Syntheses and structures of azol-1-yl derivatives of nitronyl and imino nitroxides

2-(Pyrazol-1-yl)-, 2-(imidazol-1-yl)-, 2-([1,2,4]triazol-1-yl)-, and 2-(benzotriazol-1-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl were prepared by reactions of 2-bromo-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (NIT-Br) with the corresponding sodium azolides. In prepared 2-(azol-1-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyls, the NIT-N_Het bond is readily hydrolyzed. Reduction of imidazole-3-oxide-1-oxyls leads to corresponding 2-(azol-1-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyls, which are much more stable against hydrolysis. The structures of spin-labeled imidazoles, [1,2,4]triazoles and benzotriazoles are confirmed by X-ray analysis, showing that the paramagnetic molecules form packings with motifs from centrosymmetric dimers to topologically linear chains.     

XPS spectra of free nitroxyl radicals and their electronic structure

The N(1s) and O(1s) XPS spectra of stable nitroxyl radicals and molecules with a related heterocycle structure: 4,4,5,5-tetramethyl-2-phenyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, 4,4,5,5-tetramethyl-2-(3-nitrophenyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, 4,4,5,5-tetramethyl-2-(2-hydroxyphenyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, 4,4,5,5-tetramethyl-2-(2-hydroxy-3-nitrophenyl)-4,5-dihydro-1Himidazole-3-oxide-1-oxyl, and 4,4,5,5-tetramethyl-2-phenyl-4,5-dihydro-1H-imidazole-3-oxide were studied. The possibility to apply X-ray electron spectra for investigation of the charge electron and spin density distribution on free radical atoms and at their coordination by a metal is considered.     

S<Stack><Subscript>N</Subscript><Superscript>H</Superscript></Stack> Reaction of lithiated nitronyl nitroxide with quinoline <Emphasis Type="Italic">N</Emphasis>-oxide

The S_N^H reaction of lithiated 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide with quinoline N-oxide affords 4,4,5,5-tetramethyl-2-(1-oxidoquinolin-2-yl)-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide.     

Syntheses and Study of a Pyrroline Nitroxide Condensed Phospholene Oxide and a Pyrroline Nitroxide Attached Diphenylphosphine

The reaction of a diene nitroxide precursor with dichlorophenylphosphine in a McCormac procedure afforded 1,1,3,3-tetramethyl-5-phenyl-1,2,3,4,5,6-hexahydrophospholo[3,4-c]pyrrole-5-oxide-2-oxyl. Lithiation of the protected 3-iodo-pyrroline nitroxide followed by treatment with chlorodiphenylphosphine after deprotection afforded (1-oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-yl)diphenylphosphine oxide, and after reduction, (1-oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-yl)diphenylphosphine was realized, which was also supported by X-ray single crystal diffraction measurements. This pyrroline diphenylphosphine derivative was converted to hexadecylphosphonium salt, which is an analogue of antineoplastic agent, MITO-CP.     

Heterospin complexes based on dinuclear CuII triketonate and nitroxides

Heterospin complexes of bis(μ₂-1,1,2,2,8,8,9,9-octafluorononane-3,5,7-trionato)dicopper(II) ([Cu₂L₂]) with nitronyl nitroxides 2-(1-methyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide (2) and 4,4,5,5-tetramethyl-2-(3-pyridinyl)-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide (1) were synthesized and structurally characterized. Crystals of the complexes are formed by the discrete bis[1-methyl-4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)-1H-pyrazole]-bis (μ₂-1,1,2,2,8,8,9,9-octafluorononane-3,5,7-trionato)dicopper(II) etherate (3) and bis[3-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyridine]-bis (μ₂-1,1,2,2,8,8,9,9-octafluorononane-3,5,7-trionato)dicopper(II) (4) molecules. Each Cu atom of the dinuclear chelate fragment coordinates one paramagnetic ligand through the N atom of the pyrazole or pyridine fragment, respectively. In complex 3, the paramagnetic ligands are located on one side of the plane of the chelate fragment, whereas the ligands in complex 4 are located above and below the plane of the chelate fragment. The magnetic properties of complexes 3 and 4 are determined by dominant antiferromagnetic exchange interactions between the unpaired electrons of the Cu^II atoms in the dinuclear Cu₂L₂ moiety. Dedicated to Academician O. M. Nefedov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1836–1840, November, 2006.     

