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

Abstract

The coordination polymer [Cd₂(dipic)₂(H₂O)₃]_<i>n</i> was prepared by the reaction of cadmium­(II) chloride or bromide and dipicolinic acid (dipicH₂) at 60 °C under autogenous pressure. The <i>C</i>2/<i>c</i> polymorph (<b>1</b>) was almost exclusively obtained. However, a few crystals of the <i>P</i>2/<i>c</i> polymorph (<b>2</b>) were occasionally found in the mixture with the <i>C</i>2/<i>c</i> polymorph, thus making it a disappearing and concomitant polymorph. The polymeric chains in <b>1</b> 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 <b>2</b> 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 <b>1</b>, as shown by dispersion-corrected DFT, through π–π stacking between polymeric chains, making <b>1</b> the thermodynamically favored polymorph. Polymorph <b>1</b> was characterized by IR spectroscopy, PXRD analysis, and TGA and DSC methods. The DSC analysis did not show any sign of phase transition between <b>1</b> and <b>2</b>. This was also confirmed by variable temperature PXRD, since the pattern of <b>1</b> remained unchanged until the decomposition of <b>1</b>