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Free keywords:
mixed valency; osmium compounds; radical ligands; ruthenium compounds; spectroelectrochemistry
Abstract:
The title complexes were obtained as (MM)-M-II'(II) species
[(bpy)(2)M'(mu-abpy)M'(bpy)(2)](PF6)(4), M,M' = Ru or Os, using the new
mononuclear precursor [(bPY)(2)Os(abpy)](PF6)(2) for the
osmium-containing dinuclear complexes. One-electron reduction produces
radical complexes [(bPY)(2)M(mu-abpy)M'(bpy)(2)](.3+) and
[(bpy)(2)M(abpy)](.3+) with significant contributions from the metals,
as evident from the EPR effects on successive replacement of ruthenium
by osmium with its much higher spin-orbit coupling constant. The
diruthenium and diosmium radical complexes were also studied by EPR at
high-frequency (285 GHz), the latter shows an unusually large g
anisotropy g(1) - g(3) = 0.25 in frozen solution. Further reduction was
monitored by UV/Vis spectroelectrochemistry. Oxidation produced Os-III
EPR signals for [(bpy)Os(abpy)](3+) and
[(bpy)(2)Os(mu-abpy)Ru(bpy)(2)](5+), indicating a (RuOsIII)-Os-II
species for the latter. The diosmium(III,II) and diruthenium(III,II)
mixed-valent species remained EPR silent at 4 K, however, they exhibit
weak inter-valence charge transfer (IVCT) bands at about 1460 nm.
Whereas the cyclic voltammetric response towards reduction is only
marginally different for the three dinuclear complexes, successive
replacement of ruthenium by osmium causes the first oxidation potential
to decrease. The much higher comproportionation constant K-c for the
mixed valent diosmium(III,II) state (K-c > 10(15)) in comparison to the
diruthenium(III,II) analogue with K-c = 10(10) confirms the electron
transfer alternative for the valence exchange mechanism, in contrast to
the hole transfer established for analogous dinuclear complexes with
the formally related diacylhydrazido(2-) bridging ligands. (c) 2005
Elsevier B.V. All rights reserved.
