EPR study on the ligand-exchange reaction between bis(diethyldiselenocarbamato)copper(II) and bis(octyldithiocarbonato)copper(II)

EPR study on the ligand-exchange reaction between bis(diethyldiselenocarbamato)copper(II), Cu(Et2dsc)2, and bis(octyldithiocarbonato)copper(II), Cu(octxant)2, in CH2Cl2, CHCl3, CCl4, C6H6 and C6H5.CH3 is reported for the first time. Mixing of equimolar amounts of the parents (chromophores CuSe4 and CuS4, respectively) in C6H6, C6H5.CH3 and CH2Cl2 makes EPR signals of both parents superimposed by the spectrum of a mixed-chelate Cu(xant)(dsc) complex (chromophore CuS2Se2). A new additional EPR spectrum appears in CHCl3 or CCl4 due to a five-coordinate mixed-ligand complex with the chromophore Cu(S3Se)S as follows by comparing the g-values of parents and mixed-ligand complexes. The appearance of this complex could be explained having in mind donor-acceptor properties of complexes, solvents and the resultant reaction of Cu(octxant)2 with the ester of diselenocarbamic acid yielded in Cu(Et2dsc)2 destruction by CCl4 or CHCl3.     

Diverse pathways for the palladium(II)-mediated oxidation of olefins by tert-butylhydroperoxide

[reaction: see text] New procedures are described for the palladium-catalyzed oxidation of olefins by tert-butylhydroperoxide under slightly basic conditions in CH(2)Cl(2) solution at 0-25 degrees C.     

Flash-quench technique employed to study the one-electron reduction of triiodide in acetonitrile: evidence for a diiodide reaction product

The one-electron reduction of triiodide (I(3)(-)) by a reduced ruthenium polypyridyl compound was studied in an acetonitrile solution with the flash-quench technique. Reductive quenching of the metal-to-ligand charge-transfer excited state of [Ru(II)(deeb)(3)](2+) by iodide generated the reduced ruthenium compound [Ru(II)(deeb(-))(deeb)(2)](+) and diiodide (I(2)(?-)). The subsequent reaction of [Ru(II)(deeb(-))(deeb)(2)](+) with I(3)(-) indicated that I(2)(?-) was a product that appeared with a second-order rate constant of (5.1 ± 0.2) × 10(9) M(-1) s(-1). After correction for diffusion and some assumptions, Marcus theory predicted a formal potential of -0.58 V (vs SCE) for the one-electron reduction of I(3)(-). The relevance of this reaction to solar energy conversion is discussed.     

Photoinduced one-electron reduction of alkyl halides by dirhodium(II,II) tetraformamidinates and a related complex with visible light

Various substituted dirhodium tetraformamidinate complexes, Rh(2)(R-form)(4) (R = p-CF(3), p-Cl, p-OCH(3), m-OCH(3); form = N,N'-diphenylformamidinate), and the new complex Rh(2)(tpgu)(4) (tpgu = 1,2,3-triphenylguanidinate) have been investigated as potential agents for the photoremediation of saturated halogenated aliphatic compounds, RX (R = alkyl group). The synthesis and characterization of the complexes is reported, and the crystal structure of Rh(2)(tpgu)(4) is presented. The lowest energy transition of the complexes is observed at approximately 870 nm and the complexes react with alkyl chlorides and alkyl bromides under low energy irradiation (lambda(irr) > or = 795 nm), but not when kept in the dark. The metal-containing product of the photochemical reaction with RX (X = Cl, Br) is the corresponding mixed-valent Rh(2)(II,III)X (X = Cl, Br) complex, and the crystal structure of Rh(2)(p-OCH(3)-form)(4)Cl generated photochemically from the reaction of the corresponding Rh(2)(II,II) complex in CHCl(3) is presented. In addition, the product resulting from the dimerization of the alkyl fragment, R(2), is also formed during the reaction of each dirhodium complex with RX. A comparison of the dependence of the relative reaction rates on the reduction potentials of the alkyl halides and their C-X bond dissociation energies are consistent with an outer-sphere mechanism. In addition, the relative reaction rates of the metal complexes with CCl(4) decrease with the oxidation potential of the dirhodium compounds. The mechanism of the observed reactivity is discussed and compared to related systems.     

