Electron spin resonance study of exchange and dipole–dipole coupling in copper(II) chelates of cyclopentanetetracarboxylic acid and related systems

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Abstract
Electron spin resonance spectra of frozen solutions of the copper(II) chelates of cyclopentanetetracarboxylic acid formed by the interaction of copper(II) chloride or perchlorate in various non-aqueous solvents indicate the formation of dimeric species in which exchange coupling or dipole–dipole coupling occurs. The formation of the dimeric species is dependent on the nature of the solvent and composition of the reaction mixture. In dimethylformamide the e.s.r. spectrum indicates the formation of dipole–dipole coupled copper(II) pairs when the copper(II) chloride to ligand ratio is 1 : 1, whereas when this ratio is 1 : 2 low- and high-field signals are observed such that a mixture of dimeric species exists. When copper(II) perchlorate is used, the e.s.r. spectra point to the formation of dimeric species in which exchange coupling occurs, though the magnetic parameters associated with the signal have different values from those observed for the exchange coupled species when copper(II) chloride is used. Exchange coupling in dimeric species has been observed in the copper(II) chelates of m- and p-hydroxybenzoic acids, and in the copper(II) complexes of adenine formed in both aqueous and non-aqueous solutions. To evaluate the magnetic parameters associated with the signals due to species in which exchange coupling occurs, a general spin Hamiltonian to include the effects of exchange and dipole–dipole contributions has been used. Solution of the spin Hamiltonian has made possible the computation of the lineshape due to exchange coupled copper(II) pairs. Thus sets of magnetic parameters have been collected for exchange coupled and dipole–dipole coupled copper(II) pairs and the values obtained from the dipole–dipole coupled pairs used to suggest the possible structure of the dimeric species.

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