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Inorganic Chemistry
Coordination and Spectro-electro Chemistry
Dr Graham Heath
http://rsc.anu.edu.au/research/heath.php
Electrons provide the bonding between
the atoms in any material, so it is not surprising that electrolysis
can bring about dramatic and precise chemical changes. We have
developed a conceptual approach to exploring molecular electronic
structure and reactivity that combines electrochemical and
spectroscopic methods as explained below. Redox processes in simple
coordination complexes may turn out to be mainly metal-centred, or
truly delocalised or mainly ligand-centred, as with bipyridyls. In
bimetallic clusters, intensely coloured mixed-valence states
can arise. In all these examples, electrosynthesis offers a logical
route to the unusual oxidation states, once these have been
identified by voltammetry. In particular, in-situ optical
spectro-electrochemistry enables quite elusive species to be studied
readily without the need for isolation. The resulting spectral data
are usually elegant and very informative. We regard electrochemistry
as a powerful ally for bench chemists, rather than an end in itself,
though computational simulation of electrode response is now a
vitally important aspect of validating the interpretation of redox
behaviour.
Dr Heath and Dr Otieno-Alego lectured by invitation in May at
"Conservation Science 2002" in Edinburgh. Dr Heath also
taught an optical spectrometry course to trainees in the national
Laboratory Technician scheme at Canberra Institute of Technology. Dr
Mahon remains a consultant with cap-XX Pty Ltd and Dr Webster (QEII
Fellow, 2001 -) retains his association with the group. Nicholas
Perkins was awarded his PhD and his thesis highly commended.
Alexandra Pavli (Narrabundah College) was our CSIRO student researcher
for 2002, supervised by Stephen Lee.
Convolutive Modelling of Redox Processes at Disk Electrodes
The
reliable construction of electro-analytical electrodes with disk
geometry ensures the continued popularity of this design. However,
the discontinuity at the perimeter between the active conducting
surface and insulating annulus is a great obstacle to solving the
complex differential equations. Previous mathematical treatments
relied on one or other alternative simplifications appropriate to the
short and long time limits. A hybrid analytical-numerical procedure
known as the Integral Equation Method has been used to extend the
short-time expression so that this mathematical description can be
used throughout. Convolutive Modelling is then applied to this
description in order to predict the response of the disk electrode
under either current or potential control. (with P.J. Mahon, and
D.K. Cope [North Dakota State U., USA], K.B. Oldham [Trent
U.,Ontario Canada])
Development of a Nonlinear Electrochemical Dummy Cell
The
use of small-amplitude alternating signals is a very powerful tool in
voltammetry and for this reason the acV method has been incorporated
into a bipotentiostat built in the RSC electronics workshop. A
difficulty in testing these circuits is that the passive components
in ordinary hard-wired dummy cells are linear and so they fail to
simulate the real solution-cell electrochemical response to
alternating modulation. However, in a dummy cell with two opposed
diodes connected in series, the reverse bias current of one diode or
the other is active whatever the direction of the applied voltage.
We find that, with suitably chosen elements, the response simulates
pseudo-electrochemical behaviour over a frequency range, and that a
dummy cell of this configuration enables effective and convenient
testing of electrochemical instruments with alternating signal modes.
(with P.J. Mahon)
Mixed Valency in Binuclear, Trinuclear and Tetranuclear Clusters
Metal
cluster compounds are of enduring importance in transition metal
chemistry. We have a long-standing interest in confacial bimetallic
complexes of the form L3M(µ-X)3ML3.
