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 (KL ala ), (S)-2-(1,8-naphthalimido)-3-hydroxypropanoate (KL ser ), and (R)-2-(1,8-naphthalimido)propanoate (KL ala *), enantiopure carboxylate ligands containing a 1,8-naphthalimide π···π stacking supramolecular tecton, and, in the case of L ser – , an alcohol functional group with calcium or strontium nitrate under solvothermal conditions produce crystalline [Ca(L ala )2(H2O)]·(H2O) (1); [Ca(L ser )2]·(H2O)2 (2); [Sr(L ala )2(H2O)]·(H2O)3 (3); [Sr(L ala *)2(H2O)]·(H2O)3 (3*); and [Sr(L ser )2(H2O)] (5). Placing 3 under vacuum removes the interstitial waters to produce [Sr(L ala )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 3d 8 , and the process of dehydration and rehydration of 3d 8 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.