The miracle of self-assembly: a journey of the {Ln₆F₈} core in the world of lanthanides

This work presents a unique example of the self-assembly of {Ln₆F₈} clusters composed of Ln³⁺ cations and F^- and CF₃COO^- anions. The crystal structures of new fluorotrifluoroacetate compounds with the general formula <b>A</b>_<b>2</b><b>[Ln</b>_<b>6</b><b>(μ</b>_<b>3</b><b>-F)</b>_<b>8</b><b>(tfa)</b>_<b>12</b><b>L</b>_<b>6</b><b>]</b>, where A = Na and Ln = Pr(<b>I</b>), Eu (IIa), Tb(iii), Dy(iv) and Tm(v); A = Na and H₃O⁺ and Ln = Eu (IIb); or A = K and Ln = Yb(vi); tfa = CF₃COO^- (trifluoroacetate anion); L = Htfa (CF₃COOH, trifluoroacetic acid) and H₂O, determined by the single-crystal XRD method revealed that they were constructed from the complex anion <b>[Ln</b>_<b>6</b><b>(μ</b>_<b>3</b><b>-F)</b>_<b>8</b><b>(tfa)</b>_<b>12</b><b>L</b>_<b>6</b><b>]</b>^<b>2-</b> and alkali metal (or alkali metal and hydroxonium) cations and included solvate molecules in some cases. The core of the anion presented a rhombododecahedral unit of six lanthanide atoms linked by bridging fluoride and trifluoroacetate anions. The nature of the counterion led to the formation of compounds with different dimensions: from 1D for potassium cations and 2D for sodium cations to 3D for sodium and hydroxonium cations. The {Ln₆F₈} clusters were obtained for a wide range of lanthanides, with Pr and Yb being the end members; thus, the existence of heterometallic compounds was presumed. At a temperature of 100 °C, the new compounds started to release solvent molecules, whereas the <b>[Ln</b>_<b>6</b><b>(μ</b>_<b>3</b><b>-F)</b>_<b>8</b><b>(tfa)</b>_<b>12</b><b>]</b> core was stable up to 270-300 °C according to the TG-MS data. Substances IIb, III and IV revealed metal-centered luminescence and could be used for the development of new optoelectronic materials.