Ferrocenyl-substituted nitronyl nitroxide in the design of one-dimensional magnets.

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).

High-field EPR of copper(II)-nitroxide compound exhibiting three-step phase transition: structural insights from the field-induced sample orientation.

Copper(II)-nitroxide based Cu(hfac)2LR compounds exhibit unusual magnetic behavior that can be induced by various stimuli. In many aspects, the magnetic phenomena observed in Cu(hfac)2LR are similar to classical spin-crossover behavior. However, these phenomena originate from polynuclear exchange-coupled spin clusters Cu2+-O˙-N< or >N-˙O-Cu2+-O˙-N<. Such peculiarities may result in additional multifunctionality of Cu(hfac)2LR compounds, making them promising materials for spintronic applications. Herein, we investigate the Cu(hfac)2LMeMe material, which demonstrates a three-step temperature-induced magnetostructural transition between high-temperature, low-temperature, and intermediate states, as revealed by magnetometry. Two main steps were resolved using variable-temperature Fourier-transform infrared and Q-band electron paramagnetic resonance (EPR) spectroscopies. The intermediate-temperature states (∼40-90 K) are characterized by the coexistence of two types of copper(II)-nitroxide clusters, corresponding to the low-temperature and high-temperature phases. High-field EPR experiments revealed the effect of partial alignment of Cu(hfac)2LMeMe microcrystals in a strong (>20 T) magnetic field. This effect was used to unveil the structural features of the low-temperature phase of Cu(hfac)2LMeMe, which were inaccessible using single-crystal X-ray diffraction (XRD) technique. In particular, high-field EPR allowed us to determine the relative direction of the Jahn-Teller axes in CuO6 and CuO4N2 units.

2-Imidazoline Nitroxide Derivatives of Cymantrene.

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.

Hypersensitive pressure-dependence of the conversion temperature of hysteretic valence tautomeric manganese-nitronyl nitroxide radical 2D-frameworks.

Valence tautomeric manganese(ii)-radical lamellar compounds {[Mn2(NITIm)3]X}n with NITIm a nitronyl nitroxide radical and X = ClO4- (1) or BF4- (2) show a pressure-induced increase of their conversion temperature by approximately 40 K at a mild external pressure of 0.1 GPa, shifting the transition from near room temperature to hot temperature regions.

Re(i)-nitroxide complexes.

Spin-labeled cyrhetrenes [(NNCp)Re(CO)3] and [(INCp)Re(CO)3], where NNCp is nitronyl nitroxide 2-(η5-cyclopentadienyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and INCp is the corresponding imino nitroxide, were synthesized and characterized by EPR, CV, XRD, magnetochemistry and quantum chemistry methods. The correlations between different arrangements of paramagnetic centers and the magnetic exchange interactions for three polymorphs of [(NNCp)Re(CO)3] were studied. It was concluded that high kinetic stability of nitroxide-substituted cyrhetrenes is a promising feature of compounds for the creation of multifunctional contrast agents.

Ligand Structure Effects on Molecular Assembly and Magnetic Properties of Copper(II) Complexes with 3-Pyridyl-Substituted Nitronyl Nitroxide Derivatives.

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.

Pressure-Controlled Migration of Paramagnetic Centers in a Heterospin Crystal.

A study of the single-crystal-to-single-crystal transformation induced by temperature variation for the chain polymer Cu(II) complex with nitronyl nitroxide showed that an increase in the hydrostatic pressure of up to ∼0.07 GPa completely changes the intracrystalline displacements of molecules relative to one another. This, in turn, significantly affects the interaction energy of the unpaired electrons of the paramagnetic centers and hence the form of the temperature dependence of the magnetic susceptibility χT. The cooling of crystals under normal conditions causes a rearrangement of the intrachain exchange clusters {>N-•O-Cu(II)-O•-N<} accompanied by a shortening of the distances between the paramagnetic centers. This changes the character of exchange interactions and generates multistage spin transitions. An increase in the hydrostatic pressure leads to a drastic change in the O···O distances between the nitroxyl fragments of adjacent chains, an increase in the antiferromagnetic exchange between them, and complete suppression of spin transitions.

