Electron spin resonance and electronic spectra and crystal and molecular structures of copper(II) amino acid complexes

Abstract

The crystal and molecular structures of the trans complexes [Cu(DL-alaO)₂(H₂O)], [Cu(L-alaO)₂], [Cu(DL-proO)₂(H₂O)₂], and [Cu(acpc)₂][alaO = alaninate(1–), proO = prolinate(1–), and acpc = 1-aminocyclopentane-1-carboxylate] are reported, these being redeterminations except in the case of [Cu(DL-alaO)₂(H₂O)]. The single-crystal d–d and e.s.r. spectra of the complexes were recorded and interpreted in terms of a progressive increase in the axial ligand co-ordination, accompanied by a decrease in the in-plane Cu–O(carboxylate) bond length. In agreement with simple theoretical predictions, the principal axes of the g tensors tend to lie close to the metal–ligand bond axes and the sign and magnitude of the in-plane g anisotropy of each complex are consistent with the expected difference in σ-bonding power of the amine nitrogen and carboxylate oxygen atoms and the observed bond lengths. The single-crystal e.s.r. spectrum of cis-[Cu(glyO)₂(H₂O)][glyO = glycinate(1–)] was also measured, and the small in-plane anisotropy of the molecular g tensor confirmed the interpretation of the g values of the trans amino acid complexes. Angular overlap calculations of the d–d transition energies and molecular g values of the complexes suggest metal–ligand bonding parameters consistent with their molecular structures. However, in agreement with similar findings on analogous compounds the d_z²→d_x²–y² transitions were found to lie at anomalously high energy, this discrepancy being most marked when the axial bonds are very long.