Synthesis, Structures and Reactivity of Diazabutadiene

Ligated Rare-Earth Radical Complexes Bearing Adaptable Auxiliary Ligands

Haihan Yan, Junnian Wei, and Wen-Xiong Zhang*

Organometallics 2021, 40, 3245-3252

Bis(diazabutadiene) rare-earth diradical complexes [(^MesDAD)₂REL] (1-4) were synthesized, of which the auxiliary ligand L could be borohydride, alkyl, alkoxide, or aminoxide ligands. The synthetic approach was utilized by one-pot reaction, including reduction of DAD and salt metathesis of ligands. Single crystal X-ray diffraction results showed that complexes 1-4 maintained the similar distorted square-pyramidal coordination spheres, and the DAD ligands were all traditional DAD radical anions. When the auxiliary ligand was changed to a less bulky N-pyrrolyl ligand, partial ligand redistribution product [(^MesDAD)Lu(NC₄H₄)₂]₂ (5) was found. The redox reactivity of DAD ligands in rare-earth diradical complexes was also studied. The single-electron reduction of 1 yielded [Lu(^MesDAD^·-)(^MesDAD^2-)(THF)](6), and the disproportionation reaction of [(^MesDAD)₂ScCl(THF)] in TMEDA gave 1:1 product of ^MesDAD and [(^MesDAD)ScCl(TMEDA)] (7). Furthermore, the reaction of ^MesDAD with SmI₂ resulted in an unexpected C–C coupling product 8, which is a new reaction mode for DAD radicals.

This work is highlighted by the front cover art. For most of the rare-earth metal complexes with radical ligands, the adaptability of auxiliary ligands is limited. However, with diazabutadienes acting as radical-anionic ligands, the auxiliary ligands of rare-earth metal diradical complexes could be expanded from halides, amides or boroxides to borohydride, alkyl, alkoxide, or aminoxide ligands. Different reactivity of those radical species, such as redox or ligand redistribution reactions, was also explored. The wooden cart has a similar structure to the rare-earth metal diradical complexes, on which the controlling levers of the cart represent two radicals. As an analogy to the expansion of this family of rare-earth metal diradical complexes, the wooden cart is accelerating on a highway towards a gold mine.