Bimetallic Synergy Enables Silole Insertion into THF and the Synthesis of Erbium Single-Molecule Magnets

Abstract

The potassium silole K₂ [SiC₄ -2,5-(SiMe₃ )₂ -3,4-Ph₂ ] reacts with [M(η⁸ -COT)(THF)₄ ][BPh₄ ] (M=Er, Y; COT=cyclo-octatetraenyl) in THF to give products that feature unprecedented insertion of the nucleophilic silicon centre into a carbon-oxygen bond of THF. The structure of the major product, [(μ-η⁸ : η⁸ -COT)M(μ-L¹ )K]_∞ (1_M ), consists of polymeric chains of sandwich complexes, where the spiro-bicyclic silapyran ligand [C₄ H₈ OSiC₄ (SiMe₃ )₂ Ph₂ ]^2- (L¹ ) coordinates to potassium via the oxygen. The minor product [(μ-η⁸ : η⁸ -COT)M(μ-L¹ )K(THF)]₂ (2_M ) features coordination of the silapyran to the rare-earth metal. In forming 1_M and 2_M , silole insertion into THF only occurs in the presence of potassium and the rare-earth metal, highlighting the importance of bimetallic synergy. The lower nucleophilicity of germanium(II) leads to contrasting reactivity of the potassium germole K₂ [GeC₄ -2,5-(SiMe₃ )₂ -3,4-Me₂ ] towards [M(η⁸ -COT)(THF)₄ ][BPh₄ ], with intact transfer of the germole occurring to give the coordination polymers [{η⁵ -GeC₄ (SiMe₃ )₂ Me₂ }M(η⁸ -COT)K]_∞ (3_M ). Despite the differences in reactivity induced by the group 14 heteroatom, the single-molecule magnet properties of 1_Er , 2_Er and 3_Er are similar, with thermally activated relaxation occurring via the first-excited Kramers doublet, subject to effective energy barriers of 122, 80 and 91 cm^-1 , respectively. Compound 1_Er is also analysed by high-frequency dynamic magnetic susceptibility measurements up to 10⁶ Hz.

Keywords: magnetic properties; mixed-metal synergy; potassium; rare earths; silicon.