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 <i>one</i> L molecule. The two compounds, [Fe(DABP)₃][SO₄⊂L]·10H₂O (<b>2</b>) and [Fe(bipy)₃][SO₄⊂L]·9H₂O (<b>3</b>), 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 <b>2</b> versus single water molecules, water dimers, and hexamers in <b>3</b>. 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] (<b>1</b>). 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 <b>1</b> 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 <b>2</b> and <b>3</b>, 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 <b>2</b> and <b>3</b> 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 <b>2</b> reveal that SO₄²⁻ can be selectively crystallized against NO₃⁻, OAc⁻, or ClO₄⁻.