Nitronyl Nitroxide Moieties Research Articles

5-Fluoro-1-Methyl-Pyrazol-4-yl-Substituted Nitronyl Nitroxide Radical and Its 3d Metal Complexes: Synthesis, Structure, and Magnetic Properties

The metal–radical approach is a well-established synthetic way toward multi-spin systems that relies on the coordination of stable radical ligands with transition metal ions. The advantage offered by the use of paramagnetic ligands is that metal–radical magnetic exchange coupling is direct between the magnetic orbitals of the radical and metal ion. With the aim of further exploring this approach, crystals of four heterspin complexes, [M(hfac)2LF]2 {M = Mn, Co, or Ni and hfac = hexafluoroacetylacetonate} and [Cu(hfac)2LF]n, were obtained using a new fluorinated pyrazolyl-substituted nitronyl nitroxide radical, 4,4,5,5-tetramethyl-2-(5-fluoro-1-methyl-1H-pyrazol-4-yl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LF) as a ligand. The newly synthesized complexes were fully characterized, including X-ray crystallography and magnetometry. XRD analysis revealed that complexes [M(hfac)2LF]2 have similar dimer structures in which a metal ion is in a six-coordinated environment with four O atoms from the two hfac ligands, one radical O atom, and one pyrazole N atom from ligand LF. Nonetheless, the packing patterns of the complexes were found to be considerably different. In [Mn(hfac)2LF]2, there are no magnetically important short contacts between manganese dimers. By contrast, in [Co(hfac)2LF]2 and [Ni(hfac)2LF]2, there are short contacts between non-coordinate O atoms of nitronyl nitroxide moieties. Magnetic behaviors of [M(hfac)2LF]2 showed that the M ions and the directly coordinated radicals are strongly antiferromagnetically coupled (JMn-ON = −84.1 ± 1.5 cm−1, JCo-ON = −134.3 ± 2.6 cm−1, and JNi-ON = −276.2 ± 2.1 cm−1; H^=−2JS⃑^MS⃑^NO). Notably, the magnetization of [Mn(hfac)2LF]2 having molecular structure proved to be accompanied by hysteresis. The [Cu(hfac)2LF]n complex has a chain-polymer structure with alternating magnetic fragments: three spin exchange clusters {ONO–Cu(II)–ONO} and {Cu(II)} ions. Despite the direct coordination of radicals, its magnetic properties are weakly ferromagnetic (JCu-ON = 14.8 ± 0.3 cm−1).

Overclocking Nitronyl Nitroxide Gold Derivatives in Cross-Coupling Reactions.

Nitronyl nitroxides are functional building blocks in cutting-edge research fields, such as the design of molecular magnets, the development of redox and photoswitchable molecular systems and the creation of redox-active components for organic and hybrid batteries. The key importance of the nitronyl nitroxide function is to translate molecular-level-optimized structures into nano-scale devices and new technologies. In spite of great importance, efficient and versatile synthetic approaches to these compounds still represent a challenge. Particularly, methods for the direct introduction of a nitronyl nitroxide moiety into aromatic systems possess many limitations. Here, we report gold derivatives of nitronyl nitroxide that can enter Pd(0)-catalysed cross-coupling reactions with various aryl bromides, affording the corresponding functionalized nitronyl nitroxides. Based on the high thermal stability and enhanced reactivity in catalytic transformation, a new reagent is suggested for the synthesis of radical systems via a universal cross-coupling approach.

Synthesis, Crystal Structure and Magnetic Properties of a Trinuclear Copper(II) Complex Based on P-Cresol-Substituted Bis(α-Nitronyl Nitroxide) Biradical.

