For the first time, a heterospin Cu(II) complex with a nitronyl nitroxide radical has been discovered in which a nonclassical spin transition occurs, accompanied by bistability due to the temperature-induced excited-state trapping (TIESST effect). This finding broadens the current understanding of the potential of heterospin copper complexes with nitroxyl radicals, as it had previously been assumed that the TIESST effect could not be realized in such systems because of the rapid relaxation of the excited state.
It was demonstrated that in the synthesized compound, the metastable excited state persists up to 116 K, while at 95 K its relaxation takes more than nine hours, indicating the high stability of the bistable state. An unexpected result was that, unlike previously reported spin-crossover systems, the relaxation proceeds via an autocatalytic mechanism, pointing to the cooperative nature of the spin transition within the crystal.
Additionally, it was shown that switching of the spin state in this complex can be triggered by a terahertz pulse in less than 50 ms at 113 K, demonstrating the possibility of rapid control over the magnetic state of the material. These results advance the understanding of spin-transition dynamics and open new перспективes for the development of functional molecular magnetism materials with controllable magnetic and thermodynamic properties.
New approaches have been developed for obtaining stable organic radicals based on the furazano[3,4-b]pyrazine scaffold — a novel open-shell framework that expands the class of furazanopyrazine radical compounds. Replacing the oxadiazole fragment in bisfurazanopyrazine with a triazole unit enables synthetic diversification of this class and creates opportunities for introducing a variety of functional substituents into the molecule.
A key step in the preparation of the target compounds is the cyclization reaction of bis(arylhydrazones), leading to the formation of the corresponding 6-aryltriazolo[4,5-e]furazano[3,4-b]pyrazines. The anion radicals generated through one-electron oxidation exhibit good stability and display luminescence in the near-infrared region.
These results demonstrate the feasibility of synthetically expanding the class of furazanopyrazine open-shell compounds and obtaining new stable radicals with luminescent properties based on this platform.
A polymeric chain complex of Cu(hfac)₂ with a pyridyl-substituted iminonitroxide has been obtained, in which the iminonitroxide ligand links Cu(II) ions within the chain while simultaneously coordinating to an out-of-chain copper ion, thereby forming a branched polymeric structure, [Cu₂(hfac)₄(IN)₂]ₙ.
It was established that the specific disorder of the bridging fragment {Cu⋯N=C–N–O⋯Cu}, arising from the superposition of oppositely directed chains, makes the structure of the complex closely resemble that of an analogous compound with a nitronyl nitroxide ligand, thus creating significant structural similarity between them.
The complex exhibits temperature-induced changes in its EPR spectrum and magnetic properties, accompanied by hysteresis, while hydrostatic pressure above 1.9 kbar completely suppresses the phase transition. Magnetochemical, EPR, and quantum-chemical studies made it possible to determine the origin of the magnetic anomaly: the magnetic transition is caused by a transformation of the coordination polyhedron of the out-of-chain Cu(II) ion and a shift in the position of the pyridine nitrogen atom. This leads to switching of the indirect magnetic superexchange {Cu²⁺⋯IN} through the pyridine fragment.
These results demonstrate that the magnetic properties of polymeric heterospin complexes can be controlled through changes in coordination geometry and external stimuli such as temperature and pressure.
A series of pyridyl-substituted nitronyl nitroxides has been synthesized and structurally characterized, followed by a comprehensive magnetochemical and quantum-chemical investigation. The study was aimed at establishing the relationship between molecular structure, the nature of intermolecular contacts, and the magnetic properties of nitroxyl radicals, which is essential for the rational design of organic magnetic materials.
It was shown that the temperature dependence of magnetic susceptibility is governed by short contacts between nitroxyl groups and methyl substituents of neighboring molecules, which form channels for exchange interactions. The features of spin-density distribution in the radicals were identified, and magnetic motifs were determined for compounds bearing different substituents in the pyridine fragment.
An approach was proposed for detecting temperature-induced phase transitions in nitroxides, based on comparing experimental and calculated exchange interaction parameters: a significant discrepancy between these values indicates structural transformations upon cooling. Using a methyl-substituted nitroxide as an example, structural transformations and a change in the magnetic motif were revealed, while for the Cl-substituted radical, an order–disorder transition was detected during low-temperature X-ray structural studies.
