Search

A series of pyridyl‐substituted nitronyl nitroxides was synthesized and structurally characterized. A comprehensive magnetochemical and quantum chemical study of extended raw of the nitroxides with different substituents R in the pyridine fragment was performed. It was shown, that temperature‐dependent magnetic properties are determined by the short contacts between nitroxide groups of adjacent molecules as well as between nitroxide group and methyl substituents in the pseudo axial positions of imidazoline fragments. Quantum chemistry allows to select the appropriate model of exchange cluster for analysis of experimental magnetic data and evaluation of the exchange interaction parameters. For NN–PyCl the "order‐disorder" transition was detected by means of low temperature XRD. The difference in the experimental and calculated exchange interaction energies may serve as an indicator of temperature‐induced structural rearrangements. For instance, for methyl substituted nitronyl nitroxide NN‐PyMe structural transformations and significant changes in exchange interaction energies were observed. [ABSTRACT FROM AUTHOR]

A novel synthetic approach has been employed to synthesize a series of new nitronyl nitroxides: 2‐(1‐propyl‐1H‐imidazol‐5‐yl)‐ (Ln−Pr), 2‐(1‐isopropyl‐1H‐imidazol‐5‐yl)‐ (Li−Pr) and 2‐(1‐butyl‐1H‐imidazol‐5‐yl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐3‐oxide‐1‐oxyl (Ln−Bu). The reaction of Cu(hfac)2 with LR in a 1 : 2 ratio yields mononuclear heterospin complexes [Cu(hfac)2(LR)2] (LR=Ln‐Pr, Li‐Pr, Ln‐Bu), which have a similar crystal structure to the "jumping" crystals [Cu(hfac)2(LMe)2] that exhibit chemomechanical activity. It was shown that an increase in the alkyl substituent R leads to changes in the crystal packing of the molecules and the absence of chemomechanical activity. Furthermore, it was found that two polymorph modifications of the heterospin complex [Cu(hfac)2(Ln−Pr)2] can be obtained, and magnetic properties of [Cu(hfac)2(Ln−Pr)2] strongly depend on the angle between the planes of the paramagnetic fragment O•–N‐C=N→O and the imidazole ring in Ln−Pr. [ABSTRACT FROM AUTHOR]

High‐spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but those synthesized to date possess limited stability and processability. In this work, we have designed a tetraradical based on the Blatter's radical and nitronyl nitroxide radical moieties and successfully synthesized it by using the palladium‐catalyzed cross‐coupling reaction of a triiodo‐derivative of the 1,2,4‐benzotriazinyl radical with gold(I) nitronyl nitroxide‐2‐ide complex in the presence of a newly developed efficient catalytic system. The molecular and crystal structure of the tetraradical was confirmed by X‐ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼150 °C under an inert atmosphere and exhibits reversible redox waves at −0.54 and 0.45 V versus Ag/AgCl. The magnetic properties of the tetraradical were characterized by SQUID magnetometry of polycrystalline powders and EPR spectroscopy in various matrices. The collected data, analyzed by using high‐level quantum chemical calculations, confirmed that the tetraradical has a triplet ground state and a nearby excited quintet state. The unique high stability of the prepared triazinyl‐nitronylnitroxide tetraradical is a new milestone in the field of creating high‐spin systems. [ABSTRACT FROM AUTHOR]

By the reaction of M(hfac)2 (M = Mn(II), Co(II), Cu(II), and Zn(II); hfac is the hexafluoroacetylacetonate anion) and ferrocenyl-substituted nitronyl nitroxide (L), we succeeded in the synthesis of stable heterospin complexes: mononuclear [Zn(hfac)2L], trinuclear {[Cu(hfac)2]3L2} and chain polymer [Mn(hfac)2L]n and [Co(hfac)2L]n. The specific steric bulkiness of the ferrocenyl substituent leads to the formation of trans-type coordination polyhedra in the [Mn(hfac)2L]n and [Co(hfac)2L]n chains. The introduction of the ferrocene substituent leads to an effective weakening of intermolecular or interchain magnetic exchange coupling. Ferrimagnetic ordering was observed for one-dimensional complexes [M(hfac)2L]n (M = Mn(II), Co(II)). [Co(hfac)2L]n exhibits features of single-chain magnet behaviour: slow relaxation of magnetization below 13 K is associated with a high coercive field (54 kOe at 2 K). [ABSTRACT FROM AUTHOR]

