Temperature-induced single-crystal-to-single-crystal polymorph transformation in cadmium(II) trimer with pyridine-4-propanamide

Abstract

Single-crystal-to-single-crystal (SCSC) transformations have been established as an interesting solid state phenomenon and have gained some interest, as the transformed structure can be also determined, providing insight into the possible mechanism of transformation. SCSC transformations may be induced by external stimuli like heat, light or sorption/desorption of guest/solvent molecules and usually occur in metal-organic frameworks (MOFs). The studies of SCSC polymorph transformations of metal-organic compounds are scarce, but they start to gain attention. The molecular structure of cadmium(II) coordination trimer with pyridine-4-propanamide (4-Propy), [Cd3 Cl4(H2O)4 (4-Propy)6 ](CH3 CH2 COO)2 (1), consists of three cadmium(II) ions bridged with chloride ions ; each is coordinated by two N-monodentate 4-propy ligands. Two water molecules are bound to the terminal cadmium(II) ions of the trimer, preventing further polymerization of the trimer units. High-temperature polymorph (A) of 1 crystallized in triclinic system (P-1 space group) at room temperature. SCSC transformation to low- temperature polymorph (B) of 1 occurred by cooling a single crystal to 150 K on a diffractometer. Polymorph B crystallized in monoclinic system (P2 1 /n space group). In A, trimer units are linked via propanoate ions by N–H···O interactions, forming a 1D chain along the [0 1 1] direction. However, in B, similar 1D chains are assembled into a 2D network in the plane (0 1 0) due to the additional N–H···Cl interactions, present only in B. Therefore, lowering temperature enables the assembly of the 1D chains of A into 2D network of B by introducing different type of interaction (N–H···Cl). It seems that the transformation of A into B is possible due to slight change of the molecules’ orientation in the structure of A, enabling the formation of N–H···Cl interactions in the same crystal, without a collapse of the crystal structure. This SCSC polymorph transformation was also studied by PXRD and DSC. The Hirshfeld surface analysis was performed to identify the interactions present in A and B, revealing differences between the two polymorphs.