Inversion Symmetry Breaking in Order–Disorder Transitions of Globular Ligands Coordinating to Cobalt(II) and Nickel(II) Bisacetylacetonato Complexes During Heating.

Unexpected inversion‐symmetry breaking was observed in the order–disorder phase transitions of [M(acac)2(abco)2] (1; M=Co2+, 2; Ni2+, acac−=2,4‐pentanedionato, abco=1‐azabicyclo‐[2.2.2]‐octane=quinuclidine) during heating. The isostructural, transition‐free complexes [M(acac)2(cabco)2] (3; M=Co2+, 4; Ni2+, cabco=3‐chloro‐1‐azabicyclo‐[2.2.2]‐octane=3‐chloroquinuclidine) were also studied for comparison. Complexes 1 and 2 crystallized in ordered phases in the centrosymmetric I2/m space group at 100 K, whereas they crystallized in disordered phases in the non‐symmetric I2 space group at 300 K. The 60° step rotation disordering of the abco ligands was observed in the electron density maps of 1 and 2, which was consistent with the transition enthalpies estimated by differential scanning calorimetry (DSC). Gradual phase transitions were observed for 1 and 2 by DSC and powder X‐ray diffraction (PXRD) at approximately 225 K. The inversion‐symmetry disordering was likely induced by the local pseudo‐symmetry of the abco ligands, increasing from trigonal to hexagonal and the increased steric repulsion pathways among them. [ABSTRACT FROM AUTHOR]