Trinuclear copper(II) complex [CuII3(NIT2PhO)2Cl4] was synthesized with p-cresol-substituted bis(α-nitronyl nitroxide) biradical: 4-methyl-2,6-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)phenol (NIT2PhOH). The crystal structure of this heterospin complex was determined using single-crystal X-ray diffraction analysis and exhibits four unusual seven-membered metallocycles formed from the coordination of oxygen atoms of the N-O groups and of bridging phenoxo (µ-PhO−) moieties with copper(II) ions. The crystal structure analysis reveals an incipient agostic interaction between a square planar copper center and a hydrogen-carbon bond from one methyl group carried on the coordinated nitronyl-nitroxide radical. The intramolecular Cu∙∙∙H-C interaction involves a six-membered metallocycle and may stabilize the copper center in square planar coordination mode. From the magnetic susceptibility measurements, the complex, which totals seven S = 1/2 spin carriers, has almost a ground state spin S = 1/2 at room temperature ascribed to strong antiferromagnetic interaction between the nitronyl nitroxide moieties and the copper(II) centers and in between the copper(II) centers through the bridging phenoxo oxygen atom.

Spin Alternation Rule in USCF for Through-Bond Magnetic Coupling─A New Look: Why and When Does It Arise and How To See It.

This work takes a new look at the spin alternation rule in unrestricted self-consistent-field (USCF) calculations in terms of structural characteristics such as periodicity, impurity location, and Coulomb exchange. For clarity, the systems considered are biradicals produced from linear conjugated hydrocarbons. Both site-parametrized Hamiltonian models for theoretical analysis and spin unrestricted density functional theory (DFT) calculations are used. Theoretical analysis leads to the following conclusions: (1) The diradical state is an excited state of a linear chain of N conjugated carbon atoms (when N is about ≤ 10). Spin alternation is a consequence of the (truncated) periodic symmetry combined with filling each closed-shell pi orbital with two electrons and each singly occupied molecular orbital (SOMO) with one electron. Spin polarization is evident in triplet (T) and broken symmetry (BS) solutions for an odd N and only in the T solution for an even N. Spin alternation is visible in the BS for an odd N and always remains muted in the calculated T. (2) For a doped chain with two radical centers, spin alternation is generally visible in the BS for an odd N. The sign of spin population on the radical centers in the BS indicates the stable spin. For radical centers separated by an odd (even) number of pz electrons, spin alternation favors T (S) state with FM (AFM) interaction. Spin oscillation remains less transparent for an even N without exchange. (3) In an unrestricted treatment with exchange, spin alternation becomes observable. Without SCF iterations, the more stable state can be identified from a clear spin oscillation in the BS. An irregular oscillation indicates a possible singlet ground state. These observations are supported by density functional calculations using the B3LYP functional and the 6-311+g(d,p) basis set on linear decapentaene diradicals with nitronyl nitroxide moieties substituted on two sets of conjugated atoms, (3,9) and (3,10). Because of the SCF procedure, one finds spin alternation in the T (BS) solution and erratic oscillation in the BS (T) solution of the 3,9 (3,10) diradical in respective equilibrium geometries. The ground state is T (S). DFT adiabatic coupling constants, SOMO energies, spin population plots, and SOMO lobe diagrams compare well with molecular electronic characteristics from theoretical analysis using Hamiltonian parameters.

Heteroatom-incorporated Trimethylenemethane: Synthesis and Properties of Triphenylphenylnitroxide–(Nitronyl Nitroxide) Dyad

Abstract We designed and synthesized a triphenylphenylnitroxide–(nitronyl nitroxide) dyad with a structure isoelectronic with trimethylenemethane. Crystal structure analysis demonstrated that the dihedral angle between the nitroxide and nitronyl nitroxide moieties was excessively small due to the steric effects of the triphenylphenyl moiety. The coplanarity of the two radical systems induced an excessively strong ferromagnetic intramolecular interaction (2J/kB ≥ +2000 K; H = −2JS1/2•S1/2).

