Imino Nitroxide Research Articles

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.

“Jumping Crystals”: Oxygen-Evolving Metal-Nitroxide Complexes

The crystals of heterospin complexes [M(hfac)(2)L(2)] (where M = Cu, Ni, Co, or Mn; hfac = hexafluoroacetylacetonate; and L = nitronyl nitroxide, 4,4,5,5-tetramethyl-2-(1-methyl-1H-imidazol-5-yl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl) were found to make unusual jumping motions. Under ambient conditions, the jumping and various displacements of crystals lasted for several weeks. The mechanical motion was accompanied by the cracking and disintegration of crystals, and a solid [M(hfac)(2)(L(1))(2)] complex with the corresponding imino nitroxide 4,4,5,5-tetramethyl-2-(1-methyl-1H-imidazol-5-yl)-4,5-dihydro-1H-imidazole-1-oxyl (L(1)) was detected. The jumping was accompanied by the spontaneous elimination of oxygen, the source of which was the nitronyl nitroxyl fragment of coordinated L. An X-ray study of [M(hfac)(2)L(2)] (where M = Cu, Ni, Co, or Mn) showed that the molecular structure of all [M(hfac)(2)L(2)] and their packing in the solid state were identical. The packing of [M(hfac)(2)L(2)] was concluded to be critical to the mechanical effect. In complexes with different stoichiometries or different sets of diamagnetic ligands ([Cu(hfac)(2)L](2), [Cu(hfac)(acac)L]·EtOH, [CuPiv(2)L(2)]·2CH(2)Cl(2), and [Cu(hfac)(2)L(2)Cu(2)Piv(4)]·3C(7)H(8) (where acac is acetylacetonate and Piv is trimethylacetate), or free L), the effect vanished when the packing changed.

Tuning of the nitronyl nitroxide radical magnetic and electronic properties by inclusion in cucurbit[ n]urils

A nitronyl nitroxide, 2-(benzimidazolyl)-4,4,5,5,-tetramethylimidazolidinyl-3-oxide-1-oxy, 1 and its imino nitroxide analog 2, have been included in cucurbit[ n]urils, CB n ( n = 7,8), as their hydrochlorides and neutral compounds as well. The hydrochlorides form soluble 1:1 complexes. In the solid state the paramagnetic centers are well isolated and magnetic interactions are cancelled out in contrast to what is observed for the pure nitroxides. The two inclusion complexes involving 1 were characterized by X-ray crystallography. When they were heated in the solid state, one observed a clean conversion of the nitronyl nitroxide guest, 1, into its analog 2.

New approach to synthesis of nitronyl and imino nitroxides based on SNH methodology

It is shown that SN H approach opens new possibilities in the synthesis of polyfunctional nitronyl and imino nitroxides. It is found that the interaction of 4,4,5,5-tetramethyl-4,5-dihydro-1Himidazol-3-oxide-1-oxyl lithium salt Li1 with 3,6-diaryl-1,2,4-triazines leads to formation of the corresponding triazines bearing nitronyl nitroxide or imino nitroxide substituent at position 5 of the heterocycle. The reaction of Li1 with pyridazine-N-oxide gives rise to nitroxide with buten3-ynyl substituent 5. Spin-labeled 5 could be readily transformed by the use of 1,3-dipolar and nucleophilic addition reactions, as well as oxidative coupling, that gives a large group of new paramagnets: 2-(1H-pyrazol-5-yl)vinyl-, 2-ethynylcyclopropyl-, 2-(3-(ethoxycarbonyl) isoxazol5-yl)vinyl-, 1-(pyrrolidin-1-yl)but-3-ynyl-substituted nitronyl nitroxide and a diradical – 2,2′((1E,7E)-octa-1,7-dien-3,5-diyne-1,8-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3oxide-1-oxyl). The new nitroxides were characterized by X-ray single crystal data, ESR and static magnetic susceptibility measurements.

Cucurbituril-assisted transformation of nitronyl nitroxide into imino nitroxide in the solid state

A nitronyl nitroxide, 2-(2-benzimidazolyl)-4,4,5,5-tetramethylimidazolidinyl-3-oxide-1-oxy, 1, is included in cucurbit[8]uril (CB8) and cucurbit[7]uril (CB7) as its hydrochloride to give water soluble complexes. At high temperature (>150 °C) in the solid state, these complexes are stable but the paramagnetic guest is irreversibly converted into 2-(2-benzimidazolyl)-4,4,5,5-tetramethylimidazolidinyl-1-oxy, 2.

