Imino Nitroxide Radical Research Articles

An Unprecedented Asymmetric End-On Azido-Bridged Copper(II) Imino Nitroxide Complex: Structure, Magnetic Properties, and Density Functional Theory Analysis

The dinuclear copper(II) complex [Cu2(mu(1,1)-N3)2(im-2py)2(N3)2] [im-2py = 2-(2-pyridyl)-4,4,5,5-tetramethylimidazolinyl-1-oxy] has been prepared and structurally characterized. The crystal structure consists of a dinuclear unit in which the Cu(II) ions are bridged by two azido ions in a end-on asymmetric fashion and the imino nitroxide radicals are chelating by the two imino N atoms. Accordingly, the magnetic susceptibility data were analyzed considering a linear spin-coupling scheme rad(1)-Cu(2)-Cu(3)-rad(4) (with Si = 1/2, i = 1-4), where the Heisenberg spin Hamiltonian assumes the general form -2Sigma(i)<(j)S(i)S(j). Considering only first-neighbor spin-coupling constants (J13 = J24 = J14 = 0), magnetic susceptibility measurements show that the copper(II) imino nitroxide rad-Cu-(Cu-rad)(rad-Cu)-Cu-rad exchange coupling is ferromagnetic and large (J12 = J34 = J1 > +190 cm(-1)), as is expected for copper imino nitroxide species, and the copper-copper (rad)-Cu-Cu-(rad) coupling through the asymmetric double end-on azide bridges appeared antiferromagnetic and rather large [J23 = J2 = -43(2) cm(-1)]. By contrast, a density functional theory analysis of the system through the computation of broken-symmetry-state energies resulted in J2 approximately 0 cm(-1). This apparent paradox is resolved by introducing a second-neighbor rad-(Cu)-Cu-(rad)(rad)-Cu-(Cu)-rad spin-coupling constant J13 = J24 = J3, which turns out to be antiferromagnetic both experimentally (when J2 is set equal to zero) and computationally.

Synthesis, crystal structure and magnetic properties of a one-dimensional chain polymer, [Ni(IM2-Py)2(NO3)](NO3)

A novel complex [Ni(IM2-Py)2(NO3)](NO3), where IM2-Py is 2-(2′-pyridyl)-4,4,5,5-tetramethyl-2-imidazoline-1-oxyl, has been synthesized and characterized structurally and magnetically. The structure consists of [Ni(IM2-Py)2(NO3)]+ and a anion. The nickel(II) is in a distorted octahedral environment with four nitrogen atoms from two IM2-Py units and two oxygen atoms from a anion. The units of [Ni(IM2-Py)2(NO3)]+ were connected as a one-dimensional polymer by intermolecular interactions. Variable temperature magnetic susceptibility data show antiferromagnetic interactions between the nickel(II) and imino nitroxide radicals.

A novel one-dimensional copper(II) imino nitroxide polymer

A copper(II) complex [Cu(im2-py)(4,4′-bipy)(NO3)](NO3)·1.5H2O (im2-py = 2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl; 4,4′-bipy = 4,4′-bipyridyl) has been synthesized by reaction of Cu(NO3)·3H2O with im2py and 4,4-bipyridyl in methanol solution. Its crystal structure has been determined by X-ray diffraction. The structure shows that each copper ion is coordinated by a bidentate imino nitroxide radical, two 4,4′-bipyridyl ligands and a nitrate group to form a distorted square pyramidal environment. The crystal structure consists of chains of copper ions linked by 4,4′-bipyridyl.

Radical-dependent magnetic interactions in two 1-D copper(II) complexes with nitronyl nitroxide and imino nitroxide radicals bridged by iminodiacetate anion

Two novel one-dimensional (1-D) chain complexes [Cu(NIT3Py)(ida)] n ( 1) and [Cu(IM4Py)(ida)] n ( 2) (NIT3Py = 2-(3′-pyridinyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, IM4Py = 2-(4′-pyridinyl)-4,4,5,5-tetramethylimidazoline-1-oxyl and ida = iminodiacetate anion), were synthesized and characterized structurally and magnetically. The two complexes crystallize in neutral 1-D chains, where Cu(II)–nitroxide cationic units are linked by iminodiacetate anion ligands. The magnetic measurements show that complex 1 exhibits ferromagnetic coupling, while complex 2 exhibits antiferromagnetic interactions between the Cu(II) ion and the nitroxide radical.

