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⁻.