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 Å3. 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.