Incorporation of Switchable Building-Block Complexes into Supramolecular Architectures

Abstract

The development of mononuclear spin crossover complexes is of great academic interest due to their inherent ability to act as a molecular switch. However, in order to use them to do work these molecules must be somehow organised into ordered arrays. Presented in this thesis is the development of mononuclear building block complexes specifically functionalised with secondary coordination sites such that they are primed for self-assembly into 1D, 2D and 3D supramolecular architectures. In Chapter 1 an introduction to the potential uses of spin crossover in secondary assemblies and to recent progress in the separation of gas mixtures using metal-organic frameworks is provided. Then, the fields of spin crossover and electrochemistry in iron(II) and cobalt(II) systems are discussed in a more general context. Finally, the systems targeted in the present study are introduced. In Chapter 2 the guest adsorption properties of two known isostructural metal organic frameworks, {[CoIII(dpzca)2AgI](BF4)2∙2H2O}n and {[NiII(dpzca)2AgI](BF4)∙0.5(acetone)}n, are studied in detail. The effect of anion loading on the selectivity for carbon dioxide over nitrogen and methane is investigated and a comparison of experimental and theoretical data leads to a proposed mechanism of separation by these materials. In Chapter 3 a comprehensive review of hysteretic spin crossover in cobalt(II) systems is presented. This is followed by an analysis of the scan rate dependence of the hysteretic spin crossover in the building block complex [CoII(dpzca)2]. The thermal hysteresis of spin crossover in [CoII(dpzca)2] is shown to remain even when the temperature is ramped at only 0.2 K min-1. In Chapter 4 the effect of high hydrostatic pressure on the spin crossover in [CoII(dpzca)2] is monitored by both magnetic susceptibility and X-ray crystallography studies. The abrupt spin crossover in [CoII(dpzca)2] is able to be shifted up to ambient temperature with an applied hydrostatic pressure of 0.4 GPa. In Chapter 5 the development of a synthetic route to a novel tridentate ‘terpyridine-like’ ligand, py-pzpypz, deliberately functionalised for secondary assembly, and seven 3d metal complexes of it are presented. Highlights of this chapter include; spin crossover in [CoII(py-pzpypz)2](BF4)2, reversible electrochemistry in [MII(py-pzpypz)2](BF4)2 (where MII = CoII and FeII), and interesting crystal packing interactions between the lone pair of a ‘spare’ pyrazine nitrogen atom and the electron deficient π-system of a pyrazine ring on an adjacent molecule. In Chapter 6 the serendipitous synthesis of a unique copper(I) cyanide MOF with 1D zigzag channels is reported. A deliberate synthesis of this material is subsequently developed and the high selectivity for carbon dioxide over nitrogen is investigated. In Chapter 7 the stepwise synthesis of four new bidentate 1,2,4-triazole ligands which feature a diazine, and in two cases also a 4-pyridyl group that provide secondary coordination sites is discussed. The use of two of these ligands in iron(II) complexes of the form [FeII(L)2(NCE)2], where E = S, Se, BH3, gave four spin crossover active compounds, three of which show modest thermal hysteresis loops.