Structural and DFT Studies on the Polymorphism of a Cadmium(II) Dipicolinate Coordination Polymer

The coordination polymer [Cd₂(dipic)₂(H₂O)₃]_n was prepared by the reaction of cadmium­(II) chloride or bromide and dipicolinic acid (dipicH₂) at 60 °C under autogenous pressure. The C2/c polymorph (1) was almost exclusively obtained. However, a few crystals of the P2/c polymorph (2) were occasionally found in the mixture with the C2/c polymorph, thus making it a disappearing and concomitant polymorph. The polymeric chains in 1 are connected into dimers by π–π interaction and O–H···O hydrogen bonds. These dimers are in turn connected by intermolecular O–H···O hydrogen bonds into a 2D network. The polymeric chains in 2 are connected by intermolecular O–H···O hydrogen bonds into a zigzag chain along the [001] direction. According to DFT calculations, the hydrogen bonding is of similar order in both polymorphs (∼7.5 kcal mol^–1 per hydrogen bond). However, there is additional stability imparted in 1, as shown by dispersion-corrected DFT, through π–π stacking between polymeric chains, making 1 the thermodynamically favored polymorph. Polymorph 1 was characterized by IR spectroscopy, PXRD analysis, and TGA and DSC methods. The DSC analysis did not show any sign of phase transition between 1 and 2. This was also confirmed by variable temperature PXRD, since the pattern of 1 remained unchanged until the decomposition of 1.