The main difficulty encountered in the design of purely organic magnetic materials is that of establishing suitable strategies for avoiding the direct overlap of the magnetic orbitals of the individual magnetic centers, which inevitably leads to antiferromagnetic interactions at best, or more often to complete pairing of the spins. Many ingenious strategies have now been developed, leading to bulk ferromagnetism at very low temperatures, or to strong ferromagnetic interactions in discrete molecules [1-7]. However, the dream of a room-temperature organic ferro- or ferrimagnet has not yet been achieved.

Triazinyl-N-oxy free radicals, 2-methyl-2,4,6-triphenyl-1,2-dihydro-1,3,5-triazinyl-1-oxy (6a), 2,2,4,6-tetraphenyl-1,2-dihydro-1,3,5-triazinyl-1-oxy (6b), 2,2-dimethyl-4,6-diphenyl-1,2-dihydro-1,3,5-triazinyl-1-oxy (13), and 2,6-dimethyl-2,4-diphenyl-1,2-dihydro-1,3,5-triazinyl-1-oxy (14), in which the unpaired electron is delocalized over three nitrogen atoms, have been prepared and characterized. A method has been devised for introducing an N-oxide function into the triazinyl core. Then, by using a Grignard reagent, substitution α to the N-oxide group was achieved and the resulting 1,2-dihydrotriazine-N-oxide oxidized into the corresponding nitroxide. Solution EPR spectra exhibit hyperfine splitting that confirms spin delocalization over the three nitrogen atoms of the triazinyl ring. They also show that spin delocalization diminishes with increasing distance for the coupling and is largest for nitrogen N1 and weakest for N5. Free radicals 6a and 13 are stable in the solid state and have been characterized by X-ray diffraction, but they tend to gradually degrade in solution. In the solid state, these two free radicals are arranged into antiferromagnetically exchange-coupled pairs, J=-5.2(6) for 6a and -3.7(4) cm(-1) for 13 (H=-2JS(1)S(2)).

The recognition properties of cucurbit[8]uril (CB8) toward nitronyl nitroxide 2-(2-benzimidazolyl)-4,4,5,5-tetramethylimidazolidinyl-3-oxide-1-oxy (1) and its hydrochloride have been investigated. 1·HCl led to 1:1 inclusion complex [1·HCl@CB8], which was characterized both in solution and by single-crystal X-ray diffraction. In this compound only the tetramethylimidazolidinyl fragment is included in the host. The magnetic behavior of the complex corresponds to a Curie law with a large separation of the spin carriers in the solid. In contrast, an insoluble species exhibiting ferromagnetic behavior is formed when pure 1 reacts with acid-free CB8. The formula [(1)(2)@(CB8)(3)], in which two radical guests are arranged in such a way that the phenyl groups of the benzimidazolyl substituents are both stacked into one CB8 and the tetramethyl fragments are each capped by a terminal macrocycle, is proposed, in agreement with microanalysis, spectrophotometric, EPR, and magnetic measurements. According to McConnell's rules, the alternating spin densities within the stacked aromatic fragments result in a ferromagnetic interaction (J=+2.3 cm(-1), H=-2JS(1)S(2)) and a triplet ground spin state for the inclusion complex.

A series of layered compounds of the formula {[Mn2II(NITBzIm)3](X)}n [X=ClO4 (1) or CnH2n+1SO4 with n=10 (2), 11 (3), 12 (4), 13 (5), 14 (6), 18 (7)] were obtained by the reaction of the 2-(2-benzimidazolyl)-4,4,5,5-tetramethylimidazolidinyl-1-oxy-3-oxide nitronyl nitroxide radical (NITBzImH) with manganese(II) acetate in methanol and by the successive addition of sodium perchlorate or n-alkylsulfate. The crystal structures of all compounds have been established by Rietveld refinement of their X-ray powder diffraction patterns using the previously reported analogue compound {[Mn2II(NITIm)3](ClO4)}n obtained with 2-(2-imidazolyl)-4,4,5,5-tetramethylimidazolidinyl-1-oxy-3-oxide (NITImH) as model. Accordingly, all compounds contain the 2D cationic framework of the formula {[Mn2II(NITBzIm)3]+}n exhibiting a honeycomb-like structure with alternating manganese(II) ions and radicals and crystallizing in the form of layered compound with perchlorate or n-alkylsulfate anions in between. The series of compounds with n-alkylsulfate anions evidence a linear dependence of the spacing between the layers in respect to the length of the alkyl chain. The variation is small because the chains are almost parallel to the mean plane of the layers. The study of the magnetic behaviors shows that the spacing between the layers deeply influences the magnetic properties. Compounds 1–6 behave as magnets with Curie temperature decreasing when the interlayer spacing increases [52K (1)–30K (6)]. No 3D ordering was evidenced for compound 7 with larger spacing. A close examination of the magnetization versus magnetic field indicate that compounds 1–6 are a case of weak ferromagnetism due to spin canting within the 2D framework layers. Interestingly, it is found that the canting as well as the interlayers magnetic coupling of dipolar origin increases when the layers come closer. One explanation is that an increase in the dipolar magnetic interaction causes a more accented canting. Alternatively, it may be that when the layers come closer this induces stronger anisotropy through crystal packing effects.

