Framework Complexes of Group 2 Metals Organized by
Homochiral Rods and π···π Stacking Forces:
A Breathing Supramolecular MOF
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Abstract
The
reactions of the potassium salts of the ligands (<i>S</i>)-2-(1,8-naphthalimido)propanoate (K<b>L</b><sub><b>ala</b></sub>), (<i>S</i>)-2-(1,8-naphthalimido)-3-hydroxypropanoate
(K<b>L</b><sub><b>ser</b></sub>), and (<i>R</i>)-2-(1,8-naphthalimido)propanoate (K<b>L</b><sub><b>ala</b></sub>*), enantiopure carboxylate ligands containing a 1,8-naphthalimide
π···π stacking supramolecular tecton, and,
in the case of <b>L</b><sub><b>ser</b></sub><sup><b>–</b></sup>, an alcohol functional group with calcium or
strontium nitrate under solvothermal conditions produce crystalline
[Ca(<b>L</b><sub><b>ala</b></sub>)<sub>2</sub>(H<sub>2</sub>O)]·(H<sub>2</sub>O) (<b>1</b>); [Ca(<b>L</b><sub><b>ser</b></sub>)<sub>2</sub>]·(H<sub>2</sub>O)<sub>2</sub> (<b>2</b>); [Sr(<b>L</b><sub><b>ala</b></sub>)<sub>2</sub>(H<sub>2</sub>O)]·(H<sub>2</sub>O)<sub>3</sub> (<b>3</b>); [Sr(<b>L</b><sub><b>ala</b></sub>*)<sub>2</sub>(H<sub>2</sub>O)]·(H<sub>2</sub>O)<sub>3</sub> (<b>3</b>*); and [Sr(<b>L</b><sub><b>ser</b></sub>)<sub>2</sub>(H<sub>2</sub>O)] (<b>5</b>). Placing <b>3</b> under
vacuum removes the interstitial waters to produce [Sr(<b>L</b><sub><b>ala</b></sub>)<sub>2</sub>(H<sub>2</sub>O)] (<b>4</b>) in a single-crystal to single-crystal transformation; introduction
of water vapor to <b>4</b> leads to the reformation of crystalline <b>3</b>. 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 <b>1</b>, <b>3</b>, <b>4</b>, and <b>5</b> and two-dimensional structure for <b>2</b>. Compounds <b>1</b> and <b>3</b> have open one-dimensional channels along
the crystallographic <i>c</i> axis that are occupied by
disordered solvent. For <b>3</b>, these channels close and open
in the reversible single-crystal conversion to <b>4</b>; the
π···π stacking interactions of the naphthalimide
rings facilitate this process by rotating and slipping. Infrared spectroscopy
demonstrated that the rehydration of <b>4</b> with D<sub>2</sub>O leads to <b>3d</b><sub><b>8</b></sub>, and the process
of dehydration and rehydration of <b>3d</b><sub><b>8</b></sub> with H<sub>2</sub>O leads to <b>3</b>, thus showing
exchange of the coordinated water in this process. These forms of <b>3</b> and <b>4</b> were characterized by <sup>1</sup>H, <sup>2</sup>H, and <sup>13</sup>C solid-state NMR spectroscopy, and thermal
and luminescence data are reported on all of the complexes
