The whole thesis divided into 4 parts. Chapter 1 describes the background motivation for these studies. Our aim is to develop metal coordination polymer systems in which metals and ligands are connected in open, stable 3D networks with a view to preparing functional zeolite analogues. The use of poly-carboxylate ligands has already been shown to form such stable frameworks with appropriate metal centers. In this work divalent and trivalent metals with high coordination number have been selected to increase the connectivity between the metal and ligands. Combined with the use of hydrothermal or solvothermal methods of synthesis this increases the likelihood of 3D polymer formation.
Chapter 2 reviews the metal coordination polymers constructed from Ln3+
and cyclohexane- 1,2,3,4,5,6-hexacarboylic acid. Seven phase types were isolated and five had single crystal structure determinations. These reveal as expected, that most formulas are [Ln2
and all have high connectivity between Ln and CHA ions of 6:3 or 8:4 ratio leading to 3D frameworks which display the potential for zeotype behavior. However, in general the networks are rather condensed with largest channels about 8 x 4 Ả. From the studies, change of solvent from water to a DMF:water mixture and variation of the lanthanide ion size play critical roles in determining the framework produced. In one case formate ions derived from hydrolysis of DMF are incorporated into the framework, which thus becomes negatively charged.
Chapter 3 describes the metal coordination polymers formed between metal ions and a more spatially extended ligand, naphthalene-2,6-dicarboxylate (NDA). Previous work from our group showed that NDA could form a variety of metal coordination polymers with many metals, such as Co2+
at room temperature. Among those, only Cd gave a 3D framework for α-[Cd(NDA)(H2
O)] 8., which has 7-coordinate Cd centers. Hydrothermal synthesis gave a new polymorph 9 β-[Cd(NDA)(H2
O) ] which has 5-coordinate Cd and parallel reaction with zinc gave an isostructural phase. For binding of carboxylate ligands, zinc and cadmium show a definite periodic trend. Zn2+
favors monodentate binding, while Cd is soft and has highly variable modes including bidentate, monodentate and polydentate. The novel polymorphism of [Cd(NDA)] was further studied by high temperature pXRD and revealed that form 8 converts to form 9 via a high temperature intermediate. This also prompted our interest to investigate other cadmium benzenedicarboxylate polymers. The ligands chosen are Isophthalic acid (IPT-H2), Phthalic acid (PHT-H2) and Terephthalic acid (TER-H2). Several new polymer phases were prepared hydrothermally, the majority with 1D chains and 6-coordinate Cd.
Chapter 4 describes our attempts to form chiral 3D coordination polymer frameworks, through the use of the chiral carboxylate ligand L-tartrate, which has two chiral centers with (R,R) absolute configurations. The hydrothermal products of divalent metal ions and L-tartrate were investigated, again with particular attention paid to cadmium. Three new chiral framework compounds were formed for Cd and the chirality of the tartrates supported by refinement of the Flack parameter in the single crystal structure. The retention of chirality was further supported by the fact that meso-tartaric acid (R,S) provided quite different products from hydrothermal synthesis in which (R,S) centers are retained. Finally circular dichroism studies of solids showed that optical rotations are indeed found for the L-tartrate polymers consistent with a net chirality for the entire solid batch.
The work in this thesis gives further credence to the use of both hydrothermal and solvothermal conditions for the formation of 3D network polymers, especially with high coordination number metals such as cadmium and lanthanides ions. Our recent findings that chiral L-tartrate can survive such rigorous synthetic conditions should also pave the way for future synthesis of open chiral network solids.
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