The metal–radical approach is a well-established synthetic way toward multi-spin systems that relies on the coordination of stable radical ligands with transition metal ions. The advantage offered by the use of paramagnetic ligands is that metal–radical magnetic exchange coupling is direct between the magnetic orbitals of the radical and metal ion. With the aim of further exploring this approach, crystals of four heterspin complexes, [M(hfac)2LF]2 {M = Mn, Co, or Ni and hfac = hexafluoroacetylacetonate} and [Cu(hfac)2LF]n, were obtained using a new fluorinated pyrazolyl-substituted nitronyl nitroxide radical, 4,4,5,5-tetramethyl-2-(5-fluoro-1-methyl-1H-pyrazol-4-yl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LF) as a ligand. The newly synthesized complexes were fully characterized, including X-ray crystallography and magnetometry. XRD analysis revealed that complexes [M(hfac)2LF]2 have similar dimer structures in which a metal ion is in a six-coordinated environment with four O atoms from the two hfac ligands, one radical O atom, and one pyrazole N atom from ligand LF. Nonetheless, the packing patterns of the complexes were found to be considerably different. In [Mn(hfac)2LF]2, there are no magnetically important short contacts between manganese dimers. By contrast, in [Co(hfac)2LF]2 and [Ni(hfac)2LF]2, there are short contacts between non-coordinate O atoms of nitronyl nitroxide moieties. Magnetic behaviors of [M(hfac)2LF]2 showed that the M ions and the directly coordinated radicals are strongly antiferromagnetically coupled (JMn-ON = −84.1 ± 1.5 cm−1, JCo-ON = −134.3 ± 2.6 cm−1, and JNi-ON = −276.2 ± 2.1 cm−1; H ^ = − 2 J S ⃑ ^ M S ⃑ ^ N O ). Notably, the magnetization of [Mn(hfac)2LF]2 having molecular structure proved to be accompanied by hysteresis. The [Cu(hfac)2LF]n complex has a chain-polymer structure with alternating magnetic fragments: three spin exchange clusters {ONO–Cu(II)–ONO} and {Cu(II)} ions. Despite the direct coordination of radicals, its magnetic properties are weakly ferromagnetic (JCu-ON = 14.8 ± 0.3 cm−1). [ABSTRACT FROM AUTHOR]

Bimolecular complexes [ML2(MeOH)2][ML2(EtOH)2], where M is CoII, NiII or Co II0.5 Ni II0.5 and L is a deprotonated enaminoketone 4-(3′,3,3′-trifluoro-2′-oxopropylidene)-2,2,5,5-tetramethyl-3-imidazolidin-1-oxyl, were found to be isolated in the solid phase from a solution containing MeOH and EtOH in the ratio 1 : 10. Their solids have a layered-polymer structure due to hydrogen bonds between the OH-groups of the coordinated ROH molecules and nitroxide groups of the neighboring [ML2(ROH)2] molecules. The peculiar crystal structures of [ML2(MeOH)2][ML2(EtOH)2] have ordered alternation of methanol- and ethanol-containing supramolecular layers. Detailed crystallochemical analysis showed that during the formation of [ML2(MeOH)2][ML2(EtOH)2] crystals, the MeOH containing layer is significantly deformed in comparison with the [ML2(MeOH)2] structure. This indicates that the ethanol-containing layer determines the crystal formation, while the {ML2(MeOH)2} layer "adjusts" to it. Magnetochemical study of the bimolecular complexes [ML2(MeOH)2][ML2(EtOH)2] revealed magnetic ordering at low temperatures. [ABSTRACT FROM AUTHOR]

Novel nitronyl nitroxides, namely 2-(3-iodoethynylphenyl)- and 2-(4-iodoethynylphenyl)-4,4,5,5-tetramethyl-imidazoline-3-oxyl-1-oxides, were prepared by condensation of appropriate aldehydes with 2,3-bis(hydroxylamino)-2,3-dimethylbutane followed by oxidative treatment with NaIO4. Crystal and molecular structures of the obtained paramagnets were studied by single-crystal X-ray diffraction. In the crystals, the radicals are assembled into zigzag chains in which the radical building blocks are linked by intermolecular I⋯N–O halogen bonding. Magnetic analyses revealed that in both nitronyl nitroxides, the radicals are weakly coupled. Nonetheless, spin–spin interactions in the 3-iodoethynyl isomer are antiferromagnetic, and the coupling is stronger than that in the 4-iodoethynyl derivative, in which the exchange interaction is ferromagnetic. DFT calculations in combination with MEP, NCIplot, and QTAIM analyses were used to evaluate and characterize the structure-directing halogen bonding interactions observed in the solid state of both compounds. [ABSTRACT FROM AUTHOR]

New complexes of manganese(II) hexafluoroacetylacetonate [Mn(hfac)2] with 2-(1-R-3-pyrazol-4-yl)-4,4,5,5-tetramethyl-2-imidazoline-3-oxide-1-oxyl (R = CHF2, CH2CH2F, CH2CHF2 or CH2CF3) were synthesised and characterised structurally and magnetically. All complexes were prepared under similar conditions. Nonetheless, their crystal structures were considerably different. Depending on the structure of fluorinated alkyl substituent R, the complexation reaction led to complexes of three types: chain-polymeric complexes with the head-to-head or head-to-tail motif and complexes of molecular structure. All complexes show strong antiferromagnetic behaviour in a high-temperature region (150–300 K) and weak ferro- or antiferromagnetic exchange interactions at low temperatures. The stronger antiferromagnetic exchange, −101.7 ± 1.5 or −136 ± 10 cm−1, −82.3 ± 1.3 cm−1 and −87.4 ± 1.3 cm−1, was attributed to the magnetic interaction in three- or two-spin clusters: {>N∸O–Mn2+–O∸N<} or {>N∸O–Mn2+}, respectively. The weaker antiferromagnetic interaction, −0.005, between three-spin clusters or ferromagnetic interactions, 0.18–0.81 cm−1, between two-spin clusters are realised through the pyrazole ring or intermolecular contacts. [ABSTRACT FROM AUTHOR]