Synthesis and Structure of (Nitronyl Nitroxide-2-ido)(tert-butyldiphenylphosphine)gold(I) and -(Di(tert-butyl)phenylphosphine)gold(I) Derivatives; Their Comparative Study in the Cross-Coupling Reaction

We synthesized two new organogold derivatives of a nitronyl nitroxide (NN) with phosphine ligands, namely NN-Au-PtBuPh2 and NN-Au-PtBu2Ph. They were characterized by X-ray diffraction analysis, cyclic voltammetry, and ESR, IR, and UV/Vis spectroscopy. The X-ray structural analysis revealed a tendency of the NN moiety to form a large number of short intermolecular contacts. This phenomenon is related to the anionic nature of the paramagnetic group NN, as evidenced by a significantly lower oxidation potential in comparison with purely organic derivatives of NN radicals. The cross-coupling reaction of NN-Au-PPh3, NN-Au-PtBuPh2, or NN-Au-PtBu2Ph with an activated bromoarene, namely, p-BrC6H4NO2, was investigated. It was shown that regardless of the presence of the bulky tert-butyl substituent, all gold derivatives have similar activities in the cross-coupling reaction and give a cross-coupling product, NN-C6H4NO2, with comparable yields.

Through the challenging synthesis of tetraphenylporphyrin derivatives bearing nitroxide moieties

Tetraphenylporphyrin derivatives a synthetic heterocycles with convenient preparation and a richness of properties which make them attractive in broad fields such as energy, life and materials sciences. Thus, in the quest for new radical architectures, tetraphenylporphyrins are prime candidates. To this end, we designed free-base tetraphenylporphyrins bearing nitronyl and imino nitroxide moieties covalently bonded to the para-position of the meso-phenyl substituent. Their detailed synthesis and characterization are reported here.

An approach to fluorinated phthalonitriles containing a nitronyl nitroxide or iminonitroxide moiety

A 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl lithium derivative was found to react with perfluorophthalonitrile with formation of 2-(3,4-dicyano-2,5,6-trifluorophenyl)- (1) and 2-(2-amino-4,5-dicyano-3,6-difluorophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl (2). The yields of 1 were near 15% independently of reaction time (from 2 to 6 h), whereas the yields of 2 considerably increased from a trace amount to 20%. Both nitronyl nitroxides were reduced to the corresponding iminonitroxides in the presence of NaNO2/AcOH with yields 80–95%. Molecular and crystal structures of the obtained nitroxides were solved by monocrystal X-ray diffraction analysis, and the nature of the radical was ascertained by electron spin spectroscopy (ESR) and SQUID magnetometry. Static magnetochemical measurements of 1 revealed domination of the ferromagnetic exchange interactions between the odd electrons of the paramagnetic centres. The exchange interactions probably proceed via short intermolecular contacts (3.021(3) and 3.05(2) Å) between the nitroxide O atoms and cyanide C atoms. Structural characteristics of the prepared nitroxides make them valuable precursors for the synthesis of paramagnetic macrocycles, which may serve as building blocks in the field of molecular magnetism.

Novel bis-catecholato heterospin manganese complexes

A tris-dioxolene manganese complex was synthesized and characterized (dioxolene- mono- and dianions of 3,6-DBQ-pipe, which is 3,6-di-tert-butyl-o-benzoquinone having a N,N’-piperazine bicycle as substituent in 4,5-positions). According to the bonds lengths distribution in the chelate cycles of the ligands the complex was identified as a bis-o-semiquinone-catecholato derivative of Mn(IV). Variable temperature magnetic susceptibility measurements indicate one unpaired electron per complex molecule as a result of strong intramolecular antiferromagnetic exchange interactions. The bis-o-semiquinone-catecholato manganese complex interacts with pyridine substituted by a nitronyl-nitroxide moiety forming a six-coordinate compound. The X-ray structural diffraction study of the complex executed at 100 K indicates a trans-arrangement of the pyridine ligands and the two catecholato dianionic chelates. Variable temperature magnetic susceptibility measurements of the complex with nitronyl-nitroxide give a magnetic moment of 5.5 µB which cannot be achieved in the spin system S = 3/2 (Mn4+) and 2 × S = 1/2 (two nitronyl-nitroxides). A valence tautomeric interconversion is proposed when the high multiplicity states [Mn3+(catecholate)(semiquinone)] and [Mn2+(semiquinone)2] are partly populated.