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).

Photoresponsive magnetization reversal in green fluorescent protein chromophore based diradicals

We present density functional theory based calculations on six diradicals in which imino nitroxide (IN) and IN or oxo- or phospha-verdazyl radical centres are linked by fragments of structures called cyan, blue and green fluorescent protein, respectively. In the latter two variants the indolyl fragment of cyan fluorescent protein (CFP) is replaced by phenyl and phenol moiety to obtain blue fluorescent protein Y66F GFP (BFPF) and green fluorescent protein (GFP). The photoinduced cis–trans isomerization of these diradical substituted chromophores is accompanied by changes from antiferro- to ferro-magnetic spin coupling on the radical centres. The calculations in this work provide predictions of the change of magnetic response to photo excitation. The estimated magnetic exchange coupling constants associated with cis–trans isomerization of the chromophores can be useful parameters in designing radical substituted isomeric fluorescent chromophores. Upon irradiation with light of appropriate wavelength, the dark trans diradicals turn into their fluorescent cis isomers. Therefore, photoinduced magnetic crossover from antiferromagnetic to ferromagnetic regime associated with the change in color would be noticed in all three cases. This is a novel observation in case of the systems with GFP chromophore and its variants. These diradicals are potentially very useful in different applications and biocompatibility of such systems makes them prospective in different in vivo treatments. Moreover, change in color associated with magnetic crossover for these diradicals increases their suitability as biological taggers.

ChemInform Abstract: Assembly of Imino Nitroxides with Ag(I) and Cu(I) Ions

Abstract Magnetic interactions between coordinated imino nitroxides through the diamagnetic Cu(I) and Ag(I) ions, and radical assemblies by means of coordination to Ag(I) and Cu(I) ions are discussed.

On the Distinction between Nitroxyl and Nitric Oxide Using Nitronyl Nitroxides

A better understanding of the origins of NO and HNO and their activities and biological functions requires accurate methods for their detection and quantification. The unique reaction of NO with nitronyl nitroxides such as 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (C-PTIO), which yields the corresponding imino nitroxides, is widely used for NO detection (mainly by electron paramagnetic resonance spectroscopy) and for modulation of NO-induced physiological functions. The present study demonstrates that HNO readily reacts with nitronyl nitroxides, leading to the formation of the respective imino nitroxides and hydroxylamines via a complex mechanism. Through the use of the HNO donor Angeli's salt (AS) with metmyoglobin as a competing agent, the rate constant for C-PTIO reduction by HNO has been determined to be (1.4 +/- 0.2) x 10(5) M(-1) s(-1) at pH 7.0. This reaction yields the corresponding nitronyl hydroxylamine C-PTIO-H and NO, which is trapped by C-PTIO to form (*)NO(2) and the corresponding imino nitroxide, C-PTI. (*)NO(2) oxidizes the nitronyl and imino nitroxides to their respective oxoammonium cations, which decay mainly via comproportionation with the nitronyl and imino hydroxylamines. When [AS] > [C-PTIO], the reduction of C-PTI by HNO proceeds, eventually converting C-PTIO to the corresponding imino hydroxylamine, C-PTI-H. Similar results were obtained for 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO). It is concluded that nitronyl nitroxide is readily reduced by HNO to nitronyl hydroxylamine and is eventually converted into imino nitroxide and imino hydroxylamine. The yield of the imino hydroxylamine increases at the expense of the imino nitroxide as the ratio [AS](0)/[nitronyl nitroxide](0) is increased. Since the reaction of NO with nitronyl nitroxide yields only the corresponding imino nitroxide, nitronyl nitroxide can discriminate NO from HNO only when present at a concentration much lower than the total production of HNO.