Two ruthenium nitrosyl complexes of Na[RuNOCl4L] with nitronyl nitroxide radicals coordinated to ruthenium with N-donor pyridine rings were prepared and described. The crystal structure of both complexes is 1D or 2D polymeric, due to the additional coordination of sodium cation by bridging the chloride ligands or oxygen atoms of nitroxides. Partially, the oligomeric forms remain in the solutions of the complexes in acetonitrile. The magnetic measurements in the solid state demonstrate the presence of antiferromagnetic interactions through the exchange channels, with the distance between paramagnetic centers equal to 3.1–3.9 Å. The electrochemical behavior of the prepared complexes was investigated in acetonitrile solutions. [ABSTRACT FROM AUTHOR]

The 2-imidazoline nitroxide derivatives of cymantrene—2-(η5-cyclopentadienyl)tricarbonylmanganese(I)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (NNMn) and 2-(η5-cyclopentadienyl)tricarbonylmanganese(I)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (INMn) were synthesized. It was shown that NNMn and INMn exhibit a sufficiently high kinetic stability both in solids and in solutions under normal conditions. Their structural characteristics, magnetic properties and electrochemical behavior are close to Re(I) analogs. This opens the prospect of using paramagnetic cymantrenes as prototypes in the design of Re(I) half-sandwiched derivatives for theranostics, where therapy is combined with diagnostics by magnetic resonance imaging due to the contrast properties of nitroxide radicals. [ABSTRACT FROM AUTHOR]

While developing methods for obtaining high-spin graphene nanostructures, we carried out a model cross-coupling reaction between 1,8-diiodonaphthalene and an organogold derivative of a nitronyl nitroxide. It was found that only one iodine atom reacted, leading to the corresponding 8-iodonaphthalen-1-yl-substituted nitronyl nitroxide. The latter was successfully isolated as two polymorphic modifications, whose X-ray structural analysis revealed a strong distortion of the paramagnet's geometry in comparison with a noniodinated analog. Moreover, the spatial proximity of the heavy iodine atom and the paramagnetic moiety resulted in unprecedented temperature dynamics of the EPR spectrum. To clarify these dynamics, quantum chemical calculations were performed using the exact two-component relativistic X2C method with the self-consistent account of spin-orbit coupling in conjunction with the B3LYP functional and relativistic STO-type basis set. These computations predicted a reasonable aiso(127l) value that can affect the EPR spectrum. Substitution of the second iodine atom would lead to the formation of a diradical with a 1,8-naphthalenediyl bridge, whose structure and magnetic properties were analyzed at the DFT level. Two stable conformations of the diradical with strongly distorted geometry were identified. In both conformations, calculations predict a ferromagnetic exchange between the paramagnetic centers with J=8 and 14 cm-1 (H= -2 J·S1S2). [ABSTRACT FROM AUTHOR]

Thermally stable organic diradicals with a triplet ground state along with large singlet‐triplet energy gap have significant potential for advanced technological applications. A series of phenylene‐bridged diradicals with oxoverdazyl and nitronyl nitroxide units were synthesized via a palladium‐catalyzed cross‐coupling reaction of iodoverdazyls with a nitronyl nitroxide‐2‐ide gold(I) complex with high yields. The diradicals exhibit high stability and do not decompose in an inert atmosphere up to 180 °C. For the diradicals, both substantial AF (ΔEST≈−64 cm−1) and FM (ΔEST≥25 and 100 cm−1) intramolecular exchange interactions were observed. The sign of the exchange interaction is determined both by the bridging moiety (para‐ or meta‐phenylene) and by the type of oxoverdazyl block (C‐linked or N‐linked). Upon crystallization, diradicals with the triplet ground state form unique one‐dimensional exchange‐coupled chains with strong intra‐ and weak inter‐diradical ferromagnetic coupling. [ABSTRACT FROM AUTHOR]

Functionalized diradicals are widely used in chemistry, physics, biology and material sciences because of their inherently high reactivity and magnetic activity, but the synthesis of planar diradical molecules is still a challenge. This paper describes the effective synthesis and properties of diradicals in which nitronyl nitroxide fragments and a fused-thiophene bridge are almost coplanar. The diradicals DR1, DR2, DR4 were prepared in high yield by of the (nitronyl nitroxide-2-ide)(triphenylphosphin)gold complex with 2,5-diiodothieno[2,3- b]thiophene, 2,5-diiodothieno[3,2- b]thiophene or 2,6-diiododithieno[3,2- b:2′,3′- d]thiophene correspondingly. The torsion angles between planes of the nitronyl nitroxide fragments and fused-thiophene systems lie in the range 2.6-8.5°. The planarity of DR2 and DR4 makes the Kekulé-type coupler units effective bridges to transmit magnetic interactions by a spin polarization mechanism. According to the magnetic susceptibility measurements, supported by density functional theory calculations, the singlet-triplet energy splitting (2 J/ kB) is −130 K for DR2 and close to −80 K for DR4. The singlet diradicals DR2 and DR4 are stable and demonstrate two one-electron reversible electrochemical oxidation peaks and two-electron quasi-reversible reduction peaks resulting in the formation of the corresponding bis(oxoammonium) dication or bis(aminooxidanide) dianion. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Spin-labelled compounds are widely used in chemistry, physics, biology and the materials sciences but the synthesis of stable high-spin organic molecules is still a challenge. We succeeded in synthesising heteroatom analogues of the 1,1,2,3,3-pentamethylenepropane (PMP) diradicals with two nitronyl nitroxide ( DR1) and with two iminonitroxide ( DR2) fragments linked through the C(sp2) atom of the nitrone group. According to magnetic susceptibility measurements, EPR data and ab initio calculations at the (8,6)CASSCF and (8,6)NEVPT2 levels, DR1 and DR2 have singlet ground states. The singlet-triplet energy splitting (2 J) is low ( J/ k=−7.4 for DR1 and −6.0 K for DR2), which comes from the disjoint nature of these diradicals. The reaction of [Cu(hfac)2] with DR1 gives rise to different heterospin complexes in which the diradical acts as a rigid ligand, retaining its initial conformation. For the [{Cu(hfac)2}2( DR1)(H2O)] complex, sufficiently strong ferromagnetic interactions ( J1/ k=42.7 and J2/ k=14.1 K) between two coordinating CuII ions and DR1 were revealed. In [{Cu(hfac)2}2( DR1)(H2O)][Cu(hfac)2(H2O)], the very strong and antiferromagnetic ( J/ k=−416.1 K) exchange interaction between one of the coordinating CuII ions and DR1 is caused by the very short equatorial CuO bond length (1.962 Å). [ABSTRACT FROM AUTHOR]

