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

Abstract

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.

Strong magnetic exchange coupling in Ln2 metallocenes attained by the trans-coordination of a tetrazinyl radical ligand

“…A decade ago, a unique example of a paramagnetic linker that bridged two Ln centers highlighted the importance of magnetic coupling to the SMM behaviour. 18 Consequently, research has focused on exploiting radical ligands [19][20][21] that can unlock the path towards attaining polynuclear lanthanide complexes that act as a single magnetic entity rather than a collection of magnetically uncoupled Ln III ions in a molecular unit. Many organic radicals such as nitrozyl, [22][23][24][25][26] semiquinonoids [27][28][29] and verdazyl 30 have promoted magnetic coupling between Ln III ions.…”