Organosilicon Linkers in the Construction of Metal-Organic Frameworks and Hydrogen-Bonded Assemblies
Creators: name: Delmas, Luke
Publisher: name: Imperial College London
Year: 2019

description: Metal-Organic Frameworks (MOFs) are 3D coordination polymers consisting of metal-based nodes connected together by organic linker molecules. The field of MOFs continues to flourish as these fascinating materials show great potential in molecular storage and separation applications. In addition to modification of existing MOF materials, further advancement of the field requires discovery of new materials. Such innovation is intrinsically linked to the design of new linkers of which three-dimensional molecular scaffolds are of particular interest. Chapter 1 offers a brief introduction to the field of MOFs and explores the use of organosilicon polycarboxylic acids in the discovery of new MOFs. Literature MOFs containing organosilicon struts are dominated by the use of tetraorganosilane derivatives. The dearth of literature concerned with siloxane- and silanol-based linkers in MOFs is highlighted and the aims of this project, to explore this area, are established. Chapter 2 covers the synthetic approaches towards organosilicon polycarboxylic acids and outlines the preparation of the siloxane- and silanol-based linkers synthesized in this study. Nine novel carboxylic acids have been prepared including those based on 1,2-disilylethane (L1), hexaorganodisiloxanes (L2 – L4), triorganosilanols (L5 &amp; L6) and trisiloxanes (L7 – L9). All of these linkers (and intermediate compounds) have been fully characterised by 1H, 13C, and 29Si NMR spectroscopy, mass spectrometry, and IR spectroscopy. Chapter 3 provides a brief introduction to Hydrogen-bonded Organic Frameworks (HOFs) and presents the X-ray crystal structures of those carboxylic acids prepared in this study whose hydrogen-bonded superstructures have been determined. The structures reported show the ability of organosilicon polycarboxylic acids to self-assemble through intermolecular hydrogen-bonding to afford polymeric two- and three-dimensional structures with varying degrees of interpenetration. Chapter 4 presents a structural study of the coordination behaviour of hexa(p-carboxyphenyl)disiloxane (L2) and tris(p-carboxyphenyl)silanol (L5). Nine new MOF structures with L2 have been determined through 4 treatment of the linkers with a variety of transition (Zn, Cd, Y), main group (In) and rare earth (Ce, Yb, Er, Ho) metal precursors. These MOFs all have solvent-accessible voids in their structures and show a diversity of rarely-encountered topologies. L5 has been used in the construction of a 2D Zn-based MOF material which assembles by hydrogen bonding to form a 3D supramolecular structure. Chapter 5 describes the structure and properties of Zr and Hf frameworks constructed from 1,1,3,3-tetrakis(p-carboxyphenyl)-1,3-dimethyldisiloxane (L3). These MOFs are isostructural containing the less commonly-encountered 8-connected M6O8 (M = Zr or Hf) clusters giving rise to a 3D network of scu topology. Both MOFs exhibit permanent porosity (SBET ~ 1000 m2g-1) and show modest CO2 uptake (~ 5 wt%). The Si-CH3 groups which line the pore surface are thought to impart the MOFs with hydrophobic character as suggested by the limited (&lt;4 wt%) water vapour uptake at relative humidity levels below 20%. Chapter 6 presents results of a structural study of the coordination behaviour of hexakis(p-carboxyphenyl)-3,3-dimethyltrisiloxane (L7). Three new MOFs assembled from Zn and Mn salts have been characterised. L7 is seen to be effective in cross-linking both discrete and polymeric inorganic units to afford structures with solvent-filled channels. Chapter 7 looks at the unusual structure of a Na-based MOF built from the unsymmetrical linker 1-(tris(p-carboxyphenyl)silyl)-2-(bis(p-carboxyphenyl)(methyl)silyl)-ethane (L1). The linker is seen to cross-link a series of layers of parallel sodium-carboxylate infinite chains.