Back to Table of Contents
44200-AC3
Investigation of Polynuclear Transition Metal Complexes with Spin Transition
Catalina Achim, Carnegie Mellon University
The current quest for increased
capacity of information storage devices has fuelled intense research of
molecule-based materials that can be bistable, i.e.
that have two different states accessible within the same domain of
environmental conditions. Numerous mononuclear coordination complexes of Fe(II) can adopt two d-electron configurations, and
consequently, two spin states. Some of these complexes manifest bistability underscored by strong intermolecular
interactions exerted in solid state. Our research is focused on the
investigation of molecules that can undergo spin transitions and contain multiple Fe(II)
ions, and in which the transition can be affected by intramolecular
interactions between the metal ions.
One of the focus of the research
supported by this grant is the study of pentanuclear
cyanide-bridged clusters {[M(tmphen)2]3[M'(CN)6]2} in which M and M'
are transition metal ions, and tmphen is
3,4,7,8-tetramethyl-1,10-phenanthroline).
The design of these clusters was inspired by Prussian Blue
and is based on the use of tmphen to favor the
formation of molecule-based materials over 3-D ones. The clusters consist of a trigonal
bipyramidal core with three M ions in the equatorial
positions having a [M(tmphen)2(CN)2]
coordination and two M' ions in the axial positions having a [M'(CN)6].
These complexes are prepared in the laboratory of our collaborator,
Professor Kim Dunbar at Texas A&M. In past research, we studied clusters in
which M/M' are Zn/Cr, Zn/Fe, Fe/Fe, Fe/Co, Co/Fe, and Fe/Cr and have shown the
manifestation of a spin transition correlated with intramolecular
electron transfer in Co/Fe clusters, and a temperature-induced spin transition
between high spin (HS) and low spin (LS) FeII
in the Fe/Fe and Fe/Co ones. We have
also obtained the first direct experimental observation of uncompensated spin
density at diamagnetic FeII metal ions
that bridge the paramagnetic CrIII in the
Fe/Cr clusters.
During the last year, we studied clusters
{[M(tmphen)2]3[M'(CN)6]2} in which M/M' = Fe/Ru or Fe/Os. Magnetic susceptibility studies of both
clusters showed an increase in their magnetic moment with the increasing
temperature, which is indicative of a change in spin and/or oxidation state at
the Fe sites that may or may not be correlated with a change in oxidation state
at the Ru or Os.
To investigate this possibility, we conducted variable temperature,
variable field Mössbauer studies, which provide direct information on the
electronic structure of the Fe. These
studies showed that (1) at low temperature the Fe/Os cluster has a LS FeII/ OsIII
configuration, and that at high temperature (2) there is no HS FeII being formed, which excludes a classical LS
to HS transition at the Fe sites, and (3) HS FeIII
is being formed, which implies that a charge-transfer correlated spin
transition occurs in the Fe/Os cluster.
EPR studies conducted in collaboration with Professor Doros Petasis from
Allegheny College corroborated the information
obtained from Mössbauer about the low temperature charge distribution in the
Fe/Os cluster. A manuscript presenting
this work is currently under preparation.
The Fe/Ru
cluster has a significantly more complex behavior, which includes the existence
at 4.2 K of both HS FeIII and LS FeII in the cluster and the appearance at high
temperature of HS FeII. Therefore, elucidation of the electronic
structure of the cluster requires a significant amount of work involving both
EPR and Mossbauer spectroscopy, which is currently under way in our lab.
We have also continued to study in
collaboration with the group of Professor Michael Hannon from University of
Birmingham, UK, a series of dinuclear tetracationic triple-helical complexes of FeII, [Fe2L3]X4.nH2O
where X = BF4, Cl, or PF6 and n
= 0-4 and L is a bitopic ligand with two imidazolimine coordination sites. A detailed Mossbauer study conducted a year
earlier showed that the complexes manifest a multiple-step spin transition and
allowed us to determine for the first time the electronic structure of HS FeII sites in the very same coordination as the ions
which undergo the spin transition. In
the past year we pursued the interpretation of the spectroscopic data using a
crystal field model for the electronic structure of the FeII. In particular, the fine structure parameters
for the high-spin FeII site revealed an
axial symmetry of the crystal field at FeII,
whereas analysis of the nuclear parameters revealed a significant degree of rhombicity for the same crystal field. Our goal is to explain this apparent
contradiction in the crystal-field symmetry implied by these spectroscopic
parameters. This work delayed the
submission of a manuscript which was already in advanced stages of preparation
a year ago, but it will increase the relevance and importance of the paper we
prepare for submission to Inorganic Chemistry this year.
Back to top
