Nitronyl Nitroxide Research Articles

Planar Benzo[1,2- b:4,5- b']dithiophene Derivatives Decorated with Nitronyl and Imino Nitroxides.

Four weakly antiferromagnetic interacting biradicals of benzo[1,2- b:4,5- b']dithiophene (BDT) and BDT extended with two thiophenes (BDTTh2) linked with nitronyl and imino nitroxides (NN and IN) as BDT-NN, BDT-IN, BDTTh2-NN, and BDTTh2-IN were designed, synthesized, and characterized. Short intermolecular π-π distances were found (3.42 Å) for BDT-NN, whereas larger ones were found for BDT-IN (3.54 Å) and BDTTh2-NN (3.67 Å), respectively. Intramolecular magnetic interaction ( Jintra,exp/ kB) of BDT-NN (-26 K) is much larger than for BDT-IN (-5.3 K), while it is reduced for the dithiophene-extended molecule BDTTh2-NN (-2.3 K). Intermolecular interactions ( zJinter,exp/ kB) of BDT-NN (-6.5 K) and BDT-IN (-6.0 K) are stronger than for BDTTh2-NN (-4.6 K). Such large intermolecular couplings resulting from good π-stacking mark BDT-IN and BDTTh2-NN as promising crystalline materials with similar sized Jintra and Jinter. In addition, we also extracted a coupling within the chain of Jchain/ kB = -2.2 K and a coupling between the chains of zJinterchain = -1.5 K for BDTTh2-NN by a Heisenberg chain model. Intra- and intermolecular interactions and spin densities were examined by DFT studies.

Crystal Structure of Metal Complexes with 2-Imidazoline Nitroxides and Dicyanamide

The structure of heterospin complexes[M(DCA)2L2x]·yH2O, where M = Mn, Co, Ni, Cu, Lx is the nitronyl nitroxide radical; DCA is the dicyanamide anion, y = 0, 1, or 2 is studied. It is found that the nature of[M(DCA)2L2x]·yH2O is characterized by the formation of multiple hydrogen bonds generating ribbons, layers, or frameworks in the crystals. The analysis of magnetostructural correlations by means of band calculations of the electronic structure confirms the occurrence of ferromagnetic exchange between the unpaired electrons of paramagnetic centers in solid [Cu(DCA)2L21]·2H2O when the distances between the copper ion and the coordinated N-O group are short.

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.

Rational Design and Synthesis of a Chiral Lanthanide-Radical Single-Chain Magnet.

By the reaction of an enantiopure nitronyl nitroxide radical, 2-((1 R)-(-)-myrtenal)-4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide), abbreviated as (1 R)-(-)-myrtenal-NIT, and Dy(hfac)3·2H2O (hfac = hexafluoroacetylacetonate), we successfully obtained a homochiral DyIII-radical chain compound [Dy(hfac)3(1 R)-(-)-myrtenal-NIT] n (1) with full characterization. 1 crystallizes in the chiral P21 space group and exhibits slow magnetization relaxation, holding an effective energy barrier (Δτ/ kB) of 64.6 K (44.9 cm-1) and an attempt time τ0 of 1.0 × 10-7 s. The correlation energy (Δξ/ kB) of this chain is 10.4 K (7.2 cm-1), and the effective Curie constant is 2.8 cm3·K·mol-1. Additionally, 1 exhibits temperature-dependent hysteresis loops below 4 K, with a maximum coercive field of 0.51 T at 2 K.

A novel nitronyl nitroxide and its copper complexes: Synthesis, structures and magnetic properties

A new nitronyl nitroxide NIT-5Im-3Py (1) and its copper complexes [Cu(hfac)2]4(NIT-5Im-3Py)2 (2), [Cu(hfac)2NIT-5Im-3Py]n (3) (NIT-5Im-3Py = 2-(5-(1-imidazole)-3-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; hfac = hexafluoroacetylacetonate) have been successfully synthesized and the molecular structures were elucidated by single-crystal X-ray structural analysis. In complex 2, NIT-3Py-5Im acts as bridge ligand to link four Cu(II) ions leading to {Cu4} complex while complex 3 displays a 1D-chain structure. Magnetic studies show that antiferromagnetic interactions dominate in complex 2, while there exist strong ferromagnetic interactions between Cu(II) and coordinated NO group in complex 3.

