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The structures of four products formed in the interaction of lithiated N-methylpyrazole with isoamyl nitrite are determined. One of the reaction products is found to be a 1,4-diazocine derivative–a medium-sized condensed heterocycle in which three pyrazole moieties are annulated to an eight-membered heterocycle and yet another pyrazole moiety is located in a side chain. This molecular structure of the compound blocks the transformation of the eight-membered heterocycle into the bicyclic derivative through a transannular reaction.

The Cu(hfac)2 complex with a paramagnetic ligand 2-(1-ethyl-imidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L5Et) was synthesized in the form of two polymorphs: α-[Cu(hf...

4,4′-(Triaz-1-ene-1,3-diyl)dipyridine (L) and the first transition metal complex with it are synthesized. It is found that the solid phase of isolated solvate [Cu(hfac)2L]·0.5MePh, where hfac is hexafluoroacetylacetonate, consists of polymer chains inside which L acts as a bridge, linking together Cu(hfac)2 moieties by N atoms of pyridine rings. The chains, in turn, are arranged into layers by H bonds. The result shows that L can be a bridging ligand in the synthesis of new coordination polymers.

Copper(ii) complexes with stable nitroxide radicals are capable of magnetostructural spin-crossover like anomalies induced by external stimuli. Photoswitching in such systems is particularly important; however, retrieving the properties of photoinduced states is challenging and requires development of novel approaches. In this work, we investigate the exchange interactions in metastable photoinduced states of two compounds containing copper(ii)-nitroxide dyads. Using Electron Paramagnetic Resonance (EPR) with photoexcitation we obtain temperature dependence of magnetic susceptibility in the photoinduced state and estimates for the corresponding values of exchange coupling in the studied complexes. The interplay between intra- and inter-cluster exchange couplings is considered and analyzed. The proposed methodology is applicable also to other photoswitchable exchange-coupled systems.

Reaction of Cu(hfac)2 with methyl- and bromo-3-pyridyl-substituted nitronyl nitroxides (L R ) leads to assemble a diverse set of coordination complexes: mononuclear [Cu(hfac)2L 2-Me ], binuclear [{Cu(hfac)2}2(H2O)L 2-Me ], trinuclear [{Cu(hfac)2}3(L 6-Br )2], pentanuclear [{Cu(hfac)2}5(L 2-Me )2], and [{Cu(hfac)2}5(L 2-Me )4], cocrystals [Cu(hfac)2(L 2-Br )2]·[Cu(hfac)2(H2O)2] and [Cu(hfac)2(L 2-Br )2]·2[Cu(hfac)2H2O], one-dimensional polymers [Cu(hfac)2L 2-Br ] n and [Cu(hfac)2L 6-Br ] n , and cyclic dimers [Cu(hfac)2L 5-Me ]2, [Cu(hfac)2L 5-Br ]2, and [Cu(hfac)2L 6-Me ]2. The molecular structures of the obtained complexes are strongly affected by the substituent type and its location in the pyridine heterocycle. Occupation of the second position of the pyridine ring increases the steric hindrance of both imine and nitroxide coordination sites of L 2-R , which is favorable for the formation of various conformers and precipitation of complexes with different molecular structures. The pentanuclear [{Cu(hfac)2}5(L 2-Me )2] and [{Cu(hfac)2}5(L 2-Me )4] complexes do not have prior analogues and are valuable model objects for investigation of the mechanism of formation of various coordination polymers. The arrangement of long Cu-ONO bonds in {CuO6} square bipyramids due to the weakening nitroxide donor site in complexes, based on L 2-Me , L 2-Br , and L 6-Br ligands, results in ferromagnetic exchange interactions between spins of Cu2+ ions and nitroxides. Complexes with substituents that do not considerably affect the coordination ability of ligands (L 5-Me , L 5-Br , and L 6-Me ) exhibit strong antiferromagnetic exchange interactions between spins of Cu2+ ions and nitroxides.

