Synthesis and structure of a zinc(II)-carboxylate trimer containing the π···π stacking, 1,8-naphthalimide synthon: A supramolecular metal–organic framework

The reaction of the bifunctional ligand 3-(1,8-naphthalimido)propanoate (LC2−), which combines a 1,8-naphthalimide strong π···π stacking synthon and a carboxylate donor group, with Zn(O2CCH3)2(H2O)2 in methanol yields trimetallic Zn3(LC2)6(MeOH)4. The solid state structure has a central zinc(II) linked to two equivalent outer zinc(II) by both μ-κ1 and μ-κ2 carboxylate ligands. The two equivalent five-coordinate terminal zinc centers are also bonded to a third nonbridging κ2-carboxylate and to the oxygen atom of a methanol molecule. The central zinc(II) is six-coordinate with the four bridging carboxylate oxygen atoms forming a square planar arrangement and two trans oriented methanol molecules completing the coordination sphere. These trimers are organized into an extended structure exclusively by noncovalent interactions. Two types of strong π···π stacking interactions between sets of three stacked naphthalimide rings from three different trinuclear molecules organize the structure into two-dimensional thick sheets. The third dimension is organized by intermolecular hydrogen bonding interactions between the methanol molecules bonded to the terminal zinc(II) and the free oxygen of the μ-κ1-carboxylates from adjacent trimeric units. This interaction is supported by weak π···π stacking. Overall the structure is a highly organized supramolecular metal–organic framework (SMOF) solid.