These results demonstrate the possibility of diagnosing hidden phase transitions in nitroxyl radicals and show that the comparison of quantum-chemical calculations with magnetic data can serve as an effective tool for identifying structural changes in organic magnetic materials.
Solid-State Transitions Controlled by Solvent Molecules in Heterospin Cu(II) Complexes with Nitronyl Nitroxide
A series of polymeric chain crystal solvates, [Cu(hfac)₂LPr]·0.5Solv (Solv = (CH₃)₂CO, THF, CH₂Cl₂, CH₂Br₂, CHCl₃), containing an n-propyl-imidazolyl-substituted nitronyl nitroxide ligand (LPr), was synthesized and comprehensively investigated in order to clarify the influence of solvent molecules on the structure, stability, and magnetic properties of polymeric chain heterospin copper(II) complexes. X-ray crystallographic analysis demonstrated that solvent substitution has little effect on the structure and packing of the chains, but significantly determines the stability of the crystal solvates and the nature of their solid-state transformations at room temperature.
It was established that the complexes [Cu(hfac)₂LPr]·0.5THF and [Cu(hfac)₂LPr]·0.5(CH₃)₂CO completely transform within 24 hours into the binuclear complex [Cu(hfac)₂LPr]₂ as a result of desolvation. In contrast, the solvates containing CH₂Cl₂, CH₂Br₂, and CHCl₃ undergo much slower transformations, yielding the desolvated one-dimensional polymer [Cu(hfac)₂LPr] and/or mixtures with [Cu(hfac)₂LPr]₂.
Magnetochemical studies revealed a high sensitivity of magnetic properties to the type of included solvent. The THF- and acetone-containing complexes undergo a spin transition at 190 K, caused by changes in the coordination environment of the Cu atom, whereas the solid phases [Cu(hfac)₂LPr]·0.5Solv (Solv = THF, CH₂Cl₂, CH₂Br₂, CHCl₃) enter a magnetically ordered state at low temperatures (below 4–6 K).
These results demonstrate the key role of solvent molecules in controlling solid-state transformations and magnetic properties of polymeric chain copper(II) complexes, which is important for the targeted design of responsive molecular magnetic materials.
Smirnova K.A., Golomolzina I.V., Romanenko G.V., Fokin S.V., Tolstikov S.E., Letyagin G.A., Chernavin P.A., Bogomyakov A.S. Solvent-controlled solid-state phase transitions of a heterospin Cu(II) complex with imidazolyl-substituted nitronyl nitroxide. Dalton Trans., 2024, 53, 13119–13128. DOI: 10.1039/D4DT01861C
Single-Chain Magnets Based on 3d-Metal Ion Complexes with Ferrocenyl-Substituted Nitronyl Nitroxides
Stable heterospin complexes of Mn(II), Co(II), Cu(II), and Zn(II) with a ferrocenyl-substituted nitronyl nitroxide ligand (L) were synthesized. Their structures and magnetic properties were analyzed to evaluate the influence of the bulky ferrocenyl substituent on coordination nodes and magnetic interactions.
It was established that the formation of polymeric chain complexes of Mn(II) and Co(II) occurs through the bridging coordination of the paramagnetic nitronyl nitroxide fragment. At the same time, the presence of the bulky ferrocenyl substituent promotes the formation of trans-type coordination nodes in the chains [Mn(hfac)₂L]ₙ and [Co(hfac)₂L]ₙ.
Investigation of the magnetic properties revealed ferrimagnetic ordering in these one-dimensional polymeric chain complexes. In particular, the compound [Co(hfac)₂L]ₙ exhibits the properties of a Single-Chain Magnet (SCM), including slow relaxation of magnetization below 13 K and magnetic hysteresis with a high coercive field of 54 kOe at 2 K.
K. Maryunina, D. Nigomedyanova, V. Morozov, K. Smirnova, G. Letyagin, G. Romanenko, N. Efimov, A. Bogomyakov and V. Ovcharenko. Ferrocenyl-substituted nitronyl nitroxide in the design of one-dimensional magnets. Dalton Trans., 2024, 53, 1714-1721. DOI: 10.1039/d3dt03858k
The study of the influence of the substituent R in the imidazole fragment of nitroxyl radicals LR on the chemomechanical activity of heterospin complexes [Cu(hfac)₂(LR)₂] requires establishing the relationship between ligand structure, crystal packing, and the functional properties of the complexes. Such systems are of interest as models of chemomechanically active magnetic materials that are sensitive to structural changes.
