Investigations Of Open-Framework Metal Carboxylates, Sulfates And Related Materials
Open-framework materials constitute an important area of study in materials chemistry, because of their potential applications in areas such as sorption and catalysis. Furthermore, these materials exhibit fascinating architectures with unusual bonding and coordination patterns. Besides aluminosilicate zeolites and metal phosphates, metal carboxylates constitute an important family of open-framework structures. In this thesis, results of investigations of a variety of open-framework metal carboxylates, as well as a new family of rare-earth sulfates are presented. More importantly, studies directed towards an understanding of the mechanism of formation of open-framework phosphates and carboxylates and the importance of synthesis parameters like temperature in determining the dimensionality of the products are discussed at length. After providing an introduction to open-framework compounds (Part 1), the thesis (Part 2) presents the results of the investigations of metal carboxylates employing both transition metal and rare-earth metal ions with various organic linkers, ranging from aromatic squarate dianion, aliphatic dicarboxylates like oxalate, adipate, succinate, aminocarboxylates (which contain both the amine and the carboxylate group in the same moiety) and dihydroxybenzoates. Some of these compounds have large channels, for example, both the lanthanum mixed aliphatic dicarboxylates, strontium dihydroxybenzoate etc. Some even show unusual properties, example, a mixed valent iron (II, III) glycinate has a perfect kagome structure and shows unusual magnetic properties, entirely different from those of ordinary Fe(III) kagome structures, copper derivative of 6-amincaproic acid has large channels and remains crystalline porous even after dehydration, a cobalt oxalato-squarate has both oxalate and squarate moiety in the same structure, where the oxalate is produced in situ by the oxidation of squarate under hydrothermal conditions in the basic medium. While the phosphate and the silicate anions are the most common basic building units in open-framework inorganic structures, the possibility of building open architectures with the sulfate anion as the basic building unit has been explored in Part 3. The results have been rewarding and both layered and three-dimensional rare-earth sulfates with large channels and possessing a α-Po structure or (6, 3) nets with continuous M-O-M connectivity in two dimensions have been obtained. The most intriguing question in the area of open-framework structures relates to the mode of formation. In Part 4, the mechanism of formation of both metal phosphates and metal carboxylates have been discussed. This part includes a study of the transformations of four-membered ring phosphates to higher dimensional structures (like 3D sodalite-related structure), through lower-dimensional structures, involving a progressive building-up mechanism. Similar progressive building-up mechanism has also been studied for metal carboxylates. The importance of synthesis parameters like temperature in governing the dimensionality of the products has been studied.