Imino Nitroxide Radical Research Articles

Electron Spin Resonance Spin Probe Technique for Investigating Non‐TEMPO Radicals Dispersed in Nanospaces of a Crystalline Zn Complex

An ESR spin probe technique with non‐TEMPO radicals, such as nitronyl nitroxide (NN), benzonitronyl nitroxide (BzNN), and iminonitroxide (IN) radicals, was used for a porous metal‐organic framework (MOF), [(ZnI2)3(TPT)2] (ZnTPT; TPT = tris(4‐pyridyl)‐1,3,5‐triazine), at room temperature. The principal values of g and hyperfine coupling (A) tensors estimated from spectral reproduction were different from those for organic matrices for some of these radicals. These results indicate that host‐guest interactions occur between the ZnTPT matrix and guest radicals. Thus, when using NN, BzNN, and IN radicals as spin probes for a porous MOF, the interaction between the metal atoms or organic ligands in host materials and guest radicals should be considered. The experimental ESR spectra for the derivatives of NN or BzNN radicals were reproduced only by the rigid‐limit component in the ESR time scale. However, those for the derivatives of IN radicals were approximately reproduced only by rotational diffusion around the z‐axis perpendicular to the plane in the IN group. Interestingly, this reproduction was not around the y‐axis of the principal axes of the g tensors, parallel to the molecular long axis, as previously observed in a few organic matrices. The IN radicals dispersed in the ZnTPT matrix are expected to be accommodated in cylindrical or pseudocylindrical nanospaces sandwiched by the pyridyl or triazine rings of TPT in ZnTPT. These findings show that the ESR spin probe technique using non‐TEMPO radicals can be used to investigate the chemical and biological structures of nanosized materials.

Synthesis and Crystal Structures of Two Metal-Radical Complexes

Two new M(II)-radical complexes [M(hfac)2(IM-o-QN)] (M = Ni(1), Zn(2); IM-o-QN = 4'-quinoxalinyl-substituted imino nitroxide; hfac = hexafluoroa-cetylacetonate) have been synthesized and characterized by X-ray diffraction analysis, element analyses, IR and UV-Visible spectroscopy. X-ray analysis reveal that the structures of both complexes are similar configuration and differently spatial symmetries. The complex 1 crystallizes in the triclinic Pī space group with the respective cell parameters: a = 9.1189(18) &#197, b = 9.836(2) &#197, c = 18.537(4) &#197, α = 75.92(3)°, β= 81.95(3)°, γ = 69.32(3)°, V = 1506.1(5) &#1973, Z = 2, whereas the complex 2 is in monoclinic C2/c space group with the respective cell parameters: a = 26.996(5) &#197, b = 9.5223(19) &#197, c = 23.961(5) &#197, α = 90.00°, β= 91.07(3)°, γ= 90.00°, V = 6158(2) &#1973, Z = 8. In two new M(II)-radical complexes, the central M(II)(Ni(1) and Zn(2)) ions are coordinated by four oxygen atoms from two hfac and two nitrogen atoms from imino nitroxide radicals to form a distorted octahedron. Additionally, the optical properties and thermal analysis of the two complexes are reported.

Mononuclear manganese(iii) complexes with reduced imino nitroxide radicals by single-electron transfer and intermolecular hydrogen bonds as an intramolecular structural driving force.

Manganese(iii) complexes were synthesized by one-electron transfer from a Mn(ii) ion to the imino nitroxide radical 2-(2-imidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (IMImH) in methanol. After the manganese ions attained the +III oxidation state, the imino nitroxide radicals were found to be irreversibly reduced in the complexes. Depending on the synthesis conditions, two complexes differing by their counter-anions were isolated as single crystals. These are [Mn(IMHIm)2(MeOH)2]ClO4·H2O (1) and [Mn(IMHIm)2(MeOH)2]PF6 (2), which crystallize in the monoclinic P21/n and triclinic P1[combining macron] space groups, respectively. The two complexes show Jahn-Teller distortions typical of Mn(iii) centres and only reduced radicals are coordinated, as indicated by the N-O bond lengths and electroneutrality. In addition, the crystal structure analyses reveal two intermolecular hydrogen bonding networks. One involves counter-anions, water molecules and reduced radicals, and the other involves coordinated methanol molecules and imidazole moieties. These intermolecular interactions are driving forces that stabilize the two complexes. They also suggest that the tautomer is in the amino imine-oxide form after reduction of the radical and reveal the deprotonation of the imidazole ring, which is required for electroneutrality. This assessment is supported by single-crystal X-ray diffraction, EPR and Raman spectroscopy as well as magnetic and electrochemical studies.

