Why Are Copper(II) Amino Acid Complexes Not Planar in Their Crystal Structures? An ab Initio and Molecular Mechanics Study

This paper presents geometries of copper(II) chelates with l-alanine, l-leucine, and l-N,N-dimethylvaline optimized by the hybrid density functional method B3LYP. According to the molecular quantum mechanics results, a square-planar copper(II) coordination geometry is electronically favored in vacuo. Deviations from the planar configuration observed in the crystal state should be attributed to sterical intramolecular and/or intermolecular effects. This paper proposes a new molecular mechanics model for tetracoordinated copper(II) amino acidates to investigate these effects in detail. The empirical parameter set for the selected potential energy functions was optimized both with respect to the X-ray crystal structures (internal coordinates and unit cell constants) and with respect to the quantum mechanically derived valence angles around copper. To test this newly developed force field (FF), the equilibrium geometries of 10 molecules are predicted in vacuo and in approximate crystalline surrounding. The results were compared with their ab initio and experimental crystal structures, respectively. The unit cell volumes were reproduced in a range from −7.0% to 2.1%. The total root-mean-square deviations between the experimental and FF in crystal internal coordinates were 0.017 Å in the bond lengths, 2.2° in the valence angles, and 3.6° in the torsion angles. The force field is capable of reproducing the changes in the chelate rings' torsion angles caused by the crystal packing forces and successfully explains the nonplanarity of Cu(II) amino acid complexes in their crystal structures.