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The [Mn2(2-OHsalpn)2]2-,-,0,+ System: Synthesis, Structure, Spectroscopy, and Magnetism of the First Structurally Characterized Dinuclear Manganese Series Containing Four Distinct Oxidation States
journal contribution
posted on 1997-04-23, 00:00 authored by Andrew Gelasco, Martin L. Kirk, Jeff W. Kampf, Vincent L. PecoraroThe series of complexes
[Mn2(2-OH(Xsal)pn)2]2-,-,0,+
[where 2-OH(Xsal)pn represents
substituted-phenyl-ring
derivatives (X = H, 5-Cl, 3,5-Cl2, 5-NO2) of
1,3-bis(salicylideneamino)-2-propanol] allow for the first
detailed
structural, magnetic, and spectroscopic study of a series of complexes
that are the most active functional models
for the manganese catalases. Representative examples of each
oxidation state of the series (mimicking all of the
known oxidation states for the enzyme's reaction chemistry) have been
crystallographically characterized. The
molecules presented herein are described as symmetric derivatives
because they form dimers with both of the
ligands spanning both Mn ions with the alkoxide on the backbone of the
ligand bridging the metals. The variation
in Mn−Mn separation across the four structures is 0.11 Å
[Mn(II)−Mn(II) = 3.33 Å; Mn(II)−Mn(III) =
3.25 Å;
Mn(III)−Mn(III) = 3.36 Å; Mn(III)−Mn(IV) =
3.25 Å], showing that the basic core structure is highly
invariant.
Nonetheless, significant structural changes in the polyhedra of
each manganese ion can be observed across the
range of metal oxidation states. These symmetric structures are
distinct from the previously described asymmetric
{[Mn2(2-OH(Xsal)pn)2](sol)}0,+
structures which have only one bridging alkoxide and one monodentate
solvent
bound to the Mn(III) ion. These two forms (symmetric and
asymmetric) are reminiscent of the carboxylate shift
in metal carboxylate chemistry and illustrate how alkoxide ligands can
participitate in an analogous alkoxide
shift in order to generate a binding site for an incoming ligand, such
as methanol, or substrate, such as hydrogen
peroxide. This is the first series that allows the observation of
the effect of subtle changes in geometry on the
sign if not the magnitude of magnetic exchange in dimeric systems
across a range of oxidation states. Regardless
of the symmetric or asymmetric nature of the complex, the exchange
parameter J was found to be very low;
however, both ferro- and antiferromagnetic exchange can be realized
with these dimers.