A procedure was developed for the synthesis of mononuclear first row transition metal coordination compounds with adamantane-1-carboxylate (AdCO2), which allowed the synthesis of complexes NBu4[M(AdCO2)3], where MII = Mn, Ni, Co, Zn. The X-ray diffraction study showed that all AdCO2 act as chelating ligands, which is the main distinguishing structural feature of the synthesized compounds. The formation of four-membered metallocycles leads to a small O—M—O angle (58.4–63.0°), resulting in the distorted trigonal-prismatic environment of the central atom in the coordination anions [M(AdCO2)3]−. Under similar conditions of the synthesis, CuII forms the complex (NBu4)2[Cu(AdCO2)2(SO4)] existing in two differently colored modifications (green and light blue), which is due to a small difference in the environment of CuII in the solid state. Upon heating above 150°C, the light blue modification is transformed into the green modification. Using 1,3-diphenyltriazene (HL), it was demonstrated that the developed synthetic approach is applicable to the preparation of compounds, in which the coordination anions [ML3]− also contain only four-membered chelate metallocycles with the N—M—N angle tightened at the metal atom. The compounds NBu4[ML3], where MII = Co and Ni, were isolated in individual state and were structurally characterized.

A method for the synthesis of solvates of dinuclear heterospin complexes of bis(hexafluoroacetylacetonato)copper(ii) with 2-(4-methylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LMe) and 2-(4-ethylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt) of the composition [Cu(hfac)2LR]2 · Solv (Solv = benzene, bromobenzene, toluene, o-xylene, p-xylene) was developed. It was found that a decrease in temperature induces structural transformations of the solid phases of the complexes followed by significant changes in the distances between the paramagnetic centers in the intramolecular exchange clusters {Cu2+-O·-} from 2.321 to 1.974 A. As a result, the temperature dependences of the effective magnetic moment μeff(T) exhibit magnetic features similar to spin transitions. An analysis of the shape of the μeff(T) curves suggested that the magnetic properties of the compounds under study depend primarily on the molecular packing. A comparison of the magneto-structural correlations typical of the [Cu(hfac)2LR]2 · Solv complexes studied in this work with the data obtained earlier for analogous solvates of heterospin polymer chain complexes showed that the spin transition characteristics of the [Cu(hfac)2LR]2 · Solv systems are much less sensitive to the change of the solvent than the corresponding characteristics of the heterospin polymers containing solvate molecules in the interchain space. The magnetic characteristics of the heterospin dimer molecules depend primarily on which cavity between the dimers is filled with solvate molecules. For solvates with monoclinic symmetry, changes in the solvent molecules occupying same-type cavities have almost no effect on the spin transition characteristics in the heterospin exchange cluster.

Similar to spin-crossover (SCO) compounds, spin states of copper(II)–nitroxide based molecular magnets can be switched by various external stimuli including temperature and light. Although photoswitching and reverse relaxation of nitroxide–copper(II)–nitroxide triads were investigated in some detail, similar study for copper(II)–nitroxide spin pairs was still missing. In this work we address photoswitching and relaxation phenomena in exchange-coupled spin pairs of this family of molecular magnets. Using electron paramagnetic resonance (EPR) spectroscopy with photoexcitation, we demonstrate that compared to triad-containing compounds the photoinduced weakly coupled spin (WS) states of copper(II)–nitroxide pairs are remarkably more stable at cryogenic temperatures and relax to the ground strongly coupled spin (SS) states on the scale of days. The structural changes between SS and WS states, e.g., differences in Cu–Onitroxide distances, are much more pronounced for spin pairs than for spin triads in most of ...