Novel copper(II)-nitroxide complexes exhibiting a spin-transition-like behavior have been prepared and characterized. They include meso, chiral, and racemic 2-(3-pyridyl)-nitronyl nitroxides differently substituted in positions 4 and/or 5 by ethyl groups and pyrimidyl nitroxides. Depending on the stoichiometry of the reaction, tetranuclear and binuclear complexes were obtained whose structures are cyclic. The tetranuclear species, which include two intracyclic and two exocyclic metal sites, are similar to the previously reported complex of the tetramethylated analogue, while the binuclear complexes involve only endocyclic metal ions and have uncoordinated N-oxyl groups. The tetranuclear complexes exist as two isomers depending on the temperature of crystallization: at room temperature, N-oxyl ligand coordination is axial-axial, while it is axial-equatorial at low temperature. Unexpectedly, this isomerism concerns N-oxyl bonding to the exocyclic metal centers for the derivatives of 4,5-diethyl-substituted ligands while it involves the endocyclic metal site in the complex of the monoethylated ligand, which converts reversibly from a high-spin state to a low-spin state, as observed for the complex of the tetramethylated ligand. Binuclear complexes are diamagnetic at room temperature but convert to a paramagnetic state on warming (90-110 degrees C); the transition is irreversible and sharp.

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.

The chemistry of 2,2,4,4-substituted pentane derivatives has been investigated with the aim of providing a flexible and versatile synthetic route to pyrimidinyl nitronyl nitroxides, in which the bis-N-oxy fragment is incorporated in a six-membered ring. The synthesis of 2,4-diamino-2,4-dimethylpentane and 2,4-bis(hydroxylamino)-2,4-dimethylpentane, convenient precursors of these nitroxides, is described and full characterization of a series of pyrimidinyl nitronyl nitroxides is reported, along with a preliminary study of their coordination properties.

A synthetic route to a series of homochiral (and achiral) nitro-nyl nitroxides derived from rac-(and meso)-3,4-dimethyl-3,4-dinitrohexane is described. The two forms of this precursor, meso and rac, were identified unambiguously and reduced to the corresponding meso- and rac-diamines. The rac-diamine was condensed with an enantiopure aldehyde specifically designed to give a diastereomeric mixture of imidazolidines easily separated by flash chromatography. Both enantiopure diamines were then obtained by acidic hydrolysis of these imidazolidines. The absolute configurations of the diamino precursors were determined by X-ray crystallography of single crystals of a manganese(II) complex [(R,R,R)-5E-Mn(hfaC)(2)] of a nitroxide containing a third chiral center of known configuration. A series of homochiral nitronyl nitroxides was then prepared from the enantiopure diamino pre- cursors by condensation with aldehydes, followed by oxidation. Their structural properties were compared to those of their achiral meso counterparts and it was found that the puckering of the five-membered ring is dependent on the chirality of the imidazolyl unit. The radicals show marked optical activity, as explored by polarimetry and circular dichroism spectroscopy. These paramagnetic building blocks of known absolute configuration include chiral centers adjacent to the oxyl groups, and since they exhibit C-2 symmetry they are well suited for coordination chemistry studies because stereochemical complexity is minimized. They have been specifically designed for further developments of the metal-radical approach to molecular magnetic materials. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).

