V. I. Ovcharenko

Synthetic chemistry of stable nitroxides

Introduction to Nitroxide Chemistry. Generation and Chemical Properties of the Nitroxyl Group. Synthetic Chemistry of Stable Nitroxides. Complexes with Metal-Nitroxyl Group Coordination. New Approaches to Generation of Heterocyclic Nitroxides Using Organometal Reagents. References. Index.

Nonclassical Spin Transitions

AbstractA family of heterospin polymer chain complexes Cu(hfac)2 with pyrazole-substituted nitronyl nitroxides (LR, where R = Me, Et, Pr) of the composition Cu(hfac)2LR was found. In the solid state the complexes undergo low-temperature structural rearrangements accompanied by magnetic effects analogous to spin-crossover. Polymer chains with a “head-to-head” or “head-to-tail” motif in Cu(hfac)2LR are formed as a result of the bridging coordination of LR through the imine N atom of pyrazole and one of the O atoms of the nitronyl nitroxide fragment. Despite the low-temperature structural phase transition, the single crystals retain the quality needed for an X-ray investigation, due to which the compounds may be studied at different temperatures and structural dynamics studies are possible. It was found that the major structural changes mainly occur in the coordination polyhedra, leading to phenomena analogous to spin-crossover. This rearrangement is possible due to the Jahn–Teller nature of the Cu(II) ion, which is responsible for the pulled octahedron structure of the coordination unit. The transition of the coordinated nitroxyl O atoms from the axial (dCu–O ∼ 2.2-2.4 Å) to equatorial (dCu–O ∼ 2.0 Å) position is accompanied by a transition of the exchange interaction in the Cu(II)–O•–N< exchange clusters from weak ferromagnetic (or weak antiferromagnetic) to strong anti-ferromagnetic, compensating the spins of the Cu(II) ion and the nitroxyl fragment. The motif of the polymer chain (“head-to-head” or “head-to-tail”) proved to be inessential to the occurrence of thermally induced spin transitions. In both cases, the shortening of the Cu–O distance in the Cu(II)–O•–N< exchange cluster decreases the effective magnetic moment of the complex by a factor of

Molecular magnets based on M(hfac)2 and spin-labeled nitrile

\sqrt 2

Electrochemistry of nitronyl and imino nitroxides

, because spin compensation occurs in only half of all coordination units (Cu(hfac)2LPr, Cu(hfac)2LMe, and Cu2(hfac)4LMeLEt). The low-temperature structural rearrangement in Cu(hfac)2LEt is unusual; it leads to longer Cu–O distances in the Cu(II)–O•–N< exchange cluster and to an abrupt transition from antiferromagnetic to ferromagnetic exchange. At reduced temperatures, the unit cell volume decreased by 5-6% in all of the compounds. The cell parameters are large and so is the absolute value of the decrease (up to 400 Å3). The maximal decrease in the cell dimensions was observed in the directions of the chains and in the directions of the maximal shortening of the Cu...Cu distances. Minimal compression or even extension of the cell took place in the direction of lengthening of the Cu–Ohfac distances. The single crystals of Cu(hfac)2LEt and Cu(hfac)2LPr possess high mechanical stability in repeated cooling-heating cycles.

Spirocyclic derivatives of nitronyl nitroxides in the design of heterospin Cu II complexes manifesting spin transitions

Reduction of 2,3-dimethyl-2,3-dinitrobutane with Zn in aqueous ethanol in the presence of NH4Cl affords 2,3-bis(hydroxylamino)-2,3-dimethylbutane together with 2,3-diamino-2,3-dimethylbutane and complex Zn salts. A modified procedure was developed for the synthesis of bishydroxylamine, which involves reduction in a Zn/NH4Cl/THF−H2O system.

Structure of the complex of hexanuclear manganese pivalate with isonicotinamide

Syntheses, structure, and magnetic properties of [Cu(hfac) 2 ] 2 L, Cu(hfac) 2 L, and [M(hfac) 2 ] 3 L 2 , where M = Mn, Co, are described. Special attention is paid to the unusual structure of the two polymorphs of Cu(hfac) 2 Land to the magnetic properties of [M(hfac) 2 ] 3 L 2 .0.5C 7 H 1 6 , for which a magnetic phase transition to the ferromagnetic state has been found.

1,3-Dipolar cycloaddition in the synthesis of pyrazolyl-substituted nitronyl nitroxides

The combined cyclic voltammetry method (cyclic voltammetry detected by electron spin resonance, CVA-DESR CVA) was applied to study the electrochemical reactions of nitronyl- and iminonitroxyls (NN and IN) and to estimate the diffusion and absorption characteristics of their products. The ESR method yielded the hyperfine structure constants, including 13C. Even if exposed to lower temperature, all the studied NN were shown to spontaneously lose oxygen and to convert gradually into the relevant IN. For each specific NN, the combined CVA-DESR CVA experiments found the reduction potential value at which it can be virtually completely converted into the relevant IN.

Synthesis of nitroxyl radical by direct nucleophilic functionalization of a C-H bond in the azadiene systems

Redox potentials of a wide group of azolyl-substituted nitronyl and imino nitroxides were determined by classic cyclic voltammetry (CV). Conclusions have been made for this group of compounds, and their peculiarities were emphasized in comparison with methyl-, phenyl-, iodo-, and cyano-substituted nitroxides.