Schiff base complexes of rare earth metal ions: Synthesis, characterization and catalytic activity for the oxidation of aniline and substituted anilines

Synthesis and photochromism of catechol-containing symmetrical azobenzene compounds

Symmetrical azobenzene derivatives with two catechol groups, 1d-4d, were synthesized as kinds of novel compounds, and the structures were confirmed using mass spectrometry and nuclear magnetic resonance spectroscopy. These compounds could attain photostationary state rapidly in solution upon UV irradiation, and their photochromism had good reversibility. Substituents and their positions on azobenzene chromophore had obvious influence on the maximum absorption and photochromic performances of these as-synthesized compounds. Electron-donating group on ortho positions could contribute to the redshift π-π* band. The sulfonamide group that is bonded to dopamine molecules and azobenzene rings caused a negligible n-π* transition of cis isomer, resulting in photobleaching upon UV irradiation. Among the four compounds, 4d had the strongest electron-donating ortho-methoxy substituents and lower planarity; thus its absorption could decrease more significantly upon UV irradiation of the same intensity, and its cis-to-trans conversion could be up to 63%. Furthermore, owing to the presence of catechol groups, 4d showed an effective affinity and adhesion to substrate, and on the surface of substrate, a weak colour change could be observed upon UV irradiation, but the reversibility was poorer than that in solution.

Novel and Sustainable Colorants Developed via Incorporating Azo Chromophores into Dopamine Molecules

Inspired by the application of dopamine as an "anchor" and UV absorber, novel sustainable colorants with biscatecholic structure were synthesized through a simple incorporation of simple azo chromophores with dopamine. Their structures were confirmed using MS and NMR analyses, and their application on textile materials was investigated. Compared to the simple azo chromophores with almost no coloring ability on fabrics, the biscatecholic colorants could color different fabrics effectively, mainly through self-polymerization only in the presence of a trace amount of organic base at room temperature, which is environmentally friendly in terms of saving resources and alleviating chemical pollution. Meanwhile, the UV resistance of colored fabrics was enhanced significantly, showing the advantage of protecting wearers from UV damage.

2-Imino-2,3-dihydrobenzoxazole-a useful platform for designing rare- and alkaline earth complexes with variable di- and trianionic O,N,N, ligands

The reactions of 2-imino-2,3-dihydrobenzoxazole LH with M[N(SiMe₃)₂]₂(THF)₂ (M = Yb and Ca) and Y(CH₂SiMe₃)₃(THF)₂ proceed with the opening of the dihydrobenzoxazole ring and the elimination of HN(SiMe₃)₂ or SiMe₄. Besides, in the case of Yb[N(SiMe₃)₂]₂(THF)₂, an electron transfer from Yb(II) to L takes place and Yb(III) complex 1 is coordinated by a dianionic phenolate ligand containing a pendant radical-anionic diazabutadiene fragment form. When LH is reacted with Ca[N(SiMe₃)₂]₂(THF)₂, C-H bond activation of a methyl fragment by imino nitrogen occurs and affords a dimeric calcium complex 2. In 2, the phenolate ligand is dianionic due to the presence of the amido-imino fragment [R-NC(CH₃)-C(CH₂)-NR']^- (R = 2,4-tBu₂-C₆H₂O; R' = 2,6-iPr₂C₆H₃). In situ generated ate-complex {Na(Et₂O)_n}{Ca[N(SiMe₃)₂]₃} also enables C-H bond activation, however the dianionic phenolate ligand in the resulting complex 3 contains an amido-imino fragment [R-N-C(CH₂)-C(CH₃)NR']^- featuring the sequence of N-C and NC bonds opposite to that in 2. The reaction of Y(CH₂SiMe₃)₃(THF)₂ with LH affords mono(alkyl) yttrium complex 4. 4 contains a dianionic amido-imino phenolate ligand resulting from the migration of one alkyl group to the CN bond [R-N-C(Me)(CH₂SiMe₃)-C(Me)NR']. 4 undergoes slow intramolecular C-H bond activation of the residual CH₃ group to afford a yttrium complex coordinated by a trianionic diamido-phenolate ligand [R-N-C(Me)(CH₂SiMe₃)-C(CH₂)NR'].

Novel high-gluten flour physically cross-linked graphene oxide composites: Hydrothermal fabrication and adsorption properties for rare earth ions

