Antonio Decinti

Electrochemical studies of copper(II) complexes with Schiff-base ligands

Abstract The electrochemical reduction of copper(II) complexes with salen Schiff-base ligands derived from ethylenediamine or (R,R) or (S,S)-1,2-diphenylethylenediamine and 5-methoxy, 5-bromo and 5-nitrosalicylaldehyde have been studied by cyclic voltammetry in the potential range +1 to −2.3 V in dimethyl sulfoxide (DMSO) as a solvent. The resulting voltammograms consist of a single quasi-reversible one-electron transfer attributable to the couple [Cu(II)L]/[Cu(I)L]−. Trends in cathodic peak potential (Epc) values are observed which can be correlated with the electronic effects of the 5-substituents of the Schiff-base ligands. Changes in the basicity of the ligand groups are determinant for such electrochemical trends.

SYNTHESES AND CHARACTERIZATION OF COPPER(II) COMPLEXES WITH SCHIFF-BASE LIGANDS DERIVED FROM ETHYLENEDIAMINE, DIPHENYLETHYLENEDIAMINE AND NITRO, BROMO AND METHOXY SALICYLALDEHYDE

Abstract Copper(II) quelates with Schiff bases derived from ethylenediamine, 1R,2R-(+)- or 1S,2S-(−)-1,2-diphenylethylenediamine and 5-X-salicylaldehyde (X=OCH3, H, Br, NO2) have been synthesized and characterized by elemental analysis and IR and UV-visible spectroscopies. The ORD and CD spectra of the chiral ligands and their related complexes have also been measured. The absorption and CD spectra of the metal complexes in the visible region are consistent with near square planar coordination geometries. Trends in absorption spectra over the series of complexes can be correlated with electronic effects of the substituents at position 5 of the salicylaldehyde. The sign patterns of the CD spectra in both UV and visible regions suggest the prevalence of δ and λ conformations for the chelate ring in the complexes of Schiff bases derived, respectively, from R,R- and S,S-diamines. Specific features of the CD spectra of the metal complexes are discussed in terms of the exciton chirality method.

Epoxidation of styrene with iodosylbenzene in the presence of copper(II) Schiff-base complexes

Copper(II) complexes with salen Schiff-base ligands derived from ethylenediamine or (S,S)-1,2-diphenylethylenediamine and salicylaldehyde or 5-methoxy, 5-bromo and 5-nitrosalicylaldehyde have been tested as catalysts for the epoxidation of styrene with iodosylbenzene, in dichoromethane as a solvent. The reactions were followed by gas chromatographic analysis and mass spectrometry. Catalytic activities were found to be dependent upon both the Lewis acidity of the metal complexes and the presence of phenyl substituents on the ethylene moiety. Moderate styrene conversions and epoxide yields were obtained. Pseudo-first-order kinetics was observed for the styrene conversion. Possible reaction mechanisms are outlined.

Studies of the circular dichroism spectra of dissymmetric Schiff-bases by means of the exciton chirality method

Abstract The circular dichroism spectra of some dissymmetrical Schiff-bases derived from condensation of 2 mol salicylaldehyde with 1 mol of a chiral diamine are analyzed in terms of a simple conformational model taking the angle between the aromatic chromophores as a variable. The rotatory strengths of the π→π* transitions are calculated with AM1 molecular orbitals in combination with the dipole-coupling approximation. Calculated CD curves are found to be in reasonable agreement with the experimental CD spectra. Differences in the CD spectra of the Schiff-bases are discussed in relation to variation of the theoretical CD curve of ethylenebis(salicylideneimine) as a function of conformation.

Studies of Δ,Λ-[M(1,10-phen)2(S-ala)]qX diastereoisomeric systems of nickel and zinc with different counter-anions

Abstract Δ,Λ-[M(phen) 2 ( S -ala)] + (X q- ) 1/q diastereoisomeric systems, in which M = nickel or zinc, phen = 1,10-phenanthroline, S -ala − = S -alaninate ( q = 1 and 2), with a series of counteranions have been prepared as methanolic solutions and characterized by spectral (CD, ORD, UV-vis) and conductance data. In both nickel and zinc systems the CD spectra in the β′ band region of the diimine ligand exhibit exciton splittings with sign patterns corresponding to a predominance of the Λ isomers. Analysis of the ORD spectra by means of many-term Drude equations suggests the order Zn > Ni for the absolute values of excitation resonance energy. Differences between nickel and zinc systems have been observed concerning the effects of the counter-anion nature upon the formation equilibria. In the nickel systems these effects would consist mainly of outer-sphere interactions affecting the position of the [Λ]/[Δ] equilibrium, whereas in the zinc ones ligand/counter-anion exchange equilibria would also occur to a significant degree. By using the molar-ratio method the stability orders [Zn(phen)( S -ala) 2 ] ≈ [Zn(phen) 2 ( S -ala)] + > [Zn(phen) 3 ] 2+ and [Ni(phen) 3 ] 2+ > [Ni(phen) 2 ( S -ala)] + have been established. The ion-pairing and the third stepwise stability constants for [Zn(phen) 2 ( S -ala)]Cl have also been estimated. The counteranion dependence of the apparent stability constants for the zinc systems, is discussed in terms of the counter-anion basicity and the possible “discriminating qualities” of the [Zn(phen)( S -ala)] + species.

