Pyrazine-2-amidoxime Ni(II) Complexes: From Ferromagnetic Cluster to Antiferromagnetic Layer

Synthesis, isomerization, and DFT studies of five-membered heterocyclic ketoximes

A homologous series of five-membered heterocyclic ketoximes is synthesized by condensation reactions of 2-acetylpyrrole, 2-acetylthiophene, and 2-acetylfuran with hydroxylamine hydrochloride. These compounds were fully characterized by X-ray single-crystal diffraction, elemental analysis, 1H NMR, IR, UV-Vis, and fluorescence spectra. Interesting isomerization phenomena are observed for these heterocyclic ketoximes by different characterization methods. By means of 1H NMR and X-ray single-crystal diffraction, it was found that 2-acetylpyrrole oxime (1) exists as the Z-type geometric isomer, 2-acetylfuran oxime (3) exists as the E-isomer, while 2-acetylthiophene oxime (2) is identified as a mixture of Z- and E-isomers. To study the above phenomena further, quantum chemical calculations of total energy, dipole moment, EHOMO, ELUMO, and other parameters of each heterocyclic ketoxime were carried out by Density functional theory (DFT). The theoretical calculations were in good accordance with the experimental spectroscopic results.

The Syntheses, Characterization and Crystal Structures of a Series of Heterocyclic β-Diketones and Their Isoxazole Compounds

BACKGROUND

In the field of coordination chemistry, the introduction of heterocyclic substituents into the structure of β-diketone enables ligand to produce multiple coordination sites. The adoption of small steric oxime group into the structure of heterocyclic β-diketone by Schiff-base condensation will further increase coordination sites and facilitate the generation of polynuclear structures.

OBJECTIVE

A series of β-diketones (2a-2c) containing different heterocycles such as pyridine, thiophene and furan and their corresponding isoxazole compounds (3a-3c) were synthesized.

MATERIALS AND METHODS

The Claisen condensations were investigated in a solvent-free rheological phase system at room temperature to obtain heterocyclic β-diketones 2a-2c, which further reacted with hydroxylamine hydrochloride to obtain heterocyclic isoxazoles 3a-3c. All these compounds were well characterized by EA, IR, 1H NMR and X-ray crystal diffraction to confirm the structures. Synthetic mechanisms of compounds and the effects of different heterocycles on reactivity were discussed deeply.

RESULTS

1H NMR indicated that these β-diketones do not exist as a total diketonic form but an equilibration between diketone and enol forms in CDCl3 solvent, in which the enol form accounts for 98.0% in 2a, 94.3% in 2b, 95.5% in 2c. While the crystal structures of 2a-2c showed that the reaction allows to isolate diketones in solid state. Crystal structures of 3a-3c showed that the neutral β-ketone oximes resonate and cyclize to form the target heterocyclic isoxazoles.

CONCLUSION

SN1 nucleophilic substitution mechanism of Claisen ketoester condensation was proposed for the syntheses of 2a-2c, and SN1 single molecule nucleophilic substitution reaction mechanism was put forward for 3a-3c.

Constructing CrIII-centered heterometallic complexes: [NiCrIII] and [CoCrIII] wheels

The solvothermal reaction between Cr(acac)₃, MCl₂·6H₂O (M = Ni, Co) and 2-hydroxy-4-methyl-6-phenyl-pyridine-3-amidoxime (H₂L), under basic conditions, led to the synthesis of the heterometallic heptanuclear clusters [MCr(HL_zw)₆(HL)₆]·3Cl (M = Ni, 1; Co, 2), with the nickel analogue displaying an S = 9/2 spin ground-state.

In search of molecules displaying ferromagnetic exchange: multiple-decker Ni12 and Ni16 complexes from the use of pyridine-2-amidoxime

The use of pyridine-2-amidoxime (pyaoxH₂) in Ni chemistry has provided access to a dodecanuclear complex and a hexadecanuclear Ni cluster, namely [Ni₁₂(pyaox)₆(pyaoxH)₆(MeOH)₂Cl₂]Cl₄·5MeOH (1·5MeOH) and [Ni₁₆(pyaox)₈(pyaoxH)₈(MeOH)₄](SO₄)₄·10H₂O·26MeOH (2·10H₂O·26MeOH). Complex 1·5MeOH was isolated by the reaction of NiCl₂·6H₂O, pyaoxH₂ and NaOMe in a 1 : 1 : 2 molar ratio in MeOH in 60% yield. Treatment of NiSO₄·6H₂O with pyaoxH₂ and NEt₃ in a 1 : 1 : 2 molar ratio in MeOH afforded 2·10H₂O·26MeOH in good yield (65%). The two compounds display a multi-decker configuration based on stacked Ni₄ layers, {Ni₄(pyaox)₂(pyaoxH)₂}²⁺_x (x = 3, 1·5MeOH; x = 4, 2·10H₂O·26MeOH); each deck consists of two square planar and two octahedral Ni^II centres. The number of decks observed in 1·5MeOH and 2·10H₂O·26MeOH depends on the nature of the inorganic anion that is present in the reaction system, which provides elements of synthetic control towards new high nuclearity Ni^II species. 2·10H₂O·26MeOH is the first structurally characterized complex of any metal displaying a quadruple-decker configuration, being also the highest nuclearity metal cluster bearing pyaoxH₂ and the highest nuclearity Ni^II cluster with any type of 2-pyridyl oxime. Each cluster cation displays ferromagnetic exchange between the octahedral Ni^II ions resulting in a spin ground state of S = 6 for 1 and S = 8 for 2. Magnetothermal studies have been performed and discussed for both clusters.

High-nuclear heterometallic oxime clusters assembled from triangular subunits: solvothermal syntheses, crystal structures and magnetic properties

Three series of 3d–4f oxime clusters with slightly low magnetic relaxation or large entropy change were solvothermally constructed through the flexible connectivity of triangular {Ln₃(μ₃-OH)} subunits.

Syntheses, structures and magnetic properties of nine coordination polymers based on terphenyl-tetracarboxylic acid ligands

Nine coordination polymers based on dinuclear metal units/metal-carboxylate chains and 1,1′:4′,1′′-terphenyl-2′,4,4′′,5′-tetracarboxylic acid have been synthesized and structurally characterized. Complexes 2 and 9 show the weak ferromagnetic interactions.

Crystalline pyrazine-2-amidoxime isolated by diffusion method and its structural and behavioral analysis in the context of crystal engineering and microbiological activity

The physicochemical characterizations of PAOX were obtained both in solid-state and solution, and its two anti-conformers were observed in the X-ray. Its antimicrobial properties were tested against reference strains of bacteria and yeast.

A novel oxime-derived 3d–4f single-molecule magnet exhibiting two single-ion magnetic relaxations

A weak ferromagnetic coupled {Dy₄Ni₆} cluster with a butterfly-shaped DyIII4 core and four-coordinate diamagnetic Ni^II ions shows field-induced SMM behaviour with two overlapping thermally activated relaxations.

Correlation between structural, physical and chemical properties of three new tetranuclear NiII clusters

The structural alteration in investigated tetranuclear Ni^II clusters induced only by solvent exchange resulted in significant changes in their physical and chemical properties.

Mn(ii) coordination polymers assembled from 8 or 9-connected trinuclear secondary building units: topology analysis and research of magnetic properties

Two highly-connected trinuclear Mn(ii)-cluster polymers were successfully fabricated. The FC and ZFC data of 1 show a divergence over a temperature range of 40–3.5 K. 2 features a ferrimagnetic chain with a (5/2, 10/2) spin topology.