Framework Complexes of Group 2 Metals Organized by Homochiral Rods and π···π Stacking Forces: A Breathing Supramolecular MOF

The
reactions of the potassium salts of the ligands (S)-2-(1,8-naphthalimido)­propanoate (KLala), (S)-2-(1,8-naphthalimido)-3-hydroxypropanoate
(KLser), and (R)-2-(1,8-naphthalimido)­propanoate (KLala*), enantiopure carboxylate ligands containing a 1,8-naphthalimide
π···π stacking supramolecular tecton, and,
in the case of Lser–, an alcohol functional group with calcium or
strontium nitrate under solvothermal conditions produce crystalline
[Ca­(Lala)2(H2O)]·(H2O) (1); [Ca­(Lser)2]·(H2O)2 (2); [Sr­(Lala)2(H2O)]·(H2O)3 (3); [Sr­(Lala*)2(H2O)]·(H2O)3 (3*); and [Sr­(Lser)2(H2O)] (5). Placing 3 under
vacuum removes the interstitial waters to produce [Sr­(Lala)2(H2O)] (4) in a single-crystal to single-crystal transformation; introduction
of water vapor to 4 leads to the reformation of crystalline 3. Each of these new complexes has a solid-state structure
based on homochiral rod secondary building unit (SBUs) central cores.
Supramolecular π···π stacking interactions
between 1,8-naphthalimide rings link adjacent rod SBUs into three-dimensional
structures for 1, 3, 4, and 5 and two-dimensional structure for 2. Compounds 1 and 3 have open one-dimensional channels along
the crystallographic c axis that are occupied by
disordered solvent. For 3, these channels close and open
in the reversible single-crystal conversion to 4; the
π···π stacking interactions of the naphthalimide
rings facilitate this process by rotating and slipping. Infrared spectroscopy
demonstrated that the rehydration of 4 with D2O leads to 3d8, and the process
of dehydration and rehydration of 3d8 with H2O leads to 3, thus showing
exchange of the coordinated water in this process. These forms of 3 and 4 were characterized by 1H, 2H, and 13C solid-state NMR spectroscopy, and thermal
and luminescence data are reported on all of the complexes.