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Modelling of Crystal Lattice Effects on the Copper(II) Coordination Polyhedron Geometry in Copper(II) Amino Acid Complexes

A new molecular mechanics model is proposed and a new force field derived with the aim to simulate and predict properties of tetra-coordinated copper(II) complexes with amino acids and amino acid derivative ligands. Interactions inside a copper(II) coordination polyhedron are modelled with a Morse potential (between the metal and ligand atoms), an electrostatic potential (between four ligand atoms), and a torsion-like potential (with minima at 0o and 180o) that should hold four ligand atoms in a coordination plane. Twelve X-ray crystal structures of anhydrous copper(II) amino acid complexes with the same atom types have been selected for modelling. Eleven are with trans- and one with cis-CuN2O2 coordination polyhedron, with Cu(II) having either an irregular square-planar, a distorted planar of a flattened tetrahedral coordination geometry. The conformational potential energy was minimised for isolated molecules, and so as a molecule was in the crystalline surroundings. All calculations, including the crystal simulations were performed with the Consistent Force Field (CFF) program for conformational analysis [1]. The empirical parameters of the selected potential energy functions were optimised with respect to the experimental data (bond lengths, valence and torsion angles, and unit cell dimensions) of five molecules. To assure the planarity of the copper(II) coordination geometry for isolated molecules, the parameters were also optimised with respect to the valence angles around the copper obtained by molecular quantum mechanics for three bis(aminoacidato)copper(II) complexes. According to the molecular quantum mechanics results for three copper(II) chelates, a square-planar copper(II) configuration is electronically favoured in vacuo [2]. If there is no steric hindrance present in a molecule (as can be close contacts between bulky alkyl groups replacing the hydrogens of the amino group), it remains planar. The comparison of the experimental crystalline structures with the ab initio in vacuo derived geometries pointed out that a strain imposed by crystal packing forces could be alleviated either by distortion of the copper(II) coordination polyhedron or/and by changing the geometry of the chelate rings. These changes were up to 11o for the valence angles around Cu(II), and up to 20o for the chelate rings� torsion angles. To test the newly developed force field, the equilibrium geometries of twelve molecules are predicted in vacuo and in approximate crystalline environment. The results were compared with their ab initio and experimental crystal structures, respectively. The unit cell volumes are reproduced in a range from -7.2% to 3.8%. The force field is capable of reproducing the crystal lattice effects on the changes in the chelate rings� torsion angles as well as on the distortion of the copper(II) coordination polyhedron from idealised square-planar geometries. 1. Pietilä, L.-O. and Rasmussen, Kj., J. Comput. Chem., 1984, 5, 252-260. 2. Sabolović J. and Liedl, K. R., Inorg. Chem., submitted

copper; amino acids; modelling; crystal lattice effects; moleculat mechanics

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