New stable polyfluorinated nitroxide radicals for use in cross-coupling reactions, namely, N-tert-butyl-N-oxyamino-2,3,5,6-tetrafluoro-4-iodobenzene and N-tert-butyl-N-oxyamino-2,3,5,6-tetrafluoro-4-ethynylbenzene, were prepared from perfluoroiodobenzene. The reaction of the polyfluoro derivative with tert-butylamine under autoclaving conditions leading to the formation of N-tert-butyl-2,3,5,6-tetrafluoro-4-iodoaniline proved to be the key stage of the whole process. The fluorinated tert-butyl iodophenyl nitroxide was found to form in a solid state via N–O···I halogen bonds, a one-dimensional assembly of the radicals. The acceptor role of the nitroxide group in the halogen bonding changes to a donor role when the nitroxide reacts with Cu(hfac)2. In the last case, zero-dimensional assembly prevails, giving a three-spin complex with axial coordinated nitroxide groups and, as a consequence, causing ferromagnetic intramolecular exchange interactions between Cu(II) and radical spins. [ABSTRACT FROM AUTHOR]

In contrast to diradicals connected by alternant hydrocarbons, only a few studies have addressed diradicals connected by nonalternant hydrocarbons and their heteroatom derivatives. Here, the synthesis, structure, and magnetic properties of pyrrole-2,5-diyl–linked bis(nitronyl nitroxide) and bis(iminonitroxide) diradicals are described. The diradicals show characteristic electron spin resonance spectra in dilute glassy solutions, from which conclusions about the presence of distinct conformations, their symmetry, and interspin distance were made. X-ray diffraction analysis of the diradicals revealed that paramagnetic moieties lie in the plane of the pyrrole ring, because of the formation of an intramolecular hydrogen bond, ONO...H−N, with O...H distances of 2.15–2.23 Å. The N–O groups participating in the formation of H-bonds have greater bond lengths (~1.29 Å) as compared with nonparticipating groups (~1.27 Å). The nitronyl nitroxide and iminonitroxide diradicals showed an intramolecular antiferromagnetic interaction, with J = −77.3 and −22.2 cm−1, respectively (H = −2JS1⋅S2). [ABSTRACT FROM AUTHOR]

The interaction of octafluorotoluene (1a), as well as pentafluorobenzonitrile (1b) with tert-butylamine, followed by the oxidation of thus formed tert-butylanilines (2a,b) with meta-chloroperoxybenzoic acid led to functionalized perfluorinated phenyl tert-butyl nitroxides [namely, 4-(N-tert-butyl(oxyl)amino)heptafluorotoluene (3a) and 4-(N-tert-butyl(oxyl)amino)tetrafluorobenzonitrile (3b)] with nearly quantitative total yields. The molecular and crystal structures of nitroxide 3a were proved by single crystal X-ray diffraction analysis. The radical nature of both nitroxides was confirmed by ESR data. The interaction of Cu(hfac)2 with the obtained nitroxides 3a,b gave corresponding trans-bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato-κ2O,O′)bis{4-(N-tert-butyl(oxyl)amino)perfluoroarene-κO}copper (II) complexes ([Cu(hfac)2(3a)2] and [Cu(hfac)2(3b)2]). X-ray crystal structure analysis showed square bipyramid coordination of a centrally symmetric Cu polyhedron with the axial positions occupied by oxygen atoms of the nitroxide groups. Magnetic measurements revealed intramolecular ferromagnetic exchange interactions between unpaired electrons of Cu(II) ions and paramagnetic ligands, with exchange interaction parameters JCu–R reaching 53 cm−1. [ABSTRACT FROM AUTHOR]

