IN VACUO AND IN CRYSTAL MOLECULAR-MECHANICAL MODELING OF COPPER(II) COMPLEXES WITH AMINO-ACIDS

Two molecular mechanics models, model 7 (described earlier) and model 8 (described here), have been used to calculate the geometries of nine copper(II) complexes with cr-amino acids and their N-alkylated derivatives and the diastereoselectivity in the copper(II)/N,N-dimethylvaline system. Interactions inside the copper(II) coordination sphere are modeled by a repulsive coulombic potential among nonbonded atoms of the copper(II) coordination polyhedron and a harmonic bond-stretching potential between metal and ligand atoms. The models differ in the nonbonded part of the conformational potential: a Buckingham function (model 7) is replaced by a Lennard-Jones 12-6 function and electrostatic potential in model 8, in order to perform crystal calculations by the Consistent Force Field (CFF) program, The potential energy parameter set of model 8 is optimized on X-ray crystal structures and selected vibrational frequencies. In vacuo (models 7 and 8) and in crystal (model 8) calculations of the geometries of the copper(II) complexes are compared with their X-ray crystal structures. An optimized parameter set of model 8, the M8-kr47 force field, reproduces the unit cell volumes in a range from -8% to +4%. Significantly better reproduction of the copper(II) coordination polyhedron geometries is obtained with model 8 by an in crystal than an in vacuo approximation, especially for molecules with distorted copper(II) coordination. Model 8 is better in reproducing the bond lengths (rms(Delta b) = 0.031 Angstrom) and the torsional angles (rms(Delta phi) = 8.4 degrees) but yields worse agreement between the experimental and the calculated valence angles than model 7 (rms(Delta = 2.3 degrees). The overall similarity is reproduced equally well by the both models (rms(Delta theta) = 0.163 Angstrom). The M8-kr47 reproduces the diastereoselectivity within experimental error, while model 7 overestimates it.