We synthesized 1-ethylimidazolyl-substituted nitronyl nitroxides, i.e., 2-(1-ethylimidazol-4-yl)- (L4Et) and 2-(1-ethylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide-1-oxyl (L5Et). The stable radical L5Et is an ethyl analog of 2-(1-methylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide-1-oxyl (L5Me) described earlier, the reaction of which with Cu(hfac)2 (hfac is 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) leads to the formation of the [Cu(hfac)2(L5Me)2] jumping crystals. The reaction of Cu(hfac)2 with L5Et with reagent ratios 1: 2 and 1: 1 yields heterospin complexes [Cu(hfac)2(L5Et)2] and [Cu(hfac)2L5Et]2, respectively. X-ray diffraction study of the mononuclear complex [Cu(hfac)2(L5Et)2] determined that the compound has a packing similar to that of jumping crystals studied earlier, with the only difference being that the O...O contacts between neigh- boring nitroxide groups were found to be 0.3—0.5 A longer than in [Cu(hfac)2(L5Me)2]. As a result of the lengthening of these contacts, [Cu(hfac)2(L5Et)2] crystals lack chemomechanical activi- ty. We found that when cooling crystals of binuclear complex [Cu(hfac)2L5Et]2 below 50 K, the antiferromagnetic exchange between unpaired electrons of the >N—•O groups of neighboring molecules leads to the full spin-pairing of the nitroxides, with only the Cu2+ ions contributing to the residual paramagnetism of the compound.

New nitronyl nitroxides LMe and LMe-CP containing a 4-methylpyridin-3-yl substituent were synthesized. It was found that the interaction of Сu(hfac)2 with LMe and LMe-CP gave binuclear [Cu(hfac)2LMe]2 and [Cu(hfac)2LMe-CP]2·Solv (Solv = n-C6H14, n-C10H22, n-C16H34) and chain polymer {[[Cu(hfac)2]2LMe2][Cu(hfac)2]}∞ heterospin complexes. An important structural peculiarity of LMe and LMe-CP is a large dihedral angle between the planes of the O –N−C N → O paramagnetic fragment and the pyridine ring: 55.2 and 56.1°, respectively. The presence of a methyl group in the pyridine ring of the nitroxide molecule in {[[Cu(hfac)2]2LMe2][Cu(hfac)2]}∞ proved favorable for spin transition at nearly room temperature.

The ESR spectrum of compact nitroxide (NO)-substituted nitronyl nitroxide (NN) triplet diradical N-tert-butyl-N-oxidanyl-2-amino-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (1) was recorded in solid argon matrix at 15 K. The zero-field splitting (ZFS) parameters of 1 were derived from the recorded ESR spectrum: |D| = 0.0248 cm(-1) and E = 0.0025 cm(-1). Quantum chemical calculations have been performed using DFT and multiconfigurational ab initio (CAS) methods in order to establish equilibrium geometries of the conformational isomers resulting from twisted conformations of NO and NN moieties. The ZFS parameters of 1 were calculated at these levels of theory to test validity of the calculated structures. The calculation results were analyzed using the measured ZFS parameters and magnetic and structural data from the previous studies (Suzuki, S.; et al. J. Am. Chem. Soc. 2010, 132, 15908; Tretyakov, E. V.; et al. Russ. Chem. Bull. 2011, 60, 2608). It was found that the ab initio method is most successful for accurate predictions of molecular and magnetic parameters. Diradical 1 has only one stable enantiomeric pair in pseudoeclipsed conformations. The two chiral isomers exist in racemic crystals 1 and in solid matrices with molecular parameters close to those attributed to a free molecule. The analysis of the spin density distribution suggests that one unpaired electron occupies NO group at the equilibrium geometry, whereas the torsion of NO group governs the spin density distribution of the second unpaired electron on a conjugated fragment in NN group. The increase in planarity by torsion of NO group enhances the trimethylenemethane-type properties and, therefore, gives rise to larger ferromagnetic exchange interaction. More planar equilibrium geometry and greater (three times) exchange interaction constant J were predicted for hypothetical diradical 1a, where bulky tert-butyl group is replaced by a methyl group in the nitroxide fragment.

Molecular and crystal structures are determined for amino-substituted nitronyl nitroxide 1, the products of its subsequent oxidation, acylation, and reduction: zwitter-ions 3a, 3b and salts K(4b) and K2(4b)(CF3CO2).

