Full- or Half-Encapsulation of Sulfate Anion by a Tris(3-pyridylurea) Receptor: Effect of the Secondary Coordination Sphere

Self-assembly of the [Fe(DABP)3]SO4 (DABP = 5,5′-diamino-2,2′-bipyridine) or [Fe(bipy)3]SO4 (bipy = 2,2′-bipyridine) complex with a tripodal tris(3-pyridylurea) ligand (L) results in a layered structure that includes a sulfate anion in the cleft of one L molecule. The two compounds, [Fe(DABP)3][SO4⊂L]·10H2O (2) and [Fe(bipy)3][SO4⊂L]·9H2O (3), show very similar sheets formed by the anionic units [SO4⊂L]2− and cationic building blocks ([Fe(DABP)3]2+ or [Fe(bipy)3]2+). However, there are different water clusters that link the adjacent layers in the two products, that is, water parallelograms and quasi “water cubes” in 2 versus single water molecules, water dimers, and hexamers in 3. The half-encapsulation of sulfate by a single L molecule contrasts with the previously reported full-encapsulation of the sulfate ion by two L molecules in [M(H2O)6][SO4⊂L2] (1). This different anion encapsulation is traced to the hydrogen-acceptor properties of the pyridyl groups of L together with the hydrogen-bonding properties of the cation secondary coordination sphere for a solid-state packing optimization. In 1 the direct hydrogen bonding from the secondary coordination sphere of octahedral [M(H2O)6]2+ to L-pyridyl helps in the formation of an octahedral cation−anion coordination in the NaCl-type structure. In 2 and 3, crystal water instead of the cations has to satisfy the hydrogen-accepting demands of L. Consequently, a non-spherical and only partly water-surrounded half-encapsulated [SO4⊂L]2− anion allows for a closer approach of the [Fe(DABP)3]2+ or [Fe(bipy)3]2+ cations than the [SO4⊂L2]2− anion. Then, the similar cation and anion size in 2 and 3 with the Coulomb attraction confined to a two-dimensional plane leads to the formation of a hexagonal BN (or graphite) lattice. Competition experiments with different anions for compound 2 reveal that SO42− can be selectively crystallized against NO3, OAc, or ClO4.