A Crystallographic Study of a Novel Tetrazolyl-Substituted Nitronyl Nitroxide Radical

Spin-labelled compounds are widely used in chemistry, physics, biology, and material sciences, but the directed synthesis of some functionalized organic radicals is still a challenge. We succeeded in the preparation of a tetrazolyl-substituted nitronyl nitroxide radical in pure crystalline form. According to the single-crystal X-ray data, intra- (NH…O, 2.43 Å) and inter-molecular hydrogen bonds (NH…O, 1.91 Å) are formed between NH groups of the tetrazole cycles and O atoms of the paramagnetic moieties. The intermolecular H-bonds connect the molecules forming chains along the a-axis. Moreover, there are short intermolecular contacts between the O atoms (3.096 Å) and between the O and C atoms (3.096 Å) of the nitronyl nitroxide moieties within the chain. The spin-unrestricted broken-symmetry calculations performed at the BS-UB3LYP/def2-TZVP level of theory predicted a sufficient ferromagnetic interaction (J ≈ 20 cm–1) between the adjacent radicals inside the chain, but a weak antiferromagnetic interaction (−J ≤0.2 cm−1) between the nearest radicals belonging to the different chains. Thus, a rare case when stable radicals, the tetrazolyl-substituted nitronyl nitroxides, are ordered into ferromagnetic chains was revealed; an investigation of the magneto-structural correlations inherent in the nitroxide radical will demand a special experiment in the sub-Kelvin regime.

Photo-physical properties of donor-acceptor-radical triad based on functionalized tetrathiafulvalene and nitronyl nitroxide radical

An acceptor-donor-radical triad based on tetrathiafulvalene (TTF)-fused (benzimidazol-2-yl)-pyridine (bzip) which is alkylated with a nitronyl nitroxide (NIT) radical derivative, has been designed and synthetized. Its X-ray structure has been refined showing a complex arrangement of head-to-tail dimers of the TTF-bzip moieties and a 1D network of short contacts between the NIT radicals. Its electrochemical behavior has been studied by cyclic voltammetry (CV) showing that the TTF and NIT moieties can be reversibly and independently oxidized. The occurrence of intramolecular ligand charge transfers (ILCT) of both TTF → bzip and TTF → NIT nature has been evidenced by UV–visible electronic absorption spectroscopy coupled to TD-DFT calculations. Moreover, these calculations and RPE measurements confirmed the radical form (S = ½) of the compound as well as the spin density mainly localized on the methyl-benzo-[1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl] unit. Both emissions of the NIT radical and TTF-based molecular skeleton have been observed with a tunable intensity ratio depending on the irradiation energy. Finally, this new triad may be a valuable candidate for the construction of multi-property compounds by coordination of metallic ions either on the TTF-based molecular skeleton or on the NIT moieties.

Bis-o-semiquinonato nickel complexes with pyridine and pyridine modified by nitronyl-nitroxide moiety

