A series of pseudo-octahedral metal (M = Mn, Fe, Co, Ni, Cu, Zn) complexes 4 of a new redox-active ligand, 2,4,6,8-tetra(tert-butyl)-9-hydroxyphenoxazin-1-one 3, have been synthesized, and their molecular structures determined with help of X-ray crystallography. The effective magnetic moments of complexes 4 (M = Mn, Fe, Co, and Ni) measured in the solid state and toluene solution point to the stabilization of their high-spin electronic ground states. Detailed information on the electronic structure of the complexes and their redox-isomeric forms has been obtained using density functional theory (DFT) B3LYP*/6-311++G(d,p) calculations. The energy disfavored low-spin structures of manganese, iron, and cobalt complexes have been located, and based on the computed geometries and distribution of spin densities identified as Mn(IV)[(Cat-N-SQ)](2), Fe(II)[Cat-N-BQ)](2), and Co(II)[Cat-N-BQ)](2) compounds, respectively. It has been shown that stabilization of the high-spin structures of complexes 4 (M = Mn, Fe, Co) is caused by the rigidity of the molecular framework of ligands 3 that sterically inhibits interconversions between the redox-isomeric forms of the complexes. The calculations performed on complex 4 (M = Co) predict that a suitable structural modification that might provide for stabilization of the low-spin electromeric forms and create conditions for the valence tautomeric rearrangement via stabilization of the low-spin electromer and narrowing energy gap between the low-spin ground state tautomer and the minimal energy crossing point on the intersection of the potential energy surfaces of the interconverting structures consists in the replacement of an oxygen in the oxazine ring by a bulkier sulfur atom.

The reaction of di(tert butyl) derivatives of pyrocatechol with 2,6 dihydroxyaniline afford ed 2,4,6,8 tetra(tert butyl) 9 hydroxyphenoxazin 1 one. The chemical properties of the reac tion product and its ability to form complexes with metal salts as the tridentate ligand were investigated. The structure of hydroxyphenoxazinone was established by X ray diffraction.Key words: 2,4,6,8 tetra(tert butyl) 9 hydroxyphenoxazin 1 one, tautomerism, metal che lates, tridentate ligands, ESR spectroscopy, X ray diffraction study, quantum chemical cal culations.Phenoxazine systems and their sterically hindered structural analogs can be involved in reversible one elec tron redox reactions to form paramagnetic intermediates, the presence of the tert butyl groups leading to a substan tial increase in the stability of such free radical species. 1

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