Supramolecular Architectures of Metal Complexes Controlled by a Strong π···π Stacking, 1,8-Naphthalimide Functionalized Third Generation Tris(pyrazolyl)methane Ligand

A new hetero-bifunctional ligand designed to form supramolecular structures with a combination of covalent and noncovalent forces has been prepared. The ligand, (1,8-naphthalimide)CH2C6H4CH2OCH2C(pz)3 (Ltris, pz = pyrazolyl ring), contains both a tris(pyrazolyl)methane coordination unit and a 1,8-naphthalimide strong π···π stacking unit. The reactions of iron(II), copper(II), and cadmium(II) tetrafluoroborate salts with 2 equiv of Ltris yield [Fe(Ltris)2](BF4)2 (1), [Cd(Ltris)2](BF4)2 (2), and [Cu(Ltris)2](BF4)2 (3), respectively. Vapor diffusion crystallization yielded two pseudopolymorphs for 1. In the solid-state structures of both forms, the iron is in an octahedral environment with bond distances expected for low-spin iron(II); both tris(pyrazolyl)methane donor sets are in a κ3-coordination mode with each Ltris ligand in a U-shaped, syn arrangement. In one pseudopolymorph, the 1,8-naphthalimide side chains are on the same side of the molecule when viewed down the metal axis (syn-1), twisted only 30° with respect to each other. In the second pseudopolymorph, the side chains are oriented in opposite directions at 180° (anti-1). Both structures are one-dimensional, organized by intermolecular π···π stacking interactions of the 1,8-naphthalimide units. Each naphthalimide unit reaches past the naphthalimide unit on the adjacent cation, leading to a π···π stack using the face of the naphthalimide unit directed toward its metal complex, interlocking the U-shaped groups. The interaction of the two naphthalimide groups is directionally specific; they are oriented head to tail with substantial overlap of the parallel rings. The twisting of the side chains at 30° for syn-1 leads to a helical structure, whereas the 180° orientation of the side chains in anti-1 leads to a linear structure. The cadmium(II) complex is isostructural to syn-1 and the copper(II) is isostructural to anti-1. Although the supramolecular structures of these two compounds are similar to the matched iron(II) complexes because of the similar orientations of the side-chains and the consistent noncovalent interactions of the naphthalimide synthon, the larger size of cadmium(II) leads to a highly distorted structure about the metal. The copper(II) structure is also distorted, as expected for a d9 complex.