Mononuclear copper(II) complexes of bis-triazole-based macrocyclic Schiff base hydrazones: direct synthesis,EPR studies,magnetic and thermal properties

Mononuclear copper(II) complexes of 1,2,4-triazole-based Schiff base macrocyclic hydrazones, III and IV, have been reported. The prepared amorphous complexes have been characterized by spectroscopic methods, electron spray ionization mass spectrometry, and elemental analysis data. Electrochemical studies of the complexes in DMSO show only one quasi-reversible reduction wave at +0.43 V (ΔE = 70 mV) and +0.42 V (ΔE = 310 mV) for III and IV, respectively, which is assigned to the Cu(II) → Cu(I) reduction process. Temperature dependence of magnetic susceptibilities of III and IV has been measured within an interval of 2–290 K. The values of χ_M at 290 K are 1.72 × 10^?3 cm³ mol^?1 and 1.71 × 10^?3 for III and IV, respectively, which increases continuously upon cooling to 2 K. EPR spectra of III and IV in frozen DMSO and DMF were also reported. The trend g_|| > g⊥ > g_e suggests the presence of an unpaired electron in the d_x2?y2 orbital of the Cu(II) in both complexes. Furthermore, spectral and antimicrobial properties of the prepared complexes were also investigated.     

Conditions for the formation of copper nanoparticles by reduction of copper(II) ions with hydrazine hydrate solutions

Optimal conditions were found for the preparation of copper nanoparticles in aqueous solution via reduction of copper(II) ions with hydrazine hydrate. The effects of ligand environment of copper(II) in the initial solution (hydrate, ammonia, citrate, and glycine complexes), concentration, pH, surfactants, temperature, and mode of heating were examined. The obtained colloidal systems were studied by optical spectroscopy, X-ray photoelectron spectroscopy, X-ray powder diffraction, and atomic force microscopy. The examined colloids were found to contain generally spherical copper nanoparticles with a diameter of about 10 nm, which were coated with a copper(I) or copper(II) oxide and hydroxide film.     

Dimerization of the copper(II) N-methylbenzoylhydroxamic acid complex in toluene according to EPR data

The structure of copper complexes with N-methylbenzoylhydroxamic acid in toluene at 100 K has been investigated by EPR spectroscopy. In frozen solution, copper hydroxamate exists as one dimeric and two monomeric species. The relative concentrations of these species and their magnetic resonance parameters have been determined by mathematical modeling of the shape of the EPR line.     

Copper(II)-mediated oxidative transformation of vic-dioxime to furoxan: evidence for a copper(II)-dinitrosoalkene intermediate

The mononuclear copper(II) complex [Cu(H(2)L(1))(2)(H(2)O)](ClO(4))(2) (1) (where H(2)L(1) = 1,10-phenanthroline-5,6-dioxime) reacts with copper(II) perchlorate in acetonitrile at ambient conditions in the presence of triethylamine to afford a copper(II) complex, [Cu(L(3))(2)(H(2)O)](ClO(4))(2) (2a), of 1,10-phenanthroline furoxan. A similar complex [Cu(L(3))(2)Cl](ClO(4)) (2) is isolated from the reaction of H(2)L(1) with copper(II) chloride, triethylamine, and sodium perchlorate in acetonitrile. The two-electron oxidation of the vic-dioxime to furoxan is confirmed from the X-ray single crystal structure of 2. An intermediate species, showing an absorption band at 608 nm, is observed at -20 °C during the conversion of 1 to 2a. A similar blue intermediate is formed during the reaction of [Cu(HDMG)(2)] (H(2)DMG = dimethylglyoxime) with ceric ammonium nitrate, but H(2)DMG treated with ceric ammonium nitrate does not form any intermediate. This suggests the involvement of a copper(II) complex in the intermediate step. The intermediate species is also observed during the two-electron oxidation of other vic-dioximes. On the basis of the spectroscopic evidence and the nature of the final products, the intermediate is proposed to be a mononuclear copper(II) complex ligated by a vic-dioxime and a dinitrosoalkene. The dinitrosoalkene is generated upon two-electron oxidation of the dioxime. The transient blue color of the dioxime-copper(II)-dinitrosoalkene complex may be attributed to the ligand-to-ligand charge transfer transition. The intermediate species slowly decays to the corresponding two-electron oxidized form of vic-dioxime, i.e. furoxan and [Cu(CH(3)CN)(4)](ClO(4)). The formation of two isomeric furoxans derived from the reaction of an asymmetric vic-dioxime, hexane-2,3-dioxime, and copper(II) perchlorate supports the involvement of a dinitrosoalkene species in the intermediate step. In addition, the oxidation of 2,9-dimethyl-1,10-phenanthroline-5,6-dioxime (H(2)L(2)) to the corresponding furoxan and subsequent formation of a copper(I) complex [Cu(L(4))(2)](ClO(4)) (3) (where L(4) = 2,9-dimethyl-1,10-phenanthroline furoxan) are discussed.     

Absorption spectroscopic and ESR studies on one-electron reduction products of copper(II) and oxovanadium(IV) tetraphenylporphyrins produced by γ-radiolysis in 2-methyltetrahydrofuran solutions at 77 K

Optical absorption spectra of one-electron reduced species of copper(II) and oxovanadium(IV) tetraphenylporphyrins. Cu(II)TPP and V(IV)OTPP, in 2-methyltetrahydrofuran at 77 K reveal that not the central metal but the porphyrin ligand is reduced by an excess electron. The triplet ESR spectrum resulting from the spin-spin interaction between two odd electrons located on the porphyrin ligand and the central metal is observed for the one-electron reduced species of V(IV)OTPP while not for that of Cu(II)TPP.     

