Electron paramagnetic resonance studies on copper(II)-cyclodextrin systems

Three copper(II) complexes containing beta-cyclodextrin (betaCD) derivatives as ligands [mono-6-methylamino-6-deoxy-betaCD dithiocarbamate (CDTC), mono-6-histamino-6-deoxy-betaCD (CDHM) and mono-6-Nalpha-arginyl-6-deoxy-betaCD (CDARG)] have been studied by electron paramagnetic resonance. The spectra have been recorded at X- and S-bands and different temperatures and simulated to obtain the best set of magnetic parameters. In particular, the 300 K spectra are typical of the slow motion regime, as expected considering the high molecular weight of the ligands. Some structural characteristics of the complexes are proposed on the basis of dynamic and covalency parameters obtained from simulation.     

Electron paramagnetic resonance studies on three copper(II) complexes of 3-amino-5-methylisoxazole

EPR studies have been carried out on three copper(II) complexes of 3-amino-5-methylisoxazole(3-AMI), namely(l)Cu(3-AMI)₄Cl₂,(2)Cu(3-AMI)₂(NO₃)₂and(3)Cu(3-AMI)₂Br₂, in polycrystalline, solution and frozen solution forms in order to elucidate the stereochemistry, metal-ligand bond nature and solute-solvent interactions. The magnitude of the spectral parameters obtained from polycrystalline samples are found to represent the approximate local copper(II) environment with the ground state dominated by the d_x²-_y² orbital. The molecular species obtained in pyridine and N,N '-dimethyl formamide (DMF) solutions of Cu(3-AMI)₂(NO₃), and Cu(3-AMI)₂Br₂ are similar to the corresponding species in complexes of isoxazole and its derivatives studied earlier. But, unlike the above two complexes the spectral parameters obtained in the pyridine and DMF solutions of Cu(3-AMI)₄Cl₂ are characteristic of CuN₄ and CuO₄, chromophores respectively with strong metal-ligand σ-bonding. The 4s-character in the ground state is estimated in all the complexes to indicate the axial field strength.     

Pegylation of 1,4,8,11-tetraazacyclotetradecane (cyclam) and its Cu(II) complexation

Two novel star-like cyclam derivatives appended with four amino groups and four PEG-arms have been synthesised. The complex formation of cyclam and the two cyclam derivatives with Cu(II) has been studied by UV-vis and time-resolved laser-induced fluorescence (TRLFS) measurements outlining 1:1 complexes between cyclam ligands and Cu(II). By TRLFS measurements a quench effect of the Cu(II) on the fluorescence of the ligand was also found to determine the complex formation between Cu(II) and the ligands.     

Steric effects in the complexation kinetics of copper(II) with rac-5,5,7,12,12,14-hexamethyl 1,4,8,11-tetraazacyclotetradecane in basic aqueous media

Copper(II) reacts with rac-5,5,7,12,12,14-hexamethyl-l,4,8,11-tetraazacyclotetradecane (tet-b) in strongly basic aqueous media to give [Cu(tet-b) (OH) (blue)]⁺ which contains trigonal bipyramidally co-ordinated Cu²⁺ with the tet-b ligand in its most stable, folded form. The kinetics of formation of this blue complex have been studied at 25.0° ± 0.1°C using the stopped-flow technique. Second-bond formation is proposed as the rate-determining step for tet-b reaction with Cu(OH)^-₃ and Cu(OH)^2-₄. Possible mechanisms for the reaction and the steric effects resulting from the methyl groups on the alkyl backbone of the macrocyclic ligand are considered.     

Electron spin resonance of some paramagnetic compounds of iridium(II)

This paper deals with some electronic problems suggested by Malatesta. Electron spin resonance with [IrBr₃(NO)(PPh₃)₂] shows that the electronic configuration of the paramagnetic compound is described by that of the Ir-NO moiety. Specifically the three values of g tensor components demonstrate that the unpaired electron is located in a largely comprised π* (NO) orbital, so that the electronic state of NO is similar to that of gaseous trapped NO. The overlap between Ir and NO orbitals is through the d_π (d_xz or d_yz) metal orbitals, with consequent bent coordination of NO ligand. In conclusion the stability of d⁷ configuration of Ir in this monomeric compound is due to the easy electron transfer between metal and ligand, in agreement with that described in other monomeric Ir(II) compounds.