Anion-templated self-assembly of tetrahedral cage complexes of cobalt(II) with bridging ligands containing two bidentate pyrazolyl-pyridine binding sites

Abstract

The bridging ligands L 1 and L 2 contain two N,N-bidentate pyrazolyl-pyridine units linked to a central aromatic spacer unit (1,2-phenyl or 2,3-naphthyl, respectively). Reaction with Ni(II) salts and treatment with the anions tetrafluoroborate or perchlorate result in formation of dinuclear complexes having a 2:3 metal:ligand ratio, with one bridging and two terminal tetradentate ligands. In contrast, reaction of L 1 and L 2 with Co(II) salts, followed by treatment with tetrafluoroborate or perchlorate, results in assembly of cage complexes having a 4:6 metal:ligand ratio; these complexes have a metal ion at each corner of an approximate tetrahedron, and a bis-bidentate bridging ligand spanning each edge. The central cavity is occupied by a tetrahedral counterion that forms multiple hydrogen-bonding interactions with the methylene protons of the bridging ligands. The anionic guest fits tightly into the central cavity of the cage to which it is ideally complementary in terms of shape, size, and charge. Solution NMR experiments show that the central anion acts as a template for cage formation, with a mixture of Co(II) and the appropriate bridging ligand alone giving no assembly into a cage until the tetrahedral anion is added, at which point cage assembly is fast and quantitative. The difference between the structures of the complexes with Ni(II) and Co(II) illustrate how the uncoordinated anions can exert a profound influence on the course of the assembly process. Metal-directed self-assembly has recently become a major tool by which coordination chemists can prepare large and elaborate complexes such as helicates, grids, boxes, rings, and cages from relatively simple components (1-11). Many examples are based on accurate control of metal-ligand coordinate bond formation, with the course of the assembly involving a labile metal ion and a multidentate ligand dictated by the metal͞ligand interactions. This behavior is exemplified by the formation of helical complexes with linear oligopyridines, where the partitioning of the ligand into bidentate or terdentate binding domains is dictated by the preference of the metal ion for four-coordinate or six-coordinate geometry (3). Recently, however, it has become apparent that ''innocent'' anions can dictate the course of the assembly process by acting as a template around which a particular combination of metal ions and ligand can assemble in a way which would not occur in the absence of the anion. For example, Lehn and coworkers (12) showed how a trinuclear M 3 L 3 triple helicate converted to a circular M 5 L 5 helicate in the presence of chloride ion, which was tightly bound in the center of the resulting cationic cavity. Anions that are chosen for their innocence in terms of coordinating ability can nevertheless direct the course of an assembly process via noncovalent interactions.Here we describe how the anions perchlorate and tetrafluoroborate act as templates for the formation of edge-bridged tetrahedral M 4 (-L) 6 cages from Co(II) and bis-bidentate bridging ligand...

Self‐assembly of anion‐binding supramolecular cage complexes

“…(Fig. 3) [16,18] of a type studied by Saalfrank [21] and Raymond [22], amongst others. These have a metal ion at each vertex of an approximate tetrahedron with Co•••Co separations of ca.…”

“…reaction of the known compound 3-(2-pyridyl)-1H-pyrazole with the bis(bromomethyl)-aromatic compound in a 2:1 ratio under phase transfer conditions. Thus, L 1 was prepared using 1,2-bis(bromomethyl)benzene [16,17], L 2 using 2,3-bis(bromomethyl)-naphthalene [18], and so on. These ligands are all tetradentate, and accordingly, reaction with metal cations that prefer an octahedral coordination geometry requires 1.5 ligands per metal ion to provide the necessary six donor atoms (assuming that noncoordinating anions are used).…”

“…6Å) indicating that the anion is centrally located. It appears that the anionic guest is perfectly complementary to the host cage in terms of shape, size, and charge [16,18].…”

Two new 2D coordination polymers containing bowl-shaped voids assembled from the bis(chelating) bridging ligand

“…[1]. Strategies for creating voids in the 0D architectures and 3D frameworks via coordination-driven self-assembling process have been well documented [2][3][4]. Compared with the 0D units and 3D coordination polymers widely reported, the 2D coordination polymers with voids have been scarcely studied in recent years.…”

Anion Templates in Synthesis and Dynamic Combinatorial Libraries

“…Since palladium-pyridine bonds are relatively labile, it was expected that a dynamic equilibrium of different assemblies would be established. 4 (33) [Pd(dppp)(L …”