To carry out this investigation, an efficient synthetic route to the target nitroxyl radicals LR was developed. This approach involves the introduction of the required alkyl substituents at the first position during heterocycle assembly, with the 2-imidazoline-1-oxyl fragment located at the fifth atom of the imidazole ring. The reaction of Cu(hfac)₂ with radicals LR in a 1:2 ratio yields mononuclear heterospin complexes [Cu(hfac)₂(LR)₂] (LR = Ln-Pr, Li-Pr, Ln-Bu), which possess crystal structures analogous to those of the known chemomechanically active “jumping” crystals. Comparison of the obtained compounds made it possible to trace the influence of alkyl substituent size on molecular packing in the crystal and the resulting properties.
It was shown that increasing the size of substituent R leads to a reorganization of crystal packing and the disappearance of chemomechanical activity, indicating the key role of steric factors in shaping the functional characteristics of heterospin complexes. Thus, the targeted modification of the organic radical enables control over the structure and properties of heterospin copper coordination compounds, thereby providing a foundation for the rational design of new magnetic and chemomechanically active materials with tailored characteristics.
Golomolzina, I. V.; Tolstikov, S. E.; Smirnova, K. A.; Fokin, S. V.; Letyagin, G. A.; Romanenko, G. V.; Bogomyakov, A. S. N-Alkylimidazol-5-yl-Substituted Nitronyl Nitroxides and Their Mononuclear Cu(II) Complexes: Synthesis, Structure and Magnetic Properties. Chemistry – A European Journal 2024, 30 (13), e202303499. DOI: 10.1002/chem.202303499 .
A series of chain-polymeric copper(II) complexes with pyridine-based nitroxyl radicals has been synthesized, in which reversible magnetostructural phase transitions occur, accompanied by reconstruction of the crystal structure upon changes in temperature. It was shown that in one-dimensional polymers built from centrosymmetric dimers [Cu(hfac)₂LR]₂ connected by Cu(hfac)₂ fragments, temperature variation induces a transformation in the coordination environment of the Cu atoms within the binuclear units.
A crucial role is played by the volume of the substituent (Vᵣ), located within a cavity between fragments of volume (V꜀): the greater the free space available for its movement, the lower the transition temperature. It was established that when Vᵣ ≈ V꜀, phase transitions occur at high temperatures (>320 K), whereas when Vᵣ << V꜀, the presence of free space for substituent disorder increases the entropy of the process and lowers the phase-transition temperature. Thus, a clear relationship has been established between molecular structure, crystal packing, and the magnetic properties of these compounds, enabling targeted control over phase-transition temperatures.
The obtained compounds belong to the class of “breathing crystals,” capable of reversibly changing their structure and magnetic characteristics without destruction of the crystal lattice, and they exhibit a thermochromic effect upon temperature variation. Such properties indicate the potential use of these complexes as functional magnetic and sensing materials. The established correlations provide a foundation for the rational design of molecular magnets with tailored phase-transition temperatures.
Layered polymeric complexes [ML₂(ROH)₂] (M = Co, Ni; R = Me, Et, n-Pr, i-Pr, n-Bu, i-Bu) with the 3-imidazoline nitroxide ligand L, which exhibit ferromagnetic properties at low temperatures, are readily formed from alcoholic solutions of ML₂. It was established that from a solution containing MeOH and EtOH in a 1:10 ratio, bimolecular complexes [ML₂(MeOH)₂][ML₂(EtOH)₂] precipitate in the solid phase. These compounds possess a layered polymeric structure formed through hydrogen bonds between the OH groups of coordinated alcohol molecules and the nitroxyl groups of neighboring complexes.
X-ray crystallographic analysis showed that the crystal structure features an ordered alternation of methanol- and ethanol-containing supramolecular layers. Compared with the individual [ML₂(MeOH)₂] complexes, the MeOH-containing layers undergo significant deformation and adapt to the ethanol-containing layers, which largely determine crystal formation. The ordered alternation of {ML₂(MeOH)₂} and {ML₂(EtOH)₂} layers is associated with stronger interlayer interactions. The resulting mixed-alcohol bimolecular complexes, like their monoalcohol analogues, are capable of magnetic ordering at temperatures below 5 K.
