Hydrothermal synthesis of metal coordination polymers using mixed pyridine-carboxylate ligands

Zeolites are an important class of natural and synthetic microporous solids made of aluminosilicate building blocks. They have commercial importance in the fields of selective sorption and separation of gases, cation-exchange and catalysis. Their properties result from the chemical functionality, high internal surface area and crystalline regularity of their micropores. In order to extend these properties and applications, synthesis of new zeolite analogue materials, or zeotypes, has been an active area for the past 20 years. Recently zeotypes constructed from crystalline metal-organic framework polymers (MOFs) have been prepared by our group and others that offer quite complementary properties to purely inorganic oxide framework zeotypes. Chapter 1 surveys the background to their successful design and preparation. Typically the organic components of the MOFs may be from pyridyl N-donor ligands, or carboxylate O-donor ligands. Many metals prefer mixed N, O-ligand donor sets. In this thesis we will explore the chemistry behind construction of MOFs from ligands that possess both pyridine and carboxylate functionality together. Chapter 2 explores the MOFs formed from copper, a labile and versatile metal, with the simplest archetypal ligand of this class, pyridine-4-carboxylate or isonicotinate [INA]. A total of 16 phases, mostly new, were formed by systematic variation of reaction conditions, temperature, time, pH, solvent, metal reagent and stoichiometry. Hydrothermal conditions led to polymer formation whilst ambient conditions afforded a discrete complex [Cu(INA)2(H2O)4]. Importantly two phases from hydrothermal reaction at 180°C the polymer [CU(INA)2]⋅(H2O)2 and the hybrid solid [Cu(OH)(INA)]⋅(H2O) were formed which have been shown to have stable 3D open frameworks to above 220°C. Interestingly whilst their physical stability is similar, the chemical stability of the hydroxy hybrid is much superior. In addition to Cu(II) compounds a number of phases with mixed Cu(I)-Cu(II) or Cu(I) oxidation states were also isolated. Chapter 3 explores metal-ligand systems with slight additional complexity, the pyridine dicarboxylates [PDAs], with Zn, Cd, Ni, Co and Cu metals. Synthesis and crystal structures of 14 phases are reported. Pyridine ortho-carboxylates may achieve stable chelation of various metals and lead to predictable architectural units. Two compounds with 3D open frameworks and large channels were formed using [24PDA] and neutral bipyridine spacers [Cu3(24PDA)2(44bpy)2]⋅(H2O)4 and its bpee analog. These have similar topological connectivity but the change in linker group modifies the size and shape of the channels and is a prime example of structural engineering in these types of materials through organic modification. Chapter 4 looks at MOFs formed from extended two ring systems, the bipyridine dicarboxylates [BDAs] with lanthanide metals. The synthesis and crystal structures of 10 new phases are reported. The proclivity for lanthanides to form small cluster hydroxide units has been exploited to form new hybrids such as [Ln4(μ3-OH)4(44BDA)4]. These offer prospects for thermally stable open frameworks with novel organo-ceramic pore functionality. Finally in Chapter 5 we explore the synthesis of tris-chelated [M(BDA)3] species for M = Ru with the idea to prepare thermally stable chiral building blocks for enantiopure MOFs. Several [Ru(44BDA)3H4]⋅(H2O)x pseudo-polymorphs have been found. Two routes for their successful chiral resolution have been initiated. The first is through deprotonation and resolution using chiral organic cations derived from amines. The second is through complete hexa-esterification to cations [Ru(44BDA-R6)]2+. These now await resolution by chiral anions. Finally a novel microporous solid [Ru(bpy)3][Cu4Cl6] has been serendipitously prepared through attempts to form mixed Cu-Ru MOFs. This is an unprecedented structure with a 3D open framework composed solely of metal halide. In conclusion pyridine-carboxylates offer a wide range of possibilities for MOF zeotype formation. The present work shows some of the scope for architectural design and manipulation with the caveat that reaction conditions may have a profound effect on the product outcome. The hydrothermal technique is shown to be highly successful and adaptable to MOF formation for many metal and ligand systems. Judicious use of conditions may also allow partial hydrolysis to form hybrid solids incorporating both organic ligand and metal hydroxides. The physico-chemical properties of these may in many cases prove superior to pure MOFs