Bis(1,1,1,3,5,5,5-heptafluoro-4-iminopent-2-ene-2-aminato)copper(<Emphasis Type="SmallCaps">ii</Emphasis>) — a new metal-containing matrix in the design of heterospin systems

The first heterospin compounds based on bis(1,1,1,3,5,5,5-heptafluoro-4-iminopent-2-ene-2-aminato)copper(ii) (CuL₂) and nitronyl nitroxide radicals, 2-R-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyls (NIT-R, where R = H, Me, Ph, and 1-methylpyrazol-4-yl), were synthesized and structurally characterized. An important peculiarity of the structure of synthesized solid phases is the formation of a supramolecular structure due to hydrogen bonds between the oxygen atoms of the nitronyl nitroxide fragments of NIT-R and the hydrogen atoms of the NH groups of CuL₂.     

Crystals of the CuII complex with nitronyl nitroxide and imino nitroxide exhibiting mechanical activity

As part of continuing studies of multispin compounds capable of exhibiting chemomechanical activity, a series of heterospin solids of the composition [Cu(hfac)₂L _x L′_2?x ], [Cu(hfac)₂L′], [Cu₂(Piv)₄L′₂]·0.5C₆H₁₄, and [Cu₂(hfac)₂(Piv)₂L′2], where hfac is the hexafluoroacetylacetonate anion, L is 4,4,5,5-tetramethyl-2-(1-methyl-1H-imidazol-5-yl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, L′ is the imino nitroxide analog of L, viz., 4,4,5,5-tetramethyl-2-(1-methyl-1H-imidazol-5-yl)-4,5-dihydro-1H-imidazole-1-oxyl, and Piv is the 2,2-dimethylpropionate anion, were synthesized and characterized. The packing of the synthesized crystals of the solid solutions [Cu(hfac)₂L _x L′_2-x ], where L predominates, is similar to that for [Cu(hfac)₂L₂], and these crystals are able to undergo chemomechanical motion. On the contrary, the crystals of [Cu(hfac)₂L _x L′_2?x ], where L′ predominates, have structural parameters similar to those of [Cu(hfac)₂L′₂] and do not exhibit thermally activated or photoactivated chemomechanical activity.     

Key influence of the nature of the substituent in the propynal molecule on the outcome of its reaction with vicinal di(<Emphasis Type="Italic">N</Emphasis>-hydroxyamine)

The reaction of propynals X-C≡C-CHO, where X = Alk or Ar, with 2,3-di(N-hydroxyamino)-2,3-dimethylbutane gives exclusively 1-X-2-(1-hydroxy-4,4,5,5-tetramethylimidazolidin-2-ylidene)ethanones. In the case of X = Me₃Si, the reaction affords 2-(2-trimethylsilylethynyl)-4,4,5,5-tetramethylimidazolidine-1,3-diol. The reaction of propynal containing X = Et₃Ge yields both types of the products. The resulting imidazolidine-1,3-diol can be quantitatively isomerized to imidazolidin-2-ylideneethanone, oxidized to 2-[2-(triethylgermyl)ethynyl]-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, or transformed into ethynyl-substituted nitronyl nitroxide. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 588–594, March, 2008.     

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

Cyclic dinitrones underwent nucleophilic substitution of the hydrogen atom in the reaction with a paramagnetic carbanion, the lithium derivative of 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide, to give polyfunctional nitronyl nitroxyls.     