Conjugated Oligomers with Stable Radical Substituents: Synthesis, Single Crystal Structures, Electronic Structure, and Excited State Dynamics

Understanding and controlling spin dynamics in organic semiconductors is of significant technological interest. We present a comprehensive joint experimental and computational study elucidating excited-state dynamics and kinetics of oligothiophenes covalently linked to two radicals. The synthesis, steady-state, and ultrafast photophysics, magnetic properties, computational modeling, and single crystal X-ray diffraction of a series of oligothiophenes with appended nitronyl nitroxide (NN) diradicals (RAx and RBx) are presented. We show that incorporation of the diradicals results in an intriguing molecular packing that is reminiscent of organic cages, unusual excited-state dynamics, and interesting photophysical and magnetic properties. We find an increase in the distance and dihedral angle between the diradical rings and the oligothiophene core result in weak antiferromagnetic interactions. Single crystal X-ray diffraction and computational modeling suggest that efficient conjugation along the backbone lea...

Lanthanide Tetranuclear Cage and Mononuclear Cocrystalline Nitronyl Nitroxide Complex with Single-Molecule-Magnet Behavior.

The first tetranuclear metallacage and mononuclear cocrystalline DyIII-radical complex was synthesized and characterized. The metallacage in [Dy4(hfac)8(IMPhThio)2(OH)4][Dy(hfac)3(NITPhThio)2] consists of Dy(hfac)2+ groups bridged by OH- and the IMPhThio radical. Field-induced single-molecule-magnet behavior was observed in the nitronyl nitroxide radical-bridged polynuclear complex.

Two new lanthanide-nitronyl nitroxide complexes: Magnetic and fluorescence properties

Two new lanthanide complexes of nitronyl nitroxide radical have been synthesized and characterized on the structure, magnetism and fluorescence: [Ln(hfac)3(NITPh-(OCH3)2)2](Ln = (Tb(1),Dy(2)), (NITPh-(OCH3)2 = 2-(3′,4′-dimethoxy-phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide); hfac = hexafluoroacetylacetonate). Two complexes crystallize in monoclinic space group C2/c, which have similar mononuclear structures. The central metal ion is eight coordinated by six oxygen atoms from three hfac molecules and two oxygen atoms from two nitronyl nitroxide radicals. The varying temperature magnetic susceptibility indicates that there are weak antiferromagnetic interaction between metal ions and nitronyl nitroxide radicals in complexes 1 (zJ′ = −0.145 cm−1) and 2 (zJ′ = −0.140 cm−1). Ac magnetic susceptibility studies exhibit no single-molecule magnet behavior in complexes 1 and 2. In addition, the fluorescence properties of 1 show that it is an excellent fluorescent probe of recognizing Cr2O72− anion. Because of the linear relationship at low concentration and detection limit reached 1 × 10−7 M, it can be used as a luminescence-based sensor for quantitative analysis.

Magnetic relaxation in mononuclear Tb and Dy complexes involving chelate nitronyl nitroxide ligand

Three new lanthanide-nitronyl nitroxide radical complexes [Ln(tfac)3(NIT-2thz)] (Ln(III) = Gd 1, Tb 2, Dy 3; tfac = trifluoroacetylacetonate; NIT-2thz = 2-(2′-thiazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been synthesized. Single crystal X-ray diffraction analyses reveal that complexes 1–3 are isomorphous. They all crystallized in the C2/c space groups. Each central Ln(III) ion is eight-coordinated by three bischelate tfac ligands and one radical ligand. More specifically, NIT-2thz acts as a chelate ligand to link one Ln(III) ion through its nitrogen atom of the thiazole ring and oxygen atom of the N–O group to form a two-spin system. Magnetic studies show that Gd complex exhibits ferromagnetic Gd(III)–radical coupling, Tb and Dy complexes exhibit frequency-dependent out-of-phase signals.