Previously unknown [ML2(H2O)n] bischelates, where M is Mn(II), Co(II), Ni(II), or Cu(II) and L is deprotonated triformylmethane, are studied by X-ray diffraction analysis. It is revealed that in the crystals of all compounds there are multiple hydrogen bonds linking bischelate molecules into polymer layers or a single framework. The character of the temperature dependence of the effective magnetic moment [ML2(H2O)n] indicates the existences of weak intracrystalline exchange interactions between the unpaired electrons of the paramagnetic centers.

It was shown that dipole-stabilized paramagnetic carbanion lithiated 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide can be attached in a nucleophilic manner to either isolated or conjugated aldonitrones of the 2,5-dihydroimidazole 3-oxide and 2H-imidazole 1-oxide series to afford adducts the subsequent oxidation of which leads to polyfunctional mono- and diradicals. According to XRD, at least two polymorphic modifications can be formed during crystallization of the resulting paramagnetic compounds, and for each of them, geometric parameters of the molecules are similar. An EPR spectrum of the diradical in frozen toluene has a complicated lineshape, which can be fairly well reproduced by using X-ray diffraction structural analysis and the following set of parameters: D=14.9 mT, E=1.7 mT; tensor a((14) N)=[0.260 0.260 1.625] mT, two equivalent tensors for the nitronyl nitroxide moiety a((14) N)=[0.198 0.198 0.700] mT, and g≈2.007. According to our DFT and ab initio calculations, the intramolecular exchange in the diradical is very weak and most likely ferromagnetic.

First isoxazolyl-substituted nitronyl nitroxides (L and $$L^{Me_2 }$$ ) were synthesized and characterized. Their reactions with Cu(hfac)2 and Mn(hfac)2 (hfac is hexafluoroacetylacetonate) afford the heterospin complexes [Cu(hfac)2L] n , [Cu2(hfac)4L] n , $$\left[ {Cu_2 (hfac)_4 L^{Me_2 } } \right]_n$$ , $$\left[ {Cu(hfac)_2 L^{Me_2 } } \right]_n$$ , $$\left[ {Cu(hfac)_2 L^{Me_2 } _2 } \right]$$ , $$\left[ {Cu(hfac)_2 L^{Me_2 } (MeCN)} \right]$$ , [Mn(hfac)2]3L4, and $$\left[ {Me(hfac)_2 L^{Me_2 } } \right]_2$$ . In the ligand L, the N atom of the isoxazole ring (NIz) has weak electron-donating properties. For example, the paramagnetic ligand in the chain polymer complex [Cu(hfac)2L] n acts as a bidentate bridging ligand coordinated through both O atoms of the nitronyl nitroxide group (ON-O); the NIz and OIz atoms are not involved in the coordination. The introduction of Me groups into the isoxazole substituent results in an increase in the electron density on the NIz atom and enables the synthesis of the chain polymer complex $$\left[ {Cu(hfac)_2 L^{Me_2 } } \right]_n$$ , in which the bidentate bridging ligand $$L^{Me_2 }$$ is coordinated through the ON-O and NIz atoms. In the mononuclear complexes $$\left[ {Cu(hfac)_2 L^{Me_2 } _2 } \right]$$ and $$\left[ {Cu(hfac)_2 L^{Me_2 } (MeCN)} \right]$$ , the paramagnetic ligand is coordinated only through the NIz atom. The solid heterospin Mn complexes [Mn(hfac)2]3L4 and $$\left[ {Mn(hfac)_2 L^{Me_2 } } \right]_2$$ have a molecular structure. In these complexes, strong antiferromagnetic intracluster exchange interactions arise. The residual magnetic moments of the exchange clusters in the complex [Mn(hfac)2]3L4 are ferromagnetically coupled, resulting in the increase in the effective magnetic moment (μeff) of the complex with decreasing temperature in the range of 300—30 K. The thermomagnetic study of the complexes [Cu(hfac)2L] n , [Cu2(hfac)4L] n , and $$\left[ {Cu_2 (hfac)_4 L^{Me_2 } } \right]_n$$ in the range of 2–300 K revealed the ferromagnetic ordering at temperatures below 5 K. The ESR study of the solid complex $$\left[ {Cu(hfac)_2 L^{Me_2 } } \right]_n$$ showed that the decrease in its μeff in the temperature range of 30–300 K is associated with the direct exchange interaction between the unpaired electrons of the nitronyl nitroxides of adjacent chains, whereas at temperatures below 30 K, only Cu2+ ions contribute to the magnetic susceptibility of the complex.