Triaqua(pyridine-2,5-dicarboxylato-κ2N,O2)[4,4,5,5-tetramethyl-2-(4-pyridyl)imidazoline-1-oxyl-κN2]zinc(II)

The title compound, [Zn(C7H3NO4)(C12H16N3O)(H2O)3], was prepared from the imino nitroxide radical ligand 2-(4-pyrid­yl)-4,4,5,5-tetra­methyl­imidazoline-1-oxyl, zinc(II) perchlorate and potassium pyridine-2,5-dicarboxyl­ate. The ZnII ion exists in a distorted octa­hedral coordination geometry with one pyridyl N atom from a radical ligand, one N and one O atom from a bidentate pyridine-2,5-dicarboxyl­ate ligand and three O atoms from water mol­ecules. A two-dimensional network is formed through O—H⋯O hydrogen-bonding inter­actions.

A quantum chemical study of magnetic interactions in phenazine derivatives for chemical or photo-induced organic magnets

Donor-radical model systems for chemical or photo-induced magnets are proposed, and their electronic states and intramolecular magnetic interactions are investigated by Kohn-Sham hybrid density functional theory (KS-HDFT). 5,10-dihydrophenazine is employed as a donor and nitronyl- or imino-nitroxide radicals as stable radical substituents. The models mostly provide ferromagnetic intramolecular spin alignment in hole doping. In addition, the inductive effect on intramolecular magnetic interactions is examined by substituting the 5- and 10-positions of dihydrophenazine by several electron-withdrawing and -donating groups. The model compounds exhibit appropriate magnetic interactions and ionization potentials to allow them to serve as components of chemical or photo-induced magnets.

Design, synthesis and electronic states of an iminonitroxide radical with excited quartet high-spin state and photo-induced electron transfer in its CT complex

We report the design, synthesis and electronic state of a π -conjugated stable iminonitroxide radical, 1 , the electron donor property of which is improved by an attachment of dimethylamino group. The photo-excited quartet high-spin state was observed by a time-resolved ESR experiment. In this paper, we report the design, synthesis and electronic state of a π-conjugated stable iminonitroxide radical, 1 , the electron donor property of which is improved by an attachment of dimethylamino group. The photo-excited quartet high-spin state was observed by a time-resolved ESR experiment. CV measurement has clarified that the π-HOMO of the ground state is located higher ca. 0.35 eV in energy than 2 reported previously. These results show that the electron donor property is improved and the nature of the photo-excited spin alignment is conserved. Two CT complexes ( 1 -TCNQ and 1 -BQF 4 ) were synthesized using 1 as an electron donor. Their complexes have shown strong CT bands. The time-resolved ESR signal without the fine-structure splitting was observed for 1 -TCNQ CT complex. The signal may arise from the photo-induced electron transfer from the quartet excited state of 1 to the TCNQ acceptor.

The emitting state of the imino nitroxide radical

The solid-state low-temperature luminescence spectrum of an uncoordinated imino nitroxide radical, 2-(2-imidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl, is reported. The luminescence maximum is at 16 300 cm −1 and the band has a width at half height of 3140 cm −1 with a prominent vibronic progression interval of 1150 cm −1. Corresponding spectroscopic results are shown for the nitronyl nitroxide analog, where a lower-energy luminescence maximum at 14 700 cm −1 and a narrower band with a width at half height of 1900 cm −1 are observed. These differences are analyzed in terms of the different delocalized π-systems of the radicals.

Synthesis, structure and magnetic properties of lanthanide(III) complexes with new imidazole-substituted imino nitroxide radical

Synthesis, characterization and magnetic properties of new lanthanide–radical complexes, [Ln III(hfac) 3(IM2imH)] (Ln = Gd, Tb; IM2imH = 2-(2-pyridyl)-4,5-dihydro-4,4,5,5-tetramethyl-1 H-imidazolyl-1-oxy), are described. The molecular structure of the [Tb(hfac) 3(IM2imH)] has been determined by the X-ray diffraction. The magnetic susceptibility data for [Gd(hfac) 3(IM2imH)] show that the Gd–IM2imH magnetic interaction is antiferromagnetic with an exchange coupling constant J = −2.59 cm −1 in contrast to the ferromagnetic interaction in most of Gd(III) complexes containing paramagnetic center, which will be examined in connection with planarity of the IM2imH chelate.

Crystal Structure of a Novel One-dimensional Chain Compound Formed by Zinc(II) and Imino Nitroxide Radicals

The structure of novel complex formed by zinc(II) and imino nitroxides of formula [Zn(IM2-Py)2(NO3)]·(NO3) (IM2-Py = 2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl) was determined by the X-ray diffraction method. The compound crystallizes in orthorhombic, space group P212121 with a = 11.026(2), b = 13.575(3), c = 20.435(4)A, V = 3058.6(11)A3, Z = 4. The final R value is 0.0565 for 2637 reflections (I > 2σ(I)). X-ray analysis reveals that the crystal structure consists of [Zn(IM2-Py)2(NO3)]+ moiety and a NO3- anion. The zinc(II) ion is six-coordinated with a distorted octahedral geometry. The units of [Zn(IM2-Py)2(NO3)]+ and NO3- anions were connected as a one-dimensional chain compound by the intermolecular interaction.

