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

A new approach to the synthesis of polyfunctional pyrazolyl-substituted nitronyl nitroxides was developed based on the presynthesized pyrazole derivatives prepared by 1,3-dipolar cycloaddition. The structures of the resulting mono-and biradicals were confirmed by X-ray diffraction. Dedicated to Academician A. L. Buchachenko on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2105–2116, September, 2005.     

Electrospray mass spectrometry of stable iminyl nitroxide and nitronyl nitroxide free radicals

Electrospray ionization (ESI) mass spectra have been recorded for a range of substituted nitronyl nitroxide and iminyl nitroxide monoradicals and biradicals. Secondary species formed in the ESI source were observed as the dominant ions in both the iminyl nitroxide and nitronyl nitroxide spectra. Daughter ion spectrometry was used to establish fragmentation mechanisms for the nitronyl nitroxide and iminyl nitroxide moieties as well as the secondary species under ESI conditions.     

Copper(II) complexes with pyrazolyl-substituted nitronyl and imino nitroxides

It was established that the reactions of pyrazol-3-yl-substituted nitronyl nitroxide (HL¹) and pyrazol-3-yl-substituted imino nitroxide (HL³) with Cu(II) acetate lead to self-assembly of the Cu₄(OH)₂(OAc)₄(DMF)₂(L¹)₂ tetranuclear and Cu₂(OAc)₂(H₂O)₂(L³)₂ dinuclear complexes, respectively. The reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted nitronyl nitroxide (HL²) gave unexpected solid Cu₂(H₂O)₂(L⁶)₂ · 2DMF, in which L⁶ is a deprotonated 5-carboxy-pyrazol-3-yl-substituted nitronyl nitroxide, formed as a result of cleavage of an ester bond in the starting HL². A similar transformation of the paramagnetic ligand was observed in the reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted imino nitroxide (HL⁴). It led to the formation of Cu₂(DMF)₂(L⁷)₂, where L⁷ is deprotonated 2-(5-carboxy-1H-pyrazol-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide. An X-ray diffraction study indicated that in Cu₄(OH)₂(OAc)₄(DMF)₂(L¹)₂ and Cu₂(OAc)₂(H₂O)₂(L³)₂, the L¹ and L³ paramagnetic ligands perform the bridging cyclic tridentate function, while in Cu₂(H₂O)₂(L⁶)₂ · 2DMF and Cu₂(DMF)₂(L⁷)₂, the paramagnetic L⁶ and diamagnetic L⁷ are bridging bicyclic tetradentate ligands. The magnetic behavior of complexes with coordinated nitronyl nitroxide – Cu₄(OH)₂(OAc)₄(DMF)₂(L¹)₂ and Cu₂(H₂O)₂(L⁶)₂ · 2DMF is dictated by the dominant antiferromagnetic exchange interactions, which is confirmed by quantum-chemical data. The magnetic susceptibility of Cu₂(OAc)₂(H₂O)₂(L³)₂ reflects the competition between the antiferromagnetic and ferromagnetic components, of which the latter is due to electron coupling in the Cu(II) ← N=C–N ? O exchange channels. EPR data confirm the results received from static magnetic measurements for multispin solids.     

A new method for the reduction of nitronyl nitroxides

Reduction of nitronyl nitroxides using hexamethylenetetramine is a very convenient method for preparing the corresponding imino nitroxides and 2-imidazoline N-oxides.     

Biradical with the “non-linear” diacetylene fragment

The nitronyl nitroxide biradical, 2,2′-[1,1′-(hexa-2,4-diyne-1,6-diyl)bis(1H-pyrazole-4,1-diyl)]bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide 1-oxyl), was synthesized. The structure and magnetic properties of the biradical were studied. In the solid state, this nitroxide contains the nonlinear C^S-C≡C-C≡C-C^S fragment. The C^S-C≡C and C≡C-C≡ angles are 172.4(5) and 171.1(3)°, respectively. Dedicated to Academician G. A. Tolstikov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 385–390, February, 2008. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 385–390, February, 2008.     

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.     

