Magnetic and electrochemical studies of some encapsulated metal ions
Creators: name: Martin, Lisandra Lorraine
Publisher: name: The Australian National University
Year: 1986
Subjects: subject: Transition metal complexes
subject: Transition metal complexes Magnetic properties

description: The encapsulation of transition metal ions by hexadentate ligands of the macrobicyclic type involving saturated nitrogen donor atoms is the topic of this thesis. The presence of the metals V, Cr, Mn, Fe, Co, Ni and Cu in the centre of the ligand cage, has enabled the electronic structures of these cage complexes to be probed and elucidated by techniques such as magnetism, NMR, visible and ultraviolet spectroscopy in both the solid and solution states. Magnetic susceptibilities were measured over a wide range of temperatures for the finely powdered solids. The M(II) cages were all high-spin except for Fe(II), where both high-spin (t⁴₂g e²g ) and low-spin (t⁶₂g) configurations were observed. The M(III) cages, where M = Fe + Ni, were low-spin. Non-Curie law behaviour was observed for [Mn sar]³⁺ , for which the moment at 300 K apparently confirmed a high-spin, S = 2 ground state but fell sharply below 150 K to values expected for an S = 1 ground state. Indeed, magnetization data at high magnetic field strengths confirmed that a low-spin, S=1 ground state was in fact present. The cage complexes of V(III) and Cr(III) were magnetically dilute and uncomplicated. The magnetic data for the transition metal cages were interpreted by considering the effect of structural perturbations (e.g. trigonal and tetragonal distortions) on an octahedral ligand field ground term. In addition, comparisons with the congeneric [M(en)₃]ⁿ⁺ and other hexaamine complexes have been made. Magnetic measurements in solution were obtained using the Evans NMR method, and confirmed that the magnetic moments remained unchanged in solution compared with the solid state, with the exception of the Fe(II) cages. The magnetic behaviour of the Fe(II) cages in solution was studied over the temperature range 233 + 351 K, and can be described by a model involving nearly equienergetic ¹A ₁g and ⁵T₂g states in spin equilibrium. The Fe(II) cages appear to be the first examples of a spin equilibrium system with pseudo-octahedral Fe(II) coordinated by saturated amine donor atoms. The presence of observable d-d bands in the electronic spectrum of these complexes provided an unusual opportunity to investigate the spin equilibrium by optical spectroscopy. Both the magnetic and optical data conform to the equilibrium model from which the relvant thermodynamic parameters were obtained. The ¹H NMR of the Fe(II) cages were characterized by large isotropic paramagnetic shifts which were strongly dependent on temperature. The temperature dependence of the shifts also conformed to the spin equilibrium model, with thermodynamic parameters of a similar magnitude obtained. ¹H NMR studies were also performed with the contact shifted Ni(II) cages, in order to assist in the assignment of proton resonances for the Fe(II) cages. Preliminary electrochemical studies have been used to ascertain the accessible oxidation states and to provide an indication as to the degree of reversibility and/or stability of a complex in a number of oxidation states. Measurements were performed in aqueous and nonaqueous media in order to assess the effect of solvation and/or solvent participation in the redox processes. Most complexes displayed reversible or near reversible M(II)/(III) redox processes. A correlation between the redox potential and dn electronic configuration was observed for the M(II)/(III) couples for the encapsulated metal ions, Cr + Cu. The vanadium cages, isolated in their IV oxidation state, are stabilized by deprotonation of two donor amine atoms, and show complex pH dependent equilibria which is reflected in the observed redox properties. The stabilization of labile transition metal ions by encapsulation within a cage, makes them ideal candidates for electron self-exchange studies. Furthermore, the cage complexes are nearly spherical with similar charges as required by the Marcus-Hush theories. Preliminary studies of self-exchange rate constants for some cage complexes by an electrochemical technique are briefly discussed.