Self-Assembly of Water-Mediated Supramolecular Cationic Archimedean Solids

Understanding the self-assembly of small structural units into large supramolecular assemblies remains one of the great challenges in structural chemistry. We have discovered that tetrahedral supramolecular cages, exhibiting the shapes of Archimedean solids, can be self-assembled by hydrogen bonding interactions using tricationic N-donors (1 or 2) in cooperation with water (W). Single crystal X-ray analysis shows that cage (2)₄(W)₆, assembled in an aqueous solution of cation 2 and KPF₆, consists of four tripodal trications linked by six water monomers and resembles the shape of a truncated tetrahedron. Similarly, cage (1)₄(W₆)₄ spontaneously self-assembles in an aqueous solution of cation 1 and NH₄PF₆ and consists of four tripodal cations and four water hexamers. Here, each of the four (H₂O)₆ units act as tritopic nodes between three distinct tripodal cations forming a polyhedron similar to the cantellated tetrahedron. These two well-defined cages are assembled via total of 12 and 36 hydrogen bonds, respectively. Both cages possess interior solvent-accessible volumes exceeding 1000 ų. Furthermore, each one of the (H₂O)₆ clusters in face-centered cubic structure 1b acts as a node between two distinct (1)₄(W₆)₄ units, and thus a solvent-filled tubular three-dimensional network (tube diameter of ∼6.5 Å) is generated that mimics the structure of diamond at the nanometer scale. To our knowledge, this is the first example of such species being formed entirely via hydrogen bonding interactions.