A novel TTF-based donor carrying four nitronyl nitroxides

A novel TTF-based tetraradical donor was synthesized and its electronic structure was fully characterized by spectroscopic measurements, magnetic susceptibility measurements, and electrochemical analyses.     

Esters of 4,5-dihydro-1H-imidazole-1-oxyl-3-oxide-carboximidic acid

Esters of 4,5-dihydro-1H-imidazole-1-oxyl-3-oxide-carboximidic acid were synthesized. Their structures and magnetic properties were investigated.     

Inclusion of a Nitronyl Nitroxyl Radical and Its Hydrochloride in Cucurbit[8]uril

The recognition properties of cucurbit[8]uril (CB8) toward nitronyl nitroxide 2‐(2‐benzimidazolyl)‐4,4,5,5‐tetramethylimidazolidinyl‐3‐oxide‐1‐oxy ( 1 ) and its hydrochloride have been investigated. 1? HCl led to 1:1 inclusion complex [ 1? HCl@CB8], which was characterized both in solution and by single‐crystal X‐ray diffraction. In this compound only the tetramethylimidazolidinyl fragment is included in the host. The magnetic behavior of the complex corresponds to a Curie law with a large separation of the spin carriers in the solid. In contrast, an insoluble species exhibiting ferromagnetic behavior is formed when pure 1 reacts with acid‐free CB8. The formula [( 1 )₂@(CB8)₃], in which two radical guests are arranged in such a way that the phenyl groups of the benzimidazolyl substituents are both stacked into one CB8 and the tetramethyl fragments are each capped by a terminal macrocycle, is proposed, in agreement with microanalysis, spectrophotometric, EPR, and magnetic measurements. According to McConnell’s rules, the alternating spin densities within the stacked aromatic fragments result in a ferromagnetic interaction (J=+2.3 cm^?1, H=?2 JS₁S₂) and a triplet ground spin state for the inclusion complex.     

X-ray emission and X-ray photoelectron studies of electron density distribution in Cu(II) complexes with 2-imidazoline nitroxides

Electron density distribution in Cu(II) complexes with 2-imidazoline nitroxides was investigated by X-ray emission and X-ray photoelectron spectroscopy. The structure of the highest occupied molecular orbital (HOMO) of the compounds is determined mainly by the interaction of the metal with the donor atoms of the ligands. Coordination of the nitroxide by the Cu(hfac)₂ acceptor matrix changes the shape of the CuL _α spectral line. The 3s X-ray photoelectron spectra provide evidence about the transfer of electron density to the copper atom in the complexes.     

Controlled free‐radical polymerization of 2‐vinylpyridine in the presence of nitroxides

Bulk free‐radical polymerization of 2‐vinylpyridine (2VP) in the presence of 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) was studied under different conditions (temperature and presence of additives). Linear poly‐(2‐vinylpyridine) with a narrow molecular weight distribution and controllable molecular weight was prepared in the presence of acetic anhydride at 95 °C up to a conversion of 66%. At higher conversions side reactions became very important (pseudoliving polymerization). By applying this procedure, well‐defined random copolymers of 2VP with styrene or tert‐butylmethacrylate as well as block copolymers of 2VP with styrene were synthesized. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2889–2895, 2001     

Stable Subporphyrin meso-Aminyl Radicals without Resonance Stabilization by a Neighboring Heteroatom

Most aminyl radicals studied so far are resonance-stabilized by neighboring heteroatoms, and those without such stabilization are usually short-lived. We report herein that subporphyrin meso-2,4,6-trichlorophenylaminyl radicals and a bis(5-subporphyrinyl)aminyl radical are fairly stable under ambient conditions without such stabilization. The subporphyrin meso-2,4,6-trichlorophenylaminyl radical crystal structure displays a characteristically short C_meso−N bond and a perpendicular arrangement of the meso-arylamino group. The stabilities of these radicals have been ascribed to extensive spin delocalization over the subporphyrin π-electronic network as well as steric protection around the aminyl radical center.     