Four coordinated bis-o-semiquinonato nickel complexes interact with pyridine forming five- or six-coordinated compounds. Coordination number and structure of the product depends on the nature of o-quinonato ligand and synthetic conditions. (Py)2Ni(3,6-DBSQ)2·Tol has trans-arrangement of pyridine ligands whereas in it analog – (Py)2Ni(4,5-N,N′-pipe-3,6-DBSQ)2 pyridine molecules occupy cis-positions (3,6-DBSQ – anion-radical of 3,6-di-tert-butyl-o-benzoquinone; 4,5-N,N′-pipe-3,6-DBSQ – tricyclic 5,8-di(tert-butyl)-2,3-dihydro-1,4-ethanoquinoxaline-6,7-dione). The structure of six-coordinated compounds is close to octahedral which promotes high values of effective magnetic moments and the predomination of ferromagnetic exchange interactions. In spite of different geometries complexes demonstrate very close magnetic behavior. Using 1:1 stoichiometric ratio allows to obtain five-coordinated nonsolvated square-pyramidal complex (Py)Ni(3,6-DBSQ)2 with disordered position of pyridine. The last compound has essentially lower magnetic moment compared to six coordinated analogs. Complex reveals strong metal–ligand antiferromagnetic coupling due to deviation of nickel atom out of the bis-o-semiquinonato plane. Pyridine modified in 4-position by nitronyl-nitroxide radical (4-NIT-Py – 2-(4-pyridinyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) also coordinate to bis-o-semiquinonato nickel precursors forming (4-NIT-Py)2Ni(3,6-DBSQ)2 and (4-NIT-Py)2Ni(4,5-N,N′-pipe-3,6-DBSQ)2. Nickel complexes containing different types of free radical ligands were obtained for the first time. However two additional spins do not produce significant changes in their magnetic behavior.

Reaction of Paramagnetic Synthon, Lithiated 4,4,5,5-Tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide, with Cyclic Aldonitrones of the Imidazole Series.

It was shown that dipole-stabilized paramagnetic carbanion lithiated 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide can be attached in a nucleophilic manner to either isolated or conjugated aldonitrones of the 2,5-dihydroimidazole 3-oxide and 2H-imidazole 1-oxide series to afford adducts the subsequent oxidation of which leads to polyfunctional mono- and diradicals. According to XRD, at least two polymorphic modifications can be formed during crystallization of the resulting paramagnetic compounds, and for each of them, geometric parameters of the molecules are similar. An EPR spectrum of the diradical in frozen toluene has a complicated lineshape, which can be fairly well reproduced by using X-ray diffraction structural analysis and the following set of parameters: D=14.9 mT, E=1.7 mT; tensor a((14) N)=[0.260 0.260 1.625] mT, two equivalent tensors for the nitronyl nitroxide moiety a((14) N)=[0.198 0.198 0.700] mT, and g≈2.007. According to our DFT and ab initio calculations, the intramolecular exchange in the diradical is very weak and most likely ferromagnetic.

Molecular Conformations and Magnetic Parameters of the Compact Trimethylenemethane-Type Triplet Diradical

The ESR spectrum of compact nitroxide (NO)-substituted nitronyl nitroxide (NN) triplet diradical N-tert-butyl-N-oxidanyl-2-amino-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (1) was recorded in solid argon matrix at 15 K. The zero-field splitting (ZFS) parameters of 1 were derived from the recorded ESR spectrum: |D| = 0.0248 cm(-1) and E = 0.0025 cm(-1). Quantum chemical calculations have been performed using DFT and multiconfigurational ab initio (CAS) methods in order to establish equilibrium geometries of the conformational isomers resulting from twisted conformations of NO and NN moieties. The ZFS parameters of 1 were calculated at these levels of theory to test validity of the calculated structures. The calculation results were analyzed using the measured ZFS parameters and magnetic and structural data from the previous studies (Suzuki, S.; et al. J. Am. Chem. Soc. 2010, 132, 15908; Tretyakov, E. V.; et al. Russ. Chem. Bull. 2011, 60, 2608). It was found that the ab initio method is most successful for accurate predictions of molecular and magnetic parameters. Diradical 1 has only one stable enantiomeric pair in pseudoeclipsed conformations. The two chiral isomers exist in racemic crystals 1 and in solid matrices with molecular parameters close to those attributed to a free molecule. The analysis of the spin density distribution suggests that one unpaired electron occupies NO group at the equilibrium geometry, whereas the torsion of NO group governs the spin density distribution of the second unpaired electron on a conjugated fragment in NN group. The increase in planarity by torsion of NO group enhances the trimethylenemethane-type properties and, therefore, gives rise to larger ferromagnetic exchange interaction. More planar equilibrium geometry and greater (three times) exchange interaction constant J were predicted for hypothetical diradical 1a, where bulky tert-butyl group is replaced by a methyl group in the nitroxide fragment.

