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][SO₄L]·10H₂O (2) and [Fe(bipy)₃][SO₄L]·9H₂O (3), show very similar sheets formed by the anionic units [SO₄L]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][ SO₄L₂] (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 [SO₄L]²⁻ anion allows for a closer approach of the [Fe(DABP)₃]²⁺ or [Fe(bipy)₃]²⁺ cations than the [SO₄L₂]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⁻.

In summary, we obtained two supramolecular architectures, [Fe(DABP)₃][SO₄L]·10H₂O (2) and [Fe(bipy)₃][SO₄L]·9H₂O (3), via the self-assembly of [Fe(DABP)₃]SO₄ or [Fe(bipy)₃]SO₄ with a tris(3-pyridylurea) ligand (L). The two compounds show a similar layered structure, in which each ligand molecule includes a sulfate anion in its cleft via multiple hydrogen bonds. Alternative water parallelograms and quasi “water cubes” link the 2D layers formed by [SO₄L]²⁻ anionic units and [Fe(DABP)₃]²⁺ cations into a 3D network in compound 2, while in 3 single water molecules, water dimers and hexamers are found between the layers. FT-IR, PXRD, and elemental analysis reveal that SO₄²⁻ can be selectively crystallized from an aqueous solution containing other oxoanions, such as NO₃⁻, OAc⁻, and ClO₄⁻, as competing anions. The encapsulation of sulfate by a single ligand molecule rather than two receptors is rationalized by the better solid-state packing mode of the complex cations [Fe(DABP)₃]²⁺ and [Fe(bipy)₃]²⁺ without sufficient secondary hydrogen-bond donors and half-encapsulated anions [SO₄L]²⁻.

Inorg. Chem., 2009, 48 (21), pp 10249–10256

Corresponding author: Wu, BA, Chinese Acad Sci, Lanzhou Inst Chem Phys, State Key Lab Oxo Synth & Select Oxidat, Lanzhou 730000, Peoples R China

E-mail addresses: wubiao@lzb.ac.cn, janiak@uni-freiburg.de, yangxj@lzb.ac.cn

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