Multireference (MR) configuration interaction (CI) approach for quasidegenerate systems

The multireference (MR) configuration interaction (CI) wavefunctions utilizing generalized Hartree-Fock (GHF) molecular orbitals are discussed in relation to our previous extended HF and recent full-valence MCSCF methods for quasidegenerate systems. The reaction natural orbitals (RNO) for constructing the reference functions are selected on the basis of GHF calculations. The computational procedures for the MR CI are illustrated by considering imino nitroxide (HNNO) as an example. It is found that the present MR CI provides well-balanced wavefunctions between the ground and lower-lying excited states of the species. The correlation corrections coming from higher-order excitations are examined by a Davidson-like formula. The relation of this formula to the coupled-cluster method is stressed.

New Cascade Syntheses of Nitronyl Nitroxides and a New Synthetic Approach to Imino Nitroxides

AbstractSuspensions of MxOy (MnO2, Co2O3, Ni2O3) in protic solvents (MeOH, EtOH) have been found to be suitable systems for use in a cascade transformation of 4,4,5,5‐tetramethyl‐2‐[2‐(trimethylsilyl)ethynyl]imidazolidine‐1,3‐diol (1a) into 2‐ethynyl‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐1‐oxyl 3‐oxide (3). In MnO2/MeOH, the products of further transformation of 3, (Z)‐ and (E)‐2‐(2‐methoxyvinyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐1‐oxyl 3‐oxides, were obtained by the regiospecific addition of MeOH to the triple bond of 3. In the reaction in MnO2/MeOH+H2O (1:1), the Z isomer was the sole product. A multistep one‐pot transformation of 1 into 4,4,5,5‐tetramethyl‐2‐[2‐oxo‐1‐(4,4,5,5‐tetramethylimidazolidin‐2‐ylidene)ethyl]‐4,5‐dihydro‐1H‐imidazole‐1‐oxyl 3‐oxide occurred in MnO2/EtOH. For (E)‐2‐[2‐(diethylamino)vinyl]‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐1‐oxyl 3‐oxide, it was shown that use of the MnO2/secondary amine system provided a one‐pot transformation of 1a and gave (E)‐aminovinyl‐substituted nitronyl nitroxide as the sole product. In combined MnO2/ROH/K2CO3 systems (R = Me, Et), the imidazolidine‐1,3‐diol 1a was transformed into (E)‐2‐(2‐ethoxyvinyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐1‐oxyl 3‐oxide and the acetals 2‐(2,2‐dimethoxyethyl)‐ and 2‐(2,2‐diethoxyethyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐1‐oxyl 3‐oxide. In MnO2/MeNO2 we obtained the products of the transformations of different 2‐substituted 4,4,5,5‐tetramethylimidazolidine‐1,3‐diols (R = C≡C–SiMe3, aryl, hetaryl, alkyl) or nitronyl nitroxides into the corresponding imino nitroxides (13 products); this reaction is a new method for the preparation of imino nitroxides and allowed us to synthesize the first α‐acetylenyl‐substituted imino nitroxides. The majority of paramagnetic compounds, including the derivatives of 1a, were obtained as perfect crystals, and their structures were determined by X‐ray structure analysis (14 solved structures). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Electrochemistry of nitronyl and imino nitroxides

Redox potentials of a wide group of azolyl-substituted nitronyl and imino nitroxides were determined by classic cyclic voltammetry (CV). Conclusions have been made for this group of compounds, and their peculiarities were emphasized in comparison with methyl-, phenyl-, iodo-, and cyano-substituted nitroxides.

Ab Initio Study of Some Persistent Nitroxide Radicals

Stable free radicals have a variety of applications. Particularly, the aminoxyl group is frequently used in spin-labeling experiments. Nitroxides may also exhibit intriguing chiral and magnetic properties, and can be of interest for studies of molecular parity violation. We present results for three different groups of persistent nitroxide radicals: i) acyclic: dimethylaminoxyl (Me2NO), bis(trifluoromethyl)aminoxyl (CF3)2NO, and di-tert-butyl nitroxyl [(Me3C)2NO]; ii) cyclic: aziridine-N-oxyl, azetidine-N-oxyl, pyrrolidine-N-oxyl and piperidine-N-oxyl; and iii) imino nitroxides. We used density functional and ab initio (MP2, coupled cluster) methods to obtain insight into the underlying chemistry. The molecular structures, harmonic vibrational frequencies, inversion barriers, and hyperfine coupling constants are reported. The cyclic aziridine-N-oxyl exhibits a considerable inversion barrier of ?hc 3500 cm–1 compared to only ?hc 500 cm–1 for the other examples. Stable imino nitroxides are theoretically characterized for the first time in our work. We discuss the possibilities that some of the chiral derivatives may be dominated by molecular parity violation in their dynamics.

