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

Abstract

Enantiopure, trifunctional carboxylate ligands synthesized by linking the strong π · · · 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 (L(gly)(-)), alanine (L(ala)(-)), and serine (L(ser)(-))]. These ligands have been used to form complexes of the formula [M(L(amino acid))2(4,4'-bipy)(H2O)2] · xH2O (M = Fe, Co, Ni, Cu, Zn; x = 4.25-5.52) when mixed with an appropriate metal salt and 4,4'-bipyridine by layering methods. These complexes are isostructural, with the central metal atom coordinated to two κ(1)-carboxylate ligands, two water molecules, and one end each of two 4,4'-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 π · · · 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 Å and has a diameter of ca. 40 Å. For the achiral ligand L(gly)(-), 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 water molecules held inside the framework of the complexes in isolated pockets by hydrogen-bonding interactions. For both [Cu(L(ala))2(4,4'-bipy)(H2O)2] · 4.25H2O and [Co(L(ser))2(4,4'-bipy)(H2O)2] · 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 water molecules in these closely packed solids is facilitated by the "soft" π · · · π stacking interactions organizing one dimension of the structures. The observed magnetic and Mössbauer spectral properties are typical of isolated, magnetically dilute, paramagnetic pseudooctahedral divalent transition-metal complexes.