The bistable cobalt complex containing two symmetrical 3,6 di tert butyl o benzoquinone and one 1,10 phenanthroline moieties as ligands (1) was synthesized. Complex 1 was isolated in the individual state and characterized by IR and ESR spectroscopy, X ray diffraction, magnetochemical studies, and precision calorimetry. A change in temperature causes the re versible metal-ligand electron transfer and spin crossover (redox isomerism) in complex 1 in the crystalline state. The redox isomeric transformation at ∼250 K is accompanied by the phase transition. The structural study at two temperatures confirmed the changes in the molecular and crystal structure associated with the redox isomeric transformation.

o-Semiquinonic nickel pincer complexes (R2PCP)Ni(SQ) show a versatile coordination sphere dynamics via "swing" or "fan" oscillations depending on the steric properties of the phosphorus substituents.

The properties of phenoxyl radicals generated by the oxidation of mono , di , and triphosphorus derivatives of 2,6 di(tert butyl) 4 methylphenol (ionol) were studied by ESR. These compounds exist as conformers that are interconvertible with a temperature dependent rate. Numerical processing of the ESR spectra gave the thermodynamic and activation param eters that characterize the interconversion of the conformers. The antioxidant activities of the compounds were studied in a model oxidation of oleic acid and with biological objects. These phenols efficiently inhibited radical oxidation reactions.Derivatives of 2,6 di(tert butyl) 4 methylphenol (ionol) are efficient antioxidants. 1 They are biologically active compounds capable of inhibiting lipid peroxidation and lowering the oxidative stress of the organism. In re cent years, we have proposed a convenient route to a number of ionol derivatives with one, two, or three phos phorus containing groups, starting from easily accessible 3,5 di(tert butyl) 4 hydroxybenzaldehyde and trimethyl silyl esters of phosphorus(III) acids. 2-4 Here we studied the antioxidant properties of phosphorus containing ionol derivatives 1-4.A comparative analysis of the antioxidant activity was performed against reference compound 5 containing no 2,6 di(tert butyl)phenol fragment.The antioxidant activities of 2,6 di(tert butyl)phenols are determined by the stability of the corresponding phenoxyl radicals. 1 We generated phenoxyl radicals from compounds 1-4 and studied their properties by ESR.Experimental 1 H, 13 C, and 31 P NMR spectra were recorded on a Bruker Avance 400 spectrometer (400, 100, and 162 MHz, respectively) in CDCl 3 for compounds 2 and 3, DMSO d 6 for 1, and D 2 O for 4 with Me 4 Si ( 1 H, 13 C) and 85% H 3 PO 4 in D 2 O ( 31 P) as standards. All reactions were carried out in anhydrous solvents under dry argon. ESR spectra were recorded on a Bruker ER 200 D SRC instrument fitted with an ER 4105 DR double resonator (~9.5 GHz) and an ER 4111 VT thermocontroller and on a Bruker EMX 8/2.7 instrument. Diphenylpicrylhydrazyl (DPPH, g = 2.0037) was used as a standard in the determination of the g factor. Spectra were simulated with the WinEPR SimFonia v 1.25 program (Bruker).Compounds 1-5 were prepared according to our proce dure 3,4 by reactions of trimethylsilyl esters of phosphorus(III) acids with 3,5 di(tert butyl) 4 hydroxybenzaldehyde and 3,5 di(tert butyl) 4 hydroxybenzoyl chloride. The physico chemical constants of compounds 2-4 (see Ref.3) and 5 (see Ref. 4) were identical with the literature data.O,O Diethyl [3,5 di(tert butyl) 4 hydroxyphenyl(trimethyl silyloxy)methyl]phosphonate (1) was obtained from diethyl

A number of square pyramidal o-semiquinonic nickel complexes with 2,6-bis(diphenylphosphinomethyl)phenyl were investigated by EPR spectroscopy in solution. The complexes have flexible coordination spheres and can exist in solution as two isomers rapidly (on the EPR timescale) interconverting one to another at room temperature. The equilibrium is entropy driven and does not significantly depend on the substituents in the semiquinone ring. The spin density distribution depends on mesomeric properties and bulkiness of substituents in ortho-position to oxygen atoms.

Square pyramidal o-semiquinonic nickel complexes with 2,6-bis(diphenylphosphinomethyl)phenyl have a flexible coordination sphere and exist in solution as the mixture of tautomers rapidly, according to EPR time scale, interconverting at room temperature.

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