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.     

Synthesis and Crystal Structure of Dicyanamide(5,7,7,12,14,14-Hexamethyl-1,4,8,11-tetraazacyclotetradecane) Copper(II) Percholatehydrate

1 INTRODUCTION Recently the chemistry of transition metal com- plexes containing macrocyclic ligand has become in- creasingly important[1～12]. However, the structurally characterized transition metal complexes containing macrocyclic ligand teta are stil…     

Electron paramagnetic resonance study of 3,4,5-trimethoxytetraphenyl porphyrinoxovanadium (IV) complex

3,4,5-Trimetoxytetraphenylporphyrinoxovanadium (IV) complex (3,4,5-TMVOTPP) was synthesized by a new one pot synthetic method. The complex was studied in the form of single crystal, powder (polycrystalline state), solution and frozen solution (glassy state) by electron paramagnetic resonance (EPR) between room temperature (RT) and liquid nitrogen temperature (LNT). Interestingly a well-resolved octet in the EPR spectrum at RT is observed in the pure paramagnetic state of the crystal. This observation is attributed to a greatly reduced dipolar interaction between paramagnetic vanadyl ions due to the large size of the molecule and the resultant stacking in the crystalline state. The line width of the EPR signals in single crystal at RT is approximately 3.3 mT which is more than the usual line width in diluted paramagnets ( approximately 1.5 mT) and is attributed to some kind of broadening effect akin to slow motion broadening. The line width in solvents is more than the crystal value but decreases appreciably at low temperatures. The decrease in line width at low temperature is attributed to the increase in spin-lattice-relaxation time and quenching of RT broadening motion. Only one octet is observed in the crystal EPR spectra which suggests only one formula unit per unit cell or a parallel/antiparallel ordering of V=O vectors in case the formula units per unit cell are more than one. This result needs verification by a detailed X-ray investigation. The crystalline field symmetry around the V(4+) metal ion is revealed to be axial by the observed angular dependence of the EPR spectrum and the powder EPR spectrum. No super hyperfine splitting of the hyperfine lines of the vanadyl ion is observed in solid state or diluted glass up to liquid nitrogen temperature. This suggests an expected weak in-plane pi-bonding with ligands. The spin Hamiltonian parameters for vanadyl ion in crystal, powder, diluted solutions and frozen glasses are evaluated and discussed.