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.