Recent Advances as Materials of Functional Metal-Organic Frameworks

Metal-organic frameworks (MOFs), also known as hybrid inorganic-organic materials, represent an emerging class of materials that have attracted the imagination of solid-state chemists because MOFs combine unprecedented levels of porosity with a range of other functional properties that occur through the metal moiety and/or the organic ligand. The purpose of this critical review is to give a representative and comprehensive overview of the arising developments in the field of functional metal-organic frameworks, including luminescence, magnetism, and porosity through presenting examples. This review will be of interest to researchers and synthetic chemists attempting to design multifunctional MOFs.


Introduction
Metal organic frameworks (MOFs) are a new class of crystalline materials [1], the structures of which are composed of metal-oxide units or metal ions joined by organic linkers through strong covalent bonds.It may be defined as supramolecular solids but consists of strong bonding providing robustness, linking units that are available for modification by organic synthesis, and a geometrically well-defined structure.The latter property further implies that these solids should be highly crystalline.Specifically, the chemistry of MOFs has provided an extensive class of crystalline materials with high stability, tunable metrics, organic functionality, and porosity.
An extensive body of research results [2] currently exists from the synthesis of metal-organic frameworks (MOFs), an area that has attracted widespread attention due to the facility with which well-defined molecular building blocks can be assembled into periodic frameworks and the promise that such a process holds for the logical design of materials.The synthesis of MOFs generally involves the copolymerization of organic links and metal ions in a polar solvent under mild temperatures (up to 200 ∘ C) and autogenous pressures (up to 100 atm).Since most products can be considered kinetically driven and lie on local thermodynamic minima, factors such as solubility of the organic link and metal salt, solvent polarity, ionic strength of the medium, temperature, and pressure play critical roles in determining the character of products.Indeed, slight perturbations in synthetic parameters have been the basis for the preparation of what seems to be a flood of new MOF compounds.
The designed construction of extended metal-organic frameworks from soluble molecular building blocks represent one of the most challenging issues facing synthetic chemistry today.Also metal-organic frameworks (MOFs) are widely studied because of their potential applications in many areas such as luminescence, magnetism, hydrogen storage, and gas adsorption and separation.We will give a representative and comprehensive overview of the arising developments of the functional MOFs.

Luminescent Metal-Organic Frameworks
The luminescent properties of metal-organic frameworks (MOFs) have attracted much attention for a long time, although these types of materials were frequently referred to as metal coordination polymers in the literature before the term "MOF" was widely adopted.The first report of luminescence, in which the structure was termed "MOF" that we are aware of, appeared in 2002.Since then, nearly 200 articles have appeared reporting light emission by MOFs, and a few reviews covering certain aspects of MOF luminescent properties have been published [3][4][5][6][7][8][9][10].
The hybrid nature of MOF materials, which include both an organic ligand and a metal ion, enables a wide range of emissive phenomena found in few other classes of material; the metal coordination can increase the conjugation and rigidity of the linkers.MOFs offer a unique platform for the development of solid-state luminescent materials as they have a degree of structural predictability, in addition to welldefined environments for lumophore in crystalline form.The lanthanoid (Ln) ions have spectrally narrow emission, even in solution, and nearly all of the lanthanoids exhibit photoluminescent properties [11][12][13][14][15][16][17].
He et al. [18] synthesized the first open-framework heterometallic MOF structure (Figure 1) based on the assembly of infinite rod building units from the solvothermal reaction of zinc nitrate hexahydrate, sodium hydroxide, and 1,3benzene-dicarboxylic (m-BDC) acid.The strong fluorescent emission (Figure 2) of the above MOF may make it a potential useful photoactive material.Chen et al. [19] report a prototype luminescent MOF (Figure 3), Tb (BTC)⋅G (MOF-76: BTC = benzene-1,3,5tricarboxylate, G = guest solvent), for the recognition and sensing of anions, exhibiting a high-sensitivity sensing function with respect to fluoride (Figure 4).Such a recognition event can be readily transformed into an external luminescence intensity change once luminescent metal site and/or organic linkers have been incorporated into the luminescent MOFs.
Sun et al. [43] have reported a novel 2D coordination polymer 1 (Figure 5(a)) consisting of ferromagnetic Ni(II) chain with alternating double EO-azide bridges and (EOazide)bis(carboxylate) triple bridges, and the material exhibits solvent sensitive metamagnetism; it can become 2 by dehydrating.The reversible dehydration/hydration processes are accompanied by significant changes in critical temperature, critical field and hysteresis (Figure 5(b)).
Liu et al. [44] have successfully synthesized a high nuclearity cubic cage involving 64 Fe 3+ ions (Figure 6), which displays strong antiferromagnetism (Figure 7).They also demonstrate that the combination of the small stereohindrance HCOO − and polypodal ligands can obtain high nuclearity magnetic clusters in order to explore their novel and interesting magnetism.

