Structural Variation In β-Octamolybdate Polyoxoanion Based Crystals Across Lanthanide Series Isolated from a Water/DMSO Solution

The crystallization of lanthanide-containing β-octamolybdate (β-{Mo₈O₂₆}⁴⁻) based solids from a binary 1 : 1 (water/DMSO) solution under ambient conditions is reported. A uniform synthetic protocol yielded three structurally related series of general composition {Ln(solvent)_n}[NaMo₈O₂₆] ⋅ yH₂O, with the whole lanthanide series (except for radioactive Pm). The three series are (i) {Ln(DMSO)₈}[NaMo₈O₂₆] ⋅ 0.5H₂O, Ln=La, Ce, Pr, Nd and Sm (Series C1); (ii) {Ln(DMSO)₆(H₂O)₂}[NaMo₈O₂₆] for Ln=Eu, Gd, Tb, Dy and Ho (Series C2) and (iii) [{Ln(DMSO)₇}][NaMo₈O₂₆] ⋅ H₂O for Ln=Dy, Ho, Er, Tm, Yb and Lu (Series C3). All solids reported here contain [−β-Mo₈O₂₆−Na−β-Mo₈O₂₆−] anionic chains, which is the dominant feature in the crystal packing of these solids. The chains interacted via hydrogen bonds with the solvated lanthanide counter cations. Lanthanide contraction brings about the difference in the primary coordination sphere of the Ln³⁺ cations in the three series reported here. This manifests in the crystal structure as a change in the relative orientation of chains (or clusters) and the position of Ln³⁺ centers, with water acting as the structure director. The study demonstrates Nature's parsimonious ability to optimize intermolecular interactions via diminutive deviations from recognizable packing patterns when a crystal nucleates from solution.