Magnetic and electrochemical studies of some encapsulated metal ions
Date
1986
Authors
Martin, Lisandra Lorraine
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Abstract
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.
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