Porous Metal-Organic Frameworks
The designed construction of extended porous frameworks from soluble molecular building blocks represents one of the most challenging issues facing synthetic chemistry today.A large number of wonders and advances have been made possible by the synthesis of porous metal-organic frameworks (MOFs) because they have a wide array of applications, ranging from hydrogen storage to gas adsorption and separation to catalysis and so on.Thanks to several effective engineering strategies, systematic fabrication of porous MOFs can be achieved through designer assembly from judiciously selected molecular building blocks.

Hydrogen Storage in Metal-Organic
Frameworks.New materials capable of storing hydrogen at high gravimetric and volumetric densities are required if hydrogen is to be widely employed as a clean alternative to hydrocarbon fuels in cars and other mobile applications.With exceptionally high surface areas and chemical-tunable structures, microporous metal-organic frameworks have recently emerged as some of the most promising candidate materials; they can display outstanding performance characteristics for cryogenic hydrogen storage at 77 K and pressures up to 100 bar.One of the first metal-organic frameworks investigated for hydrogen storage was the cubic carboxylate-based framework Zn 4 O(BDC) 3 (see Figure 8), and its gas storage properties were found to depend very large on the methods utilized in preparation and activation, with Langmuir surface areas ranging between 1010 and 4400 m 2 ⋅g −1 and H 2 uptake capacity varying accordingly [45][46][47][48][49]. Inspired by the performance of compounds such as Zn 4 O(BDC) 3 , researchers have thus far reported hydrogen storage data for over 150 other microporous metal-organic frameworks [50][51][52][53][54][55], containing carboxylate-based frameworks, heterocyclic azolate-based frameworks, mixed-ligand/functionality systems, metal cyanide frameworks, and so on.
Wang et al. [56] reported a porous MOF, PCN-20 (Figure 9) with a twisted boracite topology based on a designed planar TTCA ligand.PCN-20 possessed a large Langmuir surface area of over 4200 m 2 ⋅g −1 as well as demonstrated a high hydrogen uptake capacity of 6.2 wt% at 77 K and 50 bar (Figure 10).
Rowsell et al. [57,58] have obtained remarkably detailed information on the primary binding sites of hydrogen in a series of metal-organic frameworks (Figures 11, 12  building units from metal cations that have received less attention to increase the building energies for H 2 on all sites.In particular, the use of more polarizing centers or the installment of open metal sites should enhance hydrogen physisorption by this case of materials.
However, significant further advances will be required in order to meet the US DoE targets for an onboard hydrogen system.

Gas Adsorption and Separation.
In industry, the adsorptive separation needs efficient porous materials.Preparing as traditional porous solid material, the porous metalorganic frameworks with tailored structures and tunable surface properties are becoming promising candidates for gas adsorption and separations, also they have commendable thermal stability, in most cases, these structures are robust enough to allow the removal of the included guest species resulting in permanent porosity, so MOFs are ideal for research and practical applications, such as in gas separation and purification as adsorbents [59][60][61][62][63].
In 2009, Britt et al. [65] reported that a known MOF, Mg-MOF-74 (Figures 16 and 17), with open magnesium sites, rivals competitive materials in CO 2 capture, with 8.9 wt.% dynamic capacity, and undergoes facile CO 2 release at significantly lower temperature, 80 ∘ C. Mg-MOF-74 offers an excellent balance between dynamic capacity and regeneration.These results demonstrate the potential of MOFs with open metal sites as efficient CO 2 capture media.
Besides, MOFs also have some other properties that occur through the metal moiety and/or the organic ligand, such as catalysts [67], enantioselective catalysis [68], and applications in industry [69].

Conclusions Remarks
We exhibited the variety applications as materials of few MOFs here; in fact although a large number of have  been reported until now, we only have seen the tip of the iceberg with respect to the application potential of MOFs.In order to investigate more potential applications of MOFs, an extensive body of researchers should pay for further effort.

Figure 1 :Figure 2 :
Figure 1: Framework of the MOF with DMF templating cations occupying the pores.

Figure 5 :
Figure 5: (a) The repeating unit (left) and the 2D layer formed by organic ligands interlinking 1D chains (right) in 1.(b) The  and 1/versus  plots at 1 kOe for 1 and 2.

Figure 6 :
Figure 6: The structure of Fe 64 cubic.((a) and (b)) Top and side view of the Fe 8 corner.(c) The cubic cage with the corner Fe 8 O 3 cores and HCOO − edges highlighted.

Fe 8 Figure 7 :
Figure 7:  versus  plots under 1 KOe field and  versus  plot (inset) for Fe 64 and Fe 8 a.Data are for one Fe 3+ .