Methods for the synthesis of binuclear [Cu(hfac)2LEt]2 and tetranuclear [[Cu(hfac)2]4(LEt)2] heterospin compounds based on copper hexafluoroacetylacetonate [Cu(hfac)2] and 2-(4-ethylpyridin-3-yl)-4,5-bis(spirocyclopentyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt), were developed. The crystals of the complexes are elastic and do not crash during repeated cooling–heating cycles. It was found that a singlet–triplet conversion occurred in all of the {Cu(II)–O•–N<} exchange clusters in the molecules of the binuclear [Cu(hfac)2LEt]2 which led to spin coupling with cooling. The transition occurred in a wide temperature range with a maximum gradient ΔχT at ≈180 K. The structural transformation of the crystals takes place at T < 200 K and is accompanied by the lowering of symmetry from monoclinic to triclinic, twinning, and a considerable shortening of the Cu–ONO distance (2.19 and 1.97 Å at 295 and 50 K, respectively). For the tetranuclear [[Cu(hfac)2]4(LEt)2], two structural transitions were recorded (at ≈154 K and ≈118 K), which led to a considerable change in the spatial position of the Et substituent in the nitronyl nitroxyl fragment. The low-temperature process was accompanied by a spin transition recorded as a hysteresis loop on the χT(T) curve during the repeated cooling–heating cycles (T½↑ = 122 K, T½↓ = 115 K). This transition is unusual because it causes spin coupling in half of all of the {>N–•O–Cu2+} terminal exchange clusters, leading to spin compensation for only two paramagnetic centers of the six centers in the molecule. [ABSTRACT FROM AUTHOR]

Bimolecular complexes [ML2(MeOH)2][ML2(EtOH)2], where M is CoII, NiII or Co II0.5 Ni II0.5 and L is a deprotonated enaminoketone 4-(3′,3,3′-trifluoro-2′-oxopropylidene)-2,2,5,5-tetramethyl-3-imidazolidin-1-oxyl, were found to be isolated in the solid phase from a solution containing MeOH and EtOH in the ratio 1 : 10. Their solids have a layered-polymer structure due to hydrogen bonds between the OH-groups of the coordinated ROH molecules and nitroxide groups of the neighboring [ML2(ROH)2] molecules. The peculiar crystal structures of [ML2(MeOH)2][ML2(EtOH)2] have ordered alternation of methanol- and ethanol-containing supramolecular layers. Detailed crystallochemical analysis showed that during the formation of [ML2(MeOH)2][ML2(EtOH)2] crystals, the MeOH containing layer is significantly deformed in comparison with the [ML2(MeOH)2] structure. This indicates that the ethanol-containing layer determines the crystal formation, while the {ML2(MeOH)2} layer "adjusts" to it. Magnetochemical study of the bimolecular complexes [ML2(MeOH)2][ML2(EtOH)2] revealed magnetic ordering at low temperatures. [ABSTRACT FROM AUTHOR]

A new group of "breathing" crystals has been synthesized. These are aromatic solvates of the copper(II) hexafluoroacetylacetonate complex with spin-labeled pyrazole Cu(hfac)2L.0.5Solv, where L is 2-(1-butyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and Solv is benzene, toluene, ethylbenzene, propylbenzene, butylbenzene, styrene, o-xylene, m-xylene, p-xylene, 1,4-bis(trifluoromethyl)benzene, 1-methyl-4-ethylbenzene, 1-methyl-4-vinylbenzene, 1,4-diethylbenzene, 1,2,3-trimethylbenzene, or 1,2,4-trimethylbenzene. The main feature of Cu(hfac)2L.0.5Solv single crystals is their remarkable mechanical stability and ability to undergo thermally induced structural rearrangements accompanied by spin-crossover-like phenomena. The structures of Cu(hfac)2L.0.5Solv solvates are similar and based on mutually parallel {Cu(hfac)2L}∞ heterospin chains with a "head-to-head" motif. The localization of voids with guest molecules being the same in all crystals, the temperature dependence of the effective magnetic moment (μeff) for Cu(hfac)2L.0.5Solv is determined by the structure of the guest molecules, along which the polymer chains are "gliding" when the temperature changes. When the temperature decreased from 300 to 100-50 K, μeff decreased, abruptly or gradually, from ∼2.7-2.4 to 1.∞ β for the majority of Cu(hfac)2L.0.5Solv except the solvates with benzene, toluene, and 1,4-bis(trifluoromethyl)benzene. When Cu(hfac)2L.0.5C6H6 and Cu(hfac)2L.0.5CH3-C6H5 were cooled to 50 K, μeff decreased to ∼2.1-2.2 β. When Cu(hfac)2L.0.5(1,4-(CF3)2-C6H4) was cooled to 50 K, μeff initially decreased from ∼2.7 to 1.9 β and then abruptly increased to ∼2.4 β. A single-crystal X-ray diffraction analysis of each solvate within a temperature range wider than the range of magnetic anomaly temperatures revealed a complex interrelated dynamics of the aromatic solvent guest molecules and heterospin chains. The dynamics largely depended on the orientation of the solvent guest molecules relative to the polymer chains. An analysis of the thermally induced phase transformations revealed a relationship between the structural rearrangement of Cu(hfac)2L.0.5Solv and the form of the magnetic anomaly on the μeff(T) curve and between the structural rearrangement of the solvate and the temperature of the magnetic effect. [ABSTRACT FROM AUTHOR]

