Israel Zilbermann

Stabilization of nickel(III)-1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane by axial binding of anions in neutral aqueous solutions

The title tervalent nickel complex, NiIIIL5aq, is stabilized in neutral and slightly alkaline aqueous solutions in the presence of stabilizing anionic ligands via the formation of NiIIIL5X2 complexes, where X=F−, SO42−, HPO42−, HCO2−, C6H5CO2−, CH3CO2− and (CH3)3CCO2−. These complexes can be used as new powerful single electron oxidizing reagents in this medium. Some of the properties of the NiIIIL5X2 complexes are described and discussed.

Comproportionation and redox catalyzed isomerization of Cu(II)(1R,4S,8R,11S-1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane)2+ in aqueous solutions

Abstract The complex Cu(II)(1R,4S,8R,11S-tmc)2+ comproportionates and isomerizes in the presence of excess of ligand and Cu° in alkaline aqueous solutions to yield Cu(II)(1R,4R,8S,1S-tmc)2+. The visible and EPR spectra of the two complexes as well as their electrochemical properties are reported. The complexes Cu(II)(1R,4R,8S,11S-tmc)(ClO4)2 and [Cu(II)(1R,4S,8R,11S-tmc)]2N3(ClO4)3 were isolated and crystallographically characterized.

Spectroscopic and electrochemical characterization of the interaction of nitrogen monoxide and cobalt tetrasulfonated phthalocyanine in aqueous solutions and surfactant films

Abstract Cobalt(II) tetrasulfonated phthalocyanine (Co(II)TSPc) was attached to didodecyldimethylammonium (DDA) and dodecyltrimethylammonium (DTA) films. Spectroscopic data suggest that monomeric Co(II)TSPc is the major species in the surfactant films. Differential pulse voltammetry for CoTSPc/DTA films in phosphate buffer solution at pH 6 shows reduction peaks at +0.61, +0.06 and −0.64 V attributed to Co(III)/Co(II), Co(II)/Co(I) and TSPc reduction, respectively. In CoTSPc/DDA films, Co(II) is reduced at −0.03 V and TSPc at −0.68 and −0.84 V, and the peak for Co(III)/Co(II) is undetectable. The reaction of Co(II)TSPc with NO was studied for dissolved CoTSPc as well as for CoTSPc/DDA and CoTSPc/DTA films using spectroscopic and electrochemical techniques. From cyclic voltammetric data for Co(II)TSPc/DDA films in NO containing solution at pH 9, the stability constant of the Co(II)TSPc(NO) adduct was estimated to be 1.3×10 8 M −1 . A slow shift of the FTIR band located at 1646–1700 cm −1 for solid Co(II)TSPc/DDA films exposed to NO is attributed to a conformation from a bent to linear configuration for the CoTSPc(NO) adduct. The rate of this transformation as obtained from UV–Vis experiments conducted for CoTSPc dissolved in aqueous solution as well as from FTIR and electrochemical data for CoTSPc/DDA films is 0.015±0.002 min −1 . The Co(II)TSPc(NO) adduct formed in these films in solutions at pH 9 at NO concentrations −5 M is reduced at −0.17 V. Higher NO concentrations cause reductive nitrosylation and the formation of a Co(I)TSPc(NO + ) complex which is reduced at −0.72 and −0.46 V, in DDA and DTA films, respectively. The electroreduction of NO at these potentials is positively shifted by 100 and 450 mV relatively to that obtained for DDA and DTA films devoid of CoTSPc, respectively.

Measured Rates of Fluoride/Metal Association Correlate with Rates of Superoxide/Metal Reactions for Fe III EDTA(H2O) - and Related Complexes

The effects of 10 paramagnetic metal complexes (Fe(III)EDTA(H2O)-, Fe(III)EDTA(OH)2-, Fe(III)PDTA-, Fe(III)DTPA2-, Fe(III)2O(TTHA)2-, Fe(III)(CN)6(3-), Mn(II)EDTA(H2O)2-, Mn(II)PDTA2-, Mn(II)beta-EDDADP2-, and Mn(II)PO4(-)) on F- ion 19F NMR transverse relaxation rates (R2 = 1/T2) were studied in aqueous solutions as a function of temperature. Consistent with efficient relaxation requiring formation of a metal/F- bond, only the substitution inert complexes Fe(III)(CN)6(3-) and Fe(III)EDTA(OH)2- had no measured effect on T2 relaxation of the F- 19F resonance. For the remaining eight complexes, kinetic parameters (apparent second-order rate constants and activation enthalpies) for metal/F- association were determined from the dependence of the observed relaxation enhancements on complex concentration and temperature. Apparent metal/F- association rate constants for these complexes (k(app,F-)) spanned 5 orders of magnitude. In addition, we measured the rates at which O2*- reacts with Fe(III)PDTA-, Mn(II)EDTA(H2O)2-, Mn(II)PDTA2-, and Mn(II)beta-EDDADP2- by pulse radiolysis. Although no intermediate is observed during the reduction of Fe(III)PDTA- by O2*-, each of the Mn(II) complexes reacts with formation of a transient intermediate presumed to form via ligand exchange. These reactivity patterns are consistent with literature precedents for similar complexes. With these data, both k(app,O2-) and k(app,F-) are available for each of the eight reactive complexes. A plot of log(k(app,O2-)) versus log(k(app,F-)) for these eight showed a linear correlation with a slope approximately 1. This correlation suggests that rapid metal/O2*- reactions of these complexes occur via an inner-sphere mechanism whereas formation of an intermediate coordination complex limits the overall rate. This hypothesis is also supported by the very low rates at which the substitution inert complexes (Fe(III)(CN)6(3-) and Fe(III)EDTA(OH)2-) are reduced by O2*-. These results suggest that F- 19F NMR relaxation can be used to predict the reactivities of other Fe(III) complexes toward reduction by O2*-, a key step in the biological production of reactive oxygen species.