A procedure was developed for the synthesis of the kinetically stable nitroxide 2-(N-tert-butyl-N-hydroxyamino)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl containing the sterically hindered hydroxylamino group. The crystal and molecular structure of this compound was determined. The isolation of this nitroxide hydroxylamine in the individual state enabled a new procedure for the preparation of its oxidation product, viz., the corresponding nitroxide biradical. The results of quantum chemical calculations of the electron exchange in the biradical molecule are in good agreement with the experimental data on the triplet ground state of the molecule in the crystal obtained earlier.

To find the factors favorable for the appearance of chemomechanical activity of heterospin crystals, a series of new heterospin complexes were synthesized and characterized. It includes [Cu(tfac)2LIm2]·2CH2Cl2, [Cu(tfac)2LIm2]·2EtOH, [[Cu(pfu)2]2LIm3]·1/2CH2Cl2, [Cu(pfh)2LIm2]·1/2CH2Cl2, [Cu(piv)2LIm2]·2MeOH, [Co(piv)2LIm2], [\( Cu(hfac)_2 L^{CD_3 } _2 \)], [Cu(hfac)2LTr]2·CH2Cl2, and [Cu(hfac)2LTr2] (LIm, \( L^{CD_3 } \), and LTr are N-methylimidazolyl-, N-trideuteriomethylimidazolyl-, and N-methyltriazolyl-substituted nitronyl nitroxides, respectively; tfac, hfac, pfu, pfh, and piv are the charged coordinated diamagnetic ligands 1,1,5,5-tetrafluoropentane-2,4-dionate, 1,1,1,5,5,5-hexafluoropentane-2,4-dionate, 1,1,1,2,2,3,3,4,4,8,8,9,9,10,10,11,11,11-octadecafluoroundecane-5,7-dionate, 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dionate, and 2,2-dimethylpropionate, respectively). The crystal and molecular structures of all compounds were determined. The results of the X-ray diffraction study of the complex [Ni(hfac)2LIm2] synthesized earlier are reported. In the solid state of the complexes [Cu(pfh)2LIm2] and [Co(piv)2LIm2], the paramagnetic ligands are cis-coordinated to the central atom in a monodentate fashion via the donor N atom of the imidazole ring. In the dinuclear complexes [[Cu(pfu)2]2LIm3] and [Cu(hfac)2LTr]2, the paramagnetic ligands are also in cis positions but act as bridges through coordination of the donor N atoms of the azole ring and the O atom of the nitronyl nitroxide moiety to different Cu2+ ions. In the solid complexes \( Cu(hfac)_2 L^{CD_3 } _2 \) , [Cu(hfac)2LTr2], [Cu(tfac)2LIm2]·2CH2Cl2, [Cu(tfac)2LIm2]·2EtOH, and [Cu(piv)2LIm2]·2MeOH, the nitronyl nitroxide radicals in the mononuclear heterospin molecules are in trans positions. The packing motif in the crystal structures of the complexes \( Cu(hfac)_2 L^{CD_3 } _2 \) , [Cu(tfac)2LIm2]·2CH2Cl2, and [Cu(tfac)2LIm2]·2EtOH is the same as that in the previously studied complexes [M(hfac)2LIm2] exhibiting chemomechanical activity. Among the complexes under consideration, only crystals of \( Cu(hfac)_2 L^{CD_3 } _2 \) can exhibit chemomechanical activity, that is to make jumps upon heating or irradiation. The results of the present study suggest that the packing of the solid-state structure plays a key role in the generation of mechanical activity of the crystals.

