[Formulae and special characters can only be approximated here. Please see the pdf version of the Abstract for an accurate reproduction.]
The kinetics for outer-sphere electron transfer between a series of cobalt(II) poly-aza
cage ligand complexes and the iron(III) sarcophagine-type hexa-aza cage complex,
[Fe(sar)]3+ (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane), in aqueous solution
have been investigated and the Marcus correlation is used to deduce the electron self-exchange rate constant for the [Fe(sar)]3+/2+ couple from these cross-reactions. The
deduced electron self-exchange rate constant is in relatively good agreement with the
experimentally determined rate constant (k ex calc = 4 ´ 10 5 M -1 s -1 ; k ex obs = 8 ´ 10 5 M -1
s -1 ). The successful application of the Marcus correlation to the electron transfer
reactions of the Fe cage complex is consistent with the trend for the Co, Mn, Ni and Ru
cage complexes which all follow the pattern of outer-sphere electron transfer reactivity
expected from the Marcus-Hush formalism. A comparison of predictions based on the
Marcus correlation with the experimentally determined kinetics of an extended series of
cross reactions involving cobalt cage complexes with low-spin-high-spin cobalt(III)/(II)
couples shows that electron transfer reactions involving large spin changes at the metal
centre are not necessarily anomalous in the context of the adiabatic Marcus-Hush
formalism. The results of this study also show that for suitable systems, the Marcus
correlation can be used to reliably calculate the rates of outer-sphere electron transfer
cross-reactions, with reaction free-energy changes spanning the range -6 to -41 kJ mol -1
and many different combinations of initial electronic configurations. Together, these
results provide a coherent and internally consistent set of experimental data in support
of the Marcus-Hush formalism for outer-sphere electron transfer. The results with the
caged metal-ion systems also highlight the special nature of the mechanism of electron
transfer in reactions of metal-aqua ions.
¶
A new range of symmetrically disubstituted hexa-aza sarcophagine-type cage
ligand complexes are prepared in this study by the base-catalysed co-condensation of
formaldehyde and a-methylene aliphatic aldehydes with cobalt(III) tris(1,2-diamine)
precursors in acetonitrile solution. Encapsulation reactions based on the condensation
of the weak carbon di-acids propanal and decanal with formaldehyde and the cobalt(III)
tris(1,2-diamine) precursors, [Co(en)3 ] 3+ (en = 1,2-ethanediamine) and D-lel3 -[Co((R,
R)-chxn)3 ] 3+ (chxn = 1,2-cyclohexanediamine), yield unsaturated cobalt(III) cage
complexes with an endo-cyclic imine function in each cap. The Co III -coordinated endo-cyclic imine units of the cage ligands are reactive electrophiles that are readily reduced
by the BH4 - ion to give the corresponding symmetrically di-substituted hexaamine
macrobicyclic cage ligands. The nitromethane carbanion is also shown to add at the
endo-cyclic imine function to yield a novel nitromethylated cage ligand complex. The
latter reaction introduces a new method for the regioselective functionalisation of cage
ligands at sites removed from the more commonly substituted bridgehead positions.
The capping of cobalt(III) tris(1,2-diamine)-type complexes with weak CH-acids
developed in this study introduces a new and more direct route to symmetrically di-substituted cage ligand complexes.
¶
A new range of cobalt(III) surfactant cage complexes, with linear octyl, dodecyl
and hexadecyl hydrocarbon chains built directly into the bridgehead structure of the
cage ligand, have been prepared by the base catalysed co-condensation of formaldehyde
and long chain aliphatic aldehydes with the tripodal cobalt(III) hexaamine complex,
[Co(sen)]3+ (sen = 4,4',4''-ethylidynetris(3-azabutan-1-amine)), in acetonitrile solution.
Chiral surfactant cage complexes are obtained by capping reactions beginning with the
optically pure L-[Co(sen)]3+ precursor complex. The cobalt(III) cage complexes with
octyl to hexadecyl substituents are surface active and reduce the surface tension of
water to levels approaching those of organic solvents. The dodecyl substituted cage
complex forms micelles in aqueous solution when the concentration of cage complex is
> 1 ´ 10 -3 mol dm -3 at 25 °C. The cobalt(III) cage head-group of these surfactants
undergoes an electrochemically reversible one-electron reduction to the corresponding
cobalt(II) cage complex. The reduction potential of the surfactant head group can be
tuned to more positive potentials by replacing the bridgehead hydrocarbon chain
substituent with an ether linked hydrocarbon chain. The cobalt(III) surfactant-cage
complexes are biologically active and are lethal to the tapeworm Hymenolepis diminuta,
and the vaginal parasites, Trichomonas vaginalis and Tritrichomonas foetus. The
surfactant cage complexes also cause lysis in red-blood cell membranes at
concentrations as low 10 -5 mol dm -3 . Their biological activity is linked to the high
head-group charge (3+) and size which cause distortions in biological membranes when
the membrane is treated with these molecules. The combination of the chemically
reversible outer-sphere redox properties of the cobalt cage head-groups and the chirality
of the head group introduces a new and possibly unique series of chiral surfactant
coordination complexes which are also redox active.
¶
The chiral carboxylic-acid ionophore, lasalocid A, has been used to promote the
selective supramolecular transport and extraction of cobalt(III) hexa-aza cage cations
and related tripodal cobalt(III) complexes. The conjugate base anion of lasalocid A
forms stoichiometric outer-sphere complexes with the cobalt(III) cage and tripod
complexes. These outer-sphere complexes are highly lipophilic and partition strongly
from water into a chloroform phase. The extraction of the dissymmetric cobalt(III)
complexes by the chiral polyether anion is enantioselective for many systems and
results in the partial resolution of initially racemic complexes in the aqueous phase. A
strong structural preference was demonstrated by the ionophore for symmetrically
disubstituted cobalt(III) hexa-aza cage cations with a D-absolute configuration of the
ligand about the metal-ion and an R configuration of the coordinated secondary amine
N-H groups. The lasalocid A anion was also shown to promote the transport of the
complexes, intact, across a chloroform bulk-liquid membrane against an NH4 +
concentration gradient. The transport of the cobalt(III) complexes was also
enantioselective and resulted in partial resolution of the initially racemic aqueous phase.
The most efficiently transported enantiomer of each complex was also the most
efficiently extracted isomer in all systems examined, consistent with a transport process
limited by interfacial diffusion. The magnitude of the enantiomer separation obtained in
some systems was sufficient to indicate that lasalocid A mediated extraction and
transport may become a practical method for the resolution of particular types of
kinetically-inert chiral metal-amine complexes.
| Identifer | oai:union.ndltd.org:ADTP/216686 |
| Date | January 1997 |
| Creators | Walker, Glen William, not available |
| Publisher | The Australian National University. |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.anu.edu.au/legal/copyright/copyrit.html), Copyright Glen William Walker |
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