Ever since the publication of the pioneering 1990 paper by Hoskins & Robson [1], “crystal engineering of coordination frameworks” has attracted growing interest, and there are now 2882 published papers on the subject. We have synthesized large crystals of novel and potentially useful organic-inorganic compounds by hydrothermal synthesis at high temperatures (>100°C) and high autogeneous pressures (>1 atm), and slowly cooling the reaction mixture. Their structures were determined by X-ray crystallography and NMR, and their properties were examined. Until recently this synthetic approach was relatively unexplored.

A number of compounds with interesting topologies will be discussed. For example, a mixture containing 2,6-naphtalenedicarboxylic acid (H₂NDC) and a Cd²⁺ salt gives a 3D structure with diamondoid topology containing zig-zag metallic chains. A ternary reactive mixture composed of triethylamine (TEA), Cd²⁺ and H₂NDC leads, depending on experimental conditions, to a range of products. Thus large single crystals of [Cd₂(NDC)(OH)₂] are formed when the molar fraction of Cd²⁺ > 0.6. When Cd²⁺ < 0.6, large crystals result when the molar fraction of H₂NDC > 0.25. When the molar fraction of TEA > 0.7, a new layered structure is formed. A mixture containing a metal, biphenyl-4,4‘-dicarboxylic acid (H₂BPhDC) and 1,2-bis(4-pyridyl)ethane (BPE) leads to a 3D framework assembled by a centrosymmetric dimeric secondary building unit (SBU). 3D repetition of the SBU leads to a highly porous framework with large cavities filled by the interpenetration of two identical frameworks. Trimesic acid (H₃BTC) and BPE also lead to a porous 3D structure of unusal topology and containing triple metallic layers interconnected by disordered BPE ligands. Because of their interesting electronic and optical properties (photoluminescence), we are now particularly interested in metal-organic frameworks containing lanthanide metals.

[1] Hoskins & Robson, J. Am. Chem. Soc. 1990, 112, 1546.

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