Inorganic Chemistry, Vol.47, No.20, 9518-9527, 2008
Modeling of the Various Minima on the Potential Energy Surface of Bispidine Copper(II) Complexes: A Further Test for Ligand Field Molecular Mechanics
Copper(II) complexes of bispidines (bispidine = tetra-, penta-, or hexadentate ligand, based on the 3,7-diazabicyclo[3.3.1]nonane backbone) display several isomeric forms. Depending on the substitution pattern of the bispidine and the type of coligands used, the structure elongates along one of the three potential Jahn-Teller axes. In an effort to develop a computational tool which can predict which isomer is observed, 23 bispidine-copper(II) complexes with 19 different ligands are analyzed theoretically by ligand field molecular mechanics (LFMM). With two exceptions, the lowest-energy LFMM structure and the experimental solid-state structure agree concerning the Jahn-Teller axis. However, in most cases and especially for six-coordinate complexes, LFMM predicts a second local minimum within a few kilojoules per mole. Although detailed analysis reveals that the current force field is too "stiff", reasonable quantitative reproduction of the structural data is achieved with Cu-L bond length root mean square (rms) deviations for nine complexes of 0.05 angstrom or less and with 20 reproduced to a rms deviation of 0.1 angstrom or less. Across all of the complexes, the Cu-amine and Cu-pyridyl bond length rms deviations are 0.07 and 0.12 angstrom, respectively.