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]SO ₄ (bipy = 2,2′-bipyridine) complex with a tripodal tris(3-pyridyiurea) 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][SO₄CL] · 10H₂O (2) and [Fe(bipy) ₃][SO₄CL] · 9H₂O (3), show very similar sheets formed by the anionic units [SO₄CL]^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][SO₄CL₂] (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 NaCI-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 [SO₄CL]^2- anion allows for a closer approach of the [Fe(DABP)₃]²⁺ or [Fe(bipy)₃]²⁺ cations than the [SO₄CL ₂]^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 SO₄^2- can be selectively crystallized against NO₃^-, OAc ^-, or ClO₄^-.