Preparation of N-aryl-ketonitrone spin traps

The syntheses of seven N-aryl-C,C-dialkoxycarbonylnitrones 1–7, six of which were original, were achieved from the appropriate aryl-nitroso compounds. These ketonitrones were found to trap efficiently carbon-centred free radicals in aqueous media, yielding stable aminoxyl radicals whose EPR spectra lasted several days. The two penta-deuterated compounds 6 and 7 were also found to be efficient at trapping methoxyl radical. Their various spin adducts showed simple three line signals, very sensitive to the polarity of the environment. This study represents the very first use of linear ketonitrones as spin traps.     

Host–Guest Complexes of some Stable Free Radicals

Hydrophobic host–guest complexes of some water soluble stable free radicals – potassium nitrosodisulfonate (Fremy's salt), 2-$p$-phenylsulphonic acid-2-phenyl-1-picrylhydrazyl (KSO₃-DPPH^.) and 2,2-p-phenylsulphonic acid-1-picrylhydrazyl ((KSO₃)₂DPPH^.), both of them as potassium salts, with 18-C-6, B18-C-6 and DB18-C-6 crown ethers, were obtained and characterised by elemental analysis, IR, ESR and UV-Vis.     

Interaction of a lithiated nitronyl nitroxide with polyfluorinated 1,4-naphthoquinones

A 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl lithium derivative was found to react with 2-methoxypentafluoro-1,4-naphthoquinone to form a product of addition at the carbonyl function: radical 2-(3,5,6,7,8-pentafluoro-1-hydroxy-2-methoxy-4-oxo-1,4-dihydronaphthalen-1-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl. The yield of the addition product increased with temperature and reached 84% at 0?°C. The reaction of the lithium derivative with hexafluoro-1,4-naphthoquinone gave rise to a product of addition at both carbonyl groups, namely, nitronyl nitroxide diradical 2,3,5,6,7,8-hexafluoro-1,4-bis(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazole-2-yl)-1,4-dihydronaphthalene-1,4-diol in a 16% yield. The structures of both mono- and diradical were solved by X-ray diffraction analysis, which revealed formation of an intramolecular H-bond between the OH group and nitroxide oxygen. According to electron paramagnetic resonance (EPR) spectroscopy, the obtained mono- and dinitroxide are prone to spontaneous deoxygenation in a toluene solution to give corresponding iminonitroxides. In water, they are much more stable.     

Hyperfine Coupling Constants of β‐Phosphorylated Nitroxides: A Tool to Probe the Cybotactic Effect by Electron Paramagnetic Resonance

Nitrogen hyperfine coupling constants (hcc) a_N of nitroxides measured by electron paramagnetic resonance are used to probe the cybotactic effect of solvents. A few articles (Janzen et al., Can. J. Chem. in 1982 for a_Hβ and Deng et al., J. Fluorine Chem. in 2002 , both for a_Hβ and a_Fβ) dealt with the cybotactic effect on the hcc of atoms (hydrogen H, fluorine F, and phosphorus P) in β positions of nitroxides. They show either no significant change or an increase in a_β with a_N. However, an early report by Il′Yasov et al. (Theor. Exp. Chem. 1976 , 656) described a dramatic change in a_Pβ with the solvent and an anti‐correlation of a_Pβ with a_N. We decided to reinvestigate that work using the N‐(2‐methylpropyl)‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl)‐N‐oxyl radical that carries both hydrogen and phosphorus atoms on the same carbon attached to the nitroxy moiety and that is highly persistent, in contrast to the nitroxide used by Il′Yasov. We observe the same trends as those observed by Il′Yasov, except that the solvent cybotactic effect on a_N and a_Pβ is dramatically weaker in our case. We showed that a_Pβ is correlated to both the cohesive pressure c and the normalized Reichardt polar constant E_T^N, and that hydrogen‐bonding donor solvents exhibit a specific behavior.     

