Dynamic pseudo Jahn–Teller distortion in a compressed octahedral CuO6 complex

The crystal structure of cis-[Cu(C8H7O3)2(H2O)2] (115 K data) reveals bidentate vanillinate ions coordinated via methoxy and deprotonated hydroxy oxygen atoms and water molecules in a distorted octahedral CuO6 chromophore. A cis orientation of the ligands enables two non-identical O(methoxy)CuO(water) coordination axes (2.354(1) + 2.163(1); 2.151(1) + 2.020(1) Å), and the third shortest O(hydroxy)CuO(hydroxy) axis (1.919(1) + 1.914(1) Å). This 115 K coordination sphere differs importantly to the one obtained from the 293 K data of the same compound, where two long O(methoxy)CuO(water) axes are of the same length, and only minor changes at the short O(hydroxy)CuO(hydroxy) axis are noticed. An axial symmetry of the complex with an inverse g1,2(g⊥)>g3(g∥)g1,2(g⊥)>g3(g∥) pattern is observed in the temperature range from 298 to 180 K. A further decrease of temperature reveals gradual changes from axial to rhombic symmetry (g1 > g2 > g3) that is reversible. A mean-square displacement amplitude (MDSA) analysis reveals a disorder in the CuO(methoxy) bonds, but not in the other metalligand CuO(hydroxy) and CuO(water) bonds at 293 and 115 K. The disorder is significantly weaker in the 115 K structure. The MSDA analysis and the structural-EPR agreement show vibrational disorder in two coordination axes, due to the cis conformation of the complex with two O(methoxy)CuO(water) axes.

The coordination sphere of cis-[Cu(C8H7O3)2(H2O)2] at 115 K, with three rhombic coordination axes, differs importantly from the room temperature one, where compressed axial axes were determined. The MSDA analysis confirms the vibrational disorder in the structure at high temperature, and its gradual freezing by decreasing temperature. These results are in agreement with the EPR data that also show the reversibility of this change. Based on the EPR data, a {dz2}1{dz2}1 ground state is proposed, although the room temperature spectrum suggests a {dx2-y2}1{dx2-y2}1 ground state, but with a small g3 value (close to ge).Figure optionsDownload as PowerPoint slide