Soft‐X‐ray‐Induced Redox Isomerism in a Cobalt Dioxolene Complex

Abstract

Valence tautomerism (VT) defines reversible interconversions between two or more redox isomers. It is established that these interconversions can be stimulated by temperature and light irradiation. [1] For example, the diamagnetic [Co III -(Me 2 tpa)(DBCat)]PF 6 ·C 6 H 5 CH 3 complex (1) (Me 2 tpa = bis(6-methyl-(2-pyridylmethyl)) (2-pyridylmethyl)amine, DBCat = 3,5-di-tert-butylcatecholato) was found to undergo a thermally induced interconversion in the solid state yielding the redox isomer characterized by the high-spin Co II -semiquinonato (hs-Co II -SQ) charge distribution (see Scheme 1). [2,3] The observed transition can be formally described as the result of an entropy-driven intramolecular electron transfer involving the donor catecholato and the cobalt(III) acceptor. At cryogenic temperatures, laser irradiation of the solid compound at 904 nm, where a ligand-tometal charge transfer (LMCT) occurs, was found by bulk magnetic measurements [2] to induce the same process, affording the hs-Co II -semiquinonato species as a metastable phase in 90 % yield with a rather long lifetime (two weeks at 9 K).Soft X-ray absorption spectroscopy (XAS) is an elementsensitive synchrotron-based technique and provides a powerful tool to study the electronic and chemical structure of a specific atom and its coordination environment. It is particularly powerful in the magnetic study of 3d metal complexes. [4] With the additional asset of very high detection sensitivity, XAS has been effectively used in the characterization of systems with multiple quasi degenerated electronic states, [5] including very diluted and nanostructured systems. [6] We have found that for 1 this technique not only yields this important information, but also intrinsically provides the perturbation for inducing interconversion between the two redox isomers. This is an unprecedented result and we believe it to be particularly important for the study of all the complexes exhibiting photochromism. Figure 1 shows the temperature dependence of the cobalt L 3 -edge X-ray absorption spectra of 1 (the L 2,3 spectra are shown in Figure S1 of the Supporting Information). The spectra were obtained with a X-ray flux of 10 8 photons s À1 on Scheme 1. The two different electronic configurations involved in VT process. Figure 1. Temperature dependence of the normalized L 3 -edge X-ray absorption spectra of 1, acquired with a low X-ray photon flux on the sample. The underlying light gray and dark gray curves show the theoretical spectra expected for pure Co II and Co III states, respectively (see text for details).[**] We acknowledge ESRF for provision of synchrotron radiation facilities. We thank ID08 beamline staff for assistance in using the beamline and Dr. N. Brookes for fruitful discussions. This work was funded by EU through project MOLSPINQIP (STREP 211284) and NoE MAGMANet (NMP3- CT-2005-515767) and by the Italian CNR through the PROMO project and Commessa CNR PM.P05.012: "Nanosistemi organizzati di magneti molecolari".Supporting information for this article ...

Molekulare Nanomagnete als Testobjekte für die Quantenmechanik

“…[13] Die am häu-figsten untersuchten schaltbaren Magnete sind Spincrossover-, [57][58][59][60] Polycyanometallat- [61][62][63] und Metalldioxolenkomplexe, welche Redoxisomerie zeigen. [64][65][66][67][68] Zu den attraktiven Besonderheiten dieser Systeme zählen die Mçglichkeit einer optisch induzierten Umwandlung zwischen verschiedenen magnetischen Zuständen unterschiedlicher Leitfähigkeiten sowie die Mçglichkeit zur Untersuchung des Stark-Effekts. [17] Cobaltdioxolenkomplexe unterliegen Redoxisomerisierung durch einen intermolekularen Elektronentransfer zwischen dem Liganden und dem Metallion (Abbildung 3 …”

A Room‐Temperature X‐ray‐Induced Photochromic Material for X‐ray Detection

“…There have been some excellent studies on X‐ray‐induced photochromic materials, like Prussian blue analogs,3 Fe II spin‐crossover complexes,4 manganite,5 spinel,6 cobalt dioxolene,7 zinc phosphate with methylviologen cations,8 BiOI/BiI‐nylon11 composite films,9 and oxometallates with impurities (such as LiNbO 3 :Cu),10 in strict accordance with the definition of photochromism 2. However, to the best of our knowledge, examples of X‐ray‐induced photochromic materials are still rather rare in the literature.…”

“…However, to the best of our knowledge, examples of X‐ray‐induced photochromic materials are still rather rare in the literature. Even so, most of them only show X‐ray‐induced photochromism below room temperature,3–7 which is not economic and convenient for use. Moreover, in the reported studies of X‐ray‐induced photochromic species, the adopted X‐ray was either hard ( λ <1 Å)3, 4 or soft ( λ >1 Å),5–10 but not both.…”

“…Electron transfer has been found to be one of the subsequent photophysical processes in some reported X‐ray‐induced photochromic species 3. 4, 7–10 In this case, electron spin resonance (ESR) studies were carried out to make sure the presence of electron transfer during X‐ray irradiation. The experimental results showed that compound 1‐Y and its decolored samples are ESR silent but 1‐HX‐B and 1‐SX‐B display single line signals with g factors of 2.0029 and 1.9988, respectively (Figure 5).…”

“…Types of electron transfer in known X‐ray‐induced photochromic species are ligand‐to‐metal charge transfer,7, 9b metal‐to‐metal charge transfer,3 metal‐centered transitions,4 charge transfer between inorganic frameworks and organic templates,8 and charge transfer between inorganic frameworks and doped metals 10. In our case, the Br→L electron transfer is a key factor for the X‐ray‐induced photochromism of 1‐Y .…”