Selective Ion Exchange in Supramolecular Channels in the Crystalline State.

Artificial ion channels are of increasing interest because of potential applications in biomimetics, for example, for realizing selective ion permeability through the transport and/or exchange of selected ions. However, selective ion transport and/or exchange in the crystalline state is rare, and to the best of our knowledge, such a process has not been successfully combined with changes in the physical properties of a material. Herein, by soaking single crystals of Li2 ([18]crown-6)3 [Ni(dmit)2 ]2 (H2 O)4 (1) in an aqueous solution containing K+ , we succeeded in complete ion exchange of the Li+ ions in 1 with K+ ions in the solution, while maintaining the crystalline state of the material. This ion exchange with K+ was selectively conducted even in mixed solutions containing K+ as well as Na+ /Li+ . Furthermore, remarkable changes in the physical properties of 1 resulted from the ion exchange. Our finding enables not only the realization of selective ion permeability but also the development of highly sensitive biosensors and futuristic ion exchange agents, for example.

(Azulene-1,3-diyl)-bis(nitronyl nitroxide) and (Azulene-1,3-diyl)-bis(iminonitroxide) and Their Copper Complexes.

In contrast to diradicals connected by alternant hydrocarbons, only a few studies on those connected by nonalternant hydrocarbons have been reported. The syntheses, structures, and magnetic properties of azulene-1,3-diyl linked bis(nitronyl nitroxide) (NN2 Az) and bis(iminonitroxide) (IN2 Az) diradicals and their Cu(hfac)2 (hfac=hexafluoroacetylacetonate) complexes were investigated. NN2 Az was shown to have an intramolecular ferromagnetic interaction with Jobs /kB =+10.0 K (H=-2JS1 ⋅S2 ) between (nitronyl nitroxide) spins, whereas IN2 Az was estimated to have a much weaker intramolecular magnetic interaction. The reactions of NN2 Az and IN2 Az with Cu(hfac)2 gave a 1:2 [{Cu(hfac)2 }2 (NN2 Az)] complex and a 1:1 [Cu(hfac)2 (IN2 Az)]⋅C6 H12 complex, respectively. [{Cu(hfac)2 }2 (NN2 Az)] showed strong intramolecular antiferromagnetic interactions (J1-Cu-R /kB ≈-800 K, J2-Cu-R /kB ≈-500 K) between the CuII spins and the coordinating NN spins, whereas [Cu(hfac)2 (IN2 Az)] exhibited a ferromagnetic exchange interaction (Jobs-Cu-R /kB =+114 K) between the CuII spin and the imino-coordinated iminonitroxide spin.

Spin-state-correlated optical properties of copper(ii)-nitroxide based molecular magnets.

Molecular magnets based on copper(ii) ions and stable nitroxide radicals exhibit promising switchable behavior triggered by a number of external stimuli; however, their spin-state-correlated optical properties vital for photoinduced switching have not been profoundly investigated to date. Herein, the electronic absorption spectra of single crystals of three representatives of this unique family are studied experimentally and theoretically in the visible and near-IR regions. We established that the color of the complexes is mainly determined by optical properties of the nitroxide radicals, whereas the Cu(hfac)2 fragment contributes to the near-IR range with the intensity smaller by an order of magnitude. The thermochromism of these complexes evident upon thermal spin state switching is mainly caused by a spectral shift of the absorption bands of the nitroxides. The vibrational progression observed in the visible range for single crystals as well as for solutions of pure nitroxides is well reproduced by DFT calculations, where the C-C stretching mode governs the observed progression. The analysis of the spectra of single crystals in the near-IR region reveals changes in the energy and in the intensity of the copper(ii) d-d transitions, which are well reproduced by SOC-NEVPT2 calculations and owe to the flip of the Jahn-Teller axis in the coordination environment of copper. Further strategies for designing bidirectional magnetic photoswitches using these appealing compounds are discussed.

FTIR study of thermally induced magnetostructural transitions in breathing crystals.

