Mapping the thermo-induced valence tautomerism in cobalt dioxolane single crystals
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Universidade Federal de Minas Gerais
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The development and investigation of smart materials, which present bistability when exposed to external stimuli is a key challenge to material physics and chemistry. Among the various types of these materials, the valence tautomers are compounds which switch
between different electronic and spin states and can be used as sensors, signal processors and memory storage [1] since their solid structure
does not present substantial rupture during the valence tautomerism (VT) interconversion. The VT has been studied in molecules
with a cobalt metal center, nitrogen based ancillary ligands and semiquinone radicals [2-3], and it was observed that it is modulated by the
ancillary ligand. For these cobalt complexes, the VT takes place in a reversible fashion [4], in both liquid state and solid state, as single
crystals, being possibly dependent on the solid-state arrangement of the complexes and on solvation [5-6]. The VT in such molecules can be
induced by temperature as first and second order transitions with a wide range of characteristic T1/2
according to the ancillary ligand. In the low temperature regime, the VT is also shown to be induced with photo irradiation in multiple
wavelengths. Interestingly, it can also be induced with soft and hard X-rays irradiation with high yield of metastable isomers [7-8].
Among the cobalt complexes that display VT, the cobalt 3,5-di-tert-butyl semiquinone pyridine complex is a particularly interesting
tautomer, because not only its valence tautomerism can be thermo and photo-induced, but also turned on or off by the presence of solvent
molecules in the crystal lattice [5]. It can be crystallized in two different forms, with and without a solvent molecule in the crystal lattice.
The first shows no temperature dependence of its magnetic susceptibility, and in the second, the same dependence indicates that
only half of the cobalt centers in the unit cell present VT, which we confirmed in X-ray diffraction (XRD) experiments. This, along with
results of density functional theory (DFT) calculations, raised an interesting possibility of studying the behavior of particular sites of the
crystal separately, utilizing X-ray energies around the cobalt K-edge to understand how each particular site responds to the
temperature and how the total VT interconversion takes place within the crystal lattice. In our work we combine the site selectivity of XRD
and the characteristic resonant X-ray absorption by cobalt atoms in different oxidation states, in order to spatially map the thermo-
induced valence tautomerism within the crystal, and also within the cobalt complexes
Abstract
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Tautomeria, Cobalto, Raios X, Difração
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Valence tautomerism, Spatial mapping, Cobalt complexes, X-ray diffraction, X-ray absorption
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https://scripts.iucr.org/cgi-bin/paper?S0108767321084415