Chiral discrimination in diasteroisomeric systems of cadmium with 1,10-phenanthroline and S-amino acidates

Abstract Δ,Λ-[Cd(phen) 2 (S-aa)] + NO − 3 diastereomeric systems (where phen = 1,10-phenanthroline and S-aa = S -alaninate, S -serinate, S -threoninate, S -prolinate, S -valinate, S -leucinate and S -phenylalaninate) have been prepared in the form of methanolic solutions and characterized by spectral (UV, CD, ORD) and conductance measurements. The CD spectra of these systems in the β′ band region of the diimine ligand exhibit residuary exciton splittings, whose intensities and sign patterns suggest the following decreasing order of [Λ]/[Δ] diastereoisomeric equilibrium constants: S -prol > S -phenala > S -val > 1 > S -ala > S -leu > S -ser > S -threo. The same order is inferred from the ORD data over the range 400–600 nm. Comparison with the selectivities observed for some zinc analogues and the analysis of molecular models suggest that in the cadmium systems the chiral discriminations are governed mainly by a balancing between intramolecular steric repulsion, differential solvation and discriminating ion association. Solvent effects on the optical activity of some cadmium and zinc systems in methanol/n-propanol and methanol/benzene mixtures have also been studied. The corresponding results are discussed in terms of the contributions of the above discriminating forces.

Ion-pairing and optical activity of diastereomeric zinc(II) 1, 10-phenanthroline, S-valinate systems

A series of S-valinatobis(1, 10-phenanthroline)zinc salts in methanolic solution have been prepared and their conductivities and ORD spectra measured. A positive linear correlation of optical rotation with conductance was found for salts of non-chelating counter anions. From the application of the Yoe-Jones method on the system Zn, phen, S-val−, Cl− it was concluded that the counter anion dependence of the optical activity is connected mainly with heterocyclic ligand counter anion exchange equilibria.

ELECTROTOPOLOGICAL STATE MODELING OF STABILITY CONSTANTS OF BINARY AND TERNARY COPPER(II) COMPLEXES WITH A-AMINO ACIDS

The second stepwise formation constants of binary and ternary copper(II) complexes with a-amino acidate ligands are modeled by using four-descriptor sets consisting of three indexes defined as linear combinations of the E-states of some skeletal groups of the different species involved in the formation equilibria, and the logarithm of the statistical factor. Hydrophobicity and basicity properties of the ligands are also described. Results indicate that both hydrophobic interactions and ligand basicities, as differential factors over the sequence of copper(II) complexes, would operate mainly through changes in the stability of the copper(II)-carboxylate bonds

ELECTROTOPOLOGICAL STATE STUDIES OF COPPER(II) COMPLEXES WITH alpha-AMINOACIDATES

Descriptor combinations, consisting of the electrotopological state (E-state) indexes of some skeletal groups, the first-order 1c molecular connectivity index and the logarithm of the statistical factor, are employed to describe the second stepwise formation constants of binary and ternary copper(II) complexes with a-aminoacidate ligands. Descriptor sets formed by E-state values of skeletal groups belonging to the chelate rings, 1c and the logarithm of the statistical factor lead to the best regression equations. Results suggest that differences in stability over the series of metal chelates depend mainly upon variations in the coordination tendency of the carboxylate ligand groups

Catalytic behavior of some copper(II) complexes with mixed ligands in the decomposition of hydrogen peroxide

Abstract A comparative study of the catalyzed decomposition of hydrogen peroxide using the following copper(II) complex salts, Cu(bipy)( S -threo)Cl · 3H 2 O, Cu(phen)( S -threo)Cl · 2H 2 O, Cu(bipy)( S -prol)Cl · 2H 2 O and Cu(phen)( S -prol)Cl · 2H 2 O has been made. Kinetic parameters were experimentally determined by the polarographic method at 25°C, pH 7.7 and constant ionic strength (μ = 0.1 M NaNO 3 ). The catalytic behavior of the chelate with 2,2′-bipyridine and S -prolinate was also studied at pH 6.5 and 8.5. The reactivity follows the sequence: [Cu(bipy) S -prol] > [Cu(phen) S -prol] > [Cu(bipy) S -threo] > [Cu(phen) S -threo]. Activation energies are very similar to each other. The pH-dependent exchange of the amino acid ligand with hydrogen peroxide seems to be a critical factor in the reaction pathway. Several reaction mechanisms are proposed.