Nanoscale Assembly of Paramagnetic Organic Radicals on Au(111) Single Crystals

The successful thin‐film deposition of a pyrene‐substituted nitronyl nitroxide radical under controlled conditions has been demonstrated. The electronic properties, chemical environment at the interface, and morphology of the thin films have been investigated by a multitechnique approach. Spectroscopic and morphological analyses indicate a Stranski–Krastanov growth mode and weak physisorption of molecules onto the metallic surface. Electron spin resonance (ESR) spectroscopy shows that evaporation processes and deposition do not affect the paramagnetic character of the molecules. Useful concepts for the engineering of new, purely organic‐based magnets, which may open the way to fruitful exploitation of organic molecular‐beam deposition for assembly on solid surfaces in view of future technological applications, are presented.     

Three new lanthanide compounds based on nitronyl nitroxide radical: Crystal structure,magnetic properties,and luminescence properties

Three new lanthanide compounds were obtained using 2-(3-methylthiophene)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) (NIT-3Methien). These compounds, [Gd(hfac)₃(NIT-3Methien)₂]?0.5CH₃(CH₂)₅CH₃ (1: Half n-heptane trihexafluoroacetylacetonate-di-2-(3-methylthiophene)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide gadolinium(III)), [Tb(hfac)₃(NIT-3Methien)₂]?0.5H₂O (2: Half Hydrate trihexafluoroacetylacetonate-di-2-(3-methylthiophene)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide terbium(III)), and [Dy(hfac)₃(NIT-3Methien)₂]?0.5H₂O (3: Half Hydrate trihexafluoroacetylacetonate-di-2-(3-methylthiophene)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide dysprosium(III)), (CH₃(CH₂)₅CH₃ = n-heptane), (hfac = hexafluoroacetylacetonate), were characterized structurally and magnetically. The three compounds crystallize in the triclinic space group P1( - ). Ln(III) ion was eight-coordinate by six oxygens from three hfac ligands and two oxygens from two radicals. In 1, direct current (DC) magnetic studies reveal ferromagnetic interactions between the Gd(III) ion and radicals with J₁ = 0.94 cm^?1. In 2 and 3, there are antiferromagnetic interactions between the Tb(III), or Dy(III) ions and radicals. The luminescence characterizations show that 2 exhibits highly selective luminescent sensing of Cr₂O₇^2? ions.     

Molecular design,synthesis, and magnetic characterization of poly(phenylacetylene) with pi-toporegulated pendant nitronyl nitroxide radicals as models for organic superpara- and ferromagnets

The present article describes the synthesis and magnetic properties of poly(phenylacetylene) ( 3 ) with pi-toporegulated pendant stable 4,4,5,5-tetramethylimidazolin-3-oxide-1-yloxyl radicals topologically participating in the pi-conjugated system of poly(phenylacetylene). Polyradical 3 was prepared by the condensation reaction of poly(p-ethynylbenzaldehyde) with 2,3-bis(hydroxylamino)-2,3-dimethylbutane followed by oxidation with lead dioxide. The spin concentration of 3 determined by the ESR spectroscopic method was 1.1 × 10²¹ spins/g. This value approximately corresponds to 0.5 unpaired electron spin per the repeating unit. A powder ESR spectrum of 3 gave a 100% Lorentzian single line showing spin-exchange narrowing. The vibrating sample magnetometer (VSM) measurement of 3 afforded a straight line with a positive slope, suggesting that 3 has predominant paramagnetic properties within an experimental error.     

Redox polymers incorporating pendant 6-oxoverdazyl and nitronyl nitroxide radicals

Polymers comprised of redox-active organic radicals have emerged as promising materials for use in a variety of organic electronics, including fast-charging batteries. Despite these advances, relatively little attention has been focused on the diversification of the families of radicals that are commonly incorporated into polymer frameworks, with most radical polymers being comprised of nitroxide radicals. Here, we report two new examples prepared via ring-opening methathesis polymerization containing 6-oxoverdazyl and nitronyl nitroxide radicals appended to their backbones. The polymerization reaction and optoelectronic properties were explored in detail, revealing high radical content and redox activity that may be advantageous for their use as semiconducting thin films. Initial studies revealed that current–voltage curves obtained from thin films of the title polymers exhibited memory effects making them excellent candidates for use in resistive memory applications. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 309–319