"Breathing crystals" based on copper(II) hexafluoroacetylacetonates and pyrazolyl-substituted nitronyl nitroxides comprise the exchange-coupled clusters within the polymeric chains. Owing to an interplay of exchange interaction between copper(II) and nitroxide spins and Jahn-Teller nature of copper(II) complex, the breathing crystals demonstrate thermally and light-induced magnetostructural transitions in many aspects similar to the classical spin crossover. Herewith, we report the first application of variable temperature (VT) far/mid Fourier transform infrared (FTIR) spectroscopy and mid FTIR microscopy to breathing crystals. This VT-FTIR study was aimed toward clarification of the transitions mechanism previously debated on the basis of superconducting quantum interference device, X-ray diffraction, and electron paramagnetic resonance data. VT-FTIR showed the onset of new vibrational bands during phase transitions occurring at the expense of several existing ones, whose intensity was significantly reduced. The most pronounced spectral changes were assigned to corresponding vibrational modes using quantum chemical calculations. A clear-cut correlation was found between temperature-dependent effective magnetic moment of studied compounds and the observed VT-FTIR spectra. Importantly, VT-FTIR confirmed coexistence of two types of copper(II)-nitroxide clusters during gradual magnetostructural transition. Such clusters correspond to weakly coupled and strongly coupled spin states, whose relative contribution depends on temperature. The pronounced difference in the VT-FTIR spectra of two states in breathing crystals is a fingerprint of magnetostructural transition, and understanding of these characteristics achieved by us will be useful for future studies of breathing crystals as well as their diamagnetic analogues.

A magnetically isolated cuprate spin-ladder system: synthesis, structures, and magnetic properties.

We synthesized and characterized two magnetically isolated spin ladders, Cu2(CO3)(ClO4)2(NH3)6 (1) and Cu2(CO3)(ClO4)2(H2O)(NH3)5 (2), which are the first examples of carbonate bridging molecular spin ladders. Compounds 1 and 2 form a ladder configuration by stacking a structural unit composed of two Cu(2+) ions and one CO3(2-), where the Cu-O-Cu interactions form the rungs and legs of each ladder and the counter anions (ClO4(-)) occupy the space between the ladders and ensure their magnetic isolation. A S = 1/2 magnetically isolated spin-ladder model with a ladder-rung magnetic interaction J1/k(B) = 364 K (where J is defined as positive for antiferromagnetic interactions) and a ladder-leg magnetic interaction J2/k(B) = 27.4 K accurately predicts the temperature dependence of the molar magnetic susceptibility for 1. The ladder configuration of 2 is similar to that of 1 except that the CO3(2-) is alternately skewed in different directions in the stacked structural unit. Interestingly, this minor structural variation in 2 results in its remarkably different magnetic behavior; the magnetic susceptibility curve of 2 is accurately described by an alternating chain model with J3/k(B) = 7.26 K and J4/k(B) = 4.42 K.

Photoswitching of a thermally unswitchable molecular magnet Cu(hfac)2L(i-Pr) evidenced by steady-state and time-resolved electron paramagnetic resonance.

Most photoswitchable molecular magnets exhibit thermally induced switching, as is typical of spin crossover (SCO), valence tautomerism and SCO-like phenomena. We report a rare case of a copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) that does not exhibit quantitative SCO-like behavior in the temperature range of its chemical stability (2-350 K); however, it can be switched to a metastable thermally inaccessible spin state via visible/near-IR light at cryogenic temperatures. By means of photogeneration, unique information on this otherwise unobservable spin state has been obtained using steady-state Q-band (34 GHz) and time-resolved W-band (94 GHz) electron paramagnetic resonance (EPR) spectroscopy. In particular, we have found that the electronic structure and relaxation properties of the photoinduced state in Cu(hfac)2L(i-Pr) are very similar to those in its sister compound Cu(hfac)2L(n-Pr) that is thermally switchable and has been exhaustively characterized by many analytical methods, previously. The first observation of photoswitchable behavior in a thermally unswitchable copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) paves the way for photoswitching applications of this and similar compounds in the remarkably broad temperature range of 2-350 K.