Synthesis of New Coordination Polymers or MOFs

1.1 Design Principles

Coordination polymers also known as metal–organic frameworks (MOFs) or metal-coordination networks are compounds constructed from organic ligands and metal ions connected through coordination bonds and other weak chemical bonds which can extend infinitely into one two or three dimensions [1-3]. The arrangement of the components in coordination polymers mostly exists only in solid 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].

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There are four different kinds of building blocks of which coordination polymers are built; ligands, metal ions or clusters, counter anions and solvent molecules. Metal ions which are called nodes or connectors and ligands which act as linkers are the main components [7]. They are the starting reagents which form the principal 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 used 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 used because of the high 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 unusual network 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 provide 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). Instead of employing one transition metal ion at a network vertex, SBUs can produce extended frameworks of high structural stability. Metal-complex connectors have 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 specific sites free for linkers [25,10,11].

Linkers are categorized in to three: inorganic, organic, and organic-inorganic hybrid types. Halides (F, Cl, Br, and I) are the smallest and simplest of all linkers. CN^– and SCN^– have similar bridging ability to halides [12,13].

Typical organic ligands are shown in Fig. 2 consisting of neutral, anionic and cationic organic ligands. The organic ligands act as bridging organic groups between the metal ions. Most famous neutral ligands are pyrazine (pyz) and 4,4’-bpy [15-18]. An example 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 provide a variety of frameworks, in which they act as linear linkers [38].

Coordination polymers with cationic organic ligands are very rare, which is naturally a result of their very low coordination power for cationic metal ions [39–43]. Developed were novel cationic ligands based on N-aryl pyridinium and viologen derivatives and were successfully employed [39–41].

Counter ions are present in the coordination frameworks when neutral bridging ligands are used as linkers to keep the neutrality in the overall charge. Furthermore, other roles such 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].

2. Synthesis

Many new coordination polymers or MOFs have been synthesized in the last few years; however, their methods of preparation and synthesis were quite similar. Most of them are synthesized by employing a so called “modular synthesis”, 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 synthesis is removed by applying vacuum, heat, or exchange with volatile molecules, resulting in large pore volume and large surface area accessible to guest molecules. Synthetic methods such as solvothermal synthesis (conventional approach), 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 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 concentrations, and pH of the co-solvent solution [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 separation 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 Synthesis

Solvothermal 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 closed 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 formamide, 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].

Due 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 liquids, 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 [bmim][BF4] (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 synthesis

Microwave-assisted synthesis has attracted much 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 hour 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 obtained by the regular solvothermal processes, but much quicker [60-64].

The first 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 author 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 substrates beyond an optimal 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 μm 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 method.

1.2.3 Sonochemical Synthesis

Sonochemical methods can also achieve a reduction in crystallization time and significantly 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 adjustable 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 rates (>1010 K/s) producing fine crystallites [76].

High-quality MOF-5 crystals in the 5-25μm 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 physical 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 surface area) with particle size of ca. 0.6 μm 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 recently by a sonochemical method under the pH-adjusted synthesis conditions using NaOH and TEA [80]. Inexpensive industrial grade 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 Synthesis

The electrochemical synthesis is based on metal ions continuously supplied through anodic dissolution as a metal source instead of metal salts, which react with the dissolved 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 cell with the H₃BTC dissolved in methanol as solvent and a copper cathode. During a period of 150 min at a voltage of 12-19V and a currency of 1.3 A, a greenish blue precipitate 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, electrolyte, 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, diffuse reflectance infrared Fourier transform spectroscopy, and scanning electron microscopy.

1.2.5 Mechanochemical Synthesis

Mechanochemical synthesis involves breakage of intramolecular bonds mechanically 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]. Quantitative 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 contribution 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. While 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].