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Storage space is a perennial problem not only in macroscopic world but at the molecular level as well. Efficient storage of certain gases such as hydrogen, methane etc has futuristic technological applications. Therefore, race is on for the discovery of new materials with largest storage capacity per gram of the material. Metal organic framework (MOF) materials are a class of light weight hybrid materials with surface areas per gram that could cover an entire foot ball field. This article gives a glimpse of the recent advances in MOFs.
Introduction
Porous solids are of scientific and technological interest [1].They are classified according to their pore diameter, materials with pore size of 20 Å or less are called microporous, those with pore size in the range of 20 to 500 Å are called mesoporous and those having pore size of more than 500 Å are called macroporous. Among the well known porous materials are zeolites which may be aluminosilicates and aluminophosphates [2]. They are purely inorganic porous materials. Metal organic frameworks (MOFs) are another class of porous materials consisting of metal ions and coordinating organic spacer units. They are crystalline in nature and belong to the general family of coordination polymers. The basic building blocks for such hybrid materials are metal ions and polydentate organic ligands [3]. These open framework materials are of current interest for many technological applications such as gas storage, molecule separations and catalysis; hydrogen storage, chiral separation and heterogeneous catalysis are focus areas.
Primary Building Blocks of MOFs
The organic spacers generally contain carboxylic acid and amine functionality, although potentially any coordinating functional group such as nitrile, isonitrile and thiol can be used. The organic spacers and the metal ions form the primary building blocks in the synthesis of MOFs resulting in certain motifs which are the secondary building units (SBUs). Propagation of the SBUs into two- and three-dimensional network results in the formation of MOFs. With the proper choice of the metal ion and the organic spacers, it is possible to tailor-make MOFs of specific dimension, pore size and functionalities. According to Yaghi [3] the synthesis of MOFs is termed as reticular synthesis. The metal ion controls the coordination geometry (square planar, tetrahedral, octahedral etc) and organic spacer controls features such as the directionality of propagation and size of the pores formed in such materials. Some of the commonly employed organic spacers and building units are shown in Scheme 1.
Based on the structural analysis of several MOFs, Ramanan and Whittingham [4] have suggested that the formation of the point zero charge (pzc) molecule at the isoelectric point is crucial for the formation of neutral MOFs with well defined structures. Formation of ionic MOFs would primarily be controlled by electrostatic interactions between the ions. The formation of pzc molecule occurs by the displacement of water molecules (in aqueous medium) from the aqua complex of the metal ions by the organic spacers. The initial interaction would be hydrogen bonding between the coordinated water molecules and the hetero atom of the organic spacers. Formation of well-defined geometries of pzc molecule is controlled by the coordination geometry of the metal ion itself.
Secondary Building Units (SBUs)
Coordination of carboxylate ion to a metal center can result in many different SBUs, of which paddle wheel and M4O(CO2)6 motifs will be discussed here [5]. Coordination of four carboxylate groups around two metal centers result in the formation of the paddle wheel motif with two free coordination sites on the metal centers that can propagate this SBUs into linear (one-dimensional) network (Scheme 2).
If a dicarboxylic acid is used instead of a monocarboxylic acid, it could result in the formation of a two-dimensional network and with an additional ligand coordinated on the metal center (L-L), a three-dimensional network can be generated (Scheme 3).
Typical examples of L-L ligands are 4,4￠-bipyridine and 1,4-diazabicyclo[2.2.2] octane (DABCO) (Scheme 1). Some of the organic linkers that can impart higher dimensionality to the MOFs are shown in Scheme 1. The shape and geometry of these linkers strongly influence the pore size, geometry and topology of the MOFs obtained. The octahedral geometry of the M4O(CO2)6 motif is defined by the four MO4 tetrahedra sharing a common vertex, and six carboxylic acid carbons occupying the vertices of the octahedron (Scheme 4) [6].
The M4O(CO2)6 SBUs are typically formed when the organic linker is reacted with the metal oxide rather than metal salts. The M4O(CO2)6 SBUs are connected by aromatic rings to yield MOFs of different structures. For example, when the SBUs are connected by para phenylene units (for example, terephthalic acid derivatives), the MOFs formed have a cubic structure as shown in Scheme 5.
Examples of carboxylate building blocks and the topology of the SBUs that can be generated using them are shown in Scheme 6. Using the reticular approach an endless array of MOFs can be generated with varied topologies, pore dimensions and pore volumes.

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