This thesis describes the synthesis of a range of novel dithiadiazolyl radicals. The structures of these compounds are discussed. The physical properties of several compounds have been investigated using EPR spectroscopy and magnetic susceptibility studies. Chapter one begins with an overview of the chemistry of 1,2,3,5-dithiadiazolyl radicals. A general discussion of the history of organic conductors and magnets, and the terms involved in some of the techniques used is given in order to provide a background to the work presented. The second chapter outlines the synthesis and general characterisation of all the dithiadiazolyl radicals discussed in this thesis. A proposed mechanism for the conversion of parent nitriles into dithiadiazolyl radicals has been included. The third chapter describes the solid state structures of three dichlorophenyl dithiadiazolyl derivatives (2,4-, 2,5- and 3,5-dichlorophenyl-l,2,3,5-dithiadiazolyl). A further polymorph of 3,5-dichlorophenyl-1,2,3,5-dithiadiazolyl has also been included. The magnetic susceptibility of 2,4- and 3,5-dichlorophenyl-1,2,3,5-dithiadiazolyl has been investigated and the EPR analysis of all three compounds has been performed. These compounds are the first examples of neutral dithiadiazolyl radicals that form evenly spaced, segregated stacks in the solid state. Chapter four describes the dimer stacking structures of two further dichlorophenyl dithiadiazolyl derivatives (2,3- and 3,4-dichlorophenyl-1,2,3,5-dithiadiazolyl). The fifth chapter discusses the association of 3,5-dibromphenyl-1,2,3,5-dithiadiazolyl in the solid state. An investigation of this compound by EPR spectroscopy is also presented. Chapter six describes the trans cofacial association of p-iodophenyl-1,2,3,5-dithiadiazolyl in the solid state, only the second published example of this mode of dimerisation. The synthesis of o- and p-iodobenzonitrile are also described. An investigation of the EPR signal of this compound has also been included. Chapter seven describes the specialised techniques used in the synthesis of all the compounds. A list of the instruments used for analysis is also included.

Thesis (MSc)--Stellenbosch University, 2012. / Please refer to full text for abstract

New 1,2,3,5-dithiadiazolyl (DTDA), 1,2,3,5-diselenadiazolyl (DSDA) radicals and their resulting metal complexes were synthesized and characterized. The overarching theme of this thesis is the utility of intermolecular interactions for facilitating previously unseen magnetic behaviours in thiazyl radical-metal complexes. This thesis contains the first examples of thiazyl radical metal complexes acting as molecule based magnets. The 4-benzoxazol-2′-yl-1,2,3,5-dithiadiazolyl (boaDTDA) radical and its selenium analogue 4-benzoxazol-2′-yl-1,2,3,5-diselenadiazolyl (boaDSDA) were coordinated to several paramagnetic metal ions including transition metal ions Mn(II), Co(II) and Ni(II). The Ni(hfac)2(boaDTDA) and Ni(hfac)2(boaDSDA) complexes are isomorphous and both demonstrate step like π-stacking leading to additional ferromagnetic (FM) intermolecular interactions. The Mn(hfac)2(boaDTDA) (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato) complex was the first DTDA metal complex to conclusively show that intermolecular S(DTDA)…O(hfac) contacts can lead to intermolecular anti-ferromagnetic (AF) interactions which, in turn, can lead to a large spin ground state. Based on the magnetic properties of the Mn(hfac)2(boaDTDA) complex, a new DTDA biradical ligand, 4,6-bisDTDApyrimidine (bisDTDApym), was developed and coordinated to Mn(hfac)2. The resulting dinuclear Mn(II) complex, [Mn(hfac)2]2(bisDTDApym), is arranged in the solid state by short S(DTDA)…O(hfac) interactions forming two dimensional ferrimagnetic sheets. These ferrimagnetic sheets AF couple to one another, giving rise to AF ordering below 4.5 K. The [Mn(hfac)2]2(bisDTDApym) is the first thiazyl metal complex to magnetically order and is a unique example of a molecular coordination complex which magnetically orders. This thesis also presents the synthesis and characterization of precursor materials of the form Ln(hfac)3(DME) (DME = dimethoxyethane) for coordination reactions to thiazyl radical ligands. The Dy(hfac)3(boaDTDA) and Dy(hfac)3(pyDTDA) (pyDTDA = 4-(2′-pyridyl)-1,2,3,5-DTDA) complexes demonstrate single molecule magnetism with energy barriers of 100 K and 70 K respectively. Ten-coordinate Ln(hfac)3(pyDTDA)2 (Ln = La, Ce, Pr) complexes demonstrate phase transition behaviour between dimerized and undimerized phases and were studied by X-ray crystallography and magnetometry. The aforementioned compounds are some of the over 50 new compounds which have been synthesized and fully characterized in this thesis.

A series of binuclear coordination complexes of 4-(2′-pyrimidal)-1,2,3,5-dithiadiazolyl and its selenium analogue have been prepared to examine their structural and magnetic properties. The zinc(II) coordination complex is the first example of a DTDA radical ligand N-coordinated to a diamagnetic metal center. The magnetic properties reveal that it exhibits Curie behaviour and can be used as a benchmark to compare the analogous coordination complexes which possess paramagnetic metal ions. The nickel(II) coordination complex of the selenium containing radical pymDSDA was shown to dimerize in the solid state and is the only binuclear complex thus far that has done so. The manganese(II) complex of pymDSDA is by far the most interesting and is isomorphous to the DTDA analogue. For both complexes, one of the two molecules in the asymmetric unit form chains in the solid state joined by intermolecular contacts between a sulfur or selenium atom from the radical, and an oxygen atom coordinated to a neighbouring molecule. This feature gives rise to a ground state spin greater than that of an individual binuclear coordination complex. The radical ligand is however disordered in the solid state and so these random chain lengths are dependent on the orientation of the ligands in adjacent complexes. The 1,2,3-DTA species examined herein are related to the 4,5-dioxo-4,5-dihydronaphtho[1,2-d][1,2,3]dithiazolyl radical and the related protonated species 4-hydroxy-5H-naphtho[1,2-d][1,2,3]dithiazol-5-one. The proton from this latter compound has shown that it can be substituted with alkyl groups and this was achieved using acetyl chloride to place an acetyl group in this position. The above radical did not exhibit the strong donor properties required for metal coordination and preliminary investigations of the radical dianionic suggest that it is chemically irreversible by cyclic voltametry. The acetyl group unfortunately did not provide the chemically reversibility of interest although has established a potential route toward the substitution chemistry of this compound. The other 1,2,3-DTA compounds discussed herein are not complete, although the data acquired on the precursor compounds leading up to the radical will be discussed.