Molecular modelling of Jahn-Teller distortions in Cu(II)N6 complexes: elongations, compressions and the pathways in between.

Ligand Field Molecular Mechanics (LFMM) parameters have been optimised for six-coordinate Cu(II) complexes containing amine, pyridine, imidazole and pyrazine donors. As found in previous LFMM applications, the new parameters automatically generate distorted structures with the magnitudes of the Jahn-Teller elongations in good agreement with experiment. Here, we explore the rest of the potential energy surface. The introduction of axial strain drives the LFMM structures via rhombic geometries to the compressed structure, the latter corresponding to the saddle point between successive elongation axes. Calculated barrier heights between compressed and elongated geometries also agree well with available experimental data. In every case bar one, the LFMM predicts that the crystallographically observed elongation axis corresponds to the overall lowest energy well. The structural predictions are confirmed by independent density functional theory (DFT) optimisations. LFMM calculations on bis(2,5-pyrazolylpyridine)copper complexes display a smooth variation in structure as a function of pyrazolyl substituent from elongated for R = H through to fully compressed for R = (t)Bu. This behaviour is driven by the steric interactions with the ground state varying smoothly as a linear combination of {d(x2-y2)}1 and {d(z2)}1.