Syntheses, structures and magnetic properties of four-spin Mn-Imino nitroxide radical complexes

Based on the nitroxide radicals, four-spin complexes [Mn(hfac)2(IMpPhCl)]2·NITpPhCl (1) and [Mn(hfac)2(IMmPhCl)]2·NITmPhCl (2) (IMpPhCl = 2-(4'-chlorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl, IMmPhCl = 2-(3'-chlorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl, hfac = hexafluoroacetylacetonate) have been synthesized and characterized by single-crystal X-ray diffraction. The X-ray crystal structure analyses show that the structures of the two compounds are similar and the imino nitroxide radical ligand acts as a bridge ligand linking two Mn(II) ions through the oxygen atom of the N–O group to form a four-spin system. Two kinds of nitroxide radicals: nitronyl nitroxide and imino nitroxide radicals coexist in the cyclic Mn(II) complexes. The magnetic studies show that there exists an antiferromagnetic interaction between Mn(II) ions and the imino nitroxide radical ligands, which is explained by spin polarization mechanism. The antiferromagnetic interaction of Mn−Rad in complex 2 (J1 = −9.36 cm−1) is stronger than that in complex 1 (J1 = −9.19 cm−1), which is consistent with crystal structure of complexes (The bond length of the shortest Mn-O in complex 2 (2.1625 Å) is smaller than complex 1 (2.1898 Å)).

Synthesis and Straightforward Quantification Methods of Imino Nitroxide-Based Hexaradical Architecture on a Cyclotriphosphazene Scaffold.

The synthesis of a homogeneous neutral hexaradical architecture consisting of six imino nitroxide radical moieties covalently bonded on a cyclotriphosphazene scaffold was reported. The synthesis of hexaradical imino nitroxide compounds follows the Ullman procedure involving the condensation of 2,3-bis(hydroxylamino)-2,3-dimethylbutane with hexa-(4-formylphenoxy)cyclotriphosphazene (3) followed by oxidation of the condensation product hexa-[4-(1-hydroxy-4,4,5,5-tetramethyl-2-imidazoline-2-yl)phenoxy]cyclotriphosphazene (2) by NaIO4. Characterization of hexaradical was performed by X-ray and SQUID in solid state and by EPR, absorption spectroscopy, and electrochemistry in solution. CV of 1 shows an oxidation peak at 1.184 V (vs SCE) and a reduction peak at -0.883 V, both characteristics of the presence of phenyl imino nitroxide (7) moieties, suggesting that the contribution of the cyclotriphosphazene core is negligible. Attention was particularly focused on developing methods, UV-vis spectroscopy and square-wave voltammetry, to quantify the number of radicals in a way to confirm easily and rapidly the polyradicals' structure.

Heterospin MnIII complex as a reaction product of the redox-induced change in the ligand coordination mode

The reaction of MnII 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 MnIII complex [MnL22L3]·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 MnII to MnIII 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 MnIII ion forms with L2 six-membered metallocycles typical of Schiff bases, whereas with L3 MnIII 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 NiII chloride with imino nitroxide HL2 that no oxidation of the metal occurred and bis(chelate) [NiL22(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.

Stereo sensitivity of exchange interactions in NiII and CuII heterospin complexes with 5-formylpyrrolyl-substituted nitroxides

5-Formylpyrrolyl-substituted nitronyl and imino nitroxide radicals HL1 and HL2 were synthesized. Their solid phases are formed by packing pairs of the molecules. In the {HL1...HL1} pairs, the dominant interaction is the ferromagnetic exchange with J/kB = 8.8 K (Hamiltonian \(H = 2J\left( {\overrightarrow {{s_1}} \overrightarrow {{s_2}} } \right)\)). The ferromagnetic exchange occurs also in the heterospin molecules [Ni(L1)2], [Cu(L1)2], and [Ni(L2)2(MeOH)2]. In the complexes [Ni(L1)2] and [Cu(L1)2], a small change in the mutual orientation of the coordinated ligands has a considerable effect on the value and the sign of the energy of exchange interactions between the unpaired electrons of the metal ion and paramagnetic ligands.

