A mononuclear 6-coordinate hs-Fe(ii)–radical complex exhibits single-molecule magnet behaviour.

Preorganization of a supramolecular unit composed of two π-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 [π-radical]2 dimer. Synthesis and characterization of the new π-radical bitaDTDA (4-(3′-benzisothiazolyl)-1,2,3,5-dithiadiazolyl) is reported. Unlike most DTDA-based ligands in which a supramolecular [π-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.