Dr Nick Perkins devised a strategy for constructing extended
confacial arrays by attaching two RuL3 fragments to an MX6
or M2X9 core. For example, linear
tetrametallics of the type
[(R3P)3Ru(µ-X)3Os(µ-X)3Os(µ-X)3Ru(PR3)3]+
with a II, III, III, II resting state show two Os-centred reductions
and two Ru-centred oxidations. The pattern of metal-metal bonding is
predicted to change systematically as a function of oxidation level,
in accord with spectro-electrochemical data spanning five oxidation
states. (with C. Barlow, L. Dubicki,
A. J. Edwards, M. Gupta, D. L. Kay, S. B. Lee, N. E. Perkins, R. D.
Webster, A. C. Willis)
Development of NMR Spectro-electrochemistry
Marriage
of electrolysis with potentially incompatible NMR spectroscopy has
largely eluded chemists, despite the attractions of high-field NMR
spectroscopy as a structurally informative solution-phase technique.
In 2000, we reported a successful electrogenerative cell for
practicable in-situ NMR detection. This assembly fits in a normal
spinning sample tube in a standard multinuclear, FT spectrometer.
Refinement of the Bramley/Prenzler in-situ electrochemical NMR cell
has been taken up by Dr Richard Webster as part of his QEII
Fellowship program (see elsewhere). Promising targets for
complementary NMR/EPR characterisation include the reversible
[Co(dtc)3]0/+ couple, and a family of
Ru(acac)2L2 complexes in oxidation states II to
IV. (with R. Bramley, M. A. Keniry, R. D. Webster)
Porphyrin Dimers Linked by Conjugated Poly-Alkyne Bridges
Meso-linked
'Arnold bisporphyrins' have an extended poly-alkynyl bridge of the
form MOEP-X-MOEP where X = C4, C6, or even C8.
The conjugated dimer anions are accessed by chilled in-situ
spectro-electrochemistry. Mixed-valence [P2]1-
and doubly reduced [P2]2- have very intense NIR
absorption bands beyond 1000 nm. These distinctive spectra in three
oxidation states are well fitted by a generic 'two-porphyrin
eight-orbital' scheme, initially containing 8, then 9, then 10
electrons.
In
a long-sought advance, the new generation of sterically protected
Arnold bisporphyrins can be reversibly oxidised as well as reduced.
The stable, EPR-active p-radical ions [P2]1+
and [P2]1- of tri-aryl
[Zn(tr-ArP)]-C4-[Zn(tr-ArP)] reveal almost superimposable
two-band near-IR spectra. The eye-catching homology between 7- and
9-electron cases was actually predicted by the original MO scheme
[New J. Chem. 1998]. (D. P. Arnold,
R. D. Hartnell [Queensland U. Technology], and G.A. Heath, D. L. Kay,
R.D. Webster)
Redox Activation of the Iron-Molybdenum Cofactor of Nitrogenase
The
remarkable mechanism of biological nitrogen fixation is being
elucidated by direct electro-modulation of the catalytic Fe/Mo/S
cluster, extruded intact from its protein. This collaboration
involves ferrying the precious material from the UK Nitrogen Fixation
Laboratory for spectro-electrochemistry in Melbourne (FTIR) and later
at ANU (EPR, UV-NIR). (with S. P. Best [Melbourne U.], L. L.
Martin [Flinders U.], C. J. Pickett [John Innes Centre, Norwich,
UK])
Corrosion Studies
This
collaborative activity is concerned with the deterioration of metals
and other materials (i.e., with deleterious redox processes). We are
concerned with the scientific conservation of historic items, as well
as technological problems such as pitting corrosion of dairy
equipment and unwelcome inorganic deposits in internal combustion
engines. Surfaces are studied by many methods, including Raman
imaging microscopy, X-ray single-crystal and powder diffraction, and
impedance spectroscopy. Further studies of aero-engine combustion
deposits were commissioned by the Australian Transport Safety Bureau
(ATSB). (with A.J. Edwards, S. B. Lee, P.J. Mahon, L. Norén,
R. D. Webster, and M. Sterns, [Chemistry, ANU], G. L. Bailey
[Australian War Memorial], D. C. Creagh, V.Otieno-Alego [U.
Canberra], D. L. Hallam, R. Tait [National Museum of Australia], A.
Romeyn [Commonwealth Dept of Transport and Regional Services,
Canberra])
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