These results demonstrate that the supramolecular architecture and magnetic properties of metal-organic radical complexes can be regulated through the choice of solvent and crystallization conditions. The ordered alternation of mixed-alcohol layers and their mutual adaptation govern the structure and magnetic behavior of the compounds, making such systems promising candidates for the development of functional materials with controllable magnetic properties.
Crystalline cesium salts containing the difurazanopyrazine anion radical were synthesized with the following compositions: Cs(L•)(H₂O)₂-I, Cs(L•)(H₂O)₂-II, Cs₂(L•)₂(HL•)(H₂O), and Cs₃(L•)₃(L••)₂(H₂O)₃. In the solid state, Cs ions coordinated by water molecules are positioned between stacks of difurazanopyrazines, forming a framework structure.
Investigation of the structures and magnetic properties of these compounds revealed that Cs(L•)(H₂O)₂-I is a metastable modification that transforms upon cooling into Cs(L•)(H₂O)₂-II, which differs in the magnitude of the relative shift of the anion radicals within the stacks. It was established that in the Cs(L•)(H₂O)₂-II phase, strong antiferromagnetic exchange occurs between the paramagnetic centers.
These results demonstrate that even minor changes in the relative arrangement of radicals within the crystal can significantly affect the nature of magnetic interactions, while phase transitions lead to substantial reorganization of these interactions. This creates opportunities for controlling the magnetic properties of such compounds through variation of synthesis conditions and crystal lattice structure.
The observed relationships broaden current understanding of exchange-interaction mechanisms in organic radical systems and suggest that these compounds may serve as promising platforms for the development of functional molecular magnetic materials, including applications in molecular electronics and spintronics, where controlled magnetic ordering at the molecular level is required.
Magnetostructural Single-Crystal-to-Single-Crystal Phase Transition Coupled with Ligand Transformation
It was discovered that the heterospin solid phases of the chain-polymeric [Cu(hfac)₂LEt]∞ and the binuclear molecular [Cu(hfac)₂LEt]₂-I spontaneously transform into the binuclear molecular complex [Cu(hfac)₂LEt]₂-II. The single-crystal-to-single-crystal (SC–SC) transition [Cu(hfac)₂LEt]₂-I → [Cu(hfac)₂LEt]₂-II was monitored over time by X-ray crystallographic analysis and was found to proceed within 12–18 hours in the temperature range 255–277 K.
The transformation of the polymeric [Cu(hfac)₂LEt]∞ into the molecular [Cu(hfac)₂LEt]₂-II is accompanied by pronounced macroscopic changes in the crystals, including alterations in shape, spontaneous mechanical displacements, and a color change from orange to dark green. This process begins, to some extent, even in crystals kept under the mother liquor, and after separation from solution, the SC–SC transformation proceeds completely at room temperature within 4 hours.
A similar dynamic behavior was observed for the propyl derivative complex [Cu(hfac)₂LPr]₂-I, which under ambient conditions transforms into [Cu(hfac)₂LPr]₂-II. At the macroscopic level, the transition [Cu(hfac)₂LPr]₂-I → [Cu(hfac)₂LPr]₂-II is accompanied by spontaneous crystal fragmentation, visually perceived as the scattering of small particles of the newly formed phase in different directions. The reverse transformation, [Cu(hfac)₂LPr]₂-II → [Cu(hfac)₂LPr]₂-I, occurs upon cooling below 225 K. Upon heating [Cu(hfac)₂LPr]₂-II above 300 K, an irreversible SC–SC phase transition, [Cu(hfac)₂LPr]₂-II → [Cu(hfac)₂LPr]∞, was observed, causing a pronounced color change from dark green to orange.
Thermal treatment of a single crystal of [Cu(hfac)₂LPr]∞ at 303 K on a diffractometer for 24 hours led to the formation of a new polymeric complex, [Cu(hfac)₂LPr]∞, where LPr is a product of radical transformation involving oxidation of LPr and migration of the O atom of the nitroxyl group to the pyrazole ring.