[Display omitted] A one-stage synthesis of paramagnetic Re(I) compounds based on the reaction of [Re(CO) 5 Br] with spin-labeled N -donor heterocycles was developed. Nitronyl nitroxide derivatives of 1 H- imidazole (L1), 1 H- pyrazole (L2), di(1 H -pyrazol-1-yl)methane (L3) and 4-methyl-2-(1 H -pyrazol-1-yl)quinoline (L4) can easily substitute two coordinated CO molecules in [Re(CO) 5 Br], resulting in a series of paramagnetic fac- [Re(CO) 3 (L i) n Br]· x Solv complexes. Single crystal XRD studies showed that the paramagnetic L1-L4 ligands are coordinated by Re(I) through the imine atoms N of the heterocycles. According to SQUID measurements and EPR spectroscopy data, magnetic behavior of [Re(CO) 3 (L1) 2 Br] (1), [Re(CO) 3 (L2) 2 Br] (2), and [Re(CO) 3 (L3)Br] (3) complexes are typical for biradicals, while [Re(CO) 3 (L4)Br] (4) is typical for monoradical. The combination of Re(I) therapeutic properties and contrasting characteristics of nitroxide radicals in diagnostic magnetic resonance imaging, stability and simplicity of synthesis make [Re(CO) 3 (L i) n Br] valuable model objects for material design for theranostics purposes. [ABSTRACT FROM AUTHOR]

Hitherto unknown 4-substituted TEMPO derivatives bearing β-(oxy)acrylate moiety have been synthesized via nucleophilic addition of 4-hydroxy-TEMPO to methyl and ethyl propiolate, respectively. The reaction of these radicals (L) with Cu(hfac) 2 produces heterospin chain-polymers [Cu(hfac) 2 L] n , which contain alternating { N – · O–Cu–O – · N } and { C O–Cu–O C } fragments. The magnetic behavior of these coordination polymers have been studied using SQUID magnetometry to reveal that the complex based on 4-[ E -(3-methoxy-3-oxoprop-1-en-1-yl)oxy]-TEMPO does not show significant exchange interactions between the spins of the Cu 2+ ion and the nitroxides. By contrast, complex with 4-[ E -(3-ethoxy-3-oxoprop-1-en-1-yl)oxy]-TEMPO features ferromagnetic exchange interaction between the paramagnetic centers. [ABSTRACT FROM AUTHOR]

Four coordinated bis-o-semiquinonato nickel complexes interact with pyridine forming five- or six-coordinated compounds. Coordination number and structure of the product depends on the nature of o-quinonato ligand and synthetic conditions. (Py) 2 Ni(3,6-DBSQ) 2 ·Tol has trans-arrangement of pyridine ligands whereas in it analog – (Py) 2 Ni(4,5-N,N′-pipe-3,6-DBSQ) 2 pyridine molecules occupy cis-positions (3,6-DBSQ – anion-radical of 3,6-di- tert -butyl-o-benzoquinone; 4,5-N,N′-pipe-3,6-DBSQ – tricyclic 5,8-di( tert -butyl)-2,3-dihydro-1,4-ethanoquinoxaline-6,7-dione). The structure of six-coordinated compounds is close to octahedral which promotes high values of effective magnetic moments and the predomination of ferromagnetic exchange interactions. In spite of different geometries complexes demonstrate very close magnetic behavior. Using 1:1 stoichiometric ratio allows to obtain five-coordinated nonsolvated square-pyramidal complex (Py)Ni(3,6-DBSQ) 2 with disordered position of pyridine. The last compound has essentially lower magnetic moment compared to six coordinated analogs. Complex reveals strong metal–ligand antiferromagnetic coupling due to deviation of nickel atom out of the bis-o-semiquinonato plane. Pyridine modified in 4-position by nitronyl-nitroxide radical (4-NIT-Py – 2-(4-pyridinyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) also coordinate to bis-o-semiquinonato nickel precursors forming (4-NIT-Py) 2 Ni(3,6-DBSQ) 2 and (4-NIT-Py) 2 Ni(4,5-N,N′-pipe-3,6-DBSQ) 2 . Nickel complexes containing different types of free radical ligands were obtained for the first time. However two additional spins do not produce significant changes in their magnetic behavior. [ABSTRACT FROM AUTHOR]

The {[Cu(hfac) 2 LR] 2 [Cu(hfac) 2 ]} ∞ chain-polymer complexes with alkyl-pyridyl-substituted nitroxides LR exhibit phase transitions. The transition temperature increases, while the ratio of the volume of cavity, occupied by R, to the volume of R decreases in the series of compounds with R = i -Pr, n -Pr, Et, CP, n -Bu, Me, MeO. [Display omitted] Reaction of Cu(hfac) 2 with pyridyl-substituted nitronyl nitroxides yielded chain-polymer complexes {[Cu(hfac) 2 LR] 2 [Cu(hfac) 2 ]} ∞ , where LR – 2-(4-R-pyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H -imidazole-3-oxide-1-oxyl, R = Me, MeO, Et, n -Pr, i -Pr, n -Bu, and CP (cyclopentyl). X-ray crystallography experiments proved that the solid state of the investigated compounds has centrosymmetric dimers [Cu(hfac) 2 LR] 2 bonded by the Cu(hfac) 2 fragments into chain polymers. The environment of the Cu atoms in the dimeric fragments undergoes structural rearrangement upon temperature variation. Such transformations cause anomalous temperature dependences of µ eff. We established the relationship between the phase transition temperature and the amount of space accessible for pyridine substituent R. In the complexes containing LMe, LMeO, L n-Bu, and LCP substituents R occupy relatively small amount of space, which results in ordering of R and high transition temperatures of these complexes, whereas the presence of the accessible volume for the substituent disordering leads to increased entropy of the high temperature phase, and, therefore, decreased transition temperature of the complexes with LEt, L i-Pr, and L n-Pr. [ABSTRACT FROM AUTHOR]