Syntheses, structures, and magnetic characterizations of five Mn(II) and Co(II) complexes based on 2,6-Pyridyl-substituted nitronyl and imino nitroxide radical

The reaction of biradical 2,6-NITpy (1) (2,6-NITpy=2,6-bis (3′-oxide-1′-oxyl-4′,4′,5′,5′-tetramethylimidazolin-2′-yl) pyridine), 2-NIT-6-IMpy (2) (2-NIT-6-IMpy=2-(3′-oxide-1′-oxyl-4′,4′,5′,5′-tetramethylimidazolin-2′-yl)-6-(1′-oxyl-4′,4′,5′,5′-tetra-methylimidazolin-2′-yl) pyridine), or 2,6-IMpy (3) (2,6-IMpy=2,6-bis(1′-oxyl-4′,4′,5′,5′-tetra-methylimidazolin-2′-yl) pyridine) with M(hfac)2·XH2O (M=MnII, CoII) were investigated. These reactions were found to be related to the type of the metal ion employed. Interestingly, redox reactions involve in the synthesis process of the complexes. Structures of the five complexes were elucidated by X-ray analysis, and their magnetic properties were investigated in detail. The temperature dependence of χMT for all the five complexes at high temperature range exhibited antiferromagnetic interactions between the transition metal spin (MnII or CoII) and the nitroxide radical spin. At low temperature, the χMT curves exhibit different magnetic behaviors because of the different crystalline fields and intermolecular interactions.

An Azide-Functionalized Nitronyl Nitroxide Radical: Synthesis, Characterization and Staudinger–Bertozzi Ligation Reactivity

An azide-functionalized nitronyl nitroxide was synthesized and its reactivity towards the Staudinger–Bertozzi ligation was explored. Whereas a model reaction in solution showed the conversion of nitronyl nitroxide to an imino nitroxide radical, the same reaction at the interface of gold nanoparticles allowed for successful covalent incorporation of the nitronyl nitroxide radical onto the nanoparticles.

Crystal Structures and Magnetic Properties of Two New Cobalt(II) Complexes with Triazole-Substituted Nitronyl and Imino Nitroxide Radicals

Two new cobalt(II) complexes of formula [Co(hfac)2(NITphtrz) 1 and Co(hfac)2(IMphtrz) 2] have been prepared and characterized structurally [where NITphtrz = 4,4,5,5-tetramethyl-2-(2-phenyl-1,2,3-triazole-4-yl)imidazoline-1-oxyl-3-oxide and IMphtrz = 4,4,5,5-tetramethyl-2-(2-phenyl-1,2,3-triazole-4-yl)imidazoline-1-oxyl]. All of the complexes crystallize in an isomorphous triclinic space group P1- with the Co(II) ion octahedrally coordinated via the bidentate chelating mode. The magnetic measurements show that two complexes exhibit antiferromagnetic interactions between the metal ions and the nitroxide radicals.

Crystal structure of Synthesis and crystal structure of a new mercury(II) complex with imino nitroxide radical, C30H38Cl6Hg3N8O2

C30H38Cl6Hg3N8O2, triclinic, P1̄ (no. 2), a = 7.3649(5) Å, b = 11.4075(8) Å, c = 13.0120(9) Å, α = 100.454(1)°, β = 102.089(1)°, γ = 103.229(1)°, V = 1010.0 Å3, Z = 1, Rgt(F) = 0.0202, wRref(F2) = 0.0502, T = 291 K.

Molecular System Based on Novel Photochromic Biindenylidenedione Derivative Demonstrating Photomodulation of Magnetism

AbstractStable nitronyl nitroxide radical and imino nitroxide radical were incorporated into the benzene rings of novel photochromic 7,7′‐dimethyl‐[2,2′‐bi‐1H‐indene]‐3,3′‐diethyl‐3,3′‐dihydroxy‐1,1′‐dione (1), leading to the synthesis of novel multifunctional compounds 4 and 5. The photochromic properties, ESR spectroscopy and magnetic properties of the title compounds were investigated. Compounds 4 and 5 possess visible photochromism upon photoirradiation, and their ESR signals undergo secular broadening after photoirradiation. The magnetic susceptibility measurement shows that the antiferromagnetic interaction of 4 and 5 becomes stronger after photoirradiation. In compounds 4 and 5 there are two kinds of spin centers after photoirradiation: one is nitroxide radical, and the other is photo‐generated radicals from two indanone moieties. Our results demonstrated that the colour and magnetic properties of compounds 4 and 5 could be modulated by photoirradiation.

