to Mg2+ ions 79.ZIF-8 was prepared belatedly by a sonochemical method under the pH-adjusted synthesis conditions using NaOH and TEA 80. Inexpensive industrial manikin DMF was employed as a solvent. A small amount of TEA as a deprotonating agent was necessary to obtain ZIF-8 crystals when the resulting solution was subjected to an ultrasonic treatment for 1 h at a 60% power level.1.2.4 Electrochemical SynthesisThe electrochemical synthesis is based on metal ions continuously supplied through anodic dissolution as a metal source instead of metal salts, which react with the dissolve linker molecules and a conducting salt in the reaction medium. The metal deposition on the cathode is avoided by employing protic solvents, but in the process H2 is generated 81. The electrochemical route is also possible to run a continuous process to obtain a higher solids content compared to normal batch reactions 76.The first electrochemical synthesis of MOFs was reported in 2005 by researchers at BASF 82 for HKUST-1. Bulk copper plates are used as the anodes in an electrochemical kiosk with the H3BTC dissolved in methanol as solvent and a copper cathode. During a period of one hundred fifty min at a voltage of 12-19V and a currency of 1.3 A, a greenish blue descend was formed. After activation, a dark blue colored powder having surface area of 1,820m2/g was obtained. This work was further used in the ZIFs syntheses 81,83.Recently, HKUST-1, ZIF-8, Al-MIL-100, Al-MIL-53, and Al- MIL-53-NH2 were synthesized via anodic dissolution in an electrochemical cell 84. The synthesis parameters such as solvent, electrol yte, voltage-current density, and temperature on the synthesis yield and textural properties of the MOFs obtained, was investigated and the produced MOF structures were characterized by X-ray diffraction, gas adsorption, atomic force microscopy, send reflectance infrared Fourier transform spectroscopy, and scanning electron microscopy.1.2.5 Mechanochemical SynthesisMechanochemical synthesis involves breakage of intramolecular bonds automatically followed by a chemical transformation 80. Synthesis of porous MOF by mechanochemical reaction was reported first in 2006 89. Mechanochemical reactions can occur at room temperature under solvent-free conditions, which has an advantage in avoiding organic solvents 90. numerical yields of small MOF particles can be obtained in short reaction times, normally in the range of 10-60min. Mostly, metal oxides were found to be preferred than metal salts as a starting material, which results in water as the only side product 80. The critical contri bution of moisture in mechanochemical synthesis of pillared type MOFs was recently reported by Kitagawa group 91.Liquid-assisted grinding (LAG) involves addition of small amounts of solvents which can lead to acceleration of mechanochemical reactions due to an increase of mobility of the reactants on the molecular level 92,93. The liquid can also work as a structure-directing agent. However, mechanochemical synthesis is limited to specific MOF types only and large amount of product is difficult to obtain. dapple a mechanochemical reaction between H3BTC and copper acetate produces HKUST-1, reaction using copper formate resulted in a previously unknown phase, potentially due to templating effects of the different acid byproducts formed 93. Recently, a mechanochemical approach was also applied for ZIF synthesis using combinations of ZnO and imidazole (HIm), 2-methylimidazole (HMeIm), and 2-ethylimidazole (HEtIm) as the starting material within 30-60min reaction time 94.