A Supramolecular [pi-radical](2) Unit Acts as a Ligand: Coordination of La and Ce by a Pancake-Bonded Pair

Preorganization of a supramolecular unit composed of two pi-radicals generates a ligand with an unusual square-shaped tetradentate binding site, ideal for coordination to early lanthanide ions. The supramolecular unit is assembled via a pairwise interaction between two neutral radicals, creating a pancake-bonded diamagnetic [pi-radical](2) dimer. Synthesis and characterization of the new pi-radical bitaDTDA (4-(3'-benzisothiazolyl)-1,2,3,5-dithiadiazolyl) is reported. Unlike most DTDA-based ligands in which a supramolecular [pi-radical](2) pair dissociates to generate a bidentate, paramagnetic ligand, the pancake-bonded [bitaDTDA](2) unit apparently remains intact upon coordination to early lanthanide ions capable of supporting a 10-coordinate ligand sphere. The Ln(hfac)(3)[bitaDTDA](2) complexes (Ln = La, Ce; hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato) are readily prepared by manual mechanochemical methods, indicating a low energy barrier to coordination. Only minor conformational changes are evident in the supramolecular [bitaDTDA](2) ligand upon coordination. A structural comparison of the free [bitaDTDA](2) ligand and Ln(hfac)(3)[bitaDTDA](2) coordination complexes indicates favorable preorganization of the ligand.