Novel Metal-Organic Frameworks: Microwave Crystallization and Structure

Abstract

Recently, a considerable interest has been directed towards the synthesis of hybrid organic-inorganic solids, which exhibit an enhanced structural versatility, original electronic, magnetic and optical properties. Also, some of these crystalline metal-organic frameworks can reversibly occlude large amounts of gases, which makes them suitable for gas storage materials for hydrogen-fueled vehicles and portable electronics. Rigid aromatic dicarboxylates have been explored as appropriate building units for construction of organic backbone, while zinc ions because of their ability to accommodate various coordination modes, have usually been selected to generate open architecture. Here we report the synthesis and structures of two metal-organic frameworks ZnBDC-I and ZnBDC-II (BDC - 1,4-benzenedicarboxylate). Both compounds have been isolated after crystallization of a nlixture containing zinc nitrate, N a2BDC and water. The crystallization was performed under microwave heating for 1 hour at 180 deg. C. The structure of I consists of metal-organic zigzag chains in which tetrahedral Zn centres are linked by BDU acting as bridging bidentate ligands. The structure of II is a 3-D network arising through interconnection of the square-pyramidal Zn units and BDC ions. Each BDC is coordinated to four Zn atoms. It seems likely that interactions between zinc centres and the terminal C=O groups of the chains in I lead to the formation of the 3-D open framework II. This structural relationship between I and II gives an insight into the possible pathways in the building process of metal-organic networks. In accord with this observation it includes a transformation of I-D chain to 3-D open framework. (Crystal data for I are as follows: monoclinic system, C 2/c, a=1.50262(6), b=0.50422(2), c=1.21132(6) nm, beta=103.900(2)deg.; for II - monoclinic system, C 2/c, a=1.79978(5), b=0.63647(1), c=0.72657(2) nm, beta=91.467(1)deg.).