Metal Organic Frameworks: History, Synthesis, and Topological approach

Doctoral thesis, 2009

The storage, separation, purification and catalytic reactions of small molecules are important research domains in both pure and applied chemistry, where porous materials play a major role. Among the different possible options for the preparation porous materials a recent approach is to build structures based on metal ions, metal complexes, or metal clusters, bridged by organic ligands. Porous compounds constructed with such patterns are called Metal Organic Frameworks (MOF). However, this term stands not only for the composition of the material but also for the simplification and the topological analysis of such structures represented by 3D-nets. This topological description is an evolution of the Schläfli symbols used to describe regular polytopes. An important property of some MOF:s is that they can absorb and release hydrogen by a physisorption process. Unfortunately, not with standing that the results for absorption at low temperature (i.e. 78 K) are relatively good compared to other systems, at room temperature the amount of hydrogen that can be stored decreases dramatically. Concerning other small molecules, the MOF:s can be very selective; depending on the molecular function in the cavity and the size of the channels, that separation of two different gases such as carbon dioxide and methane can be obtained. More complicated molecules can also be stored or purified. These molecules can, for example, be pharmaceutical drugs where constant low concentration needs to be delivered over time, or the compound needs to be separated into its enantiomers. For both purposes, an adapted MOF can be used. This thesis will present the syntheses, structural characterisation and topological analysis of a number of coordination compounds, which form one-, two-, or three-dimensional systems. The compounds obtained are based on: (a) [M(CN)6]3- bridged by phenanthrolinum type cations, M=Co3+ (2) or Fe3+ (3); (b) [Co(H2biim)]3+ and different types of hydrogen bonded carboxylates (compounds 5-8); (c) oxalate coordination polymers, 1, and 9-11. Notably, in the case of 9 a new five-connected topology was discovered. A family of so-called scu-nets with general formula [MII[CoIII(ethylenediamine)(oxalato)2]2]n·xH2O are also presented, forming porous compounds by coordination and hydrogen bonds, 12-15. Theses materials have potential void volumes of 20%, and the 1D channels have a tunable diameter of 3.7- 4.1 Å. The cavities formed are suitable for small molecule storage, and in the case of the calcium compound 12, the hydrogen absorption measurement reveal a relatively high absorption at room temperature and a pressure of eight bars compared to other, previously described, materials.