Coordination compounds based on imidazole and benzimidazole substituted nitronyl nitroxide radicals with transition metal ions and trivalent lanthanide ions are described from the perspective of their magnetic properties.For the transition metal compounds the crystal structures show various metal-nitroxide dimensionalities including mononuclear (0D), one-dimensional (1D) and two-dimensional (2D) complexes. The mononuclear complexes were isolated with most metal ions of the first transition series. One copper(II) complex shows a copper(II)-radical ferromagnetic coupling (J= +75 cm(-1)) while for the other mononuclear compounds, mainly with manganese(II), the metal-radical interactions are antiferromagnetic. The one-dimensional and two-dimensional complexes are manganese(II) compounds which show canting effects leading to weak ferromagnetism.For the trivalent lanthanide ions [La(III), Gd(III) and Eu(Ill)], three series of mononuclear complexes were obtained in which the metal center is bound to four, two or one nitroxide radicals depending on the counter ions and ancillary ligands. Unexpectedly, in most gadolinium(III) complexes, the Gd(III)-radical interactions were found to be antiferromagnetic in contradiction with other foundings and previous theoretical models. In support to the magnetic studies, the optical properties of the lantanide complexes have also been investigated and are briefly described. (c) 2005 Elsevier B.V. All rights reserved.

Nitronyl nitroxides are stable free radicals that have been used to prepare new molecular solids with intriguing magnetic properties. These properties have been probed for the electronic ground state using a large number of different physical techniques and theoretical methods. In contrast, the excited states and optical spectroscopy of these compounds and their metal complexes have received little attention until recently. in this overview, we present their absorption and luminescence spectra. Luminescence is observed between 700 nm and 1100 nm, and the lowest-energy absorption bands occur between 500 run and 700 nm. Several excited electronic states are in the red to near-infrared wavelength range, leading to a wide variety of interesting spectroscopic features.

Pyrimidinyl nitronyl nitroxides where the bis-N-oxy fragment is included in a six-membered ring were prepared from diacetonamine by a sequence of reactions including a Grignard reaction, a Ritter reaction and oxidation of the intermediate pyrimidine; the properties of the 2-phenyl-substituted representative are fully described.

This paper reports the structural, magnetic and optical properties of three series of lanthanide complexes [Ln(radical)(4)](ClO4)(3), [Ln(radical)(2)(NO3)(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. (C) 2003 Elsevier Ltd. All rights reserved.

This paper reports the synthesis, structures, and magnetic and optical properties of a series of gadolinium(III) (1a-4a) and europium(III) (1b-4b) complexes with nitronyl or imino nitroxide radicals. The crystal structures of compounds 1a and 1b consist of [Ln(III)(radical)(2)(NO3)(3)] entities in which the gadolinium(III) (1a) or europium(III) ion (1b) is 10-coordinated to two nitronyl nitroxide radicals and three nitrato ligands. The crystal structures of compounds 2a-4a and 2b-4b consist of [Ln(III)(hfac)3(radical)] entities in which the gadolinium(III) (2a-4a) or europium(III) ion (2b-4b) is 8-coordinated to one nitronyl (2a and 2b) or one imino (3a, 4a and 3b, 4b) nitroxide radical and three hexafluoroacetylacetonato ligands. The gadolinium(III) complexes (1a-4a) are isostructural with their europium(III) analogues (1b-4b). The magnetic properties of the gadolinium complexes were studied. Along the series 1a-4a only compound 2a exhibits a ferromagnetic Gd-III-radical coupling (J(Gd-rad) = +1.7 cm(-1)), while for the others this coupling is antiferromagnetic (1a: J(Gd-rad1) = -4.05 cm(-1) and J(Gd-rad2) = -0.80 cm(-1); 3a: J(Gd-rad) = -2.6 cm(-1); 4a: J(Gd-rad) = -1.9 cm(-1)). The first full luminescence spectra of lanthanide complexes with free radical ligands are reported between 650 and 1200 nm. 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 fine structure of the title compounds.