Structure and magnetic properties of Cu(II) bischelate with spin-labeled aminoenal

The molecular and crystal structure of Cu(II) bischelate with the deprotonated stable nitroxide radical 4,4,5,5-tetramethyl-2-(2-oxo-1-(4,4,5,5-tetramethylimidazolidine-2-ylidene)ethyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxylom (HL) are studied. It is revealed that the complex packing is formed of separate CuL₂ molecules. Oxygen atoms in >N∸O groups do not participate in Cu²⁺ ion coordination, and so only weak exchange interactions are observed between paramagnetic centers in CuL₂.     

Seven-membered metallocycle in the CuII complex with deprotonated 2-(2-hydroxy-3-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide

The reaction of CuCl₂ with deprotonated 2-(2-hydroxy-3-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide afforded a bis-chelate complex containing seven-membered metallocycles.     

2D and 3D Cu(hfac)2 complexes with nitronyl nitroxide biradicals

Reactions between Cu(hfac)2 and nitronyl nitroxide biradicals 1,4-bis[4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyrazol-1-yl]butane (L4) and 1,8-bis[4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyrazol-1-yl]octane (L8) gave respectively a framework compound [Cu(hfac)2]2L4 and a layered polymer compound [Cu(hfac)2]2L8. The framework of [Cu(hfac)2]2L4 consists of 66-membered condensed metallocycles. Inside the framework, the structure has macrohelixes (pitch approximately 25 A) extending along the [001] crystallographic direction. All the helixes have the same direction of winding; the crystals, therefore, are optically active, the structure corresponding either to P-isomer (P4(1)2(1)2) or to M-isomer (P4(3)2(1)2). The long distances between the Cu atoms and the O atoms of the coordinated >N-O groups (Cu-O 2.351-2.467 A) are responsible for ferromagnetic exchange interactions in Cu2+-O-N< and >N-O-Cu2+-O-N< exchange clusters.     

Reaction products of nitronyl nitroxyl radicals with acids

Study of the structures of compounds generated by the reactions of 4,4,5,5-tetramethyl-2-phenyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide with trifluoromethanesulfonic and picric acids demonstrated that these reactions produced salts of disproportionation products of nitronyl nitroxyl.     

Phase transitions in the solvate of the heterospin complex Cu(hfac)2 with the nitronyl nitroxide radical

The temperature dependence of the heat capacity of a polycrystalline sample of the heterospin solvate [Cu(hfac)₂L]·0.5[1,4-(CF₃)₂C₆H₄] (L is 2-(1-butyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide 1-oxyl) was studied in the range of 5–298 K. The results of the thermochemical study of this compound were shown to be in agreement with the data from magnetochemical and diffraction measurements providing evidence that, as the temperature changes, the complex undergoes phase transitions at 59 and 123 K.     

Heterospin complexes based on cobalt semiquinolate with nitroxides

Heterospin complexes based on cobalt semiquinolate with stable nitroxides of composition [Co(SQ)₂(NIT)₂]·8CH₂Cl₂, [Co(SQ)₂(NIT)₂]·4MeCN, [Co(SQ)₂(IN)₂]·5Me₂CO, [Co(SQ)₂(IN)₂]·4MeCN, and [Co(SQ)₂(IN)₂]·2CH₂Cl₂·C₆H₁₄, where SQ is 3,6-di-tert-butyl-o-benzosemiquinolate, NIT is 4,4,5,5-tetramethyl-2-(1-methyl-1H-imidazol-5-yl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, and IN is the imine analog of NIT, viz., 4,4,5,5-tetramethyl-2-(1-methyl-1H-imidazol-5-yl)-4,5-dihydro-1H-imidazole-1-oxyl, were synthesized. The characteristic feature of these compounds is that they contain two different types of paramagnetic organic ligands. The X-ray diffraction study showed that the solid phases of the complexes have a molecular structure. According to the data from thermomagnetic analysis (2–350 K), all complexes exhibit effects of valence tautomerism at ∼20 °C. It was found that incorporated solvent molecules have an effect on the magnetic anomaly temperature.     

