engleski

Simulation of copper(II) coordination polyhedron distortion: An improvement of the molecular mechanics model based on coulombic interactions

In order to reproduce the shapes of distorted coordination polyhedra of tetra-coordinated copper(II) complexes by means of a molecular mechanics method, an electrostatic model was proposed (J. Sabolović and N. Raos, Polyhedron 1990, 9, 2419). With the model a square-planar complex was defined as an octahedron consisting of four charges situated in the coordination plane, and two charges (denoted as X and X′) above and below the plane. The model proved good in reproducing the geometry of both planar and non-planar copper(II)-aminoacidates. The analysis of the potential energy surface for the coordination polyhedron distortion showed, however, that besides two “real” (trans-and cis-planar) minima, a large series of “false” minima (distorted pyramids, trigonal bipyramids) appeared with a lower conformational energy than the “real” ones. The reason for their occurrence was the lack of a repulsive potential between apically placed charges, so the system was able to degenerate easily to the “penta-coordinated” complex (X-M-X′ = 0°). The repulsive coulombic interaction between the apically placed charges is now introduced. The empirical parameters used are the same except the bond-stretching parameters kb, MX is chosen in such a way as to give the same M-X value as before. The new model reproduces the copper(II)-aminoacidato geometries in the same manner as the old model or slightly better. From the shape of the potential energy surface in the space of trans-valence angles for the “complex” CuX2N2O2 it can be concluded that: the degeneracy to the “penta-coordinated” complex is suppressed but three “false” minima that correspond to three distorted pyramids (X-M-X = 105°) are left ; while in the previous model the energy difference between the trans-planar minimum and the pyramid with the lowest conformational energy was 70.61 kJ mol−1, in the new model the corresponding difference is 0.78 kJ mol−1. Tetrahedral configuration would be possible to achieve only by choosing a very small value of kb, MX. The appearance of the distorted pyramid minima opens a possibility of the application of the new model to penta-coordinated copper(II) complexes.

copper ; coordination geometry plasticity ; amino acids ; molecular mechanics model ; force field

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