Neutral Inverse-Sandwich Rare-Earth Metal Complexes of the Benzene Tetraanion

Abstract

Rare-earth metal complexes of the parent benzene tetraanion and neutral inverse-sandwich rare-earth metal arene complexes have remained elusive. Here, we report the first neutral inverse-sandwich rare-earth metal complexes of the parent benzene tetraanion supported by a monoanionic β-diketiminate (BDI) ligand. Reduction of the trivalent rare-earth metal diiodide precursors (BDI)MI₂(THF) (BDI = HC(C(Me)N[C₆H₃-(3-pentyl)₂-2,6])₂; M = Y, 1-Y; M = Sm, 1-Sm) in benzene or para-xylene by potassium graphite yielded the neutral inverse-sandwich rare-earth metal arene complexes [(BDI)M(THF)_n]₂(μ-η⁶,η⁶-arene) (M = Y, Sm; arene = benzene, 2-M; arene = para-xylene, 3-M). Single crystal X-ray diffraction, spectroscopic and magnetic characterization, together with density functional theory (DFT) calculations confirm that these neutral rare-earth metal arene complexes possess an [M³⁺‒(arene)^4‒‒M³⁺] electronic structure with strong metal‒arene δ interactions. The arene exchange reactivity shows that 2-Sm has higher stability than 3-Sm. Furthermore, 2-Sm can behave as a four-electron reductant to reduce unsaturated organic substrates. Particularly, while the reaction of 2-Sm with 1,3,5,7-cyclooctatetraene (COT) yielded (BDI)Sm(η⁸-COT) (4-Sm), 2-Sm reacted with 1,4-diphenylbutadiyne to afford (BDI)Sm(η⁴-C₄Ph₂) (5-Sm), the first rare-earth metallacyclopentatriene complex.