Layer or Tube? Uncovering Key Factors Determining the Rolling-up of Layered Coordination Polymers

Abstract

Nanotubular materials have garnered considerable attention since the discovery of carbon nanotubes. Although the layer-to-tube rolling up mechanism has been well recognized in explaining the formation of many inorganic nanotubes, it has not been generally applied to coordination polymers (CPs). To uncover the key factors that determine the rolling-up of layered CPs, we have chosen the Co/<i>R</i>-, <i>S</i>-Xpemp [Xpemp = (4-X-1-phenylethylamino)methylphosphonic acid, X = H, F, Cl, Br] systems and study how the weak interactions influence the formation of layered or tubular structures. Four pairs of homochiral isostructural compounds <i>R</i>-, <i>S</i>-Co(Xpemp)(H₂O)₂ [X = H (<b>1H</b>), F (<b>2F</b>), Cl (<b>3Cl</b>), Br (<b>4Br</b>)] were obtained with tubular structures. The inclusion of 3,3'-azobipyridine (ABP) guest molecules led to compounds <i>R</i>-, <i>S</i>-[Co(Xpemp)(H₂O)₂]₄·ABP·H₂O with layered structures when X was Cl (<b>5Cl</b>) and Br (<b>6Br</b>), but tubular compounds <b>1H</b> and <b>2F</b> when X was H and F. Layered structures were also obtained for racemic compounds <i>meso</i>-Co(Xpemp)(H₂O)₂ [X = F (<b>7F</b>), Cl (<b>8Cl</b>), Br (<b>9Br</b>)] using racemic XpempH₂ as the reaction precursor, but not when X = H. A detailed study on <i>R-</i><b>6Br</b> revealed that layer-to-tube transformation occurred upon removal of ABP under hydrothermal conditions, forming <i>R-</i><b>4Br</b> with a tubular structure. Similar layer-to-tube conversion did not occur in organic solvents. The results demonstrate that weak interlayer interactions are a prerequisite but not sufficient for the rolling-up of the layers. In the present cases, water also provides a driving force in the layer-to-tube transformation. The experimental results were rationalized by theoretical calculations.