The psuedo‐michael reaction of 2‐aminoimidazolines 2. Part 1. Synthesis and structure assignment of isomeric 5(1H)‐Oxo and 7(1H)‐Oxo‐2,3‐dihydroimidazo[1,2‐a]pyrimidine‐6‐carboxylates

The pseudo‐Michael reaction of 1‐aryl‐2‐aminoimidazolines‐2 with diethyl ethoxymethylenemalonate (DEEM) was investigated. Extensive structural studies were performed to confirm the reaction course. For derivatives with N1 aromatic substituents, it was found that the reaction course was temperature dependent. When the reaction temperature was held at ?10 °C only the formation of 1‐aryl‐7(1H)‐oxo‐2,3‐dihydroimi‐dazo[1,2‐a]pyrimidine‐6‐carboxylates ( 4 ) was observed in contrast to earlier suggestions. Under the room temperature conditions, the same reaction yielded mixtures, with varying ratio, of isomeric 1‐aryl‐7(1H)‐oxo‐ ( 4a‐4f ) and 1‐aryl‐5(1H)‐oxo‐2,3‐dihydroimidazo[1,2‐a]pyrimidine‐6‐carboxylates ( 5a‐5f ). The molecular structure of selected isomers, 4b and 5c , was confirmed by X‐ray crystallography. Frontal chro‐matography with delivery from the edge was applied for the separation of the isomeric esters. The isomer ratio of the reaction products depended on the character of the substituents on the phenyl ring. The 1‐aryl‐7(1H)‐oxo‐carboxylates ( 4a‐4f ) were preferably when the phenyl ring contained H, 4‐CH₃, 4‐OCH₃ and 3,4‐Cl₂ substituents. Chloro substitution at either position 3 or 4 in the phenyl ring favored the formation of isomers 5a‐5f . The isomer ratios were confirmed both by ¹H NMR and chromatography. The reaction of the respective hydrobromides of 1‐aryl‐2‐aminoimidazoline‐2 with DEEM, in the presence of triethylamine, gave selectively 5(1H)‐oxo‐esters ( 5a‐5f ).     

Structure of 2,4,4,5,5‐pentamethyl‐2‐imidazoline‐1‐oxyl‐3‐oxide

The crystal and molecular structures of the stable nitroxide radical 2,4,4,5,5pentamethyl2imidazoline1oxyl3oxide was determined. The N—O bond lengths are 1.279(2) and 1.280(2), respectively. The O^-—N⁺=C—N— O fragment is nearly planar with carbon atoms of the ethyl fragment that deviated from the O—N⁺=C—N—O plane by –0.204(5) and +0.176(5). The minimum intermolecular distance between the oxygen atoms of NO groups is 4.094.     

Complex of bis(hexafluoroacetylacetonato)copper(II) with a stable acyclic nitroxide (tert-butyl)(3-keto-2-methylbutyl-2) nitroxyl oxime

A complex of bis(hexafluoroacetylacetonato)copper(II) with a stable acyclic nitroxide (tert-butyl)(3-keto-2-methylbutyl-2)nitroxyl oxime (L), Cu(hfac)₂L, has been synthesized. The structure of the complex was studied by X-ray diffraction analysis. The compound has a molecular structure with chelate coordination of the nitroxide. The tetragonally distorted octahedral environment of the copper(II) ion is formed by the oxygen atoms of the hfac anions and by the nitrogen and oxygen atoms of the oxime and nitroxyl groups of L, respectively. The nitroxyl group lies in the equatorial plane of the octahedron (d_Cu?O=1.907 Å). This type of N?O coordination leads to strong antiferromagnetic exchange interactions between the unpaired electrons of the copper(II) ion and the coordinated nitroxyl group and, as a consequence, to diamagnetism of Cu(hfac)₂L.     

EPR Study of Ligand Effects in Spin Triads of Bis(o-Semiquinonato)Copper(II) Complexes

We report the X-band (9 GHz) electron paramagnetic resonance (EPR) study of series of bis(o-semiquinonato)copper(II) complexes with different ligands. It was found previously, that exchange interactions in spin triads of these compounds are very sensitive to the structure of the ligand coordinated to the central copper(II) ion. Ligand moderates the copper–radical and radical–radical exchange interactions and strongly changes the magnetic properties of the compound. Depending on a ligand, ferromagnetic or antiferromagnetic character of exchange dominates in the system. The EPR study of these complexes allowed us to obtain information on zero-field splitting parameters and their distributions in the studied compounds. The EPR results compliment previously obtained spectroscopic data on these compounds and suggest the pronounced plasticity of the clusters manifested in the broad distributions of their rhombicity parameters.     

Spin transition in the molecular heterospin complex of Cu(hfac)2 with 4,4,5,5-tetramethyl-2-(1-methylpyrazol-5-yl)-4,5-dihydroimidazole-1-oxyl 3-oxide

The synthesis, structure, and magnetic properties of the products of the reaction for Cu(hfac)₂ (hfac is hexafluoroacetylacetonate) with spin-labeled nitronyl nitroxides 4,4,5,5-tetramethyl-2-(1-R-1H-pyrazol-5-yl)-3-imidazoline-1-oxyl 3-oxides L^5/R (R = Me, Et, Pr, Bu), viz., binuclear complex [Cu(hfac)₂L^5/Me]₂ and chain polymer complexes [Cu(hfac)₂L^5/R]_n, are described. The polymer heterospin chains are built according to “head-to-head” (R = Me, Et, Pr, Bu) and “head-to-tail” (R = Pr, Bu) motifs. Compound [Cu(hfac)₂L^5/Me]₂ is characterized by the ability to reveal the reversible effect of thermally induced spin transition at a temperature about 75 K (without hysteresis). In the set of heterospin Cu^II compounds with spin-labeled pyrazoles, this is the earlier unknown example of a molecular complex exhibiting a similar magnetic anomaly.     