Tertiary-poly-amine ligands as stabilisers of transition metal complexes with uncommon oxidation states

Abstract Tertiary-amine ligands are known to be poorer [sgrave] donor ligands than the corresponding primary- or secondary-amine ligands. They are known to shift the redox potentials of given couples to the anodic direction relative to the corresponding complexes with primary- or secondary-amine ligands. A review of data in the literature and of recent results on nickel complexes with tetra-aza-macrocyclic ligands and copper complexes with open chain polyamine ligands suggests that the major source for these effects is the poorer solvation of the complexes with the tertiary-amine complexes due to the lack of hydrogen bonding between the complexes and the solvent, or the counter ions. Thus the stabilisation of low valent transition metal complexes by tertiary-amine ligands is due to thermodynamic reasons. On the other hand, tertiary-amine-macrocyclic ligands stabilise high valent complexes because the route to the formation of imine groups is kinetically inhibited in these complexes.

Properties of monovalent nickel complexes with tetraaza-macrocyclic ligands in aqueous solutions☆

The effects of N-alkylation on the redox potential of the couples NiLi2+/NiLi+, L = tetraaza-14-membered-macrocyclic ligands, and on the properties of the monovalent nickel complexes in aqueous solutions are reported for 14 complexes. The spectra and lifetimes of the NiLi+ complexes are reported. The self-exchange rates for the couples NiLi2+/NiLi+ were determined. Two of the ligands were synthesized for the first time for this study. Cyclic voltammetry and pulse radiolysis were used. The results point out that: (i) N-alkylation always shifts the redox potential to a less cathodic one; this effect stems to a large degree from the decrease in the solvation energy of the complex caused by the N-alkylation of the ligand. (ii). The lifetime of the monovalent complexes is not linearly related to the redox potential of the NiLi2+/NiLi+ couples. (iii) The NiLi+ complexes exist in several isomeric forms; the rate of the isomerization depends on the structure of the ligand. (iv) Different isomers of NiLi+ may be formed when the complex NiLi2+ is reduced by different reagents; therefore, the pulse-radiolytically formed NiLi+ complexes might have different properties than those formed electrochemically.

Ligation and Mediated Oxidation of Nitrogen Monoxide by Nickel(II) Tetrasulfonated Phthalocyanine

Ligation and mediated electro-oxidation of NO by the water soluble Ni(II) tetrasulfonated phthalocyanine (Ni(II)TSPc) at pH 7 to 9 is demonstrated using spectroscopic (ultraviolet-visible, Fourier transform infrared, and electron spin resonance) and electrochemical (cyclic voltammetry) techniques. The experimental results are in favor of the formation of a Ni(III)TSPc (NO - ) complex in solution.

Coating Platinum Nanoparticles with Methyl Radicals: Effects on Properties and Catalytic Implications

It was recently reported that the reaction of methyl radicals with Pt(0) nanoparticles (NPs), prepared by the reduction of Pt(SO4)2 with NaBH4, is fast and yields as the major product stable (Pt(0)-NPs)-(CH3)n and as side products, in low yields, C2H6, C2H4, and some oligomers. We decided to study the effect of this coating on the properties of the Pt(0)-NPs. The results show that the coating can cover up to 75% of the surface Pt(0) atoms. The rate constant of the reaction, k((.)CH3+Pt(0)-NPs), decreases with the increase in the surface coverage, leading to competing reaction paths in the solution, which gradually become dominant, affecting the composition of the products. The methyl coating also affects the zeta potential, the UV spectra, and the electrocatalytic reduction of water in the presence of the NPs. Thus, the results suggest that binding alkyl radicals to Pt(0) surfaces might poison the NPs catalytic activity. When the Pt(0)-NPs are prepared by the reduction of a different precursor salt, PtCl6(2-), nearly no C2 H4 and oligomers are formed and the methyl coating covers a larger percentage of the surface Pt(0) atoms. The difference is attributed to the morphology of the Pt(0)-NPs: those prepared from Pt(SO4)2 are twinned nanocrystals, whereas those prepared from PtCl6(2-) consist mostly of single crystals. Thus, the results indicate that the side products, or most of them at least, are formed on the twinned Pt(0) nanocrystal edges created between (111) facets. In addition, the results show that Pt(0)-NPs react very differently compared with other noble metals, for example, Au(0) and Ag(0); this difference is attributed in part to the difference in the bond strength, (M(0)-NP)-CH3, and should be considered in heterogeneous catalytic processes involving alkyl radicals as intermediates.

Role of Radicals in the Lipid Peroxidation Products of Commercial Infant Milk Formula

Four commercial brands of infant milk formula were investigated, and the amount of shelf lipid peroxidation was determined by measuring malondialdehyde (MDA) or, more exactly, thiobarbituric acid reactive substance (TBARS) levels, which are the end products of lipid peroxidation. TBARS level, indicated by MDA concentration, was higher in the vegetarian brands. The vegetarian brands are, as expected, more prone to oxidation than dairy brands, as they contain more unsaturated fats. The introduction of formate anion diminishes the peroxide amounts initially present in the milk samples as well as those induced by radiation-induced oxidation.

Tautomerism in N-confused porphyrins as the basis of a novel fiber-optic humidity sensor

It is shown that N-confused porphyrins exhibit tautomerism not only in organic solvents, as already reported, but also after incorporation in dry/humid Nafion films. This allows the development of a new fiber-optic humidity sensor which exhibits long-term stability and a linear response over the humidity range 0 to at least 4000 ppm.