Flexible Reactivity of the Benzene Tetraanion in a Neutral Inverse-Sandwich Lanthanum Complex

Neutral inverse-sandwich lanthanum arene complexes with the parent benzene tetraanion are still limited. Here, we report the neutral inverse-sandwich lanthanum benzene complexes [(Cp^Ar5)La(THF)_n]₂(μ-η⁶:η⁶-C₆H₆) (Cp^Ar5 = η⁵-C₅Ar₅, Ar = ⁱPr₂-C₆H₃-3,5; n = 0, 2; n = 1, 2-THF) supported by a superbulky penta-arylcyclopentadienyl ligand. Complex 2 was isolated from the reduction of the half-sandwich lanthanum diiodide precursor [(Cp^Ar5)LaI₂(THF)₂] (1) in benzene by the K/KI reductant, yielding 69%. The reaction of complex 2 with 0.5 equiv of (HBBN)₂ enabled C-H bond functionalization of the benzene tetraanion, generating a novel borylated product [(Cp^Ar5)La]₂(μ-η⁶:η⁶-C₆H₅BBN) (3). NMR analyses, single-crystal X-ray diffraction, and UV-vis spectroscopic studies demonstrated that complexes 2, 2-THF, and 3 share a [La³⁺-(arene)^4--La³⁺] electronic structure, which was further confirmed by density functional theory (DFT) calculations. Moreover, treatment of complex 2 with Me₃SiN₃ afforded a product [(Cp^Ar5)La{N(SiMe₃)₂}]₂(μ-N₃)₂ (4) via four-electron reduction and subsequent Si-N bond activation. Additionally, the dimeric peroxo complex [(Cp^Ar5)La(THF)]₂(μ-η²:η²-O₂)₂ (5) was detected in the reaction of 2 with O₂. The redox reactivity of 2 shows its great potential in the multielectron reduction of unsaturated substrates, functioning as a La(I) synthon.