Functionalized Tris(anilido)triazacyclononanes as Hexadentate Ligands for the Encapsulation of U(III), U(IV) and La(III) Cations

Abstract

Tripodal multidentate ligands have become increasingly popular in f-element chemistry for stabilizing unusual bonding motifs and supporting small molecule activation processes. The steric and electronic effects of ligand donor atom substituents have proved crucial in both of these applications. In this study we functionalized the previously reported tris-anilide ligand {tacn(SiMe2NPh)3} (tacn = 1,3,7-triazacyclononane) to incorporate substituted aromatic rings, with the aim of modifying f-element complex solubility and ligand steric effects. We report the synthesis of two proligands, {tacn(SiMe2NHAr)3} (Ar = C6H3Me2-3,5 or C6H4Me-4), and their respective group 1 transfer agents—{tacn(SiMe2NKAr)3}, M(III) complexes [M{tacn(SiMe2NAr)3}] for M = La and U, and U(IV) complexes [M{tacn(SiMe2NAr)3}(Cl)]. These compounds were characterized by multinuclear NMR and FTIR spectroscopy and elemental analysis. The paramagnetic uranium complexes were also characterized by solid state magnetic measurements and UV/Vis/NIR spectroscopy. U(III) complexes were additionally studied by EPR spectroscopy. The solid state structures of all f-block complexes were authenticated by single-crystal X-ray diffraction (XRD), together with a minor byproduct [U{tacn(SiMe2NC6H4Me-4)3}(I)]. Comparisons of the characterization data of our f-element complexes with similar literature examples containing the {tacn(SiMe2NPh)3} ligand set showed minor changes in physicochemical properties resulting from the different aromatic ring substitution patterns we investigated.