Spectral and Electron Paramagnetic Resonance Investigations of Copper(II) Complexes of Linear-Chain Fatty Diacids

Electron paramagnetic resonance spectra of polycrystalline copper complexes of butanedioic, pentanedioic, hexanedioic, heptanedioic, and decanedioic acids are presented, together with 77 K electronic spectra. The complexes are formulated as dimeric copper carboxylate units linked into infinite chains. Monomer impurities are also present and increase in quantity with the length of the diacid. The monomer and dimer signals occur at very different field strengths, but the g values calculated from the S = $\text{1}\text{2}$ spectra are similar to those calculated from the S = 1 spectra. The EPR method can thus be used to locate copper ions in possible biological frameworks and to study the geometry around the metal sites. The distortion from axial symmetry around the copper increases with the length of the diacid, as shown by the observed zero-field splitting parameters. Gaussian analysis of the optical absorptions yields information used with EPR data to calculate covalency and Fermi contact terms. Sodium, potassium, and lithium salts transform the dimeric polymers into monomeric polymers. The presence of magnetic exchange interactions in copper dicarboxylates is discussed and thereby shown to be of interest in the study of copper ions in molecules of biological importance containing carboxylate groups.