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 [(CpAr5)­La­(THF)n]2(μ-η6:η6-C6H6) (CpAr5 = 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 [(CpAr5)­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 [(CpAr5)­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 [(CpAr5)­La­{N­(SiMe3)2}]2(μ-N3)2 (4) via four-electron reduction and subsequent Si–N bond activation. Additionally, the dimeric peroxo complex [(CpAr5)­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.