NCN Pincer–Pt Complexes Coordinated by (Nitronyl Nitroxide)-2-ide Radical Anion

New pincer-Pt complexes coordinated by (nitronyl nitroxide)-2-ide radical anion were prepared as stable compounds in high yields. The structures of these Pt complexes and the oxidized complexes were unequivocally determined by spectral and crystal structure analyses. The oxidation potential of the nitronyl nitroxide moiety in these complexes was shifted in the negative direction by ~0.6 V as a result of coordination to the Pt(II) atom.

“Jumping” crystals: structures and properties of CuII complexes with N-methylimidazolyl- and N-methyltriazolyl-substituted nitronyl nitroxides

To find the factors favorable for the appearance of chemomechanical activity of heterospin crystals, a series of new heterospin complexes were synthesized and characterized. It includes [Cu(tfac)2LIm2]·2CH2Cl2, [Cu(tfac)2LIm2]·2EtOH, [[Cu(pfu)2]2LIm3]·1/2CH2Cl2, [Cu(pfh)2LIm2]·1/2CH2Cl2, [Cu(piv)2LIm2]·2MeOH, [Co(piv)2LIm2], [\( Cu(hfac)_2 L^{CD_3 } _2 \)], [Cu(hfac)2LTr]2·CH2Cl2, and [Cu(hfac)2LTr2] (LIm, \( L^{CD_3 } \), and LTr are N-methylimidazolyl-, N-trideuteriomethylimidazolyl-, and N-methyltriazolyl-substituted nitronyl nitroxides, respectively; tfac, hfac, pfu, pfh, and piv are the charged coordinated diamagnetic ligands 1,1,5,5-tetrafluoropentane-2,4-dionate, 1,1,1,5,5,5-hexafluoropentane-2,4-dionate, 1,1,1,2,2,3,3,4,4,8,8,9,9,10,10,11,11,11-octadecafluoroundecane-5,7-dionate, 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dionate, and 2,2-dimethylpropionate, respectively). The crystal and molecular structures of all compounds were determined. The results of the X-ray diffraction study of the complex [Ni(hfac)2LIm2] synthesized earlier are reported. In the solid state of the complexes [Cu(pfh)2LIm2] and [Co(piv)2LIm2], the paramagnetic ligands are cis-coordinated to the central atom in a monodentate fashion via the donor N atom of the imidazole ring. In the dinuclear complexes [[Cu(pfu)2]2LIm3] and [Cu(hfac)2LTr]2, the paramagnetic ligands are also in cis positions but act as bridges through coordination of the donor N atoms of the azole ring and the O atom of the nitronyl nitroxide moiety to different Cu2+ ions. In the solid complexes \( Cu(hfac)_2 L^{CD_3 } _2 \) , [Cu(hfac)2LTr2], [Cu(tfac)2LIm2]·2CH2Cl2, [Cu(tfac)2LIm2]·2EtOH, and [Cu(piv)2LIm2]·2MeOH, the nitronyl nitroxide radicals in the mononuclear heterospin molecules are in trans positions. The packing motif in the crystal structures of the complexes \( Cu(hfac)_2 L^{CD_3 } _2 \) , [Cu(tfac)2LIm2]·2CH2Cl2, and [Cu(tfac)2LIm2]·2EtOH is the same as that in the previously studied complexes [M(hfac)2LIm2] exhibiting chemomechanical activity. Among the complexes under consideration, only crystals of \( Cu(hfac)_2 L^{CD_3 } _2 \) can exhibit chemomechanical activity, that is to make jumps upon heating or irradiation. The results of the present study suggest that the packing of the solid-state structure plays a key role in the generation of mechanical activity of the crystals.