A novel rare-earth nitronyl nitroxide radical complex as a high-efficiency sensor for Cr3+ and Cr2O72− ions in aqueous solutions

In this article, a new complex with formula [Eu(hfac)3(NITPh-DOMe)2] (1) (hfac = hexafluoroacetylacetonate, NITPh-DOMe =2-(3′,4′-dioxylmethylene-phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) has been successfully synthesized. Central Eu(III) ions of complex 1 are eight coordinated by two NITPh-DOMe radicals and three hexafluoroacetylacetonate ligands. The luminescent explorations show that complex 1 with high sensitivity, selectivity, almost linear at low concentration and the low detection limit for detection of Cr3+ (1 μM) and Cr2O72− (0.1 μM) ions in aqueous solutions.

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.

Theoretical insights into the magneto-structural correlation: Comparison between series of copper(I) and silver(I) metal complexes with nitronyl nitroxide radicals

Using nitronyl nitroxide radicals, we presented a comparative study of magneto-structural correlations between series of CuI and AgI models (Cu(a) ∼ Cu(d) and Ag(a) ∼ Ag(d), modified from [CuI(immepy)2]PF6 and [AgI(impy)2]PF6 (Oshio et al., 1997). Cu(a) ∼ Cu(d) possess three characteristics that are significantly different from Ag(a) ∼ Ag(d) for the more utilization of CuI than that of AgI in the ferromagnetic joints of radical networks. (1) Structural shorter Cu-N length in Cu(a) ∼ Cu(d) than corresponding Ag-N length in Ag(a) ∼ Ag(d) leads to the quite larger Wiberg bond index (WBI), ∇2ρ(r) values, and metal atomic spin densities between triplet (T) and broken-symmetry (BS) states (ΔρT-BS) of the former than that of the latter. (2) From each of AgI (Ag(a) ∼ Ag(d)) to the corresponding one of CuI (Cu(a) ∼ Cu(d)), the spin density increases up to one or two order of magnitudes, which indicates the more notable spin delocalization from ONCN groups (paramagnetic centers) to CuI than that to AgI. Changing the dihedral angle (θ) between two radical components, the variation of the magnetic coupling constants (Jab) is directly proportional to the spin density of CuI but inversely proportional to that of one ONCN group. (3) Due to the larger d orbital distributions of CuI than that of AgI in the SOMOs, large metal-ligand charge transfer (MLCT) occurs in Cu(a) ∼ Cu(d) but little in Ag(a) ∼ Ag(d). The broken orthogonality is not evidently associated with the significantly different magnetic coupling interactions between CuI and AgI models.

Nitronyl nitroxide radicals at the interface: a hybrid architecture for spintronics

Cross-fertilization between molecular magnetism and organic spintronics is leading to the development of concepts based on the use of molecules as active elements to influence spin-related transport processes. The research on hybrid devices, where the magnetic molecules in contact with the electrodes influence the spin and charge injection and transport, is moving its first steps but is expected to quickly expand the technological potential of molecular spintronics and quantum computing. New exciting possibilities, linked to the individual properties of these molecular units and to their interaction with novel substrates, are getting disclosed. The chemical functionalization of these molecules is the tool which allows to tune their electronic and magnetic properties and to directly create these hybrid architectures. However, the coupling of molecules with the spin transport phenomena is far from being trivial. First, the stability of molecules in the device environment must be tested and, subsequently, the organization of molecules in the desired architectures must be mastered permitting a careful control of the interactions between inorganic substrates and molecular layers. Here we summarize how this research activity can be developed in the case of one of the simplest magnetic molecules, an organic radical. We will start from an innocent surface, such as gold, to move then toward a real-device environment. We evidence how these efforts can result in a surface-specific molecular-based method to influence the spin injection and transport phenomena, paving the way for developing new devices in which a fine-tuning of magnetic features is required.

