Search

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]

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]

This study explores the antiarrhythmic and hypotensive potential of pyridyl-substituted nitronyl nitroxides derivatives, uncovering the crucial role of a single carbon moiety of the pyridine cycle alongside radical and charged oxygen centers of the imidazoline fragment. Notably, the introduction of fluorine atoms diminished the antiarrhythmic effect, while the most potent derivatives featured the nitronyl nitroxide pattern positioned at the third site of the pyridine cycle. Gender-dependent responses were observed in lead compounds LCF3 and LMe, with LMe inducing temporary bradycardia and hypotension specifically in female rats, and LCF3 causing significant blood pressure reduction followed by rebound in females compared to milder effects in males. Mechanistic insights point towards β 1 adrenoceptor blockade as an underlying mechanism, supported by experiments on isolated rat atria. This research underscores the interplay between structure, cardiovascular effects and gender-specific responses, offering insights for therapeutic strategies for treating free radical-associated cardiovascular disorders. [Display omitted] • Distinctive antiarrhythmic and hypotensive properties. • The third-positioned nitronyl nitroxide fragment is a must. • Gender variations in compound activity are observed. • Stable aqueous solutions and lower toxicity compared to established HNO donors. [ABSTRACT FROM AUTHOR]

It was shown that dipole-stabilized paramagnetic carbanion lithiated 4,4,5,5-tetramethyl-4,5-dihydro-1 H-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 2 H-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(14N)=[0.260 0.260 1.625] mT, two equivalent tensors for the nitronyl nitroxide moiety a(14N)=[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. [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]

Using single crystal XRD, it was proved that the cyclocondensation of 3,4-diaminofurazan with diethyl-2-oxosuccinate gives two products belonging to different heterocyclic systems, being structural isomers -derivatives of furazano[3,4-b]pyrazin-6-one (2) and furazano[3,4-b][1,4]diazepin-7-one (6). The simultaneous formation of compounds 2 and 6 in this reaction provokes also the generation of a third solid product, an unusual co-crystal (2)0.40(6)0.60. This is a rare example of co-crystallization of two isomers, the molecules of which occupy the same position in the crystal structure, despite the differences of the heterocyclic systems. [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]

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]

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]

Spontaneous solvent-controlled solid-state transformations were observed for a series of polymeric chain solvates [Cu(hfac)2LPr]·0.5Solv (Solv = (CH3)2CO, THF, CH2Cl2, CH2Br2, CHCl3) with 2-(1-propyl-1H-imidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LPr) under ambient conditions. The conversion of powdered polymeric chain [Cu(hfac)2LPr]·0.5THF and [Cu(hfac)2LPr]·0.5(CH3)2CO complexes, accompanied by the loss of solvent molecules, occurs completely to binuclear [Cu(hfac)2LPr]2 within a day. On the other hand, in the case of [Cu(hfac)2LPr]·0.5Solv (Solv = CH2Cl2, CH2Br2, CHCl3), the partial transformation into a desolvated 1D polymer [Cu(hfac)2LPr] or its mixture with [Cu(hfac)2LPr]2 takes much longer time. The magnetic behavior of the isostructural solvates is highly sensitive to the included solvent molecules. The complexes with THF, CH2Cl2, CH2Br2, and CHCl3 undergo a transition to a magnetically ordered state below 4 K, a phenomenon reported for the first time for the polymeric chain Cu(II) complexes with a "head-to-tail" motif. [Cu(hfac)2LPr]·0.5THF and [Cu(hfac)2LPr]·0.5(CH3)2CO undergo a spin transition at 190 K, which is induced by the transformation of the Cu atom environment. In the case of the THF solvate, the increase in the distances between the Cu and oxygen atoms leads to enhanced ferromagnetic exchange interactions, while in the acetone solvate, the nitroxide coordination type changes from axial to equatorial at certain coordination sites, resulting in the emergence of strong antiferromagnetic exchange. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nitronyl and imino nitroxide diradicals,  being stable heteroatom analogues of the 1,1,2,3,3‐pentamethylenepropane, have been synthesized and investigated. Despite the very close proximity of two radical moieties, the diradicals have almost degenerate singlet and triplet states (J/k=−7.4 and −6.0 K). The main reason for this is the disjoint nature of their nonbonding molecular orbitals. For more details see the Full Paper by N. Gritsan, V. Ovcharenko et al. on page 2793 ff. [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]

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-l], consisting of Mn(hfac): 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-buty!-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 = l). 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 EC) 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. [ABSTRACT FROM AUTHOR]

TheESR spectrum of compact nitroxide (NO)-substituted nitronylnitroxide (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 recordedin solid argon matrix at 15 K. The zero-field splitting (ZFS) parametersof 1were derived from the recorded ESR spectrum: |D| = 0.0248 cm–1and E= 0.0025 cm–1. Quantum chemical calculations havebeen performed using DFT and multiconfigurational ab initio (CAS)methods in order to establish equilibrium geometries of the conformationalisomers resulting from twisted conformations of NO and NN moieties.The ZFS parameters of 1were calculated at these levelsof theory to test validity of the calculated structures. The calculationresults were analyzed using the measured ZFS parameters and magneticand 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 thatthe ab initio method is most successful for accurate predictions ofmolecular and magnetic parameters. Diradical 1has onlyone stable enantiomeric pair in pseudoeclipsed conformations. Thetwo chiral isomers exist in racemic crystals 1and insolid matrices with molecular parameters close to those attributedto a free molecule. The analysis of the spin density distributionsuggests that one unpaired electron occupies NO group at the equilibriumgeometry, whereas the torsion of NO group governs the spin densitydistribution of the second unpaired electron on a conjugated fragmentin NN group. The increase in planarity by torsion of NO group enhancesthe trimethylenemethane-type properties and, therefore, gives riseto larger ferromagnetic exchange interaction. More planar equilibriumgeometry and greater (three times) exchange interaction constant Jwere predicted for hypothetical diradical 1a,where bulky tert-butyl group is replacedby a methyl group in the nitroxide fragment. [ABSTRACT FROM AUTHOR]

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]