Simulation of plasticity of a copper(II) coordination polyhedron with a force field based on coulombic interactions: Conformational analysis of copper(II) chelates with α-amino acids

Abstract

In order to simulate the plasticity of the copper(II) coordination sphere with the empirical force-field method (molecular mechanics), a square planar complex was defined as an octahedron consisting of four charges situated in the coordination plane, and two charges above and below the plane. Between these point charges only repulsive coulombic interactions take place besides harmonic bond-stretching interactions with the central atom. The model was parametrized on tetrahedrally distorted bis( l-N, N-dimethylvalinato) copper(II) ( 1). The obtained force field reproduced angles around copper in both planar and non-planar copper(II)-aminoacidates with r.m.s.-deviation from 0.15 to 3.4°. The enantioselectivity effect [energy difference between Cu( l-ligand)( l-ligand) and Cu( l-ligand) ( d-ligand)] for compound 1 was reproduced within experimental error. The shape of the potential energy surface was analysed using the CuN 2O 2 “complex”; it yielded cis and trans conformations, but also many “false” minima. The model appears to be reliable for not too great deviations from planarity (15–30° for trans valence angles around copper).