Recrystallization of [Cu 4 (hfac) 4 (MeO) 4 ] and [Cu 11 (hfac) 10 (OH) 8 (O) 2 ]·2C 7 H 8 and their interaction with nitroxides in acetone containing media led to self-assembling of complexes containing fragment {Cu 4 (hfac) 2 (LHF) x (LF) 2-x } (x = 0–2), with triply-charged tetrahydropyrantriol-anions. The LHF/LF ratio is determined by the presence of base and acetone in the reaction mixture and the duration of crystallization. [Display omitted] It was shown experimentally that interaction of polynuclear hexafluoroacetylacetonates [Cu 4 (hfac) 4 (MeO) 4 ] and [Cu 11 (hfac) 10 (OH) 8 (O) 2 ]·2C 7 H 8 with nitronyl nitroxide radicals as well as their recrystallization in the presence of acetone led to complexes containing tetranuclear fragment {Cu 4 (hfac) 2 (LHF) x (LF) 2- x } (x = 0–2), where LHF and LF are triply charged anions of 2-methyl-4,6-bis(trifluoromethyl)tetrahydropyran-2,4,6-triol and 2,4,6-tris(trifluoromethyl)tetrahydropyran-2,4,6-triol, respectively. The main factors contributing to the formation of the 4-nuclear fragment and trioxanes LHF and LF were identified. It is established that self-assembly of {Cu 4 (hfac) 2 (LHF) x (LF) 2- x } is observed only in the case of mono- and polynuclear CuII hexafluoroacetylacetonates. For CoII and NiII compounds the formation of such a fragment is not registered. [ABSTRACT FROM AUTHOR]

• Stable nitronyl nitroxides bearing N -fluoroalkylpyrazolyl substituents. • Reversibility of electrochemical oxidation and reduction. • Crystal structures analyzes based on pairwise interaction energies and NO...ON distances. • Packing Motifs and Magneto-Structural Correlations. The aim of the study was to prepare and investigate structure and properties of a new series of stable nitronyl nitroxides bearing N -fluoroalkylpyrazolyl substituents. According to cyclic voltammetry data, a paramagnet with a difluoromethyl substituent can reversibly undergo electrochemical oxidation and reduction. In contrast, radicals with fluorinated ethyl substituents are electrochemically oxidized reversibly, but their electrochemical reduction is quasi-reversible. Molecular and crystal structures of the nitroxides were confirmed by single-crystal X-ray diffraction analysis. The crystal structures of the radicals were studied quantitatively in terms of pairwise interaction energies and energy framework analysis. Analysis of the energies of molecular pairs revealed that in crystalline phases of all nitroxides, the binding is the strongest in dimers assembled via C–H...O interactions of nitroxide O atoms with pyrazole and/or alkyl H atoms. Other dimers are formed predominantly by C–H...π interactions, and in some cases, their binding energy is almost equal to that for the aforementioned ones. The above contacts are complemented short contacts C–H...A and others, which predetermine the final crystal packing of radicals. A consequence of their packing is the geometry of relative arrangement of nitroxide groups and, above all, distances O...O between the oxygen atoms of the paramagnetic centers. These distances were found to exceed the sum of van der Waals radii; consequently, in solid phases of the paramagnets, weak antiferromagnetic exchange interactions are predominant (| J/k B | ≤ 2.74 K, H = – 2J·S 1 S 2. [ABSTRACT FROM AUTHOR]

Abstract: The synthesis, structure and photoluminescence of a group of substituted 2-pyrazolylquinolines, including a spin-labelled derivative, are described. A comparative study of photoluminescence from the synthesized compounds demonstrates that introduction of a nitronyl nitroxide radical group results not only in the expected reduction in photoluminescence from the pyrazolylquinoline moiety, but also gives rise to a new red photoluminescence band (maximum at 692nm in acetonitrile) from the radical fragment. [Copyright &y& Elsevier]

Breathing crystals based on polymer-chain complexes of Cu(hfac)2 with nitroxides exhibit thermally and light-induced magnetostructural anomalies in many aspects similar to a spin crossover. In the present work, we report the synthesis and investigation of a new family of Cu(hfac)2 complexes with tert-butylpyrazolylnitroxides and their nonradical structural analogues. The complexes with paramagnetic ligands clearly exhibit structural rearrangements in the copper(II) coordination units and accompanying magnetic phenomena characteristic for breathing crystals. Contrary to that, their structural analogues with diamagnetic ligands do not undergo rearrangements in the copper(II) coordination environments. This confirms experimentally the crucial role of paramagnetic ligands and exchange interactions between them and copper(II) ions for the origin of magnetostructural anomalies in this family of molecular magnets. [ABSTRACT FROM AUTHOR]

