Formation Mechanism and Reversible Expansion and Shrinkage of Magnesium-Based Homochiral Metal-Organic Nanotubes.

ABSTRACT

Two pairs of homochiral magnesium phosphonates, namely (R)-, (S)-[Mg(pemp)(H2 O)2 ] (1) and (R)-, (S)-Mg10 (pemp)10 (H2 O)10 ]⋅3 H2 O (2) are reported (pemp2- =(R)- or (S)-(1-phenylethylamino) methylphosphonate). Compounds 1 show one-dimensional tubular structures. The tube wall is purely inorganic, containing six-membered rings made up of corner-sharing {MgO5 N} octahedra and {PO3 C} tetrahedra. The organic groups reside outside the tube. A bottom-up "direct growing" mechanism is proposed for the formation of the nanotubular structures of 1, based on the electrospray ionization mass spectrometry studies. Compounds 2 display two-dimensional layered structures containing cross-linked squashed nanotubes. Interestingly, the two structures can interconvert reversibly upon temperature and pH modulation. This is the first report of a metal-organic nanotube that can shrink and recover in a reversible manner.