Coordination geometries and cooperative ordering effects in copper(II) complexes with tridentate Schiff base dianions 5 E.p.r. spectra of [N-salicylidene-(R,S)-alaninato]copper(II) complexes with derivatives of imidazole and pyrazole and structure of 1-methylimidazole[N-salicylidene-(R,S)-valinato]copper(II)

Abstract

1-Methylimidazole[N-salicylidene-(R,S)-valinato]copper(II) shows square-planar coordination around the Cu2+ ion, coordinated in the basal plane by O,N,O atoms from the tridentate Schiff base dianion and by N from the coordinated 1-methylimidazole. In the crystal structure, enantiomers related by an inversion centre are bound by H bonds to units with a CuċCu distance of 8.039Å. Intermolecular H bonds stabilize stacking of such units, which are propagated along the [010] axis. Two such chain systems in the unit cell are related by a glide plane parallel to (010). The basal planes of two magnetically non-equivalent complexes are canted by an angle of 74.64∘. The shortest CuċCu distance between non-parallel-aligned coordination polyhedra is 10.518Å. On the other hand, the closest CuċCu spacing of parallel-orientated coordination polyhedra is 5.628Å. Hence, in the e.p.r. powder spectrum only the coupling between ferrodistortively ordered coordination polyhedra is manifested.

E.p.r. and ligand field spectra of a series of [N-salicylidene-(R,S)-alaninato]copper(II) complexes with imidazole, pyrazole and some of their derivatives provide evidence for coordination geometries ranging from square-pyramidal, with various strengths of axial interaction, to square-planar. The cooperative ordering phenomena in the structure are discussed on the basis of the e.p.r. powder spectra. When the e.p.r. spectra yield crystal g values from two coupled molecular g tensors of orthorhombic symmetry, a simple model based on consistency of G parameters was developed for evaluation of the canting angle 2γ between aligned coordination polyhedra and molecular g values, respectively. The data obtained from the e.p.r. powder measurements are in excellent agreement with the results of single X-ray structural studies.