Cationic Methyl Complexes of the Rare-Earth Metals: An Experimental and Computational Study on Synthesis, Structure, and Reactivity

Synthesis, structure, and reactivity of two families of rare-earth metal complexes containing discrete methyl cations [LnMe_(2−x)(thf)_n]^(1+x)+ (x = 0, 1; thf = tetrahydrofuran) have been studied. As a synthetic equivalent for the elusive trimethyl complex [LnMe₃], lithium methylates of the approximate composition [Li₃LnMe₆(thf)_n] were prepared by treating rare-earth metal trichlorides [LnCl₃(thf)_n] with 6 equiv of methyllithium in diethyl ether. Heteronuclear complexes of the formula [Li₃Ln₂Me₉L_n] (Ln = Sc, Y, Tb; L = Et₂O, thf) were isolated by crystallization from diethyl ether. Single crystal X-ray diffraction studies revealed a heterometallic aggregate of composition [Li₃Ln₂Me₉(thf)_n(Et₂O)_m] with a [LiLn₂Me₉]²⁻ core (Ln = Sc, Y, Tb). When tris(tetramethylaluminate) [Ln(AlMe₄)₃] (Ln = Y, Lu) was reacted with less than 1 equiv of [NR₃H][BPh₄], the dimethyl cations [LnMe₂(thf)_n][BPh₄] were obtained. The coordination number as well as cis/trans isomer preference was studied by crystallographic and computational methods. Dicationic methyl complexes of the rare-earth metals of the formula [LnMe(thf)_n][BAr₄]₂ (Ln = Sc, Y, La−Nd, Sm, Gd−Lu; Ar = Ph, C₆H₄F-4) were synthesized, by protonolysis of either the ate complex [Li₃LnMe₆(thf)_n] (Ln = Sc, Y, Gd−Lu) or the tris(tetramethylaluminate) [Ln(AlMe₄)₃] (Ln = La−Nd, Sm, Dy, Gd) with ammonium borates [NR₃H][BAr₄] in thf. The number of coordinated thf ligands varied from n = 5 (Ln = Sc, Tm) to n = 6 (Ln = La, Y, Sm, Dy, Ho). The configuration of representative examples was determined by X-ray diffraction studies and confirmed by density-functional theory calculations. The highly polarized bonding between the methyl group and the rare-earth metal center results in the reactivity pattern dominated by the carbanionic character and the pronounced Lewis acidity: The dicationic methyl complex [YMe(thf)₆]²⁺ inserted benzophenone as an electrophile to give the alkoxy complex [Y(OCMePh₂)(thf)₅]²⁺. Nucleophilic addition of the soft anion X⁻ (X⁻ = I⁻, BH₄⁻) led to the monocationic methyl complexes [YMe(X)(thf)₅]⁺.