Anomalous Diamagnetic Behaviour of the 1H NMR Spectra of a Coordinated Imino Nitroxide Radical in Diamagnetic and Paramagnetic Lanthanide(III) Complexes

AbstractDiamagnetic lanthanide(III) complexes with an imino nitroxide radical IM2py [2‐(2′‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazol‐1‐oxyl], [Ln(hfac)3(IM2py)] (Ln = Y and Lu; hfac = hexafluoroacetylacetonate), exhibit 1H NMR chemical shifts for the pyridyl protons of the coordinated IM2py suggesting it behaves as if it were diamagnetic. To a series of paramagnetic lanthanide complexes of this type, the structural analyses of the lanthanide induced shifts (LIS) of the hfac 3‐proton and pyridyl protons in IM2py were successfully applied, showing not only the diamagnetic behaviour of the pyridyl protons in the IM2py group, but also the dominant dipolar contribution and isostructural nature with apparent or effective axial symmetry in solution irrespective of the asymmetry in the solid state. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2004)

2,6‐Bis(pyrazolyl)pyridine Functionalised with Two Nitronylnitroxide and Iminonitroxide Radicals

AbstractDuring the last two decades, growing interest in the field of material chemistry has been focused on the synthetic design of organic high‐spin molecules for use as novel magnetic materials. Conjugated radicals based on the bis(pyrazolyl)pyridine core are very attractive in principle, since they per se combine optical, chelating and magnetic properties on a single molecular entity. The functionalisation of this core via the bis(4′‐formyl) derivatives with Ullman radicals results in novel functional biradicals, which may further be used for supramolecular architectures. In this paper we describe the synthesis, structure and properties of 2,6‐bis[4′‐(3‐oxo‐1‐oxyl‐4,4,5,5‐tetramethylimidazolin‐2‐yl)pyrazol‐1′‐yl]pyridine (Pz2PyNN) and 2,6‐bis[4‐(1‐oxyl‐4,4,5,5‐tetramethylimidazolin‐2‐yl)pyrazolyl]pyridine (Pz2PyIN). The two biradicals can clearly be differentiated by UV/Vis, IR and EPR spectroscopy. The optical absorptions of the blue Pz2PyNN appear around λmax= 610 nm (ϵ ≈︁ 1700 M−1 cm−1) while for Pz2PyIN the orange‐red colour arises from absorptions at λmax= 468 nm (ϵ ≈︁ 1400 M−1 cm−1). A triplet ground state for the bis(nitronylnitroxide) Pz2PyNN with ΔES‐T ≈︁ 11.8 ± 4.8 cm−1 can be deduced from the EPR studies because of the increase of the double integrated intensity times temperature at very low temperatures, indicating complete population of the triplet state. For the related bis(iminonitroxide) Pz2PyIN a much smaller (at least one order of magnitude lower) singlet−triplet energy gap with an upper limit of 0.7 cm−1 has been derived (0.7 ⪖ ΔES‐T ⪖ −0.07 cm−1). The new multifunctional biradicals represent the first example of high‐spin molecules in the bis(pyrazolyl)pyridine family. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2004)

Synthesis and characterization of [Co(NO2)2(acac)(IMH2py)] with one-electron-reduced imino nitroxide radical associated with unusual displacement of acetylacetonate in the starting complex trans-Na[Co(NO2)2(acac)2

A reaction of trans-Na[Co(NO2)2(acac)2] with IM2py(2(2′-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl) in methanol afforded trans-[Co(NO2)2(acac)(IMH2py)](IMH2py=1-hydroxyl-2(2′-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole); one-electron reduction of the N–O radical moiety in IM2py and displacement of one of the two acac ligands with retention of two nitrito ligands in the starting complex during the reaction. This new complex was characterized by UV–Vis, 1H NMR spectra and X-ray analysis.

A one-dimensional polymeric nickel(II) complex of an imino nitroxide radical with thiocyanate bridges

The title compound, catena-poly­[[[2-(2-pyridinyl)-4,4,5,5-tetra­methyl­imidazolin-1-oxyl]­thio­cyanato­nickel(II)]-μ-thiocyanato], [Ni(NCS)2(C12H16N3O)]n, was prepared from the imino nitroxide radical (IM2py), nickel(II) nitrate and potassium thio­cyanate. The crystal structure consists of neutral chains of [Ni(IM2py)(SCN)] units linked by further thio­cyanate as bridges. The Ni—NCS—Ni chains form a zigzag structure. Each NiII ion is located in a distorted square-pyramidal environment. The basal plane positions are occupied by two N atoms from the imino nitroxide radical and two thio­cyanate N atoms, one from the terminal thio­cyanate group and the other from the bridging thio­cyanate group. The apical position is occupied by an S atom from the bridging thio­cyanate group.

