Efficient and Tunable One-Dimensional Charge Transport in Layered Lanthanide Metal-Organic Frameworks

The most conductive MOFs are those made from organic ligands and square-planar transition metal ions connected into two-dimensional (2D) sheets that stack in a similar manner to the graphene sheets in graphite. Their electrical properties are thought to depend critically on the covalency of the metal-ligand bond. Much less importance is given to charge transport normal to the 2D sheets, not least because there is little synthetic opportunity to control their stacking sequence or distance. Here, we report exquisite control over the stacking sequence and distance in a series of materials made from 2D sheets of organic ligands connected in the third dimension by infinite lanthanide-oxygen chains. Contrary to transition metal MOFs, efficient charge transport leading to conductivity values of up to 0.5 S/cm in the lanthanide materials occurs primarily normal to the 2D sheets. We further show that the smaller lanthanides Yb³⁺ and Ho³⁺ enforce a shorter stacking distance of only 3.002(6) Å and afford consistently higher conductivity than the larger lanthanides Nd³⁺ and La³⁺, which distend the sheets up to 3.068(2) Å. This first systematic study of structure-function relationships in layered conductive MOFs is enabled by the high degree of crystallinity afforded by the relatively ionic lanthanide-ligand bonds. These results demonstrate that increasing the covalency of the metal-ligand bond is not the only viable path to achieve record conductivity in 2D MOFs, and that the interactions of the organic ligands alone can produce efficient charge transport pathways.