Rare‐earth cyclobutadienyl sandwich complexes: Synthesis, structure and dynamic magnetic properties

The potassium cyclobutadienyl [K2{η4‐C4(SiMe3)4}] (1) reacts with MCl3(THF)3.5 (M = Y, Dy) to give the first rare‐earth cyclobutadienyl complexes, i.e. the complex anions [M{η4‐C4(SiMe3)4}{η4‐C4(SiMe3)3‐κ‐(CH2SiMe2}]2–, (2M), as their dipotassium salts. The tuck‐in alkyl ligand in 2M is thought to form through deprotonation of the sandwich complexes [M{η4‐C4(SiMe3)4}2]– by 1. Complex 2Dy is a single‐molecule magnet, but with prominent quantum tunnelling. An anisotropy barrier of 323(22) cm–1 was determined for 2Dy in an applied field of 1 kOe, and S‐shaped magnetic hysteresis loops were observed up to 7 K