Slow magnetic relaxation in lanthanide complexes with chelating imino nitroxide radicals

Abstract Two lanthanide-radical complexes [Tb(hfac)3(IM-2py)] (1) and [Tb(hfac)3(IM-2thz)] (2) (hfac = hexafluoroacetylacetonate; IM-2py = 2-(2′-pyridyl)-4,4,5,5-tetramethyl- imidazoline-1-oxyl; IM-2thz = 2-(2′-thiazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl) have been synthesized. Single crystal X-ray diffraction analysis reveals that complexes 1 and 2 are mononuclear complexes in which the IM-2py/IM-2thz radical is coordinated to the Tb(III) ion in a chelating way. The similar magnetic behavior is observed for two compounds through direct current magnetic measurements, implying antiferromagnetic coupling between Tb(III) and imino nitroxide radical. Alternating current magnetic susceptibility studies show that both complexes exhibit frequency-dependent out-of-phase signals indicative of a slow relaxation of magnetization.

Syntheses and Crystal Structures of Two Transition Metal Complexes (M = Mn and Co) Containing Malonate and Reduced Imino Nitroxide Radicals

Two novel transition metal complexes with malonate and reduced imino nitroxide radicals, [Co(mal)(Him2-py)2] (ClO4) 1 and [Mn (mal)(Him2-py)2] (H2O) 2 (Him2-py = 1-hydroxy-2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline) have been synthesized and their crystal structures were determined by X-ray diffraction method. During the reaction, one-electron reduction of the N–O radical moiety in IM2py has been reviewed. The structural analyses reveal that two title complexes are isostructural and crystallize in monoclinic space group C2. For the complex 1, a=17.004(9), b=10.753(5), c=9.207(5) Å with β=113.856(8)∘. For the complex 2, a=16.721(5), b=10.897(5), c=9.253(3) Å with β=120.807(6)∘. In two complexes, the coordination number around the metal ion is six, and the coordination sphere is a distorted octahedron. Two nitrogen atoms from Him2-py and two oxygen atoms from malonate are in the basal plane, and two nitrogen atoms from pyridyl rings of Him2-py at the axial position.

Crystal structure of diaquabis[2-{2′-[(1′-methyl)benzimidazolyl]}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide]nickel(II)bisperchlorate dihydrate, [Ni(H2O)2(C15H19N4O2)2][ClO4]2 · 2H2O

Article Crystal structure of diaquabis[2-{2′-[(1′-methyl)benzimidazolyl]}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide]nickel(II)bisperchlorate dihydrate, [Ni(H2O)2(C15H19N4O2)2][ClO4]2 · 2H2O was published on December 1, 2010 in the journal Zeitschrift für Kristallographie - New Crystal Structures (volume 225, issue 4).

Synthesis and Crystal Structure of Copper(II)-Hexafluoro-Acetylacetonate Complexes with Pyridyl-Substituted Nitronyl and Imino-Nitroxide Radicals

A one-dimensional chain compound was formed by copper (II) hexafluoroacetylacetonate (hfac) and nitroxide radical 2-(4-pyridyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-oxide (NITpPy). The product was structurally characterized by single crystal X-ray diffraction. [Cu2(hfac)4NITpPy] n crystallizes in the monoclinic C2/c space group with a = 19.70(2), b = 16.834(19), c = 15.274(18) A, β = 119.073, V = 4427(9) A3, Z = 4, R = 0.0699 wR 2 (all data) = 0.1752. In the compound, two different Cu(hfac)2 units are bridged by NITpPy. The nitrogen atom of pyridine ring is coordinated with Cu(II) ion to afford a three-spin complex. Cu(II) ions of Cu(hfac)2 and the oxygen atoms of nitronyl nitroxide formed weak coordination bonds with the bond length 2.495(8) A. A one-dimensional chain compound [Cu2(hfac)4NITpPy]n was formed by copper (II) hexafluoroacetylacetonate (hfac) and nitroxide radical 2-(4-pyridyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-oxide (NITpPy). In the compound, two different Cu(hfac)2 units are bridged by NITpPy forming a 1D zigzag chain with the weak coordination bond Cu–O bond length 2.495(8) A.

