Specific Features of EPR Spectroscopy of Organotin Compounds with Paramagnetic Ligands of the o- Iminobenzosemiquinone T
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ific Features of EPR Spectroscopy of Organotin Compounds with Paramagnetic Ligands of the o-Iminobenzosemiquinone Type M. G. Chegereva, K. V. Arsenyevab, A. V. Cherkasovb, and A. V. Piskunovb, * a
Research Institute of Physical and Organic Chemistry, Southern Federal University, Rostov-on-Don, Russia Institute of Organometallic Chemistry, Russian Academy of Sciences, Nizhny Novgorod, 603600 Russia *е-mail: [email protected]
bRazuvaev
Received May 5, 2020; revised June 5, 2020; accepted June 10, 2020
Abstract—New paramagnetic tin(IV) derivatives containing the radical-anion of 2,4,6,8-tetra-tert-butylphenoxazin-1-one redox-active ligand (phenox-imQ) are synthesized and studied by EPR spectroscopy. Complexes I–V are paramagnetic in the crystalline state and in solution. The geometry of the obtained paramagnetic species and spin density distribution in them are studied by the Density functional theory. Diamagnetic complex VI is synthesized from the phenox-imQ dianion, and its one-electron oxidation is studied. The molecular structure of complex VI · ТHF is determined by X-ray diffraction analysis (CIF file CCDC no. 2000962 (VI · ТHF)). Keywords: phenoxazin-1-one, redox-active ligand, tin, EPR, coordination polyhedron DOI: 10.1134/S1070328420110019
INTRODUCTION The chemistry of redox-active ligands is being developed during the recent 50 years. When just discovered, similar ligands and their metal complexes were objects of EPR spectroscopic studies only [1]. However, the true potential of these ligands turned out to be much wider, and at present they are unique tools for the extension of the reactivity of both transition and main-group metal complexes. The use of the derivatives of the redox-active ligands in the coordination and organoelement chemistry of metals made it possible to accomplish transformations that earlier considered to be unfeasible: the facilitation of redox reactions of the s-, p-, and d-metal derivatives; crosscoupling reactions without a change in the oxidation state of the metallocenter; and a phenomenon of redox isomerism and its practical use [2–8]. At the moment, many works devoted to the synthesis and study of the structures and catalytic or magnetic properties of the transition metal complexes based on redox-active ligands of various nature, o-quinones (Q), o-iminoquinones (imQ), α-diimines (DAD), and their substituted analogs, have been published [9–11]. The introduction of the radical-anionic (o-semiquinone, SQ) ligand into metal complexes provides opportunities for studying the dynamics of the coordination sphere of the complex-forming agent in situ using EPR spectroscopy. Unpaired electron localized on the paramagnetic ligand exerts no appreciable effect on the metal and its environment. This feature means that o-semiquinones can be used as spin labels. The parameters of the EPR spectra of the
semiquinone complexes (g factor, hyperfine structure, and hyperfine coupling (HFC) constants) are very sensitive to the nature of the metal atom, ligand environment and geometric configuration of
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