Graphene oxide (GO) nanosheets were immobilized and cross-linked by high-gluten flour (HGF), and a series of biomass-GO composites with various HGF-to-GO mass ratios were fabricated through a one-step hydrothermal method. The HGF-GO composites were used as novel adsorbents to adsorb rare earth ions (REE³⁺: La³⁺, Yb³⁺, Y³⁺, Er³⁺ and Nd³⁺) from aqueous solutions, and their adsorption properties were also investigated detailly. To evaluate the physicochemical properties of HGF-GO composites and further understand the mechanisms of adsorption of REE³⁺ onto HGF-GO composites, the HGF-GO composites were characterized by scanning electron microscopy (SEM), thermal gravimetric analyzer (TGA), Raman spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. Several important condition parameters including contact time, initial REE³⁺concentrations, solution pH values and temperature that might affect the adsorption process were studied in detail. The maximum adsorption capacities of HGF-GO_1:1 composite toward La³⁺, Yb³⁺, Y³⁺, Er³⁺ and Nd³⁺ were 30.32, 36.64, 32.84, 42.36 and 48.68 mg g^-1, respectively. The experimental data indicated that the adsorption of REE³⁺ onto HGF-GO_1:1 was well fitted by the pseudo-second order kinetic model and the Langmuir isotherm model, and the adsorption process was a spontaneous and endothermic reaction. The HGF-GO_1:1 composite could be well regenerated and reused after five adsorption-desorption cycles, and its removal efficiency for Yb³⁺ remained as a constant of 100%.

Azobenzenes and catalysis

Azobenzene is the most extensively used class of chromophore in a large variety of applications.

Oxidative coupling of anilines to azobenzenes using heterogeneous manganese oxide catalysts

Cryptomelane-type manganese oxide shows high efficiency for the NN formation reaction in the oxidative coupling of anilines to azobenzenes. The difference of the Hammett constants (Δσ) of two substituted groups determines the selectivity to unsymmetric azobenzenes in cross-coupling reactions, which are favored at Δσ < 0.32.

A new Salen-type azo-azomethine ligand and its Ni(II), Cu(II) and Zn(II) complexes: Synthesis, spectral characterization, crystal structure and photoluminescence studies

A novel Salen-type azo-azomethine ligand H2agen, 2,2'-{ethane-1,2-diylbis[nitrilomethylylidene]}bis{4-[ethylphenyldiazenyl]phenol}, formed by the 1:2M condensation of ethane-1,2-diamine with 5-[(4-ethylphenyl)diazenyl]-2-hydroxybenzaldehyde and its nickel(II), copper(II), and zinc(II) complexes were synthesized and characterized by the spectroscopic and analytical methods. The UV-vis spectra of the ligand were investigated in three organic solvents (DMSO, DMF and CHCl3). The ligand shows two absorption bands assigned to π-π(∗) and n-π(∗) transitions in the solvents used. Cu(II), and Ni(II) are tetra-coordinate binding to two phenolic oxygens and two imine nitrogens in approximate square planar geometry. Zn(II) also coordinates using the same sites like other metals but gave tetragonal configuration. Molecular structure of the Cu(II) complex [Cu(agen)] was determined by single crystal X-ray diffraction study. The X-ray data revealed that crystallographic imposed symmetry was absent for the complex molecule. In the structure, the Cu(II) ion is coordinated to two phenolate oxygen atoms and two imine nitrogen atoms of the azo-azomethine ligand with approximate square planar geometry. The ligand H2agen and its metal complexes exhibit strong blue emissions with irradiation. Fluorescence quantum yields and excited-state lifetimes for the ligand and its complexes were obtained. The H2agen ligand had a 35% quantum yield and a 3.27 ns excited-state lifetime. Complexation with metal ions caused reductions in intensities and quantum yields.

Synthesis, characterization and properties of salicylhydrazide-salicylacylhydrazone derivatives and their terbium complexes

A series of terbium complexes with salicylhydrazide-salicylacylhydrazone derivatives were synthesized and characterized by elemental analysis, IR spectra, UV/vis spectra and thermal analysis. The luminescence and electrochemical properties of the terbium complexes were investigated. The results show that all the target complexes exhibited characteristic emissions of terbium ions and the complex substituted by the chlorine has the strongest luminescence intensity with the highest quantum yield at 0.609. The introduction of donating electron groups could increase the oxidation potential and the highest occupied molecular orbital energy level of the terbium complex; however, the introduction of accepting electron groups gave the opposite result.

Synthesis and luminescence properties of salicylaldehyde isonicotinoyl hydrazone derivatives and their europium complexes

Four novel salicylaldehyde isonicotinoyl hydrazone derivatives and their corresponding europium ion complexes were synthesized and characterized, while the luminescence properties and the fluorescence quantum yields of the target complexes were investigated. The results indicated that the ligands favored energy transfers to the emitting energy level of europium ion, and four target europium complexes showed the characteristic luminescence of central europium ion. Besides the luminescence intensity of the complex with methoxy group, which possessed the highest fluorescence quantum yield (0.522), was stronger than that of other complexes. Furthermore, the electrochemical properties of the target complexes were further investigated by cyclic voltammetry, the results indicated that the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels and the oxidation potential of the complexes with electron donating group increased, however, that of the complexes with accepting electron group decreased.

Graphene oxide supported MnO2 nanorods: an efficient heterogeneous catalyst for oxidation of aromatic amines to azo-compounds

Graphene oxide supported MnO₂ nanorods (GOnc), a composite material, has been synthesized and characterized by XRD, FE-SEM, EDX, BET surface area measurement, FTIR and Raman Spectroscopy.