Cis-Dichlorido[4,4,5,5-tetramethyl-2-(2-pyridyl)-2-imidazoline-1-oxyl]palladium(II) tetrahydrofuran hemisolvate

The asymmetric unit of the title complex, [PdCl2(C12H16N3O)]·0.5C4H8O, consists of one palladium complex in a general position and one half tetra­hydro­furan (THF) solvent mol­ecule, with the O atom lying on a twofold rotation axis. The PdII atom is bound to one chelating imino nitroxide radical through two N atoms, one from the pyridyl ring and the other from the imidazoline ring. The coordination of the metal centre is completed by two Cl atoms in a cis configuration, leading to a quasi-square-planar coordination of the metal centre. The four atoms that define the PdII coordination environment and the eight atoms that belong to the pyridylimine fragment are coplanar, with no deviation larger than 0.087 (5) Å. In the crystal structure, inter­molecular inter­actions shorter than the corresponding van der Waals radii sum are observed only between PdII complexes, and no short contact is observed around the THF mol­ecule. Weak C—H⋯O and C—H⋯Cl inter­actions yield a two-dimensional network of complexes in the (101) plane.

Synthesis, Crystal Structures and Magnetic Properties of Copper(II) Complexes with Nitronyl Nitroxide Radicals

AbstractTwo new copper(II) compounds with imino nitroxide radicals [Cu(IM‐MeImz)2 · (SCN)] 0.5[Cu(SCN)4] (1) and [Cu(IM‐MeImz)2 · (SCN)]ClO4 · H2O (2) (IM‐meImz = 2‐(5‐methylimidazol‐4‐yl)‐4,4,5,5‐tetramethyl‐2‐imidazoline‐1‐oxyl) have been synthesized and characterized structurally and magnetically. X‐ray analysis demonstrates that complex 1 contains CuII ions in both square‐pyramidal and square planar coordination. There are complete charge separation into [Cu(IM‐MeImz)2(SCN)]+ cations and 0.5[Cu(SCN)4]− anions, in a 2:1 ratio. The complex 1 was connected as a one‐dimensional polymer by intermolecular interactions. In complex 2, the coordination around the copper atom is distorted square pyramidal and the apical position is occupied by one nitrogen atom of SCN− anion. The 2‐D network structure was formed and arranged through intermolecular H‐bonds interactions. The complex 1 exhibits intramolecular weak ferromagnetic coupling between CuII ion and the radicals.

Magnetic interactions in a series of paramagnetic Ln(III) complexes with a chelated imino nitroxide radical

The magnetic interactions in a new series of isostructural imino nitroxide radical lanthanide(III) complexes, [Ln(hfac) 3(IM2py)] (Ln = Gd–Yb: IM2py = 2-(2′-pyridyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazoline-1-oxy; hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione), are examined by considering the intrinsic paramagnetic contribution of the Ln(III) ion from the corresponding [Ln(hfac) 3(pybzim)] with a diamagnetic pybzim(2-(2-pyridyl)benzimidazole) ligand; the Ln(III)–IM2py interaction being antiferromagnetic for the 4f 7 to 4f 13 Ln(III) complexes and negligibly small for the other complexes. This series is the first example reverse to the previous cases for the series of Ln–Cu or Ln–aminoxyl(NIT) radical (4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazoline-3-oxide-1-oxy) complexes, other than only a few examples of semiquinone Ln complexes. This reverse nature of the magnetic interaction, as compared with the NIT complexes, validates the empirical approach by O. Kahn et al. [Inorg. Chem. 38 (1999) 3692; J. Am. Chem. Soc. 122 (2000) 3413] in the spin-coupled systems for a series of Ln(III) complexes.

Synthesis and magneto-optical properties of a series of bis(β-diketonato) Co(II) complexes with imino nitroxide radical chelate

A series of bis(4,4,4-trifluoro-1-R-1,3-butanedinato) Co(II) complexes with imino nitroxide radical(IM2py), [Co(Rtfc) 2(IM2py)] (R = trifluoromethyl, phenyl, naphthyl, thienyl), was prepared and characterized by the X-ray analysis which demonstrated the stereospecific formation of the trans(CF 3)-isomer with the trans disposition of the CF 3 groups. The significant substituents effect on the UV–Vis spectra and magnetic properties were discussed in consideration of the inversion of the d π(t 2g) orbital levels leading to the orbital orthogonality or overlap with the SOMOπ ∗ of IM2py in terms of the AOM parametrizations or spin-paring energy.