Structure directing role of amines and water molecules in the self-assembly of polyoxomolybdates

Abstract

Six supramolecular complexes of charged aliphatic/aromatic amines and polyoxomolybdate anions (β-Mo₇O₂₄⁶⁻ and β-Mo₈O₂₆⁴⁻) are reported in an effort to study the ‘Shrink wrapping’ effect of these amines on the formation of small sized unsymmetrical polyoxomoybdates (POMos). The single crystal X-ray diffraction structures of complexes I–II (with tetramethylammonium (TMA⁺)), III (with tetraethylammonium (TEA⁺)), IV (with tris(2-aminoethyl)ammonium (TREN³⁺)), V (with 3-aminopyridinium (AP⁺)), and VI (with 2-aminomethylpyridinium (AMP⁺)) as cations, show that water, amine based cations and small size (POMo) anions are assembled to form various supramolecular architectures running from infinite 1D chains to 3D networks. Unique and interesting water clusters having W₈ to W₂₁ members are formed, having 3, 4, 6 and up to 11 neighboring H-bonded water molecules in them. Steric shielding and/or “Shrink-wrapping” effect of organic cations is neither according to the number of amine⋯POMo interactions nor as their ability to wrap around the POMo. Therefore the effect has been analyzed by considering an accumulative outcome of the water clusters and the amines. Various water clusters help to envelop the POMos very effectively to prevent anion⋯anion interactions, helping them ‘shrinked wrapped’ to smaller β-Mo₇O₂₄⁶⁻ species in complexes I–II and IV–VI. Complex III however, has only one water molecule, encapsulation by water is minimal and it results in the highest number of anion⋯anion interactions and yields bigger β-Mo₈O₂₆⁴⁻ species. The crystallographic results have been amply supplemented by Hirshfeld surface analysis and other solid state studies.