A dynamic Jahn-Teller effect and destructive phase transition of [hexakis(3-bromo-2-pyridone)copper(II)] perchlorate polymorphs

Two polymorphs of hexakis(κ(O)-3-bromo-2-pyridone)copper(II) bis(perchlorate) (1 = (Formula presented.) 2 = (Formula presented.)) have been prepared and characterized by variable temperature single-crystal and powder X-ray diffraction studies. Compound 1 undergoes a size dependent destructive phase transition near 200 K where the Cu(II) ion shifts from a general position to an inversion center. The halogen bond angle O1•••Br13—C13 increases from 156.88 at 200 K to 171.45 at 100 K during the transition. Compounds 1 and 2 have a ground state orthorhombic geometry at 100 K and the Cu(II)—On2 (n = 1-6) bond lengths begin to converge with increasing temperature indicative of a dynamic Jahn-Teller effect. Both forms exhibit a temperature-dependent color change, in agreement with the temperature dependence of the Jahn-Teller distortion. © 2025 Informa UK Limited, trading as Taylor & Francis Group.