A reaction of a lithiated derivative of 4,4,5,5-tetramethyl-3-oxido-4,5-dihydro-1H-imidazole 1-oxyl with 3,6-di-tert-butyl-o-benzoquinone at −80 °C predominantly gave spin-labeled dihydrooxepine via nucleophilic 1,2-addition of the paramagnetic carbanion to the CO group of the benzoquinone followed by insertion of the O atom into the ring. At lower temperatures, this reaction was accompanied by 1,4-addition of the organolithium compound to the benzoquinone followed by oxidation of the resulting adduct into spin-labeled o-benzoquinone. This “one pot” process is a novel approach to organic derivatives that can further be converted into di- and polyradicals combining nitronyl nitroxide and semiquinolate fragments.

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 C C fragments, with the distances between the terminal carbon atoms of the neighboring diacetylene groups (T and d) being 6.170 and 4.466 A, respectively, and the angle between the translation vector and the median line passing through the C C C C fragment 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 373 K for 1 h 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 400–420 K the product explodes, releasing about 360 kJ/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 A4N = 0.35 mT), indicating the efficiency of the C C C C fragment as an exchange channel. The character of the experimental μeff(T) dependence for 4 indicates a strong intramolecular antiferromagnetic-type exchange interaction (J/kB ∼ −104 K) and the dominating weak intermolecular ferromagnetic exchange.

Polymer chain complexes [Cu(hfac)2LR] n exhibit thermally and light-induced magnetic anomalies in many aspects similar to a spin crossover. These compounds attracted significant attention in the field of molecular magnetism and have been extensively studied by electron paramagnetic resonance (EPR) during the last several years. All compounds studied so far were based on copper(II) ions bridged by pyrazolyl-substituted nitronylnitroxides. The present work reports the first EPR study of complexes of Cu(hfac)2 with tert-butylpyrazolylnitroxides—a new type of nitroxide ligand expected to modify exchange interaction pathways and physical properties of the crystals. The Q-band EPR spectra of three representative novel compounds are principally different from those studied previously, supporting the assumption that the magnetic motif of the compound has changed. Dominant intercluster exchange interactions are now found along the structural polymer chains. This complicates the EPR detection of phase transitions to some extent; however, theoretical modeling of the observed spectral changes allows for unambiguous assignment of different spin states and transitions between them. The magnitudes of intercluster exchange interaction were estimated to be ca. 0.1–1.5 cm−1 for the studied compounds.

Suspensions 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)

It was established that the reactions of pyrazol-3-yl-substituted nitronyl nitroxide (HL1) and pyrazol-3-yl-substituted imino nitroxide (HL3) 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 (HL2) gave unexpected solid Cu2(H2O)2(L6)2 · 2DMF, in which L6 is a deprotonated 5-carboxy-pyrazol-3-yl-substituted nitronyl nitroxide, formed as a result of cleavage of an ester bond in the starting HL2. 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 (HL4). It led to the formation of Cu2(DMF)2(L7)2, where L7 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 L1 and L3 paramagnetic ligands perform the bridging cyclic tridentate function, while in Cu2(H2O)2(L6)2 · 2DMF and Cu2(DMF)2(L7)2, the paramagnetic L6 and diamagnetic L7 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.

The nitronyl nitroxide biradical, 2,2′-[1,1′-(hexa-2,4-diyne-1,6-diyl)bis(1H-pyrazole-4,1-diyl)]bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide 1-oxyl), was synthesized. The structure and magnetic properties of the biradical were studied. In the solid state, this nitroxide contains the nonlinear CS-C≡C-C≡C-CS fragment. The CS-C≡C and C≡C-C≡ angles are 172.4(5) and 171.1(3)°, respectively.

The reactions of bis(hexafluoroacetylacetonato)copper(II) [Cu(hfac)2] with the nitronyl nitroxide biradicals bis[4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyrazol-1-yl]alkanes (L6, L10, and L12) produced the framework heterospin complex [Cu(hfac)2]2L6 and the layer-polymeric heterospin complexes [Cu(hfac)2]2L10 and {[Cu(hfac)2]2L12} [Cu(hfac)2(PriOH)2], respectively. In the solid state of these compounds, the stereochemical nonrigidity is manifested as a deformation of the polymethylene fragments-(CH2)n-.