Ultrafast Photoswitching in a Copper‐Nitroxide‐Based Molecular Magnet

Molecular compounds with photoswitchable magnetic properties have been intensively investigated over the last decades due to their prospective applications in nanoelectronics, sensing and magnetic data storage. The family of copper‐nitroxide‐based molecular magnets represents a new promising type of photoswitchable compounds. We report the first study of these appealing systems using femtosecond optical spectroscopy. We unveil the mechanism of ultrafast (<50 fs) spin state photoswitching and establish its principal differences compared to other photoswitchable magnets. On this basis, we propose potential advantages of copper‐nitroxide‐based molecular magnets for the future design of ultrafast magnetic materials.     

Synthesis,Structure, and Magnetic Properties of Heterospin Polymers MI[MII(Hfac)L2]

Russian Journal of Coordination Chemistry - Heterospin polymeric complexes based on Ni(II) and Co(II) hexafluoroacetylacetonates and deprotonated iminonitroxide,...     

Multinuclear compounds of s-elements with sterically hindered <Emphasis Type=Italic>о</Emphasis>-semiquinonates and catecholates

Alkali metals form mutinuclear compounds with redox-active singly and doubly reduced derivatives of sterically of hindered о-benzoquinones. Complexes [Na₆(Cat)₃(THF)₇]?4THF, [Na₅(Cat)₂(SQ)(THF)₅], [Na₄(SQ)₄(THF)₄]?THF?C₆H₁₄, [Na₄(3,5-SQ)₄(THF)₄], [K₄(SQ)₄(THF)₄]?4THF and [Li₃(SQ)₃(THF)₃]?THF (Cat is catecholate, SQ is semiquinonate) possessing high sensitivity to atmospheric oxygen were synthesized and their structures were determined. Magnetochemical measurements and quantum chemical calculations revealed that variation of the alkali metal and/or change in the structure of the polymetallic core significantly affect the energy of exchange interaction between the unpaired electrons of the paramagnetic ligands framing the polynuclear metal core formed by s-elements.     

Heterospin complexes based on dinuclear CuII triketonate and nitroxides

Heterospin complexes of bis(μ₂-1,1,2,2,8,8,9,9-octafluorononane-3,5,7-trionato)dicopper(II) ([Cu₂L₂]) with nitronyl nitroxides 2-(1-methyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide (2) and 4,4,5,5-tetramethyl-2-(3-pyridinyl)-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide (1) were synthesized and structurally characterized. Crystals of the complexes are formed by the discrete bis[1-methyl-4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)-1H-pyrazole]-bis (μ₂-1,1,2,2,8,8,9,9-octafluorononane-3,5,7-trionato)dicopper(II) etherate (3) and bis[3-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyridine]-bis (μ₂-1,1,2,2,8,8,9,9-octafluorononane-3,5,7-trionato)dicopper(II) (4) molecules. Each Cu atom of the dinuclear chelate fragment coordinates one paramagnetic ligand through the N atom of the pyrazole or pyridine fragment, respectively. In complex 3, the paramagnetic ligands are located on one side of the plane of the chelate fragment, whereas the ligands in complex 4 are located above and below the plane of the chelate fragment. The magnetic properties of complexes 3 and 4 are determined by dominant antiferromagnetic exchange interactions between the unpaired electrons of the Cu^II atoms in the dinuclear Cu₂L₂ moiety. Dedicated to Academician O. M. Nefedov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1836–1840, November, 2006.     

Copper(II) bis(hexafluoroacetylacetonate) complexes with 2-(2-methyltetrazolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 1-oxides

An efficient procedure was developed for the synthesis of alkyltetrazolyl-substituted nitronyl nitroxides (L ¹ and L ²). These compounds were used to prepare the first alkyltetrazolyl-substituted imino nitroxides (L ³ and L ⁴). The molecular structures of L ³ and L ⁴ were confirmed by X-ray diffraction. Investigation of the products prepared by the reaction of copper(II) bis(hexafluoroacetylacetonate), Cu(hfac)₂, with nitroxides made it possible to divide ligands L ¹—L ⁴ into two groups. The reactions of spin-labeled tetrazoles L ¹—L ³ with Cu(hfac)₂ afford the heterospin complexes Cu(hfac)₂L₂, whereas L ⁴ serves as a bidentate ligand in the Cu(hfac)₂ L ⁴ complex. In the solid Cu(hfac)₂L₂ complexes, antiferromagnetic exchange interactions between the unpaired electrons of the nitroxide fragments of the adjacent molecules prevail, due to which μ_eff decreases with decreasing temperature, and the spins of nitroxides are completely compensated at 5–10 K. The Cu(hfac)₂ L ⁴ complex displays strong intramolecular ferromagnetic coupling, due to which μ_eff at room temperature is close to 2.9 μ_B. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 64–70, January, 2006.