Abstract: An approach to the synthesis of nitronyl nitroxide 2,2′-(buta-1,3-diyne-1,4-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide 1-oxyl) (4) was developed. Compound 4 is the first diradical with nitronyl nitroxide groups directly linked through a diacetylene fragment. In solid phase the diradicals are arranged in stacks with parallel Cts, with the distances between the terminal carbon atoms of the neighboring diacetylene groups (T and d) being 6.170 and 4.466Å, respectively, and the angle between the translation vector and the median line passing through the Ch name="sbnd" />Ct of 45.9°. The values of T and d are outside the range of structural criteria allowing a topochemical reaction. Thus UV irradiation does not initiate solid phase polymerization of 4. After exposure at 373K for 1h the crystals of 4 turn dark-brown, become X-ray amorphous and lose the majority of their paramagnetic centers without significantly changing their mass. Upon further heating up to 400–420K the product explodes, releasing about 360kJ/mol of heat. A diluted solution of 4 in 1,4-dioxane produces an EPR spectrum typical of a strong exchange (a multiplet of nine broadened lines with A 4N =0.35mT), indicating the efficiency of the Ch name="sbnd" />Ct as an exchange channel. The character of the experimental μ eff(T) dependence for 4 indicates a strong intramolecular antiferromagnetic-type exchange interaction (J/k B ∼−104K) and the dominating weak intermolecular ferromagnetic exchange. [Copyright &y& Elsevier]

The crystals of heterospin complexes [M(hfac)2L2J (where M = Cu, Ni, Co, or Mn; hfac = hexafluoroacetylacetonate; and L = nitronyl nitroxide, 4,4,5,5-tetramethyl-2-(l-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 accom?anied by the cracking and disintegra- tion of crystals, and a solid IM(hfac)2(L¹ )2] complex with the corresponding imino nitroxide 4,4,5,5-tetramethyl-2-(1-methyl-lH-imidazol-5-yl)-4,5-dthydro-1H-imidazole-1-oxyl (L¹) 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)2L2] (where M = Cu, Ni, Co, or Mn) showed that the molecular structure of all [M(hfac)2L2] and their packing in the solid state were identical. The packing of [M(hfac)2L2] was concluded to be critical to the mechanical effect. In complexes with different stoichiometries or different sets of diamagnetic ligands ([Cu(hfac)2L]2, [Cu(hfac)(acac)L]· EtOH, [CuPiv2L2]· CH2CI2, and [Cu(hfac)2L2Cu2Piv4]· 3C7H8 (where acac is acetylacetonate and Piv is trimethylacetate), or free L), the effect vanished when the packing changed. [ABSTRACT FROM AUTHOR]

Abstract: Solid heterospin compounds based on Cu(hfac)2 complexes with a new group of nitronyl nitroxides bearing different azine-N-oxide substituents at position 2 of the 2-imidazoline ring (L n ) were studied. The major factor responsible for the change in the magnetic characteristics of the [Cu(hfac)2L1] complex with triazine nitronyl nitroxide with temperature was shown to be the specific pairwise packing of heterospin molecules with the dominant antiferromagnetic exchange between the radical fragments of adjacent molecules. For complexes of Cu(hfac)2 with 1-oxoazin-2-yl-substituted nitronyl nitroxides L2 and L4, 7-membered metallocycles were obtained, although they form rarely. It was shown that polymer chains formed in the solid complex with spin-labeled pyrazine-N-oxide [(Cu(hfac)2)3(L3)2] due to the cross-linking of {(Cu(hfac)2)2(L3)2} binuclear fragments via the bridging [Cu(hfac)2]. [Copyright &y& Elsevier]

It is shown that SNH 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-1H-imidazol-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 buten-3-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) isoxazol-5-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-3-oxide-1-oxyl). The new nitroxides were characterized by X-ray single crystal data, ESR and static magnetic susceptibility measurements. [ABSTRACT FROM AUTHOR]

Abstract: 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 Cu4(OH)2(OAc)4(DMF)2(L1)2 tetranuclear and Cu2(OAc)2(H2O)2(L3)2 dinuclear complexes, respectively. The reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted nitronyl nitroxide (HL 2) gave unexpected solid Cu2(H2O)2(L6)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 Cu2(DMF)2(L7)2, where L 7 is deprotonated 2-(5-carboxy-1H-pyrazol-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide. An X-ray diffraction study indicated that in Cu4(OH)2(OAc)4(DMF)2(L1)2 and Cu2(OAc)2(H2O)2(L3)2, the L 1 and L 3 paramagnetic ligands perform the bridging cyclic tridentate function, while in Cu2(H2O)2(L6)2 ·2DMF and Cu2(DMF)2(L7)2, the paramagnetic L 6 and diamagnetic L 7 are bridging bicyclic tetradentate ligands. The magnetic behavior of complexes with coordinated nitronyl nitroxide – Cu4(OH)2(OAc)4(DMF)2(L1)2 and Cu2(H2O)2(L6)2 ·2DMF is dictated by the dominant antiferromagnetic exchange interactions, which is confirmed by quantum-chemical data. The magnetic susceptibility of Cu2(OAc)2(H2O)2(L3)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. [Copyright &y& Elsevier]