Overcoming the Complex Excited-State Dynamics of Colloidal Cadmium Selenide Nanocrystals Involved in Energy Transfer Processes

Semiconductor nanocrystals are often characterized by complex excited-state dynamics which reflect the inhomogeneous character of ensemble of nanocrystals. A new hybrid inorganic–organic donor–acceptor system involving CdSe nanocrystals and paramagnetic nitronyl nitroxide free radicals is shown to lead to efficient Forster (dipolar) resonance energy transfer. This transfer process, which is monitored by steady-state and time-dependent photoluminescence quenching experiments, occurs on a time scale similar to that of the intrinsic recombination in CdSe nanocrystals, allowing to unravel some of the complexity associated with the excited-state photophysics of semiconductor nanocrystals. A Stern–Volmer formalism that can handle the multiexponential nature of the time-dependent excited-state kinetics of CdSe nanocrystals is developed, leading to excellent agreement between steady-state and time-dependent photoluminescence data when a log-normal distribution model is used for the intrinsinc recombination rate c...

Construction and Magnetic Study of One‐Dimensional Lanthanide–Radical Chains Involving Pyridinone‐Substituted Nitronyl Nitroxide Radicals

A new nitronyl nitroxide radical‐containing pyridinone group (NIT‐PhPyO, 1) has been synthesized {NIT‐PhPyO = 2‐[4‐(4‐pyridinone)phenyl]‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide}. By using this functionlized nitronyl nitroxide, two isomorphic lanthanide–nitronyl nitroxide radical compounds [{Ln(hfac)3}3(NIT‐PhPyO)2]n (Ln = Gd 2 and Dy 3; hfac = hexafluoroacetylacetonate) have been successfully obtained. X‐ray spectroscopic analysis reveals that complexes 2 and 3 possess a 1D chain structure with Ln2O2 binuclear units, which represent the first examples of Ln–radical chains involving binuclear Ln2O2 units. Analysis of the magnetic data indicates that a weak antiferromagnetic coupling exists between two Gd3+ ions, while the magnetic interaction between the GdIII ion and the coordinated nitroxide group is ferromagnetic. Alternating current (ac) magnetic studies indicate that compound 3 displays slow magnetic relaxation behavior at low temperature.

Radical Cation π-Dimers of Conjugated Oligomers as Molecular Wires: An Analysis Based on Nitronyl Nitroxide Spin Labels.

Nitronyl nitroxide (NN)-substituted conjugated oligomers, which were expected to self-associate in biradical cation states, were designed to analyze the capability of π-dimers as molecular wires. The oligomer moieties were composed of dithienyl-N-methylpyrrole with methoxy substituents at the inner β-position of thiophene rings (DTP-NN. ) and its propylenedioxythiophene (ProDOT) inserted derivative (DTP-P-NN. ), or two ethylenedioxythiophene (EDOT) and two ProDOT units (E2 P2 -NN. ). Among them, chemical one-electron oxidation gave biradical cations (DTP-P).+ -NN. and (E2 P2 ).+ -NN. that formed π-dimers (DTP-P-NN. )22+ and (E2 P2 -NN. )22+ in dichloromethane at low temperatures. ESR studies of (DTP-P-NN. )22+ and (E2 P2 -NN. )22+ showed the presence of a relatively strong exchange interaction between two NN radicals through the radical cation π-dimer moieties. DFT calculations supported these experimental results and predicted that exchange interactions between two NN radicals were comparable or stronger than those through covalently linked quaterthiophene. Thus, the conjugated oligomer radical cation π-dimers acted as efficient molecular wires for electronic communication.