Diels-Alder reactions of (-)-thebaine with N-substituted maleimides have been used to obtain cycloadducts possessing α-facial stereoselectivity. The 3-O-and 6-O-demethylated derivatives as well as the products of complete and partial reduction of carbonyl groups in the pyrrolidinedione moiety were also obtained. Some of the synthesized compounds have proved to be promising analgesics. The introduction of the bromine atom into the aryl moiety favors prolongation of the analgesic effect.

A new approach to the synthesis of polyfunctional pyrazolyl-substituted nitronyl nitroxides was developed based on the presynthesized pyrazole derivatives prepared by 1,3-dipolar cycloaddition. The structures of the resulting mono-and biradicals were confirmed by X-ray diffraction.

Metal complexation reactions of N-t-butyl-N-oxidanyl-2-amino(nitronyl nitroxide) diradical (1) with M(hfac)2 (M: Mn or Cu) were investigated. These reactions were found to be very sensitive to the type of metal ion employed. Complex [Mn(hfac)2·1], consisting of Mn(hfac)2 and diradical 1, was readily prepared by mixing the components. However, the reaction of Cu(hfac)2 with 1 or N-t-butyl-N-oxidanyl-2-amino(iminonitroxide) diradical (2) involved the reduction of the diradical to the N-t-butyl-N-oxidanide-2-amino(iminonitroxide) radical anion (3) and finally produced the polymer-chain complex [Cu2(hfac)2·32·Cu(hfac)2]n. The structures of these complexes were elucidated by X-ray analysis, and their magnetic properties were investigated in detail. The temperature dependence of χpT (χp: magnetic susceptibility) for [Mn(hfac)2·1] exhibited a strong antiferromagnetic interaction (H = -2JS1·S2, J/kB = -217 K) between the Mn(II) spin (S = 5/2) and the diradical 1 spin (S = 1). However, the χpT-T plots for [Cu2(hfac)2·32·Cu(hfac)2]n indicated the presence of several magnetic interactions: a large ferromagnetic interaction (J/kB = 510 K) between iminonitroxide 3 and the imino-coordinating Cu(II) atom, a moderately large ferromagnetic interaction (J/kB = 58 K) between the iminonitroxide and (iminonitroxide oxygen)-coordinating Cu(hfac)2, and a weak antiferromagnetic interaction (J/kB = -1.4 K) between the two Cu(hfac)-3 moieties within a Cu2O2 square.

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 (2J) 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 CuO bond length (1.962 A).

We report the study of light-induced magnetostructural anomalies in a polymer chain complex of Cu(hfac)2 (hfac = hexafluoroacetylacetonate) with an unusual acyclic tert-butylpyrazolylnitroxide radical (Ltert(Me)) using EPR. This complex ([Cu(hfac)2Ltert(Me)]n) belongs to the family of thermo- and photoswitchable molecular magnets "breathing crystals". Compared to previously studied breathing crystals with nitronyl nitroxides, [Cu(hfac)2Ltert(Me)]n shows much weaker absorption bands in the visible spectral region and therefore is superior for optical manipulation of the spin states. Illumination with light (λ ≈ 540 nm) at cryogenic temperatures leads to formation of a metastable weakly coupled spin state, which relaxes to the ground strongly coupled spin state on a time scale of hours. These phenomena are in many aspects similar to the light-induced excited spin state trapping (LIESST) well-known for spin-crossover compounds. Remarkably, the photoinduced spin state in [Cu(hfac)2Ltert(Me)]n is metastable at temperatures up to TLIESST ≈ 60 K, which is a significant improvement compared to that of previously studied breathing crystals with nitronyl nitroxides (TLIESST ≈ 20 K). We describe LIESST-like behavior observed in [Cu(hfac)2Ltert(Me)]n and discuss possible reasons for the increased stability of the photoinduced spin state.

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.

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-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. © ARKAT USA, Inc.