Very Strongly Ferromagnetically Coupled Diradicals from Mixed Radical Centers. II. Nitronyl Nitroxide Coupled to Tetrathiafulvalene via Spacers

We predict large and positive intramolecular magnetic exchange coupling constants (J) for coupled diradicals constructed from nitronyl nitroxide and tetrathiafulvalene monoradical moieties. These diradicals have the general formula TTF-coupler-NN, where the couplers are mostly aromatic systems. Unrestricted density functional methodology (UB3LYP) has been used to optimize the molecular geometries of the triplet diradicals using the 6-311 g(d,p) basis set. This has been followed by single-point UB3LYP calculations for triplet and broken symmetry (BS) states using 6-311++g(3df,3pd) basis and the optimized triplet geometries. We find that the species comprising of ethylene (geminal coupling) and pyridine as couplers have singlet ground states whereas the other species have triplet ground states. These findings are in support of the spin alternation rule. The largest J value we predict is 648.6 cm(-1) for the molecule with the spacer pyrrole. We also determine the percent weightings of triplet and singlet components in the BS state, estimate the diradical nature, and calculate the relative weights of different singlet and triplet component functions in the BS solution in each case.

2-(1H-Indol-3-yl)-4,4,5,5-tetra-methyl-imidazolidine-1-oxyl 3-oxide.

In the title compound, C15H18N3O2, the plane of the indole ring system is twisted with respect to the plane of the nitronyl nitroxide moiety, exhibiting a dihedral angle of 21.61 (6)°. The crystal packing is stabilized by N—H⋯O hydrogen bonds and weak C—H⋯O inter­actions.

Magnetic and structural properties of monoradicals and diradicals based on thienyl-substituted nitronyl nitroxide

Magnetic properties of monoradicals and diradicals based on thienyl-substituted nitronyl nitroxide (NN) were investigated by measuring temperature and magnetic-field dependences of magnetization. The magnetic behavior of the monoradical 3-benzo[b]thienyl NN (3-BTHNN) is interpreted in terms of the quasi-2D ferromagnetic Heisenberg model with the coupling constant J/k=+0.16 K and J′ /k=+0.02 K, while the alternating antiferromagnetic (AFM) intermolecular interactions are observed in the monoradicals, 2- and 3-thienyl NN (2- and 3-THNN). In contrast, the magnetic behavior of the diradicals, 2,3- and 2,5-bis(NN)thiophene (2,3- and 2,5-THBNN), is explained by quite strong AFM intramolecular interactions between the two spin centers, each located on the two NN moieties. We discuss here the origin of the magnetic interactions observed in these radicals by considering their molecular and crystal structures determined by the X-ray diffraction techniques.

Electronic structures and magnetic properties of copper(II) complexes with axially coordinated nitronyl nitroxide radicals

Electronic structures and magnetic properties of Cu(II) complexes with axially coordinated nitronyl nitroxide radicals are discussed on the basis of computational study using density functional theory (DFT) calculation. The tendency of the effective exchange integrals J is examined by changing the position of the nitronyl nitroxide moiety. The spin density distributions for these complexes are also estimated in order to understand the mechanism of the ferromagnetic interaction. The relationship between the geometrical parameters and the magnetic interaction indicates the delocalization of the unpaired electron from the π ∗ orbital of the nitronyl nitroxide group to the 3 d z 2 atomic orbital of the Cu(II) ion. On the other hand, there is practically no dependence of the overlap between the 3 d x 2 - y 2 atomic orbital of the Cu(II) ion and the π ∗ orbital of the nitronyl nitroxide group on the magnitude of the ferromagnetic coupling. It is revealed that the delocalization mechanism is important for the ferromagnetic properties in Cu(II) complexes with axially coordinated nitronyl nitroxide radicals.