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

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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 dz2→dx2–y2 transitions were found to lie at anomalously high energy, this discrepancy being most marked when the axial bonds are very long.

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