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Two six-coordinate Sn(IV) complexes, bis-[2,4,6,8-tetra-(tert-butyl)-9-oxyphenoxazinyl-1-onolate]Sn(IV) 4 and bis-[2,4,6,8-tetra-(tert-butyl)-phenoxazinyl-1-onolate]Sn(IV) dichloride 5 with redox-active tridentate 2,4,6,8-tetra(tert-butyl)-9-hydroxyphenoxazin-1-one 2 and bidentate 1-H-1-oxo-2,4,6,8-tetra(tert-butyl) phenoxazine-1-one 3 ligands were synthesized and their molecular structures determined with the use of X-ray crystallography. Electronic structures of the complexes were investigated with DFT B3LYP*/6-311++G(d,p) calculations, SQUID magnetometry and EPR spectroscopy and assigned to the biradical Sn(IV)(CatNSQ)2 and Sn(IV)(ISQ)2Cl2 electromeric forms, where (CatNSQ)2− and (ISQ)− are radical-anions formed by the ligands 2 and 3, correspondingly. Weak ferromagnetic coupling of the two paramagnetic centers was established in complex Sn(IV)(CatNSQ)2 with ligands 2, whereas in the same type complex formed by ligands 3 the coupling of the paramagnetic centers bears weak antiferromagnetic character. The calculated exchange coupling constants J for 4 and 5 (5 cm−1 and −8 cm−1, respectively) are in sufficiently good agreement with the values obtained from analysis of μeff(T) dependences (3.5 cm−1 and −1 cm−1).

A stable biradical bis-chelate complex 3, bis-[2,4,6,8-tetra-(tert-butyl)phenoxazinonato] Zn(II), based on redox-active ligands has been prepared and structurally characterized by X-ray crystallography. Lengths of the coordination Zn–O bonds are equal to 1.938 and 2.008 A and those of the intraligand C–N and C–O bonds (1.356 and 1.308 A, respectively) point to radical-anion form of the phenoxazinone ligand. Effective magnetic moment of a microcrystalline sample of 3 measured at 70 K is equal 2.53 μB, and steadily decreases to 2.39 μB upon elevation of temperature. EPR, UV–Vis measurements and DFT B3LYP∗/6-311++G(d,p) calculations show that 3 has the ground triplet electronic state characterized with weak ferromagnetic coupling of the unpaired electrons localized on the ligands.

Chain complex of copper(II) hexafluoroacetylacetonate with pyrazole-substituted nitronyl nitroxide containing exchange-coupled copper(II)–nitroxide clusters has been studied using electron paramagnetic resonance (EPR) spectroscopy. The structural rearrangements at low temperatures induce spin transitions in one half of the copper(II)–nitroxide clusters and effectively lead to reversible diamagnetic dilution. This results in significant changes of EPR spectra and allows us to obtain the value of dipole–dipole interaction in exchange-coupled spin pair as well as the value of exchange interaction between neighboring pairs.