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.

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

It was established that the reactions of pyrazol-3-yl-substituted nitronyl nitroxide ( HL 1) and pyrazol-3-yl-substituted imino nitroxide ( HL 3) with Cu(II) acetate lead to self-assembly of the Cu 4(OH) 2(OAc) 4(DMF) 2(L 1) 2 tetranuclear and Cu 2(OAc) 2(H 2O) 2(L 3) 2 dinuclear complexes, respectively. The reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted nitronyl nitroxide ( HL 2) gave unexpected solid Cu 2(H 2O) 2(L 6) 2 · 2DMF, in which L 6 is a deprotonated 5-carboxy-pyrazol-3-yl-substituted nitronyl nitroxide, formed as a result of cleavage of an ester bond in the starting HL 2. 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 ( HL 4). It led to the formation of Cu 2(DMF) 2(L 7) 2, where L 7 is deprotonated 2-(5-carboxy-1 H-pyrazol-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole 3-oxide. An X-ray diffraction study indicated that in Cu 4(OH) 2(OAc) 4(DMF) 2(L 1) 2 and Cu 2(OAc) 2(H 2O) 2(L 3) 2, the L 1 and L 3 paramagnetic ligands perform the bridging cyclic tridentate function, while in Cu 2(H 2O) 2(L 6) 2 · 2DMF and Cu 2(DMF) 2(L 7) 2, the paramagnetic L 6 and diamagnetic L 7 are bridging bicyclic tetradentate ligands. The magnetic behavior of complexes with coordinated nitronyl nitroxide – Cu 4(OH) 2(OAc) 4(DMF) 2(L 1) 2 and Cu 2(H 2O) 2(L 6) 2 · 2DMF is dictated by the dominant antiferromagnetic exchange interactions, which is confirmed by quantum-chemical data. The magnetic susceptibility of Cu 2(OAc) 2(H 2O) 2(L 3) 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.

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.

Computational Chemistry

Stable free radicals have a variety of applications. Particularly, the aminoxyl group is frequently used in spin-labeling experiments. Nitroxides may also exhibit intriguing chiral and magnetic properties, and can be of interest for studies of molecular parity violation. We present results for three different groups of persistent nitroxide radicals: i) acyclic: dimethylaminoxyl (Me2NO), bis(trifluoromethyl)aminoxyl (CF3)2NO, and di-tert-butyl nitroxyl [(Me3C)2NO]; ii) cyclic: aziridine-N-oxyl, azetidine-N-oxyl, pyrrolidine-N-oxyl and piperidine-N-oxyl; and iii) imino nitroxides. We used density functional and ab initio (MP2, coupled cluster) methods to obtain insight into the underlying chemistry. The molecular structures, harmonic vibrational frequencies, inversion barriers, and hyperfine coupling constants are reported. The cyclic aziridine-N-oxyl exhibits a considerable inversion barrier of ∼hc 3500 cm-1 compared to only ∼hc 500 cm-1 for the other examples. Stable imino nitroxides are theoretically characterized for the first time in our work. We discuss the possibilities that some of the chiral derivatives may be dominated by molecular parity violation in their dynamics.

A one-dimensional zigzag chain involving a cadmium(II)-imino nitroxide: structure and magnetic properties

A novel one-dimensional complex salt with the formula [Cd(IM2py)2][Au(CN)2]2 (IM2py = 2-(2′-pyridinyl)-4,4,5,5-tetramethylimidazoline-1-oxyl), has been synthesized and its structure was determined by single-crystal X-ray diffraction methods. The coordination geometry around Cd(II) ion is a distorted octahedral. IM2py coordinates to Cd(II) via pyridyl- and imino-N atoms to form a planar, five-membered chelate ring. [Cd(IM2py)2][Au(CN)2]2 units are linked through weak Au ··· Au interactions, generating an infinite 1D zigzag chain. Magnetic studies show weak antiferromagnetic coupling between two radicals through the diamagnetic Cd(II) ion.

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.