Continuous-wave and pulse EPR study of the copper(II) complex of N-confused tetraphenylporphyrin: direct observation of a sigma metal-carbon bond

N-confused or inverted porphyrins, a family of porphyrin isomers that contain a confused pyrrole ring connected through its alpha and beta' positions in the macrocycle, exhibit unique physical and chemical properties, like, for instance, the ability to stabilize unusual oxidation states of metals due to the reactivity of the inverted pyrrole. In this Article, a combined multifrequency continuous-wave and pulse electron paramagnetic resonance (EPR) study of the copper(II) complex of N-confused tetraphenylporphyrin (TPP) is presented. By use of pulse EPR methods like ENDOR and HYSCORE, the magnetic interactions between the unpaired electron of the compound and the surrounding nitrogen nuclei were revealed. Through 13C labeling of the macrocycle, a detailed study of the carbon hyperfine interaction became possible and provided further insight into the character of the metal-carbon bond. The observed hyperfine couplings of the ligand atoms in the first coordination sphere showed the presence of a remarkably strong sigma Cu-C bond and allowed for a detailed analysis of the spin delocalization over the porphyrin macrocycle. Interestingly, it was found that the observed delocalization is approximately 11% larger than the corresponding one for CuTPP.     

Modular iminopyridine ligands. Application to the enantioselective copper(II)-catalyzed Henry reaction

Chiral iminopyridines prepared in a modular fashion from monoterpenic (camphor-derived) ketones and pyridinylalkylamines catalyze the enantioselective Henry (nitro aldol) reaction between nitromethane and o-anisol in the presence of copper(II) acetate, with high yields and good ee (up to 86%) under straightforward experimental conditions without the need for air or moisture exclusion.     

Regioselective radical cyclization initiated by the reaction of allylic hydroperoxides with iron(II) sulfate

Treatment of 1-methyl-2-methylene-1-cyclohexyl hydroperoxide with a mixture of FeSO₄/CuCl₂ yielded 1-(1-chlorocyclohexyl)ethanone as the major product consistent with 6-endo-trig cyclization of the intermediate 5-acetylhex-5-enyl radical. This strategy was extended to the ring enlargement of a series of 1-isopropenylcycloalkyl hydroperoxides. Regioselective 7- or 8-endo-trig cyclization reactions could be achieved by treatment of the corresponding cyclopentyl or cyclohexyl hydroperoxides with either a mixture of FeSO₄/CuCl₂ or with FeSO₄ only. The influence of substituents on the efficiency of the 8-endo-trig cyclization process was also explored.     

Reduction of the copper(II) cation in acetonitrile by trimethyl phosphite. Evidence for an intermediate copper(II) complex

Reduction of the solvated copper(II) cation by trimethyl phosphite in acetonitrile occurs via a short-lived purple intermediate believed to be a copper(II)-phosphite complex.     

EPR study of copper(II) complexes of hydroxysalen derivatives in order to be used in the DNA cleavage

Two series of functionalized hydroxy-salen-copper(II) complexes with various side chain lengths have been synthesized. The first one is characterised by amino side chain protected by the tert-butyloxycarbonyl group (Boc) whereas, the second series is obtained by removal of the Boc-protecting group under acidic conditions and formation instead of it an ammonium salt. EPR studies were carried out on the copper(II) complexes. EPR signals attributed to monomers and dimers of Cu2+ species were evidenced. Determination of the copper(II) environment in each complex was attempted using all the experimental results. Square planar and tetrahedral symmetries were found for the copper(II) monomers. From the fine structure observed for the pair signal, the distance between the Cu2+ ions in the pair has been calculated (3.9-4.3A). From these values, it seems that the formation of pairs is obtained by a face-to-face bimolecular association.     

Rate constant for the reaction of the t-butoxyl radical with Fe(II) ion

The rate of the reaction of the tert-butoxyl radical (t-BuO^√) with Fe²⁺ was measured using laser flash photolysis of methanolic solutions at room temperature. t-BuO^√ were generated by homolytic photodecomposition of di-tert-butyl peroxide. The rate constant for oxidation of Fe²⁺ with t-BuO^√ radicals was studied under pseudo-first order conditions. On the basis of competitive kinetics the quantum yield of oxidation, Φ(Fe³⁺), was determined as function of Fe²⁺ concentration by measuring the absorbance of Fe³⁺ as [FeCl]²⁺ complex. By using the literature values of the rate constants of relevant competing reactions, the desired rate constant was determined to be 3.0×10⁸ M⁻¹ s⁻¹.