A series of F-Element chelators; diaza crown ethers functionalized with catecholate binding substituents

Abstract

Reported here is the preparation of azacrown ethers functionalized with catechol groups. The synthetic approach was (1st) novel in that it made use of the Mannich reaction and (2nd) valuable in that it provided an improved synthesis (in terms of practical deployment) of the known N,N′-bis(2,3-dihydroxybenzyl)-4,13-diaza-18-crown-6, H4ChaCha. Moreover, it demonstrated potential application of the synthetic method for accommodating a wide range of catecholate functionalities by using the synthetic strategy to prepare N,N′-bis(2,3-dihydroxy-5-tert-butylbenzyl)-4,13-diaza-18-crown-6 (H4tBu2ChaCha) for the first time. These H4ChaCha and H4tBu2ChaCha macrocycles offer exciting opportunity to expand redox chemistry for the f-elements. As “proof-of-principle,” we isolated the unusual tetrameric cluster [La2(tBuChaCha)2]2 from reactions between H4tBu2ChaCha and La[N(SiMe3)2]3. Characterization of [La2(tBuChaCha)2]2 by elemental analysis, single crystal X-ray diffraction, IR, and UV–vis–NIR spectroscopy suggested that the complex represented a rare example of an f-element semiquinone. It further demonstrated that the combination of La3+ and H4tBu2ChaCha provided access to one-electron oxidation chemistry within redox potential windows that were amenable to mild reaction conditions.