This paper reports the synthesis, crystal structures, and magnetic properties of a series of lanthanide complexes with nitronyl nitroxide radicals of general formula [[Ln(III)(radical)(4)] x (ClO(4))(3) x (H(2)O)(x) x (THF)(y)] (1-4) and [Ln(III)(radical)(2)(NO(3))(3)] (5, 6) [Ln = La (compounds 1, 3, 5) or Gd (compounds 2, 4, and 6); radical = 2-(2'-benzymidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITBzImH, compounds 1, 2, 5, 6) or 2-[2'-[(6'-methyl)benzymidazolyl]]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITMeBzImH, compounds 3, 4)]. (1) C(64)H(88)Cl(3)LaN(16)O(24), fw = 1710.76, orthorhombic, Fddd, a = 11.0682(8) A, b = 34.240(3) A, c = 42.787(3) A, V = 16215(2) A(3), Z = 8, R = 0.0876, R(w) = 0.2336. (2) C(64)H(88)Cl(3)GdN(16)O(24), fw = 1729.10, tetragonal, P 4 macro 2c, a = 16.0682(4) A, b = 16.0682(4) A, c = 18.7190(6) A, V = 4833.0(2) A(3), R = 0.0732, R(w) = 0.2218. (3) C(68)H(94)Cl(3)LaN(16)O(23), fw = 1742.80, tetragonal, P 4 macro 2(1)m, a = 21.125(3) A, b = 21.125(3) A, c = 10.938(2) A, V = 4881.5(14) A(3), R = 0.1017, R(w) = 0.3126. (5) C(28)H(34)LaN(11)O(13), fw = 871.57, orthorhombic, Pna2(1), a = 19.5002(12) A, b = 13.0582(8) A, c = 14.5741(9) A, V = 3711.1(4) A(3), R = 0.0331, R(w) = 0.1146. (6) C(28)H(34)GdN(11)O(13), fw = 889.91, orthorhombic, Pna2(1), a = 19.1831(10) A, b = 13.1600(7) A, c = 14.4107(7) A, V = 3638.0(3) A(3), Z = 4, R = 0.0206, R(w) = 0.0625. Compounds 1-4 consist of [M(III)(radical)(4)](3+) cations, uncoordinated perchlorate anions, THF, and water crystallization molecules. In these complexes, the coordination number around the lanthanide ion is eight, and the polyhedron is either a distorted dodecahedron (1) or a distorted cube (2, 3). The crystal structures of 5 and 6 consist of independent [M(III)(radical)(2)(NO(3))(3)] entities in which the lanthanide is ten-coordinated and has a distorted bicapped square antiprism coordination polyhedron. For the lanthanum(III) complexes, the temperature dependence of the magnetic susceptibility indicates that radical-radical magnetic interactions are negligible either for compounds 1 and 3, while for compound 5 it is simulated considering dimers of weakly antiferromagnetically coupled radicals (J(rad-rad) = -1.1 cm(-1)). In the case of the gadolinium(III) compounds (2, 4, 6), each magnetic behavior gives unambiguous evidence of antiferromagnetic Gd(III)-radical interaction (2, J(Gd-rad) = -1.8 cm(-1); 4, J(Gd-rad) = -3.8 cm(-1); 6, J(Gd-rad1) = -4.05 cm(-1) and J(Gd-rad2) = -0.80 cm(-1)), in contrast to the ferromagnetic case generally observed. The nature of the Gd(III)-radical interaction is explained in relation to the donor strength of the free radical ligand.

An extensive investigation of the magnetic properties of three series of biradicals (bis-nitronyl nitroxides diNN-R, bis-imino nitroxides diIN-R and mixed INNN-R, where R is either hydrogen, a triple bond or trimethylsilylacetylenic group) has been carried out to give clear values of the intramolecular interactions through the m-phenylene coupling unit with alpha-nitronyl nitroxides (NN) or alpha-imino nitroxides (IN). An EPR study of the molecules in the isolated state is validated by ab initio calculations, which show the respective influence of spin polarisation and molecular conformation on the singlet-triplet gaps. All these results indicate that the triplet state is the ground state for such biradicals, except when the imidazolyl cycles are orthogonal to the phenyl ring. The magnetic properties of the biradicals in the solid state can be rationalised by examination of the short contacts produced between the ONCNO and ONCN groups. EPR studies on single crystals of the H-substituted series have confirmed the presence of a structural distortion for diNN-H whereas diIN-H and INNN-H do not exhibit such a peculiarity. The magnetic behaviour of diIN-H is described well by a four-spins model, with a strong intermolecular antiferromagnetic interaction of -90 K, whereas in the case of the two other compounds, a supplementary contact involves more complex interactions between the dimers. The compound diNN-tmsa exhibits a ferromagnetic intermolecular interaction of +11 K within the dimers, and this could be attributed to the relative disposition of the imidazolyl rings. Compound diNN-tr reveals a chain-like behaviour, whereas diIN-tr shows a predominant antiferromagnetic interaction within the dimers. The values for the intramolecular interactions in the solid state are in good agreement with those found for the isolated molecules.

