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 degrees 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.

Automated summary

Unexpected inversion-symmetry breaking was observed during the order-disorder phase transitions of [M(acac)(2)(abco)(2)]. The crystal structures, electron density maps, and thermodynamic properties of the complexes were thoroughly studied.