C O MMU N I C A T I O N S
the hypothesis that the evacuated network is structurally similar to
the filled network and that the matrix maintains its integrity with
empty channels. Indeed, only a handful of materials has been
observed that have a similarly robust lattice structures in the
presence and absence of guests.10
In conclusion, a large, highly symmetric bis-urea macrocycle 1
was easily synthesized from commercially available materials. The
NMR, TGA, and X-ray diffraction data show conclusively that 1
assembles to form columnar nanotubes, which is driven by the
strong urea-urea self-association and π-stacking of the rigid
spacers. The formation of these noncovalent porous solids creates
a robust cavity that can reversibly bind and exchange guest acetic
acid molecules. Their modular construction from rigid spacers and
protected ureas should enable the large-scale synthesis of similar
tubular materials and enhance their viability in practical applications.
We are currently probing the affinity and binding specificity of
the cavity of 1, and preliminary studies show that the nanotubes
can reversibly bind a wide range of guest molecules just like
ordinary molecular sieves. We will report these data in due course.
Figure 3. Three successive cycles of TGA of assembly 1‚AcOH (23 to
100 °C at 1 deg/min), followed by reloading of acetic acid guest.
Acknowledgment. This work was supported by the NSF (Grant
No. 9973132.).
Supporting Information Available: Detailed descriptions of the
synthesis and characterization of key compounds and crystal data,
atomic coordinates, bond lengths and angles of 1 and 4 (CIF), and
experimental and calculated PXRD of filled and empty 1 (PDF). This
Figure 4. PXRD patterns for assembled 1 with encapsulated AcOH
(bottom). Assembled 1 after AcOH removal by heating to 120 °C (middle).
Assembled 1 after AcOH reinsertion by treatment with AcOH vapor (top).
temperature of 150 °C, and the white crystals retained their shape
and showed no sign of cracking. We then sought to rebind the acetic
acid guest by treatment of the empty self-assembled network with
acetic acid. The crystals were exposed to acetic acid vapor in a
sealed vessel for 72 h and then reexamined by TGA (Figure 3,
cycle 2). A nearly identical weight loss curve was observed (11.02%
weight loss). Repetition of acetic acid guest removal and rebinding
of the same crystals a third time established that the inclusion
behavior is reversible and that the channels retain their inclusion
ability over time (Figure 3, cycle 3).
Powder X-ray diffraction (PXRD) provided further evidence for
the structural stability of the empty nanotube assembly and the
reversibility of the binding process. The guest-filled crystals were
ground to a powder and examined by PXRD (Figure 4). The guest
was then removed by heating to 120 °C for 1.5 h, and the powder
was reexamined by PXRD. The PXRD pattern of empty assembly
shows a slightly different but well-defined structure. Upon treatment
of the evacuated solid with acetic acid vapor, the powder exhibits
a PXRD pattern with peak positions and intensities nearly identical
to the original acetic acid bound structure.
The single crystal data was used to calculate PXRD patterns for
the AcOH bound and evacuated solids. Not surprisingly, the
calculated PXRD for the bound material closely matched the
observed PXRD, supporting the hypothesis that the bulk material
shows a single phase product that has a similar structure to the
single crystal. For the evacuated solid, a PXRD was calculated
simply by omitting the acetic acid guests from the single crystal
X-ray structure. Again this calculated structure is very close to the
experimentally observed PXRD of the empty assembly, supporting
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