The synthesis of nitronyl and imino nitroxides has been reexamined with the aim of both increasing yields and of offering opportunities for new structures. The conditions for the formation of 2,3-bis(hydroxyamino)-2,3-dimethylbutane, the key intermediate of Ullman's route, have been carefully studied, and a new procedure is proposed, which affords the free base in a very pure form and up to 60% yield. Full characterization of this intermediate including an X-ray crystal structure is presented. An alternative synthetic route through 2,3-diamino-2,3-dimethylbutane and the corresponding imidazolidines which bypasses the delicate synthesis of the bis(hydroxyamino) compound is described. It is shown that 3-chloroperbenzoic acid is an effective oxidant for the transformation of adequately substituted imidazolidines into nitronyl nitroxides, which are obtained in high yield. An illustration of the potentialities of this new route, a new nitronyl nitroxide with two ethyl substituents in positions 4 and 5 of the imidazoline ring, is reported. The scope and limitations of the two routes are discussed.

Copper(II) complexes of a racemic mixture of a chiral nitronyl-nitroxide are characterised. One, A, is a centro-symmetrical species where two enantiomers are coordinated to a metal centre. The second, B, is a 1D compound comprising bridging ligands through the oxyl and pyridyl donor sites. One observes that, although the crystals are racemic, within a chain, all ligands and metal centres have respectively the same chirality. Possibilities of obtaining optically active extended structures through chiral induction from nitroxide ligands to metal ions are discussed in relation with molecular spin transition species.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Pure organic ferromagnets, i.e. materials which contain no metallic centers, are very rare. We report here results of very low temperature AC susceptibility and DC magnetization measurements made on one of these exotic ferromagnets (formula C13H17N2O3) based on the spin S=12 free radical NIT (nitronyl nitroxide). A very weak hysteresis has been observed with a coercive field Hc=0.15Oe at 0.1K. (Tc⋍0.44mK) The single crystal sample appears to be perfectly isotropic, so that measurements along the three crystal axis allow for a self-consistent way to correct for demagnetization effects. Scaling of the data allows us to estimate values for critical exponents γ=1.35 and β=0.33, and a very large critical region is observed, extending up to nearly 10 times Tc.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTA S = 7 Ground Spin-State Cluster Built from Three Shells of Different Spin Carriers Ferromagnetically Coupled, Transition-Metal Ions and Nitroxide Free RadicalsKira E. Vostrikova, Dominique Luneau, Wolfgang Wernsdorfer, Paul Rey, and Michel VerdaguerView Author Information CEA-Département de Recherche Fondamentale sur la Matière Condensée Service de Chimie Inorganique et Biologique Laboratoire de Chimie de Coordination (URA CNRS 1194), 17 rue des Martyrs F38054 Grenoble Cedex 09, France Laboratoire Louis Néel, CNRS, 25 rue des Martyrs F38054 Grenoble Cedex 09, France Laboratoire de Chimie Inorganique et Matériaux Moléculaires (URA CNRS 7071) Université Pierre et Marie Curie 4 place Jussieu, F75252 Paris Cedex 05, France Cite this: J. Am. Chem. Soc. 2000, 122, 4, 718–719Publication Date (Web):January 19, 2000Publication History Received7 September 1999Published online19 January 2000Published inissue 1 February 2000https://pubs.acs.org/doi/10.1021/ja993210ohttps://doi.org/10.1021/ja993210orapid-communicationACS PublicationsCopyright © 2000 American Chemical SocietyRequest reuse permissionsArticle Views711Altmetric-Citations163LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Cluster chemistry,Ligands,Magnetic clusters,Magnetic properties,Noncovalent interactions Get e-Alerts