Detection of nitric oxide from pig trachea by a fluorescence method

A nitronyl nitroxide radical covalently linked to an organic fluorophore, pyrene, was used to detect nitric oxide (NO) from freshly excited tissues. This approach is based on the phenomenon of the intramolecular fluorescence quenching of the fluorophore fragment by the nitroxide. The pyrene-nitronyl (PN) reacts with NO to yield a pyrene-imino nitroxide radical (PI) and NO 2. Conversion of PN to PI is accompanied by changes in the electron paramagnetic resonance (EPR) spectrum from a five-line pattern (two equivalent N nuclei) into a seven-line pattern (two nonequivalent N nuclei). The transformation of the EPR signal is accompanied by an increase in the fluorescence intensity since the imino nitroxide radical is a weaker quencher than the nitronyl one. The results indicate that the fluorescence measurements enable detection of nanomolar concentrations of NO compared to a sensitivity threshold of only several micromolar for the EPR technique. The method was applied to the determination of NO and S-nitroso compounds in tissue from pig trachea epithelia. The measured basal flux of S-nitroso compounds obtained from the tissues was about 1.2 nmol/g × min, and NO-synthase stimulated by extracellular adenosine 5 ′-triphosphate produced NO flux of 0.9 nmol/g × min.

Magnetic and optical properties of nitroxide radicals and their lanthanide complexes

This paper reports the structural, magnetic and optical properties of three series of lanthanide complexes [Ln(radical) 4](ClO 4) 3, [Ln(radical) 2(NO 3) 3] and [Ln(radical)(hfac) 3] (Ln=Gd(III), La(III) or Eu(III)) with nitronyl or imino nitroxide radicals. The magnetic properties of the gadolinium complexes were studied. Along the series, most gadolinium(III) complexes exhibit antiferromagnetic Gd III-radical interaction. These results are discussed. The full absorption and luminescence spectra of some lanthanide complexes and their uncoordinated free radical ligands were measured. The rich vibronic structure in luminescence and absorption spectra indicates that several excited states define the absorption spectra between 400 and 800 nm. Qualitative trends can be established between magnetic ground state properties and the energies and vibronic structure of the title compounds.

Ferromagnetic interactions of μ-NO 3− bridged 2-D silver (I) complex with imino nitroxide radical

A first μ-NO 3 − bridged and self-assembled 2-D silver (I) complex [Ag(I)(im-mPy)NO 3)] ( 1) (im-mPy=(2-(3-pyridyl)-4,4,5,5-tetramethyl-2-imidazolin-1-ol), has been prepared and structurally characterized. Temperature dependence of susceptibility measurements shows a weak ferromagnetic coupling between im-mPy radicals with J=3.3 cm −1.

The first azide(mu1,1)-bridged binuclear cobalt(II)-imino nitroxide complex with ferromagnetic behavior.

The first azide(mu1,1)-bridged binuclear cobalt(II) complex with a chelated imino nitroxide radical, [Co2(immepy)2(N3)(4)].2EtOH, was structurally and magnetically characterized, where immepy = 4,4,5,5-tetramethyl-2-(6'-methyl-2'-pyridyl) imidazoline-1-oxyl. Five nitrogen atoms complete the coordination sphere of the Co(II) ion, showing a distorted trigonal bipyramid geometry. Two N(3)(-) anions act as bridges between cobalt ions in the mu1,1 coordination mode, resulting in a binuclear structure with an inversion center. Magnetic studies show that ferromagnetic couplings occurred between the adjacent cobalt(II) ions through N3(-)(mu1,1)) bridges, and antiferromagnetic couplings between the cobalt(II) ions and organic radicals.

Crystal Structure and Magnetic Properties of A One-Dimensional Polymer [Mn(im2-py)(tp)(H2O)2]·1.25H2O

A new manganese(II) complex [Mn(im2-py)(tp)(H2O)2]·1.25H2O (im2-py=2-(2'-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl and tp=terephthalato dianion) has been synthesized and characterized structurally and magnetically. The crystal structure consists of neutral chains of manganese(II) ions linked by the terephthalate ligand. Variable temperature magnetic susceptibility data exhibit a weak antiferromagnetic coupling between the manganese(II) ion and the imino nitroxide radical.

A nickel(II) complex with imino nitroxide radicals: structure and magnetic properties

A new nickel(II) complex [Ni(im2-py) 2(N 3) 2] (im2-py=2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl) has been synthesized and characterized structurally and magnetically. The structure of this complex is mononuclear with nickel(II) ion octahedrally coordinated and bound to the imino nitroxide radicals through the pyridyl and imino nitrogen atoms. The azide groups are coordinated to the metal as terminal ligands. The variable temperature magnetic susceptibility data show that a strong ferromagnetic coupling between the nickel(II) ion and imino nitroxide radical, and weak intermolecular antiferromagnetic interaction.