Comparison of the obtained data showed that these multiple phase transformations occur only when both a Cu(II) ion and a coordinated nitroxyl O–N group are simultaneously present, whereas replacement of the radical with a diamagnetic analogue or the use of other metals suppresses such effects. These findings identify the structural factors governing the dynamic behavior of the crystals and open opportunities for the development of functional molecular materials capable of responding to temperature and external stimuli through changes in structure, color, and magnetic properties.
In the study of a mixed-ligand copper(II) hexafluoroacetylacetonate complex with an imidazolyl-substituted nitronyl nitroxide ligand (L), it was established that the compound crystallizes in two polymorphic modifications — α- and β-[Cu(hfac)₂L]. Each modification consists of polymeric chains with cis-coordination of the hfac ligands, forming blue-green and brown crystals, respectively. Despite the structural similarity of these forms, their magnetic properties differ significantly.
It was found that upon the first cooling of the metastable α-[Cu(hfac)₂L] below 75 K, a complete irreversible transformation into the stable β-[Cu(hfac)₂L] occurs, accompanied by a sharp decrease in the effective magnetic moment. In subsequent cooling/heating cycles within the temperature range 260–180 K, a reversible spin transition characteristic of the β-form is reproduced. This transition arises from substantial changes in the Cu–ONO distances within heterospin exchange clusters and, consequently, from switching of ferromagnetic exchange to antiferromagnetic exchange.
Such an effect was observed for the first time in heterospin transition-metal complexes with nitroxides. The experimental μ_eff(T) dependence observed during the first cooling of α-[Cu(hfac)₂L] was termed the “spin transition of the emerging phase”, in order to distinguish it from the μ_eff(T) curve of β-[Cu(hfac)₂L], which corresponds to the typical reversible spin transition of a heterospin transition-metal complex with an organic radical.
The discovered phenomenon highlights the importance of detailed investigation of magnetic behavior under different temperature regimes in order to avoid misinterpretation of observed effects.
This work demonstrates the possibility of forming metal–organic coordination polymers based on a hexanuclear manganese pivalate cluster and ligand H, in which the ligand can perform either ditopic or tritopic functions. It was established that the composition and structure of the resulting products are strongly influenced by the alcohol solvents ROH (R = Me, Et, i-Pr, n-Bu) used and by the reaction temperature. This makes it possible to deliberately control the architecture of the resulting coordination structures by varying the synthesis conditions.
It was shown that such ligands can act as “bridges” linking metal centers into extended polymeric chains and layered structures, which is a key strategy in the design of coordination polymers and metal–organic frameworks. These systems are of particular interest as functional materials with tunable magnetic, sorption, and catalytic properties. The obtained results expand current understanding of the principles underlying coordination polymer construction and open new opportunities for the targeted synthesis of materials with predefined structures and properties.
The first spin-labeled cyrhetrenes, [(NNCp)Re(CO)₃] and [(INCp)Re(CO)₃] (nitronyl nitroxide NNCp and iminonitroxide INCp), have been synthesized. The resulting complexes are organometallic rhenium systems in which a stable free radical—serving as a carrier of an unpaired electron and thus responsible for magnetic properties—is attached to a robust molecular “framework.”
It was shown that the spin density is predominantly localized on the nitroxide fragment, while the magnetic response is governed by weak intermolecular interactions. This combination of stability and paramagnetic behavior makes these compounds a promising platform for theranostics—an approach that integrates diagnosis and therapy—particularly for the development of magnetic resonance imaging contrast agents capable of simultaneously monitoring the distribution of the substance in the body and exerting a therapeutic effect.
A study of the crystal structure of 1D transition-metal complexes with nitroxyl radicals, Cu(hfac)₂LR, revealed significant differences between phase transformations induced by temperature changes and those induced by external hydrostatic pressure. It was shown that even a slight increase in pressure can drastically alter the nature of thermally induced magnetic effects: raising the pressure to just ~0.03 GPa leads to the disappearance of the magnetic anomaly observed under normal conditions. Such a substantial change in the magnetic response of the system is achieved through the realization of a thermally inaccessible low-spin state of the copper(II) ion, driven by the stabilization of strong antiferromagnetic interactions within exchange clusters. Thus, due to their extreme sensitivity to pressure, such compounds can serve as highly sensitive sensors of external stimuli. This also makes them promising functional molecular materials for spintronics, where controlled switching of magnetic states under external factors is required, as well as for the development of systems sensitive to pressure at the molecular level.
