Synthesis of rare-earth borolide sandwich complexes by multi-electron borole reduction

The limited redox chemistry of rare-earth (RE) elements means that multi-electron transfer reactions with RE compounds are a major synthetic challenge. Here, we demonstrate multi-electron reduction of a borole in two complementary RE reaction systems. In the first approach, the dimetallic lanthanum(iii) compound [(η ⁵-C₅ ⁱ Pr₅)La(µ:η ⁶:η ⁶-C₆H₆)La(η ⁵-C₅ ⁱ Pr₅)], containing a benzene tetra-anion ligand, reduces two equivalents of the ferrocenyl-borole FcBC₂(SiMe₃)₂C₂Me₂ (1) to give the double sandwich complex [{(C₅ ⁱ Pr₅)RE}FcBC₂(SiMe₃)₂C₂Me₂] (3-RE, RE = La), containing a 6π-aromatic borolide di-anion. In the second, complementary reaction type, combining KC₈ with 1 and the half-sandwich complexes [(C₅ ⁱ Pr₅)RE(BH₄)₂(THF)] (RE = Y, La, Dy) also gives 3-RE but with a broader range of rare-earth elements. In 3-RE, evidence for two-electron reduction of the borole is supported by comparisons with the magnesium borolide [Mg(FcBC₂(SiMe₃)₂C₂Me₂)(THF)₃] (2). In contrast, salt-metathesis approaches to 3-RE were ineffective, with alkali metal reduction of the borole proving to be unsuccessful, and 2 does not undergo salt metathesis with rare-earth precursors. The dysprosium analogue 3-Dy exhibits single-molecule magnet behaviour, with an effective barrier of 1222(8) cm^-1 and magnetic hysteresis up to 60 K.