Homochiral, Helical Supramolecular Metal-Organic Frameworks Organized by Strong π···π Stacking Interactions: Single-Crystal to Single-Crystal Transformations in Closely Packed Solids

Enantiopure, trifunctional carboxylate ligands synthesized by linking the strong π3 3 3 π stacking 1,8-naphthalimide
supramolecular synthon to three naturally occurring amino acids using the azide/alkyne click reaction have been prepared
[amino acid = glycine (Lgly
-), alanine (Lala
-), and serine (Lser
-)]. These ligands have been used to form complexes of the
formula [M(Lamino acid)2(4,40-bipy)(H2O)2] 3 xH2O (M = Fe, Co,Ni, Cu, Zn; x = 4.25-5.52) when mixed with an appropriate
metal salt and 4,40-bipyridine by layeringmethods. These complexes are isostructural, with the centralmetal atomcoordinated
to two κ1-carboxylate ligands, two water molecules, and one end each of two 4,40-bipyridine ligands in a distorted octahedral
environment. Each ligand is oriented in a trans arrangement. These complexes all have homochiral, helical, supramolecular,
three-dimensional metal-organic framework structures, with the helical organization of the individual metallic units held
together solely by strong, noncovalent π3 3 3 π stacking interactions of the naphthalimide; the other two dimensions are
organized mainly by the bipyridine ligands. The helices are extremely large; one turn of the helix travels ∼60 and has a
diameter of ca. 40 . For the achiral ligand Lgly
-, the nickel complex forms two types of homochiral crystals in the same tube, a
clear example of spontaneous resolution. Despite the large size of the individual helices, they are tightly interconnected and
nestled closely together. Part of the interconnection comes from the interstitial watermolecules held inside the framework of the
complexes in isolated pockets by hydrogen-bonding interactions. For both [Cu(Lala)2(4,40-bipy)(H2O)2] 3 4.25H2O and
[Co(Lser)2(4,40-bipy)(H2O)2] 3 4.68H2O, the interstitial water molecules can be removed by placing the crystals under a
vacuum for several hours, a process that can be reversed by exposure to atmospheric moisture. This removal/reintroduction of
the interstitial water molecules takes place with no loss of crystallinity, representing dramatic examples of single-crystal to
single-crystal transformations. The structures undergo little change other than the pockets holding the interstitial water
molecules in the hydrated complexes become void spaces in the dehydrated complexes. The removal/reintroduction of the
watermolecules in these closely packed solids is facilitated by the “soft”π3 3 3 πstacking interactions organizing one dimension
of the structures. The observed magnetic and M€o
ssbauer spectral properties are typical of isolated, magnetically dilute,
paramagnetic pseudooctahedral divalent transition-metal complexes.