ABSTRACT
Stable cyclotriphosphazenes 4 and 5, incorporating one and two carbon radical centers, respectively, have been easily prepared and characterized. EPR spectroscopic studies in fluid solution at room temperature were carried out for both compounds and also for diradical 5 in frozen solvent matrixes. Spectral results are consistent with a triplet or degenerate singlet triplet ground state for 5. Reductive cyclic voltammetry shows a redox couple, being monoelectronic for 4 and bielectronic for 5.
ABSTRACT
The electrochemical synthesis of magnetic nanoparticles of new Sr-Fe and Sr-Co-Fe oxides using an undivided cell with two Fe electrodes is reported in this work. These materials are collected as precipitates by electrolyzing acidic solutions containing mixtures of chlorides and nitrates of Sr2+, Fe3+ and, optionally, Co2+ at temperatures between 40 degrees C and 80 degrees C. Sr-Fe oxides are produced with energy costs lower than 2.7 kWh kg-1 in the pH range 2.0-6.0 at 50 mA cm-2, whereas Sr-Co-Fe oxides are obtained with a cost of 3.0 kWh kg-1 at pH 1.5 and at 35 mA cm-2. Inductively coupled plasma analysis of materials and energy dispersive X-ray microanalysis of single particles confirm that they are composed of pure mixed oxides, without metallic Fe impurities. All synthesized compounds crystallize as inverse cubic spinels, with structures similar to those of maghemite and magnetite. They are formed by round-shape nanoparticles with sizes lower than 50 nm, as observed by transmission electron microscopy. Thermal desorption spectrometry allows us to detect the presence of hydrogen and volatiles proceeding from water decomposition in their lattices. After heating the electrogenerated materials at 300 degrees C during 1 h to eliminate such species, Sr-Co-Fe oxides with similar magnetic properties to those of hard ferrites are obtained, but magnetic Sr-Fe oxides only behave as soft ferrites.
Subject(s)
Cobalt/chemistry , Electrochemistry/methods , Iron/chemistry , Magnetics , Oxides/chemical synthesis , Strontium/chemistry , Crystallography/methods , Electrolysis/methods , Materials Testing , Microspheres , Molecular Conformation , Nanotechnology/methods , Oxides/chemistry , Particle Size , Quality Control , Surface Properties , TemperatureABSTRACT
The condensation reactions between (4-amino-2,6-dichlorophenyl)bis(2, 4,6-trichlorophenyl)methyl radical and acetylacetone or 1, 4-bis(5-methyl-2-thienyl)-1,4-butanedione yield [2,6-dichloro-4-(2, 5-dimethyl-1-pyrrolyl)phenyl]bis(2,4,6-trichlorophenyl)methyl radical (3(*)()) and [2,6-dichloro-4-[2, 5-bis(5-methyl-2-thienyl)-1-pyrrolyl]phenyl]bis(2,4, 6-trichlorophenyl)methyl radical (4(*)()), respectively. EPR studies of both radicals 3(*)() and 4(*)() in CH(2)Cl(2) solution suggest a weak electron delocalization with coupling constant values of 1.25 and 1.30 G, respectively, with the six aromatic hydrogens. Their electrochemical behavior was analyzed by cyclic voltammetry. Both radicals show reversible reduction processes at E degrees = -0.69 V and -0.61 V versus SSCE, respectively, and anodic peak potentials at E(p)(a) = 1.10 and 0.72 V, respectively, versus SSCE at a scan rate (nu) of 200 mV s(-)(1), being reversible for radical 4(*)(). X-ray analysis of radical 3(*)() shows a high value (65 degrees ) of the dihedral angle between the 2,5-dimethylpyrrolidyl moiety and the phenyl ring. Smooth oxidation of radical 4(*)() in CH(2)Cl(2) containing trifluoroacetic acid gives an ionic diradical species with a weak electron interaction (|D/hc| = 0.0047 cm(-)(1)). A Curie plot of the Deltam(s)() = +/-2 signal intensity versus the inverse of the absolute temperature in the range between 4 and 70 K suggests a triplet or a nearly degenerate singlet-triplet ground state.