New 2p‐3d‐4f Chain Compounds [LnZn(hfac)5(NIT‐Pyrim)2] constructed from Pyrimidine based Nitronyl Nitroxides

The 1:1:2 mixture of Ln(hfac)3, Zn(hfac)2, and NIT‐Pyrim (hfac = hexafluoroacetylacetonate, NIT‐Pyrim = 2‐pyrimidine‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide) afforded a series of 2p‐3d‐4f magnetic chains [Ln(hfac)3Zn(hfac)2(NIT‐Pyrim)2] [LnIII = Gd (1), Ho (2), Yb (3)], in which Zn(hfac)2 and Ln(hfac)3 units are bridged by pyrimidine substituted nitronyl nitroxides through their NO moieties and pyrimidine nitrogen atoms. These complexes represent the first examples of 2p‐3d‐4f complexes with ZnII ions. Magnetic studies show that there exist ferromagnetic exchange couplings between the coordinated NO groups of radical ligands and the GdIII ions.

Three cyclic lanthanide-radical complexes syntheses, crystal structures and magnetic properties

Three lanthanide–nitronyl nitroxide radical complexes [Ln(hfac)3(NIT-2Br-4Py)]2 (Ln(III) = Gd (1), Tb (2), Dy (3); hfac = hexafluoroacetylacetonate; NIT-2Br-4Py = 2-(2′-bromo-4′-pyridyl)-(4,4,5,5-tetramethyl-3-oxylimidazoline-1-oxide)) have been synthesized. Single crystal X-ray diffraction analysis reveals that complexes 1–3 are isomorphous in which NIT-2Br-4Py acts as a bridging ligand linking two Ln(III) ions through the nitrogen atom of pyridine ring and the oxygen atom of NO group to form a four-spin system. The magnetic studies show that ferromagnetic coupling interaction between Ln(III) ions and nitronyl nitroxide radical. Moreover, ac measurements show that Tb(III) complex exhibits frequency-dependent signals at low temperature.

Lanthanide-Nitronyl Nitroxide Chains Derived from Multidentate Nitronyl Nitroxides.

Unprecedented lanthanide (Ln)-radical loop-chain coordination polymers were achieved using multidentate pyridyl- or triazole- substituted nitronyl nitroxide ligands. Their magnetic units consist of ferromagnetic [Ln2Radical] moieties, leading for the dysprosium(III) derivatives to slow relaxation of magnetization, which was found to be dependent on the heterocyclic ligands.

Electron Spin Resonance Study of Molecular Orientation and Dynamics of Phenyl Imino and Nitronyl Nitroxide Radicals in Organic 1D Nanochannels of Tris( o-phenylenedioxy)cyclotriphosphazene.

Imino and nitronyl nitroxide (IN and NN, respectively) radicals such as phenyliminonitroxide (PhIN) and phenylnitronylnitroxide (PhNN), respectively, were dispersed in the organic 1D nanochannels of tris( o-phenylenedioxy)cyclotriphosphazene (TPP). Electron spin resonance (ESR) measurements were conducted on these inclusion compounds (ICs) in the temperature range 4.2-300 K. The modulated-septet ESR spectra of TPP ICs using PhIN observed in the range 165-258 K were reproduced with the EasySpin program package using a model in which some of the PhIN molecules underwent uniaxial rotational diffusion in the TPP nanochannels around the molecular long axis corresponding to the principal y-axis of the g tensor. However, for the TPP IC using PhNN, complicated ESR spectra were observed, which were not consistent with the modulated quintet observed in solution or in the fast-motion limit in solids. These spectra were reproduced by the superposition of a quintet originating from rotational diffusion of PhNN molecules and a septet based on rotational diffusion of PhIN molecules generated in the synthetic process. The rotational diffusion activation energies of PhIN and PhNN in the TPP nanochannels were estimated to be 19 and 45 kJ mol-1 using an Arrhenius plot, respectively. These were consistent with that for NN radicals in the 1D nanochannels of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) (37-54 kJ mol-1) with a larger pore diameter than TPP reported in our previous study, or with that for 4-substituted-2,2,6,6-tetramethyl-1-piperidinyloxyl (4-X-TEMPO) in TPP nanochannels (5-26 kJ mol-1) with regard to molecular size or host-guest or guest-guest interactions. These results indicate that not only the NN group but also IN may be used for the clarification of chemical or biological structures of nanomaterials such as nanosized cavities.