Extended metal-organic frameworks

Members: Sandra Arcediano, Garikoitz Beobide, Oscar Castillo, Fabio Scé, Javier Cepeda, Mónica Lanchas, Antonio Luque, Sonia Pérez, Pascual Román, Jintha Thomas and Daniel Vallejo

Introduction: Metal–organic frameworks (MOFs) are a class of hybrid materials comprising metal ion-based vertices and organic ligands (linkers) that serve to connect the vertices into two or three-dimensional periodic structures. The structures and properties of MOFs can be carefully tailored by judicious selection of metal ion and organic linker building blocks. A hallmark property of MOFs is their intrinsic porosity, which renders them potentially useful for gas storage, separations, catalysis, and a variety of additional applications that rely on highly specific host–guest interactions. Their promising properties coupled with the ease by which their structures can be modified make MOFs one of the most exciting, diverse, and rapidly growing areas of modern chemistry research.

Detailed research lines:

A. Design of new metal-nucleobase (MBioFs and supraMBioFs) porous materials, it makes use of the coordinative versatility of the nucleobases to act as bridging ligands and their ability to establish strong supramolecular interactions. Examples of such materials are scarce, especially in the case of supraMBioFs, although these are more economical than those MOFs obtained through complicated connector-ligands.[Coord. Chem. Rev. 2013, 257, 2716]

Focusing on the synthesis of the scarce supraMBioFs (only one example published by us: CrystEngComm 2011, 13, 3301), the synthetic strategy to achieve this goal is the use of building blocks consisting of discrete metal complexes where the nucleobase is attached to the metal center by at least two donor atoms. The geometric constraints achieved among the coordinated nucleobases that this coordination mode imposes are otherwise difficult to obtain, specially the non-coplanar disposition of the nucleobase synthon groups. Given that multiple hydrogen bonding donor/acceptor positions are accessible, these discrete entities may assemble together by means of double or triple complementary hydrogen bonds to generate a supramolecular solid. Porosity comes from the previously mentioned geometric constraints that hinder an effective packing of the metal-nucleobase entities leading to the presence of great voids in the crystal structure.

B. Increase of the MOFs intrinsic porosity. It is accomplished by means of templating micelles of small organic molecules that increase the adsorptive capacity of the starting material. Despite MBioF are an emerging type of relatively cheaper porous materials, in most cases, are still far from the most outstanding MOFs with surface areas comprised between 3000-7000 m2/g. We aim to markedly enhance the intrinsic adsorption capacity by interfering the self-assembling of the MBioF with nanometric micelles during the crystal growth.

The key factor for the success of the methodology is the use of small surfactant molecules that provide stable microemulsions containing small micelles with diameters below 2 nm or near this edge that determines the limit between micro- and mesoporosity. In a previous work, we have been able to duplicate the intrinsic adsorption capacity of [Cu2(µ3-adeninato)2(µ-butanoato)2]n compound by means of using a microemulsion of butyric acid.[Chem. Commun. 2012, 48, 907]

C. High-scale solvent-free synthetic route of MOFs and MBioFs. Our experience in these kind of reactions has shown us that the use of a low temperature melting reagent allow the synthesis with a high yield of a great variety of MOFs in absence of any solvent (Figure 4a). Taking into account that commonly the synthesis of MOFs requires the use of toxic and expensive amidic solvent in a close-vessel at high temperature and that the product yield is far from being quantitative, solvent-free synthesis is very promising alternative. It provides the products in a high yield, with high atomic efficiency at low cost, with environmentally compatible byproducts (water, acetic acid…) and it is easily scalable.[Chem. Commun.

2012, 48, 907]