Magnetic tuning of all-organic binary alloys between two stable radicals

Mixtures of 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN) and 2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (BImNN) crystallize as solid solutions (alloys) across a wide range of binary compositions. (F4BImNN)(x)(BImNN)((1-x)) with x < 0.8 gives orthorhombic unit cells, while x ≥ 0.9 gives monoclinic unit cells. In all crystalline samples, the dominant intermolecular packing is controlled by one-dimensional (1D) hydrogen-bonded chains that lead to quasi-1D ferromagnetic behavior. Magnetic analysis over 0.4-300 K indicates ordering with strong 1D ferromagnetic exchange along the chains (J/k = 12-22 K). Interchain exchange is estimated to be 33- to 150-fold weaker, based on antiferromagnetic ordered phase formation below Néel temperatures in the 0.4-1.2 K range for the various compositions. The ordering temperatures of the orthorhombic samples increase linearly as (1 - x) increases from 0.25 to 1.00. The variation is attributed to increased interchain distance corresponding to decreased interchain exchange, when more F4BImNN is added into the orthorhombic lattice. The monoclinic samples are not part of the same trend, due to the different interchain arrangement associated with the phase change.     

New derivatives of 4,4,5,5-tetramethyl-4,5-dihydro-1-H-imidazol-1-oxyl 3-oxide as monomers for polyradicals with special magnetic properties

The preparation of new derivatives of 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide as monomers for purely organic polymers with special magnetic properties is described. 3,5-Diethynylbenzaldehyde and 3,5-bis(trimethylsilylethynyl)benzaldehyde were converted to the corresponding 1,3-dihydroxy-4,4,5,5-tetramethylimidazolidine derivatives which were oxidized to stable nitronyl nitroxide radicals.     

A novel route to spin-labeled dihydrooxepines and o-benzoquinones

A reaction of a lithiated derivative of 4,4,5,5-tetramethyl-3-oxido-4,5-dihydro-1H-imidazole 1-oxyl with 3,6-di-tert-butyl-o-benzoquinone at ?80 °C predominantly gave spin-labeled dihydrooxepine via nucleophilic 1,2-addition of the paramagnetic carbanion to the CO group of the benzoquinone followed by insertion of the O atom into the ring. At lower temperatures, this reaction was accompanied by 1,4-addition of the organolithium compound to the benzoquinone followed by oxidation of the resulting adduct into spin-labeled o-benzoquinone. This ??one pot?? process is a novel approach to organic derivatives that can further be converted into di- and polyradicals combining nitronyl nitroxide and semiquinolate fragments.     

Heterospin Mn<Superscript>III</Superscript> complex as a reaction product of the redox-induced change in the ligand coordination mode

The reaction of Mn^II chloride with imino nitroxide radical, 2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (HL²), affords the Mn^III complex [MnL² ₂L³]·Me₂CO, a distinctive feature of which is the simultaneous presence in the ligand shell of both the initial imino nitroxide and the product of its reduction 2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide (HL³). The reaction involves the oxidation of Mn^II to Mn^III and the reduction of the imino nitroxide radical to the corresponding amidine oxide along with a change in the coordination mode of the heterocyclic ligand on going from L² to L³. The Mn^III ion forms with L² six-membered metallocycles typical of Schiff bases, whereas with L³ Mn^III forms a seven-membered metallocycle due to the coordination of L³ by oxygen atoms of the phenol and N-oxide It was found in a similar reaction of Ni^II chloride with imino nitroxide HL² that no oxidation of the metal occurred and bis(chelate) [NiL² ₂(H₂O)₂]·2Me₂CO was formed in the solid phase. The molecular and crystal structures of the compounds were determined, and their magnetic properties were studied.     

Crystallography and magnetism of 2-amido-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyls

Two 2-Ar-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl radicals were synthesized (Ar = para-acetamidophenyl, and para-(tert-butyloxycarbonyl)aminophenyl, pAPNN, pBPNN) that incorporate amide-type NH moieties as potential hydrogen bonding sites. Both form hydrogen bonded cyclic dyads, but pAPNN exhibits spin pairing (2J/k = (−)5 K) due to a close, antiparallel nitroxide-nitroxide stacking contact, while pBPNN is essentially paramagnetic.