Reversible Temperature-Stimulated Single-Crystal-to-Single-Crystal Conformational Polymorph Transformation in Cadmium(II) Coordination Trimer with a Water Vapor Sorption/Desorption Potential

A crystalline cadmium­(II) coordination trimer with pyridine-4-propanamide (4-propy), [Cd3Cl4(H2O)4­(4-propy)6]­(CH3CH2COO)2, readily responds to external stimuli while preserving crystal integrity. Two polymorphic forms, Polymorph-I (triclinic form) and Polymorph-II (monoclinic form), undergo a reversible single-crystal-to-single-crystal conformational polymorph transformation upon temperature change, while the triclinic form (Polymorph-I) additionally shows substantial reversible sorption/desorption potential via two pathways, in response to stimulation by (i) elevated temperature, and (ii) reduced pressure. The elucidation of the conformational transformation via a variety of ex situ and in situ methods (SCXRD, VT-PXRD, TG/DSC and VT-IR), complemented by in-depth theoretical studies, suggested that the conformational transformation might be a process that occurs slowly and continuously over a wider temperature range, with different processes (distinctive conformational changes) taking place over different temperature intervals.