Metal-organic framework

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Metal-Organic Frameworks (MOFs) are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. In some cases, the pores are stable to elimination of the guest molecules (often solvents) and can be used for the storage of gases such as hydrogen and carbon dioxide. MOFs are also known as hybrid frameworks and coordination polymers, although these terms are not strictly identical.

Other possible applications of MOFs are gas purification, gas separation, catalysis and sensors.

The prototypical MOF-5 ^[1] is built up by Zn₄O groups on the corners of a cubic lattice, connected by 1,4-benzenedicarboxylic acid (alternatively known as terephthalic acid). In this way the organic molecules are referred to as the ligands between the metal centers.

MOF-74 is a matrix composed mainly of carbon and zinc, and can adsorb hydrogen to a greater density than in its frozen form. In appearance it is a porous crystalline powder and shows great promise as a practical method of storing large volumes of hydrogen. This brings fuel-cell powered cars a step closer to being realised.^[2]

Classification of hybrid materials based on dimensionality ^[3]

	Dimensionality of Inorganic
Dimensionality of Organic		0	1	2	3
	0	Molecular Complexes	Hybrid Inorganic Chains	Hybrid Inorganic Layers	3-D Inorganic Hybrids
	1	Chain Coordination Polymers	Mixed Inorganic-Organic Layers	Mixed Inorganic-Organic 3-D Framework
	2	Layered Coordination Polymer	Mixed Inorganic-Organic 3-D Framework
	3	3-D Coordination Polymers

Contents 1 Preparation of Framework Structures 2 Common Ligands in MOFs 3 See also 4 References

[edit] Preparation of Framework Structures

Frameworks have been prepared under reflux, precipitation, and recrystallization, the vast majority of compounds have been discovered using hydrothermal_synthesis techniques. Reagents are typically sealed in a PTFE lined autoclave with water or some other solvent and heated to between 100°C and 250°C where autogenous pressure maintains the aqueous reaction environment. This allows for the activation energy needed to assemble the complex framework structures to be achieved without the solvent evaporating. It is a process that often results in single crystals suitable for single crystal X-ray diffraction, which can reveal complete structural information. Structural data for crystals too small for standard diffractometry can sometimes be accessed using synchrotron X-ray sources.

[edit] Common Ligands in MOFs

Common name	IUPAC name	Chemical formula	Structural formula
Bidentate Carboxylics
Oxalic acid	ethanedioic acid	HOOC-COOH
Malonic acid	propanedioic acid	HOOC-(CH2)-COOH
Succinic acid	butanedioic acid	HOOC-(CH2)2-COOH
Glutaric acid	pentanedioic acid	HOOC-(CH2)3-COOH
Phthalic acid	benzene-1,2-dicarboxylic acid o-phthalic acid	C6H4(COOH)2
Isophthalic acid	benzene-1,3-dicarboxylic acid m-phthalic acid	C6H4(COOH)2
Terephthalic acid	benzene-1,4-dicarboxylic acid p-phthalic acid	C6H4(COOH)2
Tridentate Carboxylates
Citric Acid	2-Hydroxy-1,2,3-propanetricarboxylic acid	(HOOC)CH2C(OH)(COOH)CH2(COOH)
Trimesic acid	benzene-1,3,5-tricarboxylic acid	C9H6O6
Imidizoles
1,2,3-Triazole	1H-1,2,3-triazole	C2H3N3
pyrrodiazole	1H-1,2,4-triazole	C2H3N3
Other
Squaric acid	3,4-Dihydroxy-3-cyclobutene-1,2-dione	C4H2O4

[edit] See also

• Zeolitic imidazolate frameworks
• Omar M. Yaghi

[edit] References